CO2 Reduction on Transition Metal (Fe, Co, Ni, and Cu) Surfaces: In Comparison with Homogeneous Catalysis

Reduction of CO2 to CO on Fe, Co, Ni, and Cu surfaces has been studied using density functional theory (DFT) methods. Three reaction steps were studied: (a) adsorption of CO2 (M + CO2 = CO2/M) (M = transition metal surface), (b) decomposition of CO2 (CO2/M = (CO + O)/M), and (c) desorption of CO ((CO + O)/M = O/M + CO). Binding energies and reaction energies were calculated using the generalized gradient approximation (GGA) via the Perdew-Burke-Emzerhof (PBE) functional. Calculations show an interesting trend for reaction energies and total reaction barriers, as a function of metal: from Fe to Cu, reactions tend to be less exergonic; the metals earlier in the 3d series have lower total barriers for CO2 reduction. However, "overbinding" of CO2 on Fe causes a thermodynamic sink on the reaction coordinate, and Co and Ni are more favorable in terms of a smaller fluctuation in reaction energies/barriers for these elementary catalytic steps. A Bronsted-Evans-Polanyi (BEP) relationship was analyzed for C-O bond scission of CO2 on the metal surfaces. Heterogeneous catalysis is also compared with the homogeneous models using transition metal beta-diketiminato complexes, showing that both heterogeneous and homogeneous catalysis of CO2 reduction display the same energetic trend as a function of metal.