Oligomerization of chromium(iii) and its impact on the oxidation of chromium(iii) by hydrogen peroxide in alkaline solutions

Abstract

Monomeric, dimeric and trimeric chromium(III) species in solution were separated by ion exchange and characterized with UV/Vis absorption and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The kinetics of the oxidation of the separated species by hydrogen peroxide in alkaline solutions were studied by conventional and stopped-flow UV/Vis absorption spectroscopy. Results indicate that the intensity of Cr–Cr scattering in the EXAFS spectra (d_Cr–Cr ∼ 2.99 Å), a measure of the degree of oligomerization, increases as the solution alkalinity is increased. As the oligomerization proceeds, the rate of oxidation by hydrogen peroxide in alkaline solutions decreases in the order: monomer > dimer > trimer > aged/unseparated alkaline chromium(III) solution where higher oligomers dominate. The dominant redox pathway has an inverse order with respect to C_NaOH. The data suggest that the rate-determining step involves the weakening of the bridging bonds in the oligomer and a concomitant release of one hydroxyl group from the chromium(III) moiety upon the attack by hydrogen peroxide.