Alexander Hofmann

Ab initio study of hydrogen adsorption in MOF-5.

Metal-organic frameworks (MOFs) are promising adsorbents for hydrogen storage. Density functional theory and second-order Møller-Plesset perturbation theory (MP2) are used to calculate the interaction energies between H(2) and individual structural elements of the MOF-5 framework. The strongest interaction, DeltaH(77) = -7.1 kJ/mol, is found for the alpha-site of the OZn(4)(O(2)Ph)(6) nodes. We show that dispersion interactions and zero-point vibrational energies must be taken into account. Comparison of calculations done under periodic boundary conditions for the complete structure with those done for finite models cut from the MOF-5 framework shows that the interactions with H(2) originate mainly from the local environment around the adsorption site. When used within a Multi-Langmuir model, the MP2 results reproduce measured adsorption isotherms (the predicted amount is 6 wt % at 77 K and 40 bar) if we assume that the H(2) molecules preserve their rotational degrees of freedom in the adsorbed state. This allows to discriminate between different isotherms measured for different MOF-5 samples and to reliably predict isotherms for new MOF structures.

Hydrogen adsorption on the tetragonal ZrO2(101) surface: a theoretical study of an important catalytic reactant

In order to understand the fundamental properties of the activation of zirconia by hydrogen in relation to the dehydrogenation of hexane, we have performed first-principles calculations using the density functional formalism (PW91 functional) and a plane wave basis set to describe the valence electronic wavefunctions. The interaction of hydrogen atoms and molecules with the thermodynamically most stable, stoichiometric (101) surface has been examined in detail. Three main stages of the hydrogen–ZrO2(101) interaction can be found: a weak interaction corresponding to molecular adsorption of H2 on the top of one surface Zr atom (Ead = −7.1 kJ mol−1), dissociative adsorption, which leaves H atoms on top of one Zr and one O atom (−17.8 kJ mol−1), and, finally, a repulsive interaction (+81.0 kJ mol−1) as precursor of water formation when hydrogen atoms are located above oxygen positions. Desorption of water (+179.9 kJ mol−1) forms a defect at the surface and creates therefore a zirconia suboxide. To separate these effects, atomic hydrogen adsorption has also been considered. Changes in the geometry and charge are discussed as well as the band structures of the adsorbates relative to the vacuum energy. The results are discussed and compared with the available experimental data.

Activation and isomerization of n-butane on sulfated zirconia model systems—an integrated study across the materials and pressure gaps

Butane activation has been studied using three types of sulfated zirconia materials, single crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO(3) and water was established by DFT calculations, which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H(2)O and SO(2), hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with <5 micromol g(-1) sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na = 35. Future research will have to clarify what distinguishes these few sites.

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.

A Boradiselenirane and a Boraditellurirane: Isolable Heavy Analogs of Dioxiranes and Dithiiranes

The isolation of BE2 heterocycles (E = Te, Se, S) from the reaction of a manganese borylene complex with elemental chalcogens is reported. The BTe2 and BSe2 cycles-a boraditellurirane and a boradiselenirane, respectively-are the first analogs of dioxiranes based on heavy chalcogens. While the BTe2 unit is still found datively bound to manganese, the Se and S analogs were isolated in their free forms. All heterocycles have been shown to transfer a chalcogen atom, allowing for the isolation of novel borachalcones and their dimerization products.