(569cu) 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. The high TMC activity, however, contrasts with their highly oxophillic nature, which should cause rapid deactivation by strongly adsorbed O* atoms. Understanding the effect of surface oxidation on catalytic behavior and material stability is key to resolving this TMC catalysis puzzle and enabling TMC usage in electrochemical operations.

Here, we elucidate the surface structure and catalytic activity of β-Mo₂C and α-MoC phases under electrochemical conditions using density functional theory and kinetics-informed ab initio thermodynamics calculations. The β-Mo₂C(011)-C surface was investigated for hydrogen evolution reaction (HER) and α-MoC(311)-Mo for formic acid electrooxidation (FAEO). Kinetics-informed Pourbaix diagrams reveal that both surfaces are extensively oxidized with the α-MoC phase showing greater stability with respect to surface C removal through CO/CO₂ release (onset potential 0.72 V_­) over β-Mo₂C (onset potential 0.46 V)_­.

Surface oxidation on β-Mo₂C is found to be detrimental towards HER^[1] since all metal sites are predicted to be covered with O*/OH* species. In contrast, partial surface oxidation on α-MoC has a promoting effect towards FAEO, as the surface exposes undercoordinated Mo sites with a weakened binding affinity toward reactive intermediates due to repulsive lateral O* interactions. The α-MoC-catalyzed FAEO is predicted to proceed through OH*-mediated C-H scission as a rate-limiting step, similar to FAEO on Pt(111). Surface stability and electrocatalytic activity trends are confirmed experimentally using CV and XPS measurements. The computational evidence for the phase-dependent promoting effect of surface O* on TMC catalytic activity opens up new opportunities for the wider use of TMCs in electrocatalysis.

[1] Yu, Siying et al. ‘Implication of Surface Oxidation of Nanoscale Molybdenum Carbide on Electrocatalytic Activity’ under review