Environmental Photochemistry Part III

A proposal for modelling photo-Fenton reactors for water pollution remediation is presented. Reactor models, based on chemical reaction engineering principles and radiative energy transport fundamentals in homogeneous systems, are derived at both laboratory and solar pilot-plant scales. The proposed methodology is illustrated by presenting an example on the modelling and scaling up of a solar reactor for degradation of a model pollutant in aqueous solution: the herbicide 2,4-dichlorophenoxyacetic acid. Firstly, a kinetic model derived from a reaction sequence is proposed and its kinetic parameters estimated, using an isothermal, well-stirred tank laboratory photoreactor. Afterwards, the kinetic model is employed to predict the reacting species concentrations during the photo-Fenton degradation in a pilot-plant, nonisothermal solar reactor designed to capture the UV/Visible/IR solar radiation. This approach has proved to be appropriate to simulate the behaviour of the photo-Fenton reactor under different experimental conditions.

The surface properties of TiO₂ play a very important role in determining photocatalytic reaction efficiencies because heterogeneous photocatalytic reactions take place on the surface. Various parameters such as composition, phase structures, surface hydroxyl group, particle size, crystallinity, surface defects, and adsorbates or surface complexes play a key role. TiO₂ surfaces have been actively modified through manipulating the above parameters to enhance the photocatalytic performance. Here the main effects that influence the surface electron transfer are reported.

The use of photocatalysis for the photosplitting of water to generate hydrogen and oxygen has gained interest as a method for the conversion and storage of solar energy. The application of photocatalysis through catalyst engineering, mechanistic studies and photoreactor development has highlighted the potential of this technology, with the number of publications significantly increasing in the past few decades. In 1972 Fujishima and Honda described a photoelectrochemical system capable of generating H₂ and O₂ using thin-film TiO₂. Since this publication, a diverse range of catalysts and platforms have been deployed, along with a varying range of photoreactors coupled with photoelectrochemical and photovoltaic technology. This chapter aims to provide a comprehensive overview of photocatalytic technology applied to overall H₂O splitting. An insight into the electronic and geometric structure of catalysts is given based upon the one- and two-step photocatalyst systems. One-step photocatalysts are discussed based upon their d⁰ and d¹⁰ electron configuration and core metal ion including transition metal oxides, typical metal oxides and metal nitrides. The two-step approach, referred to as the Z-scheme, is discussed as an alternative approach to the traditional one-step mechanism, and the potential of the system to utilise visible and solar irradiation. In addition to this the mechanistic procedure of H₂O splitting is reviewed to provide the reader with a detailed understanding of the process. Finally, the development of photoreactors and reactor properties are discussed with a view towards the photoelectrochemical splitting of H₂O.

Over the past decade, the doping of nonmetal elements in wide band-gap semiconductors such as TiO₂ has been intensively investigated as an effective strategy of expanding the responsive solar spectrum of pristine semiconductors toward visible region. This chapter gives a review of this highly hot research topic. The fundamental principles involved and basic approaches are initially described. A range of nonmetal dopants are subsequently detailed with examples showing their effect on the photocatalytic performance such as pollutant degradation and water splitting under visible light. The problems simultaneously introduced by doping are also discussed.

Photochemical reactions induced by TiO₂ nanoparticles share common mechanistic features where electron and hole pairs are formed, migrate to the surface, and their recombination competes with their reaction with various substrates. The main interest in TiO₂ photocatalysis is related to its potential application for decontamination of water and air. However, the absorption of TiO₂, which is limited to UV light, does not enable the use of natural or cheap light sources, and therefore tremendous effort has been invested in inducing visible-light activity via modification of TiO₂ including doping with nonmetals and metals, surface coating, and bi- and multicomponent assembling. In addition, much research has been carried out to inhibit the electron–hole recombination and enhance the reactions of holes and electrons with substrates. The basic mechanism of bare and modified TiO₂ and the main principles of the photocatalytic processes remain similar, although the excitation energy is different and the energies of the electrons and holes and their reaction kinetic parameters may vary. These photocatalytic processes are reviewed and discussed.

This review article presents an overview of the application of ultraviolet light-emitting diode (UV LED) sources in heterogeneous photocatalysis within the context of artificial UV sources. The feasibility of UV LEDs as a source of UV irradiation in heterogeneous photocatalysis was first demonstrated almost a decade ago; however, for the most part, photocatalytic experimental set-ups utilise artificial light sources in the form of conventional UV lamps to initiate the desired photocatalytic transformations. A look at all sources of UV irradiation used in heterogeneous photocatalysis is taken with a focus on the growing importance of solid-state lighting devices such as UV LEDs. UV LEDs have higher external quantum efficiency and a lifetime of over 100,000 h; they are small in size and produce directional UV light which can be of the desired wavelength. In recent times, these UV LED sources have become widely applied in heterogeneous photocatalysis studies in the research literature and are fast becoming a viable alternative to conventional UV lamps.

Based on preceding work on photoelectrochemistry at semiconductor single crystal electrodes the field of photocatalysis at semiconductor powders has experienced a tremendous growth in the last three decades. The reason for this is the genuine property of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a great variety of electron donor and acceptor substrates. Whereas high attention was paid to water splitting and exhaustive aerobic degradation of pollutants, only a small part of research explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts and for the investigation of reaction mechanisms are discussed. Novel atom-economic C–C and C–N coupling reactions illustrate the relevance for organic synthesis. They exemplify that the multidisciplinary field of semiconductor photocatalysis combines classical photochemistry with electrochemistry, solid state chemistry, and heterogeneous catalysis.

Wastewater effluents from industry, at times, contain toxic organic chemicals that need to be treated prior to the effluent disposal. Advanced oxidation processes (AOPs) are efficient techniques that can potentially destroy a wide range of organic molecules. The choice of these techniques is based on their great potential for the complete mineralization of organic compounds present in these effluents that is readily explained by the generation of strongly oxidizing free radical intermediates, e.g., hydroxyl radicals. Photocatalysis is one of these AOPs where these radicals are generated on the surface of the illuminated semiconductor particles. Mesoporous metal oxides and mixed metal oxides have been receiving considerable attention in recent years due to their scientifically interesting properties and possible industrial applications in the fields of catalysis, adsorption, separation, ion exchange, and chemical sensing. This chapter covers recent developments in the syntheses of mesoporous semiconductor materials and presents applications of mesoporous semiconductors materials as efficient photocatalysts. The underlying reaction mechanisms will be explained and discussed.

This chapter reviews the use of infrared spectroscopy and electron paramagnetic resonance (EPR) spectroscopy to investigate reaction pathways in photocatalysis. In the case of infrared spectroscopy, examples are given from four different experimental methods for obtaining spectra of photocatalysts and adsorbed species: transmission, diffuse reflectance IR Fourier transform (DRIFT), attenuated total reflectance (ATR) and reflection–absorption infrared spectroscopy (RAIRS),which is applicable to single-crystal surfaces. EPR spectroscopy has been employed to observe trapped charge species (electrons and holes) and radical intermediates produced by reaction of electrons or holes with adsorbed species. Examples of both are given.

Among Advanced Oxidation Processes (AOPs), the Fenton process and the photochemically enhanced or assisted Fenton process, commonly called photo-Fenton, are considered to be among the most efficient for the oxidative degradation of a large variety of organic contaminants in aqueous systems. These processes, based on the generation of highly oxidizing species (hydroxyl radicals and possibly others) from hydrogen peroxide and Fe ions, may be counted among the few methods that are actually applied on a technical scale for an abiotic (pre-)treatment of wastewaters. With close to 5,000 articles published on this topic during the last decade, covering both fundamental aspects and applications, this chapter is restricted to a selective overview of the photo-Fenton process applied to water treatment. It briefly recalls the fundamentals of the Fenton reaction, describes the main lines of research for process enhancement and economic feasibility, summarizes the essentials determining the primary process parameters, and discusses the present state of technical development and its priorities.