Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru-Doped TiO₂ /RuO₂ Electrocatalysts

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru-doped TiO₂ /RuO₂ (abbreviated as Ru/TiO₂ ) as a proof-of-concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ru^{δ +} can function as the main active center for the NOR process. Remarkably doping Ru into the TiO₂ lattice can induce an upshift of the d-band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N₂ to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO₂ on the TiO₂ surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO₂ in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h^-1 g_cat ^-1 (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO₂ catalyst (with the hybrid composition of Ru_x Ti_y O₂ and extra RuO₂ by 2.79 wt% Ru addition amount) in 0.1 m Na₂ SO₄ electrolyte.