A Survey of Catalytic Materials for Ammonia Electrooxidation to Nitrite and Nitrate

Studies of the ammonia oxidation reaction (AOR) for the synthesis of nitrite and nitrate (NO_2/3 ^- ) have been limited to a small number of catalytic materials, majorly Pt based. As the demand for nitrate-based products such as fertilisers continues to grow, exploration of alternative catalysts is needed. Herein, 19 metals immobilised as particles on carbon fibre electrodes were tested for their catalytic activity for the ammonia electrooxidation to NO_2/3 ^- under alkaline conditions (0.1 m KOH). Nickel-based electrodes showed the highest overall NO_2/3 ^- yield with a rate of 5.0±1.0 nmol s^-1  cm^-2 , to which nitrate contributed 62±8 %. Cu was the only catalyst that enabled formation of nitrate, at a rate of 1.0±0.4 nmol s^-1  cm^-2 , with undetectable amounts of nitrite produced. Previously unexplored in this context, Fe and Ag also showed promise and provided new insights into the mechanisms of the process. Ag-based electrodes showed strong indications of activity towards NH₃ oxidation in electrochemical measurements but produced relatively low NO_2/3 ^- yields, suggesting the formation of alternate oxidation products. NO_2/3 ^- production over Fe-based electrodes required the presence of dissolved O₂ and was more efficient than with Ni on longer timescales. These results highlight the complexity of the AOR mechanism and provide a broad set of catalytic activity and nitrate versus nitrite yield data, which might guide future development of a practical process for the distributed sustainable production of nitrates and nitrites at low and medium scales.