(52g) Phase-Dependent Promoting Effect of Surface Oxygen on Molybdenum Carbide Catalysts during Formic Acid Electrooxidation

Transition metal carbides (TMC) are abundant and promising low-cost electrocatalyst as they exhibit Pt-like catalytic activity. When combined with metal catalysts, TMCs demonstrate excellent performance towards C-H and O-H bond breaking in an oxidizing environment. Their paradoxically high activity, however, contrasts with their highly oxophilic nature, which should cause rapid deactivation by strongly adsorbed O* atoms. A key requirement for resolving the TMC catalysis puzzle and enabling their use in electrochemical operations is to understand the effect of surface oxidation on their catalytic behavior and material stability.

Here, we elucidate the surface structure and catalytic activity of α-MoC and β-Mo₂C phases under electrochemical conditions using density functional theory and kinetics-informed ab initio thermodynamics. Both the surfaces were found to be extensively oxidized with the α-MoC phase showing greater stability, consistent with experimental CV predictions. Our oxidation model accurately predicts the extent of charge transfer associated with surface oxidation and also provides a mechanistic explanation for the surface species observed in XPS.

The effect of the resulting surface oxidation was found to be detrimental towards HER [1] on the β-Mo₂C(011)-C since all metal sites O*/OH* covered. In contrast, partial surface oxidation on the α-MoC-(311)-Mo has a promoting effect towards the formic acid electrooxidation [2], as the surface oxygen weakens otherwise strongly bound intermediates. The α-MoC exposes H₂O-protected [MoC₂O₂] and [MoC(CO)O₂] oxycarbidic motifs available for catalysis in a wide potential window. It shows Pt-like O*-mediated O-H and C-H bond activation pathway. The α-MoC hosts active sites such as O*/Mo_top which are, however, blocked by H₂O and HCOOH yielding no electrooxidation current. Pt doping was investigated computationally making the surface less oxophilic, suggesting possible design strategy towards active and selective carbide electrocatalyst.

[1] Yu et al. J. Mater. Chem. A, 2024, doi: 10.1039/D4TA01746C

[2] Gautam et al. ACS Catalysis, 2025, accepted doi: 10.1021/acscatal.4c06544