Reduction of the photoexcited uranyl ion by water1. UIV and H2O2 formation

Photooxidation of water by the uranyl ion was studied. Solutions of uranyl in 0.01–4.0M H2SO4, HClO4, or 0.1–1.0M Na2SO4 and NaClO4 containing “lacunary” heteropolytungstate (HPT) K10P2W17O61 or K8SiW11O39 were irradiated with a nitrogen laser, a mercury or xenon lamp, or visible light. Spectrophotometric analysis showed that the irradiation results in the accumulation of UIV. Simultaneously the formation of H2O2 proceeds. The quantum yield Φ of the reaction increases as the concentration of the acid or salt increases. For aerated solutions of 1M H2SO4 or 1M HClO4, irradiation by light with λ=337.1 Φ is close to (1.5–2)·10−3. The irradiation of solutions with pH −4 for many days leads to an almost quantitative transformation of UO22+ into UIV. When the irradiation was carried out in the absence of HPA, UIV was not detected, although hydrogen peroxide was observed in the solution.