Surface MoOx residues from Mo(CO)6 capping enhance ammonia oxidation on Pt nanocubes

Ammonia is emerging as a promising carbon-free energy carrier. The electrocatalytic ammonia oxidation reaction (AOR), highly structure-sensitive, exhibits exceptional activity on Pt nanocubes (NCs) with (100) facets. Yet, synthesis methods employing metal carbonyl capping agents inevitably introduce residual metals, whose catalytic effects are often neglected. Here, Pt NCs synthesized with Mo(CO) 6 were found to contain trace Mo both in the bulk and at the surface. Surface Mo species, partially oxidized to MoO x , substantially modify the electronic structure of Pt(100), lowering activation barriers for rate-determining AOR steps and enhancing resistance to nitrogen poisoning. These effects collectively boost AOR performance, enabling a direct ammonia fuel cell with a Pt-154 anode to achieve a peak power density of 26.7 mW cm −2 . These results reveal the overlooked impact of capping-agent-derived contaminants in AOR and point to the deliberate incorporation of heterometallic modifiers as a viable strategy for advanced electrocatalyst design.