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

The reduction of nitrate (NO₃ ^-) 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 NO₃ ^- 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 NH₃ 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 NO₃ ^-, enhancing the catalytic activity. We also show how the back oxidation of the intermediate NO₂ ^- 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 NO₃ ^- to NO₂ ^- reduction step, the identified bottleneck of the overall process.