J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2119-2124 Hydrogenation Behaviour over Si0,-supported Lanthanide-Palladium Bimetallic Catalysts with Considerable Hydrogen Uptake Hayao Imamura," Koji Igawa, Yoshie Kasuga, Yoshihisa Sakata and Susumu Tsuchiya Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University,2557 Tokiwadai, Ube 755 Japan Novel lanthanide-containing catalysts prepared by the use of dissolution of lanthanide metals in liquid ammonia have been studied. The hydrogenation of propene was carried out at 193-263 K over Si0,-supported lanthanide-palladium bimetallic catalysts (Ln-Pd/SiO,; Ln = Eu and Yb) obtained when the dissolved lanthanide in liquid ammonia reacted with 5 wt.% Pd/SiO, . Ln-Pd/SiO, showed remarkable synergetic effects between the lantha- nide and palladium metal involving considerable hydrogen uptake during the hydrogenation of propene.The rapid hydrogen uptake by Ln-Pd/SiO, occurred at the start of the reaction, followed by the hydrogenation of propene with a definite induction period. The catalyst contained reactive hydrogen species which were able to efficiently hydrogenate the adsorbed propene. The kinetic studies indicated that the hydrogenation predomi- nantly proceeds through a reaction path using hydrogen taken up by the catalyst. The presence of adsorbed propene on the catalyst surface was important to induce a promoting effect towards hydrogen uptake with subsequent hydrogenation of propene. Since lanthanide (Ln) elements have specific electron configu- Ind.Ltd.) was dried through a calcium oxide column and rations based on 4f orbitals, they are expected to catalyse through a sodium hydroxide column before use. Propene was various reactions that cannot be achieved with d-block tran- of research purity and further purified by triple distillation. sition metals. Recently, there has been increasing interest in the specific properties of lanthanides and related compounds as heterogeneous catalysts.' Procedures of Catalyst Preparation and Catalytic Reactions Eu and Yb readily dissolve in liquid ammonia to yield a The Si0,-supported Pd catalyst was prepared by impregnat- homogeneous solution containing solvated electrons.' When ing SiO, with aqueous solutions of PdCl, to 5 wt.% palla- transition-metal powder is added to this solution, the dis- dium. After drying and subsequent reduction at 623 K withsolved lanthanide metal in liquid ammonia is found to react flowing hydrogen as a standard pressure, palladium waswith the transition metal to form novel lanthanide metal detected in the form of small particles with a mean diameter overlayers and lanthanide-containing bimetallic compounds.of ca. 9 nm with the surface area of 55.8 m2 g- '. Reduced Pd By using the solvating ability of liquid ammonia for the powders were conventionally prepared by the incipientlanthanide metals, we have recently developed methods for wetness technique. the preparation of new catalysts containing lanthanides and The method of the lanthanide addition to the Pd catalyst have demonstrated that they exhibit specific catalytic proper- was similar to that previously described for the Ni catalyst8 ties. Our aim in the study of interactions of the lanthanide 5 wt.% Pd/SiO, that had been reduced was placed in awith the transition metal has been to reveal the correlation of Schlenk tube containing a solution of liquid ammonia (15-20 the electronic and geometric effects of bimetallic catalysts cm3) at 198 K. Eu or Yb was added to the Pd catalyst sus- with the catalytic pended in liquid ammonia with vigorous stirring.Upon dis- We now report catalytic behaviour of novel Si0,-solution of the lanthanide metal in liquid ammonia solvent, a supported Ln-Pd bimetallic catalysts (Eu-Pd/SiO, and blue homogeneous solution was immediately formed, which Yb-Pd/SiO,) obtained when the lanthanide metal dissolved was characteristic of solvated electrons., The blue colour in liquid ammonia reacts with 5 wt.% Pd/SiO, .Ln-Pd/SiO, gradually disappeared as a result of the reaction of the dis- shows synergetic effects between the lanthanide and palla- solved lanthanide metal with the Pd catalyst. On disap- dium metal involving considerable hydrogen uptake during pearance of the blue colour, the reaction tube was allowed to the hydrogenation of propene. The effects of the lanthanide warm to room temperature and the excess of ammonia was metal overlayer on hydrogen uptake and the related catalytic pumped off leaving Si0,-supported Eu-Pd and Yb-Pd behaviour over the surfaces of Si0,-supported palladium are bimetallic catalysts.Unsupported Ln-Pd catalysts were simi- investigated. Much attention is devoted to the phenomena larly prepared using the reduced Pd powders instead of associated with the action of hydrogen taken up in the cata- Pd/SiO,. The content of lanthanides in the bimetallic cata- lyst and their catalytic consequences in connection with the lyst was represented by the fraction of the at.%. All sample synergism of this bimetallic system. preparation steps were carried out in an atmosphere of dry argon without exposure to air, otherwise the catalysts became Experimental unreactive. Materials The catalytic reactions were performed on a recirculation reactor constructed of Pyrex glass and equipped with a Eu and Yb ingots (99.9%, Shin-Etsu Chemical Co.Ltd.) were mercury manometer. Prior to the reaction the catalyst was used in the form of turnings or granules. SiO, (380 m2 g-') subjected to evacuation treatment at 293-723 K for 2 h, set was the commercially available Degussa Aerosil 380. PdC1, at 193-263 K of the reaction temperature and then the (99.9% ; Rare Metallic Co. Ltd.) was commerically obtained hydrogenation was initiated by admitting H, and C,H,. The and used without further purification. Ammonia (Iwatani reacting gas in the system was periodically collected by a gas sampler and analysed by a Shimazu TCD gas chromato- graph with a column of active alumina. The gas composition during the reaction was determined by the mass balance between the quantities of propene and propane evaluated by gas chromatography (GC) and from the changes in pressure in the gas phase.Since the uptake of propene and propane by the catalyst was too small to be identified by manometric techniques, the hydrogen uptake was estimated from differ- ences between the quantities of the hydrogen obtained from the drop in pressure and the hydrogen used in the formation of propane. The hydrogenation rates per g of catalyst were found by measuring the rate of appearance of propane. The accuracy of the measurements on the recirculating reactor using GC and pressure changes was confirmed in the hydro- genation of propene which was carried out for reference over 5 wt.% Ni/SiO, (JRC-S3-5Ni; Reference Catalyst of the Catalysis Society of Ja~an).~ Analyses IR spectra were recorded on a JASCO FT-IR 7000 spectrom-eter equipped with a MCT detector.The samples for IR studies were prepared according to the method described above. To the solution of Eu or Yb in liquid ammonia was added, at 198 K, a disc of 5 wt.% Pd/SiO, that had been reduced at 623 K for 5 h. In a dry-argon glove bag the disc thus treated was carefully loaded into the IR cell of variable temperatures (193-673 K). IR spectra were obtained from the ratio of the background spectrum of catalysts to that of the species adsorbed on the catalysts. Extreme care was taken to prevent contamination by air. The X-ray diffraction patterns of the samples were obtained with a Shimazu X-ray diffractometer (VG-107R) using Cu-Ka radiation.Results and Discussion Hydrogenation of Propene over LR-Pd/SiO, :Influence of Lanthanide Addition 5 wt.% Pd/SiO, was highly active for the hydrogenation of propene at 193 K, whereas lanthanide metals exhibited very low or negligible activity under similar conditions.” Upon introduction of the lanthanide metal onto the Pd surface, the catalytic properties were markedly changed (Fig. 1). The hydrogenation was performed at 193 K over Yb-Pd/SiO, by the admission of a gaseous mixture of propene (35 Torr) and hydrogen (35 Torr). Yb-Pd/SiO, with lanthanide loading of 30 at.% [Yb(30”/.)-Pd(70%)/Si02] showed the rapid hydro- gen uptake over a period of ca. 10 min at the start of the reaction, followed by the hydrogenation of propene. No reac- tion of propene with hydrogen was observed during the initial hydrogen uptake; there was an obvious induction period for the hydrogenation. As shown in Fig.1, the catalyst contained reactive hydro- gen species able to hydrogenate propene efficiently as a means of extracting hydrogen. All of the hydrogen taken up by the catalyst was consumed, but the hydrogenation still continued (Fig. 1). From an analysis of the material balance between hydrogen and propene as reactants and the propane product, it seems that the hydrogenation of propene proceeds by consuming both the hydrogen taken up by the catalyst and the hydrogen in the gas phase. However, this does not necessarily imply that propane is produced through separate reaction paths, considering rapid hydrogen uptak-xtract cycles as shown later.Under the reaction conditions exam- ined, the hydrogen was not left in the catalyst when the reac- tion was completed. The general trend in the effects of 40. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 20 t 0 20 40 60 t/min Fig. 1 Hydrogenation of propene at 193 K over Yb(30”/,)-Pd(70’/,)/Si02. 0, Gaseous hydrogen ; 0, hydrogen uptake; a,propane. The catalyst (0.042 g) was evacuated at 633 K for 2 h before the reaction. P(C,H,) = 35 Torr, P(H2)= 35 Tom. lanthanide metals on the catalytic behaviour over the Pd metal surface was reproducible in runs using Ln-Pd/SiO, obtained from palladium samples prepared separately, although there were differences in degree.The Eu-Pd and Yb-Pd systems exhibited similar behaviour in the composi- tion ranges investigated. It has often been shown that Pd catalysts simultaneously absorb hydrogen during the reactions in which hydrogen par- ticipates.’ ’ However, X-ray diffraction spectra of Yb(30?Ao)-Pd(70%)/Si02 showed only the existence of metal- lic palladium in the cubic structure with non-distinguishable changes in the lattice parameters and showed no indication of PdH after the hydrogen uptake. The amounts (0.115 mmol) of hydrogen species taken up by Yb(30%)-Pd(7O%)/SiO2 (0.042 g) (shown in Fig. 1) exceeded those corresponding to stoichiometric palladium hydride (PdH) by about 12-fold.Even if hydrogen is fleetingly absorbed in the Pd metal, the absorbed hydrogen is so stable under these reaction conditions that it cannot be thermody- namically desorbed to react rapidly with propene as shown in Fig. 1.’’ Consequently, no absorption of hydrogen by palla- dium as an acceptor site is expected. The lanthanide com- ponent present on the catalyst can also absorb hydrogen to form more stable metal hydrides (YbH, and YbH3),I3 but the circumstances are similar for the lanthanide. Unsupported Yb-Pd catalysts prepared by the reaction of Pd metal powders with dissolved Yb in liquid ammonia were examined. Fig. 2 shows the progress of the hydrogenation with time; substantial bulk uptake of hydrogen occurred during the reaction and the formation of PdH was confirmed by XRD.The hydrogen taken up here was too stable to react with propene and remained intact in the catalyst, as opposed to the results observed for Ln-Pd/SiO, . Some questions still remain as to which sites the hydrogen is accepted in the Ln-Pd/SiO, catalyst system. The rates of hydrogen uptake (vH) as a function of Eu or Yb content in Ln-Pd/SiO, are shown in Fig. 3. The rate showed a tendency to decrease with increasing loading of the lanthanide metal on the Pd surface. For the separate constit- uent, Pd/SiO, or Ln/SiO,, the hydrogen uptake hardly occurred under the same reaction conditions. Thus the Ln-Pd bimetallic system is an interesting example of syn- J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 L$ 20 10 0 0 100 200 300 400 500 t/mi n Fig. 2 Hydrogenation of propene at 193 K over unsupported Yb(3%)-Pd(97%). 0,Gaseous hydrogen; 0,hydrogen uptake; 0, propane. The catalyst (0.098 g) was evacuated at 633 K for 2 h before the reaction. P(C,H,) = 34 Torr, P(H,) = 35 Torr. ergism involved in an enhancement of the ability to take up hydrogen. The existence of some synergetic effects between lanthanide and palladium metals rather than individual com- ponent elements constitutes active sites for the hydrogenation with considerable hydrogen uptake. However, the reasons for the hydrogen uptake and the composition dependence caused by this synergism have not yet been clarified. The initial rates of hydrogenation as a function of lanthanide content simi- larly decreased with increasing lanthanide loading (Fig. 4).This similarity between the dependence of hydrogen uptake and hydrogenation strongly suggests that a possible path for propene hydrogenation is associated with the hydrogen species taken up by the catalyst. For the hydrogenation of the olefin, the Ln-Pd system exhibited a composition dependence which was different from other lanthanide-containing bimetallic systems4-’ so far studied. This might be attribut- able to a reaction scheme in which such hydrogen species strongly participate. In addition, the thermal pre-treatment of Ln-Pd/SiO, under vacuum conditions affected the catalytic behaviour. For the catalyst pre-treated at a lower temperature (293 K), no hydrogen uptake was observed and the slow hydro- 1 1 I I 1 I 0 20 40 60 Eu, Yb (Yo) Fig.3 Rates of hydrogen uptake us. lanthanide content in Eu-Pd/SiO, (0)and Yb-Pd/SiO, (0).The catalysts were evac- uated at 633 K for 2 h before the reaction. P(C,H,) = 35 Torr, P(H,) = 165 Torr. 0.4-I 0, r I .-C -E g 0.2 E -2 0 0 20 40 60 content (Yo) Fig. 4 Rates of propene hydrogenation us. lanthanide content in Eu-Pd/SiO, (0)and Yb-Pd/SiO, (0). genation occurred without any induction period. The rates of hydrogen uptake and hydrogenation varied markedly with changes in evacuation temperature of Ln-Pd/SiO, and they increased with increasing temperatures (293-723 K). The hydrogenation activity of Yb(3O%)-Pd(7O%)/SiO, evacuated at 723 K was ca.five-fold greater than that of the catalysts treated at 293 K. It was also found that for Ln-Pd/SiO, the number of active sites, evaluated from CO chemisorption, increased with increasing evacuation temperature (293-723 K). As reported previously for Ln-Co,’ Ln-Ni,4i6 Ln-Cu’ and Ln-Ag5 bimetallic systems, we have demonstrated that such thermal treatments result in rearrangement of surface morphology or structure, leading to appearance of enhanced catalytic activity. For Nd/Cu(l 11)14 and Yb/Ni(100),15 it has been previously found that the overlayer-to-intermetallic transition occurs by heating at elevated temperatures. Effects of Co-existent Propene There was no indication of hydrogen uptake by the catalyst even when Ln-Pd/SiO, was brought into contact with only hydrogen at 193 K.A very important feature of the present catalyst system is that hydrogen uptake occurred only in the presence of propene; the effect of co-existent propene is strongly suggested. To determine how the hydrogen uptake is affected by presence of a second gas, we carried out two types of reaction differing in sequence of hydrogen and propene addition. As shown in Fig. 5, the first reaction method con- sisted of exposing Yb(30%)-Pd(70%)/Si02 to 35 Torr of hydrogen at 193 K for 30 min, followed by addition of propene (35 Torr) (referred to as H, +C,H,). The addition of propene resulted in rapid hydrogen uptake by the catalyst, but for the reaction between propene and hydrogen there was also a definite induction period.The uptake of hydrogen by the catalyst was observed during this induction period. However, its observation only in the presence of propene does not imply that a hydrogenated intermediate whose surface concentration builds up before substantial desorption of the propane product. This is because the carbon balance between propene and propane was always maintained during the reaction; hence the possibility of a decrease in pressure due to its disappearance from the gas phase can be ruled out. Thus it is to be expected that the adsorbed propene plays an important role in the rapid initiation of hydrogen uptake. When the pressure of propene varied from 6 to 67 Torr under a fixed pressure of hydrogen (169 Torr), the results obtained at higher pressures of propene showed higher rates of hydro- gen uptake (Fig.6), followed by faster hydrogenation with a definite induction period. The rate of hydrogen uptake was J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20 I I t10 t 0 20 40 60 80 t/min Fig. 5 Effect of co-existent propene (introducing propene later; H, + C,H,). An arrow indicates the introduction of 35 Torr of propene. 0,Gaseous hydrogen; 0,hydrogen uptake; 0,propane. Yb(30%)-Pd(7O%)/SiO2 (0.044 g) was evacuated at 633 K for 2 h before the reaction. The hydrogenation was carried out at 193 K by introducing hydrogen (35 Tom) and subsequently propene (35 Torr). approximately proportional to the pressure of propene, reaching a value of 5.4 x lo-' mmol min-' g-' at 67 Torr.The rate of hydrogenation was equal to 2.0 x lo-' mmol min-' g-' at 6 Torr of propene, and increased in proportion to the propene pressure to reach a value of 1.1 mmol min-l g-at 67 Torr. Furthermore, at constant propene pressure, the reaction was performed at hydrogen pressures of 34-489 Torr. The rates of hydrogen uptake (Fig. 6) and hydro-genation were inversely proportional to the pressure of hydrogen. Upon addition of propene, hydrogen uptake immediately occurred; the adsorbed propene preferentially induces the hydrogen uptake irrespective of changes in its pressure. In marked contrast to this, there were still variable induction periods to initiate the hydrogenation.Thus it is reasonable to consider that there exists at least two separate sites on the surface of Ln-Pd/SiO, with which the adsorbed propene species are strongly associated: (i) hydrogen uptake and (ii) hydrogenation. c 1 0.1 11 I1111 I I I I11111 I I I I L 10 100 P/Torr Fig. 6 Dependence of hydrogen uptake on pressures of hydrogen(0)and propene (0).Yb(30%)-Pd(7O%)/SiO2 was evacuated at 633 K for 2 h before the reaction. The reaction was carried out at 193 K by introducing hydrogen and subsequently propene. $ 301 ;\In a second reaction, propene (35 Torr) was circulated over40 1 addition of C3H6 the catalyst at 193 K for 30 min and then hydrogen (35 Torr) was added into the circulating propene (C3H6+ H,; Fig.7). The induction period almost disappeared and immediately the hydrogenation of propene occurred. The production of propane usually agreed with the consumption of hydrogen in I quantities during the reaction; thus, little hydrogen uptake was observed in this case. In marked contrast to the case of the previous run (H,+C3H6), the rates of hydrogenation tended to increase proportionally with an increase in the pressure of hydrogen, leading to speculations of a different reaction pathway in this reaction condition, as described in the following section. It now seems certain that the predominant presence of propene on the catalyst surface is closely involved in occurrence of immediate hydrogenation as well as the ability to take up hydrogen.Upon addition of hydrogen and sub-sequent propene (H, -+ C,H,), rapid hydrogen uptake occurred but not in the reverse order (C3H, -+ H,); the sites for hydrogen uptake are formed in the presence of both hydrogen and propene, while the formation of sites for hydrogenation is possibre in the presence of propene alone. In the (H, 4C,H,) run, the active sites for hydrogenation are probably formed by the adsorption of propene on vacant sites of the catalyst surface mostly covered with hydrogen; therefore the induction period is required to a certain extent to form such catalytically active sites. IR spectra of adsorbed carbon monoxide on Yb-Pd/SiO, in the presence of co-adsorbed propene, which directly reflect variations in the nature of a catalyst surface, strongly sup-ported this view (Fig. 8).IR studies of adsorbed CO on Ln-Ni/SiO, have been reported.* Carbon monoxide was che-misorbed at 193 K with a pressure of 10 Torr. IR spectra of CO adsorbed on Pd/SiO, and Yb(20%)-Pd(8O%)/SiO2, which were evacuated at 673 K for 2 h as a pre-treatment, are shown in Fig. 8(a) and (b), respectively. Mainly two kinds of bands were observed at 2097 and 1993 cm-', which were assigned to the two adsorption states as linear and bridged adsorbed CO, respectively.16 IR spectrum of CO adsorbed on Yb-Pd/SiO,, which was exposed to propene (10 Torr) at 40 Jaddition of H2 30 L 5 *O 10 0 0 20 40 60 80 f/min Fig. 7 Effect of co-existent propene (introducing propene first; C,H,+H,).An arrow indicates the introduction of 35 Torr of hydrogen. 0,Gaseous hydrogen; 0,hydrogen uptake; 0,propane. Yb(30%)-Pd(7O%)/SiO2 (0.061 g) was evacuated at 633 K for 2 h before the reaction. The. hydrogenation was carried out by intro-ducing propene (35 Torr) and subsequently hydrogen (35Torr). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 L 2097 Q) CQ e8D 0.1 I 2071 II 2200 2000 1800 wavenumber/cm-’ Fig. 8 IR spectra of adsorbed CO on (a) Pd/SiO,, (b) Yb(20%)-Pd(8O%)/SiO2 and (c) propene-pre-adsorbed Yb(20%)-Pd(80%)/Si02 Fig. 9 Dependence of hydrogen uptake (0”) on pressures of hydro-..and propene 0,gen, Yb(3O%-Pd(7O%)/SiO, was evacuated at 193 K after the pretreatment, was shown in Fig. 8(c).It is noticed that the shapes of the spectrum in Fig. 8(c) were markedly changed compared to those in Fig. 8(a)and (b),e.g., the bands at 2097 and 1993 cm-’ decreased and the rela- tively low-frequency bands at ca. 2071 and 1925 cm-’ increased in intensity. It has been often argued from spectral studies that the presence of co-adsorbed hydrocarbons of dif- ferent types simply causes the frequencies of the CO peaks to shift to lower frequency.” In this case, it can be explained as follows: propene is an electron donor to the present bimetal- lic surface. Its pre-adsorption decreases the sites of CO adsorption attributable to the high-frequency bands and makes a greater charge density available for backdonation into the antibonding x* orbital of the adsorbed CO molecule, leading to the increase in intensity of the bands with such lower frequencies.Kinetic Behaviour over Ln-Pd/SiO, The reaction was carried out by introducing simultaneously a mixture of hydrogen and propene with varying partial pres- sures. Fig. 9 shows the initial rate (uH) of hydrogen uptake for Yb(30%)-Pd(7O%)/SiO2 as a function of C,H, and H, pres-sure. The initial rate of hydrogen uptake was proportional to the pressure of propene, rather than that of H, . The observed rate dependence on pressure may be described in the follow- ing form : kPPt)H = -(1) PH where k is the rate constant, and P, and P, the pressure of hydrogen and propene, respectively. Taking into consider- ation that the hydrogen uptake occurred only in the presence of propene and was in proportion to its pressure, it is reason- able to expect that the number involved in sites for the hydrogen uptake is proportional to coverage of adsorbed propene (O,), which is represented by assuming the Langmuir adsorption here.The number of the sites for hydrogen uptake is proportion- al to 0, KP PP8-‘-(l + KHPH + KpPp) where K, and K, are the adsorption coefficient of hydrogen and propene, respectively. The fraction (0,) of the surface 633 K for 2 h before the reaction. The reaction was carried out at 193 K by introducing a mixed gas of hydrogen and propene. covered with hydrogen is KH ‘H -(1 + KHPH + KpPp) The rate (uh) of hydrogen uptake seems to be proportional to the product of the concentrations of the sites (0,) and the surface hydrogen species (OH), which leads to where k’ is the rate constant.If the hydrogen adsorption is strong and the propene weak here, KH P, $ 1, K,Pp, the rate expression can reduce to k‘K, P, k”Ppuh=----(3)KHPH PH which is identical to U, [eqn. (l)] obtained experimentally, where k”( =k‘Kp/KH) is the constant. Fig. 10 shows the initial rate (up) of propene hydrogenation as a function of H, and C,H, pressure. As shown in Fig. 10, the rate of hydrogenation of propene over Yb-Pd/SiO, showed a negative dependence on the H, pressure and a 10 I I I I Ill11 I Ill 10 100 Pflorr Fig. 10 Dependence of propene hydrogenation (up) on pressures of hydrogen, 0and propene 0 I I \ 0,\o \ r \ I \ m \ 7 I C.- h E \ \ 0 c 0.1 4.0 4.5 5.0 103 KIT Fig.11 Arrhenius plots for the hydrogen uptake (0)and hydro- genation (0).Yb(30~o)-Pd(70'/,)/Si0, was evacuated at 633 K for 2 h before the reaction. The reaction was carried out by introducing a mixed gas of hydrogen (165 Torr) and propene (35 Torr). (---) See text. positive dependence on the C3H6 pressure, which was very similar to that observed for the hydrogen uptake (Fig. 9). The pressure dependence of vp was quite different from that for the lanthanide-free catalyst; it is generally accepted for con- ventional supported palladium catalysts that the hydro-genation is approximately first order with respect to hydrogen pressure and has little dependence upon alkene pressure.l8 Furthermore, the apparent activation energy for the hydrogen uptake and hydrogenation was almost equal between 193 and 263 K and was no more than ca. 3 kJ mol-' for some reason (Fig. 11). The close similarity in kinetic behaviour of hydrogen uptake and hydrogenation implies that the hydrogen taken up by the catalyst is strongly involved in a possible path for propene hydrogenation. In the (C3H6-+ H2)run which showed a different kinetic behaviour, an activation energy of 46 kJ mol- was determined for the hydrogenation of propene (see a dashed line in Fig. 11) and was significantly different from the above value. From Fig. 1 it seems as if some of the propane is produced by reacting propene with the hydrogen taken up by the cata- lyst and the rest arises from the reaction with gaseous hydro- gen.However, considering the similarity of the pressure dependence and the activation energy to those for the hydro- gen uptake, it can be concluded that the hydrogenation of propene over Ln-Pd/SiO, is by a rate-limited hydrogen uptake. Thus, the gaseous hydrogen is necessarily taken up to react with propene in this catalyst system and the reactivity of the hydrogen species taken up is high. Once the cata- lytically active sites for hydrogenation are formed, the hydro- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 gen uptake and subsequent hydrogenation process is rapid, and phenomena similar to spillover may be contained.The hydrogenation exclusively proceeds through a reaction path using the hydrogen taken up in the catalyst. 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