The pH dependence of the electrocatalytic nitrate reduction by tin-modified palladium(100) electrodes: Effects of structures of tin species and protonation of nitrite

Nitrate is a common pollutant in groundwater and endangers drinking water safety. The electrocatalytic nitrate reduction using tin-modified palladium electrode (Sn/Pd) provided a promising alternative for nitrate removal. However, the pH effect on the nitrate reduction activity of Sn/Pd and its mechanistic insights are still unclear. In this study, Sn/Pd(100) single crystalline electrodes were used as a model electrode to understand the pH effect on its nitrate and nitrite reduction activity and mechanism. Sn/Pd(100) electrodes showed the nitrate reduction activity at pH < 3.8. DFT calculations suggest that Sn2+ site can be found on Pd(100) at pH < 3.8 and promote nitrate adsorption. The copresence of Sn and Pd sites can reduce the LUMO energy of the adsorbed nitrate molecule and decrease the van der Waals electrostatic potential of the O-N bond, which promotes electron injection and protonation to nitrate. At pH > 3.8, on the other hand, the formation of Sn(OH)(+) or Sn(OH)(2) can inhibit the adsorption of NO3- on Sn atoms, resulting in the disappearance of the electrocatalytic activity. Sn atomic sites mainly promote the reduction step of NO3- -> NO2-, and the subsequent NO2- reduction reaction occurs on Pd sites. The protonation of NO2- was crucial to the nitrite reduction on Pd(100) surface. The findings in this study would be useful to develop highly active electrocatalysts based on Sn and Pd materials for the nitrate reduction.