J. H. SINFELT
856
Kinetics of Ethylene Hydrogenation over a Platinum-Silica Catalyst
by J. H. Sinfelt Esso Research and Engineering Co., Linden, New Jersey
(Received October 30, 1963)
The kinetics of ethylene hydrogenation have been investigated over a silica-supported platinum catalyst in the temperature range 45-93’ using a flow reactor system. The reaction rate was found to be proportional to the square root of the hydrogen partial pressure. With respect to ethylene partial pressure, the rate was observed to decrease with increasing pressure a t the lower ethylene pressures studied (0.007 to 0.10 atm.), but to approach a zero-order dependence a t sufficiently high pressures. The apparent activation energy was 16 kcal./mole. The rate data support a mechanism involving adsorption of both hydrogen and ethylene on the active catalyst sites, the rate being determined by the rate of reaction between hydrogen atoms and ethylene molecules adsorbed on the surface.
The kinetics of ethylene hydrogenation over nickel catalysts have been studied extensively, but much less has been reported on the kinetics over other metals, such as platinum. Furthermore, kinetic studies over supported metals have received much less attention than studies over metal films, wires, or powders. In the case of supported platinum catalysts, the reported data on the kinetics of ethylene hydrogenation appear to be limited to those of Bond,2 along with some very briefly reported results of Schuit and van Reijena3 The latter results were obtained a t low temperature ( -40’), while the former were obtained in the temperature range 0-50’. The supported platinum catalysts employed by these investigators differed markedly in platinum content, the catalyst studied by Schuit and van Reijen containing about 50% Pt on SiO, as compared to 1-5y0 Pt on several different supports in the studies of Bond. Furthermore, these investigators observed marked differences in the way in which the reaction rates varied with the partial pressures of ethylene and hydrogen. This is not unreasonable, considering the differences in conditions and the nature of the catalysts employed. However, differences such as these observed over wide ranges of conditions can be illuminating with regard to the over-all mechanistic picture of the system and how it can vary with conditions. In the present paper, the kinetics of ethylene hydrogenation over supported platinum have been extended The Journal of Physical Chemistry
to higher temperatures using a Pt-Si02 catalyst of much lower platinum content (0.0573. Reaction rates have been measured over a wide range of hydrogen and ethylene partial pressures (40-60-fold variation), and some significant differences from what has been reported a t lower temperatures have been observed.
Experimental Apparaius and Procedure. A flow reactor system a t atmospheric pressure was used in this work. The reactor consisted of a stainless steel tube approximately 1.0 cm. in diameter and 8.0 cm. in length. In place, the reactor was held in a vertical position and was surrounded by an electrical heater. A small electrically heated coil was placed in the inlet line to the reactor to serve as a preheater. The catalyst was located midway along the length of the tube and occupied a space amounting to about 20% of the volume of the tube. The catalyst was supported on a fritted stainless steel disk and was held in place by packing quartz wool on top. The reaction temperature was measured with an iron-constantan thermocouple housed in a 3mm. axial thermowell extending upward through (1) G. C. Bond. “Catalysis by Metah,” Academic Press Inc., New York, N. Y., 1962, p. 240. (2) G. C. Bond, Trans. Faraday Soc., 5 2 , 1235 (1956). (3) G. C. A. Schuit and L. L. van Reijen, Advan. Catalysis, 10 242 (1958).
KINETICS OF ETHYLENE HYDROGESATION
the fritted steel disk to the center of the catalyst charge. The reactant gases, ethylene and hydrogen, were passed downflow through the reactor in the presence of helium as a diluent. The flow rates of the individual gases were measured with orifice-type flow meters. The total gas flow rate was maintained a t 1 l./min. throughout, and the partial pressures of the hydrogen and ethyllene could be varied individually by appropriate adjustment of the helium flow rate. The reaction products were analyzed by a chromatographic unit coupled directly to the outlet of the reactor. The chromatographic column, 0.6 cm. in diameter and 1 m. in length, was packed with 100-mesh silica gel and operated a t 40’. Excellent resolution of the ethylene and ethane was obtained. A thermal conductivity detector was used with the column, and helium was employed as the carrier gas. The run procedure involved passing the reactant gases over the catalyst for a period of 3 min. prior to sampling the products for chromatographic analysis. The ethylene was then cut out and the hydrogen flow continued for 10 min. prior to another reaction period. Most of the reaction periods were bracketed by periods at a standard set of conditions, as an insurance against possible complications due to changing catalyst activity. Materials. Research grade ethylene was obtained from the Matheson Co. A chromatographic analysis of the ethylene showed no detectable hydrocarbon impurities such as ethane or methane, although it is estimated that levels of the order of 100 p.p.m. would have been detected. High purity hydrogen obtained from the Linde Co. was further purified by passing it through a Deoxo unit containing palladium catalyst to remove trace amounts of oxygen as water and then drying with a molecular sieve. The Pt-SiOz catalyst used in this work contained 0.050 wt. % Pt and was prepared by impregnating calcined silica gel with aqueous chIoroplatinic acid, and then calcining the finished catalyst a t 538’ in air for 1 hr. The silica gel, obtained from Davison Chemical Co., was calcined for 4 hr. at 538’ prior to impregnation with platinum. The B.E.T. surface area of the calcined silica gel was 388 m.”g. The particle size of the Pt-SiOz catalyst as used in the rate studies was approximately 0.3 mm. The catalyst was reduced in place in the reactor a t 500’ for 3 hr. in flowing hydrogen (200 cc./min.) prior to making any reaction rate studies.
Results
857
temperature range 45 to 93’. The reaction occurred very selectively, no side products being detected. The approach in studying the kinetics was to measure initial reaction rates, and 90% of the data was obtained at conversion levels less than 5%. The rates were calculated using the relation 1’
r = - x W where F represents the ethylene feed rate in gram-moles per hour, W represents the weight in grams of platinum on the catalyst charged to the reactor, and x represents the fraction of the ethylene converted to ethane. The reaction rate r is then expressed as grammoles of ethylene hydrogenated per hour per gram of platinum. The effect of temperature on reaction rate is shown in Fig. 1. The data in the plot were obtained in successive reaction periods in a falling temperature sequence. During the measurements a t the different temperatures, the hydrogen partial pressure PH and the ethylene partial pressure p~ were maintained constant a t 0.20 and 0.030 atm., respectively. From the slope of the Arrhenius plot of the data, the apparent activation energy is 16 kcal./mole. While the data in Fig. 1 show satisfactorily the effect of temperature on reaction rate, the absolute values of the rates are less meaningful. It has been observed that the activity of the catalyst can vary markedly over an extended series of measurements and is also sensitive to the length of time that the catalyst is reduced.
Y a3
2 0.7 0.5
0.4 0.3
I 2.6
2.8
I
I
3.0
3.2
1 / T X 108,
The hydrogenation of ethylene to ethane over the Pt-SiOz catalyst used in this work was studied over the
3.4
OK.-’.
Figure 1. Effect of temperature on the rate of CzHa hydrogenation; pa = 0.20 atm., p~ = 0.030 atm.
Volume 68, Number 4
April, 1964
J. H. SINFELT
858
In determining the effects of hydrogen and ethylene partial pressures on the rate of hydrogenation, it was decided to bracket all the reaction periods with periods at a standard set of conditions. This made it possible to detect variations in catalyst activity during the course of the measurements. The effect of hydrogen or ethylene partial pressure, varied one at a time away from the standard conditions, was then determined by simply comparing the rate, r , at the particular set of conditions with the average, ro, of the rates at the standard conditions immediately before and after the period in question. This procedure serves to minimize the complications due to varying catalyst activity. The results on the effects of hydrogen and ethylene partial pressures on the relative rates, r/ro,are presented in this form in Fig. 2 and 3. In the logarithmic plot of r/ro us. p~ at constant p~ in Fig. 2, a single line with a slope of approximately 0.5 fits the data for both temperatures. This indicates that the reaction rate is proportional to the square root of the hydrogen pressure over the range of pressures investigated. In Fig. 3, a similar logarithmic plot of r/ro us. p~ a t constant p~ shows that the effect of ethylene partial pressure varies somewhat with temperature. However, a t both temperatures the rate falls with increasing ethylene partial pressure at the lower ethylene pressures, but tends to become independent of ethylene partial pressure a t the higher pressures. Thus, at sufficiently high ethylene partial pressures, the reaction approaches zero order with respect to ethylene.
3l 2
0.4
-
0.3
-
0.2
-
0.5
0.1
I
“ 1 ”
I
I
I I I l l 1
I
I
I
Discussion The kinetic data on ethylene hydrogenation over the Pt-SiOz catalyst used in this work appear to be best explained by a mechanism involving adsorption of both reactants on the active catalyst sites. On the assumption that the hydrogen is chemisorbed dissociatively to atoms on the platinum surface, the surface reaction can be written
where the symbol (a) denotes an adsorbed species. The reaction rate is given by r = ~CBHOE
(2)
where OH and BE represent the fractions of the active surface covered by hydrogen and ethylene, respectively. Assuming that BH can be expressed in terms of a Langmuir isotherm, and that hydrogen is chemisorbed in competition with ethylene, we can write
If the term bH”apH1/ais small compared to unity, corresponding to weak adsorption of hydrogen, the denominator of eq. 3 can be ignored, and eq. 2 for the rate expression becomes 0.1
0.2
0.3
0.5 0.7 1 PH X 10, atm.
2
3
5
Figure 2. Effect of p~ on rate of CZH, hydrogenation at p~ = 0.030 atm.: 0, 77’; A, 45”.
The Journal of Physical Chemistry
7 1 0
r
=
kbH’”pH’/’(1 - BE)&
(4)
This equation accounts for the observed dependence of the rate on the square root of the hydrogen partial
KINETICS OF ETHYLENE HYDROGENATION
859
pressure. The equation also predicts that at constant ' 4 3 hydrogen pressure the rate will go through a maximum at BE = 0.5 and then decrease with further increase of 2 BE. Since BE is determined by the ethylene pressure p ~ the , rate should then decrease with increasing p ~ : for values of PE above that corresponding to BE = 0.5. 1 The results of the present study show that the rate decreases with increasing PE even a t the lowest ethylene p 0.7 pressures investigated, and to make the results con'= sistent with eq. 4 it must be postulated that an ethylene 0.5 coverage of 0.5 is attained a t ethylene pressures lower 0.4 than any covered in this work. This could well 0.3 be the case. However, eq. 4 cannot readily account for the observation that the rate becomes essentially 0.2 zero order with respect to ethylene at the higher ethylene pressures investigated, unless one assumes that BE approaches a maximum value less than unity as 0.1 I I I I 1 p~ is increased without limit. It is conceivable that 10 -4 10--3 10-2 lo-' 1 PE, atrn. the packing of ethylene molecules on the surface of the Figure 4. Comparison of eq. 7 with observed rate data; platinum crystallites is such that the ethylene cannot curve calculated from eq. 7; points represent d a t a a t 77". completely cover the platinum sites; i e . , a certain fraction of the sites is not accessible to ethylene, but is as illustrated in Fig. 4. We note that all the data were accessible to hydrogen. Making an assumption of obtained in the region to the right of the maximum in this nature, we can write a modified Langmuir adsorpthe curve in Fig. 4. I t was not possible to obtain tion expression for BE data a t low enough ethylene pressures to observe the maximum. In Fig. 3, the data indicated that the curve of r/ro us. p~ was flatter a t the lower temperature. This would be predicted by eq. 7 provided that where c represents the maximum degree of coverage a higher value of b E was assumed a t the lower temperaby ethylene as PE is increased without limit. On subture. Such an assumption is reasonable since adsorpstitution of expression 5 for & in eq. 4,the rate exprestion constants should increase with decreasing temperasion becomes ture. The position of the maximum in the curve of r/rOus. p~ would be expected to shift to a lower value of p~ at the lower temperature. The apparent activation energy of 16 kcal./mole obEquation 6 indicates that, a t very low ethylene partial served in this work is somewhat higher than has generpressures, the rate will increase with increasing ethylene ally been observed for ethylene hydrogenation (about pressure up to a maximum value, beyond which the 11 kcal./mole). However, the catalyst employed in rate declines with further increase in ethylene pressure. the present study represents a marked departure from As the ethylene pressure is increased without limit, the usual catalyst employed, e.g., metal films, or in however, the rate approaches a constant limiting value. the case of supported metals, much higher metal conApplying eq. 6, we can write an expression for r/ro, tents than the 0.05% used in this work. the rate at a particular set of conditions relative to the The kinetic analysis presented in this paper is unrate at standard conditions. Arbitrarily taking c = doubtedly oversimplified, but it does appear to account 0.90, and noting that p~ = 0.030 atm. a t the standard reasonably well for the negative order with respect to conditions employed in this study, we obtain for r/rO ethylene at low ethylene pressures and for the shift toward zero order a t the higher pressures. This type of analysis could tend to reconcile some of the apparent discrepancies in the literature regarding the effect of Taking a value of bE equal to 500 atm.-l, we obtain ethylene pressure, since part of this might arise simply satisfactory agreement between eq. 7 and the data a t from the fact that rate measurements were obtained 77' for the range of ethylene pressures investigated, in different ranges of conditions. However, it is likely Volume 68,Number
4 April,
1.9f74
WAYNEL. WORRELL AND JOHN CHIPMAN
860
that the kinetic treatment presented here would not be satisfactory for situations where a large excess of ethylene relative to hydrogen is employed, since in such cases side reactions involving excess decomposition and possibly polymerization of unsaturated fragments could lead to irreversible poisoning of the surface. The activity of the 0.05% Pt on Si02 catalyst used in this study, expressed as rate per unit weight of platinum, appears to be much lower than has been observed by Bond2 for platinum catalysts containing 1 to 5% platinum. While this might merely reflect differences in catalyst preparation techniques, it might also indicate
heterogeneity of platinum centers, such that successive increments of platinum have different intrinsic catalytic activities. In support of this hypothesis, it has been observed that for a series of platinum-on-charcoal catalysts prepared in the same way, the rate of benzene hydrogenation per unit weight of platinum shows a sharp decrease below 0.1% p l a t i n ~ m . ~ It is conceivable that this effect may be associated with an interaction between the platinum and support. (4) A. M. Rubinshtein, K. M. Minachev, and N. I. Shuikin, Dokt. Akad. Nauk S S S R , 67, 287 (1949).
The Free Energies of Formation of the Vanadium, Niobium, and Tantalum Carbides'
by Wayne L. Worrel12 and John Chipman Department of Metallurgy. Massachusetts Institute of Technology, Cambridge, Massachusetts (Received October $0, 106$)
By measuring the pressure of carbon monoxide in equilibrium with graphite-carbideoxide mixtures at temperatures of 1180 to 1370°K., the free energies of formation of the carbon-rich carbides V c , NbC, and TaC have been determined. The notations MC and G C are introduced to denote carbide compositions, respectively, a t the carbon-rich and a t the metal-rich boundaries of the homogeneous field. The thermodynamic properties of other group VA carbides (RC and M2C) are estimated from the MC data and the phase diagrams.
Introduction With the increasing interest in refractory metal carbides, any determination of their thermodynamic properties at elevated temperatures should be very useful. In this investigation, the thermodynamic properties of vanadium carbide, niobium carbide, and tantalum carbide were determined by bringing a graphite-carbide-oxide pellet into equilibrium with a carbon monoxide atmosphere. Since the metal carbide is in equilibrium with carbon, the experimental results are valid for only the most carbon-rich composition of the carbide (MC). The Journal of Physical Chemistry
Because the group Va carbides exhibit a homogeneity range,a the simple formula MC is insufficient to designate a particular carbide composition. In this study, the formula for the carbide is modified to indicate precisely the carbide under discussion even when its exact (1) This paper is based on a thesis suhmitted by Wavne L. Worrell in partial fulfillment of the requirements for the dmree of Doctor of
Philosophy at the Massachusetts Institute of Technology, May, 1963. (2) Inorganic Materials Research Division, Lawrence Radiation Laboratory, University of California, Berkeley, Calif (3) E. K. Storms, Los Alamos Scientific Lab Report LAMS-2674, 1962.
