Photocatalysis case studies

In the introduction, the model systems will be defined to familiarize the reader with the approach in order to appreciate the connection to real-world catalysis. Following the introduction, we will demonstrate via four case studies various fundamental aspects in thermal and photocatalysis whereby studies on model systems might become important to unravel the foundations of reaction mechanisms. 

Case Study 6.39 Photocatalysis - oxidation of lactams and /V-acylamines... 

To this purpose, in a study on the photocatalytic degradation of 4-chlorophenol, Camera-Roda and Santarelli [89] proposed an integrated system in which photocatalysis is coupled with pervaporation as process intensification for water detoxification. Pervaporation represents a useful separation process in the case of the removal of VOCs and in this study it is used to remove continuously and at higher rate the organic intermediates that are formed in the first steps of the photocatalytic degradation of the weakly permeable 4-CP. 

In this work the authors summarize their own studies of photoprocesses on CdS colloids with particles of various size. In these studies, attention was given precisely to photocatalytic reactions on CdS, the photocatalytic reactions on TiC>2 were considered concurrently with the reported ones. In most cases photocatalytic reactions on semiconductors are the redox reactions. So of special interest was to study the regularities of reactions of interfacial transfer of photoexcited electron by the pulse photolysis and luminescence quenching methods. Many interesting phenomena were found while studying the model photocatalytic reactions by the method of stationary photolysis, i.e., under the conditions of real photocatalysis. 

Photocatalysis, i.e., using semiconductor particles under band gap irradiation as little micro reactors for the simultaneous reduction and oxidation of different redox systems, has been intensively studied during the last 25 years since the pioneering work of Carey et al [1]. The main focus of these studies seems to be the investigation of the principal applicability of photocatalytic systems for the efficient treatment of water and air streams polluted with toxic substances. Several review articles on this topic have recently been published [2]. In some cases, pilot-scale or even commercially available reactors have already been constructed, especially when titanium dioxide is used as the photocatalyst [3]. 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

Minero, C. Aliberti, C. Pelizzetti, E. Terzian, R. Serpone, N. Kinetic studies in heterogeneous photocatalysis. 6. AMI simulated sunlight photodegradation over titania in aqueous media A first case of fluorinated aromatics and identification of intermediates, Langmuir 1991, 7, 928. 

Application of heterogeneous photocatalysis to water and air purification appears a particularly convenient method in the cases of easily accessible UV light (mainly solar, but also artificial) and diluted pollutants. H02-based photocatalysis offers unique possibilities of mineralization of most organic compounds and, in addition, sterilization. This type of AOP useful for water and air purification has been studied and developed extensively over the last few decades. UV light, especially as a part of solar radiation, must be used to activate this process. In addition oxygen and water are indispensable. 

Having demonstrated the feasibility of using photocatalysis for controlling the valence state of the Pu simulant, Ce, we next investigated the possibility of controlling the valence state of a non-simulated actinide system. In this case, we decided to study the reduction of U022+. 

McLoughlin et al. (2004b) have also made a similar comparison between a VTC, a PTC, and a CPC for disinfection of water heavily contaminated with Escherichia coli, both by photocatalysis and by UV irradiation without catalyst. In this case the collectors did not track the sun but were inclined at local latitude with reactor tubes running east-west. It was also found that the CPC had the best performance, followed by the PTC and the VTC, which showed comparable results. It is necessary to point out that the PTC studied was of the nonconcentrating type with a very different configuration with respect to the one used by Bandala et al. (2004). 

Layered compounds provide unique character for electron-transfer processes owing to their low dimensionality. Especially layered materials with ion-exchange and/or intercalation capabilities show behavior that is not seen in so-called bulk-type materials. Layered materials, which have been often used in studies of photoelectron transfer as well as photocatalysis, may be classified into two groups compounds in which the host layers work as an active component for the photoexcitation and electron-transfer reactions, and materials in which the layers are inert for electron-transfer processes. Examples of the former are layered titanates and niobates and of the latter are clays. In the latter case, photoactive materials are intercalated in the interlayer spaces. Recently, the exfoliation of various layered compounds has become possible and artificial assemblies consisting of these exfoliated sheets have been formed. Electron transfer in such assemblies is also an attractive subject in this field. 

The study of the photochemical activity of FeCl4 in alcohol by Brealey and Uri (6) revealed an interesting case of photocatalysis. The postulated mechanism is... 

Accordingly, many of these studies examined the primary photocatalytic events that involve (a) absorption of light, (b) formation of free carriers (electrons and holes) and/or trapped carriers (electrons as Ti in the case of Ti02, and holes as OH radicals), and (c) reaction of preadsorbed acceptor or donor molecules with the relevant trapped carriers. This approach was reasonable when the only purpose of photocatalysis was elimination of undesirable environmental pollutants. However, when we queried how to render a process more efficient, it became evident that we needed to address more fundamental questions, namely the nature of the primary events that follow photoexcitation of the photocatalyst. In most instances, our... 

Studies in photocatalysis have focused on either the oxidation of organic compounds case of phenol, aromatics, herbicides, pesticides, solvents and aliphatics in the absence of inorganic cations (Barbeni et al., 1984 Lu and Chen, 1997 Mills et ai, 1996 Ollis et al., 1984) or the photoreduction of inorganic ionic species in water free of... 

---