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

Abstract

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 CH 3 OH vapor in contact with coatings of particulate Pt (0−8 wt %)/TiO 2 is combined with time-resolved photocharge (TRPC) measurements. Preliminary studies were also performed on several commercially available cofumed Fe 2 O 3 −TiO 2 (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/TiO 2 catalysts displaying the largest CCSD values do not correspond with highest photocatalytic efficiency for the oxidative degradation of CH 3 OH. 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 . O 2 •- and HOCH 2 (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 HCOOCH 3 product. This process competes with charge carrier recombination at small CCSD values, which yield the highest rates of CH 3 OH consumption and HCOOCH 3 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 sol − gel prepared Fe 2 O 3 −TiO 2 mixed oxide photocatalysts as a function of weight percent Fe.