Dr. Wolfgang Schmidt
Max-Planck-Institut für Kohlenforschung
- Catalytic Hydrodesulfurization of Gaseous Fuels with Autogenously Formed Hydrogen
Optenhostert, T. and Puthenkalam, S. and Stegmann, N. and Steffen, M. and Schmidt, W.
Chemie-Ingenieur-Technik 93 (2021)Hydrogen production from gaseous fuels, such as natural or liquefied petroleum gas, for fuel cell application requires feedstock free of catalyst poisons. As sulfur is one of the main poisons, different methods are employed to remove sulfur species. We propose a process not requiring external hydrogen feed to the feedstock. The hydrogen for the hydrogenation of the sulfur species is obtained directly from the hydrocarbon matrix reacting on a zeolite catalyst. The result is a desulfurization process with considerably simplified process management and a superior process startup. © 2021 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH
view abstract 10.1002/cite.202000173
- In Situ Synchrotron X-ray Diffraction Studies of the Mechanochemical Synthesis of ZnS from its Elements
Petersen, H. and Reichle, S. and Leiting, S. and Losch, P. and Kersten, W. and Rathmann, T. and Tseng, J. and Etter, M. and Schmidt, W. and Weidenthaler, C.
Chemistry - A European Journal (2021)Mechanochemistry, as a synthesis tool for inorganic materials, became an ever-growing field in material chemistry. The direct energy transfer by collision of the educts with the milling media gives the possibility to design environmental-friendly reactions. Nevertheless, the underlying process of energy transfer and hence the kinetics of mechanosynthesis remain unclear. Herein, we present in situ synchrotron X-ray diffraction studies coupled with pressure measurements performed during the formation of ZnS and the subsequent phase transition (PT) from the hexagonal to the cubic modification. Milling Zn and S8 results in the sublimation of S8, observed by a sudden pressure increase. Simultaneously, the hexagonal metastable ZnS-modification (wurtzite) forms. Via detection of the pressure maximum, the exact start of the wurtzite formation can be determined. Immediately after the formation of wurtzite, the structural PT to the thermodynamic stable cubic modification sphalerite takes place. This PT can be described by the Prout-Tompkins equation for autocatalytic reactions, similar to thermally induced PT in sulfur vapor at high temperatures (T>1133 K). The increase in the reactivity of the wurtzite formation is explained by the reaction in sulfur vapor and the induction of defect structures by the collisions with the milling media. © 2021 The Authors. Published by Wiley-VCH GmbH
view abstract 10.1002/chem.202101260
- In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: From highly dilute solutions via cluster formation to nanoparticles
Onur Şahin, E. and Tüysüz, H. and Chan, C.K. and Moon, G.-H. and Dai, Y. and Schmidt, W. and Lim, J. and Scheu, C. and Weidenthaler, C.
Nanoscale 13 (2021)The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta2(OEt)10), the formation of hydrolysed TaxOyHz clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of TaxOy octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the TaxOyHz clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous TaxOyHz transforms directly into crystalline orthorhombic L-Ta2O5 without formation of any crystalline intermediate structures. © 2021 The Royal Society of Chemistry.
view abstract 10.1039/d0nr07871a
- Monitoring the Structure Evolution of Titanium Oxide Photocatalysts: From the Molecular Form via the Amorphous State to the Crystalline Phase
Onur Şahin, E. and Dai, Y. and Chan, C.K. and Tüysüz, H. and Schmidt, W. and Lim, J. and Zhang, S. and Scheu, C. and Weidenthaler, C.
Chemistry - A European Journal (2021)Amorphous TixOy with high surface area has attracted significant interest as photocatalyst with higher activity in ultraviolet (UV) light-induced water splitting applications compared to commercial nanocrystalline TiO2. Under photocatalytic operation conditions, the structure of the molecular titanium alkoxide precursor rearranges upon hydrolysis and leads to higher connectivity of the structure-building units. Structurally ordered domains with sizes smaller than 7 Å form larger aggregates. The experimental scattering data can be explained best with a structure model consisting of an anatase-like core and a distorted shell. Upon exposure to UV light, the white TixOy suspension turns dark corresponding to the reduction of Ti4+ to Ti3+ as confirmed by electron energy loss spectroscopy (EELS). Heat-induced crystallisation was followed by in situ temperature-dependent total scattering experiments. First, ordering in the Ti−O environment takes place upon to 350 °C. Above this temperature, the distorted anatase core starts to grow but the structure obtained at 400 °C is still not fully ordered. © 2021 The Authors. Published by Wiley-VCH GmbH
view abstract 10.1002/chem.202101117
- S-PEEK as a Catalyst for Gas Phase OME Synthesis
Kley, K.S. and Grünert, A. and Schmidt, W. and Schüth, F.
ChemCatChem 13 (2021)Oxymethylene ethers are a diesel additive/alternative, which reduce pollutant emissions. Based on methanol, it is possible to produce them from renewable energy and CO2. Sulfonated ion exchange resins are known to catalyze the synthesis of OME in liquid phase. Here, we report that the sulfonated polymer polyether-ether-ketone (S-PEEK) exhibits outstanding catalytic properties in the gas phase synthesis of OMEn. PEEK was reacted with gaseous SO3 to a degree of 22 % sulfonated monomer units. Using this material as a catalyst in the direct conversion of formaldehyde and methanol, an extraordinary selectivity to OME1 and OME2 of 93 % at equilibrium conversion of 55 % was obtained. Further, it was shown that the catalyst is stable for more than 60 h, and that with increasing reaction time selectivity to OME2 increases. © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH
view abstract 10.1002/cctc.202100191
- Synthetic ferripyrophyllite: Preparation, characterization and catalytic application
Qiao, Y. and Theyssen, N. and Spliethoff, B. and Folke, J. and Weidenthaler, C. and Schmidt, W. and Prieto, G. and Ochoa-Hernández, C. and Bill, E. and Ye, S. and Ruland, H. and Schüth, F. and Leitner, W.
Dalton Transactions 50 (2021)Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5]2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles. © The Royal Society of Chemistry.
view abstract 10.1039/d0dt03125a
- Carbon Supported Phosphoric Acid Catalysts for Gas-Phase Synthesis of Diesel Additives
Grünert, A. and Schmidt, W. and Schüth, F.
Catalysis Letters (2020)Abstract: Carbon supported phosphoric acid (H3PO4/C) was found to be a more productive catalyst for the gas-phase synthesis of the diesel fuel additive/substitute oxymethylene ethers (OME) as compared to benchmark zeolite catalysts. In this contribution, the performance of catalysts H3PO4/C and related H2PO4 −/C and HPO4 2−/C materials in OME synthesis from methanol and formaldehyde is described. Graphic Abstract: [Figure not available: see fulltext.]. © 2020, The Author(s).
view abstract 10.1007/s10562-020-03200-4
- Direct Atomic-Level Imaging of Zeolites: Oxygen, Sodium in Na-LTA and Iron in Fe-MFI
Mayoral, A. and Zhang, Q. and Zhou, Y. and Chen, P. and Ma, Y. and Monji, T. and Losch, P. and Schmidt, W. and Schüth, F. and Hirao, H. and Yu, J. and Terasaki, O.
Angewandte Chemie - International Edition 59 (2020)Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra-framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron-beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra-framework Na cations, are successfully observed in one of the most electron-beam-sensitive and lowest framework density zeolites, Na-LTA. Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
view abstract 10.1002/anie.202006122
- Effect of water leaching on biochar properties and its impact on organic contaminant sorption
Schreiter, I.J. and Schmidt, W. and Kumar, A. and Graber, E.R. and Schüth, C.
Environmental Science and Pollution Research 27 (2020)When biochar (BC) is applied to soil, one process that can alter its properties and contaminant sorption is the leaching of minerals and dissolved organic carbon (DOC). This study investigated changes in properties of three BCs (cattle manure, grain husk, and wood chips), due to leaching, and the subsequent impact on sorption of trichloroethylene (TCE) and tetrachloroethylene (PCE). The manure-derived BC released 27.4 mg g−1 DOC, which is over ten times more than that measured for the two plant-based BCs (2.5 and 1.5 mg g−1 DOC for grain husk and wood chips, respectively). In all leachates, potassium is the dominant cation, whereas chloride, sulfate, and phosphate are the main anions. In total, the manure-derived biochar released the highest sum of total ions (73.1 mg g−1), followed by BC produced from grain husk (15.5 mg g−1) and wood chips (1.2 mg g−1). Leaching increased external surface area, mesopore volume, and hydrophobicity of the manure-derived BC and decreased its polarity. This enhanced sorption via partitioning. In plant-based BCs, micropore volume and size distribution were altered, most likely through the un-blocking of pores, causing increased sorption via pore-filling for both TCE and PCE. The results indicate that, depending on feedstock material, BC leaching can alter the environmental fate of organic compounds. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
view abstract 10.1007/s11356-019-06904-2
- High Dynamics of Vapor Adsorption in Ordered Mesoporous Carbon CMK-5: A Small Angle X-ray Scattering Study
Schmidt, W. and Amenitsch, H.
Journal of Physical Chemistry C 124 (2020)Adsorption of dichloromethane in CMK-5 is a highly dynamic process going along with substantial rearrangement of both the adsorbent phase and the adsorbate phase. The carbon nanotubes of CMK-5 expand and contract during the process which is accompanied by increase and decrease in average distances between the tubes. Adsorption starts preferentially within the interstitial pores and then continues in the tubular ones. The complete process was monitored by small-angle X-ray diffraction. The SAXS data provided structural information on the carbon material and quantitative information on the adsorption process, which allowed drawing a precise picture of the complete adsorption process. © 2020 American Chemical Society.
view abstract 10.1021/acs.jpcc.0c05356
- Insights into the mechanochemical synthesis of Sn-β: Solid-state metal incorporation in beta zeolite
Joshi, H. and Ochoa-Hernández, C. and Nürenberg, E. and Kang, L. and Wang, F.R. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
Microporous and Mesoporous Materials 309 (2020)Sn-β zeolite is an active material for the isomerization of glucose to fructose, which is one of the critical reactions for the valorization of biomass. The material is synthesized either by a top-down or bottom-up approach. In this work, we use a top-down approach for the synthesis of Sn-β to incorporate the tin atoms into the *BEA framework. As compared to the literature, we replace the process of manual grinding with the use of ball milling to make the process reproducible, flexible, and scalable. The primary focus of this work is to investigate the processes occurring during the synthesis by a variety of characterization tools. These techniques include thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), physisorption, X-ray diffraction (XRD), and chemisorption monitored by Fourier-transform infrared spectroscopy (FTIR). The synthesis is followed by characterizing the material at various stages of synthesis. Finally, the materials are tested for the isomerization of glucose to fructose to assess the chemical nature of Sn-β zeolites. The results of this investigation provide several insights into the mechanochemical process for the incorporation of atoms in a zeolite framework. For instance, the importance of the size of precursors, distribution of Sn atoms during synthesis, and chemical changes occurring during milling are highlighted. These insights could produce a blueprint for the synthesis of a variety of solid catalysts. © 2020
view abstract 10.1016/j.micromeso.2020.110566
- Self-organization of silicates on different length scales exemplified by amorphous mesoporous silica and mesoporous zeolite beta using multiammonium surfactants
Castro, M. and Losch, P. and Farès, C. and Haouas, M. and Taulelle, F. and Breynaert, E. and Kirschhock, C. and Park, W. and Ryoo, R. and Schmidt, W.
RSC Advances 10 (2020)In this study the structure directing effect of a gemini-type piperidine-based multi-ammonium surfactant during hydrothermal zeolite synthesis was investigated for two cases: with and without a source of aluminum. The absence of an aluminum source led to the formation of an amorphous mesoporous MCM-48 type silica material, while the presence of aluminum guaranteed the formation of zeolite beta with a hierarchical pore system. The two opposing cases were studied in a time and temperature-dependent manner. The mobility and through space interaction of these large surfactant molecules were studied by liquid state nuclear magnetic resonance (NMR) at a temperature relevant to hydrothermal synthesis (363 K) in pure water and upon addition of an aluminum and silicon source. In the gel state, at different stages of aging and hydrothermal synthesis, low angle X-ray diffraction (XRD) and solid state magic angle spinning nuclear magnetic resonance (1H MAS NMR) spectrometry determined the developing order within the system. At each of these different synthesis steps the respective intermediate materials were calcined. Transmission electron microscopy then allowed closer inspection of the locally developing mesoscopic order, while N2physisorption was used to follow the evolution of porosity. © The Royal Society of Chemistry 2020.
view abstract 10.1039/d0ra03828h
- Encapsulation of sub-micrometer sized zeolites by porous silica – towards a rational design strategy for functional yolk-shells
Joshi, H. and Schmidt, W. and Schüth, F.
Microporous and Mesoporous Materials (2019)Catalysis often requires spatial separation of active centres. In material science this translates to a challenge in the synthesis of such materials, namely core-shells. Yolk-shell materials, a type of core-shell materials, possess a void between the core and shell that can be advantageous in catalysis. Yolk-shell materials with zeolitic core have not been reported extensively, despite their potential applicability in catalysis. This stems from the non-spherical morphology and surface properties of the zeolites, which makes controlled coating without defects difficult. Herein, we report a strategy for the encapsulation of beta zeolite (HBEA) with disordered mesoporous silica shell (HBEA@void@mSiO 2 ). HBEA is chosen as the centre (yolk) due to its cuboidal shape. The process involves creation of two shells, (a) sacrificial shell composed of resorcinol and formaldehyde, and (b) mesoporous silica shell. The result is an organic@inorganic hybrid that is thermally treated to obtain the corresponding hybrid. Polyvinylpyrrolidone (PVP) is an important component of the synthesis which assists in obtaining a uniform coating around the core. Thorough morphological, structural, fractal and textural characterization of this material was performed by electron microscopy, XRD, SAXS and sorption techniques. The hybrid possesses a hierarchical structure with an increasing porosity and spatial isolation of the core by the presence of a void. The siliceous nature of HBEA@void@mSiO 2 also enables a post-synthesis treatment for functional modification with mercaptosilane groups. The synthesis process shown here is highly controllable and has laid a solid foundation for a generalized synthesis strategy to build functional yolk-shell materials based on zeolites. © 2019
view abstract 10.1016/j.micromeso.2019.03.013
- Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion
Losch, P. and Huang, W. and Vozniuk, O. and Goodman, E.D. and Schmidt, W. and Cargnello, M.
ACS Catalysis 9 (2019)Complete catalytic oxidation of methane in the presence of steam at low temperatures (T < 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C-H bond of methane at low temperature is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technological applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure-property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from commercially available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepared. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T50 of 355 °C and T90 of 375 °C (where T50 and T90 are temperature values at which the samples reach 50% and 90% methane conversion, respectively) in steam-containing reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications. © 2019 American Chemical Society.
view abstract 10.1021/acscatal.9b00596
- Studying proton mobility in zeolites by varying temperature infrared spectroscopy
Losch, P. and Joshi, H. and Stegmann, N. and Vozniuk, O. and Schmidt, W.
Molecules 24 (2019)We report a varying temperature infrared spectroscopic (VTIR) study with partial deuterium isotopic exchange as a method for characterizing proton mobility in acidic materials. This VTIR technique permits the estimation of activation energies for proton diffusion. Different acidic materials comprising classical proton-conducting materials, such as transition metal phosphates and sulfonated solids, as well as different zeolites, are tested with this new method. The applicability of the method is thus extended to a vast library of materials. Its underlying principles and assumptions are clearly presented herein. Depending on the temperature ranges, different activation energies for proton transfer are observed irrespective of the different materials. In addition to the well-studied transition metal phosphates, Si-rich zeolites appear to be promising proton-transfer materials (with Eact < 40 kJ mol−1) for application in high-temperature (>150 ◦C) PEM fuel cells. They significantly outperform Nafion and sulfonated silica, which exhibit higher activation energies with Eact ~ 50 and 120 kJ mol−1, respectively. © 2019 by the authors.
view abstract 10.3390/molecules24173199
- Gas-phase synthesis of oxymethylene ethers over Si-rich zeolites
Grünert, A. and Losch, P. and Ochoa-Hernández, C. and Schmidt, W. and Schüth, F.
Green Chemistry 20 (2018)Oxymethylene ethers are a class of synthetic fuels that allows significantly reduced levels of pollutant emissions from compression-ignition engines. Conventionally they are synthesized in liquid-phase. As a new approach for the production of oxymethylene ethers the continuous gas-phase synthesis from methanol and formaldehyde was studied. A broad range of zeolites has been studied as the catalysts for the reaction and a relationship between reactivity and silica-to-alumina ratio was established. Moderate acidity as found in silicon-rich zeolites proved to be advantageous. Even aluminum-free zeolite shows high selectivity and activity to OME indicating that silanol groups as found on the external surface or in defects provide sufficient acidity for the reaction. The zeolitic catalysts deactivate with time but can be fully regenerated with common regeneration protocols. © 2018 The Royal Society of Chemistry.
view abstract 10.1039/c8gc02617c
- Ozone Treatment: A Versatile Tool for the Postsynthesis Modification of Porous Silica-Based Materials
Joshi, H. and Jalalpoor, D. and Ochoa-Hernández, C. and Schmidt, W. and Schüth, F.
Chemistry of Materials 30 (2018)Facile synthesis of silica-based functional materials at low temperatures has remained a challenge in materials science. To this end, we demonstrate the use of a gaseous ozone stream, generated via an electric discharge method, as a versatile tool for the postsynthesis modification of silica-based functional nanomaterials. First, a parametric study is conducted with a mesoporous model material to obtain basic insights into the reaction of the organics with ozone. The study is then extended to a number of distinct silica-based inorganic materials. The scope of ozone treatment can be broadly classified into three categories: (a) elimination of templates or structure directing agents (SDAs) from materials with pore sizes ranging from 0.5 to 10 nm, (b) selective transformation of organic groups functionalized on the mesoporous silica, and (c) simultaneous elimination of intermediate polymeric shells and template from the outer shells to obtain yolk-shell type materials. Each material studied here requires different parameters (temperature, time, and concentration of ozone) depending on its physical and chemical properties which have been carefully examined. Overall, the study demonstrates the potential of ozone treatment in tailoring functional materials at low temperature and provides vital insights into the reaction of ozone with silica-based materials. The study shows that gaseous ozone treatment is not limited to only one type of materials but can be applied to many systems, and we are convinced that the methodology can be applied to a multitude of organic@inorganic systems way beyond the scope of materials presented here. © Copyright 2018 American Chemical Society.
view abstract 10.1021/acs.chemmater.8b04113
- Proton Mobility, Intrinsic Acid Strength, and Acid Site Location in Zeolites Revealed by Varying Temperature Infrared Spectroscopy and Density Functional Theory Studies
Losch, P. and Joshi, H.R. and Vozniuk, O. and Grünert, A. and Ochoa-Hernández, C. and Jabraoui, H. and Badawi, M. and Schmidt, W.
Journal of the American Chemical Society 140 (2018)The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. < 40 kJ mol -1 ) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O-(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O-D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique. © 2018 American Chemical Society.
view abstract 10.1021/jacs.8b11588
- Scalable One-Pot Synthesis of Yolk-Shell Carbon Nanospheres with Yolk-Supported Pd Nanoparticles for Size-Selective Catalysis
Wang, G.-H. and Chen, K. and Engelhardt, J. and Tüysüz, H. and Bongard, H.-J. and Schmidt, W. and Schüth, F.
Chemistry of Materials 30 (2018)
view abstract 10.1021/acs.chemmater.8b00456
- Sorption mechanisms of chlorinated hydrocarbons on biochar produced from different feedstocks: Conclusions from single- and bi-solute experiments
Schreiter, I.J. and Schmidt, W. and Schüth, C.
Chemosphere 203 (2018)Biochar is increasingly deemed a potential sorbent for contaminants in soil and water remediation. We tested three biochars from different feedstocks (cattle manure, grain husk, and wood chips) produced at relatively low pyrolysis temperature (450 °C), for their sorption behavior towards trichloroethylene (TCE) and tetrachloroethylene (PCE) in single- and bi-solute systems. In single-solute experiments, all biochars show stronger sorption for TCE (about 50% based on solubility-normalized Freundlich coefficients). The lower sorption of PCE is attributed to steric effects, e.g. size exclusion in small micropores and specific interactions. Plant-derived, carbon-rich biochars with high specific surface area and microporosity predominantly sorb via pore-filling, as also observed in activated carbon. Biochar produced from manure, with higher ash content and polarity, and smaller total pore volume (PVtot), shows significant contribution of partitioning. These findings also apply to bi-solute systems. TCE and PCE show different competition behavior depending on biochar properties. Plant-based biochars are pore-filling-dominated and show strong competition. However, competition behavior in microporous biochars depends on the concentration range. Manure biochar with high polarity and low PVtot shows significant partitioning and therefore less competition. Compared to the plant-based chars competition in manure biochar is not concentration-dependent. These results indicate that biochars with a large fraction of non-carbonized phase facilitate non-competitive sorption and might be a valuable sorbent in mixed contaminant systems. © 2018 Elsevier Ltd
view abstract 10.1016/j.chemosphere.2018.03.173
- Unraveling Direct Formation of Hierarchical Zeolite Beta by Dynamic Light Scattering, Small Angle X-ray Scattering, and Liquid and Solid-State NMR: Insights at the Supramolecular Level
Castro, M. and Losch, P. and Park, W. and Haouas, M. and Taulelle, F. and Loerbroks, C. and Brabants, G. and Breynaert, E. and Kirschhock, C.E.A. and Ryoo, R. and Schmidt, W.
Chemistry of Materials 30 (2018)A case study on the understanding of the formation of hierarchical Beta zeolites using gemini-type piperidine based multiammonium surfactant (N6-diphe) is reported. Complementary techniques were used to investigate N6-diphe's structure-directing effect at the molecular level. Combining characterization of the resulting zeolite materials with the toolboxes herein developed for studying clear solutions and dense gels discloses self-assembly processes that govern the growth (and growth inhibition) of nano-Beta zeolite crystals. In clear solution, small-angle X-ray scattering and liquid-state NMR provide insights about the formation of nanoparticles and their degree of order. 14N and 1H-DOSY NMR probe the dynamics and mobility of soluble species. In a dense gel, on the other side, 27Al- and 29Si-(MAS) NMR elucidate the varying local connectivity between initial nano-objects and the final solid products. It has been found that cylindrical micelles control the transformation of solubilized silica and alumina during the formation of zeolite nuclei and guide their crystal growth to nano-Beta rods with bimodal mesoporosity. The predominant smaller mesopores (6 to 8 nm) originate from the template's hydrophobic alkyl chains, while larger mesopores (10 to 30 nm) are supposed to result from a spinodal decomposition-type segregation of phases consisting of as-formed hydrophobic zeolite rods and an aqueous solution. © 2018 American Chemical Society.
view abstract 10.1021/acs.chemmater.8b00109
- Surface-Casting Synthesis of Mesoporous Zirconia with a CMK-5-Like Structure and High Surface Area
Gu, D. and Schmidt, W. and Pichler, C.M. and Bongard, H.-J. and Spliethoff, B. and Asahina, S. and Cao, Z. and Terasaki, O. and Schüth, F.
Angewandte Chemie - International Edition (2017)About 15years ago, the Ryoo group described the synthesis of CMK-5, a material consisting of a hexagonal arrangement of carbon nanotubes. Extension of the surface casting synthesis to oxide compositions, however, was not possible so far, in spite of many attempts. Here it is demonstrated, that crystalline mesoporous hollow zirconia materials with very high surface areas up to 400m2g-1, and in selected cases in the form of CMK-5-like, are indeed accessible via such a surface casting process. The key for the successful synthesis is an increased interaction between the silica hard template surface and the zirconia precursor species by using silanol group-rich mesoporous silica as a hard template. The surface areas of the obtained zirconias exceed those of conventionally hard-templated ones by a factor of two to three. The surface casting process seems to be applicable also to other oxide materials. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/anie.201705042
- Co3O4 Nanoparticles Supported on Mesoporous Carbon for Selective Transfer Hydrogenation of α,β-Unsaturated Aldehydes
Wang, G.-H. and Deng, X. and Gu, D. and Chen, K. and Tüysüz, H. and Spliethoff, B. and Bongard, H.-J. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
Angewandte Chemie - International Edition 55 (2016)A simple and scalable method for synthesizing Co3O4nanoparticles supported on the framework of mesoporous carbon (MC) was developed. Benefiting from an ion-exchange process during the preparation, the cobalt precursor is introduced into a mesostructured polymer framework that results in Co3O4nanoparticles (ca. 3 nm) supported on MC (Co3O4/MC) with narrow particle size distribution and homogeneous dispersion after simple reduction/pyrolysis and mild oxidation steps. The as-obtained Co3O4/MC is a highly efficient catalyst for transfer hydrogenation of α,β-unsaturated aldehydes. Selectivities towards unsaturated alcohols are always higher than 95 % at full conversion. In addition, the Co3O4/MC shows high stability under the reaction conditions, it can be recycled at least six times without loss of activity. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/anie.201604673
- Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
Journal of the American Chemical Society 138 (2016)Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
view abstract 10.1021/jacs.6b04251
- Influence of the degree of infiltration of modified activated carbons with CuO/ZnO on the separation of NO2 at ambient temperatures
Sager, U. and Däuber, E. and Bathen, D. and Asbach, C. and Schmidt, F. and Tseng, J.-C. and Pommerin, A. and Weidenthaler, C. and Schmidt, W.
Adsorption Science and Technology 34 (2016)The reduction of NO2 in air at ambient temperatures with activated carbons can be increased by the infiltration of metal oxide nanoparticles into the sorbents. The NO2 is first adsorbed to the activated carbon and subsequently catalytically reduced to physiologically neutral substances by the metal oxides. The catalytic reduction at ambient temperatures is rather slow. In a former study concerning the application in cabin air filters, it was shown that the modification of activated carbon with 5 wt% CuO/ZnO leads to reduced breakthrough of NO2 and that the adsorbent was able to regenerate between repeated NO2 adsorption cycles. Here we show that the efficiency of the sorbent can be more than doubled by increasing the metal oxide infiltration to 20 wt% whereas a further increase in loading yields no additional improvement, due to a partial transformation of the oxidic compounds. © 2016, © The Author(s) 2016.
view abstract 10.1177/0263617416653120
- Nitrogen-Doped Ordered Mesoporous Carbon Supported Bimetallic PtCo Nanoparticles for Upgrading of Biophenolics
Wang, G.-H. and Cao, Z. and Gu, D. and Pfänder, N. and Swertz, A.-C. and Spliethoff, B. and Bongard, H.-J. and Weidenthaler, C. and Schmidt, W. and Rinaldi, R. and Schüth, F.
Angewandte Chemie - International Edition 55 (2016)Hydrodeoxygenation (HDO) is an attractive route for the upgrading of bio-oils produced from lignocellulose. Current catalysts require harsh conditions to effect HDO, decreasing the process efficiency in terms of energy and carbon balance. Herein we report a novel and facile method for synthesizing bimetallic PtCo nanoparticle catalysts (ca. 1.5 nm) highly dispersed in the framework of nitrogen-doped ordered mesoporous carbon (NOMC) for this reaction. We demonstrate that NOMC with either 2D hexagonal (p6m) or 3D cubic (Im3m) structure can be easily synthesized by simply adjusting the polymerization temperature. We also demonstrate that PtCo/NOMC (metal loading: Pt 9.90 wt %; Co 3.31 wt %) is a highly effective catalyst for HDO of phenolic compounds and “real-world” biomass-derived phenolic streams. In the presence of PtCo/NOMC, full deoxygenation of phenolic compounds and a biomass-derived phenolic stream is achieved under conditions of low severity. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/anie.201511558
- Zeolite Beta Formation from Clear Sols: Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods
Castro, M. and Haouas, M. and Lim, I. and Bongard, H.J. and Schüth, F. and Taulelle, F. and Karlsson, G. and Alfredsson, V. and Breyneart, E. and Kirschhock, C.E.A. and Schmidt, W.
Chemistry - A European Journal 22 (2016)The formation of silicate nanoaggregates (NAs) at the very early stages of precursor sols and zeolite beta crystallization from silicate nanoparticles (NPs) are investigated in detail using a combination of different analysis methods, including liquid-state29Si,27Al,14N, and1H NMR spectroscopy, mass spectrometry (MS), small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and transmission electron microscopy at cryogenic temperatures (cryo-TEM). Prior to hydrothermal treatment, silicate NAs are observed if the Si/OH ratio in the reaction mixture is greater than 1. Condensation of oligomers within the NAs then generates NPs. Aluminum doped into the synthesis mixtures is located exclusively in the NPs, and is found exclusively in a state that is fourfold connected to silicate, favoring their condensation and aggregation. These results are in agreement with general trends observed for other systems. Silicate NAs are essential intermediates for zeolite formation and are generated by the aggregation of hydrated oligomers, aluminate, and templating cations. Subsequent further intra-nanoaggregate silicate condensation results in the formation of NPs.1H and14N liquid NMR as well as diffusion ordered spectroscopy (DOSY) experiments provide evidence for weakly restricted rotational and translational mobility of the organic template within NAs as a consequence of specific silicate–template interactions. NAs thus appear as key species in clear sols, and their presence in the precursor sol favors silicate condensation and further crystallization, promoted either by increasing the Si/OH ratio or by heating. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/chem.201600511
- Controllable Synthesis of Mesoporous Peapod-like Co
3O 4@Carbon Nanotube Arrays for High-Performance Lithium-Ion Batteries
Gu, D. and Li, W. and Wang, F. and Bongard, H. and Spliethoff, B. and Schmidt, W. and Weidenthaler, C. and Xia, Y. and Zhao, D. and Schüth, F.
Angewandte Chemie - International Edition 54 (2015)Abstract Transition metal oxides are regarded as promising anode materials for lithium-ion batteries because of their high theoretical capacities compared with commercial graphite. Unfortunately, the implementation of such novel anodes is hampered by their large volume changes during the Li+ insertion and extraction process and their low electric conductivities. Herein, we report a specifically designed anode architecture to overcome such problems, that is, mesoporous peapod-like Co<inf>3</inf>O<inf>4</inf>@carbon nanotube arrays, which are constructed through a controllable nanocasting process. Co<inf>3</inf>O<inf>4</inf> nanoparticles are confined exclusively in the intratubular pores of the nanotube arrays. The pores between the nanotubes are open, and thus render the Co<inf>3</inf>O<inf>4</inf> nanoparticles accessible for effective electrolyte diffusion. Moreover, the carbon nanotubes act as a conductive network. As a result, the peapod-like Co<inf>3</inf>O<inf>4</inf>@carbon nanotube electrode shows a high specific capacity, excellent rate capacity, and very good cycling performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/anie.201501475
- Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
Gu, D. and Jia, C.-J. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Schüth, F.
Journal of the American Chemical Society 137 (2015)Co<inf>3</inf>O<inf>4</inf> with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H<inf>2</inf> reduction of nanocast Co<inf>3</inf>O<inf>4</inf> at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3¯m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo<inf>2</inf>O<inf>4</inf>, CoCr<inf>2</inf>O<inf>4</inf>, and CoFe<inf>2</inf>O<inf>4</inf>) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe<inf>2</inf>O<inf>4</inf> with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation. © 2015 American Chemical Society.
view abstract 10.1021/jacs.5b06336
- Microstructure analysis of complex CuO/ZnO@carbon adsorbers: What are the limits of powder diffraction methods?
Tseng, J.C. and Schmidt, W. and Sager, U. and Däuber, E. and Pommerin, A. and Weidenthaler, C.
Physical Chemistry Chemical Physics 17 (2015)Activate carbon impregnated with a mixture of copper oxide and zinc oxide performs well as active adsorber for NO<inf>2</inf> removal in automotive cabin air filters. The oxide-loaded activated carbon exhibits superior long-term stability in comparison to pure activated carbon as has been shown in previous studies. The carbon material was loaded only with 2.5 wt% of each metal oxide. Characterization of the oxide nanoparticles within the pores of the activated carbon is difficult because of the rather low concentration of the oxides. Therefore, a systematic study was performed to evaluate the limits of line profile analysis of X-ray powder diffraction patterns. The method allows evaluation of crystalline domain size distributions, crystal defect concentrations and twinning probabilities of nanoscopic materials. Here, the analysis is hampered by the presence of several phases including more or less amorphous carbon. By using physical mixtures of defined copper oxide and zinc oxide particles with activated carbon, potential errors and limits could be identified. The contribution of the activated carbon to the scattering curve was modeled with a convolution of an exponential decay curve, a Chebyshev polynomial, and two Lorentzian peaks. With this approach, domain size distributions can be calculated that are shifted only by about 0.5-1.0 nm for very low loadings (≤4 wt%). Oxide loadings of 4 wt% and 5 wt% allow very reliable analyses from diffraction patterns measured in Bragg-Brentano and Debye-Scherrer geometry, respectively. For the real adsorber material, mean domain sizes have been calculated to be 2.8 nm and 2.4 nm before and after the NO<inf>2</inf> removal tests. © the Owner Societies 2015.
view abstract 10.1039/c5cp01135c
- Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
ChemCatChem 7 (2015)A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/cctc.201500703
- Selective activators of protein phosphatase 5 target the auto-inhibitory mechanism
Haslbeck, V. and Drazic, A. and Eckl, J. M. and Alte, F. and Helmuth, M. and Popowicz, G. and Schmidt, W. and Braun, F. and Weiwad, M. and Fischer, G. and Gemmecker, G. and Sattler, M. and Striggow, F. and Groll, M. and Richter, K.
Bioscience Reports 35 (2015)Protein phosphatase 5 (PP5) is an evolutionary conserved serine/threonine phosphatase. Its dephosphorylation activity modulates a diverse set of cellular factors including protein kinases and the microtubule-associated tau protein involved in neurodegenerative disorders. It is auto-regulated by its heat-shock protein (Hsp90)-interacting tetratricopeptide repeat (TPR) domain and its C-terminal alpha-helix. In the present study, we report the identification of five specific PP5 activators [PP5 small-molecule activators (P5SAs)] that enhance the phosphatase activity up to 8-fold. The compounds are allosteric modulators accelerating efficiently the turnover rate of PP5, but do barely affect substrate binding or the interaction between PP5 and the chaperone Hsp90. Enzymatic studies imply that the compounds bind to the phosphatase domain of PP5. For the most promising compound crystallographic comparisons of the apo PP5 and the PP5-P5SA-2 complex indicate a relaxation of the auto-inhibited state of PP5. Residual electron density and mutation analyses in PP5 suggest activator binding to a pocket in the phosphatase/TPR domain interface, which may exert regulatory functions. These compounds thus may expose regulatory mechanisms in the PP5 enzyme and serve to develop optimized activators based on these scaffolds.
view abstract 10.1042/BSR20150042
- Effect of preparation of iron-infiltrated activated carbon catalysts on nitrogen oxide conversion at low temperature
Busch, M. and Schmidt, W. and Migunov, V. and Beckel, A. and Notthoff, C. and Kompch, A. and Bergmann, U. and Winterer, M. and Atakan, B.
Applied Catalysis B: Environmental 160-161 (2014)Nitrogen oxides are toxic and their concentration in human workspace should be reduced to a minimum level. Among the possible catalyst materials activated carbon based catalysts are a cheap and non-toxic alternative of high availability. In this paper we investigate two different methods for the preparation of iron-infiltrated activated carbon catalysts: chemical vapor infiltration (CVI) and the incipient wetness method (IWM). The effects of the preparation method on the structure and catalytical performance are compared with the effects of infiltration load and co-deposition of silicon dioxide. The study elucidates profound differences in the nitrogen dioxide adsorption and catalytic nitrogen oxide decomposition, depending on the catalyst preparation technique. Samples prepared by chemical vapor infiltration exhibit well dispersed iron/iron oxide particles all over the sample cross section. Crystalline iron oxide is only detected in the samples prepared via the gas phase and not in samples prepared by IWM. The nitrogen dioxide adsorption is notably enhanced in samples with a large accessible micropore volume. All samples containing iron catalyze the conversion of nitrogen oxides into nitrous oxide and carbon monoxide, but especially the co-deposition of silica enhances the nitric oxide conversion into less harmful species. The iron/silica-co-deposited activated carbon catalyst prepared via incipient wetness method exhibits the best catalytical performance of all investigated catalysts at 425. K. © 2014 .
view abstract 10.1016/j.apcatb.2014.05.010
- Flexibility versus rigidity: What determines the stability of zeolite frameworks? A case study
Verheyen, E. and Joos, L. and Martineau, C. and Dawson, C.J. and Weidenthaler, C. and Schmidt, W. and Yuan, R. and Breynaert, E. and Van Speybroeck, V. and Waroquier, M. and Taulelle, F. and Treacy, M.M.J. and Martens, J.A. and Kirschhock, C.E.A.
Materials Horizons 1 (2014)All silica COK-14/-COK-14 with OKO topology is the first case of a zeolite which reversibly transforms from a systematically interrupted to a fully connected state and back. Analysis of the opening/closing behavior allowed the study of entropy and framework flexibility as determinants for the stability of zeolite topologies, which, until now, has been experimentally inaccessible. Interconversion of the all-silica COK-14 zeolite with fully connected OKO topology and its -COK-14 variant with systematic framework interruption was investigated using high-temperature XRD, thermogravimetric analysis, 29Si MAS NMR, nitrogen adsorption and a range of modelling techniques. Specific framework bonds in the OKO framework can be reversibly hydrolyzed and condensed. Structural silanols of the parent -COK-14, prepared by degermanation of the IM-12 zeolite, were condensed by heating at 923 K, and hydrolyzed again to the initial state by contacting the zeolite with warm water. Molecular modelling revealed an inversion of the relative stabilities for both variants depending on temperature and hydration. Condensation of the structural silanols in -COK-14 to COK-14 is entropy driven, mainly resulting from the release of water molecules. Framework reopening in the presence of water is spontaneous due to the high rigidity of the fully connected OKO framework. Isomorphous substitution was demonstrated as a viable option for stabilization of the fully connected OKO framework as this renders the closed framework flexible. © the Partner Organisations 2014.
view abstract 10.1039/c4mh00127c
- Highly microporous monodisperse silica spheres synthesized by the Stöber process
Bazuła, P.A. and Arnal, P.M. and Galeano, C. and Zibrowius, B. and Schmidt, W. and Schüth, F.
Microporous and Mesoporous Materials 200 (2014)Silica spheres that are prepared by the Stöber process are usually considered non-porous. Here we report on a slightly modified synthesis protocol that allows preparation of microporous Stöber particles. Successive treatment with water and alcohol at room temperature results in substantial reorganization of the silica within the Stöber particles. Hydrolysis of alkoxy groups as well as condensation and re-esterification of silanol groups upon re-immersion in alcohol are crucial for that process. As the result of the silica reorganization, micropore channels are formed within the particles. After a final washing with water to remove all alkoxy groups from the particles, pure microporous silica spheres are obtained. The total pore volumes of these materials are comparable to those of zeolitic materials. © 2014 Elsevier Inc. All rights reserved.
view abstract 10.1016/j.micromeso.2014.07.051
- Impacts of geometry, symmetry, and morphology of nanocast Co3O4 on its catalytic activity for water oxidation
Deng, X. and Schmidt, W. and Tüysüz, H.
Chemistry of Materials 26 (2014)Herein, we report a systematic study on the synthesis of ordered mesoporous Co3O4 nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N2 sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co3O4. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co3O4 replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co3O4 replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co3O4 replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co3O4 shows superior activity to the cubically ordered one. © 2014 American Chemical Society.
view abstract 10.1021/cm5023163
- Multidiagnostic analysis of silicate speciation in clear solutions/sols for zeolite synthesis
Castro, M. and Haouas, M. and Taulelle, F. and Lim, I. and Breynaert, E. and Brabants, G. and Kirschhock, C.E.A. and Schmidt, W.
Microporous and Mesoporous Materials 189 (2014)The formation of zeolites in presence of tetraalkylammonium cations from so-called clear solutions using silicon alkoxides is a highly complex process which challenges experimental chemistry. Most clear solutions are better described as clear sols as they contain nanosized silicate particles, which are formed during hydrolysis of the Si source before self-assembly into the zeolite framework. This process spans multiple time- and length-scales and only a combination of different analysis methods allows revelation of molecular level zeolite formation mechanisms. On the example of the early stages of the formation of zeolite beta from clear solutions/sols the different windows of observation of liquid-state 29Si and 27Al nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and mass spectrometry (MS) are demonstrated. Each diagnostic means by itself needs to be carefully assessed for its window of temporal and spatial resolution which can be achieved by exploiting the overlapping information available from their combination. © 2013 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.micromeso.2013.08.027
- Ordered mesoporous Cu-Ce-O catalysts for CO preferential oxidation in H2-rich gases: Influence of copper content and pretreatment conditions
Gu, D. and Jia, C.-J. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
Applied Catalysis B: Environmental 152-153 (2014)Highly ordered mesoporous Cu-Ce-O catalysts with different Cu contents have been synthesized by using ordered mesoporous silica KIT-6 as a hard template. The mesostructural order of the negative replica is influenced by the ratio of Cu to Ce. Using XRD, HR-SEM, TEM and EDX analysis, it was found that the ordered mesostructures of the nanocomposites degenerate with increasing Cu concentration, due to CuO leaching during the template removal process and a phase separation at high Cu concentration. Cu ions can replace Ce-ion in the structure of CeO2 at Cu concentrations below 40mol%. However, the Cu concentration in the final materials is lower than expected from the ratio used in the synthesis. The activity in preferential oxidation of CO in H2-rich gases (PROX) was tested at a space velocity of 60,000mLh-1gcat -1. The activity of the mesoporous catalysts increases with the concentration of Cu and becomes stable for Cu concentrations higher than 20mol%. A CO conversion around 100 % can be attained with Cu0.20Ce0.80O2 as catalyst at 160°C. The exit CO concentration can be as low as 70ppm under these conditions. The CO2 selectivity can reach 100 % at low temperature (60- 80°C). Direct loading of CuO on the surface of mesoporous CeO2 leads to large CuO crystals and correspondingly low activity. The influence of the pretreatment atmosphere on activity was also studied. Oxidation-reduction-reoxidation cycling can improve the catalytic activity of the catalysts. © 2014 Elsevier B.V.
view abstract 10.1016/j.apcatb.2014.01.011
- Catalytic reduction of nitrogen oxides via nanoscopic oxide catalysts within activated carbons at room temperature
Sager, U. and Schmidt, W. and Schmidt, F. and Suhartiningsih
Adsorption 19 (2013)Cabin air filters consisting of activated carbon infiltrated with nanoscopic metal oxide particles as catalysts have been investigated for the reduction of nitrogen oxides within motor-car cabins. In that concept, nitrogen dioxide is adsorbed on the activated carbon during operation conditions of the car and then reduced by the catalysts within the pores. The conversion has to take place at ambient temperature during the relatively long standstill periods of motor-cars. In this article we are going to discuss the manufacturing of the adsorbents by "liquid phase infiltration" and their characterization by techniques, such as nitrogen sorption analysis, X-ray diffraction, thermogravimetry, energy dispersive X-ray spectroscopy, and electron microscopy. The new adsorbents were evaluated in repeated breakthrough tests using NO 2 (4 ppmV as feed concentration) in humid air as the adsorptive. In the intermittent rest periods of varying duration the volume flow through the fixed bed of adsorbent was stopped. The measured breakthrough curves indicate a catalytic conversion of the nitrogen dioxide in the filter beds. © 2013 Springer Science+Business Media New York.
view abstract 10.1007/s10450-013-9521-8
- Diffraction and Spectroscopy of Porous Solids
Schmidt, W. and Schüth, F. and Weidenthaler, C.
Comprehensive Inorganic Chemistry II (Second Edition): From Elements to Applications 5 (2013)Porous solids are in the first place solids and thus all methods for the analysis of common solids can be also applied on porous ones. Structural information and bulk properties of the solid are accessible as for any other material. In addition, specific information on pore sizes, pore shapes, and properties of pore surfaces of such material can be obtained by different methods as well as information on guest species within the pores and on host-guest interactions. In the present chapter, diffraction and spectroscopic methods for the analysis of porous solids will be described and the type of information that can be achieved by the different methods will be illustrated. Diffraction and scattering of x-rays, neutrons, and electrons by a porous solid will be introduced as well as the application of infrared and nuclear magnetic resonance spectroscopy for the analysis of surface properties, host-guest interaction, and diffusion studies that are complemented by interference microscopy. © 2013 Elsevier Ltd. All rights reserved.
view abstract 10.1016/B978-0-08-097774-4.00501-5
- One-pot synthesis of mesoporous Cu-γ-Al 2O 3 as bifunctional catalyst for direct dimethyl ether synthesis
Jiang, H. and Bongard, H. and Schmidt, W. and Schüth, F.
Microporous and Mesoporous Materials 164 (2012)Mesoporous copper-alumina (Cu-Al 2O 3) with different copper contents was synthesized in a one-pot reaction via the evaporation-induced self-assembly of Pluronic P123 and the corresponding metal precursors in ethanolic solution in the presence of nitric acid. Mesoporous Cu-Al 2O 3 calcined at 400°C exhibits a large BET surface area of 265 m 2/g and a pore volume of 0.48 cm 3/g. XRD results indicate that the wall of mesoporous Cu-Al 2O 3 calcined at 400°C is amorphous, and that it is transformed to crystalline material by further thermal treatment at 800°C. Copper was formed as very small particles in the composite under 5% H 2 flow at high temperature. Moreover, the mesoporous structure did not collapse after the sample was reduced at 650°C for 4 h, and the copper particles with sizes of around 6 nm were well distributed through the entire mesoporous γ-Al 2O 3 network. Using the mesoporous Cu/γ-Al 2O 3 as a bifunctional catalyst for one-step dimethyl ether synthesis from synthesis gas, a CO conversion of 72% and a DME selectivity of 69% were obtained at 50 bar and 310°C. © 2012 Elsevier Inc. All rights reserved.
view abstract 10.1016/j.micromeso.2012.08.004
- One-pot synthesis of polyhedron-like hollow aluminosilicate with mesoporous shells
Gu, X. and Tao, H. and Schmidt, W. and Lu, G. and Wang, Y.
Journal of Materials Chemistry 22 (2012)Hollow mesoporous aluminosilicate particles have been synthesized through a one-pot synthesis without using any cavity-forming template. The hollow particles have mesoporous shells with surface areas up to 1300 m 2 g -1. The thickness of the shell can be adjusted from about 30 nm to 50 nm by tuning the molar Si/Al ratio and/or variation of the time of hydrothermal treatment. EDX analysis (element mapping) revealed a certain zoning of aluminum in the amorphous aluminosilicate particles investigated. There is an enrichment of Al on the outside of the particles close to the surface. The presence of aluminum appeared to be the key factor for the inhomogeneous dissolution of the aluminosilicate (dissolution of silicon-enriched cores) that leads to the formation of hollow particles. © 2012 The Royal Society of Chemistry.
view abstract 10.1039/c1jm13400k
- Ordered mesoporous materials with MFI structured microporous walls - Synthesis and proof of wall microporosity
Reichinger, M. and Schmidt, W. and Narkhede, V.V. and Zhang, W. and Gies, H. and Grünert, W.
Microporous and Mesoporous Materials 164 (2012)Ordered mesoporous materials (OMMs) of 1-dimensional hexagonal and 3-dimensional cubic symmetry of the pore systems were synthesized via well-established soft templating routes starting from precursor solutions of MFI-type zeolites (Silicalite-1, TS-1). The products were characterized by XRD, nitrogen and argon physisorption, DTG/DTA, IR, UV-vis spectroscopy, XANES, TEM, 119Xe NMR, and determination of the pair distribution function (PDF) in order to elucidate their structure, in particular to prove the presence of microporosity in arrays smaller than the coherence lengths of XRD, i.e. in the pore walls. The mesoporosity of the OMMs was well supported by physisorption studies and by TEM while the regularity of the structure was documented by XRD, which also served to exclude the presence of microporous crystalline grains. Instead, microporosity was detected by adsorption/desorption of water on tetrahedrally coordinated Ti-sites (XANES), by 119Xe NMR, by the comparison of the PDF with those of amorphous and of MFI-type solids, and by sequential decomposition of the structure directing agents for meso and micropore systems. From comparison of XRD and physisorption data and from the TEM micrographs, the thickness of the microporous mesopore walls was concluded to be ≈1.5 nm. Therefore, the failure of Ar physisorption to detect pores of sizes typical of MFI structures was attributed to the small micropore volume and the very short pore extension. The structural integrity of OMMs with 1-dimensional hexagonal pore system could be improved by a hydrothermal post-treatment despite the microporous nature of their pore walls, which resulted in more narrow mesopore size distributions peaking at somewhat larger pore sizes. © 2012 Elsevier Inc. All rights reserved.
view abstract 10.1016/j.micromeso.2012.07.025
- Size-controlled synthesis and microstructure investigation of Co 3O 4 nanoparticles for low-temperature CO oxidation
Dangwal Pandey, A. and Jia, C. and Schmidt, W. and Leoni, M. and Schwickardi, M. and Schüth, F. and Weidenthaler, C.
Journal of Physical Chemistry C 116 (2012)Noble-metal-free functional oxides are active catalysts for CO oxidation at low temperatures. Spinel-type cobalt oxide (Co 3O 4) nanoparticles can be easily synthesized by impregnation of activated carbon with concentrated cobalt nitrate and successive carbon burn off. Mean size and particle size distribution can be tuned by adding small amounts of silica to the carbon precursor, as witnessed by whole powder pattern modeling of the X-ray powder diffraction data. The catalytic tests performed after silica removal show a significant influence of the mean domain size and of size distribution on the CO oxidation activity of the individual Co 3O 4 specimens, whereas defects play a less important role in the present case. © 2012 American Chemical Society.
view abstract 10.1021/jp306166g
- Synthetic inorganic ion exchange materials
Ion Exchange Technology I: Theory and Materials (2012)Synthetic ion exchangers are used for a wide range of different applications, ranging from water softening, water remediation, catalysis, molecular sieving, and selective adsorption to medical applications. Their well-defined compositions and tunable structural features allow minute tailoring of their specific properties. A selection of the most prominent ion exchangers for aqueous ion exchange is introduced in this chapter, and the ion exchange process is illustrated for selected materials. Furthermore, structural features of individual materials are discussed and typical preparation methods and applications are addressed. © Springer Science+Business Media B.V. 2012.
view abstract 10.1007/978-94-007-1700-8_8
- Co3O4 - SiO2 Nanocomposite: A very active catalyst for co oxidation with unusual catalytic behavior
Jia, C.-J. and Schwickardi, M. and Weidenthaler, C. and Schmidt, W. and Korhonen, S. and Weckhuysen, B.M. and Schüth, F.
Journal of the American Chemical Society 133 (2011)A high surface area Co3O4 - SiO2 nanocomposite catalyst has been prepared by use of activated carbon as template. The Co3O4 - SiO2 composite, the surface of which is rich in silica and Co(II) species compared with normal Co 3O4, exhibited very high activity for CO oxidation even at a temperature as low as '76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H2O content 3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co 3O4 - SiO2 catalyst led to the conclusion that adsorption of H2O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions. © 2011 American Chemical Society.
view abstract 10.1021/ja2028926
- Hydrothermally highly stable acidic mesoporous aluminosilicate spheres with radial channels
Gu, X. and Jiang, T. and Tao, H. and Zhou, S. and Liu, X. and Ren, J. and Wang, Y. and Lu, G. and Schmidt, W.
Journal of Materials Chemistry 21 (2011)Hydrothermally highly stable mesoporous aluminosilicate spheres with radial channels were synthesized in the CTAB-NaF-TPAOH system through a one-step procedure at high aging temperature. The characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption analysis, 27Al MAS solid state NMR spectroscopy, pyridine adsorption FT-IR combined with the typical hydrothermal treatments showed that this kind of material exhibited large surface area, specific pore arrangement, strong acidity and high hydrothermal stability. Detailed studies suggest that F- ions direct the perpendicular arrangement of aluminosilicate clusters during the hydrothermal treatment at 160°C, while TPA+ stabilized the structure. Both F- and TPA+ ions are considered to improve the acidity and hydrothermal stability of this material through coordination of framework atoms, thus, enhancing the condensation of Si-O-Si bonds in the amorphous pore walls. Due to the accessible radial pore arrangement and high acidity, the catalytic activity for Friedel-Crafts alkylation of toluene with benzyl alcohol was excellent with 100% conversion of benzyl alcohol.
view abstract 10.1039/c0jm01973a
- Synthesis and microwave absorbing properties of highly ordered mesoporous crystalline NiFe2O4
Gu, X. and Zhu, W. and Jia, C. and Zhao, R. and Schmidt, W. and Wang, Y.
Chemical Communications 47 (2011)Highly ordered mesoporous NiFe2O4 with excellent microwave absorption properties has been synthesized by using mesoporous silica (KIT-6) as a hard template. © 2011 The Royal Society of Chemistry.
view abstract 10.1039/c0cc05800a
- Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
Journal of Nanoscience and Nanotechnology 11 (2011)Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
view abstract 10.1166/jnn.2011.5074
- Accessing ultrashort reaction times in particle formation with SAXS experiments: ZnS precipitation on the microsecond time scale
Schmidt, W. and Bussian, P. and Lindén, M. and Amenitsch, H. and Agren, P. and Tiemann, M. and Schüth, F.
Journal of the American Chemical Society 132 (2010)Precipitation of zinc sulfide particles is a very rapid process, and monitoring of the particle growth is experimentally very demanding. Applying a liquid jet flow cell, we were able to follow zinc sulfide particle formation on time scales down to 10 -5 s. The flow cell was designed in such a way that data acquisition on the microsecond time scale was possible under steady-state conditions along a liquid jet (tubular reactor concept), allowing SAXS data accumulation over a time scale of minutes. We were able to monitor the growth of zinc sulfide particles and found experimental evidence for very rapid particle aggregation processes within the liquid jet. Under the experimental conditions the particle growth is controlled by mass transfer: i.e., the diffusion of the hydrogen sulfide into the liquid jet. © 2010 American Chemical Society.
view abstract 10.1021/ja101519z
- Alkene epoxidation with mesoporous materials assembled from TS-1 seeds - Is there a hierarchical pore system?
Reichinger, M. and Schmidt, W. and Berg, M.W.E.v.d. and Aerts, A. and Martens, J.A. and Kirschhock, C.E.A. and Gies, H. and Grünert, W.
Journal of Catalysis 269 (2010)Hexagonal mesoporous solids were synthesized from solutions containing TS-1 seeds. The products were characterized by XRD, nitrogen and argon physisorption, TEM, TG/DTA of template decomposition (also after extraction of the mesopore template), UV-Vis and IR spectroscopy, and XANES at the TiK edge. Their catalytic activities were assessed for cyclohexene epoxidation in hydrophilic and hydrophobic environment (CH3OH/water, with H2O2 oxidant, and decane, with tert-butyl hydro-peroxide oxidant, respectively) and for n-hexene epoxidation in hydrophilic environment. The mesopore system was clearly documented by XRD, physisorption measurements, and TEM, whereas evidence for micropores by physisorption proved elusive. However, the micropore template was detected in the solids by TG/DTA even after extraction of the mesopore template, and among the Ti sites, which were confirmed to be tetrahedrally coordinated by UV-Vis and XANES, a clear majority was able to coordinate two water molecules. It was concluded that the pore walls had been built up from nanoparticulate TS-1 precursors resulting in walls of ca. 1.5 nm thickness, which resemble rather the exterior layers of a TS-1 crystallite than its (hydrophobic) interior. In cyclohexene epoxidation, the micro-mesophases were by 1-2 orders of magnitude more active than TS-1 and outperformed also Ti-MCM-41, at similar selectivity in hydrophobic medium. With 1-hexene in hydrophilic medium, however, the micro-mesophases failed completely whereas TS-1 exhibited high activity. © 2009 Elsevier Inc. All rights reserved.
view abstract 10.1016/j.jcat.2009.11.023
- Comparing the statistics of interstellar turbulence in simulations and observations Solenoidal versus compressive turbulence forcing
Federrath, C. and Roman-Duval, J. and Klessen, R. S. and Schmidt, W. and Mac Low, M. M.
Astronomy & Astrophysics 512 (2010)Context. Density and velocity fluctuations on virtually all scales observed with modern telescopes show that molecular clouds (MCs) are turbulent. The forcing and structural characteristics of this turbulence are, however, still poorly understood. Aims. To shed light on this subject, we study two limiting cases of turbulence forcing in numerical experiments: solenoidal (divergence-free) forcing and compressive (curl-free) forcing, and compare our results to observations. Methods. We solve the equations of hydrodynamics on grids with up to 1024(3) cells for purely solenoidal and purely compressive forcing. Eleven lower-resolution models with different forcing mixtures are also analysed. Results. Using Fourier spectra and A-variance, we find velocity dispersion-size relations consistent with observations and independent numerical simulations, irrespective of the type of forcing. However, compressive forcing yields stronger compression at the same rms Mach number than solenoidal forcing, resulting in a three times larger standard deviation of volumetric and column density probability distributions (PDFs). We compare our results to different characterisations of several observed regions, and find evidence of different forcing functions. Column density PDFs in the Perseus MC suggest the presence of a mainly compressive forcing agent within a shell, driven by a massive star. Although the PDFs are close to log-normal, they have non-Gaussian skewness and kurtosis caused by intermittency. Centroid velocity increments measured in the Polaris Flare on intermediate scales agree with solenoidal forcing on that scale. However, A-variance analysis of the column density in the Polaris Flare suggests that turbulence is driven on large scales, with a significant compressive component on the forcing scale. This indicates that, although likely driven with mostly compressive modes on large scales, turbulence can behave like solenoidal turbulence on smaller scales. Principal component analysis of G216-2.5 and most of the Rosette MC agree with solenoidal forcing, but the interior of an ionised shell within the Rosette MC displays clear signatures of compressive forcing. Conclusions. The strong dependence of the density PDF on the type of forcing must be taken into account in any theory using the PDF to predict properties of star formation. We supply a quantitative description of this dependence. We find that different observed regions show evidence of different mixtures of compressive and solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells. Finally, we emphasise the role of the sonic scale for protostellar core formation, because core formation close to the sonic scale would naturally explain the observed subsonic velocity dispersions of protostellar cores.
view abstract 10.1051/0004-6361/200912437
- From glycerol to allyl alcohol: Iron oxide catalyzed dehydration and consecutive hydrogen transfer
Liu, Y. and Tüysüz, H. and Jia, C.-J. and Schwickardi, M. and Rinaldi, R. and Lu, A.-H. and Schmidt, W. and Schüth, F.
Chemical Communications 46 (2010)Using iron oxide as catalyst, glycerol can be converted to allyl alcohol through a dehydration and consecutive hydrogen transfer. © 2010 The Royal Society of Chemistry.
view abstract 10.1039/b921648k
- Numerical and semi-analytic core mass distributions in supersonic isothermal turbulence
Schmidt, W. and Kern, S. A. W. and Federrath, C. and Klessen, R. S.
Astronomy & Astrophysics 516 (2010)Context. Supersonic turbulence in the interstellar medium plays an important role in the formation of stars. The origin of this observed turbulence and its impact on the stellar initial mass function (IMF) still remain open questions. Aims. We investigate the influence of the turbulence forcing on the mass distributions of gravitationally unstable cores in simulations of isothermal supersonic turbulence. Methods. Data from two sets of non-selfgravitating hydrodynamic FLASH3 simulations with external stochastic forcing are analysed, each with static grid resolutions of 256(3), 512(3) and 1024(3) grid points. The first set applies solenoidal (divergence-free) forcing, while the second set uses purely compressive (curl-free) forcing to excite turbulent motions. From the resulting density field, we compute the mass distribution of gravitationally unstable cores by means of a clump-finding algorithm. Using the time-averaged probability density functions of the mass density, semi-analytic mass distributions are calculated from analytical theories. We apply stability criteria that are based on the Bonnor-Ebert mass resulting from the thermal pressure and from the sum of thermal and turbulent pressure. Results. Although there are uncertainties in applying of the clump-finding algorithm, we find systematic differences in the mass distributions obtained from solenoidal and compressive forcing. Compressive forcing produces a shallower slope in the high-mass power-law regime compared to solenoidal forcing. The mass distributions also depend on the Jeans length resulting from the choice of the mass in the computational box, which is freely scalable for non-selfgravitating isothermal turbulence. If the Jeans length corresponding to the density peaks is less than the grid cell size, the distributions obtained by clump-finding show a strong resolution dependence. Provided that all cores are numerically resolved and most cores are small compared to the length scale of the forcing, the normalised core mass distributions are close to the semi-analytic models. Conclusions. The driving mechanism of turbulence has a potential impact on the shape of the core mass function. Especially for the high-mass tails, the Hennebelle-Chabrier theory implies that the additional support due to turbulent pressure is important.
view abstract 10.1051/0004-6361/200913904
- Small gold particles supported on MgFe2O4 nanocrystals as novel catalyst for CO oxidation
Jia, C.-J. and Liu, Y. and Schwickardi, M. and Weidenthaler, C. and Spliethoff, B. and Schmidt, W. and Schüth, F.
Applied Catalysis A: General 386 (2010)We present the study on the catalytic performance of gold particles supported on spinel type MgFe2O4 nanocrystals (Au/MgFe2O4) which exhibit high activity for low temperature CO oxidation. Using XRD, TEM, XPS and CO titration techniques, we investigated the effect of the pretreatment atmosphere on the structure and catalytic properties of the Au/MgFe2O4 catalyst in CO oxidation. TEM, XPS and XRD showed that the pretreatment atmosphere had a negligible effect on the particle size distribution, chemical states of the gold, and the structure of the support. Among the various pretreated catalysts, O2-Au/MgFe2O4 exhibits superior activity, indicating that pretreatment in oxidative atmosphere induced the high capability of the catalyst to activate CO and supply active oxygen for CO oxidation as confirmed by CO titration experiments. © 2010 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.apcata.2010.07.036
- Small-sized HZSM-5 zeolite as highly active catalyst for gas phase dehydration of glycerol to acrolein
Jia, C.-J. and Liu, Y. and Schmidt, W. and Lu, A.-H. and Schüth, F.
Journal of Catalysis 269 (2010)The catalytic properties of nanocrystalline HZSM-5 catalysts with high Si/Al molar ratio (ca. 65) were investigated in the gas phase dehydration of aqueous glycerol. Compared with bulk HZSM-5, the small-sized catalyst exhibits greatly enhanced catalytic performance in glycerol dehydration even with very high GHSV (=1438 h -1). Catalysts with different Si/Al ratios were studied, but it is difficult to separate the influence of Si/Al ratio from that of particle size. However, by varying the proton exchange degree for one mother batch of zeolite, a series of H xNa 1-xZSM-5 catalysts with same particle size and different Brønsted acid site densities was prepared. The catalytic results for this series of samples show that high density of Brønsted acid sites favors the production of acrolein. Based on these results, small-sized HZSM-5 with high aluminum content appears to be most promising for gas phase dehydration of glycerol. © 2009 Elsevier Inc. All rights reserved.
view abstract 10.1016/j.jcat.2009.10.017