The kinetics and mechanism of photo-assisted Ag(I)-catalysed water oxidation with S2O8(2-).
The kinetics of photo-assisted Ag(I)-catalysed water oxidation into O2 with S2O8(2-) 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 S2O8(2-), respectively. The rate law is expressed as -dc(S2O8(2-))/dt = 2dc(O2)/dt = kLc(S2O8(2-))c(Ag(+)), where kL is 12.4 ± 1 mol(-1) L h(-1) at 24.5 °C and the activation energy is 3.7 × 10(4) 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}(-), Ag(2+), 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 S2O8(2-) 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.
Institute of Chemistry for Functionalized Materials, Liaoning Normal University, Dalian, 116029, P. R. China. wsyou@lnnu.edu.cn.
Lihong
L
Yu
Jidan
J
Wang
Dan
D
Guo
Wansheng
W
You
Meiying
M
Liu
Lancui
L
Zhang
Can
C
Li