Photocatalysis of Adsorbed Catechol on Degussa P25 TiO₂ at the Air-Solid Interface

Semiconductor photocatalysis with commercial TiO₂ (Degussa P25) has shown significant potential in water treatment of organic pollutants. However, the photoinduced reactions of adsorbed catechol, a phenolic air pollutant from biomass burning and combustion emissions, at the air-solid interface of TiO₂ remain unexplored. Herein we examine the photocatalytic decay of catechol in the presence of water vapor, which acts as an electron acceptor. Experiments under variable cut-off wavelengths of irradiation (λ_cut-off ≥ 320, 400, and 515 nm) distinguish the mechanistic contribution of a ligand-to-metal charge-transfer (LMCT) complex of surface chemisorbed catechol on TiO₂. The LMCT complex injects electrons into the conduction band of TiO₂ from the highest occupied molecular orbital of catechol by visible light (≥2.11 eV) excitation. The deconvolution of diffuse reflectance UV-visible spectral bands from LMCT complexes of TiO₂ with catechol, o-semiquinone radical, and quinone and the quantification of the evolving gaseous products follow a consecutive kinetic model. CO₂(g) and CO(g) final oxidation products are monitored by gas chromatography and Fourier-transform infrared spectroscopy. The apparent quantum efficiency at variable λ_cut-off are determined for reactant loss (Φ_{- TiO2/catechol} = 0.79 ± 0.19) and product growth Φ_CO2 = 0.76 ± 0.08). Spectroscopic and electrochemical measurements reveal the energy band diagram for the LMCT of TiO₂/catechol. Two photocatalytic mechanisms are analyzed based on chemical transformations and environmental relevance.