Isolated Rhodium Atoms Activate Porous TiO₂ for Enhanced Electrocatalytic Conversion of Nitrate to Ammonia

The direct electrochemical reduction of nitrate to ammonia is an efficient and environmentally friendly technology, however, developing electrocatalysts with high activity and selectivity remains a great challenge. Single-atom catalysts demonstrate unique properties and exceptional performance across a range of catalytic reactions, especially those that encompass multi-step processes. Herein, a straightforward and cost-effective approach is introduced for synthesizing single-atom dispersed Rh on porous TiO₂ spheres (Rh₁-TiO₂), which functions as an efficient electrocatalyst for the electroreduction of NO₃ ^- to NH₃. The synthesized Rh₁-TiO₂ catalyst achieve a maximum NH₃ Faradaic efficiency (FE) of 94.7% and an NH₃ yield rate of 29.98 mg h^-1 mg_cat ^-1 at -0.5 V versus RHE in a 0.1 M KOH+0.1 M KNO₃ electrolyte, significantly outperforming not only undoped TiO₂ but also Ru, Pd, and Ir single-atom doped titania catalysts. Density functional theory calculations reveal that the incorporation of Rh single atom significantly enhances charge transfer between adsorbed NO₃ ^- and the active site. The Rh atoms not only serve as the highly active site for electrochemical nitrate reduction reaction (NO₃RR), but also activates the adjacent Ti sites through optimizating the electronic structure, thereby reducing the energy barrier of the rate-limiting step. Consequently, this results in a substantial enhancement in electrochemical NO₃RR performance. Furthermore, this synthetic method has the potential to be extended to other single-atom catalysts and scaled up for commercial applications.