Sung June Cho

Combined DRS-RS-EXAFS-XANES-TPR study of supported chromium catalysts

The surface chemistry of supported chromium catalysts (Cr/SiO2· Al2O3) has been systematically investigated as a function of the support composition (Si:Al ratio) and the Cr oxide loading by a combination of diffuse reflectance spectroscopy (DRS), Raman spectroscopy (RS), X-ray Absorption spectroscopy (EXAFS–XANES) and temperature-programmed reduction (TPR). Combination of all the obtained results by these characterization techniques leads to a uniform interpretation and general picture of surface Cr. On hydrated surfaces, the molecular structure of the Cr oxide species depends on the isoelectric point of the oxide support and the Cr loading: more polymerized Cr oxide species correspond to higher Cr loading and silica content of the support. After calcination, the Cr oxide species are anchored onto the surface by reaction with surface hydroxy groups of the supports. On alumina the reaction starts with the most basic OH groups on alumina. This suggests that the anchoring process is an acid–base reaction. On calcined surfaces, the polymerization of the anchored Cr oxide species and the amount of Cr2O3 clusters increases with silica content and Cr loading. Reduced Cr samples possess both Cr2+ and Cr3+, the relative concentrations of which are support and loading dependent: lower Cr2+:Cr3+ ratios correspond to higher Cr loading and alumina content of the support.

Lanthanum-catalysed synthesis of microporous 3D graphene-like carbons in a zeolite template

Three-dimensional graphene architectures with periodic nanopores—reminiscent of zeolite frameworks—are of topical interest because of the possibility of combining the characteristics of graphene with a three-dimensional porous structure. Lately, the synthesis of such carbons has been approached by using zeolites as templates and small hydrocarbon molecules that can enter the narrow pore apertures. However, pyrolytic carbonization of the hydrocarbons (a necessary step in generating pure carbon) requires high temperatures and results in non-selective carbon deposition outside the pores. Here, we demonstrate that lanthanum ions embedded in zeolite pores can lower the temperature required for the carbonization of ethylene or acetylene. In this way, a graphene-like carbon structure can be selectively formed inside the zeolite template, without carbon being deposited at the external surfaces. X-ray diffraction data from zeolite single crystals after carbonization indicate that electron densities corresponding to carbon atoms are generated along the walls of the zeolite pores. After the zeolite template is removed, the carbon framework exhibits an electrical conductivity that is two orders of magnitude higher than that of amorphous mesoporous carbon. Lanthanum catalysis allows a carbon framework to form in zeolite pores with diameters of less than 1 nanometre; as such, microporous carbon nanostructures can be reproduced with various topologies corresponding to different zeolite pore sizes and shapes. We demonstrate carbon synthesis for large-pore zeolites (FAU, EMT and beta), a one-dimensional medium-pore zeolite (LTL), and even small-pore zeolites (MFI and LTA). The catalytic effect is a common feature of lanthanum, yttrium and calcium, which are all carbide-forming metal elements. We also show that the synthesis can be readily scaled up, which will be important for practical applications such as the production of lithium-ion batteries and zeolite-like catalyst supports.

Self-assembled peptide architecture with a tooth shape: folding into shape.

Molecular self-assembly is the spontaneous association of molecules into structured aggregates by which nature builds complex functional systems. While numerous examples have focused on 2D self-assembly to understand the underlying mechanism and mimic this process to create artificial nano- and microstructures, limited progress has been made toward 3D self-assembly on the molecular level. Here we show that a helical β-peptide foldamer, an artificial protein fragment, with well-defined secondary structure self-assembles to form an unprecedented 3D molecular architecture with a molar tooth shape in a controlled manner in aqueous solution. Powder X-ray diffraction analysis, combined with global optimization and Rietveld refinement, allowed us to propose its molecular arrangement. We found that four individual left-handed helical monomers constitute a right-handed superhelix in a unit cell of the assembly, similar to that found in the supercoiled structure of collagen.

Synthesis of Aluminosilicate and Gallosilicate Zeolites via a Charge Density Mismatch Approach and Their Characterization

Aluminosilicate and gallosilicate zeolite syntheses via a charge density mismatch (CDM) approach are compared at intermediate-silica compositions (Si/Me = 5-16, where Me is Al or Ga). With a variation of the crystallization temperature and of the type and/or concentration of alkali metal ions added as a crystallization structure-directing agent (SDA) to tetraethylammonium-tetramethylammonium, tetraethylammonium-hexamethonium, and strontium-choline mixed-SDA systems, we were able to obtain 11 different zeolite structures. However, only 5 out of a total 40 pairs of aluminosilicate and gallosilicate synthesis runs at otherwise identical chemical compositions were found to give the same zeolite product with no detectable impurities, suggesting that the structure-directing ability of Ga is quite different from that of Al even in intermediate-silica synthesis conditions. The CDM approach to offretite synthesis led to hexagonal plate-like crystals with aspect ratios lower than 0.3, and UZM-22 exhibited no significant preference of Al substitution for particular tetrahedral sites, especially for site T1, unlike its framework type material ZSM-18. More interestingly, the EU-1 zeolite obtained from an aluminosilicate synthesis mixture containing Li(+) as an inorganic crystallization SDA in the tetraethylammonium-hexamethonium double-organic additive system has been characterized to locate about half of its Li(+) ions in the framework, while the Li distribution over the 10 topologically different tetrahedral sites is nonrandom in nature.

Synthesis of heat-resistant mesoporous SiOC ceramic and its hydrogen adsorption

High-temperature stable mesoporous SiOC ceramic was prepared by the self-assembling reaction between an anionic surfactant and the inorganic precursor N-trimethoxylsilylpropyl-N,N,N-trimethylammonium chloride, followed by calcination at 600 °C under nitrogen. The preferred structure was adjusted by varying the co-structure directing agent (CSDA)/surfactant ratio, which allows for better control of the nanoscale morphologies of mesoporous SiOC. The microstructural change was thoroughly investigated by BET, SAXS, TEM, and 29Si NMR. The obtained SiOC products exhibited a high surface area, up to 1762 m2 g−1, with uniform pore size distribution, which was only reduced to 1403 m2 g−1 when exposed to 800 °C in air. In particular, the mesoporous product showed 1.1 wt% of H2 sorption at 298 K and 25 atm, which could be extended into hydrogen storage materials.

Suppressed N2O formation during NH3 selective catalytic reduction using vanadium on zeolitic microporous TiO2.

Emission of N2O from mobile and off-road engine is now being currently regulated because of its high impact compared to that of CO2, thereby implying that N2O formation from the exhaust gas after-treatment system should be suppressed. Selective catalytic reduction using vanadium supported TiO2 catalyst in mobile and off-road engine has been considered to be major source for N2O emission in the system. Here we have demonstrated that vanadium catalyst supported on zeolitic microporous TiO2 obtained from the hydrothermal reaction of bulk TiO2 at 400 K in the presence of LiOH suppresses significantly the N2O emission compared to conventional VOx/TiO2 catalyst, while maintaining the excellent NOx reduction, which was ascribed to the location of VOx domain in the micropore of TiO2, resulting in the strong metal support interaction. The use of zeolitic microporous TiO2 provides a new way of preparing SCR catalyst with a high thermal stability and superior catalytic performance. It can be also extended further to the other catalytic system employing TiO2-based substrate.

Synthesis of Silicate Zeolite Analogues Using Organic Sulfonium Compounds as Structure‐Directing Agents

A microporous crystalline silica zeolite of the MEL structure type and three other zeolite analogues composed of germanosilicate frameworks were synthesized using tributylsulfonium, triphenylsulfonium, or tri(para-tolyl)sulfonium as the structure-directing agent. The germanosilicates thus obtained had ISV, ITT, or a new zeolite structure depending on the synthesis conditions. The structure of the new germanosilicate was solved using X-ray powder diffraction data with the aid of a charge-flipping method. The solution indicated a crystal structure belonging to the P63/mmc space group with cell parameters of a=16.2003 Å and c=21.8579 Å. After calcination, the new germanosilicate material exhibited two types of accessible micropores with diameters of 0.61 and 0.78 nm.

Synthesis of manganese substituted hexaaluminate and its fabrication into monolithic honeycombs for catalytic combustion

Abstract Hexaaluminate powder has been synthesized with the impregnation of La(NO 3 ) 3 and Mn(NO 3 ) 2 onto γ-Al 2 O 3 and subsequent calcination at 1400°C. The hexaaluminate powder was formed into monolithic honeycomb of 200 cpi. The results of X-ray diffraction of the samples showed that the powder and the monoliths consisted of LaAl 11 O 19 and LaAlO 3 phases. The BET surface area of the manganese substituted powder and honeycomb decreased dramatically to ∼13 m 2 g −1 at 1200°C. Further heat treatments up to 1400°C did not cause a significant reduction of the surface area, ∼7 m 2 g −1 for both sample. The bending strength of the monolithic honeycomb decreased significantly when the temperature jumped about 300°C as a thermal shock was applied. The manganese inserted as a catalyzing component in the magnetoplumbite structure allowed lean methane combustion, 1% over the honeycomb heated at 1400°C, to start at 480°C, comparable to that of the corresponding powder.

Directed C−H Activation and Tandem Cross-Coupling Reactions Using Palladium Nanocatalysts with Controlled Oxidation

Controlled oxidation of palladium nanoparticles provided high-valent PdIV oxo-clusters which efficiently promote directed C-H halogenation reactions. In addition, palladium nanoparticles can undergo changes in oxidation states to provide both high-valent PdIV and low-valent Pd0 species within one system, and thus a tandem reaction of C-H halogenation and cross-coupling (C-N, C-C, and C-S bond formation) was successfully established.

EXAFS AND CATALYTIC ACTIVITY OF AGPT BIMETALLIC CLUSTER SUPPORTED ON NAY ZEOLITE

Platinum clusters with ca. 1 nm in size have been prepared as encaged in supercages of NaY zeolite by the reduction of platinum after ion exchange of Pt(NH3)42+ into NaY. A series of bimetallic AgPt/NaY samples with Ag/Pt atomic ratios of 0, 0.11, 0.43 and 1.0 have been obtained by the reduction of silver after subsequent ion exchange of Ag+ into the Pt/NaY sample. The result from measurements for 129Xe NMR spectrum, and hydrogen chemisorption has indicated that the Ag atoms were put together onto the Pt cluster, with the surface enriched by Ag atoms. The extended X-ray absorption fine structure (EXAFS) of the bimetallic cluster measured above Pt LIII and Ag K edges have been analyzed with reference EXAFS spectra which were experimentally obtained from the metal foils and those for AgPt pair which were theoretically prepared by FEFF codes distributed by Rehr et al. The catalytic activity in ethane hydrogenolysis over the AgPt bimetallic cluster has also been measured.