Revealing nitrogen-containing species in commercial catalysts used for ammonia electrosynthesis

Stimulated by the growing demand for sustainable and/or economical distributed ammonia synthesis, the electrochemical nitrogen reduction reaction has attracted considerable interest. The nitrogen-containing impurities in commercial metal-based nitrogen reduction reaction catalysts such as metal oxides and metallic irons have, however, been overlooked. Herein we report the presence of nitrogen-containing species in NOx- or nitrides at substantial levels revealed from many commercial catalysts. We call attention to the necessity to screen the NOx-/nitrides impurities in commercial catalysts, as the nitrogen impurities are not commonly listed in vendors' assay documents. A simple two-step procedure (alkaline/acidic treatment followed by HPLC/UV-vis analysis) is recommended as a reliable protocol for screening NOx-/nitrides impurities in catalyst materials. A case analysis is also carried out on the previously reported H2O-NaOH-KOH system with both N-15-isotopic labelling and nitrogen elemental tracking, reassigning the true nitrogen source of the electrochemically produced NH3 from gaseous N-2 to nitrogen-containing impurities in catalysts.