Photocatalytic kinetics

Establishing Photocatalytic Kinetic Rate Equations Basic Principles and Parameters... 

Fig. 35 TEM images of the forming process of urchin-like mischcrystal Ti02 at different reaction temperatures and crystallization times. A HATP B 70 "C for 0 h C 85°C for 1 h, D and E 85°C for 2 h F HRTEM of panel D. (G) XPS curves of Ti02/HATP (mass ratios of 0.7) and Ti02 (H) Photocatalytic kinetic simulation of the different samples on RhB. Reproduced from ref. 68. Copyright (201D, with permission from Elsevier.

Analysis of photocatalytic kinetics in various reactions over solid catalysts is based essentially on the classical Langmuir approach, assuming one relatively rapid reaction achieving adsorption equilibrium followed by a single, slow surface reaction step... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

It is well known that photocatalytic oxidation of or nic pollutants follows Langmuir-Hinshelwood kinetics[6]. Therefore, this kind of reaction can be represented as follows. 

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

The photocatalytic decomposition of 4-NP can be written in terms of Langmuir-Hishelwood (L-H) kinetics [3]. 

Besides these chemical effects, which are understood in terms of the established theories in semiconductor physics and chemical kinetics, new physico-chemical phenomena are observed in the case of extremely small particles. The metal or semiconductor behavior is gradually lost with decreasing size, the consequences being drastic changes in the optical properties of the materials and also in their photocatalytic effects. 

Many physical-chemical processes on surfaces of solids involve free atoms and radicals as intermediate particles. The latter diffuse along the adsorbent-catalyst surface and govern not only kinetics of catalytic, photocatalytic, or some heterogeneous radiative processes, but also creation of certain substances as a result of the reaction. 

Chen J, Ollis DF, Rulkens WM (1999) Kinetic processes of photocatalytic mineralization of alcohols on metallized titanium dioxide. Water Res 33 1173-1180... 

Tang WZ, An H (1995) Photocatalytic degradation kinetics and mechanism of acid blue 40 by UV/Ti02 in aqueous solution. Chemosphere 31 4171-4183... 

Yatman HC, Akyol A, Bayramoglu M (2004) Kinetics of photocatalytic decolorization of an azo reactive dye in aqueous ZnO suspensions. Ind Eng Chem Res 43 6035-6039... 

Fig. 16.2 Simplified kinetic model of the photocatalytic process. ps represents the light absorbed per unit surface area of the photocatalyst, e b and h+b are the photogenerated electrons and holes, respectively, in the semiconductor bulk, kR is the bulk recombination rate constant and /R the related flux, whatever recombination mechanism is operating A is the heat resulting from the recombination kDe and kDh are the net first-order diffusion constants for fluxes Je and Jh to the surface of e b and h+b in the semiconductor lattice, respectively e s and h+s are the species resulting from...

Macroscopic n-type materials in contact with metals normally develop a Schottky barrier (depletion layer) at the junction of the two materials, which reduces the kinetics of electron injection from semiconductor conduction band to the metal. However, when nanoparticles are significantly smaller than the depletion layer, there is no significant barrier layer within the semiconductor nanoparticle to obstruct electron transfer [62]. An accumulation layer may in fact be created, with a consequent increase in the electron transfer from the nanoparticle to the metal island [63], It is not clear if and what type of electronic barrier exists between semiconductor nanoparticles and metal islands, as well as the role played by the properties of the metal. A direct correlation between the work function of the metal and the photocatalytic activity for the generation of NH3 from azide ions has been made for metallized Ti02 systems [64]. 

I. Bouzaida, C. Ferronato, J.M. Chovelon, M.E. Rammah and J.M. Herrmann, Heterogeneous photocatalytic degradation of the anthraquinone dye, Acid Blue 25 (AB25) a kinetic approach. J. Photochem. Photobiol.A Chem., 168 (2004) 23-30. 

Okamoto, K., Yamamoto, Y, Tanaka, H., Itaya, A. 1985. Kinetics of heterogeneous photocatalytic decomposition of phenol over anatase TiO, powder. Bull Chem Soc Jpn 58 2023-2028. 

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