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/O ⁻₃ 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) + O ⁻₃ , O ⁻₃ + H⁺ → HO₃, HO₃ → OH + O₂, OH + HSO ⁻₄ → H₂O + SO ⁻₄ , OH + Ce(III) → OH⁻ + Ce(IV), and SO ⁻₄ + Ce(III) → SO_4/2− + 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.