Kinetic study of reduction reaction for supported PdO species by means of dispersive XAFS method

Abstract

Time-resolved dispersive XAFS experiments have been performed to study the reduction reaction of supported PdO species on γ-Al2O 3. It has been clarified that the reduction of PdO by H2 proceeds with an unique kinetic behavior, i.e., the initial part of the reduction is expressed by the first-order kinetics on Pd and the reaction order is changed to be zero as the progress of the reduction. The observation is different from that for the other supported metal oxides, for which the reduction is first order through the reaction. The reduction process of PdO is divided into the surface reduction to form the surface Pd(0) species and the succeeding oxygen exchange between inner PdO and surface Pd(0). When the latter step is faster than the former, the reduction rate of PdO is explained in terms of the zero-order kinetics. The pressure dependences of the conditional first- and zero-order rate constants suggest that the Langmuir-type molecular adsorption of H2 precedes the surface reduction of PdO particles.