Kinetics of alkane reactions on metal catalysts: activation energies and the compensation effect

Compensation phenomena in heterogeneous catalysis take the form of a linear correlation between activation energy E and pre-exponential factor In A; its occurrence may, however, be due to the use of 'apparent' rather than 'true' Arrhenius parameters. It is shown by model calculations using the Langmuir-Hinshelwood bimolecular rate equation, and experimentally using Pt-catalysed hydrogenolysis of C3H8 and of n-C4H10, that E-app increases with P-H, as expected. These values of E-app show compensation. Apparent Arrhenius parameters for many different Pt and Pd catalysts and for various reactions lie close to a common compensation line. Alkane activation requires dehydrogenative chemisorption, which is endothermic; this causes E-app to exceed E-true, and accounts for the dependence of E-app on alkane chain length, and the very high values sometimes observed. Compensation occurs as a result of using constant reactant pressures, which give inconstant surface concentrations. (C) 1999 Elsevier Science B.V. All rights reserved.