Simultaneous electronic and ionic surface conduction of catalyst supports: A general mechanism for spillover: The role of water in the Pd-catalyzed hydrogenation of a carbon surface

Abstract The experimental evidence on hydrogen spillover from catalyst to support is briefly reviewed. A double-layer charging mechanism for spillover in the solid-gas system is suggested by a gradual experimental shift from an ordinary electrode-liquid electrolyte system to a solid-gas one. The mechanism is essentially dissociation of H 2 on the catalyst to H + + e. Both species migrate onto the support surface by means of protonic and electronic conduction which must be exhibited by the support. The protonic conduction is the property of a surface electrolyte composed of surface OH and adsorbed hydrogen bonding molecules. The driving force for the observed fast migration is the electrochemical potential gradient. The observed long-range spillover, together with low-coverage saturation effects, are explained by the proposed mechanism. The main rate-determining steps for spillover are shown to be (a) H 2 dissociation to H + + e on the catalyst surface; (b) transfer of electronic charge through the contact between catalyst and support particles; (c, d) electronic and protonic conductance.