Highly Active and Selective Electroreduction of N₂ by the Catalysis of Ga Single Atoms Stabilized on Amorphous TiO₂ Nanofibers.

Abstract

The electroreduction of N₂ under ambient conditions has emerged as one of the most promising technologies in chemistry, since it is a greener way to make NH₃ than the traditional Haber-Bosch process. However, it is greatly challenged with a low NH₃ yield and faradaic efficiency (FE) because of the lack of highly active and selective catalysts. Inherently, transition (d-block) metals suffer from inferior selectivity due to fierce competition from H₂ evolution, while post-transition (p-block) metals exhibit poor activity due to insufficient "π back-donation" behavior. Considering their distinct yet complementary electronic structures, here we propose a strategy to tackle the activity and selectivity challenge through the atomic dispersion of p-block metal on an all-amorphous transition-metal matrix. To address the activity issue, lotus-root-like amorphous TiO₂ nanofibers are synthesized which, different from vacancy-engineered TiO₂ nanocrystals reported previously, possess abundant intrinsic oxygen vacancies (V_O) together with under-coordinated dangling bonds in nature, resulting in significantly enhanced N₂ activation and electron transport capacity. To address the selectivity issue, well-isolated single atoms (SAs) of Ga are successfully synthesized through the confinement effect of V_O, resulting in Ga-V_O reactive sites with the maximum availability. It is revealed by density functional theory calculations that Ga SAs are favorable for the selective adsorption of N₂ at the catalyst surface, while V_O can facilitate N₂ activation and reduction subsequently. Benefiting from this coupled activity/selectivity design, high NH₃ yield (24.47 μg h^-1 mg^-1) and FE (48.64%) are achieved at an extremely low overpotential of -0.1 V vs RHE.