Photocatalytic Nitrate Reduction to Ammonia via Zr‐Mediated Proton‐Coupled Electron Transfer

Abstract The reduction of nitrate (NO 3 − ) is a fundamentally exciting reaction with important environmental implications. From a mechanistic perspective, it involves the transfer of 8 e − and 9 H + , with the initial activation of NO 3 − representing a significant challenge. Here we propose a distinct and competitive mechanism for the redox activation of this inert anion based on photocatalytic proton‐coupled electron transfer (PCET). The use of a PCET mediator based on a Zr coordination cage enabled formation of NH 3 under visible light irradiation (440 nm). Importantly, the incorporation of Li + as a Lewis acid within the cage structure further generated highly reactive sites that pre‐associate and activate NO 3 − , enhancing the catalytic activity. We also show how the back oxidation of the intermediate NO 2 − has a dramatic impact in the efficiency and highlight the role of the sacrificial electron donor in outcompeting this side reaction. These aspects were finally combined with the use of silver as a d‐block metal catalyst to facilitate the NO 3 − to NO 2 − reduction step, the identified bottleneck of the overall process.