DFT Insights into NO Electrochemical Reduction: A Case Study of Pt(211) and Cu(211) Surfaces

Abstract

Electrochemical reduction of nitrate and nitrite represents a promising technology for wastewater treatment, in which NO reduction largely determines the reaction selectivity. While tremendous efforts have been made, there is a lack of fundamental understanding regarding the selectivity toward value-added products (e.g., NH4+ and NH3OH+) through NO* protonation and toward water purification (e.g., N2O or N2 as the product) through NO*–NO* dimer formations. In this contribution, we employed periodic density functional theory (DFT) calculations to investigate NO electrochemical reduction mechanisms on Pt(211) and Cu(211). We found that both NO* protonation and cis-(NO–NO)* formation are possible on Cu with weak NO bonding, while only NO* protonation is favorable on Pt with strong NO binding. The selective formation of cis-(NO–NO)* on Cu rather than Pt shows that NO* binding has a crucial role in determining the formation of cis-(NO–NO)*. The correlation between the free energy of the cis-(NO–NO)* formation and NO* binding energy, on the Cu as well as on the Ag, Au, and PdCu surfaces at various coverages, shows that the cis-(NO–NO)* formation becomes more favorable with the weakening of NO adsorption and vice versa. Thus, our results indicate that the selectivity of complex NO electrochemical reduction can be described by the simple NO binding energy. This trend-based analysis may ultimately lead to the design of electrocatalysts with the desired performance.