Reaction Mechanism of the Reverse Water-Gas Shift Reaction Using First-Row Middle Transition Metal Catalysts L′M (M = Fe, Mn, Co): A Computational Study

The mechanism of the reverse water gas shift reaction (CO2 + H-2 -> CO + H2O) was investigated using the 3d transition metal complexes L'M (M = Fe, Mn, and Co, L' = parent beta-diketiminate). The thermodynamics and reaction barriers of the elementary reaction pathways were studied with the B3LYP density functional and two different basis sets: 6-311 +G(d) and aug-cc-pVTZ. Plausible reactants, intermediates, transition states, and products were modeled, with different conformers and multiplicities for each identified. Different reaction pathways and side reactions were also considered. Reaction Gibbs free energies and activation energies for all steps were determined for each transition metal. Calculations indicate that the most desirable mechanism involves mostly monometallic complexes. Among the three catalysts modeled, the Mn complex shows the most favorable catalytic properties. Considering the individual reaction barriers, the Fe complex shows the lowest barrier for activation of CO2.