A DFT study of the effect of copper promotion upon iron oxide surface species

Cluster models for sites on the {1 1 1} 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 strengths 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 they decreased by 60-80 kJ mol^-1 when copper substituted in the surface of the catalyst. A minimum energy structure for molecularly adsorbed water was not identified because all attempts to do so resulted in dissociation.