(68c) Kinetics of Organic Dye Removal From Photocatalytic, Self-Cleaning Titania Surfaces

Photocatalyzed removal of thin organic films has been explored for dyes and long chain fatty acids, as well as various synthetic polymers, bacterial cells, and soot.   We demonstrate here simple kinetic models for dye removal, and show examples which exhibit intrinic and disguised kinetics, depending on the porosity of the photocatalyst.

            We earlier explored decolorization kinetics of organic dyes deposited on TiO2 powder layers and illuminated under air. We modeled the dye bleaching as a simple kinetic network consisting of two reactions in series:

            Dye(colored) => Intermediate (colored) => Product(s) (clear)

To describe the time evolution of optical absorbance data in these reactions, we assumed two successive first order reactions, and successfully fit our model to data for four different organic dyes.(1-2).

           We have subsequently examined  the total oxidation kinetics for removal of deposited films of stearic acid, palmitic acid, soot, and sulfur. In contrast to the dye model, all could be described as zero order kinetics when the physical influences of light absorption or surface diffusion were taken into account, as appropriate.(3)

           When the apparent kinetic order differs from the true order of the chemical reaction due to physical (rather than chemical) phenomena, we say that the chemical kinetics are “disguised” by the simultaneous influence of chemical reaction and physical processes.  As our dye systems operate when reactant,  intermediate and photocatalyst may act to absorb light, we have a potential multiplicity of physical effects to (re)consider.

           Accordingly we revisit our dye bleaching kinetic data to explore a reinterpretation, that of intrinsic zero order kinetics (multilayer initial coverage) or first order kinetics (submonolayer initial coverage)  disguised by light absorption throughout the dye coated photocatalyst layers,  and we identify the circumstances under which true zero versus first order kinetics may be observed.  We also show that in porous, optically thick photocatalysts, the apparent reaction order is larger by 1 than the true order: thus an intrinsically  zero order reaction appears to be first order, and an intrinsically first order reaction may be represented by second order kinetic rate forms.   The generality of these results is shown through consistency demonstrations for eight different dyes.

 References

 

1. A.J. Julson , D. F. Ollis ,  Applied Catalysis B: Environmental,  65 (2006) 315.
2. P. Chin and D. Ollis, Catalysis Today, 123 (2007) 177-188.
3. D. F. Ollis, Applied Catalysis B-Envronmental, 99 (2010) 478.