Photocatalysis reaction kinetics

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

Chapter 8 addresses the treatment of contaminated air streams using photocatalysis. Special attention is given to the distinction between reaction kinetics and mass transport processes. The reviewed studies show the evolution from the early days of Ti02 photocatalysis, where the aim was to understand the basic process parameters, to today s development of phenomenological models assisting in the scaling-up of units. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

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. 

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. 

In order to understand the reaction mechanism of photocatalysis, a simple kinetic model can be presented as follows. 

Pervaporation - photocatalysis In the described systems the membrane usually permeates water and rejects the reactants, enhancing their residence time in the photoreactor. However, it is known that some intermediate products of the photo-catalytic degradation of organic compounds can negatively affect the reaction rate, therefore, in some cases it is useful to eliminate these by-products in order to improve the thermodynamic and/or the kinetics of the reaction. 

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. 

On the other hand, kinetics of reactions occiuring on a solid surface, that is, catalysis or photocatalysis, must be significantly different. There may be two representative extreme cases. One is so-called a diffusion controlled process, in which siuface reactions and the following detachment process occur very rapidly to give a negligible surface concentration of adsorbed molecules, and the overall rate coincides with the rate of adsorption of substrate molecules. In this case, the overall rate is proportional to concentration of the substrate in a solution or gas phase (bulk), that is, first-order kinetics is observed IS). The other extreme case is so-called surface-reaction limited, in which surface adsorption is kept in equilibrium during the reaction amd the overall rate coincides with the rate of reaction occurring on the surface, that is, reaction of e and h+ with surface-adsorbed substrate (l9). Under these conditions, the overall rate is not proportional to concentration of the substrate in the bulk unless the adsorption isotherm obeys a Henry-type equation, in which the amount of adsorption is proportional to concentration in the bulk (20). In the former case, the rate... 

In most photocatalysis, Langmuir-Hinshelwood kinetics is observed, suggesting the importance of substrate adsorption in key steps of the reaction [83]. That is, the rate of photocatalysis is proportional to the concentration (or pressure) of the reactant, which in turn can be related to the surface coverage by the Langmuir expression, Eq. 8,... 

Excited-state electron transfer (ET) is a fundamental complex phenomenon playing a crucial role in a variety of photophysical, photochemical and biochemical reactions (for reviews see, for example. Refs. [ I - 3 ). Owing to the essential role of ET in many processes and in photochemical applications (e.g., solar energy conversion and storage [4-6], photocatalysis [7], photopolymerization [8], information processing and storage [9] and photomedicine [10]) the understanding of the factors which determine the thermodynamics, kinetics and dynamics of the ET processes is very important. 

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