The current electrochemical method for H2 production is water electrolysis, a process with a high energy demand, which is limited by the oxygen evolution reaction (OER). One way to handle the problems related to the OER is to use other oxidative reactions by so-called coupled water electrolysis. One option is the SO2 oxidation reaction (SO2OR), a process that generates H2SO4, which has industrial use, by the consumption of an abundant pollutant that demands, under standard conditions, 0.70 V less than the OER according to theoretical predictions. On the basis of theoretical calculations the mechanism is expected to be the same for a range of metallic catalysts, with the best ones being Pt and Au, in order. Here, the SO2 electro-oxidation on Pt and Au electrodes was investigated by in situ infrared reflection–absorption (IRRA) spectroelectrochemistry, aiming to elucidate the mechanism. On Pt, species such as dithionate, S2O6 2–, not commonly cited in the literature, were found as intermediates, and PtOH and PtO were suggested as oxidative species. On Au electrodes, the situation observed was completely different. The electrolyte chaotropicity strongly influenced the adsorption of SO2 on Au, changing from Au–O for highly kosmotropic media to Au–S for more chaotropic systems. When the Au–S bond is formed, dithionate and S2O5 2– species were simultaneously present along with the Au­(SO3) complex in solution. The observation of these two species was accompanied by potential oscillations, and an HN-NDR (hidden N-shaped negative differential resistor) oscillator was observed. Different mechanisms for different electrolytes are proposed for Au electrodes.

SO2 oxidation has been proposed as an alternative pathway for the electrochemical generation of H2 as it requires lower potentials than water splitting and at the same time consumes an atmospheric pollutant. Theoretical predictions suggest that gold and platinum are the most active catalysts for this reaction. This work presents experimental evidence that, contrary to the predictions, SO2 oxidation starts at less positive potentials on Au electrodes (ca. 0.60 V (vs. RHE)) than on Pt. It is found further that the observed current densities on Au are one order of magnitude higher than on Pt. In addition, the SO2 oxidation mechanism depends on the chemical nature of the electrolyte used: a kosmotropic anion (HSO4-) results in lower currents than a chaotropic one (ClO4-) and the latter displays oscillatory reaction rates under both potentiostatic and galvanostatic regimes.

The electrochemical SO 2 oxidation reaction (SO 2 OR) is a promising alternative to the O 2 evolution reaction for H 2 electrochemical generation. The SO 2 OR exhibits a low thermodynamic potential and small kinetic barrier, and Au has been observed to be a very active catalyst. For this material, nonlinear dynamic behavior is observed, which is related to the electrolyte chaotropicity at the solid-liquid interface. In this work, the influence of the chaotropicity was investigated by electro-chemical and spectroelectrochemical techniques, and the main reasons for the dependence on the chaotropicity were investigated. The electrochemical response as a function of the chaotropicity was determined as a function of the mechanism observed, and the reaction pathway selection was related to the intermediate adsorption, which in turn was controlled by the interfacial water structure. The "neutral" chaotropicity was found to be the optimum condition for SO 2 OR.[a] Dr.

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