Photocatalysis and Water Purification

From Fundamentals to Recent Applications

Pierre Pichat, Max Lu

• Publisher: Wiley-VCH
• Year: 2013
• Print ISBN: 9783527331871
• eBook ISBN: 9783527645411
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Environmental Management
• Index: Technology & Engineering

Water is one of the essential resources on our planet. Therefore, fresh water and the recycling of waste-water are very important topics in various areas. Energy-saving green technologies are a demand in this area of research.

Photocatalysis comprises a class of reactions which use a catalyst activated by light. These reactions include the decomposition of organic compounds into environmental friendly water and carbon dioxide, leading to interesting properties of surfaces covered with a photocatalyst:
they protect e.g. against incrustation of fouling matter, they are self-cleaning, antibacterial and viricidal. Therefore, they are attractive candidates for environmental applications such as water purification and waste-water treatment.

This book introduces scientists and engineers to the fundamentals of photocatalysis and enlightens the potentials of photocatalysis to increase water quality. Also, strategies to improve the photocatalytic efficacy are pointed out: synthesis of better photocatalysts, combination of
photocatalysis with other technologies, and the proper design of photocatalytic reactors. Implementation of applications and a chapter on design approaches for photocatalytic reactors round off the book.

'Photocatalysis and Water Purification' is part of the series on Materials for Sustainable Energy and Development edited by Prof. G.Q. Max Lu. The series covers advances in materials science and innovation for renewable energy, clean use of fossil energy, and greenhouse gas mitigation and associated environmental technologies.

Part I

Fundamentals: Active Species, Mechanisms, Reaction Pathways

1

Identification and Roles of the Active Species Generated on Various Photocatalysts

Yoshio Nosaka and Atsuko Y. Nosaka

TiO₂ photocatalysts have been utilized for the oxidation of organic pollutants [1–5]. For further practical applications, the improvement in the photocatalytic efficiency and the extension of the effective wavelength of the irradiation light are desired. From this point of view, better understanding of the primary steps in photocatalytic reactions is prerequisite to develop prominent photocatalysts. The properties of TiO₂ and the reaction mechanisms in molecular level have been reviewed recently [6]. Therefore, this chapter describes briefly active species involved in the photocatalytic reactions for bare TiO₂ and TiO₂ modified for visible-light response, that is, trapped electrons, superoxide radical (O₂^⋅−), hydroxyl radical (OH^⋅), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂).

1.1 Key Species in Photocatalytic Reactions

Since the photocatalytic reactions proceed usually with oxygen molecules (O₂) in air, the reduction of oxygen would be the important process in photocatalytic reduction. On the other hand, taking into account that the surface of TiO₂ photocatalysts is covered with adsorbed water molecules in usual environments and that photocatalysts are often used to decompose pollutants in water, oxidation of water would be the important process in photocatalytic oxidation. As shown in Figure 1.1, when O₂ is reduced by one electron (Eq. (1.1)), it becomes a superoxide radical (O₂^⋅−) that is further reduced by one electron (Eq. (1.2)) or reacts with a hydroperoxyl radical (HO₂^⋅, i.e., protonated O₂^⋅−) to form hydrogen peroxide (H₂O₂). The latter reaction is largely pH dependent because the amount 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^⋅). In the field of radiation chemistry, it is well documented that OH^⋅ is produced by one-electron oxidation of H₂O with ionization radiation. However, the formation of OH^⋅ in the photocatalytic oxidation process has not been confirmed, as described later.

1.1

1.2

1.3

Figure 1.1 One-electron reduction steps of oxygen to OH radical and two-electron oxidation step of water to H₂O₂ observed in the TiO₂ photocatalyst.

Figure 1.2 shows the standard potentials [8] for the one-electron redox of active oxygen species as a function of pH of the solution. The conduction band (CB) bottom for anatase and rutile TiO₂ along with valence band (VB) top of TiO₂ is also depicted. The pKa values for H₂O₂ and OH^⋅ are 11.7 and 11.9, respectively [7]. Therefore, the linear lines showing pH dependence in Figure 1.2 change the inclination at the individual pH. It is notable that in the pH range between 10.6 and 12.3, one-electron reduction resulting in OH^⋅ formation (Eq. (1.3)) occurs at a higher potential than that resulting in H₂O₂ formation (Eq. (1.2)). As commonly known, the potential of the VB of TiO₂ is low enough to oxidize H₂O, suggesting the possibility of the formation of OH^⋅. However, the potentials in the figure are depicted 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 VB holes at the surface of TiO₂ solid takes place in the heterogeneous system. Since the oxidation of H₂O to H₂O₂ and O₂ is also possible, only the potential difference between VB and OH^⋅ should not be used easily for explaining the possibility of the formation of OH^⋅. The competition between OH-radical-mediated reaction versus direct electron transfer has been studied as the effect of fluoride ions on the photocatalytic degradation of phenol in an aqueous TiO₂ suspension [9]. Under a helium atmosphere and in the presence of fluoride ions, phenol is significantly degraded, suggesting the occurrence of a photocatalytically induced hydrolysis [9].

Figure 1.2 The standard potentials for the one-electron redox of active oxygen species along with the energy bands of TiO₂ as a function of pH of the solution. All redox couples are one-electron process except for those indicated with 2e and 4e.

Primary intermediates of water photocatalytic oxidation at the TiO₂ in aqueous solution were investigated by in situ multiple internal reflection infrared (MIRIR) absorption combined with the observation of photoluminescence from trapped holes [10]. The reaction is initiated by a nucleophilic attack of a H₂O molecule on a photogenerated hole at a surface two hold coordinated O site to form [TiO^⋅HO–Ti]. A plausible reaction scheme is shown in Figure 1.3. Detailed investigations revealed the presence of TiOOH and TiOOTi as primary intermediates of the oxygen photoevolution reaction. This means that water is oxidized to form hydrogen peroxide adsorbed on TiO₂ surface, but the formation of OH radical in the oxidation process of water was denied.

Figure 1.3 Reaction scheme for the oxygen photoevolution reaction on TiO₂ (rutile) in contact with an aqueous solution of pH 1–12.

(Source: Reprinted with permission from Nakamura et al. [10]. © 2004 American Chemical Society.)

Ultraviolet photoelectron spectroscopy (UPS) studies showed that the top of the O-2p levels for surface hydroxyl groups (Ti–OH) at the rutile TiO₂ (100) face is about 1.8 eV below the top of the VB at the surface [11]. This implies that surface hydroxyl groups cannot be oxidized by photogenerated holes in the VB. On the basis of the electronic structure of surface-bound water obtained from the data reported in the literature of X-ray photoelectron spectroscopy (XPS) study, it is evidenced that water species specifically adsorbed on terminal (surface) Ti atoms cannot be photooxidized under UV illumination [12]. The photogenerated VB free holes are favorably trapped at the terminal oxygen ions of the TiO₂ surface (O²⁻)_s to generate terminal (O⁻)_s radicals, rather than being trapped at adsorbed water species to produce adsorbed OH^⋅. As discussed later, when OH^⋅ is detected in photocatalytic reactions, it should be formed by photocatalytic reduction of H₂O₂ (Eq. (1.3)).

1.2 Trapped Electron and Hole

Different from the semiconductor bulk, many electronic energy states may be formed within the band gap at the solid surface. These energy levels are capable of trapping VB holes and CB electrons. The trapped energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for carriers to travel from the bulk to the surface [13]. At the surface, the trapping sites generally correspond to five-coordinated Ti⁺ and two-coordinated O⁻ surface ions. When an appropriate acceptor (a scavenger), such as O₂ for electrons or methanol for holes, is adsorbed on the surface, it was suggested that the carriers should be preferentially transferred to the adsorbate rather than remain trapped at the surface sites [13].

When there are no molecules that can suffer the reaction, the existence of electrons and holes can be detected at a low temperature such as 77 K. To detect such paramagnetic species, electron spin resonance (ESR) spectroscopy is a valuable method [14, 15].

Holes and electrons could be observed by the absorption spectra just after the short pulse excitation under ambient temperature [16]. Trapped holes show that the absorption peaked at about 500 nm [17] and disappeared by the further reactions. On the other hand, trapped electrons show a broad absorption band that peaked at about 700 nm [18], which react mainly with oxygen molecules in air. Trapped electrons are so stable in the absence of O₂ that the kinetics can be explored by means of a stopped flow technique [19]. The reduction kinetics has been investigated through the electron acceptors such as O₂, H₂O₂, and NO₃⁻, which are often present in photocatalytic systems. The experimental results clearly showed that the stored electrons reduce O₂ and H₂O₂ to water by multielectron transfer processes [19]. Moreover, NO₃⁻ is reduced via the transfer of eight electrons evidencing the formation of ammonium ions. On the other hand, in the reduction of toxic metal ions, such as Cu(II), two-electron transfer occurs, indicating the reduction of the copper metal ion into its nontoxic metallic form.

1.3 Superoxide Radical and Hydrogen Peroxide (O₂^⋅− and H₂O₂)

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Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Editorial Board
• Series Editor Preface
• Preface
• About the Series Editor
• About the Volume Editor
• List of Contributors
• Part I: Fundamentals: Active Species, Mechanisms, Reaction Pathways
• Part II: Improving the Photocatalytic Eff icacy
• Part III: Effects of Photocatalysis on Natural Organic Matter and Bacteria
• Part IV: Modeling. Reactors. Pilot plants
• Index