Modeling Photocatalytic Reactions

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. 

Modeling photocatalytic reaction processes requires careful consideration of reaction and adsoiption phenomena. In order to establish the importance of these matters, experiments can be developed using model pollutants such as methylene blue, phenol, 2-chlorophenol, 2,4-dichlorophenol, catechol (or 1,2 benzenediol), andpyrogallol (or 1,2,3 benzenetriol), each having quite different behaviours of adsorption and reaction. 

Although it has been assumed that oxidative and reductive photocatalytic reactions take place simultaneously on Ti02 particles, it is very difficult to confirm this assumption experimentally, because the products of both reactions mix immediately. However, it is possible to set up a simple system that can be used to model individual particles and is compatible with microscopic detection of reaction products.65-685 Such a system, shown in Fig. 2.8, involves a Ti02 film that has metallic regions, e.g., Pd or ITO, and a scanning microelectrode. The microelectrode can be positioned as close as 20 fim to the surface. Its potential can be set at values at which either 02 or H202, for example, can be monitored as a function of time. 

Fig. 5.4 Two-dimensional (2D) ladder model for the photocatalytic reactions at small semiconductor particles. X"m represents the distribution of particles containing n electrons and m holes at some instant.

Figure 17.2 illustrates our model for splitting water by solar energy. I" is important that all the redox reactions involved in thf system be reversible. The quinone compound in the organic solvent combines the two photocatalytic reactions, and its function can be compared to the electron relaying molecules in thylakoid membranes of chloroplasts. Electron transfer reactions via quinone compouncs in artificia systems have been studied as a model of photosynthesis22-23 and in an electrochemical system for acid concentration.24 ... 

In another study, Tsum et al. [80] reported the use of porous Ti02 membranes having pores of several nanometers for a gas-phase photocatalytic reaction of methanol as a model of volatile organic component (VOC). In this system, the titanium dioxide is immobilized in the form of a porous membrane that is capable of selective permeation and also a photocatalytic oxidation that occurs both on the surface and inside the porous Ti02 membranes. In this way, it is possible to obtain a permeate stream oxidized with OH radicals after one-pass permeation through the Ti02 membranes. 

From simple measurements of the rate of a photocatalytic reaction as a function of the concentration of a given reactant or product, valuable information can be derived. For example, these measurements should allow one to know whether the active species of an adsorbed reactant are dissociated or not (22), whether the various reactants are adsorbed on the same surface sites or on different sites (23), and whether a given product inhibits the reaction by adsorbing on the same sites as those of the reactants. Referring to kinetic models is therefore necessary. The Langmuir-Hinshelwood model, which indicates that the reaction takes place between both reactants at their equilibrium of adsorption, has often been used to interpret kinetic results of photocatalytic reactions in gaseous or liquid phase. A contribution of the Eley-Rideal mechanism (the reaction between one nonadsorbed reactant and one adsorbed reactant) has sometimes been proposed. 

The main difference between photochemical and thermal reaction is the presence of a radiation-activated step. The rate of reaction of this step is proportional to the local volumetric rate of energy absorption (LVREA). For any emission model, the LVREA is a function of the spatial variables, of the physical properties and geometrical characteristics of the lamp-reactor system, and some physicochemical properties of the reacting mixture. The most important design parameter that is pertinent in photochemical and photocatalytic reactions is the effective attenuation coefficient. 

Ti(>2 is a basic and simple material, and more than thirty years have passed since the discovery of the Ti(>2 photocatalytic reaction. The wettability of solid surfaces is also a basic and familiar property. It is amazing that the excellent characteristics (high hydrophilicity among other properties) of the already well understood TiC>2 were discovered only recently. In this paper, we introduced the surface reconstruction model of hydroxyl groups as the mechanism for the highly hydrophilic conversion. However, areas for further research still remain. 

Photocatalytic reaction assisted with Fe ions High-performance liquid chromatography Kinetic model 1 Kinetic model 2... 

Figure 1 shows a semibatch swirl-flow monolith-t) e reactor that has been designed to study purely the kinetics of photocatalytic reactions for any model components (Ray and Beenackers, 1997). Monoliths are unique... 

Brandi, R.J., Alfano, O.M., and Cassano, A.E. Rigorous model and experimental verification of the radiation field in a flat plate solar collector simulator employed for photocatalytic reactions . Chem. Eng. Sci. 54, 2817 (1999). 

Three-dimensional CFD-coupled with radiation field modeling and photocatalytic reaction dynamics was employed by Salvado-Estivill et al. (2007b) to model the decomposition of TCE in a flat-plate, single-pass photocatalytic reactor containing immobilized P25. The outcome was pollutant-specific kinetic rate parameters, which were independent of the reactor geometry, radiation field, and fluid dynamics. This was followed by... 

Chapter 1 examines the phenomenological principles involved in the modeling of photocatalytic reactions including the photo-adsorption of chemical species. This chapter proposes a method to quantify photo-adsorbed species onto irradiated TiOi. The technique is applied to the oxidation of phenol and benzyl alcohol. 

The principle of photocatalysis is often explained with an illustration like Fig. 2, a schematic representation of the electronic structures of semiconducting materials, a band model. An electron in an electron-filled valence band (VB) is excited by photoirradiation to a vacant conduction band (CB), which is separated by a forbidden band, a band gap, from the VB, leaving a positive hole in the VB (Section III.B). These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst. Such an interpretation accounts for the photocatalytic reactions of semiconducting and insulating materials absorbing photons by the bulk of materials. In the definition of photocatalysis given above, however, no such limitation based on the electronic structure of a photocatalyst is included. For example, isolated... 

Fig. 9. (a) Model interpreting a rate-determining step in a four-step series reaction. Step 3 with largest activation energy determines the overall rate, (b) Simplified scheme of photocatalytic reaction with a photocatalyst pc. It is clear that this process is not a series reaction since a deactivation step is included. 

Yan, X. Ohno, T. Nishijima, K. Abe, R. Ohtani, B. Chem. Phys. Lett. 2006, 429, 606 This paper claimed inappropriate use of organic dyes as test compounds for visible light-sensitive photocatalysts. Citation of this paper was not expected at all, since the authors using methylene blue (MB) as a model compound for photocatalytic reaction never want to refer to this and, on the other hand, those who do not use MB need not to refer to this. However, there has been an appreciable number of citations and, to the author s surprise, approximately half of the citations of this paper were for reasonable use of MB, indicating that authors of those papers did not read the paper. 

Photoreactions of organic compounds over model surfaces of wide band-gap oxide semiconductors have received considerable attention recently [43, 79-82]. The most-studied photocatalytic reactions on rutile TiO lllO) single-crystal surfaces include ethanol [43], acetic acid [78], trimethyl acetic acid [80, 81], and acetone [82]. In this section, we will focus on the photoreaction of ethanol over TiOj(llO). Ethanol is dissociatively adsorbed via its oxygen lone pair on fivefold coordinated Ti atoms to produce adsorbed ethoxide species (Fig. 7.6). STM studies of the adsorption of ethanol on TiO2(110) demonstrated the presence of both alkoxides and surface hydroxyls [83] confirming the adsorption is dissociative. Figure 7.11 is the XPS Cls spectra after the exposure of ethanol (9=0.5 with respect to Ti atoms). 

This chapter reviews the basic principles involved in modeling the rates of photocatalytic reactions. These matters require clarification in order to proceed with the successful design, simulation and scale-up of photo catalytic reactor units. 

The modeling of photocatalytic reaction rates is essentially based on a number of mathematical statements, which can be expressed by a set of ordinary differential... 

The consideration of equations (1-12) and/or (1-13) leads to the advancement of photocatalytic conversion rate models, such as the series-parallel model proposed by Salaices et al. (2004) where the derived kinetic parameters are based on the iiradiated weight of catalyst. As such, these can be considered as intrinsic parameters with phenomenological meaning pertinent to the photocatalytic reaction. 

Kinetic Modeling of the Photocatalytic Reaction Network The Parallel-Series Approximation... 

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