Enhancing the photocatalytic removal of toluene by modified porous TiO2 with internal hydrophobic interface

Abstract

The efficiency and stability of photocatalytic oxidation techniques in the treatment of volatile organic compounds (VOCs) in the air are affected by the adsorption performance at the reaction interface and the photogenerated electron–hole separation efficiency. Carbon-loaded porous TiO2 with an internal hydrophobic interface was prepared by finely controlling the calcination time and temperature of a titanium-based metal–organic framework (MIL-125). The carbon load in the porous TiO2 presents a gradient from the pore surface to the interior. The formed hydrophobic interface in the channel avoids competitive adsorption between gas-phase toluene and H2O and effectively improves photogenerated electron transfer. Experiments and comprehensive analysis by using techniques such as thermogravimetric analysis, Raman spectroscopy, and ultraviolet diffuse-reflectance spectroscopy clarified the mechanism of gradient carbon formation in the porous TiO2, in which the organic ligands in MIL-125 are carbonized during calcination because of the low oxygen content of the pores. The sample prepared by pyrolyzing MIL-125 at 400 ​°C for 1 ​h showed the best activity in the photocatalytic degradation of toluene; the activity was twice that achieved with the commercial photocatalyst P25. Surface photovoltage spectroscopy and photoelectrochemical measurements proved that the gradient carbon load enhances photogenerated electron transfer in the porous TiO2. In-situ diffuse-reflectance infrared Fourier-transform spectroscopy was used to investigate the processes of photocatalytic degradation of toluene, in which the generation of C–H-containing intermediates is a crucial step. This research provides a novel concept for the future design and modification of photocatalyst structures for the effective photocatalytic degradation of VOCs.