RESUMEN
The kinetics of the removal of stearic acid (SA) islands by photocatalytic coatings is controversial, with some reporting that the islands fade as their thickness, h, decreases with the irradiation time, t, but maintain a constant area, a, -da/dt = 0, and others reporting that -dh/dt = 0 and -da/dt = -constant, i.e., the islands shrink, rather than fade. This study attempts to understand the possible cause for these two very different observations through a study of the destruction of a cylindrical SA island and an array of such islands, on two different photocatalytic films, namely, Activ self-cleaning glass, and a P25 TiO2 coating on glass, which have established uniform and heterogeneous surface activities, respectively. In both cases, using optical microscopy and profilometry, it is shown that, irrespective of whether there is as a single cylindrical island or an array of islands, h decreases uniformly with t, -dh/dt = constant, and -da/dt = 0, so that the SA islands just fade. However, in a study of the photocatalyzed removal of SA islands with a volcano-shaped profile, rather than that of a cylinder, it is found that the islands shrink and fade. A simple 2D kinetic model is used to rationalize the results reported in this work. Possible reasons for the two very different kinetic behaviors are discussed. The relevance of this work to self-cleaning photocatalytic films is discussed briefly.
RESUMEN
The photocatalytic oxidation of stearic acid, SA, by O2 is a common test method used to assess the activity of new materials and underpins a standard test for self-cleaning activity. The kinetics of this process have been well-studied and are often interpreted using one of two simple models, which are revisited here in this overview. The first model is based on the common scenario of a SA layer on top of an all-photocatalyst layer which yields zero order kinetics, for which it is suggested that all the reaction sites are occupied by SA during the bulk of the photocatalytic process. An important, but rarely noted feature of this system is that the rate of SA removal depends directly upon the fraction of absorbed ultra-bandgap radiation, which suggests that the photocatalyst particles are extensively networked, thereby allowing the photogenerated electrons and holes to move rapidly and efficiently to the surface to effect the destruction of SA. The second kinetic model has been used to describe the first order kinetics of SA removal observed for mesoporous photocatalytic films comprised of isolated photocatalyst particles, in which the SA is inside (rather than on top) of the photocatalytic film, and is developed further here. It is shown that, contrary to previous reports, this model is not appropriate for porous photocatalytic films in which the particles are extensively networked, such as ones based on powders or sol-gel films, even though they too may exhibit decay kinetics where the order is > 0. The reason for the latter kinetics appears to be a distribution of reactivities through such films, i.e. high and low activity sites.