Strong Photon Energy Dependence of the Photocatalytic Dissociation Rate of Methanol on TiO₂(110)

Photocatalytic dissociation of methanol (CH₃OH) on a TiO₂(110) surface has been studied by temperature programmed desorption (TPD) at 355 and 266 nm. Primary dissociation products, CH₂O and H atoms, have been detected. The dependence of the reactant and product TPD signals on irradiation time has been measured, allowing the photocatalytic reaction rate of CH₃OH at both wavelengths to be directly determined. The initial dissociation rate of CH₃OH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm, suggesting that CH₃OH photocatalysis is strongly dependent on photon energy. This experimental result raises doubt about the widely accepted photocatalysis model on TiO₂, which assumes that the excess potential energy of charge carriers is lost to the lattice via strong coupling with phonon modes by very fast thermalization and the reaction of the adsorbate is thus only dependent on the number of electron–hole pairs created by photoexcitation