Dissociative adsorption of H2O2 on TiO2(110) surface for advanced oxidation process

Abstract

Advanced oxidation processes (AOPs) involving the conjugation of H2O2 with metal oxide catalysts such as TiO2 have been studied for a long time because they enable efficient degradation of pollutants in wastewater. The combination of H2O2 and TiO2 is well known to generate oxidizing agents such as (OH)-O-center dot and0( center dot)O(2)(-) radicals by catalytic reactions. However, the reaction mechanism for the production of these radicals is controversial. Here, we investigated the H2O2-dosed surface of rutile TiO2(110) by low-temperature scanning tunneling microscopy (LT-STM). We successfully probed the intermediate step of AOP at the single-molecule level. Ti-O-O-Ti peroxides were formed on the surface of aqueous H2O2 vapor-dosed TiO2 (110), whereas H2O molecules were hardly found on the surface until the dosing concentration of H2O2 exceeded 0.04 Langmuir, although 30% aqueous H2O2 solution was used. H2O2 could be adsorbed only at the oxygen vacancy, which limited the number of Ti-O-O-Ti peroxide molecules. The formed peroxides could generate (OH)-O-center dot radicals by further reaction with H2O molecules. Direct observation of the intermediate step of AOP upon adsorption of H2O2 molecules enabled us to understand the mechanism of the (OH)-O-center dot radical-generating reaction.