Behavior of Cupric Single Atom Alloy Catalysts for Electrochemical Nitrate Reduction: An Ab Initio Study

Electrochemical nitrate reduction (E-NRR) powered by renewable electricity is a sustainable method of converting toxic nitrate into benign products (N-2) or value-added products (NH3). Recently, single atom substitutions of Pd and Ru in inexpensive Cu have shown high activity and selectivity of E-NRR over hydrogen evolution and toward the desired product. Here, we investigate the E-NRR pathway of nine single atom substitutions in Cu to understand the relationship between single atom identity and the activity and selectivity of E-NRR using Density Functional Theory (DFT). We find that while reaction and adsorption energy trends are strongly correlated to the d-band center of the single atom substitution, the single atom catalyst surfaces do not neatly follow scaling relationships as transition states do not occur in the same configuration on all the substitutions. Of the metals investigated, we predict that Ti-, Ru-, Ni-, and Pd-single atom alloys (SAAs) improve the selectivity and activity of E-NRR, while Mo-SAA favors HER, and W-, Pt-, Au-, and In-SAAs do not alter the Cu activity. Ti-, Ru-, and Ni-SAAs selectively reduce nitrate into NH3, whereas Pd-SAA reduces nitrate to N-2 at high pH. The selectivity of the SAA for NH3 or N-2 arises from the preference of adsorbed N* for either the SAA element or Cu. Ni- and Pd-SAA performances are predicted to be more sensitive to potential conditions than Ti and Ru. Overall, this work provides a framework for the design of SAA E-NRR catalysts that are selective to the desired N product.