J . Phys. Chem. 1992, 96, 1880-1888
1880 WE
0
.ABD
Distance
-
Figure 12. (top) Schematic diagram of the ordered ABD monolayer film
structure on a SnO, glass electrode. (bottom) Potential profile through the electrolyte solution in an electrochemical system. state with an sp-hybridized nitrogen atom would be accompanied by a loss of molecular freedom.12-ls~20 In addition, it seems worthwhile to study the effect of an electrochemically-produced electric field on the isomerization kinetics. Obviously interactions between electric field and molecular dipoles would occur in the present system. Since the transition states derived by rotation and inversion mechanisms have distinctly different dipole moments,'4J9reaction kinetics based on the two schemes would be affected differently in response to the change of electric field. Such an approach is particularly
possible using the present experimental arrangement as schematically demonstrated in Figure 12, where the electric field is generated electrochemically between working and counter electrodes. Since the potential gradient mainly occurs in the double-layer vicinity of the working electrode?' the necessarily strong electric field can be easily achieved using the ordinary potential bias. And further, the highly-ordered arrangement of ABD molecules in the electric field may prevent molecular dipoles from compensation with each other and thus enhance the effect of electric field. Work is in progress on this theme.
Conclusion The kinetic parameters for the thermal isomerization of cisABD molecules in the assembled monolayer film are precisely determined using the electrochemical method for the first time. The activation enthalpy in such a reaction environment is found to be distinctly smaller than that of azobenzene in crystal, but quite comparable with that of cis-ABD in chloroform solution. The kinetic behavior observed is attributed to the specific isomerization environment effect created by the assembled monolayer film. The present results suggest the inversion mechanism for the thermal isomerization and also imply that studies on the electrochemically-generated electric field effect may offer more useful information on the reaction mechanism. In addition, from a direct comparison with the conventional spectrophotometric method, the electrochemical approach employed in this study is evidenced to be considerably effective for quantification of the ultrathin monolayer film. Acknowledgment. We thank Drs. S. Suzuki, K. Imura, K. Yamamoto, and K. Hyodo of Mitsubishi Paper Mills for their helpful discussions and gratefully acknowledge the financial support from Mitsubishi Paper Mills. Registry No. ABD, 112360-09-5;SnO,, 18282-10-5.
Microkinetic Analysis of Diverse Experimental Data for Ethylene Hydrogenation on Platinum James E. Rekoske, Randy D. Cortright, Scott A. Goddard, Sanjay B. Sharma, and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received: September 9, 1991)
Kinetic analysis employing a mechanism that captures the essential surface chemistry of the reaction allows quantitative interpretation of diverse experimental data. This approach is used with a Horiuti-Polanyi mechanism, modified by hydrogen activation steps, to describe the surface chemistry for ethylene hydrogenation over platinum catalysts. In this investigation, kinetic analysis provides a quantitative means of comparing, contrasting, and consolidating results from steady-state kinetic studies, deuterium tracing measurements,vibrational spectroscopy,and temperature programmed desorption. A noncompetitive pathway is dominant at low temperatures, involving sites for hydrogen adsorption that are not blocked by carbonaceous species. At higher temperatures and lower ethylene pressures, more surface sites become available for hydrogen adsorption, and the reaction shifts to a pathway involving competitive hydrogen and ethylene adsorption.
Introduction The quantification of chemical kinetic phenomena on heterogeneous catalysts is an important aspect of assessing the performance of these materials and for understanding the essential surface chemistry that controls catalyst behavior. For example, it is common to describe steady-state reaction kinetics data in terms of rate expressions based on various catalytic reaction mechanisms (e&, Boudartl). Quantitative studies of this type are vital for the comparison of different catalytic materials studied over a range of reaction conditions and for elucidation of relationships between catalyst performance and catalyst chemical properties.
* To whom correspondence should be addressed. 0022-3654/92/2096-1880%03.00/0
A key problem faced in the analysis of kinetic data is that studies of catalytic and surface chemistry are generally conducted over a variety of reaction conditions, as dictated by the limitations of the various experimental techniques used in these studies. Therefore, it is difficult to bring this information together for the purpose of extracting quantitative kinetic information. Accordingly, an important problem in chemical kinetics studies of heterogeneous catalysts is to interpret, coordinate, and generalize results from diverse experimental studies to create a basis for quantitative description of kinetic phenomena. We attempt in ( 1 ) Boudart, M. The Kinetics of Chemical Processes; Butterworth-Heinemann: Boston. 1991.
0 1992 American Chemical Society
Ethylene Hydrogenation on Platinum the present paper to address this problem through kinetic analysis of diverse experimental data for ethylene hydrogenation on platinum. Kinetic analysis is based on experimental data to guide the selection of plausible elementary reaction steps. For example, spectroscopic studies identify surface adsorbed species likely to be active, steady-state kinetic data suggest kinetically significant reaction paths, isotopic scrambling experiments give information about the relative rates of elementary reactions, and temperature programmed desorption studies provide information about adsorption/desorption processes. An appropriate mechanism must contain sufficient detail to describe all of the available experimental data; however, excessive detail leads to ambiguity in the estimation of kinetic parameters. In general, the level of chemical detail that may be incorporated in kinetic analysis of experimental data in terms of reaction mechanisms involves a compromise between the amount of detail that is accessible to experimental observation and the desirability of writing all reactions as elementary steps. Accordingly, the present paper addresses the following question: What level of complexity is required for kinetic analysis to capture the essential quantitative features of the surface chemistry? We have chosen to focus the present investigation on ethylene hydrogenation over platinum. This reaction has been the subject of numerous spectroscopic and kinetic studies. We have supplemented the literature with our own steady-state kinetic studiesZ and deuterium tracing experiments3 over a wide range of conditions. In addition, we report the results of in situ infrared spectroscopy to probe the reactivity of ethylidyne species during ethylene hydrogenation. We show that kinetic analysis of these experimental data, combined with temperature programmed desorption data from the literature, can be conducted effectively using a Horiuti-Polanyi mechanism, modified to include competitive and noncompetitive adsorption of hydrogen and ethylene. This analysis suggests that adsorbed hydrogen must be activated before it may react with hydrocarbon species on the surface. Essential Chemistry of Ethylene Hydrogenation Reaction Kinetics Studies. Numerous steady-state kinetic studies, each conducted over a limited range of conditions, have been reported in the literature for the hydrogenation of ethylene over platinum c a t a l y ~ t . ~ - The ' ~ following general results have been found in these investigations: first-order hydrogen kinetics are observed at temperatures above 273 K, slightly negative ethylene kinetic orders are observed at low ethylene pressures and temperatures above 273 K, and activation energies between 8.8 and 10.8 kcal/mol are observed between 170 and 360 K. In our laboratory we measured catalytic activities and kinetic orders over a wide range of temperatures and pressures for ethylene hydrogenation over Pt/Cab-0-Si1 and Pt wire catalysts.* The major conclusions from this study are as follows. (a) The catalyst exhibits an initial deactivation period in which the turnover frequency decreases to about 75% of that for the fresh catalyst. (b) The hydrogen order increases with increasing temperature from 0.5 order at 223 K to 1.1 order at 333 K, as shown in Figure
1. (c) The ethylene order is nearly zero at low temperatures and high ethylene pressures. (2) Cortright, R. D.; Goddard, S. A.; Rekoske, J. E.; Dumesic, J. A. J . Catal. 1991, 127, 342. (3) Goddard, S. A.: Cortriaht. R. D.: Dumesic. J. A. Submitted for publication in J. Cafal.,1991.(4) Sinfelt, J. H. J. Phys. Chem. 1964, 68, 856. (5) Bond, G. C. Trans. Faraday SOC.1956, 52, 1235. (6) Bond, G. C.; Phillipon, J. J.; Wells, P. B.; Winterbottom, J. M. Trans. Faraday SOC.1964,60, 1847. (7) Dorling, T. A.; Eastlake, M. J.; Moss, R. L. J . Caral. 1969, 14, 23. (8) Horiuti, J.; Miyahara, K. Hydrogenation of Ethylene on Metallic Catalysts; NBS-NSRDS No. 13; Government Printing Office: Washington, DC. 1968. (9) Kazanskii, V. B.; Strunin, V. P. Kiner. Catal. (Engl. Trans/.) 1960,1, 517. (10) Schlatter, J. C.; Boudart, M. J. Coral. 1972, 24, 482. (11) Schuit, G. C. A.; Reijen, L. L. V. Adu. Cural. 1958, 10, 242. (12) Zaera, F.; Somorjai, G. A. J . Am. Chem. SOC.1984, 106, 2288.
Prediction
100.0
336 K
p
1
298 K
1.01
0
@
273 K
"j
248 K
233 K
0.0'
'
I
I " " I
100 1000 Hydrogen Pressure (Torr)
Figure 1. Comparison of experimental and predicted ethylene hydrogenation turnover frequencies versus hydrogen partial pressure. Experimental hydrogen kinetic orders: at 223 K, 0.47; at 248 K, 0.53; at 273 K, 0.66; at 298 K, 0.76; at 336 K, 1.10.
60 50
m (II
c
5
40
c
8
30
m
P
20 10
n 0
1
2
3
4
5
6
Number of Deuterium Atoms
Figure 2. Experimental ethane isotopic distributions resulting from reaction of 150 Torr of deuterium with 25 Torr of ethylene at 248 and 293 K.
(d) The turnover frequency for ethylene hydrogenation increases with temperature with a constant apparent activation energy of 8.6 kcal/mol. The results of this kinetic study are in general agreement with the previous steady-state kinetic studies, but, more importantly, the results provide consistent reaction kinetics over a wide range of conditions. Deuterium Tracing Experiments. Isotopic tracing experiments have been conducted for the hydrogenation of ethylene in which ethylene was reacted with deuterium and the isotopic compositions of gaseous ethane, ethylene, and dihydrogen were recorded.56812,13 * Horiuti and Polanyi14investigated the rate of exchange between ethylene and heavy water. These authors proposed a mechanism in which ethylene is associatively adsorbed and hydrogen is disd a t i v e l y adsorbed on the surface. The adsorbed ethylene species reacts with adsorbed hydrogen atoms to form a half-hydrogenated state which subsequently reacts with another adsorbed hydrogen atom to form ethane. Sato and MiyaharaI3 concluded from their investigation that the desorption of ethane was irreversible and that the first hydrogenation was faster than the other steps. In our laboratories a deuterium tracing study was performed over the same range of conditions as our steady-state kinetics inve~tigation.~ One of the major conclusions of this investigation was that the ethane isotopic distributions observed from the reaction of ethylene with deuterium broadened with increasing temperature. Figure 2 shows this broadening when the temper7
1.
9
( 1 3) Sato, s.;Miyahara, K. J. Res. Inst. Catal., Hokkaido Uniu. 1975, 23,
(14) Horiuti, J.; Polanyi, M. Trans. Faraday SOC.1934, 30, 1164.
1882 The Journal of Physical Chemistry, Vol. 96, No. I , 1992
Rekoske et al.
I
I
.01 Absorbance
2917
............................... 3100
3000
2900
1
.-r
2800
1560
1520
1400
1440
1400
1360
1320
'
Wavenumber ("1)
Wavenum ber (cm- )
Figure 3. Infrared spectra of ethylene adsorbed on 10% Pt/Cab-0-Si1 catalyst at 298 K.
ature is increased from 248 to 293 K at 25 Torr of ethylene and 150 Torr of deuterium. Surface Spectroscopy. Infrared spectroscopy experiments were conducted in our laboratory over a Pt/silica catalyst at 300 K. These transmission infrared studies were conducted on a 2.5-cmdiameter catalyst disk containing approximately 0.15 g of a 10% Pt/Cab-0-Si1 catalyst. A stainless steel cell fitted with two CaFz windows and ports for gas inlet and outlet was used in these experiments. Gases were purified by passage over Oxytrap (Aldridge) and molecular sieve traps. A Nicolet FX70 FTIR system was used to collect and process the infrared spectra. Figure 3 shows a typical infrared spectrum obtained in these experiments. Analysis of the spectrum based on the literature15 indicates that three distinct surface species are formed upon adsorption at room temperature: r-adsorbed ethylene (1500 an-'), di-a-adsorbed ethylene (1420 cm-I), and a partially dehydrogenated ethylidyne species (1342 cm-I). Ethylidyne, as identified through LEED,I6 is oriented perpendicularly to the surface in a 3-fold hollow site. Mohsin et al.15 used transmission I R spectroscopy to investigate ethylene adsorbed at low temperatures on supported Pt catalysts. Adsorption at 150 K yielded primarily the r- and di-a-adsorbed ethylene species. Upon warming the sample to room temperature, these authors observed a transformation of the di-a-adsorbed species to ethylidyne. Upon hydrogen addition, Mohsin and co-workers found that the r- and di-u-adsorbed ethylene species reacted much faster with hydrogen than did ethylidyne. Somorjai and co-workers1' reported that the rate of ethylene hydrogenation over Pt(ll1) at 298 K was about lo6 times faster than the rate of ethylidyne removal or the deuteration of its methyl group. Beebe and Yatesi8 showed that ethylidyne was not essential to ethylene hydrogenation. The authors used in situ IR spectroscopy to monitor the surface of a Pd/alumina catalyst during ethylene hydrogenation. These authors did not detect ethylidyne formation under hydrogen-rich conditions, and they observed no difference in the reaction rate with or without ethylidyne on the surface. These results suggest that ethylidyne is not a reactive intermediate but it is instead a spectator in the hydrogenation of ethylene. Similar in situ IR studies were, therefore, conducted in our laboratories over silica-supported Pt to investigate the reactivity of ethylidyne. These studies were conducted at room temperature and at atmospheric pressure. (15) Mohsin, S. B.; Trenary, M.; Robota, H. J. J . Phys. Chem. 1988, 92,
5229. (16) Kesmdel, K.; Dubois, L.; Somorjai, G. A. J . Chem. Phys. 1979, 70,
2180. (17) Somorjai, G. A.; VanHove, M. A.; Bent, B. E. J . Phys. Chem. 1988,
92, 973. (18) Beebe, T. P., Jr.; Yates, J. T., Jr. J . Am. Chem. SOC.1986, 108, 663.
'I
"f
cna?ed,
1390
1380
1370
1360
1350
.
,
-
1340
,
1 1330
Wavenumber (cm'') Figure 4. Infrared spectra of 10% Pt/Cab-0-Si1 during transient ethylene hydrogenation conditions at 298 K: ethylene/hydrogen pressure ratio of 5.0. The characteristicadsorption band for ethylidyne is at 1342 cm-I.
In the in situ IR studies, helium and hydrogen flows were first established and the system was allowed to reach steady state. A background spectrum of the hydrogen-covered surface was collected to use as a reference for all spectra collected during subsequent transient experiments. An ethylene transient experiment was initiated by a step change in the ethylene flow. Once the steady-state condition was reached, the ethylene flow was stopped and the surface was again monitored. At a high CzH4:H2pressure ratio of 5.0, ethylidyne surface species were observed, as indicated in Figure 4. Upon introduction of ethylene, the partially dehydrogenated ethylidyne species appears. The ethylidyne surface coverage reaches steady state in about 1 min. When the ethylene flow is stopped, ethylidyne persists on the surface even after 5 min. In contrast, no ethylidyne surface species were observed at a lower C2H,:Hz pressure ratio of O S , as shown in Figure 5 . Thus, one can conclude that the ethylidyne does not participate actively in the reaction, since the reciprocal of the turnover frequency for ethylene hydrogenation is approximately 1 s at the conditions of the IR experiments. It should be noted that r- or di-a-adsorbed ethylene species could not be observed during these experiments because of interference from gaseous ethylene. The shifts in the base lines in both Figures 4 and 5 were due to heating of the silica support from the exothermic hydrogenation reaction. The same steady-state surface coverage was observed when ethylene was introduced first followed by hydrogen. Initially, before hydrogen was introduced, the surface contained ethylidyne
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1883
Ethylene Hydrogenation on Platinum
1390
1380
1370
1360
1350
1340
1330
Wavenumber (cm") Figure 5. Infrared spectra of 10% Pt/Cab-0-Si1 during transient ethylene hydrogenation conditions at 298 K ethylene/hydrogen pressure ratio of 0.5. The characteristic adsorption band for ethylidyne is at 1342 cm-I.
species. In the case of low hydrogen pressures, the introduction of hydrogen decreased the intensity of the ethylidyne band to its steady-state value. At higher hydrogen pressures, the ethylidyne band was completely removed. Thus, at room temperature, the ethylidyne species is reactive; however, it appears to be much less reactive than either the A- or di-o-adsorbed ethylene, as indicated by the study of Mohsin et al.I5 The spectator role of the ethylidyne species is consistent with the results of Beebe and Yates.18 Another important surface species that has been identified under reaction conditions is the ethyl radical (-CzH5). Backman and MaselI9 used electron energy loss spectroscopy (EELS)to examine the surface intermediates formed during ethylene hydrogenation on Pt( 1 1 1) at 298 K. They found the surface to be covered with a mixture of ethyl radicals and ethylidyne. The ethyl radicals were easily hydrogenated and removed from the platinum surface, which suggests that these species are reactive intermediates. Adsorption/Desorption Studies. The zero-coverage sticking coefficient for dissociative hydrogen adsorption has been reported to be between 0.06 and 0.45, depending on the surface of Pt under investigati~n.~"-~~ Also, the sticking coefficient of hydrogen on platinum surfaces does not generally exhibit a temperature dependenceeZ4 The desorption kinetics of hydrogen from Pt surfaces have been extensively investigated using temperature programmed desorption.22~a~z7*31~3z It has been indicated by some authors22,24~28~33 that the desorption peak is significantly wider than expected, suggesting that the preexponential factor for the reaction is lower than the normal value of 1013s-'. The values of the desorption activation energy with a normal preexponential factor were estimated to be between 17 and 20 kcal/mol. A lower preexponential factor of lo6 s-I gives an activation energy of 5-12 kcal/mol. The adsorption of ethylene on R(100) surfaces has been studied by Fisher and K e l e m e ~ ~ . These )~ authors measured a sticking (19) Backman, A. L.; Masel, R. I. J . Vac. Sci. Technol. A 1991, 9, 1789. (20) Christmann, K.; Chehab, F.; Penka, V.; Ertl, G. Surf. Sci. 1985, 152, 356. Christmann, K. Surf. Sci. Rep. 1988, 9, 1 . McCabe, R. W.; Schmidt, L. D. Surf.Sci. 1977, 65, 189. Netzer, F.P.; Kneringer, G. Surf. Sci. 1975, 51, 526. Christmann, K.; Ertl, G.;Pignet, T. Surf. Sci. 1976, 54, 365. Chappell, R.; Hayward, D. 0. J . Vac. Sci. Technol. 1972, 9, 1052. Lu,K. E.; Rye, R. R. Surf. Sei. 1974, 45, 677. Lu, K. E.; Rye, R. R. J . Vac. Sci. Technol. 1975, 12, 334. McCabe, R. W.; Schmidt, L. D. Surf. Sci. 1976, 60, 85. Norton, P. R.; Richards, P. J. Surf. Sei. 1974, 44, 129. SalmerBn, M.; Gale, R. J.; Somorjai, G.A. J . Chem. Phys. 1977, 67, Baldwin, V. H.; Hudson, J . B. J . Vac. Sci. Technol. 1971, 12, 49. Poelsema, B.; Mechtersheimer, G.; Comsa, G. Surf.Sci. 1981, I l l , (33) McCabe, R. W.; Schmidt, L. D. Proc. 7th Int. Vac. Congr. 1977,2, 120.
coefficient of 0.2-0.7, depending on the structure and temperature of the Pt surface. Smith and Merri1135found that the adsorption kinetics for ethylene on Pt( 100) and Pt( 1 1 1 ) crystals could be satisfactorily explained by Langmuirian site blocking with a zero-coverage sticking coefficient of unity. Weinberg and cow o r k e r ~also ~ ~reported a sticking coefficient of unity for ethylene adsorbed on Pt( 1 1 1). Temperature programmed desorption studies of ethylene on Pt show both desorption and decomposition of ethylene. Salmerdn and Somorjai3' investigated the desorption of ethylene from Pt( 1 1 1). Molecular ethylene desorbed from the surface at low temperatures or decomposed to yield adsorbed atomic hydrogen. This hydrogen desorbed as Hz or hydrogenated the remaining adsorbed ethylene to ethane. At higher temperatures the partially decomposed olefin underwent further dehydrogenation, producing coke and gaseous hydrogen. The kinetic parameters for ethylene desorption obtained by Salmerdn and Somorjai were adjusted to 9 kcal/mol and 107.0S-I by Godbey et al.38 Other author^^^*^ have found similar desorption kinetics for ethylene from Pt surfaces. Temperature programmed desorption studies provide information on the desorption rate constants of a co-adsorbed system of hydrogen and ethylene. Berlowitz et al.41co-adsorbed ethylene and hydrogen on Pt( 11 1) and performed temperature programmed reaction (TPR) studies. They found that ethane and ethylene desorbed at higher temperatures from a clean Pt( 11 1 ) surface compared with desorption from a surface that had hydrogen preadsorbed onto it. Since a catalyst surface during ethylene hydrogenation is expected to be predominantly covered with hydrocarbon species, the results of Berlowitz et al. have important consequences, as shown later in this paper. Quantitative Analysis of Kinetic Data Formulation of Reaction Mechanism. In this kinetic analysis of ethylene hydrogenation on Pt, it is desired to develop a chemically reasonable mechanism with a consistent set of kinetic parameters that describes the observed steady-state kinetic dataZ and deuterium tracing result^.^ In addition, we wish to reconcile these results from high surface area Pt catalysts with the position and relative heights of the various desorption peaks from the temperature programmed reaction of co-adsorbed ethylene and hydrogen on Pt single crystals observed by Berlowitz et As outlined below, the Horiuti-Polanyi mechanism can be modified to explain these observed experimental data. The initial catalyst deactivation indicates the formation of inactive surface species that block the active sites on the catalyst surface. The ethylene kinetic order suggests that hydrocarbon species are the most abundant reactive intermediates on the catalyst surface and that hydrogen competes with ethylene for adsorption on active sites at higher temperatures and lower ethylene pressures. The change in the hydrogen kinetic order with temperature suggests a change in the nature of the rate determining or slow steps in the reaction mechanism. The breadth of the deuterium distribution in ethane suggests that at least one of the surface hydrogenation steps is reversible, while the relatively low levels of deuterium incorporation into ethane indicate that the hydrogen adsorption/desorption step is not very fast. The spectroscopic studies suggest that three adsorbed ethylene species are present on the catalyst surface. While reaction through ethylidyne or similar species may be possible at high temperatures, (34) Fischer, T. E.; Kelemen, S. R. Surf. Sci. 1977, 69, 485. (35) Smith, D. L.; Merrill, R. P. J . Chem. Phys. 1970, 52, 3588. (36) Weinberg, W. H.; Deans, H. A,; Merrill, R. P. Sur/. Sci. 1974, 41, 312. (37) Salmerbn, M.; Somorjai, G. A. J. Phys. Chem. 1982, 86, 341. (38) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G.A. Surf. Sci. 1986, 167, 150. (39) Windham, R. G.; Bartram, M. E.; Koel, B. E. J . Phys. Chem. 1988, 92, 2862. (40) Windham, R. G.;Koel, B. E. J . Phys. Chem. 1990, 94, 1489. (41) Berlowitz, P.; Megiris, C.; Butt, J. B.; Kung, H. H. Langmuir 1985, 1, 206.
Rekoske et al.
1884 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
our infrared studies indicate the reactivity of ethylidyne with hydrogen is lower than the reactivity of the ?r- and di-u-forms of adsorbed ethylene at room temperature. The di-u-adsorbed ethylene species should occupy two surface adsorption sites. Similarly, from geometric considerations it is expected that the *-adsorbed species also would occupy two surface sites. Further, simple thermodynamic bond breaking/forming calculations suggest that the energetic differences between these two forms of adsorbed ethylene are small. We thus do not attempt to differentiate between A- and di-u-adsorbed ethylene species in our kinetic analysis. A competitive Horiuti-Polanyi mechanism can be used to explain kinetic and isotopic data from ethylene hydrogenation at temperatures above 300 K.13 In addition, the presence of a half-hydrogenated or ethyl species, as predicted by the HoriutiPolanyi mechanism, has been observed spectroscopically.’9 This mechanism is shown schematically as follows. H2 2* c* 2H*
+ C2H4 + 2* *C2H4* + H * *C2H5* + H *
--
*C2H4*
*C2H5*
+
C2Hs
+*
+ 3*
At temperatures below 300 K, however, this mechanism is incapable of showing half-order hydrogen and zero-order ethylene kinetics over a wide range of pressures. Accordingly, this mechanism does not have sufficient detail to reproduce the changes in the hydrogen and ethylene reaction orders observed at low temperature in the kinetic experiments. The above problem can be alleviated using a noncompetitive Horiuti-Polanyi mechanism involving sites that adsorb hydrogen but are not blocked by hydrocarbon species. It may be assumed that the catalyst surface becomes highly covered with adsorbed hydrocarbon at lower temperatures, approaching saturation ~ o v e r a g e . ’ ~ However, , ~ ~ ~ ~ * the smaller hydrogen molecule may adsorb on a site that is inaccessible to hydrocarbon molecules. The presence of this hydrogen adsorption site, S,gives rise to half-order hydrogen kinetics according to the Langmuir isotherm for dissociative, equilibrated hydrogen adsorption. The concept of noncompetitive adsorption of hydrogen on metal surfaces has been discussed in the literature. M e ~ a khas i ~ sug~ gested competitive/noncompetitive adsorption for olefin hydrogenation from the work of Hougen and co-~orkers.“~Sinfelt has also discussed the possibility of noncompetitive adsorption sites for hydrogen chemisorption on pt/silica during the hydrogenation of ethylenen4Boudart4 has discussed the possibility of noncompetitive adsorption of ethane and hydrogen for the hydrogenolysis of ethane. The noncompetitive reaction mechanism can show a change in hydrogen order by decreasing the reversibility of hydrogen adsorption as temperature is increased. As this step becomes irreversible, first-order hydrogen kinetics are observed; however, a reaction order greater than unity cannot be predicted by this model. More importantly, the sticking coefficient of hydrogen on platinum does not generally exhibit a temperature suggesting that the reaction rate should become independent of temperature as first-order kinetics are reached. This behavior is not observed experimentally; therefore, the noncompetitive Horiuti-Polanyi mechanism may lack sufficient detail to reproduce the ethylene kinetic data over the entire range of temperatures and pressures. A combination of competitive and noncompetitive pathways for ethylene hydrogenation allows prediction of the kinetic orders and the overall catalytic activity within the temperature range where the steady-state kinetic studies were conducted. In particular, the failure of the noncompetitive mechanism a t higher (42) Mezaki, R. J . Cafal. 1968, IO, 238. (43) Rogers, G. B.; Lin, M. M.; Hougen, 0. A. AIChE J . 1966,12,369. (44) Boudart, M.; DjBga-Maridasou. The Kinetics of Heferogeneous Cafalytic Processes; Princeton University Press: Princeton, NJ, 1984.
temperatures is corrected by increasing contributions of a competitive pathway as temperature increases. It is important to note that the competitive and noncompetitive pathways involve the same basic surface chemistry, differing primarily in the blocking of hydrogen adsorption sites by carbonaceous species. The noncompetitive/competitivemechanism suggests that the adsorption of hydrogen is reversible at low temperatures to explain half-order hydrogen kinetics. However, the description of the deuterium tracing studies requires the adsorption of hydrogen to be less reversible than is necessary to predict half-order hydrogen kinetics at 248 K. Specifically, analyses of the deuterium tracing experiments at these temperatures predicted ethane deuterium distributions that had less ethane-do and -dl than those observed experimentally. This deficiency in the analyses can be eliminated if the mechanism is altered by addition of a step wherein adsorbed hydrogen becomes incorporated slowly into the pool of hydrogen involved in the hydrocarbon exchange process. Similar reasoning led Sat0 and M i ~ a h a r to a ~suggest ~ a “hydrogen activation” step to explain the steady-state kinetics and deuterium tracing observed for the hydrogenation of ethylene over nickel. Kinetic analysis suggests that both the noncompetitively and competitively adsorbed hydrogen requires activation to react with the hydrocarbon surface species. The TPR experiments probe the competitive portion of the mechanism, since these steps are expected to be more characteristic of the lower surface coverage conditions of the surface science experiments. It was necessary in the TPR analyses to include the formation of ethylidyne species from adsorbed ethylene: *C*H4* + S’- *CH3C* HS’
+
This step was necessary since it provides a substantial amount of surface hydrogen under the transient conditions of the temperature programmed process. The addition of the ethylidyne formation step to the analyses of the steady-state kinetic and deuterium tracer studies is not necessary in view of its low reactivity at the temperatures of interest. Eptimetimof Kinetic and Thermodynamic Parameters. On the basis of the above arguments, the following working mechanism for the hydrogenation of ethylene is proposed: H2 + 2 s 2HS noncompetitive hydrogen adsorption (1)
HS
--
+ S’
HS’ + S
C2H4 *C2H4*
+ HS’
*C*HS* + HS’
--
+ 2* H* + S’ H2
-
+ 2*
*C2H5* + S’
-
C2H6 + 2 *
2H*
HS’
noncompetitive hydrogen activation (2) *C2H4* ethylene adsorption (3)
+ S’
initial hydrogenation (4) second hydrogenation
competitive hydrogen adsorption
+*
(5) (6)
competitive hydrogen activation (7)
In this mechanism, ethylene adsorbs associatively and occupies two surface sites. Hydrogen can adsorb dissociatively either on surface sites (*) in competition with ethylene or on noncompetitive adsorption sites (S). Hydrogen adsorbed competitively (H*) and noncompetitively (HS) can be converted to activated surface hydrogen (HS’) that can subsequently react with the adsorbed hydrocarbon. In this investigation, Langmuirian adsorption kinetics with second-order coverage effects, (1 - e)*, were assumed for adsorption steps 1,3, and 6. The ethylene adsorption rate constant can be estimated from the surface science l i t e r a t ~ r e ~using ” ~ ~a sticking coefficient of 1.0 with no activation energy. The desorption kinetics are estimated from the TPD spectra of ethylene from Pt(ll1) observed by Godbey et al.38(a preexponential factor of 107.0s-I and an activation energy of 9 kcal/mol). The competitive hydrogen adsorption rate constant can be estimated from the surface science literature using a sticking coefficient of 0.1 (45) Sato, S.; Miyahara, K. J . Res. Inst. Catal., Hokkaido Uniu. 1974, 22, 172.
Ethylene Hydrogenation on Platinum
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1885
TABLE I: Kinetic Parameters of the Modified Horiuti-Polanyi Mechanism for the Hydrogenation of Ethylene on PlatinumQ
80 1
I
I
I
60
1 2 3 4 5 6 7
9.00 2.21 4.00 3.41 3.60
X lo-*
0.00
10" x 105 X 1OIo
10.60
X
x
1015
0.00 9.43 9.00
1.50 X 10'
0.00
7.17 X 10'O
10.62
5.87 3.25 1.2s 3.58 6.00 1.39 1.13
x 109 X
x X X
x X
lo9 107 lOI4 lo4 109 lOI3
6.00 10.00 9.00 10.70 26.66 6.00 10.00
40
20
"Preexponential factors are in units of Torr-' s-I for adsorption steps and s-l for surface reactions and desorption steps.
0 0
TABLE II: Comparison of Predicted and Experimental Ethylene Kinetic Orders at 150 Torr of Hydrogen over Platinum temp/K 223 248 273
exutl -0.17 -0.17 -0.17
uredicted -0.09 -0.20 -0.35
temu/K 298 333
exutl -0.20 -0.43
uredicted -0.52 -0.65
with no activation energy.24*29v46An activation energy of 6 kcal/mol was chosen for the desorption of both the competitive and noncompetitive desorption of hydrogen. This activation energy corresponds to the hydrogen state observed by Christmann et al.24 at higher coverages. In addition, over the temperature range of this investigation the second hydrogenation step can be consider irreversible. The other kinetic parameters must be adjusted such that an accurate description of the kinetic information can be obtained with a consistent set of physically realistic kinetic parameters. An optimization procedure was undertaken in which the values of the kinetic parameters were adjusted within physically realistic limits until a consistent set could explain the steady-state kinetics, deuterium tracing, and TPR results. The analysis of steady-state kinetic data involves the simultaneous solution of the five surface steady-state equations for reactive adsorbed species based on the working mechanism, the three site balances for the catalyst surface, and the reactor design equations for each gas-phase The deuterium distributions are described by solving the equations given by material balances for each isotopic gas-phase and surface species. The parameters in these equations are the forward and reverse reaction rates for each elementary step. These reaction rate parameters are determined from preexponential factors and activation energies for the forward and reverse reactions of each step of the mechanism. The specific details of this method and the equations used to describe the deuterium distributions are discussed in more detail el~ewhere.~ Finally, steps 3-7 along with the ethylidyne formation step are used to describe the TPR peaks. The rate equations for the desorption process are solved as a function of time for the experimental heating rate and initial coverages. Results of Kinetic Analyses. The modified Horiuti-Polanyi mechanism describes quantitatively the results from steady-state kinetic studies and deuterium tracing measurements, while also incorporating information from surface science studies. The parameters used in the kinetic analyses are shown in Table I. A key experimental result from steady-state kinetic studies of ethylene hydrogenation was the apparent temperature dependence of the hydrogen reaction order. Figure 1 shows a comparison between the experimental and predicted steady-state kinetics. The kinetic analyses predict the experimentally observed change in hydrogen ord,er with temperature and the turnover frequency at different temperatures. The apparent activation energy of reaction (46) Nishiyama, Y.; Wise, H. J . Caral. 1974, 32, 50.
(47) Amiridis, M. D.; Rekoske, J. E.; Dumesic, J. A.; Rudd, D. F.; Spencer, N. D.; Pereira, C. J. AIChE J . 1991, 37, 87. (48) Dumesic, J. A.; Milligan, B. A.; Greppi, L. A.; Balse, V. R.; Sarnowski, K. T.; Beall, C. E.; Kataoka, T.; Rudd, D. F.; Trevino, A. A. Ind. Eng. Chem. Res. 1987, 26, 1399. (49) Goddard, S. A.; Amiridis, M. D.; Rekoske, J. E.; Cardona-Martinez, N.; Dumesic, J. A. J . Curul. 1989, 117, 155.
1
2
3
4
5
6
Number of Deuterium Atoms 60 1
II
I
50 40
30
20 10
0 0
1
2
3
4
5
6
Number of Deuterium Atoms
Figure 6. Comparison of experimental and predicted ethane isotopic distributions resulting from reaction of 150 Torr of deuterium with 25 Torr of ethylene at 248 (a) and 293 K (b).
is predicted to be 8.8 kcal/mol, which is in good agreement with the experimentally determined value of 8.6 kcal/mol. The ethylene reaction order was found experimentally to be negative at low ethylene partial pressures and approaches zero order at higher pressure. In addition, this reaction order becomes more negative as the reaction temperature increases. Table I1 compares the experimentally determined ethylene orders between 5 and 75 Torr and the predicted orders over the same pressure range; good agreement is observed between the trends of the experimental and predicted reaction orders. Figure 6 shows a comparison between the experimental and predicted ethane isotopic distributions at 150 Torr of hydrogen and 25 Torr of ethylene and temperatures of 248 (a) and 293 K (b). The predicted results capture the broadening of the ethane isotopic distribution with increasing temperature shown in the experiments. Compared to the experimental data, the kinetic model exhibits similar distributions with maxima at ethane-& The experimental errors in the ethane distribution were f4 mol 9% for ethane-do to ethane-d2 and fl mol % for ethane-d3 to ethanad,. The higher error associated with ethanado to ethanedz was due to the overlap of ethylene in the mass spectrometer spectra for these masses. Figure 7 shows predictions of the temperature programmed reaction spectra of Berlowitz et al.41for ethylene and hydrogen co-adsorbed on clean Pt( 111). The predicted TPR peaks show general agreement with the observed TPR spectra of Berlowitz et al. The rate constants for steps 3-5 that were used to describe the steady-state kinetic data and deuterium tracer studies are also used to describe the TPR peaks. The preexponential factor and activation energy for ethylene decomposition to form ethylidyne were 4 X 10" and 14 kcal/mol, as reported in the literat~re."~' (50) Mohsin, S . B.; Trenary, M.; Robota, H. J. J. Chem. Phys. Lett. 1989, 154. 511.
Rekoske et al.
1886 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 hydrogen
TABLE III: Predicted Elementary Reaction Rates (Turnover Frequencies) for Ethylene Hydrogenation over Platinum
a
0
temp/K ethylene pressure/Torr hydrogen pressure/Torr
c
U
Rl
R-I
R2
R-2
150
200
250 Temperature
R3 R-3 R4 R-4
350
300
RS
(K)
R-5 Rb R-5
R7
0.05
.......... -
hydrogen ethylene
R-7
b
/ht\
248 25 150 1.41 1.26 0.30 0.01 0.29 0.07 0.31 0.09 0.22 0.00 0.65 0.59 0.14 0.01
333 25 150 1.47 0.08 3.97 1.19 30.64 7.59 44.21 21.16 23.05 0.00 68.93 47.28 65.36 22.04
248 5 150 1.41 1.26 0.31 0.01 0.39 0.07 0.42 0.10 0.32 0.00 4.39 4.22 0.38 0.04
understanding the quantitative surface chemistry of ethylene hydrogenation over platinum. This table shows the relative contributions of the competitive and noncompetitive portions of the reaction mechanism, and it presents the reversibilities of each step. The overall turnover frequency for ethylene hydrogenation is equal to the rate of step 5, which is irreversible. In addition, the net rates (Le., forward minus reverse rate) of steps 3 and 4 are equal to the turnover frequency. Furthermore, the overall turnover frequency is equal to the sum of the net rates of steps 1 and 6, where these net rates represent the contributions to the overall reaction of the noncompetitive and competitive pathways, respectively. The net rates of steps 2 and 7 are twice as large as the net rates of steps 1 and 6, respectively. The ratio of the rate of an individual step i to the overall rate is equal to Ri/RS. A key finding from the experimental kinetic studies was the variation of the hydrogen reaction order with temperature. For simplicity,we focus discussion here on the noncompetitive pathway, which is dominant below 273 K. If the adsorption of hydrogen is equilibrated, then the observed rate would be half-order with respect to hydrogen; if this step is irreversible, then first-order
kinetics are expected. We observe in Table I11 that as the temperature increases from 248 to 333 K, the reversibility of the noncompetitive adsorption of hydrogen, R , / R5,decreases from 6.4 to 0.06. In addition, the competitive nature of the production of ethane also increases with temperature. At low temperature, the surface is nearly saturated with hydrocarbon species, thus allowing hydrogen to adsorb only on the noncompetitive adsorption sites. As the temperature increases, the surface coverage decreases and more competitive adsorption sites are available, increasing the rate of the competitive pathway, as shown in Figure 8. This figure shows as the temperature increases, the competitive nature of the production of ethane increases and the kinetic orders reflect those of a competitive mechanism. Both the decrease in reversibility of the noncompetitive adsorption of hydrogen and the increase in the rate due to the competitive pathway contribute to the predicted increase in hydrogen reaction order with increasing temperature. The ethylene order becomes more negative as the reaction temperature increases? In addition, the rate is nearly independent of ethylene pressure above 75 Torr. These results are explained in the kinetic analysis as a shift in the reaction pathways. As the pressure of ethylene decreases, the surface coverage by carbonaceous species decreases, and the competitive pathway also becomes more important. The competitive nature of the production of ethane increases from 0.27 to 0.53 as the ethylene pressure decreases from 25 to 5 Torr at 248 K. The agreement in Figure 6 between the experimental and predicted ethane distribution trends indicates that the essential chemistry of the significant steps is contained in the modified Horiuti-Polanyi mechanism. As the temperature increases from 248 to 333 K, the relative reversibility of the formation of the half-hydrogenated state, R4J RS,increases from 1.4 to 1.9, respectively. This reversibility increases the number of hydrogen or deuterium atoms that exchange with an ethylene molecule, which allows the ethane isotopic distributions to become broader. One important aspect of this work is the need to allow the rate constants for hydrogen desorption to be different for the temperature programmed desorption and the steady-state analyses. Seebauer et al. have compiled a listing of desorption processes for which the preexponential factor and the desorption energy vary with coverage.52 In our case, a higher rate constant was used for the steady-state kinetic analyses. Since the steady-state surface is known to be highly carbon covered, the increase in the rate constant may be attributed to the existence of interactions between adsorbed species. In fact, SalmerBn and Somorja?' found that D2 madsorbed with ethylene on Pt( 1 1 1) desorbs at substantially lower temperatures than on a clean Pt( 11 1) surface at the same hydrogen coverage. The effects of the adsorbed hydrocarbon species are also evident in the present study from the low sticking coefficient required for the noncompetitive adsorption of hydrogen.
(51) Pettiette-Hall, C. L.;Land, D. P.;McIver, R.T.; Hemminger, J. C. J . Phys. Chem. 1990, 94, 1948.
417.
c
U
OmO'l
ethane
Temperature (K) Comparison of predicted temperature programmed reaction (TPR)of coadsorbed ethylene and hydrogen on Pt(ll1) with results of Berlowitz et al.: (a) experimental; (b) predicted. Figure 7.
In these TPR analyses, a hydrogen desorption energy of 14.5 kcal/mol and a preexponential factor of lOI3 s-I were used, which are different from the values used in the analyses of the steadystate and deuterium tracing results. In general, we note that the rate constant for the hydrogen desorption step must be higher for the steady-state and deuterium tracing analyses compared to the TPR analyses. We must, therefore, conclude that the adsorption/desorption kinetics of hydrogen on a surface highly covered by carbonaceous species are different from those on a cleaner metal surface.
Discussion Table I11 shows the elementary reaction rates predicted by the kinetic analysis at 248 and 333 K. These rates are useful in
( 5 2 ) Seebauer, E. G.; Kong, A. C. F.; Schmidt, L. D. Surf. Sci. 1988, 193,
Ethylene Hydrogenation on Platinum
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1887
1 .o 0.9 0.8
e
0.7
m
5 w
0.6
c
0.4
I
0.3
.e LL
0.5
0.2 240
260
280
300
320
340
Temperature (K)
Figure 8. Contribution of competitive mechanism to the overall ethylene hydrogenation reaction rate with temperature. Reaction conditions: 25-Torrethylene pressure and 1SO-Torr hydrogen pressure.
It is noteworthy that kinetic parameters obtained a t UHV conditions can be used at pressures many orders of magnitude higher for kinetic analyses. It is expected that this usefulness occurs in cases in which the surface coverage regimes of the surface science measurements and the kinetic studies are similar. The inability of our kinetic analyses to use the measured parameters for hydrogen desorption is an illustration of this concept. Since the hydrogen adsorption/desorption studies have been camed out on surfaces containing little or no carbonaceous species, it is unreasonable to expect that these parameters would be valid for hydrocarbon-covered surfaces over which ethylene hydrogenation results were obtained. The number of S-sites used in the simulations was set at 0.3 times the number of competitive adsorption sites. This is an approximate number based on Monte Carlo simulations of the number of pair sites available for adsorption of dihydrogen on a R(111) surface saturated with hydrocarbon species. This number was found to have no effect on the steady-state kinetic analysis, provided that the preexponential factor for hydrogen adsorption was changed in such a manner as to compensate for the change in the number of S-sites. The essential surface chemistry must include two distinct pools of surface hydrogen that participate in exchange processes, and the S’-site incorporates this chemistry in the mechanism. Another way to explain the chemistry could be the slow incorporation of adsorbed hydrogen atoms into the carbonaceous surface layer, after which relatively faster hydrogen exchange takes place between adsorbed hydrocarbon species. The number of S’-sites was arbitrarily set at 0.5 times the number of competitive adsorption sites. While we have employed a modified Horiuti-Polanyi mechanism to describe the diverse experimental results, other possible modes of hydrogen exchange within the hydrocarbon layer were considered involving steps previously proposed in the l i t e r a t ~ r e . ~ J ~ For example, BondS employed a mechanism based on the mechanism suggested by Wilsons3 to explain his ethane deuterium distributions. This mechanism incorporated the following three steps: a step involving exchange of hydrogen atoms between two adsorbed ethyl species, a step involving an exchange of hydrogen atoms between adsorbed ethylene and ethyl surface species, and a step in which two adsorbed ethyl species react in a disproportionation reaction to give adsorbed ethylene and gaseous ethane. Kinetic analyses were performed on a mechanism that contained these three steps with a noncompetitive Horiuti-Polanyi mechanism. The distributions predicted from this seven-step mechanism contain amounts of ethane-do, -d2, and -d3that are near those observed experimentally; however, these predicted values do not agree with the experimental results as well as those values predicted from the elementary rates of the present model. Moreover, while the mechanism containing these three steps explain the surface chemistry rather well at temperatures below 273 K, the (53) Wilson, J. N.; Otvos, J. W.; Stevenson, D. P.; Wagner, C. D. J . Chem. Phys. 1952, 20, 1331.
competitive Horiuti-Polanyi mechanism would need to be added to explain the surface chemistry at higher temperatures. Therefore, a total of ten reaction steps would be needed to describe the surface chemistry as accurately as the present model does with only seven steps. It may appear that the modified Horiuti-Polanyi mechanism needed in the present study to explain the steady-state kinetics, deuterium tracing results, and temperature programmed studies is complicated, especially in view of the relatively simple chemistry expected for ethylene hydrogenation. While it is true that the mechanism contains seven steps, it must be stressed that the chemistry describes by these steps is simply that contained in the four-step Horiuti-Polanyi mechanism. The seven steps are needed because noncompetitive and competitive pathways are needed at different conditions. It would have been possible, for example, to have used fewer steps and a more complex site blocking/generation expression for hydrogen adsorption. Finally, we note that Langmuirian kinetics were used throughout this work, and this assumption may be at least partially responsible for the apparent complexity of the mechanism and the differences in rate constants used for studies at high pressures (steady-state kinetics and deuterium tracing) from those used for studies at vacuum conditions (temperature programmed desorption and reaction). The hydrogen adsorption/desorption steps showed the largest variation in kinetic parameters between high- and low-pressure conditions. It is possible that coverage-dependent kinetic parameters could have been used to consolidate these measurements at different surface coverages. We chose, instead, to define Langmuirian kinetic parameters characteristic of different coverage regimes. We reach the conclusion, however, that extrapolation of kinetic constants from surface science measurements at low pressures to catalytic studies at higher pressures should be carried out with caution by attempting to compare surfaces at comparable coverages.
Conclusions Kinetic analysis employing a reaction mechanism that captures the essential surface chemistry of the reaction allows quantitative interpretation of diverse experimental data. The present study probed the utility of this approach in describing steady-state kinetic data, isotopic tracing results, and temperature programming spectra related to ethylene hydrogenation over Pt. The kinetic analysis used a competitive/noncompetitive Horiuti-Polanyi mechanism modified by hydrogen “activation” steps previously proposed in the l i t e r a t ~ r e . ~ ~ The noncompetitive pathway is dominant at low temperatures, involving sites for hydrogen adsorption that are not blocked by carbonaceous species. At higher temperatures and lower ethylene pressures, more surface sites become available for hydrogen adsorption, and the reaction shifts to a more conventional competitive pathway. The steady-state kinetics provide information primarily regarding the relative rates of hydrogen adsorption and ethane formation. Deuterium tracing provides complementary information about these steps, as well as vital information about the rates of hydrogen exchange on the surface between adsorbed ethylene and ethyl species. The rate constants estimated for hydrogenation of surface ethylene and ethyl species were in agreement with those determined from temperature programmed reaction studies on single crystals at ultrahigh vacuum conditions. The primary difference between studies on high surface area catalysts at high pressures from those on single crystals at vacuum conditions involved the hydrogen adsorption/desorption processes. These differences may be related to different surface coverage regimes in these studies. In particular, the Pt surface is highly covered by carbonaceous species at high-pressure reaction conditions leading to non-Langmuirian site blocking (Le., noncompetitive adsorption between hydrogen and ethylene) and faster hydrogen desorption compared to clean Pt surfaces. Finally, we suggest that kinetic analysis of diverse experimental data represents a further step in the quantification of kinetic phenomena on catalytic surfaces, beyond the analysis of steadystate kinetic data alone. Moreover, we suggest that this approach
1888
J. Phys. Chem. 1992,96, 1888-1894
provides a quantitative means to compare, contrast, and consolidate results obtained on high surface area catalysts as well as on single crystal surfaces. Acknowledgment. We acknowledge financial support from the National Science Foundation. We thank Dr. R. J. Madon (En-
gelhard) for helpful discussions during the initial stages of this work, Professor W. E. Stewart for providing us with his general regression analysis software (GREG), and Rod Bain for providing us with his nonlinear equation solver. Registry No. Ethylene, 74-85-1; Pt, 7440-06-4.
Kinetic Study of the Initial Stages of Dehydrogenation of Cyclohexane on the Pt( 111) Surface Deborah Holmes Parker,+Claire L. Pettiette-Hall,$Yunzhi Li, Robert T. McIver, Jr.,* and John C. Hemminger* Institute for Surface and Interface Science and Department of Chemistry, University of California, Irvine. California 9271 7 (Received: September 30, 1991; In Final Form: December I O , 1991)
The initial dehydrogenation in the conversion of cyclohexane to benzene has been studied by laser-induced thermal desorption (LITD) combined with Fourier transform mass spectrometry (FTMS). Our previous work has shown that the initial dehydrogenation of cyclohexane occurs at 180 K while benzene is not formed at a substantial rate until -280 K. A stable surface intermediate exists in the temperature range 180-280 K. We have studied the kinetics of the dehydrogenation of cyclohexaneto form this intermediate in the temperature range 175-215 K. LITD combined with FTMS was used to measure the surface concentration of cyclohexane as a function of time. From the kinetic data we find that the initial dehydrogenation is first order in cyclohexane coverage for all cyclohexane starting coverages studied. The activation energy at low coverage (0.05 ML) is 9.5 f 0.5 kcal/mol and increases to 13.5 f 0.9 kcal/mol at -0.30 ML. Over this same coverage range the rate constant preexponential factors increase from 2.2 X lo9 s-' at 0.05 ML coverage to 7.4 X lo1*s-l at 0.30 ML coverage. We discuss the coverage-dependent rate in terms of a simple site-blocking model and pseudo-first-order kinetics, recognizing that the total hydrocarbon coverage on the surface does not change as the reaction proceeds. At higher coverages (0.50 ML), the desorption of cyclohexane contributes significantly to the observed rate. At these high coverages, the rate constant measured by LITD is the sum of the desorption rate constant and the reaction rate constant. The kinetic analysis at this coverage is complicated by the changing surface coverage due to desorption of cyclohexane. Our data also show that the reaction rate is not substantially inhibited by the surface hydrogen which is a product of the reaction.
-
I. Introduction The reactivity of hydrocarbons on metal surfaces is an important area of catalysis. The reactivity of cyclohexane and other c6 hydrocarbons on Pt surfaces in particular has been the topic of a number of worksl-20 due to the technological importance of cyclohexane dehydrogenation on platinum catalysts in relation to petroleum processing.21 The work presented here reports the kinetics of the initial dehydrogenation of cyclohexane on Pt( 111). This work is part of an ongoing study in our laboratory to understand the chemisorption and reactivity of c6 hydrocarbons on platinum surfaces. Early studies of the reactivity of C6hydrocarbons on a variety of platinum and other transition-metal catalysts have proposed several different cyclic intermediates involved in the hydrogen exchange and reactivity of these molecule^.^^-^^ In one of the first studies of hydrocarbon adsorption on a single-crystal Pt surface,' Gland et al. studied the adsorption and reactivity of cyclohexane, cyclohexene, and 1,3-cyclohexadiene on Pt( 11 1). Their results suggested that cyclohexene is an intermediate in the dehydrogenation reaction and the dehydrogenation of cyclohexene is the rate-limiting step in the reaction. Near-edge X-ray absorption fine-structure (NEXAFS) experiments11J2show the C< bond of cyclohexane lies parallel to the Pt surface and has a bond length of 1.51 i 0.03 A (compared to the gas-phase bond length of 1.535 A). Electron-stimulated desorption ion angular distribution (ESDIAD) experiments by Madey and Yates26also report cyclohexane bonded parallel to the Pt( 11 1) surface and conclude that three of the 12 C-H bonds were directed into the surface. Present address: Chemistry Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL 60439. 'Present address: TRW, Inc. Space and Technology Group, Redondo Beach, CA 90278. *Authors to whom correspondence should be addressed.
0022-3654/92/2096-1888$03.00/0
Demuth et a1.4 report a new C-H stretching frequency observed with HREELS (high-resolution electron energy loss spectroscopy) for adsorbed cyclohexane which is unusually low in frequency and broadened and attribute it to H-bonding-like interactions of the adsorbate with the metal. Hoffman and Upton'O propose a dif(1) Gland, J. L.; Baron, K.; Somorjai, G. A. J . Catal. 1975, 36, 305. (2) Blakely, D. W.; Somorjai, G.A. J. Catal. 1976, 42, 181. (3) Firment, L. E.; Somorjai, G. A. J . Chem. Phys. 1977, 66, 2901. (4) Demuth, J. E.; Ibach, H.; Lehwald, S. Phys. Rev. Lett. 1978,40, 1044. (5) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf Sci. 1978, 78, 577. (6) Smith, C. E.; Biberian, J. P.; Somorjai, G. A. J . Catal. 1979, 57, 426. (7) Davis, M.; Somorjai, G. A. Surf Sci. 1980, 91, 73. (8) Tsai, M.-C.; Muetterties, E. L. J . Am. Chem. SOC.1982, 104, 2534. (9) Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. J . Am. Chem. Soc. 1982, 104, 2539. (10) Hoffman, F. M.; Upton, T. H. J . Phys. Chem. 1984, 88, 6209. (1 1) Stbhr, J.; Sette, F.; Johnson, A. L. Phys. Rev. Lett. 1984, 53, 1044. (12) Hitchcock, A. P.; Newbury, D. C.; Ishii, I.; Stohr, J.; Redwing, R. D.; Johnson, A. L.; Sette, F. J . Chem. Phys. 1986,85, 4849. (13) Abon, M.; Bertolini, J. C.; Billy, J.; Massardier, J.; Tardy, B. Sur!. Sci. 1985, 162, 395. (14) Kang, D.B.; Anderson, A. B. J. Am. Chem. SOC.1985, 107. 7858. (15) Campbell, C. T.; Campbell, J. M.; Dalton, P. J.; Henn, F. C.; Rodriguez, J. A.; Seimanides, S. G.J . Phys. Chem. 1989, 93, 806. (16) Campbell, J. M.; Seimanides, S.; Campbell, C. T. J . Phys. Chem. 1989, 93, 815. (17) Rodriguez, J. A.; Campbell, C. T. J . Phys. Chem. 1989, 93, 826. (18) Campbell, C. T.; Rodgriguez, J. A.; Henn, F. C.; Campbell, J. M.; Dalton, P. J.; Seimanides, S. G. J . Chem. Phys. 1988, 83, 6585. (19) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500. (20) Chesters, M. A.; Gardner, P.Specrrochim. Acta 1990, 46A, 1011. (21) Davis, S. M.; Somorjai, G. A. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A,, Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984; Vol. 4. Chapter 7. (22) Maatman, R. W.; Mahaffy, P.; Hoekstra, P.; Addinck, C. J. J. Catal. 1971, 23, 105. (23) Smith, G. V.; Swoap, J. R. J . Org. Chem. 1966, 31, 3904. (24) Erkelens, J. J . Catal. 1967, 8, 212. (25) Seigel, S. Adu. Caral. 1966, 16, 124. (26) Madey, T. E.; Yates, Jr., J. T. Surf, Sci. 1978, 76, 397.
0 1992 American Chemical Society
