DENSITY-FUNCTIONAL STUDY ON THE ACTIVATION OF METHANE OVER PD-2, PDO,AND PD2O CLUSTERS

Simple functional models for elementary steps in the total oxidation o f methane over supported palladium catalysts were investigated using d ensity functional theory. Three simple cluster models were proposed, n amely, the palladium dimer and PdO diatomic and linear Pd2O, to probe the mechanism of the methane activation on metallic and oxidized palla dium phases, respectively. The strongest adsorption was found on Pd-2, where also the C-H bond became easily activated; however, no stable p roduct of the C -H bond scission was indicated. Similar hydrogen activ ation took place on Pd2O and, in addition, adsorbed methyl and OH spec ies formed the most stable system after crossing a moderate energy bar rier. The same product was previously found stable also in the case of PdO dimer but the activation barrier was high. On the Pd2O cluster, t he process of energy barrier crossing was accomplished in two steps: e asy formation of a free hydrogen moiety and actual oxidation, which ma de the overall process less demanding energetically. (C) 1997 John Wil ey & Sons, Inc.