Electrocatalytic reduction of HNO3 at polyaniline filmed electrodes.

The electrochemical reduction of HNO3 has been studied at vitreous carbon, Au and Pt, both bare and filmed with polyaniline. Filmed electrodes compare favourably with bare ones because they allow HNO3 reduction at acid concentrations where the latter are inactive (3 M HNO3) and they do not undergo passivation or inhibition in more concentrated acid media (4 M H2SO4 + 0.5 M HNO3). The likely intermediates HNO2 and NO are reduced even more easily than HNO3 at polyaniline electrodes in a mediated reaction controlled by the kinetics of the cross-exchange between reduced polyaniline and redox species in the solution. The overall HNO3 reduction at polyaniline is, however, an autocatalytic process, as on the bare electrodes.