Electrocatalytic reduction of nitrocompounds on gold UPD modified electrodes: Part 1. Reduction of nitrobenzene and 3-nitro-1,2,4-triazole on Au and Au/M(UPD) (M = Pb, Tl)

Abstract

The underpotential deposition (UPD) of Pb and Tl has been found to exhibit a marked influence on the performance of the Au electrode for the electroreduction of nitro groups. In acid solutions, the UPD of Pb shifts the four-electron reduction wave of nitrobenzene to phenylhydroxylamine towards more negative potentials and also causes a complete inhibition of the reduction step of phenylhydroxylamine to aniline. However, Pb(UPD) and Tl(UPD) adatom layers on Au catalyse the four-electron reduction of 3-nitro-1,2,4-triazole in acid solutions. The catalytic effect is characterized by a positive shift of the reduction wave and the attainment of diffusion-controlled limiting currents at medium degrees of UPD adatom coverages. At complete UPD coverages, the catalytic activity decreases and the limiting current becomes kinetically controlled as on bare Au. The kinetic character of the limiting current was attributed to the dehydration reaction of a dihydroxylamine intermediate in an ECCE reaction sequence. The catalytic effect observed at medium degrees of UPD adatom coverages was interpreted in terms of strong adsorption of the nitro group with a bridged complex involving a substrate (Au) and an adsorbate atom (Pb,Tl) which favour NO bond cleavage. Finally, in alkaline solutions it has been found that Pb(UPD) and Tl(UPD) monolayers catalyse the four-electron reduction of nitrobenzene and 3-nitro-1,2,4-triazole.