Electrocatalytic oxidation of ammonia is an appealing, lowtemperature process for the sustainable production of nitrites and nitrates that avoids the formation of pernicious N 2 O and can be fully powered by renewable electricity. Currently, however, the number of known efficient catalysts for such a reaction is limited. The present work demonstrates that copperbased electrodes exhibit high electrocatalytic activity and selectivity for the NH 3 oxidation to NO 2 À and NO 3 À in alkaline solutions. Systematic investigation of the effects of pH and potential on the kinetics of the reaction using voltammetric analysis andin situ Raman spectroscopy suggest that ammonia electrooxidation on copper occurrs via two primary catalytic mechanisms. In the first pathway, NH 3 is converted to NO 2 À via a homogeneous electrocatalytic process mediated by redox transformations of aqueous [Cu(OH) 4 ] À /2À species, which dissolve from the electrode. The second pathway is the heterogeneous catalytic oxidation of NH 3 on the electrode surface favoring the formation of NO 3 À . By virtue of its nature, the homogeneous-mediated pathway enables higher selectivity and was less affected by electrode poisoning with the strongly adsorbed "N" intermediates that have plagued the electrocatalytic ammonia oxidation field. Thus, the selectivity of the Cu-catalyzed NH 3 oxidation towards either nitrite or nitrate can be achieved through balancing the kinetics of the two mechanisms by adjusting the pH of the electrolyte medium and potential.
Studies of the ammonia oxidation reaction (AOR) for the synthesis of nitrite and nitrate (NO2/3−) 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 NO2/3− under alkaline conditions (0.1 m KOH). Nickel‐based electrodes showed the highest overall NO2/3− 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 NO2/3− yields, suggesting the formation of alternate oxidation products. NO2/3− 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.
As the global demand for fertilisers and other nitrogenous products increases, so does the demand for robust, cost-effective and sustainable alternatives to the Ostwald process for the oxidation of ammonia...
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