1,720,998 research outputs found
Synthesis of Mesoporous Silicas of Controlled Pore Wall Thickness and Their Replication to Ordered Nanoporous Carbons with Various Pore Diameters
No full textThis work was supported in part by the
Creative Research Initiative Program of the Korean Ministry of Science and Technology, and by the School of Molecular Science
through the Brain Korea 21 project.
Synthesis of ordered mesoporous carbon molecular sieves CMK-1
No full textHighly ordered mesoporous carbon molecular sieves designated as CMK-1 were synthesized using mesoporous MCM-48 silica with various pore diameters as templates. sucrose as a carbon source and sulfuric acid as the carbonization catalyst. The effect of the sucrose-to-sulfuric acid ratio on the structure of the resultant carbon, the effect of the pyrolysis temperature, and the optimum amounts of sucrose for MCM-48 with various pore diameters have been investigated. These results show that the synthesis method can be accurately controlled to obtain the highly ordered mesoporous carbon molecular sieves in a large scale. (C) 2001 Elsevier Science B.V. All rights reserved.This work was supported by KOSEF
Plastic Deformation and Computer Simulations of Equal Channel Angular Pressing
No full textSevere Plastic Deformation (SPD) is a generic designation of a group of metal-working techniques involving very large shear strains which are imposed, in combination with hydrostatic pressure, without any significant change in the overall dimensions of a specimen or a workpiece. This technique can produce miraculously fine grained structures below submicron scale. Equal channel angular pressing (ECAP) is a unique and relatively simple metal forming process to the other metal forming processes, but still complex under coupled effects with multi-factors, such as geometric factors, material factors and processing conditions.
Investigating the plastic deformation behavior in the deformation zone during ECAP is crucial for predicting the metal flow, microstructural evolution, controlling the quality of deformed workpiece and optimizing the ECAP process. In this review, modelling of ECAP using various numerical methods are reviewed. The papers from literature analyzing ECAP processing by the FEM are summarized and compared by software, mesh size, dimension, and analyzed results. It can be found that not only stress, strain, velocity, and fracture tendency but also their distributions in terms of the effects of the processing variables (speed, size, friction, die geometries, etc.) can be predicted.X1123Nsciescopuskc
Characterization of regular and plugged SBA-15 silicas by using adsorption and inverse carbon replication and explanation of the plug formation mechanism
No full textA series of SBA-15 silicas was synthesized using triblock copolymer templates with the same length of poly(propylene oxide) block (POm; m = 70) and different average lengths of poly(ethylene oxide) blocks (EOn, n = 17-28). The average EOn length was varied by mixing EO20PO70EO20 copolymer with either EO5PO70EO5 or EO106PO70EO106 copolymer, whereas the tetraethyl orthosilicate (TEOS)/EO-unit molar ratio was kept constant. In addition, samples with the higher ratios were synthesized for the EO20PO70EO20 template. In all cases, 2-D hexagonally ordered SBA-15 silicas with interconnected primary pore structure (as inferred from carbon inverse replication) were obtained, but higher TEOS/EO-unit molar ratios resulted in the formation of samples with plugged pore structures, which were recently reported by others and referred to as plugged hexagonal templated silicas (PHTSs). Nitrogen adsorption data showed that n equal to about 19 was optimal from the point of view of formation of high-pore-volume, large-pore SBA-15 structure. All of the replicas exhibited structures with high specific surface area and pore volume and tended to have a similar pore diameter but more optimized SBA-15 structures tended to afford better ordered inverse carbon replicas. Regular and PHTS SBA-15 samples were additionally studied using argon adsorption at 77 K, which provided an important insight into the plugged structure. In particular, a PHTS sample that on the basis of nitrogen adsorption at 77 K appeared to be fully plugged, that is, with all primary mesopore channels closed by finely porous plugs, was found to be only partially plugged based on argon adsorption at 77 K, although there was evidence for the presence of some constrictions in all of the channels. A mechanism of the plug formation at high TEOS/copolymer molar ratios is proposed on the basis of similarity of the PHTSs with as-synthesized SBA-15 subjected to postsynthesis modification with TEOS. It is proposed herein that when the TEOS/EO-unit molar ratio is excessively high, only a part of TEOS present initially interacts with the EOn blocks of the copolymer template and thus hydrolyzes and condenses faster to form the SBA-15 structure. Subsequently or concurrently, the remaining TEOS, which hydrolyzes and condenses more slowly, solubilizes in the copolymer template in the SBA-15 pores (perhaps in the poly(propylene oxide) core of the micelles), or displaces the template, and then condenses to form the plugs.M.J. acknowledges support by NSF
Grant CHE-0093707. The donors of the Petroleum Research Fund administered by the American Chemical Society are also
gratefully acknowledged for a partial support of this research.
R.R. acknowledges support in part by the Ministry of Science and Technology through Creative Research Initiative Program,
and by School of Molecular Science through Brain Korea 21 Project
Detailed structure of the hexagonally packed mesostructured carbon material CMK-3
No full textDetailed investigation of the ordered mesoporous CMK-3 carbon using XRD structural modeling based on the continuous electron density representation and the Rietveld technique allowed deriving comprehensive and consistent information on the material anatomy. The electron density distribution map agrees with carbon 'bridges', which seem to be attributed to the material interconnecting carbon nanorods in the CMK-3 mesostructure. These carbon 'bridges' are supposed to be derived from former complementary mesopores of the SBA-15 template used. (C) 2002 Elsevier Science Ltd. All rights reserved.We are grateful to the INTAS Fellowship grant for Young Scientists YSF 2001/ 2-3, INTAS grant (proposal 2283) and joint grant KRSF-RFBR 02-03-97704. R. Ryoo gratefully acknowledges that this work was supported in
part by the Ministry of Science and Technology through Creative Research Initiative Program, and by School of Molecular Science through Brain Korea 21 Project
Pore structure and graphitic surface nature of ordered mesoporous carbons probed by low-pressure nitrogen adsorption
No full textOrdered mesoporous carbons (OMCs) were produced by pyrolysis of sucrose adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in hydrofluoric acid. Subsequently, some of these OMCs were heat-treated at temperatures of up to 1600 degreesC. The ONIC pore structure was studied by low-pressure nitrogen adsorption. Information on the graphitic order of the surface of the mesopore walls was also obtained from the nitrogen adsorption data. These results were correlated to the order of the graphene layers at the outer surface, which was studied by X-ray photoelectron spectroscopy (XPS). The OMCs were predominantly mesoporous, but they also contained micropores. For OMCs produced in an SBA- 15 matrix, the micropore volume decreased upon heating. After heating to 1600 degreesC, nearly all micropores had disappeared. Furthermore, upon heating the width of the mesopores increased from 35 to 50 Angstrom. All these changes can be explained by a shrinking of the OMC framework upon heating. A different behavior was found for OMCs derived from MCM-48. Upon heating these materials at increasingly high temperatures, the width of the mesopores first decreased, and for temperatures above 1100 degreesC it increased again. For all OMCs studied the graphitic order of the mesopores and the order of the graphene layers at the outer surface increased upon heating. For a given temperature, the graphitic surface order of OMCs derived from SBA-15 and MCM-48 was similar. (C) 2003 Elsevier Science Inc. All rights reserved.The authors are thankful to Dr. Annette
Schwerdtfeger for reviewing the manuscript. R. Ryoo gratefully acknowledges that this work was supported in part by the Korean Ministry of Science and Technology through the Creative Research Initiative Program, and by the School of Molecular Science through the Brain Korea 21 Project
Ordered mesoporous carbons
No full textOrdered mesoporous carbons have recently been synthesized using ordered mesoporous silica templates. The synthesis procedure involves infiltration of the pores of the template with appropriate carbon precursor, its carbonization, and subsequent template removal. The template needs to exhibit three-dimensional pore structure in order to be suitable for the ordered mesoporous carbon synthesis, otherwise disordered microporous carbon is formed. MCM-48, SBA-1, and SBA-15 silicas were successfully used to synthesize carbons with cubic or hexagonal frameworks, narrow mesopore size distributions, high nitrogen Brunauer-Emmett-Teller (BET) specific surface areas (up to 1800 m(2) g(-1)), and large pore volumes. Ordered mesoporous carbons are promising in many applications, including adsorption of large molecules, chromatography, and manufacturing of electrochemical double-layer capacitors.RR and SHJ thank the Korea Science and Engineering Foundation for financial support (2000-1-30700-002-3). MK and MJ gratefully acknowledge the Donors of the Petroleum Research Fund administrated by the American Chemical Society for partial support of this research
Understanding Deactivation of Ru Catalysts by In-situ Investigation of Surface Oxide Stability under CO Oxidation and Oxidative/Reductive Conditions
No full textIn addition to the catalysts' activity and selectivity, the deactivation of catalysts during use is of practical importance. It is crucial to understand the phenomena of the deactivation to predict the loss of activity during catalyst usage so that the high operational costs associated with catalyst replacement can be reduced. In this study, the activity of Ru catalysts, such as nanoparticles (3~6 nm) and polycrystalline thin film (50 nm), have been investigated under CO oxidation and oxidative/reductive reaction conditions at various temperatures with the ambient pressure X-Ray photoelectron spectroscopy (APXPS). With APXPS, the surface oxides on the catalyst are measured and monitored in-situ. It was found that the Ru film exhibited faster oxidation-and-reduction compared to that of nanoparticles showing mild oxidative-and-reductive characteristics. Additionally, the larger Ru nanoparticles showed a higher degree of oxide formation at all temperatures, suggesting a higher stability of the oxide. These observations are in agreement with the catalytic activity of Ru catalysts. The loss of activity of Ru films is correlated with bulk oxide formation, which is inactive in CO oxidation. The Ru nanoparticle, however, does not exhibit deactivation under similar conditions, suggesting that its surface is covered with a highly active ultrathin surface oxide. Since the active oxide is more stable as nanoparticles than as a film, the nanoparticles showed mild oxidative/reductive behavior, as confirmed by APXPS results. We believe these simultaneous observations of both the surface oxide of Ru catalysts and the reactivity in real time enable us to pinpoint the deactivation phenomena more precisely and help in designing more efficient and stable catalytic systems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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