Assessing the Activity of Transition Metal Oxides for the Electrochemical N<sub>2</sub> Oxidation to Nitrate
The electrochemical oxidation of dinitrogen (N<sub>2</sub>) to nitrate (NO<sub>3</sub><sup>–</sup>) is an attractive method for decentralized fertilizer production. Yet, scarce experimental evidence with trace NO<sub>3</sub><sup>–</sup> pro...
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| مؤلفون آخرون: | , , , , , , , , , |
| منشور في: |
2025
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| الملخص: | The electrochemical oxidation of dinitrogen (N<sub>2</sub>) to nitrate (NO<sub>3</sub><sup>–</sup>) is an attractive method for decentralized fertilizer production. Yet, scarce experimental evidence with trace NO<sub>3</sub><sup>–</sup> produced in reported catalysts hints at the kinetics challenge and motivates a search for reliable electrocatalysts. We addressed the gaps in the understanding of N<sub>2</sub> oxidation by computing three pathways: the (1) direct electrochemical pathway that extends all the way to NO<sub>3</sub><sup>–</sup>, (2) surface lattice oxygen pathway on perovskites, and (3) surface-adsorbed oxygen pathway. These computations revealed the unfavorable trade-off between N<sub>2</sub> activation and NO<sub>2</sub> desorption/O vacancy filling step energies, which potentially limit the N<sub>2</sub> oxidation activity and render the parasitic OER dominant. However, several oxides which possess reactive surface oxygen and inert/moderate OER activity were identified as more promising for experimental assessment. We then experimentally examined 20+ transition metal oxides, namely, ABO<sub>3</sub> perovskites (A = La, Sr, Ca, Bi and B = Co, Mn, Fe) and MO<sub>2</sub> rutiles (IrO<sub>2</sub>, RuO<sub>2</sub>, TiO<sub>2</sub>, SnO<sub>2</sub>, and Fe- and Ir-doped TiO<sub>2</sub> and SnO<sub>2</sub>) in alkaline and neutral electrolytes. Electrochemical measurements via up to 22 h chronoamperometry showed minimal NO<sub>3</sub><sup>–</sup> concentrations of <1 ppm<sub>N</sub> via UV–vis spectroscopy, which were comparable to those measured in the absence of N<sub>2</sub>. Time-dependent investigations of different substrates (i.e., carbon paper and Ti foil), increasing catalyst loadings in H-cells and flow cells, as well as high-surface-area La<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>3</sub> and La<sub>0.5</sub>Sr<sub>0.5</sub>MnO<sub>3</sub> showed that the observed NO<sub>3</sub><sup>–</sup> concentrations were not greater than those measured without N<sub>2</sub> with experimental certainty. This work underscores the need for proliferating NO<sub><i>x</i></sub> production (mass<sub>prod</sub>) beyond system size (mass<sub>sys</sub>) or rigorous quantitative <sup>15</sup>N<sub>2</sub>-labeling to provide concrete evidence for true N<sub>2</sub> oxidation and encourages exploration of ambient N<sub>2</sub> oxidation beyond conventional approaches. |
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