THE ELECTROCATALYTIC REDUCTION OF NO3- ON PT, PD AND PT+PD ELECTRODESACTIVATED WITH GE

The electrocatalytic reduction of nitrate has been investigated on Pt, Pd and Pt + Pd electrodes covered with a submonolayer of germanium. P t + Pd electrodes were prepared by electroless deposition of submonola yers of Pd on Pt by exchange of PdCl2 for preadsorbed copper. Underpot entially deposited germanium enhances the reduction rate of nitrate st rongly. The reduction of nitrite is enhanced to a lesser extent, where as 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 electrod es. Amperometry shows that the current densities for nitrate reduction at 0.1V depend strongly on the composition of the electrode surface. The activities increase in the order Pd, Pt and Pt + Pd and all electr odes display a proportional relation between the activity and the germ anium coverage. This shows that germanium is involved in the rate dete rmining step, which is the reduction of nitrate to nitrite and its rol e is to bind the oxygen atom of nitrate. The higher activities for Pt + Pd electrodes can be understood in terms of changes in the electroni c structure of the metals as a result of alloying. Selectivity measure ments with a rotating ring-disk electrode have shown for all electrode s that the hydroxylamine selectivity increases for increasing germaniu m 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. (C) 1997 El sevier Science S.A.