Kinetic compensation effects observed during oxidation of carbon monoxide on γ-alumina supported palladium, platinum, and rhodium metal catalysts:: Toward a mechanistic explanation

The kinetic compensation effect (KCE) is a well-known behavior pattern wherein a set of related reactions show a linear relationship between the calculated Arrhenius parameters, log(10)A and E-a. Although various theoretical explanations have been advanced, none has yet found general acceptance. The present paper reports multiple rate measurements for the heterogeneously catalyzed oxidation of CO on several identically prepared samples of supported noble metal (Pd, Pt, or Rh) catalysts. Distinctive KCEs were observed for all three metals; these are discussed with reference to a new parameter, the degree of compensation (kappa), which quantitatively measures deviation from the ideal isokinetic relationship (kappa = 1.00 KCE). For carbon monoxide oxidation on each of the three metals, kappa was greater than 0.85 KCE, regarded as significant compensation. These KCEs are discussed in the context of published kinetic and mechanistic studies of CO oxidation, from which a theoretical explanation of the observed pattern of rate characteristics is proposed. overall kinetic control is ascribed to a common dominant, rate-determining process on each metal, resulting in approximately isokinetic behavior. Temperature-dependent secondary controls are identified as modifying the kinetics of surface reactions involving adsorbed intermediates, and thus the apparent magnitudes of Arrhenius parameters. Different contributions from secondary controls, between the individual reactions of each set, are attributed to temperature-determined variations of concentrations, equilibria, and mobilities of adsorbed participants in the (dominant) rate-controlling process. A modified Arrhenius equation, applicable to KCE reaction sets, is suggested in which the preexponential term varies with temperature. (c) 2006 Wiley Periodicals, Inc.