Influence of the Electrode and Chaotropicity of the Electrolyte on the Oscillatory Behavior of the Electrocatalytic Oxidation of SO₂

Abstract

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.

Influence of Anion Chaotropicity on the SO2 Oxidation Reaction: When Spectator Species Determine the Reaction Pathway

“…Additionally, the reaction pathway on gold electrodes is completely different from that observed on Pt. Oscillatory behavior has also been observed due to the different intermediates formed . Oscillatory behavior on Pt has also been reported for the SO 2 OR but for a modified Pt electrode .…”

“…Oscillatory behavior on Pt has also been reported for the SO 2 OR but for a modified Pt electrode . Recently, it was pointed out that in addition to the electrode material determining the reaction pathway and activity, the chaotropic nature of the supporting electrolyte plays a pivotal role for Au electrocatalysts . The oscillatory behavior and increased activity were observed in chaotropic media and not in kosmotropic electrolytes .…”

“…One interesting process for this assisted water electrolysis is the SO 2 oxidation reaction (SO 2 OR) . This pathway is not only an alternative pathway with a lower overpotential than the OER; it also presents an opportunity to consume SO 2 , a pollutant emitted by fossil fuel combustion, by generating H 2 SO 4 , a chemical of great industrial interest.…”

“…In previous publications, it has been shown that some of these statements are incorrect. The catalyst plays an important role in the process, with Au being the most active among the reported catalysts since it can achieve current densities orders of magnitude higher than Pt . Additionally, the reaction pathway on gold electrodes is completely different from that observed on Pt.…”

“…Recently, it was pointed out that in addition to the electrode material determining the reaction pathway and activity, the chaotropic nature of the supporting electrolyte plays a pivotal role for Au electrocatalysts . The oscillatory behavior and increased activity were observed in chaotropic media and not in kosmotropic electrolytes . Unexpected results were obtained for solutions of “neutral” chaotropicity because, in addition to the oscillatory behavior, the activity was the highest, indicating a volcano‐like profile for the SO 2 OR with respect to the electrolyte chaotropicity …”