In Situ Spectroscopic Probing of the Hydroxylamine Pathway of Electrocatalytic Nitrate Reduction on Iron‐Oxy‐Hydroxide

Abstract Crystalline γ‐FeO(OH) dominantly possessing ─ OH terminals (𝛾‐FeO(OH) c ), polycrystalline γ‐FeO(OH) containing multiple ─ O, ─ OH, and Fe terminals (𝛾‐FeO(OH) pc ), and α‐Fe 2 O 3 majorly containing ─ O surface terminals are used as electrocatalysts to study the effect of surface terminals on electrocatalytic nitrate reduction reaction (eNO 3 RR) selectivity and stabilization of reaction intermediates. Brunauer‐Emmett‐Teller analysis and electrochemically determined surface area suggest a high active surface area of 117.79 m 2 g −1 (ECSA: 0.211 cm 2 ) for 𝛾‐FeO(OH) c maximizing the surface accessibility for nitrate adsorption and exhibiting selective eNO 3 RR to NH 3 at pH 7 with a yield rate 18.326 mg h −1 cm −2 , >85% Faradaic efficiency (FE), and at least nine‐times catalyst‐recyclability. 15 N‐ and D‐labeling combined with in situ IR and Raman studies validate the adsorption of nitrate ions on the ─ OH terminals of 𝛾‐FeO(OH) c and the generation of nitrite and hydroxyl amine as eNO 3 RR intermediates. A kinetic isotope effect (KIE) value of 2.1 indicates H 2 O as the proton source and proton‐coupled electron transfer as the rate‐limiting step. The rotating‐ring disk electrochemical (RRDE) study and subsequent Koutecký‐Levich analysis reveal the electron‐transfer rate constant (k) for the 2e‐ reduction of nitrate to nitrite is 5.7 × 10 −6 cm s −1 . This study provides direct evidence of the hydroxyl amine formation as the dominant pathway of eNO 3 RR on γ‐FeO(OH).