Kinetics And Mechanism Of Photo-assistant Ag（Ⅰ）-catalysed Water Oxidation With S₂O₈^2-

The kinetics of photo-assistant Ag（I）-catalysed water oxidation into O2 with S₂O₈^2-has been investigated, We found that when the concentration of Ag+ is less than 7.06 × 10-3 mol L-1, the O2-evolution rate under visible light illumination（λ ≥ 400 nm） is a first-order law with 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-1L h-1at 24.5oC and the activity energy is 3.7 × 104 J mol-1. Without illumination, k0 is 8.5±1 mol-1 L h-1 at 24.5 oC and the activity energy is 6.5 × 104 J mol-1. Compared with those without illumination, the rate constants under visible light are increased by ca. 3.8 mol-1L h-1at 4.5, 11.5, 17.5 and 24.5oC,it is found that the visible light can improve the evolution of O2 remarkably and which are hardly effected by the reaction temperature. Employing MS/MS, ESR, XRD and UV-visible spectroscopy, the intermediate species of {AgS2O8}-, Ag2+, OHË™, Ag2O3 and AgO+ in the process of water oxidation have been detected. Based on the experimental evidences, the mechanism of Ag（I）-catalysed water oxidation with S₂O₈^2- has been developed, in which the reaction（AgO+ + H2 O â†' Ag+ + H2O2） is considered as the rate-determined step. The increase of the O2-evolution rate under visible light illumination results from the absorbance of the AgO+ species at 375 nm.The relative average deviation for the detection of the O2 under the atmosphere of Ar and air has been also investigated. It is proved that the relative average deviation of the former is smaller than the latter, especially when the less O2 is generated.Ethylenebisbiguanide 1 and ethylenebisbiguanidesilver（ Ⅲ） 2 have been synthesized.Employing IR and NMR, the structure of 1 has been characterized. The electrochemistry property of 2 in 0.5 mol L-1H2SO4 solution has been investigated. At the potential of 0.4 – 1.0V（vs. Ag/AgCl）, a pair of reversible redox peaks is obtained, which is attributed to an electron redox process of Ag3+/Ag2+. At the potential of-0.4 – 0.5 V（vs. Ag/AgCl）, an anode peak is obtained and the electron transfer number n ≈ 2, which is attributed to two electrons oxidation process of Ag2+/Ag.