Formation of excited uranyl ion in oxidation of U(IV) with xenon difluoride in aqueous solutions of sulfuric and deuterated sulfuric acids

Abstract

Kinetic features of the chemiluminescence accompanying oxidation of U(IV) with xenon difluoride in solutions of 0.2 M H₂SO₄ in H₂O and 0.2 M of D₂SO₄ in D₂O at temperatures within 283–313 K and concentrations (M) 10⁻⁶ ≤ [U(IV)] ≤ 10⁻⁴ and 10⁻⁴ ≤ [XeF₂] ≤ 10⁻³ were examined. The shape of the kinetic curve depends on the concentration of the reactants and the solution temperature. At [U(IV)] = 10⁻⁶ and [XeF₂] = 10⁻⁴ M, the curve exhibits a maximum, more prominent at low temperatures. At relatively high concentration ([U(IV)] = 10⁻⁴ and [XeF₂] = 10⁻³ M), self-acceleration of the reaction is observed: The mechanism of oxidation of U(IV) turns into a branched-chain mode owing to a higher concentration in the solution of radicals ^·OH, HO ^·₂ , and XeF ^·₂ , whose additional amounts are yielded by hydrolytic reduction of XeF₂. A kinetic isotope effect of the solvent k _H/k _D was revealed, which attains a maximum of 1.8 at 283 K. The activation energies of the oxidation of U(IV) with xenon difluoride in H₂SO₄ and D₂SO₄ solutions were estimated at E _a,H = 12 and E _a,D = 13 kcal mol⁻¹, respectively. The occurrence of the isotope effect is an indirect evidence of participation in the reaction of the OH (or OD) groups. The rate of hydrolytic reduction of XeF₂ in deuterated solvent (0.2 M D₂SO₄ in D₂O) in its photochemical stage is several times lower, and the luminescence accompanying the reaction is by an order of magnitude smaller than that in 0.2 M H₂SO₄.