Suib New and Future Developments in Catalysis

Catalysis for Remediation and Environmental Concerns
1. Auflage 2013
ISBN: 978-0-444-53871-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 630 Seiten

New and Future Developments in Catalysis is a package of seven books that compile the latest ideas concerning alternate and renewable energy sources and the role that catalysis plays in converting new renewable feedstock into biofuels and biochemicals. Both homogeneous and heterogeneous catalysts and catalytic processes will be discussed in a unified and comprehensive approach. There will be extensive cross-referencing within all volumes.The various sources of environmental pollution are the theme of this volume. The volume lists all current environmentally friendly catalytic chemical processes used for environmental remediation and critically compares their economic viability. - Offers in-depth coverage of all catalytic topics of current interest and outlines future challenges and research areas - A clear and visual description of all parameters and conditions, enabling the reader to draw conclusions for a particular case - Outlines the catalytic processes applicable to energy generation and design of green processes

Autoren/Hrsg.

Suib, Steven L

Leseproben

Chapter 1 Photocatalysts for Elimination of Toxins on Surfaces and in Air Using UV and Visible Light
Kenneth J. Klabunde and Manindu N. Weerasinghe,     Kansas State University, Department of Chemistry, CBC Building, Manhattan, KS 66506, USA Acknowledgments
The support of the Army Research office and the Department of Energy is gratefully acknowledged. 1.1 Introduction
Over the last several decades there has been great concern about environmental pollution due to the fact that it is one of the primary causes for various health problems as well as for possible changes in the global climate. Generally, environmental pollution can be defined as contamination of air, water, and land due to manmade waste and can be divided into three major groups: air pollution, water pollution, and soil pollution. From these three major groups, air pollution has received the attention of many researchers due to the seriousness of the impact on climate change, acid rain, smog, and human and animal health. Air pollution can be subgrouped into two main categories: as indoor and outdoor. Both categories are equally important as they can create very unhealthy conditions to humans as well as to animals and plants [1]. These environmental problems are related to energy use, and clean renewable energy is needed, such as solar energy, wind, geothermal, etc. Out of all these renewable energy sources, solar energy has the most potential. In fact, it has been calculated that the amount of solar energy arriving at the earth’s surface in a minute is sufficient to meet the energy demand of the world for a year. But, the lack of efficient solar energy harvesting and storing methods is one of the main drawbacks that we face. So, there are thousands of researchers around the globe experimenting on efficient methods to harvest and store solar energy. Solar energy can be used to heat or to produce electricity. Solar energy can also be converted into chemical energy or can be used to catalyze important reactions [2]. Photocatalysis is one of the very successful and active areas of research that have provided important ways to harvest readily available solar energy to destroy harmful organic air contaminants to overcome environment pollution. Usually, any chemical reaction requires a certain amount of activation energy to initiate the reaction. In normal chemical reactions the activation energy will usually be supplied by simple methods such as heating, mechanical stirring, etc. But, in photochemical reactions, light is used for this purpose. Upon exposure to certain wavelengths of light, photocatalytic material can be used to catalyze specific chemical reactions based on the oxidation and reduction potentials of the photogenerated charge carriers. Thus, in photocatalytic reactions, the catalytic material plays an intermediate role in absorbing light energy and promoting desired chemical reactions. According to the literature, various photocatalytic materials have been employed to drive water splitting to produce hydrogen and oxygen gases, mineralizing harmful organic pollutants, as well as to remove organic dye molecules from industrial effluents. Even though, there are many materials that have been reported, the number of materials that have become successful on an industrial scale is very limited [3]. There are various factors that determine the efficiency of a photocatalyst. These are efficiency of charge separation, energy range of the solar spectrum suitable for the excitation of the material, optimum intensity of the light photons, environment of active sites, etc. Usually, during the preparation of photocatalytic materials the energy levels of the conduction and valance bands of the materials are modified, or the chemical environment of the active site is changed by doping with suitable doping agents. These changes to photocatalytic systems usually enhance the light absorption, electron-hole pair generation, and the overall activity. Surface acidity is another important factor that determines the specificity, efficiency, and the mechanism of action of a photocatalytic material. For example, acidity of titania-based materials is strongly related to the amount of surface hydroxyl groups present on the surface and these groups play a major role in trapping photogenerated holes and thereby decrease the recombination of electron-hole pairs, which in turn increase the quantum efficiency of the photocatalyst [4]. Most of the successful photocatalytic materials that have been reported consist of a supporting base material. Usually compounds such as zeolite, titania, and silica are popular as successful base materials due to their high stability under high temperature and pressure conditions, low toxicity, low cost, and the ability to obtain various physico-chemical properties simply by changing particle dimensions. Usually the supporting material facilitates the catalytic activity of the catalytic site by enhancing charged carrier separation, providing reduced electron-hole recombination, and facilitating charge transfer to an adsorbed species [5]. On the other hand, most of the catalytic systems reported in the past are primarily based on at least one semiconducting base material. Semiconducting materials are required to obtain good photocatalytic activities due to the ability of semiconductors to create reactive electron-hole pairs upon irradiation of UV or visible light. But, whether comparable photocatalytic activity can be obtained without using semiconducting base materials is an important question that still remains unanswered. Thus, it is very important to directly compare other available options, such as insulator-based materials, in order to determine the photocatalytic activities of these materials. 1.2 Titanium Dioxide-Based Photocatalysis
Titanium dioxide photocatalysis is the most studied and well-understood photocatalytic system. Thus, studying the mechanistic details of how titania behaves is important. Titanium dioxide, also known as Titania, is a white-colored compound that is widely used as a photocatalyst, catalytic support, sensor material, and hydrogen adsorber. Titania is a semiconductor with a band gap of 3.2 eV, and has been shown to promote mineralization of organic pollutants, water splitting, and carbon dioxide reduction upon exposure to UV light. Titanium dioxide occurs in nature in three well-known mineral forms known as anatase, rutile, and brookite. Among these mineral forms, anatase typically exhibits higher photocatalytic activity than the other two forms. But, in some cases it has been reported that even higher photocatalytic activity is possible with precise mixtures of both anatase and rutile. One such example is commercially available Degussa P25 TiO2, which consists of 80% anatase phase and 20% rutile phase. Because of a relatively wide band gap, titania absorbs light corresponding to wavelengths shorter than 388 nm, which is only 3–4% of the solar energy that reaches the earth. Thus, in principle, photocatalytic activity should be enhanced by adjusting the band gap toward visible light energies by doping, since visible light is readily available in the solar spectrum. Doping has been carried out in earlier research using various methods and materials. Common doping materials used have been inorganic compounds, noble metals, transition metal oxides, organic dye molecules, anionic compounds, etc. [5–7]. 1.2.1 Non-Metal Doping
Doping with various non-metallic compounds has been carried out to obtain visible light photoactivity of titania photocatalysts usually by introducing new energy states in between the band gap. Low-band gap, nitrogen-modified titania-based visible light photocatalysts prepared by Kisch and coworkers and Panayoto and coworkers are good examples of photocatalytic materials based on titania that has been doped with non-metallic material. According to these reports, nitrogen-doped titania photocatalyst clearly shows an intense band-to-band absorption in the range of 400–500 nm visible range of the solar spectrum, which brings the modified band gap of titania to 2.46–2.20 eV and very high photocatalytic activity toward formic acid mineralization under visible light (Figure 1.1) [8,9]. Figure 1.1 Diffuse reflectance spectra of (a) TiO2, (b) TiO2–N, (c) TiO2–N1 calcined 1 h, and (d) TiO2–N2 calcined 0.5 h [8]. Tang and coworkers also reported on highly crystalline and ordered mesoporous TiO2 thin films doped with carbon, synthesized via a highly cost-effective route, that exhibit high photocatalytic activity. In this material carbon inclusion plays a major role to stabilize the framework of titania during thermal crystallization process. Moreover, according to their findings high crystallinity and ordered mesoscopic structures always help to enhance the efficiency of photocatalysis [10] (see Figure 1.2). Figure 1.2 TEM images of a TiO2 thin film crystallized at 550 °C with pure post-induced carbon as the confining material. The zoom-in image is also shown on the right. The inset is a selected area electron diffraction pattern (SAED) indexed as the anatase phase [10]. Further, there are reports about titania-based photocatalytic materials co-doped with several non-metallic compounds. Xiang and coworkers as well as Hamal and coworkers have separately reported successful preparation methods for visible light active titania photocatalysts using more than one non-metal...