Photocatalysis

Photocatalysis can be defined as follows A change in the rate of chemical reactions or their generation imder the action of light in the presence of substances called photocatalysts - that absorb light quanta and are involved in the chemical transformations of the reactants [4]. Typical photocatalysts or photosensitisers are semiconductor materials. There are many chemical compounds which can act as photocatalysts, but only a very few of these materials are photochemically and chemically stable semiconductor photocatalysts, one compound dominates titania (titanium dioxide) Ti02. 

Induslrial Catalysis A Practical Approach, Second Edition. Jens Hagen Copyright 2006 WILEY-VCH Verlag GmbH Co KGaA.Weinheim ISBN 3-527-31144-0 

Electron-hole recombination usually dominates semiconductor photosensitation so the overall process is often not very efficient (typically 1%) with respect to 

Titania exists mainly as two crystalline forms, anatase and rutile. Anatase is generated by the usual low temperature production methods, such as alkaline hydrolysis of titanium(IV) compounds followed by calcination at moderate temperatures (400-500 °C). Anatase readily converts to rutile at elevated temperatures ( 700 °C) although this phase change is often accompanied by extensive sintering. As a consequence, rutile usually has a much lower specific surface area (by a factor of 10 or more) than the anatase from which it was derived. 

Titania only absorbs 2-3 % of the solar spectrum so is of limited use as a photosen-sitiser for any solar-driven system. Despite this, much research has been carried out 

The reaction rate of photocatalysis is commonly slow because of the phenomenon of electron-hole recombination. The limited activating UV spectrum in sunlight for Ti02 also explains why solar photocatalysis has a low photonic efficiency. A number of strategies have been developed to modify photocatalysts for visible-light response, such as using metal (Lin et al., 2010), metal oxide (Peng et al., 2011), and metal sulfide (Li et al., 2010) as modifiers. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The more efficient system of genera ting OH radicals in the homogeneous phase is H2O2/uv, where the quantum yield, 254 0.50 (20,21). There 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

The most interesting observation has been that in the sol-gel matrices, the keto-enol proton-transfer dynamics in photoexcited BP(OH)(2) is faster than the dynamics in pure silica sol-gel glass and even fester than in liquid solution. 

The successful entrapment of enzymes within alkoxysilane-polymerized sol-gel matrices has been perhaps the most nontrivial molecular doping of these materials. The expected pressure development upon polymerizations, the release of alcohols, and the low probability that the active site will remain open to the pore size all made it quite improbable that this entrapment will succeed. Following the report of Braun et al. in 1990 [39] that in fact the opposite is true, and not only that such entrapments keep the enzymes alive, but their stability is also 

Last but not least, we wish to highlight that biocatalysis with sol-gel materials should not be limited to enzymes only - entrapped catalytic antibodies maybe applied as well. An example is the study by Shabat et al [48] who entrapped the 

Finally, the first commercialized sol-gel-entrapped catalyst is due to Reetz [55]. A number of lipases entrapped in hydrophobic sol-gel matrices can be found that are marketed by Fluka high activity and stability characterize these entrapped enzymes. 

We deeply thank our many collaborators on this catalysis project, the names of which appear in the reference list as our coauthors - without their deep understanding of the complex issues involved in catalysis, none of these achievements could have been reached. We thank Prof. Mario Pagliaro for help in writing Section 31.10. 

The oxidation of acetate by O2 is a downhill reaction, which is catalysed by Ti02 in the presence of light absorbed by the semiconductor. Corresponding reactions are performed with stable oxides, primarily with Ti02 particles. The basic processes are illustrated in Fig. 2.34 for dispersed small semiconductor particles as typically used in this application. The holes produced by light excitation are used for the oxidation of the acetate, whereas electrons are transferred to O2. 

Whether the anodic or the cathodic reaction is rate-limiting has not yet been determined. Recently, it has been shown that the role of oxygen is actually twofold  

This chapter is intended to serve as a basis for understanding the reviews given throughout this volume. It has provided an overview of essentially all the fundamental processes governing the conversion of solar energy into other forms of energy based on exploiting semiconductor photochemistry. 

These microscopic details have to be viewed in the context of the entire electrochemical system, as outlined in Section 2.3. The energetics of the interface in relation to electrochemical measnrements and solid-state conventions have been given. The energetics provide a gnideline for determining the maximnm energy 

The authors submitted the final version of this manuscript in 2001. Owing to subsequent delays in the preparation of this book series, most of the references in this chapter date from this time. This chapter provides the fundamental photophysics and photoelectrochemical basics that are still relevant. Updated references, where needed, have been provided by the editors. 

Japanese companies have pioneered a number of interesting and valuable commercial applications. Cleaning windows is time consuming 

Catalysts are extensively used and have played a huge role in making bulk chemical manufacturing technology more competitive and environmentally fnendly. Undoubtedly catalysis will continue to provide the answer to many economic and environmental challenges currently faced by industry. As indicated above catalysts are now needed by the fine chemical and pharmaceutical industries, and they need to be robust, selective, recoverable and reusable. 

Photoassisted catalytic hydrogenation reactions invariably involve promoted loss of a ligand and generation of coordinatively unsaturated species. Examples in the literature are becoming increasingly common. 

Olefin hydrogenation catalyzed by Fe(CO)s normally requires somewhat severe conditions, typically 150°C and 10 atm H2 (/, p. 64). With near-ultraviolet irradiation the carbonyl becomes effective at ambient conditions for hydrogenation (and isomerization) of olefins (448, 449). Photoinduced labilization of carbonyls is thought to give tricarbonyl species as the active catalysts, e.g., 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem- 

Titania acts as an electrode or electrode coating in photoelectrolysis cells to enhance the efficiency of electrolytic splitting of water into hydrogen and oxygen (the Honda-Fujishima effect see below). The most important use of the photocatalytic functional performance of titania (in particular anatase) has been found in the 

Electrode Ultrafine particle gas-sensitive Ti02 film 

The outstanding photocatalytic activity of the anatase polymorph of titanium dioxide (Ti02) was discovered and studied by Fujishima and Honda (1972). The use of n-type Ti02 as an anode and Pt as a cathode, and irradiation by sunlight or UV light, led to the evolution of hydrogen at the Pt electrode and oxygen at the TiOj site. A comprehensive account on the surface science of titanium dioxide, based on almost 800 references, has been produced by Diebold (2003). 

While the oxidation reactions initiated by the photo-generated holes are  

The photocatalysts that are commonly used for CDC transformations typically fall into one of three categories  

It will be demonstrated in later sections that there are several eompli-mentary catalytie systems that can be implemented for the same transformation. For example, nitro-Mannich, Strecker-type, and alkynylation reactions can be performed under a variety of photochemical conditions. Other reactions require more specific conditions to bias pathways involving discrete intermediates, which in some cases may depend upon the use of terminal oxidants or specialized coupling partners. However, controlling the outcome of a photochemical reaction is challenging and may be influenced by certain thermodynamic or electronic properties of the molecule targeted for C-H activation (see the following section). 

---

Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

---