Semiconductor photocatalysis kinetics

A variety of models have been derived to describe the kinetics of semiconductor photocatalysis, but the most commonly used model is the Langmuir-Hinshel-wood (LH) model [77-79]. The LH model relates the rate of surface-catalyzed reactions to the surface covered by the substrate. The simplest representation of the LH model [Eq. (7)] assumes no competition with reaction by-products and is normally applied to the initial stages of photocatalysis under air- or oxygen-saturated conditions. Assuming that the surface coverage is related to initial concentration of the substrate and to the adsorption equilibrium constant, K, tire initial... 

In this chapter, an attempt has been made to address fundamental mechanistic and kinetic aspects of TiO2 photocatalysis of organophosphorus compounds. Comparisons between homogeneous (radiolysis) and heterogeneous (photocatalysis) hydroxyl-generating processes have helped to elucidate the reaction pathways and led to number of important mechanistic conclusions. From the various kinetic parameters, the overall rates and efficiencies for the degradation of organophosphorus compounds can be predicted and may find direct application in evaluation and implementation of semiconductor photocatalysis. 

Kinetic Modeling of Electron Transfer Processes in Colloidal Semiconductor Photocatalysis... 

This review concentrates on John Albery s work in the field of colloidal semiconductor photoelectrochemistry. John s major contributions to this area, as in so many others, have been through his astounding facility for generating useful asymptotic solutions for highly complex kinetic models of electrochemical systems. So as to put John s work in colloidal photoelectrochemistry into context. Sections 9.1-9.3 of this chapter provide a review of the more salient kinetic models of semiconductor photocatalysis developed over the last 20 years or so. Section 9.4 then concentrates on the Alberian view and presents, for the first time, John s model of the chronoamperometric behaviour of colloidal CdS. 

This review has been written in order to clarify fundamental aspects of photocatalysis, an important subject in inorganic and material chemistry, not to present a list of studies on photocatalysis reported so far, since it seems rather difficult to make a complete review by introducing all or a large part of the reported studies on photocatalysis of relatively long history. This review is based on the author s experience in studies on photocatalysis and topics are limited to so-called semiconductor photocatalysis definition and examples of photocatalysis, its principle and kinetics, visible light-induced photocatalysis, and design of active photocatalysts are discussed in detail. 

Ray, A. K., and Beenackers, A., 1997, Novel swirl-flow reactor for kinetic studies of semiconductor photocatalysis. AIChEJ., 43(10) 2571-2578. 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Keywords Kinetics, mechanism, charge separation, Titanium dioxide, nanocrystals, photocatalysis, semiconductors, Magnetic resonance... 

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

Martyanov, Igor N. is a Ph.D. researcher and recently worked in the laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, SB RAS, Novosibirsk, Russia. The title of his PhD Thesis (1998) was Kinetics of photocatalytic redox reactions of organic molecules in semiconductor suspensions (CdS and TiC>2) . Areas of his interests kinetics of photocatalytic reactions in liquid phase at deep conversion the influence of the surfactants. 

This review is a discussion of the kinetic modelling of the photoelectrochemistry of colloidal semiconductor systems. This area is currently attracting significant attention from the scientific community due to the applications of colloidal semiconductors within two rapidly advancing research fronts heterogeneous photocatalysis and nanocrystalline particle technology. 

Heterogeneous photocatalysis on semiconductor particles has been shown to be an effective means of removing toxic organic pollutants as well as toxic metal ions from water. In the first part of this chapter, systematic kinetic... 

---