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

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 (VO) 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 VO, resulting in Ga-VO 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 VO 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.