The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.

Carbon Dioxide Capture: Prospects for New Materials

Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Review and analysis of molecular simulations of methane, hydrogen, and acetylene storage in metal-organic frameworks.

The controllable synthesis of nanostructured CeO2-based materials is an imperative issue for environment- and energy-related applications. In this review, we present the recent technological and theoretical advances related to the CeO2-based nanomaterials, with a focus on the synthesis from one dimensional to mesoporous ceria as well as the properties from defect chemistry to nano-size effects. Seven extensively studied aspects regarding the applications of nanostructured ceria-based materials are selectively surveyed as well. New experimental approaches have been demonstrated with an atomic scale resolution characterization. Density functional theory (DFT) calculations can provide insight into the rational design of highly reactive catalysts and understanding of the interactions between the noble metal and ceria support. Achieving desired morphologies with designed crystal facets and oxygen vacancy clusters in ceria via controlled synthesis process is quite important for highly active catalysts. Finally, remarks on the challenges and perspectives on this exciting field are proposed.

Nanostructured ceria-based materials: synthesis, properties, and applications

Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman spectroscopy and O2 adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen...

Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption.

Metal-organic frameworks (MOFs) are promising adsorbents for hydrogen storage. Density functional theory and second-order Moller−Plesset perturbation theory (MP2) are used to calculate the interaction energies between H2 and individual structural elements of the MOF-5 framework. The strongest interaction, ΔH77 = −7.1 kJ/mol, is found for the α-site of the OZn4(O2Ph)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 H2 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 H2 molecules preserve their rotational degrees of freedom in the adsorbed state. This allows to discrimina...

Ab initio study of hydrogen adsorption in MOF-5.

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.

Oxidative activation of n-butane on sulfated zirconia.

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.

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

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+,SO42-], [H+,HS2O7-], and [2H+,S2O72-]. 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, [SO42-,2H+,3H2O], into pyrosulfate structures, [S2O72-,2H+,H2O], during ...

Alexander Hofmann

Papers

Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Ab initio study of hydrogen adsorption in MOF-5.

Oxidative activation of n-butane on sulfated zirconia.

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

Surface structure of hydroxylated and sulfated zirconia. A periodic density-functional study