Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Abstract

Selective oxidation of ammonia to nitric oxide over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process. Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we propose and demonstrate a chemical looping ammonia oxidation catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed process exhibit near complete NH ₃ conversion and exceptional NO selectivity with negligible N ₂O production, using nonprecious V ₂O ₅ redox catalyst at 650 ^oC. Operando spectroscopy techniques and density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V ⁵⁺/V ⁴⁺ 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 participate in the hydrogen transfer process. The outstanding performance originates from the low activation energies for the successive hydrogen abstraction, facile NO formation as well as the easy regeneration of V = O species. Our results highlight a transformational process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.

Abstract

This study reports a fundamentally new and commercially promising chemical looping ammonia oxidation scheme using an inexpensive transition metal oxide, and provides mechanistic insight into the oxygen species governing ammonia oxidation.