Electrocatalytic reduction of nitrite and nitric oxide to ammonia with iron-substituted polyoxotungstates

Abstract

Heteropolytungstates in which one of the positions normally occupied by a tungsten cation is occupied instead by an iron cation are shown to be catalysts for the electroreduction of nitrite to ammonia. The lacunary derivatives in which the empty tungsten site is unoccupied show no catalytic activity. The catalytic mechanism involves the intermediate formation of a nitrosyl complex of the Fe(II) form of the catalyst. The pH dependence of the rate of formation of the nitrosyl complex shows that nitrous acid is the reactive form of nitrite between pH 2 and 8. The catalyzed reduction does not produce hydroxylamine as an intermediate and appears to depend upon the ability of the multiply reduced heteropolytungstates to deliver electrons to the NO group bound to the iron center in a concerted, multiple-electron step. The iron-substituted heteropolytungstates are not degraded by repeated cycling between their oxidized and reduced states. A particularly valuable feature of the heteropolytungstate is the ease with which the formal potentials of the several redox couples they exhibit may be shifted by changing the identity of the central heteroatom. Exploitation of this feature provides diagnostic information that can be decisive in establishing the mechanism of electrocatalytic processes.