No abstract

Cerium oxide is an important material for catalytic and fuel cell applications. We present an ab initio density functional theory (DFT) study of the vibrational properties of ceria focusing on the interpretation of Raman spectra of polycrystalline powder samples, with vibrational bands in the frequency region between 250 and 1200 cm −1 . The model systems include the oxidized CeO 2 as well as the reduced CeO 2−x and Ce 2 O 3 bulk materials together with the CeO 2 (111) and oxygen defective CeO 2−x (111) surfaces. The experimentally observed band at 250 cm −1 is assigned to a surface mode of the clean CeO 2 (111) surface, in agreement with our Raman spectra of ceria (CeO 2 ) powders with varying crystal size (Filtschew, A.; Hofmann, K.; Hess, C., J. Phys. Chem. C 2016, 120, 6694). The reduced model systems display signature vibrational bands in the 480−600 cm −1 region associated with the presence of oxygen defects and reduced Ce 3+ ions. In the high-frequency region between 800 and 900 cm −1 , characteristic peroxide (O 2 2−) stretching vibrations at the oxidized and defective ceria surfaces are obtained, and a systematic study with respect to the peroxide coverage provides the basis for a correlation between the position of the peroxide stretching mode and its adsorption geometry and concentration. The present theoretical analysis allows for a consistent description of the experimental Raman spectra of polycrystalline ceria. The outlined approach serves as a reference for the description of vibrational properties of other metal oxides.

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.

The surface structure of sulfated zirconia (SZ) is examined by density-functional theory (DFT) with periodic boundary conditions. Adsorption of H2O and SO3 (or H2SO4) on the (101) surface of tetragonal zirconia is studied for different loadings up to H2SO4·3H2O and 2H2SO4·2H2O per two surface unit cells (four Zr surface sites). The considered surface species include H2O, [H+,OH-], SO3, [H+,HSO4 -], [2H+,SO4 2-], [H+,HS2O7 -], and [2H+,S2O7 2-]. Statistical thermodynamics is used to evaluate the relative stability of different surface structures for different temperatures and pressures of H2O and SO3 (or H2SO4). The simulated surface phase diagrams show a strong dependency on the considered sulfur species (H2SO4 or SO3) as well as on pressure and temperature. Monosulfates and pyrosulfates may occur, but higher condensated sulfates are not observed. In agreement with infrared experiments, we predict transformation of water-rich structures, [SO4 2-,2H+,3H2O], into pyrosulfate structures, [S2O7 2-,2H+,H2O], during calcination. Further increase of the temperature yields adsorbed SO3 before the clean surface is reached. Water adsorbed on the t-ZrO2(101) surface leaves in three steps upon heating from 250 to 730 K at 0.01 bar pressure:  physisorbed water below room temperature, the first chemisorbed water at about 440 K and the last water at about 730 K.

Catalytic activation and conversion of light alkanes by sulfated zirconia is unequivocally shown to be initiated by producing small concentrations of olefins. This occurs via stoichiometric oxidative dehydrogenation of butane by SO3 or pyrosulfate groups to butene (present mostly as alkoxy groups), water, and SO2. Thermal desorption and in situ IR spectroscopy have been used to determine all three reaction products. The concentration of butene formed determines both the catalytic activity of sulfated zirconia as well as the deactivation via formation of oligomers. The thermodynamics of the oxidative dehydrogenation of n-butane by different SZ surface structures has been examined by density functional (DFT) calculations. The calculations show that pyrosulfate or re-adsorbed SO3 species have the highest oxidizing ability.