No-Kuk Park

A Study on Selective Oxidation of Hydrogen Sulfide over Zeolite-NaX and -KX Catalysts

Selective oxidation of hydrogen sulfide (H2S) was studied on zeolite-NaX and zeolite-KX. Elemental sulfur yield over zeolite-NaX was achieved about 90% at 225 °C for the first 4 hours, but it gradually decreased to 55% at 40 hours after the reaction started. However, yield of elemental sulfur on zeolite-KX was obtained within the range of 86% at 250 °C after 40 hours. The deactivation of the zeolite-NaX and -KX catalysts was caused by the coverage of a sulfur compound, produced by the selective oxidation of H2S over the catalysts. The coverage of a sulfur compound over the zeolite-NaX and -KX was confirmed by the TPD (temperature-programmed desorption) tests utilizing thermogravimetric analysis and FT-IR analysis. Even though high temperature was required to prevent the deactivation of zeolite-NaX, the temperature cannot be raised to 250 °C or above due to the SO2 production and the decrease of thermodynamic equilibrium constant. Zeolite-KX was superior to the zeolite-NaX for both its selectivity to elemental sulfur and its resistance to deactivation in the selective oxidation of H2S.

Dynamic Hydrogen Production from Methanol/Water Photo-Splitting Using Core@Shell-Structured CuS@TiO2 Catalyst Wrapped by High Concentrated TiO2 Particles

This study focused on the dynamic hydrogen production ability of a core@shell-structured CuS@TiO2 photocatalyst coated with a high concentration of TiO2 particles. The rectangular-shaped CuS particles, 100 nm in length and 60 nm in width, were surrounded by a high concentration of anatase TiO2 particles (>4~5 mol). The synthesized core@shell-structured CuS@TiO2 particles absorbed a long wavelength (a short band gap) above 700 nm compared to that pure TiO2, which at approximately 300 nm, leading to easier electronic transitions, even at low energy. Hydrogen evolution from methanol/water photo-splitting over the core@shell-structured CuS@TiO2 photocatalyst increased approximately 10-fold compared to that over pure CuS. In particular, 1.9 mmol of hydrogen gas was produced after 10 hours when 0.5 g of 1CuS@4TiO2 was used at pH = 7. This level of production was increased to more than 4-fold at higher pH. Cyclic voltammetry and UV-visible absorption spectroscopy confirmed that the CuS in CuS@TiO2 strongly withdraws the excited electrons from the valence band in TiO2 because of the higher reduction potential than TiO2, resulting in a slower recombination rate between the electrons and holes and higher photoactivity.

Effect of oxidation states of Mn in Ca1−x Li x MnO3 on chemical-looping combustion reactions

We investigated the effect of the oxidation state of Mn in CaMnO3 perovskite particles to improve their oxygen transfer performance for chemical-looping combustion (CLC). Li was introduced in the Ca site of CaMnO3 to increase the Mn oxidation state. Ca1−xLixMnO3 particles were synthesized by the solid-state method, and the amount of Li added ranged from 0 to 0.015 mol. The structure of the synthesized Ca1−xLixMnO3 particles was examined using XRD, and all particles were confirmed to have a CaMnO3 perovskite structure. The shape and chemical properties of the prepared particles were characterized by using SEM and CH4-TPD. The binding energy and oxidation state of the different elements in the Ca1−xLixMnO3 particles were measured by XPS. When Li was added, the oxidation state of Mn in Ca1−xLixMnO3 was higher than that of Mn in CaMnO3. The oxygen transfer performance of the particles was determined by an isothermal H2-N2/air and CH4-CO2/air redox cycle at 850 °C, repeated ten times, using TGA. All particles showed an oxygen transfer capacity of about 8.0 to 9.0 wt%. Among them, Ca0.99Li0.01MnO3 particles had the best performance and the oxygen transfer capacity under H2-N2/air and CH4-CO2/air atmosphere was 8.47 and 8.75 wt%, respectively.

Catalytic reduction of sulfur dioxide with carbon monoxide over tin dioxide for direct sulfur recovery process

SO(2) reduction by CO over SnO(2) catalyst was studied in this work. The parameters were the reaction temperature, space velocity (GHSV) and [CO]/[SO(2)] molar ratio. The optimal temperature, GHSV and [CO]/[SO(2)] molar ratio were 550 degrees C, 8000 h(-1) and 2.0, respectively. Under these conditions, the SO(2) conversion and sulfur selectivity were about 78% and 68%, respectively. The following reaction pathway involving two mechanisms was proposed in SO(2) reduction by CO over SnO(2) catalyst: in the first step involving Redox mechanism, the elemental sulfur was produced by the mobility of the lattice oxygen between SO(2) and SnO(2) surface. In the second step, COS was formed by the side reaction between elemental sulfur and CO or metal sulfide and CO. In the third step involving COS intermediate mechanism, the abundant elemental sulfur was produced by the SO(2) reduction by COS which was produced in the second step and was more effective reducing agent than CO.

The study on the selective oxidation of H2S over the mixture zeolite NaX-WO3 catalysts

At temperatures lower than 250 °C the deactivation of zeolite NaX catalyst occurred in the presence of water vapor. The gradual accumulation of water vapor on the surface of catalyst could cause deactivation of catalyst. The zeolite NaX-WO3 catalysts were prepared to study a method preventing deactivation of catalysts from the adsorption of water vapor. The zeolite NaX-WO3 (9 : 1) with a low content of WO3 showed the highest conversion of H2S. It is believed that the addition of WO3 caused either a decrease of the strong adsorption of water vapor on the zeolite NaX or an increase of the reducibility of WO3 by some interactions between zeolite NaX and WO3.

Preparation of Nanosized -Al2O3 Particles Using a Microwave Pretreatment at Mild Temperature

This study investigated the effect of microwave pretreatment to reduce the growth temperature of α-Al2O3. The microwave pre-treating of the synthesized powders at 1,000∘C produced rhombohedral structured α-Al2O3 with high specific surface area and dispersion; however the structure accumulated among the particles was seen above 1,200∘C when the microwave did not pretreated.

Synthesis of Macro-Porous SnO2 for Dye-Sensitized Solar Cells

Macro-porous SnO2 was synthesized using the template method and its electrical properties were investigated for applications as an electrode in dye-sensitized solar cells (DSSCs). Polystyrene (PS) spherical nano beads in a colloidal solution were used as a template for the synthesis of macro-porous SnO2. Macro-porous SnO2 was synthesized by mixing the precursor and polystyrene colloid. The concentration of tin chloride (SnCl4) used as the precursor ranged from 0.025 to 0.1 M. The surface area and crystallinity of macro-porous SnO2 increased with increasing concentration of the SnCl4 solution. The crystalline SnO2 electrode in a DSSC produced a high short circuit current (Jsc) owing to its high surface area. Overall, macro-porous SnO2 can be applied to electronic devices because of its high performance, such as electron mobility.

Size Control of Particles with Changing Concentration of TIP used as a Precursor for Synthesis of TiO2 by Ultra-Sonic Spray and Pyrolysis Method

The nano sized titanium dioxide was synthesized by the ultra-sonic spray method and pyrolysis of TIP (titanium iso-propoxide), which was employed as the precursor solution. The fine liquid drop of the TIP sprayed by the ultra-sonic spray nozzle was conversed into TiO2 nano particles by pyrolysis through the hot tubular reactor. The concentration of TIP solution was controlled in the range of 0.1∼1.0 M and the morphology, the particle size, the crystal structure, and the surface area of synthesized TiO2 were investigated by SEM, TEM, XRD and N2-adsorption method, respectively. DSSC (dye sensitized solar cells) was also prepared by employing the synthesized TiO2 nano powder as the electrode and the photo-electrical property of TiO2 was compared in terms of efficiency of solar cells. It was further confirmed that the particle size of TiO2 was controlled with changing concentration of TIP solution. When 0.1 M TIP was used as the precursor solution, the size, the surface area and the crystal structure of synthesized TiO2 were approximately 10 nm (by TEM), 56 m2/g and the anatase structure, respectively. The particle size of TiO2 decreased with decreasing concentration of TIP solution. Therefore, it was concluded that the efficiency of DSSCs can be improved by employing high surface area and nano-sized TiO2 as the electrode.

Application of ZnO Single Crystal Rods as Anti-Reflective Material for PV Cells

In this study, ZnO nanorods for antireflection coatings application to the silicon solar cells were epitaxially grown over the Si substrate using both thermal evaporation method and wet chemical method and their characteristics were investigated. A precursor for the thermal evaporation method was prepared by impregnating zinc acetate over the activated carbon at three different loading amounts: 10, 20, and 30 wt%. The temperature for the epitaxial growth of ZnO nanorods over the substrate was fixed to 850°C. The diameter of the synthesized single crystalline ZnO nanorods was about 1 μm. The shape and size of the ZnO nanorods varied with the content of the precursors loaded over the activated carbon. The length of the ZnO nanorods increased as the loading amount of the zinc was increased. Single crystalline ZnO nanorods were grown over the substrates by a hydrothermal synthesis in an autoclave reactor. Zinc nitrate was used for the precursor material in the wet chemical method. The length and diameter of the ZnO nanorods synthesized by the wet chemical method were about 200 nm and a few nm, respectively. The surface morphology of the ZnO nanorods prepared by two growth methods was observed by scanning electron microscope. Its crystal structure was analyzed by an X-ray diffraction spectrometer. The optical properties, such as reflectance, were measured by the UV-Visible spectrophotometer. It was confirmed that ZnO nanorods decrease the reflectance of UV and Visible lights. Therefore, it was concluded that ZnO nanorods can be used as the antireflective material for photo voltaic cells.

Surface modification of layered perovskite Sr 2 TiO 4 for improved CO 2 photoreduction with H 2 O to CH 4

Layered perovskite Sr2TiO4 photocatalyst was synthesized by using sol-gel method with citric acid. In order to increase the surface area of layered perovskite Sr2TiO4, and thus to improve its photocatalytic activity for CO2 reduction, its surface was modified via hydrogen treatment or exfoliation. The physical and chemical properties of the prepared catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, elemental mapping analysis, energy-dispersive X-ray spectroscopy, N2 adsorption-desorption, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, photoluminescence, and electrophoretic light scattering. CO2 photoreduction was performed in a closed reactor under 6 W/cm2 UV irradiation. The gaseous products were analyzed using a gas chromatograph equipped with flame ionization and thermal conductivity detectors. The exfoliated Sr2TiO4 catalyst (E-Sr2TiO4) exhibited a narrow band gap, a large surface area, and high dispersion. Owing to these advantageous properties, E-Sr2TiO4 photocatalyst showed an excellent catalytic performance for CO2 photoreduction reaction. The rate of CH4 production from the photoreduction of CO2 with H2O using E-Sr2TiO4 was about 3431.77 μmol/gcat after 8 h.