The Electrocatalytic Reduction of Nitrate Mediated by Underpotential-Deposited Cadmium on Gold and Silver Electrodes in Acid Media

Rotating ring-disk electrode techniques have been employed for the investigation of the electrocatalytic reduction of nitrate induced by underpotential-deposited cadmium on Au and Ag surfaces. Based on Albery's kinetic analysis it has been concluded that for concentrations of nitrate in the millimolar range, the reaction products depend on the pH of the solution. Specifically, for pH = 3, the reaction yields predominantly nitrite, whereas for pH = 1, the process proceeds beyond the nitrite stage to generate products which do not undergo oxidation on a Au ring electrode polarized at potentials as high as 1.0V vs. SCE. Additional evidence in support of these results was provided by exhaustive bulk electrolysis experiments. The lack of linearity of some of Albery's diagnostic plots could be accounted for quantitatively by a numerical integration of the differential equations which govern disproportionation-type reactions at rotating ring electrodes. Good agreement was found between theory and experiment using independently determined values for the rate constant of disproportionation of NO₂ in solution.