Photocatalysis, despite its advantages, presents slow reaction rates. Thus efforts have focused on the improvement of photocatalytic reactors designs and on the enhancement of photocatalytic activity through various techniques, which include the addition of metallic ions to reduce the recombination of charges (h+/e-).

Some metals used as reaction enhancers, including Cd, Cr, Ag, among others, show a significant improvement by trapping the photo generated electron e-. These metals however, are invariably equally or more harmful than the targeted pollutants and their effect on the rate of reaction stops when they are reduced to the next oxidation state (e.g. Ag+1 to Ag0).

In previous studies (Ortiz-Gomez et al 2006), it was demonstrated that Fe can be used as reaction enhancer in photocatalytic reactions when added in low concentrations.

In the present study it is shown that Fe behaves differently from other metals used as reaction enhancers in photocatalytic reactions (Cd, Cr, Ag, etc), given that the effect of Fe as Fe3+ is the same as that of Fe as Fe2+. Moreover, given that the starting oxidation state does not affect its effect on the oxidation rate, it is observed that the effect of Fe lasts throughout the reaction time, unlike the other metals that inhibit the reaction once they are reduced and deposited over the catalyst surface. The effect of Fe ions was observed on the oxidation of a wide variety of organic compounds, including phenolic compounds, carboxylic acids and pesticides. This reveals yet another advantage of Fe over other metals aside from being much more environmentally friendly and relatively inexpensive.

The experimental data reveal as well that when Fe is added as Fe3+, it is quickly reduced to Fe2+ once the photocatalytic reaction starts. Also, when added as Fe2+, it remains in the same oxidation state. Additionally, colorimetric and spectroscopic analyses (EDX, XPS) show that Fe is adsorbed on TiO2 during the “dark period” (a period given for adsorption of reactants onto catalyst surface). Furthermore, these analyses reveal that a fraction of Fe remains adsorbed onto the catalyst surface throughout the reaction period.

Based on all the above observations, a mechanism for the effect of Fe on photocatalytic reactions is proposed. It is hypothesized the Fe enhances the reaction rate by modifying the mechanism that takes place over the catalyst surface. At low concentrations, Fe facilitates the transfer of the photo generated electron to an electron scavenger (e.g. O2) by undergoing a quick reduction-oxidation reaction. The overall effect is a reduction of the h+/e- recombination rate. At high concentrations, Fe ions adsorbed on catalyst surface become recombination centers, thus increasing the h+/e- recombination rate and reducing the overall reaction rate. All experimental observations support this hypothesis. A similar mechanism was proposed for the effect of Pt over TiO2 (Hufschmidt et al, 2002).

Hufschmidt et al (2002). Journal of Photochemistry and Photobiology A: Chemistry 148, 223-231.

Ortiz-Gomez et al 2006, Enhancement of Photcatalytic Mineralization of Phenol in the Presence of Ferric Ions: Kinetic Modelling and Parameter Estimation, AIChE Annual Meeting, San Francisco, Nov 12-17, 2006.

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