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Author: search.filters.author.Kanchanawarin, Jarin

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    Local structure of stoichiometric and oxygen-deficient A 2Ti6O13 (A = Li, Na, and K) studied by X-ray absorption spectroscopy and first-principles calculations
    (2018-10-21)
    Kanchanawarin, Jarin
    ;
    Limphirat, Wanwisa
    ;
    Promchana, Pratya
    ;
    Sooknoi, Tawan
    ;
    Maluangnont, Tosapol
    Oxygen vacancy defects (V<inf>O</inf>) in Ti-based oxides play important roles in catalytic processes despite limited knowledge regarding their formation and characterization. Here, we demonstrate the use of X-ray absorption spectroscopy (XAS) measurements to compare the relative proportion of V<inf>O</inf> defects in as-grown alkali hexatitanate A<inf>2</inf>Ti<inf>6</inf>O<inf>13</inf> (A = Li, Na, K). Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions were studied. The similarity of measured XANES spectra of Ti K-edge in all samples indicates the presence of (Ti<sup>4+</sup>)O<inf>6</inf> units in good agreement with reported X-ray diffraction results. The small influence of cations A at the tunnel was observed and can be well reproduced in the simulated spectra. In addition, we present a semi-quantitative approach to intuitively determine the content of V<inf>O</inf> defects in oxygen-deficient K<inf>2</inf>Ti<inf>6</inf>O<inf>13-x</inf> by in situ time-resolved XAS measurements under reducing conditions (10%H<inf>2</inf>/Ar, 50-650 °C). The in situ XANES measurements indicate that the oxidation state of bulk Ti remains the same as the as-grown sample, i.e., 4+, at elevated temperatures. By in situ EXAFS measurements, the relative number of V<inf>O</inf> defects is highest at a reduction temperature of ∼550 °C and slightly decreases after that. To confirm the formation of V<inf>O</inf> defects, first-principles calculations were independently carried out using a 126-atom K<inf>2</inf>Ti<inf>6</inf>O<inf>13</inf> supercell with V<inf>O</inf> at various positions. Based on calculated EXAFS, the removal of the oxygen atom nearest to the tunnel, which is the lowest energy structure, provides a good match to the experimental spectra.
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    Production of liquid fuel from palmitic acid over nanocrystalline CeO2-based catalysts with minimal use of H2
    (2017-01-01)
    Maluangnont, Tosapol
    ;
    Dararat, Chalinee
    ;
    Kulrat, Teerapong
    ;
    Soontontaweesub, Surachet
    ;
    Anothaiwalaikul, Thitima
    The deoxygenation of palmitic acid into diesel-range hydrocarbons can be promoted over nanocrystalline ceria-based catalysts under atmospheric N<inf>2</inf> or 10% H<inf>2</inf>/N<inf>2</inf> in a fixed-bed flow reactor at 400 °C. Oxygen vacancy sites are active for ketonization of palmitic acid to C<inf>31</inf> ketone and also subsequent cracking of the formed ketone to hydrocarbons. The 22–31% selectivity of C<inf>9</inf> to C<inf>17</inf> liquid hydrocarbons can be achieved at 100% palmitic acid conversion. The deoxygenation under N<inf>2</inf> can be facilitated, presumably by hydrogen transfer from coke precursors. Catalytic activity of ceria-based catalysts can be tuned by pretreatment conditions, type of a carrier gas, or lattice modification.