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)