fundamentals active species mechanism reaction pathways, science
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//-->1Part IFundamentals: Active Species, Mechanisms, Reaction PathwaysPhotocatalysis and Water Purification: From Fundamentals to Recent Applications,First Edition. P. Pichat.©2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.31Identification and Roles of the Active Species Generated onVarious PhotocatalystsYoshio Nosaka and Atsuko Y. NosakaTiO2photocatalysts have been utilized for the oxidation of organic pollutants[1–5]. For further practical applications, the improvement in the photocatalyticefficiency and the extension of the effective wavelength of the irradiation lightare desired. From this point of view, better understanding of the primarysteps in photocatalytic reactions is prerequisite to develop prominent photo-catalysts. The properties of TiO2and the reaction mechanisms in molecularlevel have been reviewed recently [6]. Therefore, this chapter describes brieflyactive species involved in the photocatalytic reactions for bare TiO2and TiO2modified for visible-light response, that is, trapped electrons, superoxide radical(O2•−), hydroxyl radical (OH•), hydrogen peroxide (H2O2), and singlet oxygen(1O2).1.1Key Species in Photocatalytic ReactionsSince the photocatalytic reactions proceed usually with oxygen molecules (O2)in air, the reduction of oxygen would be the important process in photocatalyticreduction. On the other hand, taking into account that the surface of TiO2pho-tocatalysts is covered with adsorbed water molecules in usual environments andthat photocatalysts are often used to decompose pollutants in water, oxidationof water would be the important process in photocatalytic oxidation. As shownin Figure 1.1, when O2is reduced by one electron (Eq. (1.1)), it becomes asuperoxide radical (O2•−) that is further reduced by one electron (Eq. (1.2)) orreacts with a hydroperoxyl radical (HO2•, i.e., protonated O2•−) to form hydro-gen peroxide (H2O2). The latter reaction is largely pH dependent because theamount of HO2•, whose pKa is 4.8, changes largely at pH around neutral [7].One-electron reduction of H2O2(Eq. (1.3)) produces hydroxyl radical (OH•). Inthe field of radiation chemistry, it is well documented that OH•is producedby one-electron oxidation of H2O with ionization radiation. However, the for-mation of OH•in the photocatalytic oxidation process has not been confirmed,Photocatalysis and Water Purification: From Fundamentals to Recent Applications,First Edition. P. Pichat.©2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.41 Identification and Roles of the Active Species Generated on Various PhotocatalystsOxygenO2e−LightReductionO2−e−e−Superoxide radical(Disproportionation)reductionTiO2excitationH2O2H2O2Hydrogen peroxideh+Reductionh+OHHydroxyl radicalWaterH2OFigure 1.1One-electron reduction steps of oxygen to OH radical and two-electronoxidation step of water to H2O2observed in the TiO2photocatalyst.as described later.•O2+e−→O2–•O2–+2H++e−→H2O2(1.1)(1.2)(1.3)H2O2+H++e−→OH+H2O•Figure 1.2 shows the standard potentials [8] for the one-electron redox of activeoxygen species as a function of pH of the solution. The conduction band (CB)bottom for anatase and rutile TiO2along with valence band (VB) top of TiO2isalso depicted. The pKa values for H2O2and OH•are 11.7 and 11.9, respectively[7]. Therefore, the linear lines showing pH dependence in Figure 1.2 change theinclination at the individual pH. It is notable that in the pH range between 10.6and 12.3, one-electron reduction resulting in OH•formation (Eq. (1.3)) occurs at ahigher potential than that resulting in H2O2formation (Eq. (1.2)). As commonlyknown, the potential of the VB of TiO2is low enough to oxidize H2O, suggestingthe possibility of the formation of OH•. However, the potentials in the figure aredepicted based on the free energy change in a homogeneous aqueous solution.Therefore, it does not always mean that the one-electron oxidation of H2O by VBholes at the surface of TiO2solid takes place in the heterogeneous system. Sincethe oxidation of H2O to H2O2and O2is also possible, only the potential differencebetween VB and OH•should not be used easily for explaining the possibility ofthe formation of OH•. The competition between OH-radical-mediated reactionversus direct electron transfer has been studied as the effect of fluoride ions on thephotocatalytic degradation of phenol in an aqueous TiO2suspension [9]. Under ahelium atmosphere and in the presence of fluoride ions, phenol is significantlydegraded, suggesting the occurrence of a photocatalytically induced hydrolysis [9].Primary intermediates of water photocatalytic oxidation at the TiO2in aqueoussolution were investigated byin situmultiple internal reflection infrared (MIRIR)absorption combined with the observation of photoluminescence from trappedholes [10]. The reaction is initiated by a nucleophilic attack of a H2O molecule on aphotogenerated hole at a surface two hold coordinated O site to form [TiO•HO–Ti].1.1 Key Species in Photocatalytic Reactions5−1.06 CB (Anatase)−0.86 CB (Rutile)−0.23+O2,H /HO2−0.046−0.03e(2)e)(4−0.33 O2/O2−−0.08 O2,H2O/HO2−,OH−+0.184HO2−/OH,2OH−+0.20O2−/HO2−,OH−+0.40O2,2H2O/4OH−O2,2H+/H2O2+0.69H2O2,H+/OH +1.14O2,4H+/2H2O +1.23HO2,H+/H2O2+1.44O2−,2H+/H2O2+1.59O−, H2O/2OH−+2.14 VB (TiO2)OH,H+/H2O +2.38+3.034.8pH11.7 14Figure 1.2The standard potentials for the one-electron redox of active oxygen speciesalong with the energy bands of TiO2as a function of pH of the solution. All redox couplesare one-electron process except for those indicated with 2e and 4e.A plausible reaction scheme is shown in Figure 1.3. Detailed investigations revealedthe presence of TiOOH and TiOOTi as primary intermediates of the oxygenphotoevolution reaction. This means that water is oxidized to form hydrogenperoxide adsorbed on TiO2surface, but the formation of OH radical in theoxidation process of water was denied.Ultraviolet photoelectron spectroscopy (UPS) studies showed that the top of theO-2p levels for surface hydroxyl groups (Ti–OH) at the rutile TiO2(100) face isabout 1.8 eV below the top of the VB at the surface [11]. This implies that surfacehydroxyl groups cannot be oxidized by photogenerated holes in the VB. On thebasis of the electronic structure of surface-bound water obtained from the datareported in the literature of X-ray photoelectron spectroscopy (XPS) study, it isevidenced that water species specifically adsorbed on terminal (surface) Ti atomscannot be photooxidized under UV illumination [12]. The photogenerated VB freeholes are favorably trapped at the terminal oxygen ions of the TiO2surface (O2−)s
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