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

This paper reinvestigates the mechanism of HNO ₂ 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 NH ₂OH detection), OLEMS (for volatile products) and FTIRS. In acidic media, HNO ₂ 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 N ₂O, while HNO ₂ is reduced in a following diffusion-limited wave to mainly NH ₂OH. The two waves display different reaction orders. Loss of HNO ₂ (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 (NH ₂OH) has been proposed. A general mechanistic scheme, including NO, HNO ₂ and NO2-, is discussed.