Electrochemical Reactivity of Strong Sulfuric Acid: Some Unusual Behavior at Pt Electrodes

Abstract

In very strong, 98%, sulfuric acid cyclic‐voltammetry experiments at Pt reveal unusual reduction and oxidation processes as distinct from those associated with electrodeposition and ionization of atomic H, and surface oxide formation and reduction, which are familiar in dilute aqueous solutions of at Pt. No reduction processes are observed on a less electrocatalytic metal, Au, under the same conditions. Holding the potential of a Pt electrode near the potential in 98% gives rise to reduction of the acid, producing a species that is immediately chemisorbed and becomes oxidized in the adsorbed state in a following anodic potential sweep over the “surface oxide formation” potential region. In the succeeding cathodic sweep, a large cathodic current peak follows surface oxide reduction. By means of chemical simulation experiments, it is shown that the reduction behavior is accounted for first by formation of or at potentials near , followed by oxidation to a species, possibly adsorbed dithionate , or some other chemisorbed sulfur‐oxygen species, in a following anodic sweep. This oxidation process cannot be from or back to or since, in the next cathodic sweep, a re‐reduction occurs in a well‐defined peak, a little positive to . Experiments at a rotated Pt disk electrode enable distinctions to be made between processes associated with strongly bound chemisorbed species and other species that can be spun off into solution, giving diminished currents in successive cyclic voltammograms. Additions of small quantities of water diminish the reduction reaction observed in 98% .