摘要:

FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO. 89 1990 Structure of Sufaces and Intefaces as studied using Synchrotron Radiation THE FARADAY DIVISION THE ROYAL SOCIETY OF CHEMISTRY LONDONOrganising Committee Professor J. N. Sherwood (Chairman) Mrs Y. A. Fish Professor D. A. King Dr G. C . King Dr C. Norris Dr R. Oldman Dr C. Thornton ISBN: 0-85186-997- 1 ISSN: 0301-7249 Printed in Great Britain by J. W. Arrowsmith Ltd, BristolA GENERAL DISCUSSION ON S'truct tire of'Stirf&ces and Interfaces us st tidied tising Sinchrotron Radiation 4th, 5th and 6th April, 1990 A General Discussion on Structure of Surfaces and Interfaces as studied using Syn- chrotron Radiation was held at the University of Manchester on 4th, 5th and 6th April, 1990. The President of the Faraday Division, Professor R.H. Ottewill, F.R.S., was in the chair: about 90 members of the Faraday Division and visitors from abroad attended the meeting. Among the visitors were: Prof. 1. Abbati, Italy Mr J-L. Allain, France Dr H. Bach, West Germany Dr N. Barrett, France Prof. A. M. Bradshaw, West Germany Dr G. Comelli, Italy Dr G. Contini, Italy Prof. P. T. Dawson, Canada Dr A. Fontaine, France Dr S. M. Heald, USA Dr J. L. Hogan, USA Dr J. Jupille, France Dr K. Kern, West Germany Prof. D. C. Koningsberger, The Netherlands Mr P. Lambooy, The Netherlands Prof. K. A. R. Mitchell, Canada Dr G. Paolucci, Italy Prof. P. Pershan, USA Dr K. C. Prince, Italy Prof. S. A. Rice, USA Dr A. Robinson, West Germany Dr I. K. Robinson, USA Dr K. M. Robinson, USA Prof. R. Rosei, Italy Dr R.G. van Silfhout, The Netherlands Prof. J. V. Smith, USA Prof. J. F. van der Veen, The NetherlandsA GENERAL DISCUSSION ON S'truct tire of'Stirf&ces and Interfaces us st tidied tising Sinchrotron Radiation 4th, 5th and 6th April, 1990 A General Discussion on Structure of Surfaces and Interfaces as studied using Syn- chrotron Radiation was held at the University of Manchester on 4th, 5th and 6th April, 1990. The President of the Faraday Division, Professor R. H. Ottewill, F.R.S., was in the chair: about 90 members of the Faraday Division and visitors from abroad attended the meeting. Among the visitors were: Prof. 1. Abbati, Italy Mr J-L. Allain, France Dr H. Bach, West Germany Dr N. Barrett, France Prof. A. M. Bradshaw, West Germany Dr G. Comelli, Italy Dr G.Contini, Italy Prof. P. T. Dawson, Canada Dr A. Fontaine, France Dr S. M. Heald, USA Dr J. L. Hogan, USA Dr J. Jupille, France Dr K. Kern, West Germany Prof. D. C. Koningsberger, The Netherlands Mr P. Lambooy, The Netherlands Prof. K. A. R. Mitchell, Canada Dr G. Paolucci, Italy Prof. P. Pershan, USA Dr K. C. Prince, Italy Prof. S. A. Rice, USA Dr A. Robinson, West Germany Dr I. K. Robinson, USA Dr K. M. Robinson, USA Prof. R. Rosei, Italy Dr R. G. van Silfhout, The Netherlands Prof. J. V. Smith, USA Prof. J. F. van der Veen, The NetherlandsCONTENTS 1 21 31 41 51 65 77 91 107 119 137 143 159 169 181 191 20 1 21 1 23 1 247 259 275 29 1 Introductory Lecture. Structural Investigations of Adsorbed Layers using Syn- chrotron Radiation A. M. Bradshaw Glancing Angle XAFS and X-Ray Reflectivity Studies of Transition- metal/Aluminium Interfaces In Situ Structural Studies of the Passive Film on Iron and Iron/Chromium Alloys using X-Ray Absorption Spectroscopy M.Kerkar, J. Robinson and A. J. Forty Electrochemical Inclusion of Copper and Iron Species in a Conducting Polymer observed In Situ using Time-resolved X-Ray Absorption Spectroscopy D. Guay, G. Tourillon and A. Fontaine X-Ray Absorption Spectroscopy under Conditions of Total External Reflection: Application to the Structural Characterisation of the Cu/GaAs( 100) Interface S. Pizzini, K. J. Roberts, G. N. Greaves, N. T. Barrett, I. Dring and R. J. Oldman General Discussion Sulphur-induced Structural Chemistry of Oxide Surfaces C. A. Muryn, D. Purdie, P.Hardman, A. L. Johnson, N. S. Prakash, G. N. Raiker, G. Thornton and D. S-L. Law I n Situ Studies of Supported Rhodium Catalysts P. Johnston, R. W. Joyner, P. D. A. Pudney, E. S. Shpiro and B. P. Williams Characterisation of Oxide-supported Alkene Conversion Catalysts using X-Ray Absorption Spectroscopy Structural Studies of High-area Zeolitic Adsorbents and Catalysts by a Combina- tion of High-resolution X-Ray Powder Diffraction and X-Ray Absorption Spec- troscopy E. Dooryhee, G. N. Greaves, A. T. Steel, R. P. Townsend, s. W. Carr, J. M. Thomas and C. R. A. Catlow EXAFS Study of the Influence of Hydrogen Desorption and Oxygen Adsorption on the Structural Properties of Small Indium Particles Supported on A1203 F. W. H. Kampers and D. C. Koningsberger General Discussion Structure and Roughening of the Pt( 110) Surface I.K. Robinson, E. Vlieg and K. Kern Structure of the Ge( 11 1)-c(2 x 8) Surface as determined from Scattered X-Ray Intensities along Crystal Truncation Rods R. G. van Silfhout, J. F. van der Veen, C. Norris and J. E. Macdonald X-Ray Scattering from Surfaces and Interfaces R. A. Cowley and C. A. Lucas X-Ray Scattering from Semiconductor Interfaces J. E. Macdonald General Discussion Langmuir Monolayers: Structures and Phase Transitions Z-h. Cai and S. A. Rice Structure of Surface and Interfaces as studied using Synchrotron Radiation: Liquid Surfaces P. S. Pershan Genera 1 Discuss ion Core-level Shift Spectroscopy on Tungsten Surfaces: Overlayer and Underlayer Adsorption Characterization of Metal/Organic Molecule and Metal/ Polymer Interfaces by NEXAFS Spectroscopy G. Tourillon, D. Guay, A. Fontaine, R. Garrett and G. P. Williams Structure of the Surface Methoxy Species on Cu{ 11 1) D. E. Ricken, J. Sorners, A. W. Robinson and A. M. Bradshaw s. M. Heald, E. V. Barrera and H. Chen J. Evans, J. T. Gauntlett and J. F. W. Mosselmans G. P. Derby and D. A. King301 Normal-incidence Standing X-Ray Wavefield Adsorption and SEXAFS Studies of Adsorption Structures on Cu and Ni Surfaces N. P. Prince, M. J. Ashwin, D. P. Woodruff, N. K. Singh, W. Walter and R. G. Jones Photoelectron Diffraction Study of 0, N and C Adsorption Structures on Ni( 100) and Cu(ll0) A. L. D. Kilcoyne, D. P. Woodruff, A. Robinson, Th. Lindner, J. Somers, D. E. Ricken and A. M. Bradshaw Study of Xenon Layers on a Cu( 11 1) Surface J. Jupille, J-J. Ehrhardt, D. Fargues and A. Cassuto 31 1 323 329 General Discussion 341 Index of Names

ISSN:0301-7249    

DOI:10.1039/DC99089FP001

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Furaday Discuss. Chern. SOC., 1990, 89, 21-30 Glancing Angle XAFS and X-Ray Reflectivity Studies of Transitionmetal/ Aluminium Interfaces Steve M. Heald, Enrique V. Barrera and Huaiyu ChenT Brookhaven National Laboratory, Upton, N Y 11973, USA The combination of glancing angle X-ray absorption fine structure (XAFS) and X-ray reflectivity measurements provide detailed structural information about interfaces. Results from transition-metal/Al interfaces are used to illustrate the capabilities of the techniques. Reflectivity measurements are used to determine the thickness and density profile of the films, along with the amount of interfacial or surface roughness. The XAFS measurements provide information about the local bonding at the interface, and are well suited for studying the interfacial chemistry.Of interest is the role of impurities such as oxygen in modifying interface structure and thermal stability. For Cu/AI, Ni/AI and Cr/AI a small amount of interfacial oxygen is found to affect strongly both the initial interface structure, and the response of the interface t o thermal annealing. For NiiAl it is also found that interfacial oxygen afiects primarily the interface reaction, while oxygen within the A1 layer suppresses the grain boundary reaction channel. The study of surfaces and interfaces by X-rays is greatly facilitated by using glancing incidence angles.” At small angles of incidence, X-rays can undergo total external reflection, resulting in an extremely small penetration depth. This small penetration greatly reduces the bulk contribution to the signal, allowing the small interface or surface signal to be studied in detail.Such an approach, combined with synchrotron radiation, has allowed detailed X-ray studies of a variety of surface reconstructions and ad layer^.^-' In this paper we report the application of glancing angle X-rays and X-ray absorption fine structure measurements (XAFS) to study buried interfaces. Examples are given f o r transition-metal/Al interfaces. X-ray reflectivity measurements are sensitive to the density profile with depth. The method can provide nanometer depth resolution and is sensitive to elemental concentra- tions as low as 1%.’ From the density profile it is straightforward to calculate the penetration of the X-rays to determine the region of the sample being probed by XAFS.The density profile can also be used as a basis for correcting the anomalous dispersion distortions present in the XAFS as a consequence of the glancing angle geometry.’ Such a correction is necessary for detailed quantitative analysis, although, as shown in this paper, qualitative examination of the raw spectra can provide much information. pi-eviously these techniques were used to make a detailed study of the influence of interfacial oxygen on the interface structure and thermal stability of AI/Cu interfaces.”’ Small amounts cjf oxygen were found to have a strong effect on both properties. This work has been extended to Al/Ni and AI/Cr interfaces, where again oxygen is found to have a strong influence on interface structure and stability.In particular, for Ni/AI we can distinguish between the effects of 0 impurity in the layer and 0 impurity at the i n t e r fa c e . +Current address: Physics Dept., Illinois Inst. of Tech., Chicago, IL 60616, USA.22 Glancing Angle XAFS and X-Ray Rejlectivity Studies - 1 g O L : Interface + c t P? PI V l 2 O O O L ; , _1.- 0 ZOO LOO 600 800 depth/ A Fig. 1. Steps in the analysis of reflectivity data as illustrated for a Cu/AI bilayer annealed at 120" C. ( a ) Reflectivity data (points) and a least-square fit (line). V i . PI and P2 refer to angles of the first minimum, first peak and second peak, respectively. ( b ) Cu density profile derived from the fit in ( a ) . ( c ) Calculated contribution to the Cu fluorescence signal from the three angles shown in ( a ) .Background The index of refraction for X-rays is slightly less than one, leading to total external reflection a t small incident angles below the critical angle, 8,. This leads to a very small penetration depth ( a few nm), and it was Parratt' who demonstrated the potential for using glancing angle X-rays to probe surface structure. For bilayer systems the X-rays can be made to reflect from the interface if the overlayer is less dense than the substrate. Interface reflection occurs when the incident angle is greater than 8,- for the surface, but less than 8, for the substrate. Measurements of the substrate absorption can then be used to determine the bonding environment of the atoms within the penetration depth of the interface. The penetration depth can be varied by varying the incident angle to look at the depth dependence of the bonding.These ideas were beautifully verified by extensive studies of the interfaces in AI/Cu bilayers. 273.7.8 Some examples drawn from this work are shown in fig. 1 and 2 to demonstrate the basic experimental procedure. First. the reflectivity is measured asS. M. Heald, E. V. Barrera and H. Chen 23 2 4 6 8 10 12 k / A - ' Fig. 2. Extended fine structure data for the three angles shown in fig. l ( a ) compared to CuAI, and C u standards; ( a 1 CuAI,, ( h ) V1, ( c ' ) P1, ( d ) P 2 , ( e l Cu. The standards have been multiplied by 1/2 to approximate the anomalous dispersion distortion of the glancing angle data. shown in fig. l ( a ) , usually at X-ray energies above and below the absorption edge of the substrate (9.2 and 8.6 keVf for Cu ).From this data a density profile can be determined by fitting. An example of the quality of fit is also shown in fig. l ( a ) . Fig. l t h ) shows the Cu density profile derived from fig. l ( a ) . The derived density profile can then be used to calculate the contribution to the XAFS from the Cu atoms at various depths, as shown in fig. l ( c ) , for the three angles indicated in fig. l ( a ) . The fluorescence contribution is calculated by multiplying the Cu concentration by the depth-dependent electric field intensity. The oscillatory behaviour comes from interference in the electric field due to the surface and interface contributions to the reflectivity. The reflectivity oscillations in fig.l ( a ) come from the same source. Different parts of the sample are probed for different angles. This is confirmed by the extended region of the XAFS (EXAFS) spectra shown in fig. 2. At small angles the spectra are similar to those of CuAI,, indicating substantial Cu-AI bonding, while for larger angles more Cu-Cu bonding from the substrate is evident. These measurements allow us to deduce the amount of chemical interaction at the interface and the types of bonds present even for small amounts of reaction. Note, the interface width derived from reflectivity can also be due to a rough interface with little intermixing, and from the EXAFS measurements it is simple to distinguish between the two cases. In the single-scattering approximation the EXAFS spectrum can be written as" .;-I e V = 1.60218~ 10 "'J.24 Glancing Angle X A FS and X-Ray Reflectivity Studies where p is the total absorption and po is the smooth background characteristic of an isolated atom. The sum is over the neighbouring atomic shells, where N is the number of atoms in a shell, R is their average distance, and CT' is the mean-square displacement of the N atoms about R.A ( k ) and @ ( k ) depend on the types of atoms involved, and are usually calibrated using standard compounds such as Cu for Cu-Cu bonding and CuAI, for Cu-AI bonding. Each nearest neighbour distance gives rise to a distinct frequency of oscillation and using Fourier analysis along with fitting techniques it is possible to extract the structural parameters N, R and CT for the first few nearest neighbour shells.Since EXAFS is a local probe, structural information can be gained even if no long-range order exists. The other component of the XAFS is the near-edge structure which is more sensitive to the chemical state of the absorbing atom. In the glancing angle mode, the absorption coefficient, p, is not directly measured. Either the reflectivity or the fluorescence signal can be measured, and they must be corrected to obtain the absorption. In the present work the fluorescence signal was found to be of higher quality. For very thin layers or dilute systems there is a direct proportionality to the absorption, but in other cases anomalous dispersion distortions become important.' However, once a model for the reflectivity has been obtained it is a simple matter to make the corrections.The distortion affects primarily the amplitude of the XAFS, and for qualitative comparisons as in fig. 2 corrections are not necessary. Experiment a1 The samples were prepared in an ion-pumped UHV chamber with a base pressure of 5 x 1OP"' Torr.? Cu and Ni were evaporated using alumina crucibles and Cr was evapor- ated from a W basket. Al was deposited two ways. For the Cu/Al samples, W boats were used while for the Ni/AI and Cr/AI samples an alumina crucible was used. Subsequent Auger analysis showed that the alumina deposited aluminium had a small (2-3%) amount of 0 contamination and second set of Ni/AI samples was prepared using a W boat for the aluminium. This resulted in clean samples and allowed us to make a comparison of the two cases.Float glass substrates (2.5 x 5 cm) were used, and the substrate holder was cooled to keep the samples at or below room temperature during deposition. In each case two samples were made: one with a clean interface and one in which the metal surface was exposed to 0, gas prior to Al deposition. For C u the exposure was 1000 L$ and for Ni and Cr the exposures were 600 L. The X-ray measurements were made at the Cornell High Energy Synchrotron Source (CHESS) beamline C-2, and the National Synchrotron Light Source (NSLS) beamline X-1 1A. The samples were mounted on a precision goniometer and gas ionization chambers were used to measure the incoming, reflected and fluorescence X-rays. The entrance slit was 50-100 p m which resulted in a flux on the sample of ca.10' photons s I . This resulted in a fluorescence signal of ca. lo7 which allowed high quality XAFS data to be obtained in ca. 15 min. Measurements were made at room temperature on as prepared and annealed samples. The samples were annealed in air for the Cu/Al samples at temperatures ranging from 65-2OO0C, and in a getter-purified Ar atmosphere for temperatures ranging from 230- 340 "C for the Ni/AI samples and 240-390 "C for Cr/Al. The same sample was annealed sequentially at successively higher temperatures. The annealing time was 5 min at temperature with ca. 10 min needed to reach temperature. Above ca. 350 "C the float glass substrates had a tendency to break into two pieces, but measurements could be made on one of the pieces and there seemed to be no degradation of the substrate flatness.; 1 Torr i 1 L - 1 0 "Torrs '. 101 325/760 Pa.S. M. Heald, E. V. Barrera and H. Chen 25 2.5 2.0 x Y .- > .- ti 1.0 2 5 0.5 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 angle/" Fig. 3. X-Ray reflectivity data obtained at 8.633 keV for: (a) a Ni/Al bilayer with a clean interface and ( b ) a Ni/AI bilayer with an interface exposed t o 600 L of 02. The A1 layer for these samples had a small amount of 0 contamination as described in the text. 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 angle/" Fig. 4. X-Ray reflectivity data obtained at 6.189 keV for: ( a ) a Cr/Al bilayer with a clean interface and ( b ) a Cr/AI bilayer with an interface exposed to 600 L of 0:. The Al layer for these samples had a small amount of 0 contamination as described in the text.26 Glancing Angle XAFS and X - Ray Reflectivity Studies 2 0 1 5 1 0 0 5 0 0 0 200 400 600 800 0 200 400 600 800 depth/A Fig.5. Ni density profiles derived from fitting the data in fig. 3 aiong with the corresponding reflectivity data obtained at 8.033 keV. Reflectivity Data Fig. 3 and 4 present some of the reflectivity data obtained from the Ni/Al and Cr/AI interfaces. The Ni/Al data in this case was taken on the sample which showed a small amount of 0 contamination in the Al layer. Detailed results for Cu/AI can be found in a previous publication.3 The figures show the data obtained for energies above the Ni and Cr K-edges, respectively, which is the most sensitive to changes in the interfacial mixing. It is clear that interfacial 0 has a strong effect on the annealing behaviour in both systems.For Ni/AI the reaction for a clean interface begins at ca. 250 "C, while for the oxygen exposed interface the reaction begins near 310°C. For Cr/Al the corresponding temperatutes are found to be 330 and 420°C. Fig. 5 shows the Ni concentration profiles obtained from fitting the Ni/Al data in fig. 3. For the clean interface sample the reaction proceeds from the interface, generating a well defined intermediate layer with a density consistent with NiAI3 formation. Other workers''-" have also found that NiAI, is the first compound to form. Only at 310 "C does significant Ni reach the near-surface region (the fall off at the surface is due to an air-formed A1203 layer). This is likely to be due to compound growth along the Al grain boundaries.For the oxygen-exposed interface the intermediate layer is not as well defined, although the Ni concentration near the surface begins increasing at temperatures similar to the clean sample. It appears that the interfacial 0 acts to block the interface reaction, but the reaction in the grain boundaries takes place at similar temperatures for both samples. As mentioned both of these samples had small amounts of 0 in the A1 layer. To check whether this was affecting the annealing behaviour, a second sample was measured which had a clean Al layer and a clean interface. The concentration profiles for this sample are shown in fig. 6. Again the reaction begins at 25OoC, and a well defined NiAI, layer is formed. However, there is also more Ni reaching the surface. This indicates that the grain boundary reaction was inhibited for the first sample.Thus, theS. M. Heald, E. V. Barrera and H. Chen 27 2 C 0 m .C 4-8 L != e, C 4-8 8 .- z 1 .o 0.5 0.0 0 2 00 400 600 800 depth/ A Fig. 6. Ni density profles obtained for a Ni/AI bilayer with a clean interface and clean A1 layer. effect of 0 in the A1 layer is localized to the grain boundaries, while the 0 placed at the interface affects only the interfacial reaction. Note that for the contaminated sample, the 0-induced reaction barrier is overcome at approximately the same temperature (310 "C) for both the interface and grain boundaries. Detailed analysis has not yet been carried out on the Cr/Al reflectivity data. The analysis is complicated by the high degree of roughness of the surface and interface.This can be seen especially in the data for the clean sample where it is obvious that the reflectivity oscillatio!s are washed out. Model calculations indicated that the r.m.s. roughness is ca. 150 A. This is quite smooth in an optical sense (i.e. the samples still had a mirror-like appearance), but is large when compared to the Al layer thickness of ca. 500 A. By comparison the Ni/AI and Cu/Al samples had r.m.s. roughnesses in the range of 20-30 A before annealing, and the float glass substrates have a roughness in the range of 5-10 8,. The roughness for C r seems to be due to the deposition process, since as shown in the next section, the XAFS results indicate that little mixing has occurred.It is interesting that the 0-exposed sample seems to have a somewhat smoother interface, although comparison with calculations reveals that it is still substantially rougher than for Cu/AI or Ni/AI. XAFS Results Some examples of the extended fine structure are shown in fig. 7 and 8 for Ni/AI and Cr/Al, respectively. The Ni/AI results demonstrate the sensitivity of the EXAFS measurements. Even though the reflectivity showed a similar interface width for the two samples, it requires the EXAFS to observe the room temperature reaction occurring at the interface. The sample with impure Ai shows no reaction to within our sensitivity of 5-10 8, for the reacted layer thickness. Further analysis is required to quantify the amount of reaction in the clean sample, but from comparison with previous Cu/Al28 I I I I 1 1 1 1 I l l ! Glancing Angle XAFS and X - Ray ReJZectivity Studies I I I I I l l 1 1 I l l 1 I I I I 1 1 1 1 I I 1 I I I 1 1 0 - 0 -1 10 12 2 4 6 k 1 A - I 8 Fig.7. EXAFS data for the Ni/AI bilayers with a clean interface, ( a ) Ni; ( b ) C1; ( c ) C2; ( d ) C2, 250 "C; (e) NiAI,. C1 has an 0-contaminated A1 layer and C2 has a clean Al layer (see text). Ni and NiAI, are standard materials. 1 Fig. 8. EXAFS data for the Cr/Al bilayer with a clean interface unannealed ( h ) and after a 330 "C anneal ( c . ) compared with Cr ( a ) and Cr,AI,, ( d ) .S. M. Heald, E. V. Barrera and H. Chen 29 analysis we estimate the reacted region to be 20-30 A thick. Similar amounts of reaction have been observed by photoemission in UHV-prepared interfaces." For Cr/ Al no reaction is observed. However, for this case a second sample with clean Al was not prepared, and the same suppression of the spontaneous interface reaction may be occurring.The reacted samples give spectra similar to the expected reaction products NiAI, and Cr2AIl3 . 1 4 1 For Cu/Al the corresponding compound is CuAI,, and detailed quantita- tive analysis for the reacted spectra gave results which were similar, but not identical to the CuAI, spectrum. From the angle dependence of the spectra there were indications of additional Cu-rich phases (most likely CuAI) near the Cu layer. Detailed analysis is continuing to see if the same effect is occurring for the Ni and Cr cases. Discussion and Conclusions The influence of impurities on the Ni/AI reaction has been studied by Zhao et al." for large-grained Al substrates, and by Ma e f al." for thin-film couples.For clean material they reported planar NiAI3 growth. For both a contaminated interface (6 x 10' L exposure on the Al surface) and a contaminated Al layer (deposited at 8 x lo-' Torr), the reaction rate was reduced and a rough reaction front was created. These results seem to contradict our findings somewhat. In particular, we found planar growth only for a contaminated Al film. However, the previous measurements were based on Rutherford backscattering (RBS) analysis, which does not have the same depth resolution as th? glancing angle technique. Typically, the reaction layers were much thicker (> 1000 A). Examination of the spectra indicate an interface width of several hundred Angstroms which is not inconsistent with the concentration tails observed in our data.Also, the rough interfaces reported in contaminated samples were found for an annealing temperature of 324 "C, a temperature at which we also observe growth along the grain boundaries. Thus, a detailed comparison of our results with past work does not reveal any inconsistencies. Previous work on clean samples found an initial reaction at 250 "C for the thin film couples," and 330°C for the large-grained Al substrates. However, as pointed out in ref." the cleaning procedures used for the large-grained material may have left a small amount of contamination. In the present work we have shown that very small amounts of interface impurities can inhibit reaction.For the case of Cu, the 1000 L exposure has been shown to give 1/2 monolayer coverage on single-crystal surfaces." On polycrys- talline surfaces the coverage could be greater, but probably not substantially so. Auger analysis is planned to estimate the amount of 0 deposited for exposures employed. These will be made on the metal surface prior to Al deposition. Detection of such small amounts of impurities is difficult for buried interfaces, and could have been missed in some previous studies. Indeed, Auger sputter profiling of the 0-exposed Cu/Al did not reveal any excess 0 at the interface. For Ni/AI the addition of 0 impurities to the Al film did not affect the temperature dependence of the interface reaction, but only the grain boundary diffusion path.This allowed the formation of a well defined NiAI, layer, without extensive Ni diffusion ahead of the reaction front. The impurity 0 is apparently strongly bound up and cannot diffuse back to the interface to inhibit the reaction. This is not unexpected since from the phase diagram 0 in Al should be in the form of Al2O3. This has been observed in contaminated Al thin films,lh and perhaps it is the formation of A1,03 in the grain boundaries which is acting as a diffusion barrier. The EXAFS measurements did reveal that the 0 impurities in the Al inhibit the initial interface reaction. Since the annealing behaviour is essentially unchanged, this implies that the amount of 0 necessary to ; In some references this compound i s rel'erred to as CrAl,30 Glancing Angle XAFS and X - Ray Reflectivity Studies quench the initial reaction is much smaller than that necessary to affect subsequent annealing reactions. This paper has demonstrated some of the features of the glancing angle X-ray techniques for studying buried interfaces.Both chemical and compositional information can be obtained with nanometer resolution. The methods seem especially well suited for studying the initial stage of interface reaction, where the reacted layer is thin and may not have a well defined crystalline structure. In many respects the methods complement the traditional techniques of RBS and TEM. RBS is excellent for following the later stages of reaction in which the thick reacted layers would be more difficult to probe by the glancing techniques. TEM sample preparation is difficult, but for cases such as epitaxial films on semiconductors, atomic-scale resolution can be obtained.However, for systems with more disorder, the results from TEM may not be so clear cut. Also, for cases such as Cu/Al we have found that the sample preparation necessary for TEM can cause enhanced reaction at the interface, obscuring the original structure. In contrast, the glancing angle techniques require little sample preparation aside from the requirement of a flat substrate. In many cases this can be achieved by deposition onto flat substrates, although as seen for the Cr/Al case, work is sometimes needed to establish the proper deposition conditions. We would like to thank J. K. D. Jayanetti and M. W. Ruckman for their help in making the measurements. This work is supported by the US Dept. of Energy, Basic Energy Sciences under contract nos. DE-AS05-ER10742 and DE-AC02-76CH00016. References 1 L. G. Parratt, Phjx Rev., 1954, 95, 359. 2 S. M. Heald, H. Chen and J. M. Tranquada, Phjx Reti. B, 1988, 38, 1016. 3 H. Chen and S. M. Heald, J. Appl. Phj,.~., 1989 66, 1793. 4 W. C. Marra, P. Eisenberger and A. Y. Cho, J. Appl. Phjx, 1979, 50, 6927. 5 G. H. Vineyard, Phys. Rev. B, 1982, 26, 4146. 6 I . K. Robinson, Phys. Rev. Lett., 1983, 50, 1145. 7 H. Chen and S. M. Heald, Solid State lonics, 1989, 32/33, 994. 8 H. Chen, Ph.D. Thesis, (City University of New York, 1989). 9 E. A. Stern and S. M. Heald, in Handbook c!f'Sj.nchrotron Radiation, ed. E. E. Koch (North-Holland, Amsterdam, 1982), vol. 1 b, p. 955. 10 E. C. Colgan, M. Natasi and J. W. Mayer, J . Appl. Phj-s., 1985, 58, 4125. 1 1 X. A. Zhao, H-Y. Yang, E. M a and M-A. Nicolet, J . Appl. Ph-vs., 1987, 62, 1821. 12 E. Ma and M-A. Nicolet, J. Appl. PIij.s., 1989, 65, 2703. 13 M. W. Ruckman, L. Jiang and M. Strongin, J. Vac. Sci. Technol., to be published. 14 A. E. Gershinskii, G . V. Timofeeva and N. A. Shalygina, Thin Sol. Films, 1988, 162, 171. 15 ,4. P. Raddorf and J. F. Wendelken, J. Vac. Sci. Techriol., to be published. 16 M. J. Verkerk and G . J. vander Kolk, J . Vat. Sci. Twhnol. A, 1986. 4, 3101. Paper 9/05381 F; Rc>ceiwtl 14th I>ecember, 1989

ISSN:0301-7249    

DOI:10.1039/DC9908900021

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Furadav Discuss. Chem. Soc., 1990, 89, 31-40 In Situ Structural Studies of the Passive Film on Iron and Iron/ Chromium Alloys using X-Ray Absorption Spectroscopy Moussa Kerkar and James Robinson* Department of' Physics, Universit-v of Warwick, Coventry, CV4 7AL A. John Forty Universit-v of Stirling, Stirling, Scotland The structure of the passive film that protects iron and iron/chromium alloys from corrosion has been investigated in siru in an aqueous environment using X-ray absorption spectroscopy, with fluorescence yield detection. From an analysis of the EXAFS and XANES it has been shown that the film formed on iron consists of FeO, octahedra, linked together by sharing edges to form what are probably sheets, or chains. When the iron is alloyed with chromium, or when it is exposed to a solution containing chromate ions, the passive film becomes more disordered and the distance between neighbouring iron atoms increases slightly.More significantly, from the passivation point of view, it has been shown that the chromium is incorpor- ated into the passive layer as a phase essentially identical to Cr(OH),, i.e. CrO, octahedra linked by hydrogen bonds into an amorphous three- dimensional phase. I t is this phase that appears to give rise to the enhanced corrosion resistance that chromium imparts to iron alloys. The corrosion of iron and mild steels (rusting) is of course very familiar to us all and it is a major economic, safety and environmental problem. It is well known that by alloying the iron with various metals, in particular chromium, to form steels this corrosion can be inhibited in all but the most aggressive media.Similar inhibition can also be achieved by immersion in certain solutions containing, for example, chromate ions. This inhibition of corrosion, or passivation, is brought about by the formation of a thin 'oxide'-containing passive layer on the metal surface which prevents further chemical attack on the metal. Surprisingly, despite its obvious importance, the chemical nature and structure of this film are still not well understood and are the subject of considerable debate. Clearly an understanding of the structure of the passive film and particularly of the role played by alloying elements in modifying this structure would be very beneficial. The establishment of these properties of the passive film has been hindered by the extreme thinness of the film (typically 2 nm), and also because it is probably significantly disordered.Until very recently such studies as have been made have largely been conducted ex situ, using electron diffraction and X-ray photoelectron spectroscopy (XPS). The results of these studies have been far from conclusive. Most electron diffraction data' ' suggest that the film is mainly y-Fe,O, with possibly an inner layer of Fe,O, (duplex model). The XPS data,".' on the other hand, suggest that the film is either y-Fe,O, or y-FeOOH, the latter being observed when there were iron(ii) ions present in solution, a conclusion also reached by some of the electron diffraction studies.' A major problem with these electron diffraction and XPS studies, however, is that they require that the sample is transferred from the corrosive environment to ultrahigh vacuum ( U H V ) for investigation. It is very likely that this will lead to dehydration and recrystalli- sation of the film, particularly in the presence of an electron beam which can cause local heating, therefore there must be considerable doubt that the phases investigated 3132 Passive Films ex situ are the same as those present in situ.Clearly there is a need for in situ structural investigations if this problem is to be resolved. Probably the first technique capable of providing in situ structural information to be applied to this problem was Mossbauer spectroscopy. Recent in situ studies of the passive film formed on Fe at controlled electrode potentials,' and ex situ studies of dried films,'*' using this technique, have demonstrated that there do indeed appear to be structural changes on drying, indicating the importance of in situ investigation.The in situ Mossbauer spectroscopy study concluded that the passive film had a disordered iron oxyhydroxide-like structure, possibly consisting of chains or sheets of FeO, octahedra linked by their edges. Mossbauer spectroscopy does not, of course, provide direct structural information, and therefore the above conclusions were largely drawn from comparisons with data from known structures. Direct structural information can come only from diffraction or extended X-ray absorption fine structure (EXAFS) studies. Whilst in situ X-ray diffraction studies of thin surface films at the electrode/solution interface are possible,"." the disordered nature of the passive layer on Fe means that this approach is probably not well suited to the study of these films.EXAFS studies are, however, feasible. A particular advantage of the EXAFS technique is that it is possible to investigate the local structure about a particular atom type by analysing the fine structure above its X-ray absorption edge. Thus, for example, in a sample containing both Fe and Cr it is possible to investigate the structure about each of these atoms. In addition to the structural information that may be obtained from an analysis of the EXAFS it is often also possible to obtain useful information from the near-edge region (X AN ES).The possibility of using EXAFS to probe the structure of the passive film on iron was first demonstrated in 1982."' This first study was conducted ex situ, but there have subsequently been a few in situ investigation,"'" including two where the sample has formed part of an electrochemical cell. Some of these studies have merely demonstrated that there is a change in the X-ray absorption spectrum on passivation, without any serious attempt at obtaining structural information (probably largely due to the poor quality of the data). Indeed, on occasion this system seems to be being used as a test of experimental, particularly detection, techniques rather than as the subject of a structural study. Such analyses ' O . ' ' as have been performed indicate Fe--0 bond lengths lying between those of y-Fe,O, and y-FeOOH with a suggestion that the presence of Cr increases this bond length. On the basis of these results the authors proposed that the film was not a stoichiometric oxide, or hydroxide, and that water was incorporated into the film. The uncertainties associated with the bond lengths in these studies were, however, very large, in part due to the data quality, and in part to the presence of a contribution to the EXAFS from the unpassivated metal.The recording of the X-ray absorption spectrum of a passive film, on iron or one of its alloys, of a quality adequate for reliable analysis of the EXAFS, presents a number of problems in regard to the design of the experiment. It is clearly not possible to use conventional transmission techniques to record the X-ray absorption spectrum of a passive layer.Ex situ studies of surface films, including those of passive layers on Fe,I5 have largely relied on electron-yield detection to record the spectra, thus taking advantage of the limited sampling depth of this technique (50 nm) to achieve a degree of surface selectivity. For in situ studies this approach cannot easily be followed, and therefore fluorescent photon detection, which is well suited to the study of what are effectively dilute systems, is used. The sampling depth of this technique is, however, very much greater than the thickness of any passive layer and therefore, typically, for a bulk sample with a thin passive layer on the surface, fluorescent photons will arise from both the bulk metal and the passive layer.The absorption spectrum will thus be a mixture of contributions from the passive layer and the metal, and it is then very difficult, if not impossible, to analyse. There are two ways this problem may be overcome.M. Kerkar, J. Robinson and A. J. Forty 33 Firstly, angles of incidence less than the critical angle may be used and in this way the X-ray beam is totally externally reflected after sampling only a thin surface layer. However, even with very flat samples, the sampling depth would still be greater than the thickness of the passive layer. An additional problem for in situ work is that the glancing-incidence configuration leads to long solution pathlengths for the X-ray photons, and at the iron and chromium K-edges, in which we are interested, the X-rays are not very penetrating. The second approach, first adopted by Long et af."' and used in the work to be described here, is to use very thin layers of metal so that when they are passivated essentially all the metal is converted to the passive film, thereby minimising the contribution from the metal substrate.In this paper we describe how X-ray absorption spectroscopy (both EXAFS and XANES), at both the Fe and Cr K-edges, may be used to study, in situ, the structure of the passive film that forms on iron, and some iron/chromium alloys, prepared as thin films. These films were passivated both chemically and electrochemically and X-ray absorption data have been obtained as a function of alloy composition, electrode potential and solution composition, with a view to determining the structure of the film and identifying the role played by Cr in improving the corrosion resistance of steels.In order to study the electrochemically passivated samples a cell design that reconciles the requirements of a free path for the X-ray photons to, and from, the sample, and a good electrochemical configuration is required and this is also discussed. Experimental Thin films of iron and its alloys with chromium can be easily produced by thermal evaporation onto a variety of substrates. Glass microscope slides have been used by others,"'-" but in our experience these contain a small, but significant, iron impurity. The samples used in this study were therefore prepared by thermal evaporation, under vacuum, onto Mylar.This plastic contains no iron or chromium impurities, it scatters X-rays only weakly, and metals adhere well to it. The film thickness was continuously monitored during deposition and the typical thickness used was between 1.2 and 2.0 nm. The composition of the films was determined by subsequent EDAX analysis in a transmission electron microscope. For the electrochemical work a thin film of gold was evaporated onto the Mylar prior to depositing the Fe film. This was to ensure that good electrical contact to the film was maintained. The gold does not absorb X-rays sig- nificantly in the energy range of interest. 7 keV photons do not penetrate far in water, and therefore electrochemical cells must be designed with this in mind. We have used two types of electrochemical cell that permit the electrode potential to be maintained whilst recording X-ray spectra, and these designs are shown in fig.1. In the first [fig. l ( a ) ] the electrode (alloy on gold on Mylar) was mounted in such a way that it could be floated on the solution surface whilst it was being passivated. When passivation was complete the electrode was raised, whilst maintaining the electrode potential, against a vertical Mylar window, thus trapping a thin capillary layer between the electrode and the window. This thin solution layer was easily penetrated by the incident and fluorescent X-ray photons and minimised the amount of scattered radiation reaching the fluorescence detector. In the second design [fig. l ( h ) ] the electrode was also the window through which the X-rays entered and left.It had better electrochemical characteristics, did not require the electrode to be moved and it permitted much easier solution deoxygenation. In both cells the secondary electrode was a platinum gauze, whilst the saturated calomel reference electrode (SCE) was mounted in a separate compartment. For the electrochemical work the electrode potential was maintained with a potentiostat (Hi-Tek DT 2101) in conjunction with a function generator (Hi-Tek PPR1) and all potentials are referred to the SCE. All34 Passive Films C 0 .- c W aJ J) \ IM. Kerkar, J. Robinson and A. J. Forty 35 solutions were prepared from AnalaR reagents with distilled, deionised, water and solutions were deoxygenated with argon.In situ X-ray absorption spectra at both the Fe and Cr K-edges were recorded for Fe and Fe/Cr alloys (up to 25% Cr) passivated by immersion in 0.1 mol dm-3 sodium nitrite or 0.005 mol dmP3 potassium chromate, and samples electrochemically passivated in borate buffer (pH 8.4) and 0.1 mol dm-' sodium perchlorate. The metal films to be passivated by immersion were placed in solution immediately on removal from the vacuum evaporation chamber and left for several days prior to their spectra being recorded. Electrochemical samples were mounted in the cell and were cathodised at negative potentials, to remove any air-formed oxide, prior to anodic passivation. When the cell shown in fig. 1 ( b ) was used, then prior to anodisation it was flushed with clean electrolyte, whilst maintaining the electrode potential and purging with argon, in order to eliminate any iron(l1) ions that had been introduced into the solution by the cathodic pretreatment.All the X-ray absorption results described here were obtained on beamline 8.1 at the S.R.S. Daresbury. The monochromator on this station" was a double-crystal instru- ment with a bent first crystal and a post-monochromator Pt coated mirror to provide a high photon flux at the sample. The spectra of model compounds (Fe and Cr oxides and oxyhydroxides) were obtained in the conventional transmission mode whilst fluores- cence detection was used for the passive films. Whilst initial work was conducted with a scintillation detector the majority of the results were, however, obtained with a 13-element solid-state Ge detector (Canberra). Each element of this detector was equipped with its own amplifier and discriminator and, to prevent pile-up effects, the count rate in any single channel was limited to <lo4 counts s-'.The fluorescence yield X-ray absorption spectra of the passive films were obtained by mounting the sample vertically at an angle of ca. 45" to the incident X-ray beam and positioning the detector in the plane of the storage ring making an angle of 45" to the sample. For most of this work the storage ring was operating at 2 GeV with a current of 200 mA and good-quality spectra were obtained by summing ca. 8 scans, each of 45 min. duration. Results and Discussion Whilst evaporated thin films have been used before for EXAFS studies of passive films, there do not appear to have been any comparisons of the behaviour of this type of sample and bulk material. In particular, it is not clear whether it is justifiable to extend conclusions drawn from studies of thin films to bulk alloys.Fig. 2 shows polarisation curves obtained in borate buffer for bulk iron, an evaporated gold film and a 2 nm film of iron evaporated onto gold. I t can be seen straight away that the curve for the Fe film on Au is essentially a combination of the curves for Au and Fe. In other words, whilst some of the gold is clearly exposed to the solution, the behaviour of the iron is essentially the same as that of bulk iron. The polarisation curves for alloy films have also been compared with those of bulk material and again the behaviour is essentially identical.In addition to recording polarisation curves these film electrodes have also been studied using photocurrent spectroscopy," and again the behaviour of the films i s almost identical to that of electrodes made from bulk alloys. On the basis of these results it can therefore be concluded that the thin films appear to behave in the same way as bulk alloys, and that structural information obtained by studying these films should be equally applicable to bulk alloys. The solutions used for immersion passivation, potassium chromate and sodium nitrite, were chosen as it has been shown that films formed in these solutions appear to have structure, composition and formation kinetics similar to those formed by anodic oxidation in solutions free from iron( 1 1 ) ions.' These films were therefore studied prior to undertaking the rather more difficult electrochemical experiments. In general terms36 Passive Films /' 1., 1 1 I I I -0.4 0.0 0.4 0.8 E / V us. SCE Fig. 2. Polarisation curves in borate buffer solution (pH 8.4) for, ( a ) iron, ( b ) gold and ( c ) a 0.20 nm film of iron on gold. I I 7100 7120 7140 7160 7180 energy / e V Fig. 3. XANES spectra at the Fe K-edge for ( a ) a 0.2 nm film of Fe at 0.8 V in 0.1 mol drn.-, sodium perchlorate solution, ( h ) y-FeOOH, ( c ) a-FeOOH, ( d ) y-Fe203 and ( e ) a-Fe,O,. these immersion passivated films were indeed found to have a structure similar to that of the electrochemically formed ones, and it is therefore convenient to discuss both types of sample together.Fig. 3 shows the near-edge (XANES) spectrum, at the Fe K-edge, of an electrochemi- cally passivated Fe film, and for comparison the spectra for the model compounds. It can be seen that the spectrum for the passive film is more like that for the oxyhydroxides than the oxides (in fact detailed examination shows it to be most like that for the y-oxyhydroxide). These results therefore lend support to the idea that the passive film has a structure like that of an oxyhydroxide. More detailed structural information, however, can be obtained from an analysis of the EXAFS.M. Kerkar, J. Robinson and A. J. Forty 37 -6 t 1 ' 1 I I 1 1 0.4 0.6 0.8 wavevector ( k )/nm ~ ' 0 0.2 0.4 0.6 0.8 1.0 r/nm Fig. 4. ( a ) k3-weighted EXAFS spectrum for an Fe film at 0.8 V in 0.1 mol dm-3 sodium perchlorate and the best-fit theoretical curve.( b ) The corresponding Fourier transforms. The theoretical curves are shown dashed. X-Ray absorption spectra at the Fe K-edge were recorded for a wide range of samples, and the EXAFS oscillations were extracted in the usual way. In order to permit the analysis of the EXAFS it was first necessary to obtain accurate phase-shift data. Using the program E X C U R V ~ ~ , which implements full curved wave theory,18 the EXAFS shown by a-Fe203 was modelled using known crystallographic parameters, and ab initio calculated phase shifts were refined to optimise the fit to the experimental data. These refined phase shifts were then checked by analysis of the EXAFS for the other model compounds, whose structures were also well known, before being used to deter- mine the structures of the passive films.Fig. 4 shows, for example, the best fit between experimental and calculated data for an Fe film passivated at 0.8 V in sodium perchlorate solution. To fit the EXAFS two shells were required, six oxygen atoms at 0.201 nm and six iron atoms at 0.302 nm. These shell radii do not of themselves permit easy iden- tification of the structure as in most iron oxides and oxyhydroxides the basic building unit is an octahedron of oxygen atoms surrounding the iron centre, and therefore the radii of the first shells are very similar. Whilst the radii of the second shells range from 0.295 nm for cr-Fe20, to 0.308 nm for y-FeOOH the greatest variation occurs in the third shell. This arises from the different ways the FeO, octahedra are joined to form the extended three-dimensional structure.For the a-oxyhydroxide, and both forms of the oxide, there is a shell containing iron atoms lying between 0.33 and 0.35 nm and its presence is clearly observed in the EXAFS. For the y-oxyhydroxide no such shell exists, nor was any evidence for such a shell obtained for the passive films. This therefore lends further support to the idea that the structure of the passive film on iron is similar to that of y-FeOOH, i.e. FeO, octahedra linked together by sharing edges and not faces or corners. Fe/Cr alloy samples were studied in a similar way to the pure iron films and the results of the analysis of the Fe K-edge EXAFS for immersion passivated samples are given in table 1, as are those for 7-FeOOH.The data presented are the shell radii and the value of A, which is defined as twice the Debye-Waller factor, in all cases there were six atoms of oxygen in the first shell and six of iron in the second. It can be seen38 Passive Films Table 1. Results of the analysis of the EXAFS of immersion passivated sample sample Fe-O/nm A,/10-4 nm' Fe-Fe/nm A,/10-4 nm' y- FeOO H 0.204 0.8 0.309 1.4 Fe Fe 5%Cr Fe 10% C r Fe 15% C r Fe 25% Cr samples passivated in 0.1 mol dm- NaNO, 0.199 1.9 0.303 5.7 0.200 2.2 0.302 3.9 0.200 1.9 0.305 3.9 0.201 2.6 0.305 5.0 0.202 2.8 0.307 5.8 samples passivated in 0.005 mol dm-j K'CrO, Fe 0.200 2.2 0.302 4.8 Fe 5%Cr 0.199 1.8 0.302 4.6 Fe 10% Cr 0.199 2.1 0.302 5.1 Fe 15% Cr 0.200 2.7 0.304 4.5 Fe 25% C r 0.200 2.5 0.302 5.8 Table 2.Results of the analysis of the EXAFS for samples passivated electrochemically sample Fe-O/nm A,/ 10- nm' Fe-Fe/nm A,/ lo-" nm' Fe Fe 25% Cr Fe Fe 25% C r samples passivated in borate solution 0.202 2.0 0.305 3 .O 0.202 2.0 0.305 4.4 0.201 2.4 0.302 3.6 0.202 2.1 0.305 4.6 samples passivated in perchlorate solution that the only significant changes with composition are for samples passivated in sodium nitrite solution where the second shell, initially much shorter than that in 7-FeOOH, increases in radius with increasing chromium content, whilst the first shell radius also increases slightly. No similar trend is seen for passivation in potassium chromate (note that whilst absolute radii are subject to an error of ca.0.002 nm any trends are significant at least at the 0.001 nm level). Note also that the disorder, particularly of the second shells, as indicated by the Debye-Waller factors, is significantly greater in the passive films than in the crystalline model compound. The results of analysis of EXAFS data for Fe and alloys containing 25% Cr electro- chemically passivated in borate buffer and sodium perchlorate solutions are shown in table 2. As was pointed out earlier it has been suggested',' that the passive film formed in iron( 1 1 ) ion-free solutions may be different from that formed in the presence of these ions. The results obtained with both types of cell described earlier were identical, even with electrolyte flushing, and therefore no evidence was found to support this hypothesis.From table 2 it can be seen that the samples passivated in perchlorate solution behave in a similar way to those passivated in sodium nitrite, i.e. the radius of the second shell increases with increasing Cr content. For samples passivated in borate buffer; however, the film structure seems to be independent of the Cr content, the second shell radius being long even for the pure Fe film. This lack of sensitivity to the Cr content is probably due to the incorporation of B into the film (such incorporation is well established). Thus the effect of the introduction of either Cr- or B-containing species into the passive film is t o slightly increase the Fe-Fe distance in the second shell with an accompanying increase in the Debye-Waller factors.The films on pure Fe have also been studied asM. Kerkar, J. Robinson and A. J. Forty 39 10 5 - 4 G O 4 - 5 -1 0 1 I 1 I I I I 0.4 0.6 0.8 1 .o wavevector ( k ) / n m - ' 0.4 0.6 0.8 1 . o wavevector ( k )/nm ' Fig. 5. ( a ) The k3-weighted EXAFS spectrum at the Cr K-edge of an Fe foil passivated in 0.005 mol dmP3 potassium chromate and the best-fit theoretical curve. ( b ) The corresponding curves for Cr(OH),. The theoretical best fit curves are shown dashed. a function of the electrode potential and there appears to be no variation in structure with potential, though of course there is an increase in the film thickness with increasing passivation voltage. The overall conclusion from these results is that for the passive film on Fe the basic structure is similar to that of y-FeOOH except that the second shell radius is shorter and the overall structure is rather disordered. The contraction of this second shell radius may be due to the lack of long-range, three-dimensional order in the passive film, i.e.the octahedra are joined into chains, sheets or clusters but not into an extended three-dimensional structure. The incorporation of other species into the film e.g. Cr or B, leads to a lengthening of the second shell radius but no other detectable changes. However, it is unlikely that this dilation of the second shell alone explains the improved corrosion resistance imparted by alloying with Cr or treating with chromate-containing solution, and therefore to investigate this further the EXAFS at the Cr K-edge spectra were recorded.For all samples passivated in potassium chromate solution it was necessary to wash and dry the films prior to analysis otherwise chromate ions in solution would contribute to the X-ray absorption spectrum, otherwise the spectra were recorded in situ. The results obtained with in situ and ex situ samples were, however, essentially the same whilst the structure of the Cr-containing part of the film was also independent of whether the Cr was incorporated from the solution, or from the alloy. XANES spectra at the Cr K-edge of these passive films show clearly that the bulk of the Cr in the passive layer is present in the 3+ oxidation state, and from the similarity in the spectra in the near-edge region it appears that the passive film has a similar structure to Cr(OH), , rather than Cr20,.To obtain further information on the structure of the Cr(OH),-like phase in the passive film the EXAFS were analysed, using a procedure similar to that already described for the Fe K-edge results, and using Cr,O, to determine the phase shifts. Fig. 5 shows the best fit to the data for the passive film and for commercial Cr(OH),. I t can be seen that the spectra are very similar and that40 Passive Films the parameters used to obtain the best fits were, within experimental error, identical except for some variation in the Debye-Waller factors for the second and third shells, implying a possible variation in the disorder. It can therefore be concluded that the structure of the passive film and Cr(OH), are very similar.The material known as Cr(OH), is in fact an amorphous hydrous oxide, of variable water content, in which CrO, octahedra are linked together by hydrogen bonds. This therefore appears to be the structure of the Cr-containing part of the passive film and it is the formation of this amorphous phase that seems to be responsible for the improved corrosion resistance of Cr-containing Fe alloys. Conclusion These in siru measurements have shown that the structure of the passive film on iron may be regarded as a disordered y-FeOOH, which is consistent with that first suggested by O’Crady,‘ on the basis of Mossbauer results. The results presented here, however, represent the first direct in situ structural confirmation of this suggestion. This observa- tion must be reconciled with the conclusion drawn from ex situ electron diffraction studies that the passive film is y-Fe,O,.y-FeOOH can be transformed into y-Fe,O, but this requires elevated temperatures therefore, whilst it would not be expected to occur on drying, it is quite possible that it will occur when the sample is exposed to an electron beam and local heating occurs. The results presented here anyway show that it no longer is necessary to rely on ex situ investigations to determine the structure of thin surface films. The structure of the chromium containing phase in the passive film on Fe/Cr alloys had not previously been identified and it was generally referred to merely as an oxide, or hydrous oxide, phase. The structure of this phase is now established. We are grateful to the S.E.R.C.for the support of this work and to the director and the staff of the SRS, Daresbury, for the provision of synchrotron radiation. References 1 2 3 M. Nagayama and M . Cohen, J. Electrochem. Soc., 1962, 109, 781. C. L. Foley, J. Kruger a n d C. J. Bechtoldt, J. Electrochem. Soc., 1967, 114, 994. K. Kuroda, B. D. Cahan, Gh. Nazri, E. Yeager a n d T. E. Mitchell, J. Electrochem. Soc., 1982, 129, 2163. 4 H. Konno and M. Nagayama, in The C’orrosion and 0.uidation of Metals, ed. U. R. Evans (Arnold, London, 1968), p. 585. 5 M. E. Brett, K. M . Parkin a n d M. J. Graham, J. Electrochem. Soc., 1986, 133, 2031. 6 W. F. O’Grady, J. Electrochem. Soc., 1980, 127, 555. 7 J . Eldridge, M. E. Kordesch a n d R. W. Hoffman, J. Vac. Sci. Technol., 1982, 20, 934. 8 M. Fleischmann, A. Oliver a n d J. Robinson, Electrochim. Acta., 1986, 31, 899. 9 M. G. Samant, M. F. Toney, G. L. Borges, L. Blum a n d 0. R. Melroy, Sue$ Sci., 1988, 193, I29 10 G. G. Long, J. Kruger, D. R. Black and M. Kuriyama, J. Electrochem. Soc., 1982, 129, 240. 1 1 J. Kruger, G . G. Long, M. Kuriyama, I>. R. Black, E. N. Farabaugh, D. M. Sanders and A. I . Goldman, Proc. Inr. Congr. Met. Corros. NRCC: Ottaua, Ont., 1984, 419. 12 G . G. Long, J. Kruger, D. R. Black and M. Kuriyama, J. Electroanal. Chem., 1983. 150, 603. 13 M. E. Kordesch and R. W. Hoffman, Nucl. Inst. Meth., 1984, 222, 347. 14 J. M . Fine, J. J . Rusek, J. Eldridge, M. E. Kordesch, J. A. Mann, R. W. Hoffman and I>. R. Sandstrom, J. Vac. Sci. Technol., 1983, Al, 1036. 15 G. G. Long, I>. A. Fischer, J. Kruger, D. R. Black, I>. K . Tanaka and G. A. Danko, Phj,.s. ReL. 8, 1989, 39, 1651. 16 M. J . Van Der Hoek, W. Werner, P. Van Zuylen, B. R. Dobson, S. S. Hasnain, J. S. Worgan and G. Luijckx, Nucl. Inst. Method., 1986, 246, 380. 17 M. Kerkar a n d J. Robinson, J. Electroanal. Cheni., to be published. 18 S. J. Gurman, N . Binstead a n d I . Ross, J. Phj.s. <-, 1YX4, 17, 143. Puper 9/05380H; Recvired 14th Decrmher; 1989

ISSN:0301-7249    

DOI:10.1039/DC9908900031

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faraday Discuss. Chern. SOC., 1990, 89, 41-50 Electrochemical Inclusion of Copper and Iron Species in a Conducting Polymer observed In Situ using Time-resolved X-Ray Absorption Spectroscopy Daniel Guay, Gerard Tourillon and Alain Fontaine" LURE ((CNRS, CEA, MEN) Ba^t 2090, 91405 Orsay, France A comparative in situ time-resolved X-ray absorption study of the electrochemical inclusion of copper and iron species in poly(3-methyl- thiophene) (PMeT) is reported. For both treatments, incorporation of metallic ion and complexation with sulphur atoms of the polymer backbone lead to a significant increase of the ex situ macroscopic conductivity. However, the detailed mechanisms and kinetics of the various processes are specific to the metallic ion incorporated in the polymer. The conductivity of organic conducting polymers can be varied from insulating/semi- conducting to conducting when they are doped.From a technical point of view, polythiophenes and their derivatives' appear to be the most promising organic conducting polymers, owing to their high stability against oxygen and moisture in both their doped and undoped forms. Therefore, practical applications in the fields of organic batteries, display devices and photovoltaic systems have been tested. in organic conducting polymers, the macroscopic conductivity is generally limited by both intra- (presence of structural defects) and inter-chain (lack of ordering) defects. Intrachain conduction can be controlled and modified by varying the structure of the monomer units'" and the nature of dopant,' to produce a more regular polymer with a metallic-like behaviour.An elegant way of increasing the contact between the polymeric chains is to bridge them with metallic ions. To this end, copper species were included in doped PMeT and an increase of the macroscopic conductivity (from 50 to 150 K' cm-') was observed after cathodic polarization.' Energy dispersive X-ray absorption spectroscopy is a particularly well suited method for investigating the electrochemical inclusion of metallic ions in conducting polymers' because it provides information on the oxidation state, the coordination geometry and the bonding angles and time-resolved in situ investigation can be achieved. This spectroscopy is therefore a powerful technique for obtaining information on growth mechanisms and kinetics of various processes.In this paper, we report a comparative in situ time-resolved X-ray absorption spectroscopic study of the electrochemical inclusion of copper and iron metallic species in doped PMeT under continuous cathodic polarization. Electrochemical Conditions Grafted polymer on a platinum wire (500 p m diameter) is obtained by the oxidation, at + 1.35 V uersus SCE, (saturated calomel electrode), of 3-methylthiophene (5 x 10 ' mol dm ') in CH,CN with N(Bu),SO,CF, (5 x 10 ' mol dm - 7 ) as the electrolytic salt.< The polymer is obtained directly in its doped conducting state with a typical 25-30°/0 doping level and its thickness is controlled by the electrolysis time. 4142 !n situ XAS ~f Conducting Polymers For the X-ray absorption spectroscopy experiments, a layer of PMeT, 3 mm thick, is grafted on the Pt wire. The polymer is rinsed with acetone, and dried and immersed in an aqueous solution containing the metallic ions (CuCI, and FeCI,, the concentrations of both solutions in the range 10 -'-lo ' mol dmp3).Measurements of the time-depen- dent electrolysis current and polarization time-dependent macroscopic conductivity were obtained with more concentrated solutions (0.1 - 1 .0 mol dm- '). The electrolytic cell used for the X-ray absorption study has a thickness of 3 mm. It is composed of a teflon ring covered by two kapton windows. This cell is inserted in the X-ray beam with the grafted polymer at the focusing spot of the optical system. A two-electrode configuration (the grafted electrode and a Pt wire) is used to reduce the metallic species (-5 V).The volume occupied by the polymer fibres (40% of the investigated volume) is estimated from the decrease in the CuCI, solution absorption coefficient observed when the grafted polymer is introduced into the electrolytic cell. X-Ray Absorption Spectrometer The studies were conducted at Lure-Orsay by using the synchrotron radiation from the DCI storage ring (1.85 GeV-I- 250 mA). Time-resolved X-ray absorption spectroscopy uses an elipsoidally bent crystal for X-ray dispersive optics. XANES spectra were recorded with Si (31 1 which yields a resolution of 0.9 eV at the copper K-edge, while the Si ( 1 11) was used to obtain the EXAFS data (resolution ca. 1.9 eV at the copper K-edge).The Bragg-reflected beam is focused on the sample and subsequently detected by a 1024-element photodiode array. The whole spectrum of a thin iron or copper reference foil is recorded in ?8 ms with a good signal-to-noise ratio. The size of the focused beam is 0.5 x 0.5 mm- and so a very small volume of the sample is analysed. All the measurements have been made to probe a region of the polymer close to the Pt wire. A number of reference compounds were used in order to obtain the backscattering amplitude and phase-shift functions of the various pairs of atoms, namely Cu,O (two oxygen atoms at 1.85 A), CuO (four oxygen atoms at 1.96 A), metallic Cu, FePO, (four oxygen neighbours at 1.88 A), ferrocene (ten carbon atoms at 2.03 A)" and FeS, (pyrite, six sulphur atoms at 2.259 A).In the case of the Fe-0 pair, phase-shift and amplitude functions extracted from FePO, were cheocked by fitting the fi:st coordination shell of a-Fe203 (six oxygen atoms: three at 1.945 A and three at 2.1 16 A). No suitable reference compound was found for the Cu-S pair. Results Electrolysis Current and Conductivity Characteristics The time-dependent evolution of the electrolysis current when cathodic polarization is applied to the grafted PMeT immersed in the CuCI, aqueous solution is characterized by a sharp initial drop and a subsequent increase of the electrolysis current. For longer polarization times (more than 6 min) the current decreases slowly. This behaviour is different from that observed when the polymer is soaked in a FeCI, aqueous solution: no increase in the electrolysis current is observed after the initial sharp drop.Instead, a slow decreasing current is measured over more than 2 h. Fig. 1 curves A and B show the ex situ macroscopic conductivity of the grafted PMeT measured at various times after cathodic polarization was applied to the polymer soaked in CuCI, and FeCI, aqueous solutions, respectively. Curve A is characterized by a steady increase of the macroscopic conductivity from 50 to 150 0 ' cm I . Longer polarization times do not induce further changes in the ; I e V = 1 . 6 0 2 1 8 ~ 1 0 "'J.D. Guay, G. Tourillon and A. Fontaine 43 200 160 120 80 40 0 Fig. 1. Ex situ macroscopic conductivity of PMeT measured at various times after cathodic polarization is applied to the polymer soaked in a CuCI, (0) and FeCI, (0) solution.1 ( a ) 1 0 1 I I I I I 1 1 1 -10 0 10 20 30 40 5 b 60 7 b 80 energy/eV Fig. 2. Cu K-edge absorption spectrum ofCuC1, solution ( a ) , Cu'-pyridine complex in acetonitrile ( h ) and 0.5 mm thick CuO metallic foil ( c ) . conductivity. Curve B shows first an increase of the macroscopic conductivity from 40 to 75 0-l cm-'. For longer polarization times, 0- values decrease slowly to 0.5- 1.0 0 cm I . At the end of the electrochemical process, the value of the macroscopic conductivity is less than that characteristic of the untreated polymer. Cu K-edge Features of Basic Compounds Fig. 2 shows the Cu K-edge absorption spectrum of three copper species: curve ( a ) , SO mmol dm ' CuCI, aqueous solution; curve ( b ) , Cu'-bipyridine complex in44 In situ XAS of Conducting Polymers - 4.0 III 3 .- c f 4 3.0 - c G .C - 2 D 2.0 1 .0 1 I 1 I I 0 20 40 60 energy/eV Fig. 3. Time-dependent evolution of the Cu K-edge absorption spectrum of cathodic ly polarized PMeT soaked in a CuCI, solution. The zero reference energy corresponds to 8979.12 eV and the polarization time is indicated on the right-hand scale. acetonitrile; curve ( c ) , metallic copper. Both the energy position and the shape of the absorption edge conveys chemical information on the absorbing atom. Compared to Cu", the shift in the energy position of the absorption edge is +1.2 eV for the Cu'- bipyridine complex and +8.5 eV for the Cu" ions. The white line observed at 15 eV in the absorption spectrum of Cu" (peak 1) is due to a transition to an antibonding resonance, reflecting the presence of six oxygen atoms in a regular octahedral configur- ation.The interpretation of the edge features of the Cu'-bipyridine complex is similar to that of Cu,O, where Cu' has a linear configuration with two oxygen atoms at 1.85 A. If the z-axis is chosen to lie along the two Cu-N bonds, the 4p,, 4,). and 4,: metallic orbitals are no longer degenerated (as they are in the isolated atom), due to the lowering of the site symmetry. Peak 2 of curve ( 6 ) corresponds, therefore, to the IS-^^,,, non- bonding Cu orbitals. Multiple-scattering calculations were also conducted on Cu metallic clusters of varying dimensions and the shoulder in the middle of the rise of the absorption cross-section (peak 4) and the first three oscillations centred at 13.4, 22.4 and 43.8 eV (peak 5,6 and 7, respectively) have been reproduced when the fourth shell surrounding a given copper atom is included.' The edge and pre-edge features of the absorption spectra of the compounds in fig.2 can be used as fingerprints of the chemical state of copper. Time-resolved X-Ray Absorption Spectroscopy and Inclusion of Copper Species in PMeT Electronic Structure: X A NES In a first set of experiments, X-ray absorption spectra were continuously recorded while cathodic polarization was applied to a PMeT grafted polymer soaked in a CuCI, aqueous solution, using a Si (311) crystal. Fig. 3 shows a set of selected Cu K-edge spectra measured during the reduction process. The time elapsed from the beginning of the experiment is indicated on the right-hand scale and t = 0 denotes the absorption spectrum of the Cu" ions in the polymer before application of cathodic polarization.D.Guay, G. Tourillon and A. Fontaine - li 45 0 2000 4000 t l s Fig. 4. Time-dependent evolution of the relative proportions of Cu" (- . -), Cu' (- - - ) and Cu" (-) in a cathodically polarized PMeT polymer. There is a rapid shift of the absorption edge towards lower energy and the appearance of a small bump in the rise of the absorption cross-section, along with a decrease in the white line intensity after the onset of polarization (curve t = 6 5 0 s ) . Cu' is therefore formed during the first step of the reduction process (fig. 2). The presence of Cu' ions is also confirmed by the red coloration appearing when the polymer is treated with a bipyridine solution (Cu" ions give a blue coloration).Further modifications are observed in the absorption spectra upon prolonged cathodic polarization. The bump in the rise of the absorption edge is displaced a little more towards low energy and oscillations characteristic of the metallic state are observed (curve t = 1656 s). All these changes in the absorption spectrum are consistent with the reduction of the Cu' species to Cuo metallic copper. The overall electrochemical inclusion of copper species in PMeT proceeds, therefore, via a series of subsequent reduction steps: Cu'' -+ Cu' -+ Cu". The inclusion process is fully reversible. Curves A, B and C of fig. 4 represent the time evolution of the Cu", Cu' and Cu" concentrations under continuous cathodic polarization, respectively, as determined by curve fitting of the absorption spectra of fig.3 with a linear combination of the absorption spectra of the three various copper species (fig. 2). The electrochemical time-dependent evolution of the various concentrations exhibits three kinetic domains. ( i ) At the beginning, there is a sharp drop of the Cull concentration with a concomitant increase in the Cu' concentration. (ii) This prompt change is followed by a slow increase of the Cu' concentration (time constant of ca. 600s). ( i i i ) Finally, the CuO concentration increases rapidly. Changes in the Cu", C u t and CuO concentrations are rather slow at the end of the electrochemical process. One should also note that, at the beginning of the reduction process ( t < 600 s), small initial metallic clusters are not well imaged by the absorption edge spectrum of the bulk Cu foil, as it has been demonstrated by multiple-scattering calculations on clusters of Cu atoms of various dimensions.' That is to say that, for small metallic clusters, the shape of the copper absorption edge is different from that of the metallic foil, and therefore a linear combination making use of this last absorption edge profile introduces uncertainties in the concentration determination of small metallic clusters of a few atoms.At t = 600 s, curve C of fig. 4 should be smoother than what was actually found.46 In situ XAS of Conducting Polymers 0.0 10.0 distance/A Fig. 5. Fourier transforms (k3-weighted) of Cu K-edge spectra: curves ( a ) - ( f ) are for the cathodically polarized PMeT with polarization times t = 0,160, 168, 184,200 and 460 s, respectively.Structural Characteristics: EXAFS A second set of experiments (same electrochemical procedure) was performed with a newly grafted polymer, using this time an Si (1 11) crystal in order to obtain a full energy domain to collect EXAFS data. Small differences may be observed in the time constants of the various processes, owing to small variations in the size of the probed region of the polymer and in its location with respect to the Pt wire. Fig. 5 shows a set of k3-weighted Fourier transforms (FT) at different polarization times. Curve ( a ) is for polarization time t = 0 and is typical of the Cu" ions after their inclusion in the polymer.The first peak of the FT is symmetric. No peak is observed at larger distances, owing to the poor atomic organization, characteristic of a solution. Curve fitting (FIFT) of the filtered inverse Fourier transform of the first peak shows that the first-neighbour shell is composed of six oxygen atoms at 1.95 A. The first-neighbour shell therefore makes a regular octahedron around the Cull ions, as expected from the appearance of a strong white-line resonance in the near-edge absorption spectrum. At the beginning of the electrolysis (polarization time between 0 and 150s), curve fitting of the filtered inverse Fourier transform of that peak shows that the first coordina- tion shell of the copper species is comprised only of oxygen atoms.For t = 160 s [curve (6) of fig. 61, a shoulder is observed on the side of the peak corresponding to larger distances. For longer polarization times [curve (c), ( d ) and ( e ) ; t = 168 s, 184 s and 200 s, respectively], the first peak is composed of two well resolved maxima, the intensity of the second peak increasing at the expense of the first. The FT obtained for still longer polarization times [curve (f), t = 460 s] shows the presence of peaks corresponding toD. Guay, G. Tourillon and A. Fontaine 47 0 0 7 7 0 c 'C ' 0 3 c 3 3 700 0 7000 0 0 700 0 0 0 energylev Fig. 6. First peak filtered inverse Fourier transform: curves ( a ) - ( e ) are for the cathodically polarized PMeT with polarization times t = 0, 160, 168, 200 and 460 s, respectively.Curve ( f ) is for CuCI, reference compound. large distances (associated with second and third coordination shells): copper atoms are in a well organized crystalline environment. Fig. 6 shows the filtered inverse Fourier transforms (FIFT) of the first shell of various FT curves; ( a ) to ( e ) are the FIFT of the t = 0, 160, 168, 200 and 460 s FT curves, respectively, while curve (f) is that of the Cu2S reference compound. In curve ( a ) , the oscillations are regularly distributed around the zero value and vanish after ca. 400 eV. This situation is characteristic of a first coordination shell where the atoms are of low 2 value. Upon cathodic polarization [curves ( b ) , (c) and ( d ) ] the FIFT of the first shell evolves to a situation where (i) the overall amplitude of the oscillations is reduced and (ii) the high energy oscillations (energy> 150 eV) are almost totally attenuated or show a beat node around 250 eV.For longer polarization times [curve ( e ) , t = 460 s], the oscillations in the 200eV region recover their amplitude and now extend into the 600 eV region. Comparison of curves ( a ) and (f) (CU" ions in the polymer and Cu2S, respectively) shows that the oscillations are almost of opposite phase in the 200 eV region. Therefore, a Cu atom surrounded by both sulphur and oxygen atoms will have a first shell FIFT showing reduced amplitude of oscillation or a beat node in that energy region, depending on the relative proportions of the atoms. The behaviour of the first shell FIFT at various polarization times demonstrated that both oxygen and, later on, sulphur atoms are included in the first-neighbour shell of the Cu' ions.The polarization time dependent evolution of the first peak of the FT curves reflects the increasing proportion of sulphur atoms in the first coordination shell of the Cu' species. Prolonged cathodic polarization induces the formation of Cuo metallic clusters, which are responsible for the second and third coordination shells.48 In situ XAS of Conducting Polymers Table 1. Fit parameters of the first shell FIFT of cathodically polarized PMeT soaked in a FeCI, aqueous solution oxygen atoms sulphur atoms polarization ~ time/s N" R h / A u"/A AEIeV N R I A &/A2 AEIeV in solution - 4.0 2.04 -0.001 0.0 - 2.0 2.14 0.006 - - - - - - - - - in the polymer 0 4.0 2.04 0.005 0.0 2.0 2.37 -0.005 - 1350 3.7 2.10 0.006 0.0 2.3 2.43 0.004 - 7695 6.0 2.19 0.004 0.0 - - - - '' Coordination number; ' distance; ' Debye- Waller factor.Electrochemical Inclusion of Iron Species in PMeT Electronic and Structural Characteristics: X A NES and EXAFSt Using FePO, and FeSz as reference compounds, reasonably good fits of the various first shell FIFT were obtained. Characteristic steps of the reduction process have been analysed and values of the fitting parameters are listed in table 1. Fe"' ions in solution are surrounded by six oxygen atoms; four of them are at 2.04 A and two at 2.14 A. The first coordination shell around the Fe"' ions is a slightly distorted octahedron. This structure is consistent with the shape of the absorption edge.In a distorted octahedron (with six identical oxygen atoms), the x, y and z axes are no longer equivalent and the interaction energies between the metallic ion orbitals and the ligand orbitals vary according to the distance between the atoms. This gives rise to the splitting observed in the absorption maximum and the reduction of its intensity. Incorporation of Fe'" ions in the polymer causes major changes in the FT curve. The first coordination shell of the Fe'" ions in PMeT evolves rapidly to four oxygen atoms at 2.04 %, and two sulphur atoms at 2.37 A. This Fe-S distance corresponds to what is observed in a number of heme-iron enzymes.' Owing to the slow kinetics, the cathodic potential applied to the grafted polymer had to be increased gradually during the course of the experiment to allow the reduction process to go to completion.(More than 140 min elapsed between the start and the end of the experiment.) On the basis of the observed spectra, several observations can be made. There is a gradual shift of the absorption edge towards lower energy under cathodic polarization. After completion of the reduction process, the absorption edge is displaced by 4.1 eV from its initial position, as observed between FeCI,.6H20 and FeCI,.4H,O powders and should therefore be attributed to the reduction of Fe"' to Fe". The 24.5 and 20.7 eV peaks which are due to the elongated octahedron around Fe"' become less and less important as the reduction proceeds. After ca. 1395 s of electrolysis, the first coordination shell of the iron species is composed of 3.7 oxygen atoms at 2.10 A and 2.3 sulphur atoms at 2.43 A.The number and type of the first coordination shell atoms are identical but the Fe-0 and Fe-S distances are increased (compare to the t = 0 coordination shell). The absorption maximum evolves from a broad to a narrow peak with an increase in intensity, along with a shift to lower energy. -k For the sake of brevity we d o not include the figures, giving results in an unusually straightforward presentation. They can be found in a more extensive paper submitted by D. Guay ei a/.D. Guay, G. Tourillon and A. Fontaine 49 At the end of the reduction period, the first coordination :hell surrounding the reduced Fe" ions is only composed of six oxygen atoms at 2.19 A and makes a regular octahedron around the ions.This geometry is consistent with the strong resonance observed in the near-edge spectrum. Complete reduction of the Fe"' ions requires more than 2 h. The presence of Fe" ions at the end of the reduction process is also confirmed by the red coloration observed after treatment with bipyridine. Discussion In situ X-ray absorption spectroscopy investigations of the electrochemical treatment of PMeT with aqueous iron- and copper-ion solutions reveal that there are major differences between the treatments. The present discussion illustrates the roles played by both ions in promoting an enhancement of the macroscopic electrical conductivity. Before electrochemical reduction, Cu" ions are surrounded by oxygen atoms, coming from the surrounding water molecules.At the commencement of Cu' formation (first kinetic domain), the rate of Cull reduction is large. Since Cu' ions are not stable in water, they interact with the oxygen atoms of the S03CF, doping salt: Cu'S0,DF.; is already known to be stabilized by benzene, giving rise to a crystallized compound. Within the second kinetic domain, stabilization of the Cu' ions by the S atom of the polymer occurs because the oxygen compiexing sites of the doping salt are already occupied. Two sources of sulphur atom are available in the doped PMeT: i.e. the doping salt SO,CF, and the heteroatom of the monomer unit of the polymer. Because of the steric hindrance due to the oxygen and carbon atoms, the sulphur atom of the salt cannot be part of the first coordination shell of any other element.Therefore, Cu' ions interact with the sulphur atom of the thiophene unit. The 40% Cu' concentration limit of the second kinetic domain should correspond to the easily accessible S sites of the polymer. Finally, metallic copper is formed with a well organized platelet crystalline structure. The CuO concentration increases rapidly with a concomitant increase in the rate of Cu" reduction. These changes can be understood if the formation of CuO metallic clusters proceeds from Cu' ions through the fGllowing disproportionation mechanism c u ' + c u ' --* c u l l + CUO. According to that scheme, formation of CuO acts as a source of Cu" ions and electronic charges from the external source are only needed for the Cu" reduction step (Cul'+ e -+ Cu').The time dependent derivative of the Cu" concentration curve is found, therefore, to be similar to the time-dependent electrolysis current curve (fig. 1A) (if the sign of the slope is neglected and with the possible exception of a difference in the timescale): the increase of the electrolysis current curve corresponds to the increase of the Cu" reduction rate observed in the third kinetic domain. The presence of Cu' ions at the end of the electrochemical reduction treatment is consistent with the fact that Cu" formation begins after all the stabilizing sites are already occupied. Comparison of the ex situ macroscopic conductivity of the cathodically polarized PMeT in CuCI, solution (fig. 1 ) with the time-dependent concentration curve of the various copper species (fig.4) shows that the increase of conductivity is not correlated to the formation of CuO metallic clusters. The CuO concentration increases at the end of the electrochemical reduction process while the change in the conductivity occurs at the beginning of the electrochemical treatment. In fact, the conductivity increase is correlated to the appearance of a Cu'-S complex (second kinetic domain). The slight difference in the timescale is not a key issue. I t should be due to differences in the CuCI, solution concentration and position-dependent electric field vector intensity in a cylindrical geometry. The u = 150 0 cm ' conductivity observed for long polarization times corresponds to the constant Cu' ion concentration.50 In situ XAS of Conducting Polymers Inclusion of Cut-S bounded ions is therefore seen to be an effective way to increase the conductivity. According to a previously proposed scheme,? the Cu' interaction with two sulphur atoms on different polymeric chains bridges them together and reduces the interchain hopping time.In this model, the copper atom is linked to two sulphur atoms which should be in a linear configuration. Transmission electron microscopy and X-ray diffraction data obtained on those systems has revealed the existence of well defined crystalline patterns, suggesting the evolution of the material towards an organized system.' The situation of the PMeT-FeCl, system is somewhat different. There is a spon- taneous association of Fe"' with the sulphur atoms of the polymer upon incorporation of the ions in the polymer. As Cul-S, this bridging causes a reduction of the interchain hopping time and an increase of the macroscopic conductivity.This is what is observed at the very beginning of the ex situ macroscopic conductivity curve of a cathodically polarized PMeT in FeCI, aqueous solution. However, for long polarization times, two phenomena act to reduce the conductivity. First of all, as it was demonstrated by EXAFS and XANES measurements, Fe" ions are surrounded by six oxygen atoms and they do not interact with the polymer backbone. Upon reduction, the bridging of the polymeric chains by Fe"' ions becomes less strong and the macroscopic conductivity should decrease to its initial value. However, (T decreases to a value lower than that of the untreated polymer.That is tc say that, for long polarization times, Fell ions induce a dedoping effect. This obserr.dtion has already been made in the case of polypyrrole synthesized" in the presencz of Fe(CN),". Both factors contribute, therefore, to a decrease in the (T values from 75 to 1.0 Ki cm * and may be thought to be responsible for the slow kinetics of reduction (more than 2 h even if the applied potential was increased in the course of :he experiment), the potential drop in the polymer being important due to the high resistivity. Iron clusters do not grow into PMeT, probably because the reduction potential of the Feii+2e - Fe" reaction is too negative compared to the reduction potential of the polymer. Conclusion Time-dependent structural investigations can be correlated to electrical measurements to derive fundamental explanations for the increase of the macroscopic conductivity of conducting polymers upon electrochemical inclusion of metallic species. The inclusion of metallic ions in polymers is an efficient way of increasing their conductivity by bridging together different chains and reducing the interchain hopping time. However, choice of the bridging ions is not a straightforward matter since solubility and redox properties as well as complexation processes should be considered. References I C. Tourillon, in Hotidhook of ('otitfirctitig P i i / ~ > t n e r ~ ed. T. A. Skotheim (Marcel Dekker, New York, 19861, vol. I , chap. 9, p. 393. 2 M. Kobayashi, N . C'olaneri, M. Hoqssel, F. Wudl and A. J . i-leeger. ('hem. PIiI.\., 198S, 82, 5717. 3 I). C o u r i e r and G . Tourillon J. P / i j , \ . ('hetw, 1986, YO, 5561. 4 G. Tourillon, E. I h r t y g e , A. t-ontaine and .A. Jucha, P/i\.\. Rep. Lt.//., 1986, 57, 603. 5 C . Tourillon, and F. Gamier, J . E / w f t - ( i ~ i t i t i / . ( ~ h e u i . , 1982, 135, 173. 6 N . N . Greenwood and A. E a r n r h a u . < ' / i e t ? i i s / t . ! , o f ' / / i r ~ E/et?ien/\. ( Pergamon Press. Nen York, IY86). 7 C;. N . Greaves, P. J . Durham, G. Diakun a n d P. Quinn, Ncr/irr.u, I9tII. 294, 139. 8 L. S . Kau, E. W. Svastitr, J . H. I h w s o n and K . 0. Hodgson, Itiorg. C-hetri., 1986, 25, 4307. 9 LJ. J . C'alvo, M . L. I h r o u x and t. 13. Yeaper, h o c . E / ~ . c . t r . o c . l i r i r i i . Soc... 1985, 85, 125.

ISSN:0301-7249    

DOI:10.1039/DC9908900041

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1990, 89, 51-63 X-Ray Absorption Spectroscopy under Conditions of Total External Reflection: Application to the Structural Characterisation of the Cu/GaAs( 100) Interface S. Pizzini and K. J. Roberts*+ Department of Pure and Applied Chemistry, Uniuersity of Strathclyde, Glasgow GI 1 X L G. N. Greaves SERC Daresbury Laboratory, Warrington WA4 4 A D N. T. Barrett LURE Bztiment 209D, Centre Uniuersitaire, 91 405 Orsay Cedex, France I. Dring and R. J. Oldman ICI Chemicals and Pol-ymers plc, The Heath, Runcorn WA7 4QE The theoretical principles and instrumentation requirements of X-ray absorp- tion spectroscopy under conditions of total external reflection are described with particular reference to their application in the characterisation of solid/solid interfaces.The advantages of combining XANES, EXAFS and reflectivity in a single series of measurements are highlighted through the structural characterisation of oxide layers on Cu and GaAs( 100). The data reveal the surface of Cu deposited on float-glass to comprise a macroscopic mix of metallic C u and structurally disordered oxide. The latter appears to have a local structure close to that observed in Cu10 but with a higher oxygen coordination af, the surface. The surface of commercial-grade GaAs( 100) has ca. 7-9 A of disordered oxide in which G a coordinates to oxygen both tetrahedrally and octahedrally whilst As is only found in tetrahedral sites. Strong correlated second-shell cation coordinations around both G a and As central atoms reveal a single non-stoichiometric surface oxide depleted of As at the surface.10 and 100 A thick Cu films deposited on GaAs(100) are found to be completely oxidised with the oxide being more disordered and having a higher oxygen coordination than those deposited on float-glass. For 10 A thick films, Cu appears coordinated to Ga and As through the oxygen which decorates the inner surfaces of micro-voids and fissures in the disordered oxide on GaAs. 1. Introduction Solid/solid interfaces play a key role in many physical and chemical processes and their non-destructive structural characterisation is of significant technological interest. The structural nature of the interface between metals and semiconductors is of fundamental importance to device technology. The formation of a passivated oxide layer at this interface either during or after fabrication can have a significant impact on electrical conductivity and hence device performance and reliability.Additionally, the structural control of such interfaces is important in MOS technology for the preparation of well defined Schottky barriers. ; Also at SEKC Daresbury Laboratory, Warrington WA4 4AD. 5152 The Cu/GaAs( 100) Interface Experimental studies have so far concentrated mostly on metallic depositions on clean GaAs surfaces, usually in UHV conditions.'-' The chemistry at such clean interfaces is now reasonably well understood' and predicts the formation of alloyed phases associated with topochemical reaction between the semiconductor and its overlay- ing metal. Photoemission studies'-' of Cu/GaAs show evidence for intermixing between Cu with the substrate, resulting in the formation of copper arsenides and the segregation of metallic gallium.However, commercial-grade GaAs has a native oxide coating,x and thus in practical electronic devices the presence of a thin insulator between the metal and the semiconductor9 is assumed. Although some XPS studies'" of the Cu/GaAs( 100) system have shown the oxide layer on GaAs wafers to be unaffected by the metal overlayer, the understanding of the whole interface region including the extent and form of the oxide layer on the substrate, the reaction of the metal overlayer with the substrate and the structure of the metal overlayer is far from complete. X-Ray absorption spectroscopy under conditions of total external reflection' ' - I z allows surface sensitive studies to be carried out under realistic operational conditions (i.e.in air, under dense gases or under liquid). In this paper we outline the theoretical principles and instrumental requirements for this technique and describe its application to the structural characterisation of the Cu/GaAs( 100) interface. The latter is presented in three stages: first, to examine the surface structure of Cu thin films on an inert glass substrate; secondly, to determine the local atomic structure of the oxide layer on a polished GaAs( 100) wafer;14 and finally to determine the Cu environment within Cu overlayers on GaAs( 100). 2. Glancing-angle X-Ray Absorption Spectroscopy 2.1. Background Theory X-Rays can be made surface-sensitive by using glancing-angle geometry.When a monochromatic X-ray beam is incident on a flat surface there is a well defined angle of incidence 4c below which the angle of refraction is zero and total external reflection takes place. Total external reflection is possible at the interface between air and a condensed medium because the refractive index of most materials is less than 1 in the X-ray part of the electromagnetic spectrum." The critical angle 4c is related to the refractive index of the condensed medium ( n 2 ) by Snell's law: (1) where 6 = (4nNe2/m)1/E2, p = pA/4n; N is the surface atomic density (atoms cm-'), E is the X-ray photon energy and p is the absorption coefficient of the medium. As p and S are of the order of 10~'-lO-h the numerical value of 4c is small (typically of the order of a few mrad for most materials).Far from an absorption edge p can be neglected cos 4c = n, = 1 - 6 - ip and cos 4c can be expanded to give: 4 , - ( 2 6 4,( mrad) = 20p I where p is the density of the sample (g cm - 3 ) . or in practical units: ?/ E (keV) ( 3 ) From electromagnetic theory, for glancing angles 4<< bc the refracted wave does not penetrate the medium but propagates parallel to the surface, vibrating in a plane perpendicular to it, and is coupled to the standing wave generated by the incident and refractive waves." The penetration depth z of X-rays in the medium is governed by the damping of the evanescent wave in the direction perpendicular to the surface. As 4 decreases z falls to a minimumih such that: (4) Zm,n = A / (47dh.I.S. Pizzini et al.53 zmin is typically ca. 20-50 A for most materials for hard X-ray photons. As 4 is increased past +c the penetration rapidly increases, approaching the limiting case for 4 >> & c , where z = (sin 4 ) p . From the Fresnel equations, the X-ray reflectivity coefficient for photons of energy E incident on a perfectly smooth surface with glancing angle 4 is given by:'6 h - ~ [ 2 ( h - 1)]"2 R( E, x) = h + x [ 2 ( h + l)]''? where x = 4/qhc, h = x'+(x'- 1)'/?+ Y' and Y = P / S . This equation thus shows that the reflectivity R ( E ) is related to both the real (8) and imaginary (P) parts of the refractive index. Thus the reflected X-rays scattered from a sample at energies greater than a component absorption edge will contain extended absorption fine structures (ReflEXAFS) in both P ( E ) and S( E ) , related by the Kramers-Kronig equations.R ( E ) can be written" as: R(P, 6 ) = R(P0, &J+AR(P -Po, -80) (6) where Po and 6,) refer to the non-oscillatory part of p and 6 and ( P - Po) and (6 - &,) are, respectively, the P - and S-EXAFS. The difference between these two components ( A R ) can be expressed as: (7) The relative weights of the p- and S-EXAFS are determined by the function (dP/dS). Martens and Rabe" have shown that the latter approaches zero for glancing angles 4 << 4c. In this condition, the 6-EXAFS can be neglected and the ReflEXAFS are essentially P-EXAFS. By inverting eqn ( 5 ) , the relation between P ( E ) [ e . g . p ( E ) ] and A R ( p -Po, S-Sd = (d~/dP).~CO,,\,[(P -Po)+(dp/dS).,c"",,(S -&)I- R ( E ) can Since the (1 - W/(I be extrapolated.In the case 4 << &c we obtain:" oscillations in S are small compared with the oscillations in the function + R ) , 6( E ) can be approximated to a monotonic function beyond an absorp- tion edge and the ReflEXAFS are therefore quite simply related to the EXAFS oscillations in the absorption coefficient. The same treatment cannot be applied to the case @<< 4c, where the oscillations in S dominate." In this case a Kramers-Kronig analysis must be applied to derive the 6-EXAFS from the experimental R ( E ) . " In this work we restrict ourselves to the case where 4<< 4', where we can use the approximation p == 1 - R, derived from eqn (8) by assuming A R << R.The resulting absorption spectra were normalised, background-subtracted and modelled using the curved-wave least-squares fitting routine EXC'URVXX. I' 2.2. Instrumentation Working at the small glancing angles required for total external reflection geometry imposes tight constraints on incident-beam collimation and sample alignment. We have developed a dedicated instrument"' for Refl EXAFS investigations on the Daresbury SRS (fig. 1 ) ; it has the following features: ( a ) beam definition is provided by precision tungsten carbide slits 50pm thick which are positioned before and after the sample; ( h ) the sample is set on a horizontal goniometer (step size 0.25 mdeg) on which is mounted a motorised translation stage (step size 1 p m ) which is used to align the sample precisely at the focus on the goniometer arc, ( c ) the incident ( I < ) ) and reflected ( I , ) intensities as function of photon energy are measured with ion chambers.A third ion54 I _ _ _ _ _ _ The Cu/GaAs( 100) Interface Fig. 1. Schematic diagram of the experimental set-up for the glancing-angle instrument. chamber ( I , ) is available to record the transmission spectrum of a model compound simultaneously and so provide for precise energy calibration and measurement of any chemical shift. Experimentally the sample is first accurately aligned on the goniometer and a reflectivity curve is collected at a fixed energy close to the absorption edge of interest. From this R ( 4 ) dependence suitable angular positions, subject to the restrictions imposed by the approximations discussed in section 2.1, are chosen.R( E ) is then measured at the desired glancing angles ( 4 ) . ReflEXAFS measurements are carried out on station 9.2"' at the Daresbury SRS, which can provide photons in the 5-35 keV energy range. An Si(220) harmonically rejecting double-crystal monochromator is used; when convoluted with the extended source height of 300pm and the height of the precision entrance slits, it provides an angular resolution of<0.01 mrad. This corresponds to an energy resolution of ca. 5 x lop5, which is more than adequate for these kind of measurements. 3. Application to Cu/GaAs( 100) 3.1. Materials LEC Czochralski grown polished GaAs( 100) wafeTs, cut 2" off-axis, were obtained from ICI Wafer Technology.Cu thin films, ca. l000A thick, on float-glass substrates and 100 and 10 A thick on GaAs( 100) were vacuum evaporated at base pressures typically ca. 10- ' Tort-.? The evaporation rate was monitored using a quartz thickness monitor. 3.2. Cu on Float-Glass The reflectivity curve of the Cu film on glass, recorded at an energy of 8.68 keV is shown in fig. 2( a ) and reveals a critical angle ( 4, = 0.33') and hence a film density less than that expected for pure Cu (4,=0.36"). ReflEXAFS spectra at three glancing angles (4, = 0.3+,, b2 = 0.64, and 4l = 0.754,), corresponding to penetration depths of ca. 18, 21 and 26 A, respectively, were recorded." Fig. 3 compares the near-edge absorption spectrum of the Cu film (angle &) with the transmission spectra of some model compounds and shows that the surface of the C u film is partially oxidised.The edge position and characteristic edge feature indicates that oxidised Cu is present at the surface predominantly as C u ' . Fig. 2 ( b ) and ( c ) show the Cu K-edge k'-weighted experimental ReflEXAFS spec- trum (angle 4?) and the corresponding Fourier transform. The fitted parameters to the .; 1 Torr 1 0 1 315/760 PaS. Plzzini et al. 55 1.0 0.8 L .- > L 0 . 4 .- - (d E 0.2 C 0. 0 0. 00 0. 15 0. 30 0. 45 0. 60 (PIa 4 6 8 10 k l k ' 1.2 0. 0 0 2 4 6 8 10 R I A Fig. 2. Experimental data for 1000 A thick Cu film deposited on float-glass. ( a ) Reflectivity curve recorded at an energy of 8.92 keV also showing angular positions used to record ReflEXAFS spectra.( h ) k'-weighted ReflEXAFS spectrum recorded for glancing angle 4:. ( c ) Fourier transform of ( h ).56 1.2 0. 8 * .- c a & 0.4 m e, v 2 0. 0 .fl B 2 -0.4 -0. 8 The Cu/GaAs( 100) Interface -. .. -. -. .- .- -20 0 20 40 60 80 100 energy/ eV Fig. 3. The near-edge spectra of model compounds Cu ( a ) , Cu,O ( b ) , CuO ( c ) tecorded in transmission compared with the ReflEXAFS spectra of Cu/glass at angle & ( d ) , 100 A Cu/GaAs ( e ) and 10 8, Cu/GaAs d f ) (cf: section 5). The energy values have been scaled to the approximate position of the absorption K-edge for metallic Cu at 8.779 keV. data together with the crystallographic parameters of the model compounds are given in table 1. These show that whilst the film surface is oxidised a substantial amount of residual mTtallic Cu is also present.The latter is evidenced by the strong coordination at ca. 2.5 A which is due to the first shell of f.c.c. metallic Cu. This shell is ca. 50% of the amplitude for bulk Cu and is complemented by an oxygen shell present at ca. 1.85 A for the angles 4 , and 42 and at ca. 1.90 A for &. The former is close to the first oxygen shell in Cu20. The Cu-Cu distances at 2.8-3.0 A are probably due to cation shells of the oxide phase and should be compared with the second-neighbour shell at 3.00 A in bulk Cu20. The splitting of this shell on the ReflEXAFS data suggests that the surface layer has a more distorted structure than bulk Cu20. The weightings of the coordination numbers of the first (oxygen)- and second (metallic copper)-neighbour shells are consistent with a mixed metal-oxide surface layer with each phase giving a contribution of ca.50%. However, the angular dependence of the coordination numbers of the 0 and Cu cation shells are indicative of a composition which varies slightly with depth, the oxide phase being more pronounced in the outmost surface layer. The longer Cu-0 distance typical of CuO, obtained for the lowest penetration depth (angle 4 , ) confirms this trend by being indicative of a higher associ- ation to oxygen in the outmost surface layer. 3.3. The Oxide Coating on GaAs(100) Fig. 4(a) shows the reflectivity curve from a polished 3 in? GaAs( 100) wafer at an energy of 11.78 keV, which is close to the ASK absorption edge. Associated ReflEXAFS spectraS.Pizzini et al. 57 Table 1. Shell distances R ( A ) , coordination numbers N and Debye-Waller factors 2 0 2 (Az) obtained from least-squares fits of the ReflEXAFS spectra of the Cu film deposited on glass atom N 0 1.4 c u 5.2 c u 3.9 c u 6.1 c u 3.7 0 1.2 c u 6.6 c u 2.6 c u 5.0 c u 1.3 0 1.1 c u 6.7 c u 2.3 c u 3.2 c u 3.0 R 2rr2 41 1.91 2.53 2.81 3.05 3.43 42 1.85 2.52 2.77 2.98 3.45 43 1.86 2.54 2.81 3.01 3.49 0.010 0.018 0.015 0.027 0.029 0.0 10 0.018 0.015 0.027 0.029 0.010 0.018 0.015 0.027 0.029 ~~ ~ Cu (metal) c u z o CUO atom N R atom N R atom N R c u 12 2.556 0 2 1.841 0 4 1.947 c u 6 3.615 c u 12 3.007 0 2 2.766 0 6 3.526 c u 4 2.884 c u 4 3.071 c u 2 3.159 c u 2 3.410 c u 2 3.727 The three glaqcing angles + I , 4z and 43 are as indicated in fig.2(a). The crystallographic values of N and R ( A ) for Cu, Cu20 and CuO are also reported. at two glancing angles ( 4 , = 0.604, and 42 = 0.704,), corresponding to penetration depths of ca. 25 and 30 A, respectively, were recorded above the ASK and GaK absorption edges. The ASK-edge k'-weighted ReflEXAFS spectrum (angle 4,) and the correspond- ing Fourier transform are shown in fig. 4( b ) and ( c ) . The structural parameters obtained from the least-squares fitting of these data are given in table 2 together with the crystallographic parameters of some model compounds. These results show the surface layer to be partially oxidised whilst the persistence of correlations characteristic of the GaAs substrate indicates that the oxide thickness is less than the X-ray penetration depth.The closeness of the ReflEXAFS fits to the GaAs bulk structure confirms that phase separation between the GaAs substrate and its oxygen-rich surface phase occurs. The approximate thickness of this layer, estimated from the penetration depth and the observed coordination numbers, is ca. 7-9 A." Ga and As atoms have different oxygen environments in the 2urface oxide. There is an oxygen neighbour shell for both Ga and As atoms at ca. 1.7 A, close to that found for the tetrahedral coordination in GaAsO, or three-fold coo!dination in As,03. However, Ga atoms have an additional oxygen shell at ca. 1.95 A which is typical of58 The Cu/GaAs( 100) Interface 1.0 2 0.8 .- > .- * 2 0. 6 2 B g 0. 2 vl 0. 4 .- - 2 c 0. 0 0. 0 0. 1 0. 2 0. 3 cpl" I 4 6 8 10 k1A-I 0 2 4 6 8 10 R I A Fig.4. Experimental data for polished GaAs. ( a ) Reflectivity curve recorded at an energy of 11.78 keV, showing the positions of the two glancing angles 4, and 4: at which ReflEXAFS spectra were recorded. ( h ) As K-edge k'-weighted ReflEXAFS spectrum recorded at glancing angle 4 , . ( c ) Fourier transform of ( h ) .Faraday Discuss. Chem. SOC., 1990, vol. 89 S. Pizzini et ai. (Facing p . 5 9 )S. Pizzini et al. 59 Table 2. Shell distances R ( A ) , coordination numbers N and Debye-Waller factors 2 a 2 (A’) obtained from least-squares fits of Ga and ASK-edge ReflEXAFS spectra of the GaAs( 100) wafer for two glancing angles 4 , and 42 < 4?) (M stands for As or Ga) Ga-0 Ga-0-M Ga-As Ga-Ga R N 2u’ R N 2u’ R N 2u2 R N 2a2 __ 41 1.72 0.6 0.001 2.88 3.1 0.006 2.46 3.0 0.008 3.92 10.1 0.026 1.95 2.2 0.002 3.15 3.4 0.005 42 1.66 1.3 0.008 2.81 3.0 0.005 2.49 2.4 0.022 3.97 4.5 0.018 1.96 2.7 0.003 3.10 5.4 0.008 As As-0 As-0-M As-Ga As- As R N 20’ R N 2a’ R N 2a’ R N 2u2 __ - ____________ 41 1.68 0.9 0.002 2.84 1.1 0.012 2.43 3.1 0.012 3.95 10.4 0.028 3.09 1.0 0.012 42 1.68 1.1 0.001 2.84 1.0 0.006 2.44 3.0 0.012 3.97 9.0 0.026 3.10 1.2 0.007 Ga-0 As-0 Ga- As Ga-Ga As- As R N R N R N R N R N GaAs 2.44 4 3.99 12 GaAsO, 1.77 4 3.16 4 1.72 4 3.16 4 Ga,O, 1.92 3 2.83 1 2.08 3 2.94 3 3.3 1 3 As20, ‘ I 1.79 3 3.22 3 The crystallographic values of R ( A ) and N for GaAs and some Ga and As oxides are also reported.I’ Arsenolite phase of As,O,. the octahedral coordination of Ga,O,. Thus while As in the surface oxide seems to be exclusively in a tetrahedral environment, Ga exists in both tetrahedral and octahedral coordinations.The Ga (As) coordination shell distances at ca. 3 A resemble the second-she11 distances of Ga and As oxides (table 2). Two cation shells are present, at the same distances (ca. 2.8 and ca. 3.1 A) for both Ga and As central atoms. These are probably associated with the two different oxygen coordinations for Ga. The shell at ca. 2.8 A possibly arises from a coordination between cations both in tetrahedral environment, whilst the shell at ca. 3.1 A shell is related to the coordination between octahedral Ga and a tetrahedral cation (As or Ga). The matching of these second-shell distances for both Ga and As central atoms show the oxide to be a single component which can thus be modelled as a microscopically random mixture of tetrahedral and octahedral sites,60 The Cu/GaAs( 100) Interface with Ga occupying both and As only the former four-fold site.Conversely, if the surface comprised a macroscopic mixture of the separate oxide phases, as suggested before," then only one second shell would be present to complement the single oxygen shell for As. Comparison between the Ga and As environments as a function of glancing angle shows no significant variation for As concentration while indicating a distinct increase in Ga coordination. Together with the angle dependence of the 0-shell amplitudes, this trend indicates a greater Ga association to 0 in the outmost oxide layer and a correspond- ing As depletion.Calculation of the oxygen-bridging cation-cation bond angles for the ca. 2.8 A correlation show this to be close to tetrahedral, which indicates that oxygen atoms may initially enter the GaAs structure in substitutional sites. The depletion of As atoms from the GaAs( 100) surface suggests that these are the sites where the oxygen substitution preferentially takes place. However, the weighting of the coordination numbers of the oxygen and the Ga (As) oxide shells suggest that this substituted structure is less well packed than the substrate. Also, a consistent number of octahedral sites is present in the oxide layer, especially in the near surface. Plate 1 shows a structural model based on this analysis. This was constructed as a continuous random network (CRN)".'" built on a GaAs(100) substrate.Oxygens were attached to the dangling bonds at the surface and the CRN was built up from a mixture of octahedral and tetrahedral Ga atoms, tetrahedral As atoms and bridging 0 atoms. Bond lengths were closely constrained to the measured oxygen distances, but the tetrahedral bond angle was allowed to vary. An open structure results. The presence of two-fold coordinated oxygens and the variable cation coordination combine to encourage the formation of micro-voids (typical size ca. 5 A ) and fissures whose internal surfaces are oxygen-rich. 4.3. Cu Overlayers on GaAs(100) Fig. 5 ( a ) compares the reflectivity curves for 100 and 10A thick Cu overlayers on GaAs( 100) at an energy of 8.68 keV. The measured critical angles (4cl =0.310° and Scz = 0.278") should be compared with those calculated from the electron density for a clean surface which for Cu, Cu,O and GaAs are 0.364, 0.310 and 0.284", respectively.The relationship between 4c and density [eqn (3)] shows the thicker Cu overlayer to have a density similar to that of Cu,O. As the thickness of the lOA Cu overlayer is less than the penetration depth of X-rays, the reflectivity process is expected to be determined by the GaAs substrate interface, with the difference in 4,.. with respect to the calculated value being probably due to the presence of a graded interface." Comparison of the near-edge spectra of the two Cu overlayers on GaAs( 100) [fig. 3( e ) and (f)] with the model-compound data show subtle differences with respect to the Cu/glass data discussed in section 3.2.Whil2t the position of the edge feature indicates the presence of Cu' cations in both the 10A and the 1008, overlayers, the additional presence of a shoulder at ca. 7 eV above the edge, close to that observed in CuO, suggests a non-stoichiometric oxide comprising both Cu' and Cu" cations. Additionally, the XANES spectra for Cu/GaAs are typical of a completely oxidised surface, in contrast to Cu/float-glass which show residual features typical of metallic Cu. Background-subtracted ReflEXAFS spectra for 100 A ( 4 , = 0.44,) and 10 8, (& = 0.5 thick Cu films on GaAs are overlayed and presented together with their corresponding Fourier transforms in fig. 5 ( 6) and ( c ) . The structural parameters obtained from least-squares fits of the data are resumed in table 3.In both the thinner and the thicker overlayer we find that Cu is completely oxidised. In the thicker layer no significant long-range order is observed. This is consistent with a disordered oxide overlayer being formed from the amorphous oxide covering GaAs( 100).S. Pizzini et al. 61 1.0 3 0.8 .- > .- c) 2 0. 6 -0 2 % 0.4 8 0.2 .- - c 0. 0 0. 6 15 10 5 M h -Y * s o -5 -10 2 4 6 8 10 k / k ’ 0 1 2 3 4 5 R / A Fig. 5. Experimental data for Cu on GaAs showing overlayed data from 100 A (-) and 10 8, thick (. - - * . * ) Cu overlayers on GaAs(100). ( a ) Reflectivity curves recorded at an energy of 8.68 keV. ( b ) Cu K-edge k3-weighted ReflEXAFS spectra. ( c ) Fourier transforms of ( b ) . The C u - 0 distances in the Cu oxide are longer than the more ordered phase observed on glass and are more typical of four-fold coordination in CuO.The more extensive Cu oxidation on GaAs compared to float-glass substrate is presumably due to the open structure of the oxide layer on GaAs. The cracks and micro-voids in this layer will encourage mass-transport and hence the reactivity of this interface, Cu initially bonding covalently via lone-pair electrons of the 0 atoms decorating the internal surfaces. In the thinner Cu layer, additional cation correlations are found at higher distances. Whilst the similar back-scattering amplitudes of Cu, Ga and As make an unambiguous assignment of these correlations very difficult, the amorphous nature of the oxide62 The Cu/GaAs( 100) Inferface Table 3.Shell distances R ( A ) , coordination numbers N and Debye- Waller factors 2a' (A') obtained from least-squares fits of Fourier-filtered CuK-edge ReflEXAFS spectra of Cu/ GaAs( 100) atom N R 2 2 100 A 0 3.2 1.95 0 2.3 2.60 0.01 1 0.008 10 A 0 4.3 2.00 0.009 0 2.4 2.8 I 0.009 M 3.0 (10.2) 3.00 (kO.003) 0.010 M 5.3 (k0.5) 3.80 (10.004) 0.010 M is Cu, As or Ga. The values reported in parentheses refer to the variations in the parameters obtained for M = Cu, As or Ga. discussed above seems to preclude the presence of Cu cations at these larger distances. Additionally, as the first Cu-M (M = Ga, As or Cu) correlation is calculated at a distance (ca. 3.00*0.03 A) close to the cation-cation distance in the oxide coating on GaAs (table 2); this suggests that Cu trapped in the voids in the surface oxide correlates with the surrounding cations.The open oxide structure on GaAs should promote Cu inter- diffusion into the oxide. Such cation diffusion is commonly observed in chalcogenide glasses." Alternative models suggesting the formation of arsenides and alloys can also be ruled out. Cu-As first-shell distances in Cu,As or Cu,As are typically ca. 2.45-2.5 A, whilst Cu-Ga distances in Cu-Ga alloy are expected at ca. 2.55 A. The absence of coordination shells at such short distances tends to support previously published data"' in excluding the formation of Cu arsenides or Cu-Ga alloy at the Cu/GaAs(100) interface. In the thicker Cu overlayer Cu-M correlations are not present and the oxygen coordination number is smaller.I t would seem that the reactivity of Cu in the structure of the thinner Cu overlayer catalyses extensive oxidation when more Cu is present. Where Cu passivates on the surface of float-glass, leaving approximately half of the Cu as metal, the opposite appears to be the case for GaAs( 100) where all the Cu becomes oxidised. As the Cu coverage builds up it would appear that the surface oxide generated is increasingly disordered, explaining the absence of Cu-M correlations and the reduced oxygen coordination numbers. 4. Conclusions X-Ray absorption spectroscopy under conditions of total external reflection provides a useful technique for the structural characterisation of 'real' surfaces and interfaces. Structural information from XANES, EXAFS and reflectivity can be combined in a single series of measurements to provide a cross-correlated structural probe.By varying the glancing angle within the total external reflection region, quite subtle depth-resolved structural variations can be characterised. This technique has been applied to a structural investigation of the Cu/GaAs( 100) interface. Examination of Cu thin films deposited on float-glass substrates reveals the surface to consist of a mixture of metallic and oxide (predominantly Cu') phases. The oxide is found to have fairly long-range structural order but with a local environmentS. Pizzini et a]. 63 distorted with respect to that expected for a bulk Cu' oxide. The surface oxide of GaAs( 100) appears to have an open amorphous structure consisting of microscopically random distributions of octahedrally coordinated Ga and tetrahedrally coordinated Ga and As.Cu deposited onto the GaAs surface is completely oxidised, producing an amorphous phase of four-fold Cu. At the interface correlations are seen, which suggest the Cu bonds to the oxygen-rich internal surfaces present in the voids and micro-fissures which characterise the natural oxidised surface of GaAs( 100). No trace of arsenides or Cu-Ga alloy formation is present at the interface. Further applications of these techniques can be envisaged to other 'real' interfaces such as those involved in catalysis, electrochemical processes, ion-implantation and low-dimensional solids. Developments associated with these techniques are also cur- rently in p r ~ g r e s s , ~ ~ * ~ ' notably the incorporation of powder X-ray diffraction within our general set-up, which is directed towards a complete characterisation of interfaces using synchrotron radiation.This work has been supported by an S.E.R.C. cooperative research project in collabor- ation with ICI Chemicals and Polymers plc. We gratefully acknowledge S.E.R.C. for provision of beam time on the Daresbury SRS, and K. Singer who prepared the Cu overlayers on GaAs. S.P. acknowledges ICI Chemicals for the financial support of a research assistantship. References 1 S. H. Pan, D. Mo, W. G. Petro, I . Lindau and W. E. Spicer, J. Vac. Sci. Technol. B, 1983, I , 593. 2 W. G. Petro, T. Kendelewicz, I . Lindau and W. E. Spicer, P h j ~ . Rev. B, 1986, 34, 7089. 3 W. G.Petro, I. A. Babalola, P. Skeath, C. Y. Su, I . Hino, I. Lindau and W. E. Spicer, J. Vac. Sci. Technol., 1982, 21, 585. 4 I . Lindau, T. Kendelewicz, N. Newman, R. S. List, M. D. Williams and W. E. Spicer, Surj: Sci., 1985, 162, 591. 5 R. E. Viturro, C. Mailhiot, J. L. Shaw, L. J. Brillson, D. La-Graffe, G. Margaritondo, G. D. Pettit and J. M. Woodall, J. Vac. Sci. Technol. A, 1989, 7, 855. 6 R. Cao, K. Miyano, T. Kendelewicz, I . Lindau and W. E. Spicer, Appl. Phj-s. Lett., 1988, 53, 210. 7 J. F. McGilp and A. B. McLean, J. Phjx C, 1988, 21, 807. X K. A. Bertness, J . J. Yeh, D. J. Friedman, P. H. Mahowald, A. K. Wahi, T. Kendelewicz, I . Lindau and W. E. Spicer, Phj~s. Rec. B, 1988, 38, 5406. 9 S. M. Sze, Phj~sics of.Semicotiductor Detliceq (Wiley, New York, 1981). 10 S. P. Kowalczyk, J. R. Waldrop and R. W. Grant, J. Vae. Sci. Technol., 1981, 19, 611. 1 1 G. Martens and P. Rabe, PI?j*s. Star. Sol. ( a ) , 1980, 58, 415. 17 S. J . Gurman and R. Fox, Philos. Mag. B, 1986, 54, L45. 1.1 S. M. Heald, H. Chen and J . M. Tranquada, Phj>s. Rev. B, 1988, 38, 1016. 14 N. T. Barrett, G . N. Greaves, S. Pizzini and K. J. Roberts, Surf Sci., 1990, 227, 337. I 5 R. W. James, The Crj.stalline State, Vol. / I , The Optical Principles of the Djfraction ot'x-raj-s ( G . Bell and Sons, 1969). I6 L. G. Parratt, Ph.rs. Rer., 1954, 95, 359. 17 L. Bosio, R. Cortes, A. Defrain and M. Froment, J. Electroanal. Chem., 1984, 180, 265. I X S. J. Gurman, J. fhj*s. C', 1988, 21, 3699. 19 S. Pizzini, K. J. Roberts, G. N. Greaves, N. Harris, P. Moore, E. Pantos and R. J. Oldman, Rec. Sci. /t?s/runi., 1989, 60, 2525. 2t) G . P. Diakun, G . N. Greaves, S. S. Hasnain and P. D. Quinn, Daresbury Laboratory Report, I)L/SC'I/TM38E (1984). 21 S. Pizzini, K. J . Roberts, I . Dring, R. J . Oldman and G. N. Greaves, P/ij.sica B, 1989, 158, 676. 22 W. M. Lau and R. P. Bult, Mater. Lett., 1987, 5, 88. 23 I). E. Polk and D. S. Roudreaux, Phj-s. Rep., 1973, 31, 92. 24 G. N . Grea\,es and E. A. Davis, Philos. MU,^., 1974, 29, 1201. 75 A. T. Steel, G. N . Greaves, A. P. Firth, A. E. Owen, J. Non-C'rj-st. Solids, 1989, 107, 1 5 5 . 26 N . Barlow. C. Hrennan, S. E. Doyle, G. N. Greaves, M. Miller, A. H. Nahle, K. J. Roberts, J. Robinson, J . N. Sherwood and F. C. Walsh, Reil. Sci. Instrum., 1989, 60, 2386. 27 <;. Derbyahire, B. Dobson, G . N. Greaves, N. Harris, P. Mackle, P. R. Moore, K. J . Roberts, N . Allinson, J . Nicoll, S . Doyle and R. J . Oldman, Rer. Sci. Inctrirm., 1989, 60, 1897. Paper 0/00293C; Receiued 18rh Jatiuar!., 1990

ISSN:0301-7249    

DOI:10.1039/DC9908900051

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1990, 89, 65-75 GENERAL DISCUSSION Dr A. Fontaine ( LURE, Orsay) addressed Professor Bradshaw: Electron energy-loss spectroscopy (EELS) is a competitor of X-ray absorption spectroscopy: What should be the future of X-ray inelastic diffraction in order to probe core-level excitation? In other words, using a very high energy beam monochromatized very carefully ( lOP6-lO-') coupled to an analyser with the same resolution, it should be possible to investigate the oxygen, N, C, . . . (all the low-2 elements) using a very thick sample. Can you comment on that? Prof. A. M. Bradshaw (Fritz-Haber-Institut der MPG, Berlin) replied: I agree with Dr Fontaine that there are areas in which EELS is a serious competitor of X-ray absorption spectroscopy in the study of core-level excitations.One very topical example is provided by recent work on high-T, superconductors. This is not necessarily true, however, for studies of surfaces, in particular of adsorbed layers. Although there have been measurements of SEXAFS-type wiggles in EELS of adsorbed layers, such investiga- tions are confined to very stable chemisorption systems not susceptible to electron beam damage. The particular experiment which Dr Fontaine describes (inelastic X-ray scatter- ing) has already been used to examine near-edge excitations in low-2 materials by Schulke et al.' It might prove to be a useful method for bulk samples and could avoid some of the problems of sample preparation associated with XUV absorption experi- ments. It might conceivably provide the possibility of investigating low-2 adsorbates on bulk catalyst samples; otherwise I cannot see any surface structural applications.I . W. Schulke, U. Bonse, H. Nagasawa, A. Kapolat and A. Berthaud, P h j ~ Rec. A, 1988. 38, 2112. Prof. D. P. Woodruff (University of Warwick) said: Although I believe extended fine structure in electron energy loss has been investigated only for atomic oxygen and carbon in the chernisorbed state, Hitchcock and co-workers have shown that near-edge structure in adsorbed benzene can be studied by this method, although the signal-to-noise ratios of their data are not very good so the experiment may be rather marginal. Prof. Bradshaw replied: That is correct. Hitchcock and co-workers' have recently published some EELS near-edge spectra for adsorbed benzene and pyridine.The S / N ratio is, however, considerably worse then in a state-of-the-art soft X-ray experiment with synchrotron radiation. Furthermore, we should remember that these molecular resonances in the near-edge region are at least a factor ten stronger than the SEXAFS wiggles at higher energies. 1 T. Tylisic7ak, F. Epo4to and A. P. Hitchcock, Phi,\. Rev. Lett., 1989, 62, 2551 Dr K. Prabhakaran ( University ofManchester) communicated: This is just a comment on the Cu( 110) (2 x I ) - 0 system. We found that oxygen is adsorbed molecularly in a peroxo form on a clean Cu( 110) surface at low temperature. Adsorption of molecular oxygen on a Cu( 110) ( 1 x 1 ) surface results in the formation of a diffuse ( 1 x 1 ) pattern, whereas when the adsorption is done on a Cu(ll0) ( 2 x I ) - 0 surface, it transforms back to the Cu( 110) ( I x 1) pattern.' This suggests that in the presence of peroxo species the missing row reappears on the surface.I Surf. Sci.. 1986, 177, L971 6566 General Discussion Prof. Bradshaw replied: I have no real explanation for this interesting result. One possibility is that the molecular oxygen requires a particular surface site associated with the unreconstructed surface. Molecular oxygen on Ag( 110) is known'.2 to be adsorbed in a lying-down configuration with its axis parallel to the (1 10) azimuth, presumably in the troughs of a non-restructured surface. The energy required to lift the reconstruction of the Cu(ll0) surface could also in this case be provided by the molecular oxygen. The amount is probably comparable with the energy difference of ca.40 kJ mol-' between Cu( 110)(2 x 1)-0 with and without the missing row, at least according to Nlbrskov's calculations.3 It is, however, remarkable that the atomic oxygen, which should still give the ( 2 x 1) structure, then forms a disordered layer. 1 K. C. Prince, G. Paolucci and A. M. Bradshaw, SurJ Sci. 1986, 175, 101. 2 D. A. Outka, J. Stohr, W. Jark, P. Stevens, J. Solomon and R. J. Madix, Phys. Rev. B, 1987, 35, 4119. 3 K. W. Jacobsen and J . K. N~rskov, to be published. Prof. R. W. Joyner ( University qf Liverpool) addressed Dr Heald: To the uninitiated, the growth mode shown in fig. 5 ( a ) of your paper for the Ni/AI layer is unexpected. Why does the Ni/AI, grow to a constant thickness (of ca.150 A ) in the range 250-310 "C, and why does a layer of apparently different composition grow on top with a thickness which is temperature dependent? Do you have independent corroboration, e.g. by electron microscopy, for the model you propose? Dr S. M. Heald (Brookhaven National Laboratory, New York) replied: It is quite common for reactions in thin-film couples to form predominantly one phase (in our case NiA1,). The uniformity of the growth depends on the dominant diffusion path. Our results show that the presence of 0 impurities in the AI suppresses the grain-boundary diffusion channel. This results in the growth of a uniform layer during the initial reaction at lower temperatures. For higher temperatures the blocking of the grain boundaries is overcome and the reaction also takes place outwards from the grain boundaries as well as from the interface.In both cases NiAI3 is being formed. The changing average composition at higher temperatures is due to a changing ratio of unreacted A1 to NiAI3. In response to your second question: We have not applied other techniques to Ni/AI. For the case of Cu/AI RBS and TEM studies were in good agreement with the X-ray results for both the composition and roughness of the interface in unreacted samples. Dr 1. K. Robinson ( A T & T Labs, Murrajj Hill, N J ) asked: With regard to your fig. 3, can you elaborate on the number of free parameters and the uniqueness of the fitting of reflectivity data? In particular, can you distinguish between a double step interface and a single broad interface (i.e.very 'rough')? Dr Heald replied: For each layer there are three parameters: thickness, density and roughness. Usually three or four layers are needed to obtain a good fit. However, we found that simultaneously fitting the spectra obtained above and below the transition- metal K-edge dramatically reduces the correlations between the parameters. It is true that different models (for example an error-function roughness compared to a linear transition region) can be used to fit the reflectivity, but if the density profiles from these are compared, they are quite similar. Thus, I believe that the density profiles are uniquely determined. Regarding the approximation of the double step with a single rough interface, this fails completely to explain the data.In fact, there are distinct indications of a double step in the 250 "C data in fig. 3. Note how the lowest-angle peak is significantly higher than the second and third, which are nearly equal. This results from a critical angle intermediate between Al and Ni, and shows up only if the intermediate layer is well defined.General Discussion 67 Dr R. J. Oldman (ICI Chemicals and Polymers, Runcorn) asked: Do you have an independent check on the roughness values derived from modelling reflectivity data or are they just modelling parameters necessary to get a good fit? Dr Heald replied: We do not have an independent check for the roughness of the Ni/AI and Cr/Al samples. I believe the roughness parameters are meaningful, however, since they were checked by TEM and surface profilometry in our earlier work on Cu/AI and Au samples.In those cases good agreement was found. Prof. J. F. van der Veen (FOM Institute for Atomic and Molecular Physics, Amsterdam) said: It seems to me that the Ni density profiles in your fig. 5 could just as well have been derived from a Rutherford backscattering analysis. RBS is much easier to perform. In the discussion section of your paper you mention that RBS does not have the same depth resolution as the glancing-angle reflectivity technique. What is the depth resolution in your experiments, and what specific advantages in your view does the reflectivity technique have over RBS? Dr Heald replied: We have sent out scme of the samples our for RBS analysis, and the typical depth resolution was ca.100A, while we can easily achieve sensitivities of 10-20 A in the interface width. I disagree with the notion that RBS is much easier to perform. We are dealing with reflectivities greater than I%, which means the experiments could be easily performed using laboratory X-ray sources. We used a synchrotron source as we were also making EXAFS measurements. The reflectivity also has the advantage that it can be applied in situ to gas/solid and liquid/solid interfaces. Prof. van der Veen then said: In RBS, one can achieve a depth resplution of ca. 50 A with the use of small exit angles. Even a depth resolution of s 10 A can be achieved, though I admit that the latter value is non-standard. [See e.g. ref. (1) for a review of high-resolution RBS.] 1 J .F. \an der Veen, Surf. Sci. Rep., 19S5, 5. 199. Prof. P. Pershan (Haruard Uniuersit-y) said: I would like to support the claim that the resolution of X-ray reflectivity is generally superior to that of Rutherford backscatter- ing. Although Dr Heald did not measure reflectivity to large angles, we have measured specular reflectivity of the Si(100) face past the [400] position. Resolution in this case is better than 1 and certainly superior to anything possible by Rutherford backscattering. Prof. D. C. Koningsberger (Eindhoven Uniuersitj? of Technology) said: You did not show any coordination parameters in your paper. The quality of your data looks reasonably good. So, I would like to ask you whether it is possible really to analyse the data, which you collected with glancing-angle EXAFS and what is the accuracy you can achieve? Dr Heald replied: The comparisons in the paper are qualitative since that was all that was needed to demonstrate the growth of NiAlz.It is certainly possible to analyse the data quantitatively, as we have demonstrated for Cu/AI interfaces [see ref. ( 2 ) of our paper]. To do this the data must first be corrected for anomalous dispersion affects. For the present data anomalous dispersion reduces the EXAFS amplitude by 30-50%, and distorts the phase such that analysis would result in about a 0.03 A error. Both the Ni and NiAI, contributions have nearly the same distortions, and, therefore, the qualita- tive comparisons in the paper are still valid. We have demonstrated previously that the distortions can be corrected sufficiently to have only a small additional contribution to the normal errors encountered in the EXAFS analysis.68 General Discussion Dr J.Jupille ( Laboratoire Maurice Letort, Villiers-les- Nancy) said: The EXAFS analysis is performed at different angles of incidence of the X-ray beam, so as to vary the part of the sample which is probed. However, the energy scan corresponding to the EXAFS study will change the depth of the analysis (as in the combination of EXAFS with standing waves). Is that problem accounted for during the collection of EXAFS data in the present case? Dr Heald replied: The penetration depth into the interface does vary as the energy is changed. However, for angles near the first peak, as was used for collecting the present EXAFS data, the change is quite small.It does need to be taken into account when correcting the data for anomalous dispersions distortions. For cases where the change in penetration is important, we have shown [see ref. (2) in our paper] that it is possible to vary the incidence angle as the energy is changed to maintain an approximately constant penetration. Dr K. M. Robinson ( US Naval Research Laboratory, Washington D.C.) said: Because a , varies as a, the penetration depth varies as a function of E. If you do no correction for a , ( E ) , are you not continually probing a different section (depth) of your sample? Even if you do a linear correction, the penetration will vary and the same question applies. Dr Heald replied: As mentioned in the previous response this was not a serious problem for the present measurements.It is true that for the near-edge region where the anomalous dispersion is large, additional changes in 6 occur which are not so easy to remove. However, we have not used the near-edge region in the present work. Dr K. Prabhakaran said: Could you predict anything about the reactivity of the interface, say, for example, with regard to the adsorption behaviour? The reason why I am asking is, we observed that the adsorption behaviour of a polycrystalline Ni surface can be altered by depositing Al. We also observed that Al is helping to fill the d-band holes. On the Al-deposited Ni surface we observed a precursor to dissociation of CO. Dr Heald replied: I do not believe that our results reveal much about this question aside from the observation of a spontaneous reaction between Ni and Al at room temperature.This is consistent with your observation of AI altering the chemistry of an Ni surface. Professor J. M. Thomas (The Royal Znstitution) addressed Dr J. Robinson: Whilst one is impressed by the strength of your argument (and the cogency of your fig. 5 in particular) that the structure of the passive film is, compositionally at least, given by Cr(OH), , I would be surprised if this were still the composition at modest temperatures. The hydrogen-bonded network that you talk of is hardly likely to survive more than a few hundred degrees Celsius. And I suppose that even at room temperature some ‘fluxionality’ of the OH groups might be expected.My second point refers to your remark, with which I agree, that the passive film on iron may be regarded as disordered y-FeOOH. My colleague, Dr Tricker when he worked with me in the University College of Wales, in the mid 1970s, showed by conversion electron Mossbauer spectroscopy-which, incidentally, via energy analysis, is capable of depth profiling-was among (if not) the first to show that y-FeOOH is present at a partially oxidized iron surface.’ 1 M . J. Tricker and J . M . Thomas, App/. Sur/. Sci., 1978, 1, 388. Dr J. Robinson (University of Warwick) replied: No we have not investigated what happens to the passive film on heating. We have, however, investigated the effect ofGeneral Discussion 69 transfer of the passive film to high vacuum and this does not appear to alter the structure significantly.Presumably at sufficiently high temperatures one might expect the hydroxides and oxyhydroxides to decompose to the corresponding oxides. Prof. J. V. Smith (University of Chicago) said: My first question addresses the interpretation of interatomic distances. A metal-oxygen distance is ca. 0.05 8, shorter than an M -OH distance. The interpretation of distances in an ordered crystal structure, as studied by Bragg diffraction, depends on the relation of the centroid of measured electron density to the disorder caused by both thermal vibration and chemical substitu- tion in the same crystallographic site. Depending on the vibration model the distances from the centroids may be increased by 0.01-0.03 8, in many inorganic materials at room temperature.The interference phenomena in EXAFS involve a different adjust- ment for the disorder. Hence, should your interpretation of the Cr(OH),-like phase consider these subtle effects? A second question concerns whether the experimental data will allow a mixture of other coordinations than the value of six. For example, the possibility of some five- coordination might allow a better spatial fit on a surface. Dr Robinson replied: In the case of the Cr(OH),-like phase I do not think there is any need to consider these subtle effects. The EXAFS and XANES of the passive film and commercial Cr( OH), are essentially identical with the corresponding bond lengths, refining to within 0.01 8, of each other. It was found that the phase shifts were transferable between Cr,O, and the hydroxide and therefore it is unlikely that anharmonicity needs to be considered in this case.On your second point, whilst the analysis of EXAFS is rather insensitive to coordina- tion numbers I would expect a significant reduction in the Fe-0 and Cr-0 bond lengths on going from six- to five-fold coordination. In addition, the XANES region is particularly sensitive to the coordination and I would again expect significant differences on going to five-fold coordination. In view of the similarities between the XANES of the passive film and that of the six-fold coordinate oxyhydroxide model compounds 1 think it is clear that the passive film does not contain significant quantities of five- coordinate metal centres.In view of the strong correlation between coordination numbers and Debye- Waller factors we used a coordination number of six in all our analyses of the Fe K-edge EXAFS to make it easier to identify any possible trends in the disorder with changing alloy composition. Prof. Koningsberger then said: An apparent deviation of the real coordination distance can be caused by not including in the EXAFS analysis an asymmetric distribution function. The result of not including an asymmetric distribution function leads to an apparent contraction of the coordination distance. Did you look in your analysis for the presence of an asymmetric distribution function? Dr Robinson replied: Given the signal-to-noise level of the data and the rather limited data range it is quite difficult to test for anharmonicity.We have, however, checked the phase differences between the passive film and several model compounds, as a function of k, and have been unable to detect any non-linearity. This suggests that the distribution function is symmetric. Prof. R. Parsons ( University ojSouthampton) said: Does the structural investigation give any clue as to why the film is passive? Presumably it must transfer electrons either by conduction or by tunnelling. Does the chromium hydroxide have such properties? Dr Robinson replied: The Cr(OH),-like phase is amorphous and I think it is this that gives the film its good passivating properties.70 General Discussion In answer to your second question. We have conducted photocurrent spectroscopy studies of the passive film on Cr-rich alloys.As with the film on pure iron, the passive layer behaves like an n-type semiconductor, but with the apparent band gap increased to ca. 2.5 eV. Given the thinness of the passive film, however, tunnelling seems highly probable. It should be possible to investigate this by studying electron-transfer reactions at the passive film surface, though I am not aware of any such studies having been made. Dr G. Thornton (University of Munchester) said: In the experiments you describe the metal/alloy film is consumed. Do you expect your conclusions to be valid for a system in which the metal/alloy substrate is present? Dr Robinson replied: The experiments are designed to consume all the metal film so as to facilitate the analysis of the EXAFS.If there is any unconverted metal present then this also contributes to the EXAFS. In particular the first shell of the metal overlaps with the second shell of the passive film (both of these shells contain metal atoms), making the analysis difficult. We have, hdwever, analysed films where there is ca. 20 A of metal protected by a similar thickness of passive layer and our conclusions remain the same. We therefore consider our approach to be valid. Dr J. W. Couves ( The Royal Institution ) addressed Dr Fontaine: Is there any evidence for the degradation of the polymer integrity during the reduction of the PMeT-FeCI, system? This could account for the decrease in macroscopic conductivity at long polarisation times, as well as the proposed loss of bridging Fe3+ and de-doping of the polymer.Dr Fontaine replied: In this case the distinction between incoming neighbours during the polarization is very favoured by the (.-) out of phase backscattering amplitudes of S and 0. Dr Jupille then said: The increase of conductivity of the polymer is related to the appearance of Cu' ions interacting with the sulphur atoms of the thiophene unit. This is in a second stage of the reduction process. In the first stage of that process, the Cu' ions are surrounded by oxygen. Do these environments lead to differences within the Cu K-edge absorption spectrum? Dr Fontaine replied: I t is true that the increase in conductivity is correlated with interchain connection, which occurs through Cu' bridging. Cu' in a linear configuration is identified in the Cu K-edge because of the non-bonding orbitals.Therefore XANES is not sensitive to the nature of the ligand. However, this is no longer the case when using EXAFS because Cu-S and Cu-0 appear as oscillations out of phase. Prof. Koningsberger said: The S / N value of your experimental data collected with the dispersive EXAFS method seems to be excellent, certainly taking into account the short data collecting time. Is it possible really to analyse these data and to obtain reliable coordination parameters? Dr Fontaine replied: I should have given more details about the EXAFS analysis, but because of the lack of time, I didn't show slides. But the paper contains figures showing how sensitive the dispersive geometry can be to trace tiny changes in short time.The lack of mechanical movement during data collection is of great advantage. It eliminates one of the main sources of noise given by the scanning mode.General Discussion 71 Dr I. K. Robinson then asked Dr Roberts: First, can you summarise what is learned about the depth distributions obtained by variation of the grazing angle? In your experience, is this as useful a tool as advertised? Secondly, could you elaborate on the surface preparation? You might expect to see As-As distances in an interface made with freshly grown GaAs. Dr K. J. Roberts (University of Strathclyde) replied: As far as the Cu films are concerned, no major structural changes were observed by changing the glancing angle below the critical angle. However, the oxide phase appeared to be more pronounced at the outermost surface and there was an indication of a greater proportion of Cu" in this top layer. The angle-dependent ReflEXAFS spectraoprobed the surface structure from a minimum of ca.15 A to a maximum of only ca. 30 A below the air/film interface. These penetration depths are imposed by the reflection geometry, since we are forced to work below the critical angle if we want to avoid correcting the measured data for the distortions introduced by anomalous dispersion. Nevertheless this is the most interesting region, and we have shown that structural changes can be recorded even in such a small range of penetration depths. Of course, whether angle-dependent measure- ments will be useful or not very much depends on the system and on the depth scale for which structural changes are expected to occur.The variation in the glancing angle was particularly useful in our ReflEXAFS study of GaAs(100). Here dramatic changes were observed in the environment of Ga atoms. In particular the coordination of Ga to 0 increases considerably at the outermost surface. The surface is observed to be depleted of As atoms in the surface, as evidenced by a comparison of the step edge thresholds (arbitrary units) which were: Z / A As K-edge Ga K-edge 41 25 0.14 0.16 42 30 0.2 1 0.18 In answer to your second question, surprisingly the surface science community has, as far as we know, yet to see the advantages of total external reflection geometry for SEXAFS-type measurements. Should such measurements be carried out one might well expect to see the formation of As-As dimers on such clean GaAs surfaces.For our work we examined GaAs( 100) as-received from ICI Wafer Technology. A native oxide ca. 10 A thick was therefore expected on the surface. As-As distances are unlikely at the GaAs/oxide interface but we cannot show this conclusively as, owing to the closeness in the atomic numbers of G a and As and in the corresponding backscattering amplitudes, it is difficult to distinguish unambiguously between these two atoms. This is certainly one of the limitations of the EXAFS technique. Were we to examine other III-V compounds such as InP or GaSb then the analysis of group V dimer formation would be easier. Dr Heald asked: Did you have a problem with Bragg reflections interfering with the single crystal EXAFS? Dr Roberts replied: We did not have any problem with the GaAs( 100) measurements since we recorded the spectrum in the reflected signal.The angle (typically 7") subtended by the detector window is too small to enable the first diffraction peak expected from the GaAs substrate to be recorded.7 2 General Discussion The same is not true when working in fluorescence geometry where to maximise count rate from dilute analytes a detector subtending a large solid angle is used. So far we have adopted two approaches to get round this problem: when working at glancing angles greater than +c we can spin the sample to 'average' out these effects;',' for total reflection geometry the goniometric requirements are too severe to enable sample rotation at constant 2.Recently, in collaboration with Professor Regnard's group in Grenoble, we have examined the local environment around ion-implanted As in crystalline Si. On that occasion a multi-element solid-state detector was used to discriminate in terms of energy between Bragg reflections and fluorescent radiation. Even in this case there have been problems as the Bragg peaks can saturate the individual detectors and the resulting non-linearities in the detector response (due to count pile-up) can make the data unanalysable. In such cases we eliminated the non-linear channels from the final summed up data set. 1 G. N. Greaves, P. J. Halfpenny, G. M. Lamble and K. J. Roberts, J. Phj2.y. (Paris), 1986, 12,901, C8 suppl. 2 N. Barrett, G. M. Lamble, K. J. Roberts, J.N. Sherwood, G. N. Greaves, R. J. Davey, R. J. Oldman and D. Jones, J. Crystal Growth, 1989, 94, 689. Prof. R. W. Joyner said: The 'catalytic' effect for copper oxidation, which you ascribe to the GaAs surface may be morphological. The reactivity of oxygen with copper is very surface-specific. Thus the oxygen sticking probability varies from ca. unity on Cu(l10) to < lo-' on CU( I 11). Dr Roberts replied: This might be the case if the Cu films had a preferred orientation. Texture measurements have been made on the Cu/float glass samples and no preferred orientation was found. Given the disordered nature of the native oxide on GaAs( loo), we doubt also that there is a preferred orientation on the Cu/GaAs samples, but we have no evidence as yet. This question highlights the obvious need to carry out diffraction measurements at the same time as the ReflEXAFS measurements.Whilst such an experiment has yet, to our knowledge, to be performed, there is no technical reason why it could not be carried out. In our view the combination of diffraction measurements (to reveal long-range order as well as texture) is the next priority in the development of this technique. Prof. J. M. Thomas said: Your suggestion that the surface of commercial-grade GaAs(100) has an oxide in which Ga coordinates to oxygen both tetrahedrally and octahedrally whilst As is only tetrahedrally bonded is eminently reasonable in the light of recent work by my Chinese collaborator Prof. Ruren Xu and his colleagues in Jilin University. AIAsO, (which is similar to GaAsO,) forms a number of open, continuous network structures.Ruren Xu and co-workers' have determined the structure of this zeolite-like solid, and indeed they see four- and six-coordinated Al but only four- coordinated As, with the tetrahedral and octahedral arrangements involving bridging oxygens. My colleague Dr Richard Jones at the Davy Faraday Laboratory has recently established' that AIPO, sometimes crystallizes with this same structure. 1 G. Yang, L. Li, J. Chen, R. Xu, J. Chem. Soc., C'hem. C'ommun., 1989, 810. 2 R. H. Jones, Ruren Xu, J . M. Thomas, Yan Xu and A. K. Cheetham, J. Chem. Sor., Chem. Comrnun., 1990, in press. Dr Roberts replied: It is most interesting to hear of this corroborating evidence from Professor Thomas's group. The structure reported by Yang et al.is for AlzAslO,-ethanolamine (AAE) which seems to exhibit essentially the same local structure as we observed in the surface oxide on GaAs(100). In AAE the Group I 1 1 aluminiumGenera 1 Discussion 73 is both tetrahedrally and octahedrally coordinated whilst the Group V arsenic is only tetrahedral. The bond lengths too are comparable with our data, e.g.: our work Yang et al. Group 111-0 (tetrahedral) 1.69 1.718- 1.75 1 Group I 11-0 (octahedral) 1.95 1.844- 1.947 Group V - 0 (tetrahedral) 1.68 1.649- 1.698 However, in a more direct comparison one should be more cautious. AAE is a highly crystalline structure exhibiting a well defined long-range order whilst our material is only ca. lOA thick and variable in stoichiometry. It is quite different from AAE or GaAsO,.The surface shows significant As depletion and there is a general lack of the long-range order of AAE. Also the kind of cage-like structure described by Yang et al. for AAE would, we believe, be easily detected by EXAFS through the backscattering enhancement provided by multiple scattering effects in ring-like structures such as this. The Chairman then invited discussion on all the papers. Prof. S. A. Rice (71ie James Franck Institute, Chicago) asked Prof. Bradshaw: Do you think it possdble to extend the study of elastic light scattering to the soft X-ray range, say 10-20 A, so as to be able to study the configurations of, say 20-30 long linear hydrocarbons or partially fluorinated hydrocarbons adsorbed at an interface? The issue turns on the available X-ray flux and the efficiency of the detectors, and on suppression of background scattering.Have you any feeling for what is possible now, or with new synchrotrons, or with new detectors? Prof. Bradshaw replied: In principle, I think it is already possible to perform such measurements at existing synchrotron radiation facilities, although the spectral brilliance of undulators at sources such as the ALS or BESSY I1 ( c a . 10" photons s-' mm-' mrad-', 0.1 % bw - ' ) would be of clear advantage. The main problem will be finding position-sensitive detectors. I t would appear that phosphor- coated CCD detectors' are at present most suitable. These have recently been used for applications in soft X-ray microscopy with Gd20,S: Tb as phosphor.' However, at the relatively large scattering angles expected for such photon energies, these detectors of typically < 1 cm2 surface area will cover only a relatively small solid angle.1 S. M. Grunner, Rev. Sci. Instr., 1989, 60, 1545. 2 W. Meyer-Ilse, in X-Ray Microscopy 11, ed. D. Sayre, M. Howells, J. Kirt and H. Harbach (Springer- Verlag, Berlin, 1988), p. 124. Dr Fontaine asked Dr Bradshaw: What forthcoming developments can you envisage in connection with spin-polarized photoemission and with circularly polarized X-rays to investigate the magnetism of interfaces and surfaces? Prof. Bradshaw replied: There is a whole variety of possible experiments here, some of which are already being performed, or at least planned, at several synchrotron radiation facilities.The availability of elliptically polarised undulator radiation will promote considerable activity in this area in future. The first photoemission measure- ments on non-magnetic materials using circularly polarised light were performed some years ago by Heinzmann, Kirschner and co-workers.' The emission from a particular74 General Discussion spin-orbit split component of a bulk electronic band shows a preferential spin orienta- tion, parallel or anti-parallel to the electron momentum. From the sign of the spin polarisation and the helicity of the light the double-group symmetry of the occupied bands can be determined. The effect has proved useful in band-mapping studies of heavier metals. Similar experiments have also been performed on adsorbed rare gases.’ Another interesting result from non-magnetic systems is the observation of circular dichroism in the angular distribution of photoelectrons from both oriented molecules and crystalline solids3.The effect requires no spin detection and is not restricted to systems showing strong spin-orbit coupling. In connection with magnetic materials there are two recent results which I think have important implications for future work. The first is the measurement by Schiitz et aL4 of different absorption coefficients for left- and right-hand polarised synchrotron radiation in the K and L near-edge spectra of ferromagnets. This technique probes the spin density of unoccupied valence bands and might be described as magnetic circular dichroism. The second result, again from Kirschner’s group, is the observation of exchange splitting in Fe 2p core-level photoemission.Depending on the helicity of the light and the sample magnetisation the two spin-orbit split components are each resolved into lines of different intensity. Spin detection of the photoemitted electrons is not required. Similar results have apparently also been obtained in the valence region. 1 A. Eyers, F. Schafers, G. Schonhense, U. Heinzmann, H . P. Oepen, K. Hiinlich, J . Kirschner and G. Borstel, Phys. Reu. Lett., 1984, 52, 1559. 2 G. Schonhense, A. Eyers, U. Friess, F. Schafers and U. Heinzmann, P h j x Rev. Lett., 1985, 54, 547. 3 G. Schonhense, Phys. Scr., 1990, T31, 255. 4 G. Schutz, W. Wagner, W. Wilhelrn, P. Kienle, L. Zeller, R. Frahm and G. Materlik, P h j x Rec.Lett., 5 L. Baumgarten, L. M. Schneider, H . Petersen, F. Schafers and J . Kirschner, Phjss. Rec. Lett., 1990,65,492. 1987, 58, 737. Prof. G . N. Greaves (SERC, Daresbury) then said: I would like to pick up Stuart Rice’s remark about light scattering from surfaces by drawing attention to the possible utility of combining X-ray glancing-angle geometry with a two channel-cut-crystal arrangement to record surface diffuse scattering. In principle this would enable ang!es as small as the crystal rocking curve to be approached (ca. arc sec or 8 x lop5 A at 1.5 A). In practice it has been used by Bonse, Hart, Bordas and others to measure conventional small-angle scattering at normal incidence. By using near-perfect crystals and multiple reflections, X-ray scattering can be extended into the Brillouin scattering regime.The use of grazing incidence would facilitate the study of molecules on surfaces and their aggregation with coverage. SR offers the advantage of choosing the wavelength to match the size of the scattering centre and the k range to determine the dimensionality of the assembly. Dr Roberts concluded: Over the past few years glancing-angle X-ray absorption spectroscopy has developed into an extremely promising technique for the structural characterisation of condensed interfaces. Structural information on solid-liquid inter- faces (e.g. electro-chemical, crystal growth interfaces) and solid-solid interfaces (e.g. surface-reacted layers, thin films and coatings) is at present extremely limited. Glancing- angle X-ray techniques have the potential to form a bridge between the classical surface science (UHV) techniques and the more realistic interfaces as typified by many techno- logical and industrial systems.However, this technique is still in its infancy and significant further developments can be expected; these are likely to include: improve- ments in X-ray collimation and reduction in air scatter paths to enable data with improved signal-to-noise ratios to be collected; fluorescence data collection using multi-element solid-state detectors to enable dilute analytes to be examined; total electron yield detection to enable data collection of concentrated analytes to be collected at glancingGeneral Discussion 75 an alvse r two-axis monochromator detector Fig. 1.Schematic representation of a four-axis scattering system. The two-axis monochromator provides a ‘tail-less’ tunable and non-deviating incident beam and the fourth axis (analyser 1 provides an exact definition of the scattered X-ray beam direction. angles greater than the critical angle, 4c; fast scanning monochromators and high- resolution position-sensitive solid-state detectors for time-resolved X-ray absorption spectroscopy measurements; environmental cells for in situ measurements. These developments will be significantly aided by moving from the current double-axis to the quadruple-axis scattering geometry shown in fig. 1 . This set-up has the following features: an undeviating, tail-less tunable X-ray beam is provided by a pair of oppositely handed Si channel-cut monochromators mounted on the first two axes; the scattered beam is collimated by a three-bounce Si analyser mounted on a scanable 28 arm. Such a set up will enable the separation of the small-angle/diffuse scattering components from the reflected signals used in the ReflEXAFS technique. Such an instrumental development will enable high-resolution X-ray diffraction and reflectivity data to be collected in the same series of measurements that are used to collect the glancing angle X-ray absorption spectroscopy data. It will also, by virtue of a better definition of the scattered beam path, enable rougher interfaces to be examined. Such an instrument potentially provides a research capability which directly mirrors but complements that routinely available (e.g. ESCALAB) to surface scientists. Improvements in source characteristics can also be expected in the next generation of electron storage rings. The proposed Daresbury Advanced Photon Source (DAPS) will provide a projected reduction in source size and resultant improvement in spectral brightness of two orders of magnitude. This has the following implications for ReflEXAFS measurements: currently the large source size provided by first-generation storage rings has to be significantly collimated to satisfy glancing-angle requirements and typically ca. 95% of available beam flux cannot be used; the projected DAPS facility will provided high-intensity photons around 1OOOeV and we can expect to be able to study light elements such as Mg, Al, P, S and C1 in adsorbates and surface coatings. Thus the future for the continuing development of glancing-angle synchrotron X-ray techniques for the structural characterisation of interfaces looks good. We can expect that these will provide information to extend and complement that provided by conven- tional surface science techniques.

ISSN:0301-7249    

DOI:10.1039/DC9908900065

出版商:RSC    

年代:1990

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Faraday Discuss. Chem. SOC., 1990, 89, 77-89 Sulphur-induced Structural Chemistry of Oxide Surfaces Christopher A. Muryn, Douglas Purdie, Peter Hardman, Allen L.. Johnson,? N. S. Prakash,$ Ganesh N. Raiker and Geoffrey Thornton Interdisciplinary Research Centre in Surface Science and Chemistry Department, Manchester University, Manchester M13 9PL Daniel S-L. Law SERC Daresbury Laboratory, Warrington WA4 4AD Surface EXAFS, NEXAFS and photoemission have been used to study the structure of surface phases resulting from reaction of H,S with NiO( 100) and SO2 with TiO,( 110). S K-edge SEXAFS data, in conjunction with the LEED pattern symmetry, suggest that the NiO( 100)/H2S reaction at 570 K results in reduction of the substrate to form an azimuthally aligned Ni( 100)c(2 x 2 ) s raft, with S occupying the fourfold Ni( 100) hollow site.S K-edge NEXAFS results for SO, adsorption on low-temperature TiO,( 110) identify two surface SO, species: chemisorbed SO, and SO:-. The polarisa- tion dependence of the NEXAFS suggests that the SO2 molecular plane lies close to parallel t o the surface. Photoemission data also evidence chemisorp- tion at 110 K and show that this precursor phase reacts further in a thermally activated process to form a stable surface sulphate-like species. A mechanism is suggested which involves SO, bonding to a terrace Ti site, with an activated hop t o an adjacent site in which sulphur bonds to two oxygen atoms on the raised row. 1. Introduction Although the reactivity of transition-metal oxide surfaces has received increased attention in recent years, the underlying physics and chemistry remain poorly undzrstood.’ In particular, the lack of structural information about clean substrates and adsorbate complexes provides a major hinderence to theoretical studies aimed at understanding factors which influence the reactivity.With the limited information available, it has been thought that surface defects in the form of steps and oxygen vacancies play a major role. More recent work on well defined single-crystal surfaces has borne out this idea in some cases; for instance we found H,O dissociation on SrTi0,(001) to be catalysed by step sites,’ and steps on a TiO,( 110) surface have been found to activate SO, dissociation.3 There are, however, examples of reactions in which defects do not play a significant part, such as in the reaction of HzO with TiOJ In this work our interest lies in the structural chemistry associated with the reaction of S-containing molecules with transition-metal oxides.Sulphur poisoning of oxide and oxidised metal catalysts and catalyst supports, and the potential use of oxide gas sensors provide a motivation for these studies over and above their intrinsic interest. Previous work in this area includes X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED) studies of SO, reaction with TiO,( 1 10),3*5 a-Fe_703(0001),h Ti,03( lOl2),’ and an LEED study ? Present address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 :I: Present address: Department Chemie Appliquee et Genie Chimique, CNRS UA417, 43 Bd du 1 1 1 EW.Novembre 191 8, F-69622 Villeurbanne Cedex, France. 7778 Sulphur-induced Structural Chemistry of Oxide Surfaces of the reaction of H,S with NiO( loo).' The chemistry observed is, as might be expected, strongly substrate dependent. On Ti203( 10i2), SO2 oxidises the substrate to a mixture of TiS2 and TiO,;' a-Fe203(0001) forms a sulphate-like adsorbate complex;' and on room-temperature Ti02( 110) there have been reports of both minimal reaction' and reaction to form a surface sulphite species.' Here we focus on the reaction of NiO( 100) with H2S, and the reaction of SO2 with Ti0,(110). In the case of the TiO1(110)-SO2 reaction we explore to a limited extent the influence of 0 vacancies on the reactivity.We employ three synchrotron radiation techniques: surface extended X-ray absorption fine structure ( SEXAFS);9 near edge X-ray absorption fine structure (NEXAFS);"' and photoemission. The first is used to determine the surface structure resulting from the H2S surface reduction of NiO( 100); NEXAFS is used to fingerprint the adsorbate complexes formed on reaction of SO2 with Ti02( 110); and valence-region photoemission is used to follow the temperature dependence of this reaction. 2. Experimental X-ray absorption measurements employed a Ge( 11 1) crystal pair in the monochromator of station 6.3 at the SRS, Daresbury Laboratory, giving an energy range 2000 d hv/eVJ- d 8400.'' During this work the base pressure of the instrument was ca.4 x loplo mbar. At the S K-edge (2472eV) the monochromator resolution is ca. 3 eV full width at half maximum (FWHM) and the degree of polarisation is close to 100%. A double-pass cylindrical mirror analyser (CMA) (Physical Electronics Inc.) was used to record the yield of S KLL Auger electrons (kinetic energy = 2100 eV) as a monitor of the surface absorption coefficient above the S K-edge. Using this Auger yield method necessarily limits the data range to ca. 150eV above the S K-edge, because at this photon energy the substrate 0 1s photoelectron peak enters the analyser energy-window. Normalisation to the incident X-ray flux was accomplished by measuring the drain current from a thin Al foil placed between the monochromator and the sample.The photoemission measurements were performed using the grazing incidence mono- chromator ( 3 0 s hv/eVc 200) and CMA on station 6.1 at the SRS," the instrument base pressure during this work being ca. 1 x 10p'O mbar. The valence-band photoemission experiments were carried out at 110 eV, the combined resolution (monochromator+ analyser) being ca. 0.7 eV (FWHM). To prevent sample charging effects, the rutile TiQ2 sample had been pre-reduced so that it was blue-grey in colour, indicating an n-type carrier concentration of about 10" cmp3. Ti0,(1 10) surfaces were prepared in si?u by cycles of Ar' bombardment (800 eV, 17 PA, 5 min) and annealing at 925 K for 20 min. The surface stoichiometry could be restored by additional annealing in 10p6mbar O2 (BOC, research grade) at 925 K for 20 min.The success of this procedure was determined by noting the lack of band-gap photoemission," although this method could not be used in the NEXAFS experiments. NEXAFS experiments investigated only an 0, annealed surface, photo- emission measurements being carried out on both a stoichiometric surface and a reduced surface which contains 0 point defects." These cleaning procedures resulted in a surface contamination level below the detection level of Auger spectroscopy, and a 1 x 1 LEED pattern. SO, (K&K, 99.98%) exposure was carried out at 100 K (NEXAFS) and 110 K (photoemission). After subsequent annealing, NEXAFS spectra were recorded at 100 K and photoemission spectra were recorded at the anneal temperature. The LEED pattern was monitored during exposure in the case of the photoemission measurements, becom- ing more diffuse on adsorption.The pattern gradually returned towards that of the clean surface on annealing. .; 1 eV- 1.602 x 10 "'J.C. A. Muryn et al. 79 0 p I [oil] Fig. 1. Schematic LEED pattern resulting from reaction of HIS with NiO(100) at 570 K. The interpretation of the pattern is that given in ref. (8). (a) NiO( 100); (0) Ni( 100)c(2 x 2)s. A clean NiO( 100) 1 x 1 surface was prepared by cleaving in situ. Exposure to 2 x lo4 L of H2S (B.D.H. 99.6%) was carried out with the sample at 570 K,* the measurements being performed at 155 K. Model compound NEXAFS data shown in fig. 6 (later) were obtained using a Ge(ll1) crystal pair in the monochromator of SRS station 3.4 (SOXAFS)14 and a fluorescence detector.Except for NiS the compounds were obtained commercially and were of at least 96% purity. a-NiS was kindly supplied by Dr R. A. D. Pattrick of the Geology Department, Manchester University. Spectra were calibrated in energy by aligning the SO:- white line to 2482.7 eV." 3. Results and Discussion Ni0(100)+ H2S A number of surface phases can be obtained by reacting NiO( 100) with H2S at elevated temperatures, ultimately resulting in the formation of a surface sulphide.' Reaction at 570 K results in the LEED pattern shown schematically in fig. 1. If the pattern is taken to represent a c(2 x 2) overlayer, the overlayer principal-order beams have position vectors which are 1.19 times larger than those of the NiO(100) substrate. Since this corresponds to the ratio of NiO and Ni metal cell constants, the LEED pattern can be interpreted in terms of an azimuthally aligned Ni( 100)c(2 x 2)s overlayer.x The Auger spectrum is consistent with this interpretation.Fig. 2 shows the corresponding S K-edge SEXAFS spectrum recorded in a normal incidence geometry after background subtraction and normalisation of the data to the edge jump. The experimental EXAFS oscillations x ( k ) with a k' weighting and their Fourier transform are shown as solid lines in fig. 2. EXAFS spectra were analysed using a curve-fitting routine based on the rapid curved-wave computational scheme.16 The phase shifts used in the data analysis were obtained by adjusting theoretical S-Ni phase shifts to fit a S K-edge EXAFS spectrum of bulk NiS, for which the bond distances are known accurately." This model compound data set was obtained by Brennan et a/." and used in their SEXAFS study of Ni( 100)c(2 x 2)S, and subsequently by us in a SEXAFS study of Ni( 110)c(2 x 2)s." The best theoretical fit to the SEXAFS data is shown in fig.2 as dashed lines. This fit was achieved with ,a single-shell nearest-neighbour S-"i bond distance of 2.26* 0.04A (using a 0.06 A correction to the theoretical phase shiftix) and an effective80 Sulphur-induced Structural Chemistry of Oxide Surfaces 1.0 - I 0.0 2 Y, * -1.0 0.50- - 5 4 - 0.2 5 - I I I 4 5 6 k l A ' Fig. 2. S KLL Auger yield SEXAFS spectra of NiO( 100) following H2S adsorption at 570 K and cooling t o 155 K, recorded at normal incidence. The EXAFS function x ( k ) weighted by k 2 (solid line) and the best theoretical fit (dashed line) are compared in the upper part of the figure.The lower section contains the modulus of the Fourier transform. coordination number of 3.8 f 1.5. It is of particular note that the analysis routine rejected models which included an oxygen backscattering atom, the presence of which would strongly influence our short k-range data. The bond distance and the coordination number results are consistent with those found in a SEXAFS study of Ni( 100)c(2 x 2)S, where S was found to occupy the fourfold hollow site: S- Ni bond distance 2.23 f 0.02 A; effective coordination number 3.94 f 0.75." This suggests the model shown in fig. 3 for the surface structure resulting from the NiO( 100)/H2S reaction at 570 K, where S lies in the fourfcdd hollow of an Ni(!OO) plane in an Ni(lOO)c(2x2)S raft.As for the thickness of the raft, this cannot be obtained from our limited SEXAFS data. However, preliminary Stacding X-ray Wavefield (SXW)'" data point to a single Ni layer thickness of the raft. An interesting comparison can be made between this result and a surface structure obtained on reacting H,S with Ni( 111). In the latter case, reaction at 440 K produces a (5J3 x 2 ) s overlayer, which SEXAFS data" show to have the pseudo-Ni( 100)c(2 x 2 ) s structure shown in fig. 4. In this case the S-Ni bond distance is 2.27i0.02 A.'' It seems that the Ni( 100)c(2 x 2)s structure offers particular stability, providing the thermodynamic driving force for the reaction.Hence, it is not surprising that the H2S reduction of NiO( 100) would limit to one monolayer given the relatively mild reaction conditions employed. Since this overlayer is incommensurate with the substrate,C. A. Muryn et al. 81 Fig. 4. The pseudo-Ni( 100)c(2 x 2 ) s overlayer model of Ni( 11 1 ) ( 5 J 3 x 2).*’ Ni( 11 1) substrate atoms are shown as open circles, ‘surface’ Ni atoms as grey and S atoms as black. azimuthal alignment could arise from pinning of the structure at steps, as suggested for dioxygen physisorbed on the basal plane of graphite.22 This behaviour differs from the (5J3 x 2)s overlayer on Ni( 11 I), where a relatively small super-cell can be formed by a distortion of the overlayer from fourfold symmetry. Ti02( 110) + SO2 The results of previous work on this system are to some extent contradictory.In a room-temperature adsorption study using UPS, XPS and LEED, Smith et ~ 1 . ~ found essentially no reaction with the nearly perfect surface. This is in contrast with the conclusions reached in a similar study, where surface sulphite (SO:-) was evidenced following room-temperature adsorption.’ Formation of a surface sulphide phase was detected when SO2 was reacted with a defected surface created by Ar+ b~mbardrnent.~ In contrast to the earlier work, we have concentrated on lower temperature adsorption and the effect of thermal activation. S K-edge NEXAFS spectra recorded at a 20” (grazing) angle of incidence are shown in fig. 5 . Condensing SO2 at 100 K gives rise to NEXAFS identical to that obtained in the gas-phase spectrum, from which the two major features observed can be assigned to an S 1s -+ TT* (2473 eV) bound-state resonance and a (T* (2478 eV) shape resonance.23 On annealing the sample incrementally, the SO2 features decrease in intensity and appear to be replaced by similar peaks shifted to ca.1-2 eV lower photon energy (the (T* component is observed at normal incidence) and a new resonance at 2482 eV. At higher anneal temperatures the ‘shifted SO, features’ decrease in intensity leaving the 2482 eV resonance. Peaks due to condensed SOz appear Fig. 3. The structural model for NiO( 100) + HIS at 570 K derived from SEXAFS data and the LEED pattern. In this model substrate Ni atoms are represented by the small shaded circles, 0 atoms by the lightly shaded large circles.Ni atoms in the Ni( 100) plane are darkly shaded and S atoms are black.82 Sulphur-induced Structural Chemistry of Oxide Surfaces 1 7-- -i T-1 -~ --i -__I- -2 247 0 2490 2510 photon energy/eV Fig. 5. S KLL Auger yield NEXAFS spectra of TiO,( 110) following SO2 adsorption at 100 K and annealing to the indicated temperatures. All measurements were recorded with the sample at 100 K and at 20" (grazing) angle of incidence. in all spectra because of re-adsorption from the residual vacuum at the measurement temperature (100 K). A significant contribution to the shifted SO, features will also arise from re-adsorption. We can identify the surface species responsible for the 2482 eV resonance by com- parison with S K-edge NEXAFS spectra of model compounds shown in fig.6. The Na,S,O, spectra are consistent with those previously reported by Sekiyama et ~ l . , ~ ~ differing only in the relative intensities and a 2 eV shift of the energy scale. The latter arises from the use in the earlier work of NiS to calibrate the energy scale. Sekiyama et al.'4 interpret the rich structure of these spectra in terms of bound-state transitions to antibonding orbitals derived from S 3p, and d-type shape resonances, through which a chemical-shift trend can easily be discerned. When compared with our 500 K data in fig. 5, the model compound spectra clearly fingerprint sulphate (SO: ) species as the origin of the 2482 eV resonance. Although the two features between 2490 and 2500 eV in the Na2S04 spectrum are not reproduced in the surface spectrum, this region of the spectrum matches that of single-crystal CuSO, ." The SO, features which are shifted to lower photon energy of the condensed SO, peaks indicate the presence of a chemisorbed SO2 species with a longer S-0 bond distance than that in the gas phase.25 These peaks are obscured by the condensed SO, features in the spectra recorded after lower-temperature anneals.However, in the 300 K anneal spectrum the u* peak is sufficiently clear to allow its polarisation dependence to be investigated. Spectra recorded at X-ray incidence angles of 20" and 90" with the photon E-vector in the [OOl] azimuth are shown in fig. 7. In order to clarify the polarisation dependence, we have attempted to subtract the background due to the SO: 2482 eV resonance.Although this procedure is not likely to be particularly accurate,C. A. Muryn et al. 83 / // I I 2480 2 500 2520 photon energy/eV Fig. 6. S K-edge NEXAFS spectra of ( a ) Na2S0,, ( b ) Na,SO,, ( c ) Na2S20, and ( d ) a-NiS recorded at ca. 120 K. the data clearly indicate that the higher-energy resonance is considerably reduced at grazing incidence. We can first interpret this data on the basis of the commonly employed building-block model of molecular resonances.Io This would consider the low-( high)- energy near edge features to be due to scattering into S-0 T* (S-0 a*) final-state resonances. In this model a transition into T* (a*) from S 1s is allowed if a component of the E-vector lies perpendicular (parallel) to the molecular bond axes. On this basis the a* polarisation dependence indicates that the SO2 molecular plane is close to parallel to the surface.The building-block model can give rise to errors;'6 the data are better analysed in terms of excitation into SO, molecular orbitals and by employing standard group-theory techniques. The gas-phase SO, NEXAFS spectrum has been assigned in these terms by comparison with electronic structure c a l c ~ l a t i o n s . ~ ~ This identifies the 2473.8 eV peak with excitation into the 3b, virtual orbital, with the 'a*' feature comprising two peaks due to excitation into 9a, and 6b, lying at 2478.4 and 2478.9 eV, respectively. The appropriate C2\. molecular coordinate system has the x axis perpendicular to the SO2 plane, with the z axis parallel to the C', rotation axis.In C2\, which we will assume to be the effective symmetry of the surface species, x, y and z transform as B , , B2 and A , , respectively. From group theory the small 9a, and 6b, intensity at grazing incidence (20") (fig. 7 ) indicates that in this geometry the E-vector must lie close to x, i.e. perpendicular to the molecular plane. At normal incidence we expect to observe the 9a, and/or the 6b, transition, depending on whether the molecule is azimuthally ordered on the surface. The width of the 2475eV peak in fig. 7 compared with the 9a,/6b2 doublet of condensed SO, indicates that only one component is present. However, this84 Sulphur- induced Structural Chemistry of Oxide Surfaces Fig. 7. Polarisation dependence of the S K-edge NEXAFS spectra recorded from Ti02( 1 10)-S02 at 100 K after annealing to 300 and 500 K.Data are shown for angles of X-ray incidence of 20" and 90" with the E-vector in the [OOl] azimuth, the data being normalised to the edge step. The lower part of the figure shows the 300 K annealed data after removing a fourth-order polynomial background fitted to the region 2461-2481 eV. Fig. 8. Model of Ti02( 110) with the suggested site of SO2 chemisorption shown on the left and surface sulphate on the right. The black circles represent Ti atoms, large circles represent 0 atoms with the open large circles being the bridging oxygen atoms of the raised row. The hatched circles represent SO2. component cannot be identified in the absence of normal incidence data recorded with the E-vector in the [ l i O ] azimuth.If the molecule was parallel to the surface then no 3b, contribution is expected at normal incidence. That there is indicates that the molecule is tilted upwards from the surface in a geometry such as that shown in fig. 8. The anomalously large 3b, contribution in this geometry (it should have a c o d 8 dependence, where 8 is the angle between the E-vector and x ? ~ ) may well arise from a condensed SO? peak contribution. Having gleaned an idea of the orientation of SO2 on Ti02( 1 lo), albeit in the presence of a co-adsorbed SO:- phase, we can consider possible adsorption sites. In principle, SO2 could bond either to the fivefold coordinate Ti site on the terrace or to the terraceC. A. Muryn et al. 85 or bridging oxygen atoms (see fig.8). However, the minor perturbation apparent in the NEXAFS suggests the Ti site since SO, bonding to 0 atoms would be expected to result in the formation of a complex anion, which as we have seen above have very different NEXAFS signatures. Although there is no direct analogy in the molecular coordination chemistry of SOz,28 the surface geometry proposed probably corresponds to a type of '77'-pyramidal' coordination, as shown in fig. 8. A similar SOz coordination has been proposed for Ag(l10).29 In this bonding configuration SO, would bond to a Ti atom through the S atom, the M-S bond lying along the surface normal. In transition-metal square-planar complexes which incorporate SO, in this fashion, the bond angle between M-S and the sum of the S - 0 vectors is typically 120"." With this type of SO, coordination on TiO,( 1 lo), the molecular plane would be tilted at 30" up from the surface.On the basis of ionic bonding of SO:- to the surface, the 2482 eV resonance of the surface sulphate, which would correspond to an S 1s- t? transition," is not expected to show a polarisation dependence even with the 'adsorbate' oriented on the surface. In a tetrahedral geometry all three Cartesian axes are identical, transforming as t2, which gives rise to the polarisation independence. Covalent bonding will lower the symmetry, for instance bidentate bonding of SO:- to Ti will reduce the symmetry at least to CZb, which will split the t: resonance and introduce a polarisation dependence. Assuming the presence of an ordered SO:- species on the surface, a close to Td geometry is suggested by the polarisation independence of the 500 K annealed TiO,( 1 lO)-SO, spectra shown in fig.7. Although the NEXAFS cannot help us in identifying the SO:- adsorption site, it seems likely that it involves incorporation of SO, into a bridge site on the raised row of 0 atoms, as shoown in fig. 8. Assuming bulk termination, the 0-0 distanceoon the raised row is 2.96 A, somewhat larger than that in uncoordinated SO:-, 2.43 A.30 This difference could be accommodated by moving together the two raised 0 atoms bound to SOz, putting an upper limit of the SO%- occupation on the raised row of one half, which corresponds to a coverage of 1/6 of the surface. The edge jump observed in the '500 K annealed' NEXAFS spectrum is consistent with this figure.Valence region photoemission data for low-temperature SO, adsorption on both a reduced and stoichiometric TiOJ 1 10) surface are consistent with molecular adsorption on a Ti site, with SO, bonding through S. Fig. 9 shows photoemission spectra of TiO,( 110) before and after adsorption of 100 L of SO2 at 110 K on the stoichiometric surface, along with the difference spectrum and the gas-phase He" ( h v = 40.8 eV) spectrum.3'.32 A band-bending correction was not needed to align the surface spectra. Essentially the same result is achieved on the reduced surface (see fig. 10) except that the additional Ti 3d contribution to the valence-band and band-gap regions are quenched, giving rise to band bending and indicating preferential bonding of SO, to the 0 vacancy sites.Aligning the difference and gas-phase spectra at the 0 lone pair la2/5b, peak indicates a relaxation/polarisation shift of 2.1 eV to lower ionisation energy, and a bonding shift of 0.4eV for the 2bI/7a,/4b, features with the same shift for 6al. These shifts are consistent with the orbital the 2b,/7al/4b2 set containing a significant contribution from the S 3p orbitals, which would form CT and T bonds with substrate Ti atoms. The 6aI is the S 3s-0 2s antibonding orbital, the S 3s component of vrhich could provide additional CT bonding to the surface. This feature is larger relative to the other components in the 110 eV difference spectrum, as expected from the variation in the S 3s and 0 2p photoionisation cross-section.'3 The 0 2p,-containing 8a, orbital is probably represented by the shoulder at ca.6 eV in the difference spectrum, although its position is difficult to determine accurately in the presence of the 5bz/la, features. The feature in the difference spectrum at 4.1 eV binding energy has no counterpart in the gas-phase spectrum and probably evidences formation of surface sulphate species. Annealing both SO,-adsorbed surfaces results in a clear change to the difference spectra. Fig. 10 shows the results of annealing the initial SO, phase on the reduced86 Sulphur-induced Structural Chemistry of Oxide Surfaces '1 a2:5b, I I r~~ -~1 .----.T--d 30 20 10 EF binding energy/eV Fig. 9. Photoelectron spectra ( h v = 110 eV) of ( b ) clean stoichiometric Ti02( 110) at 110 K, ( a ) the clean surface dosed with 100 L SO, at 110 K, (c) the difference of the dosed and clean spectra, and ( d ) an He" ( h v = 40.8 eV) spectrum of gas-phase SOz aligned at the la2/5b7 peak position.surface to 150 K. The data are presented in the form of difference spectra, where we have simplified the spectra by subtracting a spectrum of the clean stoichiometric surface rather than that of the reduced surface. This only has the effect of elivinating the substrate-related changes described above and removing the need for a band-bending correction t o the alignment of data. The difference spectra in fig. 10 are compared with XPS ( h v = 1486.6 eV) spectra of Li,S04 and Li2S03 34 and an He" spectrum of Li2S04." Being guided by the NEXAFS data, we will interpret the 150 K difference spectrum in terms of a surface SO:--like species. However, we cannot rule out the possibility that the different reaction conditions employed in the NEXAFS and photoemission experi- ments give rise to different products.This is a point to s t r w s i n c e the difference between the valence region spectra of SO: and SO: is sufficiently small to prevent a positive assignment on the basis of these spectra alone. Indeed, the match between the surface spectrum and that of 'ionic' SOf- is better than that for sulphate. Aligning the difference spectra at the 2t2 S - 0 bonding-orbital peak of SO: ' 5 ~ 2 h is the only arrangement which does not give a negative bonding shift for this orbital. This corresponds to a negligible polarisation/relaxation shift when compared with the He" spectrum of Li2S04," and is the alignment employed for sulphate on ~ - F e ~ 0 , ( 0 0 0 1 ) .~ On this basis the I t , mainly 0 2s orbital experiences a bonding shift of ca. 0.9 eV, in line with our expectation that the negative charge on the 0 atoms bound to Ti will be less than in an ionic sulphate. Similar arguments can be used to explain the 1.1 eV bonding shift of the 0 2s S 3s, 3p 2a, orbital. Unfortunately, there does not appear to be a published theoretical analysis of the effect on the electronic structure of the covalent coordination of SO: , such as that carried out for the NO, ion." It is therefore difficult to make a detailed assignment ofC. A. Muryn et al. 87 30 20 10 E F binding energy/eV Fig. 10. The photoemission ( h v = 110 eV) difference spectrum (S02-clean) for a reduced Ti02( 110) surface ( a ) after exposure to 200 L at 100 K ( b ) after warming to 150 K (see text for details).Surface spectra are compared with suitably aligned XPS ( h v = 1486.6 eV) spectra of ( d ) Li2S04 and ( e ) Li2S03 from ref. (34) and ( c ) an He" ( h v = 40.8 eV) spectrum of a Li2S0, from ref. (35). The orbital assignments are from ref. (36). discrepancies between the difference spectra and the spectrum of 'isolated' SO:-. We would, for instance, expect 3d-orbital density to be introduced by adsorption since formally two electrons are transferred to the surface as SO2 is oxidised to SO:-. This charge can be nominally assigned to create one Ti3+ species and two Ti3.'+ species per SO:- moiety created. With saturation coverage of SO:-, all Ti atoms under the raised rows would be in oxidation state 1 1 1 .At least one of the low binding-energy peaks should carry this 3d density since it does not go into band-gap states. Otherwise, one is forced to consider a model in which raised row 0 atoms carry a formal charge of -1. Annealing the surface to higher temperatures simply causes the adsorbate-induced features to decrease in intensity; no further change to the shape of the spectrum is observed above 150 K. The intensities of adsorbate features decreased uniformly up to room temperature, falling to ca. 50% of the 150 K values. All adsorbate features could be removed by heating to 800 K. Taken together, the NEXAFS and photoemission data indicate chemisorption at 110 K with a further, activated reaction to form a stable surface sulphate-like species.The mechanism suggested would involve SO2 bonding to a terrace Ti site, with an activated hop to an adjacent site in which sulphur bonds to two oxygen atoms on the raised row. In other words, the reaction proceeds from the left to the right of fig. 8.88 Sulphur-induced Structural Chemistry of Oxide Surfaces In the low-temperature adsorption regime examined here, the effect of oxygen vacancies on the reaction mechanism appears to be small. While this in line with results for the Ti02( and TiOJ 1 10)"-H20 reaction, there is an essential difference. Whereas SO2 quenches the defects on Ti02( 1 lo), oxygen vacancies are relatively unaffected by H 2 0 adsorption on the (100) and (1 10) surfaces.Some knowledge of the initial species formed on reacting SO2 with Ti02( 110) oxygen vacancies would provide valuable insight into this difference in behaviour. The He' difference spectrum reported for SO2 adsorption on room temperature Ti02( 110)' is not dissimilar to that of the annealed surface in fig. 10, although the He' spectrum was interpreted as indicating surface SO:- formation. Given the discussion above, it is possible that the species formed in the earlier work' was SO:-; this could be tested by NEXAFS measurements. Observation of room-temperature reaction is restricted to the work of Onishi et al.' In our work, reaction of a stoichiometric surface was found to be minimal at this temperature, consistent with the report of Smith et aL5 A possible explanation for this discrepancy lies in the method of sample preparation; Onishi et al.' annealed only in vacuum which leaves the surface with a small concentration of 0 vacancies.Such defects are known to activate H,O dissociation at room temperature, in contrast to their irlert character at 160 K.13 4. Summary This paper has described the application of synchrotron radiation techniques in the study of two oxide surface reduction reactions. A NEXAFS and synchrotron radiation photoemission study of the low-temperature Ti02( 1 lo)/ SO2 reaction indicates molecular adsorption at ca. 110 K, being a precursor to incorporation of SO2 into the bridging oxygen atoms of the raised row to form a sulphate-like species. A SEXAFS study of the Ni0(100)/H2S reaction at 570K suggests the reduction of the substrate to form an azimuthally aligned Ni( 100)c(2 x 2)s raft, with S in the fourfold hollow site of Ni( 100).In general terms, this work illustrates the potential of synchrotron radiation tech- niques in studies of the quite complicated structural chemistry asociated with oxide surface reactions. We plan future NEXAFS and SEXAFS work which will focus on a structure determination of the separated precursor SO2 and product SO:- phases on Ti02( 1 lo), and the adsorbate structure associated with 0 vacancies. We are grateful to P. A. Cox for useful discussions, R. G. Egdell for the loan of the Ti02(110) sample and to J. R. Drabble for the NiO crystal. This work was funded by the S.E.R.C. (U.K.), including the provision of studentships to C.A.M., D.P. and P.H.Additional support was provided by Johnson Matthey PIC. References 1 V. E. Henrich, in Surface and Near-Surface Chemistry of Oxide Materials, ed. J. Nowotny and L-C. Dufour (Elsevier, Amsterdam, 1988) pp. 23-60; Rep. Prog. fhys., 1985, 48, 1481. 2 N. B. Brookes, F. M. Quinn and G. Thornton, Solid State Commun., 1987, 64, 383. 3 H. Onishi, T. Aruga, C. Egawa and Y. Iwasawa, S u r - Sci., 1988, 193, 33. 4 C. A. Muryn, G. Tirvengadum, J. Crouch, D. R. Warburton, G. N. Raiker, G. Thornton and D. S-L. Law, J. Phys.: Condensed Matter, 1989, I , SB127. 5 K. E. Smith, J. L. Mackay and V. E. Henrich, fhys. Rev. B, 1987, 35, 5822. 6 R. L. Kurtz and V. E. Henrich, Phys. Rev. B, 1987, 36, 3413. 7 K. E. Smith and V. E. Henrich, fhys.Reu. B, 1985, 32, 5384. 8 A. Steinbrunn, P. Dumas and J. C . Colson, Surf: Sci., 1978, 74, 201. 9 P. H. Citrin, J. Physique, 1986, 47, C8-437. 10 J. Stohr and D. A. Outka, Phj1.Y. Rev. B, 1987, 36, 7891. 1 1 A. A. MacDowell, D. Norman and J. B. West, Ret.. Sci. Instr., 1986, 57. 2667. 12 M. R. Howells, D. Norman, G. P. Williams and J. B. West, J . Phjx E, 1978, I I , 199.C. A. Muryn et al. 89 13 R. L. Kurtz, R. Stockbauer, T. E. Madey, E. Roman and J. L. De Segovia, Surf: Sci., 1989, 218, 178. 14 A. A. MacDowell, J. B. West, G . N. Greaves and G . van der Laan, Rev. Sci. Instr., 1988, 59, 843. 15 T. A. Tyson, L. Roe, P. Frank, K. 0. Hodgson and B. Hedman, Phys. Rev. B, 1989. 39, 6305. 16 S. J. Gurman, N. Binsted and 1. Ross, J. Phys. C: Solid State 1984, 17, 143. 17 J. Trahan, R. G. Goodrich and S. F. Watkins, Phjx Rev. B, 1970, 2, 2859. 18 S. Brennan, J. Stohr and R. Jaeger, Phys. Rev. B, 1981, 24, 4871. 19 D. R. Warburton, G . Thornton, D. Norman, C. H. Richardson, R. McGrath and F. Sette, Surf Sci., 1987, 189/190, 495. 20 D. P. Woodruff, D. L. Seymour, C. F. McConville, C. E. Riley, M. D. Crapper, N. P. Prince and R. G . Jones, Phys. Rev. Lett., 1987, 58, 1460. 21 D. R. Warburton, P. L. Wincott, G. Thornton, F. M. Quinn and D. Norman, Surf Sci., 1989, 2111212, 71. 22 M. F. Toney and S. C . Fain Jr, Phys. Rev. B, 1984, 30, 1115. 23 A. P. Hitchcock, S. Bodeur and M. Tronc, Chem. Phys., 1987, 115, 93. 24 H. Sekiyama, N. Kosugi, H. Kuroda and T. Ohta, Bull. Chem. Soc. Jpn, 1986, 59, 575. 25 J. Stohr and R. Jaeger, PhJx Rep. B, 1982, 26, 4111. 26 J . Somers, A. W. Robinson, Th. Lindner and A. M. Bradshaw, Phys. Rev. B, 1989, 40, 2053. 27 S. Bodeur and J. M. Esteva, Chem. P h p . , 1985, 100, 415. 28 R. R. Ryan, G . J. Kubas, D. C. Moody and P. G. Eller, Strucr. Bond., 1981, 46, 47. 29 D. A. Outka and R. J. Madix, SurJ Sci., 1983, 137, 242. 30 A. F. Wells, Structural Inorganic Chemistry (Oxford University Press, Oxford, 5th edn, 1986). 31 I . H . Hillier and V. R. Saunders, Mol. Phys., 1971 22, 193. 32 I>. R. Lloyd and P. J. Roberts, Mol. Phys., 1973, 26, 225. 33 S. M . Goldberg, C. S. Fadley and S. Kono, J. Electron Spectrosc. Relat. Phenom., 1981, 21, 285. 34 R. Prins, J. Chem. Phys., 1974, 61, 2580. 35 J . A. Connor, I . H. Hillier, M. H. Wood and M. Barber, J. Chem. Soc., Faraday Trans. 2, 1974,6, 1040. 36 N. Kosuch, G. Wiech and A. Faessler, J. Electron Spectrow. Relat. Phenom., 1989, 20, 11. 37 A. A. MacDowell, C. D. Garner, I . H. Hillier, C. Demain, J. C. Green, E. A. Seddon and M. F. Guest, J. Chem. Soc., Chem. Cornmun., 1979, 427. Paper 0/0050l K; Received 2nd Fehruary, 1990

ISSN:0301-7249    

DOI:10.1039/DC9908900077

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faruday Discuss. Chem. SOC., 1990, 89, 91-105 In Situ Studies of Supported Rhodium Catalysts Peter Johnston, Richard W. Joyner,* Paul D. A. Pudney, Efim S. Shpirot and B. Peter Williamsf Leverhulme Centre for Innovative Catalysis, Department of Chemistry, and Surface Science Research Centre, Uniuersity of Liverpool, PO Box 147, Grove St, Liverpool L69 3BX Rhodium catalysts have been prepared, supported on y-alumina, vanadium( I 1 1 ) oxide, chromia and molybdena. The activity and selectivity of these materials in the conversion of synthesis gas, ( H2/C0 : 1/1, 523 K and 5 bar pressure), to methanol, ethanol and hydrocarbons has been studied. All catalysts showed similar activity except for chromia, which was almost inactive at the temperature chosen. Selectivity to higher oxygenates followed the trend: V203 > Moo3 > A1203 > Cr203.The alumina supported catalysts had the highest selectivity to methanol. Catalysts have been characterised in situ by extended X-ray absorption fine structure (EXAFS) and the alumina-supported material has been examined in the greatest detail. On reduction at 473 K, rhodium particles with diameter (1 nm are observed by electron microscopy, and EXAFS indicates that these contain cu. 10 rhodium atoms on average. There is good evidence that these particles may be bonded to oxygen of the support or residual chlorine from the impregnation pro- cedure. X-ray photoelectron spectroscopy shows that the surface stoichiometry is ca. C1/ Rh : 1.3/ 1. On exposure to carbon monoxide at room temperature the metal particles break up completely and form entities with the composition (CO),.Rh.C12 and bond lengths similar to those in crystalline Rh2CI,(C0)4.On exposure to synthesis gas at 323 K metallic particles are partly reformed. Under conditions of catalytic relevance only metallic particles are observed although some Rh-CI bonding persists on alumina. No rhodium- chlorine bonding is observed on vanadia-supported catalysts, although the metal particles have a similar average size to those found on alumina. There is some evidence that the particle size is much less uniform on vanadia than on alumina. Exposure to synthesis gas causes small changes in particle parameters, which may be related to the chemisyption of carbon monoxide. The average particle size is much larger, >30A diameter, on the chromia support.The relation between catalyst structure and performance is discussed. Rhodium probably has the most interesting catalytic chemistry of any metal, playing a major role both in homogeneous and heterogeneous catalysis. it is the catalyst of choice for the reaction CO+NO -+ N,+CO, which is the most difficult of the reactions carried out in auto-exhaust treatment, and it is also excellent in the homogeneous carbonylation of methanol. It is an important component of the metal gauzes used for the oxidation of ammonia, and its performance in catalytic CO hydrogenation is fascinating. It is the only metal which produces significant yields of oxygenates other than methanol without promotion. Rhodium is also one of the most studied metals in catalysis. +Permanent Address: Zelinsky Institute for Organic Chemistry, U.S.S.R. Academy of Sciences, 47 Lenin $: Now at: ICI Chemicals and Polymers, Billingham, Cleveland.Prospect, Moscow, U.S.S.R.92 In situ Studies of Supported Rhodium Catalysts An aspect of major interest concerns the relative importance of zerovalent metal compared with either Rh' or Rh"'. This is of particular importance in synthesis gas chemistry, as indicated by several studies.' Using extended X-ray absorption fine structure, (EXAFS) spectroscopy, it was demonstrated that small rhodium particles were converted to isolated Rh' (CO)? entities on exposure to carbon monoxide. The present study was undertaken with two aims. The first was to probe the equilibrium: "2 Rhf nRh'(CO)? ('0 in the presence of mixtures of carbon monoxide and hydrogen.This question is of special interest in oxygenate synthesis, since Ponec and co-workers have argued strongly that positively charged metal ions are the active species in alcohol synthesis.' The catalytic significance of Rh' has also been stressed in a recent infrared study by Knozinger and co-workers.3 A preliminary account of part of our work has been p ~ b l i s h e d . ~ We are also interested in the role of the support in determining selectivity in synthesis gas chemistry. We distinguish three classes of product which are formed by rhodium catalysts, methanol, higher oxygenates and hydrocarbons. This ability of rhodium to synthesize each of these is interesting from a mechanistic standpoint.The Fischer- Tropsch synthesis, where the first step is thought to be cleavage of the carbon-oxygen bond, provides a route to hydrocarbons. In contrast, the integrity of this bond is maintained in methanol synthesis, as demonstrated by Takeuchi and Katzer for a palladium catalyst.6 The mechanistic pathway to ethanol and other higher oxygenates is unclear. It is most easily pictured as insertion of CO into a surface methylene or methyl species,' but this remains a conjecture. It is widely recognised that the support has a major influence on both activity and selectivity, but the reasons are again unknown. The extent to which well characterised rhodium surfaces dissociate carbon monoxide is a subject of controversy.x High-dispersion rhodium catalysts have therefore been prepared on a range of supports, characterised in situ by EXAFS and their activity in synthesis gas conversion measured.Experimental Catalysts with 1% rhodium loading by weight have been prepared by impregnation of rhodium trichloride (Johnson Matthey PLC) onto commercially available supports (Strem Chemicals), using the incipient wetness method. The requisite amount of RhCI,.xH,O was dissolved in the minimum volume of distilled water. Supports were used as received, except for alumina which was calcined at 873 K before impregnation. Following impregnation, the catalysts were dried in air, heated in vucuo for 12 h and pressed into discs 13 mm in diameter and cu. 1 mm thick. Some discs were used directly for EXAFS analysis while the remainder were sieved (600-1000 p m mesh), for catalytic activity measurements.Activity Testing A charge of 0.20 g was tested in a laboratory scale microreactor (Labcon Ltd, Croft-on- Tees), having a diameter of 3 mm. The reactor was purged with hydrogen and the catalyst reduced in situ at 673 K for 20 h (1 bar,? GHSV 3000 h-'). The catalyst was cooled to 523 K under hydrogen and the atmosphere changed to an equimolar CO-H2 mixture. Reaction conditions were either 5.0 or 30.0 bar pressure, with GHSV of ca. 3000 or ca. 18 000 h - I , respectively. .) 1 bar= 10' Pa.0.8 - 0.6 v) c1 .- C e 2 0.4 v x C .- C C .- 0.2 0.0 n 2 4 6 0 10 R I A Fig. I . ( a ) The EXAFS spectrum of dirhodium tetracarbonyl dichloride: (-) experiment, weighted by multiplication by k ' ; ( - - - ) calculated using the parameters given in table 1 .( b ) Fourier transform of the spectrum shown in fig. l ( a ) : (-) experiment, weighted by multiplication by k'; ( - - - ) calculated using the parameters given in table 1 .94 In situ Studies of Supported Rhodium Catalysts Table 1. Parameters used to fit the EXAFS spectrum of Rh,CI,(CO), shown in fig. 1 neighbour coordination number interatomic distance/ A Debye- Waller factor/ A' carbon carbon chlorine oxygen rhodium rhodium 1 .o 1 .o 2.0 5.8" 1 .o 1 .o 1.77 1.85 2.35 2.84 3.12 3.3 1 0.015 0.015 0.009 0.005 0.0 14 0.020 " See text. On-line product analysis was performed with a dual-column, parallel-injection gas chromatograph (Pye Unicam 4500 series). Permanent gases were separated by a 1 m x 3 mm column packed with Carbonsieve S (100-120 p m mesh) and detected by thermal conductivity.The hydrocarbon and oxygenate products were separated on a 30 m megabore capillary column and detected by flame ionisation. The peak areas were corrected using literature detector response factors.y Catalyst Characterisation EXAFS measurements were performed using station 9.2 on the wiggler beamline at the Daresbury synchrotron radiation source and an in situ apparatus which has been described previously.'" Catalysts in the form of 13 mm diameter discs were treated in appropriate gas mixtures at pressures up to 1 bar. During these experiments the source operated at 2 GeVt with circulating electron currents in the range 150-250 mA. Data collection procedures have been described previously,"' and data were anal ysed by standard procedures, which are discussed in detail elsewhere.' I The significance of all shells reported has been confirmed using a previously published statistical test,'? and random errors are quoted.Phase shifts have been obtained from the Daresbury data base and optimised by study of suitable standard compounds (rhodium metal, RhCl,, Rh2(C0)"C12 and Rh203). Our fit for metallic rhodium has been publi~hed.~ Fig. 1 shows that obtained for the dicarbonyl dichloride using the crystallographic parameters from Dahl et a l l 3 Six shells of neighbours were included, as indicated in table 1; the values used are taken from the crystal structure with the exception of the oxygen coordination number and the Debye- Waller factors. The linear Rh-C-0 bonding cause a 'focussing effect' and results in an exaggerated Rh-0 coordination number.Thermal vibrational parameters are not the same in EXAFS, where correlation of atomic motions is included, and X-ray diffraction, where it is not. X-ray photoelectron spectroscopy measurements were performed on a commercial spectrometer (Vacuum Generators ESCA3), using A1 K,, radiation. The adventitious carbon peak was assumed to have a binding energy of 285.0 eV and used for calibration purposes. Catalysts could be pretreated at pressures up to 1 bar in suitable environments. Relative stoichiometries were obtained from standard equations using excitation cross- sections due to Scofield.'" Results Catalytic Performance The performance of four 1% rhodium catalysts on different supports is summarised in table 2.Catalysts showed similar activity, with the exception of the chromia-supported + I eV.= 1 . 6 0 2 ~ 10 ' " J .Faraday Discuss. Chem. Snc., 1990, vol. 89 Plate 1. Scanning transmission electron micrograph of the as-received Rh/AI,O, catalyst. The bar at the top of the picture represents 100 A. P. Johnston ef al. ( Facing p. 9 5 )P. Johnston et al. 95 Table 2. Performance of rhodium catalysts in synthesis gas conversion, H,/CO:l/l at 523 K Plbar G H SV/ h- ' time on stream/h conversion'"'( Yo) selectivity' " ) oxygenates: methanol ethanol ethanal C, oxygenates others total oxygenates hydrocarbons: methane ethane ethene c, others total hydrocarbons 5 2920 0- 144 7 16.0 16.3 4.3 0.6 4.5 41.7 43 .O 2.3 0.9 4.3 6.2 56.7 30 17 960 144-216 3 45.2 5.6 7.8 4.3 0.1 63.0 35.5 0.2 0.4 0.9 37.0 - 30h 5 19 320 3000 216-300 0-36 S 6 42.8 9.7 13.1 29.3 3.5 9.1 1.5 1.6 0.5 0.4 61.7 so. 1 25.7 20.4 2.1 4.4 0.3 4.5 3.1 8.1 3.1 12.5 38.3 49.9 5 S 2559 3 000 0- 240 144- 200 11 6 - 3.5 - 3.1 18.6 2.8 8.2" 0.3 30.3 9.7 3.5" 8.0 40.4 57.3 16.0 13.4 0.2 4.4 9.5 9.5 3.6 5.7 69.7 90.3 I' Conversions and selectivities are averages over the times indicated.under standard conditions. ' Entirely propanal. '' Butanal and acetic acid. '' T = 623 K. Following re-reduction, material, which was almost inactive at 523 K. At 5 bar pressure, selectivity to higher oxygenates follows the sequence: V203 > Moo3 > Al2O3 > Crz03. The vanadia catalyst had the lowest selectivity to hydrocarbons at this pressure.The trend in selectivity to methanol was: A1203 > V203 > Cr203 > MOO,. The alumina supported catalyst was tested at both 5 and 30 bar pressure, with the space velocity adjusted to maintain constant residence time. The results show that the activity of this catalyst decayed with time, but that it could be regenerated by reduction in hydrogen at 673 K. At the higher pressure there is a marked increase in selectivity to methanol, largely at the expense of hydrocarbons and in particular methane. Catalyst Characterisation The alumina-supported catalysts have been studied in greatest detail and shall be considered first. Our preliminary study showed that after reduction in hydrogen at 473 K these are highly dispersed materials, with a nearest-neighbour coordination number of 4.8 * 0.5, corresponding to a mean particle size of ca.10 atoms. The scanning transmission electron micrograph shown in plate 1 confirms the highly dispersed nature of these materials and indicates considerable uniformity in particle size. These small particles are markedly disrupted by exposure to carbon monoxide at room temperature, as observed previously by Prins and co-workers.' Detailed analysis of the EXAFS spectrum, shown in fig. 2 in both reciprocal and real space, indicates the presence of three main shells of neighbours, carbon, chlorine and oxygen and some weak, residual Rh-Rh bonding. The best-fit parameters are given in table 3(6). Because of the obvious contribution of chlorine to this spectrum we have reanalysed that of the reduced catalyst, which is shown in fig.3, with the fitting parameters given in table 3 ( a ) . The result of exposure to synthesis gas (H,/CO ratio = 2/ 1) at 373 K and 1 bar pressure,1 2 r, I > z o Y v * - 2 - 4 0.30 h CI v) ..- c 4 0.20 m x c 0) c v Y v) .- CI .- 0.10 0.00 c t t 2.0 3.0 4.0 5.0 6.0 7.0 R I A Fig. 2. ( a ) The EXAFS spectrum of the Rh/AI,O, catalysts after reduction at 473 K and exposure to carbon monoxide, ( 1 bar), at 298 K: (-) experiment, weighted by multiplication by k 3 ; ( - - - ) calculated using the parameters given in table 3 ( a ) . ( b ) Fourier transform of the spectrum shown in fig. 2 ( a ) : (-) experiment, weighted by multiplication by k 3 ; (- - -1 calculated using the parameters given in table 3( b ) .P.Johnston et al. 97 Table 3. Best-fit parameters obtained from analysis of the EXAFS spectra of the 1% Rh/A1203 catalyst neighbour coordination number interatomic distance/A ( a ) reduced in hydrogen (1 bar) at 473 K oxygen 1.4 f 0.5 1.79 * 0.04 chlorine 1.5*0.5 2.28 * 0.03 rhodium 5.5 * 0.4 2.64 * 0.01 ( b ) exposed to carbon monoxide (1 bar, 298 K), after reduction carbon 1.6 f 0.4 1.8 1 * 0.04 chlorine 1.8*0.3 2.33 * 0.02 rhodium 0.3 f 0.2 2.77 * 0.04 oxygen 4.2'"' 3.08 f 0.03 ( c ) after exposure to CO/H2 (ratio 1/2, at 323 K, 1 bar) carbon chlorine rhodium 2.0 * 0.4 1.9 * 0.3 2.6 * 0.3 1.87 * 0.04 2.25 * 0.03 2.73 * 0.02 0.25 0.20 0.15 0.10 - 0.05 ? Y, 0.00 4 * 0.05 0.10 0.15 0.20 0.25 '' Values not accurate owing to multiple scattering. k l k ' Fig. 3. The EXAFS spectrum of the Rh/A1203 catalysts after reduction at 473 K in hydrogen, ( 1 bar): (-) experiment, weighted by multiplication by k ; ( - - - ) calculated using the parameters given in table 3( a ) and including a rhodium-chlorine shell.98 0.15-- 0.10 In situ Studies of Supported Rhodium Catalysts P, I \ I \ -- 0.30 0.20 Fig.4. The EXAFS spectrum of the Rh/AI2O3 catalysts after reduction at 473 K in hydrogen, (1 bar), and exposure to synthesis gas at 323 K, (CO/H,: 1/2, 1 bar): (-) experiment, weighted by multiplication by k ; (- - - ) calculated using the parameters given in table 3(c). Table 4. XPS studies of Rh/AI2O3 catalysts pretreatment rhodium 3d,,, binding energy/eV Cl/Rh ratio as received 308.8 f 0.1 reduced in hydrogen, 1 bar, 473 K 307.6 f 0.1 exposed to carbon monoxide, 1 bar, 298 K 308.3 f 0.2 exposed to H,/CO 308.5 f 0.2 1.2 f 0.1 1.7 f 0.1 1.0*0.1 1.1 fO.1 after previous exposure to carbon monoxide, is shown in fig.4; fitting parameters can be found in table 3(c). An analogous series of experiments has been performed monitoring the catalysts by X-ray photoelectron spectroscopy (XPS). Of interest is the rhodium 3d5,, binding energy and the presence of chlorine, which had a constant binding energy of 198.8+0.2 eV. The results are listed in table 4. To parallel the catalytic studies, a Rh/AI2O3 catalyst disc has been pre-reduced at 673 K and examined by EXAFS, the structural parameters determined are listed in table 5. This catalyst has then been exposed to synthesis gas for 4 h at 523 K, and the EXAFS spectrum measured after cooling to room temperature in CO/H2 is shown in fig.5. Subsequently the catalyst was re-reduced at 523 K and then exposed to carbon monoxide at the same temperature. The EXAFS spectrum was measured at each stage and the results are also listed in table 5.P. Johnston et al. 99 Table 5. Best-fit parameters obtained from EXAFS analysis of Rh/AI2O3 under catalytically relevant conditions neighbour coordination number interatomic distance/ 8, ( a ) reduced in hydrogen 1 bar, 673 K oxygen 1.4 * 0.6 1.77 f 0.03 chlorine 1.2 * 0.6 2.27 f 0.03 rhodium 4.1 f0.5 2.66 f 0.01 ( b ) Exposed to synthesis gas (1 bar, H2/CO:2/1, 523 K), after reduction oxygen" 2.1 *0.5 1.88 f 0.03 chlorine 1.3 k 0.6 2.19f0.03 rhodium 5.0 * 0.5 2.74 * 0.02 ( c ) as ( b ) , after evacuation and exposure to hydrogen at 523 K o x y g e n ' I 1.8k0.5 1.91 *0.03 chlorine 1.5f0.6 2.25 * 0.03 rhodium 5.0 * 0.5 2.69 * 0.02 ( d ) as ( c ) , but after exposure to carbon monoxide at 523 K ( i ) fit with a carbon nearest neighbour carbon 2.8 * 0.5 1.92 f 0.03 chlorine 1.5k0.6 2.25 * 0.03 rhodium 5.1 k0.5 2.72 k 0.01 oxygen 1.5 * 0.05 1.75 * 0.03 rhodium 5.12k0.5 2.73 f 0.01 (ii) fit with an oxygen nearest neighbour chlorine 1.5 * 0.6 2.20 * 0.03 " This shell may represent both oxygen and carbon neighbours, see discussion in text.X-ray absorption studies have also been performed for the vanadia- and chromia- supported materials and the results are given in tables 6 and 7. A theory-experiment comparison for a vanadia-supported catalyst is given in fig.6. Discussion Structural Aspects of Alumina-supported Catalysts We consider first the information on structure of the catalysts derived from the X-ray absorption measurements and the attempt to relate these to the activity patterns observed. As noted already, small rhodium particles are susceptible to attack by carbon monoxide, with severe disruption of the metallic structure. The resulting entity has previously been analysed by Koningsberger and co-workers,' who concluded that the species formed ds (CO)?Rh03, where the oxygen is from the support and the Rh-0 distance is 2.12 A. We believe that this is not correct. The data are more accurately described by the proximity of chlorine rather than oxygen and the deduced RhzCI distance is very similar to that in the crystalline compound (CO),RhCI, at 2.33 A.The coordination numbers observed in the presence of carbon monoxide also show that the environment adopted by rhodium atoms is very similar to that in the cry$talline gem-dicarbonyl species. Not surprisingly, only a single Rh-C distance (1.81 A), is required to fit the data, while packing ic the crystalline state requires two rather different Rh-C separ- ations, ( 1.77 and 1.85 A ). The presence of chlorine on the catalyst surface is demonstrated by XPS, which indicates an average Cl/ Rh ratio of 1.3. Thus, almost half of the chlorine from the impregnation step is retained on the alumina-supported material. The import- ance of residual halogen in catalysis is widely recognised but has been emphasised by100 In situ Studies of Supported Rhodium Catalysts 0.25 0.30 t Fig.5. The EXAFS spectrum of the Rh/AI2O3 catalysts after reduction at 673 K in hydrogen ( 1 bar), and exposure to synthesis gas at 523 K, (CO/H2: 1/2, 1 bar): (-) experiment, weighted by multiplication by k ; (- - -) calculated using the parameters given in table 3 ( c ) . Bond et aZ.15 The results in table 2 spotlight the role of chlorine ions in stabilising the rhodium-carbon-monoxide species on the catalyst surface. Previous studies have emphasised the role of surface hydroxyl species in oxidising rhodium to the + 1 oxidation While this may be correct, the EXAFS results suggest that the presence of state.3. I k l 7 the chlorine ion may be necessary to stabilise Rh’ entities. Where the surface mono- rhodium dicarbonyl has been prepared in halogen-free conditions, Frederick et al. have shown that it is unstable at 298 K in uacu0.I’ It is surprising that the structural role of surface chlorine has not been recognised previously.In all cases where the disruption of rhodium particles has been established by structural techniques, rhodium chloride has been used as the catalyst precursor. The EXAFS spectra reported by Koningsberger and coworkers are essentially similar to those presented here and show the peak in the Fourier transform which we assign to Rh-CI bonding [see e.g. ref. ( l c ) , fig. 3 , peak B ) . These workers also detected the presence of considerable residual chlorine on the catalyst surface.’ ’ After reduction in hydrogen at 473 K, and before exposure to carbon monoxide, EXAFS indicates that the rhodium is present as small metallic particles.The pre- domiaant bonding is rhodium-rhodium, with an interatomic distance contracted by 0.05 A compared with the clean metal. Such contractions are common in small metal particles, and may be significantly larger than that reported here.’” The Rh-Rh coordi- nation number indicates that the average particle contains ca. 10 atoms, in other words somewhat smaller than the smallest quasi-spherical particle, which contains 13 atoms. Because the analysis of the catalyst in carbon monoxide highlighted the presence of chlorine, the spectrum of the reduced catalyst has been reanalysed. A statisticallyP. Johnston et al. 101 Table 6. Best-fit parameters obtained from analysis of the EXAFS spectra of the 1% Rh/V,03 catalyst.neighbour coordination number interatomic distance/ A ( a ) reduced in hydrogen (1 bar) at 473 K oxygen 1.2f0.6 1.98 * 0.04 rhodium 5.9 f 0.4 2.68 f 0.02 rhodium 2.0 f 0.5 3.69 f 0.03 rhodium 4.8 f 1 .O 4.67 f 0.04 rhodium 6.2" 5.29 f 0.1 ( b ) reduced in hydrogen, (1 bar) at 673 K oxygen 1.4 f 0.6 1.93 f 0.04 rhodium 6.8 f 0.4 2.68 f 0.02 rhodium 2.0 f 0.5 3.72 f 0.03 rhodium 4.4 f 1 .o 4.64 f 0.04 rhodium 7.3" 5.26 f 0.1 ( c ) exposed to synthesis gas, ( H2/CO: 2/1) at 523 K, 1 bar oxygen 0.7 f 0.4 1.99 f 0.04 rhodium 6.3 f 0.4 2.69 f 0.02 rhodium 2.0 f 0.05 3.72 f 0.03 rhodium 4.8 * 1.0 4.67 * 0.04 rhodium 5.7" 5.28 f 0.1 '' Values not accurate due t o multiple scattering. Table 7. Best-fit parameters to EXAFS spectra of 1% Rh/Cr203 catalysts reduced in hydrogen (1 bar) at 673 K ~~ ~ neighbour coordination number interatomic distance/ A rhodium rhodium rhodium rhodium 10.1 f0.5 5.5 * 1.0 1 1 .O * 3.0 12.7" 2.69 f 0.01 3.74f0.05 4.67 f 0.04 5.28" No change in parameters was observed after exposure to synthesis gas at 523 K.'' Subject to errors due to multiple scattering. significant improvement in the fit is obtained if two non-rhodium shells are included, as shown in fig. 2. The Rh-0 and Rh-CI distances calculatedoare both shorter than those observed in the bulk compounds, in each case by ca. 0.1 A. This contraction is thought to be real and not due to systematic errors in the phase shifts. The phase shifts employed give a good description of bonding in bulk rhodium oxide and rhodium chloride, with nearest-neighbour distances accurate to k0.02 A.Reduction at 473 K is therefore ineffective in removing residual chlorine from the proximity of the catalyst particles. The continued presence of chlorine on the catalyst is confirmed by XPS (table 4). Fig. 4 and table 3(c) indicate that exposure of the Rh/A1203 to synthesis gas at a temperature as low as 323 K is sufficient to start the restoration of the metallic structure. Rhodium-rhodium bonding with a coordination number about half of that in the reduced catalyst is observed. The table also indicates a general feature of the rhodium particloes in synthesis gas, irrespective of the support, namely a significant expansion (ca. 0.07 A) of the Rh-Rh distance with respect to the reduced particle.This is not observed in0.30 "'it ( a ) 2 2 p \ r . /-_,-. 0.0 - I 10 2.0 3.0 4.0 5.0 6.0 7.0 R / A Fig. 6. ( a ) The EXAFS spectrum of the Rh/V203 catalysts after reduction at 673 K, ( 1 bar): (-) experiment, weighted by multiplication by k (an experimental glitch has been removed from the spectrum at ca. 10.2 A I ) ; ( - - -) calculated using the parameters given in table 6. ( b ) Fourier transform of the spectrum shown in fig. 6 ( a ) : (-) experiment, weighted by multiplication by k ; ( - - - ) calculated using the parameters given in table 3 ( b ) .P. Johnston et al. 103 the presence of hydrogen alone and is therefore a result of carbon monoxide chemisorp- tion. Although not shown in the figure, significant EXAFS oscillations were observed out to k > 14 k’ (750 eV above the absorption edge), and the full data range was used in determining nearest neighbour Rh-Rh distances.A more restricted data range was used in the analysis where low Z scatterers such as oxygen, carbon and chlorine were of interest. At this point it is appropriate to comment on a problem involving fitting of the EXAFS data where coordination to both oxygen of the support and carbon monoxide is suspected. Because of the low scattering cross-section of carbon and oxygen, there is insufficient information contained in the spectra to allow sensible fitting of both Rh-C and Rh-0 shells with interatomic distances <2 A. Table 5 ( d ) shows alternative best fits with carbon or oxygen as the nearest neighbour.The interatomic distances deduced from the carbon neighbour appear more realistic and are preferred; both sets of parameters give similar values of the fitting index. It is nonetheless unlikely that this shell represents bonding both from adsorbed carbon monoxide and from oxygen of the support. We now consider the results of EXAFS analysis where the Rh/AI2O3 catalyst has been examined under conditions of catalytic relevance. The EXAFS results are sum- marised in table 5. The first thing to note is that reduction at the higher temperature, 673 K compared to 473 K, does not appear to cause sintering. If anything, EXAFS shows that the particles are smaller as a result of the higher temperature reduction. The most significant result is that on exposure to synthesis gas at 523 K there is no disruption of the small particle structure, even though there is evidence for some residual chlorine.We therefore conclude that rhodium metal and not Rh’ is responsible for all of the catalytic activity observed. There appears to be an increase both in the Rh-0 and Rh-Rh coordination numbers. Although this is within the absolute error bars in both cases, it is believed that the relative trends are real. We believe that the increases are the signature of chemisorbed carbon monoxide on the catalyst particles. As commented above, there is insufficient information in the spectra to allow reliable fitting of both carbon and oxygen near-neighbour2hells. It seems, however, to be significant that the ‘Rh-0’ distance increases by 0.1 A in the presence of synthesis gas, and we suggest that this is due to carbon monoxide chemisorbed on the small particles.It does not mean that the Rh-C distance is 1.9 A, because this distance is calculated with Rh-0 and not Rh-C phase shifts. The suggestions that the changes reflect chemisorbed CO are reinforced by the results in table 5 ( d ) , where the largest ‘Rh-0’ coordination number is noted in the presence of CO. The absence of a major Rh-0 contribution requires comment, especially since this shell is strong in the gem-dicarbonyl spectrum. The weakness of this feature probably reflects a combination of static and dynamic disorder in the adsorbed layer. It is known that the wag mode of CO adsorbed on metal surfaces is quite soft and this will significantly weaken the Rh-0 contribution to the spectrum.It is disappointing that carbon monoxide adsorbed on these very small particles cannot be studied with greater precision. The behaviour of this catalyst on exposure to hydrogen and subsequently to carbon monoxide at 523 K has been studied and the results are summarised in tables 5 ( c) and ( d ) . The only changes noted are those thought to reflect increased CO chemisorption, as discussed above. Structural Aspects of Other Supports The EXAFS data obtained on vanadia- and chromia-supported catalysts are more limited and also less interesting than obtained from the alumina-supported catalysts. Two features of the vanadia-supported materials are worthy of note; the absence of any evidence for chlorine, and the presence of non-nearest-neighbour rhodium shells.The average particle size on vanadia is similar to that on alumina, so that chlorine should104 In situ Studies of Supported Rhodium Catalysts be observed if present. The catalysts show evidence only for some bonding to oxygen of the support. The presence of higher Rh-Rh shells can be seen most clearly from the peaks at R> 3.5 8, in the Fourier transform [fig. 6( 6)], and suggests that the particle size distribution is less uniform than on the alumina support. Greegor and Lytle'" have calculated the way in which coordination numbers vary for nearest-neighbour shells with particle size. For particles containing 10-15 atoms we expect only a very weak contribution to the EXAFS from non-nearest shells. This is the result obtained from the alumina-supported materials, where the contribution from these shells is below statistical significance over the range analysed.The ability to detect these shells in the vanadia-supported catalyst data suggests that there is a fraction of larger particles present. This would of course imply that particles smaller than the 12 atom average must also be present, to yield the observed Rh- Rh nearest-neighbour coordination number. Electron microscopy studies are planned to probe this point. The results on the chromia-supported catalyst show that the mean particle size is much larger than on either of the other supports. EXAFS becomes much less useful at large particle size, but the observed coordination numbers suggest that the average particle diameter is 17 f ': A, containing 200-300 atoms.The observed relatively large non-nearest-neighbour coordination numbers support this estimate of particle size. Structure-Performance Relationships The results in table 2 show that the activity pattern of the catalysts is: MOO, > Al2O3 = Vz03 >> Cr203. No data are available on the dispersion of the molybdena-supported material, but the activities of the other catalysts are broadly in line with the EXAFS observations on particle size. The technique is not very sensitive for large particle sizes, such as are found in the chromia-supported catalysts, so we cannot be certain that the low activity of the chromia catalysts is entirely due to poor dispersion. Because of the poor activity of the chromia-supported catalyst, realistic selectivity comparisons can be made only for the other three materials.Of these, alumina and vanadia show rather similar patterns, but molybdena is very different, with much higher hydrocarbon yields, complete suppression of methanol and ethanol and the production of significant quantities of aldehydes. These changes may suggest the presence of stronger acid sites on molybdena than for the other two supports, perhaps due to some reduction of Mo"' local to the rhodium particles. The behaviour of the molybdena- supported material will be the subject of further investigation. The characterisation studies provide no evidence for the presence of Rh' species under catalytically relevant conditions. However, there are three interesting differences between the alumina- and vanadia-supported catalysts: that on alumina yields more methanol but much less ethanol than that on vanadia, but hydrocarbon chain growth appears more significant on vanadia.Before ascribing a role to the surface of the support, two factors relating to the rhodium particles must be considered. The presence of chlorine rather than oxygen, proximate to the metal particles on alumina, may have an influence on selectivity. Nearby chlorine could alter the electronic environment over much of the surface of the small particles present here. We have calculated that its range of operation as a catalyst poison or promoter is ca. 3.5 A'' very similar to the radius of the metal particles involved here. 4lternatively, if ethanol synthesis involves carbon monoxide bound to both metal and support, as envisaged by Sachtler et al.," the proximity of chlorine could change alcohol selectivity. I t should also be remembered that, although the average particle size is quite similar on alumina and vanadia, the distribution of sizes may be much broader on vanadia, as indicated by the strength of higher Rh-Rh shells in the EXAFS spectra.At the very small particle sizes involved, some structure sensitivity in the selectivity would not be unexpected. This is particularly the case where CO dissociation is involved, which is known to be structure sensitive.''P. Johnston et a]. 105 Thus Lin et al. have demonstrated that the extent of hydrocarbon chain growth on small ruthenium particles supported on alumina depends on the particle size, with smaller particles favouring longer chains.24 At the present stage it is not fruitful to speculate further on the origins of the observed selectivity differences, especially since the change in the reaction pressure so markedly affects the selectivity of the alumina-supported catalyst.We are grateful to the S.E.R.C. for their support of this work, to Mr R. Billsborrow of the S.E.R.C. Daresbury Laboratory for experimental advice, to Johnson Matthey PLC for the loan of precious metals and to BP Research, Sunbury-on-Thames, for the use of their in situ cell. Mr R. Devenish (Dept of Materials Sciences and Engineering, University of Liverpool), took the beautiful micrograph which is plate 1 . R.W.J. acknowl- edges a helpful conversation with Professor D.C. Koningsberger. References 1 ( a ) H. F. T. Van’t Blik, J. B. A. D. Van Zon, T. Huizinga, J. C. Vis, D. C. Koningsberger and R. Prins, J. Phys. Chem., 1983, 87, 2264; ( h ) H. F. T. Van’t Blik, J. B. A. D. Van Zon, T. Huizinga, D. C. Koningsberger and R. Prins, J. Mol. Catal., 1984, 25, 379; ( c ) H. F. T. Van’t Blik, J. B. A. D. Van Zon, T. Huizinga, D. C. Koningsberger and R. Prins, J. Am. Chem. Soc., 1985, 107, 3139; ( d ) H. F. T. Van’t Blik, J. B. A. D. Van Zon, T. Huizinga, D. C. Koningsberger and D. E. Sayers, J. Chem. Phys., 1985, 82, 5742. 2 G. van der Lee, B. Schuller, H. Post, T. L. F. Favre and V. Ponec, J. Catal., 1986, 98, 522. 3 M. I. Zaki, G. Kunzmann, B. C . Gates and H. Knozinger, J. Phys. Chem., 1987, 91, 1486. 4 P. Johnston, R. W. Joyner and P. D. A. Pudney, J. Phys. Condensed Matter, 1989, 1, SB171. 5 R. W. Joyner, Vacuum, 1988, 38, 309. 6 A. Takeuchi and J. R. Katzer, J. Phys. Chem., 1981, 85, 937. 7 M. lchikawa and T. Fukushima, J. Chem. Soc., Chem. Commun., 1985, 321. 8 J. T. Yates, E. D. Williams and W. H. Weinberg, Surf: Sci., 1980, 91, 562; D. G. Castner, L. H . Dubois, B. A. Sexton and G. A. Somorjai, Surf Sci., 1981, 103, L134. 9 W. A. Dietz, J. Gas Chromatogr., 1967, pp. 68. 10 R. W. Joyner and P. Meehan, Vacuum, 1983, 33, 691. I I S. J. Gurman, N. Binstead and I . Ross, J. Phjqs. C, Solid State Phys., 1984, 17, 143. 12 R. W. Joyner, K. J. Martin and P. Meehan, J. Phys. C, Solid State Phys., 1987, 20, 4005. 13 L. F. Dahl, C. Martell and D. L. Wampler, J. Am. Chem. Soc., 1961, 83, 1761. 14 J. H. Scofield, J. Electron. Spectrosc., 1976, 8, 129. 15 G. C. Bond, R. R. Rajaram and R. Burch, Appl. Catal., 1986, 27, 379. 16 A. K. Smith, F. Hugues, A. Theolier, J. M. Basset, R. Ugo, G. M. Zanderighi, J. L. Bilhou, V. Bilhou- 17 P. Basu, D. Panayotov and J. T. Yates, J. Phys. Chem., 1987, 91, 91. 18 B. G. Frederick, G. Apai, and T. N. Rhodin, J. Am. Chem. Soc., 1987, 109, 4797. 19 E. S. Shpiro et a/., submitted. 20 R. B. Greegor and F. W. Lytle, J. Catal., 1980, 63, 476. 31 R. W. Joyner and J. B. Pendry, Catal. Lett., 1988, I , 1. 12 W. M. H . Sachtler, Proc. 8th Int. Congr. Catal. (Verlag Chemie, Weinheim, 19841, 1, pp. 151; W. M. H. Sachtler and M. Ichikawa, J. Phys. Chem., 1986, 90, 4752; M. Ichikawa, P. E. Hoffmann and A. Fukuoka, J. Chem. Soc., Chem. Commun., 1989, pp. 1395. Bougnal and W. F. Graydon, Inorg. Chem., 1979, 18, 3104. 23 W. Erley, H. Ibach and H. Wagner, Su~-/:f: Sci. 1979, 83, 585. 24 Z - Z . Lin, T. Okuhara, M. Misono, K. Tohji and Y. Udagawa, J. Chem. Soc. Chem. Commun., 1986, 1673. Paper 0/00324G; Receiced 22nd Januar?,, 1990

ISSN:0301-7249    

DOI:10.1039/DC9908900091

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faraday Discuss. Chern. SOC., 1990, 89, 107-117 Characterisation of Oxide-supported Alkene Conversion Catalysts using X-Ray Absorption Spectroscopy John Evans, 3. Trevor Gauntlett and 3. Frederick W. Mosselmans Department of Chemistry, The University, Southampton, SO9 5NH An X-ray absorption spectroscopy (XAS) cell has been developed to allow the loading of air-sensitive catalyst precursors in a glove box and their in situ activation and operation to be monitored. This has been used to investigate two alumina-supported propene metathesis catalysts derived from Mo,(OAc), and Li,[Mo,Me,]. The EXAFS data so derived has been ana- lysed by spherical wave methods using ab initio phase shifts and back- scattering factors. The detailed structural features obtained are sample- history dependent, but the following general observations can be drawn about the mean surface species present.In all cases the quadruple Mo-Mo bond is cleaved on the surface; some samples show evidence of one (or fewer) metal atoms at longer distances. For the latter sample activation in uucuo at 70°C yielded sites approximating to [MoOz(Me),(O-)]” . The addition of propene caused a change in the X g S spectra. There was evidence of a shell with Mo=C distance of ca 1.8 A that can be attributed to a terminal carbene ligand. For one of the Mo,( OAc),-derived catalysts there was also evidence for a molybdacyclobutane unit. The Li4[ Mo,Me,]-based materials also showed evidence of residual Mo=O bonds after exposure to propene. The Zr K-edge EXAFS of Zr(ally1)Joxide (oxide = silica and alumina) were very similar for a range of sample conditions, showing evidence for a residual ally1 group and Zr-0 bonds.X-ray absorption (XAS) has been successfully applied to provide structural detail on oxide-supported transition metal catalysts that had hitherto been unobtainable.’ It is one of the few techniques than can provide information about the local order of a metal site on the surface of an amorphous material, and has the added advantage that experiments may be performed in situ. Several different designs for a catalysis cell have been published, including that of Lytle’ using a BN or Be boat to contain a powdered sample, and the Koningsberger design employing pressed It is known that pressing discs modifies the surface structure of the support3 and also creates voids.‘ Since our test catalysis experiments were performed on free powders it seemed appropri- ate to carry out XAS measurements on samples in the same state.Hence we report a cell design which allows air-sensitive samples to be loaded in a glove box, activated and then exposed to a flow of reagent gas. The samples chosen for this study were two sets of alkene conversion catalysts that had been prepared from organometallic or coordination complex precursors.5 These were molybdenum-based propene metathesis and zirconium-centred ethene polymerisation catalysts. Olefin metathesis has been catalysed by supported-metal catalysts for many years. Using NM R, homogeneous catalysts have been characterised supporting the Chauvin mechanismh involving the transformation via a metallacycle of one alkylidene to another.’ The supported catalysts have generally been less well characterised, partly due to the harsher conditions under which they operate.Supported-molybdenum organometallics were first reported as being active in olefin metathesis by Whan and co-workers.8 However, no work on the structure of such catalysts was carried out at that time. XPS studies by Walton and co-workers were108 XAS Studies of Alkene Conversion Catalysts. inconclusive as to the nature of the catalyst derived from dimolybdenum tetra-acetate.' Iwasawa and co-workers have studied the interaction of ally1 molybdenum compounds with silica and alumina.lo The XAS studies, however, were undertaken on species after oxidation or reduction at high temperature, since their techniques did not enable them to study the highly unstable supported complexes." In the present work, using dimolyb- denum tetra-acetate and the more reactive tetralithium octamethyldimolybdenum as precursors, propene metathesis catalysts active at room temperature have been prepared.These have been activated in situ on an EXAFS beam line and then spectra of the active catalyst under propene have been recorded. Organozirconium-based polymerisation catalysts were reported by Yermakovi2 and Ballard." Again the metal centres were structurally ill-characterised, and similar experi- ments have been performed in the XAS cell, with tetra-allylzirconium as precursor. Experimental The catalysts are extremely oxygen-sensitive thus all experimental procedures, unless otherwise stated, were carried out under nitrogen using standard Schlenk techniques or in a nitrogen dry-box.The support employed for the metathesis reactions was aluminium oxide grade C (Degussa AG); this was preheated to 650 "C in air for 18 h prior to use. The surface area of aluminium oxide grade C is loo* 10 m' g-' and the surface density of hydroxyl groups was determined (by CH, evolution from MeMgCI) as 0.8 nm-I. The Zr experiments also used aluminium oxide C and Aerosil 200 silica; both were dried in vacuo at 200 "C for 12 h.14 Propene Metathesis The metathesis experiments were performed in a silica vessel (volume 128.4 cm3) attached to a conventional vacuum line which had an ultimate vacuum of 1 x lop5 mbar. The catalysis vessel was heated by a conventional oven.A typical catalytic run was initiated by putting approximately 0.1 g of pretreated support into the reaction vessel. This was then heated under vacuum at 200 "C for 1 h. The support was then cooled, under vacuum, to room temperature prior to the introduc- tion of 1 atmt nitrogen. The amount of molybdenum precursor introduced was normally sufficient to give a molybdenum: support mass ratio of 1 : 20. The tetralithium octamethyldimolybdenum was injected in a solution of Et,O (freshly distilled from sodium benzophenone) onto the support and left for 1 h. Then the solvent was removed by careful pumping, during which there were regular admissions of nitrogen to prevent the support being deposited on the sides of the reaction vessel.The catalyst was then pumped on at room temperature for 1 h before activation. However, for Mo,(AcO),, the substrate and the support were ground together in a nitrogen dry-box before being loaded into the catalysis vessel and transferred back onto the vacuum line. Activation of the catalyst was achieved by heating the catalysis vessel to the desired temperature in vacuo for 1 h. After cooling to room temperature, 40mbar of propene was admitted to the reaction chamber. Then after 1 h the pressure in the reactor was increased to 1 atm with nitrogen and samples were then injected into a Pye Unicam GCD chromatograph. This instrument was calibrated using a standard gas mixture of 1 .O% methane, 0.94% ethene, 1.05% propene and 1 .O3% 2-butene in nitrogen.The chromatograph contained a 2 m column filled with 'Porapak Q' and was operated at 150 "C using a flame ionisation detector.J. Evans, J. T. Gauntlett and J. F. W. Mosselmans Table 1. Propene metathesis catalysis results precursor (solvent) activation temperature turnover no. / "C /min-' 3 00 20 60 70 75 100 150 200 250 0.3 0.004 0.06 0.2 0.08 0.04 0.008 0.008 0.0006 109 The turnover numbers, expressed as molecules of propene disproportionating per molybdenum atom per min, for the metathesis of propene at room temperature (20 "C) for the precursors after activation at the most favourable temperature, are shown in table 1. The turnover rates are accurate to *5%. The turnover rates for the octamethyldimolybdenum( 1 1 ) salt and dimolybdenum tetra-acetate are of the same order as those found by Iwasawa et al.for tetra-allyldimolyb- denum on aluminaI5 (0.3 molecules min-'). X-Ray Absorption Spectra XAS spectra were recorded in fluorescence mode on Station 9.2 of the S.R.S., at the S.E.R.C. Daresbury Laboratory, using a double-crystal Si (220) monochromator. For the Mo K-edge data, a zirconium filter was used in front of a thallium-doped potassium iodide scintillation counter. The spectra were calibrated by the use of a molybdenum foil monitor, whose edge was taken to be at 20,003.9eV.t The Zr K-edge data were similarly recorded and calibrated, but without a filter. Background subtraction was achieved using the IBM PC resident program PAXAS,'~ and curve fitting was performed using the program EXCURVE" on the Daresbury Convex c220.Initially the spectra were aligned, averaged and background subtracted in PAXAS. They were then Fourier filtered in EXCURVE before refinement of trial structures. The type of atom of each shell, its abundance and the total number of shells are then progressively varied until a good model has been obtained. The relative importance of each additional shell can be validated by two methods. First, as a rough guide, its contribution to the total EXAFS can be calculated using the 'Fitstat' option in EXCURVE, in which the integral of the EXAFS due to each individual shell is obtained.'* More rigorously the statistics test" of Joyner et al. may be applied as each shell is added. Thus, by a trial and error procedure the final model or models are obtained. There is a similarity in the backscattering properties of the carbon and oxygen atoms, but in most cases the differences were sufficient to allow them to be distinguished.When there are two shells at similar distances high (>0.8) correlations between the parameters of the separate shells might be expected. This was observed for two of the samples described below viz. Zr( allyl),/silica and Li4[ Mo,Me,]/alumina. Thus the statistical tests are essential to check the validity of each shell and this is used to justify some of the models despite some high correlations between parameters. However, in this situation the accuracy of some of the parameters may be less than normally achievable by EXAFS. Unless otherwise stated, the shells were found to have less than 1% probability of being + 1 e V = 1.602 I8 x 1 O - ' " J .110 XAS Studies of Alkene Conversion Catalysts.W///' "14 Fig. 1. Diagram of the XAS cell for the catalytic studies; ( a ) outer-housing with two small (25 mm outer diameter) Be windows for transmission measurements and a larger one at which a single scintillation counter may be placed. There is one vacuum connection flange, one for electrical connections and the third for the sample-bearing stem. ( b ) Central stem in which the sample is mounted between Be windows (4mm pathlength) in a stainless-steel block. The sample sits on a silica fritte and gas may be admitted through the central tube to flow down through the sample. The two smaller tubes are part of cooling coils. Heating elements and thermocouple connections are on the stainless-steel block.insignificant." The value of AFAC, the proportion of absorption resulting in EXAFS, was maintained at 0.85, the refined value for Mo foil. The values of the Debye-Waller factors are 2 u 2 , where u is the mean-square deviation in interatomic distances. Statisti- cally derived errors on the determined distances were all <0.01 A, but a more realistic estimate is 1.5%, or 0.03-0.04 A, for the distances quoted in this paper.,' The error bounds on coordination numbers are probably of the order of *30%. XAS Samples For the Mo EXAFS experiments a complex: support ratio of between 0.5 : 100 and 1 : 100 was used in order to try to achieve maximum uniformity in the state of the molybdenum atoms. The Li,[ Mo2Me8]-derived catalyst was prepared as above, however, after the solvent had been removed the catalyst was transferred to the XAS cell in a dry nitrogen glove box.The Mo,(AcO),-based catalyst was prepared in the same manner as for the catalytic reactions, with the mixed catalysts placed in the XAS cell. The sample is held between two beryllium windows 4mm apart in the controlled environment EXAFS cell (fig. 1) which allows the powder sample to be heated and cooled under vacuum (lo-' mbar) and for gas to be passed through the sample. Samples were evacuated for 1 h and then the activation was carried out by heating the samples to 300 "C [for Mo,(OAc),] and 70 "C (for Li4[Mo2Me8]) for 1 h. The cell was then allowed to cool to ambient temperature under vacuum, before spectra were collected on the activated catalyst prior to the introduction of propene.Finally, around 40 mbar of propene was admitted to the cell and spectra of the catalyst were recorded once more. Zr(allyl), was adsorbed from a pentane solution to saturate the available surface sites," ca. 8% by weight of complex. The materials were also dried in uucuo at room temperature. In situ reduction by H2 wasJ. Evans, J. T. Gauntlett and J. F. W. Mosselmans 1 1 1 Fig. 2. In situ monitoring of the activation of the 0.6% Moz(AcO),/alumina sample at 300°C. These Mo K-edge XANES spectra have been shifted along the energy scale and down the absorbance axis to facilitate comparison. The time intervals between runs was ca. 7 min. Results Mo,(AcO),-derived Catalysts Two experiments were performed using concentrations of molybdenum of 0.52 and 0.60% by weight.Since the molybdenum(i1) acetate catalyst is prepared by a dry mix method, only two states of the catalyst were studied in detail: after activation and under propene. There is a noticeable change after the addition of propene, which suggests that the coordination sphere of the molybdenum is being affected by the propene in some manner. XANES spectra were recorded continuously during thermal activation to monitor the progress of the reaction; these spectra, which had a duration of ca. 7 min, are shown in fig. 2 (for the 0.6% sample). The elevated sample temperature produces a broader spectrum but changes in the XANES are apparent as the activation progresses. The most notable of these changes is the enhancement of the pre-edge peak.This is generally associated with a ls-5p/4d electronic transition. This implies that during heating the coordination environment of the molybdenum centres is changing, with a resultant loss of symmetry to allow a low-lying metal-centred orbital of mixed p/d character. This is most accentuated for molybdenum in high-oxidation-state tetrahedral centres. The EXAFS results for the two ‘post-propene’ experiments are shown in table 2. A solution of the ‘pre-propene’ EXAFS has proved impossible to solve for both experi- ments; an R factor of 40% could not be achieved for either set of data. This probably indicates a large disorder in the coordination environment of the molybdenum atoms after activation. Another possibility is that one or other of these collections of spectra may be contaminated by oscillations due to beam movement.** Both of these spectra, however, have a component at high k.It is possible that after activation there are molybdenum neighbours at between 2.7 and 3 A. There is no sign of any molybdenum atoms within bonding distance in the EXAFS data of the ‘post-propene’ samples, suggesting that on activation the multiple molyb- denum-molybdenum bonds are broken as the metal atoms are dispersed around the surface of the alumina. The data from the more dilute sample could be satisfactorily fitted by a three-shell model (C, 0 and C, in increasing radius). As discussed below, this first shell is at a distance expected for a terminal alkylidene ligand. The second sample, though, provided evidence for six shells (fig. 3 ) , including a molybdenum atom112 XAS Studies of Alkene Conversion Catalysts. Table 2.Results of EXAFS analysis for the Mo2( AcO), system after exposure to 40mbar of propene Debye- Waller atom type shell radius/A coordination number factor/A2 C 0 C C 0 C C C Mo 1.88 2.06 2.58 1.81 2.04 2.26 2.64 2.98 3.01 sample I, 0.52% Mo" 2.0 2.0 3.2 sample 11, 0.60% Mot' 0.8 1.6 1.9 2.5 2.3 0.5 0.013 0.003 0.012 0.004 0.008 0.007 0.017 0.008 0.012 ~~ " Fourier window 1.3-3.0 A, fit index 0.37, R = 24.8%, E, = 27.9 eV, VPI = -6.0 eVh Fourier window 0.6-3.7 A, fit index 0.05, R = 10.7%, E, = 14.6 eV, VPI = -5.0 eV. - 4 1 4 6 8 10 '12 v14 ' 161 shell radius/ A Fig. 3. The best fit for the Mo K-edge EXAFS of sample I1 of the Mo2(AcO),/alumina sample after exposure to propene (40 mbar).(a) k3-weighted EXAFS and (b) Fourier transforms, phase corrected for the first shell. (-) Observed data; (- - -) curved wave theory (a). at around 3 A. This is too long for a metal-metal bond but could be indicative of an oxygen-bridge dimer. The inner three shells could be fitted as either all 0 sites, or the chemically more plausible C , 0, C sequence, as for the first sample. The remaining two carbon shells only passed a statistical test (on the raw data) at the level of a less than 5% probability of being insignificant. Li,Mo2Me, .4Et20 as a Catalyst Precursor Here also, two separate sets of data were obtained. In each case spectra were recorded after activation at 70°C in vacuo and under propene.The two experiments involved 0.87 and 0.70% by weight of molybdenum, respectively. The results of the EXAFS analyses are shown in table 3, and the fits for the sample I are presented in fig. 4. InJ. Evans, J. T. Gauntlett and J. F. W. Mosselmans 113 Table 3. Results of EXAFS on Li,Mo2Mex .4Et20-derived catalysts Debye- Waller atom type shell radius/ 8, coordination number factor/A2 0 C C Mo 0 c 0 Mo 0 C Mo pre-propene exposure sample I, 0.87% Mo" 1.71 (1.72) 2.6 1.86 (1.88) 1.0 (2.0) 2.1 1 (2.08) 2.0 ( 1.4) 1.75 2.8 2.03 3.3 2.20 2.2 2.60 0.8 sample 11, 0.70% Mob post-propene exposure sample I, 0.87% Mo' 1.66 1 .o 1.80 1.8 2.44 2.1 3.08 0.9 sample 11, 0.70% Mo" 1.75 3.9 2.09 2.4 2.59 0.9 0.017 0.009 (0.0 17) 0.013 (0.010) 0.006 0.006 0.006 0.016 0.008 0.005 0.016 0.023 0.015 0.0 14 0.015 " Fourier window 0.5-2.5 A, fit index 0.92 (0.60), R = 23.5 (24.2)%, E,, = 33.8 (31.5) eV, VPI = -3.0 eV.h Fourier window 0.6-3.2 A, fit index 0.96, R = 16.0%, E,, = 23.6 eV, VPI = -5.0 eV.' Fourier window 1.3-3.5 4, fit index 0.74, R = 20.6%, E, = 28.1 eV, VPI = -5.0 eV." Fourier window 0.7-2.8 A, fit index 0.84, R = 19.0%, 15,) = 32.4 eV, VPI = -5.0 eV.this table more than one fit is quoted for the spectrum of the first sample, as it proved impossible to distinguish whether one of the shells consisted of carbon or oxygen atoms ( o r a mixture of the twoj. For this catalytic system the pre-propene data could be solved suggesting a more ordered structure after activation. After activation the catalyst appears to consist of molybdenum $toms tethered by relatively short molybdenum-oxygen bonds at between 1.7 and 1.9 A.There may also be some carbon ligands a t distances that vary from 2.03 through 2.1 1 to 2.20 A. While the last is indicative of a molybdenum-carbon single bond, the first two may represent either a strong single bond or a weak double bond. Coordination numbers are less than precise for these systems and, in particular, the model for the second experiment has rather more atoms than are feasible in the coordination sphere. This experiment also shows indications of a molybdenum atom at a distance similar to a molybdenum- molybdenum single bond length. In the first experiment when propene is added there are signs of a carbene group (1.80 A ) similar to that in the acetate catalyst, though the molybdenum-oxygen bonds are still shorter than in that system (1.70 * 0.0: A).In the second experiment, however, there are carbon atoms at a distance of 2.09 A. There are signs in the first experiment of a molybdenum atom outside the immediate coordination sphere but possibly linked by an oxygen bridge. In the second there is a nearby molybdenum atom, which could either be directly bonded or linked by an oxygen bridge; in this model there are a high number of oxygens in the coordination sphere.114 XAS Studies of Alkene Conversion Catalysts- 0 1 2 3 4 5 shell radius/ A B , I shell radius/a Fig. 4. The best fit for the Mo K-edge EXAFS of sample I of the Li,[ Mo2Me,]/alumina sample (A) after activation at 75 "C; k'-weighted EXAFS and ( b ) Fourier transforms, phase corrected for the first shell.(-) Observed data; (- - -) curved wave theory. ( B ) Similarly presented data on sample I after exposure t o propene (40mbar). Zr(allyl),-derived Catalysts Several series of spectra were recorded on alumina and silica at differing zirconium concentrations. All spectra were very similar, both with and without hydrogen reduction, showing low EXAFS amplitude. This indicates low coordination numbers and/or high disorder in the coordination sphere. A fit ofoone of these data sets is presented in fig. 5. This shows ca. three Zr-0 bonds at 2.0 A, and 3 C atoms at ca. 2.25 A, suggesting that in these samples, on average, one of the ally1 groups is retained on the metal centre. Discussion There were two aims to this project viz.to identify the species present after catalyst activation and during catalysis. The first aim was only achieved to a limited extent for one example, namely the Li,[ Mo,Me,]-derived catalyst. The likely presence of some carbon ligands on the activated state strongly suggests that on impregnation some methyl groups remain on the molybdenum centres. Methane gas was found to be evolvedJ. Evans, J. T. Gauntlett and J. F. W. Mosselmans 115 shell radius1 8, Fig. 5. The best fit for the Zr K-edge EXAFS of a saturated sample of Zr(allyl),/silica in uacuo. ( a ) k3-weighted EXAFS and ( b ) Fourier transforms, phase corrected for the first shell. (-) Observed data; (- - -) curved wave theory ( a ) . Fit index 1.53, R = 33%, E,= 26.6 eV, VPI = -1.0 eV, Debye-Waller factors 0.017 and 0.018 A* for 0 and C shells, respectively.during the absorption, thus it seems probable a partial hydrolysis by hydroxyl groups occurs Mo-Me+S-0-H + Mo-0-S+MeH. This tethering is probably by two or perhaps three hydroxyl groups, though on activation some of these may become terminal ligands. The range of Mo-O,,,~ bond distances given by Iwasawa" is large, ranging from 1.72 to 2.10 A, hence definite assignment of the bond types would seem difficult, especially as not all single molybdenum-oxygen bonds are necessarily tethering, they may be part of an oxygen bridge. Something of the range of Mo-0 distances may be identified from the complex K,[Mo,05(oxa- late)z(OH2)2].24 Each Mo centre cpntains two terminal M=O near 1.70A, with an Mo-0-Mo bond length of 1.88 A, and terminal Mo-0 distances of ca.2.15 8, to oxalate and 2.33 8, to the water ligands. A very tentative description of the mean site might be taken as [MoO,(M~)~(O-)]" . It is worth noting that neither in this nor in the acetate system could any surface aluminium atoms be seen in the EXAFS, though had the spectra been recorded at lower temperatures this situation might have been different. The second question then concerns the catalytic species during metathesis. Modifying the Chauvin mechanism to allow for the Schrock observation of the co-existence of two types of metallacyclobutane structures during alkene metathe~is,~' the possible observ- able species may be described as 1-3 in scheme 1. Although there are virtually no structural data on model Mo compounds, there is now a representative set of structure determinations on W complexes; the close similarity of the atomic radii of these elements allows the latter to be acceptable guides of interatomic distances, as presented in the The two sets of spectra for the Mo,( AcO),-derived catalyst in the presence of propene are not identical, but the results are similar; there are carbon atoms at between 1.8 and 1.9 A in both experiments, the distance expected for a carbene carbon.The molybdpum atoms are probably tethered to the surface by two oxygen atoms at around 2.05 A. In sample I the third shell at 2.58 A is difficult to assign. This distance is too long for a direct bond between the two atoms but too short for there to be a single atom between these two groups; it would imply an angle of around 100" at the carbene carbon.A mean structure of 4 may represent these results. The differences between sample I116 XAS Studies of Alkene Conversion Catalysts- R OH" ' 0 (4) Scheme 1 and sample I1 might be rationalised if one of the carbene units is transformed into a metallacyclobutane ( 5 ) ; presumably a slightly different coordination site alters the position of the surface equilibria shown in scheme 1. The carbons at 2.98 A may be attached to the carbene carbon, suggesting a Mo-C-C angle of around 126", which is in thc expected range. Although there is evidence of another molybdenum atom at 3.01 A in one of the spectra, there are not enough oxygens around the centres for a bridge between these atoms to be likely.The Li4[ Mo,Me,]-based catalysts differed in that there was strong evidence of Mo=O bonds (ca. 1.7 A) after exposure to propene. Again carbene formation is likely (the first shell for sample I1 may well have both 0 and C components). There is reasonable evidence of paired species present in the catalyst, though whether both molybdenum atoms play a part in the reaction is undetermined. The systems that have been studied here are perhaps not ideal subjects for XAS studies in that they are not clean and well ordered. However, without oxidation or reduction at elevated temperatures many adsorbed systems are complicated and dis- ordered. No assignment can be made about the oxidation state of the catalyst. Although the results obtained are not as clear-cut as might be desirable, a few common threads run through them which provide some insight into the metal coordination spheres.The actual structure of the catalyst is possibly dependent on the precursor, but the active catalyst is probably tethered by two oxygen atoms to the alumina surface and in some cases is part of a paired species. The pre-edge peak in the active catalyst XANES indicates a significant distortion from a centro-symmetric species. The use of promoting agents such as Sn( CH3)4 to activate heterogeneous metathesis catalysts" has provided strong evidence for the Chauvin metathesis mechanism in the heterogeneous reaction but there appear to be no previous reports of the presence of carbenes in active heterogeneous metathesis catalysts. The presence of carbon atoms at distances appropri- ate for carbenes from 1.80 to 1.96 A is thus the most important finding of this work.Although on this limited evidence it is impossible to comment on the mechanism of carbene formation or on the step from alkene and carbene to metallacycle and vice versa, the presence of such species on a heterogeneous catalyst strongly suggests that this is the propagating step in such catalysis. We thank the S.E.R.C. for support (to JTG and JFWM). Both the S.E.R.C. and the staff of the Daresbury Laboratory are thanked for providing the facilities at the S.R.S. We are grateful to Degussa AG for providing the oxide supports.J. Evans, J. T. Gauntlett and J. E W. Mosselmans 117 References 1 ( a ) Characterisation of Catalysts, ed. J.M. Thomas and R. M. Lambert (Wiley, Chichester, 1980); ( b ) Bimetallic Catalvsts: Discoveries, Concepts and Applications, J. H. Sinfelt (Wiley, New York, 1983); ( c ) X-Ray Absorption: Principles, Applications and Techniques of EXAFS, SEXAFS and X A NES, ed. D. C. Koningsberger and R. Prins (Wiley, New York, 1988); ( d ) J. Evans, in Catalysis, ed. G. C. Bond and G . Webb (Royal Society of Chemistry, Cambridge, 1989), vol. 8, p.1. 2 F. W. Lytle, G. H . Via and J. H. Sinfelt, in Synchrotron Radiation Research, ed. H. Winich and S. Doniach (Plenum Press, New York, 1980), p. 49. 3 J . L. van d e Venne, J. P. M. Rindt and G. J. M. M. Coenen, J. Colloid Interface Sci., 1980, 74, 287. 4 W. C. Conner, E. L. Weist, T. Ito and J. Fraissard, J. Phys. Chem., 1989, 93, 4138.5 Catalysis by Supported Metal Complexes, ed. Yu. I . Yermakov, B. N. Kuznetsov and V. A. Zhakarov, (Elsevier, Amsterdam, 1981). 6 J. L. Herrison and Y. Chauvin, Makromol. Chem., 1970, 141, 161. 7 R. R. Schrock, J. Organomet. Chem., 1986, 300, 249. 8 J. Smith, W. Mowat, D. A. Whan and E. A. V. Ebsworth, J. Chem. Soc., Dalton Trans., 1974, 1742. 9 S. A. Best, R. G . Squires and R. A. Walton, J. Catal., 1979, 60, 171. 10 Y. Iwasawa, S. Ogasawa and M. Soma, Chem. Lett., 1987, 1039; Y. Iwasawa, H. Ichinose, S. Ogasawa and M. Soma, J. Chem. Soc., Faraday Trans. I , 1981, 77, 1763. I I Y. Iwasawa, N. Ito, H. lshii and H . Kuroda, J. Chem. Soc., Chem. Commun., 1985, 827. 12 Yu. I . Yermakov and V. A. Zhakarov, Adv. Catal., 1975, 24, 173. 13 D. G . Ballard, Adv. Catal., 1973, 23, 263. 14 J. Schwartz and M. D. Ward, J. Mol. C'atal., 1980, 8, 465. 15 Y. lwasawa and S. Ogasawa, J. Chem. SOC., Farada-v Trans. 1 , 1979, 75, 1465. 16 N. Binsted, PAXAS, Programme for the Analysis of X-Ray Abssorption Spectra (University of 17 N. Binsted, J. Campbell, S. J. Gurman and I . Ross, EXCURVE, (S.E.R.C. Darebury Laboratory, 1988). 18 N Binsted, S. L. Cook, J . Evans, G. N. Greaves and R. J. Price, J. Am. Chem. Soc., 1987, 109, 3669. 19 R. W. Joyner, K. J. Martin and P. Meehan, J. Phys. C, 1987, 20, 4005. 20 J . M. Corker, J. Evans, H . Leach and W. Levason, J. Chem. Soc., Chem. Commun., 1989, 181. 21 V. A. Zhakarov, V. K. Dudchenko, E. A. Paukshtis, L. G. Karakchiev and Yu. I . Yermakov, J. Mol. Catal., 1977, 2, 421. 22 Phantom Oscillations, K. I . Pandya, J. van Grondelle, A. MuSoz Paez and D. C. Koningsberger (Eindhoven University of Technology, 1989). 23 Y. Iwasawa, Adv. Catal., 1987, 35, 187. 24 F. A. Cotton, S. M. Woodhouse and J. S. Wood, Inorg. Chem., 1964, 3, 1603. 25 J. Feldman, W. M. Davis and R. R. Schrock, Organometallics, 1989, 8, 2266. 26 R. R. Schrock, R. T. DePue, J . Feldman, C. J . Schaverian, J. C. Dewan and A. H . Liu, J. Am. Chem. Sor.., 1988, 110, 1423; M. T. Youinou, J. Kress, J. Fischer, A. Aguero and J. A. Osborn, J. Am. Chem. Soc., 1988, 110, 1488; M. R. Churchill, A. C. Rheingold, W. J. Youngs, R. R. Schrock and J. H. Wengrovius, J. Organornet. Chem, 1981, 204, C17; J. Feldman, J. S. Murzdek, W. M. Davis and R. R. Schrock, Organometallics, 1989, 8, 2260. 27 K-I. Tanaka and K. Tanaka, in Homogeneous and Heterogeneous Catalysis, ed. Yu. 1. Yermakov and V. Likholobov (VNU Science Press, Utrecht, 19861, p. 245. Southampton, 1989). Paper 9/05425A; Received 18 th December, 1989

ISSN:0301-7249    

DOI:10.1039/DC9908900107

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1990, 89, 119-136 Structural Studies of High-area Zeolitic Adsorbents and Catalysts by a Combination of High-resolution X-Ray Powder Diffraction and X-Ray Absorption Spectroscopy Eric Dooryheet and G. Neville Greaves S. E. R. C. Daresbury Laboratory, Warrington WA4 4AD Andrew T. Steel, Rodney P. Townsend and Stuart W. Carr Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebbington, Merseyside L63 3JW John M. Thomas* and C. Richard A. Catlow Davy Faraday Research Laboratory, The Royal Institution, 21 Albermarle Street, London W l X 4BS We have characterized at high temperature a model uniform heterogeneous catalyst for the oligomerization of hydrocarbons (a nickel-exchanged zeolite of initial composition Na59A159Si,330384 - xH,O treated with an aqueous solution of NiC1, so as to yield a homogeneous distribution of Ni, with Si/ Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) abov? the Ni edge and also its high-resolution diffraction pattern (at A = 1.5486 A).We have obtained unique insights into the microenviron- ment of the Ni'+ ions in the as-prepared and the dehydrated as well as the reduced state of the catalyst; in particular, values of atomic coordinates, site-occupancy and bond lengths, have been obtained. High-resolution X-ray diffraction, using a specially constructed environmental cell, has enabled the precise location of sorbed Xe atoms in a sodium-exchanged zeolite X adsorbent (1.5 atoms of Xe per unit cell) under a pressure of 1.75 bar at room temperature to be determined from a Rietveld refinement of the powder diffraction pattern. Direct proof of the role of the strongly polarizing Na+ ion located at the Sn site in firmly binding the Xe to the inner wall of the supercage is obtained. X-ray absorption near-edge structures (XANES) as well as parallel EXAFS studies above the Al, Si and Ga edges on a variety of other zeolites are also reported. Combined (Cia edge) XANES and EXAFS studies show that gallosilicate networks akin to those present in crystalline (gallo) zeolite X and Y are already formed in the precursor gel and are also present in the aqueous solution in contact with the colloidal-amorphous material, which together constitute the gel. We also report one of the first applications of A1 EXAFS for the semi-quantitative 3;Jdy of the dealumination of faujasitic zeolites.Introduction Highly microporous adsorbents and catalysts, typified by zeolites, possess the convenient attribute that all, or nearly all, of their bulk atoms are, at one and the same time, surface atoms, accessible to reactant molecules of diameter up to ca. 8 A. Their surface area, which may frequently be in excess of 600m2gp', is thus internal. When, therefore, $ Also at: Davy Faraday Research Laboratory. 119120 High-area Zeolitic Adsorbents and Catalysts Fig. 1. Structure of zeolite-Y, with extraframework cation sites shown. gaseous species, enter the interior of such solids and are either adsorbed or subsequently converted catalytically into products, the fate of those bound species as well as the (internal) surface characteristics of the microporous host, may be probed by the majority of the techniques available to the solid-state chemist and physicist.’-4 Bulk powder diffraction (XRD) s t ~ d i e s , ” ~ along with bulk absorption fine structure (XAFS) studies, both of which are optimally carried out using synchrotron radiation sources, therefore yield, in combination, unique insights into the surface and catalytic properties of such high-area solids.Study A concentrates on the characterization of the microenvironment and migration of nickel ions at the internal surfaces of a zeolite Y catalyst5-’ in which some of the Na’ ions have been exchanged by Ni’+ ions. The resulting NaNi-Y catalyst has been probed both by XRD and XAFS prior to and after dehydration and also after reduction in hydrogen.Study B aims to determine the location of sorbed xenon inside a Na+ ion-exchanged zeolite X adsorbent. Study C is a quantitative study of the dealumination of a zeolite by comparing the changes in XAFS of aluminium and silicon edges. Study D is a detailed analysis of the microenvironment of a typical, tetrahedrally bonded atom (in this case gallium) in the course of formation of a (gallo) zeolite from its amorphous precursor gel to the final crystalline state. We have previously reported briefly on preliminary results obtained in study A.” Study B is an extension of earlier using laboratory X-ray sources, aimed at locating within zeolite cages, important adsorbents such as xenon 12-14 (which simulates the behaviour of methane) and methyl chloride.Study C, although complementary to solid-state NMR studies of dealumination, ‘’,16 can yield quantitative values for bond distances such as A1-0 in the dealuminated zeolite. Study D is, to our knowledge, the first report of an analysis which charts the local environment of a tetrahedrally bonded element from the pre-crystalline state to the crystalline solid. We have briefly reported” these results previously. The structure of zeolite-Y, with which we are mainly concerned in this paper, is shown in fig. 1 , which displays the customary polyhedral framework structure of the In this communication we focus upon four distinct categories of study.E. Dooryhee et al. ( a ) 121 1 6 1 2 0 8 0 4 0 2 4 6 8 1 0 Fig. 2. Background-subtracted Ni-edge EXAFS spectrum and ( b ) Fourier transform for hydrated Ni-Y.Dotted lines show calculated spectrum using structural parameters given in table 1. zeolite, each full line representing a T-0-T bond (T= Si or Al) and each vertex being a T site. S, sites are situated at the centre of the double-six rings (D6R), i.e the hexagonal prisms connecting two sodalite cages, and the S1, and Sill sites are at the walls of the supercage. S,, and SII, sites are mirror images, inside the sodalite cages, of the S, and SII sites, respectively. Experimental All our X-ray absorption and diffraction studies were carried out using the synchrotron radiation source (SRS) at the S.E.R.C. Daresbury Laboratory. For study A, three different samples were used. The first was a hydrated nickel-ex- changed zeolite-Y catalyst containing some un-exchanged sodium ( Na,7Ni21A159Si ,330384 exchanged with 0.005 mol deg-3 NiC12 solution, henceforth labelled NaNi-Y); the second was the same material dehydrated in vacuo at 300 "C for 14 h; and the third was the product of reducing the dehydrated sample in a stream of hydrogen at 450°C for 32 h.The EXAFS data for the Ni K-edge (8333 eVt) were collected in transmission using station 7.1 at the SRS Daresbury Laboratory. As noted, the measurements on the dehydrated sample were collected at room temperature in situ in a Lytle cell in which the dehydration had been effected. The hydrated and reduced samples may be safely exposed to the atmosphere and the data were obtained under normal ambient conditions.The spectra were background ~ubtracted'~ and analysed using the standard Daresbury EXAFS software package ( E X C U R V S ~ ~ ~ ) . Least-square fitting of the data was undertaken using phase shifts that had been refined by employing NiO and Ni metal as model122 High-area Zeolitic Adsorbents and Catalysts 0 8 0 4 0 0 4 Fig. 3. ( a ) Background-subtracted Ni-edge EXAFS spectrum and ( b ) Fourier transform for dehydrated Ni-Y. compounds; these yield information on backscattering from respectively surrounding 0 and Ni atoms, the latter being relevant to the reduced compounds. Calculated phase shifts for Si backscattering proved to be adequate. Background subtracted data and Fourier transforms for the three samples are shown in fig 2-4. The diffraction studies used the high-resolution Debye-Scherrer diffractometer of Station 9.1 on the hard X-ray (Wiggler) line at the SRS, Daresbury.Data were collected on the dehydrated sample, in situ in the GTP furnace. The pattern which was measured using a wavelength of 1.5486 A between angles 26, of 6 and 70°, is shown in fig. 5. The data were refined using the Rietveld profile refinement technique, employing the PDPL programme suite developed by Fitch and Murray. For study B, prior runs were carried out, using a conventional volumetric apparatus for studies of gas uptaken by porous solids, on a zeolite-X sample (Si/AI = 1.15) prepared at the Port Sunlight Laboratories of Unilever plc. An environmental chamber, capable of accommodating a flat plate on which the sample was mounted, permitted us to record high-resolution (Debye-Scherrer) diffraction data, again using Station 9.1 at the SRS Daresbury.Xenon could be admitted to the environmental cell at a variety of pressures up to 1.75 bar.? For study C, three samples of ammonium-exchanged zeolite-Y (prepared at the Unilever Port Sunlight Laboratories) were used, as well as a-A1203 and y-A1203, as references. The first of these, labelled 252, was a standard zeolite (Si/Al ratio of 2.52). The second and third, C2 (Si/Al=5.2) and D2 (Si/Al=5.6), respectively, had both been subjected to steam dealumination. XAFS data were collected at the SRS using the soft X-ray EXAFS (SOXAFS) Station 3.4. A double crystal quartz monochromator was employed, enabling the A1 K-edge (1559 eV) to be conveniently scanned.Si K-edge (1839eV) data were obtained in a separate set of experiments in which an InSbE. Dooryhee et al. 123 0 4 Fig. 4. ( a ) Background-subtracted Ni-edge EXAFS spectrum and ( b ) Fourier transform for reduced Ni-Y. 16000 Fig. 5. Diffraction pattern for dehydrated Ni-zeolite-Y (300 "C). monochromator was used. The use of soft X-rays necessitates that the samples are under a high vacuum. The EXAFS signal is measured straightforwardly by monitoring the total photo-electron yield. To prevent charging of the samples, they were mixed with graphite and supported on a copper plate. For study D, the following gallium-containing zeolites were prepared (as described in the respective cited reference): Na(GaY)," Na(GaX)," gallosodalite." Results on studies of zeolite g a l l ~ - o m e g a , ~ ~ - ~ ~ and K(GaL) 2224 will be described elsewhere.The synthesis of all these zeolites uses a two-stage process, i.e. the formation of an amorphous alkaline slurry or gel (on mixing) of gallate solution and a source of silica, followed by prolonged hydrothermal crystallization.124 High-area Zeolitic Adsorbents and Catalysts The ammonium form of the gallo-Y zeolite, NH,(GaY) was prepared by ion exchange (10 times) of Na(GaY) with 0.1 mol dm-' NH,NO,. Gallium-exchanged NH,(AIY) and NH,(GaY) were prepared from 0.1 mol dm ' aqueous solution of gallium(II1) sulphate (adjusted to a pH of 3 by addition of conc. ammonia). l o g of the zeolite NH,(AIY) or NH,(GaY) was slurried with 100 cm' of this solution; these mixtures were set aside at 25 "C for 16 h.The zeolites were recovered by filtration, washed with distilled water and dried in air. NH,(GaY) was treated with 0.1 mol dm' NH4N0, acidified to a pH of 3 with nitric acid. This sample was set aside at 25 "C for 24 h and the zeolite collected by filtration. The precursor gels were prepared as for the crystalline zeolites, except that they were separated from solution at an early stage either by filtration or by centrifuging at 4000r.p.m. for ca. 1 h. EXAFS spectra at the Ga K edge (10367 eV) were recorded on Station 7.1 of the Daresbury SRS. The spectra were collected in transmission mode. Solid samples were finely ground and supported between strips of adhesive tape. Solutions and gels were housed within heat-sealed polythene containers. Normalization and background sub- traction were undertaken and the data were analysed using the EXCURVE programme."' As well as using gallosodalite as a reference, we also used two aqueous solutions of gallium to pin point the differences between 4- and 6-coordinated gallium.(Both these solutions exhibited (see below) a single EXAFS oscillation with no 'longer range' interactions.) Sodium gallate solutions (made up of 10,8 and 82% respectively of NaOH, Ga203 and water) showed a tetrahedral Ga-0 distance of 1.80 a; gallium sulphate [made up from 0.1 mol dm-3 Ga,(SO,), buffered with 0.1 mol dm-3 NH,NO, to pH 3) had a Ga-0 distance of 1.93 A, which is characteristic of 6-coordination. Results and Discussion Study A: Microenvironment and Migration of Nickel Ions at the Internal Surfaces of NaNi-Y Catalyst From the EXAFS studies several conclusions may be drawn.The EXAFS spectrum, fig. 2, of the as-prepared NaNi-Y sample clearly indicates the presence of a hydrated Ni2+ cation. As is evident from the Fourier transform of the data, there is one shell of oxygen atoms surrounding the central Ni; the least-squares fit to the data summarised in table 1 shows a coordination number of 6 and an Ni--.O bond length of 2.06 A, identical to that in the hydrated Ni cation in Ni( N03)2 - 6H20. We should note, however, that the oxygen backscattering peak is not entirely symmetrical which may indicate some partial hydrolysis of the hydrated cation. In addition we observe smaller peaks at larger distances due to backscattering from more distant shells; these almost certainly arise from framework atoms.We conclude, therefore, that Ni is present as a hydrated, and possibly partially hydrolysed cation, occupying sites in the supercage where it is loosely bonded to the framework. Fig. 3 reveals a much more complex structure after dehydration. The main feature is a peak corresponding to a Ni...O bond at 2.02 A, which is slightly less than in the hydrated systems. In addition, there is a peak corresponding to a short bond length of 1.87 A. Clearly, Ni is present in a variety of environments in this system, as shown in table 1 . Reduction of the material gives rise to additional features, in particular the peak at 2.50 A (see fig. 4) which can clearly be attributed to backscattering by surrounding Ni atoms, hence indicating the presence of metallic Ni particles on reduction.The Ni.-.Ni spacings at 4.29 and 5.09 A lie close to the second and third shells in the f.c.c. structure. Note also the disappearance of the short 1.87 8, Ni.S-0 bond length, suggesting that this site is a precursor of the metal sites in the reduced material.E. Dooryhee et al. 125 Table 1. Bond lengths between nickel and: ( i ) oxygen (ii) tetrahedral (Si or Al) atoms ( i i i ) other Ni atoms as revealed by EXAFS studies Ni-T Ni-Ni Ni-0 sample Nl R, 2af N, R, 2ai N3 R3 2 4 hydrated 6 Ni-Y dehydrated 1 Ni-Y 3 2 2 reduced 5 Ni-Y - 2.06 0.010 4 1.87 0.005 4 2.02 0.009 2 2.49 0.037 - 2.68 0.017 - 2.07 0.015 - 2 - - - - - 3.30 0.014 - - - - - 3.15 0.023 - 3.69 0.019 - - - - - - - - - - 1 2.50 0.013 3.41 0.020 1 4.20 0.016 - - 1 5.09 0.006 T = framework (Al-Si), R is the bond length, N is the coordination number and 2 a ' is the Debye- Waller factor.Table 2. Atomic positions, temperature factors and site occupancies of hydrated Ni-Y zeolite at 300 "C atom X Y Z B A' N comments Si -0.05302 0.12297 0.03562 1.49 137.0 Ni(1)-Si = 3.37 At -0.05302 0.12297 0.03562 1.49 55.0 0 1 -0.10930 0.10930 0.00000 2.01 96.0 Ni( 1)-01 = 3.77 0 2 -0.00195 -0.00195 0.14766 0.65 86.0 Ni( 1)-02 = 3.60 0 3 0.18785 0.18785 -0.02923 1.49 96.0 Ni( 1)-03 = 2.25 0 4 0.16719 0.167 198 0.3 13 15 2.75 96.0 Ni( 1)-04 = 4.80 Nil 0.23378 0.23378 0.23378 3.3 19.6 Sll. site Ni (1) 0 0 0 1.7 13.9 SI site Ni (2) 0.07058 0.07058 0.07058 5.8 1.4 S18 site Ni (3) 0.10570 0.10570 0.10570 5.8 2.4 SI.site Ni (4) 0.1973 1 0.1973 1 0.19731 5.8 2.1 SII site o w 1 0.08 155 0.03539 0.12309 5.8 35.9 sodalite cage o w 2 0.40970 - 0.283 15 -0.21 161 5.8 1 1.6 supercage Ow 3 0.173 -0.197 0.048 5.8 10.9 supercage Space group Fd3m; lattice parameters: 24.3661, 24.3661, 24.3661, 90, 90, 90; R factors: R , = 4.41, R,, = 12.14, R, = 7.77; note refinement used a variable Lorentzian peak shape. The XRD study of the dehydrated material provided important additional structural information. The diffraction pattern (fig. 5) was successfully refined using the Rietveld technique (yielding a weighted profile 'R factor' of 12.14% as shown in table 2). Evidently, as the EXAFS also show, there is a variety of sites which the Ni2+ ions occupy. The conclusions drawn from the results are that on dehydration there is migration of the Ni'+ ions from their original position in the supercage into the hexagonal prism, S , sites.This is in line with the earlier work of Olson.'' However, our refinements also show that Nil+ ions occupy two types of S , . site (see fig. 6 ) . The S , , sites are close to the so-called six-rings but project into the sodalite cage, the two S , , sites being distinguished from one another by their different distances along the (111) axes from the centre of the six-ring. Our XRD data also show that heating at 300 "C for 14 h does126 High-area Zeolitic Adsorbents and Catalysts Fig. 6. Ni and Na sites in zeolite-\(. Number convention is as in table 2. Fig. 7. Configuration of the proposed partial hydrated Ni S , , site.not fully remove the water from the hydration shell of all the Ni2+ ions, with Nil+ ions at S,, sites persisting. Comparing tables 1 and 2, there is an apparent contradiction between our EXAFS and XRD results, especially in regard to Ni-0 bond lengths for the dehydrate! samples, which is, at first, puzzling. The N i - 0 bond length for the S , site of 2.25 A , derived from the diffraction data, is significantly longer than the value of 2.02 A obtained from analysis of the EXAFS results. This apparent discrepancy may be rationalised by noting that only about three-quarters of the S , sites are occupied. In an unyccupied S, site the distance of the surrounding oxygen atoms from the centre is 2.6 A , where as in the occupied sites the 0 atoms relax inwards.EXAFS will measure the relaxed Ni.q.0 bond length, whereas diffraction will see an average 0 position. I t is reasonable to postulate, therefore, that the diffraction bond length is an aveEage of the relaxed and unrelaxed bond lengths. If we use the EXAFS value of 2.02 A for the former and the value of 2.6 A referred to above for the latter togtther with an occupation number of 0.75 for the S , site we obtain an estimate of 2.17 A for the average Ni...O bond length, which is in acceptable agreement with the diffraction value of 2.25 A. An alternative explanation that may be advanced postulates asymmetric relaxations of the Ni” ions away from the centres of the hexagonal prism. However, we would expect such relaxations to lead to enhanced temperature factors which are not observed.The explanation in terms of relaxation of the surrounding 0 atoms seems therefore to be the more plausible.E. Dooryhee et al. i I 8 0 127 1 i 4 ’ I I I I I 1 I 6 1 0 1 4 18 22 26 30 34 201’ Fig. 8. Diffraction pattern of unloaded zeolite-)< (outgassed at 31 “C). There is good evidence that the non-S, Ni ions are all partially hydrated species. Of the two S,, sites, the first is bonded to the framework oxygens but with additional coordinating water molecules as illustated in fig. 7; the second, which is further towards the centre of the sodalite cage, is largely hydrated. The positions of the oxygen atoms (of the water molecules) located in the sodalite cage are consistent with these models for hydration of the S,, Ni2+.The Ni in the SI, sites is also directly bonded to the framework oxygen, and it is also likely that these cations are partially hydrated. Study B: Location of Xenon in the Cavities of Na+-exchanged Zeolite-X Previous studies of xenon sorbed within the intrazeolite cavities of zeolite-p were carried out by Gameson et alx using laboratory X-ray sources. These studies, along with comparable ones9 involving sorption of methyl chloride by zeolite-p, were carried out at low temperatures ( 10-200 K). Although valuable structural data pertaining to the location and dynamics of physically adsorbed Xe were obtained, complementing data derived from solid-state NMR,”.“ it is clear that the flux and monochromaticity of synchrotron sources yield quantitatively improved data at higher temperatures. This is especially important in obtaining basic information about guest species sorbed at ambient temperatures and around atmospheric pressures, the conditions most frequently used in gas separations.We also note that high-quality structural data on guest species sorbed within zeolites may be derived from neutron powder diffraction studies, as exemplified by the work of Fitch et a1.26 on the benzene-zeolite-Y system, and the work of Wright and coworkers” on Xe in zeolite-p, and Wright and coworkers’x on pyridine in zeolite-L, Williams” and Newsam3(’ on benzene in zeolite-L. From our raw data (see fig. 8 for the ‘unloaded’ and fig. 9 for the ‘loaded’ zeolite-)< samples) it is immediately apparent that the diffraction patterns of the two ‘loaded’ samples are significantly different from one another and from that of the ‘unloaded’ sample. Using the Rietveld-profile-refinement procedure, we find that for both loaded samples there is one predominant site for the sorbed Xe as shown in fig. 10.The latter is situated close to Naf ions in the SI1 site, and it is clear that the highly polarizable Xe atom is bound to the polarizing Nat cation. Study C: XAFS of Dealuminated Zeolites Fig. 1 l(a)-(c) show the Al edge data collected for the three zeolite-\( samples; corre- sponding Si edge data are shown in fig. 12(a)-(c). In addition, data were obtained for128 High-area Zeolitic Adsorbents and Catalysts - ( a ) 20000 15000 i ' 3 10000 4500L 3000 - 1 5 0 0 . I 1 1 1 I 1 I 1 2 14 16 1 8 20 6 8 1 0 2 0 / O Fig.9. Diffraction pattern of the Xe loaded zeolite-)< (at 31 OC): ( a ) data for 1 atm; ( b ) data for 1.75 atm. Xe atom Na+ ions Na'(4) ion Fig. 10. Site occupied by Xe in zeolite-><.E. Dooryhee et al. 129 1 5 5 0 ‘ 5 7 0 *, 590 :61C 1630 1550 energy/eV 1 0 0 8 0 2 0 0 1590 :510 :530 : 550 : 550 1570 energy/eV 0 0 I I , I ‘550 ‘ 5 7 3 : 59c ‘ 5 ’ 0 ‘530 ‘ 6 5 3 energy/eV Fig. 1 1 . A1 XANES for three ammonium-exchanged zeolite-\( samples. ( a ) sample 252 (before dealumination) ( h ) sample ‘C2’ (after dealumination) ( c ) sample ‘D2’ (after dealumination).130 1 0 0 8 0 6 E! =r 0 4 0 2 0 0 High-area Zeolitic Adsorbents and Catalysts . ( a ) I .- i I I - I I 1 I I I 0 6 O C 0 3 : 8 3 5 l o r ( c ) ‘885 1 3 9 5 ‘ 8 4 5 1 8 5 5 1 8 6 5 : 8 7 5 energy/eV Fig.12. Si XANES of zeolite-Y samples ( a ) before (sample 252) and ( h ) and ( c ) after dealumina- tion. ( b ) sample C2, ( c ) sample D2.E. Dooryhee et al. 131 Table 3. First coordination shell parameter obtained from analysis of EXAFS data for A1 edge compound A l m a -0 bond length/ A coordination number Debye- Waller factor/A2 252 D2 c 2 1.7 (6) 1.8 (8) 1.8 ( 3 ) 5 7 6 0.02 ( I ) 0.03 (8) 0.01 (3) several Al- and Si-containing model compounds, including cu-A1203. There are clear qualitative differences between the A1 spectra of the dealuminated samples in fig. 11 ( b ) and (c) compared with the spectrum in fig. l l ( a ) . In contrast no important change is observed in the Si edge data in fig. 12(a)-(c). These observations confirm the known fact that there are substantial structural changes on dealumination and that aluminium jettisoned from the framework ends up mainly in the intrazeolitic cavities.Quantitative analysis of the data, showed that, in all samples, there was little well defined structure beyond the first shell. The results for the first shell for the A1 data are, however, of considerable interest and are show! in table 3. It is clear that there is a marked change in the AI...O bond length (ca. 0.1 A); moreover, this is accompanied by an increase in the analysed coordination number of the Al. Neither effect is observed in the silicon environment. The results on the bond lengths which indicate a change in coordination are of greater quantitative significance, as it is known that the extraction of precise coordination numbers from EXAFS studies is difficult owing to the high correlation between the coordination number and the Debye- Waller factor.The change in the Al-0 bond lengths indicates a change in the coordination of the Al. The values for the C2 and De samples are towards the range of those observed for octahedrally coordinated AI(AI2O3 , Y = 1.91). The results therefore provide strong corroborating evidence that steam dealumination results in the deposition of some octahedrally coordinated A1 (probably in the form of a hydroxy species) in the channels of the zeolite. The results also show the general value of SOXAFS studies of aluminosilicate systems. EXAFS studies of light elements such as A1 and Si have not been extensively pursued.Our results present some of the first quantitative studies of the EXAFS above the A1 edge. Study D: Microenvironment of Ga in Proceeding from the Precursor, Dispersed State to the Crystal line Gall-zeolite The principal aim here was to ascertain the resemblance between the local environment surrounding G a atoms in the precursor gels and the gallo-framework of the final crystalling zeolite. Fig. 13-16 show a representative selection of XAFS data and the respective Fourier transforms for the precursor gels and crystalline gallo-zeolites-)< and -Y. The results are also summarized in table 4. Not unexpectedly, EXAFS spectra for certain pairs of solids, e.g. Na(GaY) and Na(GaX) are almost identical, the domin?ting influence of the first-shell Ga-0 backscattering, at the tetrahedral distance of 1.8 A, being apparent. The refined outer-$hell distances (Ga-Si) of these two crystalline gallozeolites fall within 3.10 * 0.05 A, the variation being considerably less than expected from the semi-empirical calculations based on the correlations between NMR "Si chemical shifts and T-0-T angles.The XANES spectra of a number of crystalline gallozeolites (see fig. 15) show some distinct differences, reflecting the variety of the local stereochemistry rather than the coordination number of the gallium atom in the framework. in general we find that the EXAFS spectrum of the G a at the gel stage is very similar to that of the Ga in the crystalline zeolite, indicating that the gallo-silicate networks are132 High-area Zeolitic Adsorbents and Catalysts 4 6 a 1 0 1 2 k / A I I 1 I I 1 3 5 9 R I A Fig.13. Ga-edge EXAFS data and ( b ) corresponding Fourier transform for crystalline Na(GaX) zeolite. comparable in the respective states. We also find that the EXAFS fingerprint of the fourfold coordinated Ga in the separated aqueous phase and the separated, semi- colloidal, suspended solid phase of the zeolite precursor gel is almost identical. There are, however, some discernible differences in the XANES fingerprints for the correspond- ing samples. Evidently, the local environment of the Ga changes slightly between the supernatant liquid and the dried gel; but, by and large, similar local environments exist in the gallosilicate networks present in solution and in the amorphous solid phase. Upon crystallization, however, the Ga XANES and EXAFS show small but distinct differences from those of the dispersed aqueous or amorphous suspended phases. More details of similar studies on other gallosilicate zeolite precursors and crystalline phases, along with quantitative analyses of the various associated XAFS spectra, will be pub- lished elsewhere.3'E.Dooryhee et al. 133 0 4 - 0 Y, * - 4 - 8 . 1 2 2 A c.l .- E l c) C .- 0 I I I I I 4 6 a 1 0 1 2 k/ A I I I I 1 1 3 5 7 9 R I A Fig. 14. ( a ) Ga-edge EXAFS data and ( b ) corresponding Fourier transform for crystalline Na(GaY) and its precursor gel (broken line). Na(Ga X) NaNMe(Ga-Omega) 0 Gallo sodalite a 1 - 0 Na(Ga Y) I 1 I I I I 0 20 4 0 6 0 80 100 1 ----- 4 0 2 0 energy / e V Fig. 15. Ga-edge XANES data for a range of crystalline gallium-containing zeolite K(GaL), Na NMe,(G-IR) and gallosodalite shown for comparison.134 High-area Zeolitic Adsorbents and Catalysts 4 6 8 1 0 1 2 k l A I I I I I 1 3 5 7 9 R I A Fig.16. ( a ) Ga-edge EXAFS data and ( b ) corresponding Fourier transform for the supernatant liquid (broken line) and the dried gel of zeolite-Y. Conclusions Study A In an as-prepared nickel-exchange zeolite--Y catalyst, hydrated Ni'+ ions are shown (by EXAFS) to be situated within the supercages either as Ni(H,O)i+ species or as solvated ions attached to the walls of the supercage via the oxygen atoms of the framework. On heating to 300°C, water is progressively removed from the Ni2+ ions, which migrate out of the supercages into the double-six rings (S,-sites).There is an apparent discrepancy between the Ni-0 bond distances obtained from EXAFS and XRD. But this is resolved in terms of the partial occupancy of the SI sites. Some 30-40% of the Ni'+ ions in the dehydrated state are situated, still partially hydrated, in sodalite cages where they are accessible to attack by hydrogen, thereby yielding small crystallites of metallic nickel.E. Dooryhee et al. Table 4. EXAFS-derived gallium-oxygen distances 135 type number radius/A U 2 / P GaA Gallate (as) Na[GaY] Na[GaX] whole gel dried gel supernatant Ga-exchanged [ Al YI Ga-exchanged [Gay1 0 0 0 Si Na Si/ Na 0 Si Na Si/ Na 0 Si Na Si/ Na 0 Si Na Si/ Na 0 Si Na Si/ Na 0 0 Si Na Si/ Na 0 0 Si 6 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 2 4 4 4 4 2 4 1.92 1.80 1.78 3.14 3.40 4.34 1.78 3.14 3 -40 4.32 1.79 3.1 1 3.41 4.36 1.79 3.1 1 3.40 4.35 1.80 3.1 1 3.41 4.37 1 .80 1.99 3.18 3.41 4.3 1 1.83 1.98 3.27 (0.009) (0.005) 0.006 0.017 0.018 0.023 0.005 0.017 0.017 0.024 0.006 0.025 0.020 0.03 1 0.006 0.023 0.020 0.029 0.006 0.028 0.02 1 0.029 0.012 0.020 0.027 0.016 0.03 1 0.015 0.016 0.023 Study B At room temperatute and under an equilibrium pressure of 1-1.75 bar, atoms of xenon are firmly bound in the intrazeolitic supercage of a Na'-exchanged zeolite-)<.These atoms of Xe, of which there are 1.5 per cent cell on average, are closely associated with the extraframework Na+ cations situated at the S,, site. Study C We report one of the first applications of A1 EXAFS, and in particular its use in elucidating the products of dealumination of a faujasitic zeolite.The average Al-0 bond length of a dealuminated zeolite has been determined demonstrating the creation of octahedral sites.136 High-area Zeolitic Adsorbents and Catalysts Study D Combined XANES and EXAFS beyond the Ga edge of a number of precursor aqueous colloidal and crystalline gallozeolites have been carried out. It is demonstrated that, both in the aqueous solution and amorphous gel, gallosilicate networks very similar to those that exist in the final crystalline gallozeolite are already pre-formed; and there is no evidence of islands or rafts of gallo-containing species. We are grateful for the support of S.E.R.C. and unilever plc and for the cooperation of many of our colleagues, notably Carol Williams, Andrew Fitch, Peter Maddox, John Couves, Richard Jones, Robert Cernik, Steven Pickett, David h, Sheehy. References 1 J.M. Thomas, Proc. 8th Int. Symp. Catal., Berlin, July, 1984, (Verlag Chemie 2 K. Klier, Langmuir, 1988, 4, 13. 3 C. R. A. Catlow and J. M. Thomas, Prog. Inorg. Chem., 1987, 35, 1. 4 J. M. Thomas, Angew. Chemie Intl. Edn. Eng., 1988, 27, 1673. 5 P. J. Maddox. J. Stachurski and J. M. Thomas, Catal. Lett., 1988, I , 191. adill and Michael 1 vol 1. 6 T. Rayment, J. M. Thomas and C. Williams, J. Chem. Soc. Faraday Trans. 1, 1988, 84, 2915. 7 J. W. Couves, R. H. Jones, B. J. Smith and J. M. Thomas, Adu. Materials, 1990, 2, 181. 8 I . Gameson, T. Rayment, J. M. Thomas and P. A. Wright, Chem. Phys. Lett., 1986, 123, 145. 9 I . Gameson, J. M.Thomas and P. A. Wright, J. Phys. Chem., 1988, 92, 988. 10 C. R. A. Catlow, J. W. Couves, E. Dooryhee, G. N. Greaves, P. J. Maddox, A. T. Steel, J. M. Thomas and R. P. Townsend, in preparation. 11 C . R. A. Catlow, E. Dooryhee, G . N. Greaves, A. T. Steel, J. M. Thomas and R. P. Townsend, Int. Zeolite Association Conf: Specialist Research Reports. 12 J. Fraissard and T. Ito Zeoliter, 1988 8, 350. 13 S. Ramdas, J. M. Thomas and P. A. Wright, J. Chem. Soc., Chem. Commun., 1984, 1338. 14 A. K. Cheetham, A. K. Nowak, J. van der Ouden, B. Petersen, S. D. Pickett and M. M. F. Post, J. 15 C. A. Fyfe, G. C. Gabbi, J. Klinowski and J. M. Thomas, Nature 1982, 296, 533. 16 J. Klinowski and J. M. Thomas, Adv. Catal., 1985, 33, 197. 17 S. W. C a n , C. R. A. Catlow, E. Dooryhee, G. N. Greaves, A. T. Steel, J. M. Thomas, and R. P. 18 S. W. Lytle, J. H . Sinfelt and G. H . Via, S~nchrotron Radiation Rerearch, ed. H . Winick and S. Doniach, 19 S. K. Harbron e t a l . , Inorg. Chem., 1986, 25, 1789. 20 N. Binstead, S. J. Gurman and I . Ross, J. Phjx C., 1984, 17, 143. 21 E. Oldfield and H. K. C. Timkin, J. Am. Chem. Soc., 1987, 109, 7699. 22 J. D. Jorgensen and J . M. Newsam, Zeolites, 1987, 7, 569. 23 J. M. Thomas and Xing Sheng Lui, J. Phys. Chem., 1986, 90, 4843. 24 Xing Sheng Lui, Ph.D. Theyis, (University of Cambridge, 1985). 25 D. H. Olson, J. Phys. Chem. 1968, 72, 652. 26 A. N. Fitch, J. Jobic and A. Renouprez, J. Chem. Soc., Chem. Commun. 1985, 284. 27 A. K. Cheetham, S. Ramdas, J. M. Thomas and P. A. Wright, J. Chem. Soc., Chem. Commun.., 1984, 1338. 28 A. K. Cheetham, A. K. Nowak, J. M. Thomas and P. A. Wright, Nature, 1985, 318, 611. 29 C. Williams, Ph.D. Thesis (University of Cambridge, 1987). 30 J. M. Newsam, in press. 31 S. W. Carr, C. R. A. Catlow, E. Dooryhee, G. N. Greaves, A. T. Steel, J. M. Thomas and R. P. Townsend, Phys. Chem., 1990, 94, 1233. Townsend, 8th Int. Zeolite Auociation Donf: (Amsterdam, 19891, Specialist Research Reports. (Plenum Press, New York, 19801, p.401. in preparation. Paper 0/00350F; Received 23rd January, 1990

ISSN:0301-7249    

DOI:10.1039/DC9908900119

出版商:RSC    

年代:1990

数据来源: RSC