Subnanometer-sized CuO_x Clusters on TiO₂ as Active Photocatalysts for Ammonia Production from Photocatalytic Nitration Reduction Reaction

The Haber-Bosch process is commonly used to produce ammonia while consuming energy and yielding huge emissions of carbon dioxide. Finding an alternative way to produce ammonia sustainably is a promising research direction with less CO₂ emission. Photochemical nitrate reduction reaction (NtRR) to ammonia (NH₃) could solve the nitrate pollutant problem in water and approach to produce ammonia with a more environmentally friendly process and lower energy consumption. In this work, the subnanometer-sized copper oxide clusters decorated on titanium oxide nanoparticles (CuO_x-TiO₂) were synthesized through the hydrothermal method and calcination treatment. CuO_x-TiO₂ demonstrated impressively conversion rate of NO₃⁻ to NH₃ with a yield of 153.09 μg-g_cat⁻¹-h⁻¹ in KNO₃ solution. The characteristic structure analysis revealed CuO_x clusters with ~less than 1 nm decorated on TiO₂ nanoparticles. From in-situ X-ray absorption fine structure (XAFS) spectroscopic technique, the transformation of the oxidation state of Cu clusters and the changes of local structure in the CuO_x-TiO₂ were observed. The photocatalytic reaction mechanism of nitrate reduction on the CuO_x-TiO₂ was successfully revealed. These discoveries have broad implications for the functional advancement of catalysts based on subnanometer-sized clusters. Our findings may pave the way for exploring advanced ammonia synthesis routes with reduced energy consumption and carbon emissions.