The Journal of Physical Chemistry, Vol. 82, No. 21, 1978 2323
Hydrogenation of Ethylene in Granular Copper
tronic polarization is neglected on the grounds that the electronic polarizability of the complex is equal to the sum of the electronic polarizabilities of the individual molec u l e ~ . ~As J ~can be seen from the above data and figures, wherever there is any interaction between the two nonpolar molecules, the electronic polarizability exceeds the additive values. Therefore the contribution of the electronic polarizability cannot be neglected if one wants to determine the exact dipole moment of the complex. Thus, the refractive index measurement method can be used for detecting the formation of a complex and to determine its equilibrium constant, even for a weak complex.
2.04
2 03
-4 2.02
Acknowledgment. The authors are thankful to Dr. C. M. Pathak, Department of Physics, for allowing us to use his laboratory facilities and to Professor 0. P. Malhotra, I
I
4
or
0.2
03
Co
04
I
Head, Chemistry Department, for providing the facilities and to U.G.C. for financial support.
MI1
Flgure 1. Plot of na2 against the concentration of benzene: (1) 0.5 mol L-' total concentration; (2) 0.3 mol L-' total concentration.
References and Notes R. Foster, "Organic Charge Transfer Complexes", Academic Press, New York, N.Y., 1969. P. H. Emsiie, R. Foster, C. A. Fyfe, and I. Hormann, Tetrahedron, 21, 2843 (1964). A. K. Colter and E. Grunwaid, J . Phys. Chem., 74, 3637 (1970). M. E. Baur, D. A. Horsma, C. M. Knobier, and P. Perez, J . Phys. Chem., 73, 641 (1969). (a) E. H. Lane, S. D. Christian, and J. D. Childs, J . Am. Chem. Soc., 96, 38 (1974); (b) S. D. Christian, J. D. Chiids, and E. H. Lane, ibM., 94, 6861 (1972). H. C. Tse and M. Tamres, J . Phys. Chem., 81, 1367, 1376 (1977). M. Tamres, "Molecular Complexes", Vol. I, R. Foster, Ed., Elek. Science, London, 1973. J. A. Riddlck and W. B. Bunger, "Techniques of Chemistry", Voi. 11, "Organic Solvents", A. Weissberger, Ed., Wiiey-Interscience, New York, N.Y., 1970. R. Foster and N. Kulevsky, J . Chem. Sm., Faraday Trans., 69, 1427 (1973). 2 . Yoshida and E. Osawa, Bull. Chem. SOC. Jpn., 38, 140 (1965). N. B. Jurinski and P. A. D. de Maine, J . Am. Chem. Soc., 86, 3217 ( 1964). M. Muanda, J. B. Nagy, and 0. B. Nagy, Tetrahedron Lett., 38,3421 (1974). N. Kulevsky, "Molecubr Association", Vol. I, R. Foster, Ed., Academic Press, New York, N.Y., 1975.
only a 1:l complex is formed in HMB-TCNE, is much lower than the value of K , which includes other types of complexe~.~ Here it must be noted that the literature data are under different sets of conditions;'J2 the value of K , depends on the experimental conditions such as whether A >> D, D >> A, or A = D, and also on the nature of the solvent. A change in electron cloud density in the neutral atom or molecule will lead to a change in the polarizability. Since the complex is generally more polar than the components, the electronic polarizability or the refractive index increases and the deviation depends upon the extent of interaction between the donor and acceptor. Therefore the stronger the complex, the larger the deviation in the refractive index values. The dipole moment of a complex depends on the total polarizability of the complex. However in the calculation of the dipole moment of the complexes, so far, the elec-
Hydrogenation of Ethylene in Granular Copper. Promoter Effect of Hydrogen P. Gajardo," M. C. Lartiga, and S. C. Droguett Depatiamento de 06mica, Facuffadde Ciencias &&as y MatemCiticas, Universidad de Chile, Casiiia 2777, Santiago, Chile (Received October 26, 1977; Revised Manuscript Rqceived May 31, 1978)
The promoter effect of hydrogen, in granular copper, on the hydrogenation of ethylene has been studied at 0 and 252 "C. The promoter effect observed at 252 "C is due to the presence of hydrogen occluded in the metal bulk, probably located near the reaction surface. At 0 "C, an increase in the catalytic activity of the copper is also observed after hydrogen pretreatment of Cu at 150 "C. The nature of this phenomenon is discussed in both cases. The effect of ethylene on copper is studied at both temperatures and self-hydrogenation of this gas is observed. The role of hydrogen adsorbed on the metal is discussed in relation to the reaction mechanism. The activation energy was determined at temperatures ranging from -21 to 0 "C for samples freshly pretreated
with hydrogen and for samples without treatment. Introduction The promoter effect of dissolved gases on the activity of metallic catalysts has been studied since 1935.1 Several *Address comeswndence to authorat theuniveniG Catholique de Louvain, Groupe de Physic0 Chimie Minikaleet de Catalyse, Plaee Croix du Sud, 1, B-1348Louvain-la-Neuve, Belgium.
0022-3654/78/2082-2323$01 .OO/O
investigations designed to clarify the nature of the active sites and the catalytic role of the dissolved gases within the metal have been carried Some interesting phenomena have been observed such as the existence of a narrow maximum in the curve of activity vs. concentration of gas in the metal.6 Attempts have been made to specify the step in the total process where the promoter 0 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 21, 1978
gas exerts its a ~ t i 0 n . l l - l ~ Interest in this problem persists because of its relation to the basic theory of catalysis16and its possible effect on data reproductibility and measured rate constants. Furthermore, it is important to know the influence of hydrogen occluded in metallic membrane catalysts1' upon their activities. Hydrogen has received the most attention as a promoter gas probably because this phenomenon has been observed on repeated occasions in hydrogenation reactions and because of the ease with which hydrogen is sorbed by many metals. The hydrogenation of ethylene at low temperatures and the promoter effect produced by sorption of hydrogen in metals of group 1B contradicts the d-character theory.ls According to this theory, these metals (Cu, Ag, and Au) should neither adsorb hydrogen nor hydrogenate ethylene since they do not have d-band vacancies indispensable in hydrogenation reactions. Experimental evidence has been r e p ~ r t e d for '~~ hydrogenation ~~ of ethylene on copper even at 0 "C. In addition, Mc Cabe and Halseyll observed that hydrogen adsorbed at 120 "C produces an enhancing effect on the hydrogenation of ethylene at 0 "C. Hall and HasselP have shown that this phenomenon also occurs when the reaction is carried out at temperatures of 27-50 "C, but Hall et al.I5 did not detect it between 75 and 125 "C. It was postulated that at higher temperatures the hydrogen responsible for the promoter effect is desorbed from the reaction surface. However, we have shown21that the effect occurs in copper membranes when ethylene is hydrogenated between 180 and 256 "C. It is necessary to know the location of the promoter hydrogen within the metal in order to determine the mechanism of this phenomenon. Some result^^-^ indicate that in the case of nickel the hydrogen is located in the bulk of the metal. We also determinedz1that in copper membranes, at high temperatures, the hydrogen is located in the bulk of the metal near the reaction surface. Other authors have pointed out that for nickello and coppernickel alloys13this promoter hydrogen would be adsorbed on the metal surface. The goal of this work is to study the promoter phenomenon in granular copper a t 0 and 252 "C. We also intend to clarify the existing contradictions about the location of the promoter hydrogen in the metal. Finally, some experimental results have allowed us to shed some light a t the role played by sorbed hydrogen in the hydrogenation of ethylene on copper.
Experimental Section The experiments were carried out under static conditions. Product analysis was done by chromatography. The reactor volume was 26 cm3, Liquid nitrogen and turnings of gold were used as traps to avoid poisoning the catalyst by mercury vapors. In the high temperature region, the temperature was controlled by means of a thermostated oven with a variation of f0.25 "C. For experiments below 0 "C, baths of alkaline salts solutions were used. The maximum temperature variation in this range was f0.15 "C during the time of the experiment. The molar composition of the reacting mixture was always constant at C2H4:H2= 1:l. Gases. The hydrogen was produced electrolitically and later purified. The purification train included copper turnings at 400 "C, silica gel, phosphorus pentoxide and liquid nitrogen traps, and a commercial purifier DEOXO. The ethylene employed (Matheson Gas Product, Research grade) contained 10 ppm of N2, 5 ppm of 0 2 , and 5 ppm of CH4. Catalyst. The granular copper was prepared as de-
P. Gajardo, M. C. Lartiga, and S. C. Droguett
scribed by Best and Russellz2 starting from nitrate trihydrated (Analar) and ammonium carbonate (Analar). The copper oxide obtained was reduced for 17 h at 400 "C with a hydrogen flow rate of 30 cm3/min at atmospheric pressure. Afterward, the 28-35 mesh fraction was separated and used for the experiments. The area of the catalyst, determined23by argon adsorption (15.2 A2), was 5 m2/g. Because the copper surface oxidizes rapidly in contact with air at room temperature, each sample was reduced again in situ when placed in the reactor (24 h under the same conditions). The amount of catalyst employed in the high temperature experiments was 0.2 g while at low temperatures (below 0 "C) it was 10.8 g. Method. Before starting each series of experiments, the catalyst was evacuated for 10 min and then the reactant mixture was introduced at a selected pressure. After the desired reaction time, the products were extracted and analyzed. Between experiments the reactor was evacuated for 5-10 min.
Results Separate experiments, designed to detect the enhancing effect of hydrogen and the effect of ethylene on the catalyst, were performed at 0 and 252 "C. In some experiments a mixture of CzH4 + H2 (PH2= PC2H4= 90 Torr) was diluted with helium to 1atm. The reaction rate was found to be independent of the concentration of He at both temperatures indicating that the reaction rate is not controlled by diffusion of the reactants. Promoter E f f e c t of Hydrogen and Effect of Ethylene at 0 "C. To verify the existence of the promoter effect at 0 "C, a treatment with hydrogen was performed. The catalyst was heated for 30 min at 150 "C at a hydrogen pressure of 200 Torr. After cooling to 0 "C, the reactor was evacuated for 5 min. The reaction was carried out by introducing the reaction mixture into the reactor at 104 Torr. After 2 min, the gas was removed and analyzed. The activity, A , is defined as the number of moles of ethylene converted per gram of catalyst per minute. It thus corresponds practically to the initial reaction velocity. Due to the slow rate, after 2 min there is still a linear relationship between the amount of ethane produced and the time of reaction. The temperature of this treatment (150 "C) was selected by taking into account the results obtained by Mc Cabe and Ha1sey.l' These authors reported that the promoter hydrogen is adsorbed above 120 "C. A higher temperature was not employed in order to avoid the penetration of the hydrogen into the copper bulk which starts to be observed around 190 0C.24 It should be noted that since the Cu was heated at 400 "C for 24 h during its previous reduction, hydrogen is occluded in the Cu bulk. Since this hydrogen can escape from the bulk only at a high degasification temperature,21* its concentration in the Cu bulk must remain constant in the experiments carried out at 150 "C. Therefore, if a change in the catalytic activity is observed after hydrogen treatment at 150 "C, it must be connected to the action of hydrogen occurring only on the Cu surface. In general, the results were reproducible within experimental error. Nevertheless, as observed by Mc Cabe and Halsey,'l in the first experiment carried out with a freshly prepared sample, the catalyst has a high activity that decreases abruptly in the next experiments and cannot be restored to the original value with hydrogen. After several experiments, the catalyst activity reaches a steady state. In Figure la, only before experiment 1was the catalyst
The Journal of Physical Chemistry, Vol. 82, No. 21, 1978 2325
Hydrogenation of Ethylene in Granular Copper ,
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TABLE I: Effect of Ethylene on the Catalytic Activity of Copper at 0 Oca
I
P c ~ H ~ Pmix, ! A , molk Torr Torr treact x 106 105 2 min 1.29 44 l h 1.49 105 2 min 0.97 44 l h 1.26 105 2 min 0.85 6 44 l h 1.14 7 150 over night 1.30 Pc2H, is the pressure of ethylene; Pmix the pressure of the ethylene-hydrogen mixture (C,H,:H, = l:l), treact, the reaction time, and A , the number of moles of ethane produced per gram of catalyst. no. 1 2 3 4 5
't O
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Number of experiment
Figure 1. Dependence of the catalytic activity on the number of experiments at 0 "C (PH1= Pep, = 52 Torr).
given the preliminary treatment with hydrogen at 150 "C. Experiments 2-7 were carried out successively, the reactor being evacuated for 5 min at the reaction temperature (0 "C) prior to each experiment. The activity decreases strongly during the first two experiments, reaching a steady state after a few experiments. These results show that the catalyst treated with hydrogen possesses effectively a higher activity probably due to the existence of hydrogen chemically adsorbed on the reaction surface. In order to elucidate the role of ethylene in this phenomenon, later experiments were carried out with the same sample (Figure lb). The same method as employed for curve a was used, except that in this case the catalyst was treated with ethylene a t 0 "C and a pressure of 44 Torr for 1h, after the previous treatment with hydrogen (before experiment 1,curve b). The results show that the ethylene produces a strong decrease in the catalytic activity (experiment 1). The activity is increased in experiment 2 reaching a steady value slightly lower than that in curve a (experiments 3-6). The reactor was then filled with ethylene (150 Torr) and held at 0 "C overnight. Experiments 7 and 8 (curve c) were performed successively without previous treatment with hydrogen. A new decrease in the catalytic activity is observed in experiment 7 (compared to that of experiment 1,curve b) followed by an increase in experiment 8. The catalyst was then treated with hydrogen at 150 "C and as usual an increase in the catalytic activity was observed (experiment 9). The activity level obtained was the same as that in experiment 3, curve a. The reactor was then evacuated for 71 h at 150 "C and experiment 10 was performed under conditions similar to those of the previous experiments. As shown in Figure IC,the activity in this case reaches a value similar to that in experiment 8, creating a new lower stationary state curve c. Table I shows the results of a series of experiments designed to study in more detail the action of ethylene over copper. Initially, the catalyst was exposed to the Hz CzH4 mixture (104 Torr) for 2 days at 0 "C to allow the surface of the catalyst to reach the most stable working condition. Each experiment in Table I was performed after evacuation for 5 rnin at 0 "C with the exception of the first where the catalyst was held for 39 h under vacuum at room temperature. Experiments 1-6 were alternately carried out with a mixture of hydrogen + ethylene and ethylene alone. The amount of ethane (the only detected product) formed per gram of catalyst, Al, is shown in the fifth column. In order to see if the activity is constant when the copper is treated with ethylene for longer times, ex-
+
periment 7 was conducted after exposing the catalyst to ethylene overnight at 0 "C and 150 Torr. The results of this experimental series show the existence of so-called (C2H2)ads+ CzHs), also self-hydrogenation (2C2H4 observed with other metals,25~26,~30 except that in this case this phenomenon produces an irreversible poisoning of only a small part of the copper surface. The subsequent addition of Hz a t 0 "C to this catalyst (after experiment 7) resulted in the formation of a considerable amount of ethane. This type of experiment was repeated on the other granular copper samples and the same phenomenon was observed. It is important to point out that the amount of ethane produced by self-hydrogenation is independent of both the exposure time and the pressure of ethylene (experiments 2 and 7 in Table I) which may indicate that the surface where the reaction takes place is approximately the same in both cases. Activation Energy. The activation energy of the reaction was determined between 0 and -21 "C. The results of these experiments are shown in Figure 2. The apparent activation energies found for samples freshly treated with hydrogen (curve 1) and for samples without treatment (curve 2) were 7 and 11 kcal/mol, respectively. Hydrogen Promoter Effect and the Action of Ethylene ut 252 "C. As already mentioned, when the catalyst is placed under a hydrogen atmosphere at 252 "C the hydrogen can penetrate into the bulk of the ~ o p p e rbut ~~i~~ any hydrogen chemisorbed at the surface desorbs quickly when the reactor is evacuated. It is then possible to obtain working conditions such that the copper surface has no chemisorbed hydrogen but hydrogen is dissolved in the bulk of the metal. If any enhancing effect is observed under these conditions, it must be due to the action of the hydrogen in the copper bulk. The first experiments at this temperature were performed to determine if it is possible to obtain reproductibility with a fresh catalyst previously treated with hydrogen. The pressure of the ethylene-hydrogen mixture was 180 Torr and the reaction time was 2 min. The results are shown in Table 11. Listed in the second column are the times employed for the treatment with hydrogen carried out before some of the experiments. During this treatment, the catalyst was placed under hydrogen at 282 Torr and 252 "C. Before each experiment and after the hydrogen treatments, the reactor was evacuated for 10 min at the same temperature. The decrease in the activity can be better viewed by the use of the relation ( A ' - A'?/A' X 100,i.e., the percentage decrease in the activity, as shown in the fourth column; A' being the activity of one experiment and A" that of the next experiment. In contrast to the experiments carried out a t 0 "C, a continuous decrease in the catalytic activity was observed
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The Journal of Physical Chemistry, Vol. 82, No. 21, 1978
TABLE 11: Influence of Hydrogen Treatment on the Activity of a Freshly Prepared Samplea no.
t, min
104A mol/g min
(A' - A")/A' x 100
t, min
no.
104A mol/g min
(A' - A " ) / A ' x 100
15 0 3.37 2.5 16 1020 3.45 - 2.5 17 0 3.41 1.2 18 0 3.19 6.3 0 3.19 0 19 20 90 2.89 9.5 21 0 2.76 4.5 22 0 2.68 3.1 23 1072 2.76 3.2 24 0 2.72 1.6 11 25 0 2.68 1.6 12 26 90 2.42 9.6 13 27 0 2.33 3.6 14 28 0 1.80 1.8 The reaction temperature was 252 C. t is the time of previous hydrogen treatment at 282 Torr and 252 C; A , the catalytic activity, ((A' - A " ) / A ' )X 100, the percentage decrease of the catalytic activity (see text); and Pc,H, = PH, = 90 Torr. 1 2 3 4 5 6 7 8 9 10
273 0 0 0 90 0 0 0 0 0 0 90 0 0
6.99 5.87 5.35 5.01 5.70 4.49 4.68 4.58 4.58 4.32 4.14 3.63 3.54 3.45
16.0 7.1 6.4 -13.8 21.2 -4.3 0.2 0.0 5.7 4.0 12.5 2.4 2.5
O
O
TABLE 111: Effect of Hydrogen Treatment on the Catalytic Activity of the Sample Used in Obtaining Table I1 after Its Torra Degasification at 413 "C and 104~ (A' - A")/A' t, min mol/g min x 100 29 0 0.43 39 0 0.34 8.7 30 0 0.39 8.7 40 0 0.33 2.9 31 0 0.37 5.7 41 60 0.39 -13.1 32 5 0.37 0 42 0 0.37 5.7 33 0 0.35 4.4 43 1200 0.44 - 20.7 34 0 0.34 1.7 44 0 0.40 9.9 35 10 0.40 - 12.7 45 0 0.41 - 2.8 0.84 - 105.4 36 0 2.5 46b 900 0.39 9.0 37 0 0.37 47 0 0.77 1.8 38 15 0.38 0.73 4.8 - 1.6 48 0 a The reaction temperature was 252 "C. t is the hydrogen treatment time at 282 Torr and 252 "C;A, the catalytic activity, ((A' - A " ) / A ' )x 100, the percentage decrease of the catalytic activity (see text);PHz = Pc,H, = 90 Torr. The temperature of hydrogen treatment was 413 "C. no.
t, min
104A mol/g min
(A' - A")/A' x 100
at 252 "C. This was probably due to the poisoning effect of the CzH4which could be decreased only slightly by hydrogen treatments. It is knowna that sorbed hydrogen is removed with great difficulty from the bulk of the copper. At 250 'C, only 46% of the total is eliminated by evacuation. It is likely, therefore, that during the experiments in Table I1 the copper still retained some occluded hydrogen in its bulk from the initial reduction. For this reason, a previous elimination of occluded hydrogen is necessary to detect the promoter effect at 250 "C. This operation was carried out after experiment 28 (Table 11) by evacuating the reactor for 14 h at 413 "C. Table I11 shows the results obtained at 252 "C under conditions similar to those of Table 11. The hydrogen treatments were performed at 252 "C except for experiment 46 in which the catalyst was treated at 413 "C. A continuous decrease in the catalyst activity was observed but under these conditions it was enhanced by the hydrogen treatment. It appears, however, that in these experiments the activity depends on the time and temperature of the treatment with hydrogen. Using the same sample as before, experiments similar to those of Table I at 0 "C were carried out at 252 "C to study the effect of ethylene on the activity of the catalyst. This time the catalyst was treated for only 2 rnin with ethylene. Self-hydrogenation of ethylene was observed (Table IV, experiments 4 and 5), together with a decrease in the activity for the next reactions. The ethane produced in this case could not have come from the interaction of ethylene with hydrogen left on the surface from the
no.
TABLE IV: Effect of Ethylene on the Catalytic Activity of Copper at 252 "Ca no.
PClH4>
Pmix*
Torr
Torr 170 170 170
1 2 3
4 5 6 7
186 186 170 170
1 0 5 ,~ mol/g 4.15 4.15 4.19 3.55 1.74 3.62 3.65
a Pc,H,is the pressure of the ethylene; Pmix is the pressure of the ethylene-hydrogen mixture (C,H,:H, = 1:l). The reaction time was 2 min. A , is the number of moles of ethane produced per gram of catalyst.
previous experiment because the surface hydrogen desorbes on evacuating the reactor at 252 "C for 10 min. Ethane formation was also observed when hydrogen was added to the reactor after ethylene had been introduced and the reactor had then been evacuated for 15 min at the same temperature (252 "C). Discussion Results of Experiments at 0 "C. These results (Figure 1) show that the hydrogen pretreatment enhances the catalytic activity (experiment 1, curve a) as has been observed by other authors.ll The decrease of the catalytic activity with the experiment number could be explained assuming that the "promoter" hydrogen is desorbed into the gaseous phase or it reacts with ethylene. However this
Hydrogenation of Ethylene in Granular Copper
effect can be associated not only with the physicochemical changes of the catalyst produced by the sorption of hydrogen in the metal (Le., the promoter effect of the hydrogen), but it may be also be linked to other phenomena. For example, the subsequent decrease of the catalytic activity with the experiment number suggests that the promoter effect of hydrogen involves removal of a carbonaceous residue from the surface by hydrogen as is observed with other metals.25 Moreover, this promoter effect may result from the reaction of ethylene with the hydrogen that is chemisorbed at 150 "C. Indeed, supposing that each cm2of Cu contains 1.4 X 1015active sites on the in 10.8 g of catalyst the number of hydrogen atoms chemisorbed in order to form a monolayer is about 7.6 X lozo. If this hydrogen reacts with ethylene, 6.3 X mol of ethane should be formed which is an amount greater than those shown in the Figure 1. The nature of hydrogen adsorption on copper and its reaction with ethylene at room and lower temperatures has not been satisfactorily explained. As pointed out by Beek,26it is not enough for a catalyst to have optimal geometrical conditions in order to adsorb hydrogen or hydrogenate ethylene, but a suitable electronic configuration is also required. Thus, this author has reported that pure copper films, for instance, neither adsorb hydrogen a t -183 "C nor hydrogenate ethylene at room temperature even though the crystal parameters are near to those of nickel. Nevertheless, in a study carried out with spectroscopically pure copper films,27it has been shown that hydrogen and ethylene are adsorbed reversibly at low temperatures and hydrogen reacts with ethylene at 0 "C. Attempts to explain the activity of the copper are based on the assumption that some oxygen or nickel impurities are present on the surface, thus allowing the adsorption of hydrogen; but it has been s h o ~ nthat ~ ~ the * ~con~ tamination of a copper surface with oxygen does not alter its ability to adsorb hydrogen. The absence of the irreversible adsorption of ethylene on the catalyst under study (which is a characteristic of nickel metal) shows that the surface of the copper catalyst does not contain any trace of nickel. In fact, the behavior of our catalyst toward ethylene (Table I) differs from that known for nickeLZ5 A possible explanation of the mechanism for the adsorption of hydrogen and hydrogenation comes from our results. The existence of self-hydrogenation (Table I) and the irreversible poisoning of only a small part of the surface by ethylene gives us the basis to assume that the mechanism of the hydrogenation occurs via formation of the radicals C2H3 and/or CzH2 adsorbed on the metallic surface: CZH4
+
(CZH3)ads
+ (H)ads
The hydrogen adsorbed on the catalyst would increase the rate of adsorption of new hydrogen molecules, the step that is assumed to be rate-limiting in the hydrogenation of ethylene on copper.26 The enhancing effect of pretreatment with hydrogen at 150 "C in the reaction carried out at 0 "C is probably due to the increase in the number of atoms adsorbed on the surface and thus the reaction rate. The preexponential factor of the rate constant decreases by a factor of -700 as a result of hydrogen pretreatment of the catalyst, but this is more than compensated for by
The Journal of Physical Chemistty, Vol. 82, No. 21, 1978 2327 I
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Figure 2. Temperature dependence of the reaction rate: A , InRial rate of reaction; pressure of reaction mixture, 104 Torr. Conditions of the hydrogen pretreatment were as follows: hydrogen pressure; 200 Torr; temperature; 150 "C; time; 1 h: (0)with H2 pretreatment: (0) without H2 pretreatment.
a decrease in the activation energy (Figure 2). Consequently, the promoter effect of hydrogen is not due to the formation of new active sites, but to a decrease in the energy barrier of the reaction pathway. Sato and M i ~ a h a r ahave ~ ~ reported that the hydrogenation of ethylene on copper films occurs via associative adsorption of ethylene according to the Horiuti-Polanyi mechanism. The self-hydrogenation of ethylene (e.g., via dissociative adsorption) was considered to be a side reaction in the hydrogenation of ethylene on Ni, Pt, and Rh. In the case of copper films, self-hydrogenation was not considered at all. From the results in Table I we can assume that the self-hydrogenation of ethylene on granular copper can neither be ignored nor considered as a side reaction in hydrogenation at 0 "C. In fact, it takes place on the "carbide" surface of the catalystm i.e., a catalyst that has been in contact with a CzH4 H2mixture and/or C2H4 alone for a long time at the same temperature. Moreover, the formation of a considerable amount of ethane upon the introduction of Hzinto the evacuated reactor where the self-hydrogenation took place indicates that the adsorbed species on copper do not behave like a poison, but rather like active species in relation to hydrogen. Along this line, it is interesting to note that Franken and Ponec31 pointed out a reversible character of the adsorption of ethylene on copper film at room temperature. The hydrogen sorbed by the metal has been reported to influence the rates of adsorption of hydrogen and ethylene and of other reactions on other metals. Thus, Ablesova et alS5pointed out that hydrogen and also oxygen sorbed in nickel cause an increase in the rate of adsorption of hydrogen and ethylene. Wood32 observed that the presence of small amounts of hydrogen atoms on the surface of Pd-Ag alloys is necessary for the dehydrogenation of cyclohexane. The hydrogen sorbed by the metal seems to be essential in the dehydrogenation of some hydrocarbons at high t e m ~ e r a t u r e . ~ ~ Results of Experiments at 252 "C. The hydrogen promoter effect cannot be explained by the removal of the carbonaceous residue from the surface of the catalyst. Indeed the results shown in Table I1 demonstrate that hydrogen pretreatment cannot counteract the continuous drop in catalytic activity. In contrast, the catalytic activity of copper which has a smaller amount of hydrogen in the bulk, is increased by the hydrogen occluded in the bulk of the metal. The location of the hydrogen in the metal is concluded from the following facts, deduced from Table 111.
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(i) The experiments were carried out a t 252 "C, a temperature where the hydrogen is rapidly desorbed from the surface when the reactor is evacuated, leaving a small probability that significant amounts of hydrogen atoms remained adsorbed on the surface at the start of each experiment. (ii) The activity increases with increasing contact time during the hydrogen pretreatment indicating that this process is slow. Hydrogen diffusion in copper has already been reported to be a slow process.34 The fact that even after 10 min of contact of the hydrogen with copper (Table 111, experiment 35) there is a considerable increase in the reaction rate indicates that the hydrogen responsible for this effect is near the catalyst surface. (iii) This effect is higher the higher the hydrogen treatment temperature (Table 111, experiment 46) which shows that the process, like the diffusion of H2 in copper, is also activated. The conclusions concerning the location of hydrogen in the metal are in agreement with those obtained with copper membranes at the same temperature.21 The elimination of occluded hydrogen from the bulk by evacuation of the catalyst at 413 " C explains the abrupt decay of the catalytic activity between experiment 28, Table I1 and experiment 29, Table 111. As the catalyst was treated at 400 "C for a long time before activity measurements, we feel that the sintering of copper is not responsible for this phenomenon. The existence of the self-hydrogenation of ethylene on copper a t 252 "C and subsequent ethane formation by addition of hydrogen into the evacuated reactor suggests that dissociative adsorption of ethylene takes place during the hydrogenation reaction, as was indicated at 0 "C. Along this line, it is interesting to note that Alexander et al.27 pointed out that the adsorption of hydrogen and ethylene in the catalytic reaction on copper has the same characteristics at low and high temperature. However, in contrast to the study carried out at 0 "C, it is not possible to preclude the hypothesis which considers the dissociative adsorption of ethylene as a side reaction in the hydrogenation process. Indeed, the experimental results shown in Tables 11-IV were obtained on a copper surface which had not reached its steady ("carbide") state. Its catalytic activity continuously dropped, probably due to the poisoning effect of the ethylene.
P. Gajardo,
M.C. Lartiga, and S. C.
Droguett
Finally, considering that the location of the promoter hydrogen is different for low temperature and high temperature hydrogenation, the mechanisms which bring about the enhancement of catalytic activity must also be different in these two cases.
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