Inhibit the formation of toxic methylphenolic by-products in photo-decomposition of formaldehyde–toluene/xylene mixtures by Pd cocatalyst on TiO₂

Photocatalytic removal of single volatile organic compounds (VOCs) has been widely investigated; however, photodecomposition of VOC mixtures has been rarely addressed, which may bring safety doubts in indoor air purification due to possible formation of harmful compounds. Here we show that in photocatalytic oxidation of formaldehyde–toluene and formaldehyde–xylene mixtures, the introduction of Pd cocatalyst on TiO₂ photocatalyst successfully inhibits the formation of toxic methylphenols, thus promoting the complete mineralization of VOC mixtures into CO₂ via the harmless benzaldehyde intermediates. Mechanistic analysis reveals that the loading of Pd cocatalyst effectively removes the inherent surface −OH groups of TiO₂, which significantly promotes the activation of O₂ into ^[rad]OH radicals. The Pd cocatalyst also directs the ^[rad]OH radicals to attack the methyl group instead of the aromatic ring for the formation of benzaldehyde and its further oxidation to CO₂, thus yielding a better overall photocatalytic performance.