DFT models for active sites on high temperature water-gas shift catalysts

A simple two-step regenerative model for the kinetics of high temperature water-gas shift provided a good fit to published experimental data for a commercial ferrochrome catalyst. The fitting process yielded an enthalpy of localization of oxygen adatoms on the surface equal to -611 kJ mol^-1, and it predicted that virtually all adsorption sites were covered by oxygen adatoms at reaction conditions. Cluster models were created to represent possible active sites on {1 0 0}, {1 1 0} and {1 1 1} surfaces of Fe₃O₄, the active state of the catalyst. Energies of localization of oxygen adatoms on exposed cation sites were calculated using density functional theory. The computed energies were found to vary in proportion to the number of oxygen anions missing from the normal octahedral coordination of the cation adsorption sites. Comparing the results from the kinetic modeling to the computed energies of localization suggests that on average, the active site on the working catalyst is coordinated to 3.2 oxygen anions, not counting the oxygen adatom.