Solid Phase Photocatalytic Reaction on the Soot/TiO₂ Interface:  The Role of Migrating OH Radicals

The photocatalytic degradation of flame-deposited hexane soot on TiO 2 films was carried out under UV illumination (λ > 300 nm) in air in order to investigate the behavior of photooxidants at the solid/solid (soot/TiO 2 ) interface. The photodegradation of the immobile soot layer on TiO 2 film was monitored by performing soot weight loss measurement, scanning electron microscopic (SEM) analysis, and gas-chromatographic CO 2 production measurement. The bulk of soot layer with ∼2 μm thickness was completely oxidized to CO 2 over 30 h irradiation. The presence of O 2 was essential in the photocatalytic soot oxidation. The SEM images of the irradiated soot films revealed that the gap distance between the edges of soot on glass and TiO 2 domains continuously increased with UV illumination up to 80 μm, implying that the active oxidants generated on the TiO 2 surface migrated across the gap into a remote soot domain to initiate the photocatalytic oxidation. The apparent activation energy for the photocatalytic soot oxidation was estimated to be 18.7 kJ/mol, which is largely ascribed to the diffusional activation of active oxidants because the reactant (soot) is immobile. Introducing water or 2-propanol vapor efficiently enhanced or reduced the photooxidation rate of soot since water or 2-propanol was the precursor or the scavenger of the main oxidant species, OH radical, respectively. The migrating nature of photooxidants (including OH radicals) should be taken into account in quantitative understanding of photocatalytic reaction mechanisms in all media (gas, liquid, and solid phases) in general.