(296b) Role of Copper as a High Temperature Shift Promoter

Cluster models for sites on the {111} surface of Fe₃O₄ were used to study the strength of bonding of water-gas shift intermediates using density functional theory. Three site models were used, representing an unpromoted catalyst, a catalyst where copper cations substitute for iron cations below the surface and a catalyst where copper cations substitute in the surface. The strength of bonding of oxygen, carbon dioxide, dissociated water and dissociated formic acid were all observed to decrease by less than 20 kJ mol^-1 when copper substituted below the surface, but it decreased by 60 to 80 kJ mol^-1 when copper substituted in the surface of the catalyst. A minimum energy structure for molecularly adsorbed water was not identified; attempts to do so resulted in dissociation. Mechanistic models for high temperature shift were fit to data for an unpromoted catalyst and a copper-promoted catalyst. The same mechanistic model, with only small variations in the energetics was found to describe the kinetic of shift over both catalysts. Together the results suggest that copper substitutes within the iron oxide causing changes in the electronic structure of the oxide that affect shift kinetics.