Hydrogenation of Acetylene over Supported Metal Catalysts Part 1 .-Adsorption of [14C]Acetylene and [14C]Ethylene on Silica Supported Rhodium, Iridium and Palladium and Alumina Supported Palladium BY ASAD S. AL-AMMAR AND GEOFFREY WEBB* Department of Chemistry, The University, Glasgow, G12 SQQ Received 28th April, 1977 The adsorption of [' 4C]ethylene and [ 14C]acetylene on supported palladium, rhodium and iridium catalysts occurs irreversibly at 298 K in two distinct stages ; a non-linear primary region, in which the species are predominantly dissociatively adsorbed, and a linear secondary region. Hydrogenation catalysis is associated with the hydrocarbon species adsorbed on the secondary region. From [ L4C]carbon monoxide adsorptions it is concluded that the hydrocarbon primary region is associated with the metal, whilst the secondary region probably involves the formation of overlayers on the primary adsorbed species.The co-adsorption of ethylene and acetylene shows that, under acetylene hydrogenation conditions, both are adsorbed at separate sites and undergo hydrogenation inde- pendently of each other. The relevance of these observations to the selective hydrogenation of acetylene is discussed. The metal-catalysed hydrogenation of acetylene has been the subject of a large number of studies which have been reviewed by Bond and Wells and, more recently, by Wells and by Webb.3 In general the selectivity for ethylene formation has been considered to be a consequence of a thermodynamic factor, which controls the relative surface coverages of acetylene and ethylene, the same surface sites being assumed to be involved in the adsorption of both hydrocarbons, and a mechanistic factor, which determines the extent to which the ethylene is further hydrogenated to ethane without undergoing intermediate de~orption.~'~ In a recent study7 of the hydrogenation of acetylene in the presence of large excesses of ethylene over palladium + alumina catalysts it has been suggested that the ethane is formed by the hydrogenation of ethylene, rather than directly from acetylene, and that this hydrogenation may occur at sites which are independent of those involved in the adsorption and hydrogenation of acetylene.Similarly, in the selective hydro- genation of propadiene over a series of silica-supported metals * and of buta- 1,3-diene over alumina-supported palladium, rhodium and platinum, it has been suggested that the sites responsible for alkene formation may be distinct from those involved in alkane formation.It has also been suggested that the availability of hydrogen at these distinct sites may be different, possibly due to the occlusion of hydrogen within the metal. In a previous study of the adsorption of [14C]acetylene and [14C]ethylene on supported rhodium catalysts,10 it was observed that the adsorption isotherms showed two distinct regions : a steep primary region which, from related [14C]carbon monoxide adsorption, was considered to represent monolayer coverage of the metal by hydrocarbon, and a linear region thought to be associated with the support.It was also observed that acetylene could only displace 3 10% of the preadsorbed ethylene from the surface. 195196 ADSORPTION OF [14C]ACETYLENE AND [14C]ETHYLENE The present studies were undertaken in an attempt to determine (a) the relevance of the primary and secondary adsorption regions to the hydrogenation of ethylene and acetylene, and (b) to examine the co-adsorption of ethylene and acetylene under hydrogenation conditions. From such studies it was hoped to gain a further insight into the factors which determine the selective behaviour of metal catalysts in hydro- genation reactions. EXPERIMENTAL CATALYSTS The catalysts containing 5 % w]w metal supported on Aerosil silica (Degussa) or y- alumina (Degussa) were prepared by adding an aqueous solution of the metal chloride, containing the required weight of metal, to an aqueous suspension of the support.Excess water was evaporated and the catalyst finally dried in an air oven at 393-413 K. The catalysts were stored as the supported salt until required. Before use the supported salt was reduced in a stream of hydrogen ( m 10 cn13 min-') at 473 K for 12 h. Activation was completed by heating in an atmosphere of hydrogen at 623 K for a further 6 h. Finally the catalyst was evacuated at 623 K for 6 h and cooled to ambient temperature in vacuo. This procedure lead to reproducible activity from sample to sample. MATERIALS Acetylene (B.O.C.) contained both acetone and air. These were removed by a series of bulb to bulb distillations; the purified acetylene contained no impurities detectable by gas chromatography. Ethylene (Matheson) contained no detectable impurities and was merely degassed before use.[14C]labelled hydrocarbons (Radiochemical Centre, Amersham) were diluted to the required specific activity with the purified nonactive hydrocarbon before use. [14C]~arbon monoxide was prepared by the reduction of [14C]carbon dioxide (Radio- chemical Centre, Amersham) with metallic zinc.'l Cylinder hydrogen was purified by diffusion through a heated palladium thimble. APPARATUS The principal feature of the apparatus was that it permitted the direct observation of the adsorbed species during the adsorption process and in subsequent reactions. The reaction vessel is shown in fig. 1. The catalyst was spread uniformly on the glass boat (B), which FIG.1.-Reaction vessel used for the direct monitoring of surface adsorbed species. (GM, Geiger- Muller counter ; F, furnace region ; B, catalyst boat). could be moved into the furnace region (F) for activation and then withdrawn into position under the intercalibrated Geiger-Muller counters (Mullard MX 168/01) during the reaction. By suitable placement of the boat, the gas phase and the combined gas phase and surface count rates could be determined simultaneously.A . S . AL-AMMAR AND G . WEBB 197 The reaction vessel (volume 533 cm3) was connected to a mercury-free conventional high vacuum system, Pressures were measured using a calibrated differential transducer (Akers Electronics). The vessel was also connected to a gas sampling system (volume 5.5 cm3) which was then coupled to a combined gas chroinztograph-proportional c0unter.l This permitted the sampling and analysis of the reaction products throughout the course of the reaction.Analysis was performed using a 1 m column packed with 30-60 mesh activated silica ge!. The column was operated at 333 K with helium as carrier gas at a flow rate of 60 cm3 niin-l. On elution from the column the eluant was mixed with the required amount of methane before entering the gas proportional counter. EXPERIMENTAL PROCEDURE Adsorption of [14C]labelled species was investigated by admitting small aliquots of the labelled adsorbate to the reaction vessel and determining the surface and gas phase count rates after each addition. In the hydrogenation reactions a premixed sample of hydrogen and acetylene, in the ratio 3 : 1, was admitted to the reaction vessel to the required pressure.The progress of the reaction was monitored by the pressure fall observed with the transducer. At the desired pressure fall the reaction vessel was connected to the sample volume and a sample of the reaction mixture analysed. This procedure was repeated several times during the course of each reaction. RESULTS All adsorption and hydrogenation measurements were carried out at 294-298 K using hydrocarbon pressures in the range 0 to 12.5 Torr. On freshly reduced catalysts the adsorption isotherms for [14C]acetylene, ['"C]- ethylene and [14C]carbon monoxide were of the form shown in fig. 2. The shapes II 53 12 W 0 1 2 gas-phase count rate/10-3 min-' FIG.2.-Adsorption isotherms for [14C]carbon monoxide (@), [14C]acetylene (0) and [14C]ethylene (B) on freshly reduced (a) 0.20 g Rh/silica ; (b) 0.51 g Ir/silica ; (c) 0.10 g Pd/silica and (d) 0.10 g Pd/alumina. { [14C]specific activity was 0.10 mCi (mmol)-l in each case).2 98 ADSORPTION OF [14C]ACETYLENE AND [l4C]ETHYLENE of these isotherms are similar to those observed previously,1o the hydrocarbon isotherms consisting of a steep non-linear primary region followed by a linear second- ary region. With both acetylene and ethylene the gas in contact with the surface during the build up of the isotherm was analysed. This showed that with both acetylene and ethylene over rhodium and iridium, and with ethylene over palladium the gas in equilibrium with the surface primary adsorbed species consisted entirely of ethane.With acetylene over both palladium catalysts ethane was the sole gaseous species present up to 80% of the total primary region. During the subsequent build up of the isotherms the gas in contact with the surface wasethane, formed in the primary region, together with the adsorbate hydrocarbon. No ethylene was ever observed as a self-hydrogenation product from the adsorption of acetylene on any of the catalysts. gas-phase count rate/10-3 min-I FIG. 3.-Adsorption isotherms for [14C]carbon monoxide (E) ; [14C]acetylene (0) and [14C]ethylene (e) on steady state catalysts (a) 0.20 g Rh/silica ; (6) 0.51 g Ir/silica and (c) 0.10 g Pd/silica. ([14C]carbon specific activity was 0.10 mCi (mrnol)-l in each case). Catalysts which had been activated as stated above, but which had been allowed to cool to ambient temperature in hydrogen before evacuation, showed similar acetylene and ethylene isotherms to those shown in fig.2, except that the extent of the primary region was substantially reduced as shown in table 1. The gradients of the secondary isotherms were independent of the cooling-evacuation procedure. In the hydrogenation of acetylene over each catalyst it was observed that, from reaction to reaction, the catalyst became progressively deactivated until eventually a constant activity was attained. Thus, for example, with Pd/silica (0.10 g), Pd/alumina (0.10), Rh/silica (0.25) and Ir/silica (0.51 g) the activities, expressed as the first order rate constant (klmin-I), decreased from initial values of 1.4 x lo-', 1.45 x 2.70 x and 8.35 x to constant limiting values of 1.59 x 1.50 x 0.62 x and 2.70 x respectively.For catalysts which had been deactivated to this constant limiting activity, which will be termed the catalyst steady state, adsorption isotherms of [14C]acetylene, [14C]ethylene and [14C]carbon monoxide were determined. Investigations with [14C]acetylene showed that, although only a part of pre-adsorbed [14C]acetylene participated in a subsequent hydrogenation reaction, a further fraction of the pre-adsorbed acetylene could be removed by allowingA . S . AL-AMMAR AND G . WEBB 199 TABLE 1 .-EXTENT OF PRIMARY ADSORPTION OF [' 4C]ACETYLENE AND [' 4C]ETHYLENE ON CATALYSTS EVACUATED AT 623 K AND 298 K FOLLOWING A ~ A T I O N adsorbate temperature/K (0.1 g) (0.1 g) (0.2 g) (0.51 9) surface count rate at turning point/min-1 evacuation Pd/A1203 Pd /SiO z Rh/SiOz Ir/SiO2 C2H2 623 2610 293 3 5688 801 2 C2H2 298 1975 2442 3867 6391 C2H4 623 500 848 5232 6706 CzH4 298 190 325 3497 533 1 the catalyst to stand under either hydrogen or the hydrogen-rich reaction product mixture following the hydrogenation of 12.5 Torr acetylene with 37.5 Torr hydrogen.Consequently, the adsorption isotherms for the deactivated steady state catalysts, shown in fig. 3, were determined after the catalyst had been allowed to stand under the hydrogen-rich reaction product mixture for 6 h, this being the time for the com- plete removal of all of the " removable " preadsorbed acetylene. Comparison of theiisotherms shown in fig. 2 and 3 shows that catalysts in their steady states showed TABLE 2.-vARIATION OF f14C]ACETYLENE AND [ 14C]ETHYLENE SURFACE COUNT RATES WITH VARIOUS TREATMENTS AT 298 K gas pressure /Torr 1.98 0.51 1.12 5.07 6.46 1.37 3.77 0.63 0.67 0.59 0.61 0.46 0.71 2.00 0.3 1 0.59 surface count ratelmin-1 initial evacuation CzHz C& 2225 2179 2195 - 3518 3457 3463 - 7117 7090 7082 - - 820 782 - 1248 1212 1248 1248 4513 4369 4513 4513 2322 2267 2320 - 5655 5508 5655 - 482 482 481 - 2887 2823 2799 2887 4943 4921 4841 - 5112 5009 5112 5112 10350 10173 10350 10350 150 150 - - 180 178 82 180 1546 1528 - 1545 1612 1600 1612 - 2135 2112 1667 2135 3313 3267 2831 3313 9315 9298 9156 9315 1835 1794 1835 1835 3350 3297 3350 3350 Hz - I - 710 - 2176 4370 5513 - - - 3760 9023 - - - 1187 - - 1315 50 H2+C2H2 1060 1818 3557 - - - - - 115 2212 3 843 - - 25 - - - 875 1363 3838 - [l4C1 specific activities : 0.025 mCi(mmoI)-' (Rh), or 0.10 mCi (mmol)-l ( F W ) ; 0.05 mCi (mmol)-l (Pd) and 0.10 mCi (mmol)-l (Ir).200 ADSORPTION OF ['4C]ACETYLENE AND [l4C]ETHY LENE either a much reduced (- 50% with acetylene over palladium) or absent primary region in acetylene or ethylene adsorption. The steady state catalysts also showed a much reduced capacity for carbon monoxide adsorption compared with those of the freshly reduced catalysts. Evacuation for 1 h of steady state surfaces which had been equilibrated with varying pressures of [14C]acetylene or [I4C]ethylene had little effect on the surface count rates, as shown in table 2 (column 5 ) .Subsequent admission of 12.5 Torr non-radioactive acetylene to the [14C]acetylene precovcred surfaces, or of 12.5 Torr ethylene to the [14C]ethylene precovered surfaces, also had little effect on the surface count rate (table 2, column 6).Admission of 12.5 Torr acetylene to [l 4C]ethylene precovered surfaces, following evacuation of the gas phase [14C]ethylene, resulted in the displacement of small amounts of radioactivity, as shown in table 2 (column 6). With all catalysts this displacement was complete within 1 h and the amount of radioactivity displaced was independent of the surface concentration of the [14C]ethylene. I I I I 0 20 LO 60 80 103 time/& FIG. 4.-Effect of hydrogen on the adsorbed [14C]acetylene adsorption isotherms for Rh/silica (e), Ir/silica (H) and Pd/silica (0) in the steady state.The effects of hydrogen (37.5 Torr) on surfaces which had been equilibrated with various pressures of [14C]acetylene, the pressures being such that each steady state surface was covered to beyond the primary region, are shown in fig. 4. With each catalyst there is a rapid (10-20 min) initial removal of the secondary adsorbed species, followed by a further slow (- 2 h) decrease in the surface count rate. The decrease in count rate at the end of the rapid stage is independent of the initial surface con- centration of the secondary adsorbed acetylenic species, as shown in table 2 (column 8). Admission of 50 Torr of a hydrogen : acetylene (3 : 1) reaction mixture to a [ 4C]acetylene precovered steady state surface, following evacuation of the residual gas phase [l 4C]acetylene, yeilded similar results to those observed when hydrogen alone was used (table 2, column 9).In order to examine the adsorption of ethylene in the presence of acetylene, thereby obtaining information regarding the possible thermodynamic effect in acetylene hydrogenation, [14C]ethylene was admitted to a steady state catalyst in the presence of 12.5 Torr of gas-phase acetylene. As shown in fig. 5, the extent of adsorption of the ethylene on the steady state palladium and iridium catalysts wasA . S . AL-AMMAR AND G . WEBB 201 almost the same as that on a similar catalyst in the absence of acetylene. With rhodium, the presence of the acetylene reduced the adsorptive capacity of the surface for ethylene by z 30 %, although this value is much larger than the amount of [‘“CI- ethylene displaced by the admission of 12.5 Torr acetylene to an ethylene precovered steady state rhodium surface.The effect of acetylene hydrogenation upon the surface concentration of [‘“CI- ethylene was investigated by admission of 50 Torr of a hydrogen : acetylene (3 : 1) reaction mixture to a steady state catalyst which had been previously exposed to various pressures of [‘“Clethylene. When, before admission of the reaction mixture, the residual C1”C]ethylene in the gas phase had been evacuated from the reaction vessel, gas-phase count rate/10-3 min-I FIG. 5.-Adsorption of [14C]ethylene on (a) 0.51 g Irlsilica, (b) 0.20 g Rh/silica and (c) 0.10 g Pdlsilica in the steady state in the absence (0) and presence of 12.5 Torr acetylene (0).the surface count rate decreased quite rapidly on addition of the reaction mixture to a limiting value as shown in table 2 (column 9). However, when the residual [l”C]ethylene was allowed to remain in the reaction vessel, or 5 Torr of [14C]ethylene was admitted to the vessel after the reaction mixture, the surface count rate decreased only fractionally until the pressure fall in the system corresponded to the virtual removal of all of the acetylene. At this point, as shown in fig. 6 , the surface count rate decreased quite rapidly. During the acetylene hydrogenation the selectivity was 0.94 (Pd), 0.75 (Rh) and 0.16 (Ir), values similar to those reported previous1y.l Throughout the acetylene hydrogenation small yields of [I4C]ethane, which increased linearly with pressure fall, were observed.With a gas pressure of 5.0Torr [l4“C]- ethylene, the [I4C]ethane yield constituted 12.7 % (Pd), 9.5 % (Rh) and 2.8 % (Ir) of the total ethane yield. The reactivity of the acetylenic species adsorbed on the primary region of a freshly reduced catalyst was examined by covering the surface with [14C]acetylene to a point corresponding to the completion of the primary region. A mixture of hydrogen and202 ADSORPTION OF ['4C]ACETYLENE AND ['4C]ETHYLENE timelmin (0 and 0 ) 0 40 80 120 160 200 240 280 320 I I 8 1 01 I I I I I I I I I 0 20 40 60 80 100 120 140 160 timelmin FIG. 6.-Variation of the [14C]ethylene surface count rate during the hydrogenation of 12.5 Torr acetylene and 37.5 Torr hydrogen in the presence of 5 Torr [l4C]ethylene for silica-supported rhodium (O), iridium (a) and palladium (U).acetylene (3 : 1 ratio) was admitted to the catalyst to a pressure of 50 Torr and the surface and gas phase monitored for radioactivity. No change in surface count rate was detectable over a period of 6 h (time of reaction, 2-4 h), with any of the catalysts, although when the reaction product mixture was left in contact with the surface for 72 h, some of the surface species had been hydrogenated to [I4C]ethane, as shown in table 3. TABLE 3 .-EXTENT OF PERMANENT RETENTION OF [14C]ACETYLENE IN THE PRIMARY ADSORPTION REGION AT 298 K time of treatment in hydrogen = 72 h surface count ratelmin-1 % catalyst initial final retention PdIf3J203 2595 63 1 24 Pd/Si02 2905 491 17 Rh/SiOz 5696 1821 32 Ir/Si02 8007 3904 49 DISCUSSION From the results presented above a number of features emerge regarding the adsorption of acetylene and ethylene, both separately and in competition, on sup- ported palladium, rhodium and iridium catalysts.In particular, these results show that, in agreement with our previous observations using supported rhodium catalysts,l O over all the catalysts studied the hydrocarbon adsorption isotherms show two distinct regions : a steep primary region followed by a linear secondary region. The relevance of these two regions to hydrogenation will be discussed below. The results also show that when acetylene and ethylene are allowed to compete for the same surface,A . S . AL-AMMAR AND G . WEBB 203 each is adsorbed independently of the presence of the other.This observation sug- gests that, contrary to earlier suggestion^,^-^ thermodynamic factors may not play an important role in determining the selectivity observed in acetylene hydrogenation. From the adsorption isotherms shown in fig. 3 and 5 it is apparent that on steady state catalysts the adsorption of ethylene is limited to the secondary region. Similarly, for steady state rhodium and iridium catalysts the adsorption of acetylene is also limited to the secondary region, although with palladium there appears to be a part of the primary region which can be regenerated with hydrogen at ambient temperature. These observations, together with the low reactivity of the primary adsorbed acetylenic species and the independent hydrogenation of ethylene and acetylene, during the hydrogenation of the latter, indicate that the species of importance from the stand- point of hydrogenation is located on the secondary region.The question arises as to the nature of the species which constitute the primary and secondary regions and as to their location on the surface. The absence of an ethylene primary region on the steady state catalysts, suggests that in this region the acetylene and ethylene are adsorbed at the same surface sites, although as shown in table 1 the actual amounts of ethylene and acetylene adsorbed on these sites on freshly reduced catalysts differ widely from metal to metal. From the amounts of ethane formed during the primary adsorption of acetylene, the average composition of the adsorbed acetylenic species in this region can be calculated as C2Hle4(Pd) ; CzHl .,(Rh) and C2Hl.&r), assuming that, as observed previ- ~ u s l y , ~ ~ * l4 the amount of catalyst hydrogen available for hydrogenation is negligible. The composition of the primary adsorbed species, calculated as above, is also inde- pendent of the extent of coverage of the primary region. These results indicate that the species adsorbed on the acetylenic primary region are predominantly dissociatively adsorbed. The amounts of carbon monoxide adsorbed on the freshly reduced catalysts correspond to CO to total metal atom ratios of 4.27 (Pd), 2.44 (Rh) and 1.88 (ir, these values being similar to those expected from the particle sizes as determined by electron microscopy, which showed that the average particle sizes were 85 A (Pd), 25 A (Rh) and 18 A (Ir), assuming that the carbon monoxide is adsorbed in the linear mode.15 Further, in all cases the relative amounts of carbon monoxide and acetylene adsorbed on the primary region are in the approximate ratio of 2 : 1.These observations are consistent with the postulate that the primary adsorption occurs directly on the exposed metal and that, as suggested previously,1° the turning point in the adsorption isotherms corresponds to monolayer coverage of the metal with hydrocarbon. The small amounts of carbon monoxide adsorbed on the steady state rhodium and iridium catalysts, 5.9% of that adsorbed on the freshly reduced surfaces, can be interpreted in terms of the adsorption of carbon monoxide on single metal sites left vacant following the dissociative adsorption of acetylene. With the steady state palladium catalysts the extent of carbon monoxide adsorption is 59% of that of the freshly reduced catalysts, in agreement with the observation that, with this metal, the primary region is reduced to - 50% by the deactivation process.According to the above suggestions, the secondary adsorbed species arise from the adsorption of amounts of ethylene and acetylene in excess of that required for monolayer coverage. Indeed from the adsorption isotherms it can be deduced that, for example, at a gas pressure of 5 Torr of acetylene, the ratio of the adsorbed acetylene to the total number of metal atoms is in excess of unity. It was suggested previously lo that the secondary adsorption arose from " spill-over " of the hydrocarbon from the metal on to the support.However, the present results show that the extent of secondary adsorption is similar over all the catalysts and, with palladium-silica and204 ADSORPTION OF [14C]ACETYLENE AND [l4c]ETHYLENE palladium-alumina, appears to be independent of the support. Furthermore, on catalysts which have been treated with hexamethyldisilazane, which effectively covers the support surface with trimethylsilyl groups, the extent of secondary adsorption is the same as on untreated catalysts.16 Since, as noted by Levy and Boudart,17 the phenomenon of " spill-over " requires suitable sites on the support, it seems unlikely that the present observations can be satisfactorily interpreted in terms of " spill-over ".It has recently been suggested from LEED studies of acetylene adsorption on Pt(l11) surfaces that, at high pressures, the acetylene forms an overlayer on the initially adsorbed acetylene on the meta1.l' Although, in the present studies, the precise chemical nature cf the secondary adsorbed acetylene or ethylene could not be deduced, the results are not inconsistent with the suggestion that the secondary adsorbed species arise from the formation of overlayers on the primary adsorbed material. The results also lend support to the recent suggestions of Thomson and Webb regarding the importance of the formation of dissociatively adsorbed carbonaceous species in catalytic hydrogenation of unsaturated hydrocarbons by metals.Their model for hydrogenation depends upon hydrogen transfer be tween adsorbed C&, which is permanently retained on the surface, and the hydrocarbon undergoing hydrogenation. There is in this paper what seems to be convincing evidence for formation of the primary adsorbed species followed by further hydrocarbon adsorp- tion on that layer and the subsequent hydrogenation of this secondary species. The observation that, even with the secondary adsorbed ethylene or acetylene, the adsorbed hydrocarbon would not undergo molecular exchange with gaseous hydrocarbon, nor could it be removed by evacuation, shows that the adsorption was effectively irreversible. Whilst this conclusion is in agreement with those drawn far acetylene adsorption from studies of acetylene-deuterium exchange,20 it is in sharp contrast with the behaviour of ethylene adsorption over alumina-supported rhodium and palladium catalysts at low temperature,21 as deduced from ethylene- deuterium exchange studies, where it was concluded that ethylene adsorption was readily reversible.These observations, together with the lack of displacement of any appreciable amounts of ['4C]ethylene by acetylene, may suggest, at first sight, that the species formed during the adsorption process are of little relevance to hydro- genation. However, this is not the case, since the results obtained are independent of whether or not the catalyst was evacuated at high temperature following activation, or, more importantly, whether or not hydrogen was present during the adsorption of the [14C]ethylene.The results obtained from the admission of [14C]ethylene to a catalyst in contact with an acetylene + hydrogen reaction mixture were identical with those obtained when the ethylene was admilted first. Rather surprisingly, the only effect of preadsorbed hydrogen, assumed to be present after the catalyst was allowed to cool to ambient temperature in hydrogen before evacuation, was to reduce the extent of the primary region. Whilst the reasons for this are not readily apparent, it would appear that the presence of preadsorbed hydrogen in some way acts as a poison for the formation of the dissociatively ad- sorbed primary species. From the behaviour of added [I Tlethyiene during acetylene hydrogenation we conclude that the adsorptions of acetylene and ethylene occur at independent sites on the secondary region.Since, with palladium and iridium, the extent of ethylene adsorption is the same in the presence or absence of acetylene, it may be concluded that the differences in the rates of ethylene hydrogenation in the presence and absence of acetylene, observed previously 5 3 and in the present work,16 are not due to any variation in the surface coverage of the hydrocarbon, but are probably due to differ- ing hydrogen availabilities in the two systems. Similar effects have been observedA . S . AL-AMMAR AND G. WEBB 205 in the competitive hydrogenation of cycloalkenes over metal catalysts.22 This conclusion will be discussed more fully in a subsequent paper, when the effects of added ethylene and of the poisoning effects of carbon monoxide on acetylene hydro- genation will be discussed.In conclusion, the studies reported in this paper have shown that on the catalysts studied, the hydrogenation of acetylene and ethylene is associated with the secondary adsorbed species, which are not adsorbed directly on the metal, but which probably exist as an overlayer on dissociatively adsorbed hydrocarbon. Further, under acetylene hydrogenation conditions, the adsorption isotherms show that the con- centration of adsorbed species is directly proportional to the gas phase pressure up to pressures > 22 Torr. The competitive adsorption of ethylene and acetylene also shows that in the secondary region on the surface, the adsorption of ethylene is not affected by the presence of acetylene and, therefore, that thermodynamic effects of the type suggested previously are unlikely to play any part in determining the selectivity in acetylene hydrogenation.The authors are grateful to the Government of Iraq for financial support to one of us (A. S. Al-M.). We are also grateful to Prof. S. J. Thomson for many helpful and stimulating discussions. G. C. Bond and P. B. Wells, Adv. Catalysis, 1963, 15, 91. P. B. Wells, Surface and Defect Properties of Solids (Specialist Periodical Reports, Chemical Society, London, 1972), vol. 1, p. 24. G. Webb, in Comprehensive Chemical Kinetics, ed. C. H. Bamford and C. F. H. Tipper (Elsevier, Amsterdam), vol. 20, chap. 1, in press. G. C. Bond, D. A. Dowden and N. Mackenzie, Trans. Faraday Soc., 1958,54,1537. G. C. Bond and P. B. Wells, J. Catalysis, 1965, 4, 211 ; 1966, 5, 65 ; 1966, 5, 419. G. C. Bond, G. Webb, P. B. Wells and J. M. Winterbottom, J. Catalysis, 1962, 1, 74. C. Kemball, W. T. McGown, D. A. Whan and M. S . Scurrell, J.C.S. Faraday I, 1977,73,632. * K. C. Khulbe and R. S. Mann, Proc. 6th Irzt. Congr, Catalysis (London, 1976), preprint no. A35. A. J. Bates, Z. K. Leszeynski, J. J. Phillipson and P. B. Wells, J. Chem. Soc. (A), 1970, 2453. R. B. Bernstein and T. I. TayIor, Science, 1947, 21,498. lo J. U. Reid, S. J. Thomson and G. Webb, J . Catalysis, 1973, 30, 372. l2 G. F. Taylor, S. J. Thomson and G. Webb, J. Catalysis, 1968, 12, 191. l3 J. A. Altham and G. Webb, J. Catalysis, 1970, 18, 133. l4 Z . Paal and S. J. Thomson, J. Catalysis, 1973, 30, 96. l 5 G. Blyholder and C. Marvin, J. Amer. Chem. Soc., 1969,91, 315. l 6 A. S. Al-Ammar and G. Webb, unpublished results. R. B. Levy and M. Boudart, J. Catalysis, 1974, 32, 304. E. L. Kesmodel, P. C . Stair, R. C . Baetzold and G . A. Somorjai, Phys. Rev. Letters, 1976,36, 1316. l 9 S. J. Thomson and G. Webb, J.C.S. Chem. Comm., 1976, 526. 2o G. C. Bond and P. B. Wells, J. Catalysis, 1966, 6, 397. 2 1 G. C. Bond, J. J. Phillipson, P. B. Wells and J. M. Winterbottom, Trans. Faraday SOC., 1966, 22 J. K. A. Clarke and J. J. Rooney, Adu. Catalysis, 1976, 25, 125. 62,443. (PAPER 7/714)