The electrocatalytic reduction of NO^-₃ on Pt, Pd and Pt+Pd electrodes activated with Ge

The electrocatalytic reduction of nitrate has been investigated on Pt, Pd and Pt + Pd electrodes covered with a submonolayer of germanium. Pt + Pd electrodes were prepared by electroless deposition of submonolayers of Pd on Pt by exchange of PdCl₂ for preadsorbed copper. Underpotentially deposited germanium enhances the reduction rate of nitrate strongly. The reduction of nitrite is enhanced to a lesser extent, whereas germanium is inactive for NO and hydroxylamine reduction. Further, cyclic voltammetry shows that the well known inhibition of the nitrate reduction at low potentials is absent for germanium-modified electrodes. Amperometry shows that the current densities for nitrate reduction at 0.1 V depend strongly on the composition of the electrode surface. The activities increase in the order Pd, Pt and Pt + Pd and all electrodes display a proportional relation between the activity and the germanium coverage. This shows that germanium is involved in the rate determining step, which is the reduction of nitrate to nitrite and its role is to bind the oxygen atom of nitrate. The higher activities for Pt + Pd electrodes can be understood in terms of changes in the electronic structure of the metals as a result of alloying. Selectivity measurements with a rotating ring-disk electrode have shown for all electrodes that the hydroxylamine selectivity increases for increasing germanium coverage. Pd displays higher hydroxylamine selectivities than Pt and Pt + Pd electrodes. No gaseous products were observed for Pt, whereas for Pt + Pd and Pd N₂O selectivities up to 8% were found.