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.