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Photocatalysis is currently an extremely active and constantly growing research area. When thinking about the concepts of this book we realized that it will be impossible to produce a comprehensive overview of this research field. Hence, we decided to ask selected experts to provide state-of-the-art contributions focusing on specific aspects of photocatalysis which they consider to be important. Looking at the result of this compilation of knowledge we are confident that it provides both an in depth coverage of the most important fundamental aspects of photocatalysis and some of the major concepts of future activities and thus the perspectives of this field.

This first part aims to provide an overview of the physical and chemical processes involved in photocatalytic reactions. Starting with the “mother of photocatalysis”, i.e., the photoelectrochemistry of semiconductors, it will be shown how charge carriers are generated upon the interaction of light with suitable semiconductors. Following their separation in the field of the space charge layer the original physical nature of the charge carriers is then converted into chemical entities through trapping processes at surface sites. The subsequent reactions of these often very reactive free radical species is covered in Chapter 2, following the oxidative and the reductive paths separately. Photocatalytic systems often consist of nanoparticulate semiconductors, which besides exhibiting a very high surface area for the desired reactions also interact with one another, leading to phenomena such as the antenna effect and the cooperative action within photonic structures. These mechanistic aspects are covered in Chapter 3. Chapter 4 shows how photocatalytic systems can be designed to exhibit high specificity for certain desired reactions, thus avoiding the need to separate the reaction products at the end of the process. Various ways to improve the photocatalytic activity of “standard” photocatalyst materials such as titanium dioxide are presented in Chapter 5: impurity-doped semiconductors, coupled semiconductors, dye-sensitized semiconductors, LMCT-sensitized semiconductors, and metal/semiconductor interfaces. New concepts in photocatalysis are presented in the last chapter of this part (Chapter 6), including graphene and carbon nitrides as potential photocatalysts. The so-called Z-scheme photocatalytic system is described in detail and the features of plasmonic photocatalysis are presented and critically discussed.

The principal objective of this part is to present the most important fundamental studies concerning the underlying photocatalytic processes, since knowledge of these processes is of utmost importance in understanding the photocatalytic reaction mechanism and thus for a better design of photocatalytic systems. The trapping of the photogenerated charge carriers reaching the surface of the photocatalyst at suitable defect sites is one of the primary processes in photocatalysis which can, on the one hand, underpin the photocatalytic activity, whereupon the trapping sites act as docking centers for the charge carrier transfer between the semiconductor surface and the adsorbed molecules. However, on the other hand, a very high number of these trapping sites can lead to an enhanced recombination of the charge carriers resulting in a drastic decrease of the photocatalytic activity. Different techniques for the detection of the trapped charge carriers such as transient absorption spectroscopy and electron spin resonance spectroscopy as well as photo-electrochemical experiments will be presented here. Moreover, this part will also highlight the interplay phenomena taking place between physical and chemical events in heterogeneous photochemistry and photocatalysis through an examination of some relevant processes such as the physical and chemical decay of the active state of surface-active centers through charge carrier recombination and chemical interaction, as an effect of catalytic and non-catalytic surface photochemical processes on the photostimulated formation of defects.

In this part, a brief introduction to the general guideline for designing new materials will be given (Chapter 10), then the state-of-the-art research activities of new materials development will be reviewed in detail, which cover those designed for the three major photocatalytic reactions, i.e., degradation of various organics in gaseous and liquid phases (Chapter 11), pure water-splitting or hydrogen evolution/oxygen evolution from aqueous solution containing a sacrificial reagent (Chapter 12), and CO2 reduction and production of hydrocarbon fuels (Chapter 13). The crucial issues that should be addressed in future research activities in new materials development will also be highlighted.

This part describes the fundamentals of radiation transport in absorbing and scattering media (Chapter 14), the evaluation of the rate of photon absorption by the photocatalyst, the application of radiation transport theory for photoreactor design (Chapter 15), and the modeling and experimental validation of laboratory- and pilot-scale photoreactors (Chapter 16). The models developed consider the optical properties of the photocatalytic materials, the emission of radiation from different light sources (solar radiation and artificial radiation sources), the photon absorption and scattering effects in photocatalytic reactors, the elementary reaction kinetics of photocatalytic processes, the fluid-dynamics in the reactors, and the mass balances on the reacting species. Both rigorous and simplified methods for evaluation of the rate of photon absorption (full solution of the radiative transfer equation and two- and six-flux models) are presented. The models are applied to case studies of photocatalytic oxidation of water contaminants in both laboratory-scale reactor and pilot-scale solar photoreactors, including compound parabolic collectors (CPC), flat-plate photoreactors (FPRs) and annular geometries.

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