Photocatalytic reduction of [RhCln(H2O)6−n]3−n(n = 0–6) in a titanium dioxide suspension: the role of structurally different sacrificial reducing agents

Abstract

The photocatalytic reduction of rhodium(III) chloro–aqua complexes, [RhCln(H2O)6−n]3−n (n = 0–6), was carried out employing formic acid, ethanol and sucrose as hole scavengers or sacrificial reducing agents (SRA). The investigation was conducted in air equilibrated titanium dioxide (TiO2) suspensions at 25 °C and pH 1employing UV light within a closed-loop flow-through reactor. Varying the sodium chloride concentration, according to speciation data, individual solutions of rhodium(III) chloro–aqua complexes were prepared at those Cl−-concentrations coinciding with the maximum of a specific species, and to that regard the photocatalytic reduction behaviour of Rh(III) was related to the complex present in greater abundance. The fastest photocatalytic reduction rate was observed for those solutions having neutral and anionicrhodium(III) chloro–aqua complexes dominant in solution. No photocatalytic reduction, of any of therhodium(III) chloro–aqua complexes, was effected in the absence of an organic SRA, which is in contrast to [PdCln(H2O)4−n]2−n (n = 0–4) and [PtCln(H2O)4−n]2−n (n = 0–4) that do show photocatalytic reduction in the absence of an added organic SRA. At low concentrations sucrose was found to be a superior SRA compared to both ethanol and formic acid. This behaviour is attributed to the structural differences of the SRAs as sucrose has more oxidizable electron donor sites, in the form of hydroxyl groups, compared to both ethanol and formic acid. However, at higher concentrations ethanol was found to be a more effective SRA than both formic acid and sucrose. This is due to coagulation of the photocatalyst (TiO2) setting in at higher sucrose concentrations, which causes the photocatalyst to settle out from suspension