Cover Page

Table of Contents

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

1: Identification and Roles of the Active Species Generated on Various Photocatalysts

1.1 Key Species in Photocatalytic Reactions

1.2 Trapped Electron and Hole

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

1.4 Hydroxyl Radical (OH)

1.5 Singlet Molecular Oxygen (1O2)

1.6 Reaction Mechanisms for Bare TiO2

1.7 Reaction Mechanisms of Visible-Light-Responsive Photocatalysts

1.8 Conclusion

References

2: Photocatalytic Reaction Pathways – Effects of Molecular Structure, Catalyst, and Wavelength

2.1 Introduction

2.2 Methods for Pathway Determination

2.3 Prototypical Oxidative Reactivity in Photocatalytic Degradations

2.4 Prototypical Reductive Reactivity in Photocatalytic Degradations

2.5 The Use of Organic Molecules as Test Probes for Next-Generation Photocatalysts

2.6 Modified Catalysts: Wavelength-Dependent Chemistry of Organic Probes

2.7 Conclusions

References

3: Photocatalytic Mechanisms and Reaction Pathways Drawn from Kinetic and Probe Molecules

3.1 The Photocatalyic Rate

3.2 Surface Speciation

3.3 Multisite Kinetic Model

3.4 Conclusion

References

Part II: Improving the Photocatalytic Eff icacy

4: Design and Development of Active Titania and Related Photocatalysts

4.1 Introduction – a Thermodynamic Aspect of Photocatalysis

4.2 Photocatalytic Activity: Reexamination

4.3 Design of Active Photocatalysts

4.4 A Conventional Kinetics in Photocatalysis: First-Order Kinetics

4.5 A Conventional Kinetics in Photocatalysis: Langmuir–Hinshelwood Mechanism

4.6 Topics and Problems Related to Particle Size of Photocatalysts

4.7 Recombination of a Photoexcited Electron and a Positive Hole

4.8 Evaluation of Crystallinity as a Property Affecting Photocatalytic Activity

4.9 Electron Traps as a Possible Candidate of a Recombination Center

4.10 Donor Levels – a Meaning of n-Type Semiconductor

4.11 Dependence of Photocatalytic Activities on Physical and Structural Properties

4.12 Synergetic Effect

4.13 Doping

4.14 Conclusive Remarks

Acknowledgments

References

5: Modified Photocatalysts

5.1 Why Modifying?

5.2 Forms of Modification

5.3 Modified Physicochemical Properties

Summary

References

6: Immobilization of a Semiconductor Photocatalyst on Solid Supports: Methods, Materials, and Applications

6.1 Introduction

6.2 Immobilization Techniques

6.3 Supports

6.4 Laboratory and Industrial Applications of Supported Photocatalysts

6.5 Conclusion

References

7: Wastewater Treatment Using Highly Functional Immobilized TiO2 Thin-Film Photocatalysts

7.1 Introduction

7.2 Application of a Cascade Falling-Film Photoreactor (CFFP) for the Remediation of Polluted Water and Air under Solar Light Irradiation

7.3 Application of TiO2 Thin-Film-Coated Fibers for the Remediation of Polluted Water

7.4 Application of TiO2 Thin Film for Photofuel Cells (PFC)

7.5 Preparation of Visible-Light-Responsive TiO2 Thin Films and Their Application to the Remediation of Polluted Water

7.6 Conclusions

References

8: Sensitization of Titania Semiconductor: A Promising Strategy to Utilize Visible Light

8.1 Introduction

8.2 Principle of Photosensitization

8.3 Dye Sensitization

8.4 Polymer Sensitization

8.5 Surface-Complex-Mediated Sensitization

8.6 Solid Semiconductor/Metal Sensitization

8.7 Other Strategies to Make Titania Visible Light Active

8.8 Conclusions

Acknowledgment

References

9: Photoelectrocatalysis for Water Purification

9.1 Introduction

9.2 Photoeffects at Semiconductor Interfaces

9.3 Water Depollution at Photoelectrodes

9.4 Photoelectrode Materials

9.5 Electrodes Preparation and Reactors

9.6 Conclusions

References

Part III: Effects of Photocatalysis on Natural Organic Matter and Bacteria

10: Photocatalysis of Natural Organic Matter in Water: Characterization and Treatment Integration

10.1 Introduction

10.2 Monitoring Techniques

10.3 By-Products from the Photocatalytic Oxidation of NOM and its Resultant Disinfection By-Products (DBPs)

10.4 Hybrid Photocatalysis Technologies for the Treatment of NOM

10.5 Conclusions

References

11: Waterborne Escherichia coli Inactivation by TiO2 Photoassisted Processes: a Brief Overview

11.1 Introduction

11.2 Physicochemical Aspects Affecting the Photocatalytic E. coli Inactivation

11.3 Using of N-Doped TiO2 in Photocatalytic Inactivation of Waterborne Microorganisms

11.4 Biological Aspects

11.5 Proposed Mechanisms Suggested for Bacteria Abatement by Heterogeneous TiO2 Photocatalysis

11.6 Conclusion

References

Part IV: Modeling. Reactors. Pilot plants

12: Photocatalytic Treatment of Water: Irradiance Influences

12.1 Introduction

12.2 Reaction Order in Irradiance: Influence of Electron–Hole Recombination and the High Irradiance Penalty

12.3 Langmuir–Hinshelwood (LH) Kinetic Form: Equilibrated Adsorption

12.4 Pseudo-Steady-State Analysis: Nonequilibrated Adsorption

12.5 Mass Transfer and Diffusion Influences at Steady Conditions

12.6 Controlled Periodic Illumination: Attempt to Beat Recombination

12.7 Solar-Driven Photocatalysis: Nearly Constant nUV Irradiance

12.8 Mechanism of Hydroxyl Radical Attack: Same Irradiance Dependence

12.9 Simultaneous Homogeneous and Heterogeneous Photochemistry

12.10 Dye-Photosensitized Auto-Oxidation

12.11 Interplay between Fluid Residence Times and Irradiance Profiles

12.12 Quantum Yield, Photonic Efficiency, and Electrical Energy per Order

12.13 Conclusions

References

13: A Methodology for Modeling Slurry Photocatalytic Reactors for Degradation of an Organic Pollutant in Water

13.1 Introduction and Scope

13.2 Evaluation of the Optical Properties of Aqueous TiO2 Suspensions

13.3 Radiation Model

13.4 Quantum Efficiencies of 4-Chlorophenol Photocatalytic Degradation

13.5 Kinetic Modeling of the Pollutant Photocatalytic Degradation

13.6 Bench-Scale Slurry Photocatalytic Reactor for Degradation of 4-Chlorophenol

13.7 Conclusions

Acknowledgments

References

14: Design and Optimization of Photocatalytic Water Purification Reactors

14.1 Introduction

14.2 Catalyst Immobilization Strategy

14.3 Synergistic Effects of Photocatalysis and Other Methods

14.4 Effective Design of Photocatalytic Reactor System

14.5 Future Directions and Concluding Remarks

Acknowledgments

References

15: Solar Photocatalytic Pilot Plants: Commercially Available Reactors

15.1 Introduction

15.2 Compound Parabolic Concentrators

15.3 Technical Issues: Reflective Surface and Photoreactor

15.4 Suspended or Supported Photocatalyst

15.5 Solar Photocatalytic Treatment Plants

15.6 Specific Issues Related with Solar Photocatalytic Disinfection

15.7 Conclusions

Acknowledgments

References

Index

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Title Page

Editorial Board

Members of the Advisory Board of the “Materials for Sustainable Energy and Development” Series

Professor Huiming Cheng

Professor Calum Drummond

Professor Morinobu Endo

Professor Michael Grätzel

Professor Kevin Kendall

Professor Katsumi Kaneko

Professor Can Li

Professor Arthur Nozik

Professor Detlev Stöver

Professor Ferdi Schüth

Professor Ralph Yang

Series Editor Preface

The Wiley Series on New Materials for Sustainable Energy and Development

Sustainable energy and development is attracting increasing attention from the scientific research communities and industries alike, with an international race to develop technologies for clean fossil energy, hydrogen and renewable energy as well as water reuse and recycling. According to the REN21 (Renewables Global Status Report 2012 p. 17) total investment in renewable energy reached $257 billion in 2011, up from $211 billion in 2010. The top countries for investment in 2011 were China, Germany, the United States, Italy, and Brazil. In addressing the challenging issues of energy security, oil price rise, and climate change, innovative materials are essential enablers.

In this context, there is a need for an authoritative source of information, presented in a systematic manner, on the latest scientific breakthroughs and knowledge advancement in materials science and engineering as they pertain to energy and the environment. The aim of the Wiley Series on New Materials for Sustainable Energy and Development is to serve the community in this respect. This has been an ambitious publication project on materials science for energy applications. Each volume of the series will include high-quality contributions from top international researchers, and is expected to become the standard reference for many years to come.

This book series covers advances in materials science and innovation for renewable energy, clean use of fossil energy, and greenhouse gas mitigation and associated environmental technologies. Current volumes in the series are:

Supercapacitors. Materials, Systems, and Applications
Functional Nanostructured Materials and Membranes for Water Treatment
Materials for High-Temperature Fuel Cells
Materials for Low-Temperature Fuel Cells
Advanced Thermoelectric Materials. Fundamentals and Applications
Advanced Lithium-Ion Batteries. Recent Trends and Perspectives
Photocatalysis and Water Purification. From Fundamentals to Recent Applications

In presenting this volume on Supercapacitors, I would like to thank the authors and editors of this important book, for their tremendous effort and hard work in completing the manuscript in a timely manner. The quality of the chapters reflects well the caliber of the contributing authors to this book, and will no doubt be recognized and valued by readers.

Finally, I would like to thank the editorial board members. I am grateful to their excellent advice and help in terms of examining coverage of topics and suggesting authors, and evaluating book proposals.

I would also like to thank the editors from the publisher Wiley-VCH with whom I have worked since 2008, Dr Esther Levy, Dr Gudrun Walter, and Dr Bente Flier for their professional assistance and strong support during this project.

I hope you will find this book interesting, informative and valuable as a reference in your work. We will endeavour to bring to you further volumes in this series or update you on the future book plans in this growing field.

Brisbane, Australia

G.Q. Max Lu

31 July 2012

Preface

Thousands of articles, patents, and reviews have appeared about heterogeneous photocatalysis using semiconductors, mainly TiO2. Among them, many deal with the purification of water, which is obviously a very important problem for our environment and poses a challenge. Because of this abundance of publications, it is very difficult even for the experts to get a clear view of “photocatalysis and water purification”. In my opinion, this book – which gathers reviews from eminent scientists with a long experience in the field – will allow the readers to gain the general view they expect. The knowledge on the fundamentals is integrated with the potential implementation of photocatalytic water purification using UV lamps or solar light. To that end, this book is structured in four sections with a total of 14 chapters.

The three opening papers explore the fundamentals. Chapter 1 focuses on the active species, especially the oxygen species, which follow the excitation of the photocatalyst by band gap irradiation, and provides information on the roles of these species. Chapter 2 identifies the degradation pathways of organic pollutants and indicates how these pathways are influenced by the pollutant molecular structure, the photocatalyst, and irradiation characteristics. Dealing also with pathways, Chapter 3 presents a synthetic and critical view of the kinetic models and shows how the effects of various parameters can be rationalized.

Improving the photocatalytic efficacy is obviously an extremely important question that is addressed in Chapters 4–9. Understanding how the diverse characteristics of the photocatalyst intervene and whether these characteristics can be tailored to increase the efficacy is considered in Chapters 4 and 5. On the basis of a critical appraisal of the present knowledge it is concluded in Chapter 4 that it is, unfortunately, premature to indicate uncontestable guidance for photocatalyst design. Chapter 5 discusses in detail the various ways by which the photocatalyst can be modified. The use of composite materials is also taken into account. The conclusion is similar to that of Chapter 4. As the photocatalyst is in practice rarely dispersed in the water to be treated, Chapter 6 covers the main methods used to immobilize the photocatalyst and details the nature, shapes, and advantages and disadvantages of the supporting materials. Chapter 7 focuses on the interest of TiO2 thin films and gives examples of the types of reactors in which they can be employed; some of the means allowing one to make these films sensitive in the visible region of the solar spectrum are also discussed. Chapter 8 provides a detailed and critical review of all the methodologies utilized for visible-light sensitization of photocatalysts and indicates how to properly evaluate the results. Finally, as the recombination of the photoproduced charge carriers is the main flaw of heterogeneous photocatalysis, there is also the possibility to effect the separation of these charge carriers by use of an applied potential. Chapter 9 reports on this technique, called photoelectrocatalysis, and concludes that the design of reactors needs further progress to achieve effective water purification.

Most studies regarding water photocatalytic purification refer to both organic pollutants representative of the different categories of organic compounds and to great classes of exogenous pollutants such as pesticides, dyes, and pharmaceuticals. It was therefore deemed necessary to also include in this book a section on the effects of photocatalysis on natural organic matter (NOM) and bacteria which are found in many waters. In some cases eliminating NOM in the course of the purification is desirable; NOM can also hinder the removal of pollutants. Consequently, Chapter 10 is devoted to photocatalysis and NOM; it is concluded that, in general, photocatalysis cannot be applied as a stand-alone process, but its coupling with another technique can be beneficial. Chapter 11 covers the photocatalytic inactivation of bacteria and explains in depth how the active species issued from excitation of the photocatalyst can damage the membrane and degrade the organic constituents of bacteria.

The last section deals with modeling and reactors. Chapter 12 focuses on the influence of irradiation, which is the initiation step of photocatalysis and stresses that irradiance measurement is essential; however, the observed kinetics is also influenced by other factors whose relative importance must be determined for reactor design. Chapter 13 demonstrates that rigorous mathematical modeling based on data collected at the laboratory and bench scales enables one to simulate the performance of photocatalytic slurry reactors of different shapes, sizes, irradiation systems, and configurations. Chapter 14 includes examples of reactors using photocatalysis as a stand-alone process or combined with another AOP; it also emphasizes that immobilization of the photocatalyst must be carefully studied and that further progress in photocatalyst sensitization to visible light is highly desirable. Chapter 15 reviews the characteristics of diverse types of solar reactors including full-size demonstration plants with a detailed presentation regarding the utilization of sun light; it is concluded that appraisal of the possibilities for both pollutants removal and disinfection requires further comparison with other technologies.

I think this book covers all the main topics of “photocatalysis and water purification”. It will be helpful for students beginning research in the field and for technicians and scientists involved in the treatment of water by other technologies, who would like to learn about photocatalysis and its potentialities. Even senior scientists in the field will certainly be interested in the opinion of colleagues about some of the issues.

I sincerely thank the contributors for their response to my solicitation, their time, and efforts. I am also very grateful to the reviewers and the Wiley staff.

Pierre Pichat

About the Series Editor

1

Professor Max Lu

Editor, New Materials for Sustainable Energy and Development Series

Professor Lu's research expertise is in the areas of materials chemistry and nanotechnology. He is known for his work on nanoparticles and nanoporous materials for clean energy and environmental technologies. With over 500 journal publications in high-impact journals, including Nature, Journal of the American Chemical Society, Angewandte Chemie, and Advanced Materials, he is also coinventor of 20 international patents. Professor Lu is an Institute for Scientific Information (ISI) Highly Cited Author in Materials Science with over 17 500 citations (h-index of 63). He has received numerous prestigious awards nationally and internationally, including the Chinese Academy of Sciences International Cooperation Award (2011), the Orica Award, the RK Murphy Medal, the Le Fevre Prize, the ExxonMobil Award, the Chemeca Medal, the Top 100 Most Influential Engineers in Australia (2004, 2010, and 2012), and the Top 50 Most Influential Chinese in the World (2006). He won the Australian Research Council Federation Fellowship twice (2003 and 2008). He is an elected Fellow of the Australian Academy of Technological Sciences and Engineering (ATSE) and Fellow of Institution of Chemical Engineers (IChemE). He is editor and editorial board member of 12 major international journals including Journal of Colloid and Interface Science and Carbon.

Max Lu has been Deputy Vice-Chancellor and Vice-President (Research) since 2009. He previously held positions of acting Senior Deputy Vice-Chancellor (2012), acting Deputy Vice–Chancellor (Research), and Pro-Vice-Chancellor (Research Linkages) from October 2008 to June 2009. He was also the Foundation Director of the ARC Centre of Excellence for Functional Nanomaterials from 2003 to 2009.

Professor Lu had formerly served on many government committees and advisory groups including the Prime Minister's Science, Engineering and Innovation Council (2004, 2005, and 2009) and the ARC College of Experts (2002–2004). He is the past Chairman of the IChemE Australia Board and former Director of the Board of ATSE. His other previous board memberships include Uniseed Pty Ltd., ARC Nanotechnology Network, and Queensland China Council. He is currently Board member of the Australian Synchrotron, National eResearch Collaboration Tools and Resources, and Research Data Storage Infrastructure. He also holds a ministerial appointment as member of the National Emerging Technologies Forum.

About the Volume Editor

1 Professor Pierre Pichat

Pierre Pichat, as “Directeur de Recherche”, with the CNRS (National Center for Scientific Research, France), has been active in heterogeneous photocatalysis for many years and has founded a laboratory principally based on this field in Lyon, France. His research activity has concerned both basic investigations and applications regarding not only the photocatalytic treatment of water but also self-cleaning materials and the purification of indoor air. He has been invited to publish reviews or these domains. He has received an Appreciation Award acknowledging his pioneering contributions.

List of Contributors

Orlando M. Alfano
Universidad Nacional del Litoral and CONICET
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Paraje El Pozo. Colectora Ruta Nacional N° 168
3000, Santa Fe
Argentina
Rossano Amadelli
University of Ferrara
CNR-ISOF, U.O.S. of Ferrara
Department of Chemistry
Via L. Borsari 46
I-44121 Ferrara
Italy
Rose Amal
The University of New South Wales
School of Chemical Engineering
ARC Centre of Excellence for Functional Nanomaterials
Tyree Energy Technologies Building
Sydney NSW 2052
Australia
Masakazu Anpo
Osaka Prefecture University
(Vice President & Executive Director)
Research Organization for the 21st Century
1-1 Gakuencho, Naka-ku, Sakai
599-8531 Osaka
Japan
Rodolfo J. Brandi
Universidad Nacional del Litoral and CONICET
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Paraje El Pozo. Colectora Ruta Nacional N° 168
3000, Santa Fe
Argentina
Alberto E. Cassano
Universidad Nacional del Litoral and CONICET
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Paraje El Pozo. Colectora Ruta Nacional N° 168
3000, Santa Fe
Argentina
Chuncheng Chen
Institute of Chemistry
Chinese Academy of Sciences (CAS)
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Photochemistry
No. 2, 1st North Street, Zhongguancun
Beijing 100190
China
Pilar Fernández-Ibáñez
Plataforma Solar de Almería (CIEMAT)
Carretera Senés, km 4
04200, Tabernas (Almería)
Spain
Akira Fujishima
Kanagawa Academy of Science and Technology
Photocatalyst Group
KSP East 421, 3-2-1 Sakado, Kawasaki
Kanagawa 213-0012
Japan
and
Tokyo University of Science
Division of Photocatalyst for Energy and Environment
Research Institute for Science and Technology
1 – 3 Kagurazaka, Shinjuku-ku
Tokyo 162-8601
Japan
Yu Horiuchi
Osaka Prefecture University
Department of Applied Chemistry
1-1 Gakuencho, Naka-ku, Sakai
599-8531 Osaka
Japan
William S. Jenks
Iowa State University
Department of Chemistry
3760 Gillman Hall
Ames, IA, 50011-3111
USA
Nicolas Keller
Laboratoire des Matériaux
Surfaces et Procédés pour la Catalyse
Ecole Européenne de Chimie
Polymères et Matériaux
CNRS and Strasbourg University
25 rue Becquerel
67087, Strasbourg Cedex
France
Valérie Keller
Laboratoire des Matériaux
Surfaces et Procédés pour la Catalyse
Ecole Européenne de Chimie
Polymères et Matériaux
CNRS and Strasbourg University
25 rue Becquerel
67087, Strasbourg Cedex
France
May Lim
The University of New South Wales
School of Chemical Engineering
ARC Centre of Excellence for Functional Nanomaterials
Tyree Energy Technologies Building
Sydney NSW 2052
Australia
Sanly Liu
The University of New South Wales
School of Chemical Engineering
ARC Centre of Excellence for Functional Nanomaterials
Tyree Energy Technologies Building
Sydney NSW 2052
Australia
Wanhong Ma
Institute of Chemistry
Chinese Academy of Sciences (CAS)
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Photochemistry
No. 2, 1st North Street, Zhongguancun
Beijing 100190
China
Sixto Malato
Plataforma Solar de Almería (CIEMAT)
Carretera Senés, km 4, 04200
Tabernas (Almería)
Spain
Maneil Ignacio Maldonado
Plataforma Solar de Almería (CIEMAT)
Carretera Senés, km 4, 04200
Tabernas (Almería)
Spain
Masaya Matsuoka
Osaka Prefecture University
Department of Applied Chemistry
1-1 Gakuencho
Naka-ku, Sakai
599-8531 Osaka
Japan
Valter Maurino
Univ. degli Studi di Torino
Dipt. di Chimica, Analitica
Via Pietro Giuria 5
10125 Torino
Italy
Claudio Minero
Univ. degli Studi di Torino
Dipt. di Chimica, Analitica
Via Pietro Giuria 5
10125 Torino
Italy
Atsuko Y. Nosaka
Nagaoka University of Technology
Department of Materials Science and Technology
Kamitomioka-cho 1603-1
Nagaoka, 940-2188
Japan
Yoshio Nosaka
Nagaoka University of Technology
Department of Materials Science and Technology
Kamitomioka-cho 1603-1
Nagaoka, 940-2188
Japan
Tsuyoshi Ochiai
Kanagawa Academy of Science and Technology
Photocatalyst Group
KSP East 421, 3-2-1 Sakado
Kawasaki
Kanagawa 213-0012
Japan
and
Tokyo University of Science
Division of Photocatalyst for Energy and Environment
Research Institute for Science and Technology
1 – 3 Kagurazaka
Shinjuku-ku
Tokyo 162-8601
Japan
Bunsho Ohtani
Catalysis Research Center
Hokkaido University
North 21, West 10
Sapporo 001-0021
Japan
Isabel Oller
Plataforma Solar de Almería (CIEMAT)
Carretera Senés, km 4, 04200
Tabernas (Almería)
Spain
David Ollis
North Carolina State University
Department of Chemical and Biomolecular Engineering
911 Partners Way
Raleigh, NC 27695-7905
USA
Yaron Paz
Technion-Israel Institute of Technology
Department of Chemical Engineering
3200, Haifa
Israel
Maria Inmaculada Polo-López
Plataforma Solar de Almería (CIEMAT)
Carretera Senés, km 4, 04200
Tabernas (Almeréa)
Spain
Cesar Pulgarin
Ecole Polytechnique Fédérale de Lausanne (EPFL)
Institute of Chemical Science and Engineering, GGEC 1015 Lausanne
Switzerland
Julián Andrés Rengifo-Herrera
Universidad Nacional de La Plata-UNLP CCT La Plata-CONICET
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J.J. Ronco”- CINDECA
Departamento de Química
Facultad de Ciencias Exactas
Calle 47 No. 257, 1900 La Plata
Buenos Aires
Argentina
Angela Giovana Rincón
University of Cambridge
Department of Chemistry
Lensfield Road
Cambridge, CB2, 1EW
UK
Didier Robert
Laboratoire des Matériaux
Surfaces et Procédés pour la Catalyse
CNRS and Strasbourg University
Antenne de Saint-Avold
Université de Lorraine
12 Rue Victor Demange
57500, Saint-Avold
France
Luca Samiolo
University of Ferrara
CNR-ISOF
U.O.S. of Ferrara
Department of Chemistry
Via L. Borsari 46
I-44121 Ferrara
Italy
Maríia L. Satuf
Universidad Nacional del Litoral and CONICET
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Paraje El Pozo. Colectora Ruta Nacional N° 168
3000, Santa Fe
Argentina
Nurit Shaham-Waldmann
Technion-Israel Institute of Technology
Department of Chemical Engineering
3200, Haifa
Israel
Masato Takeuchi
Osaka Prefecture University
Department of Applied Chemistry
1-1 Gakuencho
Naka-ku, Sakai
599-8531 Osaka
Japan
Takashi Toyao
Osaka Prefecture University
Department of Applied Chemistry
1-1 Gakuencho
Naka-ku, Sakai
599-8531 Osaka
Japan
Davide Vione
Univ. degli Studi di Torino
Dipt. di Chimica Analitica
Via Pietro Giuria 5
10125 Torino
Italy
Zhaohui Wang
Institute of Chemistry
Chinese Academy of Sciences (CAS)
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Photochemistry
No.2, 1st North Street, Zhongguancun
Beijing 100190
China
and
Donghua University
College of Environmental Science and Engineering
No.2999, North Renmin Road
Shanghai 201620
China
Jincai Zhao
Institute of Chemistry
Chinese Academy of Sciences (CAS)
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Photochemistry
No. 2, 1st North Street, Zhongguancun
Beijing 100190
China

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.

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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.

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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 [TiOHO–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.)

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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)

Since photocatalysts are usually used in air, photoexcited CB electrons transfer to the oxygen in air to form superoxide radical O2⋅−. The highly sensitive MIRIR technique was applied and surface intermediates of the photocatalytic O2 reduction were directly detected. Figure 1.4 shows the proposed mechanism of the reduction of molecular oxygen at the TiO2 surface in aqueous solutions [20]. In neutral and acidic solutions, CB electrons reduce the surface Ti4+ that adsorbs H2O, and then O2 attacks it immediately to form superperoxo TiOO as shown in path A in Figure 1.4. This superperoxo is reduced to peroxo Ti(O2), which is equivalent to hydroperoxo TiOOH, when it is protonated (Figure 1.4). The hydroperoxo has the same structure with the hydrogen peroxide adsorbed on TiO2 surface. On the other hand, in the alkaline solution, as shown in path B, the adsorbed O2 receives a photogenerated CB electron to produce O2⋅−. If it is not used for reactions or oxidized, the produced O2⋅− is converted to H2O2 by disproportionation with protons. Although the reaction rate for molecules having higher electron affinity is usually large, the reactivity of O2⋅− is generally weak. At pH lower than 4.8, it takes the form of HO2 by the protonation, whose lifetime is short owing to the rapid reaction with O2⋅− or HO2 to form stable H2O2 [7], as stated above.

Figure 1.4 Reaction paths for the photocatalytic reduction of O2 at the TiO2 surface, suggested from IR measurements in neutral and acidic aqueous solutions (path A) and in an alkaline solution (path B).

(Source: Reprinted with permission from Nakamura et al. [20]. © 2003 American Chemical Society.)

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Since the lifetime of O2⋅− is long in alkaline solution [21], it can be detected after stopping the irradiation. To detect O2⋅−, a chemiluminescence method with luminol or luciferin analog (MCLA) has been used [22]. Figure 1.5 shows the reaction scheme for luminol chemiluminescence reactions. Luminol (LH) is easily oxidized in alkaline solution under air forming one-electron oxidized state (L⋅−), and reacts with O2⋅− to form unstable peroxide (LO2H). This species releases N2 to form the excited state of 3-aminophthalate (3-APA), which emits light at 430 nm. When L is oxidized further, a two-electron oxidation form of luminol (L), or a kind of diazo-naphthoquinones, is formed. It can react with H2O2 to form peroxides to proceed the same chemiluminescence reaction. Thus, using an oxidant, H2O2 could be separately detected by a luminol chemiluminescence method [23].

Figure 1.5 Chemiluminescence reactions for detecting O2⋅− and H2O2. The excited state of 3-aminophthalate (3-APA) is formed by two different reactions.

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The decay profile of O2⋅− concentration does not obey first- or second-order kinetics, but obeys fractal-like kinetics, namely, the distribution of the distance between holes and adsorbed O2⋅− governs these decay kinetics [21]. For anatase thin film photocatalysts irradiated with very weak (1 µW cm−2) UV light, the quantum yields of O2⋅− were reported to be 0.4 and 0.8 in air and water, respectively [24].

As suggested in Figure 1.2, O2⋅− may be produced by the photocatalytic oxidation of H2O2 (Eq. (1.4)).

1.4

Figure 1.6 shows the amount of O2⋅− formed after 10 s in the presence of H2O2 of various concentrations. Increase in O2⋅− was observed with a small amount of H2O2, indicating the oxidation of H2O2 with photogenerated hole h+ (Eq. (1.4)) or the increase in the reduction of O2 owing to the suppression of photogenerated e from the recombination. When the amount of H2O2 was larger than 0.2 mmol l−1, the formation of O2⋅− decreased, indicating that the adsorption of H2O2 on the whole surface blocks the access of O2, which would increase the electron–hole recombination rate.

Figure 1.6 The effect of H2O2 on the concentration of O2⋅− measured after 100 s irradiation of the TiO2 suspension of AMT-600 (TAYCA Corp.) and P25 (Nippon Aerosil Co., Ltd).

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1.4 Hydroxyl Radical (OH)

Although OH has been usually recognized as the most important active species of the photocatalytic oxidation, recent reports confirmed that the contribution of OH in the photocatalytic oxidation process is not usually dominant [6]. It should be emphasized that OH has been referred too easily to be involved in the oxidation mechanism of photocatalytic reactions.

Several methods to detect OH in photocatalytic reactions have been reported. Usually, the spin trapping reagents, such as DMPO (5,5-dimethyl-1-pyrroline-N-oxide), have been used to detect OH radicals (Figure 1.7a). However, it is not a molecule stable enough in aerated aqueous solutions and can be easily oxidized. In many reports, the possibility of the other reactions for DMPO than the OH radical adduction has not been anticipated. Based on the detailed study in [25], it was indicated that the amount of radical adduct in the photocatalytic reaction was increased with DMPO concentration and that no saturation was observed, whereas OH formed by photolysis of H2O2 could be trapped by excess amount of DMPO. This means that the OH radical adduct DMPO–OH was formed by the photocatalytic reaction of DMPO itself and not through OH radicals. Thus, spin trapping experiments for detecting OH must be carefully performed to prove the presence of OH [25, 26].

Figure 1.7 (a) The spin trapping reaction for OH with DMPO. (b) The reaction of terephthalic acid with OH forming fluorescent 2-hydroxy terephthalate.

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A fluorescence probing method, based on the reaction of OH with stable molecules seems more suitable than those with unstable spin trapping regents. In the field of radiation chemistry, the reactions of OH with terephthalic acid (TA) and coumarin have been used because these products show strong fluorescence aiding in sensitive detection [27]. Therefore, this method has been adopted to detect OH in photocatalytic reactions in aqueous suspension systems [28, 29]. The quantum yield of OH in TiO2 aqueous suspension was on the order of 10−5 [30]. Kinetic analysis for the formation rates of the OH adduct (DMPO–OH) along with the competitive adsorption of phosphate showed that, at a pH = 4.25, phthalic acid that was adsorbed on TiO2 surface was oxidized directly by VB holes, with a quantum yield of 0.08 [31]. This high quantum yield could be attributed to the direct oxidation of adsorbed TA with VB holes.

Since radicals can be sensitively analyzed with ESR, nitroxide radical (3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxy) has been used as a probe to detect OH radicals [32]. The quantum efficiencies of OH for several TiO2 photocatalysts were measured by the TA fluorescence method (Figure 1.7b) and compared with those obtained with the spin-trap and spin-probe ESR methods stated above [29]. The OH yields measured by the TA fluorescence method were smaller by a factor of about 100, showing no correlation with those obtained by the DMPO spin trapping and the TA spin probing methods. Although the formation of OH has been reported mainly using the spin trapping method, the contribution of the free OH may be very small when the reactant is readily oxidized. Thus, the OH should be distinguished from that generated by the trapped holes in photocatalytic reactions.

OH was expected to be directly detected by means of ESR spectroscopy at low temperature. However, actually the OH was not detected by ESR spectroscopy at 77 K, but only trapped holes were detected for hydrated TiO2 particles [33]. Under hydrated conditions, when the frozen trapped holes were partly melted, they oxidized the adsorbed molecules [33]. Thus, the involvement of OH in the oxidation process was not proved by direct detection with ESR.

Another definite method to confirm the presence of OH is the observation of the optical absorption spectrum in gas phase. By scanning the excitation wavelength (282–284 nm) and monitoring the fluorescence at 310 nm, the spectrum could be identified as the absorption lines of OH. This highly sensitive and selective technique is called as the laser-induced fluorescence (LIF) method. Using this method, the first direct observation of the presence of OH in TiO2 photocatalytic systems was reported [34]. The quantum yield of OH calculated from the LIF intensity was about 5 × 10−5. When the O2 gas of low partial pressures was flowed, the formation of OH was clearly enhanced. Since the addition of H2O2 on the TiO2 surface increased the LIF intensity, H2O2 molecules were also considered to form by the reduction reactions of O2. The addition of methanol (a scavenger of hole) decreased significantly the LIF signal intensity, suggesting the formation of H2O2 by the oxidation of surface OH groups by holes. This mechanism of OH formation is illustrated in Figure 1.8 [34]. With a similar reaction system, the formation and diffusion of H2O2 have been reported using the LIF method [35]. Consequently, it was proved that OH radicals are mainly formed by the reduction of H2O2, which is formed by the two-electron reduction of O2 and/or two-electron oxidation of H2O.

Figure 1.8 A plausible reaction scheme for the OH radical formation on the irradiated TiO2 surface.

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Using a molecular fluorescence marker, the diffusion of OH from TiO2 surface during UV irradiation has been verified [36]. The detected amount of OH decreased with decreasing the concentration of oxygen, that is, at [O2] = 0.2 vol%, no significant amount of OH was detected. This result indicates that the OH formation is very sensitive to the oxygen concentration, and the reduction process of oxygen, which results in the formation of O2⋅− leading to H2O2, is a key process in the formation of OH.

The effect of H2O2 addition on the rate of OH formation in aqueous suspension systems was measured for various TiO2 [37]. As shown in Figure 1.9, the OH formation rates were increased with the addition of H2O2 for P25 (Nippon Aerosil Co, Ltd) and F4 (Showa Titanium Co., Ltd) TiO2, which are rutile-containing anatase, and for rutile TiO2 (MT-500B, TAYCA Corp.). The quite opposite tendency was observed for AMT-600 (TAYCA Corp.) and ST-21 (Ishihara Sangyo Co., Ltd), which consist of 100% anatase TiO2, where the OH formation rate decreased on H2O2 addition. The increase of OH is attributable to the photocatalytic reduction of H2O2 (Eq. (1.3)). Since the rutile-containing anatase increased the OH generation, the structure of H2O2 adsorbed on the rutile TiO2 surface is likely preferable to produce OH.

Figure 1.9 The formation rates of OH measured by a fluorescence probe method plotted for several TiO2 photocatalysts as a function of the concentration of H2O2.

(Source: Reprinted with permission from Hirakawa et al. [37]. © 2007 Elsevier.)

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1.5 Singlet Molecular Oxygen (1O2)

To explain the formation of singlet oxygen, the disproportionation of O2⋅− was proposed through the intermediate formation of HO2 as shown by Eq. (1.5) [38]. Since the energy difference of HO2 → O2 from HO2 → H2O2 at a pH = 0 is calculated to be +1.49 V from Figure 1.2, O2 may be excited to 1O2. But, it becomes 0.53 V at pH = 14, which is smaller than the excitation energy of 0.98 eV (or 1270 nm in wavelength).

Alternatively, the oxidation of O2⋅− as indicated by Eq. (1.6) has been proposed as the formation mechanism [39]. Since O2⋅− is formed by the electron transfer of photoexcited CB electrons at the surface, it may be easily oxidized.

1.5

1.6

Figure 1.10 shows the plausible pathways for the consecutive reduction and oxidation of O2. Since three electrons in the π* state of O2⋅− cannot be distinguished from one another, three electronic states may be produced depending on the removed electron. These are 3Σg ,1Δg, and 1Σg+ states in the order from the lower energy. The last two states are electronic excited states of molecular oxygen and named as singlet oxygen. The lifetime of the 1Σg+ state is very short and immediately transfers to the 1Δg121g2