Advancing the Understanding of Photocatalytic Nitrate Reduction for Ammonia Synthesis: Copper Catalysts and Formic Acid Synergy

Abstract

In recent years, photo-and electrocatalytic nitrate reductions have emerged as promising methods for sustainable ammonia production. Among the catalysts studied, copper-based systems have shown superior efficiency and selectivity in nitrate conversion, especially in the presence of sacrificial formic acid. However, the catalytic activity of copper is often hindered by its unstable oxidation state, primarily due to surface oxidation, and the underlying reaction mechanism remains poorly understood. This study reports that the scavenger formic acid plays a versatile role in the CuO x :TiO 2 catalytic system: first, it stabilizes the Cu 2 O phase on the TiO 2 surface. Second, it acts as a hole scavenger, reducing electron−hole recombination. Third, as formic acid undergoes oxidation, it produces CO 2•− radicals with significant reduction potential. These radicals effectively facilitate the reduction of nitrate ions, resulting in ammonia production. Moreover, copper's unique ability to degrade formic acid contributes to the creation of bound atomic hydrogen on its surface. This, in turn, promotes selective nitrate hydrogenation and subsequent formation of ammonia within the catalytic system. The CuO x :TiO 2 produced 1.639 mmol/g/h ammonia with an 82% nitrate-to-ammonia conversion rate. These findings not only deepen our comprehension of photocatalytic nitrate reduction processes but also highlight the potential of Cu:TiO 2 /Cu 2 O catalysts for sustainable ammonia production and environmental remediation applications.