Reaction: Earth-Abundant Metal Catalysts for Energy Conversions
2017; Elsevier BV; Volume: 2; Issue: 4 Linguagem: Inglês
10.1016/j.chempr.2017.03.019
ISSN2451-9308
Autores Tópico(s)Advanced battery technologies research
ResumoMorris Bullock is a laboratory fellow at Pacific Northwest National Laboratory and director of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the Department of Energy. He is a fellow of the AAAS, American Chemical Society, and Royal Society of Chemistry. He received his PhD while working for Prof. Charles Casey at the University of Wisconsin-Madison and worked as a postdoc for Prof. Jack Norton at Colorado State University. He edited the book Catalysis Without Precious Metals (Wiley-VCH, 2010). He has been a cheapskate for a long time and is glad to have an excuse to practice that habit in his research. Morris Bullock is a laboratory fellow at Pacific Northwest National Laboratory and director of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the Department of Energy. He is a fellow of the AAAS, American Chemical Society, and Royal Society of Chemistry. He received his PhD while working for Prof. Charles Casey at the University of Wisconsin-Madison and worked as a postdoc for Prof. Jack Norton at Colorado State University. He edited the book Catalysis Without Precious Metals (Wiley-VCH, 2010). He has been a cheapskate for a long time and is glad to have an excuse to practice that habit in his research. Precious metals are the workhorses of catalysis—they play a key role in refining crude oil into fuels and transform organic starting materials into high-value pharmaceutical products and agrochemicals. Rhodium, ruthenium, rhenium, palladium, and platinum are more than the workhorses—having contributed enormously to the development of modern chemical processes that improve our lives in many ways, they are the heroes of catalysis. As noted by Ludwig and Schindler in the March issue of Chem, however, precious (noble) metals also present some drawbacks: high cost, low abundance, and toxicity. Earth-abundant metals are much more sustainable and offer appealing attributes that address the disadvantages of precious metals. Catalysis is carried out industrially on scales that vary by orders of magnitude. The scale of pharmaceutical manufacturing is dwarfed by the scale of refining fossil fuels. Yet at all scales, there are advantages to replacing precious metals with earth-abundant metals. The high value of pharmaceutical products might suggest that using precious metals is not a drawback, given that the cost of chiral phosphine ligands can easily exceed that of the rhodium or ruthenium in the catalyst. Nevertheless, using precious metals is not sustainable in comparison with using earth-abundant metals that are available in orders-of-magnitude higher quantities. Moreover, cost savings from using abundant metal catalysts instead of familiar precious-metal catalysts can still be substantial, considering the billions of dollars spent annually in the manufacture of pharmaceuticals. The scale needed for energy conversions worldwide, however, is enormous, and thus the motivation to focus on abundant metal catalysts is compelling. Solar and wind are sustainable, renewable energy sources, but their intermittency is a drawback, because power is generated only when the sun is shining or the wind is blowing. The mismatch between the demand for energy and the time at which it is generated leads to the requirement for large-scale energy storage. There is also a spatial mismatch: much of the energy is needed in locations remote from those where it originates. Both the temporal and the spatial mismatches of energy supply and demand indicate that energy storage is necessary to facilitate the increasing usage of renewable energy. When excess electrical energy is generated, conversion of that energy to a fuel stores the energy in chemical bonds. We intuitively recognize that large amounts of energy can be stored in chemical bonds. Every time we drive a car with an internal combustion engine, chemical energy stored in C–H and C–C bonds of hydrocarbons is converted into energy, but the greenhouse gas CO2 is also produced. Carbon-neutral, renewable energy sources are appealing alternatives. Intense efforts worldwide have focused on the design and development of electrocatalysts for the interconversion of electrical energy and chemical energy. Recent discoveries have shown that essentially all of the critical reactions needed for a sustainable-energy future can be accomplished with catalysts based on earth-abundant metals. The production of H2 by proton reduction, the oxidation of H2 as catalyzed in fuel cells, the reduction of O2, and the oxidation of water are prominent examples of reactions that are traditionally catalyzed by precious metals and that have recently been shown to be catalyzed by earth-abundant metals (Figure 1). Fuel cells convert the chemical energy in the H–H bond to electrical energy. Platinum is the standard catalyst in proton-exchange membrane fuel cells; it oxidizes H2 at the anode and catalyzes the reduction of O2 to water at the cathode. Is there enough platinum on the planet for everyone to drive a car powered by fuel cells? Even if there were, is it wise to mine it (causing more environmental impact) and use it for that purpose? The amount of platinum required in fuel cells has decreased by large amounts in the last two decades, but an even more appealing alternative is to avoid the need for precious-metal catalysts entirely. Considering the persistent use of platinum in fuel cells, even after many technological improvements, why should we think the oxidation of H2 can be catalyzed by earth-abundant metals? Because biology accomplished it long ago! Surprising results from protein crystallography5Fontecilla-Camps J.C. Volbeda A. Cavazza C. Nicolet Y. Chem. Rev. 2007; 107: 4273-4303Crossref PubMed Scopus (1123) Google Scholar revealed that hydrogenases in nature have active sites composed of organometallic complexes with CO ligands, as well as the complexity of the surrounding proteins. Hydrogenases,6Lubitz W. Ogata H. Rüdiger O. Reijerse E. Chem. Rev. 2014; 114: 4081-4148Crossref PubMed Scopus (1362) Google Scholar enzymes with iron and/or nickel at the active site, catalyze the evolution of H2 and the oxidation of H2 with remarkable rates and energy efficiency. Nature had a head start of a few billion years, but synthetic inorganic chemists are working fastidiously to make up for lost time and have made substantial progress recently as the interest in developing earth-abundant metal catalysts has burgeoned. Biology provides inspiration for the design of synthetic catalysts. The recognition of the role of the pendant amine in the structure of the active site of [FeFe]-hydrogenase (Figure 1A) prompted the synthesis of synthetic functional models that incorporated pendant amines as proton relays. Nickel complexes with pendant amines (Figure 1B) function as electrocatalysts1Cardenas A.J.P. Ginovska B. Kumar N. Hou J. Raugei S. Helm M.L. Appel A.M. Bullock R.M. O'Hagan M. Angew. Chem. Int. Ed. 2016; 55: 13509-13513Crossref PubMed Scopus (46) Google Scholar for H2 production with turnover frequencies greater than 107 s−1, far exceeding the rates of natural enzymes. These synthetic electrocatalysts, however, have much higher overpotentials than the hydrogenases in nature, so the energy efficiency falls short of that achieved by the enzymes. Figure 1D shows an iron electrocatalyst for the reduction of O2 to water;3Pegis M.L. McKeown B.A. Kumar N. Lang K. Wasylenko D.J. Zhang X.P. Raugei S. Mayer J.M. ACS Cent. Sci. 2016; 2: 850-856Crossref PubMed Scopus (127) Google Scholar it has optimized turnover frequencies greater than 106 s−1, yet this complex also functions at a high overpotential. The iron complex shown in Figure 1C, also containing a pendant amine functioning as a proton relay, is an electrocatalyst for oxidation of H2.2Liu T. DuBois D.L. Bullock R.M. Nat. Chem. 2013; 5: 228-233Crossref PubMed Scopus (198) Google Scholar Oxidation of water, studied for decades with ruthenium catalysts, can be catalyzed by the iron complex shown in Figure 1E.4Fillol J.L. Codolà Z. Garcia-Bosch I. Gómez L. Pla J.J. Costas M. Nat. Chem. 2011; 3: 807-813Crossref PubMed Scopus (637) Google Scholar This progress is encouraging because it shows that cheap metals can accomplish the reactions once thought to be solely in the domain of precious metals, but it is only the first step. Precious-metal catalysis has benefitted from decades of optimization; compared with earth-abundant metal catalysts, they almost invariably have faster rates, longer lifetimes, and often superior tolerance to impurities. Many scientific and technological barriers remain before the achievements obtained through basic research on earth-abundant metals can be translated into practical catalysts for energy conversions. Recent discoveries of new chemical reactivity in earth-abundant metals might encourage others to apply the diverse skills needed in synthesis, mechanisms, spectroscopy, theory, and catalyst evaluation to produce a new generation of faster, more energy-efficient catalysts for energy-conversion reactions. Our research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy (DOE) Office of Science through the Basic Energy Sciences program. Pacific Northwest National Laboratory is operated by Battelle for the US DOE. Catalyst: Sustainable CatalysisLudwig et al.ChemMarch 09, 2017In BriefJacob R. Ludwig received his BSc in chemistry in 2014 from Michigan State University, where he performed research in the laboratory of Jetze Tepe. After graduation, he joined the Schindler lab at the University of Michigan to pursue his PhD degree. Corinna S. Schindler received her diploma in chemistry from the Technical University of Munich. After a research stay with K.C. Nicolaou at the Scripps Research Institute, she joined the group of Erick Carreira at ETH Zürich for her graduate studies. She then returned to the US to conduct postdoctoral studies with Eric Jacobsen at Harvard before starting her independent career at the University of Michigan in 2013. Full-Text PDF Open ArchiveReaction: Sustainable Catalysis without MetalsDavid J.C. ConstableChemApril 13, 2017In BriefDavid J.C. Constable is the science director of the American Chemical Society's Green Chemistry Institute. In this role, he works to catalyze and enable the implementation of sustainable and green chemistry and engineering throughout the global chemistry enterprise. Full-Text PDF Open ArchiveReaction: Opportunities for Sustainable CatalystsPatrick L. HollandChemApril 13, 2017In BriefPatrick Holland pursued doctoral training at the University of California at Berkeley with Robert Bergman and Richard Andersen and postdoctoral training with William Tolman at the University of Minnesota. As a faculty member at the University of Rochester, he began his independent research in 2000 by focusing on the study of three-coordinate Fe-Co complexes. Since then, his research group has addressed Fe-N2 chemistry, reactive metal-ligand multiple bonds, engineered metalloproteins, redox-active ligands, solar H2 production, and the mechanisms of organometallic transformations at base-metal complexes. In 2013, he moved to Yale University, where he is a professor of chemistry. Full-Text PDF Open Archive
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