Transient photocatalytic oxidation of toluene vapor via nanostructured layers prepared by sol-gel on anatase particles

Abstract

Gas-phase photocatalytic degradation of toluene mixed with wet oxygen was performed in a continuous reactor using anatase nanoparticles, home-prepared by the sol-gel method combined with peptization treatments. Titanium (IV) isopropoxide was used to synthesize amorphous titania precursor, partially transformed into crystalline nanoparticles by hydrothermal or thermal treatments. The main oxidation product was carbon dioxide, but traces of benzene and benzoic acid were also detected. In order to characterize the catalysts, the evolution of titanium (IV) alkoxide to form anatase nanoparticles was studied using 1H-MAS NMR, EPR, TPD, and FTIR techniques. The results indicate that the four coordinated Ti4+ cations of the alkoxide induce the formation of amorphous aggregates of polymeric layers. The organics removal by peptization originates thin nanostructured titania layers that cover anatase particles, forming TiO2 homojunction. Photoactivity runs showed that both toluene retention and mineralization occurred at the start of irradiation, both processes exhibiting a long transient. After different times, retention stopped while mineralization reached steady-state conditions. Hydrated proton structures are considered to be the origin of both processes. Under irradiation, water photodesorption from these structures favors both the interaction of nanostructured layers with anatase particles, determining hole transfer to external surfaces and the generation of hydrated hydronium cations which, depending on the water loss, are centers able to carry out toluene mineralization or retention. After the mineralization step, photocatalytic centers must be reconstituted by recovering water, a process whose rate becomes the rate-determining one.