INITIAL REACTION STEPS IN PHOTOCATALYTIC OXIDATION OF AROMATICS

Transient reaction at 273 and 300 K was used to study the initial step s in the photocatalytic oxidation (PCO) of benzene, toluene, p-xylene, mesitylene, benzyl alcohol, benzaldehyde, and m-cresol adsorbed on a thin film of TiO2 catalyst. Adsorbed aromatics were oxidized by O-2 ph otocatalytically in the absence of gas-phase aromatics, and the compou nds remaining on the surface were characterized by temperature-program med oxidation and desorption (TPO, TPD). Benzene and methyl benzenes o xidize rapidly at 273 or 300 K to form adsorbed intermediates that are more strongly adsorbed and much less reactive than the original aroma tic, which is relatively weakly adsorbed on TiO2. The catalyst is expe cted to be covered with these intermediates during steady-state reacti on. The rates of PCO of benzene and methyl benzenes to CO2 are slow re lative to complete oxidation of alcohols or chlorinated hydrocarbons. The intermediates do not appear to be alcohols or aldehydes formed by oxidation of a methyl group, nor do they correspond to addition of an hydroxyl to the aromatic ring. Benzyl alcohol oxidizes photocatalytica lly to benzaldehyde and then to CO2 and H2O during PCO, but adsorbed m -cresol does not photocatalytically oxidize.