The kinetics and mechanism of photo-assisted Ag(i)-catalysed water oxidation with S2O82−

Abstract

The kinetics of photo-assisted Ag(I)-catalysed water oxidation into O₂ with S₂O₈²⁻ has been investigated. When the concentration of Ag⁺ is less than 7.06 × 10⁻³ mol L⁻¹, O₂-evolution under visible light illumination (λ ≥ 400 nm) obeys the first-order rate law with respect to the concentrations of Ag⁺ and S₂O₈²⁻, respectively. The rate law is expressed as −dc(S₂O₈²⁻)/dt = 2dc(O₂)/dt = k_Lc(S₂O₈²⁻)c(Ag⁺), where k_L is 12.4 ± 1 mol⁻¹ L h⁻¹ at 24.5 °C and the activation energy is 3.7 × 10⁴ J mol⁻¹. It is found that visible light can improve the evolution of O₂ remarkably. Compared with those without illumination, the rate constants under visible light are increased by ca. 3.8 mol⁻¹ L h⁻¹ at 4.5, 11.5, 17.5 and 24.5 °C, which are hardly affected by the reaction temperature. Employing MS/MS, ESR, XRD and UV-visible spectroscopy, the intermediate species {AgS₂O₈}⁻, Ag²⁺, OH˙, Ag₂O₃ and AgO⁺ in the process of water oxidation have been detected. Based on the experimental evidence, the mechanism of Ag(I)-catalysed water oxidation with S₂O₈²⁻ has been developed, in which the reaction (AgO⁺ + H₂O → Ag⁺ + H₂O₂) is considered as the rate-determining step. The increase of the O₂-evolution rate under visible light illumination results from the absorbance of the AgO⁺ species at 375 nm, promoting the rate-determining step.