The Influence of Solution-Phase HNO2 Decomposition on the Electrocatalytic Nitrite Reduction at a Hemin-Pyrolitic Graphite Electrode

The mechanism of nitrite electroreduction by hemin adsorbed at pyrolitic graphite is investigated. Two main issues are addressed: the effect of the medium and the selectivity of the reaction, which was determined by the combined use of the rotating ring disk electrode (R R DE) and online electrochemical mass spectroscopy (OLEMS). In acidic media, the behavior observed is indicative of the presence of NO, as the main reactant, generated from the solution-phase decomposition of HNO2. Reduction of the NO-heme complex shows a Tafel slope of 59 mV/dec(-1) and a pH dependence of 42 mV/pH, indicative of a so-called EC mechanism. In acidic media, HNO2 and NO are reduced to hydroxylamine (NH2OH) with almost 100% selectivity at low potentials, nitrous oxide (N2O) being only a minor side product. In neutral media, the hemin is largely unresponsive to the presence of nitrite, giving only a very small reduction current. The comparison of our simple heme catalyst to the behavior of the naturally occurring heme-containing nitrite reductases, which operate under biological conditions, suggests that these enzymes dissociate nitrite at neutral pH either via a complexation step favored by a specific ligating environment or by locally regulating the pH to induce HNCO2 dissociation.