Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites

Electrocatalytic reduction of waste nitrates (NO3-) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts with varying metal centers uniquely favor mono-nitrogen products (e.g., NH3), as well as provide synergistic supports for nanoparticle catalysts. But the reaction fundamentals remain largely underexplored. Herein, we report a set of 3d-, 4d-, 5d- and f-block atomically dispersed M-N-C catalysts with a well-defined M-Nx coordination. The selectivity and activity of NO3- reduction to NH3 in neutral media were thoroughly studied, with a specific focus on deciphering the role of the NO2- intermediate in the reaction cascade, wherein strong correlations (R=0.9) were found between the NO2- reduction activity and NO3- reduction selectivity for NH3. Moreover, theoretical computations identified the associative/dissociative adsorption pathways for NO2- evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for certain oxyphilic metals. The free energies for the reductive adsorption of nitrate [ ∗ + NO3− → ∗NO2 ], correlated strongly with experimental NH3 selectivity. This work provides a platform for designing multi-element NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.