Chanho Pak

Synthesis and characterization of mesoporous carbon for fuel cell applications

The synthesis and application aspects of ordered mesoporous carbon (OMC) as a novel material for fuel cell catalysts are reviewed in this paper. The synthesis and structural characterization of OMC is outlined and the recent advances in the synthesis of OMC relevant to fuel cell technologies is presented. Recent examples of the application of OMC as a support for fuel cell catalysts are summarized and practical approaches for the application of OMC for the fuel cell systems are discussed. Future perspectives on the use of OMC in energy conversion and storage devices are also suggested.

Cerium impregnated H-mordenite as a catalyst for shape-selective isopropylation of naphthalene. Selective deactivation of acid sites on the external surface

Abstract The impregnation of cerium is the effective method for the deactivation of external acid sites of H-mordenite. The selectivity of 2,6-DIPN in the isopropylation of naphthalene was enhanced by the impregnation with such a large amount as 30–50 wt.-% of cerium without significant decrease of catalytic activity. The highest selectivity of 2,6-DIPN was achieved up to 70% over a highly dealuminated H-mordenite, (HM(128); SiO 2 /Al 2 O 3 = 128, with 30 wt.-% of cerium. The enhancement of the selectivity is ascribed to the deactivation of external acid sites judging from the activity of the cracking reaction of 1,3,5-triisopropylbenzene. The effective pore radius was not reduced by the modification. The ceria is highly dispersed only on the external surface of H-mordenite without any formation of new kinds of acid sites. The 129 Xe NMR observation suggfests that cerium is not in the pores, but on the external surfaces. The deactivation of the external acid sites is a characteristic property for cerium. Lanthanum and neodymium inhibited catalytic activity of the isopropylation because the pores were narrowed by their impregnation. A possible reason of the deactivation is ascribed to the amphoteric property of ceria.

Morphology-selective synthesis of mesoporous SBA-15 particles over micrometer, submicrometer and nanometer scales

Mesoporous silica structures are of increasing importance as supports for enzymes and molecular organometallic catalysts. For high-surface-area, porous 3-d catalytic supports, the relationship between the exterior particle morphology and the 3-d mesopore structure is of particular significance. This paper describes the designed synthesis of selected morphologies of mesoporous SBA-15, which can be chosen from micrometer sized spheres to hundreds or tens of nanometers sized monodispersed particles such as platelets, hexagonal columns, rice-shapes, rods with tunable aspect ratios, and donuts. These are directly synthesized via control of the fundamental synthesis factors, including initial temperature, stirring rate and micelle packing parameter, rather than by the use of additives that have been generally utilized for specific morphologies in previous reports. The relationship between these basic synthesis parameters and morphologies provides insights into the formation of mesostructured materials.The SBA materials with various morphologies are expected to be useful in applications that require anisotropic or path-length-controlled diffusion.

Ultrastable Pt nanoparticles supported on sulfur-containing ordered mesoporous carbonvia strong metal-support interaction

Sulfur-containing ordered mesoporous carbon (S-OMC) material was successfully obtained from a mesoporous silica template through a nano-replication method using p-toluenesulfonic acid as the framework source. The S-OMC material thus obtained could be utilized as an excellent support for Pt nanoparticles of size 3.14 nm, even though the Pt loading was 60 wt%. The Pt nanoparticles supported on the S-OMC material (Pt/S-OMC) exhibited excellent thermal stability compared with those supported on sulfur-free ordered mesoporous carbon and Vulcan carbon. XPS analysis indicated that the strong metal-support interaction between the sulfur atoms in the S-OMC support and the surface atoms of the loaded Pt nanoparticles played an important role in stabilization of the Pt nanoparticles against the Ostwald ripening process during the thermal treatments. Cyclic voltammogram results revealed that the Pt/S-OMC material exhibited reasonably high electrochemically active Pt surface areas before and after thermal treatments at 600 °C, indicating that the surface of Pt nanoparticles was not poisoned by the sulfur atoms of the S-OMC support.

Durable cross-linked copolymer membranes based on poly(benzoxazine) and poly(2,5-benzimidazole) for use in fuel cells at elevated temperatures

Cross-linked benzoxazine–benzimidazole copolymer membranes with interpenetrating network structures containing a large amount of phosphoric acid (PA) were prepared by a direct casting method for polymer electrolyte membrane fuel cells at elevated temperatures. The direct casting solution was prepared by adding 6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine into the polymerization solution of 3,4-diaminobenzoic acid in poly(phosphoric acid) (PPA). Once the PPA in the membranes has been hydrolyzed to PA, the membranes can contain large amounts of PA and showed high proton conductivity, ca. 0.14 S cm−1, at 150 °C under anhydrous conditions. Membrane–electrode assemblies (MEAs) prepared using the copolymers showed high operating voltages of 0.69 V at 0.2 A cm−2, long-term durability for up to 1584 cycles, with much slower performance decay than those prepared using poly(benzimidazole) homopolymers, such as poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly(2,5-benzimidazole), from the in situ accelerated lifetime test.

Systematic phase control of periodic mesoporous organosilicas using Gemini surfactants

Highly ordered periodic mesoporous organosilica (PMO) materials with various mesostructures, including lamellar, bicontinuous cubic Ia3d, 2D hexagonal (P6mm), 3D hexagonal (P63/mmc) and cubic Pm3n, have been synthesized using Gemini surfactants with general formulas of [CnH2n+1N(CH3)2(CH2)sN (CH3)2CnH2n+1]Br2 (n = 6–18 and s = 3–12, Cn-s-n). The nature of the Gemini surfactant such as alkyl chain length (n) and spacer length (s), and the synthetic conditions such as reaction temperatures and molar compositions are controlling parameters for desired mesostructures. The PMO materials, synthesized at room temperature from Cn-6-n, exhibit phase transition from lamellar to bicontinuous cubic Ia3d, 2D hexagonal, 3D hexagonal and cubic Pm3n as the chain length decreases, whereas only the lamellar and 2D hexagonal PMO materials with different lattice parameters depending on the chain length are obtained at high reaction temperature (373 K). The Cn-8-n and Cn-10-n surfactants also yield 2D hexagonal PMO material in a very wide range of synthetic condition at 373 K. The PMO materials with various mesostructures thus obtained exhibit high BET surface areas in the range of 900–1500 m2 g−1 and total pore volumes of about 0.5–1.4 cm3 g−1.

Ordered mesoporous carbon–carbon nanotube nanocomposites as highly conductive and durable cathode catalyst supports for polymer electrolyte fuel cells

Ordered mesoporous carbon–carbon nanotube (OMC–CNT) nanocomposites were prepared and used as catalyst supports for polymer electrolyte fuel cells. The OMC–CNT composites were synthesized via a nanocasting method that used ordered mesoporous silica as a template and Ni–phthalocyanine as a carbon source. For comparison, sucrose and phthalocyanine were used to generate two other OMCs, OMC(Suc) and OMC(Pc), respectively. All three carbons exhibited hexagonally ordered mesostructures and uniform mesopores. Among the three carbons the OMC–CNT nanocomposites showed the highest electrical conductivity, which was due to the nature of their graphitic framework as well as their lower interfacial resistance. The three carbons were then used as fuel cell catalyst supports. It was found that highly dispersed Pt nanoparticles (ca. ∼1.5 nm in size) could be dispersed on the OMCs via a simple impregnation–reduction method. The activity and kinetics of the oxygen reduction reaction (ORR), measured by the rotating ring-disk electrode technique revealed that the ORR over the Pt/OMC catalysts followed a four-electron pathway. Among the three Pt/OMC catalysts, the Pt/OMC–CNT catalyst resulted in the highest ORR activity, and after an accelerated durability test the differences in the ORR activities of the three catalysts became more pronounced. In single cell tests, the Pt/OMC–CNT-based cathode showed a current density markedly greater than those of the other two cathodes after a high-voltage degradation test. These results were supported by the fact that the Pt/OMC–CNT-based cathode had the lowest resistance, which was probed by electrochemical impedance spectroscopy (EIS). The results of the single cell tests as well as those of the EIS-based measurements indicate that the rigidly interconnected structure of the OMC–CNT as well as their highly conductive frameworks are concomitantly responsible for the OMC–CNT nanocomposites exhibiting higher current density and durability than the other two carbons.

Nano-propping effect of residual silicas on reversible lithium storage over highly ordered mesoporous SnO2 materials

Highly ordered mesoporous SnO2 materials with residual silica species were successfully synthesized from a mesoporous silica template (SBA-15) via nano-replication and simple etching processes. A tin precursor, SnCl2·2H2O, was infiltrated spontaneously within the mesopores of the silica templates by melting the precursor at 353 K without using a solvent. After the heat-treatment of composite materials at 973 K under static air conditions, the controlled removal of silica templates using NaOH or HF solutions with different concentrations results in the successful preparation of mesoporous SnO2 materials, where the amounts of residual silica species are in the range 0.9–17.4 wt%. The residual silica species induce a nano-propping effect enabling the mesoporous SnO2 material (containing 6.0 wt% of silica species) to remain stable up to 973 K without any significant structural collapse. More importantly, the optimum amount of residual silica species (3.9–6.0 wt%) results in a dramatic reduction in capacity fading after prolonged charging–discharging cycles in Li-ion battery. The mesoporous SnO2 material with 3.9 wt% of silica species still exhibits a large capacity (about 600 mAh g−1) after the 30th cycle, which is probably because the residual silica species act as a physical barrier to suppress the aggregation of Sn clusters formed in the mesoporous SnO2 materials during the reversible lithium storage.

Synthesis and characterization of Pt/MCM-41 catalysts

Abstract The preparation of Pt/MCM-41 catalysts was studied. Three different precursors were used: platinum acetyl acetonate Pt(AcAc) 2 , tetraamine platinum (II) nitrate Pt(NH 3 ) 4 (NO 3 ) 2 , and hydrogen hexachloroplatinate (IV) hydrate H 2 PtCl 6 . With these precursors, the catalysts were prepared by three different methods: equilibrium adsorption, ion exchange, and a new method, which we call vacuum evaporation impregnation (VEI). Through the characterizations by XRD, physisorption, chemisorption, XAFS, and XANES-TPR, it was demonstrated that a structured, high loading (0.5–2 wt.%) and high dispersion (>1.0 calculated from CO and/or H 2 chemisorption) Pt/MCM-41 catalysts was obtained by the new preparation method VEI.

Reversible coordination change of chromium in Cr-MCM-41 and Cr-MCM-48 studied by X-ray absorption near edge structure

Chromium-substituted MCM-41 and MCM-48 mesoporous molecular sieves have been prepared via a direct hydrothermal synthesis. A change of structure of mesoporous molecular sieves and coordination of chromium was investigated by X-ray diffraction (XRD), nitrogen adsorption and X-ray absorption measurements after various redox gas treatments at high temperature. It was determined from the XRD patterns that the Cr containing material has a hexagonal MCM-41 and a cubic MCM-48 structure, respectively, and the structure is not affected significantly by redox gas treatments. The pore structure and surface area, analyzed from nitrogen isotherms, of Cr-substituted MCM-41 and MCM-48 are maintained after reduction and re-oxidation, which were performed sequentially after calcination. It is suggested from X-ray absorption near edge structure that the local environment of Cr in the MCM-41 and MCM-48 structures was transformed from tetrahedral to octahedral coordination by reduction, and vice versa by re-oxidation.