(337d) A Computational Study of Fe3o4 (100) Surface Species Related to Water-Gas Shift

The mechanism of the water-gas shift reaction over ferrochrome catalysts is still unresolved. Surface reduction/oxidation pathways, surface carbonate pathways, surface bicarbonate pathways and surface formate pathways have all been suggested or advocated to describe the kinetics of the reaction. In the present investigation, a small cluster is cut from the bulk structure of the catalyst so as to represent the (100) surface. Density functional theory is used to examine the structure of a wide range of potential water-gas surface intermediates on this surface, along with the energetics associated with their formation. Intermediates that have been considered here include oxide, hydroxide, carbonates, bicarbonates and formats as well as the molecularly adsorbed forms of each of the reagents. Both neutral adsorption as well as oxidative/reductive adsorption are considered. The most favorable reactions pathways are then identified, and the calculated energies are compared to experimental kinetic data via mechanistic kinetic modeling. The effects of substitution of copper cations and silicon cations as promoters are studied in an analogous fashion. The consideration of different substitutional promoters is used to identify coupling between energetic quantities that are relevant to catalytic performance. By identifying such coupling, model guided catalyst development can be performed in a much more realistic manner.