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

The kinetics of photo-assisted Ag(I)-catalysed water oxidation into O-2 with S2O82- has been investigated. When the concentration of Ag+ is less than 7.06 x 10(-3) mol L-1, O-2-evolution under visible light illumination (lambda >= 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(O-2)/dt = k(L)c(S2O82-)c(Ag+), where k(L) is 12.4 +/- 1 mol(-1) L h(-1) at 24.5 degrees C and the activation energy is 3.7 x 10(4) J mol(-1). It is found that visible light can improve the evolution of O-2 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 degrees 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 O-2-evolution rate under visible light illumination results from the absorbance of the AgO+ species at 375 nm, promoting the rate-determining step.