Titanium dioxide excitation

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). 

A mechanism which has been proposed for the operation of this type of photocell is illustrated in Figure 10.7, although it is not fully established in detail. In the proposed mechanism, it is suggested that absorption of light by the dye (or sensitiser, S) raises the dye to its first excited state S. In the excited state, S releases an electron into the conducting band of the titanium dioxide electrode, at the same time forming oxidised sensitiser, S +. At the counter-electrode, an electron is transferred to the... 

Since the discovery of photoelectrochemical splitting of water on titanium dioxide (TiOj) electrodes (Fujishima and Honda, 1972), semiconductor-based photocatalysis has received much attention. Although TiO is superior to other semiconductors for many practical uses, two types of defects limit its photoeatalytic activity. Firstly, TiO has a high band-gap (E =3.2 eV), and it can be excited only by UV light (k < 387 nm), which is about 4-5% of the overall solar spectmm. Thus, this restricts the use of sunlight or visible light (Kormann et al., 1988). Secondly, the... 

Jacoby et al. (1994) studied the photocatalytic reaction of gaseous trichloroethylene in air in contact with UV-irradiated titanium dioxide catalyst. The UV radiation was kept less than the maximum wavelength so that the catalyst could be excited by photons, i.e., X <356 nm. Two reaction pathways were proposed. The first pathway includes the formation of the intermediate dichloroacetyl chloride. This compound has a very short residence time and is quickly converted to the following compounds phosgene, carbon dioxide, carbon monoxide, carbon dioxide, and hydrogen chloride. The second pathway involves the formation of the final products without the formation of the intermediate. 

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

In the photooxidation of phenol induced by excited titanium dioxide, the hydroxy radical was directly implicated as the reactive species, the observed organic products having incorporated oxygen, Eq. (16) On further photolysis, aldehydes, acids, and CO2 could be obtained. 

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The direct charge transfer to dichloroacetate proposed in reaction (7.21) requires that the scavenging molecules are adsorbed on the Ti02 surface prior to the adsorption of the photon. Otherwise, this reaction could not compete with the normal hole-trapping reactions (7.9) and (7.10). So the adsorption of the model compound DCA on the titanium dioxide surface prior to the bandgap excitation appears to be a prerequisite for an efficient hole scavenging. 

Abstract This article updates the one on the same topic published in this series in 1999. The photochemistry of PAHs and PCBs in liquid water and on ice and other solids such as silica, soil and titanium dioxide continues to be actively studied. The photochemistry of PAHs in all phases continues to be dominated by oxidation by O2, with superoxide (O2- ), excited singlet oxygen (102), and hydroxyl radical ( OH) being the active agents. The recent photochemistry of PCBs has been dominated by practical considerations, i.e. how to use photochemistry to clean up environmental problems involving PCBs. The use of surfactants, the semiconductor TiC>2, and various sources of the powerful oxidant, the hydroxyl radical, in this regard has received considerable attention. 

In catalyzed photolysis either the catalyst molecule (Fig. 5-11, situation B) or the substrate molecule (Fig. 5-11, situation C), or both, are in an electronically excited state during the catalytic step. The electronically excited catalyst molecule is produced via photon absorption by a nominal catalyst (Fig. 5-11, situation B). The reaction of substrate to product is catalytic with, respect to the concentration of the electronically excited catalyst species. It is non-catalytic in photons and therefore, continuous irradiation is required to maintain the catalytic cycle. The quantum yield of product formation Product is equal to or less than unity. Titanium dioxide photocatalysis is the most widely applied example of this type, with Ti02 representing the nominal catalyst that must be electronically excited by photon absorption with formation of the electron hole pair Ti02 (hvb + cb), being the active catalytic species (cf Fig. 3-17 and Fig. 5-9, reaction 1). The oxidation of substrates by the combination of UV/VIS radiation and an appropriate photocatalyst is often called photocatalytic oxidation (PCO). 

Fig. 5.16 Classification of photo-initiated AOPs according to active wavelength ranges and spectral domains of excitation. designates modified titanium dioxide, for...

Figure 12. a) Transient absorption spectra obtained upon nanosecond pulsed laser excitation of 1) cw-[Ru"(dcbpy)2(NCS)2] dye in ethanolic solution, and 2) a sensitized Ti02 transparent film. Spectra were recorded 50 ns (la, 2a) and 0.5 ps (lb, 2b) after the laser excitation pulse (A = 605 nm, 5 ns pulse duration), b) Transient absorption spectra recorded 6 ps after ultrafast laser excitation (A = 605 nm, 150 fs pulse duration) of 1) c -[Ru (dcbpy)2(NCS)2] dye in ethanol, and 2) a fresh sensitized titanium dioxide film. Insert, the temporal behavior of the absorbance of the latter system, measured at A = 750 nm with sub-picosecond time resolution. 

The fastest kinetic phase of electron injection in c/j-[Ru (dcbpy)2(NCS)2]-sensitized nanocrystalline titanium dioxide films apparently takes place in the femtosecond regime. Besides, the vibrational relaxation of the dye excited state is expected to occur typically within 0.4-1 ps k 10 s ) [57, 58]. Observed injec-... 

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