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-Fe2O3 majorly containing ─O surface terminals are used as electrocatalysts to study the effect of surface terminals on electrocatalytic nitrate reduction reaction (eNO3RR) selectivity and stabilization of reaction intermediates. Brunauer-Emmett-Teller analysis and electrochemically determined surface area suggest a high active surface area of 117.79 m2 g-1 (ECSA: 0.211 cm2) for 𝛾-FeO(OH)c maximizing the surface accessibility for nitrate adsorption and exhibiting selective eNO3RR to NH3 at pH 7 with a yield rate 18.326mg h-1 cm-2, >85% Faradaic efficiency (FE), and at least nine-times catalyst-recyclability. 15N- 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 eNO3RR intermediates. A kinetic isotope effect (KIE) value of 2.1 indicates H2O 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 × 10-6cm s-1. This study provides direct evidence of the hydroxyl amine formation as the dominant pathway of eNO3RR on γ-FeO(OH).
Published Version
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