The Electron Economy: Oxidation Catalysis for Energy Management

The energy-efficient removal of electrons is a key chemical step in an efficient electron economy. The objective of this project to develop efficient electrocatalysts for two important classes of oxidative chemical transformations. The first transformation is the oxidative conversion of methane to alcohols or higher hydrocarbons that would convert natural gas into valuable liquid fuels without the release of any carbon dioxide. The second transformation is the oxidation of water that is critical to any energy system that uses electrochemistry as an intermediary between electricity and stored fuels. Introduction The objective of this project is to develop new classes of supported molecular electrocatalysts for the oxidative conversion of alkanes and the electrooxidation of water. A specific objective is to develop electrocatalysts for the selective conversion of methane to value-added feedstocks and to illuminate the fundamental chemical and electrochemical steps required for the selective and energy-efficient oxidative conversion of hydrocarbons. Our objectives for the electrocatalytic oxidation of water are to develop efficient catalysts and new mechanistic insights that would enable the efficient extraction and harvesting of electrons from water. Background Our current electron economy is fueled in large part by the free energy available at high densities in hydrocarbon fuels. Photosynthesis has been the source of electrons to fuel this economy; these electrons have been stored over millennia as hydrocarbon deposits (oil, natural gas and coal) which provide both the bulk of the world’s energy needs as well as chemical feedstocks that drive our modern economy. Combustion has served as the primary means of converting the stored chemical energy of hydrocarbons into work; when fuels are combusted, much of the free energy is lost and CO2 is released into the environment. In the near future, the free energy of the electrons in fossil carbon sources must be harvested much more efficiently. Further out, entirely carbon-neutral sources of high free-energy electrons will be required. Methane has a high free energy per carbon atom. However, unlike higher hydrocarbons, methane is difficult to ship unless nearby to markets serviced by pipeline and is a relatively inefficient chemical feedstock. Absent an economical pipeline, methane is cryogenically liquefied at great energetic expense for shipment in insulated tanker ships to distant markets. Where is it used as a feedstock, it is partially combusted to syngas from which more valuable products are generated by energetically wasteful processes. Thus, it would be highly desirable to find routes to efficiently convert methane by partial oxidation to higher hydrocarbons and alcohols, which are more readily transported and are better chemical feedstocks than methane itself. The partial oxidation of methane by two electrons proceeds by the general reaction: The reaction products (CH3OH, C2H6, etc) contain the majority of the high free-energy electrons of the initial methane. Considerable effort has been devoted to the development of new catalyst systems that can effect these selective oxidative conversions of methane to more readily utilized energy or chemical feedstocks. A general consensus has evolved that the basic steps of the catalytic cycle would entail: (1) binding and C-H activation of CH4 by a metal complex to generate the critical M-CH3 intermediate, (2) oxidation of the M-CH3 intermediate and (3) elimination of CH3-X to regenerate the active metal species. Of the small number of systems that achieve a complete catalytic cycle for the partial oxidation of methane, one of the most successful is the "Catalytica" system developed by Periana and coworkers in which homogeneous mononuclear (bipym)PtCl2 complexes (bipym = bipyrimidine) catalyze the oxidative conversion of methane to methane bisulfate in fuming sulfuric acid at 220°C. Some of the limitations of the mononuclear (bipym)PtCl2 system were proposed to be the slow oxidation of the Pt(II) complex to the critical Pt(IV) intermediate and the inhibition of the catalytic system by water. These advances highlight the critical challenge in developing catalytic systems that can couple hydrocarbon activation with the removal of electrons. Water is the most abundant source of electrons. However, these electrons have low free energy and, after extracting them from water by the reaction: they must be electrically pumped into other species. Photosynthesis accomplishes the reaction at a polynuclear manganese cluster, from which the electrons are photochemically pumped into higher free-energy species culminating in the synthesis of carbohydrates. If chemical storage of significant amounts of solar energy is to be realized, energy efficient, scalable water oxidation catalysts will be required. The oxidation of water to dioxygen is an exceedingly difficult, multielectron reaction with a standard thermodynamic potential of 1.23 V vs NHE. While precious metal electrodes are able to perform this reaction, significant overpotentials are necessary with the attendant energy loss. By contrast, the oxygen evolving complex (OEC) of photosystem II (PSII) of photosynthetic organisms is far more energy efficient. Results In the first three months, we have focused on three primary, complementary objectives: (1) the development of methods to generate molecular catalysts supported on electrode surfaces, (2) the development of supported complexes that can extract electrons from water, and (3) the development of new catalyst systems that can activate and oxidize hydrocarbons. We have extended our previously developed protocols to attach molecular electrocatalysts to indium-tin-oxide (ITO) electrodes utilizing catalytic 5+2 cycloaddition reactions of surface-bonded azides and alkyne-functionalized ligands ("click" chemistry). The higher oxidative resistance of the ITO electrodes relative to the graphite electrodes we have previously used is critical to our efforts to develop electrocatalysts for water oxidation. We have developed new synthetic methods utilizing either azide-functionalized trialkoxysilanes or azide-functionalized phosphates to anchor catalysts to indium-tin oxide (ITO). These studies, in combination with our previous studies on graphite electrodes, have enabled us to anchor and characterize several representative molecular catalysts of Cu, Fe, and Ru to graphite and ITO electrodes (Figure 1). Current studies are focused on the ability of these supported catalysts to extract electrons from water to generate reactive metal oxo complexes.