Surface Valence State Effect of MoO_2+x on Electrochemical Nitrogen Reduction

The valance of Mo is critical for FeMo cofactor in ambient ammonia synthesis. However, the valence effect of Mo has not been well studied in heterogeneous nanoparticle catalysts for electrochemical nitrogen reduction reaction (NRR) due to the dissolution of Mo as MoO₄^2- in alkaline electrolytes. Here, a MoO_2+x catalyst enriched with surface Mo⁶⁺ is reported. The Mo⁶⁺ is stabilized by a native oxide layer to prevent corrosion and its speciation is identified as (MoO₃ )_n clusters. This native layer with Mo⁶⁺ suppresses the hydrogen evolution significantly and promotes the activation of nitrogen as supported by both experimental characterization and theoretical calculation. The as-prepared MoO_2+x catalyst shows a high ammonia yield of 3.95 µg mg_cat^-1 h^-1 with a high Faradaic efficiency of 22.1% at -0.2 V versus reversible hydrogen electrode, which is much better than the MoO₂ catalyst with Mo⁶⁺ etched away. The accuracy of experimental results for NRR is confirmed by various control experiments and quantitative isotope labeling.