Introduction: Oxygen Reduction and Activation in Catalysis.

T 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 Coor 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,