The influence of solution-phase HNO2 decomposition on the electrocatalytic nitrite reduction at a hemin-pyrolitic...

The mechanism of nitrite electroreduction by hemin adsorbed at pyrolitic graphite is investigated. Two main issues are addressed: the effect of the medium pH and the selectivity of the reaction, which was determined by the combined use of the rotating ring disk electrode (RRDE) 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 HNO(2). 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, HNO(2) and NO are reduced to hydroxylamine (NH(2)OH) with almost 100% selectivity at low potentials, nitrous oxide (N(2)O) 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 HNO(2) dissociation.