Highly Selective Ammonia Oxidation on BiVO₄ Photoanodes Co‐catalyzed by Trace Amounts of Copper Ions

Abstract

High‐efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5–10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm−2 at 1.23 VRHE, being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm−2). The surface charge‐separation efficiency has reached 90 % under a low bias of 0.8 VRHE. This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non‐aqueous solutions. Through mechanistic studies, we find that the formation of Cu−NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu‐induced coordination effect activates N−H bonds and accelerates the first rate‐limiting proton‐coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

Cooperative oxidation of NH3 and H2O to selectively produce nitrate via a nearly barrierless N–O coupling pathway

“…We noticed that the pH-dependent AOR selectivity is almost universal, regardless of the electrode material (α-Fe 2 O 3 , BiVO 4 , etc. ), 8,9 which exhibits a higher selectivity to NO 2 − at high pH but a higher selectivity to NO 3 − at low pH. Therefore, we believe that the possible source of such pH-dependent AOR selectivity is the (iii) solution-phase reactions, although it may be more complex due to the coupling of (i), (ii) and (iii) in the actual situation.…”

“…3d and Table S2, ESI†). 8,9,32–34 Moreover, the NiCuO x /Ni/ n -Si photoanode exhibits a photovoltage of ∼450 mV at 1 mA cm −2 under AM 1.5G illumination (Fig. S19, ESI†), which surpasses the performance of most non-noble-metal-based electrocatalysts for the AOR that are operated at a higher potential 2,3,24,25 (Table S2, ESI†).…”

“…6 Nevertheless, it is conducted at elevated temperatures (600–800 °C) and pressures (410–1000 kPa) to facilitate the conversion rate, employing noble metal catalysts and emitting highly potent greenhouse gases such as N 2 O. 7 As an alternative, the photoelectrochemical (PEC) ammonia oxidation reaction (AOR) provides a sustainable solution for preparing NO x − under ambient conditions, 8,9 and the produced hydrogen at the cathode can feed back upstream NH 3 synthesis (Scheme 1a). Specifically, through the PEC AOR, as depicted in Scheme 1a, up to 3 moles of H 2 (for the NO 2 − product, 6 e − ) or 4 moles of H 2 (for the NO 3 − product, 8 e − ) can be generated per mole of NH 3 consumed in theory.…”