A grand canonical study of the potential dependence of nitrate adsorption and dissociation across metals and dilute alloys

Aqueous nitrate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>NO</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn></mml:mrow> <mml:mrow><mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> ) can be electrocatalytically reduced to value-added or benign products. However, the impact of the electrochemical potential on key reaction steps remains poorly understood. Using explicit and analytical grand-canonical density functional theory (eGC-DFT and aGC-DFT), we investigate the potential dependence of nitrate adsorption and dissociation on pure metals and Cu-based single-atom alloys (SAAs). With aGC-DFT, we find that the nitrate adsorption free energy on stable/metastable SAAs and pure metals varies linearly with the applied potential, indicated by constant slopes (electrosorption valencies) of -0.60 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:mrow><mml:mi>eV</mml:mi></mml:mrow> <mml:mrow><mml:mi>V</mml:mi></mml:mrow> </mml:mfrac> </mml:math> to -0.80 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:mrow><mml:mi>eV</mml:mi></mml:mrow> <mml:mrow><mml:mi>V</mml:mi></mml:mrow> </mml:mfrac> </mml:math> . The nitrate dissociation barrier exhibits weak, but linear potential dependencies across metals with slopes of -0.04 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:mrow><mml:mi>eV</mml:mi></mml:mrow> <mml:mrow><mml:mi>V</mml:mi></mml:mrow> </mml:mfrac> </mml:math> and -0.20 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:mrow><mml:mi>eV</mml:mi></mml:mrow> <mml:mrow><mml:mi>V</mml:mi></mml:mrow> </mml:mfrac> </mml:math> . The potential dependence for both reaction steps correlates with the change in the surface normal dipole moment, resulting largely from partial charge transfer during adsorption or N-O bond cleavage during dissociation. We demonstrate aGC-DFT predicts potential-dependent adsorption and activation energies that can differ significantly from conventional approximations (e.g., the computational hydrogen electrode model). However, aGC-DFT computed energies and these differences vary with the assumed double-layer properties. This work clarifies potential-dependent nitrate adsorption and dissociation trends across SAAs and pure metals, emphasizing the need to account for electrochemical conditions in mechanistic studies of nitrate reduction.