Insights into Designing Photocatalysts for Gaseous Ammonia Oxidation under Visible Light

Excessive emission of ammonia (NH3) gives rise to a number of negative effects on the environment and human health. Photocatalysis is an efficient method to eliminate gaseous NH3; however, photocatalytic oxidation (PCO) of NH3 in the visible light region has not been achieved to date. Herein, we test a set of typical visible-light-sensitive photocatalysts (N-TiO2, g-C3N4, and Ag3PO4) for NH3 oxidation and reveal for the first time that the semiconductor Ag3PO4 can harness visible light to realize ambient NH3 oxidation. Combining the activity testing results with the photochemical properties of samples, we confirm that photoexcited holes are responsible for triggering the initial key step of NH3 oxidation (NH3 to •NH2), and therefore, the redox potential of photoexcited holes plays the decisive role in the reaction. We propose that an active visible light photocatalyst for NH3 oxidation requires both a suitable band gap for visible light response and a low valence band edge associated with a high oxidation potential for activating NH3 to •NH2. Our findings provide new insights into the PCO of pollutants under visible light and will benefit future design of more efficient visible-light-sensitive photocatalysts.