(697a) Strategies for Steering Selectivity during Catalytic Nitrate Reduction

Nitrate (NO₃^-) is a pervasive contaminant in groundwater that is harmful to human health. In this talk, we will discuss strategies for developing (electro)catalysts that are selective to dinitrogen (N₂), which is a more desirable product than ammonia (NH₃) for drinking-water applications. Using density functional theory (DFT), we found that product selectivity to NH₃ versus N₂ is controlled by competition between N-H bond formation and N-N bond formation on Cu and Pd surfaces. On the Cu(100) surface, N-H bond formation is favorable in the hydrogenation step of surface-bound NO*. This leads to the formation of an HNO* intermediate that is readily reduced to NH₃. In contrast, on the Pd(100) surface NO* is hydrogenated to form an NOH* intermediate that is readily reduced to N*. N₂ then is generated through a N*-NO* coupling step. Importantly, we found that the N-N bond formation barrier on Pd is highly dependent on the NO* surface coverage, where a high NO* coverage lowers the N*-NO* coupling barrier. As such, increasing the NO* coverage is essential for enhancing N₂ selectivity. Using this concept, we worked with collaborators to design a Pd nanocube electrocatalyst with partial Cu coverage that achieves high NO₃^- reduction activity and N₂ selectivity by increasing NO* coverage through a Cu-to-Pd spillover mechanism (ACS Catal. 2023, 13, 1, 87–98). This work highlights the importance of NO* surface coverage for controlling product selectivity, explains the different selectivities observed for Cu and Pd, and demonstrates strategies for boosting N₂ selectivity through NO* spillover.