摘要:

J. MATER. CHEM., 1994, 4( 12), 1921-1926 Investigations of Structure and Protonic Conductivity in Composites of Hydrous Antimony(v) Oxide and Mordenite Gary 6.Hix,*" Yves Rouillard," Robert C. T. Slade" and Bernard Ducourantb a Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX# #OD Laboratoire des Agregats Moleculaires et Materiaux Inorganiques, URA CNRS 79, Universite Montpellier 2, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France Composites prepared by hydrolysis of antimony pentachloride in the presence of the zeolite mordenite have been characterised by X-ray powder diffractometry (XRD), 29Si and 27AI MAS NMR, "'Sb Mossbauer spectroscopy and investigations of ac conductivity as a function of relative humidity (RH). Both XRD and MAS NMR studies show degradation of the zeolite framework, the extent of the damage depending upon the amount of antimony pentachloride used in the preparation.Mossbauer spectra are consistent with the antimony-containing component being amorphous antimonic acid, as anticipated from the preparative conditions. The composites are protonic (H+) conductors, but do not show the enhanced conductivities characteristic of 'tin mordenites', which show the same disruption of the zeolite framework. Composite proton-conducting solid electrolytes are formed by the bulk mixing of known proton conductors and inert oxide dispersed Enhancements in conductivities over those exhibited by the original conductors are dependent upon the nature of the oxide and its particle size.Zeolites are known to be proton conductor^.^ The protonic conductivity of the zeolite mordenite has been show to be enhanced by the inclusion of a dispersed tin@) oxide pha~e,~-~ giving 'tin mordenites'. The exact nature of these materials is unclear, but the bulk of the evidence indicates that they are composite in nat~re.~ In those materials both the matrix and the dis- persed oxide are proton conductors. The enhancement in conductivity may be due to both an increased number of acid sites in the zeolite, this being due to fragmentation of the framework by dealumination, and the decreased particle size of the microdispersed tin(1v) oxide.7 In terms of the model of a hydrous oxide proposed by England et a[.,' the decreased Sn02 particle size leads to an increased surface area and hence a larger number of charge carriers on the acidic surface of those particles.Antimonic acid (HSbO, .nH,O), sometimes termed 'hydrous antimony(v) oxide' (Sb20,.xH20, x =2n + l), can exist in a variety of structures (ilmenite, cubic, monoclinic, pyrochlore and amorphous forms) within which Sb occurs in SbVOb octahedra. The 'acid' formulation is correct for crystalline materials, protons (H + ) and H,O molecules being present within the crystallites. All forms exhibit proton conduction to a greater or lesser extent. The amorphous form exhibits the highest conductivity,' an observation which may indicate that a component of conduction occurs via hydrated acidic particle surfaces (as in the hydrous oxides').The conductivity of amorphous antimonic acid at ambient temperature is higher than that of hydrous tin(1v) oxide under the same conditions.8 For materials functioning as protonic conductors at near- ambient temperatures, the mechanisms of protonic conduction form a spectrum ranging from mechanical (Grotthus) transfer of H', through vehicular (H,O+) charge transport to liquid- like charge migration." The level of hydration of particle surfaces depends on relative humidity (RH). In systems in which conduction via these surfaces is significant, conductivi- ties increase at high RH as conduction is then more liquid-like. Most materials that conduct protons at ambient tempera- ture dehydrate at ca.60 "C [this is true of both hydrous tin(1v) oxide and antimonic acids8]. Loss of water removes the conduction pathway and leads to drastic reductions in pro- tonic conductivity. The conductivity of 'tin mordeni te' com- posites (0z lop2S cm-I at 100"C and 100% RH4-7)IS among the highest known at temperatures a little above that thresh- old. With that in mind, and noting that antimonic acid is normally considered more conductive than hydrous tin(1v) oxide, we have now undertaken a study of composites formed from mordenite and 'hydrous antimony(v) oxide'. The com- posites have been studied by X-ray powder diffractometry, 29Si and 27Al MAS NMR, "'Sb Mossbauer spectroscopy and electrical (conductivity) measurements. Experimental Zeolites (Na mordenite and H mordenite) were supplied by Laporte Inorganics.Varying quantities (given in Table 1) of antimony(v) chloride (BDH Chemicals) were added clropwise to aqueous suspensions of zeolite (8.0 g in 200 an3), the presence of the zeolite being a modification of the conditions for production of amorphous antimonic a~id.~,~ The resulting suspension was, in each case, freeze-dried and then heated at 100°C under dynamic vacuum for 3 h. The product was re- suspended three times in de-ionised water for 23 h, and samples were stored in desiccators of controlled relative humidity (RH =20, 60, 70 and 80%, above aqueous solutions of sulfuric acid of appropriate concentrations). X-Ray Powder Diffraction ( XRD) XRD profiles were recorded on a Philips diffraytometer (PW1050 goniometer, Cu-Km radiation, i= 1.54178 A).Data were collected over a 10 s period for every 0.1' of 20. Observed profiles were compared with computer-generated simulations for zeolite frameworks. Table 1 Masses of mordenite and SbC15 used in syntheses of composites mass of mordenite/g sample 1 8.0 3.7 sample 2 8.0 8.0 sample 3 8.0" 10.2 " H-Mordenite. MAS NMR 29Si (59.584 MHz) and 27Al (78.152 MHz) MAS NMR spectra at ambient temperature were recorded on a Varian VXR 300 spectrometer by the SERC Solid State NMR Service (Durham). Relaxation delays in recording 29Si spectra were 30s, determined as sufficiently long to avoid all saturation effects and spin rates were ca. 5 kHz. The corresponding delays and spin rates for 27Al spectra were 0.5 s and ca.5 kHz, respectively. "Si spectra were deconvoluted assuming gaussian line- shapes, thereby giving spectral parameters for the different Si sites. "'Sb Mossbauer Spectroscopy Spectra were recorded using a constant-acceleration spec-trometer, with both the sample and the Cal2lrnSnO3 source maintained at T=4.2 K [He (1) cryostat]. Powder samples (containing ca. 45 mg Sb) were dispersed in Apiezon grease. The sample holder presented a geometrical area of 3 cm2 to the oscillator. Data were acquired through a multi-channel scaling analyser as two spectra related through a mirror plane; folding these two spectra improved the counting statistics. Experimental spectra were fitted to calculated model spec- tra, using a program described by Ruenbauer and Birchall.12 Isomer shifts are expressed with respect to the source.Electrochemical Impedance Measurements Powder samples were compressed in a 13 mm die to give pellets of cu. 1 mm thickness. A small quantity of water was added prior to pressing (to aid the binding of the pellets). Pellet faces were coated with conductive silver paint (Acheson Electrodag 915) and copper or nickel discs were then attached using the silver paint as adhesive. Pellets were equilibrated in desiccators over aqueous solutions of sulfuric acid (RH =20, 60, 70 and 80% at ambient temperature). Sample pellets were mounted in a brass cell which held six samples simultaneously. The temperature (233 <T/K <3 13) was controlled by immersion of the cell in controlled-tempera- ture baths, allowing at least 30min for equilibrium to be achieved (this period was found by experience to be in excess of the minimum necessary to give temporal stability of impedance spectra). The relative humidity in the cell was J.MATER. CHEM, 1994, VOL. 4 controlled by placing the appropriate acid solution (as described previously) in the bottom of the cell. ac Impedance spectra (admittance or impedance plane) were collected in the range 5 Hz-1 MHz (with an oscillation voltage of 100mV) using a Hewlett-Packard 4192A LF impedance analyser controlled by an TBM-compatible com- puter using a program embedding the EQUIVCRT program (fitting impedance spectra to a user-defined equivalent circuit13).Results and Discussion Chemical Analyses The parent zeolites and the composites were analysed by X-ray fluorescence (XRF). The Si:Al ratio can be calculated simply from the XRF results. 29Si NMR spectra allow the Si:A1 ratio for a zeolite framework to be calculated; appli- cation of Loewenstein's rule, which disallows Al-0-A1 linkages in zeolite frameworks, gives m =0.4 m=0.4 where 14,,, is the relative intensity of a line assigned to a Q4(mAl) silicon site (rn is the number of -OAl linkages around the Si atom). The results of the various analyses are given in Table 2, and 29Si NMR line intensities are given in Table 3. The Si:Al ratios for the composites (from both XRF and NMR) are greater than those of the parent materials.This shows that more of the Si nuclei in the remaining zeolite framework in composites have fewer Als as nearest neighbour tetrahedrally coordinated atoms and indicates a degradation of the zeolitic framework by acid-leaching of framework aluminium. The acid responsible, HC1, is generated by the reaction of the antimony(v) chloride with water. X-Ray Powder Diffraction Studies The XRD profiles obtained for composites (Fig. 1) show the zeolite component in composites to be distinctly less crystalline than the pristine zeolites. The decrease in crystallinity is seen to become more pronounced as the amount of antimony(v) chloride used in the preparation increases. In the case of sample 1, the material with the lowest antimony content, the Table 2 Chemical analyses of composites and parent zeolites XRF Si :Al mass of SiO, (%) A1203 (Yo) Sb,O, (Yo) XRF NMR product/g ~~ ~ sample 1 60.38 7.13 3 1.08 7.18 7.04 9.35 sample 2 47.95 5.57 45.72 7.26 6.85 11.95 sample 3 Na-mordenite 42.67 81.80 4.91 10.90 52.15 - 9.37 6.15 9.44 6.11 13.56 ~ H-mordenite 90.90 7.31 - 9.15 8.91 ~ Table 3 29Si MAS NMR spectral parameters for composites and parent zeolites Si (OAl) Si (1Al) Si (2x1) sample 1 sample 2 sample 3 Na-mordenite H-mordenite 112.6 288 43 105.7 434 52 98.3 346 5 112.9 298 48 105.8 418 45 99.1 465 7 112.6 292 49 105.8 420 46 100.1 27 1 6 112.6 293 40 105.9 355 55 98.9 245 5 112.7 313 58 105.3 359 39 98.1 314 3 J.MATER. CHEM., 1994, VOL. 4 Fig.1 X-Ray powder diffraction profiles of Na mordenite and composites: (a) Na-mordenite, (b) sample 1, (c) sample 2 and (d) sample 3 principal zeolite peaks are seen, though much reduced in intensity. The sample with the highest antimony content (sample 3, made from H-mordenite) can be seen to be almost completely amorphous, with no discernible peaks. Increasing the amount of antimony(v) chloride used in the preparation increases damage to the zeolitic framework. There are no peaks due to the presence of antimonic acid or Sb205 in any of the profiles, this indicating that the non- zeolitic component has a very small particle size. MAS NMR Studies The results of NMR studies are fully consistent with those from chemical analysis and XRD studies.The 29Si MAS NMR spectra of the composites (Fig. 2) show Si peaks at chemical shifts similar to those observed for the parent mordenite phases.14 The line assigned to the Si( 1Al) site remains largely unchanged with respect to the parent zeolite, but its relative intensity changes (Table 4).For samples 1 and 2 the intensity of the line due to the Si(lA1) site decreases on composite formation, this being consistent with preferential removal of Als linked to Sis of type Si( 1Al) in Na mordenite. The case of sample 3 (prepared from H-mordenite) is different, the near-total disruption of the zeolite being evident in the XRD profile [Fig.l(d)]. The lines due to Si sites with A1 tetrahedral neighbours are broader than those of the host zeolites, the largest difference being in excess of 200 Hz. This line broadening is a result of the fragmentation of the zeolite microcrystals into smaller particles by acid leaching, resulting in a range of environments for the Si nuclei and a distribution of chemical shifts. 27Al spectra (Fig. 3) are dominated by the signal of tetra- hedrally coordinated A1 (in the zeolite framework), but also indicate the presence of some octahedrally coordinated alu- minium, i.e, a peak at 6~0typical of Al"' octahedrally coordinated by 0 atoms [c.f A1(H20),3+(aq), which is the reference]. Peak positions are given in Table 5.The octahedral A1 is that acid leached from the framework. It can again be seen that the extent of leaching is more pronounced with higher antimony content, this being evident in the increase in the intensity of the line corresponding to six-coordinate aluminium (Table 4). I2'Sb Mossbauer Studies In 12'Sb Mossbauer spectroscopy, eight quadrupole trans- itions are expected for the 7/2* 5/2 decay. Quadrupole split- 50 Fig. 2 Proton-decoupled high resolution 29Si MAS NMR spectra of (a) sample 1, (b)sample 2 and (c) sample 3. Spectra were recorded at ambient temperature, with spin rate ca. 5 kHz and 42 rf pulses. The recorded spectrum is given as a dotted line and the fitted gaussians are shown by the solid lines. The difference between the observed and calculated spectrum is given as a solid line below each spectrum.Table4 "A1 MAS NMR spectral parameters for composites and parent zeolites sample 1 53.2 82.2 -0.9 17.8 sample 2 53.4 70.1 -0.9 29.9 sample 3 53.4 65.6 -0.6 34.4 ~Na-mordenite 54.1 100 -H-mordenite 54.7 100 -- 00 Fig. 3 Proton-decoupled high resolution 27Al MAS NMR spectra of composites: (a) sample 1, (h)sample 2 and (c) sample 3. Spectra were recorded at ambient temperature, with spin rates of ca. 5 kHz and employing ~16rf pulses. *Indicates spinning side bands. Table 5 '"Sb Mossbauer parameters for composites (numbers in brackets are standard deviations arising in the fitting of individual spectra) 4," I mm s-' AEq/ mms-' FWHM/ mm s-' intensity x2 sample 1 sample 2 0.61(1) 0.74(3) -0.1 l(2) 5.20(2) 9.84(4) 4.88(2) 1.06(1) 0.76(4) 0.76(2) 1.00 0.75 0.25 1.36 1.52 sample 3 0.73(2) 6.92(3) 1.20(1) 1.00 1.79 tings are detected in spectra of antimony compounds, but line splittings are less than experimental 1ine~idths.l~ The '"Sb spectra for the composites in this work (Fig.4) all showed a single line at close to zero velocity, consistent with Sb present in SbV06 octahedra (as in the source; diso z0mm s-' is typical of that environment16). The anti- mony-containing component of the composites is amorphous and SbV06octahedra are therefore likely to be present with a distribution of local environments and distortions. It is therefore impossible to be completely non-arbitrary in fitting of the individual spectra.The approach adopted was to assume the minimum number of 'Sb sites' (each giving a contribution to the experimental line) necessary to give an adequate fit (judged visually) to each spectrum. The spectra of samples 1 and 3 were satisfactorily fitted with the inclusion of only one SbV site type, giving Mossbauer parameters (Table 5) similar to those for SbVO, in Sb205.16-18 The likely Sb environment in the composites is, however, in amorphous antimonic acid; Sb20, is fabricated from antimonic acids only on heating to higher temperatures (275 "C) than used in this work. The spectrum of sample 2 was, however, satisfactorily fitted only as the sum of a minimum of two contributions (Table 5).That is likely to reflect differences in the distri- butions of local geometries for SbVO, octahedra. J. M'4TER. CHEhl., 1994, VOL. 4 150 000 140000 130 000 120 000 130 000 c 120000v) 3 8 110000 120 110 100 90 ,II I I,, I II, I I1 <*--20 0 velocity/mm s-' Fig. 4 '"Sb Mossbauer spectra of composites: (a)sample 1, (6)sample 2 and (c) sample 3 The '"Sb Mossbauer spectra are consistent with description of the antimony component in the composites as amorphous antimonic acid. ac Conductivity Studies Impedance spectra consisted of a high-frequency arc and a sloping rise of reactance with respect to resistance at lower frequency, this being typical of ionic conduction and blocking electrodes. The conductance of the sample was extracted as the intercept of the low-frequency line with the resistance (real) axis.Over the experimental temperature range (233 < T/K <3 13), and at all relative humidities, all three samples showed Arrhenius-like behaviour in the temperature dependence of their conductivities (Fig. 5). Empirical acti- vation energies E, for protonic conduction are given in Table 6. The conductivity for a given composite and tempera- ture increases with RH, as discussed in the Introduction. The degree of hydration of the conducting surfaces will vary with temperature and it follows that E, values contain a contri- bution from this variation and are best treated as empirical parameters only. Sample 1 has conductivities of the same order of magnitude as those for the parent zeolite (Na mordenite, 0% 10-8-10-5 S cm-' at 50% RH3)under similar conditions.Table 6 Activation energies for protonic conduction in composites at different relative humidities EJkJ mol-' RH (Yo) sample 1 sample 2 sample 3 20 38k 1 45+1 23k2 60 42k 1 45k1 38f I 70 44+2 42f1 38k1 80 29k2 45f2 39f2 J. MATER. CHEM., 1994, VOL. 4 -2 r -3 --4--5 --6-\. --2 --3 2 7 ‘E -4-0 9 I-b -5-v +\ +\+ -2 r --5 I 1 I I I Fig. 5 Temperature-dependent protonic conductivities for (a)sample 1. (b)sample 2 and (c) sample 3, at RH=20 (+), 60 (x), 70 (0)and 80% ( ). Lines shown are the linear regressions and correspond to the activation energies given in Table 6.All experimental values are less than those observed for antimonic acid (including the less-conductive crystalline forms), investigations of ~hi~h~,~~-~~have found 0% 10-3-10-4 S cm-l at ambient temperature. Sample 2 is similar in nature to sample 1 (though less crystalline zeolite is present) and has conductivities similar to those of sample 1. Sample 3 (in which disruption of the zeolite framework is most extensive) exhibits conductivities of the same order of magnitude as the parent H-mordenite, except at RH =80% where the conductivity of sample 3 is higher. The composites in this work do not show the enhancement of protonic conductivities (relative to those of parent zeolites) exhibited by ‘tin m~rdenites’,~-’ but do show disruption of the zeolite framework, as in the case of ‘tin mordenite’ and other ‘tin zeolite^'.^ Antimonic acid [‘hydrous antimony(v) oxide’] itself differs from hydrous tin(1v) oxide in having an intraparticle protonic conduction pathway in addition to surface conduction [in hydrous tin(1v) oxide the intraparticle structure is that of SnO,]. The formation of composites in this work was aimed at giving a high surface area for the antimonic acid component.The results in this study may indicate that the intraparticle conduction pathway is of much greater significance in amorphous antimonic acid than in crystalline antimonic acids (reflecting a disordered/glassy intraparticle structure for the amorphous form) and that surface conduction is much less significant than has been supposed previously (see Introduction).Alternatively. the anti- monic acid may have been deposited in particles too large for a continuous conduction pathway via them alone. Conclusions In ‘hydrous antimony(v) oxide/mordenite’ compo4tes, the zeolite component exhibits extensive damage to the zeolite framework. The extent of that damage increases with the amount of antimony pentachloride used in the preparation. This is evident both from the XRD profiles, in which there is a broadening or disappearance of peaks due to the zeolite and from the MAS NMR studies, in which there is a reduction in the intensity of the line corresponding to the Si(1Al) site at the same time as octahedrally coordinated A1 sites are detected.”‘Sb Mossbauer spectra are consistent with the aiitimony- containing component of the composites being amorphous antimonic acid, as would be anticipated from the prcparative conditions. ‘Hydrous Sb20,-mordenite’ composites do not show the enhanced protonic conductivities characteristic of ‘tin morden- ite~’,~-~even though they do show the framework disruption found to be characteristic of those system^.^ The chemistry of the ‘oxide’ component in composites formed from zeolites and ‘hydrous oxides’ is therefore of considerable Significance in determining the protonic (H’ ) conductivity of those composites. We thank Dr D. J. Jones of 1’Universite Montpellxer 2 for useful discussions on the fitting of Mossbauer spectra.We thank Laporte Inorganics for provision of zeolite samples. We thank the SERC National Solid State NMR service (Ihrham) for recording of the NMR spectra and for subsequent decon- volutions. We thank SERC for a studentship for G.B.H. We thank NATO for a travel grant enabling joint studies at Exeter and Montpellier. This work was part-funded under the Commission of the European Communities JOULE programme. References 1 R. C. T. Slade, H. Jinku and J. A. Knowles. Solid Sttrte lonics, 1992,50,287. 2 R. C. T. Slade and J. A. Knowles, Solid State Ionics, 199i. 46,45. 3 E. Krogh-Andersen, 1. G. Krogh-Andersen and E. Skou, in Proton Conductors: Solids, Membranes and Gek-Materials untl Devices, ed.Ph. Colomban, Cambridge University Press, Climbridge, 1992. ch. 14 and references therein. 4 N. Knudsen, E. Krogh-Andersen, I. G. Krogh-Andersen and E. Skou, Solid State Zonics, 1988,28-30, 621. 5 N. Knudsen, E. Krogh-Andersen, I. G. Krogh-Andersen and E. Skou, Solid State Zonics, 1989,35. 51. 6 N. Knudsen, E. Krogh-Andersen, I. G. Krogh-Andersen and E. Skou, Solid State Ionics, 1991,46, 89. 7 G.B. Hix, R. C. T. Slade, K. Molloy and B. Ducourant, J. Mater. Chem., 1994,4, 1913. 8 W. A. England, M. G. Cross, A. Hamnett, P. J. Wiseman and J. B. Goodenough, Solid State lonics, 1980, 1, 23 1. 9 M. Abe and T. Ito, Bull. Chem.Soc. Jpn, 1968.41,333. 10 Ph. Colomban, in Proton Conductors: Solidy, Membranes and Gels-Materials and Devices, ed.Ph. Colomban, Cambridge University Press, Cambridge, 1992, ch. 3. 1926 J. MATER. CHEM., 1994, VOL. 4 11 R. von Ballmoos, Collection of Simulated XRD Powder Patterns 18 G. G. Long, J. G. Stevens and L. H. Bowen, Inorg. Nucl. Chem. for Zeolites, Butterworth, Guildford, 1984. Lett., 1969, 5, 799. 12 13 K. Ruenbauer and T. Birchall, Hyperjne Interactions, 1979,7,125. B. A. Boukamp, Solid State lunics, 1986, 18& 19 136; 1986,20, 31. 19 X. Turrillas, G. Delabouglise, J. G. Joubert, 1 Fournier and J. Muller, Solid State lonics, 1987, 17, 169. 14 15 16 G. R. Hays, W. V. Van Erp, N. C. M. Alma, P. A. Couperus, R. Huis and A. E. Wilson, Zeolites, 1984,4,377. S. L. Ruby, G. M. Kalvius, R. E. Snyder and G. B. Beard, Phys. Rev., 1966, 148, 176. N. N. Greenwood and T. C. Gibb, in Mossbauer Spectroscopy, Chapman and Hall, London, 1972, p. 444. 20 21 D. J. Dzimtrowicz, J. B. Goodenough and P. J. Wiseman, Muter. Res. Bull., 1982, 17, 971. S. Yde-Andersen, J. S. Lundsgaard, J. Malling an4 J. Jensen, Solid Slate Iunics, 1984, 13, 81. 17 V. S. Shpinel, V. A. Bryukhanov, V. Kothekar, B. Z. Iofa and S. I. Semenov, Symp. Faraday Soc., 1968, I, 69. Paper 41036625; Received 16th June, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401921

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12j, 1927-1928 MATERIALS CHEMISTRY COMMUNICATIONS Behaviour of Ceria under Hydrogen Treatment: Thermogravimetry and in sifuX-Ray Diffraction Study C. Lamonier, G. Wrobel" and J. P. Bonnelle Laboratoire de Catalyse Heterogene et Homogene, URA CNRS No 402, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France Ceria reduction under hydrogen leads to a fluorite lattice expansion between 573 and 1073 K. Both bulk reduction of Ce4' ions and insertion of hydrogen are thought to be responsible for this expansion in the 573-843 K temperature range. Cerium oxide, CeO,, crystallizes in a fluorite structure in which each cerium ion is coordinated by eight oxygen neigh- bours. When treated in a reducing atmosphere, CeO, is known to form non-stoichiometric oxides, CeO, -x.These oxides upon exposure to oxidizing environment, even at room tem- perature, can be reoxidized to CeO,. These properties make CeO, a very interesting component of catalysts in the treat- ment of automotive exhaust gas, as well as in hydrogenation or other oxidation reactions. Numerous experimental tech- niques have been used to study the interaction of reducing agents such as hydrogen with CeO,: gravimetry, H NMR,' in situ magnetic susceptibility, temperature-programmed reac-tions (TPR, TPO).2p4 Most authors reported the presence of two main peaks of hydrogen consumption at 773 or 853 K and above 1073 K in TPR. Several crystallographic studies of the Ce-0 system have also been made.2~5~6 Nevertheless, none of them was carried out under reducing conditions.Our purpose is to characterize the evolution of ceria during the reduction of the oxide under hydrogen by using both thermo- gravimetry and in situ X-ray diffraction (XRD) in the 300-1073 K temperature range. A cerium hydroxide gel was prepared by dropwise addition of a 0.5 mol dm-3 cerium nitrate solution to an excess of hydroxide potassium solution (1.5 mol dmP3) with constant stirring. The resulting precipitate was washed with hot distilled water, dried overnight at 373 K and calcined at 573 K for 4 h. XRD showed the presence of crystalline CeO, with the fluorite structure. The BET surface area was 135 m2 g-'. Reduction experiments were carried out under hydrogen with a Sartorius microbalance connected to a flow gas system for the gravi- metric measurements and in an Anton Paar chamber adapted to a Siemens D5000 diffractometer for the XRD in situ analysis.The heating and flow rates chosen were 100K h-' and 5 dm3 h-', respectively. Fig. 1 shows the weight loss of the CeO, sample when in contact with H, as a function of temperature (black curve). The derivative curve (in grey) exhibits three inflexion points at 573, 736 and 863 K. Before 473 K, the weight variation is attributed to the loss of chemisorbed water. The reduction is performed up to 1173 K and no plateau is attained. Three diffraction patterns are reported in Fig. 2 of ceria heated to 1073 K under various atmospheres.Owing to the use of a platinum holder, spectra present platinum diffraction peaks. In Fig. 2(u) the diffraction lines correspond to CeO,. When ceria is reduced under pure hydrogen, neither the hexagonal Ce203 nor the rhombohedra1 Ce01,82 phase reported by Bevan5 is formed, even at 1073 K. The whole spectrum has the same trend as in Fig. 2(u);however, each ceria diffraction line is shifted towards lower angles from Fig. 2(a) to (b), corresponding to a lattice expansion of the Or 10 --2 h v -82 -4--0 E.$ 4-3 -8-1-0.020 273 473 673 873 1073 reduction TIK Fig. 1 Thermogravimetric profile and relative derivative curve (in grey) of CeO, treated under pure hydrogen 30 40 50 60 2tYdegrees Fig. 2 XRD patterns of CeO, heated to 1073 K in (a) flowing 20% O,-N,, (b) flowing pure hydrogen, (cj flowing pure hydrogen and cooled to room temperature.(a) CeO,, (A) platinum, (H)expanded phase. fluorite structure. In Fig. 2(c) two compounds are obtained: one is that evidenced in Fig. 2(h), the other is CeO, which comes from the partial reoxidation of the latter compound. In order to specify the conditions in which the expanded phase has been formed during the hydrogen treatmcnt, the fluorite peak positions for the most intense lines have been plotted as a function of the reduction temperature (T,). In Fig. 3, A( 28) is the difference between the initial peak position (300 K) and its position at T,, corrected from the thermal expansion factor.Above 973 K, it is not possible to follow the (220) line because of the superposition of this diffraction peak with the (200) peak of platinum. Note that each curve 1928 1.2- tone I zone II ,, n zone 111 L3 .-I A I 400 600 800 1000 reduction TIK Fig.3 Shift of the main CeO, diffraction peaks during the in situ reduction treatment under hydrogen. 0,(311); *, (220); 17,(111). has the same profile and three zones can be distinguished. In the first zone (I, 300-593 K) the cell parameter is unmodified, while in zones IT (593-843 K) and I11 (843-1073 K) there is lattice expansion. Indeed, A(28) values can be considered to be within the experimental error (from 0.1" for T,<673 K to 0.03" for T,>873 K) for zone I, but this is not the case for zones I1 and 111 where the angular deviations are important enough, especially for the (220) and (31 1) lines, to be linked to a bulk phenomenon.So the expansion coefficient of the fluorite structure reaches 0.6% in zone I1 at 723 K, increases abruptly in zone I11 (1.6%) and reaches 2.1% at 1073 K. When compared with results reported in literature, it appears that the two peaks deduced from TPR experiments correspond to the derivative peak at 863 K of Fig. 1 and to the beginning of another one near 1170 K. On the other hand, the analysis of the structure under H, at temperatures up to 1070K, which reveals the presence of a cubic fluorite phase only, is largely in agreement with the results reported in ref. 2. However, this study reveals that the expansion of the CeO, lattice has already commenced by 570 K, which corresponds to the first-derivative peak in Fig.1. As it is generally admitted that ceria reduction begins at 473 K,2 it can be deduced that zone I is concerned mainly with superficial reduction and zones IT and I11 with bulk reduction. Moreover, taking into account the results of Fierro et uL1 who show the incorpor- ation of hydrogen into ceria during reduction under H, in the 573-773 K range, and considering previous studies on cerium-nickel oxides' extended to cerium-copper oxides, in which we have measured the amount of hydrogen occluded in the hydride form in doped ceria treated at 573-673 K under H,, we propose that the expansion of the lattice in J.MATER. CHEM., 1994, VOL. 4 zone I1 is due to a bulk reduction of Ce4+ ions and insertion of hydrogen as H-ions, according to the following scheme: 2Ce4++2O2-+H2+2Ce3++20I1-(1) 20H-+H,O +0,---0 (2) H,+02-+U+OH-+H-(3) in which 0,-, OH-and H- species hold anionic positions and represents anionic vacancies within the fluorite structure. In zone I, reactions (1) and (2) are thought to take place at the surface leading to the creation of the vacancies necessary to dissociate molecular hydrogen. In zone IT, owing to migration phenomena, the reduction spreads to the bulk, giving rise to the expansion of the ceria lattice up to 723 K, the temperature at which the dehydroxylation [reaction (2)] becomes important.'.' Therefore, the second peak in Fig.1 is related to this last phenomenon, the next peak (863 K) undoubtedly being connected with a fresh increase in the cell parameter due to a further reduction in zone 111. In conclusion, over the whole range of temperature studied, cerium oxide retains the fluorite structure and can be described as Ce4+xCe3+1-x02-y(OH-)zH-tEl", keeping in mind th:t 02-,OH- and H-are about the same size (1.32, 1.76, 1.54 A, respectively). This formulation accounts for the easy reoxi- dation of the solid when subjected to an oxygen atmosphere. Moreover, in situ XRD analysis of ceria reduction under H, allows the zone in which the insertion of hydrogen into the structure occurs to be delimited. This is particularly important in hydrogenation catalysis, when ceria is doped or supported, as will be shown in a following paper. References 1 J. L. G. Fierro, J. Soria, J. Sanz and J. M. Rojo, J. Solid State Chem., 1987,66,154. 2 V. Perrichon, A. Laachir, G. Bergeret, R. Frety, L. Tournayan and 0.Touret, J. Chem. Soc., Faraduy Trurts., 1994, YO, 773. 3 H. C. Yao and Y. F. Yu Yao, J. Catal., 1984,86,154. 4 L. Tournayan, N. R. Marcilio and R. Frety, Appl. Cutal., 1991, 78,31. 5 D. J. M. Bevan, J. Inorg. Nucl. Chem., 1955,1,49. 6 J. Barrault, A. Alouche, V. Paul-Boncour, L. Hilaire and A. Percheron-Guegan, Appl. Cutal., 1989,46,261). 7 M. P. Sohier, G. Wrobel, J. P. Bonnelle and J. P. Marcq, Appl. Catal., 1992,84, 169. 8 C. Binet, A. Jadi and J. C. Lavalley, J. Chim. Phjs., 1992.89, 31. Communication 4/06133K: Received lOih October, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401927

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1929 CORRIGENDUM Corrigendum to Novel Aromatic Poly(ether ketone)s Part 3.-Synthesis of Diamine Precursors with 4-8 Benzene Rings Linked by Ether, Ketone and Sulfone Groups Anthony J. Lawson, Peter L. Pauson and David C. Sherrington, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G7 IXL, Stella M. Young and (in part) Niall O’Brien, ICI Wilton Materials Research Centre, Middlesbrough, Cleveland, UK TS6 8JE J. Mater. Chem., 1994,4,1527. Please note that the structure given for compounds 14A and 14N in Table 1 on p. 1529 should be: 0 0 14A: ~oyJpo~14N:X = H0 \ \ d‘o-””. 0

ISSN:0959-9428    

DOI:10.1039/JM9940401929

出版商:RSC    

年代:1994

数据来源: RSC

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J. MATER. CHEM., 1994, 4( 121, 1931-1932 Chemical Technology in Printing and Imaging Systems. Edited by J. A. G. Drake. Royal Society of Chemistry, 1993. Pp. viii +188. Price €39.50 ISBN 0-851 86-655-7. This book is based on the proceedings of a symposium held by the North East Region Industrial Division of the Royal Society of Chemistry held in York in October 1992. It contains 14 chapters, each dealing with a specific theme within the general scope of the symposium. The chapters vary consider- ably in quality, length and in scientific/technological content. The editor states that the objective of the symposium was ‘to highlight the advanced chemistry present in all aspects of the printing process.’ In reality, this text fails to meet this objective. Eight of the chapters carry no bibliography and four of the chapters contain no chemistry related components and little science.In the outline below, the figures in brackets relate to the number of pages and references, respectively. Chapter 1, Future trends in printing, J. W. Birkenshaw (5.1, 0), gives the opinion of the author on six factors that are rather loosely connected and not particularly informative; the content is weak. Chapter 2, Environmental management of processing solu- tions, J. F. Langford (10.3, 0), is totally concerned with the Kodak Chemical Management System (CMSI) and so is of limited value. Other strategies for the management of pro- cessing solutions should have been considered. Little or no consideration is given to the underlying chemistry associated with either the problems or to the solutions to problems.The content is of limited value. Chapter 3, Environmental aspects of solvent use in printing, A, S. McCormick (4.5,0), is largely concerned with definitions of terms, description of acronyms and compliance schedules. In this instance, the lack of specific facts and the lack of a bibliography are serious omissions. Chapter 4,General aspects of flexographic plate technology, J. W. Goodin (9.5, 0), has the oddity of a reference number in the text and no bibliography. It is with this chapter, the ‘chemistry’ content of the book begins, but hardly at the ‘advanced’ level. The content is restricted to definitions and descriptions of particular processes, The grammar used has some peculiarities that can lead to misinterpretation, e.g., ‘After development the plate is dried, Figure 8, which takes about 15 minutes.’ The diagrams given are clear and the explanation meaningful. Chapter 5, Recent developments in negative lithographic plate technology (16, 16), M.J. Pratt, contains much useful information in a well presented format. The examples quoted and the explanation provided herald the beginning of the ‘valuable/useful’ sections of the book. Chapter 6, A review of direct-to-plate systems in litho- graphic printing, P. J. Watkiss (23, 71), is of value. The bibliography is up-to-date, the scientific/chemical principles are cogently delivered and the examples are meaningful.More attention could have been given to the protocol in polymer and monomer identification [e.g., poly(viny1 alcohol), poly- (vinyl butyrol) and poly(methy1 methacrylate)]. Chapter 7, Dye diffusion, thermal transfer (D2T2) printing, R. A. Hann (13, 9), is a useful chapter in an all-round sense. The essential principles of the process are clearly outlined and the physical-chemical aspects of the system are dealt with in some depth. Chapter 8, Fountain solutions, S. Dyster (5.6, 0), is a very brief account of what is a complex component of the litho- graphic printing system. The approach used is very general in nature and rather trivial in style. An example is given by the heading of a section as ‘The Use of Alcohol’ and the reference, continuously, to the use of alcohol in fountain solutions.Chapter 9, General aspects of organic pigment technology, P. Sayer (15, 6), is one of the better chapters but somewhat limited in scope. Thus, the perylenes, perinomes, thioi ndigoids, quinophthalones, etc., are not considered. Greatest attention is given to phthalocyanine pigment class and to the azo-pigment class. Chapter 10, Control of the physical character and perform- ance of organic pigments for inks, R. B. McKay (20 11), has some overlap with Chapter 8, but here the emphasis is placed more in understanding the significance of the physical proper- ties of pigments with respect to their overall performance. This is a strong chapter, containing much useful information. Chapter 11, The recycling generation, S.D. Lillcy (9, 0), covers the regulatory aspects to a limited extent, but is largely geared to the author’s employer from the point-of-view of strategy adopted. Thus, it is largely an expression of opinion and lacking in specific detail. Chapter 12, Current developments in UV-printing inks, G. Webster (14, 4), gives a reasonable account of recent developments. The author has an unacceptable, irregular use of monomer naming and of acronyms, e.g., ethoxylatedpenta-erythritoltetraacrylate (ATTA). Some useful information is provided, but the chapter is not easy to read. Chapter 13, Colorants for electronic printers, P. Gregory (18, 0), will be of use to students and the less aware technol- ogists. The style is variable but several examples of systems are provided.The naming of dyes is not consistent (use of capital letters) and the diagram showing the principle of ink jet printing is a gem (but of little meaning). Chapter 14, Recent developments in water-based inks, B. Hancock (15,O). For a chapter to be so titled and yet have no bibliography seems odd. None-the-less, the coverage is broad and the use of examples skillfully handled. Problems relate to lack of units for rates, the use of non-ST terminology and the unfortunate naming systems for polymers [c .g., poly-(vinyldichloride) (PVdC)]. The total volume has some oddities arising from a lack of editoral control. Chapter numbers are not provided. The table of contents indicates the page at which a chapter starts, but the page is not numbered in the text.Nomenclative and unit symbolism varies considerably. This can lead to confusion for the reader and should be avoided. There are examples in which symposium series hdve been transcribed into useful books. Such examples are not common and this volume is not one of them. It does not meet the market’s needs (students, technologists, research and develop- ment scientists) and at the quoted price cannot be considered to be value for money. J. T. Guthrie Received 25th August, 1994 Statistical Mechanics of Polymers. Symposium Editor, T. A. Vilgis; Editor-in-chief, Hartwig Hocker. Macromolecular Symposia Series, Volume 87. Huthig & Wepf Verlag, 1994. Pp. x + 382. Price DM 152.00; US$105.00.ISBN 3-85739-278-9. This volume contains a collection of lectures given at the international conference Statistical Mechanics of C ondensed Polymer Systems: Theory and Simulation, held at the 1,niversity of Mainz, Germany, on October 4-6, 1993. Altogether there are 37 contributions, each one averaging about ten pages. The topics cover a wide range of theoretical work and there is a nice balance between pure theory and computer simu- lation, with also a few contributions in the field of theoretical material science. Subjects treated include studies on the glass transition, the static and dynamical properties of polymers in melts and in networks, the behaviour of polymers at surfaces, copolymeric systems and liquid-crystalline phases.Reviews of existing theories are given as well as recent theoretical advances, new techniques in computer simulations as well as simulation results are described and molecular modelling also makes an appearance. This mixture very much appeals to me, for it does serve to link up microscopic, atomic views of polymers to the coarse-grained models frequently used to obtain scaling laws and universal behaviour. Naturally, with only a few pages available for each contri- bution, this in no sense could be a reference book. What it does provide, however, is an insight into where the activity in theoretical polymer research is concentrated and the kinds of ideas and theoretical approaches that are in the air. This is not to decry, by any means, the quality of the articles in their own right.Many were written by acknowledged experts in the field and are very instructive and interesting reading. It is just that they are, of necessity, too short to go very deeply into the subject matter. The volume was produced directly from the authors’ type- scripts and this has led to a slightly irritating mixture of fonts, spacings and layouts. Indeed a couple of contributions are tiresome to read, simply because of the poor quality of the layout. This, however, is a relatively minor gripe. All in all I like the book and I learned a lot from it. I would, however, hesitate to buy the book myself at the price. It would, on the other hand, be a useful volume to have in the library.A. J. Masters Received 7th September, 1994 Plastics: Surface and Finish, 2nd Edition. Edited by W. Gordon Simpson. Royal Society of Chemistry, 1993. Pp. xvi +328. Price €47.50. ISBN 0-851 86-209-8. The surface and finish of plastics has always been an important subject for both industry and academia. For industrialists the marketability of their products depends on having the required surface finish, be it decorative or functional, and for academics the scope for studying the fundamentals of surface science is almost limitless. Because in everyday life we are now sur-rounded by so many plastic products it is also a subject of considerable public interest. Since most of the authors con- tributing to this book have apparently worked in manufactur- ing during their lives it is biased much more towards industry than academia but, also has a wider appeal.It gives an J. MATER. CHEM., 1994, VOL. 4 enjoyable overview of many aspects of the subject, some in depth and others more superficially. The whole subject area is of increasing interest to the manufacturing industry and the book has come at an oppor- tune time since, with the end of the recession, many new recruits will, hopefully, be entering the field. These newcomers will find the book a very readable introduction to this area of the plastics industry. For example, the chapter on Surface Recognition introduces the terminology used to describe surfaces and their defects and clearly covers the modern technology that is available for monitoring and quantifying these defects.Similarly, there is an excellent chapter by Sherman and Garrard on surface treatments for plastic films and containers, with many references to the wider literature for those wishing to go into the subject in more depth. Another chapter that provides a fairly comprehensive intro- duction to the subject is that on calendered thermoplastics by Fairbairn. In addition to these and other chapters that cover different aspects in some depth, there are those that are of general interest only. There is, for example, an enjoyable and informa- tive account of basic finishing techniques by Simpson, who offers much advice on how to obtain different finishes on many plastics, ranging from natural products through thermo- sets to thermoplastics.A number of other important subject areas such as vinyl wallcoverings, printing processes for plastics and extruded surfaces are covered in an informative and general way. The chapters Painting Plastics and Adhesives for Plastics Fabrication are on areas of considerable industrial importance and interesting overviews are given. These overviews, however, appear to be based solely on personal experience and opinion since there is not a single reference to the wider literature in either chapter. There is a wealth of scientific and patent literature on the subject of adhesion and yet, when dealing with it under the heading Theoretical Aspects of Adhesion, Counsel1 covers the subject in just one page with no references being given. To be of real value as a textbook for people entering the field, something more than a personal overview of such an important field is required. These aspects of surface and finish are becoming of increased industrial relevance as more plastics and composites are introduced to the aerospace, automotive, packaging and domestic markets and innovative developments are most likely to come from an in-depth scientific knowledge. Too few references (there are only 61 in the whole book and many of these are to British Standards etc.) would be the one main criticism. Apart from this it represents very good value for money and will not only be of interest to those well versed in the field but will certainly whet the appetite of newcomers to the area of plastics surface and finish. P. T. McGrail Received 12th September, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401931

出版商:RSC    

年代:1994

数据来源: RSC