Activation of periodate by self-recycled Ru(III)/TiO2 for selective oxidation of aqueous organic pollutants: Essential role of homogeneous Ru(V)=O species

Abstract

Recently, metal catalysts have received much attention in periodate (PI) activation for production of reactive oxygen species to efficiently degrade organic contaminants, yet activation efficiency is cumbered with their sluggish catalytic cycle. In the study, TiO2 supported Ru(III) catalysts (Ru(III)/TiO2, 0.45 wt% as Ru) were used to catalyze PI activation for oxidation of aqueous organic pollutants. Ru(III)/TiO2 exhibited comparable performance with homogeneous Ru(III) for phenol degradation via PI activation at initial pH = 4, 7 and 10. Moreover, the Ru(III)/TiO2-PI system showed high selectivity for degradation of electron-rich organic contaminants. In comparison, benzoic acid and ibuprofen were hardly oxidized in the system. Scavenging tests, H218O-isotope labeled methyl phenyl sulfoxide (PMSO) probe experiments, X-ray absorption near-edge structurespectra and Electron Spin Resonance analysis confirm formation of Ru(V) = O species as dominant oxidant in the process with little involvement of IO3, OH and O2−. The formed Ru(V) = O species were further confirmed to be water-soluble via analysis of Ru distribution between aqueous solution and TiO2surface. They can be reduced to Ru(III) species by reacting with organics, realizing the catalytic cycle of Ru(III) → Ru(V) = O → Ru(III) and self-recycle of Ru(III) onto TiO2surface. In addition, PI was decomposed in stoichiometry into IO3–without generation of undesired iodine species (i.e., HOI and I2). This study advances understandings of PI activation mechanism by heterogeneous Ru(III) catalysts, and also provides a promising oxidation technology for wastewater purification.