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2013 | Buch

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Design of Advanced Photocatalytic Materials for Energy and Environmental Applications

herausgegeben von: Juan M. Coronado, Fernando Fresno, María D. Hernández-Alonso, Raquel Portela

Verlag: Springer London

Buchreihe : Green Energy and Technology

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

Research for the development of more efficient photocatalysts has experienced an almost exponential growth since its popularization in early 1970’s. Despite the advantages of the widely used TiO2, the yield of the conversion of sun power into chemical energy that can be achieved with this material is limited prompting the research and development of a number of structural, morphological and chemical modifications of TiO2 , as well as a number of novel photocatalysts with very different composition. Design of Advanced Photocatalytic Materials for Energy and Environmental Applications provides a systematic account of the current understanding of the relationships between the physicochemical properties of the catalysts and photoactivity.

The already long list of photocatalysts phases and their modifications is increasing day by day. By approaching this field from a material sciences angle, an integrated view allows readers to consider the diversity of photocatalysts globally and in connection with other technologies. Design of Advanced Photocatalytic Materials for Energy and Environmental Applications provides a valuable road-map, outlining the common principles lying behind the diversity of materials, but also delimiting the imprecise border between the contrasted results and the most speculative studies. This broad approach makes it ideal for specialist but also for engineers, researchers and students in related fields.

Inhaltsverzeichnis

Frontmatter

Chapter 1. A Historical Introduction to Photocatalysis

Abstract

As early as in 1911, the term photocatalysis appeared in several research communications, but for more than 50 years it was as much a scientific curiosity and, frequently, an annoyance. In the 1970s the interest in this technology boosted due to the pioneering works of Fujishima and Honda (1972) among others, but also as a consequence of the increasing concern of the society about energy supply and the environmental issues.

Juan M. Coronado

Chapter 2. Photons, Electrons and Holes: Fundamentals of Photocatalysis with Semiconductors

Abstract

Although not all the heterogeneous photocatalysts are semiconductors, this type of solids represents, by far, the most representative and widely investigated photoactive materials. For that reason, the fundamentals of the electronic structure of semiconductors, as well as the mechanism of their interaction with light and the relevance of their surface properties will be accounted for in this chapter.

Juan Manuel Coronado

Chapter 3. Environmental Applications of Photocatalysis

Abstract

Extensive research continues to optimize and widen the spectrum of potential applications of semiconductor photocatalysis. The applications are based on photooxidation, photoreduction, photosterilization and photoinduced superhydrophilicity phenomena or a combination thereof, and related with air and water treatment, active surfaces, green chemistry and energy conversion. The commercial potential of photocatalysis is impressive, including hygiene and sanitation; architecture and construction; automotive, food, chemical and textile industries; or environmental protection.

Raquel Portela, María Dolores Hernández-Alonso

Chapter 4. Turning Sunlight into Fuels: Photocatalysis for Energy

Abstract

The solar radiation can be collected and employed in different passive and active technological processes, which include the thermochemical, electrochemical, and photochemical/photobiological routes for production of storable and transportable fuels. Today, the conversion of solar radiation into electric and thermal energy is a reality. Meanwhile, CO2 photocatalytic reduction and the production of solar fuels remain the big challenges for the forthcoming years.

María Dolores Hernández-Alonso, Raquel Portela, Juan Manuel Coronado

Chapter 5. The Keys of Success: TiO2 as a Benchmark Photocatalyst

Abstract

Titanium dioxide is a material with a huge technological relevance because of its numerous applications but, since the seminal work by Fujishima and Honda, this solid is attracting the interest of the scientific community mainly because of its capacity to be photoactivated. This property is not only exploited in heterogeneous photocatalysis or in self-cleaning coatings, but it can also be applied directly to the production of electrical energy in dye-sensitized solar cells. Accordingly, despite the continuous quest for more efficient photoactive materialsTiO₂ remains as the benchmark photocatalyst. This chapter explores the physicochemicals basis of the remarkable properties of this material.

Juan M. Coronado, María Dolores Hernández-Alonso

Chapter 6. Alternative Metal Oxide Photocatalysts

Abstract

Some metal oxide semiconductors attract attention for its better performance in certain reactions compared to TiO₂, due to a more suitable band gap, higher adsorption of the reactants on their surface and to a more convenient product distribution. For instance, the interest on ZnO lays on its high quantum efficiency and for its higher electron mobility which is interesting for dye-sensitized solar cells. WO₃ is a visible light-response photocatalyst and under certain conditions may present higher photoactivity than titania under solar irradiation. It is a runner up for the water splitting reaction. Recent applications of WO₃ exploit its storage ability as an electron pool for metal protection and bactericidal activity in the dark.

Sergio García-Rodríguez

Chapter 7. The New Promising Semiconductors: Metallates and Other Mixed Compounds

Abstract

In this chapter, specific examples of both UV- and visible light–absorbing metallates and their photocatalytic activities for different reactions are described, not trying to be exhaustive though, but remarking only the most relevant photocatalysts to the best of the authors’ knowledge. At the end of the chapter, a mention to a new type of photocatalysts, bismuth oxyhalides, that is currently attracting a good deal of attention, is included.

Fernando Fresno

Chapter 8. Chalcogenides and Other Non-oxidic Semiconductors

Abstract

Although oxidic materials constitute the main group of semiconductors employed for photocatalytic applications, some non-oxide compounds have been used for these purposes too. After oxides, sulphides are the most widely used photocatalysts, with special importance in water splitting applications. CdS and ZnS are two of the “classical” photocatalysts, which have been used since the early beginning of the research in this field. In particular, CdS presents the advantage of absorbing visible light (E _g = 2.4 eV), and it has become somewhat a “standard” photocatalyst for reactions undertaken under visible irradiation. More recently, some other sulphides, like ternary sulphides and solid solutions, have shown interesting photocatalytic properties. However, sulphides present the drawback of suffering anodic photocorrosion unless prevented with certain reaction conditions or with special modifications, which limits their applications as we will see below. At the end of the chapter, selenides and other non-oxidic semiconductors will be overviewed.

Fernando Fresno

Chapter 9. Single-Site Photocatalysts: Photoactive Species Dispersed on Porous Matrixes

Abstract

Photocatalytic processes can take place at isolated photoactive centres dispersed on materials with high surface area and pore volume, such as zeolites and mesoporous materials. This special configuration is generally referred to as single-site photocatalyst and, because of its special properties, it should be differentiated from conventional photocatalysts based upon semiconductor materials. Characterisation techniques at atomic scale such as XANES, EXAFS, FTIR, UV-Vis or EPR allows the identification of these unique structures. In this chapter, the main characteristics of single-sites photocatalysts, aspects related to the synthesis routes, the catalytic properties and the benefits of their use in different applications will be analysed and discussed.

Silvia Suárez

Chapter 10. The Role of Co-catalysts: Interaction and Synergies with Semiconductors

Abstract

It is well known that the selection of an appropriate semiconductor is the key to obtain the desired results in a photocatalytic reaction. In the previous chapters, it has been shown which are the requirements for several photocatalytic processes, including pollutant removal, water detoxification or solar fuel production (from water splitting or CO₂ photoreduction), and how semiconductor engineering can help to improve the intrinsic physico-chemical and catalytic properties of a single-phase photocatalyst attending to solve the main drawbacks in photocatalytic reactions. However, in some cases, the modification of the photocatalyst with an appropriate co-catalyst leads to an improvement in the photoactivity. The use of co-catalysts leads to (1) a better charge separation and a decrease in the recombination rate, because these co-catalysts act as electron attractors; (2) an enhancement in the activity or a selectivity control in the redox reactions, because of their performance as specific catalytic active sites for reaction evolution; and, in some cases, (3) a broadening in the light operating conditions from UV to visible wavelengths.

Víctor A. de la Peña O’Shea

Chapter 11. Shaping Photocatalysts: Morphological Modifications of Semiconductors

Abstract

Tuning the size and shape of semiconductor particles to improve the performance and widen the range of application of these materials has attracted an extraordinary interest in nanotechnology, and it is matter of extensive research. Besides composition and structure, at the nanometre scale, material properties have proved to be very sensitive not only to the size but also to the shape of the particles. Control over nanocrystal architecture has opened the range of applications of semiconductors from light-emitting diodes (LEDs), medical diagnosis and energy storage, to information processing, besides their conventional uses in catalysis.

María Dolores Hernández-Alonso

Chapter 12. Immobilised Photocatalysts

Abstract

The use of immobilised photocatalysts on a substrate allows to overcome some of the main drawbacks associated to fluidised bed systems. This chapter will address the most important aspects to be considered for the preparation of immobilised photocatalysts, the techniques available for the immobilization of the active phase and the description of some of the most commonly used substrates, paying special attention to a novel family of bi-functional composites in which adsorption and photocatalytic properties are combined, the so-called adsorbent-photocatalyst hybrid materials.

Silvia Suárez

Chapter 13. Metal Doping of Semiconductors for Improving Photoactivity

Abstract

Doping the structure of TiO₂ has been extensively used as an approach for improving its photocatalytic properties. In particular, metal doping has proved to be a successful approach for obtaining photocatalysts with improved photonic efficiencies. With this strategy, it is possible to shift the optical response toward higher wavelengths. Besides optical absorption, other properties that strongly influence the photocatalytic activity, such as surface area, pore size, density of hydroxyl groups, surface acidity, and adsorption/desorption properties, can also be modulated by incorporating a dopant into the photocatalyst structure.

María Dolores Hernández-Alonso

Chapter 14. Non-metal Doping for Band-Gap Engineering

Abstract

Non-metal doping appeared in 2001 as a promising alternative strategy to red-shift the absorption edge of TiO₂ in photocatalytic applications. Since then, nearly all non-metals have been explored in single-element doping or codoping with other non-metal, a metal, or even a metalloid. The chemical state, amount, and position of the non-metal depend on the dopant selection and the synthesis method, which determine the success of the doping strategy. A big amount of theoretical and experimental research is underway, being the achievement of higher activity under visible than under ultraviolet light the main challenge.

Raquel Portela

Chapter 15. Heterojunctions: Joining Different Semiconductors

Abstract

Coupling different semiconductors is one of the strategies that have been successfully followed in order to obtain more active photocatalysts. Here we will concentrate on materials composed of clearly distinct crystalline phases and, especially on the role of the interface between these different phases, which is the feature that characterizes this type of systems and provides improved photocatalytic (and other) properties. TiO₂-based coupled systems are by far the most studied ones, and hence, they will be covered to a higher extent.

Fernando Fresno

Chapter 16. Sensitizers: Dyes and Quantum Dots

Abstract

Several approaches aimed at extending the spectral activity range of wide-band-gap semiconductors have been already reviewed in previous chapters of this book. In addition to structural modifications that may lead to different electronic effects, such as anionic or cationic doping and solid solution formation, there is a possibility of modifying the surface of the semiconductor with a substance that absorbs light energy and transfers it, under favourable conditions, to an otherwise photochemically inactive or less active substrate. This substance is referred to as photosensitizer.

Fernando Fresno, María Dolores Hernández-Alonso

Chapter 17. Future Perspectives of Photocatalysis

Abstract

After an already long history, which has experienced remarkable progress in the last 25 years, photocatalysis still has a brilliant future ahead. Thus, it seems pertinent to explore the tendencies of this technology in an attempt to anticipate the developments to come in the following decades. This is obviously not a trivial task because of the considerable uncertainties inherent to any projection, but some trends can be outlined with a reasonable degree of confidence in two main aspects: new applications and design of novel materials.

Juan M. Coronado, María D. Hernández-Alonso, Fernando Fresno, Raquel Portela

Titel: Design of Advanced Photocatalytic Materials for Energy and Environmental Applications
herausgegeben von: Juan M. Coronado
Fernando Fresno
María D. Hernández-Alonso
Raquel Portela
Verlag: Springer London
Electronic ISBN: 978-1-4471-5061-9
Print ISBN: 978-1-4471-5060-2
DOI: https://doi.org/10.1007/978-1-4471-5061-9