Saudi Cultural Missions Theses & Dissertations
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Item Restricted Investigation of Inorganic Photocatalytic Systems(Queen’s University Belfast, 2024-11) Alofi, Saleh; Mills, AndrewThis thesis details the improvement of our current understanding of the photocatalytic process through a series of detailed studies of the SA removal by a variety of different photocatalytic films. Chapter 1 and 2 provide an introduction to the subject and experimental techniques commonly employed. The first result, Chapter 3 revisits two kinetic models used to explain the photocatalytic oxidation of stearic acid (SA) by O₂, a process commonly used to assess the activity of new materials for self-cleaning. The first model, based on zero-order kinetics, describes a SA layer on top of a photocatalyst, 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 model, explaining first-order kinetics, applies to mesoporous films where SA is within the photocatalytic film. 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. The second result chapter, Chapter 4 discusses the efficiency of electron and hole transport (EHT) in TiO₂ films during the photocatalytic oxidation of stearic acid (SA) by ambient air. The EHT model is rigorously tested on highly absorbent TiO₂ films, with absorbance values much greater than 2. These films include mesoporous (sol-gel) and microporous (P25 TiO₂) types, approximately 0.5–1 μm thick, coated with a layer of (SA). Each film produced identical [SA] vs irradiation time decay profiles when irradiated from the front or the back. These results suggest that the transport of the photogenerated holes, and most likely electrons too, in these films is very efficient. When the SA layer is replaced with an Ag photocatalyst indicator ink, Ag paii, on a P25 TiO₂ film, and irradiated from the back, the ink changes colour quickly due to the reduction of Ag+ in the ink, indicating that the transport of photogenerated electrons through the film is also efficient. The third result, Chapter 5 The kinetics of stearic acid, SA, removal by photocatalytic films are modelled using a log-normal distribution in surface reactivity, which shows that as the distribution in reactivity, ρ, increases, so too does the apparent order of reaction, m, of the observed variation in SA concentration, [SA], versus irradiation time, t. The model is used to fit the observed [SA] vs. t profiles exhibited by three very different TiO₂ films, namely Activ™ self-cleaning glass, and prepared sol-gel TiO₂ and P25 TiO₂ films. These films show decay kinetics, with m = 0.3, 0.44 and 0.6, respectively, which fit the model predicted decay profiles with distribution width, ρ, values, 0.4, 0.54 and 0.7, respectively. Finally, the fourth result Chapter 6 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, and others reporting that., 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. It is shown that, irrespective of whether there is 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.26 0