271736 Kinetics of Fatty Acid Film Removal From Photocatalytic, Self-Cleaning Titania Surfaces
The photocatalyzed oxidation of stearic acid (SA) on photocatalysts, especially those composed of titania, has been explored as an indicator of photocatalyst activity and as a potential field test for residual photoactivity on installed self-cleaning glasses (1-4). The quantitation of observed reaction kinetics in such organic films requires an understanding of both the chemical mechanism and the SA-photocatalyst configuration. We have recently modeled a uniform layer configuration for this system, including uniform SA multi-layers on top of non-porous titania (ActivTM) and SA uniformly distributed throughout a porous titania film (thickness order of 100 nm) (5). Our uniform layer approach assumed intrinsic zero order SA conversion but demonstrated a kinetic disguise: non-porous surfaces with uniform illumination exhibited zero order behavior, but porous, optically thick films were predicted to exhibit approximately first order behavior, in agreement with prior experimental literature (3,4).
However, reports by Sawunyama et al (6,7) and Ghazzal et al (8) have used atomic force microscopy (AFM) to characterize SA deposits, finding that their preparation methods and catalysts yielded island-like SA deposits for submonolayers(6) and multi-layers(8) . Individual islands showed disappearance kinetics to be zero order with respect to island area (6) but apparently first order with respect to total SA remaining (7,8).
We demonstrate here kinetic models which are consistent with these “island” findings and establish the following:
- Island deposits of a single size will exhibit apparent zero order kinetics.
- Island deposits with a flat distribution of island sizes will exhibit an apparent first order behavior.
- Island deposits with a non-uniform size distribution will exhibit an apparent reaction order , n, between zero and unity.
The conclusion is that meaningful kinetic analysis of SA island photocatalyzed disappearance, and thus of any pollutant which deposits in island or particulate form, rather than a continuous film, requires knowledge of the initial island size distribution. The SA deposits also undergo significant rearrangement as the titania surface, when illuminated, transforms quickly from hydrophobic to hydrophilic. Thus it is the film and/or island distribution after initiation of illumination which is most relevant to modeling photocatalyzed removal of fatty acids such as stearic acid.
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