Catalysts for Emerging Energy Applications
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talysts for Emerging Energy Applications Bruce C. Gates (University of California–Davis, USA) George W. Huber (University of Massachusetts–Amherst, USA) Christopher L. Marshall (Argonne National Laboratory, USA) Phillip N. Ross (Lawrence Berkeley National Laboratory, USA, retired) Jeffrey Siirola (Eastman Chemical Company, USA) Yong Wang (Pacific Northwest National Laboratory, USA) Abstract Catalysis is the essential technology for chemical transformation, including production of fuels from the fossil resources petroleum, natural gas, and coal. Typical catalysts for these conversions are robust porous solids incorporating metals, metal oxides, and/or metal sulfides. As efforts are stepping up to replace fossil fuels with biomass, new catalysts for the conversion of the components of biomass will be needed. Although the catalysts for biomass conversion might be substantially different from those used in the conversion of fossil feedstocks, the latter catalysts are a starting point in today’s research. Major challenges lie ahead in the discovery of efficient biomass conversion catalysts, as well as in the discovery of catalysts for conversion of CO2 and possibly water into liquid fuels.
What Is a Catalyst?
Catalysts make chemical reactions go faster without being substantially consumed themselves. Although a catalyst changes the rate at which reactant molecules are converted into product molecules, it does not change the reaction equilibrium. If a reaction were to proceed all the way to equilibrium, the products and product distribution would be the same whether or not a catalyst were involved. However, many reactions do not proceed to equilibrium, and so a catalyst influences not just how fast the chemical change occurs, but also the distribution of products that are formed. A catalyst works by opening a new reaction pathway that is impossible without it—that is, by combining chemically with reactant molecules and forming intermediates that are converted into products more rapidly than would be possible if the reactants were present alone. A catalyzed reaction is almost always characterized by a lower energy barrier than the same reaction when not catalyzed. When a catalyst combines chemically with a reactant molecule, it forms an intermediate, which is usually converted in a sequence of steps into other intermediates and finally into products. Then the catalyst releases the product molecules and thereby becomes restored to its initial state, allowing it to combine with another reactant molecule and function again. Good catalysts undergo this process over and over again. Thus, catalytic reactions are cyclic, switching between the initial and final states with intermediate states along the way that involve partially or fully converted reactants. Ideally, a catalytic cycle continues without limit, but in reality, undesired changes render catalysts less active with continued use, and so the catalysts must be regenerated or replaced—to the benefit of the catalyst manufacturing industry. Some catalysts increase rates of
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