Surface modification of rhodium-doped strontium titanate by adsorption of cobalt clathrochelates for water photodissociation

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.

Photo-electrochemical water splitting was first put into evidence and investigated by Honda and Fujishima in 1972. Since then, a large variety of semi-conducting materials, that can absorb either UV or visible light and that have appropriate band edge features, have been synthesized and characterized. The purpose of this communication is to report on the photodissociation of water using Strontium Titanate as photomaterial. Rhodium doping has been used to modify the band structure of the perovskite and extend its light absorption range towards visible light. Methanol has been used as sacrificial agent (the overall reaction under investigation is the endergonic methanol steam photo-reforming reaction). Experiments have been performed at close-to-room temperature under visible light irradiation. The rates of water photo-reduction into hydrogen and methanol oxidation into carbon dioxide have been determined by gas chromatography analysis. Negligible amounts of hydrogen and carbon dioxide were found to form on pristine samples. However, the kinetics of the reaction was found to increase by surface deposition of metallic platinum nano-particles that act as surface co-catalyst for either water reduction into hydrogen or methanol oxidation reaction, or both. This is an indication that charge transfer processes at the semi-conductor/electrolyte interface can be considered (at least partly) as rate-determining. In addition to hydrogen and carbon dioxide, other oxidation by-products have been detected but not carbon monoxide, a poison that could potentially adsorb on Pt surfaces and inhibit reaction sites. In order to gain a better understanding of co-catalytic phenomena, we also investigated the co-catalytic effect of cobalt-clathrochelates that are known to be efficient electrocatalysts for the hydrogen evolution reaction but poor electrocatalysts for methanol oxidation. It is found that that the presence of cobalt complexes has also a positive catalytic effect on the kinetics of the reaction but a loss of performance has been observed with time. This can be attributed to either inhibition of methanol oxidation or surface contamination by photo-degradation products.