Catalytic Behaviour of Mesoporous Cobalt-Aluminum Oxides for CO Oxidation

Ordered mesoporous materials are promising catalyst supports due to their uniform pore size distribution, high specific surface area and pore volume, tunable pore sizes, and long-range ordering of the pore packing. The evaporation-induced self-assembly (EISA) process was applied to synthesize mesoporous mixed oxides, which consist of cobalt ions highly dispersed in an alumina matrix. The characterization of the mesoporous mixed cobalt-aluminum oxides with cobalt loadings in the range from 5 to 15 wt% and calcination temperatures of 673, 973, and 1073 K indicates that Co2+ is homogeneously distributed in the mesoporous alumina matrix. As a function of the Co loading, different phases are present comprising poorly crystalline alumina and mixed cobalt aluminum oxides of the spinel type. The mixed cobalt-aluminum oxides were applied as catalysts in CO oxidation and turned out to be highly active.Fil: Bordoloi, Ankur. Indian Institute of Petroleum; IndiaFil: Sanchez, Miguel Dario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Noei, Heshmat. Research Group X-Ray Physics and Nanoscience Deutsches Elektronen-Synchrotron; AlemaniaFil: Kaluza, Stefan. Fraunhofer Institute of Environmental, Safety, and Energy Technology; AlemaniaFil: Großmann, Dennis. Ruhr Universität Bochum; AlemaniaFil: Wang, Yuemin. Ruhr Universität Bochum; AlemaniaFil: Grünert, Wolfgang. Ruhr Universität Bochum; AlemaniaFil: Muhler, Martin. Ruhr Universität Bochum; Alemani

Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase

Synthesis and analysis of adaptive Pd-integrated perovskite catalysts for effective NO_x-reduction under lean conditions

The literature reports a series of precious metal integrated perovskite based catalysts revealing intelligent properties [1]. Lanthanum based perovskites are among these catalysts that are able to stabilize precious metal ions such as Pd, Pt, Rh, Ru, etc. in their crystal lattice. Precious metal ions in the catalyst respond reversibly to the changes in the exhaust gas composition by diffusing out of the perovskitic lattice as metallic nanoparticles under reducing conditions and by re-adsorbing into the crystal structure under oxidizing conditions. This behaviour improves the sintering resistance of precious metal particles and leads to enhanced NOx-reduction capability of the catalysts. Moreover, additional active species are formed in these catalysts which require less precious metal consumption in automotive catalytic converters. The present study is devoted to the synthesis of La-based perovskite catalysts by the polymeric citrate route and their analysis to establish the adaptive behaviour of the precious metal ions. In order to investigate the state and behaviour of the palladium ions in the perovskite, the catalysts were calcined under redox conditions at different temperatures and analyzed via XRD, SEM, TEM and XPS. XRD analysis showed that the La-based perovskites form an orthorhombic phase above 700°C and palladium ions are soluble in the perovskite lattice up to this temperature. FE-SEM observations displayed that no segregation of the palladium particles or agglomerates occurs in the oxidized conditions of the perovskite based catalyst. TEM-analysis of the as-prepared perovskite confirmed the presence of palladium in the matrix (or perovskitic crystal lattice), but small oxidic palladium particles on the perovskite surface were found as well. This observation agrees well with the XPS analysis showing signals with a Pd 3d5/2 binding energy of 336.5 eV, which correspond to Pd2+ in PdO, together with signals at higher binding energies, which can be assigned to intra-crystalline Pd2+at the surface an in the bulk of the crystalline [1]. TEM-EDX mapping analysis showed that lanthanum and iron were homo-geneously distributed in the perovskite matrix, however, few cobalt- and palladium- rich zones were also found. On reduction of a Pd-integrated perovskite catalyst in (20:1) N2:H2 atmosphere, palladium agglomerates in 10-15 nm sizes were detected indicating the diffusion of palladium ions out of the crystal structure to form Pd° as reported by Tanaka et al [1]. Binding energies of ~335.1 eV corresponding to Pd 3d5/2 were measured by XPS thus supporting the TEM and SEM observations. A Pd-supported perovskite Pd-LaFe(1-x)CoxO3 which was prepared by the classical impregnation method, clearly showed formation of palladium agglomerates in sizes up to 50 nm upon reduction treatment in 20:1 N2:H2 atmosphere. The XPS study suggested that some of the Pd-ions in the supported catalyst may have entered the first surface-layers of the perovskite lattice during the calcinations performed. The catalytic experiments demonstrated that these catalyst offer higher effi-ciency in NOx-conversion than typical SCR-catalysts under lean conditions employing either hydrogen or propene as reducing agents. The Pd-integrated perovskite e.g. LaFe0.65Co0.3Pd0.05O3 displayed a maximum NOx-conversion of 74 % and N2-selectivity of 60 % at 206°C in the reduction of NO by H2 in the presence of 5 vol-% O2 which is better than the performance of typical platinum supported catalysts i.e. Pt/SiO2 [2] which produce mostly N2O under similar reaction conditions. These NOx-conversions values were maintained in the presence of H2O + CO2. The challenge of the H2-SCR of NOx technology is to reduce NOx at a relative wide temperature window less than 300°C. This issue can be solved by substitution of other elements at the A-site of the host perovskite structure. In fact, the addition of ceria into the A-site of the La-based perovskite resulted in improvement of the N2-selectivity (73.7 %) at the maximum NOx-conversion (72 %) in the presence of H2O and CO2 in the feed. Furthermore a prototype was developed for catalytic testing under near real reaction conditions. For this purpose, the Pd-integrated perovskite with composition LaFe0.65Co0.3Pd0.05O3 was coated on cordierite substrates. The catalytic converter displayed a NOx-conversion of 20 % at 400°C even in the presence of 4 vol. % water vapour in the feed at high space velocity SV = 60000 h-1 during the C3H6-SCR of NOx reaction. [1] H. Tanaka, M. Misono, Curr. Opinion in Sol. State and Mat. Sci. 5 (2001) 381-387. [2] R. Burch, M. D. Coleman, Appl. Catal. B: Environ. 23 (1999) 115-121

X-ray absorption spectroscopy principles and practical use in materials analysis

The X-ray Absorption Fine Structure (XAFS) with its subregions X-ray Absorption Near-edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) is a powerful tool for the structural analysis of materials, which is nowadays a standard component of research strategies in many fields. This review covers a wide range of topics related to its measurement and use: the origin of the fine structure, its analytical potential, derived from the physical basis, the environment for measuring XAFS at synchrotrons, including different measurement geometries, detection modes, and sample environments, e. g. for in-situ and operando work, the principles of data reduction, analysis, and interpretation, and a perspective on new methods for structure analysis combining X-ray absorption with X-ray emission. Examples for the application of XAFS have been selected from work with heterogeneous catalysts with the intention to demonstrate the strength of the method providing structural information about highly disperse and disordered systems, to illustrate pitfalls in the interpretation of results (e. g. by neglecting the averaged character of the information obtained) and to show how its merits can be further enhanced by combination with other methods of structural analysis and/or spectroscopy

Surface Alloy or Metal–Cation Interaction-The State of Zn Promoting the Active Cu Sites in Methanol Synthesis Catalysts

Model catalysts containing disordered CuO and ZnO species in the pores of SBA-15 were reduced under different conditions (standard: H2, 513 K; severe: CO/H2, 673 K), studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) including operando work and used as catalysts for methanol synthesis at pressures up to 8 bar, where they were comparable with a commercial reference in terms of reaction rates related to Cu surface area. Severe reduction caused significant activation of the Cu sites beyond the sizeable level achieved already after standard reduction. In XRD, which failed to detect most of the active components, owing to small particle sizes, weak indications of alloy formation were found after severe reduction. XAFS, which averages over all species present, showed the interaction of Cu with zinc ions to increase by severe reduction, while formation of Zn0 was below detection limit. On this basis, the promoting interaction of Zn to Cu was ascribed to an interaction of zinc cations with Cu0

"Dirty nanostructures" : aerosol-assisted synthesis of temperature stable mesoporous metal oxide semiconductor spheres comprising hierarchically assembled zinc oxide nanocrystals controlled via impurities

Structural disintegration or the loss of accessible surfaces of functional nanostructures due to processes involving mass transport (e.g. sintering) is a serious problem for any application of these materials at elevated temperatures, like in heterogeneous catalysis or chemical sensing. Phases with low sintering temperatures, e.g. some metals or metal oxides like zinc oxide (ZnO), are very sensitive in this respect. Therefore, it is not only relevant to prepare important materials with refined morphologies, but the desired features need to be stable under real conditions. In this study, we describe the preparation of mesoporous ZnO nano-/microspheres by means of a template-assisted aerosol technique. Furthermore, by intentional introduction of impurity elements as dopants, specific surface areas and porosities of the prepared materials can be increased significantly. The impurities also strongly improve the thermal stability of the described ZnO nanostructures against thermal sintering. Although the pure ZnO material suffers from a complete loss of porosity, the structures of the impure (”dirty”) materials change only negligibly. Even at 500 °C morphology and porosity are preserved. The latter advantageous property was used for testing the novel nanocatalysts in heterogeneous catalysis

Preparation of Colloidal Nanoparticles of Mixed Metal Oxides Containing Platinum, Ruthenium, Osmium, and Iridium and Their Use as Electrocatalysts

The hydrolysis of H2PtCl6, RuCl3, OsCl3, and H2IrCl6 under basic conditions in the presence of a water-soluble betaine stabilizer affords aqueous colloidal solutions of the mixed metal oxide PtRuOsIrOx displaying a particle size of 1.3-1.6 nm. The ratio of the four metals can be adjusted by choosing the appropriate relative amounts of metal salts. Characterization was accomplished by TEM, XPS, and XRD. Immobilization of the reduced form on high surface conducting carbon leads to materials which are excellent electrocatalysts showing unusually high resistance to CO poisoning