Lars Löwendahl

Cobalt-promoted palladium as a three-way catalyst

Fifteen catalysts were prepared by intermittently impregnating alumina washcoats with water solutions containing La3+, Co2+ and PdCl42- ions/complex and calcining them at 550-820 degrees C. The catalysts were evaluated with respect to light-off performance, at stationary and transient feed gas stoichiometry, respectively, and redox characteristics, using NO/CO/C3H6/O-2/N-2 gas mixtures to simulate car exhaust. Alumina supported Pd exhibited three-way activity, i.e., simultaneous oxidation of CO and C3H6 and reduction of NO in a narrow interval around stoichiometric composition of the feed gas. Compared to Pd alone, addition of La or Co caused a widening of the interval under net reducing conditions. Addition of Co to Pd caused a significant increase in the activities for oxidation of CO and C3H6 under stoichiometric conditions. The conversions of CO and C3H6 started at about 100 degrees lower temperatures over Co-promoted Pd compared to unpromoted Pd. A marked increase in the activity for the reduction of NO at transient conditions was observed over Co-promoted Pd compared to unpromoted Pd. The catalysts were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy combined with energy-dispersive spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements. Only Co2+ could be detected by XPS in the surface layers of the Co-containing sample. A significant part of the cobalt is present in forms which can be oxidized and reduced under synthetic car exhaust conditions. These oxidizable/reducible cobalt sites are predominantly active for oxidation of CO and C3H6, hence promoting the reduction of NO over Pd by initiating these exothermic reactions in the catalyst.

Combinations of platinum and palladium on alumina supports as oxidation catalysts

The effects of the total content of Pd+Pt, the mol-% ratio of Pd to Pt, hydrothermal treatment of the catalyst wash-coat, and the oxygen concentration on the light-off temperature, T50, for the complete oxidation of xylene isomers, propene, and carbon monoxide were studied. The alumina wash-coats were thermally treated at 500°C for 2 h in air or after the thermal treatment, hydrothermslly treated at 814°C for 2 h in 100-% steam. The catalysts were impregnated with 5, 10, and 20 micromol of Pd+Pt per gram of catalyst. The mol-% ratio of Pd to Pt was varied in the range from 0:100 to 100:O. The light-off temperatures had a minimum at a Pd:Pt mol-% ratio of 80:20, and decreased with increasing total metals concentration, increasing oxygen concentration and hydrothermal treatment of the alumina wash-coat.

Characterization and catalytic properties of perovskites with nominal compositions La1-xSrxAl1-2yCuyRuyO3

Nine different metal oxide catalysts were prepared by impregnating alumina washcoats with water solutions containing La3+, Sr2+, Cu2+ and Ru3+ ions and calcining them at 900-degrees-C. The produced samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies combined with energy-dispersive spectroscopy (EDS) analysis, X-ray powder diffraction and specific surface area measurements. A perovskite phase of the nominal composition La1-xSrxAl1-2yCuRuyO3 was found in all samples, in increasing amount in the samples with increasing contents of strontium and ruthenium. The catalysts were evaluated with respect to light-off temperatures and redox characteristics using two gas mixtures, one containing NO/CO/C3H6/O2/N2 and the other NO/CO/N2. The light-off temperatures for nitric oxide reduction decreased from 534 to 333-degrees-C for the catalysts without and with strontium and ruthenium, respectively. In the presence of oxygen the conversion of nitric oxide declined rapidly under oxidative conditions whereas in absence of oxygen this decline was less pronounced and found to be linear over the entire redox interval studied. These studies suggest that the perovskite phase takes an active part in the conversion of nitric oxide and carbon monoxide to nitrogen and carbon dioxide.

Hydrothermal stability of silica as a support for platinum in an oxidation catalyst

Abstract Silica and alumina were subjected to hydrothermal treatment at 750°C for up to 48 h. The sodium content of silica had a strong influence on the stability towards sintering. Silica, containing 0.05 wt.-% sodium, lost less of its original surface area than did alumina during the hydrothermal treatment. Light-off temperatures, and dependence of conversion of carbon monoxide to carbon dioxide over catalysts with platinum on wash-coats of alumina and silica on flow-rate, were determined before and after deactivation. Platinum on sodium-free silica had higher activity than platinum on alumina both before and after deactivation.

Sintering of nickel particles supported on γ-alumina in ammonia

Sintering of nickel on gamma-alumina in hydrogen, ammonia, ammonia + hydrogen and ammonia + nitrogen was investigated isocoric in the temperature range 483 to 523 K. The sintering process was followed by hydrogen chemisorption, specific surface area measurements, X-ray diffraction and transmission electron microscopy. After exposure to ammonia + hydrogen, hydrogen chemisorption revealed a fast decline of the nickel surface area. The results from X-ray diffraction and transmission electron microscopy are consistent with the result from the chemisorption experiments. The BET analysis showed that there are no losses of the total surface area. The sintering rate was significantly slower if the heat treatment was performed in the presence of only one of the gases, ammonia or hydrogen, or in ammonia + nitrogen. This indicates that the sintering in the temperature range 483 to 523 K only occurs with reduced nickel particles in ammonia + hydrogen. The prevailing mechanism for the sintering seems to be sintering by particle migration. The kinetics of the sintering process indicates that the rate constant is correlated to more factors than the temperature.

Conversion of dehydroascorbic acid to a branched hexaric acid in neutral and alkaline aqueous solution

Dehydroascorbic acid is shown to be converted to 2-(threo-1,2,3-trihydroxypropyl)tartronic acid in aqueous alkaline solutions. The structure of the acid was determined by mass spectrometry of its acyclic Me3Si derivative. Mass spectrometric and chromatographic data are compared with those of related compounds. The acid is formed by a benzilic acid rearrangement of the intermediate 2,3-hexodiulosonic acid. The rate of formation at 38°C was studied quantitatively by glc. It increases at increased alkalinity but is significant even at physiological pH. The presence of oxygen does not substantially influence the reaction.

Sol-gel coating of alumina fibre bundles

Abstract A procedure for coating alumina fibre bundles with fibrillar boehmite particles is described. Fibrillar boehmite particles, having a specific surface area of 150 m2/g, were adsorbed onto the surface of the alumina fibres. The surface of the alumina fibres was first charge reversed by adsorption of a charge reversing agent. Polyvinylsulphate (PVS) proved to be an excellent charge reversing agent, whereas citric acid and Tiron (3,5-pyrocatecholdisulphonic acid disodium salt) were unable to reverse the surface charge of the alumina fibres. By using polyvinyl-sulphate (PVS), it was possible to adsorb an arbitrary number of layers of fibrillar boehmite particles. The coating of the alumina fibre bundles resulted in an increase in specific surface area from 0.09 m2/g for the uncoated fibres, to 0.7 m2/g for alumina fibres coated with four layers of fibrillar boehmite particles. The adsorbed boehmite particles were fixed onto the alumina fibre surface by calcination at 550 °C for 120 min, during which the boehmite was transformed into γ-Al2O3. The coated alumina fibre bundles were characterized by krypton adsorption and scanning electron microscopy (SEM).

Characterization of a Pt-Pd combustion catalyst on an alumina washcoat, with and without prior hydrothermal treatment of the washcoat

Two Pt/Pd catalysts on cordierite monoliths were prepared by impregnating two differently treated alumina washcoats with 10 μmol [Pt+Pd] per gram catalyst in the atomic ratio Pt/Pd=4.0. Both washcoats were first thermally treated, calcined, for 2 h at 550 °C in air and one of them was additionally treated, hydrothermally, in 100% steam for 2 h at 814 °C. The hydrothermally treated catalyst was more active for complete oxidation of xylene in air: its light-off temperature was 232 °C, compared to 259 °C for the sample calcined only. To explain this higher activity, both catalysts were characterized by BET surface area, pore-size distribution, hydrogen chemisorption, X-ray diffraction, TEM/STEM/EDS and low-energy ion scattering spectroscopy (LEIS). The catalyst with a hydrothermally treated washcoat had 30% lower surface area, larger alumina crystal size, higher degree of crystallization of alumina and larger average catalyst pore size (11 nm vs. 6 nm), than the one with the washcoat, treated only thermally. The LEIS results indicated a surface enrichment of Pd on both catalysts. The Pt signal in LEIS was higher for the hydrothermally treated sample.

Effect of hydrothermal treatment on alumina as support for noble metal catalysts

Abstract Boehmite was calcined with and without steam at temperatures in the range 550°-100°. Hydrothermal treatments yielded structures with more well-developed crystallinity and pore sizes distributed around larger average pore sizes than did thermal treatments. Pt and Pt+ Rh were deposited on hydrothermally and thermally treated alumina by direct impregnation and ion exchange. The chemisorption of hydrogen on these metals was about 50% lower on hydrothermally treated than on thermally treated supports. Platinum applied by direct impregnation on hydrothermally treated alumina had lower light-off temperatures and were more efficient catalysts for oxidation of carbon monoxide, propene, xylene and propane than on thermally treated alumina.

Carbohydrate stabilization with anthraquinone during alkaline pulping

SummaryTreatment of wood meal with aqueous alkali in the presence of anthraquinone at 80°C leads to a conversion of reducing hexose end groups in the polysaccharides to mannonic and gluconic acid end groups. Xylose end groups are oxidized to xylonic and lyxonic acid groups. This leads to a stabilization of the polysaccharides against endwise alkaline degradation.