Band photocatalytic reactions

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Upon UV light illumination, the photocatalytic reactions were initiated at the surface of the Pt/Ti02 catalyst, resulting in the formation of CO2, H2O, and intermediate species. Because of the overlap of the bands of CH3CH20Had (adsorbed ethanol) with the bands of the intermediate species, the intensity variations in the 1300-1750 cm region can be revealed through the difference spectra obtained by subtracting the spectrum at 0 min (i.e., before the reaction) from the subsequent... 

Formation of products and intermediate species, as well as disappearance of reactants during the photocatalytic reactions can be discerned by the evolution of positive (i.e., concave shape) bands and negative (i.e., convex shape) bands, respectively. 

Fig. 16.1 Processes involved in photocatalytic reactions, exemplified for some reactions for Ti02 in the presence of oxygen. When both redox states of acceptor A and donor D are energetically inside the band gap, the reaction of charge carriers is thermo-...

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

The photocatalytic reaction occurs by the formation of an electron-hole pair in a semiconducting material when the photon energy exceeds the band gap. Thus the photogenerated holes react readily with water and hydroxyl ions adsorbed in forming hydroxyl radicals. The hydroxyl radicals in turn act as oxidizing agents (97,98). 

Photocatalytic reactions at the semiconductor surface can be described by the following six steps as shown in Fig. 5.3. (D Absorption of a unit of light associated with the formation of a conduction band electron and a valence band hole in the semiconductor. (2) Transfer of an electron and a hole to the surface. (D Recombination of electron-hole pairs during the reaction processes. Stabilization of an electron and a hole at the surface to form a trapped electron and a trapped hole, respectively. (D Reduction and oxidation of molecules at the surface. (6) Exchange of a product at the surface with a reactant at a medium. Among these reaction steps, the absorption of light in the bulk (step CD) and... 

Figure 17.11 shows the spectral changes of DDQ in an n-butyronitrile solution as the result of the redox reactions with Fe(II) and bromine in aqueous solutions. The spectra are for just the DDQ solution (A), and after successive contacts first with an aqueous solution of Fe(II) chloride (B) and then with an aqueous solution of bromine (C). When the DDQ solution was brought into contact with an Fe(II) solution, an absorption band appeared at 352 nm, completely agreeing with that of the reduced form (DDHQ) of DDQ. This absorption band decreased by bringing the solution into contact with a bromine solution, as shown in Fig. 17.11, The reproducible spectral changes indicate the applicability of DDQ as the mediator between the two photocatalytic reactions producing Fe(II) ions and bromine, respectively.

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

Murabayashi coworkers (Nakajima et al., 2001, 2002) investigated the PL properties of rutile and anatase powders at room temperature in air in the absence or presence of reactants such as methanol or ethanol, with the goal of understanding the mechanisms of gas-phase photocatalytic reactions. In experiments with rutile, they observed that the PL intensity in the presence of methanol or ethanol increased linearly with the square root of UV-irradiation time, as shown in Figure 15. They also found a linear time dependence of the integrated amount of photodesorbed O2 (Nakajima et al., 2002). The time dependence of the PL intensity and the effect of O2 photodesorption were explained (as mentioned above) in terms of band bending of the powder. These results, however, were not observed for anatase powders. The authors explained that such a difference in PL behavior was related to the difference in photocatalytic activities of these alcohols. 

Thin-lilm photoelectrodes are needed in photoelectrocatalytic systems to apply a bias potential, either for the photoelectrode characterization or to facilitate the photocatalytic reactions. However, to be able to present a more comprehensive view on the performance of different materials, our subsequent discussions will focus on particulate semiconductor photocatalysts since the latter have been much more extensively investigated. Their electronic band structure (i.e., both the bandgap energy and the positions of CB and VB) is the key factor to determine whether or not a semiconductor material is suitable for a specific photocatalytic reaction, as will be demonstrated by reviewing a number of selected metal oxides and cou-pled/composite materials based on various semiconductors. 

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

The principle of photocatalysis is often explained with an illustration like Fig. 2, a schematic representation of the electronic structures of semiconducting materials, a band model. An electron in an electron-filled valence band (VB) is excited by photoirradiation to a vacant conduction band (CB), which is separated by a forbidden band, a band gap, from the VB, leaving a positive hole in the VB (Section III.B). These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst. Such an interpretation accounts for the photocatalytic reactions of semiconducting and insulating materials absorbing photons by the bulk of materials. In the definition of photocatalysis given above, however, no such limitation based on the electronic structure of a photocatalyst is included. For example, isolated... 

As has been discussed in Section III.E, a photocatalytic reaction can proceed if the CB bottom and VB top are more cathodic and anodic than the standard electrode potentials of electron acceptors and donors, respectively. Therefore, band-edge position... 

The determination of the photoluminescence parameters (excitation frequency, emission frequency, Stokes shift, fine structure parameter, and lifetime) can lead to information which, at the simplest level, indicates the presence of an electronically excited state of a species, but which can be sufficiently detailed so as to lead to a clear identification and characterization of the photoluminescent sites(J6-44). Moreover, measurements of the variations in the intensity and positions of the bands as a function of time (time-resolved photoluminescence) provide valuable kinetic data representing the reactions occurring at the surface. Although most of the photoluminescence measurements have been carried out at low temperatures for specific reasons (see Section III.C.2), there is much evidence that some of the excited states involved are present even at higher temperatures and that they play an important role in catalytic and photocatalytic reactions. Therefore, it is clear that the information obtained by photoluminescence techniques is useful and important lor the design of new catalysts and photocatalysts. 

Chemical differentiation between two oxidizable sites in the same molecule can also be achieved in organic photocatalytic reactions by choice of a different semiconductor and thus adjustment of the electrochemical band-edge positions. Consistent with this idea, the photocatalytic oxidation of lactic acid on UV-irradiated plati-nized-Ti02 leads to decarboxylation, presumably through the singly oxidized carboxyl radical. In contrast, the same reagent on irradiated platinized CdS leads to pyruvic acid by oxidation of the alcohol group (Eq. 10) [96]. 

Photoreactions of organic compounds over model surfaces of wide band-gap oxide semiconductors have received considerable attention recently [43, 79-82]. The most-studied photocatalytic reactions on rutile TiO lllO) single-crystal surfaces include ethanol [43], acetic acid [78], trimethyl acetic acid [80, 81], and acetone [82]. In this section, we will focus on the photoreaction of ethanol over TiOj(llO). Ethanol is dissociatively adsorbed via its oxygen lone pair on fivefold coordinated Ti atoms to produce adsorbed ethoxide species (Fig. 7.6). STM studies of the adsorption of ethanol on TiO2(110) demonstrated the presence of both alkoxides and surface hydroxyls [83] confirming the adsorption is dissociative. Figure 7.11 is the XPS Cls spectra after the exposure of ethanol (9=0.5 with respect to Ti atoms). 

Unfortunately, to date no data are available for the quantitative estimation of the role of photogenerated catalysis and photocatalytic reactions over halides and oxides with wide band gaps in the global chemistry of the atmosphere. Therefore, in the estimates made above, only photocatalytic reactions over the oxide semiconductors Fc203, Ti02, and ZnO with relatively narrow band gaps were taken into account. However, new compounds may be added in the future to the list of photocatalysts that are important to the chemistry of the atmosphere. 

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