W. F. TAYLOR, D. J. C. YATES,A N D J. H. SINFELT
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Catalysis over Supported Metals.
11. The Effect of the Support on the
Catalytic Activity of Nickel for Ethane Hydrogenolysis
by W. F. Taylor, D. J. C. Yates, and J. H. Sinfelt Esso Research and Engineering Co., Process Research Division, Linden, AVew Jersey
(Received A p r i l SO, f964)
The kinetics of hydrogenolysis of ethane to methane mere studied over a series of nickel catalysts in which the nickel was supported on three different oxides: silica, alumina, and silica-alumina, The rate measurements were made in a differential flow reactor a t temperatures in the range of 175 to 275”. The nickel surface areas of the catalysts were determined by hydrogen chemisorption. This niade it possible to determine the specific catalytic activity of the nickel in terms of rate of hydrogenolysis per unit area of nickel surface. Effects of the support over and above those due simply to differences in the degree of dispersion of the nickel could therefore be determined. The specific catalytic activity of the nickel varied over 50-fold for the various supports. The activity was highest for the silica support and lowest for the silica-alumina. The results of these studies suggest a specific interaction between the nickel and support.
I. Introduction In supported nietal catalysts, the effect of the support on the properties of the catalyst has con~nionlybeen assumed to be physical in nature. For example, it is clear that the support disperses the metal and leads to higher metal surface areas (although the cause of this high dispersion is not clear). The dispersion retards crystal growth by sintering and hence stabilizes the high surface area of the nieta1.l These are important advantages for supported metal systems and account for their wide use as catalysts in actual practice. If we adopt the view that the nietal and support do not interact chemically, the catalytic properties of a metal should be essentially independent of the support, apart from effects arising from diff erences in dispersion of the metal. However, evidence for specific nietalsupport interactions in supported systems has recently emerged from infrared studies of molecules adsorbed on supported n ~ e t a l s ~and - ~ from studies on the electronic properties of supported Effects of the support on the catalytic activities of metals have also been rcported.5-’ These observations indicate that supported metal catalysts are more complex than has conimonly been thought. I n this connection, however, it should be realized that the phenomenon of bifunctional catalysis, in which the metal and support have The J o i ~ r n a lof Phusical Chemistrv
separate catalytic properties of their own, has been recognized for some time in the reactions of hydrocarbons over metal-acidic oxide catalysts.*-10 This phenomenon is not necessarily related to a inetalsupport interaction, since the role of the support in such systems may be simply one of catalyzing the further reaction of an intermediate which forms on the metal sites and migrates to acidic sites on the support. That is, the metal and acidic sites can act independently; such an effect has been shown by several investigators. - I 3 (1) G. C . Bond, “Catalysis by Metals,” Academic Press, Inc., New York, N . Y., 1962, pp. 38-42. (2) R . P. Eischens and W. A. Pliskin, Advan. Catalysis, 10, 2 (1958). (3) C . E . O’Xeill and D. J. C . Yates, J . Phys. Chem., 65, 901 (1961). (4) C. E. O’Neill and D. J. C. Yates, Spectrochim. Acta, 17, 953 (1961). (5) G. 11.Schwab, J. Block, W. Muller, and D. Schultae, Naturwiss., 44, 582 (1957). (6) E. B . Maxted and S . Akhtar, J. Chem. Soc., 1995 (1960). (7) E. B . Maxted and J. S . Elkins, ibid., 5086 (1961). ( 8 ) F. G. Ciapetta and J. B . Hunter, I n d . Eng. Chem., 45, 147 (1953); 4 5 , 155 (1953). (9) F. G. Ciapetta, ibid., 45, 159 (1953); 45, 162 (1953). (10) G . A. Mills, H . Heinemann, T. H. Milliken, and A. G. Oblad, ibid., 45, 134 (1953). (11) P. B. Weisr and E. W. Swegler, Science, 126, 31 (1957).
CATALYSIS OVER SUPPORTED METALS
While there is eridence for specific nietal-support interactions in supported metal catalysts, there are no accurate data available to demonstrate an effect of the support on the specific catalytic activity of a metal, i e . , the activity per unit surface area of the metal. This is due primarily to the fact that data on the surface area of the supported metal itself are seldom available. Inforination of this kind is absolutely essential in defining the effect of the support, and in the absence of such inforiiiatioii any effect on catalytic activity could be entirely confused by possible differences in dispersion of the metal. In thc present investigation, the cffrct of the support on the specific catalytic activity of nickel for ethane hydrogenolysjs has been determined, using alumina, silica, and silica-alumina as supports. The surface areas of the supported nicltel were deterniined by hydrogen cheniisorptioii, aiid the data 011 the ltinetics of ethane hydrogenolysis were obtained in a differential flow reactor. This approach inalies it possible to obtain a clear defini1,lon of the effect of the support on the intrinsic catalytic activity of the metal. The particular supports chosen are very representative of the supports coiiiiiionly used in nickel catalysts. Consequently, the results of this investigation have a definite bearing on the functioning of real catalyst systenis. 11. Experimental
Appayatus and Pi o c e d w e . The apparatus used for the hydrogen chemisorption work mas a conventional glass vacuuni systein with an 80 l./sec. oil diffusion pump. 13y use of a trap cooled in liquid nitrogen, ultiiiiate dynamic vacua of about lo-' torr were obtained. The saiiiplc cells were made of Pyrex glass and had two stopcoclis to perniit hydrogen to flow through the bed of material. Saivples of each of the three catalyst preparations, weighing about 2 g., were put in a vacuum apparatus. Aftei evacuation at 1010" for a short time, hydrogen was passed through the bed of the sampIe at a flow rate of ,500 ~ i i i . ~ / m i nThe . temperature of the sample was then increased, in the flowing hydrogen, to 370'. This teniperature was niaintained overnight, and then the sample was evacuated for 1 hr. a t the sanic temperature. After cooling to 18', a hydrogen isotherm was measured. Kickel surface areas mere calculatcd from the isothernis, assuming that thc aiiiount adsorbed a t I O cm. reprrsented a monolayer I n the nieasureiiient of an isotherm, four or five points up to a pressure of about 30 ciii. were usually obtained, as discussed in a previous paper.I4 The nickel surface areas were calculated on the basis that each nicliel atom in the surface adsorbs one hydrogen atom
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and that8each hydrogen atom occupies 6.5 & 2 . 1 5 n'ickel surface areas of the catalysts are given in Table I. It is of interest to note that the support affects quite niarltedly the degree of metal dispersion, despite the siiidarity of preparation and reduction techniques.
Table I : Summary of Sickel Surface Areas and Kinetic Parameters for Ethane Hydrogenolysis
Ni on Si02
Ni on AlzOs
Ni on Si02 AlzOa
Kickel surface area, m.2/g. of catalyst 13.3 15.6 6.83 Rate of ethane hydrogenolysis a t 191' ( P E = 0.030 atm., PH = 0.20 atrn.), 20.6 X 10-9 12.8 X 10-4 0.20 X IO-@ mole,'hr./g. of catalyst Specific catalytic activity, moles of ethane converted/hr./m.? of Ni a t 1SlOb 1 5 . 5 x 10-5 8 . 2 5 x 10-5 0.293 x 10-5 Apparent activation energy, kcal./mole 40.6 41 8 39.2 a Value extrapolated from an Arrhenius plot of data in the range 272 to 233". * p~ = 0.20 atin., p~ = 0.030 atm.
Jleasurements of the extent of reduction of the catalysts using a vacuuni inicroba1a;ice indicated that reduction of the nickel took place at teiiiperatures as low as 250°, a,nd that the reduction u7as extensive a t this temperature. Consequently, the 370" reductioii teiiiperature eniployed in the present studies should have resulted in essentially coiiiplete reduction of the different nickel catalysts. The ethane hydrogenolysis data were obtained in a flow reactor systeiii a t atmospheric pressure, by use of a vertically iiiounted stainless steel reactor tube 1.0 ciii. in diameter and 8.0 cin. in length. Details of the reactor asseinbly, flow rate nieasureineiits, and the gas chroiiiatographic analysis of the reaction products have beeti reported previously.1n The ethane and hydi-ogen were iiiixed with heliutii and passed don-iiflonr through a bed containing 0.20 g. of catalyst diluted uniformly with 0.30 g. of ground Yycor glass. By appropriate adjustiiient of the helium flow rate, it was possible to vary the partial pressures of ethane and hydrogen individually. The total gas flow was main(12) S. G. Hindin, 244 (1958).
s.it-.Weller, and G. A. hZills, J.Phys. Chcm., 62,
(13) J. H. Sinfelt, H. Hurwit,z, and J. C. Rohrer, ibid., 64, 892 (1960). (14) D. J. C. Yates, W. F. Taylor, aiid Soc., 86, 2986 (1964).
J. H. Sinfelt, J . Am. Chem.
(15) D. F. Kleinperer aiid F. 8.Stone, Proc. Roy. SOC.(London), A243, 375 (1958). (16) J. H. Sinfelt, J . l'hys. Chem., 68, 344 (1964).
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tained a t 1 l./niin. throughout. In a typical run, the reactant gases were passed over the catalyst for 3 min. prior to sampling products for analysis. The ethane was then cut out and the hydrogen flow continued for 10 niin. prior to another reaction period. As an insurance against possible coiiiplications due to changing catalyst activity, iiiost of the reaction periods were bracketed by periods a t a standard set of conditions, so that the kinetic data could be expressed as rates relative to the rate at the standard conditions. Prior to any reaction rate ineasureiiients, the catalysts were reduced overnight in flowing hydrogen at 370’ in the reactor. M a t e k l s . The nickel catalysts used in the present work contained 10 wt. yo nickel impregnated on three different supports : silica, alumina, and silica-alumina. The silica used was Cabosil HS 5 (340 m 2 / g . surface area), obtained from the Cabot Corp., Boston, Mass. This is a very finely powdered nonporous form of silica (150-200 8. particle size) prepared by burning silicon tetrachloride in air; small amounts of chlorine inipurities are present in the resulting silica. The alumina was prepared by heating p-aluniina trihydrate, obtained from Davison Cheiiiical Co., for 4 hr. at 600’. The surface area of the alumina was 295 ni.2/g., and the pore volume 0.30 cc./g. The principal impurities in the alumina were silicon, iron, sodium, caIciuni, and magnesium. The total aniount of all these impurities, as estimated from eiiiission spectral analysis, was less than 0.1 wt. %. The silica-alumina used in the present work was Type DA-1 cracking catalyst (noininally 13% A1203, 87% SiO,), also obtained from the Davison Chemical Co. The surface area was 450 m 2 / g . and the pore voluiiie 0.40 cc./g. Principal inipurities in the silica-alumina are alkali and alkaline earth metals, principally sodium, and iron. The total aiiiount of such impurities is estiiiiated to be less than 0.1 w t . yo. In the process of iiiipregnating the various supports with nickel, the support was first wetted with deionized mater prior to adding I\Ti(N03),.6Hn0 dissolved in deionized water. In the case of the aluiiiina and silicaalumina, 100 g. of the support was wetted with 50 cc. of deionized water. I n the case of Cabosil, it was necessary to add much more deionized water, 800 cc. to 100 g. of Cabosil, to obtain satisfactory wetting of the support. In all cases the impregnation with nickel mas accoiiiplished by adding 55 g. of N i ( S 0 3 ) 26‘ H z 0 dissolvcd in 30 cc. of deionized water to the wet support. The material mas then dried overnight a t 103’, after which it was pressed a t about 10,000 p.s.i. into wafers which were subsequently crushed and screened to a size between 45 and 60 mesh. I n the The Joiirnal of Physical Chemistry
W. F. TAYLOR, D. J. C. YATES,AXD J. H. SISFELT
case of the Xi on Alz03 catalyst, it was not necessary to press the material into wafers prior to crushing and screening. Visual inspection of the catalysts indicated uniforin impregnation by the nickel in all three preparations. The ethane used in this work was obtained from the Matheson Co. A chromatographic analysis showed no detectable impurities. It is estimated that an impurity, e . g . , methane, would have been detected by the chromatographic analysis if it were present a t a concentration above 0.01 wt. 70. High purity hydrogen was obtained froin the Linde Co., Linden, K.J. It was further purified in a “Deoxo” unit containing palladiuni catalyst to remove trace amounts of oxygen. The water formed was then removed by a trap cooled in Dry Ice or by a molecular sieve dryer, the latter having been employed for the hydrogen used in the kinetic measurements.
111. Results The hydrogenolysis of ethane to methane was studied a t low conversion levels (0.02 to 8%). Rates were calculated froin the relation
r
=
(F/W)x
where F represents the feed rate of ethane to the reactor in moles/hr., W represents the weight in grains of the catalyst, and x: represents the fraction of ethane converted to methane. The reaction rate T is thus expressed as moles of ethane converted to methane/ hr./g. of catalyst. In a run to measure reaction rates, the catalyst was first prereduced with hydrogen with the identical schedule of temperatures used in the hydrogen cheniisorption equipment. This was done to ensure that the nickel surface area of the freshly reduced catalyst would correspond exactly to that nieasured by the hydrogen chemisorption technique. Then the temperature was lowered in flowing hydrogen, and at a standard set of hydrogen and ethane partial pressures (pH = 0.20 atiii., p E = 0.030 atni.) the activity of the freshly reduced catalyst was measured. Then rates were measured a t a series of temperatures in a rising temperature sequence. The data for the three catalysts are shown in the Arrhenius plots in Fig. 1. From the slopes of these plots, the apparent activation energies of the ethane hydrogenolysis reaction were determined and are given in Table I. The iiiaxiiiiuiii difference between apparent activation energies is less than By0,which is within the experimental accuracy. After determining the effect of temperature on rates, the temperature was lowered to an intermediate value, and a series of measurements using the “bracketing
CATALYSIS OVER SUPPORTED XETALS
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technique" was made to determine the effects of the partial pressures of hydrogen and ethane on the rates. For each set of conditions the rate 7' relative to the rate yo a t the standard conditions ( p = ~ 0.030 atni., pH = 0.20 atin.) was expressed as a ratio T / r o . These data are presented in Table 11.
I
1
I
I
I
I
Catalyst
m
n
N i on Si02 (177-191 ") Ni on Al2O3(177')
1.0 0.9 0.9
Ni on Si02-Al20, (246")
-2.4 -2.0 -1.7
The general features of the kinetics of ethane hydrogenolysis over the catalysts eniployed in this study are in accord with the earlier studies of Taylor and coworkers,17in which the kinetics were investigated over nickel catalysts of unknown nickel area. I n order to compare the activity of nickel supported on the three different materials, the specific activity per unit area of nickel a t 191" was calculated. This was done by dividing the measured rates over the freshly reduced Ni on Si02 and Ni on A1203at 191" by their respective nickel areas. The specific activity of the Ni on Si02-A1203catalyst at 191 O had to be calculated by extrapolation of data obtained in the temperature range 233 to 272", as the rate was too low to iiieasure a t 191 ", The values are given in Table I.
IV. Discussion The results of this investigation have shown clearly that the support can have a very large effect on the 2
Table 11: Relative Rates of C Z HHydrogenolysis ~ as a Function of C2Hsand HZPartial Pressures Catalyst
Ni on SiO, (177-191')
Ni on A1203 (177")
lit3
I
I
1.9
2.0 UT
(OK)
1 2.1 x 103
I
I
2.2
2.3
Figure 1. Effect of temperature on the rate of ethane hydrogenolysis over supported nickel catalysts a t p~ = 0.030 atm. and p~ = 0.20 atni.: 0 , Ni on SiO,; 0, Ni on A1203; A, Ni on SiO2--Al2O3.
For all three catalysts the data in Table I1 show that the rat,e of ethane hydrogenolysis increases with increasing ethane partial pressure, but decreases markedly with increasing hydrogen partial pressure. The dependence of the rate on the partial pressures of ethane and hydrogen can be expressed in the form of a simple power law, T = kpEnpHm.l7 Approximate values of the exponents n and 712 as derived from the experimental data are suniniarized here.
Ni on Si02-A1203(246')
P H , atin.
PE, atin.
r/ra"
0.10 0.20 0.40
0.030 0.030 0,030
4.03 1.00 0.14
0.20 0.20 0.20
0.010 0,030 0,100
0.37 1.00 3.91
0.05 0.10 0.20 0.30
0.030 0.030 0.030 0.030
9.88 3.03 1.00 0.28
0.20 0.20 0.20
0,010 0.030 0,100
0.35 1.00 2.72
0.10 0.20 0.40
0,030 0.030 0,030
2.66 1.00 0.27
0.20 0.20 0.20
0,010 0,030 0.100
0.33 1.00 2.40
a Rate relative to the rate a t the standard conditions ( p = ~ 0.20 atm., p ~ = : 0.030 atm.) for the particular catalyst and temperature in question; the r / r O values cannot, be used by thernselves to compare the activities of the catalysts.
(17) A. Cimino, X f . Boudart, a n d H. 796 (1954).
S.Taylor, ,J. P h y s . Chem.,
Volume 68, S u m b e r 10
58,
October, 1964
7.1;. F.
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specific catalytic activity of a metal. I n considering an explanation for these results, one might inquire about differences in the accessibility of the nickel on the various supports. To some degree this has been accounted for in the hydrogen chemisorption measureinelits, which at least indicate the accessibility of the nickel surface to hydrogen. The possibility that, the accessibility might be very different for a molecule such as ethane seems unlikely, since the tvorli of Schuit and van ReijenI8 indicated that for impregnated type nickel catalysts, similar to those of the present work, the accessibility to molecules like carbon monoxide and ethylene is similar to that of hydrogen. Furthermore, calculations by the method of Weisz and Praterlg indicate effectiveness factors a t the conditions of this work to be close to unity for the Xi on Al,03 and Xi on Si02-A1203 catalysts, taking their average pore radii to be 20 and 18 as derived from the data on pore volumes and surface areas. This indicates that pore diffusion is not a limitation for these catalysts, and it is also not likely to be a limitation in the case of the Ni on SiO, catalyst, since the pores created in the pressing of the 150-200 Cabosil particles into pellets prior to crushing and screening are probably larger than those of A1203and SiO2-AI20,. It has been proposed by Reinen and SelwoodZ0in their studies of the magnetic properties of supported nickel that the effect of the support is probably one of altering either the particle size or particle size distribution of the metal crystallites. While the effect on particle size distribution may well be a factor in the inagnetic studies, it is difficult to see horn it could be of controlling importance in the present studies, for the folloming reason. If we assume that the nickel crystallites are spheres or cubes, the average particle diameter d would be inversely proportional to the surface/volume ratio S of the nickel (d = 6/'S), or, for a constant amount of nickel in the catalyst, to the surface area of the nickel per unit weight of catalyst'. As the support was varied froin silica to silica-alumina, the surface area of the nickel decreased about twofold, from 13.3 to 6.83 ni.2/g., corresponding to an approximate twofold increase in average particle size. HOWever, the rate of ethane hydrogenolysis per unit weight of catalyst mas over 100-fold higher for the silicasupported catalyst; %.e.,the effect of the support on the catalytic activity was far greater than the effect on the average particle size of the nickel crystallites. When the rates are expressed as rates per unit of niclicl surface area, so that the effects of average particle size or nickel
s.,
The Journal of Physical Chemistry
TAYLOR; D. J. c. YATES,A S D J. H.
SINFELT
surface area are accounted for, there is still more than a 50-fold difference in the activities of the Ni on SiOz and S i on SiO2-&O3 catalysts. A similar effect can be seen if the S i on SiOz and Ni on &03catalysts are compared. Despite the higher nickel area using alumina as the support (15.6 us. 13.3 m2,g.), the reaction rate per unit weight of catalyst is 1.7 tiines as high for the silica supported nickel, as coinpared with the alumina supported nicld. On the basis of rate per unit area of nickel, the rate is about 1.87 times greater for the nickel on silica. Thus, it seems reasonable that some specific chemical or electronic interaction between metal and support is responsible for the effects observed in the present work. It is interesting that the differences in the catalytic activities of the nickel over the various supports are not accompanied by significant differences in the apparent activation energy. While the data in Table I indicate sinal1 differences in apparent activation energy, there is no consistent trend in the activation energy with decreasing catalytic activity as the support is changed from silica to alumina to silica-alumina, and in any case the differences are probably within the range of experimental error. Taking the apparent activation energies to be the same over all three catalysts, the differences in catalytic activity would then have to be due to a variation in the pre-exponential factor IC0 in the Arrhenius expression k = IC0 exp ( - E / R T ) . This is similar to the situation in ethylene hydrogcnation over metal films, where the apparent activation energy is reported to be constant over a series of despite large variations in catalyst activity. The fact that the apparent activation energy for ethane hydrogenolysis is constant over the various supported nickel catalysts in the present work, hornever, does not necessarily mean that the true activation energy is constant. For instance, if the surface is only sparsely covered by the reactive intermediate, the apparent activation energy E is related to the true activation energy E , by the expression E = Et - q , where q is the heat of adsorption. It is possible that q and Et both vary by about the same amount, thus causing E I O be essentially independent of the variation in the support for the nickel. (18) G. C . A. Schuit and L. L. van Reijen, Adaan. Catalysis, 10, 242
(1958). (19) E'. B. Weisz and C. D. Prater, ibid., 6 , 143 (1954). (20) D. Reinen and P. W, Selwood, J . Catalysis, 2 , 109 (1963). (21) 0. Beeck, Disci~ssionsFaraday Soc., 8 , 118 (1950).
