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2.3 Photocatalytic reaction kinetics
Kinetic studies are important to understand the photocatalytic processes, design of photocatalytic reactors, optimization of photocatalysts as well as for efficient utilization of light. The photocatalytic reactions involves series of elementary steps such as transportation of reactant to the catalyst surface, adsorption, absorption of photon, electron-hole pair generation, charge trapping, charge carriers migration, recombination, surface reaction, diffusion and desorption of the products or intermediates. The photocatalytic reaction rate (r) is a function of initial reactant concentration (Ci), catalyst amount (Cc) and the photon absorption rate (Pa) (Lasa et al., 2005).
( ,
i c,
a) (2.6) r f C C P
The reaction rate is controlled by the surface kinetics. Mass transfer limitation can be neglected in a well mixed photoreactor system (Matthews, 1988). The most common method applied to analyze the kinetic data is Langmuir-Hinshelwood (L-H) model, which can relate the surface reactions with surface coverage of the substrate (Mathews, 1988; Turchi and Ollis, 1989). The reaction rate (r) is proportional to the surface coverage of the substrate (θx).
= / =
r x r s/ 1
s(2.7) r dc dt k k K C K C
The kr is the reaction rate constant, Ks is the adsorption equilibrium constant of the reactant and C is the concentration of the substrate ‘x’. The reaction rate is reported to
be dependent on the light intensity showing a proportionality of I0.5 and I1.0 at high and low ligh intensities, respectively (Ohtani, 2014; Li et al., 2008; Malato et al., 2009). The terms kr and Ks showed square-root dependency on light intensity.
Other kinetic models such as, Eley-Rideal, have also been applied to study the rate expressions in terms of product formation and reactant consumption (Brosillon et al., 2008). The most critical factor in the photocatalytic process is the quantum efficiency, which determines the practical importance of a photocatalytic technique. The efficiency involves the rate of photon absorbed by the photocatalyst (Sun and Bolton, 1996). Detailed kinetic studies are limited for photocatalytic reaction. Matthews (1988) investigated the kinetics of photooxidation of various organic solutes for TiO2 under UV irradiation. Li et al. (2008) estimated the kinetic parameter for degradation of rhodamine B at different light intensities and TiO2 content. The authors found that the reaction rate constant for rhodamine B increased with increase in the light intensity. They also suggested from their kinetic model that the reciprocal of the reaction rate constant and adsorption rate constant have dependency on the reciprocal of the square root of the light intensity. It was reported that reaction rate constant increased with increase in titania content and then decreased, whereas adsorption rate constant decreased with increase in the titania content. It was concluded that the rhodamine B degradation rate was determined by the titania amount and followed the first order kinetics. Lin et al. (2009) reported that the electron-hole pairs mainly resulted in chemical reactions and followed L-H kinetics for methylparaben degradation at low UV light flux. Different parameters such as pH, TiO2 amount and oxygen concentration were investigated for photodegradation processes of methylparaben. The amount of hydroxyl radicals and surface charge of the catalyst were influenced by the pH, which led to change in the overall rate. Also the removal time was observed to decrease linearly with increase in oxygen concentration. The authors showed that at higher oxygen concentration, the reaction rate increased linearly with increase in light intensity. In case of higher TiO2 loading, the authors stated that the decrease in reaction rate was due to the scattering of particles and decrease in light penetration. Turchi and Ollis (1990) proposed four modified mechanisms based on the hydroxyl radical reaction pathway. The hydroxyl group adsorbed on the catalyst surface and the holes generated in the valence band of semiconductor material were suggested to be involved in the photooxidation
reactions. The organic compounds or intermediates on the catalyst surface can be attacked by hydroxyl radical, while the valence band hole can attack the water as well as adsorbed intermediates formed during the water splitting reaction. Hydroxyl radical attack is stated as the predominant mechanism in aqueous photocatalytic processes. Ks was reported to be independent of light intensity by Turchi and Ollis (1990). On the other hand Xu and Langford (2000) investigated the degradation rate of acetophenone at different light intensities and proposed that increase in incident intensity decreased the Ks. Chen and Ray (1998) studied the photodegradation kinetics of nitrophenol as a function of different parameters such as initial pollutant concentration, light intensity, partial pressure of oxygen, catalyst concentration, pH, chloride ion and temperature.
The degradation rate was observed to be pseudo first-order as a function of concentration of nitrophenol. At both low and high pH, they observed a slower photodegradation rate. The presence of chloride ion had negative effect on the rate of degradation at low pH. The calculated equilibrium adsorption constants of oxygen and nitrophenol were 9.98 atm-1 and 0.075 ppm-1. Nomikos et al. (2014) reported the kinetic study of methanol photoreforming on Pt/TiO2 catalyst at different initial methanol concentration and light intensity. The hydrogen production rate was increased with increase in the concentration of methanol. They also observed increase in the reaction rate with increase in the light intensity (up to 2.3mW cm-2) at lower methanol concentration. The authors reported that the hydrogen evolution was accompanied with CO2 formation in the presence of methanol, while only hydrogen production was observed using pure water. Sahu et al. (2009) studied the kinetics of hydrogen production from photocatalytic splitting of water over Pt-CdS catalyst. The hydrogen production rate was reported to increase up to 2 h of irradiation period and thereafter decreased. The authors suggested that hydrogen production rate is directly proportional to the concentration of sulfide ions adsorbed on the surface.