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Subject: Free energy × WGL Institute: IOM ×

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    Dimethyl carbonate synthesis from carbon dioxide using ceria–zirconia catalysts prepared using a templating method: characterization, parametric optimization and chemical equilibrium modeling
    (London : RSC Publishing, 2016) Kumar, Praveen; With, Patrick; Srivastava, Vimal Chandra; Shukla, Kartikeya; Gläser, Roger; Mishra, Indra Mani
    In this paper, a series of CexZr1−xO2 solid solution spheres were synthesized by exo- and endo-templating methods and tested for dimethyl carbonate (DMC) synthesis using direct conversion of CO2. The synthesized catalysts were characterized by X-ray diffraction (XRD), N2-physisorption, scanning electron microscopy (SEM), and CO2/NH3-temperature-programmed desorption (TPD). Formation of CexZr1−xO2 solid solutions with tetragonal and cubic crystal structures depending on cerium/zirconium compositions was confirmed by XRD analysis. The specific surface area of the mixed oxide decreased and the average pore diameter increased with an increase in the ceria content, with the exception of the mixed oxides with x = 0.4–0.5 i.e. Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2. The basic and acidic site density of the synthesized catalysts was in the order: ZrO2 < CeO2 < Ce0.5Zr0.5O2, and the basic and acidic site density per unit area followed the same order. The best Ce0.5Zr0.5O2 catalyst was further used for the optimization of reaction conditions such as reaction time, reaction temperature, catalyst dose and reusability for DMC synthesis. Furthermore, study of chemical equilibrium modeling was done using the Peng–Robinson–Stryjek–Vera equation of state (PRSV-EoS) along with the van der Waals one-fluid reaction condition so as to calculate change of Gibbs free energy (ΔG°) and heat of reaction (ΔH°).
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    B12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption
    (London : RSC Publishing, 2021) Wulf, Toshiki; Warneke, Jonas; Heine, Thomas
    We study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of B12X11− (X = H, F, Cl, Br, I, CN) using ab initio quantum chemistry. With a Gibbs free energy of H2 attachment reaching up to 80 kJ mol−1 (ΔG at 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H2 adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol−1 between D2 and H2 and 2.7 kJ mol−1 between HD and H2, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H–H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H2 in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of B12X11− despite its negative charge. Unfortunately, this high reactivity also makes B12X11− unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H2 affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.