(Invited) Adsorbate Coverage Steers Selectivity during Catalytic Nitrate Reduction

Nitrate (NO 3 - ) is a pervasive contaminant in groundwater that is harmful to human health. In this talk, we will discuss strategies for developing (electro)catalysts that selectively reduce NO 3 - to either dinitrogen (N 2 ), which is the desirable product if the goal is to remove NO 3 - from drinking water, or to ammonia (NH 3 ), which is the desirable produce for resource recovery. Using density functional theory (DFT), we found that product selectivity to NH 3 versus N 2 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 NH 3 . On the Pd(100) surface, NO* is hydrogenated to form an NOH* intermediate that is readily reduced to N*. N 2 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, increased NO* coverage enhances N 2 selectivity. Using this concept, we worked with collaborators to design a Pd nanocube electrocatalyst with partial Cu coverage that achieves high NO 3 - reduction activity and N 2 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 selectivity observed for Cu and Pd, and demonstrates strategies for boosting N 2 selectivity through NO* spillover.