Light-induced reduction of rhodium(III) and palladium(II) on titanium dioxide dispersions and the selective photochemical separation and recovery of gold(III), platinum(IV), and rhodium(III) in chloride media

Irradiation of aqueous TiO2 dispersions containing palladium(II) or rhodium(III) chloride salts with AM1 simulated sunlight leads to the photoreduction of these metals, which are deposited on the semiconductor particle surface. Oxygen is detrimental to the photoreduction of rhodium(III) but not to the photoreduction of palladium(II). However, in both cases the reduction process is most efficient if the solution contains CH3OH, which acts to scavenge valence band holes of the illuminated TiO2 semiconductor. The selective photoreduction and recovery of precious metals from a dilute solution (as might be found in industrial wastes) have been investigated for a mixture of gold(III), platinum(IV), and rhodium(III) chloride salts as a function of various parameters (pH, presence or absence of O2, presence or absence of a hole scavenger, and the concentration of the semiconductor). At pH 0, gold is easily separated from platinum and rhodium. The rate of photoreduction of gold(III) on TiO2 is nearly independent of the concentration of the semiconductor, under the experimental conditions employed; the limiting rate is 2.7 × 10-7 M s-1. The potential utility of this selective photochemical technique is discussed. © 1986 American Chemical Society.