Istvan Halasz

Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O2 at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O2/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O2 ⇋ NO2 equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO2 over H-ZSM5 zeolites. Brønsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.

Catalytic reduction of nitric oxide on PdO—MoO3/γ-Al2O3

Abstract The activity and selectivity of PdO—MoO 3 /γ-Al 2 O 3 catalysts containing 2% Pd and 20% Mo for the reduction of nitric oxide by carbon monoxide, by hydrogen, and by a mixture of CO + H 2 was investigated in the presence of different amounts of oxygen at reaction temperatures from 25 to 550°C. Results are compared with PdO/γ-Al 2 O 3 , MoO 3 /γ-Al 2 O 3 , and a commercial Pt—Rh catalyst. The palladium determines the performance of PdO—MoO 3 /γ-Al 2 O 3 under oxygen free reaction conditions up to 400°C, notwithstanding the high loading of the molybdena. At higher temperatures the molybdena can improve the performance of the catalyst, especially at slightly oxidizing conditions. Neither the activity nor the selectivity of PdO—MoO 3 /γ-Al 2 O 3 resemble those of the monometallic catalysts in the presence of oxygen when CO + H 2 is used for reduction. In the case of the present catalyst in fresh condition, nitric oxide is reduced selectively to nitrogen and nitrous oxide with a conversion > 70% at temperatures from 300 to 550°C under either slightly reducing or slightly oxidizing conditions.

Preparation and characterization of PdO-MoO3/γ-Al2O3 catalysts

Three different procedures have been compared for the preparation of PdO-MoO3/ /gg-Al2O3 catalysts for the purpose of achieving close proximity of the palladium and molybdenum oxides. The methods are: 1. adsorption of [PdCl2(OH)2]2− anions on MoO3/γ-Al2O3; 2. coadsorption of [Pd(NH3)4]2+ cations and MoO42− anions on γ-Al2O3; 3. adsorption of Mo(CO)6 on PdO/γ-Al2O3. Twenty catalysts were prepared and characterized by temperature-programmed reduction and X-ray powder diffraction. In general, the first method resulted in large, weakly bou oxide, whereas the other two methods resulted in a strong interaction between finely divided palladium and molybdenum oxides.

Comparison of oxidation of carbon monoxide on superconducting and insulating Y-Ba-Cu-O catalysts

Investigation of the catalytic activity of Y{sub 2}BaCuO{sub 5} has shown high and stable activity for CO oxidation when the catalyst is slightly reduced, comparable with the activity of the best catalysts for CO oxidation. This oxide was found to be about two orders of magnitude more active than the superconducting perovskite YBa{sub 2}Cu{sub 3}O{sub 7-x}. Furthermore, in contrast to the perovskite catalyst, surface reduction increased the activity of Y{sub 2}BaCuO{sub 5}. Zero-order kinetics were observed for O{sub 2} on both the oxidized and reduced forms of the catalyst, but the reaction order for CO was in the range of 1.1 to 1.5. Differences between the oxidized and reduced forms of the catalyst and between the two types of Y-Ba-Cu-O catalysts are discussed.

Positron annihilation and N2 adsorption for nanopore determination in silica-polymer composites

Silica has two different, fragmented and smooth, visual appearances when deposited onto porous Amberlite polymer substrate from TEOS at acidic and basic pH, respectively. The low temperature N2 adsorption isotherms on both materials seem to be a combination of Type 1 and Type 4 isotherms with H2 hysteresis, indicating the presence of both micro (D < 2 nm diameter) and meso (2 nm < D < 50 nm) pores. Their mesopore distributions, computed by both BJH and DFT methods, show maxima near D∼5 nm with a narrower pore size distribution range than the organic support alone. For comparison, pores were also tested by positron annihilation lifetime spectroscopy (PALS) which can scan the full micro to mezo size range in one measurement without cooling and using any adsorbate molecules. These PALS results indicated the presence of D∼0.5, 0.8 and 1.5 nm diameter micropores and an average D∼5 nm diameter mesopores in both the base and the acid set materials. When the organic substrate is burned out from the two differently made composites, the new adsorption isotherms, BET surface areas, and differently measured pore sizes became distinctly different both from each other and from those of the parent materials.

Determination of the oxygen content in superconducting and related cuprates using temperature-programmed reduction

The oxygen content and reducibilities of four base metal oxides, six nonconducting cuprates, and three superconducting cuprates were investigated by temperature-programmed reduction (TPR) from 30 to 900C. It was shown that the copper, bismuth, and thallium ions of these oxides were reduced to the zero valence state. Thus, TPR is a useful technique for determining the actual oxygen stoichiometry of cuprates. For multiphase Bi{sub 2}CaSr{sub 2}Cu{sub 2}O{sub 8+x} and Tl{sub 2}CaBa{sub 2}Cu{sub 2}O{sub 8+x} superconductors pretreated in air, x was found to be 2 {plus minus} 0.2 and 1 {plus minus} 0.2, respectively. Both values are significantly higher than previously reported. The reduction of superconducting and related cuprates required higher temperatures than CuO, but the reducibility of CuO decreased when it was calcined in air at 940C. The Bi- and Y-based superconducting cuprates exhibited the lowest reducibility among the samples investigated.

Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts

Catalytic activities of five non-conducting and three superconducting cuprates were measured for the decomposition of NO and the reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions of automotive catalysts. In contrast to earlier results, obtained at 20 to 30 times higher partial pressures of NO and 20 to 100 times lower space velocities, none of the catalysts, including five perovskite-like cuprates, showed significant activity for the decomposition of NO at reaction temperatures up to 700 °C. All catalysts were fairly active for the reduction of NO. At temperatures above about 400 °C on the perovskite-like cuprates YBa2Cu3O7-x and Ba2CuO3.5-x, the rates for NO reduction were higher than on CuO. All catalysts lost activity for NO reduction in the presence of oxygen (oxidizing conditions).

Reduction of NO by CO on PdO-MoO3/γ-Al2O3 of low molybdena loading

The catalytic activity for the reduction of NO by CO of five PdO-MoO3/γ-Al2O3 catalysts is compared in the presence of varying amounts of oxygen at reaction temperatures from 25 to 550 °C. The samples were prepared by different methods and contain about 2% of Mo and 2% Pd. Results are compared with the activities and selectivities of PdO/ γ-A12O3 and PdO-MoO3/γ-Al2O3 containing 2% Pd and 2% Pd + 20% Mo, respectively. All catalysts showed appreciable activity at temperatures between 300 and 550 °C and at stoichiometric ratios,R, of the oxidizing to reducing gases of 0.1 <R < 1.1. The activity of three PdO-MoO3/ γ-A12O3 catalysts with low concentrations of Mo and Pd was found to be significantly higher than the activity of PdO/γ-Al2O3 at 1.1 <R < 1.3 and at temperatures between 300 and 500 °C. The improved activity is ascribed to the interaction of the active metals.

Insight into the structure of polymer-silica nano-composites prepared by vapor-phase

Using a new synthesis technique, in which mesoporous Amberlite XAD7HP resin beads swollen with TEOS were exposed to vapors of either (H2O+HCl) or (H2O+NH3), we obtained smooth, porous, mechanically stable silica gel spheres after burning out the sacrificial organic template. Combined N2 sorption, SEM, TEM, (29)Si NMR, and Raman measurements were used to characterize the physical properties and molecular structures of the intermediate and final gels. Our atomically resolved TEM pictures provide the first visual demonstration of the presence of 3 to 6 member siloxane rings predicted by our Raman studies and other indirect methods. It is demonstrated that the physical appearance, morphology and porosity of the acid and base set gels are different from each other and also from those silica gels that were earlier polymerized from TEOS or Na-silicate saturated Amberlite XAD7HP with aqueous NH4OH or HCl solutions in liquid phase. We show that the different physical properties of the vapor-phase set gels are associated with different gelling rates at acidic and basic conditions, which generates molecular differences both in the intermediate and the final products.

Efficient oxyfunctionalization of n-hexane by aqueous H2O2 over a new TS-PQ™ catalyst

The oxyfunctionalization of n-hexane by aqueous H2O2 was found to proceed substantially faster over a titanium-containing pentasil type catalyst denoted as TS-PQ™ (a trademark of PQ corporation for this titanium silicate material), than over other comparable titanium-containing zeolites. This catalyst has an unprecedented >99% efficiency in utilizing H2O2 in the oxidation of a paraffin. The effect of temperature, stirring rate, pH, solvent, and reactant ratios on the reaction rate, H2O2 efficiency, and product selectivity were evaluated over the TS-PQ™ silicate using a statistically designed catalytic test performed in an atmospheric batch reactor. Accurate reaction times were determined by continuously monitoring H2O2 consumption. Hydrated and in situ dehydrated TS-PQ™ titanium silicate samples were studied in a diffuse reflectance (DRIFT) cell by FT-UV and FT-IR spectroscopy. Unlike TS-1 catalysts that mainly contain isolated Ti4+ ions in tetrahedral lattice positions, TS-PQ™ silicate contains tetrahedral and higher coordination Ti4+ ions. Slightly acidic conditions affect favorably the reaction rate and C2 regioselectivity over the TS-PQ™ catalyst. In contrast to TS-1, applying methanol (MOH) as co-solvent had little or negative effect on the reaction rate and the selectivity of TS-PQ™ catalyst for the oxyfunctionalization of n-hexane.