Abstract
The application of photocatalytic processes in environmental protection has a short history and is still in its beginnings. This issue, however, provokes continuous interest of the scientific and professional world. This chapter gives the overview of scientific and expert literature related to the application of heterogeneous photocatalysis in tackling the issues related to air protection. The focus is on purification of the indoor air, which comes to abatement of various volatile organic compounds. Basic principles of photocatalytic degradation of gaseous contaminants are presented, mostly on the TiO2 photocatalyst, as well as a detailed account of the effect of various process variables on the rate of the photocatalytic reaction. The problem of TiO2 deactivation is pointed out and deactivation mechanisms are described on the selected reaction system. The chapter aims to review technical aspects of heterogeneous catalysis applied in air protection and special attention is paid to different designs of photoreactors.
TopIntroduction
Heterogeneous photocatalysis has been intensively studied over the past decade, mostly in connection with the treatment of wastewater, drinking water and air purification and its potential application in medicine, generation of energy, degradation of waste plastics and green synthesis of industrially important chemicals (Hoffmann et al., 1995; Mills & Le Hunte, 1997; Mills & Lee, 2002; Paz, 2010a, 2010b; Raupp et al., 2001; Zhao & Yang, 2003). Fujishima and Honda (Fujishima & Honda, 1972) were the first to describe the photocatalytic process, which related to electrolysis of water in a photoelectrochemical electrode with the use of TiO2 as a photocatalyst. However, the examples of commercial application of photocatalysis and their application in real systems are still insignificant compared to the interest this topic arouses among the scientists. In to date research scientists have been focused mostly on photocatalysis of water and much less of gas phase and air protection (Fujishima & Honda, 1972). However, over past few years the interest has abruptly changed what is best shown by the fact that there are currently over 120 patents based on photocatalytic air purification (Paz, 2010a).
The removal of air pollutants is a highly important application of photocatalysis. It covers the treatment of the indoor and outdoor air, of the process gases and the treatments based on the flow of a carrier gas (usually air) with obnoxious compounds through a liquid tank (usually water) (Chatterjee & Dasgupta, 2005). According to the available data, it is highly possible to improve properties of the available photocatalysts. This can be done in various ways:
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Applying different solution phase techniques of the catalyst preparation (e.g. precipitation, hydrothermal synthesis, solvothermal synthesis, sol-gel or microemulsion method, sonochemical synthesis, etc.).
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Using appropriate gas phase technique for the synthesis of thin film (including chemical vapor deposition, physical vapor deposition, spray pyrolysis deposition, etc.).
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By varying the preparation parameters like: the precursor compound, hydrolyzing agent (in the case of sol-gel synthesis), reaction temperature, etc.
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By modifying of composition and/or of the photocatalyst’s surface by additional procedures (e.g. doping of semiconductor materials with some metals, e.g. Ag, Pt, V, Cr, Mn, Fe, Ni or using anionic species doping, e.g. carbon, nitrogen and sulphur or simultaneous doping of cationic and anionic species).
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Using thermal treatment of the photocatalyst at elevated temperatures (usually at temperatures, T > 400 °C).
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Performing activation of the photocatalyst surface by chemisorption or physisorption of dyes and metal complexes, etc. (Chatterjee & Dasgupta, 2005; Diebold, 2003; Fujishima et al., 2008).
Key Terms in this Chapter
Photoreactor: Any vessel in which a photochemical reaction takes place.
Photolysis: Bond cleavage induced by ultraviolet, visible or infrared irradiation.
Advanced Oxidation Process (AOP): Process in which oxidation of organic or inorganic contaminants occurs primarily through reactions with hydroxyl radicals (OH·).
Photodegradation: Photochemical transformation of a molecule into lower molecular weight fragments.
Photocatalysis (or Photo-Assisted Catalysis): Change in the rate of chemical reactions or their initiation under the action of ultraviolet, visible or infrared irradiation in the presence of substances called the photocatalysts that absorb light and are involved in the chemical transformation of the reactants.
Photocatalyst: Substance which, due to absorption of light quantum, participates in chemical changes of reactants causing formation of transition intermediates and a subsequent recovery of its chemical composition after every cycle of interaction.
Hydroxyl Radicals, (OH·): The neutral form of the hydroxide ions (OH–); highly reactive and short-lived species with high oxidation potential.
Band Gap Energy, Ebg: Energy distance between the bottom of the conduction band and the top of the valence band in a semiconductor.
Semiconductor: A substance, usually a solid chemical element or compound that can conduct electricity more easily than insulators but less easily than conductors. Its conductance varies depending on the current or voltage applied to a control electrode or on the intensity of irradiation.
Hole Transfer: Charge migration process in which the majority carriers are positively charged.