Ind. Eng. Chem. Res. 1989,28, 379-380 Kiperman, S. L. Usp. Khim. 1978a, 47, 3. Kiperman, S. L. Znt. Chem. Ens. 197813, 18, 59. Kiperman, S. L. Foundations of Chem. Kinetics in Heteros. Catalysis, Moscow, 1979. Kiperman, S. L. Kinetics & Catalysis. In Kinetic Problems in Heteros. Oxidation Catalysis; VINITI: Mowcow, 1979b; Vol. 6. Kiperman, S. L. Problems of Kinetics and Catalysis: Nauka: Moscow, 1981; Vol. 18, p 14. Kiperman, S. L. Kinet. Katal. 1982, 23, 1429. Kiperman, S. L. Commun. Dep. Chem. Buls. Acad. Sci. 1983a, 16, 22. Kiperman, S. L. VZZ Souiet-Japan Seminar on Catalysis, Novosibirsk, 1983b, p 203, Kiuerman. S. L. I Soviet-Indian Seminar on Catalvsis. Novosibirsk. i984, p‘217. Kiperman, S. L. in Catalysis Hydrosenation. Studies in Surface Science and Catalysis; Cerveny, L., Ed.; Elsevier: Amsterdam, 1986; Vol. 27, p 1. Kiperman, S. L.; Gadji-Kasumov, S. V. Izu. Acad. Nauk SSSR, Ser. Khim. 1965, 1110. Kiperman, S. L.; Gaidaj, N. A.; Nekrasov, N. V.; Kostyukovsky, M. M. Chem. Ens. Commun. 1982,18, 39. Kiperman, S.; Shopov, D.; Andreev, A.; Zlotina, N.; Gudkov, B. S. Commun. Dep. Chem. Buls. Acad. Sci. 1971,4, 237. Koltsov, N. I.; Kiperman, S. L. J. Res. Inst. Catal., Hokkaido Uniu. 1978, 26, 85. Krasavin, S. A,; Kharson, M. S.; Kostyukovsky, M. M.; Brasin, 0. V.; Kiperman, S. L. Izu. Acad. Nauk SSSR, Ser. Khim. 1982, 1231. Levin, D. Z.; Besprozvany, M. A.; Melamed, F. A.; Kiperman, S. L. Kinet. Katal. 1971, 12, 1455. Luu Kam Lok, T.; Gaidaj, N. A.; Gudkov, B. S.; Kiperman, S. L.; Kosan, S. B. Kinet. Katal. 1986,27, 1365. Luu Kam Lok, T.; Gaidaj, N. A.; Gudkov, B. S.; Kostyukovsky, M. M.; Kiperman, S. L.; Podkletnova, N. M.; Kosan, S. B.; Bursian, N. R. Kinet. Katal. 1986,27, 1371. Mamaladze, L. M.; Nekrasov, N. V.; Gudkov, B. S.; Kiperman, S. L. Acta Chim. Acad. Sci. Hung. 1977, 92, 73. Matveeva, T. M.; Nekrasov, N. V.; Kostyukovsky, M. M.; Navalikhina, M. D.; Krichko, A. A,; Kiperman, S. L. Izu. Acad. Nauk SSSR, Ser. Khim. 1982, 243; Proc. Intern. Symp. Heteros. Catalysis, Part 1, Varna, 1983, p 207. Medvedeva, 0. N.; Badrian, A. S.; Kiperman, S. L. Kinet. Katal. 1976, 17, 1530. Nekrasov, N. V.; Gudkov, B. S.; Kiperman, S. L. Zzu. Acad. Nauk SSSR, Ser. Khim. 1974, 1262, 2458. Palazov, A.; Andreev, A.; Shopov, D. Kinet. Katal. 1971, 12, 969. Petrov, L.; Shopov, D. C. R. Acad. Buls. Sci. 1969,2,289; Commun. Dep. Chem. Bulg. Acad. Sci. 1969,2, 313, 903. Posorelov, V. V.; Visdorovich, F. L.; Gelbstein, A. I. Kinetika-2. In 2nd Conference on Kinetics of Heteros. Catal. Reactions; Novosibirsk: New York, 1975, Vol. 2, p 88. ~I
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Pomeranzev, V. M. In Processes with Participation of Molecular Hydrogen; Kiperman, S. L., Ed.; Novosibirsk: New York, 1973; p 35. Sadovskii, A. S.; Gelbstein, A. I. Partial Oxidation of Hydrocarbons. Methodical a. Mathematic Problems of Kinetics; Novosibirsk: New York, 1973; p 8. Shakhnovich, G. V.; Belomestnych, I. P.; Nekrasov, N. V.; Kostyukovsky, M. M.; Kiperman, S. L. Appl. Catal. 1984,12, 83. Shapatina, E. N.; Kuchaev, V. L.; Pencovoi, B. L.; Temkin, M. I. Kinet. Katal. 1976, 17,644; 1977, 18, 968; 1979,20, 1183. Shcheslova, G. G.; Feofanova, N. M.; Sharapova, E. D.; Bakshi, Yu. M.; Gelbstein, A. I. Kinetika-2. In 2nd Conference on Kinetics of Catal. Reacts; Slinko, M. G., Ed.; Novosibirsk New York, 1975; Vol. 1, p 85. Slouka, P.; Beranek, L. Collect. Czech. Chem. Commun. 1979, 44, 1591. Snagovsky, Yu. S.; Avetisov, A. K. Dokl. Acad. Nauk SSSR 1971, 196, 878. Snagovsky, Yu. S.; Avetisov, A. K. Kinet. Katal. 1972, 13, 1070. Talaeva, I. G.; Vasilevich, L. A.; Avetisov, A. K.; Chesnokov, B. B.; Gelbstein, A. I. Kinetika-3. In 3rd Conference on Kinetics of Heteros. Catal. React.; Slinko, M. G.; Ed.; Kalinin: New York, 1980; Vol. 2, p 441. Temkin, M. I. Adu. Catal. 1979, 28, 173. Temkin, M. I.; Pyzhev, V. M. Zh. Fiz. Khim. 1939, 13, 851. Temkin, M. I.; Morozov, N. M.; Shapatina, E. N. Kinet. Katal. 1963, 4, 260, 555. Vartanov, I. A.; Kharson, M. S.; Kostyukovsky, M. M.; Kipovich, V. G.; Kiperman, S. L. Kinet. Katal. 1984, 25, 142. Veirosta, J.; Klenha, V.; Beranek, L. Collect. Czech. Chem. Commun. 1972,32, 1097. Vigdorovich, F. L.; Kapkov, Yu. K.; Pogorelov, V. V.; Gorelik, A. G.; Babkova, P. B.; Gelbstein, A. I. Kinetika-3. In 3rd Conference on Kinetics of Heteros. Catal. Reactions; Slinko, M. G., Ed.; Kalinin: New York, 1980; Vol. 1, p 68. Voikina, N. V.; Avetisov, A. K.; Bosdanova, 0. K.; Kiperman, S. L. Kinet. Katal. 1975, 16, 1524; 1977, 18, 518. Yuskovets, Zh. G.; Nekrasov, N. V.; Kharson, M. S.; Kostyukovsky, M. M.; Shimanskaya, M. V.; Kiperman, S. L. Kinet. Katal. 1983, 24, 1524; 1984, 25, 1361. Zlotina, N. E.; Kiperman, S. L. Kinet. Katal. 1967, 8, 393, 1335. Savelii
L. K i p e r m a n *
N . D. Zelinsky Institute of Organic Chemistry USSR Academy of Science Leninsky Prospect 47, Moscow, USSR Krasimira
E. Kumbilieva, L u c h e z a r A. P e t r o v Institute of Kinetics a n d Catalysis Bulgarian Academy of Science Sofia 1040, Bulgaria
Response to “Classical Catalytic Kinetics: What Is the Point of the Matter?” Sir: The fact that surface nonuniformity has been ignored for almost 50 years in catalytic kinetics by physical chemists, chemical engineers, and, more recently, surface scientists, is puzzling. I have tried to explain this attitude by first noting that rate expressions obtained from uniform or nonuniform surface kinetics are frequently similar (Boudart, 1956,1972), by then pointing out that they may be i d e n t i c a l in the case of structure-insensitive reactions run under conditions of high surface coverage 8 (Boudart, 1985,1986),and finally by trying to explain why acceptable estimates of rates of catalytic reactions run under conditions of high 8 can be made from rate parameters determined at low 8 (Boudart, 1988). I wish to elaborate here on this last point. It follows from the theory of nonuniform surface kinetics initiated and developed by the Soviet schools of Temkin and Kiperman, as presented in my textbook (Boudart, 1968) and also in more detail in our recent monograph (Boudart and Dj6ga-Mariadassou, 1984). In this theory, the thermodynamic nonuniformity of the surface is described by a 0888-5885/89/2628-0379$01.50/0
distribution function. One of them, and the simplest, is such that the standard Gibbs free energy of adsorption divided by RT and taken with the minus sign, t, falls linearly with converage 8 except a t values of 8 near zero or unity, with values to a t 8 = 0 and tl a t 8 = 1. The breadth of nonuniformity is to - tl = f. A typical value of f is 10. It corresponds to 15 kcal mol-’ at 750 K. Next, the theory relates t to the standard free energy for adsorption by a Brernsted relation between rate constant k and equilibrium constant K , such that k = constant X K“. Empirically, a common value of a is This value will be taken in what follows. Next, it is assumed that the reaction can be described in two steps, an adsorption step and a desorption step with the same value of a. This assumption is not as limiting as it seems since a multistep reaction can frequently be represented as a two-step reaction with simplifications related to the existence of a rate-determining step and a most abundant reactive surface intermediate (Boudart, 1972). Finally, the rate is integrated between t = to where 0 is assumed to be 0 and 0 1989 American Chemical Society
380 Ind. Eng. Chem. Res., Vol. 28. No. 3, 1989
t = t , with 0 put equal to unity. The result gives the turnover rate per site, averaged over all sites, namely ( u t ) . It is revealing to compare ( u t ) to values of ut, the turnover rate for each value of t , obtained with the same assumptions as used above. When ut is plotted versus t , a symmetric bell-shaped curve is obtained with two equal smallest values, a t the two ends of the curve for t = t o and t = tl. The maximum of the bell-shaped curve for t = f / 2 is corresponding to 0 = lI2. The first ratio of importance is (Ut)/Ut,min = ( ~ / f ) e ” ~ . The second is ut,-/ ( ut) = f / ( 2 x ) (Kiperman, 1964). With f = 10, these ratios are 3.82 and 1.59, respectively. These simple general results from the Temkin-Kiperman theory are much more important, in my opinion, than the ability of the theory to provide a rate equation that fits kinetic data better than one based on the assumption of a uniform surface. Indeed, with the advent of personal computers, kinetics-assisted design of catalysts becomes a possibility. From assumed, estimated, or experimental values of rate constants of elementary steps, a yield can be computed under given process conditions in a given reactor (Dumesic et al., 1987). By successive iteration, molecularly engineered catalysts can in principle be optimized as the experimentalist pursues an experimental program with the help of the computer. Such a program requires experience and intuition. It is hard enough t o start with a set of rate constants of elementary steps. It it were also necessary to have a set of rate constants for each elementary step, corresponding to what is expected on a nonuniform surface, the Dumesic strategy would seem doomed to failure. But the above results from the Temkin-Kiperman theory give strong reasons for optimism. Indeed, suppose that a set of rate constants were chosen for sites corresponding to t = to (0 = 0 ) . The calculated rate, namely would be 3.82 times smaller than that corresponding to the nonuniform surface, ( u t ) . This would not be unacceptably bad, considering all the pitfalls of the choice of rate constants. If the calculation had been done with rate constants corresponding to half coverage, the calculated value, would have been only -60% higher than the one corre-
sponding to the nQnuniform surface, (ut). The error would have been negligible. Thus, were it not for the insight provided by the Temkin-Kiperman theory, the strategy of Dumesic would be open to the criticism of all those who militate against the use of uniform surface kinetics. Cautious optimism in the use of “classical” surface kinetics has been a t the heart of my comments over the years, concerning the ongoing debate between the pragmatic users of uniform surfaces and the rigorist champions of nonuniform surfaces. Further theoretical work from the latter would be most useful. A recent example is contained in a constructive paper from the Weller group (Corma et al., 1988), further developing the Temkin-Kiperman approach. In the meantime, the search for cases where the uniform surface approach is valid and the reasons for this validity will continue to be expIored, as experimental catalytic kinetics on well-defined surface continues to accumulate a data bank of rate constants for elementary steps at all values of surface coverage. Literature Cited Boudart, M. AIChE J. 1956,2,62. Boudart, M. Kinetics of Chemical Processes; Prentice-Hall: Englewood Cliffs, NJ, 1968. Boudart, M. AIChE J. 1972,18, 405. Boudart, M. J. Mol. Catal. 1985, 30, 27. Boudart, M. Ind. Eng. Chem. Fundam. 1986,25, 656. Boudart, M. Catal. Lett. 1988, 1 , 21. Boudart, M.; DjBga-Mariadassou, G. Kinetics of Heterogeneous Catalytic Reactions: Princeton University Press: Princeton, NJ, 1984. Corma, A.; Llopis, F.; Monton, J. B.; Weller, S. W. Chem. Eng. Sci. 1988, 43, 785. 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. Kiperman, S. L. Introduction to the Kinetics of Heterogeneous Catalytic Reactions; Nauka: Moscow, 1964.
M. Boudart D e p a r t m e n t o f Chemical Engineering S t a n f o r d University S t a n f o r d , California 94305
