ELECTROCATALYTIC REDUCTION OF NITRO-COMPOUNDS ON GOLD UPD MODIFIED ELECTRODES .2. REDUCTION OF 2-NITRO-IMIDAZOLE AND 4-NITROIMIDAZOLE AND 4-NITROPYRIDINE-N-OXIDE ON AU AND AU M(UPD) (M=PB, TL)/

The reduction of N-heterocyclic nitro compounds on Au, an sp metal elc trocatalyst, in acid solutions can follow an electrocatalytic and/or e lectron exchange mechanism. The electrocatalytic mechanism proceeds th rough chemisorption of the nitro group and reductive cleavage of one o f the two N-0 bonds and gives diffusion-controlled limiting currents. The electron exchange mechanism proceeds through a dihydroxylamine whi ch is reduced further after an irreversible loss of water to produce t he nitroso intermediate and leads to kinetically controlled limiting c urrents. The strong electron-attracting protonated heterocyclic nuclei , as well as rendering the nitro group more easily reducible, help to stabilize the hydrated form of the nitroso intermediate. On Au/M(UPD) electrocatalysts the reduction mechanism depends on the UPD adatom cov erage. On Au surfaces, with almost complete adatom layers, the reducti on proceeds through the dihydroxylamine. However, on Au partially cove red by adatoms the electrocatalytic mechanism appears to be the only r eaction path for the reduction. A bridge adsorption complex of the nit ro group [GRAPHICS] was assumed to explain its stronger chemisorption on Au partially covered by adatoms than on Au without adatoms.