Pt-Ba/alumina NOx storage-reduction catalysts: Effect of Ba-loading on build-up, stability and reactivity of Ba-containing phases

A series of Pt-Ba/Al₂O₃ catalysts with Ba-loadings in the range 4.5–28 wt.% has been prepared by wet impregnation of Pt/Al₂O₃ with barium acetate (Ba(Ac)₂) as Ba precursor. The build-up and thermal stability of the deposited Ba-containing species was followed by means of XRD and thermogravimetry (TG) combined with mass spectroscopy (MS). Samples were characterized before and after thermal treatment (calcination). The study showed that the thermal stability of the Ba-containing phases depends on their interaction with the alumina support and the presence of dispersed platinum. In calcined catalysts, three different Ba-containing species could be distinguished based on their crystallinity and thermal stability. The relative concentration of these species varied with the Ba-loading. The first layer of Ba-containing species, corresponding to saturation of the alumina surface with Ba(Ac)₂, contained up to 12.5 wt.% of Ba in the form of amorphous BaO. Increasing the Ba-loading further resulted in 5–6 wt.% of Ba in the form of amorphous carbonates with relatively low thermal stability (LT-BaCO₃). At Ba-loadings higher than about 16 wt.%, crystalline barium carbonate became discernible which exhibited remarkably higher thermal stability (HT-BaCO₃). NO_x storage tests accomplished by exposing the catalysts to pulses of NO in oxygen containing carrier gas at 300 °C indicated that from all characterized Ba-containing phases, LT-BaCO₃ possesses the highest reactivity for NO_x storage, i.e. LT-BaCO₃ is transformed most rapidly to Ba(NO₃)₂.     

Continuous oxidation of benzyl alcohol in “supercritical” carbon dioxide

Supercritical (dense) carbon dioxide has been applied as solvent for the partial oxidation of benzyl alcohol with molecular oxygen in a fixed-bed reactor. High rate and good selectivity to benzaldehyde (93–97%) has been achieved with 0.5 wt% Pd/alumina or 0.5 wt% Pd/C, at around 100C and 100 bar, using only moderate excess of oxygen. The by-product benzoic acid has an autocatalytic effect on the hydration of benzaldehyde, and the subsequent oxidative dehydrogenation leads to benzoic acid, and benzyl benzoate by esterification. Promotion of Pd by Pb improves the selectivity. No catalyst deactivation or metal leaching has been observed. The method provides reasonable yields at much lower temperature than that applied in conventional gas phase oxidation, showing a potential for the synthesis of thermolabile, water-insoluble aromatic aldehydes.     

Continuous Semihydrogenation of a Propargylic Alcohol over Amorphous Pd81Si19 in Dense Carbon Dioxide: Effect of Modifiers

A novel catalyst, Ni/Ce–ZrO₂, exhibits very high catalytic activity and stability even in the stoichiometric steam reforming of methane (H₂O/CH₄ = 1). Furthermore, when it was employed in oxy-steam reforming, it gave enhanced CH₄ conversion (99.1%) at 750°C and the activity was maintained for 100 h. The high catalyst stability is mainly ascribed to the synergistic effect of the Ce modifier resulting from high capacity to store oxygen and high ability to produce mobile oxygen.     

Enantioselective hydrogenation of trifluoroacetophenone over cinchonidine-modified platinum

The enantioselective hydrogenation of 2,2,2-trifluoroacetophenone to (R)-1-phenyl-2,2,2-trifluoroethanol represents a new application of supported Pt catalysts modified with cinchona alkaloids. Heat treatment of a 5 wt% Pt/alumina catalyst at 400°C in flowing hydrogen, followed by an air-treatment of the reaction mixture before the hydrogenation reaction improved the enantiomeric excess by a factor of 2.5. Traces of water in apolar medium, high trifluoroacetophenone and moderate cinchonidine concentrations were also advantageous for enantiodifferentiation. Both reaction rate and enantiomeric excess increased during the initial transient period of the reaction, below 10% conversion.     

Influence of Pt location on BaCO3 or Al2O3 during NOx storage reduction

Catalysts for NO_x storage–reduction (NSR) were made selectively with Pt on either the Al- or the Ba-components without altering significantly the Al₂O₃ or BaCO₃ crystal sizes, Al/Ba weight ratio, specific surface area, porosity, and Pt dispersion using a two-nozzle flame spray pyrolysis (FSP) unit. The NO_x storage performance at 300 °C was best for Pt located near Al₂O₃ as it facilitates the oxidation of NO to NO₂ during the fuel lean period but the reduction rate during the subsequent short fuel rich period was much slower resulting in incomplete regeneration. This contributed to a gradual decrease of the NO_x conversion at increasing cycling. In contrast, Pt on BaCO₃ resulted in an initially lower NO_x storage rate but during ten storage–reduction cycles a stable NO_x conversion of about 50% was reached. When using NO₂ instead of NO or higher NO_x oxidation-reduction temperatures (e.g. 350 °C) the Pt location did not affect the NSR performance of the Pt/Ba/Al₂O₃ catalysts.     

Supercritical Fluids; Opportunities in Heterogeneous Catalysis

The use of supercritical fluids (SCFs) in technology and research has been triggered by several unique features of these media. SCFs allow improvement of technical processes such as extraction, separation, particle formation and chemical reactions. However, it has been just in the last decades that SCFs have gained their importance in science and industrial applications, due to the relatively high pressures usually required when working with these media and the rigorous demands on the equipment used. Opportunities provided by the use of supercritical media in chemical reaction design have been the focus of several reviews(1-17). Here, we concentrate on a discussion of opportunities that SCFs provide in heterogeneous catalysis. First, we will consider the intrinsic properties of SCFs, then discuss the effect of these properties on chemical reactions, especially focusing on heterogeneously catalyzed reactions. Finally, we illustrate some of the opportunities of SCF applications in heterogeneously catalyzed reactions, using pertinent case studies.     

Localization and map building using laser range sensors in outdoor applications

This paper presents the design of a high accuracy outdoor navigation system based on standard dead reckoning sensors and laser range and bearing information. The data validation problem is addressed using laser intensity information. The beacon design aspect and location of landmarks are also discussed in relation to desired accuracy and required area of operation. The results are important for simultaneous localization and map building applications (SLAM), since the feature extraction and validation are resolved at the sensor level using laser intensity. This facilitates the use of additional natural landmarks to improve the accuracy of the localization algorithm. The modelling aspects to implement SLAM with beacons and natural features are also presented. These results are of fundamental importance because the implementation of the algorithm does not require the surveying of beacons. Furthermore we demonstrate that by using natural landmarks accurate localization can be achieved by only requiring the initial estimate of the position of the vehicle. The algorithms are validated in outdoor environments using a standard utility car retrofitted with the navigation sensors and a 1 cm precision Kinematic GPS used as ground truth. © 2000 John Wiley & Sons, Inc.     

Vanadia–silica aerogels. Structure and catalytic properties in selective reductionof NO by NH3

Vanadia-silica aerogels, containing 10 to 30 wt% V₂O₅, and a xerogel were prepared from vanadium(V) oxide triisopropoxide and vanadium (III) acetylacetonate (V(III)_acac) precursors using the solution-sol-gel method and different drying processes, including conventional evaporative and high-temperature and low-temperature supercritical drying. The behavior of these mixed oxides in the selective catalytic reduction of NO by NH₃ was tested and compared to that of other vanadia-silica and vanadia-titania catalysts. The structural and catalytic properties of the sol-gel derived vanadia-silica mixed oxides were found to be mainly influenced by the drying method, the vanadia content and the vanadia precursor used. For a particular vanadia content (10 wt%), low-temperature supercritical drying and evaporative drying resulted in significantly higher vanadia dispersion than high-temperature supercritical drying, which led to crystalline V₂O₅. Turnover frequencies for SCR at temperatures T < 475K were highest for low-temperature aerogels containing well-dispersed vanadium oxide species. Exposing these catalysts to higher temperatures under SCR conditions resulted in agglomeration/redispersion phenomena and at temperatures T > 550K best catalytic behavior was observed with vanadia-silica mixed oxides for which Raman spectroscopy indicated the presence of crystalline V₂O₅, as was the case for aerogels obtained by high-temperature supercritical drying and the low-temperature aerogel with the highest vanadia content (30 wt%).