Rewiring the NO-Hydrogenation Pathway via Ru Oxidation State: RuO ₂ /TiO ₂ Delivers Low-Temperature, High-Selectivity NH ₃ Synthesis

Ammonia is essential for food production and emerging energy applications, yet its conventional Haber–Bosch synthesis is highly energy- and carbon-intensive. Nitric oxide (NO), with its weaker N–O bond compared to N2, offers a more accessible nitrogen feedstock, whether captured from flue gases or generated via air fixation. Direct hydrogenation of NO to NH3 offers a promising alternative, but product selectivity is difficult to control. Here we show that tuning the oxidation state of ruthenium provides a powerful means to direct this reaction. RuO2/TiO2, dominated by Ru4+ species, achieves over 75% NH3 selectivity at 250 °C under ambient pressure. This performance is 2.5 times higher than that of Ru/TiO2 and remains stable for 48 h of continuous operation. In situ spectroscopy and density functional theory reveal a complementary mechanism: Ru0 sites facilitate low-temperature NO activation, while Ru4+ sites stabilize key hydrogenation intermediates (*NH2/*NH2O). This stabilization likely shifts the rate-determining step from N–O cleavage to NH3 desorption, suppressing N–N coupling. These findings suggest a clear structure–function relationship in Ru-catalyzed NO hydrogenation and provide a design principle for selective, low-energy ammonia synthesis.