Catalytic Mechanism of CO2 Reduction by Keggin-Type Transition-Metal-Substituted Polyoxometalates

The mechanisms of carbon dioxide reduction catalyzed by Keggin-type transition-metal-substituted polyoxometalates (TMSPOMs) have been studied by density functional theory calculations. CO2 is sufficiently activated via side-on adsorption in which the one oxygen and carbon atom in CO2 is bonded with substituted transition metals and oxygen adjacent to the transition metals, respectively. For studied TMSPOMs, [α-XM11O39Co(_)] (XM11Z, X = Si, P; M = W, Mo; Z = Mn, Fe, Co, Ni, Cu, and Zn), PMo11Co exhibits the best catalytic activity that catalyzes CO2 reduction to CH3OH, with a ΔGmax of 0.15 eV. The difference on catalytic activity among the four XM11Co (X = Si, P; M = W, Mo) catalysts originates from different d-band centers of the Co atom, which is affected by the central heteroatoms and peripheral atoms. The integrated crystal orbital Hamilton population (ICOHP) analysis reveals that the central heteroatoms (Si vs P) and peripheral atoms (W vs Mo) can tune C-O bond strength in prepotential-determining step (PDS) intermediates. The substituted transition metals can modulate the interaction between POMs and CO2, thereby affecting the catalytic activity of the POMs. During the CO2RR, POMs act as "electron sponges", accepting and donating electrons. It is expected that this work can provide important information for the development of electrochemical CO2RR catalysts featuring both high selectivity and high efficiency.