Reaction Steps in Heterogeneous Photocatalytic Oxidation of Toluene in Gas Phase-A Review
Résumé
A review of the current literature shows there is no clear consensus regarding the reaction mechanisms of airborne aromatic compounds such as toluene by photocatalytic oxidation. Potential oxidation reactions over TiO 2 or TiO 2-based catalysts under ultraviolet and visible (UV/VIS) illumination are most commonly considered for removal of these pollutants. Along the pathways from a model pollutant, toluene, to final mineralization products (CO 2 and H 2 O), the formation of several intermediates via specific reactions include parallel oxidation reactions and formation of less-reactive intermediates on the TiO 2 surface. The latter may occupy active adsorption sites and causes drastic catalyst deactivation in some cases. Major hazardous gas-phase intermediates are benzene and formaldehyde, classified by the International Agency for Research on Cancer (IARC) as Group 1 carcinogenic compounds. Adsorbed intermediates leading to catalyst deactivation are benzaldehyde, benzoic acid, and cresols. The three most typical pathways of toluene photocatalytic oxidation are reviewed: methyl group oxidation, aromatic ring oxidation, and aromatic ring opening.
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