390d Applicability of the Fe-Assisted Photocatalytic Reactions to Dissimilar Compounds: Positive Effect of Fe Ions on Mineralization Rates of Maleic Acid, Oxalic Acid, Formic Acid, Paraquat, Phenol and 4-Chlorophenol

Aaron Ortiz Gomez, Chemical and Biochemical Department, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B8, Canada, Benito Serrano, Chemical Engineering, Universidad Autonoma de Zacatecas., Campus Siglo XXI, Edificio 6, Ejido La Escondida,, Carretera Zacatecas Guadalajara, Km. 6, Zacatecas Zacatecas., 98000, Mexico, Jesus Moreira del Rio, Chemical and Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B8, Canada, and Hugo de Lasa, Chemical and Biochemical Engineering, The University of Western Ontario, London, ON N6A 5B9, Canada.

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-).

The activity of a photocatalyst is due to its ability to promote the formation of non-selective hydroxyl radicals upon being irradiated with UV light (Fujishima et al., 2000). In previous studies (Ortiz-Gomez et al, 2007) it was demonstrated that the addition of Fe ions accelerates the rate of mineralization of various hydroxylated aromatics (phenolic compounds). As a result, a reaction mechanism was proposed in which the effect of Fe ions was due to the decrease in the h+/e- recombination, which in turn leads to higher catalyst activity.

Given that hydroxyl radicals are non-selective, the same behavior of the Fe-assisted PC reactions is expected in the oxidation of other organic compounds with dissimilar molecular structure. To provide supporting evidence that the proposed mechanism applies to all photocatalytic reactions, experiments with maleic acid, oxalic acid and formic acid, as well as with a toxic herbicide, paraquat, were performed. These runs included both photocatalytic reactions with no Fe (unpromoted PC reactions) and photocatalystic reactions assisted with Fe ions (Fe assisted PC reactions). These experiments were carried out in a 6-L photoreactor, the Photo-CREC photocatalytic reactor. In order to determine that the enhancement was due to the addition of Fe and not a function of the reactor size or design, a 16-L reactor, the Photo-CREC Solar Simulator, was also employed. In this case, phenol and 4-chlorophenol were selected as model pollutants for both unpromoted and Fe-assisted PC reactions.

It is demonstrated that in all cases Fe-assisted PC oxidations using 10 ppm Fe ions were faster than the unpromoted PC reactions. This is shown using model pollutants with very different chemical structures (short-chain carboxylic acids containing 1, 2 and 4 carbons, and a pesticide with a two aromatic ring structure). It is also demonstrated that similar enhancements in the mineralization rates were obtained in reactions performed in the second reactor setup, whose dimensions and characteristics are markedly different from the Photo-CREC unit with a 6-liter capacity.

Overall quantum yields (OQY) were calculated to measure the extent of influence of Fe in these photocatalytic reactions. In all cases the OQY were calculated taking the formation of CO2 molecules at 99 % conversion of the model pollutant as a basis for comparison. For the carboxilyc acids the increase in OQY was above 50% while for the paraquat a smaller percentage is observed (between 23 and 32%). Similar results were obtained for experiments performed with phenol and 4-chlorophenol in the Photo-CREC Solar Simulator.

These results, along with colorimetric and spectroscopic analyses (EDX, XPS), confirm that Fe enhances the reaction rate by increasing the photo-catalyst activity regardless of the model pollutant. The enhancement is seen on simple molecules (carboxylic acids) and complex molecules (paraquat), and observed also on a different reactor setup up with different model pollutants (4 phenol and 4-chlorophenol). This enhancement is explained due to lower recombination rates of h+/e-, which lead to higher catalyst activities.

References.

Fujishima et al (2000), Titanium dioxide photocatalysis. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 1, 1-21.

Ortiz-Gomez et al (2007), Enhanced Effect of Fe3+ and Fe2+ ions on the photocatalytic oxidation of organic compounds: Reaction Mechanism over the TiO2 surface., AIChE Annual Meeting, Salt Lake City, Nov. 4-9, 2007.