Reaction Pathways for Reduction of Nitrate Ions on Platinum, Rhodium, and Platinum−Rhodium Alloy Electrodes

The reduction of nitrate ions on platinum, rhodium, and platinum−rhodium alloy electrodes has been investigated using differential electrochemical mass spectrometry and in situ FTIR measurements. For 3 M HNO3 concentration it has been found that nitrate starts the reduction with partial N−O bond dissociation and N−N bond formation generating NO and N2O. At potentials lower than 0.2 V the reaction proceeds forming dissolved NH4+. For potentials lower than 0 V the reduction continues via a multiple pathway reaction leading to the nonselective production of N2, NH2OH, and N2H2. On the alloyed electrodes, the production of NO and N2O has been observed in both cathodic and anodic scans, while on pure platinum and rhodium electrodes the reaction has been observed only during the cathodic scan. Contrasting with the pure platinum and rhodium alloys, where the N−O bond break starts forming NO and N2O, on the alloys HNO2 has been observed as the first reaction step. For alloys with higher rhodium composition, like Pt75Rh25, no N2 has been detected for potentials lower than 0 V.