Operando FTIR study of the photocatalytic oxidation of methylcyclohexane and toluene in air over TiO2–ZrO2 thin films: Influence of the aromaticity of the target molecule on deactivation

The main objective of the present study is to gain further insight into the mechanism of toluene and methylcyclohexane photocatalytic oxidation on TiO2–ZrO2 thin-films, in order to understand the influence of the chemical nature of the pollutants on the deactivation process. Despite the significant differences in the physicochemical characteristics of these two molecules, the parallel study of their photocatalytic degradation can contribute to ascertain relevant aspects of the mineralization mechanism. This knowledge could be used for developing feasible solutions for this deactivation problem, which hinders a wider use of photocatalysis for air purification. The operando study of toluene photooxidation, a simultaneous analysis of surface and gas phase during reaction by using two FTIR spectrometers in tandem, allowed us to correlate the deactivation process with the formation of benzoic acid, strongly adsorbed on the surface as benzoate complexes. These species block the access of new organic molecules to the active sites and simultaneously hinder charge transfer from the photoactivated surface. As a consequence, the degradation rate of toluene considerably decreases after a few minutes of reaction. In contrast, intermediate surface complexes can be almost completely removed during the photocatalytic oxidation of methylcyclohexane. Therefore, it seems that the aromaticity of toluene plays a key role in the performance of photocatalysts during its degradation. The high stability of benzyl radicals favors the photocatalytic oxidation of this volatile organic compound (VOC) and the formation of recalcitrant oxygenated aromatic molecules which accumulate on the photoactive surface.