Enhancing the Valence Band Charge Localization via Zinc Doping into MnWO₄ to Promote Selective Ammonia Electrooxidation

Abstract

Electrochemical ammonia oxidation reaction (AOR) to selectively produce nitrate/nitrite under ambient conditions can be a potential alternative to the Ostwald process. However, the potential bottlenecks of AOR are sluggish electrokinetics and involvement of multiple steps, which lead to various parallel reactions and side products. Herein, 5-14% zinc(II) ions are doped into a Wolframite-type MnWO4 nanostructure, ZnxMn1-xWO4, which significantly localizes the valence-band state and makes the manganese centers more electropositive, leading to an excellent anodic material for AOR. Among all tested variants of ZnxMn1-xWO4 and control materials, MnWO4 and ZnWO4, 15±5 nm Zn0.01Mn0.9WO4 nanoflakes show the lowest onset (0.53 V vs Hg/HgO) for the AOR in alkaline and 0.5 M Na2SO4 electrolyte of pH 11-13. Electrokinetic study via Tafel analysis and hydrodynamic electrochemistry, and corresponding Koutecký-Levich analysis indicate that the first 2e -oxidation of adsorbed ammonia to hydroxylamine is the rate-limiting step with a rate constant of 4.915±3 × 10 -3 cm s -1 . Product analyses after chronoamperometric AOR study highlight that nitrate predominantly forms with a high yield rate of 120 (±3) μmol h -1 cm -2 and a Faradaic efficiency of ~70 % at 0.9-0.95 V (vs Hg/HgO). In-situ spectrophotometric kinetics study infers [NO2] -formed as intermediate while accumulation of [NO3] -follows a pseudo-first order kinetics with a rate constant of 4.20×10 -5 s -1 . D2O labelling, control studies like electrochemical hydroxylamine and hydrazine oxidation under similar conditions, followed by subsequent product identification and in situ Infrared analysis of the electrolyte, further emphasize hydroxylamine and nitrite as the AOR intermediates. Given the excellent anodic stability of Zn0.01Mn0.9WO4, it acts as an anode for an NH3 electrolyzer to produce H2 with a rate of 1.33 mL h -1 and 94% FE, nearly three times higher than only water splitting. Thereby, doping of an atomically dilute quantity of zinc into the MnWO4 lattice tunes the valence band to enhance AOR selectivity, and the in-depth electrokinetics studies presented herein establish the AOR pathway and the nitrate selectivity, which will definitely help further interpret the AOR mechanism on various earthabundant metal catalysts.