Insights into the surface Structure-Sensitive photocatalytic oxidation of gaseous toluene on Pd/TiO2 catalysts

Indoor volatile organic compounds (VOCs), represented by toluene, can pose a serious threat to human health. In this work, a series of Pd/TiO2 catalysts with different surface structures (i.e., dispersion/valence of Pd species, surface Ti3+/oxygen defect concentration and Pd-TiO2 interactions, etc.) was constructed by various methods. It was found that the Pd/TiO2 catalyst prepared by one-step reduction of NaBH4 method (Pd/TiO2-N) with metallic Pd particles possessed more surface Ti3+/oxygen defects and stronger Pd-TiO2 interaction than Pd/TiO2 catalysts prepared by ethylene glycol reduction loading method (Pd/TiO2-E) and incipient wetness impregnation method (Pd/TiO2-I). Further photochemical characterizations and reaction mechanism study revealed that the relatively higher concentration of Pd0 species on Pd particles, stronger Pd-TiO2 interaction, and more surface Ti3+/oxygen defects on Pd/TiO2-N could facilitate the generation of more reactive oxygen species (ROS) under light irradiation. The partial oxidation of toluene to benzyl alcohol/benzaldehyde/benzoic acid and the subsequent ringopening process were found to be highly determined by the valence states of Pd particles and the concentration of surface Ti3+/oxygen defects. This work provides instructive insights into the construction of highly efficient Pd/TiO2 catalysts for the removal of indoor VOCs.