摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 1423-1430 Field-emission Study of Face-centred Cubic Group VIII Transition Metals Part 2.-Adsorption of Hydrogen, Ethylene and Acetylene on Palladium BY ISAO KOJIMA,* EIZO MIYAZAKI AND IWAO YASUMORI Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received 13th April, 1981 The adsorption of H,, C,H, and C,H, on a Pd surface has been studied by means of field emission microscope. The adsorption of ethylene at 295 K initially caused a decrease in work function. On continued exposure, the work function reached a minimum 1 min after the initial introduction and then increased to the same level as that for the hydrogen-adsorbing surface, indicating that C,H, dissociates, releasing hydrogen atoms on the surface.In contrast, the adsorption of C,H, was mainly non-dissociative and produced enhanced electron emision from the stepped area about the (1 11) face with an accqmpanying decrease in work function of ca. 1 eV. Thermal dehydrogenation of adsorbed C,H, occurred above 370 K and the carbon left on the surface was converted to surface carbide on the (21 1) and (3 1 1) faces at ca. 700 K. Excess carbon produced graphite crystallites. The features of these adsorbed and decomposed layers are discussed in comparison with those on a nickel emitter. The adsorption of ethylene and acetylene on the surface of Group VIII transition metals such as nickel, palladium and platinum is important in the understanding of the elementary process occurring in heterogeneous catalysis, since these (&)lo metals show high activities for the hydrogenation of various hydrocarbons.Recent progress in the research on catalysis using single-crystal surfaces has revealed significant influence by the surface structure on adsorption and reaction processes.2 Among the many techniques of surface analysis, field-emission microscopy (FEM) has the advantage of providing information on a large number of well-defined single-crystal surfaces under identical experimental conditions. FEM can thus be used to examine the influence of surface structure on adsorption and to supplement our knowledge of single-crystal surfaces obtained using other techniques. Despite the experimental difficulties caused by using these three metals as emitters, so far there have been several FEM studies on the adsorption of hydrocarbons such as ethylene, acetylene, etc.on Ni394 and Pt5 surfaces. Previously, we applied FEM to studies of the ‘template effect’ of acetylene adsorbed strongly on palladium surfaces6 and to the characterization of catalytically active sites on platin~m.~ In the present work we have investigated the behaviour of the palladium surface on the adsorption of ethylene and acetylene in the temperature range 295-850 K. The experiments were carried out under similar conditions to those in FEM studies of Ni by Whalley et al.394 EXPERIMENTAL The FEM tube employed was of the conventional type with a flat fluorescent screen, and the field-emission specimen was fixed on a metal flange so that it could be changed easily.The tube was attached to an ultra-high-vacuum system. Pressures below 5 x 10-lo Torr I4231424 ADSORPTION OF GASES ON Pd (1 Torr = 133.3 Pa) were attained after bakeout at 500 K for 20 h. The tip was mounted on a Pt heating loop of 0.2 mm diameter, and a Pt-Pt/Rh 13% thermocouple was spot-welded onto the loop at the site of the Pd emitter. The emitter was prepared from a 99.999% pure Pd wire 0.1 mm in diameter, obtained from the Johnson Matthey Co., by polishing electro- lytically in a mixed solution of HCl and HNO, at 1-3 V a.c.* The procedure for cleaning the emitter was as follows. The tip was first heated to 1200 K in u.h.v., followed by field-evaporation, where positive potentials between 7 and 12 kV were applied. Then in most cases stable electron emission was observed, although the emission pattern had many bright spots.To remove these spots, ion bombardment was found to be most effective. The tip was exposed to argon at a pressure of ca. Torr and bombarded with argon ions produced by collision with field-emitted electrons of ca. A. After being fully bombarded, the tip was heated at 1200 K; the pattern of a clean Pd surface then appeared. Plate 1 (b) shows a typical pattern for a (1 1 1)-oriented clean Pd surface and plates (c)-(e) show the patterns after exposure to oxygen, followed by heating at various temperatures. The emitter surfaces corresponding to situations (c) and ( d ) seem to be oxidized to a considerable extent. The extent of oxidation was greater on (c) because of the higher dose of oxygen.By heating the oxidized surface, (d), to 1090 K, many dark patches appeared on the emission pattern (e). They can be associated with the formation of islands of high-resistance materials and could not be removed by heating below 1200 K. Emitters with such an insulating layer were not stable under the applied negative field and were sometimes destroyed within a short period of electron emission. After the repetition of hydrocarbon adsorption, carbon species were formed by dehydrogenation but were removed by heating at 770 K in an oxygen atmosphere. Almost all of the remaining surface oxygen was removed by reduction in hydrogen at the same temperature. The tip surface was then subjected to the ion-bombardment procedure as described above, and subsequent heating to 1200 K restored the original pattern of a clean surface.Changes in work function of the emitter during adsorption, 9, were calculated from the slope of log (i/ V 2 ) against 1/ V plots obtained using the Fowler-Nordheim equation: i = A V2exp( - C@/pV) A = BSP/# where B and C are constants. i is the emission current and V is the applied voltage with the voltage/field proportionality factor, p, and the effective emitting area, S. If p is assumed to be constant, the change in work-function can be calculated from A# = #-#,, = [(rn/rno)r- I] where rn is the F-N slope and the subscript refers to the clean surface. For Pd #o was taken to be 4.82 eV. The change in the pre-exponential term, A , is given by A log A = log (AIA,). Field-emission patterns corresponding to an emission current of 1 .O x 35 mm Tri-X film.A were recorded on RESULTS AND DISCUSSION HYDROGEN ADSORPTION Fig. 1 shows the variations of work function and log A with time obtained when a clean (100)-oriented Pd emitter was exposed to 7 x Torr of hydrogen at 295 K. The work function first increased to 5.0 eV (A4 = 0.18 eV) in a few minutes, but continued exposure turned the change to a slow decrease. The variation of log A with time consists of two parts: a rapid decrease before 4 attains a maximum value and a subsequent monotonic slow decrease. The increase in 4, 0.18 eV, is almost the same as that reported for the Pd( 100) face, but is slightly lower than the values of 0.2-0.4 eV for other planes such as Pd(lll), stepped Pd(111)9 or an Ni tip.3 The increase inJ .Chem. Soc., Faraday Trans. I , Vol. 78, part 5 0 10 Plate 1 PLATE 1 .-Field-emission patterns of Pd: (a) arrangement of planes on the emitter, (b) (1 1 1)-oriented clean Pd emitter, (c) (b) exposed to 20 L oxygen, followed by heating at 960 K, ( d ) (1 1 1)-oriented clean tip was exposed to 1 L oxygen, followed by heading to 780 K, (e) (c) heated to 1090 K. (1 L = lop6 Torr s). I. KOJIMA, E. MIYAZAKI AND I. YASUMORI (Facing p . 1424)J. Chem. SOC., Faraday Trans. I , Vol. 78, part 5 Plate 2 ( b ) (C) PLATE 2.-Field-emission patterns of Pd after dosage of hydrogen: (a) arrangement of planes on the emitter, (b) (100)-oriented clean Pd, (c) exposed to hydrogen for 9 min at 7 x Torr. I. KOJIMA, E. MIYAZAKI AND I. YASUMORII. KOJIMA, E.M I Y A Z A K I A N D I. YASUMORI -6.4 -6.2-+ . -7.2 t . A A 0 4.8 + r I 0 time/ rnin ‘30 1425 FIG. ].-Variation of work function (0) and log A (A) with time when Pd emitter was exposed to hydrogen. work-function owing to hydrogen adsorption is attributed to the dissociation of the hydrogen molecule into adatoms on the surface. Plates 2(a) and (b) show the disposition of planes on the (100)-oriented Pd emitter and the corresponding field-emission image of the clear surface, respectively. The emission pattern after the admission of hydrogen is shown by (c); a bright ring about the (100) face was newly developed; however, the dark lines through the (2 1 1) and (1 11) faces were intensified in comparison with the pattern for a clean surface. The absolute magnitude of the decrease, A log A , approaches ca.1 after 15 min exposure. As for the Ni3 and PtlO emitters, the magnitude of A log A owing to the hydrogen chemisorbed layer was ca. 0.2. In general the magnitude of the decrease in log A on b.c.c. transition metals (e.g. A log A z 1 on Wll) is relatively larger than those for the f.c.c. metals. Also, the change in log A can be correlated with the change in effective emitting area owing to adsorption.12 The Pd emitter with adsorbed hydrogen shows an abnormally large decrease in log A , which may be associated with emission from the area around the (100) face. Since metallic Pd is known to absorb hydrogen to a considerable extent, part of hydrogen may be dissolved in the surface layers. Such absorption is probably limited to the small parts around the (100) face in a low-pressure hydrogen atmosphere < lo-’ Torr.ETHYLENE ADSORPTION The changes in 4 and log A during ethylene adsorption are shown in fig. 2. At the initial stage of exposure at 4 x lou8 Torr 4 decreased rapidly to 4.29 eV from its initial value, 4.82 eV (A4 = -0.53 eV). Continued exposure to ethylene reversed the direction of the work-function change. Afer 4 min 4 became positive. By 15 min exposure 4 had reached a maximum value, 5.0 eV (Ad = 0.18 eV). Further dosing caused a slow decrease in work function. Note that the maximum value of 4 is almost the same as that for the surface with adsorbed hydrogen, indicating that the ethylene dissociated to give hydrogen adatoms, H(a), on the surfaw. Corresponding changes in the1426 ADSORPTION OF GASES ON Pd .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I I I I 0 5 10 15 20 time/min FIG. 2.-Variation of work function (0 or 0) and log A (A or A) with time after adsorption of ethylene at 4 x Torr. Open and filled symbols correspond, respectively, to clean surface and carbon-deposited surface after the first run of adsorption and heating. emission pattern of the Pd surface are shown in plates 3(a) and (b); few changes in emission anisotropy with adsorption were seen. On the other hand, a different change of work function with ethylene adsorption was observed on the Ni emitter. An initial increase in 4 by ca. 0.2 eV was attributed to the dissociation of ethylene forming hydrogen a d a t ~ m s .~ Further, a uniquely enhanced emission from the stepped area about the (211) face was seen, suggesting that some surface reactions were taking place; from this observation, Whalley et al. concluded that the dimerization of ethylene into C , hydrocarbons could take place on the (100) plus (1 11) terrace sites3 Strien and Nieuwenhyus studied ethylene adsorption on a Pt emitter using FEM combined with probe-hole measurements. They reported that ethylene adsorption at 310 K reduced the work function by 0.9 eV and the breaking of C-H bonds in the adsorbed ethylene occurred to some extent below 300 K on the (1 11) face, and more extensively on the (533) and (210) faces. The effect of heating the ethylene-covered surface is shown in fig. 3. When the emitter was heated to 390 K, 4 decreased to 4.65 eV; the negative value of A 4 is probably due to the presence of C2H2(a) species: i.e.on heating, hydrogen adatoms are likely to combine with each other or with other hydrocarbon species and to desorb into the gas phase; then partly dehydrogenated species, probably C2H2(a), which will reduce the work function (see later), would be left on the surface. Further heating changed the direction of the work-function change to an increase, returning to a value close to that of the clean surface at 570 K. The corresponding tip pattern was also similar to that of the clean surface [plate 3 (c)]. This increase in 4 at 390-570 K suggests that breaking of the C-C bond as well as the C-H bond in adsorbed C2H2 occurs. Decomposition would be complete by 570 K.At a higher temperature, 750 K, the darkJ . Chem. SOC., Faraday Trans. I , Vol. 78, part 5 Plate 3 ( C ) ( d ) PLATE 3.-Field-emission patterns of Pd emitter after admission of ethylene: (a) (100)-oriented clean Pd tip was exposed to ethylene for 1 min at 4 x Torr, (b) 15 min, (c) (b) heated to 570 K in vacuum, ( d ) heated at 750 K. I. KOJIMA, E. MIYAZAKI AND I. YASUMORI (Facing p . 1426)J . Chem. SOC., Faraday Trans. I , Vol. 78, part 5 Plate 4 PLATE 4.-Field-emission patterns of Pd surface after introduction of acetylene : (a) (100)-oriented clean Pd emitter exposed to acetylene for 10 min, (6) (a) heated to 620 K in vacuum, (c) heated to 750 K, ( d ) heated to 370 K in acetylene atmosphere, followed by heating to 750 K. I. KOJIMA, E.MIYAZAKI AND I. YASUMORII. KOJIMA, E. MIYAZAKI A N D I. YASUMORI 1427 -7.0 t I 1 1 I 1 300 400 500 600 700 800 temperature/K FIG. 3.-Work-function and log A after heating the ethylene-covered surface at various temperatures. lines along the (loo), (311) and (211) faces became distinct [plate 3(d)]. No other patterns, such as the bright spots which appeared on the Ni emitter,3 were observed on the Pd emitter under the present experimental conditions. These results were reproducible after the treatment described above for preparing a clean surface. The re-admission of ethylene onto the tip after heating the ethylene-covered surface to 750 K resulted in a variation in the work function similar to that of the clean surface, as shown by the solid symbols in fig.2. The corresponding changes in emission pattern were similar to those of the clean surface, except that the (21 1) and (31 1) faces remained dark, suggesting that carbon deposits are present on these areas. -6.6 3 -6.8 -7'0 t 0 5 10 15 20 25 time/min FIG. 4.-Variation of work function and log A with time when Pd emitter was exposed to an equimolecular mixture of hydrogen and ethylene at 1 x lo-' Torr.1428 ADSORPTION OF GASES ON Pd The decomposition of ethylene on the Ni emitter is different from that on Pd. When the surface carbide formed on Ni at 570 K was heated to 670 K, graphite crystallites were produced on the surface, reducing the work function by ca. 1 eV.3 On heating they were concentrated towards the (1 10) face, and finally disappeared from the surface diffusing into the Ni bulk above 850 K.Further, the repeated use of the Ni emitter was found to provide a different variation in the work function from that occurring for the first use. The effect of preadsorbed hydrogen on the Pd emitter was also examined. After the hydrogen remaining in the gas phase had been evacuated the surface was exposed to ethylene at a pressure of 5 x Torr at 295 K. The resulting change in work function was reproducible and was similar to the case of the clean surface. However, the presence of hydrogen in the gas phase caused a slower variation in work function with time, as shown in fig. 4, although the trend in the change was analogous to the case without hydrogen in the gas phase (fig. 2). This shows that H(a) may easily be substituted by C,H, in the absence of gas-phase hydrogen, whereas hydrogen in the gas phase depresses the dissociation of ethylene on the surface.ACETYLENE ADSORPTION Fig. 5 shows the decrease in 4 and log A resulting from acetylene adsorption at 295 K, where the pressure of acetylene was kept at 5 x Torr. The decrease in 4 was rapid, after 2 min becoming nearly constant at 3.77 eV (A4 = - 1.05 eV). On the other hand, the decrease in logA was smaller than that for hydrogen or ethylene adsorption. Ultraviolet photoelectron spectroscopic (u . P.s.) measurements on the Pd( 1 1 1) surface showed that adsorbed acetylene exists in a non-dissociative form on the surface and reduces the work function by 1.4 eV at 180 K.13 Taking into consideration that the formation of hydrogen adatoms on the tip surface always resulted in an increase of 4 (as seen in the case of ethylene or acetylene adsorption on Ni or Pd), it is suggested that acetylene on the Pd emitter is mainly in a non-dissociative state at 295 K.As shown in the pattern of the acetylene-covered t ime/min FIG. 5.-Variation of work function and log A with time for adsorption of acetylene on Pd surface. Open and filled symbols correspond, respectively, to clean surface and carbon-deposited surface heated after adsorption of acetylene.I. KOJIMA, E. MIYAZAKI A N D I. YASUMORI 1429 surface [plate 4(a)], the stepped regions involving the (1 1 1) terrace, such as the (533), (21 1) and (221) faces, became brighter than those in the pattern for the clean surface. In contrast, the darkness of the area about the (1 10) face became noticeable.The region about the (100) face showed no significant difference. Demuth13 concluded from a U.P.S. study on the Pd( 1 1 1) surface that acetylene forms a di-a bonded complex above 200 K. On the other hand, as in the case of Pd(100) surface, Fischer and Kelernenl4 proposed an acetylenic n-bonding form for the adsorption of acetylene. This difference is adsorbed states could be responsible for the anisotropic changes in emission intensity observed for acetylene adsorption. On heating the acetylene-covered Pd tip to 670 K, 4 increased with increasing temperature (fig. 6). This &variation corresponds to the change at 370-570 K of the ethylene-covered surface, suggesting the decomposition of adsorbed C,H,(a).Above 700 K, 4 became constant and its value was almost the same as that of the clean surface. After heating to 620 K, the emission pattern resembled that of the clean surface, but became slightly granular. Also the characteristic features of pattern around the (1 11) and (110) faces disappeared [plate 4(b)]. At 750 K, dark regions across the (100)-(311) faces developed and the pattern was characteristic of the carbon-deposited surface as observed after the decomposition of ethylene [plate 4(c)]. However, in this case some bright spots were observed around the (321) faces. When the tip was heated in an acetylene atmosphere at 370 K and evacuated at increasing temperatures up to 750 K, many bright spots appeared on the image [plate 4(d)], and the value of A4 was -0.5 eV.Further heating to 850 K resulted in an increase in work function. These bright-spot images may be attributed to graphite crystallites formed by the aggregation of surface carbon left after dehydrogenation. Such dispersed carbon crystallites were also observed on the Ni emitter on heating the acetylene-covered surface to 630-770 K.4 In a thermal desorption study of Pd black catalyst it was found that carbon species to an extent of 4.4 x l O I 4 atom cm-, were left on the surface after dehydrogenation of adsorbed a~etylene.~ The amount of graphite crystallites formed on the Pd emitter was much less than that on Ni, suggesting that part of the carbon species diffused into surface Pd layers below 750 K, particularly on high-index faces such as (210) and (320).The properties of adsorbed ethylene and acetylene and of their surface decomposi tion, 1 I 1 I I 300 400 500 600 700 800 temperature/ K FIG. 6.-Variation of work function and log A during decomposition of acetylene on Pd surface.1430 ADSORPTION OF GASES ON Pd as described above, may be correlated with their hydrogenation activities. The low activity of Pd for carbon deposition may be connected with its stable hydrogenation activity, which has been attributed to the ‘surface template’ formed by strongly adsorbed a~etylene.~ Among the metals Ni, Pd and Pt, such a template effect has so far only been observed on palladium surfaces. Further comparative discussions involving FEM results for the Pt emitter will be described elsewhere. G. C. Bond, Catalysis by Metals (Academic Press, London and New York, 1962). G. A. Somorjai, Adv. Catal., 1977, 26, 2. L. Whalley, B. J. Davis and R. L. Moss, Trans. Faraday Soc., 1970, 66, 3143. L. Whalley, B. J. Davis and R. L. Moss, Trans. Faraday Soc., 1971, 68, 2445. A. J. Van Strien and B. E. Nieuwenhyus, Surf. Sci., 1979, 80, 226. Y. Inoue, I. Kojima, S. Moriki and I. Yasumori, Proc. 6th Int. Congr. Catalysis (The Chemical Society, London, 1977), vol. 7, p. 139. E. W. Muller, Adv. Electron. Electron Phys., 1960, 13, 83. H. Conrad, G. Ertl and E. E. Latta, Surf. Sci., 1974, 41, 435. ’ I. Kojima, E. Miyazaki and I. Yasumori, Appl. Surf. Sci., 1980, 6, 93. lo R. Lewis and R. Gomer, Surf. Sci., 1969, 17, 333. l 1 R. Gomer, R. Wortman and R. Lundy, J. Chem. Phys., 1957, 26, 1147. l2 R. Gomer, Field Emission and Field Ionization (Oxford University Press, London, 1961). l 3 J. E. Demuth, Chem. Phys. Lett., 1977, 45, 12. l4 T. E. Fischer and S. R. Kelemen, Surf. Sci., 1978, 74, 47. (PAPER 1 /595)

ISSN:0300-9599    

DOI:10.1039/F19827801423

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 1431-1449 Washburn Numbers Part 4.-The Erdey-Gruz Experiment. Relative Solvent Transport Numbers for Ion Constituents in Mixtures of Water with Raffinose, Glycine, Ally1 Alcohol, Dimethylsulphoxide and Dioxan BY DAVID FEAKINS,* ROBERT D. O’NEILL,~ W. EARLE WAGHORNE AND ANTHONY J. I. WARD Department of Chemistry, University College, Belfield, Dublin 4, RepublIc of Ireland Received 14th April, 1981 Erdey-Gruz’s diffusion experiment has been used to obtain the sum of the transport numbers of water by ions, relative to a second solvent component, En& = (n+)&+(n-)&. The theory of the experiment is described in detail; the corrections for the effect of salt on solvent activity, lacking in Erdey-Gniz’s work, have been made. The experimental method involved a diaphragm cell and analysis of changes in mixed-solvent concentration by interferometry.The En& are combined with Washburn numbers, w&, some of which are new values obtained from cells with ion-selective electrodes. With w& = (n+)& t+ - (n-)& r-, the ionic n& are obtained unambiguously for systems containing low concentrations of glycine, ally1 alcohol, dimethylsulphoxide (DMSO) and dioxan. These data were also calculated for a 0.75% (w/w) raffinose+water mixture using transport and diffusion data from the literature. The ng values for the raffinose + water system, which probably approach closely the dynamic solvation number, N , of the ion in pure water, are Li+( 16), Na+( lo), K+(6), Cl-(4) and H+( 1).In the remaining systems these figures are reduced by the competition of the organic molecule for solvation of the ion. In the glycine + water system the ions transport glycine preferentially. Consider a binary mixture of water with a second component, which can be another liquid such as dioxan, or a solid such as raffinose. Suppose that an electrolyte is dissolved in the resulting mixed solvent. If an electric current is passed, an ion will move in the field direction and, because of preferential solvation, will normally move one component of the solvent with respect to the other. For highly aqueous mixtures we normally discuss the movement of water with respect to the non-aqueous component of the solvent.’ When 1 mole of ions of type i crosses a hypothetical plane perpendicular to the direction of motion it carries (ni)w moles of water relative to the second component.In this paper we show how (n& can be determined. The Washburn number, ww, of a binary electrolyte in a binary aqueous mixture2 is the number of moles of water transported to the cathode in an electrolysis relative to the second solvent component per Faradayl and is given by ww = (n+)w t, - ( K ) ~ t-. (1) The t are the ionic transport numbers. Washburn numbers are available for a number ofmixtures ofhigh water content;’? 39 they depend on the electrolyte and on the concentration and nature of the non-aqueous component. t Present address: University Laboratory of Physiology, Parks Road, Oxford OX1 3PT 1 Faraday’s constant, F, i.e. 9.648 456 x lo4 C mol-I.14311432 WASHBURN NUMBERS Notional separations of ww into (n+)w and (n-)w have been attempted on extra-experimental rea~oning.~? However, we showed recently5* that a method suggested by Erdey-Gruz et al.’ for the determination of ‘solvation numbers’ can be adapted to determine (n+)w and ( K ) ~ unambiguously. In this method the sum of these numbers is measured in a diffusion experiment, giving eqn (2): Solution of eqn (1) and (2) leads to (n+)w and (n-)w. In Erdey-Gruz’s diffusion experiments, a binary aqueous mixed solvent of a given composition was separated from a solution of the electrolyte made with a binary mixture of the same composition. This can be represented as The M are the concentrations of non-electrolyte (S) and electrolyte per kg of water.Diffusion occurred and the reference solution was then analysed for solvent composition and electrolyte concentration. The details of Erdey-Gruz’s experiments are confused; for example a number of electrolyte concentrations are incorrectly quoted. In our first report5 we combined our own measurements of ww with some of Erdey-Gruz’s diffusion results; however, when we began our own diffusion experiments we realised that he had left out an important correction. This omission does not detract from the significance of his work and, indeed, prompts the following discussion of the principle involved. (a) In the diffusion experiment the electrolyte moves from the solution into the mixed solvent, carrying En, moles of water per mole of electrolyte with respect to the second solvent component, by virtue of the microscopic forces between the ions and the solvent molecules. (b) At the same time the activities of the two solvent components change when the salt is added.An additional flow of solvent will then arise from the resulting gradient in chemical potential in the cell. There is thus a macroscopic coupling of the flows of electrolyte and solvent. This effect is of the same order of magnitude as the microscopic coupling; it cannot be eliminated by extrapolation to infinite dilution because proportionate changes in the two flows occur. Following Ortmannss we use eqn (3): Studies of the free energy of transfer of the electrolyte, AG,, from water to mixed solvents tell us how the chemical potential of the electrolyte (3) varies as the proportion of non-aqueous component ( 2 ) is increased with respect to water (1).In most cases of interest, the glycine + water system being an exception, AG,, and hence the right-hand side of eqn (3), is experimentally positive. From the left-hand side we see that the activity of the non-aqueous component then increases when the electrolyte is added. In these circumstances it is a flow of the non-aqueous solvent component that accompanies that of the electrolyte in the experiment described above. In most cases the effect is larger than the true coupled diffusion; and it led Erdey-Gruz to the erroneous conclusion that the ions were preferentially solvated by the non-aqueous component.D. FEAKINS, R. D. O'NEILL, w. E. WAGHORNE AND A. J. I. WARD 1433 The distinction between the two types of coupling was clearly recognised in the treatment of diffusion in the water + glycine + potassium chloride system by Woolf et a1.,9 and we rely on their treatment here.Recently M'Halla et have carefully analysed the irreversible thermodynamics involved and have obtained En, in a number of systems using a different kind of diffusion measurement. The experiments give (rz+)w to 1 ; this is more than adequate for a preliminary interpretation of the resultski the present state of knowledge; and to know the individual numbers at all is an advance. Here we assemble (n+), for a number of systems containing relatively low concentrations of non-aqueous component. Ally1 alcohol (prop-2-en- 1-01) was chosen for comparison with Erdey-Gruz's work; he chose it originally because it can be chemically analysed.For this and glycine+water systems we present the full thermodynamic, transport and diffusion data. Measurements of w& are already available for dimethylsulphoxide (DMSO) system^;^ details of e.m.f. measurements on dioxan + water systems will be given in later papers. Data from the literature have been used to give values for the key raffinose.+ water systems."$ The overall experimental strategy is summarised in fig. 1. WASHBURN e.m.f. method (Feakins 1961) transport cell solvent activity (W or S) v.p. etc. 'W c w .. t I ww - ERDEY - GRUZ Z:nW FIG. 1 .-Overall experimental strategy. 47 FAR 11434 WASHBURN NUMBERS PRINCIPLE OF THE DIFFUSION MEASUREMENTS It is helpful to use different concentration scales for different parts of the argument; the gain from the change of variable offsets any lack of consistency.At the beginning of an experiment the contents of the diffusion cell can be represented as in cell (I) above. The diffusion is referred to the solvent frame.9 At this stage we take as our solvent pure water and not the mixed solvent; and we define the molalities M, and M3 with respect to 1 kg of water, as distinct from m2 and m3 defined with respect to 1 kg of mixed solvent. The fluxes of non-aqueous component, J 2 , and electrolyte, J3, in this ' water' frame are given in terms of the phenomenological coefficients (L,j)l by eqn (4) and (5) or, writing Xi = (dpi/ax) and dropping the subscript 1 - J2 - L22x2+L23x3 J3 L32 x2 + L33 x3 ' It is convenient to work in terms of the Erdey-Gruz number of the non-aqueous component, Cn,, given by = L23/L33* (7) It is easily shown for the individual n and hence for Cn that Y Yl Cn, = -?En, where y denotes mole fraction in the mixed solvent in place of x used otherwise above.J2/J, is the total number of moles of non-aqueous component which accompany one mole of the electrolyte as it diffuses, and was incorrectly identified by Erdey-Gruz7 as Ens. As seen above, J2/J3 would in his experiments have contained a contribution from the macroscopic coupling, since (ap2/ax) was non-zero. There are two ways of proceeding. (a) In all but one of the experiments described here we used Erdey-Gruz's original condition and determined the flux of solvent arising from the gradient (ap2/ax) separately.(b) It is also possible to arrange that (ap2/ax) is zero, and we report one result obtained with this condition, which we are now exploring more fully in this laboratory. (a) Rearranging eqn ( 6 ) we have NowD. FEAKINS, R. D. O'NEILL, W. E. WAGHORNE AND A. J. I. WARD 1435 and L23 = L32, as required by the Onsager relations. Thus The term (X,/X3)CnS is normally very small compared with 1, thus The average ratio of the fluxes (J2/13) in cell (I) was measured, following Erdey-Gruz, by analysing side B at intervals, typically 1, 2 and 3 days. If (J2lJ3) is extrapolated to zero time its value should then correspond to infinite dilution of the electrolyte, whatever the initial value of M,; this procedure should also give the initial ratio of fluxes at t = 0, (J2/J3).Note that the second term of eqn (12) does not contain the gradients of chemical potential in the cell, although the first term does. The denominator of the first term is easily obtained as the flux of electrolyte. J,, is the flux of component 2 arising from the difference in its chemical potential between sides A and B due to the presence of the electrolyte. This difference can be calculated using eqn (3) and obtained in a two-component system by changing the concentration of the second solvent component to give the cell The flux of component 2 at zero time in this cell will be expected to be close to J,, in cell (I), but only as M, --+ 0, and therefore ( M ~ ) A -+ ( M , ) ~ , will the simulation be exact. In principle, therefore, extrapolations both to zero time and to zero M, are required to obtain a meaningful Ens, which would then be the value at M, = 0.In practice, our present experiments were not sensitive enough to detect systematic variations in the at finite times and electrolyte concentrations. The results presented are therefore averages. Within the presen! error limits they are indistinguishable from the values at infinite dilution and' have been so treated. Cnw calculated from the Cn, values of eqn (13) are thus treated as CnO, for comparison with wO,, the Washburn numbers at infinite dilution. (b) If (ap2/ax) = 0, then eqn (8) becomes J2/J3 = L23/L33 = Cn,. This condition can be arranged artificially at the beginning of the experiment by having diffusion take place from a three-component mixture into a two-component mixture, the concentration of component 2 having been changed in the three-component mixture to make (p2)* = (P,)~ in cell (I).The problem of simulating the flux of component 2 separately in two-component systems is eliminated and with it, probably to a good approximation, the necessity for extrapolation to M, = 0 (see above). 47-21436 W A S H B U R N NUMBERS A non-zero value of (ap,/ax) evolves during the experiment. Its effect was eliminated by extrapolation of the apparent Zns to t = 0. This should give Zng to good approximation, since the diffusion is taking place into a solution in which M, = 0. PRINCIPLES OF THE ANALYSIS We shall consider method (a) in detail. It should perhaps be remarked that a complete analysis of the measurements to give all four practical or phenomenological diffusion coefficients is neither, for the present purpose, necessary nor would the present experimental method have been chosen if it had been necessary.The measurements are made in an apparatus-fixed frame with which the volume-fixed frame coincides13 if changes in partial molal volume with concentration are ignored, as they will be here. Our method of analysis allows a rather direct conversion from the volume-fixed to the solvent-fixed frame. Consider cell (11). Before diffusion a volume V of the contents of side B contains, say, N , moles of water and N , moles of S , and, of course where the are the partial molal volumes. In calibrating the interferometer, reference solutions were made by weighing out small amounts of component 2, AN2 moles,* and making up a solution using the original mixed solvent to the volume V.Then k ( N , 6 + N , K)+ AN, V, = V. (15) Using this calibration we can determine the AN, value appropriate to any diffusion solution by interpolation. In the original mixed solvent, N , moles of component 2 are associated with N , of component 1 in volume V ; now N,+(AN,/k) are associated with N l . In the solvent frame, then, with But AN2 &/ is small compared with unity, and then (AN,), = AN,( 1 +q). The argument can be readily extended to the analysis of the three-component system, cell (I). Here let An3 be the number of moles of electrolyte that have diffused into a total volume V ; it is determined by conductiometric titration.The quantity An, corresponding to AN, can be determined as above, but now the calibration involves solutions of equal molarity in the electrolyte. Now and ( An,K An3E) +- (An,), = An, 1 +- V V +An,..). V * The AN2 of eqn (1 5) is defined on a 'mixed-solvent volume' scale and is not the more familiar (AN2)" measured on the volume-fixed frame for which = (AN,), 1 +--= ( :;)D . FEAKINS. R. D. O’NEILL, W . E. WAGHORNE A N D A. J. I. W A R D 1437 Thus (I2/J3) = (An2)1/ = An2/An3‘ The value of (AN2), obtained from cell (11) with the molar ratio adjusted on side A to give the same initial difference as in cell (I) does not accurately simulate the average flux of component 2 in cell (I). In cell (I) the chemical potential of component 2 changes with time not only because of its own movement from A to B but also because of the flux of electrolyte.A simple correction can be made to (AN2), assuming the linearity of Ap2 in M, (see below). If (AN:)1 is this corrected value, then in the solvent frame we have (’22/’3> = (AN:)l/(An3)l - AN,* (1 + AN,* E/ V ) - An3(1+An2V,/V+An,E/V)’ To a good approximation we can take (J22/J3) = AN,*/An, especially as t -+ 0. The apparent Zns is thus given by eqn (16) : with An2 - AN,* AM Zn, = AN,* = AN2 XL An3 A% where AM, is the initial, and Am3 the mean difference in electrolyte concentration between sides A and B. CALCULATIONS OF ACTIVITY Ortmanns8 has shown how the variation in activity of the solvent components of binary systems with electrolyte activity can be calculated using eqn (3).A simpler analysis than his, which was based on the molar scale, is possible if the ‘water’ scale is used for the concentrations of components 2 and 3. Transforming eqn (3) to this scale we have, at constant p , T, and n, (95) =(%) . aM3 Mt aM2 M3 The right-hand side of eqn (18) can be obtained from measurements on the kind of electrochemical cells with glass electrodes described later in the paper; also available are amalgam ce1ls.l The information is usually obtained as the free energy of transfer of the electrolyte, AGt, from water to a mixed solvent of a particular composition, i.e. M2, and i3 AG, Two approximations were made. (i) AG, was often not available at the high M, used, but where it was, it was negligibly different, for the present purpose, from AG;, the value in the standard state; thus AG: values were normally used.(ii) It was also sufficiently accurate to take a mean value of the gradient of AG: with M,. AG; must be transformed from the usual scales to the ‘water’ scale. With the right-hand side1438 WASHBURN NUMBERS of eqn (1 9) taken independent of M, we can write dAG, &2 = (&3 for the change in chemical potential of component 2 on addition of a salt to make a concentration M, in the binary mixture. In the experiment with the binary system, M, is changed on side A to give the difference Ap,. Now dp, = RTd In a, = RT(d In M, + d In y,). Activity data (for sources see table 7) show that if AM, = ( M ~ ) A - ( M ~ ) B is small the change in y2 is negligible, thus In a typical set of experiments using method (a) cell (I) was set up with a particular mixed-solvent system and electrolyte concentration [e.g.5% (w/w) DMSO; 1.0 mol dmP3 KCl] and diffusion allowed to proceed for ca. 24 h. To analyse the solutions the cell had to be dismantled, so that separate experiments lasting ca. 48 and ca. 72 h were also set up. Cell (11) was then studied over the same time intervals. For every experiment in cell (I), the corresponding AN: for use in eqn (1 6) was calculated from an experiment in cell (11) using eqn (1 7). AN,* was adjusted for any mismatches in the diffusion time or in Ap, between cells A and B assuming linearity in the time and in AM, over small ranges of these variables. Table 1 shows results of the experiments on cell (11) and table 2 the combination of these with the results from cell (I).For method (b) the value of M, on side A of the cell (I) is adjusted to make Ap, = 0. Now J,/J, = An,/An3 (22) (An,/An,) was plotted against t , and extrapolated to t , = 0. The results are also shown in table 2. In this experiment the matching of the p, was not perfect. The appropriate small corrections were made using the data of Erdey-Gruz et al.' Erdey-Gruz numbers, En&, are collected in table 2. TABLE RESULTS FOR CELL (11) AT 25 O C diffusion time S (M2)A (M2)B b l h ANJmol dm-3 dioxan 0.7299 1.0152 24 0.0285 46.5 0.0476 71 0.0575 DMSO 0.6736 0.7738 24 0.0 129 [5% (w/w>l 48 72 0.0246 0.03 12 glycine 0.3416 0.2099 24.5 1.45 (AN,) [2.5% (w/w)lD. FEAKINS, R. D. O'NEILL, w . E. WAGHORNE AND A.J . I. WARD 1439 TABLE 2.-RFSULTS FOR CELL (I) AT 25 OC 23 44 71 24 48 23.5 47.5 71.5 23.5 46.5 71.75 24 48 16 26.5 31.5 side A: 5% (w/w) dioxan+ 1 rnol dmP3 K T l - 0.0105 0.0252 - 0.0 147 0.1727 0.0125 0.036 1 - 0.0236 0.2794 0.0076 0.041 6 - 0.0340 0.3672 0.0133 0.0238 -0.0105 0.1800 0.0086 0.0 179 -0 0093 0.1443 0.0113 0.0127 -0.0014 0.1727 0.0147 0.02 15 - 0.0068 0.2384 0.01 50 0.0241 - 0.009 1 0.3632 0.0045 0.0065 - 0.0020 0.0934 0.0057 0.0098 - 0.0041 0.1730 0.008 1 0.01 20 - 0.0039 0.1921 - 0.0020 - 0.0080 0.0060 0.2858 - 0.00 1 4 -0.0122 0.0 108 0.41 36 side A : 5 % (w/w) DMSO+ 0.5 rnol dm-3 H+CI- 0.0004 - 0.0028 0.0032 0.1036 0.0004 - 0.0042 0.0046 0.1460 - 0.0003 - 0.0044 0.0041 0.1660 side A: 5% (w/w) dioxan+ 1.0 rnol dm-3 R b T - side A: 5% (w/w) dioxan+0.5 rnol dm-3 Rb+C1- side A: 5% (wfw) DMSO+ 1.0 mol dm-3 K+CI- side A: 5% (w/w) DMSOfO.5 mol dm-3 K W - side A: 5% (w/w) DMSO+ 1.0 mol dm-3 H+C1- - 0.085 - 0.084 - 0.093 - 0.058 - 0.064 - 0.008 - 0.029 - 0.025 - 0.02 1 - 0.024 - 0.020 0.021 0.026 0.03 1 0.032 0.024 side A: 2.5% (w/w) glycine + 1 .O mol dmP3 Na+CI- 23 0.0105 1.349 - 1.244 0.1439 - 8.6 side A: 8.558% (w/w) allyl alcohol+0.5 rnol dm-3 K+Cl- side B: 10.001 % (w/w) allyl alcohol 17.5 0.275 0.0574 4.8 24 0.394 0.0792 5.0 48.25 0.890 0.1399 6.4 AnJAn, was extrapolated to t , = 0 and corrected to correspond to the system 8.847% (w/w) ally1 alcohol in side A. This gave Xn& = 2.7.1440 WASHBURN NUMBERS TABLE 2.-(continued) 0.75% 10% ally1 raffinosea 5% dioxan alcohol 5% DMSO 2.5% glycine - K+Cl- 10 8.1 2.7 1.7 Na+CI- RbfC1- H+CI- - - - - - 8.6 - - - - 5.7 - - 2.2 - - - a Ref.(1 1). THERMODYNAMICS AND WASHBURN NUMBERS (a) Previous determinations1 of Washburn numbers for alkali-metal chlorides have involved two stages. The thermodynamics of transfer of the salts from water to the mixed solvents had normally already been determined using cells with amalgam electrodes, cell (111) (III) Here m, = m,; m, is now in mol (kg mixed solvent)-'. Washburn numbers were then found by combining these results with measurements on cell (IV): (IV) In cell (IV) m, and m, are normally chosen to make (a+), = (a+),; see below. (b) At the relatively low concentrations of organic component used-here, ion-selective electrodes are expected to give reliable results, and in the present work the following cells were used instead of cell (111): (V) (VI) AE= EvI-Ev (24) Ag-AgC1 I MCl(m,), S I M(Hg) I MCl(m,), W I AgC1-Ag.Ag-AgCl 1 MCl(m,), W I MCl(m,), S I AgC1-Ag. G(M) I MCl(rn,), W 1 AgCl-Ag G(M) I MCl(m,), S I AgCl-Ag. For a few experiments m, was made equal to m,, as in cell (111); the e.m.f. when corrected for the asymmetry potentials of the glass electrodes is the same as would be obtained from cell (111) and was used in the same way. (c) In most cases the same solutions were used in cells (V) and (VI) as in cell (IV). This is the method originally described by Feakins;14 it simplifies the calculation of the Wash burn numbers. The e.m.f. of cell (IV) is given by E = (2RT/F) t, d In a _+ + (RT/F) ww d In aw. (25) JSW Here a+ is the mean ion activity of the electrolyte, referred to the molal standard state in purewater, a, is the activity of water referred to a standard state of pure water, and t , is the cationic transport number.1, = i++flx) In eqn (25) where f+ is the mean of t , on either side of the boundary and x a variable specifyingD. FEAKINS, R. D. O'NEILL, w . E. WAGHORNE AND A. J. I. WARD 1441 the position of a point in the boundary. In the present experiments t, varied negligibly across the boundary and and (a,), were equal or nearly so; this allows the term inf(x) to be neglected and eqn (25) becomes rw rw E = (2RT/F)i+ J S dlna*+(RT/F)J S w,dlna,. But - Thus writing W AE[eqn(24)] = (2RT/fl{ S dlna*. x = (RT/F)]ww,dlnuw. S E=-t+AE+x Choice of z (a+)s also makes the first term of eqn (26) small and means that t, need not be known precisely.Table 3 gives E, x and AE values from cells (1V)-(VI). Values of xo, obtained by extrapolation of x to infinite dilution,' are given in table 4. The AE values were treated in the usual way to give AE; values; these are shown in table 5. The Washburn numbers in table 6 were calculated-using the Raoult's law activity coefficients in table 7. Table 8 gives cationic transport numbers at infinite dilution determined as explained in Part 1 .' Table 9 shows values of nO, obtained by combining wO, with En;. ELIMINATION OF ASYMMETRY POTENTIALS We assume that the e.m.f. of a cell such as (V) with a glass electrode can be represented at a time t, after the immersion of the electrode as = WE&(a)+fa(t,) - (2RT/F) In (a+)%.- (28) It is assumed that is characteristic of the particular electrode a and is analogous to the same quantity with a classical electrode. The functionfa(tl) represents the tendency, small with the electrodes used here, for the potential of the electrode to drift with time. Suppose that we use a second electrode p in cell (VI), then sEp = %;(D) +fp(tl) - (2RT/F) In (a&):. (29) The cells are set up simultaneously and at the time t, the electrodes are transferred between the two cells. After a further time t, we have "EB = + f p ( t J - (2RVF) In (a*)% (30) = 'E;(a>+fa(t,)-(2RT/F)ln (a+);. (31) wEa-sEa = WE&(a)-SE0,(a)+fa(t,)-fa(t,)-(2RT/F)ln [(a*>%/(a,)El (32) wEp-sEp = WE;CB)-SE;(P)+.fB(fz)-fg(t,)-(2RT/F) In [(4w4;1 (33) Now = AEa (say) and = AEp (say).Normally t , was equal to t,, and with wE&(a) - SE&(a) = - sEz(p) = AE; (say) we expect AEa = AEB; the agreement was normally to kO.1 mV or better.1442 WASHBURN NUMBERS TABLE 3.-AE/mV FROM CELLS (v) AND (VI), EQN (24); E/mV AND )"mV FROM CELL (Iv) (a) 20% ally1 alcohol m,/mol kg-l m,/mol kg-l AE Li+Cl- 1.000 01 0.998 31 27.18 0.801 47 0.802 86 27.00 0.400 26 0.400 56 27.94 0.200 27 0.200 31 27.96 0.149 81 0.149 94 28.41 0.050 00 0.050 12 29.06 m,/mol kg-' m,/mol kg-' E X 1.299 46 1.001 39 0.501 09 0.199 90 0.147 90 0.100 06 0.050 06 Li+Cl- 0.829 39 0.99 0.98 0.619 77 1.26 1.24 0.291 65 1.53 1.54 0.110 86 1.86 1.88 0.082 32 1.84 1.87 0.054 10 2.35 2.26 0.024 49 3.54 2.05 m,/mol kg-l m,/mol kg-l AE E X 1.000 07 0.800 70 0.389 97 0.200 41 0.150 07 0.100 02 0.050 00 0.020 03 1.000 06 0.799 10 0.401 04 0.199 95 0.149 86 0.100 03 0.049 97 0.999 82 0.399 89 0.200 06 0.149 91 0.099 81 0.049 95 0.020 00 0.512 22 0.399 76 0.191 96 0.093 56 0.069 93 0.056 29 0.023 05 0.009 59 0.496 06 0.393 16 0.193 94 0.094 20 0.070 52 0.047 03 0.023 49 0.607 59 0.235 63 0.115 72 0.086 11 0.056 97 0.028 33 0.011 27 Na+Cl- 1.21 0.79 1.90 0.26 0.45 0.34 0.44 2.37 K+C1- - 0.66 - 0.47 - 0.07 -0.55 -0.51 - 0.59 -0.57 NHlCl- -0.30 0.28 0.32 0.26 0.24 0.43 0.27 1.27 1.56 1.48 2.35 2.42 2.58 2.76 2.1 1 1.73 1.74 1.85 2.39 2.42 2.68 2.94 1.43 1.37 1.48 1.59 1.64 1.74 1.87 1.72 1.86 2.22 2.45 2.60 2.71 2.93 3.05 1.41 1.51 1.81 2.12 2.17 2.39 2.66 1.28 1.51 1.64 1.72 1.76 1.95 2.01D.FEAKINS, R, D. O'NEILL, w .E. WAGHORNE AND A. J . I. WARD 1443 TABLE 3.-(continued) (6) 5% glycine m,/mol kg-l rn,/mol kg-l AE E X 0.199 98 0.160 08 0.119 93 0.100 06 0.080 01 0.049 95 0.030 01 0.010 00 0.199 96 0.160 01 0.120 05 0.010 04 0.079 99 0.050 09 0.030 04 0.010 01 0.199 97 0.158 84 0.123 04 0.102 29 0.080 14 0.044 02 0.030 35 0.010 16 0.199 94 0.157 83 0.119 26 0.098 31 0.080 50 0.050 38 0.029 87 0.204 49 0.163 78 0.123 82 0.102 82 0.082 83 0.052 02 0.031 01 0.010 47 0.204 93 0.166 23 0.124 11 0.103 62 0.083 59 0.052 59 0.031 76 0.010 71 0.206 18 0.164 13 0.124 66 0.103 54 0.082 50 0.051 61 0.030 75 0.010 05 0.204 99 0.164 05 0.123 55 0.103 11 0.083 88 0.053 89 0.031 66 Li+Cl- - 1.02 - 0.63 -0.50 - 1.05 - 1.23 - 0.93 - 1.66 - 1.09 Na+Cl- 0.02 0.44 -0.14 -0.17 0.07 -0.11 - 0.09 -0.19 K+Cl- 0.65 0.55 - 0.69 - 0.84 - 0.39 5.46 - 1.79 - 3.74 0.74 1.05 0.57 0.96 0.52 1.32 0.39 Cs+Cl- 0.05 0.09 0.04 0.16 0.14 0.19 0.41 0.35 0.04 -0.1 1 0.10 0.1 1 0.06 0.1 1 0.14 0.17 -0.21 -0.13 0.40 0.53 0.28 - 2.62 1.02 1.95 -0.24 -0.41 - 0.22 -0.39 -0.15 - 0.53 -0.10 - 0.26 -0.11 -0.12 -0.17 - 0.25 -0.10 -0.12 - 0.0 1 0.05 0.06 0.05 0.05 0.09 0.06 0.10 0.10 0.1 1 0.14 0.06 0.12 0.09 0.03 0.15 0.13 0.13 0.11 0.06 0.09 0.11 0.13 0.09 TABLE 4.-vALUES OF Xo/mV AT 25 "c Xo/mV interval water to : Li+Cl- Na+Cl- K+CI- Rb+Cl- Cs+Cl- NHaCl- 20% ally1 alcohol 2.46 3.21 2.85 - 2.09 - - 0.09 - 5% glycine -0.01 0.08 0.11 10% dioxana 1.12 1.21 0.88 -0.04 -0.12 - a Ref.(3).1444 WASHBURN NUMBERS TABLE 5.-vALUES OF AEE/mV AT 25 OC AEi/mV interval water to : Li'Cl- Na+Cl- K+Cl- Cs+Cl- NHiCl- 20% allyl alcohol 32.1 39.8 37.9 - 29.8 5% glycine - 4.92 -4.80 - 4.45 -4.34 - TABLE VALUES OF w&/mol Faraday-' w",mol Faraday-l intervala water to : Li+Cl- Na+Cl- K+C1- RbW- Cs+W NH,+Cl- H+C1- 20% allyl alcohol 1.34 1.83 1.70 - - - 5% glycine 0.00 0.15 0.18 - - 10% dioxanb 2.01 2.17 1.58 -0.08 -0.21 - - 10% DMSO" 0.87 2.24 1.50 2.01 1.35 - 1.18 - 0.15 - 0.48 at 1.4% raffinosed 2.3 1.3 0.7 - 0.3 0.8 0.4 a For comparison with En& (table 2) the w& are assumed to be applicable to the mid-point of the interval, e.g.at 10% allyl alcohol; ref. (3); ref. (4); ref. (12). TABLE 7.-RAOULT'S LAW ACTIVITY COEFFICIENTS AT 25 OC species fw 10% dioxana 20% allyl alcohol6 10% DMSO" 5% glycined 1.0040 1.007 0.998 0.989 a Ref. (3); M. Ewert, Bull SOC.Chim. Belges, 1936, 45, 493; ref. (4); H. D. Ellerton, G. Reinfelds, D. E. Mulcahy and P. J. Dunlop, J. Phys. Chern., 1964, 68, 398. TABLE 8.-cATIONIC TRANSPORT NUMBERS, t y , AT INFINITE DILUTION AT 25 OC solution L i T - Na+Cl- K T l - C s T - NH,fCl- water 0.336 0.396 0.49 1 0.501 0.490 20% (w/w) allyl alcohol 0.342 0.409 0.498 - 0.498 5% (w/w) glycine 0.326 0.390 0.488 0.495 -D. FEAKINS, R. D. O'NEILL, w. E. WAGHORNE AND A. J. I. WARD 1445 We note, though, that AE = gAEa + AEa> = AEZ -(2RT/F) In [(a*):/(a*)g] + Cfa(t1) -fa(tJ +fg(tJ -fg(tl)]- (34) The last term of eqn (34) also vanishes iffa(t) =fb(t) even if t , # t,; the mean of AEa and AEB may therefore represent an improvement on either value alone. EXPERIMENTAL Measurements of Washburn number and ionic transport number were as before.' Units with four interconnecting half-cells were used, which, in various combinations, enabled the measurement of two Washburn and two ionic transport numbers.A similar unit was used for glass-electrode measurements. With silver-silver chloride electrodes in position in two of the half-cells, the e.m.f. of cell (IV) was measured; this ensured the equilibration of these electrodes. Each silver-silver chloride half-cell could then be connected with a glass electrode half-cell in water or mixed solvent as appropriate, enabling the e.m.f. values of cells (V) and (VI) to be determined. Two glass electrodes (Corning monovalent cation electrodes) were prepared according to the manufacturers' instructions, then left soaking in 0.00 1 mol dmP3 solutions of the appropriate alkali-metal chloride overnight.They were rinsed with the cell solutions and introduced into their half-cells. After 30 min, connecting taps between glass and silver-silver chloride half-cells were opened for long enough to record the readings of e.m.f. of cells (V) and (VI). The glass electrodes were then removed and interchanged between the cell compartments after appro- priate washings; the e.m.f.s were again measured after 30 min. They were treated as explained above to eliminate asymmetry and time-dependent potentials. Cells with glass electrodes were interfaced to an impedance-matching device (Knick, Berlin) interfaced in turn to the Hewlett-Packard 3450A multi-function meter used as a digital voltmeter .DIFFUSION MEASUREMENTS Most of the details are due to Mills and W00lf.l~ The diffusion cell was mounted in a water-bath at 25.000+0.005 O C , * itself kept in a water-bath at 24.4k0.1 O C . The four pumps used to circulate the water, and the stirrer-motor for the cell, were mounted on the laboratory wall. The thermostat baths rested on a granite slab supported on an oil-drum filled with water to reduce vibrations. The design of the cell was that of Stokeslj although slightly simplified. The cylindrical cell was divided into upper and lower compartments (50 cm3) by a 4 cm, no. 4, sintered glass disc. In operation, stirrers made of lengths of iron wire encased in glass just touched the diaphragm above and below and were rotated (1 revolution per s) by magnets fixed to the external stirrer motor. The cell-holder was designed to reproduce the position of the cell with respect to these magnets.Runs involving a solution of salt in mixed solvent initially separated from mixed solvent of the same composition illustrate the general procedure. The mixed solvent, being the less dense, was used to fill the upper compartment and the frit, and the electrolyte solution the lower compartmen t. The compartments were filled in such a way as to minimise thermal gradients in the cell at the start of the experiment; this included taking the solutions from a bath at 25.05 "C where they had stood for 1-2 days to remove excess dissolved air. After diffusion, the solution (diffusion solution) in the upper compartment was removed with a pipette and titrated conductometrically against a silver nitrate solution itself just standardised against the original electrolyte solution.This gave An,. To determine the change in solvent composition a solution was prepared having the same molar concentration of electrolyte as the diffusion solution, but with the original proportions * The Celsius temperature is the excess of the thermodynamic temperature over 273.15 K.1446 WASHBURN NUMBERS of water and non-aqueous component. The difference in refractive index between this and the diffusion solution was measured using an interferometer (VEB Carl Zeiss Jena model LI3). The interferometer had previously been calibrated with solutions of known composition. Between two solutions of different solvent composition but of the same molar concentration of electrolyte the refractive index difference is, over a small range of composition, both independent of electrolyte concentration and also linear in M,.Glycine (Merck) was recrystallised twice from conductivity water. Ally1 alcohol (prop- 2-en-1-01) (Hopkin and Williams Synthesis Grade) was dried (CaSO,) then fractionally distilled. Salts were Johnson Matthey ' Specpure' or Merck ' Suprapur' grades and hydrochloric acid B.D.H. 'CVS hydrochloric acic N'. Conductance water,16 dioxan" and DMS04 were as before. DISCUSSION The n& values for the various ion constituents are shown in table 9. The systems have been selected from the results available to correspond to the lowest concentration of non-electrolyte studied in each case.In principle, if several w$ are available with one common ion, only one Zn& is needed to obtain all the ionic n&. If a second Zn& is available, tests of consistency are possible; the alternative sets generated by using nO, for Rb+Cl- for 5% dioxan and for H+Cl- for 5% DMSO are shown. The differences between these and the sets based on EnO, for K+Cl- are equivalent to an average error of 0.35 on n$. This is also roughly what would be expected from the scatter of results in table 2. We shall assume a notional precision of 1. TABLE 9.-NUMBER OF MOLES OF WATER TRANSPORTED WITH RESPECT TO THE ORGANIC COM- PONENT AT INFINITE DILUTION, n&/moll mol;' nk/mol, mol;' raffinose dioxan ally1 alcohol DMSO gl ycinea ion 0.75% 5% 10% 5% 2.5% Li + Na+ K+ Rb+ cs+ NHt H+ c1- 15.5 11.0 9.7 9.2 5.6 5.7 2.2 2.0 - - - - - 1.4 4.2 2.4 ~ ~~~ ~ 3.5 1.2 - 7.3 4.1 4.6 - 5.0 2.7 2.4 - 3.5 3.3 2.0 - 0.7 - - - - - - 2.1 - - - 0.3 - 0.7 - 3.6 The above are all based on Zn& for K+CI-. The absolute accuracy (notional) is & 1 ; the relative accuracy among the cations is much better.The following sets were found using Zn& for Rb'Cl- for 5% dioxan and Zn& for H+Cl- for 5% DMSO. n&/moll mol;' organic component Li+ Na+ K+ Rb+ Cs+ H+ CI- 2.9 - dioxan 5% 12.0 9.8 6.1 2.8 2.5 DMSO 5% 0.0 3.7 1.9 2.7 1.4 -0.9 -1.3 a Data from ref. (9) give En& = 5 +_ 1 for K+Cl-. This would raise the above values by ca. 1 unit.D. FEAKINS, R . D. O’NEILL, w . E. WAGHORNE AND A. J. I. WARD 1447 In 1959 Miller’l determined the phenomenological coefficients in the ternary system raffinose+water+K+Cl- and gave sufficient information to enable Zn& to be calculated at a raffinose concentration of 0.75 % (w/w).Previously Longsworthla had determined w& at a mean raffinose concentration of 1.4% (w/w). Any variation in w& is normally small over such a range and the values were taken to apply also at 0.75% and the individual n& calculated. The results will be discussed in more detail in later publications, but some preliminary observations can be made. When only the Washburn numbers were available it was difficult to compare the effects of the various cosolvents on the solvation of the ions, but with the individual n& much more progress can be made. We recall3 that (35) independently of any model of the flow process or solvation.A( = v,/v,) is the molar ratio of solvent components in the bulk mixture and I( = Nw/Ns) the corresponding ratio in the transported solvent; N = Nw + N,, and is the average number of solvent molecules of both kinds carried by the ion. The ratio 1/A is a measure of the extent of preferential solvation and If preferential solvation is by water, l/A > 1 and 0 < n& < N ; in the limit of strong preferential solvation by water, n& = N . If preferential solvation is by the non-aqueous component, l/A < 1, n& < 0; in the limit of strong preferential solvation by the non-aqueous component, ng = N . The value n& = 0 means that 1 = A, i.e. that the ion carries the mixed solvent exactly in the proportions present in the bulk.In the present highly aqueous media the ions will move much more water than com- ponent 2 even if 1/A -c 1. Consider now the behaviour of systems at high concentrations (A) of water. As 1/n+o ng = M l -(A/,)] (37) In an admittedly oversimplified mass-action treatment3 All is independent of A but is related to an equilibrium constant which depends on the relative strengths of interaction of the two solvent species with the ion of interest. Thus n& would not be expected to approach a common value for a given ion, independent of the non- electrolyte, as 1/A -+ 0, but rather to be characteristic of each solvent system. The following model was developed in more detail in earlier parts. For simplicity we shall discuss only cation solvation.Each organic molecule considered here has a suitable lone pair of electrons which can interact with a cation in an ‘acid-base’ type of interaction and thus compete with water for the primary solvation of the cation. Consider first the ground state of the ion or the equilibrium distribution of solvent molecules around the ion. Evidence from gas-phase studies or from the thermodynamics of ion solvation suggests that all the species here are expected to be more ‘basic’ than water in ionic s o l ~ a t i o n . ~ ~ ~ This does not necessarily mean that preferential solvation is by the organic component. The difference in strength between the ion-molecule interactions might be reduced by averaging of the basicity over both components of the mixture by a co-operative mechanismlg and the generally smaller size of water favours its incorporation in the primary solvation shell.Even where N1448 WASHBURN NUMBERS (see later) is large enough to correspond to the movement of more than one shell, the first is likely to be dominant in determining l / A . When the ion moves under the influence of a gradient in electrical or chemical potential another effect becomes important. Here it is helpful to envisage the equilibrium distribution for a given ion as being an average over a number of distributions of different energies for all the ions of that type. The co-operative mechanism referred to above may have the effect of making quite different distributions rather close in energy. Some of these distributions may lead to lower activation energies for flow than others and ions involved in these will carry the current preferentially. In particular, ion-water complexes, being the smaller, may move more readily than complexes incorporating organic molecules.The last effect may be particularly important in the key raffinose+water system. Earlier it was suggested that even though cations may well interact strongly with raffinose at equilibrium, they will be unlikely to move it on flow. Thus the transition state involves marked preferential solvation by water. Then n& approaches N closely [eqn (35)]; and if so low a concentration of raffinose has little effect on the structure of water, then the N are also the numbers of molecules of water transported by the ions in pure water, that is, the dynamic solvation numbers. TABLE IO.-VALUES OF 112 ASSUMING N = nO, FOR RAFFINOSE+ WATER ion dioxan 5% allyl alcohol 10% DMSO 5% gl ycine 2.5% Li+ 3.4 Na+ K+ CI- 2.3 - - 1.3 1.7 1.9 0.9 1.1 1.9 1.6 0.9 0.69 0.66 0.62 0.54 N cannot be less than the highest value of n& found for any system as 1/J.-+ 0. For the moment, then, the best available Nfor pure water are then; for the raffinose +water system. We note that an earlier assumption3 of N(Li+) = 7 was wrong, and in general the ions move many more solvent molecules than hitherto assumed. Thus N(C1-) = 4, and N(K+) = 6; values close to zero had been suggested earlier. The earlier arguments were speculative and can now be ignored. Turning now from raffinose to the other non-aqueous components, we see that in the series dioxan, allyl alcohol, DMSO and glycine, there is an increasing tendency for the non-aqueous component to be transported.In the case of dioxan, there is a significant drop in n& only for Li+. The incorporation of allyl alcohol is, however, significant for all three cations studied, perhaps because of its less demanding steric requirements, and DMSO emerges as a strong ligand, particularly for Li+ and H+, the latter actually transporting DMSO in preference to water. This result, which is in keeping with n.m.r. evidence, had been reached earlier4 though on weaker grounds. All ions transport glycine in preference to water. Glycine is a zwitterion and the fully developed charges at each end of the molecule are likely to interact strongly with the ions. In all cases, the degree of preferential solvation by either component is small. ThisD. FEAKINS, R. D. O’NEILL, w. E. WAGHORNE AND A. J. I. WARD 1449 may be seen by calculating the ratios l / A with the assumption that N = nO, for the raffinose+water system. The results, in table 10, show ratios that differ little from unity. We thank Prof. M. Chemla, Dr J. M’Halla and Dr T. H. Lilley for discussions. Dr Elvira Kugler kindly provided details of some of the early experiments in Budapest. We thank the Irish Department of Education and University College, Dublin, for scholarships (to R. D. OW.) D. Feakins and J. P. Lorimer, J. Chem. SOC., Faraday Trans. I , 1974, 70, 1888. J. N. Agar, in The Structure of Electrolytic Solutions, ed. W. J. Hamer (Wiley, New York, 1959), p. 218. D. Feakins, K. H. Khoo, J. P. Lorimer, D. A. O’Shaughnessy and P. J. Voice, J . Chem. Soc., Faraday Trans. I , 1976, 72, 2661. D. Feakins and D. A. O’Shaughnessy, J. Chem. SOC., Faraday Trans. I , 1978, 74, 380. D. Feakins, E. de Valera, P. J. McCarthy, R. D. O’Neill aqd W. E. Waghorne, J. Chem. SOC., Chem. Commun., 1978, 218. [Note that the numbers given in this paper are incorrect and are corrected in ref. (6) and in the present paper.] D. Feakins, R. D. O’Neill, W. E. Waghorne and A. J. 1. Ward, J. Chem. SOC., Chem. Commun., 1979, 1029. ’ T. Erdey-Gruz, A. Hunyar, E. Pogany and A. Vali, Acta Chim. Acad. Sci. Hung., 1948, 1, 7; T. Erdey-Gruz, Transport Phenomena in Aqueous Solutions (Hilger, London, 1974), p. 486 and references therein. G. Ortmanns, Ber. Bunsenges Phys. Chem., 1965, 69, 2336. L. A. Woolf, D. G. Miller and L. J. Gosting, J. Am. Chem. Soc., 1962, 84, 317. lo J. M’Halla, P. Turq and M. Chemla, J. Chem. SOC., Faraday Trans. 1, 1981, 77, 465. l 1 D. G. Miller, J. Phys. Chem., 1959, 63, 570. l2 L. G. Longsworth, J. Am. Chem. SOC., 1947, 69, 1288; [see also ref. (3)J. l 3 R. Mills and L. A. Woolf, The Diaphragm Cell (Australian National University, Canberra, 1968). l 5 R. H. Stokes, J. Am. Chem. SOC., 1950, 72, 763. D. Feakins, J . Chem. SOC., 1961, 5308. D. Feakins and P. J. Voice, J. Chem. SOC., Faraday Trans. I , 1973, 69, 171 1. H. P. Bennetto, D. Feakins and K. G . Lawrence, J. Chem. SOC. A, 1968, 1493. L. G. Longsworth, J. Am. Chem. SOC., 1947, 69, 1288. l9 D. Feakins and P. Watson, J. Chem. SOC., 1963, 4734. (PAPER 1/61 1)

ISSN:0300-9599    

DOI:10.1039/F19827801431

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1451-1455 High-resolution Auger Electron Spectra of Adsorbed NO, NH, and N, on Sulphur-segregated and Oxidized Vanadium Surfaces BY KAZUNARI DOMEN, SHUICHI NAITO, MITSUYUKI SOMA,? TAKAHARU ONISHI* AND KENZI TAMARU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 11 3, Japan Received 22nd April, 198 1 The absorption of NO, NH, and N, on oxidized and sulphur-segregated surfaces of vanadium polycrystalline foil has been studied using high-resolution Auger electron spectroscopy (HRAES). On the oxidized surface, four different types of adsorbed nitrogen species were detected, i e . NO (a), NH, (a), N, (a) and N (a). On the other hand, on the sulphur-segregated surface, NO is adsorbed easily at room temperature, but NH, and N, were not adsorbed.When electron emission from a solid surface via an Auger process involves valence electrons of a surface element, high-resolution Auger electron spectroscopy (HRAES) gives useful information not only concerning species and abundances of elements (as in the case of conventional AES), but also concerning the valence-band structure, i.e. the chemical state of a particular element on the solid surface. So far chemisorbed species on polycrystalline Mo, W, Pd and Fe have been successfully studied by HRAES.f-5 In this report the adsorption of NO, NH, and N, was investigated on oxidized and sulphur-segregated surfaces of vanadium, which is of particular interest because of the possibility of studying chemical effects in the metal HRAES, as has been demonstrated by ESCA and AES meas~rements.~-~ EXPERIMENTAL The details of the apparatus employed in this study have been reported previously.1° The minimum pressure attainable in the vacuum chamber was 7 x lo-’ Pa, and the resolution of the analyser was ca.400 meV with an electron beam current of 1 x lop6 A and a primary energy of 1.5 keV. Vanadium foil (15 x 5 x 0.01 mm) of 99.8% purity was electrolytically etched in a solution of methanol and sulphuric acid, and then flashed in u.h.v. By this treatment the vanadium surface was covered with sulphur, which segregated from the bulk as shown in fig. 1 (a). Vanadium LMM and LMV spectra are in good agreement with the X-ray excited spectra of the clean evaporated film reported by Brundle et which indicates that the metallic state of the vanadium remains the same even though much sulphur may be segregated on the surface.The peak at ca. 508 eV was assigned to the vanadium LVV transition, according to Fiermans et a1.8 Fig. 1 (b) shows the vanadium surface oxidized by 0, (6.7 x lop4 Pa, 1 min) at room temperature. It is accordingly suggested that the surface sulphur may be removed by oxygen and the vanadium peaks are broadened by oxidation. The vanadium L VV peak is obscured by the oxygen KLL peak. There should be three peaks in the oxygen KLL Auger ~pectrum.~ The peak at lowest kinetic energy, which is assigned to the KL,L, transition, seems to be hidden by the vanadium LMV transition. When the 1- Present address: National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan.14511452 NO, NH, AND N, ADSORBED ON v temperature of the foil was raised to 523 K under an oxygen atmosphere, the sulphur disappeared completely from the surface and the shoulder to the lower-energy side of the vanadium LM,,, V peak grew considerably, indicating the oxidation of vanadium metal under these conditions. When the temperature of the foil was raised to 673 K in u.h.v., the amount of surface oxygen decreased and that of sulphur increased again as shown in fig. 1 (c). I - I l l l l r C I I 1 120 170 380 5 50 FIG. I.-S(LMM), V(LMM, LMV, LVV) and O(KLL) Auger spectra of vanadium surfaces after various pretreatments: (a) flashed in u.h.v.; (b) oxidized by 0, (6.7 x Pa, 1 min) at room temperature; (c) the temperature of the foil was raised to 673 K in u.h.v., after the treatment of (6).kinetic energy/eV RESULTS AND DISCUSSIONS (I) ADSORPTION OF NO ON SULPHUR-SEGREGATED AND OXIDIZED VANADIUM SURFACES As is shown in fig. 2, upon introduction of NO onto the sulphur-segregated vanadium surface, the nitrogen KLL line emerged and sulphur LMM peak decreased. The shape of the nitrogen KLL spectrum is similar to that for the nitrogen molecule adsorbed on Fe or Mo, which is assigned to dissociatively adsorbed n i t r ~ g e n . ~ - ~ Accordingly the overall reaction of NO with the surface is considered to be the removal of sulphur by oxidation accompanied by nitrogen chemisorption. I t is notable that the sulphur-segregated vanadium surface can activate NO, in contrast to the palladium surface, where the segregated sulphur strongly poisons the dissociative adsorption of l2 Since oxygen is used for the oxidative removal of sulphur, the increase in the oxygen KLL peak is relatively small whereas the vanadium LMV peak broadens, as is the case for oxidation by O,, which is attributable to the formation of a strong V-N bond.The dissociatively adsorbed nitrogen is stable at 873 K and disappeared by flashing in uacuo. On the other hand, when NO was introduced at room temperature onto a surface oxidized by 6.7 x Pa 0, at 673 K for 2 min, no nitrogen peak was detected.DOMEN, NAITO, SOMA, ONISHI AND TAMARU 1453 However, when NO was introduced at room temperature onto a surface oxidized by 6.7 x Pa 0, at room temperature for 30 s, two peaks of the nitrogen KLL transition were observed, as shown in fig.2. By raising the temperature to 673 K, the peak to lower kinetic energy decreased and the resultant lineshape [fig. 2(d)] was the same as that for the sulphur-segregated surface [fig. 2(b)]. These results demonstrate that at a certain oxidation state the vanadium surface can accomodate both undissociated and dissociated NO. 120 170 \ 390 \ 4 90 5 40 340 440 kinetic energy/eV FIG. 2.--S(LMM), N(KLL), V(LMV, LVV) and O(KLL) Auger spectra: (a) flashed in u.h.v.; (b) NO (6.7 x Pa, 2 min) adsorbed on the sulphur-segregated surface (a) at room temperature; (c) NO (6.7 x Pa, 30 s) adsorbed on the oxidized surface (by 0, = 6.7 x lo-* Pa, 1 min, at room temperature) at room temperature; (d) temperature of the foil raised to 673 K in u.h.v., after the treatment of (c).(11) ADSORPTION OF NH, AND N, ON SULPHUR SEGREGATED AND OXIDIZED SURFACES When NH, was introduced onto the sulphur-segregated surface, only a small peak for the nitrogen KLL transition was detected initially, but this increased considerably with prolonged electron bombardment ( E = 1.5 keV). The nitrogen spectrum was similar to that of fig 2(b), suggesting the dissociative adsorption of NH, to form N. When NH, was introduced onto the oxidized surface at room temperature, a different type of nitrogen peak was detected [fig. 3(a) and (b)]. This is assignable to partially dehydrogenated NH, species, where x is 1 or 2, according to the literat~re.~ A similar spectrum was observed for the adsorption of NH, on Pd.5 When the temperature was1454 NO, NH, AND N, ADSORBED ON v raised to 873 K [fig.3(c)], the lineshape changed to the type shown in fig. 2(b); this change is probably due to the dehydrogenation of NH, to form chemisorbed nitrogen. When N, was introduced on the sulphur-segregated surface at room temperature, no nitrogen peak was detected and there was little effect produced by electron bombardment. On the other hand, by introducing a mixture of N, and 0, on the sulphur-segregated surface at room temperature, a nitrogen peak was observed [fig. 3 (d)] which is apparently similar to that of NH, adsorbed on the oxidized surface [fig. 3(b)]. However, this nitrogen KLL line disappeared completely at 523 K. This peak t 1 I I I I 3 40 3 90 kinetic energy/eV FIG.3.-N(KLL) Auger spectra: (a) NH, (6.7 x Pa, 30 s) was adsorbed on oxidized surface (by 0, = 6.7 x Pa, 1 min, at room temperature) at room temperature without electron bombardment. (b) NH, (6.7 x Pa) was adsorbed on the same surface of (a) with electron bombardment ( E = 1.5 keV) at room temperature. (c) The temperature of the foil was raised to 873 K in u.h.v., after (b). ( d ) N, + 0, Pa, 2 min) were adsorbed on sulphur-segregated surface at room temperature. (both 6.7 x is similar to that of N, adsorbed on Fe at 373 K or Mo at 83 K, and the shoulder to higher kinetic energy is more clear than that of N, on Fe and/or Mo. Consequently this species is considered to be non-dissociatively adsorbed nitrogen. N, adsorption on the sulphur-segregated surface from a mixture of N, and 0, is considered to occur on the exposed vanadium atom where sulphur has been removed by oxygen.Otherwise the sulphur atom prevents the adsorption of the nitrogen molecule. On the other hand, the adsorption of NO or NH, assisted by electron bombardment is not inhibited by sulphur on the vanadium surface. The sulphur present on the vanadium is very reactive for oxidation. Vanadium oxide is known to be a good catalyst for the oxidation of SO, to SO,. In this connection the reactivity of sulphur segregated on the vanadium surface toward oxidation isDOMEN, NAITO, SOMA, ONISHI A N D TAMARU 1455 considerable. On the oxidized surface three different nitrogen species are detected at room temperature, i.e. non-dissociatively adsorbed NO, NH, and weakly bonded N,.It is difficult to determine on which oxidation state of vanadium these species are adsorbed. Perhaps the surface is a mixture of various oxidation states, as shown in fig. 1 and pointed out by Brundle et aZ.9 It is of interest that adsorption of the test molecules on the oxidized surface at room temperature tends to be rather associative in comparison with that on the sulphur-segregated surface. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Surf. Sci., 1974, 46, 567. T. Kawai, K. Kunimori, T. Kondow, T. Onishi and K. Tamaru, Phys. Rev. Lett., 1974, 33, 533. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Chem. Lett., 1975, 12, 1303. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Surf Sci., 1976, 54, 525. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Sut$ Sci., 1976, 54, 302. F. J. Szalkowski and G. A. Somorjai, J. Chem. Phys., 1972, 56, 6097. L. Fiermans and J. Vennik, Surf. Sci., 1971, 24, 541. L. Fiermans and J. Vennik, Surf. Sci., 1973, 35, 42. C. R. Brundle, Surf. Sci., 1975, 52, 426. lo T. Kondow, T. Kawai, K. Kunimori, T. Onishi and K. Tamaru, J. Phys. B, 1973, 6, L156. I * Y. Matsumoto, T. Onishi and K. Tamaru, J. Chem. Soc., Faraday Trans. I , 1980, 76, 1116. Y. Matsumoto, M. Soma, T. Onishi and K. Tamaru, J . Chem. SOC., Faraday Trans. I , 1980,76, 1122. (PAPER 1 /642)

ISSN:0300-9599    

DOI:10.1039/F19827801451

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 1457-1463 Infrared Study of CO, Adsorption on ZnO Adsorption Sites BY JACQUESAUSSEY, JEAN-CLAUDE LAVALLEY* AND CLOTILDE BOVET Laboratoire de Spectrochimie, ERA 824, I.S.M.R.A., Universite de Caen, 14032 Caen Cedex, France Received 18th May, 1981 Adsorption of carbon dioxide on a ZnO Kadox-15 powder has been studied by Fourier transform infrared spectroscopy. The following surface species are formed : bidentate carbonates, polydentate carbonates which appear with time or heating, hydrogenocarbonates and linear CO, species. Moreover, a band at 1546 cm-l could correspond to carboxylates reversibly adsorbed at room temperature. Attention is paid to the effect of CO, addition which splits the w,(O=C=O) and d(C0,) bands due to linear species and shifts the bidentate carbonates band from I595 to 161 5 cm-'. Taking account ofthe band u,(O='~C=O) (in natural abundance), we deduce that the splitting is due to a coupling between two linear species held by the same Zn2+ ion.We propose that such Zn2+ ions that are two-fold coordinate are situated on the edges formed by the (0001) and (1010) planes. Infrared spectroscopy has been widely used to study CO, adsorption on various oxides. Knozingerl has reviewed the studies carried out on ZnO and reported that the species formed on this oxide are not so well-defined as those for, e.g., alumina, or a-chr~mia.~ It is the purpose of this paper to examine in detail the structure of the species in order to gain some information on the nature of active sites on ZnO. We have used a Fourier transform interferometer with spectral ratioing facilities and adsorbed CO, on 100 mg discs to study the 1100-600 cm-l range, which has not been investigated previously and which could provide interesting information on the structure of the species.EXPERIMENTAL The zinc oxide powder used was Kadox- 15 from the New Jersey Zinc Co. ; its surface area was 9 m2 gel. The samples were pretreated with oxygen at 723 K as described in ref. (4). The carbon dioxide used was obtained from Air Liquide with a stated purity of 99.998%. Infrared spectra were recorded on a Perkin-Elmer 580 or a Nicolet FT MX-1 at room temperature. RESULTS The spectra of chemisorbed CO, were recorded at increasing coverage. Table 1 lists the positions and the relative intensities of the bands.The bands formed at low coverage grow in intensity until ca. 20 pmol g-' of CO, are chemisorbed. Note the frequency shift with the coverage of the two main bands, initially at 1582 and 1335 cm-l. Addition of another dose (total amount: 36 pmol g-l) increases the intensity of the bands, except for those at 1000 and 848 cm-l, but they become broader. Moreover, new bands appear (table I). Addition of larger quantities (equilibrium pressure: 60 N m-,) shifts the band at 14571458 CO, ADSORPTION ON ZnO TABLE 1 .-WAVENUMBER AND RELATIVE INTENSITIES (ABSORBANCE) OF BANDS RESULTING FROM THE ADSORPTION OF SUCCESSIVE DOSES OF CO, ON ZnO equilibrium evacuation pressure (P = 2.6 x 10-4 18 pmol g-' 36 pmol g-l 60 N m-2 N m-,) - 1665 (0.04) 2358 (0.24) 2290 (vvw) 1665 (0.08) 1595 (0.58) 1546 (0.14) 1520 (sh) - 1339 (0.40) 1325 (sh) 1303 (sh) 1040 (0.02) 1000 (0.16) 848 (0.07) 841 (sh) 680 (0.01) - 1597 (1.10) 1546 (0.23) 1520 (sh) 1424 (0.03) 1370 (sh) 1340 (0.55) 1325 (sh) 1300 (sh) 1265 (sh) 1229 (0.01) 1040 (0.03) 1002 (0.18) 848 (0.09) 841 (sh) 835 (sh) 680 (0.02) 638 (0.04) -~ ~ 2369 (0.46) 2353 (0.59) 2290 (0.02) 1650 (sh, b) 1615 (1.53) 1580 (sh) 1547 (0.32) 1520 (sh) 1425 (0.06) 1369 (sh) 1346 (0.95) 1325 (sh?) 1300 (sh?) 1265 (sh) 1229 (0.03) 1039 (0.04) 999 (0.28) 852 (sh) 841 (0.18) - - - - - 1665 (sh) 1593 (0.58) 1580 (sh) 1520 (0.25) - 1338 (0.56) - - 1002 (0.15) 848 (w) - - 679 (w) I 2350 2000 1650 1300 950 600 wavenumberlcm-' FIG.1.-Infrared spectrum of adsorbed species formed by addition of CO, (equilibrium pressure: 60 N m-,) on ZnO (spectrometer: Nicolet MX-1).Note scale change at 2000 crn-'.J. SAUSSEY, J-C. LAVALLEY A N D C. BOVET 1459 1597 cm-l to 16 15 cm-l and splits the bands at 2358 and 638 cm-l (table 1). Moreover, shoulders near 1585 and 1520 cm-l appear (fig. 1). Note that the spectrum of CO, chemisorbed on ZnO pretreated with D, to exchange the hydroxyl groups is similar except that it does not exhibit the 1229 cm-l band. Room-temperature desorption allows the characterization of the bands due to irreversibly adsorbed species (table 1). Spectra relating to degassing at temperatures 1550 ' 1350 ' 1150 ' 950 ' 750 ' wavenum ber/cm-' FIG. 2.-Infrared spectrum of adsorbed species formed by heating at 723 K an activated disc of ZnO under oxygen ( P = lo3 N m-2) with a small amount of CO, in it ( P = lo2 N m-2) (spectrometer: Nicolet MX-1). higher than 360 K show only two main bands, at 1520 and 1335 cm-l.These were very strong in Taylor and Amberg's study5 when ZnO was outgassed for 18 h at 593 K then treated with oxygen at 623 K. We were able to obtain the same bands by heating a ZnO disc at 723 K in oxygen (lo3 N m-,) containing a small amount of CO, (lo2 N rn-,). In addition to the 1522 and 1327 cm-l bands (fig. 2), sharp bands at 1030 and 876 cm-l appear, which will allow us to identify the structure of the corresponding species as shown below. In one experiment, CO, was adsorbed on a ZnO sample pretreated with H,O (25 pmol g-l). Bands of medium intensity appear at 1635, 1513, 1468, 1415, 1390, 1341 and 1222 cm-l.In the final experiment, we studied the room-temperature evolution with time of the spectrum reported in fig. 1. We observe (i) an increase in the intensity of the bands at 1580, 1520 and 852 cm-l, (ii) the appearance of new bands at 1007 and 875 cm-l, and (iii) a decrease in the intensity of the bands at 1615,999, 841 and 1547 cm-l, the latter almost disappearing. DISCUSSION ASSIGNMENT CARBONATES, HYDROGENOCARBONATES A N D CARBOXYLATES Our assignments are based on the work of Little6 and Filimonov and coworkers.' In a previous experiment8 involving addition of water to ZnO treated with CO,, two of us showed that the bands at 1590, 1525 and 1335 were not assignable to a single1460 CO, ADSORPTION ON ZnO species.* From the present study, we can deduce that the bands at 1595, 1000 and 848 cm-l are connected, as they shift or broaden when a large amount of CO, is introduced.We assign them to bidentate carbonates. Desorption experiments at higher temperatures show that the 1 339 cm-l band is also connected with the other three bands. A part of this species is irreversibly adsorbed at room temperature. When large amounts of CO, are added, the 1595 cm-l band shifts to 161 5 cm-l (table I), showing a slight change in its structure. These bands tend to disappear gradually with time giving rise to another type of bidentate carbonate (bands at 1580, 1007 and 852 cm-l). The band at 1665 cm-' can be ascribed to a different bidentate carbonate with another band near 1300 cm--l (table 2). TABLE 2.-TENTATIVE ASSIGNMENT OF SURFACE SPECIES POlY- dentate hydro- linear CO, bidentate carbonates carbon- geno- species ates carboxyl- carbon- (1) (2) (3) (4) ( 5 ) ates ates (6) (7) 1595 1615 1580 1665 1522 1546? 3605 2358 (2353 2369 1339 1346 1348 1303 1327 1635 1370 1370 64 1 1030 1424 638 (636 1000 999 1007 848 841 852 876 1229 680 677 720 835 660 (1) Formed by adsorption of 18 pmol g-l of CO,; (2) formed by the transformation of species (1) and carboxylates when larger amounts of CO, are introduced; (3) appearing after a longer contact time (2 h); (4) formed by adsorption of 18 pmol g-l of CO,; the intensity of these bands does not grow noticeably with the amount of CO, added; (5) appearing after heating; (6) formed when 36 pmol g-l of CO, were added; (7) formed when the equilibrium pressure of CO, is 60 N mP2.Bands (table 2) associated with species formed at high temperature may be assigned to polydentate carbonate^.'^ They tend also to appear with time at room temperature and were assigned to carboxylates in a previous study.g It is difficult to determine the origin of the 1546 cm-l band, which characterizes species reversibly adsorbed at room temperature and which tends to disappear with time. As we were not able to correlate it with any band in the 1100-900 cm-l range, it could be assigned to carboxylates. Hydrogenocarbonates (bands at 1424 and 1229 cm-l) are formed when more than 20 pmol g-l of CO, are added. They are reversibly adsorbed at room temperature, as Borello has suggested.lo Adsorption of CO, on a sample of ZnO pretreated with H,O shows that a band near 1635 cm-l is connected with these.It is difficult to observe the corresponding v(0H) mode. However, from fig. 3 we deduce that it absorbs at 3600 cm-l and, as in the case of alumina,, that the formation of hydrogenocarbonates proceeds through an attack of the OH groups which have the highest wavenumber band (3670 cm-l). These assignments are summarized in table 2. * The spectrum and the wavenumbers were different from those given here due to the temperature effect which favoured the appearance of the more stable species (beam temperature of the Perkin-Elmer 225 spectrometer, ca. 340 K).J . SAUSSEY, J-C. LAVALLEY A N D C. BOVET 1461 wrlvenum ber/cni- ' FIG. 3.-Effect of addition of CO, (equilibrium pressure: lo3 N m-*) on the hydroxyl groups of ZnO (spectrometer: Perkin-Elmer 580; dotted line: background).LINEAR SPECIES Bands close to the gaseous CO, frequencies can be assigned to CO, weakly adsorbed onto cationic sites. Their linear structure is preserved. Due to the interaction with the surface, the CO, symmetry is lowered, which explains the appearance of the v, mode in the infrared (band at 1370 cm-l, table 2). Note that the use of a Fourier transform spectrometer allows us to observe the d(C0,) mode. It is situated 29 cm-l below the wavenumber of the gas-phase band, showing an interaction with ZnO Lewis sites. This result confirms the presence of such sites on ZnO activated at 723 K and agrees with a recent study using CD,OCD,H as a probe molecule.ll Note that, when the bands at 2358 and 638 cm-l split due to the further addition of CO,, the band due to bidentate carbonates also shifts (1595 1615 cm-l).The splitting of v,(CO,) of linear species can be explained by looking at the v(O=13C=O) band, situated at 2290 cm-l. Whatever quantity of CO, introduced, it appears sharp and unique. Its intensity increases with the amount of CO, added, even when this amount is high enough to induce the splitting of the v,(O=~~C=O) band. So we deduce that the double bands at 2369 and 2353 cm-l and at 641 and 636 cm-1 are not due to different adsorption sites but to coupling between linear species, ADSORPTION SITES ON ZnO The sharpness of the bands reported in the present study is certainly due to the presence of well-defined adsorption sites. As in the case of H,-CO interaction on Zn0,12 we could distinguish two effects due to the addition of CO,: a discrete effect, which causes the bands due to linear species to split and the band at 1595 cm-1 to shift abruptly to 1615 cm-l, and a continuous effect responsible for the continuous shift of some bands (that at 1339 cm-l for instance) with coverage.In the discussion below, we will consider only the former, which could be associated with interactions in adjacent positions; the latter presumably involves interactions at more distant positions. l2 The morphology of zinc oxide, especially that prepared by the combustion of metallic zinc, is well known:13 the surface corresponds to the (OOOl), (OOOi), (101 1) and (1010) cleavage planes of the wurtzite crystal structure. Monodentate carbonates could be formed on the (OOOT) plane, and carboxylates and linear species on the (0001) plane.Bidentate carbonates show the presence of Zn2+02- ion-pair sites. The results1462 CO, ADSORPTION ON ZnO of the present study can determine whether they are situated on the (1010) plane or not. In a recent study, Runge and Gopel14 have compared the reactivity of polycrystalline and single-crystal ZnO surfaces using 0, and CO, adsorption. A surprising corre- spondence has been observed in results on the ZnO (1010) surface and chemically clean ZnO powder under u.h.v. conditions. Therefore, the authors concluded that the Oo- 0 I - 0 co2 0" linear carbonate 0" (1595 cm-' 1 0" 2369 cm-' 2353 cm-' 641 cm-' 638 cm-l bident ate carbonate (1615 cm-'1 FIG.4.-Proposed model for CO, adsorption. properties of 0, and CO, complexes on ZnO powder surfaces are determined by the properties of ZnO (lOi0) surfaces. On these surfaces, Gopel and coworkersL5 suggest the formation of carbonate-like surface complexes. Moreover, they found that oxygen vacancies act as specific sites for strong CO, chemisorption. The present study does not confirm the formation of strongly bound carboxylates. However, our activation mode is quite different from Gopel's as we heat our powder under oxygen, which prevents the formation of point defects VOs. From the infrared study of H,-CO interaction on ZnO powder at room temperature, Zecchina and coworkersL2 deduced the presence of sites which are formed by a triplet of exposed zinc ions and at least one reactive oxygen ion.Such sites would be situated on the (0001) face, reconstructed in order to satisfy the charge compensating criterion, as suggested by Nosker.16 We could explain our results with successively (i) formation of a bidentate carbonate on the Zn-0 pair site, (ii) formation of a linear species on an adjacent Zn2+ ion and (iii) formation of a second linear species involving the thirdJ. SAUSSEY, J-C. LAVALLEY A N D C. BOVET 1463 Zn2+ ion of the site, in such a way that when a large amount of CO, is added, the bidentate carbonate species would be adjacent to two linear species held by two adjacent Zn ions. The coupling between two adjacent linear species would explain the splitting of the bands at 2358 and 638 cm-l.However, we think that three argu- ments can be put forward against this model: (i) the reconstruction of the (0001) face does not seem to have been proved,17 (ii) the three underlying Zn ions which become coordinatively unsaturated by removing one of four negative ions from the (0001) face do not seem accessible enough to adsorb three molecules of CO, and (iii) the coupling between two linear species favours two linear species held by the same Zn2+ ion rather than two linear species held by two adjacent ions. Hence we propose another model involving two-fold coordinated cations. Such Zn2+ ions could correspond to oxygen vacancies V& on the (1010) face, but we have already mentioned that such sites are very improbable due to our activation process. Zn2+ ions with two vacancies are also present on the (1011) face, which contains equal numbers of surface cations that are three- and two-fold coordinated to lattice 0~ygen.l~ However such a face, without any reconstruction, does not present any 0,-; therefore, it is difficult to correlate the splitting of the linear species with the shift of the band at 1 595 cm-l, which characterizes bidentate carbonates and thence necessitates adsorption on an 02- ion neighbouring a Zn2+ cation.We prefer to explain our results according to the scheme reported on fig. 4, involving Zn2+ ions carrying two unshared coordinative vacancies with a reactive oxygen ion in an adjacent position. In powders, the number of steps and edge positions might be extremely high. The sites which we propose from this work could correspond to the edges formed by the (0001) and (1010) planes.LEED studies have shown that both polar faces are steppedls with a step height of one unit cell (two double layers) along the c-axis, in the (1010) directions, exposing non-polar faces. According to our model, some bidentate carbonates would be formed on the non-polar faces of these steps. Gravimetric measurements have shown that ca. 1.4 CO, molecules are chemisorbed per nm2? Unfortunately, due to the multiplicity of species formed, it is not possible to determine separately the number of bidentate carbonates giving rise to the 1595 cm-I band, i.e. the number of the corresponding sites. We thank Prof. N. Sheppard and Prof. W. Gopel for helpful discussions. H. Knozinger, Adu. Catal., 1976, 25, 184. C. Morterra, A. Zecchina, S. Coluccia and A. Chiorino, J. Chem. SOC., Faraday Trans. I , 1977, 73, 1544. A. Zecchina, S. Coluccia, E. Guglielminotti and G. Ghiotti, J. Phys. Chem., 1971, 75, 2790. J. H. Taylor and C. H. Amberg, Can. J. Chem., 1961, 39, 535. L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1966). Ya. M. Grigor’ev, D. V. Pozdnyakov and V. N. Filimonov, Russ. J. Phys. Chem., 1972, 46, 186. * J. Saussey and J. C. Lavalley, J . Chim. Phys., 1978, 75, 505. F. Bozon-Verduraz, J . Catal., 1970, 18, 12. lo E. Borello, Discuss. Faraday SOC., 1971, 52, 44. J. C. Lavalley and J. Caillod, J. Chim. Phys., 1980, 77, 373. F. Boccuzzi, E. Garrone, A. Zecchina, A. Bossi and M. Camia, J . Cural., 1978, 51, 160. * T. T. Nguyen, J. C. Lavalley, J. Saussey and N. Sheppard, J. Catal., 1980, 61, 503. l 3 K. Atherton, G. Newbold and J. A. Hockey, Discuss. Faraduy SOC., 1971, 52, 33. l4 F. Runge and W. Gopel, 2. Phys. Chem., in press. l5 W. Gopel, R. S. Bauer and G. Hansson, Surf. Sci., 1980,99, 138; W. Hotan, W. Gopel and R. Haul, l6 R. W. Nosker, P. Mark and J. D. Levine, SurJ Sci., 1970, 19, 291. l7 S. C. Chang and P. Mark, Surf. Sci., 1974, 46, 293. l a M. Henzler, Surf Sci., 1970, 22, 12; 1973, 36, 109; D. Kohl, M. Henzler and G. Heiland, Surf. Sci., l9 V. Lorenzelli and G. Busca, personal communication. Sur- Sci., 1979, 83, 162. 1974, 41, 403. (PAPER 1 /796)

ISSN:0300-9599    

DOI:10.1039/F19827801457

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, 78, 1465-1472 Temperature Dependence and Diffusion Control of the Rate Constant for Energy Transfer from Decalin to Benzene BY GIORGIO ORLANDI,*-~ SERGIO DELLONTE, LUCIA FLAMIGNI AND FRANCESCO BARIGELLETTI Istituto di Fotochimica e Radiazioni d'Alta Energia del C.N.R., Via de'castagnoli I, 401 26 Bologna, Italy Received 18th May, 1981 The transfer of energy from decaiin to solute benzene has been studied in the temperature range -20 to + 57 "C. The quenching of decalin fluorescence by benzene has been monitored by performing both intensity and lifetime measurements. The temperature dependence of the quenching process is found to be consistent with a diffusion-controlled mechanism. An important source of information on the properties of the emitting excited state of saturated hydrocarbons is the study of kinetic parameters governing the quenching of their weak fluorescence.' In the last decade several experiments have been performed with the aim of measuring such parameter^.^.^ In most work, liquid alkanes containing suitable solutes at various concentrations were excited by means of ionizing radiation and the quenching of the fluorescence intensity of the alkane itself or the appearance and growth of the solute emission were monitored.The only experiment concerning the fluorescence lifetime quenching reported so far has been performed on a unique concentration of CCl, q ~ e n c h e r . ~ These experiments have shown that the emission intensities depend on the solute concentration according to the linear Stern-Volmer (SV) equation,6 at least for concentrations which are not too high, but surprisingly the SV rate parameter is one or two orders of magnitude larger than the value expected for diffusion-con t rolled processes. These results, suggesting that the emitting state has Rydberg character,' i.e. involving the excitation of one electron from a valence to a higher shell diffuse orbital, are in our opinion open to question as they could be influenced by the concomitant transient dynamic quenching effecta and by ionic reactions deriving from the high-energy ex~itation.~ To obtain information free of such spurious effects, we performed quenching experiments on both the lifetime and the intensity of the alkane fluorescence using a two-photon excitation produced by a nitrogen laser.l0 The corresponding wavelength is 168.5 nm, which is slightly above the onset of the absorbtion spectra of most saturated hydrocarbons and is thus not expected to produce ionization.The quenching rate constants obtained with this procedure for the system decalin- benzene and cyclohexane-benzene, at room temperature, are compatible with diffus- ional processes.ll We have now studied the temperature dependence of the quenching rate parameter of the system decalin-benzene in order to ascertain whether additional support can be found for our previous conclusions. t Permanent address : Istituto Chimico 'G. Ciamician', Universita di Bologna, Bologna, Italy. 1465 48 FAR 11466 ENERGY TRANSFER FROM DECALIN TO BENZENE EXPERIMENTAL Decalin (Baker, Analysed Reagent) was passed twice through a 50cm column of freshly activated silica gel; the purity was spectroscopically controlled.The cis: trans molar ratio of the decalin used was determined by viscometric measurements, taking the viscosity data of ref. (12) as standard. The cis: trans molar ratio was found to be 2: 5. Thiophene-free benzene (Baker, Analysed Reagent) was used as purchased. All samples were sealed under vacuum after repeated freeze-pumpthaw cycles in 1 cm fluorescence cells. Benzene concentrations were determined spectroscopically. The excitation was obtained from a pulsed nitrogen laser (Lambda Physik, Gottingen) with a pulse width of ca. 3.5 ns and a pulse power of 1 MW. The beam was focussed on an area smaller than 0.05 cm2 where, as previously reported,1° a two-photon excitation of the alkane molecules took place.For fluorescence intensity measurements the exciting beam was focussed on the extreme edge of the cell to prevent reabsorption. The emission at 90' to the laser pulsca was detected using an R.C.A. 1P28 photomultiplier with a five-dinode chain configuration to obtain a faster time response. The emitted light was filtered by a 5 cm cell filled with C1, at a pressure of 2 atm* and a Bausch-Lomb high-intensity monochromator. Data were acquired and reduced using an R7912 Tektronix transient digitizer interfaced to a Z-2D Cromemco microcomputer. The deconvolution analysis of the fluorescence decay curves was performed by a non-linear iterative least-squares fitting procedure over stored couples of flash and decay curves, each partitioned in 256 data.points.With a sweep rate of 2 ns per division (full time-scale of 20 ns), the instrumental time resolution is ca. 0.080 ns. Further details on the experimental set up and deconvolution procedure are reported elsewhere.lO9 l1 Lifetime values were obtained by averaging two sets of five measurements each. The scattering (i.e. )zmitX - zminl) in z values for a given c and T was < 7%. The (1,JI) data were obtained from the ratio of integrated emission signals of pure decalin and benzene solutions; any reported value is the average of two sets of ten ( I o / I ) measurements. The spread in these data for a given c and T was < 10%. The errors relevant to z and ( I o / I ) are assumed to be respectively, The measurements at 0 and 20 OC were performed using an Air Products cryostatic apparatus.Above 0 O C temperatures were regulated by water flowing through the cell block. The absolute error in temperature measurements was & 0.5 "C. 5 % and & 7%. RESULTS The decay of decalin fluorescence for various concentrations of benzene was monitored at different temperatures in the range - 20 to + 57 O C . The detected signals could be deconvoluted with a satisfactory fitting, according to a single-exponential decay, yielding the fluorescence lifetime, z, of decalin. At each temperature z followed the SV equation which holds for a collisional irreversible energy-transfer process z-l = z,'+kc (1) where z, is the fluorescence lifetime of the pure solvent, k is the emission quenching parameter and c is the concentration of benzene.In fig. 1 typical examples of plots of z-l against c, for some of the temperatures considered, are shown. The values of k and z, obtained from the plots are listed in table 1. The value of zo at room temperature is in good agreement with previously found values.l39 l4 The small difference in the present k value at room temperature compared with our previous result" reflects the improved data acquisition procedure used in this work. From the data of table 1 it can be seen that both the magnitude of k and its temperature dependence are typical of diffusion-controlled processes. l4 * 1 atm = 101 325 PaG. ORLANDI, s. DELLONTE, L. FLAMIGNI AND F. BARIGELLETTI 1467 1 0 0 3 6 9 [benzene] / 1 0-2 mol dm-3 FIG.1 .-Quenching of decalin fluorescence lifetimes as a function of benzene concentration at various temperatures: 0, 253; a, 298; ., 330 K. TABLE 1 .-DECALIN FLUORESCENCE LIFETIMES, z,,, AND FLUORESCENCE QUENCHING PARAMETERS, k AND k', AT VARIOUS TEMPERATURES The estimated errors are & 5% for z, f 8% for k and f 13% for k'. T/"C k/dm3 mol-l s-I k'/dm3 mol-l s-l - 20 3 . 3 0 ~ 109 2.89 4.2x 109 0 4.05 x 109 2.86 4.3 x 109 + 15 4.41 x 109 2.40 3.5 x 109 + 25 4.91 x 109 2.36 3.8 x 109 + 35 6.35 x 109 2.30 4.1 x 109 + 45 7 . 1 7 ~ 109 2.24 4.0 x 109 + 57 7.99x 109 2.18 5.1 x 109 Parallel measurements of the intensity of decalin fluorescence, for various benzene concentrations, were performed. The fluorescence intensity, I, follows the SV equation I,/I = 1 +k,z,c only for low values of c, but shows an upward bend for large c, as noted previous1y.l' In eqn (2), I, represents the intensity of pure decalin and k, an appropriate emission intensity quenching parameter.Plots I,/I against c for several temperatures are reported in fig. 2. The values obtained for k , from the linear part of the plots (i.e. for c < mol dmW3) are ca. 50% larger than those obtained for k at the same temperature. The difference between the intensity and the lifetime plots can be understood in the framework of diffusion-controlled p r o c e ~ s e s ~ ~ ~ l5 in terms of the transient quenching effect.8* 16+ l7 This effect, which is important for emitters of short lifetime dissolved in a (highly) viscous solvent, is due to the almost instantaneous quenching of the emitter by the initially adjacent quencher molecules (i.e.without requiring diffusion). In the lifetime measurements the almost instantaneous non-exponential decay associated with the transient quenching effect tends to escape observation (at least if the time resolution is not very good) and is overshadowed by the longer, exponential decay 48-21468 0 ENERGY TRANSFER FROM DECALIN TO BENZENE 0 2 6 10 [benzene]/ 1 O-’ mol dm-3 FIG. 2.-Quenching of decalin fluorescence intensities as a function of benzene concentration at two temperatures: (a) 330, (b) 288 K. Dashed lines are calculated from eqn (2), using k , = k reported in table 1 . due to the diffusional process. In the intensity measurements, on the other hand, the transient dynamic quenching is fully manifest.This explains the difference in behaviour between lifetime and intensity quenching. The treatment of this effect has been given by Forster and Weller17 and, more recently, by Ware and coworkers.16 Given the resolution of our apparatus, the simpler formulae of ref. (17) have been used. According to these authors, the lifetime quenching, observed after the rapid transient quenching, is described by eqn (l), while the intensity quenching is given by I,/I = (1 + kz, C) exp [k’z, C( 1 + kz, c)-i] (3) where k’ is a parameter describing the transient dynamic quenching. The values for k’ can be derived by fitting the experimental results to eqn (3) and using the parameters z, and k determined with the lifetime experiments. The values of k’ obtained for the system decalin-benzene are given in table 1.As for the accuracy of these results, the values for k tend to be overestimated because they contain a contribution from the dynamic quenching, and conversely those for k’ are underestimated. These systematic errors are not considered to exceed the uncertainties on k and k’ (reported in table 1) which, however, are small enough to allow a discussion of the mechanism of the quenching.G . ORLANDI, S. DELLONTE, L. FLAMIGNI AND F. BARIGELLETTI 1469 DISCUSSION According to the theories for diffusion-controlled processes, k and k' can be expressed in term of diffusional parameters; specifically k is given by l5 k = 4nNRD ( 4 ) k' = kR(t,D)-i ( 5 ) and k' is related to k by17 where D = D , + D, is the sum of the diffusion coefficients of the emitter (solvent, S) and of the quencher (solute, s), R is the quenching distance or interaction radius and N' = 6.02 x 1020.* By solving eqn (4) and (9, values for D and R are obtained on the basis of the experimentally determined k and k'.These values, which are reported in table 2, are of the size expected for diffusion-controlled processes. TABLE z.-EXPERIMENTAL DIFFUSION COEFFICIENTS, Dexp., AND QUENCHING DISTANCES, R, AT VARIOUS TEMPERATURES The fourth column displays theoretical diffusion coefficients, Dtheor., calculated using eqn (7) and the values of the fifth column. The estimated errors are & 2 5 % for Dexp. and f 15% for R. 7.00 x - 20 3 x 1 3 2.2 x 3.88 x 0 4 x 10-6 1 2 4.3 x + 15 6 x 10 6.4 x 2.75 x + 25 7 x 1 0 8.2 x lov6 2.22 x + 3 5 9 x 10-6 9 1 0 .2 x 1.85 x lop2 + 4 5 1 1 x 9 12.5 x 1 . 5 7 x 1.31 x + 57 1 1 x 10 15.5 x Calculated by D = D,+D, and eqn (7); b, = 3.9 A and b, = 3.3 A. The interaction radius obtained falls in the range typical of collisional interactions (6-15 AT).16u In principle, an important contribution to the quenching of alkane fluorescence by benzene is associated with the singlet-singlet energy-transfer process. This is governed by dipole-dipole interactionslg and takes place over a critical transfer distance, Ro, usually in the order of 20-50 i.e. larger than the collisional distance. We have estimated R, for the present case, using the relationshipleb where, in keeping with the notation of ref. ( l s b ) , F,(v) is the donor fluorescence spectrum normalized to the fluorescence quantum yield, E,(v) is the molar decadic extinction coefficient of the quencher, n is the solvent refractive index and N is Avogadro's number.Using F, V ) taken from ref. (1) and cS($ measured in our radii. This rather low value for R, indicates that, in this case, the coulombic interaction laboratory, we obtain R, = 8.4 8, , which is the same size as the collisional interaction * When N' = 6.02 x lo2', D is in cm2 s-' and R in cm, the units of k and k' are dm3 mol-' SKI. t 1 A = 10-10 m = 10-1 nm.1470 ENERGY TRANSFER FROM DECALIN TO BENZENE is effective only at a short range, and the reason for this is the low fluorescence quantum yield of decalin and the rather poor overlap between the &(v) and E,(v) spectra.The diffusion coefficients D, and D,, according to the Stokes-Einstein (SE) theory,15 ( 7 ) are given by where k is Boltzmann’s constant, T is the absolute temperature, q is the solvent viscosity and b,, b, the Stokes (collision) radii. Eqn ( 7 ) is the modified form of the SE relationship, derived assuming that the coefficient of sliding friction is zero, which is appropriate for molecules of size comparable or smaller than those of the solvent.18C Using the values reported for the viscosity12 for the cis: trans molar ratio of decalin used, we derived the theoretical values of D, which are shown in table 2 along with the values of q used. The correlation between the values of the diffusion coefficients calculated by eqn (7) (theoretical) and those obtained by eqn ( 4 ) and ( 5 ) (experimental) is quite good as both are of the same size and increase at a similar rate with temperature.The ‘theoretical’ D grows faster with the temperature than its ‘experimental’ counterpart, but this is not unexpected. In fact, while according to eqn ( 7 ) the product Dq/ T should be constant, quite often it is found to depend on q (or on T ) . The following empirical equation14 Dq/T = A+bqx where x is an appropriate parameter, approximately 1, has been proposed to describe the experimental behaviour of D. The best fitting of our ‘experimental’ D to eqn (8) (see fig. 3) gives A = 3.4 x 10-lo cm2 P s-’ K-l, b = 8.6 x cm2 s-l K-I and x = 1. D , = kT/4xqb, D, = kT/4nqbS (8) 0 ~ 0.02 5 0.0 50 0.075 VIP FIG. 3.-Plot of Dq/T against q for benzene in decalin.The parameter A represents the Stokes-Einstein behaviour described by eqn (7) and b describes a deviation from it arising from the contribution to the motion of the solute molecules due to their spontaneous change of position as a result of their flow into the holes of the solvent. It can be seen that the value obtained for A is in fair agreement with the theoretical estimate of 6.1 x cm2 P s-l K-l, obtained from eqn (7) for the diffusion coefficient.G. ORLANDI, S. DELLONTE, L. FLAMIGNI A N D F. BARIGELLETTI 1471 CONCLUSION The quenching of decalin fluorescence by benzene solute, as a function of temperature, has been investigated by performing both intensity and kinetic measure- ments. The temperature dependence of the quenching process is found to be fully consistent with a diffusion controlled mechanism.In fact, analysing the experimental parameters k and k’ in terms of the diffusional process, one obtains a diffusion coefficient D with the proper size and temperature dependence. We believe this result remains valid if allowance is made for the approximations involved in obtaining k and k’. Different results obtained using high-energy excitation sources2 may be explained by the presence of species, such as alkane ions, which are known to yield excited states upon rec~mbination.~ Under such conditions the quenching of the donor fluorescence intensity as well as the growing of the acceptor emission may encompass both excited states and ionic processes. Furthermore, the use of quenchers having a large spectral overlap with the donor fluorescence could cause an efficient energy-transfer via dipole-dipole interactionIg with a rate constant larger than the diffusional one.This is a well known fact and it cannot be taken as suggestive of anomalous behaviour of the alkane singlet state. In conclusion, the fluorescence quenching of decalin by benzene takes place by a diffusion mechanism in agreement with our previous findings.ll This result indicates that the nature of the emitting excited state of decalin is that of a valence state rather than that of a Rydberg state.’ This indication is supported by the shape of the vibrational envelope of decalin fluorescence, which lacks the vibrational structure1 typical of Rydberg transitions. We thank L.Minghetti and G. Mancini for their technical assistance. W. Rothmans, F. Hirayama and S. Lipsky, J. Chem. Phys., 1973, 58, 1300. L. Walter and S. Lipsky, Int. J. Radiat. Phys. Chem., 1975, 7, 175. J. H. Baxendale and J. Mayer, Chem. Phys. Lett., 1972, 17, 458. W. P. Helman, Chem. Phys. Lett., 1972, 17, 306. G. Beck and J. K. Thomas, J. Phys. Chem., 1972,76,3856; G. Beck and J. K. Thomas, Chem. Phys. Lett., 1972, 16, 318; Y. Katsumura, T. Kanbayashi, S. Tagawa and Y. Tabata, Chem. Phys. Lett., 1979, 67, 183; Y . Katsumura, S. Tagawa and Y. Tabata, J. Phys. Chem., 1980, 84, 833. In these experiments the kinetics of the appearance of the acceptor excited state were observed but, as this process also includes an ionic contribution, this was not the most suitable way to study the energy transfer.0. Stern and M. Volmer, Phys. Z . , 1919, 20, 183. M. B. Robin, Higher Excited States of Polyatomic Molecules (Academic Press, New York, 1974), vol. 1, pp. 104-155; T. Wada and Y. Hatano, J. Phys. Chem., 1975, 79, 2210; 1977, 81, 1057; G. Foldiak, personal communication. W. R. Ware and J. S. Novros, J. Phys. Chem., 1966, 70, 3246. J. K. Thomas, K. Johnson, T. Klipper and R. Lowers, J. Chem. Phys., 1968,48,1608; J. H. Baxendale and P. Wardman, Trans. Faraday Soc., 1971, 67, 2997. lo S. Dellonte, E. Gardini, F. Barigelletti and G. Orlandi, Chem. Phys. Lett., 1977, 49, 596. l 1 F. Barigelletti, S. Dellonte, G. Mancini and G. Orlandi, Chem. Phys. Lett., 1979, 65, 176. l 2 Landoft-Bornstein, Zahlenwerte und Funktionen, 6 AuJlage 1 l / 5 a (Springer, Berlin, 1969), pp. 167- 170. l 3 W. R. Ware and R. L. Lyke, Chem. Phys. Lett., 1974, 24, 195. l 4 A. H. Alwattar, M. D. Lumb and J. B. Birks, Organic Molecular Photophysics, ed. J. B. Birks (Wiley, New York, 1973), vol. 1 , p. 403. l 5 A. Einstein, Ann. Phys. (Leipzig), 1905, 17, 549; G. G. Stokes, Mathematical and Physical Papers (Cambridge University Press, London, 1903), vol. 3, pp. 1-55; M. Smoluchowski, Z . Phys. Chem., 1917, 92, 129. l6 J. C. Andre, N. Niclause and W. R. Ware, Chem. Phys., 1978,28,371; W. R. Ware and T. L. Nemzek, Chem. Phys. Lett., 1973, 23, 557; W. R. Ware, Pure Appl. Chem., 1975, 41, 635.1472 ENERGY TRANSFER FROM DECALIN TO BENZENE l7 A. Weller, Prog. React. Kinet., 1961, 1, 187 ; Th. Forster, Fluorescenz Organischer Verbindungen (Vanderhoeck and Ruprecht, Gottingen, 195 1). J. B. Birks, Photophysics of Aromatic Molecules (Wiley Interscience, New York, 1970), ( a ) p. 518; (b) p. 569; ( c ) p. 511. l9 Th. Forster, Discuss. Faraday SOC., 1959, 27, 7. (PAPER 1 /798)

ISSN:0300-9599    

DOI:10.1039/F19827801465

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 1473-1490 Mechanistic Study of Carbon Monoxide Hydrogenation over Ruthenium Catalysts BY YOSHIHIRO KOBORI, HIROFUMI YAMASAKI, SHUICHI NAITO, TAKAHARU ONISHI,* AND KENZI TAMARU Department of Chemistry, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 1 13 Japan Received 27th May, 198 1 The mechanism of the hydrogenation of carbon monoxide to hydrocarbon products over ruthenium catalysts has been investigated. By measuring the adsorption and observing the infrared adsorption spectra of the adsorbed species during the course of the reaction, the accumulation of surface hydrocarbon species on the ruthenium catalysts was confirmed, whereas most of the surface was covered by molecularly adsorbed carbon monoxide. The reaction intermediate was examined using carbon- 13.The reactivity of deposited carbon formed by the Boudouard reaction has also been studied using carbon-13. It is concluded from the behaviour of the surface species under the reaction conditions that all the hydrocarbon products are produced via dissociatively adsorbed CO with no CO insertion. The rate-determining step has been examined, leading to the conclusion that it comprises the conversion of C, intermediates to the reaction products. The hydrogenation of carbon monoxide to form hydrocarbons has been studied by many workers and various kinds of mechanisms and intermediates have been proposed. Fischer and Tropsch, the pioneers of this reaction, assumed metal carbides as a reaction intermediate.l This mechanism, however, was contradicted by Emmett and coworkers, who observed that deposited carbon atoms, which had been labelled by radioactive carbon- 14, were incorporated only slightly into the reaction products,, and various oxygen-containing surface complexes, such as CHO(ads) or HCHO(ads), were postulated3 as reaction intermediates; Blyholder and Neff observed infrared bands in the C-H and 0-H stretching region^,^ which strongly supported the existence of such oxygen-containing intermediates.Recent studies, however, have provided experimental evidence contradicting the mechanism of oxygen-containing species as an intermediate of methanation and Fischer-Tropsch synthesis. Recently, the carbon formed by the disproportionation of CO (the Boudouard 2 CO -+ CO, + C(ads) reaction) has been shown to have high reactivity toward hydrogenation5 and to be incorporated into the reaction p r ~ d u c t s .~ - ~ ~ Biloen et al. also demonstrated that not only methane formation but also the production of higher hydrocarbons can proceed via dissociated carbon.'l On the other hand, some workers have tried to characterize surface species on ruthenium catalysts in their working states for the CO + H, reaction, especially by the infrared technique,12-14 and observed the accumulation of hydrocarbon species. They concluded, however, that this species is not the reaction intermediate but a by-product on the support or the ruthenium metal. To elucidate the mechanism of heterogeneous catalysis it is necessary to investigate the properties and the dynamic behaviour of each of the surface species in the working state of the catalyst, and to examine what role each of the surface species plays in the overall reaction.15 In the present work we studied the hydrogenation of carbon monoxide on 14731474 co HYDROGENATION OVER R U CATALYSTS ruthenium-silica and ruthenium black catalysts and investigated the roles of dissociated carbon and surface hydrocarbon species in the overall reaction.Adsorption measure- ments during the reaction and methods of detecting transients, such as isotope substitution experiments, were applied to this reaction system to elucidate the mechanism of the formation of methane and higher hydrocarbon products. EXPERIMENTAL CATALYST PREPARATION A N D REACTION KINETIC STUDIES A 4.5 wt.% Ru/SiO, catalyst was prepared by impregnating silica powder (Aerosil, Degussa) with an aqueous solution of RuCI, hydrate (Wako Pure Chemicals). The slurry was dried under vacuum at 3 10-320 K to form black masses, which were further dried in air at 373 K for a week. The prepared catalyst was then put in a closed-circulation system whose dead volume is ca. 450 cm3. After a short evacuation, ca. 5 x lo4 Pa of hydrogen has was introduced at room temperature and then the catalyst was gradually heated to higher temperatures. Reduction of the catalyst was accomplished at 723 K for 20 h with a liquid-nitrogen cold trap in a closed-circulation system which collected evolved water, hydrogen chloride and so forth, followed by evacuation at 723 k for 2 h. The dispersion of Ru was determined by H, adsorption17 to be ca.25%, which only showed a small decrease during the course of repeated reactions and treatments: there was little resulting effect on the specific activity, i.e. the activity per surface metal atom. A reaction-gas mixture of known amounts of CO and H, was prepared in the circulation system and introduced onto the clean catalyst at reaction temperatures. The initial total pressure of the reacting gas was ca. 2.7 x 103-1 .3 x lo4 Pa. In most experiments the reaction products except methane were collected in a liquid-nitrogen cold trap, which made product analysis easy and prevented secondary reactions such as olefin hydrogenation and the water-gas shift reaction. The procedure for catalyst activation was reduction with H, at 573 K for 1 h followed by evacuation for 1 h at the same temperature.H, gas from a commercial cylinder was passed over an Engelhard Deoxo unit followed by passage through a liquid-nitrogen cold trap. l2C0 (Takachiho Kagaku), 13C0 (Prochem ; 90.5 % pure) and D, (Showa Denko) were passed through a liquid-nitrogen cold trap before use. 0, gas from a commercial cylinder was liquified using a liquid-nitrogen cold trap and the vapour was used for hydrocarbon oxidation. Ethene, propene (Takachiho Kagaku) and cis-oct-4-ene (Tokyo Kasei) were obtained from commercial samples after removing air completely. ADSORPTION MEASUREMENTS DURING THE REACTION AND PRODUCT ANALYSES Ruthenium catalysts (3-4 g) were employed and the composition of the adsorbed species during the course of the reaction was calculated from the mass balance, by estimating the total amounts of all the elements in the gas phase and in the trap.Reaction products collected in the trap were analysed by the gas-chromatographic method. Since they exhibited many complicated peaks of several kinds of C i hydrocarbons (those higher than C, species), the trapped products (Ci hydrocarbons and water) were all oxidized with 0, gas to CO, and H,O over Pd-black (Nippon Engelhard) at 573 K. The amount of CO, thus produced, which corresponds to the total number of carbon atoms in C l products, was measured volumetrically or gas-chromatographically. The amount of water thus formed was determined gravimetrically as the sum of that produced during the CO+H, reaction and that produced during hydrocarbon oxidation.The latter was corrected from the amount of carbon atoms in the C,+ products, assuming that all the C i products have the composition (CH,),. Such an assumption did not introduce serious errors because most of the C,+ hydrocarbon products were olefins, and moreover higher hydrocarbons were produced to a considerable extent. The amounts of each of the C l hydrocarbon products were estimated by gas chromatography, with Porapak Q or Porapak P columns for the chain-length distribution, but with sebaconitrile/simalite or dimethylsulpholane/alumina for the distribution of isomers.KOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 1475 ISOTO PE-L A B E L LE D EX PER I MEN T S Carbon-13 was employed to provide clues to the mechanism of this reaction.The reaction was carried out in the circulation system with greaseless stopcocks to avoid contamination by carbon species from grease. The isotope ratio was determined by a quadrupole mass filter (U.T.I.). Since it was difficult to determine the 13C content of every hydrocarbon product, all the trapped products were oxidized and the 13C content of the CO, thus produced was estimated. When mass spectra of a particular portion of the hydrocarbon products were needed, C l hydrocarbons were separated into each component by gas chromatography before the mass analyses. For the deuterium-substitution experiments, 10 g of ruthenium black (Nippon Engelhard) were used instead of the silica-supported catalyst because silica support contains a large amount of hydroxy groups on its surface whose hydrogen is easily exchangeable with gas-phase hydrogen in the presence of water and active metal at the reaction temperature.The isotopic distribution of gas-phase hydrogen was determined by gas chromatography I(MnCl,-Al,O, at liquid-nitrogen temperature) and that of the hydrocarbon products was determined by the infrared method, using C-H and C-D stretching vibrational modes of C t hydrocarbon products, assuming the ratio of absorption coefficients (CH/CD) to be 1.9.16 EVACUATION EXPERIMENTS During the course of the reaction the catalyst system was evacuated to collect and analyse all the desorbing gases. The apparatus for evacuation was connected to the reaction system through two traps operated at liquid-nitrogen temperature; one was a glass tube (44, 5 m) and the other a similar tube packed with active carbon.The materials, such as C l hydrocarbons, water and carbon dioxide, collected by the former trap do not have any appreciable vapour pressures at liquid-nitrogen temperature. They were analysed by gas chromatography (Porapak Q or P) or by the oxidation technique mentioned above. The latter trap was placed behind the former and collected gases such as hydrogen, methane and carbon monoxide, which were analysed by gas chromatography after desorbing gases from active carbon at room temperature. The pressure of the remaining gas not collected by the second trap amounted to 10 Pa, measured by a McLeod gauge. In this way all the desorbing gases could be analysed without losing any portion of them.RESULTS PRODUCT SELECTIVITY Fig. l(a) shows a typical product distribution of the CO+H, reaction in a Schulz-Flory plot. Note that higher hydrocarbons up to C,,-C,, were produced under lower pressures (ca. lo4 Pa), suggesting that the mechanism operating under the reaction conditions employed in these experiments is similar to that of the Fischer- Tropsch synthesis under higher pressures. Fig. 1 ( b ) shows the proportion of 1 -olefins in products of the same carbon number. Under reaction conditions where the C l hydrocarbons were trapped as soon as they were produced, 1-olefins are the primary products. Paraffins or olefins other than 1 -olefins, especially those observed in higher hydrocarbon products, would be produced by subsequent hydrogenation or isomeri- zation of 1-olefins.Compared with the hydrocarbon products, the amounts of oxygen-containing compounds, such as methanol, were negligible except for a small amount of carbon dioxide (< 1 % of total hydrocarbon production). Selectivity over the ruthenium black catalyst was also studied, but there was no great difference from that over the silica-supported catalyst. ADSORPTION MEASUREMENTS D U R I N G THE REACTION Typical results of adsorption measurements during the CO + H, reaction are shown in fig. 2. When a CO + H, mixture was introduced onto the catalyst at 423 K, water1476 co HYDROGENATION OVER RU CATALYSTS l-&-- ce .- CI 8 2 a a 0 0 1 2 3 4 5 6 7 8 9 10 carbon number FIG. 1.-Typical product distribution of the CO+H, reaction over Ru/SiO,.p(H,) = 8.0 x lo3 Pa, p(C0) = 4.0 x lo3 Pa, 423 K. (a) Schulz-Flory plot (logarithm of the number of molecules plotted against carbon number), normalised by the amount of produced methane. (b) Proportion of 1-olefin in the isomers. A small amount (0.1 g) of catalyst was used. and hydrocarons were readily produced with no induction period. The amount of adsorbed hydrogen, H(ads), at the initial stage of the reaction [ 1.4 cm3 (s.t.p.) g-l as atomic hydrogen] was much less than that in the absence of CO [2.4 cm3 (s.t.p.) g-'1, indicating the inhibition of hydrogen adsorption by CO adsorption. In contrast, the presence of hydrogen increased the amount of CO adsorption [saturated adsorption of CO corresponds to 3.4 cm3 (s.t.p.) g-l in the presence of hydrogen, but 2.4 cm3 (s.t.p.) g-l in the absence of hydrogen].This phenomenon was observed even at 298 K [3.87cm3 (s.t.p.) g-l in the presence of hydrogen, 2.81 cm3 (s.t.p.) g-l in its absence] immediately on the introduction of hydrogen, which suggests that this is apparently not due to complex formation between CO and hydrogen but rather due to an electronic interaction between two adsorbates on the catalyst surface. The amount of molecularly adsorbed carbon monoxide, CO(ads), was estimated by a rapid isotopic exchange reaction between 13CO(ads) and 12CO(gas), introducing l2C0 onto 13CO(ads) at 423 K. The amount exchangeable CO(ads) with CO(gas) was 3.3 cm3 (s.t.p.) g-l at 423 K, which agreed reasonably well within experimental error with that of total oxygen uptake, obtained from the elemental adsorption measurements 13.4 cm3 (s.t.p.) g-l].This shows that the only oxygen-containing species on the surface in its working state is CO(ads). The amount of CO(ads) remained constant with reaction time, whereas both total amount of carbon on the surface, C(ads), and H(ads) increased with time, suggesting saturated adsorption of CO(ads) and accumulation of surface hydrocarbon species,KOBORI, YAMASAKI, NAITO, ONISHI AND TAMARU 1477 m a --- 2 3 x 1 0 ~ a2 2 c 3 a 1x10 0 ( a ) L I " 0 50 100 reaction time/min FIG. 2.-(a) Gas phase: 0, H,; c), CO; 0, H,O; 0 , carbon in C: hydrocarbons; A, methane. (b) Surface composition: 0, C(ads); 0, H(ads); ., O(ads). 1 cm3 (s.t.p.) g-' in (b) corresponds to 7.4 x 10, Pa in (a). TABLE 1 .-KINETIC PARAMETERS OF THE CO + H, REACTION OVER Ru/SiO, AT 408 K" [RATE a P ( H , ) ~ x P(CO)~] X Y CH* + 1.2 - 0.6 c; + 1.5 - 0.9 c, H,(ads) + 0.8 + 0.3 a The values X and Y were determined from the initial rate of CO + H, reaction, by changing the partial pressure of CO [p(H,) = 8 x lo3 Pa constant] and the partial pressure of H, [p(CO) = 5.3 x lo3 Pa constant], respectively.3.7 g of catalyst were used. C,H,(ads), on the catalyst. This may be clearly confirmed by the infrared technique.lg The increase of adsorption of carbon and hydrogen, namely the accumulation of surface hydrocarbon species, was also observed when ruthenium black was used as catalyst. Kinetic data on the initial rates of the CO + H, reaction over the Ru/SiO, catalyst are summarized in table 1.The accumulation of surface hydrocarbon shows a positive dependence on CO pressure, whereas other values of the order of the reaction are similar to those obtained by Vannice.ls1478 co HYDROGENATION OVER RU CATALYSTS REACTIVITY OF SURFACE HYDROCARBON SPECIES Fig. 3 shows the dependence of the rates of hydrocarbon accumulation and hydrocarbon production upon the amount of surface hydrocarbons. The rate of accumulation decreased as the amount of surface hydrocarbon increased. On the other hand, the rate of hydrocarbon production was independent of the amount of surface hydrocarbon species accumulated. In order to investigate the role of such accumulated 3 x 2 x l x FIG. 3.-Dependences of hydrocarbon production 0 5 carbon atom of C,,H, (ads)/cm3 g-' the rates of hydrocarbon accumulation (rate of carbon (total amount of carbon atom) upon the amount of accumulation) and carbon of surface hydrocarbon.- 0, Surface hydrocarbon accumulation, 0, hydrocarbon production (C, + Ci). p(H,) = 8.0 x lo3 Pa, p(C0) = 4.0 x lo3 Pa, at 423 K over Ru/SiO,. hydrocarbons in the overall reaction, the following experiments using 13C0 were carried out over the Ru/SiO, catalyst. After the accumulation of 13C surface hydrocarbon species from the 13C0 + H, reaction at 423 K, l2C0 was introduced and the molecularly adsorbed CO was replaced by l2C0 at room temperature. Then a H, + l2C0 mixture was introduced, the temperature was raised rapidly to 423 K and the 13C content in the hydrocarbon products was analysed with time. As is shown in fig.4, the 13C content in the reaction products was reduced with time, but it could be extrapolated to 100% at the initial stage of the reaction. It is thus demonstrated that at least some of the accumulated hydrocarbon species behave like real reaction intermediates through which the hydrocarbon products are formed. The reactivity of surface hydrocarbon species, or the rate of production of 13C hydrocarbon from initially accumulated surface hydrocarbons, is also given on the right-hand ordinate in fig. 4. It is clear that a major part of the accumulated surface hydrocarbons exhibits rather low reactivity, whereas a lesser part has high reactivity. In this experiment the 13C content in each of the hydrocarbon products having different carbon numbers was also examined; the products were separated into each portion by gas chromatography before oxidation to CO, for mass-spectrometric measurements.The results do not exhibit any difference in 13C content among hydrocarbons of different carbon numbers; this may be extrapolated to 100% at time zero, as shown in fig. 5, suggesting that all the hydrocarbon products are formed from common building blocks and no l2C0 insertion is involved in the chain growth. The mass spectra of the hydrocarbon products were also measured without oxidation, as listed in table 2. As the 13C in the surface is consumed, 12C from COKOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 1479 amount of 'T of surface hydrocarbon left on surface (70) FIG. 4.-13C content in the Ctproducts and reactivity of surface hydrocarbon during the T O + H, reaction following the accumulation of 13C surface hydrocarbons.Initial state: carbon in 13C,H,(ads) = 1.6 cm3 (s.t.p.) g-l, p(H,) = 9.3 x lo3 Pa, p(l2C0) = 4.0 x lo3 Pa, at 423 K over Ru/SiO,. Observed values were corrected for the isotopic purity of the 13C0 used. The right-hand ordinate represents the rate of 13C-hydrocarbon production from the initially accumulated surface hydrocarbon. 12CO(ads) = 3.3 cm3 (s.t.p.) g-l, C ._ k (LI 0 X 1 1 I 0 60 120 180 reaction tinie/min FIG. 5.-13C content in each hydrocarbon product having different carbon numbers during the T O + H, reaction after the accumulation of 13C surface hydrocarbon. x , C,; 0, C,-C,; 0, C,-C,; A, C,-C,; 0, C8-. Initial state: carbon in 13C,H,(ads) = 2.3 cm3 (s.t.p.) g-l, 12CO(ads) = 3.3 cm3 (s.t.p.) g-l, p(H,) = 9.3 x lo3 Pa, P('~CO) = 6.7 x lo3 Pa, at 423 K over Ru/SiO,.Observed percentage of 13C was corrected for the isotopic purity of the 13C0 used.1480 co HYDROGENATION OVER RU CATALYSTS TABLE 2.-cARBON ISOTOPE DISTRIBUTION IN HYDROCARBON PRODUCTS DURING THE I2c0 + H, REACTION OVER Ru/SiO, CATALYST AFTER THE ACCUMULATION OF 13C SURFACE HYDROCARBONS OR THE 13C DEPOSITION BY THE BOUDOUARD REACTION reaction amount of conditions reac- isotope distribution in accumulated tion the productsa (%) carbons p(H,) p(C0) temp. time /cm3 (s.t.p.) g-’ /103Pa /K /min product l3CO lac1 13C2 13C3 13Cq 1.4 6.0 6.0 423 10 propene 13 17 29 41 - 60 propane 19 54 18 9 - 2.0 9.3 4.7 423 100 methane 86 14 - - - 80 17 3 - - hydrocarbon) - - - - propene 71 19 8 2 - - - - (deposited carbon) (surface ethene - - - - - - - - trans-but-2-ene 60 26 8 4 2 a 13Ci indicates the fraction of species in each of the products with i 13C atoms.molecules is increasingly incorporated into the reaction products, giving an almost binominal isotope distribution for 12C and 13C, which demonstrates that the 13C from the surface hydrocarbon and the 12C from the CO behave as common building blocks in the hydrocarbon formation. The behaviour of hydrogen was also investigated by substituting H, for D, during the reaction over ruthenium black catalyst. After the accumulation of C,H,(ads) species by the CO + H, reaction at 393 K, a CO+ D, mixture was reacted and the hydrogen isotope distributions of the gas phase and C l hydrocarbon products were analysed by gas chromatography and infrared spectroscopy, respectively. As is shown in fig.6, the deuterium content in the surface hydrocarbon increased with time, but that in the hydrocarbon products decreased in a manner similar to that of gas-phase hydrogen rather than to that in the surface hydrocarbons. REACTIVITY OF DISSOCIATED CARBON When CO was introduced onto the catalyst at 423 K or higher temperatures the Boudouard reaction took place, and surface carbon species were accumulated with the evolution of carbon dioxide. The reactivity of these dissociated surface carbon species was investigated over the Ru/SiO, catalyst. First, the effect of these surface carbon species accumulated by the Boudouard reaction at 573 K upon the initial rate of hydrocarbon production was examined at 423 K (fig.7). A certain amount of dissociated surface. carbon, whose concentration is higher than that in the normal reaction, accelerates the production of hydrocarbon, but too much dissociated carbon decreases the rate. To examine the role of these accumulated surface carbon species in the overall reaction, 13C was also employed for the reaction. First, 13C was deposited over Ru/SiO, catalyst by the disproportionation of 13C0 at 573 K for 1 h; then the system was cooled to 423 K, and molecularly adsorbed 13C0 was replaced by l2C0 by introducing l2C0 in the ambient gas. A 12CO+H2 mixture was introduced onto the catalyst and the 13C content in the reaction products was analysed with time. Fig. 8 indicates that at the beginning of the reaction the 13C content of the total C l hydrocarbon products may be extrapolated to 100 %, although molecularly adsorbedKOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 100 n n 1481 0 60 reaction time/min FIG.6.-Deuterium contents in the products and surface hydrocarbons during the CO +D, reaction following the accumulation of H surface hydrocarbons: 0 , hydrogen gas; 0, (2: hydrocarbon product; A, surface hydrocarbon. Initial state: hydrogen atom in C,H,(ads) = 5.0 cm3 (s.t.p.) g-l, CO(ads) = 6.0 cm3 (s.t.p.) g-l, p(H,) = 1.2 x lo4 Pa, p(C0) = 1.2 x lo4 Pa, at 393 K over ruthenium black. The surface composition was calculated by assuming that the deuterium content in the evolved water was identical with that in the gas-phase hydrogen.“ 3 Eo i . c .- c 0 3 ‘ D 2 ~~~ 0 1 2 amount of deposited carbon/cm3 (s.t.p.) g-’ FIG. 7.-Effect of the amount of accumulated surface carbons produced by the Boudouard reaction at 573 K upon the initial rate of hydrocarbon production at 423 K : 0, ethane+ethene; 0, propene. p(H,) = 6.0 x lo3 Pa, p(C0) = 5.3 x lo3 Pa, over Ru/SiO,.1482 co HYDROGENATION OVER RU CATALYSTS CO on the surface is mostly 'TO. In table 2 are given a few examples of mass analyses of the hydrocarbons produced in this experiment which show the mixing between deposited 13C and the 12C from CO. The dissociated carbon also showed high reactivity for hydrogenation, and was predominantly hydrogenated at a moderate temperature (368 K) in the presence of molecularly adsorbed T O and dissociated carbon- 13.The main product in such an experiment was methane (96 %), with small amounts of ethane and propane, whose 13C contents are given in fig. 9, indicating the higher reactivity of dissociated carbon toward hydrogenation than molecularly absorbed CO. 100 I I ' 1- 100 90 80 70 0 'v deposited carbon left on the surface (%) FIG. 8.-13C content in C: hydrocarbon products during the I2CO+H, reaction at 423 K after the deposition, of I3C by the Boudouard reaction at 573 K. Initial state: deposited 13C = 0.7 cm3 (s.t.p.) g-l, 12CO(ads) = 2.4 cm3 (s.t.p.) g-l, p(H,) = 8.0 x lo3 Pa, p(C0) = 4.0 x lo3 Pa, over Ru/SiO,. Observed percentage of I3C was corrected for the isotopic purity of the 13C0 used. 1 1 I I I I !Y 100 90 80 70 60 0 FIG. 9.-13C contents in the products of hydrogenation over a surface carrying 12CO(ads) and deposited 13C (by the Boudouard reaction at 573 K).Hydrogenation temperature = 368 K; A, methane; 0, ethane; 0, propane. Initial state: deposited 13C = 0.6 cm3 (s.t.p.) g-l, 12CO(ads) = 2.7 cm3 (s.t.p.) g-l, p(H,) = 6.7 x 10' Pa, over Ru/SiO,. deposited carbon left on the surface (70) CHANGES IN SURFACE HYDROCARBON O N EVACUATION After the accumulation of hydrocarbon species on Ru/SiO, catalyst by the reaction of CO + H, at 423 K, the system was evacuated (10 Pa) and any changes in the system followed, analysing all the desorbed materials. An infrared studylg has shown that ca. 30% of the absorption bands of the accumulated hydrocarbon species disappear on evacuation at 423 K. The desorbed material on such an evacuation contains almost0 W 0 TABLE 3.-DESORBED SPECIES FROM Ru/SiO, AFTER THE CO+H, REACTION (423 K) OR C S H , CO-ADSORPTION (298 K) BY EVACUATION [cm3 (s.t.p.) g-l] after CO + H, reaction at 423 Kb after CO-H, co-adsorption at 298 KC evacuation temperaturea H2 co CH4 c; H,O CO, H2 co CH4 c: H,O CO, 298-423 0.38 0.55 0.00 0.003 0.05 0.00 0.07 0.56 0.00 0.000 0.05 0.01 423-473 0.27 0.21 0.00 0.003 0.04 0.04 0.03 0.15 0.00 0.000 0.02 0.14 473-523 0.14 0.12 0.00 0.000 0.02 0.18 0.05 0.05 0.00 0.000 0.01 0.24 52 3- 5 7 3 0.09 0.00 0.00 0.000 0.01 0.10 0.05 0.02 0.00 0.000 0.01 0.09 a Sequential experiments on one sample.Each evacuation was carried out for 1 h at the end of the temperature range. Before evacuation, carbon atom of C,H,(ads) = 1.5 cm3 (s.t.p.) g-l, CO(ads) = 2.7 cm3 (s.t.p.) g-l.After the reaction the system was cooled to 298 K and the gas phase was evacuated. Before evacuation, CO(ads) = 2.7 cm3 (s.t.p.) g-l, H(ads) = 0.8 cm3 (s.t.p.) g-l as atomic hydrogen. k e3 0 0 2: c P 00 w1484 co HYDROGENATION OVER RU CATALYSTS no hydrocarbons, in spite of the rapid decrease in infrared absorbance of surface hydrocarbons, as is shown in table 3. On the other hand, only a small amount of hydrogen was desorbed at reaction temperatures from the surface where CO and H, were co-adsorbed at room temperature (right-hand side of table 3) where no surface hydrocarbon species was present, which suggests that most of the hydrogen evolution from the surface after the CO + H, reaction originated from the surface hydrocarbon species.These results thus indicate that the hydrocarbon species adsorbed on the catalyst surface changed into the surface species, which are not detectable by infrared spectroscopy, producing hydrogen gas. Even when the evacuation was carried out at lower pressures ( Pa), the desorption of hydrocarbon was scarcely detected, indicating no effect from decreased pressure on desorption. TABLE 4.-cOMPARISON OF THE AMOUNTS OF SURFACE SPECIES PRESENT BEFORE AND AFTER THE DESORPTION EXPERIMENT ON A SURFACE WHERE l3C SURFACE HYDROCARBONS HAVE BEEN ACCUMULATED AND 12CO(ads) IS PRESENT amounts of is0 t ope /cm3 (s.t.p.) g-l 12C 13C befcre 2.3 1.3 after 1.9 1.3 evacuationa evacuationb a Determined by material balance (the difference between the total amount of reactant introduced into the closed-circulation system and that of the circulating reactants during the reaction).Determined by hydrogenation of the surface after the evacuation. Further evidence for the surface hydrocarbon not desorbing on evacuation was obtained by the following experiment using carbon-1 3. 13C surface hydrocarbon was accumulated from the 13C0 + H, reaction, and then 13C0 was replaced by l2C0 and the system was evacuated to Pa at 423 K. The adsorbed species remaining on the surface after this evacuation were determined by subsequent hydrogenation at 573 K, and the amount of carbon isotope in the products was compared with the total amount of surface species before evacuation. Table 4 shows that the amount of 13C which originated from the surface hydrocarbon species does not change on evacuation, whereas that of 12C from the adsorbed carbon monoxide decreases.Sorrie of these surface carbon species formed by the evacuation of the accumulated hydrocarbon exhibited high reactivity for hydrogenation. When hydrogen gas was introduced into the system at room temperature after the evacuation at 423 K, a small amount of CH, [O. 15 cm3 (s.t.p.) g-'1 was readily produced, which suggests that the C-C bond scission on the surface hydrocarbon species to form monocarbon species proceeds by evacuation. Such methane production, however, was completely prevented by saturated adsorption of CO at room temperature, which indicates that some sites free from adsorbed CO are necessary for the hydrogenation of active carbon.ADDITION OF OLEFINS TO THE CO+H, REACTION 12C olefins were incorporated into the 13C0 + H, reaction over Ru/SiO, at 423 K and the isotopic distribution of carbon in the hydrocarbon products was determinedKOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 1485 by mass spectrometry. Most of the olefin added into the reaction was hydrogenated to the corresponding paraffin, but some of the carbon atoms of the olefin additives were incorporated with the reaction products. The results with various additional olefins are summarized in table 5 . The isotopic distributions of carbon in the hydrocarbon products show an almost binominal distribution in each case. Fig. 10 TABLE 5.-13C CONTENT IN THE REACTION PRODUCTS BY THE ADDITION OF 12C OLEFINS TO 13CO+H2 REACTION OVER THE Ru/SiO, CATALYST AT 423 K ~ initial gas pressure (lo3 Pa) reaction isotope distribution" (%) time additive H, CO olefin /min product l3CO 13C1 13C2 13C3 13C4 13C5 etheneb 8.2 2.8 2.9 50 ethene 8.4 3.9 3.1 30 - - - (run 1) - (run 2) - - - - - - - - - - - - propeneb 8.4 3.1 5.9 30 cis- 8.4 4.4 0.9 30 - - - - - - - oct-4-ene - - - - - propane n-butane methane propane n-butane n-pentane ethane n-butane methane ethane propane n-butane 48 29 12 11 - - 6 3 1 5 9 7 6 - 5 9 4 1 - - - - 6 7 2 4 5 4 - - 7 0 1 9 7 2 2 - 5 1 1 9 6 9 8 7 82 12 6 - - - 7 5 1 9 4 1 1 - 5 6 4 4 - - - - 58 22 20 - - - 43 21 20 16 - - 50 18 12 11 9 - a 13Ci indicates the fraction of molecules containing i 13C atoms.In this case no trap was employed during the reaction, while after the reaction time C l hydrocarbons were collected by a liquid-nitrogen cold trap for 10 min and then analysed. The rates of hydrocarbon production are given in fig.10. L n 1 2 3 4 carbon number FIG. 10.-Effect of the addition of olefins upon the rate of the CO+H, reaction over Ru/SiO, catalyst at 423 K : a, CO+H, (no olefin addition); m, CO+H,+ethene; m, Co+H,+propene. Reaction conditions are given in table 5.1486 CO HYDROGENATION OVER Ru CATALYSTS represents the change in the rate of hydrocarbon production in these experiments; this shows an increase in the rate of higher hydrocarbon production, whereas the rate of methane production decreased. DISCUSSION ADSORPTION DURING THE CO+H, REACTION OVER RUTHENIUM CATALYSTS During the CO + H, reaction molecularly adsorbed carbon monoxide, CO(ads), covers most of the catalyst surface, and it is almost the only oxygen-containing surface species.These results were demonstrated by the volumetric method and an isotope exchange reaction between CO(ads) and CO(gas), and also agreed well with infrared spectra of the adsorbed species, which exhibited no oxygen-containing surface species except CO(ads).lg The catalytic properties of the ruthenium metal surface are drastically influenced by the presence of CO(ads). For example, when ethane and deuterium were reacted on the clean Ru/SiO, catalyst at 423 K, rapid hydrogenolysis occurred with the formation of methane, but in the presence of carbon monoxide no reaction took place, and even the hydrogen exchange reaction between ethane and molecular hydrogen did not take place. Moreover, the surface concentration of hydrogen decreased in the presence of CO(ads), suggesting a decrease in the catalytic activity for hydrogenation, which may also be associated with the negative order of the overall reaction with respect to carbon monoxide pressure.Surface hydrocarbon species, C,H,(ads), accumulated in the course of the reaction in addition to CO adsorption. These surface hydrocarbon species seem to occupy only a small fraction of the metal surface, since the amount of CO(ads) remained almost unchanged even when the hydrocarbon species accumulated to an amount compatable with that of adsorbed carbon monoxide in their carbon numbers; also an infrared study has revealed that the band positions of the surface hydrocarbon species are identical to those of the liquid aliphatic hydrocarbons.lg These results suggest that most of the surface hydrocarbon species have structures similar to those of liquid aliphatic hydrocarbons.The nature of the surface hydrocarbon species has been examined under the CO + H, reaction conditions over ruthenium catalysts using an infrared technique. Dalla Betta and SheleP2 observed the growth of the C-H stretching band on Ru/A1,0, catalyst, and they concluded that it is due to a by-product held on the support on the basis of isotope-substitution experiments. Recently two groups also reported the accumulation of surface hydrocarbon by means of an infrared technique. Bell and Ekerdt thought that the hydrocarbon species were adsorbed on the ruthenium metal of Ru/Si0,,13 whereas King considers these species to be reaction products which are held on supports such as silica and a1~mina.l~ In our experiments we have carefully examined the situation and conclude that these species stay on the metal surface, on the basis of the following reasons: (1) The hydrocarbon species are accumulated during the course of the CO+H, reaction not only on Ru/SiO,, but also on ruthenium black in a similar manner although no support was employed.(2) The kinetics of the formation of the accumulated hydrocarbon species are different from those of the overall reaction, having positive order with respect to CO pressure for the former, but negative order for the latter, which suggests that the surface hydrocarbons are not part of the reaction products of the overall reaction.(3) On evacuating the reaction system, considerable amounts of CO and hydrogen were desorbed, whereas only negligible amounts of hydrocarbonKOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 1487 species were detected in the desorbed species. However, in the hydrogenation of the surface hydrocarbon species, they all reacted readily to form mainly methane, which demonstrates a high reactivity of the surface hydrocarbons in the absence of ambient CO gas. The surface hydrocarbon species are presumably bonded to surface metal atoms not through a weak single bond but through a stronger one, such as M=C or M-C; this is suggested by the behaviour of the hydrocarbon chain on various treatments, such as evacuation. REACTIVITY OF SURFACE SPECIES D U R I N G THE CO-kH, REACTION It has been repeatedly reported that the deposited carbon formed by the Boudouard reaction has a higher reactivity to hydr~genation;~.~ this was confirmed in this report by using 13C as shown in fig.9. Moreover, Araki and Ponec9 and Sachtler et a l l o showed that during the CO+ H, reaction the deposited carbon can be incorporated to the produced methane; Biloen et al." showed incorporation occurring in the higher hydrocarbon products. Our results in fig. 8 also provide additional evidence that the dissociated carbon is a reaction intermediate, where all the reaction products are produced from deposited carbon; this indicates that there is a route to the formation of not only methane, but also higher hydrocarbons through the hydrogenation of surface carbidic carbon.In this case it was demonstrated that no CO insertion takes place, because if any CO insertion were to occur in forming higher hydrocarbons, the corresponding amount of 12C would be contained in the initial hydrocarbon products. The results of olefin addition also suggest chain growth by an oxygen-free C, intermediate. We conclude that the real intermediate which exists during the reaction is a small amount of C, species formed from dissociated carbon. Our reasons are as follows: (1) All the hydrocarbon products are produced from dissociated carbon, while molecularly adsorbed CO, which covers most of the surface, does not directly associate with the overall reaction. (2) All the hydrocarbon products are produced from common building blocks, which would be oxygen-free C, intermediates. (3) Most of the surface hydrocarbon species do not directly associate with the reaction.The role of the surface hydrocarbon species is considered to be that these species, having long carbon chains, do not directly associate with the overall reaction but can donate their carbon atoms to a surface C, intermediate through the breaking of their carbon bonds if any sites free of CO(ads) are available. It is well-known that ruthenium has the ability to break carbon<arbon bonds, as observed in hydrogenolysis on a bare surface.2o When the surface with accumulated surface hydrocarbon species was evacuated, infrared absorption of surface hydro- carbons decreased, but no hydrocarbons were detected in the desorbed species, whereas considerable amounts of CO and H, were found. After this evacuation, methane production was observed on the introduction of hydrogen even at room temperature, which suggests that reactive carbon is formed on evacuation by carbon-carbon bond scission in the surface hydrocarbon species.When 12C olefins were incorporated into the 13C0 + H, reaction the reaction products contained 12C, and its distribution indicated the occurrence of carbon-carbon bond breaking in olefins. These phenomena may be attributed to the enhanced ability of ruthenium metal to break carbon+arbon bonds. EFFECTS OF OLEFIN ADDITION Emmett and coworkers studied the effects of various additives on CO hydrogenation mainly over an iron catalyst using materials labelled with radioactive carbon- 14, for1488 co HYDROGENATION OVER RU CATALYSTS example a l c o h o l ~ , ~ ~ - ~ ~ , 25 ketene26 and ethene.25 In these experiments they found that most of these additives act as intiators of chain propagation. Pichler and S c h ~ l z ~ ~ reported that when 14CH2=CH-C,,H,g was incorporated in the CO + H, reaction over a cobalt catalyst high radioactivities were observed in the hydrocarbon products which have longer carbon chains than the added olefin.Simultaneously they found that smaller hydrocarbons show lower radioactivities in proportion to the number of carbon atoms. Recently Dwyer and Somorjai28 reported that the addition of ethene or propene to the CO + H, reaction over an iron single-crystal increases the selectivity towards higher hydrocarbons.Moreover, Ekerdt and Bell29 also reported (using a ruthenium catalyst) that olefin additives such as ethene or cyclohexene act as scavengers of surface intermediates without losing their original structure to form alkylated products. The results shown in table 5 clearly indicate that during the course of the reaction carbon-carbon bond scission and formation take place simultaneously, and carbon from the olefins is incorporated randomly into the reaction products according to an almost binominal distribution ; this means that the scavenger effect of olefin additives, as reported by Ekerdt and Bell,29 seems to be rather small. Besides 1-olefins, cis-oct-4-ene, which cannot supply carbon atoms without breaking both the single and double carbon-carbon bonds, can also supply its carbon atoms to the hydrocarbon products.This mechanism would be similar to that of providing carbon from accumulated surface hydrocarbon species. The results obtained by Emmett and coworkers21-26 and Pichler and Sch~lz,,~ where added olefin mainly acted as initiator of chain growth, suggest that the dominant role of the olefin is that of a scavenger of surface intermediates, as proposed by Ekerdt and Bell.29 It is also thought that the role of added olefin may be dependent upon the reaction conditions or the catalyst employed. The radioactivities of smaller hydrocarbons in the experiment of Pichler and Sch~lz,~ are consistent with our conclusion. ACTIVE SITES FOR THE CO+H, REACTION OVER Ru/Si02 The active sites for the reaction form a limited portion of the metal surface; the deposition of a small amount ofcarbon by CO disproportionation and the accumulation of surface hydrocarbon occur predominantly at the sites for weakly held carbon monoxide.19 These sites appear to be some sort of unsaturated site, such as edge or kink, since they are the sites of twin CO, or they may be sites modified by strongly adsorbed oxygen because of the high frequency of adsorbed C0.30-32 Moreover, Singh and G ~ e n g a ~ ~ found that carbon deposition is likely to occur on sites of the high-index plane of the ruthenium surface.Taylor et showed that the activity of ruthenium catalyst for methanation is increased by pretreating the catalyst with oxygen at higher temperatures. These observations also suggest that the reaction proceeds on sites limited to those on an unsaturated edge or those modified by oxygen. MECHANISM OF THE CO+H, REACTION OVER RUTHENIUM CATALYST In the discussions described above we concluded that the reaction is initiated by dissociative adsorption of CO and that the chain growth to form higher hydrocarbon products proceeds via C , intermediates formed from the dissociated carbon.The hydrogen-exchange reaction of C , intermediates would be fast; this was shown by the experiments illustrated in fig. 6. In this case the deuterium in the gas phase would be diluted by hydrogen in the surface hydrocarbon through the exchange reaction, but the products always showed isotope contents similar to those in the gas phase rather than those in the surface hydrocarbon, which indicates that the hydrogenKOBORI, YAMASAKI, NAITO, ONISHI A N D TAMARU 1489 of the C, intermediate and that in the gas phase (and probably the free hydrogen chemisorbed on the surface) may be in equilibrium.During the reaction, although the main part of the surface is covered by molecularly chemisorbed CO, some C, intermediates and also hydrogen stay on the surface, and the hydrocarbon products are formed through the reaction from these species. These C, species may come from various sources, such as carbon monoxide, surface hydrocarbon and olefin, and mix with each other to form reaction products as shown in tables 2 and 5. These results indicate that such C, intermediates have a high enough mobility over the catalyst surface to react with each other, although the sites for forming C, species by the scission of the C-C or C-0 bonds would be limited. Several proposals have been made concerning the rate-determining step of CO hydrogenation. Dalla Betta and Shelef concluded that the CO dissociation step is rate-determining, because no H,-D, kinetic isotope effect was observed in CO hydrogenation over various transition metals.35 On the other hand, Biloen et a1.l' thought that the process by which the C, intermediate forms C, species was rate-limiting, and Happel et al.36 suggested that the rate-controlling step of methanation over a nickel catalyst involves hydrogenation rather than only the formation of carbidic carbon.This problem has been examined from various viewpoints, leading to the conclusion that methane production, the hydrogenation of C, species, should be rate-determining.This is based on the following reasons: (1) The rate expression for hydrocarbon accumulation differs from that of methane production, which suggests that methane production from C, species is strongly suppressed by the presence of gaseous CO. The kinetics of the overall rate of the reaction suggest the hydrogenation of C! species to be rate-determining. (2) The H,-D, kinetic isotope effect in CO hydrogenation, as well as the hydrogenation of deposited carbon to form methane, have been examined over a Ru/SiO, catalyst.37 An inverse kinetic isotope effect was observed, which can be interpreted by assuming the surface concentration of CD, to be larger than that of CH,, because of the higher thermodynamic stability of CD,.The rate-determining step for producing higher hydrocarbons would be the formation of higher hydrocarbon species, such as CH,(ads)+CH,(ads)+ C,H,(ads) as proposed by Biloen et al." CONCLUSIONS The mechanism of CO + H, reaction over ruthenium catalysts was studied which lead to the following conclusions : (1) All the hydrocarbon products are produced from surface carbon formed by dissociative adsorption of CO, but the active sites are limited to a small part of the metal surface, particularly in the presence of chemisorbed CO which covers most of the surface, while the mobility of the C, species on the catalyst surface is very high. (2) During the CO + H, reaction surface hydrocarbon species having long straight chains are accumulated, and the role of these species in the overall reaction is as a reservoir of carbon atoms which may be supplied as C , species through the scission of their carbon-carbon bonds.(3) A small number of C, intermediates is present on the catalyst surface during the CO+H, reaction, and the rate-determining step would be the conversion of C, intermediates to the reaction products.1490 co HYDROGENATION OVER RU CATALYSTS This work was supported by a grant from the Japenese Ministry of Education, for which the authors are grateful. F. Fischer and H. Tropsch, Brennst. Chem., 1926, 7, 97. J. T. Kummer, T. W. De Witt and P. H. Emmett, J. Am. Chem. SOC., 1948, 70, 3632. H. H. Storch, N. Golumbic and R. B. Anderson, The Fischer-Tropsch and Related Synthesis (Wiley, New York, 1951). G. Blyholder and L. D. Neff, J. Phys. Chem., 1962, 66, 1664. P. R. Wentrcek, B. J. Wood and H. Wise, J. Catal., 1976, 43, 363. J. A. Rabo, A. P. Risch and M. L. Poutsma, J. Catal., 1978, 53, 295. G. G. Low and A. T. Bell, J. Catal., 1979, 57, 397. J. G. McCarty and H. Wise, J. Catal., 1979, 57, 406. M. Araki and V. Ponec, J. Catal., 1976, 44, 439. lo J. W. A. Sachtler, J. M. Kool and V. Ponec, J. Catal., 1979, 56, 284. l1 P. Biloen, J. N. Helle and W. M. H. Sachtler, J. Catal., 1979, 58, 95. l 2 R. A. Dalla Betta and M. Shelef, J. Catal., 1977, 48, 11 1. l 3 J. G. Ekerdt and A. T. Bell, J. Catal., 1979, 58, 170. l4 D. L. King, J. Catal., 1980, 61, 77. l5 K. Tamaru, Ado. Catal., 1964, 15, 64. l6 J. A. Amick, Diss. Abs., 1955, 15, 339. l7 R. A. Dalla Betta, J. Catal., 1974, 34, 57. l8 M. A. Vannice, J. Catal., 1975, 37, 449. l9 H. Yamasaki, Y. Kobori, S. Naito, T. Onishi and K. Tamaru, J. Chem. SOC., Faraday Trans. 1, 1981, 2o J. H. Sinfelt, Adv. Catal., 1973, 23, 91. 21 J. T. Kummer, H. H. Podgurski, W. B. Spencer and P. H. Emmett, J. Am. Chem. SOC., 1951,73,564. 22 J. T. Kummer and P. H. Emmett, J. Am. Chem. SOC., 1953, 75, 5177. 23 W. K. Hall, R. J. Kokes and P. H. Emmett, J. Am. Chem. SOC., 1957, 79, 2983. 24 R. J. Kokes, W. K. Hall and P. H. Emmett, J. Am. Chem. Soc., 1957, 79, 2989. 25 W. K. Hall, R. J. Kokes and P. H. Emmett, J. Am. Chem. SOC., 1960, 82, 1027. 26 G. Blyholder and P. H. Emmett, J. Phys. Chem., 1959, 63, 962. 27 H. Pichler and H. Schulz, Chem. Zng. Tech., 1970, 42, 1162. 28 D. J. Dwyer and G. A. Somorjai, J. Catal., 1979, 56, 249. 28 J. G. Ekerdt and A. T. Bell, J. Catal., 1980, 62, 19. 30 R. A. Dalla Betta, J. Phys. Chem., 1975, 79, 2519. 31 M. F. Brown and D. Gonzalez, J. Phys. Chem., 1976, 80, 1731. 32 A. A. Davydov and A. T. Bell, J. Catal., 1977, 49, 332. 33 K. J. Singh and H. E. Grenga, J . Catal., 1977, 47, 328. 34 K. C. Taylor, R. M. Sinkevitch and R. L. Klimisch, J. Catal., 1974, 35, 34. 35 R. A. Dalla Betta and M. Shelef, J. Catal., 1977, 49, 383. 36 J. Happel, I. Suzuki, P. Kokayeff and V. Fthenakis, J. Catal., 1980, 65, 59. 37 Y. Kobori, S. Naito, T. Onishi and K. Tamaru, J. Chem. SOC., Chem. Commun., 1981, 92. in press. (PAPER 1 /856)

ISSN:0300-9599    

DOI:10.1039/F19827801473

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1491-1497 Capillary Phenomena Part 17.-Properties of Fluid Rings Between a Sphere Above a Horizontal Plane in a Gravitational Field BY ERNEST A. BOUCHER* AND TIMOTHY G. J. JONES School of Molecular Sciences, University of Sussex, Brighton BNl 9QJ Received 1st June, 1981 The prediction of liquid ring properties for the sphere/plane system has been approached by obtaining accurate fluid/fluid meridian curves and associated quantities, e.g. capillary pressure differences, fluid ring heights and volumes, and by using approximate expressions for limiting configurations. Reduced quantities, which can be related to terrestrial gravity conditions, are compared with some zero-gravity properties. Provided the horizontal solid is of large extent, fluid rings in a gravitational field (but not at zero g ) , will reach a maximum height, unless they can completely engulf the sphere.It is not possible for the sphere to be completely enveloped if its radius is r 2 0.54[2y(1 -cos8)/Apg]i. For contact angles 8 > Oo, the ring height tends to a limit as its volume increases, having passed through the height maximum. It has become common to call an amount of liquid between two touching solids a pendular ring when the meridian describing the fluid body is axially symmetric,l and to use the term fluid bridge if the solids are held apart.2 In this paper fluid constrained by a sphere resting on a horizontal solid plane is examined, extending the range of capillary phenomena for macroscopic systems of the type recently re~iewed.~ Cross and Picknett4 examined the case of a liquid ring with particular reference to the physicochemical equilibrium between water and air. Mason and Clark5 and Clark et a1.6 have examined properties of zero-gravity bridges between two spheres and a sphere and a plane.Orr et aZ.l examined rings, mostly for zero-gravity, or more strictly zero Bond number ( B = gPAp/ y, where I is a characteristic length for the system), and gave many analytical solutions. They also made some estimates of gravitational effects. Iinoya and Muramoto7 examined liquid rings between two touching spheres and between a sphere touching a cone, using 8 = 0' and arcs of circles as approximate meridians. In this paper the emphasis is on the Laplace equilibrium of systems in a gravitational field, but the differential equations whose numerical solutions give the fluid/fluid meridian curves are given in a form which allows the gravitational field strength to be varied.Approximations to the behaviour of fluid rings are also given. The fluid forming the ring is always more dense than the second fluid. THEORETICAL BACKGROUND Fig. 1 shows the geometrical quantities used to describe the generating circle, horizontal line and fluid ring meridian curve which give the three-dimensional system by rotation about the vertical Z-axis. For identical contact angles 8 at the plane and the sphere, the meridional angles to the horizontal are, respectively, $l = 1800-8, 42 = e+v (1) 1491I492 PLANE-SPHERE FLUID RINGS and the three-phase confluence (Xe, Z*) is related to ry by Computation is conveniently started at ( X e , 20) using ry as the parameter which specifies growth as the ring volume Va is increased.For computation 2 can be used as the independent variable by artificially dealing with the mirror image of fig. 1 for which d Z > 0: the arc length S was used as independent variable for configurations encountering d Z = 0 [see ref. (2) and (3) for details of computational methods]. The pressure difference across the interface at height Z is given in terms of the shape factor H , where < = 1 corresponds to terrestrial gravity and 4' = 0 to zero gravity. Xe=Rsinry, Z e = R(1-cosry). (2) AP = 2(H-[Z) (3) 0 S X' xo x FIG. 1.-Coordinate system representing a pendular ring of fluid between a plane and a sphere.NUMERICAL RESULTS The systems studied are for equal contact angles of 8 = Oo, 30°, 60°, 90' and 180' at the two contacts with reduced sphere radius R = 0.5. The ring phase a is always more dense than phase j?. When 8 = 0' the half-angle ry increases to an upper bound (denoted ymax in table 1) as the ring volume Ya increases as shown in fig. 2. The sphere will not, therefore, be enveloped by phase a. In the limit as the contact radius X' with the plane increases the pressure difference across the interface is 2H --+ 0 as Xo + a, and the meridian becomes that of a holm (e.g. the fluid body formed by dipping a rod into an infinite For all the cases 8 > O', as Ya increases ry increases to a maximum, rymax, and then decreases to a limit, ry, corresponding to the infinitely large ring indicated in fig.3. Values of rymax and woo are given in table 1. The attainment of a maximum pendular ring height, Zgax, is analogous to the existence of sessile drop height maxima (and subsequent limiting heights for large drops) which were recently discussed.8 A critical reduced sphere radius is being anticipated (see below), above which envelopment by phase a cannot occur, i.e. larger spheres will always have a dry patch provided the liquid has sufficient space to spread sideways. In contrast, Orr et a1.l pool).E. A. BOUCHER AND T. G. J. JONES 1493 TABLE ACCURATE AND APPROXIMATE ANGLES v03 AND ymax AND SHAPE FACTORS H CORRESPONDING TO THE LATTER FOR R = 0.5 ~~ ~ ~ ~~ ~~ - Wv,/" WrnaJ" H 01" acc . approx. acc. approx.acc. approx. 0 - - 99.00 98.93 0 0 30 110.0 109.6 1 1 1.28 111.45 0.398 0.395 60 117.0 117.0 120.32 120.92 0.783 0.764 90 122.5 122.3 128.33 128.40 1 .085 1.080 180 113.0 112.6 124.62 125.20 1.517 1.527 0 0.5 1 .o 1.5 2.0 X FIG. 2.-Pendular ring configurations for R = 0.5, 8 = 0' and several values of w : (a) 30°, (b) 60°, (c) 90' and (d) vmax = 99.0'. In the limit the large ring adopts the holm configuration. X 0 1.0 2 .o 3 .O L.0 X FIG. 3.-Pendular rings for R = 0.S and 8 = 30' having values of v / : (a) 60°, (b) 90°, (c) 110'; (d) 1 1 1.28' and (e) tyco = 110'. Curve (d) represents the maximum height.1494 PLANE-SPHERE FLUID RINGS found that for a reduced system under zero-gravity conditions the properties did not depend on reduced sphere radius [fig.3 of ref. (1) for 6 = 40'1. The approximate values of y/, in table I , which are in excellent agreement with accurate values, were obtained as follows. It is assumed that for the infinitely large ring, the lower portion is part of a large sessile drop8 and the upper part is a portion of a holm,g then, approximately (4) KO( 2/ 2 R sin v [ 1 + cos (8 + I,Y,)]~ R(1 -cosy/,) = (1 -cosO)Q K l ( d 2 Rsin w,) with the + sign for 42 < 180° (raised holm) and the - sign for 42 > 180' (submerged holm). In eqn (4), KO and K, are modified Bessel functions of the second kind. A first-integral approximation for holm meridians can be adapted to give the limiting ring height, Zfitax, when 0 = 0'. In the resulting expression Ko(2/2 R sin y / m a x ) / K l ( d 2 R sin u/ma,> = R(1 -COS y/max)/(I +COS vmax)' ( 5 ) vmax is found by trial-and-error solution for a given R value, and Zgax is given by The analogous approximate height can be obtained for 0 > 0' by combining holm and sessile drop first integrals.It is assumed that the sessile drop height maximum occurs in the vicinity where y/ is a maximum, and that there is negligible pressure difference across the interface where the meridian has an inflexion (fig. 3). The com bined approxima tion eqn (2). is used as before to give tymaX for use in eqn (2). The shape factor, H , for the approximation, being one-half of the pressure difference across the interface where it meets the plane solid, is given by the maximum sessile drop height. Accurate and approximate values of vmax are compared in table 1 for R = 0.5.Cross and Picknett4 measured what is in effect vmax as a function of actual sphere radius (their fig. 4). Eqn (5) provides justification for their expectation that y/y,ax -+ 180' as R -+ 0, and it furthermore shows that yma, -+ 0' as R --+ a. Eqn ( 5 ) can also be used to give the contact height Z* (and raised fluid volume) for 6 = 0' when a sphere is just touched against a large liquid surface so that a holm forms spontaneously. Cases of 0 > Oo are dealt with by replacing (1 +cos vmax); by [ 1 + cos(8 + ~max)]'. The upward forcef, exerted by the ring on the horizontal plane is f l = n X o sin 8 - 7 ~ ( X ' ) ~ H f, = nR sin ly sin (8+ u / ) - zR2 sin2 y(H+ R cos I,U - R). y a = f, - f, - p 7 s (7) (8) (9) and the downward force exerted on the sphere is The volume of fluid forming the ring is where Vals is the sphere segment volume (standard formula) surrounded by phase a.The pressure difference A P e across the fluid/fluid interface at the three-phase confluence is 2(H-Z*) = 2(H+ Rcos y/- R). Fig. 4 shows the dependence off, on y/ for R = 0.5 and several values of 6. For 6 = Oo, 30' and 60° the positive force decreases almost linearly as y/ increases to ymax. The limit for 8 = 60' is close to f 2 = 0. For 0 = 90' the force is always negative andE. A. BOUCHER AND T. G. J. JONES 1495 2 f2 0 - 2 - -4- FIG. 4.-Dependence on v/ of the forcef, exerted on the sphere by the presence of the pendular ring, for several values of the contact angle 6: (a) Oo, (b) 30°, (c) 60°, ( d ) 90' and (e) 180'.- 1 0 1 FIG. 5.-Dependence of the pressure difference A@ at the upper contact on ring volume f a for 6 values: (a) O', (b) 30°, (c) 60°, (d) 90" and ( e ) 180".1496 PLANE-SPHERE FLUID RINGS has little dependence on cy, whereas for 0 = 180' the negative force passes through a minimum and then increases, The dependence of AP@ on Va in fig 5 shows quite a different pattern of behaviour. The curve for 0 = 0' begins at A p e = - co and, while remaining negative, terminates at Va = 1.7 when the holm meridian is reached. The curve for 0 = 30'is similar in shape, but it has a very broad maximum before levelling off as Va -+ 00. The curve for 0 = 60' has a distinct maximum at A p e N + 1.5. In contrast, the curves for 0 = 90' and 180' begin at A p e = + 00 when Va = 0 and decrease smoothly to positive A p e limits as Va + GO.By taking the meridian to be an arc of a circle for small Va it can be shown that the initial portions of the curves of the dependence of A P e on Va obey (10) sin 8 cos cy R sin cy cos 0 cos O( 1 + cos v / ) + R - R(1 -cos v / ) A p e = which gives the correct limits as Va -+ 0. Fluid rings of small volume correspond to capillary condensation at relative pressures p/p' of vapour less than unity (saturation) when 0 is small or zero. The difference in some properties between rings in a gravitational field and those where g = 0 ([ = 0) is shown by the comparison made in table 2 with the data of Orr et a1.I TABLE 2.-cOMPARISON OF REDUCED QUANTITIES WITH c = 1 AND c = 0 FOR R = 0.5 AND e = 00 30 0.005" 0.005b 60 0.051 0.073 90 0.561 0.449 99 1 .652" - 140 1 82.75b 0.270 0.270 0.6 16 0.609 1.48 1 1.242 - 33.604 - 12.182 - 12.21 5 - 1.929 - 2.045 -0.187 -0.366 0 - - 0.0002 2.798 2.791 2.462 2.383 2.110 1.875 2.022 - 0.649 a First entry for c = 1, second entry for gravity-free, [ = 0, from Orr et a1.l DISCUSSION The limiting height of the large rings is well described in terms of the approximations involving Bessel functions.The maximum height of a sessile drop grown on a plane surfaces can be used to estimate whether a ring can completely envelop a sphere. It is supposed that the fluid will only completely surround the sphere if the sessile drop of volume Va has a height exceeding 2R, which to a good approximation means that envelopment will only be possible if (1 1) R < 0.54(1 -coSe)B.When R = 0.5, 0 must exceed ca. 82" before the largest ring could become a sessile drop with the sphere completely contained inside it. The possibility of an energyE. A. BOUCHER AND T. G. J. JONES 1497 barrier preventing spontaneous envelopment of this kind has yet to be investigated. The inequality eqn (1 1) implies that for R 2 0.77 a sessile drop can never develop, regardless of 8, from a ring, because the most favourable condition is 8 = 180'. For R < 0.5 the contact angle 0 required to prevent eventual engulfment decreases to the limit 8 = 0" for R = 0, i.e. no sphere of finite radius will be engulfed when 8 = 0'. When Va is small (small ty) it can be shown that the meridian becomes an arc of a circle giving the limit according to eqn (8), fz = 2ncos 8 as Va --+ 0; a conclusion reached without proof by McFarlane and Tabor,1° Cross and Picknett4 and Princen.'l Comparison can be made between the case ([ = 1) 8 = 0" and R = 0.5 and the gravity-free (c = 0) data of Orr et aZ.l.When c = 1 the limiting configuration is that of a holm of finite volume for which ty,,, N 99', whereas for [ = 0 the limiting meridian is a catenary with ly,,, N 140' and Va -+ GO. For rings having volumes such that t,u 5 30" there are no major differences in the meridian curves for the two cases and little difference in quantities such asfz, Va, AP* and X'. As ly increases some differences occur, most notably in A P and Va: for [ = 0, A P e -+ 0 and nlra --+ co, whereas for c = 1, Va tends to a finite limit and A P + - 228. There is an interesting similarity with a pendent drop grown at the end of a tip, which shows a maximum in A P and a limit in Va (detachment), but in the absence of gravity the drop can be grown indefinitely, although A P e still possesses a maximum. F. M. Orr, L. E. Scriven and A. P. Rivas, J. Fluid Mech., 1975, 67, 723. E. A. Boucher and M. J. B. Evans, J. Colloid Interface Sci., 1980, 75, 409. E. A. Boucher, Rep. Prog. Phys., 1980, 43, 497. N. L. Cross and R. G. Picknett, Trans. Faraday SOC., 1963, 59, 846. G. Mason and W. C. Clark, Chem. Eng. Sci., 1965, 20, 859. W. C. Clark, J. M. Haynes and G. Mason, Chem. Eng. Sci., 1968, 23, 810. E. A. Boucher and T. G. J. Jones, J. Chem. Soc., Furaday Trans. I , 1981, 77, 1183. E. A. Boucher and T. G. J. Jones, J. Chem. Soc., Faraday Trans. I , 1980, 76, 1419. ' K. Iionya and H. Muramoto, Zairyo, 1967, 16, 70. lo J. S. McFarlane and D. Tabor, Proc. R . SOC. London, Ser. A , 1950, 202, 224. l 1 H. M. Pnncen, in Surface and Colloid Science, ed. E. Matijevic (Wiley, New York, 1969), vol. 2, p. 1. (PAPER 1 /882) 49 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19827801491

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1982, 78, 1499-1506 Capillary Phenomena Part 18.-Conditions for the Flotation of Solid Spheres at Liquid/Liquid and Liquid/Vapour Interfaces in a Gravitational Field BY ERNEST A. BOUCHER* AND TIMOTHY G. J. JONES School of Molecular Sciences, University of Sussex, Brighton BN1 9QJ Received 19th June, 1981 Conditions for unaided sphere flotation at fluid/fluid interfaces are explicitly given in reduced terms using density parameters ka = @"-p")/Ap and kb = CpS-&/Ap, where Ap is the positive density difference between fluids a and b. The method given entails the minimum of numerical computation needed to give the limits for stable flotation: at the limits the sphere detaches and either rises or sinks. Representative accurate data illustrate the range of flotation depending on the contact angle and the sphere radius. Two approximations are also discussed.Previous studies of single solid spheres at fluid/fluid interfaces can be divided into those where the sphere is manipulated by an externally applied f ~ r c e l - ~ and where the sphere can float ~naided.~-lO The unaided flotation of a sphere at a fluid/fluid interface depends on its radius Y, the contact angle 0 (measured through the denser fluid phase), the interfacial tension yap and the densities, pa, pp and ps. The study by Boucher and Kent4? l1 of the equilibrium and stability of spheres mechanically manipu- lated at fluid interfaces showed how these quantities can be used to predict the conditions for stable flotation. Sphere flotation was shown to be a special case of an ideal stress-controlling ~ y s t e m , ~ where equilibrium is stable only between the maximum and minimum values of the applied force.The shapes and related properties of the asymptotic fluid/fluid interfaces meeting the required boundary conditions at the sphere are generated by the accurate numerical integration of the Pascal-Laplace equation.12 A large range of values of the reduced sphere radius R is examined for contact angles in the range 0-180" to determine the limiting values of the sphere density for which stable flotation can occur. Approximate solutions to the problem of sphere flotation are also discussed. THEORETICAL BACKGROUND Fig. 1 shows two different holm-sphere systems which are mirrored in the horizontal plane 2 = 0.The criteria for the stable flotation of the lower system where 0 > 90' may be directly obtained from the criteria for the upper system where 0 < 90". The contact angles of the two systems are 0 and O', where 0' is used to denote the complement of the angle 0. The density ps of the sphere is expressed as a reduced density difference4 ka = @'-p*)/Ap; kp = @'-@)/Ap (1) where Ap is the positive density difference between the two fluid phases and a is always the phase which forms the holm. Reduced quantities of dimension Lv are given by dividing the actual dimension l v by av where a2 = 2yaB/Apg. 49-2 14991500 SPHERE FLOTATION 7 V \ P FIG. 1 .-Coordinate systems for (a) raised and (b) submerged spheres giving mirror-image holm configurations. The reduced external absolute force which must be applied to the sphere to maintain it in an arbitrary position is f:?: = ~Xesind,+n(Xe)~ Z*+ kaP,S+kp5/8$S (2) where ( X e , Z*) are the coordinates of the three-phase confluence, 4 is the meridian angle at the three-phase confluence, and V and F-@3 are the volumes of the sphere in phases a and p, respectively.[The meridian angle is generally @ = arctan (dZ/dX).] The first two terms on the right-hand side of eqn (2) represent the force exerted by the fluid/fluid interface on the sphere. The first term is the force due to the vertically resolved component of the interfacial tension, and the second term is the force due to the pressure difference (Young-Laplace) across the curved interface. The two terms give the holm v01ume,~g~~ including any solid located between 0 and Z e , P = n X 0 sin d, + Z e .(3) Substitution of eqn (3) into eqn (2), noting that v7 = YS - Va, where YS is the sphere volume, gives for a raised-holm configuration f:?; = V"+kBVS- Va*s (4) with an analogous equation for submerged holms. An excess applied forcef:;; is defined as the applied force on the sphere less that, kpVs, on the sphere entirely immersed in the less dense p phase ( 5 ) fext = va- y a , s * Z" =Z0+Rcost# ( 6 ) exc The position 2" of the centre of the sphere from the level 2 = 0 is where w( = 4 - 0) is the angle made between the three-phase confluence and the axis of rotational symmetry; y~ is always measured through the denser phase. The contact radius X* is related to the angle v/ by X e = R sin t# (7)E.A. BOUCHER AND T. G. J. JONES 1501 and the volume Va,s of solid surrounded by the a phase is It can be seen from the above equations that when R and 8 are chosen for a particular system, the holm-sphere configurations depend only on the angle ly. The only unknown is the value of 20, which must generally be obtained from numerical computation. For each pair of 8 and R values and a series of values of tp the corresponding 20 values are found from the holms which satisfy the conditions X = Rsinly and (b = 6+y. FIG. 2.-Dependence of absolute externally applied force on sphere-centre position for R = 0.5,O = 0' and various kfi values: (a) 5 , (b) 0, ( c ) - 3, (d) - 8. Fig. 2 shows the dependence off.$ on 2" for R = 0.5 and 8 = 0' and several values of kb including kb = 0.The shapes of the curves are all identical and independent of the values of kb, which only determine the positions of the curves relative t0f.g; = 0. The shape of the force-displacement curves, and hence the values of Z x wheref:;; is a maximum or minimum, depends only on the values of R and 8. The force- displacement behaviour for any sphere system may, therefore, be characterized by the excess force given by eqn (9, which can be considered as the absolute force when kb = 0. Consequently it is only necessary to carry out numerical computation for this special case. For the raised holm configuration shown in fig. 1 where the contact angle is 8 while for the submerged holmI502 SPHERE FLOTATION The addition of eqn (9) and (10) for the two systems characterized by the angles v/ and t,v' gives (1 1) with Va(0) = - Va(8').Fig. 3 showsf::: as a function of 2" for spheres of reduced radius R = 0.5 and 8 = 40 and 140'. The value of 2" at the force maximum for 8 = 40' is equal to -2" at the force minimum for 8 = 140'. Similarly, 2" at the force minimum for 6 = 40' f:; (8) +f::; (8') = - v s FIG. 3.-Dependence of externally applied force on sphere centre position ( R = 0.5), showing how systems with contact angles 8 and 180'-8 are related: (a) 6 = 40, (b) 8 = 140'. is equal to - 2" at the force maximum for 8 = 140'. The force-displacement curve for 6 = 140' can be obtained from the curve for 8 = 40' by transforming Z x (8) to - 2 (8') and using eqn (1 1) to transform f: (8) to f:;; (6). The essential condition for the unaided flotation of any sphere is that the net force on the sphere is zero,fl$t = 0.It has been shown4 that stable sphere flotation can only occur for holm-sphere configurations where df,e,"dZX > 0, and the onset of instability occurs when df,e,"dZX = 0. Fig. 2 shows that for a given system defined by a pair of R and 8 values, there exists a range of ki values (i = a or /?) for which flgi = 0. The upper value of ki, ka (upper), corresponds tof:;; (min) = 0, where the holm is submerged and ps > fl > pa. From eqn (4) with f;;; (min) = 0 ka (upper) = - f:$ (min)/ VS. (12) When ka > ka(upper) the sphere will detach from the interface and sink into the denser phase. The lower value of ki, kp (lower), occurs for the raised holm where f;$k (max) = 0 and ps < pp < pa.E.A. BOUCHER A N D T . G. J . JONES 1503 From eqn (4) withf:c:(max) = 0 kP (lower) = -f:z; (max)/ VS (13) such that when kp < kp (lower), the sphere will detach from the interface and rise into the less dense p phase. The criterion for the stable flotation of a sphere is ka (upper) 3 ki 3 kp (lower). f::; (max, O)+fz:k (min, 6') = - VS (14) (1 5 ) From eqn (1 1) it can be seen that which combined with eqn (12) and (13) gives Similarly ka (upper, 0) + kP (lower, 6') = 1. kp (lower, 0) + ka (upper, 8') = 1 and consequently the values of ka (upper) and kp (lower) for 8 > 90" can be obtained from the limiting values of ki for 8 < 90". The case of 8 = 8' = 90" is unique in that 2 Vmax) = - Z (fmin) and kP (lower) = 1 - ka (upper).NUMERICAL RESULTS FOR SPHERE FLOTATION ACCURATE PARAMETERS A range of R values from to 5.0 with 8 = 0, 20, 40, 60, 80 and 90" has been studied by accurate numerical computation. For each pair of R and 8 values, the values off:;; (max) andf:;: (min) have been computed to give kP (lower) and ka (upper) using eqn (12) and (1 3), respectively. Eqn (1 5 ) and (1 6) are used to give the limiting values of ki for the range of R values with 8 = 100, 120, 140, 160 and 180". When 6 = 0' it is found that there is no minimum inf:;;, only a lower value off: = - VS, where ly = 180' and the interface is planar. Consequently ka(upper) = I for all values of R. Similarly for 8 = 180' there is no maximum in the excess force, only an upper value off:;; = 0, where ly = 0' and hence kP (lower) = 0 for all R.Table 1 shows the values of ka (upper) for a range of R for all of the values of 0 given above. To our knowledge no data of comparable range are available in the literature. The values of k.8 (lower) can be calculated from eqn (I 6) and (1 7). The table can be considered to be a compilation of the values of -f:$(min)/ Vs, wherefrom f:; (min) and f:; (max) can be obtained. APPROXIMATE PARAMETERS James13 has given an approximate first-integral holm solution for small holm-solid systems. The approximation, which has already been critically discussed,14 gives Z e as a function of 4 and X e . Using this approximation VQ = nR sin cy sin (8+ cy)+nR3 sin2 cy sin (8+ ly) (ln[22/2/(Rsin cy(1 -cos(8+cy)))]-yy) (18) where (here) y = 0.5771 6.. . is Euler's constant. The substitution of eqn (1 8) into (5) with W S given by eqn (8) gives an expression forf:$ which depends only on cy when R and 8 have been specified. For particular values of R and 8 the values of cy which correspond to the stationary values off:;;, and hence the limiting values of ki, are found by trial and error. When R < 0.1 the agreement between the stationary values off:; calculated from1504 SPHERE FLOTATION James' approximation and from accurate computation is excellent. When R = 0.1 and 8 = 0, 20, 40, 60, 80 and 90°, values off:$(max) andf::i(min) given by accurate computation and by the approximation agree to four decimal places. The accuracy of the approximation increases as R decreases. TABLE 1 .-ACCURATE VALUES OF ka(upp~~) FOR SEVERAL CONTACT ANGLES AND SPHERE RADIIa e p R + O .O ~ 0.1 0.3 0.5 1 .o 2.0 5.0 20 40 60 80 90 100 120 140 160 180 224 879 1876 3099 3751 4402 5627 6625 7277 7503 3.26 1.25 9.80 2.00 19.9 3.16 32.3 4.62 38.9 5.41 45.6 6.21 58.1 7.74 68.4 9.0 1 75.2 9.86 77.5 10.2 1.092 1.366 1.804 2.363 2.668 2.978 3.575 4.075 4.410 4.529 I .024 1.097 1.218 1.375 1.461 1.550 1.720 1.864 1.961 1.995 1.006 1.027 1.061 1.105 1.130 1.154 I .202 1.241 1.267 1.275 1.001 1.005 1.01 1 1.019 1.024 1.028 1.036 1.043 1.048 1.049 ~ ~ a Values for R = 0.001 range from 2.4 x lo4 to 7.6 x lo5 over the range of angles. For moderately large spheres, where the holm meridians are tending to be of similar shape but of varying radius at their waist, the volume Va can be obtained from the first-integral holm approximation of Boucher and Jones.l4 The approximate expression for Va is Va = nRsiny/sin(8+y/)+zR2sin2 y/KO(1/2Rsin y/)[l +cos(8+yl)]~/Kl(1/2Rsiny/) where KO and K, are modified Bessel functions of the second kind. By a similar procedure, the stationary values off::; are obtained, and hence the limiting values of ki can be calculated. A comparison of the values off:g:(max) andf::i(min) obtained from the use of eqn (19) and from accurate computation for R = 1 .O shows that the approximation is good to better than 1 % for f::i (max), and that off:;; (min) values agree to four significant figures, except that the discrepancy is -0.002 on -6.122 for 8 = 90'. The accuracy will increase as R increases. The accurate data in table 1 can be represented by approximate empirical expression^,^^ which give explicit relations between the limiting values of ki, R and 8.The expressions for the limiting reduced density differences are (19) ka (upper) = 1 + {cos [( 180' - 8)/2])2.2 R-1.925 kp (lower) = - {cos(8/2)}2.2 R-1.925. (20) (21) and Eqn (20) and (21) rearrange to give the upper values of R for which flotation occurs for given values of ki and 8 R (upper) = (ka - l)-0.519 (cos [( 1 80' - 8)/2])1.143 for p s > pb > pa (22) R (upper) = lkBl-0.51~ [cos ( 0 / 2 ) y ~ ~ for p s < pb < p a . (23) andE. A. BOUCHER AND T. G . J. JONES 1505 When R > R(upper) upper phase when ps range of 8 values for (21) when R, ka and the sphere will detach from the interface and either rise into the < pb < pa, or sink into the denser phase when ps > @ > pa.The which flotation can occur can be determined from eqn (20) and kb are known. The lower value of 0 is given by (24) cos [( 180' - 8 (lower))/2] = (ka - 1)0.455 R0.875 while the upper value is given by cos [8(upper)/2] = lkp1°.455 R0.875. When 8 > 8(upper) or 8 < 8(lower) the sphere will detach from the interface and stable flotation cannot occur. FIG. 4.-Comparison of accurate (lines) and approximate (dots) density parameter limits (see text) as a function of reduced sphere radius R for B values: (a) 180, (b) 90, (c) 40, (d) 140, (e) 0'. DISCUSSION Some of the accurate numerical data given in table 1 are shown in fig. 4, where ka (upper) and kp (lower) are plotted as a function of the reduced sphere radius R for given values of 0.The lines represent the data from accurate computation while the dots for ka (upper) and kp (lower) are given by eqn (20) and (21), respectively. The accuracy of the empirical equations has already been discussed.15 The curves in fig. 4 show that as R + 0, ka (upper) + + 00 and kp (lower) --+ - 00, indicating that spheres which are very much denser than the denser fluid phase and very much less dense than the lighter phase, respectively, can float. When R -+ 00, ka (upper) -+ 1, indicating1506 SPHERE FLOTATION that flotation can only occur by ps -+ pb, where p is the denser phase. Similarly kp (lower) -+ 0 as R -+ co and flotation is only possible as ps + pp, where p is now the less dense phase. The values that ka (upper) and kb (lower) tend to as R -+ 00 show that the surface forces are becoming insignificant in supporting the sphere.In the limit as R -+ 00, flotation can only occur when the density of the sphere is intermediate between the densities of the two fluid phases. The use of the data given in table 1 is exemplified by application to a particular system. A sphere of radius Y = 1.9 mm and density ps = 4 g ~ m - ~ is required to float at a water/air interface for which the capillary constant is a = 3.8 mm, and hence the reduced sphere radius is R = 0.5. The water/air interface makes a contact angle of 100' with the sphere. The reduced sphere density in the less dense phase is kff = 4. For a sphere with R = 0.5 and 8 = 100' table 1 shows that ka(upper) = 2.978, and hence kff > ka (upper) indicating that the sphere cannot float.For the water/air system with R = 0.5 and 8 = loo', the sphere will only float if ps d 2.978 g ~ m - ~ . The range of 8 values for floating the sphere with R = 0.5 and ka = 4 can be estimated by using eqn (24), noting that ps > pp > pa, where the water is the p phase and the air is the a phase. Eqn (24) gives 8 (lower) = 128O, and hence the sphere will only float if 8 > 128'. With 8 = 100' and ka = 4, the upper-sphere radius calculated from eqn (22) is R (upper) = 0.42 and flotation can only occur if R < 0.42 or Y d 1.6 mm. Previous studies of the flotation of spheres have in the main only considered systems of limited range and presented a small amount of numerical data, often obtained by the use of approximations.For example Tovbin et a1.' consideJed holm-sphere systems where the meridian angle <f, was everywhere close to 180°, and hence the meridian curve could be accurately represented by the (conventional) Bessel solution for shallow interfaces. The study closest to that presented here was made by Huh and Mason,s who tabulated values of what we term ka(upper) (in their table 1) from R = 0.177 to R = 1.273 for 8 = 30, 60, 90, 120, 150 and 180'. No direct comparison of the values of ka (upper) from the two studies can be made by using the two sets of published data because Huh and Mason used the capillary constant a = (y/Apg)*, and hence their sphere radii are 2/2 times bigger than the values given here. However, reasonable comparison of the two sets of data can be made using the approximate treatment: eqn (20). For example, in our nomenclature, for R = 0.707 and 8 = 90°, Huh and Mason give ka (upper) = 1.8737 while eqn (20) gives ka (upper) = 1.91. For R = 0.700 and 8 = 90' table 1 gives ka(upper) = 1.890. With R = 0.177 and 8 = 30' Huh and Mason give kff (upper) = 2.6166 while eqn (20) gives a value of 2.44. T. G. J. J. acknowledges an S.R.C. studentship. A. D. Scheludko and A. D. Nikolov, Colloid Polym. Sci., 1975, 253, 396. C. Huh and S . G. Mason, Can. J. Chem., 1976, 54, 969. C. Fieber and H. Sonntag, Colloid Polym. Sci., 1979, 257, 874. E. A. Boucher and H. J. Kent, J. ChLm. SOC., Faraday Trans. 1 , 1978, 74, 846. C . W. Nutt, Chem. Eng. Sci., 1960, 12, 133. S. Hartland and J. D. Robinson, J . Colloid Interface Sci., 1971, 35, 1971. C. Huh and S. G. Mason, J . Colloid Interface Sci., 1974, 47, 271. H. C. Maru, D. T. Wasan and R. C . Kintner, Chem. Eng. Sci., 1971, 26, 1615. ' M. V. Tovbin, I. I. Chesha and S . S . Dukhin, Colloid J. USSR, 1970, 32, 643. lo A. V. Rapachietta and A. W. Neumann, J. Colloid Interface Sci., 1977, 59, 555. l 1 E. A. Boucher and H. J. Kent, Proc. R. SOC. London, Ser. A , 1977, 356, 61. l 2 E. A. Boucher, Rep. Progr. Phys., 1980, 43, 497. l 3 D. F. James, J. Fluid Mech., 1974, 63, 657. l4 E. A. Boucher and T. G. J. Jones, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1419. l 5 E. A. Boucher and T. G. J. Jones, J. Colloid Interface Sci., 1981, 83, 645. (PAPER 1 / 1000)

ISSN:0300-9599    

DOI:10.1039/F19827801499

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1982, 78, 1507-1514 Ion-Solvent Interactions in Water-rich Binary Mixtures. Viscometric Behaviour of Sodium Salt Solutions in Water + Sulpholane Mixtures at 30, 40 and 50 O C BY ANTONIO SACCO,* GIUSEPPE PETRELLA,* ANGELO DELL'ATTI* AND ANGELO DE GIGLIO" Institute of Physical Chemistry, University of Bari, Via Amendola 173, 70126 Bari, Italy Received 911, June, 198 1 The relative viscosities of NaC1, NaBr, NaI and NaClO, have been measured at 30, 40 and 50 OC in water + sulpholane mixtures, in the water-rich region. The resulting Jones-Dole B coefficients and their dependence on temperature provide useful information as regards changes in water structure when small amount of organic cosolvent is added. The results are also discussed in terms of the transition-state treatment.Numerous studies have shown that viscosity measurements are very useful in providing information regarding ion-solvent interactions and particularly as regards the modifications induced by ions on the solvent structure. The viscometric technique has generally been used in studying the behaviour of electrolytes in pure solvents,l-10 while little research has been carried out on electrolytic solutions in mixed s o l ~ e n t s . ~ l - ~ ~ We decided to study systematically the viscometric properties of a number of electrolytes dissolved in binary water + organic-solvent mixtures in the water-rich region, since we believe such investigations can provide useful information on the effects induced by the cosolvent on the water structure. In the present study viscometric measurements are given for NaCl, NaBr, NaI and NaCIO, in water + sulpholane mixtures in mole fractions of sulpholane [X(sulpholane)] equal to 0.0208, 0.0744 and 0.1586 at 30, 40 and 50 OC.EXPERIMENTAL The apparatus and the experimental technique used for viscometric and densimetric measurements have been described el~ewhere.~~ * The estimated precision of the measured viscosity data was always better than 0.05%. NaC1, NaBr, NaI were ultrapure grade (Merck). These salts, after drying, were used without further purification. NaClO, (Fisher reagent grade) was recrystallized several times from water + methanol mixtures and dried at 150 O C in a vacuum oven for 4 days. The purification of water and sulpholane has been described previou~ly.~~ RESULTS For the calculation of the B coefficients by means of the Jones-Dole equation15 [eqn qr = 1 +AC:+BC (1) (1>1 we followed the method based on the principle of orthogonal polynomials suggested by Vincent et aZ.,16 who have studied in detail the effects on the B coefficient of 15071508 I ON-S 0 LV E N T I N T E R A C T I ON S I N BINARY MIXTURES higher-order concentration terms in ' extended ' Jones-Dole equations.We note at this point that in order to achieve the highest possible degree of uniformity among the viscometric data, it is advisable to use only one method for calculating B coefficients, and we feel that the method suggested by Vincent et al. is the most suitable for this purpose. In order to determine the B coefficients we always used the theoretical value of the A coefficients obtained by means of eqn (2)17 The conductometric data and the values of the physical properties of the solvent mixtures were taken from a previous study of ours.14 Note that the viscometric calculations were carried out at the same percentages of solvent mixtures used in measuring conductances in order to be able to use the values of limiting equivalent conductances.Moreover, since it was not possible to obtain limiting conductance values of ions at temperatures other than 30 OC, we assumed that the A coefficients did not vary significantly with temperature; therefore the same A coefficients used at 30 OC were also used at 40 and 50 OC. TABLE 1 .-THEORETICAL A COEFFICIENTS IN WATER + SULPHOLANE MIXTURES AT 30 "C A/dmt mol-4 X( sulp holane) NaCl NaBr NaI NaC10, 0.0208 0.0062 0.0062 0.0063 0.0068 0.0744 0.0062 0.0062 0.0066 0.0073 0.1586 0.0064 0.0062 0.0066 0.0073 In table 1 the A values used in the calculation are reported.The experimental results for NaCl, NaBr, NaI an NaClO, in the water+sulpholane mixtures at the three temperatures are reported in the Appendix in table 6. The calculated B coefficients at 30, 40 and 50 O C are reported in table 2. On applying transition state treatment to the relative viscosity of electrolytic an equation can be obtained from which it is possible to calculate the molar free energy of activation of the solution for viscous flow: where ApY#, the molar free energy of activation of the pure solvent, can be obtained by means of eqn (4): and and are the partial molar volumes of the solvent and solute, respectively.In order to calculate the molar volume of the solvent we treated each solvent mixture as a pure material, and thus obtained by means of eqn ( 5 ) :A. SACCO, G. PETRELLA, A. DELL'ATTI AND A. DE GIGLIO 1509 The values of the partial molar volumes of the electrolyte, K O , in solvent mixtures at different temperatures were calculated by means of precise density measurements.* Moreover, by measuring the B coefficients at different temperatures the enthalpies and entropies of activation can be obtained by means of the equations The values of the activation parameters of the solveni tables 3 and 4, respectively. L (6) and solutes are reported in TABLE 2.-vISCOSITY B/dm3 m0l-l COEFFICIENTS IN WATER -t SULPHOLANE MIXTURES AT 30, 40 AND 50 OC 30 "C 40 OC 50 "C NaCl NaBr NaI NaC10, NaCl NaBr NaI NaClO, NaCl NaBr NaI NaCIO, NaCl NaBr NaI NaCIO, 0.088 & 0.001 0.058 fr 0.001 0.014 f 0.001 0.022+0.001 0.1 05 f 0.00 1 0.076 f 0.001 0.029 f 0.001 0.041 f 0.002 0.1 77 f 0.00 1 0.152 f 0.002 0.113f0.001 0.107 f 0.002 0.285 f 0.001 0.253 & 0.001 0.205 f 0.002 0.186 +_ 0.003 X(sulpho1ane) = 0" 0.099 fr 0.002 0.069 f 0.001 0.033 f 0.001 0.044 0.001 0.1 11 fO.OO1 0.094 fr 0.001 0.048 f 0.00 1 0.055 fr 0.00 1 0.167 f 0.002 0.162 & 0.002 0.I28 f 0.002 0.1 11 f 0.002 0.276 f 0.001 0.253 f 0.003 0.209 f 0.002 0.186 fr 0.002 X(sulpho1ane) = 0.0208 X(sulpho1ane) = 0.0744 X(sulpho1ane) = 0.1586 0.109 f 0.002 0.084 f 0.002 0.048 & 0.001 0.060 f 0.001 0.121 fO.001 0.107f0.002 0.060 f 0.001 0.063 f 0.001 0.162 f 0.002 0.168 fO.OO1 0.139 & 0.002 0.126 + 0.002 0.267 f 0.00 1 0.250 f 0.003 0.2 15 k 0.003 0.1 80 k 0.00 1 a From ref.(8). TABLE 3.-sOLVENT ACTIVATION PARAMETERS AH,9$13 313 ASPf &4,$03 'p;,$13 A&,%3 X(sulpho1ane) /kJ mol-l /kJ mol-l /kJ mol-1 /kJ mol-l /kJ mol-l 0" 14.78 5.95 9.04 8.83 8.66 0.0208 15.21 5.79 9.62 9.42 9.25 0.0744 15.74 5.01 10.9 1 10.73 10.59 0.1586 16.38 4.07 12.46 12.31 12.20 a From ref. (8). * The values of the salts will be reported and discussed in a later paper.1510 ION-SOLVENT INTERACTIONS IN BINARY MIXTURES TABLE 4.-sOLUTE ACTIVATION PARAMETERS AT 40 * c salt X(su1p holane) 313 AS,"# /kJ mol-l NaCl 0.0208 0.0744 0.1586 NaBr 0.0208 0.0744 0.1586 NaI 0.0208 0.0744 0.1586 NaCIO, 0.0208 0.0744 0.1586 23.8 28.4 35.3 22.6 28.0 33.3 18.3 26.2 30.8 20.3 25.6 29.6 -40.7 12.5 17.2 - 70.4 - 26.6 - 7.8 - 64.2 - 50.1 - 23.5 - 56.4 -42.3 0.3 - 16.9 40.9 52.5 1.4 25.5 - 45.9 - 23.9 7.3 - 36.1 - 16.7 -47.8 29.9 DISCUSSION The behaviour of electrolytes in water + sulpholane mixtures in water-rich regions can first be analysed by observing their B and dB/dT coefficients.In pure water it is well-known that the Na+ ion has a low B value;' moreover, we have recently showns that its differential dB/dT, while not being virtually zero as had been thought,l is only slightly influenced by a variation in temperature between 30 and 50 OC. Therefore we can state that the noteworthy positive values shown by dB/dT for NaCl, NaBr, NaI and NaClO, salts in pure water are due exclusively to the anion. For this reason all the anions that we have taken into consideration are defined as 'sts-ucture breakers' in pure water.On the other hand, the behaviour of the Na+ ion and of C1-, Br-, I- and C10, ions in pure sulpholane is completely In fact, the high value of the B coefficient found for Na+ ion, with dB/dT< 0, shows that it is strongly solvated in sulpholane and behaves as a 'structure maker'. In contrast in the case of anions, the low B coefficient values, with dB/dT z 0, show that they interact weakly with the solvent molecules. On the basis of the behaviour of the ions in water and in sulpholane, the noticeable increase in B observed for all salts as the percentage of sulpholane is increased in the mixtures (table 2) can be explained by supposing that interactions between the Na+-sulpholane molecules become more and more important.Let us now consider the dependence of B on temperature (table 5). Up to mole fraction 0.0208 all salts show a positive dB/dT value, and the same applies in pure water, although the values are smaller. When the mole fraction rises to 0.0744, while NaCl shows a small negative dB/dT value, the dB/dT values for the other salts are still positive, although smaller when compared with the values at X(sulpho1ane) = 0.0208. Lastly, at X(sulpho1ane) = 0.1586, while dB/dTfor NaCl is appreciably less than zero, NaBr and NaClO, have dB/dT z 0, and only NaI still shows a low positive value of dB/dT. This behaviour can be explained if one bears in mind that the structure- breaking ability of the anions generally increases with increasing size of the ion, and if one supposes that sulpholane acts as a breaker of water structure.Thus as the percentage of sulpholane in the solvent increases, the anions become less effective structure breakers. It can thus be understood how the dB/dT coefficients, which areA. SACCO, G. PETRELLA, A. DELL'ATTI AND A. DE GIGLIO 151 1 positive for all the salts in pure water and X(sulpho1ane) = 0.0208, change their sign at higher mole fractions of sulpholane and how this happens for the more able structure-breaking anions only in mixtures richer in sulpholane. The hypothesis that sulpholane has hydrophilic properties is also proved by the results of studies on water + sulpholane mixtures carried out using various experimental methods.18-21 The analysis of the solute activation parameters reported in table 4 shows that in the region 0 < X(sulpho1ane) < 0.0208 both AH:# and AS:# are negative for all TABLE 5.--dB/dT COEFFICIENTS IN WATER + SULPHOLANE MIXTURES IN THE RANGE 30-50 "C (dB/dT)/dm3 mol-l K-' X(sulpho1ane) NaCl NaBr NaI NaClO, 0" 0.001 05 0.001 30 0.001 70 0.001 90 0.0208 0.000 80 0.001 55 0.001 55 0.001 10 0.000 75 0.000 80 0.001 30 0.000 95 0.0744 0.1586 -0.000 90 -0.000 15 0.000 50 -0.000 30 a From ref.(8). salts; this fact indicates that, in this range, the average transition state is associated with bond making and an increase in order. On the other hand, with an increase in the percentage of sulpholane in the mixtures, the values of AH:# and AS:# are increasingly less negative, and in some cases they are both positive at X(sulpho1ane) = 0.1586.In the latter cases and with the highest concentrations of sulpholane we can suppose that the transition state for viscous flow is accompanied by the breaking and distortion of intermolecular bonds. Moreover, the smooth progression of the values of electrolyte activation parameters found in the water + sulpholane mixtures (table 4) confirms the absence of structural enhancement of the water by the sulpholane; in fact when the water structure is increased by the addition of a cosolvent, maxima or minima in the values of activation parameters of viscous flow are obtained, as can be seen for electrolytes in water + methano112 and water + acetonez2 mixtures.In order to be able to discuss more fully the behaviour of electrolytes it would undoubtedly be useful to know the ionic values of the B coefficients in the solvent mixtures; nevertheless, we feel that it is possible to conclude that the viscometric behaviour of suitably chosen electrolytes can provide information as regards the effect caused by small additions of organic solvents on the structure of water. Lastly, note that in the above discussion the solvent mixtures are treated as average pure solvents. Obviously this does not allow us to point out the possible occurrence of preferential solvation of one of the components of the mixture toward the ions.1512 ION-SOLVENT INTERACTIONS IN BINARY MIXTURES APPENDIX TABLE 6.-cONCENTRATION, C/mOl dmP3, RELATIVE DENSITY, d,., AND RELATIVE VISCOSITY, Vr, FOR NaC1, INCLUDED IN THE CALCULATION OF THE B VALUES ARE MARKED WITH AN ASTERISK.NaBr, NaI AND NaC10, IN WATER+SULPHOLANE MIXTURES AT 3, 40 AND 50 "C. MEASUREMENTS NOT C dr tlr C dr tlr c 4- tlr 30 OC 40 OC 50 OC 0.015 948 0.030 041 0.036 61 1 0.051 526 0.068 716 0.083 613 0.018 574 0.027 863 0.033 745 0.046 294 0.064 374 0.077 613 0.012 71 1 0.020 166 0.024 727 0.034 071 0.047 181 0.056 141 0.014 730 0.021 158 0.026 178 0.038 193 0.052 644 0.064 471 0.017 728 0.029 836 0.034 899 0.047 854 0.071 724 0.080 971 0.021 357 0.030 279 0.036 170 0.050 344 0.068 827 0.083 501 0.012 991 0.020 251 0.025 591 0.033 956 0.047 484 1.000 65 1.001 18 1.001 44 1.002 02 1.002 71 1.003 25 1.001 43 1.002 11 1.002 56 1.003 50 1.004 87 1.005 87 1.001 38 1.002 21 1.002 70 1.003 73 1.005 16 1.006 11 1.001 07 1.001 54 1.001 92 1.002 80 1.003 84 1.004 75 1.000 69 1.001 11 1.001 29 1.001 77 1.002 62 1.003 05 1.001 45 1.002 11 1.002 53 1.003 57 1.004 87 1.006 13 1.001 38 1.002 05 1.002 66 1.003 51 1.004 88 0.057 304 1.005 86 1.002 38 1.004 15 1.004 95 1.006 68 1.008 82 1.010 70 1.002 25 1.003 00 1.003 61 1.004 75 1.006 61 1.007 56 1.001 07 1.001 47 1.001 78 1.002 19 1.002 73 1.003 10 1.001 35 1.001 52 1.002 20 1.002 88 1.003 78 1.004 52 1.003 98 1.006 06 1.007 21 1.009 89 1.014 52 1.016 11 1.004 02 1.005 57 1.006 35 1.009 34 1.012 11 1.014 45 1.002 23 1.003 22 1.004 00 1.005 22 1.006 75 1.008 02 X(sulpho1ane) = 0.0208 0.015 879 0.029 91 1 0.036 451 0.051 302 0.068 416 0.083 253 0.0 18 494 0.027 742 0.033 598 0.046 093 0.064 093 0.077 273 0.012 656 0.020 079 0.024 619 0.033 924 0.046 975 0.055 894 0.014 667 0.021 066 0.026 065 0.038 026 0.052 414 0.064 187 NaCl 1.OOO 66 1.001 21 1.001 42 1.002 05 1.002 70 1.003 30 NaBr 1.001 40 1.002 10 1.002 53 1.003 49 1.004 83 1.005 82 NaI 1.001 40 1.002 22 1.002 70 1.003 77 1.005 15 1.006 05 NaClO, 1.001 08 1.001 55 1.001 92 1.002 77 1.003 80 1.004 68 1.002 55 1.004 32 1.005 02 1.007 05 1.009 32 1.011 14 1.002 52 1.003 67 1.004 28 1.005 55 1.007 64 1.008 95 1.001 29 1.001 83 1.002 14 1.002 89 1.003 61 1.004 15 1.001 64 1.002 18 1.002 55 1.003 54 1.004 29 1.005 35 X(sulpho1ane) = 0.0744 0.015 801 1.000 70 0.029 763 1.001 24 0.036 273 1.001 51 0.051 049 1.002 06 0.068 079 1.002 73 0.082 838 1.003 26 0.018 402 1.001 48 0.027 605 1.002 16 0.033 433 1.002 64 0.045 866 1.003 58 0.063 777 1.004 90 0.076 889 1.005 87 0.012 594 1.001 42 0.019 980 1.002 24 0.024 498 1.002 73 0.033 756 1.003 77 0.046 742 1.005 16 0.055 625 1.006 19 0.014 595 1.001 08 0.020 963 1.00V56 0.025 937 1.001 92 0.037 837 1.002 74 0.052 154 1.003 77 0.063 867 1.004 61 0.017 628 0.029 666 0.034 700 0.047 579 0.071 314 0.080 504 0.021 236 0.030 106 0.035 963 0.050 055 0.068 439 0.083 028 0.012 917 0.020 135 0.025 444 0.033 761 0.047 212 0.056 977 NaCl 1.000 75 1.001 13 1.001 30 1.001 73 1.002 62 1.002 99 NaBr 1.001 36 1.002 02 1.002 42 1.003 45 1.004 85 1.006 07 NaI 1.001 42 1.002 05 1.002 66 1.003 50 1.004 87 1.005 90 1.003 66 0.017 523 1.005 86 0.029 488 1.006 78 0.034 491 1.009 00 0.047 293 1.013 63 0.070 888 1.015 46 0.080 016 1.004 36 0.021 107 1.005 83 0.029 924 1.006 70 0.035 744 1.009 70 0.049 754 1.012 74 0.068 026 1.015 36 0.082 551 1.002 03 0.01 2 838 1.003 26 0.020 014 1.004 42 0.025 290 1.005 75 0.033 557 1.007 64 0.046 937 1.008 42 0.056 628 1.000 88 1.001 23 1.001 39 1.001 82 1.002 75 1.003 03 1.001 36 1.002 04 1.002 39 1.003 51 1.004 88 1.006 42 1.001 41 1.002 09 1.002 70 1.003 52 1.005 13 1.005 83 1.002 70 1.004 66 1.005 60 1.007 83 1.010 01 1.011 60 1.002 98 1.004 05 1.004 94 1.006 35 1.008 30 1.009 82 1.001 47 1.002 03 1.002 40 1.003 23 1.004 15 1.004 85 1.001 71 1.002 23 1.002 63 1.003 82 1.004 81 1.005 79 1.003 31 1.005 71 1.006 64 1.008 87 1.013 08 1.014 99 1.004 35 1.005 94 1.006 95 1.009 76 1.013 03 1.015 88 1.003 15 1.003 66 1.004 63 1.005 75 1.008 28 1.008 92A.SACCO, G. PETRELLA, A. DELL’ATTI A N D A. D E GIGLIO 1513 TABLE 6.-(continued) C 4 rtr c 4 rtr C 4 4% 30 OC 40 OC 50 OC 0.014 056 0.022 348 0.026 048 0.035 979 0.049 742 0.059 997 0.011 436 0.018 985 0.032 986 0.045 083 0.049 400 0.072 926 0.019 201 0.026 431 0.034 284 0.045 723 0.063 164 *0.075 746 0.019 075 0.031 242 0.035 646 0.048 005 0.065 419 0.080 557 0.016 113 0.023 018 0.030 201 0.040 035 0.055 314 0.067 617 1.000 88 1.001 38 1.001 65 1.002 22 1.003 16 1.003 90 1.000 72 1.000 97 1.001 41 1.001 68 1.001 78 1.002 55 1.000 90 1.001 25 1.001 77 1.002 54 1.003 83 1.004 57 1.001 85 1.002 84 1.003 54 1.004 62 1.006 26 1.007 67 1.000 81 1.001 23 1.001 66 1.002 25 1.003 13 1.003 91 X(sulpho1ane) = 0.0744 NaClO, 1.002 54 0.013 976 1.000 86 1.002 52 0.013 893 1.000 99 1.003 41 0.022 221 1.001 38 1.003 44 0.022 088 1.001 49 1.004 12 0.025 900 1.001 62 1.004 28 0.025 745 1.001 74 1.005 51 0.035 775 1.002 25 1.005 36 0.035 564 1.002 41 1.006 94 0.049 457 1.003 14 1.006 85 0.049 163 1.003 26 1.007 93 0.059 653 1.003 86 1.008 48 0.059 296 1.003 94 X(sulpho1ane) = 0.1586 1.004 02 1.006 37 1.010 61 1.014 42 1.015 43 1.022 32 1.005 80 1.007 81 1.009 81 1.012 75 1.017 63 1.020 10 1.005 13 1.007 79 1.008 75 1.011 51 1.014 90 1.018 29 1.004 05 1.005 78 1.007 16 1.009 09 1.01 1 87 1.014 25 0.011 361 0.018 862 0.032 767 0.044 790 0.049 078 0.072 442 - 0.026 256 0.034 057 0.045 423 0.062 748 0.075 247 0.018 951 0.031 038 0.035 413 0.047 690 0.064 982 0.080 033 0.016 006 0.022 870 0.030 001 0.039 770 0.054 956 0.067 182 NaCl 1.000 81 1.001 11 1.001 45 1.001 84 1.001 92 1.002 58 NaBr 1.001 53 1.002 05 1.002 86 1.004 14 1.004 87 NaI 1.001 96 1.002 91 1.003 62 1.004 67 1.006 34 1.007 79 NaClO, 1.000 72 1.001 30 1.001 55 1.002 16 1.003 16 1.004 00 - 1.003 74 0.01 1 283 1.000 86 1.006 06 0.018 730 1.001 08 1.010 28 0.032 544 1.001 57 1.013 74 0.044 481 1.001 88 1.015 21 0.048 739 1.001 93 1.021 50 *0.071 943 1.002 61 - - - 1.007 67 0.026 079 1.001 48 1.009 88 0.033 832 1.002 11 1.012 90 0.045 132 1.003 14 1.017 96 0.062 328 1.004 11 1.020 17 *0.074 741 1.004 82 1.004 97 0.018 823 1.002 05 1.007 91 0.030 830 1.003 07 1.008 81 0.035 174 1.003 70 1.01 1 79 0.047 370 1.004 83 1.015 05 0.064 551 1.006 55 1.018 37 0.079 492 1.007 88 1.004 21 0.015 896 1.000 55 1.005 27 0.022 710 1.OOO 98 1.006 91 0.029 797 1.001 44 1.008 81 0.039 496 1.001 95 1.01 1 88 0.054 578 1.002 98 1.014 34 0.066 711 1.003 67 1.002 80 1.003 90 1.004 45 1.006 03 1.007 61 1.009 27 1.003 60 1.005 83 1.009 81 1.013 15 1.014 54 1.020 15 - 1.007 57 1.009 41 1.012 86 1.017 00 1.019 68 1.004 99 1.007 97 1.009 0 1 1.012 24 1.015 36 1.018 62 1.003 98 1.005 16 1.006 77 1.008 32 1.011 44 1.013 97 1 2 3 4 5 6 7 8 B 10 11 M.Kaminsky, Discuss. Faraday Soc., 1957, 24, 171. R. T. Bicknell, K. G. Lawrence, M. A. Seeley, D. Feakins and L. Werblan, J . Chem. Soc., Faraday Trans. 1, 1976, 72, 307. A. Sacco, G. Petrella and M. Castagnolo, J . Phys. Chem., 1976, 80, 749. A. Sacco, G. Petrella, M. Della Monica and M. Castagnolo, J . Chem. Soc., Faraday Trans. 1, 1977, 73, 1936. J. E. Desnoyers and G . Perron, J . Solution Chem., 1972, 1, 198. R. T. M. Bicknell, K. G. Lawrence and D. Feakins, J . Chem. Soc., Faraday Trans. 1, 1980, 76, 637. G. Petrella and A. Sacco, J. Chem. Soc., Faraday Trans. 1, 1978, 74, 2070. A. Sacco, A. Dell’Atti, A. De Giglio and M. Petrella, J . Chem. Soc., Faraday Trans. 1, 1981,77,2693. R. L. Kay, T. Vituccio, C. Zawoysky and D. F. Evans, J . Phys. Chem., 1966, 70, 2336. D. J. P. Out and J. M. Los, J . Solution Chem., 1980, 9, 19. D. Feakins, D. J. Freemantle and K. G. Lawrence, Chem. Commun., 1968, 970.1514 I 0 N-S 0 L V E N T INTER ACT I 0 N S I N B I N A R Y MIXTURES l 2 D. Feakins, D. J. Freemantle and K. G. Lawrence, J. Chem. Soc., Faraday Trans. 1, 1974, 70, 795. l 3 J. M. McDowall, N. Martinus and C. A. Vincent, J . Chem. Soc., Faraday Trans. 1, 1976, 72, 654. l4 G. Petrella, A. Sacco, M. Della Monica and M. Castagnolo, J . Solution Chem., 1977, 6, 13. l5 G. Jones and M. Dole, J . Am. Chem. SOC., 1929, 51, 2950. l7 H. Falkenagen and E. L. Vernon, Philos. Mag., 1932, 14, 537. l 9 R. L. Benoit and G. Choux, Can. J . Chem., 1968, 46, 3215. 2o A. Sacco, G. Petrella, M. Castagnolo and A.Dell’Atti, Thermochim. Acta, 1981, 44, 59. z1 M. Castagnolo, A. Inglese, G. Petrella and A. Sacco, Thermochim. Ada, 1981, 44, 67. 22 J. Padova, J . Chem. Phys., 1963, 38, 2635. N. Martinus, C. D. Sinclair and C. A. Vincent, Electrochim. Acta, 1977, 22, 1183. D. D. MacDonald, M. D. Smith and J. B. Hyne, Can. J . Chem., 1971, 49, 2818. (PAPER 1 /926)

ISSN:0300-9599    

DOI:10.1039/F19827801507

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 1515-1523 Influence of Silica and Alumina Supports on the Temperature-programmed Reduction of Copper(I1) Oxide BY STEPHEN J. GENTRY AND PETER T. WALSH Health and Safety Executive, Research and Laboratory Services Division, Sheffield Laboratories, Red Hill, Sheffield S3 7HQ Received 9th June, 1981 Temperature-programmed reduction has been used to sty..dy the reduction of copper oxide supported on silica and alumina. It has been shown that copper oxide exists in two distinct phases when supported on silica, dispersed cupric oxide and a copper silicate species. The relative amounts of the two phases depend on surface area and copper loading, while the reducibility of the silicate phase is dependent on calcination temperature. No evidence of support interaction is found for a-Al,O,, while the complex t.p.r. profiles obtained for copper on y-Al,O, suggest complex suppport interaction. Previous work in these laboratories applied the technique of temperature- programmed reduction (t.p.r.) to an investigation of the redox behaviour of Cul* ions in X and Y zeolites1 and to a study of the promoting effect of transition metal ions on the reduction of bulk cupric oxide.2 It was found that t.p.r. was a convenient and sensitive method for characterising the materials used and also enabled mechanistic and kinetic information to be obtained on the reduction processes.In the present work t.p.r. has been used to study the influence of the support on the nature of the copper species present in copper oxide on silica and alumina.There have been several studies39 of bimetallic catalysts containing copper. The reducibility and alloying behaviour of copper and nickel on silica has received a t t e n t i ~ n ; ~ however, most of the work has concentrated on the reduced state of the metals. Mixed-oxide catalysts containing copper oxide have found use as catalytic converters for carbon monoxide,6 nitric oxide7 and hydrocarbons.8 Consequently there have been many studies of the copper-oxide-support system, particularly with high-surface-area y-A1203. Various techniques have been employed in these investigations. Physical means of characterisation have included magnetic susceptibility measurement^,^-^^ X-ray diffraction,l2. l3 differential thermal analysis,12 electron paramagnetic resonance spectroscopy,13~ l4 X-ray K-absorption spectr~scopy,~~~ l5 optical and infrared spectro~copy~~~ l6 and X-ray photoelectron spectroscopy.13~ 1 7 9 Chemical methods have utilised a static hydrogen-reduction technique,lg and the catalytic activity with respect to the dehydrogenation of isopropyl alcoholg and the oxidation of carbon monoxide.lo In addition to the studies based on the catalytic applications of the copper-support systems, the high degree of dispersion of the copper attainable on these supports has lead to investigations using e.p.r. spectr~scopy~~~ 2 o y 21 of the effect of a magnetically dilute environment on the paramagnetic Cu2+ species. Not all the techniques described above are particularly sensitive to the interactions that can occur between the catalyst and the support when low loadings are used and the catalyst is highly dispersed.Therefore we have used t.p.r., which has been shown 15151516 REDUCTION OF SUPPORTED COPPER OXIDE to be a sensitive technique for catalyst characteri~ation,~ to investigate the interaction between copper oxide and four supports (two samples of amorphous silica having different surface areas, a-Al,O, and y-Al,O,). The materials were also examined by X-ray diffraction and X-ray photoelectron spectroscopy in order to complement, where possible, the t.p.r. data and also to compare the three methods of characterisation. EXPERIMENTAL APPARATUS A N D PROCEDURE The apparatus has been previously2* described. The reactor consisted of two concentric silica tubes.The gas mixture passed down through the inner silica tube (8 mm diameter) containing the sample on a coarse silica sinter and then up through the outer silica tube (14 mm diameter, 180 mm long). A thermocouple well (4 mm diameter) was positioned in the sample bed along the axis of the reactor. The reducing gas (a H,+N, mixture) was dried by magnesium perchlorate before passage through the reference arm of the katharometer and immediately after passage through the reactor. In all experiments a linear heating rate of 13.7 K min-l from 300 to 930 K was used, together with a 10% H2+N2 mixture at a flow rate of 20 cm3 min-’ and a constant weight of material, ca. 200 mg for the supported copper oxide. Two parameters are obtained from a t.p.r. profile; the temperature at which the maximum rate of reduction occurs, T,, and the area underneath the profile corresponding to the amount of hydrogen consumed in the reduction process.Prior to reduction each sample, unless otherwise specified, was pretreated in the reactor under flowing (20 cm3 rnin-l), dried air at various temperatures for a period of 1 h as specified in the Results section. PREPARATION OF MATERIALS The four supports used are listed below, together with their specific surface area as measured by the nitrogen B.E.T. method after treatment in N, at 430 K for 1 h. Also listed is a shorthand notation: amorphous silica (Gasil35, Unilever), 330 f 2 m2 ggl, ‘SiO, (330)’; amorphous silica (Gasil 200, Unilever), 540 & 2 m2 g-l, ‘ SiO, (540)’; a-alumina (Laboratory Reagent grade, B.D.H.), 25 k 2 m2 g-l, ‘a-Al,O,’, and y-alumina (Laboratory Reagent grade, B.D.H.), 70 f 2 m2 g-l, ‘ y-Al,O,’. Each support was impregnated by slurrying with an aqueous solution of cupric nitrate (AnalaR grade, B.D.H.).The loading was varied by adjusting the molarity of the cupric nitrate solution. The slurry was evaporated to dryness at 420 K in an oven and then calcined at the appropriate temperature in the t.p.r. reactor. The loading is expressed as the percentage of copper by weight of the total. Unsupported cupric oxide was prepared by calcining cupric nitrate (AnalaR grade, B.D.H.) in an oven at 630 K and then transferring to the t.p.r. reactor. The t.p.r. experiment was then conducted without further treatment. CHARACTERISATION OF MATERIALS X-RAY D I F FR A c T I o N (X.r.d.) diffractometer using the Debye-Sherrer method.Powder X-ray spectra of the supports and supported copper oxide were recorded on a Philips X-RAY PHOTO E LE c T RO N s P E c T R 0s c o P Y (X.P.S.) X-ray photoelectron spectra were obtained with a VG ESCA 3 using A1 Km radiation. Samples were mounted on double-sided adhesive tape. The measured binding energies were corrected for possible charging effects by assigning the binding energy of the C (1s) peak arising from sample contamination as 285.0 eV.22S. J. GENTRY AND P. T. WALSH 1517 SCANNING ELECTRON MICROSCOPY A N D X-RAY MICROPROBE ANALYSIS The homogeneity of the distribution of the copper species on the support was examined using a Cambridge Stereoscan 600 together with a Link Systems energy-dispersive X-ray analyser.COLOUR The colour of the materials was noted, as this gave an indication of the degree of dispersion of the copper or whether there was any chemical interaction between the support and the copper. RESULTS X-ray microprobe analysis confirmed that the method of impregnation produced a homogeneous distribution of copper throughout the support. The t.p.r. profiles for similar loadings (ca. 1.2%) of copper on the four supports and the profile for unsupported cupric oxide are shown in fig. 1. All the samples were calcined at 900 K for 1 h. Copper oxide supported on silica shows low- and high-temperature reduction regions (cf. unsupported cupric oxide). For Cu/SiO, (330) the low-temperature region consists of two processes consuming approximately three times as much hydrogen as the single high-temperature process.However, for Cu/SiO, (540) a single low- temperature peak is observed, similar in area to the high-temperature peak. When copper is supported on a-Al,O, only low-temperature reduction processes occur, and the t.p.r. profile is similar to that obtained in the low-temperature region for Cu/SiO, 400 600 800 TI K FIG. 1.-Effect of support on t.p.r. profile. Samples calcined at 900 K for 1 h. Sample weight ca. 200 mg. (a) 1.22% Cu/SiO, (330); (b) 1.24% Cu/SiO, (540); (c) 1.22% Cu/a-Al,O,; ( d ) 1.25% Cu/y-Al,O,; (e) unsupported CuO, sample weight 16 mg. The numbers under each peak denote the amount of hydrogen consumed (pmol) in the reduction process. The peak heights have been attenuated for clarity.1518 REDUCTION OF SUPPORTED COPPER OXIDE (330).The Cu/y-Al,O, t.p.r. profile is a very broad peak with reduction occurring continuously between ca. 430 and 920 K. The total amount of hydrogen consumed by all the samples was the same, within experimental error, as that calculated for the reduction of CuO to CuO. The Cu/SiO, (540) system was examined in more detail. Fig. 2 shows the effect of calcination temperature on the t.p.r. profiles of 1.24% Cu/SiO, (540), and also, for comparison, the profile of partially decomposed unsupported cupric nitrate (obtained by calcining at 570 K for 1 h). Profiles (a)-(e) show that T, for the low-temperature reduction process remains unaffected by calcination temperature, whilst Tm for the higher-temperature reduction process increases as the calcination temperature is raised.The amount of hydrogen consumed by the low-temperature process progres- LOO 600 800 TIK FIG. 2.-Effect of calcination temperature on t.p.r. profile of 1.24% Cu/SiO, (540) calcined for 1 h, sample weight ca. 200 mg: (a) 420, (b) 520, (c) 610, ( d ) 770, (e) 900 K. Also shown (f) unsupported cupric nitrate calcined at 570K for 1 h, sample weight 16mg. The numbers under each peak denote the amount of hydrogen consumed bmol) in the reduction process. The peak heights have been attenuated for clarity. sively decreases to ca. 20 pmol, whilst the amount for the high-temperature process is constant. Above a calcination temperature of 610 K the total consumption of hydrogen is the same, within experimental error, as that calculated for the reduction of CUO to CUO.The effect of copper loading on the t.p.r. profile is shown in fig. 3. Tm does not vary significantly for either the low- or high-temperature reduction processes. Fig. 4 shows the effect of the copper loading on the amount of hydrogen consumed in the low- andS. J. GENTRY AND P. T. WALSH 1519 1 I l I I I 400 600 800 TIK FIG. 3.-Effect of loading on t.p.r. profile of Cu/SiO, (540), calcined at 770 K for 1 h: (a) 0.25, (b) 0.59, (c) 1.25, ( d ) 1.91, (e) 4.67, (f) 6.63%. Sample weight ca. 200 mg. The numbers under each peak denote the amount of hydrogen consumed (pmol) in the reduction process. 2 4 6 copper loading (wt. 5%) FIG. 4.-Effect of loading on hydrogen consumption of Cu/SiO, (540), calcined at 770 K for 1 h : (0) low-temperature process; ( x ) high-temperature process; (0) total hydrogen consumption; (- - -) theoretical total hydrogen consumption.1520 REDUCTION OF SUPPORTED COPPER OXIDE high-temperature reduction processes.The total hydrogen consumption is also shown in comparison with the theoretical value, calculated assuming the stoichiometric CuO+H, + Cu+H,O. reaction The slope of the curve corresponding to the low-temperature process appears to change at ca. 2% loading whilst the amount of hydrogen consumed still increases with copper loading. The curve corresponding to the high-temperature process rises with increased loading until the amount of hydrogen consumed reaches a constant value above ca. 4 % loading. The total hydrogen consumption is directly proportional to the copper loading throughout the experimental range and deviates from the theoretical value by a maximum of 5%.Copper loadings of ca. 1% were approaching the detectable limit of the X.r.d. technique for particle sizes > 5 nm. For the supported samples presented in fig. 1 CuO was only detected on 1.22% Cu/SiO, (330). Of the samples detailed in fig. 3 CuO was detected only on 6.63% Cu/SiO, (540), calcined at 770 K. No other diffraction lines were observed in any of these samples. Loadings of ca. 1% were at the limit of detectability of the X.P.S. technique, and consequently gave poor spectra. However, for the samples presented in fig. 1 no C~(2p,/,) chemical shifts were observed for any of the supported materials relative to CuO, which has a measured binding energy of 936.2 & 0.2 eV [corrected to a C (1s) binding energy of 285.0 eV].Also the binding energy of the Cu(2p3,,) peak for 6.63% Cu/SiO, (540) calcined at 770 K was the same, within experimental error, as that for CUO. For the samples shown in fig. 1 the colours are as follows: Cu/SiO, (330), blue-grey; Cu/SiO, (540), pale blue; Cu/a-Al,O,, blue-grey ; Cu/y-Al,O,, pale blue. The 1.24% Cu/SiO, (540) samples remained pale blue for all calcination temperatures. When the loading was increased the Cu/SiO, (540) samples calcined at 770 K gradually changed colour from pale blue to turquoise. DISCUSSION Fig. 1 shows that the reduction of copper oxide is clearly affected by the support material. All the supports give rise to reduction processes occurring at lower temperatures than in unsupported CuO (1.t.processes). Cu/a-Al,O, is the only material studied which reduces in a single step; all the other materials give rise to additional reduction processes at higher temperature than that for unsupported CuO (h.t. processes). However, for Cu/y-Al,O, the h.t. and 1.t. processes are not clearly resolved. The total amount of hydrogen consumed in each sample is equal to the theoretical amount for the overall reduction CulI + CuO. There is no evidence, however, that the reduction processes, most clearly resolved in the Cu/SiO, samples, correspond to a stepwise reduction of CulI to Cul (1.t.) and Cul to Cuo (h.t.), as observed in the reduction of cupric ions in zeolites., Rate measurements and magnetic susceptibility studieslS carried out during the isothermal hydrogen reduction of CuO dispersed on several supports were consistent with the direct reduction of CulI to CuO. Furthermore, X.P.S.combined with an infrared absorption studylS following carbon monoxide adsorption failed to identify Cul as an intermediate in the hydrogen reduction of Cu/y-Al,O,. It is unlikely that the large variations in T, with calcination temperature for the h.t. process in SiO, (540) reported here (fig. 2) could arise from a stabilisation of Cul, since this species would itself be formed from CuI1 in the 1.t. process which is clearly independent of calcination temperature.S. J. GENTRY AND P. T. WALSH 1521 We conclude that silica and alumina supports do not stabilise Cul as an intermediate in the reduction of supported CuO, in line with previous observations' of a one-step reduction of CuO supported externally on X zeolite.Cu/A1,0, SAMPLES The reduction behaviour of Cu/a-Al,O, and Cu/y-Al,O, is markedly different. The t.p.r. profile of Cu/a-Al,O, [fig. 1 (c)] consists of a major peak at 493 K and a shoulder at ca. 533 K. Cu/y-Al,O,, on the other hand, has a broad t.p.r. profile [fig. l(d)] spanning 430-920 K. These results are similar to those of Voge and Atkins,lg who ascribed the non-uniformity of CuO supported on y-Al,O, compared with a-Al,O, to chemical interactions between the oxide and y-Al,O,. Cu/a-Al,O, is blue-grey, whereas Cu/y-Al,O, is a pale blue colour, indicating the presence of relatively large CuO particles in the former sample.Banerjee et aL2, in a t.p.r. study of nickel oxide supported on a-Al,O, found that the form of the Arrhenius plots for hydrogen reduction approached that for massive nickel oxide as the loading was increased. Furthermore, the activation energy for the initial reduction process, postulated as the interaction of gaseous molecular hydrogen with low-energy sites on the NiO lattice, was found to increase with loading. This, it was suggested, would be compatible with the increase in crystal size of NiO. It is thus apparent that a-Al,O, functions solely as a dispersing agent for copper oxide, whereas a considerable range of oxide-support interaction occurs in Cu/y-Al,O,. Copper oxide and alumina are known to form a mixed oxide having a spinel-type s t r u c t ~ r e , ~ ~ and a substantial amount of evidence had been accumulated for the existence of an aluminate phase when cupric ions are dispersed on y-A1,0,.99 1 1 - 1 5 9 17-19 y-Al,O, possesses a defect spinel structure, whereas a-Al,O, has a hexagonal close-packed structure. Thus it is to be expected that a spinel would be formed more easily on y-Al,O,. Indeed Friedman et aL.l3 and Wolberg and Roth15 found that the formation of a copper aluminate phase from impregnation of y-Al,O, with cupric nitrate was dependent on surface area, calcination temperature and copper concentration. Cu/SiO, SAMPLES From the results presented in fig.2 and 3 it can be seen that the position of the 1.t. peak is uninfluenced by either calcination temperature or loading.It is clear from fig. 1 (a) and (b), however, that, although the 1.t. peak is in essentially the same region, some structure is introduced as the surface area of the support is reduced. Since increasing the loading, increasing calcination temperature and decreasing support surface area would each be expected to increase the average particle size of the oxide, it appears that the 1.t. reduction process must be essentially independent of particle size. It is suggested that the degree of dispersion in the Cu/SiO, (540) samples is such that the dominant reduction mechanism is one of nucleation rather than a decaying rate mechanism, such as the contracting-sphere model appropriate for the bulk For similar copper loadings Cu/SiO, (330) is blue-grey, similar to Cu/a-Al,O,, whereas Cu/SiO, (540) is pale blue, suggesting that CuO exists as larger particles on the lower-area support as expected.The structure observed in the 1.t. reduction profile of Cu/SiO, (330) (and also in Cu/a-Al,O,) may then be attributed to a change in mechanism. The higher-temperature shoulder (Tm = 549 K) may then arise from the contribution of a decaying rate stage in the reduction of the larger CuO particles. The position of the high-temperature peak is again independent of surface area (fig. l), indicating that a similar interaction occurs between CulI and the two silica supports. For each sample the ratio of the peak areas in the h.t. and 1.t. processes is equal to the ratio of CU" combined with the support (which may be described as1522 REDUCTION OF SUPPORTED COPPER OXIDE a copper silicate species) and in a dispersed CuO phase.For Cu/SiO, (540) this ratio is significantly greater than for Cu/SiO, (330). Thus the higher degree of dispersion attainable on the higher-area support increases the relative amount of silicate species produced. The amount of silicate produced varies with loading. From fig. 3 and 4 it can be seen that below 2% Cu the amounts of both species increase approximately equally with increased loading. However, beyond this level the amount of copper silicate formed increases more slowly until a maximum is reached at ca. 4%. Above this level all additional copper is present as the 1.t. dispersed oxide form. It thus appears that an equilibrium is established between dispersed CuO, support and copper silicate.Below a loading of ca. 2%, copper exists equally as dispersed oxide and silicate, whilst a maximum level of silicate, corresponding to ca. 2% Cu for Cu/SiO, (540), is reached as the total loading is increased. This contrasts with the Cu/y-Al,O, system,', where the crystalline phase of cupric oxide, analysed by X.r.d., forms only when the adsorption sites of the support, which interact with the copper to form the aluminate, are saturated. The degree of interaction between copper oxide and silica is strongly dependent on calcination temperature as shown in fig. 2. Two features may be identified. First, the area of the low-temperature peak at T, z 535 K diminishes to a constant value, equivalent to 20 pmol H,, for calcination temperatures above 610 K.Partially decomposed copper nitrate essentially reduces simultaneously with copper oxide [fig. 2(f)]. Consequently at the lower calcination temperatures used (< 610 K) the peak at T, z 535 K contains a component for the reduction of undecomposed nitrate, and therefore the area is larger than the theoretical value for CuO reduction. Secondly, a high-temperature peak of approximately constant area emerges, shifting from 566 K after calcination at 420 K, to 910 K after calcination at 900 K, suggesting that the equilibrium concentration of the copper silicate species is established at as low a temperature as 420 K. However, the interaction becomes markedly stronger as the calcination temperature is increased. Wolberg and Roth15 found that the formation of a copper aluminate phase from copper dispersed on y-Al,O, was similarly dependent on surface area, copper concentration and calcination temperature.The behaviour of the Cu/SiO, system in having both 1.t. and h.t. processes, ascribed here to dispersed CuO and a CuO-SiO, interaction, respectively, contrasts with that of the Ni/SiO, system.26 Here only poorly dispersed NiO is formed and a NiO-support interaction only occurs when a silica-alumina support was used. The existence of an equilibrium between CuO and copper silicate may influence the activity of Cu/SiO, catalysts. If the dispersed CuO has a greater specific activity than the copper silicate phase (which may be expected, particularly for oxidation reactions, since dispersed CuO is more easily reduced), then for loadings below 2% on the higher surface area silica support only half of the available copper is in a catalytically active form.Increasing the loading to more than 2% increases the fraction of copper present as the dispersed oxide but reduces the degree of dispersion. Hence there would be an optimum loading and support surface area for maximum catalytic activity. CONCLUSIONS T.p.r. is a sensitive analytical technique for the characterisation of supported copper oxide. The deviation of the experimental points shown in fig. 4 from the theoretical curve is at most 5% over a range of conditions which precluded detailed analysis by either X.r.d. or X.P.S.S. J . GENTRY AND P. T. WALSH 1523 It has been shown, using t.p.r., that supported copper oxide can exist in two forms.These are a dispersed cupric oxide phase and a combined cupric-oxide-support phase. For Cu/ y-Al,O, the cupric-oxide-support interaction is not as clearly demonstrable as for the Cu/SiO, system, whilst no such interaction occurs for Cu/a-Al,O, under the conditions used. A detailed study of the Cu/SiO, system showed that (i) the relative amounts of dispersed cupric oxide and copper silicate are influenced by the surface area of the support and the copper loading and (ii) the reducibility of copper silicate is markedly affected by the calcination temperature. S. J. Gentry, N. W. Hurst and A. Jones, J. Chem. Soc., Faraday Trans. 1, 1979, 75, 1688. S. J. Gentry, N. W. Hurst and A. Jones, J. Chem. SOC., Faraday Trans. I , 1981, 77, 603. H. E. Swift, F.E. Lutinski and W. L. Kehl, J. Phys. Chem., 1965, 69, 3268. J. H. Sinfelt, J . Catal., 1973, 29, 308. S. D. Robertson, B. D. McNicol, J. H. de Baas, S. C. Kloet and J. W. Jenkins, J. Catal., 1975,37,424. S. Sourirajan and M. A. Accomazzo, Can. J. Chem., 1960, 38, 1990. ’ S. Sourirajan and J. L. Blumenthal, Proc. 2ndInt. c’ongr. Catal., 1960 (Editions Technip, Paris, 1961), p. 2521. S. Sourirajan, M. A. Accomazzo and K. Nobe, Proc. 2nd Int. Congr. Catal., 1960 (Editions Technip, Paris, 1961), p. 2497. P. W. Selwood and N. S. Dallas, J . Am. Chem. SOC., 1948, 70, 2145. lo J. Mooi and P. W. Selwood, J. Am. Chem. SOC., 1952,74, 2461. l 1 P. E. Jacobson and P. W. Selwood, J. Am. Chem. SOC., 1954, 76, 2641. l 2 E. D. Pierron, J. A. Rashkin and J. F. Roth, J. Catal., 1967, 9, 38. l 3 R. M. Friedman, J. J. Freeman and F. W. Lytle, J. Catal., 1978, 55, 10. l5 A. Wolberg and J. F. Roth, J. Catal., 1969, 15, 250. Is K. P. de Jong, J. W. Geus and J. Joziasse, J. Catal., 1980, 65, 437. l 7 A. Wolberg, J. L. Ogilvie and J. F. Roth, J. Catal., 1970, 19, 86. l 8 G. Ertl, R. Hierl, H. Knozinger, N. Thiele and H. P. Urbach, Appl. Surf. Sci., 1980, 5, 49. l9 H. H. Voge and L. T. Atkins, J. Catal., 1962, 1, 171. 2o R. J. Faber and M. T. Rogers, J. Am. Chem. Soc., 1959, 81, 1849. 21 Y. Matsunaga, Bull. Chem. SOC. Jpn, 1961, 34, 1291. 22 T. Robert, M. Bartel and G. Offergeld, Surf. Sci., 1972, 33, 123. 23 A. K. Banerjee, S. R. Naidu, N. C. Ganguli and S. P. Sen, Technology, 1969, 11, 162. 24 H. Hahn, G. Frank, W. Klingler, A. Storger and G. Storger, 2. Anorg. Allg. Chem., 1955, 279, 241. 25 J. Haber, J. Less-Common Metals, 1977, 54, 243. 26 M. Houalla, F. Delannay and B. Delmon, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1766. P. A. Berger and J. F. Roth, J. Phys. Chem., 1967, 71, 4307. (PAPER 1 /928)

ISSN:0300-9599    

DOI:10.1039/F19827801515

出版商:RSC    

年代:1982

数据来源: RSC