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The Mechanism of Water Oxidation

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The Mechanism of Water Oxidation DOI: 10.1002/cctc.201000126 The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis Holger Dau,*[a] Christian Limberg,*[b] Tobias Reier,[c] Marcel Risch,[a] Stefan Roggan,[b] and Peter Strasser*[c] 724 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2010, 2, 724 – 761 Striving for new solar fuels, the water oxidation reaction cur- catalytic manganese–calcium complex of photosynthetic water rently is considered to be a bottleneck, hampering progress in oxidation. Recent developments in homogeneous water-oxida- the development of applicable technologies for the conversion tion catalysis are outlined with a focus on the discovery of of light into storable fuels. This review compares and unifies mononuclear ruthenium (and non-ruthenium) complexes that viewpoints on water oxidation from various fields of catalysis efficiently mediate O2 evolution from water. Water oxidation in research. The first part deals with the thermodynamic efficien- photosynthesis is the subject of a concise presentation of cy and mechanisms of electrochemical water splitting by metal structure and function of the natural paragon—the manga- oxides on electrode surfaces, explaining the recent concept of nese–calcium complex in photosystem II—for which ideas con- the potential-determining step. Subsequently, novel cobalt cerning redox-potential leveling, proton removal, and OÀO oxide-based catalysts for heterogeneous (electro)catalysis are bond formation mechanisms are discussed. The last part high- discussed. These may share structural and functional properties lights common themes and unifying concepts. with surface oxides, multinuclear molecular catalysts and the Introduction tensified, as it can serve as an important inspiration and benchmark for the development of artificial water-oxidation In recent years, the increased interest in direct conversion of catalysts. solar energy into storable fuels has led to intensified research Herein we will discuss and compare concepts and recent efforts with respect to artificial photosynthesis.[1] A successful finding on the mechanism of water oxidation obtained in the imitation of the natural energy-storing process requires the following separate research communities: combination of several distinct processes, including light har- * Heterogeneous electrocatalysis at electrode surfaces (sec- vesting, charge separation, electron transfer, and water oxida- tion 1), * tion (O2 evolution), as well as the fuel-forming reaction. Where- heterogeneous catalysis by oxidic bulk materials in colloi- as a close adaption to the biological process of photosynthesis dal form or deposited on electrodes (section 2), would involve the transformation of carbon dioxide into a * homogeneous catalysis by transition metal complexes chemical compound of higher energy content, at present the (section 3), * generation of molecular hydrogen (H2) by means of proton re- biocatalytic water oxidation in photosynthesis (section 4). duction seems to be a more realistic proposition for storing Rather than approaching the impossible, that is, a fully com- solar energy by artificial photosynthesis. prehensive review, we focus on recent progress and strive to The electrons and protons required for fuel generation address common themes and potentially unifying concepts. should be obtained from oxidation of water to oxygen, there- by converting the whole process into an overall water-splitting scheme, where hydrogen is the targeted fuel. Hydrogen itself 1. Electrochemical Water Splitting can be utilized as a fuel. Moreover, solar hydrogen will be the key for the realization of a number of related renewable fuel Electrolysis, the electrocatalytic splitting of liquid water into hy- production processes, such as hydrodeoxygenation of lignocel- drogen and oxygen using electricity, is the longest-studied cat- lulosic biomass into fuels and chemicals, or the reduction of alytic way to oxidize water, predating even the development of the concept of chemical catalysis. The overall reaction, cata- CO2 into methanol or other carbonaceous fuels. However, not only the generation of molecular hydrogen from water re- lyzed at the electrified solid–liquid–gas three-phase interface quires water oxidation. For all substances that today are typi- of the anode and the cathode, is given by cally considered as fuels, it holds that utilization of the chemi- 1 cally stored energy involves the reduction of molecular H2OðlÞ ! H2ðgÞþ =2 O2ðgÞ ð1Þ oxygen, that is, the transfer of electrons from the fuel to oxygen. For carbonaceous fuels, this process is coupled to CO [a] Prof. Dr. H. Dau, M. Risch 2 Freie Universitt Berlin, Physics Dept., Molecular Biophysics formation and, in most cases, also to H2O formation. In fuel Arnimallee 14, 14195 Berlin (Germany) generation, a source for these electrons is needed, and water Fax: (+ 49)30-838-56299 is the one and only truly attractive candidate; every scheme E-mail: [email protected] for sustainable, large-scale production of non-fossil fuels will [b] Prof. Dr. C. Limberg, Dr. S. Roggan eventually involve utilization of water as a raw material. Humboldt Universitt Berlin, Department of Chemistry Brook-Taylor-Strasse 2, 12489 Berlin (Germany) Water oxidation is currently considered a major bottleneck, Fax: (+ 49)30-2093-6966 hampering progress in the development of applicable devices E-mail: [email protected] for the conversion of light into storable fuels. Intense research [c] T. Reier, Prof. Dr. P. Strasser efforts are being pursued to develop both heterogeneous and Technische Universitt Berlin, Chemistry Dept. molecular catalysts that enable water oxidation at potentials The Electrochemical Energy, Catalysis and Materials Science Laboratory Strasse des 17.Juni 124, 10623 Berlin (Germany) close to the thermodynamic limit. Research into the biological Fax: (+ 49)30-314-22261 process of photosynthetic water oxidation has also recently in- E-mail: [email protected] ChemCatChem 2010, 2, 724 – 761 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemcatchem.org 725 H. Dau, C. Limberg, P Strasser et al. Holger Dau received his physics diplo- Marcel Risch received his Bachelor of ma in 1985 and doctoral degree in Science (physics) at the Technical Uni- 1989 in Kiel, Germany, working with versity of Darmstadt, Germany, in 2006 U.-P. Hansen and at the Weizmann In- and a Master of Science (physics) at stitut (Rehovot, Israel, winter 1987/88). the University of Saskatchewan, After postdoctoral work with K. Sauer Canada, in 2008. In 2009, he joined the at the UC Berkeley, USA, and H. Senger Berlin International Graduate School of in Marburg, Germany, he received his Natural Sciences and Engineering (BIG- habilitation degree at the Biology De- NSE) and he is currently studying for a partment of Philipps University Mar- PhD in the group of Prof. Dau at the burg in 1994. Besides photosynthesis Free University of Berlin. The topic of research in Marburg, he developed bi- his thesis is the characterization of an otest applications at bbe Moldaenke GmbH (1997–1999). Since electrochemical cobalt catalyst. 2000, he has been a full Professor at the Physics Department of the Free University in Berlin, Germany, where he investigates bio- logical and synthetic metal sites with X-ray spectroscopy and com- plementary methods. His current focus lies on light-driven water Stefan Roggan graduated in chemistry oxidation and H2 formation in biological and biomimetic systems. in 2003 at the Ruprecht-Karls Universi- ty of Heidelberg in Germany. He then moved together with his supervisor Prof. Christian Limberg to the Hum- Christian Limberg was born in 1965 in boldt-Universitt zu Berlin, where he Essen, Germany, and studied chemistry performed his doctoral studies. In from 1985 until 1990 in Bochum, Ger- 2007, he received his PhD in chemistry many. He earned his doctorate with A. and then joined Prof. T. Don Tilley’s Haas in 1992 and concluded postdoc- group at the UC Berkeley, USA, as a toral work together with A. J. Downs postdoctoral fellow. After his return at Oxford University, UK, with a further from the US in 2009, he continued re- PhD thesis (D. Phil.). Having performed search in Christian Limberg’s group before starting to work as a his habilitation in Heidelberg (1995– scientist for Bayer Technology Services GmbH in Leverkusen, Ger- 1999) he moved to the TU Munich in many. Germany to lead the inorganic chemis- try chair on behalf of W. A. Herrmann. In 2002, he accepted an offer to become full professor at the Hum- boldt-Universitt zu Berlin, Germany. His research is mainly con- Peter Strasser is a full Professor at the cerned with oxo metal complexes, oxidation reactions and their Chemical Engineering Division of the mechanisms (surfaces of metal–oxo catalysts as inspiration for mo- Department of Chemistry at the Tech- lecular models; biomimetic O2 activation and oxidation) as well as nical University of Berlin. His research with the activation of small substrates utilizing dinuclear com- interests focus on materials and cataly- plexes. sis for electrochemical (energy) tech- nologies. Prior to his appointment, he was an Assistant Professor at the De- partment of Chemical and Biomolecu- lar Engineering at the University of Tobias Reier received his Master of Houston, USA. From 2001 to 2004, he Science (Dipl.-Ing.) in 2010 at the Tech- served as senior member of staff at nical University Berlin, Germany, in Symyx Technologies, Inc., Santa Clara, CA, developing and utilizing
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