Mechanisms of CO2 reduction into CO and formic acid on Fe (100): a DFT study

Understanding the mechanism of CO₂ reduction on iron is crucial for the design of more efficient and cheaper iron electrocatalyst for CO₂ conversion. In the present study, we have employed spin-polarized density functional theory calculations within the generalized gradient approximation (DFT-GGA) to elucidate the mechanism of CO₂ reduction into carbon monoxide and formic acid on the Fe (100) facet. We also sort to understand the transformations of the other isomers of adsorbed CO₂ on iron as earlier mechanistic studies are centred on the transformations of the C_2v geometry alone and not the other possible conformations i.e., flip-C_2v and Cs modes. Two alternative reduction routes were considered i.e., the direct CO₂ dissociation against the hydrogen-assisted CO₂ transformation through formate and carboxylate into CO and formic acid. Our results show that CO₂ in the C_2v mode is the precursor to the formation of both products i.e., CO and formic acid. Both the formation and transformation of CO₂ in the Cs and flip-C_2v is challenging kinetically and thermodynamically compared to the C_2v mode. The formic acid formation is favoured over CO via the reverse water gas shift reaction mechanism on Fe (100). Both formic acid formation and CO formation will proceed via the carboxylate intermediate since formate is a stable intermediate whose transformation into formic acid is challenging both kinetically and thermodynamically.