Theoretical insight into the NO* Coverage-Dependent selectivity of Pd and Cu electrocatalysts for nitrate reduction

Nitrate (NO3-) is a concerning contaminant in groundwater that is harmful to human health. Electrocatalytic reduction of NO3- can be achieved with transition metals such as Pd and Cu. In this work, we apply density functional theory (DFT) to discern how the electronic properties of the metal catalyst affect the selectivity of the nitrate reduction mechanism toward either N2 products (e.g., N-2 and N2O) or N1 products (e.g., NH3 and NH4+). We find that the greater d-band filling of Cu results in more electron accumulation on the N atom of adsorbed NO*, which drives N-H bond formation favoring the production of HNO* and eventually N1 products (NH3). Conversely, the more delocalized d orbitals of Pd lead to a strong adsorbate-adsorbate coverage effect that lowers key N-N coupling barriers at high NO* coverage favoring selectivity toward N2 products (N-2 and N2O). This work elucidates why, at the electronic-structure level, nitrate reduction is highly sensitive to NO* coverage on some metals (such as Pd) and less so on others (such as Cu).