Proton-coupled electroreduction of nitrate on α-SnWO4 and photon-assisted enhancement of ammonia formation

Abstract

Herein, two-dimensional α-SnWO₄ nanoflake-like particles are synthesized to investigate the cooperative redox participation of the dual metal sites during the electrocatalytic nitrate reduction reaction (eNO₃RR). Cyclic voltammetric studies reveal the reduction of the tungstate sites of α-SnWO₄ at a more anodic potential compared with that observed for the iso-stoichiometric ZnWO₄, which leads to significantly higher eNO₃RR activity of α-SnWO₄, enabling 90 ± 4% faradaic efficiency (FE) for NH₃ at a low potential of −0.2 V (vs. RHE) in an acetate buffer of pH 2.8. A high yield and NH₃ selectivity under a low operating potential on α-SnWO₄ arise due to the ease of nitrate activation through the cooperation of redox-active Sn^II and tungstate sites, whereas it happens at a more cathodic potential on ZnWO₄ due to a distinct lattice arrangement and higher reduction potential of Zn^II sites. The drop in NH₃ yield on increasing pH and the enhancement of Tafel slope are the primary indications of [NO₃]^– adsorption and a proton-assisted activation pathway. Brunauer–Emmett–Teller analysis, contact angle measurement and electrochemically determined surface area suggest a high active surface area of 30 m² g⁻¹ (ECSA: 5 cm²) for α-SnWO₄, maximizing the surface accessibility for the eNO₃RR. 15-N labelling studies confirm the nitrate as the source of nitrogen in NH₃, and rotating disk electrode (RDE) analysis confirms an ∼8e⁻ transfer process for the eNO₃RR. A variable-temperature voltammetric study indicates a low activation barrier for [NO₃]^– reduction right after the reduction of tungstate sites. Use of D₂O or t-BuOH and the concomitant suppression of NH₃ yield, together with in situ EPR analysis in the presence of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), confirms that the first 2e⁻-reduction of the adsorbed [NO₃]⁻ is the rate-limiting and a proton-coupled electron transfer (PCET) step. Detection of [NO₂]⁻ and NH₂OH intermediates through in situ IR spectroscopy further validates the NO₃RR pathway. The n-type semiconducting nature of the α-SnWO₄ with an accessible band gap leads to the photoelectrochemical NO₃RR with enhanced NH₃ yield under visible-light illumination. Consistent photocurrent switching during light-on/off studies and enhanced surface potential after photo-irradiation during in situ Kelvin probe atomic force microscopy (KPFM) corroborate the photon-induced charge-separation. Furthermore, the α-SnWO₄ is shown to be an effective electrode material for nitrate remediation, producing ∼7.7 mg L⁻¹ NH₃ from tap water itself. This study establishes α-SnWO₄ as a potential photo(electro)catalyst for nitrate-to-ammonia conversion, offering insights into the NO₃RR mechanism and its potential practical applications.