A new sol–gel synthesis route for alumina–samaria mixed aero- and xerogel catalysts based on the so-called epoxide addition method and the use of these systems as catalysts for the oxidative coupling of methane (OCM) is reported. As precursors simple chloride or nitrate salts can be used. The mesoporous materials are X-ray amorphous even after calcination to 800 °C and show an intimate mixing of Al and Sm on the nanoscale. In the case of the xerogels derived from chlorides, C2 yields comparable to pure samaria can be achieved under OCM reaction conditions with 100 % O2 conversion. Even at lower O2 conversions the activity of the xerogel is competitive with a pure samaria reference catalyst taking the lower samaria content of 20 % into account. Accordingly, the approach is suitable to reduce the costs associated with the rare earth oxide. In addition to the preparation of aerogel and xerogel particles, the presented synthesis also allows the fabrication of xerogel films which can be coated on a suitable (monolithic) support. First results of such films are presented.

Rare earth oxides (REOs), particularly the sesquioxides, such as Sm2O3 and La2O3, have been investigated as promising catalysts in the oxidative coupling of methane (OCM). Much less attention has been paid to the reducible REOs because they are expected to give oxidation products, such as CO and CO2 (COx), rather than the desirable ethane and ethylene (C2+). Because Li addition can improve the performance of Sm2O3 in the OCM reaction and Li/MgO is commonly used as a reference OCM catalyst, the effects of lithium addition to a reducible oxide, TbOx, were investigated in detail in this study and compared with a Sm2O3 catalyst, which is the best single component OCM catalyst. Because of the well-documented volatility of lithium under OCM conditions, particularly for the Li/MgO system, the stability of lithium-doped samaria and terbia catalysts was examined as a function of preparation methods in this study. As expected, terbia supported on nanoparticle magnesia (n-MgO) is not a very active or selective OCM catalyst, and most of the observed selectivity toward C2+ products is likely due to the n-MgO support. In contrast, Li-doped TbOx/n-MgO prepared using a coimpregnation method yields a highly active and selective catalyst. The Li-TbOx/n-MgO catalyst yields the same methane conversion as pure Sm2O3, and has a higher C2+ selectivity than the Li-Sm2O3/n-MgO catalyst. The stability of the Li-TbOx/n-MgO catalyst is also higher than that of the Li-Sm2O3/n-MgO catalyst, and the loss of activity for the lithium-doped terbia catalyst appears to be the same as for the undoped Sm2O3/n-MgO catalyst (and undoped TbOx/n-MgO). The characterization data indicate stronger interactions between Li and TbOx than between Li and Sm2O3, which may explain the higher stability of the Li-TbOx/n-MgO catalysts. There are also indications that Li enters the TbOx lattice and reduces TbO1.81, to Tb2O3 during reaction, which can explain the higher C2+ selectivity compared with undoped TbOx/n-MgO. Furthermore, the Li-TbOx/n-MgO catalyst in this study is active at lower temperatures (600-700 degrees C) than typically used in the OCM (around 800 degrees C). Therefore, the Li-TbOx/n-MgO catalysts have potential to be very effective OCM catalysts, even though undoped TbOx/n-MgO catalysts are more selective toward COx than C2+ products.

A new sal-gel synthesis route for rare earth (Ce and Pr) alumina hybrid aero- and xerogels is presented which is based on the so-called epoxide addition method. The resulting materials are characterized by TEM, XRD and nitrogen adsorption. The results reveal a different crystallization behavior for the praseodymia/alumina and the ceria/alumina gel. Whereas the first remains amorphous until 875 degrees C, small ceria domains form already after preparation in the second case which grow with increasing calcination temperature. The use of the calcined gels as CO oxidation catalysts was studied in a quartz tube (lab) reactor and in a (slit) microreactor and compared to reference catalysts consisting of the pure rare earth oxides. The Ce/Al hybrid gels exhibit a good catalytic activity and a thermal stability against sintering which was superior to the investigated reference catalyst. In contrast, the Pr/Al hybrid gels show lower CO oxidation activity which, due to the formation of PrAlO3, decreased with increasing calcination temperature. (c) 2014 Elsevier Inc. All rights reserved.

Teamwork of noble partners: The preparation and catalytic properties of small, highly stable, and monodisperse Au-, Ag- and bimetallic AuAg-nanoparticles with well-defined compositions are reported. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Pulsed field gradient (PFG) NMR was used to investigate the self-diffusion of carbon dioxide in alumina stabilized samaria aerogel catalyst, a promising porous catalyst for gas-phase reactions featuring high porosity and high surface area. For diffusion studies, the catalyst was prepared in two sample packing types, macroscopic monoliths (i.e., macroscopic cylindrical particles) and powder beds with particle sizes around 200 μm that are considered for catalytic applications. Studies of diffusion in these samples revealed how macroscopic packing influences the catalyst transport properties. Application of a high magnetic field of 17.6 T in the reported PFG NMR studies enabled diffusion measurements for relatively low carbon dioxide densities in the catalyst samples corresponding to a gas loading pressure of around 0.1 atm. As a result, it was possible to perform diffusion measurements for a large range of carbon dioxide loading pressures between 0.1 and 10 atm. The measured carbon dioxide diffusivities in the beds of catalyst particles are interpreted in the context of a simple diffusion-mediated exchange model previously used for zeolites and other porous materials.

Sol-gel chemistry offers versatile new ways to prepare catalysts with tunable compositions and in different forms of application. In particular the option to obtain catalytic coatings is of interest for microreactors or the deposition of catalysts on monolithic supports. The present study explored a sol-gel approach for Fe and Co Fischer-Tropsch catalysts based on the so-called epoxide addition method. As a support and structural promoter, respectively, alumina was added. In the case of cobalt, the role of different precursors was studied, whereas in the case of iron, varying Fe-Al2O3 ratios were investigated. In both cases, ambient drying resulted in xerogels with high specific surface areas. The performance of the xerogels as catalysts for the Fischer-Tropsch (FT) reaction was studied in a fixed bed reactor and compared to cobalt-and iron-based reference catalysts synthesized by established impregnation and precipitation methods, respectively. All catalysts were carefully characterized in the as-prepared state as well as after catalysis with respect to their reducibility, stability and the distribution of the FT active component. Our study proves the suitability of the applied sol-gel technique to prepare highly active alumina-promoted Fe as well as alumina-supported Co FT catalysts and provides insight into the structure-performance relationships of these systems.

Pulsed field gradient (PFG) NMR employing a high magnetic field of 17.6T was used to study self-diffusion of carbon dioxide in alumina stabilized samaria aerogel, a promising porous catalyst for gas-phase reactions. Such rare-earth aerogels exhibit high porosity and surface area with active sites directly integrated into the pore framework. In the reported diffusion NMR studies, application of a high magnetic field was essential for obtaining sufficiently high signal-to-noise ratios under conditions of relatively low CO2 densities in the primarily mesoporous catalyst particles. The diffusion studies were performed with the catalyst that was formed into the following two types of samples: macroscopic monoliths and beds of particles with sizes around 200m. The sorbate diffusivity inside the monolith was compared with the corresponding diffusivity in the bed under conditions of fast exchange of CO2 between the particles and the interparticle voids of the bed. The two-domain exchange model proposed by Karger for zeolites was used to describe the latter diffusivity. The reported results are expected to be useful for elucidating an influence of possible transport limitations under reaction conditions in aerogel catalysts.

Oxidation catalysis by gold spurred intensive research efforts over the last two decades, which is encouraged by the unparalleled activity at temperatures even below 0 degrees C. Yet, gold nanostructures are inherently prone to coalescence at elevated temperatures, which limits their application. We demonstrate that this impediment can be overcome by reversing the classical order, that is, by depositing oxide nanoparticles on a high-surface area gold support. We used atomic layer deposition and liquid phase deposition, which leads to densely arranged oxide nanoparticles on the surface of a nanoporous gold material. In the case of a titania-coated material, a catalyst with so far unprecedented high catalytic activity already at ambient temperatures and stability up to 600 degrees C could be obtained. We demonstrate its high catalytic potential for two important reactions in the context of exhaust gas treatment: the oxidation of CO and the reduction of NO already proceeding at ambient temperatures.

Monolithic reactor concepts are currently intensively discussed in the literature. For the oxidative coupling of methane relying on a balance of surface and gas phase reactions, such concepts have previously been claimed to have beneficial effects with respect to obtainable C2 yields. In order to verify the superior performance in the case of a foam catalyst, ceria and samaria foams were fabricated by a direct foaming process. In both cases, mechanically stable, homogeneous open-cell foams were obtained as revealed by 3D magnetic resonance imaging, Hg-porosimetry, and scanning electron microscopy. As a characteristic feature of the introduced foaming methodology the process resulted in bimodal pore size distributions, ensuring low pressure drops on the one hand and sufficiently large surface areas on the other hand. Oxidative coupling of methane was carried out over the samaria foams. It was possible to obtain C2 yields that were indeed higher than those obtained with the samaria powder, in contrast to honeycomb monoliths previously studied in the literature.

Lanthanide oxide-based aerogels were synthesized employing the so-called epoxide addition sol–gel method already successfully applied for main- and transition metal oxide aerogels. Using chlorides and nitrates as precursors, our aim was to test the transferability of this robust sol–gel methodology to the entire lanthanide series. By adding the proton scavenging organic epoxide, propylene oxide, to hydrated lanthanide trichloride dissolved in ethanol or methanol, uniform monolithic alcogels were obtained. Subsequent processing in supercritical CO2 resulted in monolithic aerogels. No gelling process could be induced by using nitrates, in contrast to previous results with iron oxide or alumina aerogels. All materials were characterized by nitrogen adsorption/desorption analysis, transmission electron microscopy, and powder X-ray diffraction. With the exception of cerium, for which fractions of crystalline CeO2 were found already in the as-prepared material, XRD analysis revealed that the other materials were mainly amorphous. Subsequent heat treatment of the aerogels above 650 °C resulted in nanocrystalline phases for all aerogel materials. However, except for ceria, more detailed TEM and XRD studies provided evidence that crystalline oxychloride phases are formed in addition to fractions of oxide phases. The trends and possible explanations are discussed in this contribution.

The unique properties of gold especially in low temperature CO oxidation have been ascribed to a combination of various effects. In particular, particle sizes below a few nanometers and specific particle-support interactions have been shown to play important roles. In contrast, recent reports revealed that monolithic nanoporous gold (npAu) prepared by leaching a less noble metal, such as Ag, out of the corresponding alloy can also exhibit a remarkably high catalytic activity for CO oxidation, even though no support is present. Therefore, it was claimed to be a pure and unsupported gold catalyst. We investigated npAu with respect to its morphology, surface composition, and catalytic properties. In particular, we studied the reaction kinetics for low temperature CO oxidation in detail, taking the mass transport limitation due to the porous structure of the material into account. Our results reveal that Ag, even if removed almost completely from the bulk, segregates to the surface, resulting in surface concentrations of up to 10 atom %. Our data suggest that this Ag plays a significant role in activating of molecular oxygen. Therefore, npAu should be considered a bimetallic catalyst rather than a pure Au catalyst.