Role of Photoinduced Charge Carrier Separation Distance in Heterogeneous Photocatalysis: Oxidative Degradation of CH₃OH Vapor in Contact with Pt/TiO₂ and Cofumed TiO₂-Fe₂O₃


As a means of testing the widely held premise of a positive correlation between photoinduced charge carrier separation distance (CCSD) and the photocatalytic activity of particulate photocatalysts, an investigation of the oxidative degradation of CH3OH vapor in contact with coatings of particulate Pt (0− 8 wt %)/TiO2 is combined with time-resolved photocharge (TRPC) measurements. Preliminary studies were also performed on several commercially available cofumed Fe2O3− TiO2 (0, 2, and 6− 8 wt % Fe) particulate photocatalysts. TRPC is capable of providing a noncontact electronically derived measure of CCSD. Contrary to common wisdom prevalent at the initiation of this study, Pt/TiO2 catalysts displaying the largest CCSD values do not correspond with highest photocatalytic efficiency for the oxidative degradation of CH3OH. Indeed the present investigation supports an extension of a “Russell-like” mechanism for oxidative degradation of organic molecules, where photoelectrons, e-, and photoholes, h+, each initiate the formation of interacting chemical intermediates, i.e. O2•- and HOCH2(OO), in close proximity on the catalyst surface, thereby reducing the need for surface diffusion over large distances for the reaction to proceed toward production of the observed HCOOCH3 product. This process competes with charge carrier recombination at small CCSD values, which yield the highest rates of CH3OH consumption and HCOOCH3 formation attained in this study. It is speculated that this extension of the Russell-like mechanism, as applied to the solid/vapor photocatalytic degradation of methanol, may also provide the basis for the maximum in the rate of degradation of aqueous solutions of dichloroacetic acid, using solgel prepared Fe2O3− TiO2 mixed oxide photocatalysts as a function of weight percent Fe.



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