Opportunities and challenges in aqueous nitrate and nitrite reduction beyond electrocatalysis

Abstract

Nitrate (NO₃⁻) and nitrite (NO₂⁻) ions are common health-threatening contaminants in water. To reduce nitrate and nitrite, catalytic thermal reduction using molecular hydrogen as the reducing agent is a strategy with some clear advantages. However, this method is relatively less explored, with some vital open questions still to be addressed. In this paper, we review the current stages of thermal nitrate and nitrite reduction on the aspects of catalyst synthesis, mechanistic insights, reaction activity, product selectivity, design guidelines for promising catalysts, and the efficiency comparison between thermal and electrocatalytic reduction. The main opportunities and challenges of this thermal reduction method in drinking water and wastewater treatment are discussed. Moreover, in addition to discussing this reaction's typical N₂ formation selectivity, we also discuss the possibility of forming ammonia by tuning the reaction selectivity via rational catalyst design. This review shows that in addition to the currently popular electrocatalytic nitrate reduction, scientific efforts should also be devoted towards the thermal reduction of nitrate and nitrite. We hope that this review will arouse more interest from the catalyst community to explore the mechanistic insights and design guidelines for thermal nitrate and nitrite reduction.