Water Oxidation to Dioxygen by Ru(bpy)33+ in the Presence of Colloidal Fe and Co-Hydroxide Catalysts. Kinetics Study and Mechanistic Insights
1st International Conference on Reaction Kinetics, Mechanisms and Catalysis
06-09 Jun 2018
||oxygen evolution reaction, oxidation of water, hydroxides, colloids, stop-flow
|| Chikunov Andrey Sergeevich
, Taran Oxana Pavlovna
, Kovalʹ Vladimir V.
, Parmon Valentin Nikolaevich
Boreskov Institute of Catalysis SB RAS
Novosibirsk State Technical University
Institute of Chemical Biology and Fundamental Medicine SB RAS
The development of efficient catalysts for water oxidation reaction (WOR) is a key point to a technology of the solar energy conversion into the energy of chemical bonds. Detail appreciation of WOR kinetics and mechanism is necessary to the design the efficient catalytic systems for this process . The main obstacle to study of kinetic regularities is a short reaction time (of the order of milliseconds). Here we suggest the colloidal Fe-, Co-hydroxide catalysts stabilized by dextranized starch and “stop-flow” method to overcome these difficulties. The reaction kinetics were registered on two wavelengths (452 and 675 nm) in the most efficient conditions for O2 releasing. The second order of the reaction was revealed. The observed turnover frequencies were 14 and 60 mol O2 per [catalyst] s-1 for Fe and Co-contained catalysts, respectively. The observed activation energy (278-308 К) appeared to be 10.8 and 11.1 kkal mol-1 for Fe- and Co-catalysts, respectively. After consuming of 90 % of Ru(bpy)33+ we observed the formation of long-lived intermediate on 675 nm. The detailed study by use of LED matrix in vide wavelength range (250-780 nm) revealed that observed intermediate is a product of interaction between catalyst and oxidant (absorbance maximum 550-600 nm). The constant of intermediate consumption was found to be lower by 2-3 orders than the effective constant of oxygen releasing. Thus, the observed compound cannot participate in oxygen releasing reactions. We suggested that observed intermediate is μ-peroxo species on the surface of the catalysts, which are unable to release molecular O2 but able to oxidize organic ligands of Ru(bpy)3 3+/Ru(bpy)3 2+. Based on the obtained kinetic data we proposed the reaction scheme which include two main routes of Ru(bpy)3 3+ consumption – the oxidation of water and the destruction of the organic ligands.