High-Surface-Area Synthesis of Iron-Doped CaTiO3 at Low Temperatures: New Insights into Oxygen Activation, Iron States, and the Impact on Methane Oxidation.

In the present work, a series of CaTi1-xFexO3-δ (0 < x < 0.5) materials are prepared using a modified Pechini method based on citric acid and a polyol as chelating agents. The synthesis conditions are optimized with respect to the specific surface area and phase purity by varying polyols (ethylene glycol, glycerol, and 1.6-hexanediol) and the ratio between citric acid, polyols, and cations. The impact of the polyols and the iron content (up to 40 mol % on the B site) is studied with respect to the oxygen exchange rate, reducibility using H2-TPR, and catalytic performance for methane total oxidation. A correlation between the oxygen exchange rate studied using 18O exchange in powdered samples of CaTi1-xFexO3-δ (0 < x < 0.5) and ferric sites determined using Mössbauer spectroscopy and H2-TPR is established. The oxygen activation and diffusion in CaTi1-xFexO3-δ (0 < x < 0.5) continuously increase in the studied range of Ti substitution. The methane oxidation performance does not increase above x = 0.3, showing that methane oxidation is not limited by surface oxygen activation and CH4 is activated by specific iron sites in Fe-doped perovskites.

Brownmillerites CaFeO2.5 and SrFeO2.5 as Catalyst Support for CO Oxidation.

The support material can play an important role in oxidation catalysis, notably for CO oxidation. Here, we study two materials of the Brownmillerite family, CaFeO2.5 and SrFeO2.5, as one example of a stoichiometric phase (CaFeO2.5, CFO) and one existing in different modifications (SrFeO2.75, SrFeO2.875 and SrFeO3, SFO). The two materials are synthesized using two synthesis methods, one bottom-up approach via a complexation route and one top-down method (electric arc fusion), allowing to study the impact of the specific surface area on the oxygen mobility and catalytic performance. CO oxidation on 18O-exchanged materials shows that oxygen from SFO participates in the reaction as soon as the reaction starts, while for CFO, this onset takes place 185 °C after reaction onset. This indicates that the structure of the support material has an impact on the catalytic performance. We report here on significant differences in the catalytic activity linked to long-term stability of CFO and SFO, which is an important parameter not only for possible applications, but equally to better understand the mechanism of the catalytic activity itself.

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO†Oxidation on Pt/Ceria.

Pt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The state of Pt has often been proposed to differ depending on experimental conditions, for example, metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast to all previous reports, we show herein that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of the metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and platinum's evolving nature under reaction conditions.

Ionic liquid- and surfactant-controlled crystallization of WO3 films.

WO3 films were obtained via evaporation-induced self-assembly (EISA) using ionic surfactants such as long-chain ionic liquids 1-hexadecyl-3-methyl imidazolium chloride and bromide (C16mimCl and C16mimBr, respectively) and cetyltrimethylammonium chloride and bromide (CTAC and CTAB, respectively) as additives. Owing to the presence of the ionic surfactants, WO3 films crystallize in a preferred orientation along the a-axis on different substrates, as evidenced by X-ray diffraction. WO3 films with this orientation show improved electrochromic properties when compared to films with a lower degree of crystallographic orientation, prepared in an analogue fashion.

Surface Diels-Alder reactions as an effective method to synthesize functional carbon materials.

The post-synthesis chemical modification of various porous carbon materials with unsaturated organic compounds is reported. By this method, amine, alcohol, carboxylate, and sulfonic acid functional groups can be easily incorporated into the materials. Different carbonaceous materials with surface areas ranging from 240 to 1500 m(2)g(-1) and pore sizes between 3.0 and 7.0 nm have been studied. The resulting materials were analyzed by elemental analysis, nitrogen sorption, FTIR spectroscopy, zeta-potential measurements, thermogravimetric analysis, photoelectron spectroscopy, and small-angle X-ray scattering. These analyses indicated that the degree of functionalization is dependent on the nature of the dienophile (reactivity, steric hindrance) and the porosity of the carbon material. As possible applications, the functionalized carbonaceous materials were studied as catalysts in the Knoevenagel reaction and as adsorbents for Pb(2+) from aqueous solution.

Understanding Oxygen Release from Nanoporous Perovskite Oxides and Its Effect on the Catalytic Oxidation of CH4 and CO.

The design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides toward CH4 and CO oxidation was studied here. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi0.65Fe0.35O3-δ with specific surface areas (SSA) ranging from 45 to 80 m2/g and pore sizes from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron X-ray diffraction (XRD) and TG-MS combined with H2-TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH4 oxidation is influenced more by the dioxygen release. During isothermal long-term catalysis tests, however, the continuous oxygen release from large SSA materials promoted both CO and CH4 conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties.

Direct low-temperature synthesis of rutile nanostructures in ionic liquids.

The direct synthesis of crystalline titania nanorods by sol-gel chemistry in a special ionic liquid is reported. Unexpectedly, the high-temperature modification, rutile, is obtained directly under ambient conditions. X-ray diffraction and high-resolution transmission electron microscopy measurements support the highly crystalline and structural quality of the sample. The phase-directing property of the ionic liquids is attributable to the imide group in the counter ion, which exhibits strong interaction with specific rutile faces. Lithium insertion experiments were performed and revealed high and reversible loading capacities of up to 200 mAh g(-1).

Effects of oil on the curvature elastic properties of nonionic surfactant films: thermodynamics of balanced microemulsions.

The free energy of nonionic balanced microemulsions based on nonionic surfactants are analyzed using experimental data from (i) phase behavior, (ii) osmotic compressibility of the balanced microemulsion structure, which is obtained from small angle neutron scattering (SANS) experiments, and (iii) data on interfacial tensions obtained by T. Sottmann and R. Strey [J. Chem. Phys. 106 8606 (1997)]. The balanced microemulsion, where the spontaneous curvature vanishes at equal volumes of water and oil, has a finite swelling with the solvent with a minimum surfactant volume fraction, Phi*(S). At higher surfactant concentrations the balanced microemulsion phase having the surfactant volume fraction Phi(S1) coexists with a lamellar phase of volume fraction Phi(S2). Under the constraint of Phi(W) = Phi(O), where Phi(W) and Phi(O) are the water and oil volume fractions, respectively, the free energy density can be written as an expansion in the surfactant concentration. While the phase equilibria only depend on relative values of the expansion coefficients, absolute values can be obtained from compressibility and interfacial tension data. The osmotic compressibility of the surfactant film was measured by SANS through contrast matching water and oil. The phase behavior of nonionic surfactant-water-oil systems depends strongly on the chain length of the oil, when comparing a homologous series from octane to hexadecane using the same surfactant, here being pentaethylene oxide dodecyl ether (C(12)E(5)). The three concentrations Phi*(S), Phi(S1), and Phi(S2) increase markedly as the chain length of the oil is increased. However, from the analysis of the surface tension data it is concluded that there are no major changes in the bending rigidities as the oil is changed. The data are analyzed within the model free energy densities [formula--see text] and [formula--see text]. We find that within experimental accuracy, the first of these models provides a quantitatively consistent description of the data. For the second model there is a larger discrepancy between observed and calculated values.

Templating behavior of a long-chain ionic liquid in the hydrothermal synthesis of mesoporous silica.

The long-chain ionic liquid (IL) 1-hexadecyl-3-methylimidazolium chloride (C(16)mimCl) was used as a template to prepare porous silica with a two-dimensional hexagonal p6mm mesopore structure (MCM-41-type) as well as with a cubic Ia3d (gyroid, MCM-48-type structure) framework in basic synthesis medium via a hydrothermal synthesis procedure. A systematic study was carried out addressing the influence of the relative concentration of the IL and the pH on the mesostructure. As a main result, the IL template shows a broad range of conditions that allow the synthesis of the gyroid mesostructure with improved reproducibility. Unexpectedly, the formation of the hexagonal phase is less favorable, as the latter is quite sensitive to variations in parameters. This preference for the bicontinuous structure can be attributed to the special headgroup packing of ILs containing imidazolium groups.

A pyrrole-containing surfactant as a tecton for nanocomposite SiO2 films.

A surfactant featuring a polymerizable pyrrole head group (dodecyl-dimethyl-(2-pyrrol-1-yl-ethyl)-ammonium bromide, DDPABr) was synthesized. The thermotropic behavior of the surfactant was investigated by differential scanning calorimetry (DSC) and X-ray scattering techniques, with small-angle X-ray scattering (SAXS) analysis revealing a highly ordered lamellar bilayer structure. After full characterization, DDPABr was used in the preparation of mesostructured SiO2 nanocomposite thin films via evaporation-induced self-assembly (EISA). Resulting thin SiO2-DDPABr films were studied by 1D and 2D small-angle X-ray scattering (SAXS) techniques, indicating a lamellar nanocomposite structure. Suitable theoretical SAXS models were applied to fit the experimental 1D SAXS data. The surfactant could be chemically polymerized within the lamellar domains.