The 14th International Congress on Catalysis
Are there grounds to contend that surfaces are heterogeneous in the course of stationary catalytic processes? Victor E. Ostrovskii Karpov Institute of Physical Chemistry, Vorontsovo Pole 10, Moscow, 105064 Russia Fax number: 007 (495) 917-2490, E-mail:
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Abstract: Heat effects and kinetics of chemisorption, including those measured at catalysts approached to their states occurring during catalytic processes, show that surfaces are homogeneous; the logarithmic isotherm for adsorption equilibrium and Zeldovich-Roginskii exponential equation for chemisorption rates, observance of which is considered as the indication of surface heterogeneity, are analytically explained by multi-centered chemisorption at homogeneous surfaces, and the catalytic kinetic equations (for NH3, SO3, (CO2+H2), etc syntheses) obtained earlier under the assumption of surface heterogeneity on the basis of the logarithmic isotherm are deduced from the notion on surface homogeneity. Thus, the hypothetical notions on surface heterogeneity are groundless.
Keywords: Catalyst-surface uniformity, Chemisorption heats, Chemisorption multi-centered. 1. Introduction The hypothetical notions on surface heterogeneity (at first, on biographic and, then, on induced one) of catalysts under stationary processes and of chemisorbents under chemisorption (CS) equilibriums were developed about 70 years ago to explain the empiric logarithmic isotherm proposed by Frumkin and Shlygin for description of H2-chemisorption at Pt-electrodes and to explain the NH3-synthesis and NH3-decomposition kinetics obtained by Benton and by Winter, respectively. The measurements of the heats of CS were, at that time, infrequent and, as a rule, imperfect (although the heats of CS of N2 at iron catalysts were found to be coverage-independent1), and application of the concept of surface heterogeneity (SH) bore only on the assertion2 that the observed kinetics of NH3 synthesis and the logarithmic isotherm can not be explained on the basis of Langmuir’s model of CS. The concept of SH was criticized in different years and from different points of view by De-Boer, Corma, Culver, Dumesic, Guandry, Fedianin, Keroglu, Tompkins, Weller, by us, etc (refs. see in ref. 3). At present, almost nobody uses the notions on SH for deductions of kinetic equations (KEqs); however, these notions are presented in different monographs and are widely applied for various speculations. The resolution of the issue whether or not the surfaces are homogeneous under conditions of stationary catalysis and CS equilibriums is of great importance not only for the theories of catalysis and CS and their practical applications but also for the solid-state theory. Four criteria are usually applied to solve the problem whether the surfaces are homogeneous or heterogeneous relative to their catalytic and CS activity: the forms of coverage dependences for the CS heats, of KEqs for catalytic processes, of CS isotherms, and of CS-rate equations; therewith, the coverage dependence of the molar heats of CS measured at the catalysts approached to their states occurring in the course of catalytic processes are especially informative. 2. Experimental and Theoretical The measurements of the heats of CS are performed on the basis of original microcalorimeters3 and adsorption techniques and procedures4 and are supplemented with available heats of CS; the theoretic considerations5 are based on the Bethe approximation and on analyses of available kinetic data.
The 14th International Congress on Catalysis
3. Results and discussion We consider our data on the CS heats of different gases at Cu, Ag, Au, Co, Fe/K2O/Al2O3, and ZnO/Cu4(OH2)/Al2O3 catalysts. At all systems, the molar heats of CS are coverage-independent or almost coverage-independent over a wide range of coverages. The data obtained at two last catalysts are especially important because they represent complex catalytic systems and because, before the measurements, they were approached as close as possible to the states in which they catalyze the NH3 and CH3OH syntheses, respectively. At Fe/K2O/Al2O3, the molar heats of CS of N2 and of CS of H2 by the surface containing adsorbed nitrogen and, at ZnO/Cu4(OH2)/Al2O3, the molar heats of CS of O2, H2, and CO2 are measured. For tens of different systems, CS heats coverage-independent over a vide range of coverages are obtained by Artamonov, Bagg, Boeva, Bondareva, Brennan, Bröcker, Buzek, Černi, Cortright, Czanderna, Dell, Dowben, Dumesic, Dus, Goughlin, Grunze, Guil, Hayes, Hayward, Jones, Lisowski, Malyshev, Masia, Menayo, Palfi, Pankratiev, Pientka, Randzio, Smutek, Spiewak, Stone, Tiley, Tompkins, Trapnell, Turkov, Wedler, Zakumbaeva, Zommer, etc. By no means, all other published heats of CS are also coverage-independent. The matter is that the CS measurements are often influenced by side processes of chemical and surface–bodydiffusion nature. Preclusion or consideration of these processes does not always work. However, the most reliable works lead to negation of the notions on either biographic or induced heterogeneity of surfaces. We show that KEqs deduced from the notions on SH can not be considered as the proof of actual SH by two causes. First, it is known that any set of kinetic data can be described by different KEqs and the same KEqs can often be obtained from different models (new examples will be given). Second, the KEqs obtained earlier from the model of SH on the basis of the logarithmic isotherm (for syntheses of NH3, SO3, (CO2+H2), etc) can be also deduced on the basis of notions on surface homogeneity. The assertion2 that the NH3-synthesis kinetics can not be explained on the basis of Langmuir’s model of CS is incorrect and, principally, the applicability of KEqs deduced from the model of SH does not prove that the surface is heterogeneous. It is commonly accepted that applicability of the logarithmic isotherm and the Zeldovich–Roginskii equation (the Z–R Eq) for description of CS equilibriums and rates, respectively, indicates the SH. By using the Bethe’s approximation, we deduced5 equations for equilibriums and rates of CS when each molecule occupies two and more surface centers at different crystal faces; it was stated that the θ–P and r–θ dependences translated by these equations and by the logarithmic isotherm and the Z–R Eq, respectively, are almost the same. Thus, the applicability of these equations doesn’t prove that surfaces are heterogeneous. The specific properties of very small crystals and individual crystal faces are taken into consideration.
4. Conclusion The analysis based on the four criteria shows that the notions on surface heterogeneity are groundless.
References 1. 2. 3. 4. 5.
P.H. Emmett, S. Brunauer, J. Am. Chem. Soc., 56 (1934) 35. M. Temkin, V. Pyzhev, Russ. J. Phys. Chem., 13 (1939) 851. V.E. Ostrovskii, Rev. Sci. Instr., 73 (2002) 1304. V.E. Ostrovskii, J. Natur. Gas Chem., 13 (2004) 123. V.E. Ostrovskii, Ind. Eng. Chem. Res., 43 (2004) 3113.
