Kinetics of metal-catalyzed reactions of alkanes and the compensation effect

For a bimolecular reaction A + B --> C obeying simple Langmuir-Hinshelwood kinetics, the apparent activation energy E(app) is a function of the gas-phase concentrations of each reactant; where A is the more strongly chemisorbed, the limiting values of E(app) at low and high pressures of A are given respectively by (E(t) + n(A) Delta H-A(empty set) + n(B) Delta H-B(empty set)) and (E(t) - n(A) Delta H-A(empty set) + n(B) Delta H-B(empty set)), where E(t) is the true activation energy, n(A) and n(B) are the magnitudes of the orders of reaction (each having a value of one for these limiting cases), and Delta H-A(empty set) and Delta H-B(empty set) are the enthalpies of adsorption of the two reactants. E(t) is usually greater than E(app), and values of E(app) and the corresponding values of In A(app) show a compensation effect. With alkane hydrogenolysis, the opening step is the endothermic dehydrogenative chemisorption of the alkane; this accounts for the very high values of E(app) that are often observed. In the case of Ru/Al2O3 catalysts, analysis of the rate dependence on H-2 pressure by an expression based on the subsequent rate-limiting C-C bond breaking affords values of E(t) that are generally approximate to 60 kJ mol(-1) and values of Delta H-C(empty set) (C = alkane) in the range 60-80 kJ mol(-1). At Hz pressures greater than that at which the rate is maximal, E(app) exceeds E(t); below the maximum, the reverse is the case. Compensation between E(app) and In A(app) is again found; other literature reports confirm the general validity of the model. Our analysis supports a recent speculation which reported that compensation effects originate in differences in adsorption enthalpy terms, but the sign of the term for the alkane will be positive where its dehydrogenation initiates the reaction. Such compensation effects are not however true kinetic phenomena and are better described by the term apparent. (C) 1996 Academic Press, Inc.