Chae-Ho Shin

Fabrication of Carbon Capsules with Hollow Macroporous Core/Mesoporous Shell Structures**

By Suk Bon Yoon, Kwonnam Sohn, Jeong Yeon Kim,Chae-Ho Shin, Jong-Sung Yu,* and Taeghwan Hyeon*In this report, the fabrication of carbon capsules with hol-low core/mesoporous shell (HCMS) structures will be pre-sented. Various nanoporous carbon materials have been fabri-cated using inorganic templates, including zeolites,

Development of a new mesoporous carbon using an HMS aluminosilicate template

This work was supported by the Korea Research Foundation (New Materials Research 1998) and the KOSEF (Basic Research Program #98-05-02-03-01-3).

Synthesis and characterization of spherical carbon and polymer capsules with hollow macroporous core and mesoporous shell structures

Abstract The synthesis and characterization of spherical carbon hollow capsules with mesoporous shell (HCMS) structures is reported. Silica spheres with sub-micrometer sized solid cores and mesoporous shell (SCMS) structures were used as templates in the synthesis. The resulting carbon capsules inversely replicated the morphology of the silica template and had uniform pores with a narrow pore size distribution centered at 3 nm. The carbon capsules exhibited a BET surface area of >1000 m 2 /g and a total pore volume of >1.0 cm 3 /g. The diameter of the hollow core and the mesoporous shell thickness of the carbon capsules could easily be controlled using appropriate SCMS silica materials as templates. Using a similar synthetic procedure, hollow poly(divinylbenzene) capsules were also synthesized.

Synthesis, characterization, and catalytic properties of zeolites IM-5 and NU-88

Abstract The synthesis, characterization, and catalytic properties of two high-silica zeolites IM-5 and NU-88, whose structures still remain unresolved, are presented. When the 1,5-bis( N -methylpyrrolidinium)pentane and 1,6-bis( N -methylpyrrolidinium)hexane cations are used as organic structure-directing agents, respectively, crystallization of pure IM-5 and NU-88 was possible only from synthesis mixtures with a narrow range of SiO 2 /Al 2 O 3 and NaOH/SiO 2 ratios. The overall characterization results of this study strongly suggest that IM-5 is a new multidimensional large-pore zeolite, whereas NU-88 is a nanocrystalline material that could be an intergrowth of several hypothetical polymorphs in the beta family of zeolites. Despite its nanocrystalline nature, however, no detectable extraframework Al species were found to exist in H–NU-88, revealing excellent thermal stability. H–IM-5 and H–NU-88 both exhibit a very high 1-butene conversion compared to H–ZSM-35 over the period of time studied here, whereas the opposite holds for the formation of isobutene. This reveals that their pore topologies are large enough to allow undesired side reactions such as 1-butene dimerization followed by cracking to light olefins. They also show the initial n -octane cracking activity comparable to that on H–ZSM-5. However, a notable decrease in n -octane conversion on these two zeolites with increasing time on stream is observed. When the isomerization and cracking activities of all zeolites employed in this study are correlated with their coke-forming propensities, it can be concluded that both materials present a shape-selective character falling within the category of multidimensional, large-pore zeolites.

Designing a Highly Active Metal‐Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping‐Site Position

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

Influence of framework silicon to aluminium ratio on aluminium coordination and distribution in zeolite Beta investigated by 27Al MAS and 27Al MQ MAS NMR

27Al magic-angle spinning (MAS) and triple quantum (3Q) MAS NMR spectroscopic techniques were used to characterise zeolite Beta samples with framework Si/Al ratios between 9 and 215, obtained by synthesis in fluoride medium. A carefully controlled stepwise calcination procedure was adopted to obtain H-Beta. A partial resolution of the T-sites was observed in the 27Al MAS NMR spectra, the resolution increasing with increasing the Si/Al ratios. The relative intensity of these peaks varied gradually, with Si/Al ratio showing that the relative occupancy of the crystallographic T-sites changes with Si/Al ratio. The tetraethylammonium cation, used as an organic structure-directing agent in Beta synthesis, affects the average chemical shift of aluminium atoms in different T-sites. In H-Beta, octahedrally coordinated framework-associated aluminium atoms that could be quantitatively reverted into tetrahedral coordination were observed. The amount of this octahedral aluminium species decreases with increasing Si/Al ratio and it was absent for the two high-silica H-Beta samples with Si/Al = 110 and 215. Specific framework tetrahedral T-sites tend to convert to framework-associated octahedral sites during calcination. It is suggested that two aluminium T-sites, which are adjacent or close to each other, obeying the Loewenstein’s rule (i.e., no Al–O–Al linkage), are required for the hydrolysis of a Si–O–Al bond for the formation of octahedrally coordinated aluminium. The distribution of aluminium in zeolite Beta is a function of the Si/Al ratio and is non-uniformly distributed over the crystallographic T-sites.

Zeolite synthesis in the presence of flexible diquaternary alkylammonium ions (C2H5)3N+(CH2)nN+(C2H5)3 with n = 3-10 as structure-directing agents

Abstract The use of flexible diquaternary alkylammonium ions (C2H5)3N+(CH2)nN+(C2H5)3 (Et6-diquat-n with n=3–10) as structure-directing agents for zeolite synthesis in the presence of alkali metal cation is described. Among the organic structure-directing agents studied here, a considerable diversity in the phase selectivity was observed only for the Et6-diquat-5 ion: this cation can produce five different zeolite structures (i.e., P1, SSZ-16, SUZ-4, ZSM-57, and mordenite), depending on the oxide composition of synthesis mixtures. Analysis of the variable-temperature 1H CRAMPS NMR spectra obtained from the Et6-diquat-5 molecules in these five zeolites reveals that the host–guest interactions occurring within the respective materials maintain in a manner different from one another even at 160 °C at which the zeolite hosts crystallize.

New Insights into ETS-10 and Titanate Quantum Wire: A Comprehensive Characterization

The titanate quantum wires in ETS-10 crystals remain intact during ion exchange of the pristine cations (Na(+)(0.47) + K(+)(0.53)) with M(n+) ions (M(n+) = Na(+), K(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Pb(2+), Cd(2+), Zn(2+)) and during reverse exchange of the newly exchanged cations with Na(+). The binding energies of O(1s) and Ti(2p) decrease as the electronegativity of the cation decreases, and they are inversely proportional to the negative partial charge of the framework oxygen [-delta(O(f))]. At least five different oxygen species were identified, and their binding energies (526.1-531.9 eV) indicate that the titanate-forming oxides are much more basic than those of aluminosilicate zeolites (530.2-533.3 eV), which explains the vulnerability of the quantum wire to acids and oxidants. The chemical shifts of the five NMR-spectroscopically nonequivalent Si sites, delta(I(A)), delta(I(B)), delta(II(A)), delta(II(B)), and delta(III), shift downfield as -delta(O(f)) increases, with slopes of 2.5, 18.6, 133.5, 216.3, and 93.8 ppm/[-delta(O(f))], respectively. The nonuniform responses of the chemical shifts to -delta(O(f)) arise from the phenomenon that the cations in the 12-membered-ring channels shift to the interiors of the cages surrounded by four seven-membered-ring windows. On the basis of the above, we assign delta(I(A)), delta(I(B)), delta(II(A)), and delta(II(B)) to the chemical shifts arising from Si(12,12), Si(12,7), Si(7,12), and Si(7,7) atoms, respectively. The frequency of the longitudinal stretching vibration of the titanate quantum wire increases linearly and the bandwidth decreases nonlinearly with increasing -delta(O(f)), indicating that the titanate quantum wire resembles a metallic carbon nanotube. As the degree of hydration increases, the vibrational frequency shifts linearly to higher frequencies while the bandwidth decreases. We identified another normal mode of vibration of the quantum wire, which vibrates in the region of 274-280 cm(-1). In the dehydrated state, the band-gap energy and the first absorption maximum shift to lower energies as -delta(O(f)) increases, indicating the oxide-to-titanium(IV) charge-transfer nature of the transitions.

The influence of calcination temperature on catalytic activities in a Co based catalyst for CO2 dry reforming

The carbon dioxide dry reforming of methane (CDR) reaction could be thermodynamically favored in the range of 800 to 1,000 °C. However, the catalyst in this reaction should be avoided at the calcination temperature over 800 °C since strong metal support interaction (SMSI) in this temperature range can decrease activity due to loss of active sites. Therefore, we focused on optimizing the temperature of pretreatment and a comparison of surface characterization results for CDR. Results related to metal sintering over support, re-dispersion by changing of particle size of metal-support, and strong metal support interaction were observed and confirmed in this work. In our conclusion, optimum calcination temperature for a preparation of catalyst was proposed that 400 °C showed a higher and more stable catalytic activity without changing of support characteristics.

CO oxidation over CuO catalysts prepared with different precipitants

CuO catalysts, prepared by the precipitation method using different precipitants such as ammonium hydroxide, sodium hydroxide, sodium carbonate and sodium hydrogen carbonate were applied to CO oxidation. Among the catalysts studied, CuO synthesized with sodium hydrogen carbonate showed the highest activity for CO oxidation. With the water vapor present in the feed gas, the catalytic activity decreased considerably due to reduction in the number of active sites by competitive adsorption between water vapor and CO. The H2-TPR and CO-TPD results showed that existing Na+ cations and HCO3− and CO3/2− anions on the CuO surface could weaken the copper-oxygen bond strength and accelerate the mobility of oxygen on the surface or lattice. Finally, the morphology of the CuO crystals was dependent on the precipitants, and the introduction of Na+ cations and various anions resulted in the formation of smaller crystals.