Photocatalyst particles

For the description of photocatalytic action of semiconductor particles in the reactions of electron transfer from a certain donor to acceptor, it is convenient to recognize three states of photocatalyst particle ... 

In this case, the amount of reagent ions adsorbed at the surface of a single colloidal particle in the course of the photocatalytic process MOkina(is, DWnads will be determined by both the current concentrations of reagents and the value of the photocatalyst particle surface not blocked by PAA. Assuming the values MO s. Dkinads and the effective rate constant kn to be proportional to Q, we find ... 

Figure 1 The principle of photocatalytic water splitting (a) photoelectronic excitation in the phototcatalyst-generating electron-hole pairs and (b) processes occurring on photocatalyst particle following photoelectronic excitation (Mills and Le Hunte, 1997).

The principles of photochemical water splitting can be extended to the design of systems using photocatalytic semiconductors in the form of particles or powders suspended in aqueous solutions (Bard, 1979, 1980). In this system, each photocatalyst particle functions as a microphotoelectrode performing both oxidation and reduction of water on its surface (Figure 4). 

Photocatalytic reactions are promoted by solid photocatalyst particles that usually constitute the discrete phase distributed within a continuous fluid phase in the reactor. Therefore, at least two phases, that is, liquid and solid, are present in the reactor. The solid phase could be dispersed (SPD) or stationary (SPS) within the reactor. SPD photoreactors may be operated with the catalyst particles and the fluid phase(s) agitated by mechanical or other means. Depending on the means of agitation, the photoreactor resembles that of slurry or fluidized bed reactors. In numerous investigations, an aqueous suspension of the catalyst particles in immersion or annular-type photo reactors has been used. However, the use of suspensions requires the... 

Attaching photocatalyst particles to inert domains may assist in improving the efficiency of the photocatalytic process. The basic concept, termed as Adsorb Shuttle, is based on using the inert domains for adsorbing target compoimds that otherwise hardly adsorb on the photocatalyst. That way, a reservoir of the contaminants within a small distance from the photocatalytic sites is formed. Once adsorbed in the vicinity of the photocatalyst, the target molecules may surface-diffuse to the photocatalytic sites as shown schematically in Figure 5. 

Some diverse VOCs (halocarbons, isoprene (CH2C(CH3) CH2CH2), monoterpenes, ethanol, and methyl tert-butyl ether, (CHslsCOCHs)) were found to be photooxidized efficiently on solid aerosols. Solid photocatalyst particles, such as Ti02, ZnO, and Fe20s, were here of special importance, but the VOC oxidation was photoassisted also by dessert sand, volcanic ash, or even by chalk particles (23-25). Similarly, sulfur dioxide was found to... 

Kinetics of e -h recombination may depend on its mode if one electron is excited and this is recombined with h , the recombination rate obeys the first-order rate law, while if multiple e -h+ appears at the same time within a photocatalyst particle, the rate obeys the second-order rate law. Actually, in a femtosecond pmnp-probe diffuse reflection spectroscopic analysis of tita-nia samples, photoabsorption at 620 nm by trapped electrons showed second-order decay with a component of baseline as follows ... 

Figure 9. Metal-modified semiconductor photocatalyst particle.

In this section we examine the case where there is an electric field in photocatalyst particle and ask what influence such an electric field might have on the photocatalytic behaviour of the photocatalyst particles. Accordingly, we now modify the earlier model (Fig. 5.29) to include the electric field, whose presence may affect both the magnitude and the direction of migration of the photogenerated carriers. We then query what factors govern the snrface concentration of carriers, and thus the quantum yield of the surface reaction nnder these new conditions. 

FIGURE 1.1. Schematics of the electron-hole generation in a photocatalyst particle and some of the mechanisms involved a) Ray promotes the formation of the electron-hole and electron, b) electron-hole is used in the formation of the OH groups promoting oxidation processes, c) the electron is utilized in a number of reduction processes, d) electron and electron-hole can recombine contributing to process inefficiency. 

Under certain conditions, some of these steps may not need to be considered. For instance, if the photocatalyst particle is not porous, the mass transfer process within the catalyst particle can be ignored. In reactors where a high fluid velocity is contacting the catalyst, mass transport limitations on the surface of the Ti02 particles may be neglected with the intrinsic photocatalytic reaction becoming the controlling step (Jacoby et al., 1996). 

Core SrPe Oig nanoparticles-r/02 nanocrystals were also synthesized as magnetic photocatalytic particles [366]. In this case the photocatalyst particles are recovered from the treated water stream by applying an external magnetic field (see Table 6). 

N. Kieda, T. Tokuhisa, Immobilization of Ti02 photocatalyst particles on stainless steel substrates by electrolytically deposited Pd and Cu , Journal of the Ceramic Society of Japan, 114, 42-45, (2006). 

Neglecting scattering, for parficles small enough, photoadsorption cross-section, cr, of a photocatalyst particle can be estimated as ... 

Certain inorganic materials can be employed as photocatalysts for the synthesis or degradation of compounds in heterogeneous systems. Relevant devices contain, for example, films incorporating immobilized photocatalyst particles. Typically, titania, Ti02, is used for the treatment of water contaminated with chemical pollutants and/or bacteria [9]. The contaminants are oxidized by reactive species, i.e. hydroxyl and superoxide radicals, generated by reaction of electron/hole pairs with O2 and water adsorbed at the particle surface. Electron/hole pairs are formed when UV light (>, <400 nm) is absorbed by titania (see Scheme 14.5). 

Fig. 7.7 (a) Single-bed photocatalyst particle suspension system, co-geneiating oxygen and hydrogen in the same reactor (b) two-bed photocatalyst particle suspension system, with separated oxygen-evolution and hydrogen-evolution beds... 

Type-1 Single-Bed Photocatalyst System A single electrolyte-filled reactor bed containing a colloidal suspension of PEC photocatalyst particles which produce a mixture of H2 and O2 product gases. 

Fig. 7.32 Energy band diagram of the Z-scheme showing OER and HER photocatalyst particles with functionalized surface sites hu the oxygen- and hydrogoi-evolution reactions, respectively. Concentrations of both particle types are suspended togethta- in solution with an ion-shuttling mediator, such as the Fe e couple, effectively coupling the gas evolution reactions in tandem to photosplit water...

PMRs with photocatalytic membranes are devices in which photodegradation of contaminants occurs on the external surface and within the pores of a membrane, while reactants are permeating in a one-pass flow. Therefore, the element which has to be irradiated is the membrane itself The photocatalytic membrane in a PMR acts as a support for the photocatalyst and might act as a barrier for the molecules present in the solution (initial compounds and products or by-products of their decomposition). Due to the photocatalytic degradation of organic contaminants an improvement of permeate quality is observed. Modification of a membrane with photocatalyst particles... 

During preparation of a photocatalytic membrane by the dip coating method a membrane is dipped in a 1102 suspension in water (Kim et al., 2003 Kim and Van der Bruggen, 2010 Kwak et al., 2001 Madaeni and Ghaemi, 2007), alcohol, for example, 2-propanol (Vankelecom, 2002) or other liquids. The membrane after dipping could be additionally pressurized with a compressed gas (Bae and Tak, 2005). The photocatalyst particles are self-assembled on a polymeric membrane due to a coordination of the functional groups present on the membrane surface (e.g., carbonyl or... 

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