J. H. SIXFELT
344
Hydrogenolysis of Ethane over Supported Platinum
by J. H. Sinfelt E880
Research and Engineering Co., L i n d e n , N e w Jersey
(Received September 0, 1063)
The hydrogenolysis of ethane over two supported platinum catalysts, one employing silica as a support and the other alumina, was investigated in a flow reactor over the temperature range of 343 to 401'. The rate of hydrogenolysis to methane was found to increase with ethane partial pressure to a power in the range 0.7 to 0.9. However, the rate was found to decrease markedly with increasing hydrogen pressure. This has been interpreted. in terms of a mechanism involving extensive dehydrogenation on the surface prior to a slow step in which carbon-carbon bonds are broken. The apparent activation energy of the reaction was found to be much higher over the silica-supported platinum, although the rates were not far different at the temperatures of this study. The difference in activation energies for the different supports suggests that the support interacts with platinum in some manner, and that silica differs from alumina with regard to such an interaction.
The catalytic hydrogenolysis of ethane to methane has been studied over nickel, cobalt, and iron catalysts by Taylor and c o - w o r k e r ~ . ~ - From ~ the results of these studies it was postulated4 that the initial step in the reaction involved dehydrogenation to form unsaturated surface radicals, and that the hydrogen content of the radicals varied with the metal. Since no data were reported for noble metal catalysts, it was decided to investigate the reaction over supported platinum. I n addition, it was decided to investigate the effect of the support on the catalytic properties of the platinum, and hence studies were made with platinum supported on both alumina and silica. Although there is a widely held impressions that the support does not interact with the metal in supported noble metal catalysts, data are presented here which refute this idea.
Experimental Apparatus an,d Procedure. The reaction rate measurements were carried out in a flow system a t atmospheric pressure. The reactor was a stainless steel tube approximately 1.0 cm. in diameter and 8 cm. in length. The reactor was held in a vertical position and was surrounded by a small electrical oven. The catalyst was centered with respect to the ends of the reactor, occupying a space approximately 1.5 to 4.0 cm. in length in the various runs. A fritted stainless steel disk was used to support the catalyst in the reactor, and qusrtz T h e Journal of Physical Chenkistry
wool was packed on top of the catalyst to hold it in place. A 3-mm. axial thermowell containing an ironconstantan thermocouple extended upward through the fritted steel disk into the catalyst bed, so that the tip of the thermowell was located at the center of the catalyst bed. The reaction gases were passed downflow through the catalyst bed, and the products were analyzed by a chromatographic unit coupled directly to the outlet of the reactor. The chromatographic column was 2 m. in length and 0.6 cm. in diameter. The column was packed with 100 mesh silica gel and was operated a t 40'. Helium was used as a carrier gas, and a thermal conductivity detector was used with the column. The reactant gases, ethane and hydrogen, were passed over the catalyst in the presence of helium diluent. Gas flow rates were measured using orifice-type flow meters with manometers. A total gas flow rate of 1 l./ min. was used throughout. The run procedure consisted of passing the reactant gases over the catalyst for a period of 3 min., a t which time a sample of the ~~
K. Morikawa, W. S.Benedict, and H. S.Taylor, J . Am. Chem. Soc., 58, 1796 (1936). (2) E. H. Taylor and 13. S. Taylor, i b i d . , 61, 503 (1939). (3) C. Kemball and H. S.Taylor, ibid., 70, 345 (1948). (4) A. Cimino, M. Boudart, and H. %,Taylor, J . P h y s . Chem., 58, (1)
796 (1954).
6)
G. C. Bond, "Catalysis by Metals," Academic Press, Inc.. London, 1962, p. 40.
HYDROGENOLYSIS O F ETHANE OVER
product was taken for chromatographic analysis. The ethane was then cut out and hydrogen flow was continued for a period of 10 min. a t the reaction temperature prior to another run. In this way, it was possible to minimize variation in catalyst activity from period to period. As an additional insurance against the complica Lions due to varying catalyst activity, measurements a t any given set of conditions were in most cases bracketed with runs a t a standard set of conditions. In this way, the effect of changing a variable can be determined by coimparison with the standard condition periods run immediately before and after the period in question. Before any reaction studies were made, the catalyst was pretreated in flowing hydrogen for 3 hr. a t 500". Materials. The ethane used in this work was obtained from the Matheson Co. A chromatographjc analysis of the ethane showed no detectable hydrocarbon impurities. It is estimated that a hydrocarbon impurity, e.g., methane, would have been detected by the chromatographic analysis if it were present at a concentration above 0.01 wt. %. High purity hydrogen was obtained from the Linde Co. and was further purified by passing it through a Deoxo unit containing palladium catalyst to remove traces of oxygen as water, prior to passage through a molecular sieve dryer. The platinuim catalysts used in this work were supported on either silica or alumina. The platinum content of the catslysts was 0.60% by weight. The catalysts mere prepared by impregnation of silica or alumina with aqueous chloroplatinic acid, followed by calcination in air. The Pt/SiOz catalyst was calcined a t 538" for 1 hr., whiile the Pt/A1203 was calcined a t 593" for 4 hr. Thc silica and alumina were prepared by heating either silica gel or p-alumina trihydrate, both obtained from Davison Chemical Co., for 4 hr. a t 538 or 593", respectively. The B.E.T. surface areas of the silica and alumina were 388 and 296 m.2/g., respectively. X-Ray diffraction measurements obtained in these laboralories indicated the alumina to be 7-alumina.
Results In studying the kinetics of the hydrogenolysis of ethane to methane, the approach taken was to measure initial rates of reaction a t low conversion levels (0.05 to 5.9%). Thle reaction rate r is defined by the relation
F
=
345
SUPPORTED P L A T [ N U M
w"
where F represents the feed rate of ethane to the re-. actor in g. moles/hr., W represents the weight in grams
u 0.30
h
ob . i
c
+
\
0.10
6
M
a
U
d 0.03
I I '
Orno' 1.48
1.52
1.56
1
1
1. 60
1.64
1000/T,' ( O K .
Figure 1. Effect of temperature on the rate of hydrogenoly3is of ethane; Hs pressure = 0.20 atm.; CzHs pressure = 0.030 atm.
of platinum on the catalyst charged to the reactor, and
x represents the fraction of the ethane converted to methane. Data on the effect of temperature on rates for both the Pt/SiOz and Pt/A1,03 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 over the Pt/Si02 and Pt/AlZO3 catalysts are 54 and 31 kcal./mole, respectively. Rate data showing the effects of hydrogen and ethane partial pressures, PH and PE) respectively, are summarized in Table I in the form of a ratio r/ro, where r is the rate a t any given conditions and ro is the rate a t a standard set of conditions ( p = ~ 0.20 atm., p E := 0.030 atm.). The rate measurements a t conditions different from the standard were bracketed by rate determinations a t the standard conditions. Each r/ro value in Table I was obtained by dividing the rate determined in a given reaction period a t a particular set of conditions by the average of the rate determinations a t the standard conditions immediately before and after the given reaction period. This procedure served to minimize the effects of varying catalyst activity. While the variation in activity during any one reaction period was generally not large (10-2075 a t most), the cumulative activity decline over an extended period of time could have led to erroneous conclusions about the kinetics if the above procedure had not been adopted. V o l u m e 68, ivumber 2
Februaru, 1961
J. H. SINFELT
346
-~
Table I : Relative Rates of CzHs Hydrogenolysis as a Function of CzHe and HPPartial Pressures PH
PE 2
1
Catalyst
atm.
Pt/SiOt (377")
0.10 0.20
r/?oa
0.030
3.3
1.o 0.12 0.21 0.37 1.o 2.6 2.6 1.0 0.24 0.27 0.47 1 .o 2 1
0.030 0 030
0.60
0 0050 0.010 0 030
0.20
0.20
Pt/AlzOa (366")
atm.
0.20 0.20 0.10 0.20 0.40 0.20 0.20 0.20 0.20
0.100 0.030 0,030 0.030
0.0050 0,010 0,030 0,100
a Rate relative to the rate a t the standadconditions ( p = ~ 0.20 a h . , p~ = 0.030 atm.) for the particular cat'alyst and temperature in question; the r / r O values cannot be used by thernselves to compare the activities of the catalysts.
For both the Pt/SiOz and the Pt/AlzOa catalysts, the data in Table I show that the rate 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 as a simple power law, r = kpE"pHm. Approximate values of the exponents n and m as derived from the experimental data are summarized below. Catalyst
n
m
Pt/Si02 (377") Pt/AlzOa (366")
0 9
-1 8 -1 7
0 7
These results can be accounted for satisfactorily by the mechanism proposed by Cimino, Boudart, and Tay10r.~ According to these authors, ethane hydrogenolysis over metals involves the reaction steps
Ci" J_ CzH,
+ aH,
+ Hz --+ CH, + CH, --+ H2
CzH,
The Journal of Physical Chemistru
CH,
where x, y, and x are integers and a is equal to (6 x ) / 2 by stoichiometry. The species C2H, represents a n adsorbed dehydrogenated radical which reacts with a molecule of hydrogen to form the surface fragments CH, and CH,. The latter are hydrogenated off the surface to form methane. By postulating that the initial dehydrogenation step was an equilibrated reaction and that the cracking of C2H, in the second step mas rate controlling, the authors derived an approximate rate law of the form
where 0 < n < 1. Taking the experimental values of n for the Pt/Si02 and Pt/A1203 catalysts, and assuming a value for a, we can calculate the exponent (1 nu) of p~ and compare with m, the experimental value. The best agreement is obtained if we assume a = 3, in which case the calculated exponents of p~ for the Pt/SiOz and Pt/A120, catalysts are, respectively, - 1.7 and - 1.1, which are to be compared with the corresponding experimental m values of - 1.8 and - 1.7. The agreement is good for the Pt/SiOz catalyst but only fair for the Pt/Al2O3catalyst.
Discussion On the whole, we conclude that the kinetic formulation of Cimino, Boudart, and Taylor accounts satisfactorily for the results on ethane hydrogenolysis over supported platinum. The most interesting finding of the present study is the marked difference in the apparent activation energy for the two platinum catalysts studied. While the degree of dispersion of the platinum may be different on alumina and silica, the difference in apparent activation energies cannot be rationalized readily on the basis of a difference in platinum surface areas alone, since this would be expected to affect only the magnitudes of the rates rather than the temperature dependence. It seems more likely that some specific interaction between platinum and support is involved. The widely held impression5 that supports do not interact with noble metals is thus open to considerable question.
