Ki June Yoon

Hydrogen Production by Catalytic Decomposition of Methane over Activated Carbons: Deactivation Study

The amount of deposited carbon on activated carbons was adjusted by varying the space time and the time on stream. Carbon nuclei formation appeared to occur initially but was terminated soon and then the carbon crystallite growth became dominant. The methane decomposition rate over activated carbons had a nearly linear relationship with the amount of carbon deposited. This suggests that the carbon deposition occurs uniformly and the activity decreases due to pore blocking, resulting in loss of accessibility to the active sites. The rate was also nearly proportional to the surface area for the same kind of activated carbon, which is contrasted with the previous finding that no discernible trend was observed between the initial activity and the surface area among different kinds of the activated carbons.

Removal of paraquat in aqueous suspension of TiO2 in an immersed UV photoreactor

Experiments were performed to investigate the effect of operating parameters on the photodegradation efficiency of paraquat in a TiO2-suspended photoreactor with immersed UV lamps. TiO2 particles were prepared by a hydrothermal method. The removal rate of paraquat in the reactor was 0.54 mg/l/h with only air-sparging. The removal rate in 24 h with both the UV radiation and air-sparging was 50% higher than that with only the UV radiation. Variations of the paraquat concentration at the UV intensities of 4 and 8 W/m2 decreased slowly with time, but that at 12 W/m2 decreased more rapidly. The removal efficiency at the air-sparging flow rate of 1 //min increased as a UV light intensity increased. pH value in the reactor at the UV intensity of 12 W/m2 decreased with time until 12 h and then increased with time over 12 h.

Partial oxidation of methane over nickel-added strontium phosphate

Abstract It was found that nickel-added strontium phosphate exhibited high activity and selectivity in partial oxidation of methane. The optimum nickel content could be determined. Over the optimum catalyst, methane conversions and H 2 and CO concentrations in excess of those predicted by the thermodynamic equilibrium were observed. It is believed that the catalytically active species is metallic nickel. This metallic nickel is considered to come from nickel-substituted strontium phosphate under reducing environment, giving highly dispersed nickel metal particles.

Temperature profiles of the monolith catalyst in CO2 reforming of methane within-situ combustion of methane and ethane

The temperature profiles in a monolith reactor were measured in CO2 reforming of CH4 within-situ combustion of methane and ethane in order to find out in what sequence the reactions are occurring. The reaction gas flowed both upward and downward. A hot spot was observed at low furnace temperatures, and it tended to disappear as the furnace temperature increased. This is due to natural extinguishment of the flame caused by the endothermic reforming reactions occurring. The hot spot disappeared at a lower temperature with the up-flow when compared with the down-flow. When the hot spot appears, H2O and CO2 are produced by complete oxidation and accordingly the steam reforming and the CO2 reforming occur competitively in the rear part of the monolith. If the hot spot does not appear, it is considered that the partial oxidation of methane occurs predominantly over the complete oxidation, resulting in more efficient CO2 removal.

Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Effect of composition

Nickel-calcium phosphate/hydroxyapatite catalysts have recently been reported to exhibit high activity and selectivity in partial oxidation of methane (POM). In this work the optimum composition was determined. The optimum mole ratio of Ca/PO4 was around 10/6 and that of Ni/PO4 was in a range from 1.0/6 to 3.0/6 with the optimum Ca/PO4, and the activity could be related with the amount of metallic nickel. In a temperature range from ca. 400 to 700 K, an apparent autothermal reaction was observed to occur in some cases. This is due to the fact that the actual catalyst temperature is higher than the measured temperature, which comes from the exothermic nature of the reaction. The mixing sequence of the precursors during the catalyst preparation does little affect the catalyst activity and characteristics. Deactivation of the catalyst occurred slowly, but the catalyst could easily be regenerated. Moreover, the nickel-calcium phosphate/hydroxyapatite catalyst showed higher activity than the nickel-strontium phosphate catalyst.

Effects of ceria in CO2 reforming of methane over Ni/calcium hydroxyapatite

In CO2 reforming of methane over a calcium hydroxyapatite-supported nickel catalyst, the carbon deposition occurred more severely with increase of the methane partial pressure and at temperatures below about 1,000 K. The effects of ceria that was added as a promoter to the nickel catalyst were investigated. It was observed that the ceria not only enhanced the catalyst stability but also increased the activity, and this is considered owing to the oxygen storage capacity of ceria. The TGA analysis demonstrated that the ceria promoted the removal of the deposited carbon. The optimum Ce/Ni mole ratio was ca. 0.3/2.5. The deposited carbon could easily be removed by oxygen treatment at 1,023 K and the catalytic activity could be restored.

Oxidative coupling of methane over transition-metal-substituted strontium hydroxyapatite

Lead-substituted strontium hydroxyapatite (Sr10-xPbx(OH)2(PO4)6) showed remarkably enhanced catalytic performance for the oxidative coupling of methane (OCM) when compared with the unsubstituted strontium hydroxyapatite. Other substituted transition metals such as zinc, cobalt and nickel were not so effective for improving the catalytic performance for the OCM. The Ni-substituted catalyst exhibited quite different catalytic behavior: CO and hydrogen were the major products instead of the C2 products. The catalyst with the extent of Pb substitution(x) of 0.2 showed the highest C2 selectivity and yield (about 47% and 17% at 1,023 K, respectively) and also exhibited quite stable behavior.

Promotion effects of ceria in partial oxidation of methane over Ni-calcium hydroxyapatite

Effects of ceria added as a promoter to a nickel-calcium hydroxyapatite catalyst, which has recently been reported to exhibit high activity and selectivity in partial oxidation of methane, were investigated. The ceria-promoted catalyst exhibited higher activity and stability than the unpromoted one. This is considered due to the oxygen storage capacity of ceria, which promotes easier removal of the deposited carbon. The optimum content of ceria was determined to lie in the range of the Ce/Ni ratio from 0.1/2.5 to 0.2/2.5.

METHANOL DECOMPOSITION OVER SUPPORTED PALLADIUM AND PLATINUM

Methanol decomposition over supported Pt and Pd catalysts was investigated in the temperature range from 453 to 573 K with the partial pressure of methanol up to 0.8 arm. The specific activity of Pd was higher than that of Pt, and alumina-supported Pd was more active than silicasupported Pd, although aluminasupported catalysts produced dimethyl ether as a by-product. A silica-supported Pd catalyst prepared from a tetraamine salt solution with a proper pH exhibited higher activity than the other silica-supported Pd catalysts. Dependence of the decomposition rate on the partial pressure of methanol was similar regardless of the metal, the support or the preparation method. The apparent reaction orders were near 0.3 at low pressures below about 0.4 atm and became near zero or slightly negative at higher pressures. The apparent activation energies were about the same for most of the catalysts and were in the range from 66 to 77 kJ/g-mol.

Synergism and kinetics in CO oxidation over palladium-rhodium bimetallic catalysts

The activity and kinetics of CO oxidation over alumina-supported Pd-Rh bimetallic catalysts were investigated. One bimetallic catalyst, Pd-Rh(2), was prepared by two-step impregnation and another, Pd-Rh(l), by simultaneous impregnation. Monometallic catalysts as well as a physical mixture of them were also prepared. The catalysts were characterized by selective chemisorption of both H2 and CO, and an attempt was made to determine the surface compositions of the bimetallic catalysts. The bimetallic catalysts showed different kinetic behavior, such as higher turnover frequencies (TOFs), lower apparent activation energies and/or negative reaction orders for CO which were smaller in the absolute value, from that of the monometallic catalysts as well as a physical mixture of them. It is suggested that this Pd-Rh synergism is due to an interaction on the catalyst surface, such that adsorbed CO or oxygen on one metal migrates to the other metal site so that the reaction rate is facilitated and also that the particles of Pd and Rh are located close enough to each other for the interaction to occur. On the surface of Pd-Rh (2) most of the Pd and Rh particles existed as separate entities, while a great portion of the particles on Pd-Rh(l) exhibited the surface enrichment of Pd. This explains the higher TOF and the negative reaction orders for CO over Pd-Rh(2) which were smaller in the absolute value than those over Pd-Rh(l).