Photocatalytic degradation of gaseous VOCs over Tm3+-TiO2: Revealing the activity enhancement mechanism and different reaction paths

Abstract In this work, we synthesized thulium (Tm3+) modified TiO2 nanoparticles by a facile sol-gel route. We show that Tm3+-modified TiO2 exhibit superior photocatalytic activity than pure TiO2 towards gas-phase degradation of acetaldehyde, o-xylene, and their mixture. The photocatalytic activity enhances with the increasing Tm content from 0.1 to 0.5 mol%. The 0.5 mol%Tm-TiO2 sample demonstrated the highest photodegradation efficiency (>90.0%) for acetaldehyde and o-xylene. According to the systematic characterization such as photoluminescence (PL), photocurrent, up-conversion spectra, UV–Vis diffuse reflectance spectroscopy and electron spin resonance (ESR) analysis, the enhanced photocatalytic performance can be attributed to introducing the new Tm-4f states between the original band gap of TiO2, which led to the increase of efficiency of light-induced electron-hole separation and the increase of hydrophilicity which resulted in the increased adsorption capacity towards acetaldehyde and o-xylene. Gas Chromatography-Mass Spectrometer (GC–MS) analysis suggested that o-xylene was first oxidized to o-benzaldehyde, which was eventually converted to butanol before being mineralized into CO2. However, new intermediates including phthalide, o-methyl acetophenone, naphthalene and 5-Methyl-4-octanone were detected in the mixture.