Insight into pH-dependent synergistic mechanism of the enhanced photoelectro-chlorine conversion of ammonia (NH/NH) to N coupled with H evolution on Pt-decorated black TiO nanotube arrays.

Achieving efficient NH-N degradation into harmless N and simultaneous harvesting hydrogen energy is of great significance for protecting aquatic ecosystems and alleviating energy shortages. Herein, a photoelectrochemical-chlorine (PEC-Cl) system was developed aiming to selective converting ammonia to N with simultaneous H evolution in a wide pH range of 3-13. The Pt decorated black TiO nanotube arrays (Pt/BTNAs) with sufficient oxygen vacancies defects and the enhanced PEC properties were synthesized as the bifunctional electrode. The parameters optimization illustrated high N selectivity (88 %) and H yield rate (67.5 μmol cm h) at pH 9. The ClO derived from the cross reactions between active chlorine and HO plays a predominant role in oxidizing ammonia. More importantly, the Gibbs free energy calculation indicated that Pt decorating can also significantly promote the direct electrochemical ammonia oxidation reaction (AOR) on Pt/BTNAs anode. Under alkaline conditions (pH 9-13), the efficient removal and conversion of ammonia into N in the PEC-Cl system was achieved primarily through the direct anodic AOR, with N selectivity (89 %-93 %) and H yield rate (60.9-68.6 μmol cm h) comparable to those of the ClO-mediated indirect AOR. This study presents a flexible method for treating high and medium-concentration NH-N and simultaneously producing H through the synergistic PEC-Cl and direct electrochemical AOR processes, across a wide pH range.