Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Abstract Selective oxidation of ammonia to nitric oxide (NO) over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process in chemical industry. Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we proposed and demonstrated, for the first time, a novel chemical looping ammonia oxidation (CLAO) catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed CLAO process exhibited near complete NH3 conversion and exceptional NO selectivity (99.8%) with negligible N2O production, using nonprecious V2O5 redox catalyst at a temperature up to 300 °C lower than the existing approach. Operando spectroscopy techniques complemented with density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V5+/V4+ redox cycle. The V=O sites are suggested to be the catalytically active center leading to the formation of the oxidation products. Meanwhile, both V=O and doubly coordinated oxygen (V-OII-V) participate in the hydrogen transfer process. The outstanding performance is attributed to the low activation energies for the successive hydrogen abstraction (1.06 eV), facile NO formation (0.03 eV) as well as the easy regeneration of V=O species. Our results highlight a transformational CLAO process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.