J.G. van Ommen

Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g?1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g?1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20?50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism.

Stabilized tetragonal zirconium oxide as a support for catalysts: evolution of the texture and structure on calcination in static air

Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZrO2 to form solid solutions. The samples were prepared by controlled coprecipitation from aqueous solutions of the respective metal chlorides at room temperature and at a constant pH of 10, followed by calcination at 500°C (in the case of the Y3+ -doped sample) or 600°C (in the case of the La3+ -doped sample) to effectuate the crystallization into the tetragonal phase. The process of crystallization of the hydrous zirconia precursor was found to be retarded by the incorporation of Y3+ or La3+, the latter giving the greater effect. Upon crystallization, stabilized tetragonal samples were obtained with high specific surface areas (SBET ca. 88 m2 g?1 for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y3+ - and the La3+ -alloyed ZrO2 samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 900°C). The thermal stability of the texture of zirconia was found to be considerably improved, in comparison with the undoped monoclinic material, by the stabilization of the crystal structure in the defect tetragonal form. In particular, incorporation of 5.4 mol-% of La3+ resulted in a support material which had a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processes of crystallite growth and the accompanying intercrystallite sintering and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion.

Selective gas phase oxidation of toluene by vanadium oxide/TiO2 catalysts

Several supported metal oxide catalysts were studied qualitatively in the selective gas phase oxidation of toluene. Of these catalysts the combination of vanadium oxide and TiO2 was most effective and was therefore studied more thoroughly. Two types of TiO2 were used as support. For catalysts supported on Degussa TiO2 the catalytic properties improved with increasing vanadium content up to monolayer coverage becoming constant at higher loadings, indicating that the vanadium oxide present as multilayers and/or crystallites does not affect the catalytic reaction. The activity and selectivity of catalysts supported on Tioxide TiO2 were much lower and increased continuously with increasing vanadium content (up to 4 x monolayer coverage). This different catalytic behaviour is attributed to the presence of impurities (phosphorus and potassium) in the Tioxide material, which had a large, negative effect on the catalytic properties.

Preparation of supported vanadium and molybdenum oxide catalysts using metal acetylacetonate complexes

Supported vanadium and molybdenum oxide catalysts were prepared by reaction of the corresponding acetylacetonate complex in a non-aqueous solution with the surface hydroxyl groups of the carrier. Continuous or batch adsorption of the metal acetylacetonate from toluene, as well as wet impregnation from ethanol, resulted in a uniform coverage of the support. The applied metal oxide was probably present on the surface as a monomolecular dispersion. When readsorption or reimpregnation from toluene was carried out with TiO2 as support, metal oxide crystallites were formed, which could readily be detected with laser Raman spectroscopy. Reimpregnation from ethanol led to a complete occupancy of the surface hydroxyl groups of the carrier without the formation of metal oxide multilayers or crystallites.

Oxidative coupling of methane over lithium doped magnesium oxide catalysts

Abstract Active sites are created on the surface of a Li/MgO catalyst used for the selective oxidation of methane, by the gradual loss of CO 2 from surface lithium carbonate species in the presence of oxygen. The sites created are not stable but disappear either as a result of reaction with SiO 2 to form Li 2 SiO 3 or by the formation and subsequent loss of the volatile compound LiOH. The deactivation can be reversed, at least partially, by treating the catalyst in CO 2 under reaction conditions; it can be retarded if low concentrations of CO 2 are added to the reaction mixture.

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on ?-Al2O3, TiO2 and ZrO2 are active catalysts for the CO2 reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/?-A12O3 < Pt/TiO2 < Pt/ZrO2. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO2 and Pt/ZrO2, deactivation started immediately after the start of the reaction, while the Pt/?-A12O3 catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.

Development of monolith with a carbon-nanofiber-washcoat as a structured catalyst support in liquid phase

Washcoats with improved mass transfer properties are necessary to circumvent concentration gradients in case of fast reactions in liquid phase, e.g. nitrate hydrogenation. A highly porous, high surface area (180 m2/g) and thin washcoat of carbon fibers, was produced on a monolith support by methane decomposition over small nickel particles. Carbon fibers form a homogeneous layer less then 1 ?m thin, covering the surface of the channels in the monolith. The fibers penetrated into the cordierite, which is suggested to cause a remarkable stability of the fibers against ultrasound maltreatment. The texture of the fibers is independent of both the thickness of the ?-alumina washcoat as well as the time to grow carbon fibers.

Kinetic and mechanistic aspects of the oxidative coupling of methane over a Li/MgO catalyst

The rate of reaction of methane with oxygen in the presence of a Li-doped MgO catalyst has been studied as a function of the partial pressures of CH4, O2 and CO2 in a well-mixed reaction system which is practically gradientless with respect to gas-phase concentrations. It is concluded that the rate determining step involves reaction of methane adsorbed on the catalyst surface with a di-atomic oxygen species. The adsorption of oxygen is relatively weak. Carbon dioxide acts as a poison for the reaction of methane with oxygen, this probably being caused by competitive adsorption on the sites where oxygen (and possibly also methane) adsorbs.

The synthesis of higher alcohols using modified Cu/ZnO/Al2O3 catalys

This paper gives a review of research work in the synthesis of higher alcohols over catalysts based on Cu/ZnO/Al2O3, emphasizing three main topics: (i) the effect on selectivity of the addition of several compounds to this catalyst, (ii) the effect on selectivity of the reaction conditions used, and (iii) the reaction network leading to the different products found. Although the use of alkali compounds has been studied most extensively, other compounds, for example those containing manganese, also appear to be promising additives. The process conditions are rather critical and this may cause some practical difficulties in industrial plants. An extended aldol condensation mechanism is proposed which can explain the product distribution found.

Selective oxidation of methane to ethane and ethylene over various oxide catalysts

Preliminary results are reported for the oxidative coupling of methane to give ethane/ethylene mixtures over a series of different catalyst formulations; the temperature range studied is 650–850°C. A comparison is made of the behaviour of lead/alumina and lithium/magnesia materials. It is found that the former samples give ethane and ethylene plus a predominance of CO2 whereas the latter give ethane and ethylene plus a mixture of CO, CO2 and H2; at higher temperatures, the lead materials give also H2 and CO. The lithium-containing materials are much more stable than the lead-containing ones; the latter lose lead, probably by volatilisation of the metal. A number of other oxide materials have also been examined and have been found to be less effective, having lower activities and selectivities than the lead- and lithium-containing systems.