Mechanism of hydrazine oxidation at Palladium electrodes: Long-lived radical di-cation formation

The mechanism of the catalytic oxidation of hydrazine at Palladium (Pd) electrodes was studied in aqueous solutions between pH 2 and 11. The voltammetry recorded at pH 2 and 11 revealed that both the unprotonated hydrazine N2H4 and the protonated form N2H5+ are electro-active at the Pd surface in contrast to glassy carbon (GC) where N2H4 is the only species which undergoes oxidation in the potential range of 0.2 to 1.0 V (vs the Saturated Calomel Electrode). An unexpected reductive voltammetric wave was observed during the cyclic voltammetry of the oxidation of protonated hydrazine and concluded to originate from the reduction of a radical di-cation N2H5•2+ which is stable on the voltammetric timescale. The di-cation was inferred to result from the loss of one electron from the single lone pair of electrons on N2H5+. It is suggested that, unlike the case of N2H4, the absence of a lone pair on the N adjacent to that being oxidised as a result of protonation leads to the stability of the radical di-cation whereas in the oxidation of N2H4, the available adjacent lone pair facilitates rapid follow up chemical reaction leading to nitrogen formation.