Editorial pubs.acs.org/CR
Cite This: Chem. Rev. 2018, 118, 2299−2301
Introduction: Oxygen Reduction and Activation in Catalysis
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Fe-containing pyrolytic carbon/nitrogen catalysts, especially in acid. Collectively, these classes of ORR catalysts provide a compelling foundation in the search for Pt-free ORR catalysts. The third ORR review by Mayer and colleagues focuses on O2 reduction with homogeneous metal complexes. While catalyst systems of this type generally do not exhibit ORR performance competitive with heterogeneous catalysts, their well-defined structures and reactivity make them well suited for mechanistic study and elucidation of the fundamental principles that contribute to O2 reduction. An excellent treatise on the thermodynamics of O2 reduction under different conditions is presented, together with detailed discussions of outer sphere and, more extensively, inner sphere mechanisms of O2 reduction by different transition-metal complexes. Mano and de Poulpiquet conclude the series of reviews on the ORR in fuel cells by addressing enzymatic O2 reduction and the integration of oxidases with cathodes in biofuel cells. Multicopper oxidases (MCOs) are given particular attention due to their low overpotential for O2 reduction. Efficient electron transfer between the electrode and the enzyme active sites is crucial to the success of these systems, and mechanistic issues and kinetic modeling of the process are discussed for direct electron-transfer approaches and those involving the use of redox mediators. While there are appealing features of these enzymatic systems (e.g., the low overpotential and the possibility of avoiding a proton-exchange membrane), ongoing challenges include increasing the surface coverage to achieve higher current density and enzyme instability. Both sets of issues are addressed in the context of potential applications of biofuel cells, for example, to power implantable devices. Wikström and co-workers then review cytochrome c oxidase, another class of enzymes that catalyzes O2 reduction to H2O, using a heme/Cu active site rather than the trinuclear Cu cluster used by MCOs. This enzyme is the final enzyme in the electron transport chain in cellular respiration, and it uses the energy difference between cytochrome c oxidation and O2 reduction to pump protons across the mitochondrial membrane. The electrochemical potential associated with this proton gradient is then used to drive ATP synthesis. This process has a close conceptual relationship to fuel cells, and the authors analyze the structural features of the multicomponent enzyme and the active site for O2 reduction, the energetics, and mechanistic features of the various steps in this process, including O2 reduction and proton translocation. It is anticipated that another complementary review will be linked to this thematic issue at a future date. The latter will interrogate details of the molecular mechanisms of O2 reduction at heme, heme/Cu, and Cu active sites in proteins and synthetic model systems. The remaining reviews in this thematic issue focus on O2 activation for catalytic oxidation reactions, and their topics span enzymatic and biomimetic catalysts, homogeneous catalysis,
he reduction of O2 to H2O, involving 4 electrons and 4 protons, is a crucial process in energy production, from the cathodic half-reaction in fuel cells to the pumping of protons for ATP synthesis in biological respiration. In both of these contexts, it is important to limit or avoid the formation of partially reduced intermediates, such as superoxide, hydroperoxide, and hydroxyl radical. Such “reactive oxygen species” can participate in deleterious side reactions and/or limit the energy efficiency of the O2 reduction process. In contrast, the generation of “reactive oxygen species” is the intended outcome of many enzymes and chemical catalyst systems that activate O2 to achieve selective oxidation of organic molecules. Activated oxygen intermediates, often bound to metal ions or surfaces, are key intermediates in catalytic processes that overcome the spin forbidden and often unselective reaction of triplet O2 with closed-shell organic substrates. O2 reduction to H2O and O2 activation continue to be the major focus of contemporary research efforts in heterogeneous, homogeneous, and biological catalysis, and they are the subject of this thematic issue of Chemical Reviews. It is hoped that this compilation of articles will draw attention to both parallels and distinctions that exist across the diverse fields of biochemistry, catalysis, and fuel cell technology represented in this issue. The first series of reviews in this thematic issue focus on the oxygen reduction reaction (ORR) in fuel cells, wherein the 4 e−/4 H+ reduction of O2 to water is harnessed to produce electricity. A major challenge in this field is to minimize the overpotential or activation energy required to accomplish this reaction. Fuel cells typically employ cathodes containing Pt catalysts, and the first review by Norskov and colleagues surveys computational insights into the kinetic and thermodynamic principles that account for the effectiveness of Pt for the fourelectron reduction of O2 to H2O. Analysis of “scaling relationships” and “volcano plots”, which correlate the ORR rates or overpotentials of different catalytic materials to the binding energies of ORR intermediates (e.g., •OH and •OOH) to these materials, provides a foundation for the pursuit of novel materials that could exhibit improved performance. This computational methodology is being extended to analyze new Pt alloy materials and non-transition-metal catalysts (i.e., carbon-based materials) in an effort to provide alternative catalysis strategies that could circumvent the scaling relationships. The second review by Gewirth et al. focuses on nonprecious-metal catalysts for the ORR in fuel cells, especially pyrolyzed carbon materials containing nitrogen and Fe or Co. Catalysts of this type have been the focus of considerable attention because their performance rivals that of Pt, with respect to rates and overpotentials. Catalyst preparation methods and efforts directed toward characterization of the active site(s) involved in ORR are presented, in addition to a survey of the ORR performance of the different materials. The review concludes with a consideration of Cu-based catalysts supported on carbon-based materials. These catalysts do not yet exhibit performance commensurate with that of the Co- or © 2018 American Chemical Society
Special Issue: Oxygen Reduction and Activation in Catalysis Published: March 14, 2018 2299
DOI: 10.1021/acs.chemrev.8b00046 Chem. Rev. 2018, 118, 2299−2301
Chemical Reviews
Editorial
and heterogeneous catalysis. The first review on biological O2 activation by Huang and Groves presents O2 binding and activation by heme enzymes, as well as model systems and other synthetic metalloporphyrins. Mechanisms of O 2 activation, including electronic structure contributions to reactivity, are discussed together with the different classes of activated oxygen species that are generated from these reactions (e.g., superoxo, (hydro)peroxo, and oxo species). An overarching theme in heme chemistry is the formation and reactivity of “compound I” (cmpd I), the intermediate that arises from the reaction of an FeII heme with O2, followed by reaction of an exogenous electron and two protons leading to cleavage of the O−O bond to form an S = 1 FeIVO/ porphyrin-radical-cation. The reactivities of this species and other biological and synthetic oxo-ligated metalloporhyrins and related macrocyclic complexes are surveyed, especially in the context of C−H oxidation mechanisms and reaction outcomes, ranging from oxygenation to dehydrogenation and halogenation. The following review by Jasniewski and Que addresses the complementary topic of dioxygen activation by nonheme diiron enzymes. In these enzymes, a second Fe serves the role of the porphyrin in heme oxygenases by providing a second election for O2 activation, and the review analyzes the structural and spectroscopic characterization of the diferrous sites that react with O2, together with the various activated oxygen (i.e., superoxo, (hydro)peroxo, oxo) species that have been identified in diiron-containing enzymes and synthetic model complexes. In several cases, different activated oxygen intermediates have been identified in the same enzyme. In addition to iron, copper plays a major role in O2 binding and activation in biology. An extensive overview of Cu active sites in biology, many of which are involved in O2 activation and oxidation catalysis, was the focus of a Chemical Reviews article several years ago.1 At that time, little was known about the lytic polysaccharide monooxygenases (LPMOs), a class of mononuclear Cu enzymes that play an important role in biomass conversion, specifically the oxidative depolymerization of cellulose and other polysaccarides. The review of LPMOs by Kelemen and co-workers highlights the many advances in this field in recent years including industrial efforts to employ LPMOs in biomass conversion, in addition to fundamental studies that have led to elucidation of the structure, function, and possible mechanisms of these enzymes. Homogeneous catalyst systems for aerobic oxidation reactions have been the subject of increasing attention and have begun to achieve considerable success. These reactions feature a wide range of mechanistic steps in the substrate oxidation reaction, ranging from single-electron transfer to organometallic reactions, but they are nevertheless conceptually related to enzymatic aerobic oxidations. Specifically, a majority of these reactions proceed by an “oxidase”-type ping-pong mechanism consisting of two redox half-reactions: (1) oxidation of the organic molecule by the catalyst, followed by (2) oxidation of the reduced catalyst by O2. This process avoids the requirement for a sacrificial reductant employed in “monooxygenase”-type reactions that proceed via reductive activation of O2. Many different transition metal catalyst systems have been used in homogeneous catalytic aerobic oxidation reactions, but Cu and Pd are the most extensively developed. A comprehensive overview of synthetic Cu-catalyzed aerobic oxidation reactions was published a few years ago in Chemical Reviews by Kozlowski and co-workers.2 In the present issue,
Stahl and co-workers present applications and mechanisms of ligand-promoted Pd-catalyzed aerobic oxidation reactions. This class of reactions diverge from the industrially practiced Wacker process and related reactions that employ simple Pd salts as catalysts. The invention and discovery of ligand-supported catalyst systems has greatly expanded the scope and utility of these reactions, with ligands providing the basis for improved catalyst activity and stability, in addition to catalyst control over chemo-, regio-, and stereoselectivity in these reactions. Polyoxometalates (POMs) are anionic metal oxide clusters that conceptually bridge homogeneous and heterogeneous catalysts. The article by Weinstock, Schreiber, and Neumann describes the mechanisms of reactions between POMs and O2 and the use of POMs in catalytic aerobic oxidation reactions. A wide range of POM structures and compositions have been evaluated, but the most widely used are vanadium-substituted phosphomolybdates, in particular, H5PV2Mo10O40. The reduced POM undergoes oxidation by O2, thereby providing the basis for oxidase-type aerobic oxidations involving two redox half-reactions, similar to those catalyzed by Pd and Cu. POMs are frequently used as cocatalysts to promote aerobic oxidation of homogeneous Pd or other catalysts that are responsible for the substrate oxidation half-reaction. In other cases, the POM participates directly in the oxidation of organic molecules, operating via electron-transfer, oxygen-atom-transfer, or other pathways. The thematic issue concludes with three reviews covering topics in the field of heterogeneous catalysis. The first of these, by Snyder, Bols, Schoonheydt, Sels, and Solomon, describes advances in the development and characterization of metal-ionexchanged zeolites that contain Fe or Cu ions in well-defined sites. These systems are noteworthy for their ability to react with O2, N2O, and H2O2 to generate oxidized sites that are capable of selective oxidation of methane and benzene to methanol and phenol, respectively. Considerable progress has been made in the spectroscopic characterization of the active sites in these Fe- and Cu-zeolites. The different spectroscopic methods that have been used to characterize the active sites are described, together with the emerging data, which reveal noteworthy similarities, as well as major differences, between the zeolite-based active sites and those of Fe- and Cucontaining metalloenzymes, such as the methane monooxygenases. Hermans and co-workers then survey advances in the development and understanding of heterogeneous metaloxide catalysts for aerobic oxidation of light alkanes, including supported metal oxides, mixed metal oxides, and zeolites. The emergence of shale gas as an abundant source of C1−C4 hydrocarbon feedstocks has stimulated even greater interest in catalytic processes that enable efficient conversion of light alkanes into valuable products, such as ethylene, acetic acid, acrolein, and related olefins and oxygenates. The spectroscopic and structural characterization of the different classes of oxidebased catalysts are described, together with their applications to partial oxidation of light alkanes and insights into the catalytic mechanism. These reactions predominantly occur at elevated temperatures in the gas phase and proceed by a Mars−van Krevelen mechanism involving lattice oxide migration to the active sites and regeneration by O2. The final contribution by Friend and co-workers describes the activation of O2 on metallic surfaces, including fundamental experimental and computational studies of reactions on welldefined metal surfaces (e.g., employing ultrahigh vacuum 2300
DOI: 10.1021/acs.chemrev.8b00046 Chem. Rev. 2018, 118, 2299−2301
Chemical Reviews
Editorial
techniques). Metals across the transition series are considered, with an analysis of trends in the bonding of O2 and atomic oxygen to the different surfaces together with the reactivity of these species with important substrates, such as CO, alcohols, and alkenes, among others. The mechanisms and barriers associated with O2 dissociation on different metal surfaces are considered and analyzed in considerable detail. Collectively, the articles in this thematic issue provide an extensive, if not comprehensive, survey of advances in O2 reduction, activation, and catalytic oxidations with electrochemical, biological, and chemical catalyst systems. The results from these diverse areas demonstrate both tremendous progress, but also remaining challenges, in harnessing the utility of O2 for energy conversion and chemical synthesis.
Edward I. Solomon*
Shannon S. Stahl was an undergraduate at the University of Illinois at Urbana−Champaign and a graduate student at Caltech (Ph.D., 1997), where he worked with Professor John Bercaw. He was an NSF postdoctoral fellow with Professor Stephen Lippard at Massachusetts Institute of Technology from 1997−1999, after which he began his independent career at the University of WisconsinMadison, where he is currently a Professor of Chemistry. His research group specializes in catalysis, with an emphasis on aerobic oxidation reactions and oxygen chemistry related to energy conversion. The former especially focuses on applications relevant to pharmaceutical synthesis, and his collaborations with industry in this area have been recognized by a US EPA Presidential Green Chemistry Challenge Award and the ACS Award in Affordable Green Chemistry. He is coeditor, with Dr. Paul L. Alsters (DSM), of Liquid Phase Aerobic Oxidation Catalysis (WileyVCH), a book highlighting existing applications and future opportunities for the use of aerobic oxidation in industrial chemistry.
Stanford University
Shannon S. Stahl* University of WisconsinMadison
AUTHOR INFORMATION ORCID
Edward I. Solomon: 0000-0003-0291-3199 Shannon S. Stahl: 0000-0002-9000-7665 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. Biographies
REFERENCES (1) Solomon, E. I.; et al. Copper Active Sites in Biology. Chem. Rev. 2014, 114, 3659. (2) Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.; Kozlowski, M. C. Aerobic Copper-Catalyzed Organic Reactions. Chem. Rev. 2013, 113, 6234−6458.
Edward I. Solomon grew up in North Miami Beach, FL, received his Ph.D. at Princeton (with D. S. McClure), and was a postdoctoral fellow at The Ørsted Institute (with C. J. Ballhausen) and then at Caltech (with H. B. Gray). He was a professor at the Massachusetts Institute of Technology until 1982, when he joined the faculty at Stanford University, where he is now the Monroe E. Spaght Professor of Humanities and Sciences and Professor of Photon Science at the SLAC National Accelerator Lab. He has been an invited professor in Argentina, Australia, Brazil, China, France, India, and Japan. Prof. Solomon’s research is in the fields of physical-inorganic chemistry and bioinorganic chemistry with emphasis on the application of a wide range of spectroscopic methods combined with QM calculations to elucidate the electronic structure of transition-metal sites and its contribution to physical properties and reactivity. He has received a wide range of medals and awards and is a member of the National Academy of Sciences and the American Academy of Arts and Sciences and a fellow in the American Association for the Advancement of Science and the American Chemical Society. 2301
DOI: 10.1021/acs.chemrev.8b00046 Chem. Rev. 2018, 118, 2299−2301
