Kinetic Concepts of Heterogeneous Photocatalysis

TheABCs of empirical chemical kinetics are highlighted from the scratch. Aspects of dynamics regarding charge carrier trapping and recombination are essential part of this discussion. The effect of imposition of external constrains such as temperature of the reactivity solution, concentration of the subject compound, intensity and wavelength of light on photocatalytic degradation system is emphasised. Because of the crucial position of light intensity in heterogeneous photocatalysis, the principles of operation of important chemical actinometric systems are presented. 2.1 Background Sketch Chemical kinetics addresses important issues in heterogeneous photocatalysis. Generally, it deals with the experimental determination and analysis of the quantity of a substance (usually concentration) as a function of time. In other words, chemical kinetics is concerned with reaction rate and throughput. Unlike in chemical equilibrium where the reactants and the products are in dynamic state of balance, chemical kinetics is concerned with either disappearance or appearance of chemical substances. The latter have direct consequences on reaction rate. A given chemical reaction may get accelerated or slowed down, for example by a positive catalyst or negative catalyst respectively as shown in Fig. 2.1. A simple example of this concept can be derived from common domestic techniques such as preservation using refrigeration method. The shell life of batteries can be extended by lowering the surrounding temperature. The simple reason behind this observation is that the rate of decay of the electrolytes or sensitisers in the battery is slowed down by lowering the temperature. The following discussions will cover aspects of chemical kinetics including the factors that affect the rate of photocatalytic reactions. Some aspects of catalysis such as the determination of rate are viewed from the standpoint of the principles of chemical kinetics. The rate of reaction is measured based on the property that is easiest to measure. This property can be colour, concentration or weight. As several products of varied structure and quantity can be formed during photocatalytic reactions, it is more convenient to measure reaction rates based on the disappearance of such a property. The law of mass action is the central to the expression of reaction rates. The law is based on the fact that the rate of reaction increases proportionally with concentration. U. I. Gaya, Heterogeneous Photocatalysis Using Inorganic 43 Semiconductor Solids, DOI 10.1007/978-94-007-7775-0_2, © Springer Science+Business Media Dordrecht 2014 44 2 Kinetic Concepts of Heterogeneous Photocatalysis Fig. 2.1 Types of kinetic influences. Processes can be manipulated using a suitable catalyst to decline or increase in reaction rate, depending on the type of catalyst or reaction condition rate acceleration