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Publications (3)15.07 Total impact

  • Bingjun Xu, Yashodhan Bhawe, Mark E. Davis
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    ABSTRACT: A manganese oxide-based, thermochemical cycle for water splitting below 1000 °C has recently been reported. The cycle involves the shuttling of Na+ into and out of manganese oxides via the consumption and formation of sodium carbonate, respectively. Here, we explore the combinations of three spinel metal oxides and three alkali carbonates in thermochemical cycles for water splitting and CO2 reduction. Hydrogen evolution and CO2 reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe3O4 > Mn3O4 > Co3O4, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li2CO3 > Na2CO3 > K2CO3. While hydrogen evolution and CO2 reduction reactions occur at a lower temperature on the combinations with the more reactive metal oxide and alkali carbonate, higher thermal reduction temperatures and more difficult alkali ion extractions are observed for the combinations of the more reactive metal oxides and alkali carbonates. Thus, for a thermochemical cycle to be closed at low temperatures, all three reactions of hydrogen evolution (CO2 reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na2CO3/Mn3O4 combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °C.
    Chemistry of Materials. 04/2013; 25(9):1564–1571.
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    ABSTRACT: Zeolites that contain eight-membered ring pores but different cavity geometries (LEV, CHA, and AFX structure types) are synthesized at similar Si/Al ratios and crystal sizes. These materials are tested as catalysts for the selective conversion of methanol to light olefins. At 400 °C, atmospheric pressure, and 100% conversion of methanol, the ethylene selectivity decreases as the cage size increases. Variations in the Si/Al ratio of the LEV and CHA show that the maximum selectivity occurs at Si/Al = 15–18. Because lower Si/Al ratios tend to produce faster deactivation rates and poorer selectivities, reactivity comparisons between frameworks are performed with solids having a ratio Si/Al = 15–18. With LEV and AFX, the data are the first from materials with this high Si/Al. At similar Si/Al and primary crystallite size, the propylene selectivity for the material with the CHA structure exceeds those from either the LEV or AFX structure. The AFX material gives the shortest reaction lifetime, but has the lowest amount of carbonaceous residue after reaction. Thus, there appears to be an intermediate cage size for maximizing the production of light olefins and propylene selectivities equivalent to or exceeding ethylene selectivities.
    ACS Catalysis 10/2012; 2(12):2490–2495. · 5.27 Impact Factor
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    Bingjun Xu, Yashodhan Bhawe, Mark E Davis
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    ABSTRACT: Thermochemical cycles that split water into stoichiometric amounts of hydrogen and oxygen below 1,000 °C, and do not involve toxic or corrosive intermediates, are highly desirable because they can convert heat into chemical energy in the form of hydrogen. We report a manganese-based thermochemical cycle with a highest operating temperature of 850 °C that is completely recyclable and does not involve toxic or corrosive components. The thermochemical cycle utilizes redox reactions of Mn(II)/Mn(III) oxides. The shuttling of Na(+) into and out of the manganese oxides in the hydrogen and oxygen evolution steps, respectively, provides the key thermodynamic driving forces and allows for the cycle to be closed at temperatures below 1,000 °C. The production of hydrogen and oxygen is fully reproducible for at least five cycles.
    Proceedings of the National Academy of Sciences 05/2012; 109(24):9260-4. · 9.81 Impact Factor