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 NH3 conversion and exceptional NO selectivity with negligible N2O production, using nonprecious V2O5 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 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 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.
Highlights
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
In order to validate the feasibility of the proposed chemical looping ammonia oxidation (CLAO) process, consecutive redox cycles were carried out to investigate the product distribution and the overall chemical looping process efficiency
Among many metal oxide redox catalysts investigated, V2O5 exhibited superior NH3 conversion at a substantially lower temperature with an outstanding NO selectivity (99.8%), outperforming the expensive, commercially applied Pt−Rh gauzes at a temperature up to 300 °C lower
Summary
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. 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 V = O sites are suggested to be the catalytically active center leading to the formation of the oxidation products Both V = O and doubly coordinated oxygen participate in the hydrogen transfer process. Chemical looping is a new frontier for producing valuable chemicals in a clean and efficient manner[24,25,26,27,28], and has been demonstrated for processing methane[28,29,30,31,32,33,34,35], biofuels[36,37], syngas[38,39], coal and carbonaceous feedstocks[22,40,41] with low cost, reduced emissions and higher energy efficiency. The CLAO process is potentially advantageous compared to the conventional co-feed approach where
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