Browsing by Author "Hudson, MR"

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    Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)
    (Royal Society of Chemistry, 2014-08-28) Queen, WL; Hudson, MR; Bloch, ED; Mason, JA; Gonzalez, MI; Lee, JS; Gygi, D; Howe, JD; Lee, K; Darwish, TA; James, M; Peterson, VK; Teat, SJ; Smit, B; Neaton, JB; Long, JR; Brown, CM
    Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts. © 2014, The Royal Society of Chemistry.
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    Spectroscopic identification of hydrogen spillover species in ruthenium-modified high surface area carbons by diffuse reflectance infrared fourier transform spectroscopy
    (American Chemical Society., 2012-12-27) Blackburn, JL; Engtrakul, C; Bult, JB; Hurst, K; Zhao, Y; Xu, Q; Parilla, PA; Simpson, LJ; Rocha, JDR; Hudson, MR; Brown, CM; Gennett, T
    In recent years, carbon-based sorbents have been recognized for their potential application within vehicular hydrogen storage applications. One method by which sorbents have been reported to store appreciable hydrogen at room temperature is via a spillover process: where molecular hydrogen is first dissociated by metal nanoparticle catalysts and atomic hydrogen subsequently migrates onto the carbon substrate. Many reports have invoked the spillover mechanism to explain enhancements in reversible room temperature hydrogen uptake for metal-decorated sorbents. However, there is a lack of experimental evidence for the proposed chemical species formed as well as several differing theoretical explanations describing the process. In this report, we utilize diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to identify the various chemical species formed upon room temperature H-2 charging of ruthenium-decorated high surface area carbons. Room temperature H-2 loading of a control sample with no ruthenium nanoparticles (Ru NPs) leads to broad reversible peaks in the DRIFTS spectrum that correspond to the vibration-rotation transitions of weakly bound physisorbed hydrogen molecules. In contrast, the sample modified with Ru NPs shows a variety of reversible and irreversible peaks in addition to the physisorbed H-2 peaks. Rigorous experimental and theoretical analysis enables the assignment of the peaks to ruthenium-mediated formation of water, surface hydroxyl groups (R-OH, where R = carbon or ruthenium), and C-H bonds. The low-energy DRIFTS peaks assigned to spillover C-H bonds were additionally confirmed using inelastic neutron spectroscopy. Reversible vibrational peaks consistent with ruthenium-mediated formation of C-H bonds provide much-needed spectroscopic evidence for the spillover process. The results demonstrated here should facilitate future mechanistic investigations of hydrogen sorption on transition metal nanoparticles and high surface area activated carbons. © 2012, American Chemical Society.