Competitive Non-Radical Nucleophilic Attack Pathways for NH3 Oxidation and H2 O Oxidation on Hematite Photoanodes.

Abstract

The sluggish H2 O oxidation kinetics on photoanodes severely obstructs the overall solar-to-energy efficiency of photoelectrochemical (PEC) cells. Herein, we find a 10 to 55-fold increase of photocurrent by conducting ammonia oxidation reaction (AOR) on hematite (α-Fe2 O3 ) photoanodes under near-neutral pH (9-11) and moderate applied potentials (1.0-1.4 VRHE ) compared to H2 O oxidation. By rate law analysis and operando spectroscopic studies, we confirm the non-radical nucleophilic attack of NH3 molecules on high-valent surface Fe-O species (e.g., FeIV =O) and Fe-N species that produces NOx - and N2 , respectively, which overwhelms the nucleophilic attack of H2 O on surface FeIV =O and contributes to a high Faradaic efficiency of above 80 % for AOR. This work reveals a novel non-radical nucleophilic attack strategy, which is significantly different from the conventional indirect radical-mediated AOR mechanism, for the rational design of high-performance AOR photoelectrocatalysts.