Mechanism of cerium(III) oxidation with ozone in sulfuric acid solutions

Abstract

The kinetics of Ce(III) oxidation with ozone in 0.1–3.2 M H2SO4 solutions was studied by spectrophotometry. The reaction follows the first-order rate law with respect to each reactant. The rate constant k slightly increases with an increase in the acid concentration, which is associated with an increase in the O3/O3− oxidation potential. The activation energy in the range 17–35°C is 46 kJ mol−1. With excess Ce(III), the stoichiometric coefficient Δ[Ce(IV)]/Δ[O3] increases from 1.6 to 2 in going from 0.1 to 1–3.2 M H2SO4. The extent of the Ce(III) oxidation is 78% in 0.1 M H2SO4 and reaches 82% in 1 M H2SO4. The ozonation involves the reactions Ce(III) + O3 → Ce(IV) + O3−, O3− + H+ → HO3, HO3 → OH + O2, OH + HSO4− → H2O + SO4−, OH + Ce(III) → OH− + Ce(IV), and SO4− + Ce(III) → SO4/2− + Ce(IV). Low stoichiometric coefficient of the Ce(III) oxidation is associated with the hydrolysis of Ce(IV). The excited Ce(IV) ion arising from oxidation of Ce(III) with OH radical forms with the hydrolyzed Ce(IV) ion a dimer whose decomposition yields Ce(III) and H2O2. After the ozonation termination, Ce(IV) is relatively stable in sulfuric acid solution, with only 5–7% of Ce(IV) disappearing in 24 h.