New insights into the mechanism of nitrite reduction on a platinum electrode

This paper reinvestigates the mechanism of HNO2 and NO2- reduction on polycrystalline platinum as a function of electrolyte pH and reactant concentration. Intermediates and/or reaction products were detected by means of various (combined) techniques: RRDE (for NH2OH detection), OLEMS (for volatile products) and FTIRS. In acidic media, HNO2 is depleted due to homogeneous-phase reactions (accelerated by stirring) that generate NO: the latter species, which readily forms an adlayer on Pt, is reduced in a first wave to N2O, while HNO2 is reduced in a following diffusion-limited wave to mainly NH2OH. The two waves display different reaction orders. Loss of HNO2 (as NO) in the blanketing Ar stream hampered an accurate quantitative evaluation of the product distribution. When the pH is increased, NO2- is reduced in a single peak with a much lower current density than in the previous case. In RDE experiments, the peak was found to decrease with increasing rotation rates. The presence of a sluggishly reduced intermediate (NH2OH) has been proposed. A general mechanistic scheme, including NO, HNO2 and NO2-, is discussed.