Showing papers on "Catalyst support published in 2000"

Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts

Abstract: CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering.

Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition

Abstract: Catalytic steam reforming of condensable vapors (i.e. bio-oils) derived from pyrolysis of biomass is a technically viable process for hydrogen production. In this study the aqueous fraction of bio-oil, generated from fast pyrolysis, was catalytically steam reformed at 825 and 875°C, high space velocity (up to 126,000 h−1) and low residence time (26 ms). Using a fixed-bed micro-reactor interfaced with a molecular beam mass spectrometer (MBMS), a variety of research and commercial nickel-based catalysts were tested. The catalysts were prepared by impregnation of an α-Al2O3 support with nickel and additives. Since the main constraint in reforming bio-oils is catalyst deactivation caused by carbon deposition, two strategies were applied to improve the performance of the catalysts. The first approach aimed at enhancing steam adsorption to facilitate the partial oxidation, i.e. gasification of coke precursors. The second one attempted to slow down the surface reactions leading to the formation of the coke precursors due to cracking, deoxygenation, and dehydration of adsorbed intermediates. Magnesium and lanthanum were used as support modifiers to enhance steam adsorption while cobalt and chromium additives were applied to reduce coke formation reactions. The cobalt-promoted nickel and chromium-promoted nickel supported on MgO-La2O3-α-Al2O3 catalysts showed the best results in the laboratory tests. At the reaction conditions progressive catalyst deactivation was observed leading to a decrease in the yields of hydrogen and carbon dioxide and an increase in carbon monoxide. The loss of activity also resulted in the formation of higher amounts of methane, benzene and other aromatic compounds. Commercial catalysts that were developed for steam reforming of natural gas and crude oil fractions proved to be more efficient for hydrogen production from bio-oil than most of the research catalysts mainly due to the higher water–gas shift activity.

NOx storage-reduction catalyst for automotive exhaust with improved tolerance against sulfur poisoning

Abstract: The main cause of deterioration for the NO x storage-reduction catalyst (NSR catalyst) is sulfur poisoning. On the basis of the thermogravimetric (TG) and FT-IR analyses of aged catalysts, we assumed sulfur poisoning of NSR catalyst to consist of two main factors. One is that sulfur dioxide in the exhaust gas is oxidized on precious metals and reacts with the support, forming aluminum sulfate. Another is that SO x reacts with the NO x storage components such as barium to form barium sulfate. These consequences lead to the concept that sulfur poisoning should be suppressed by the enhancement of sulfur desorption from the support and barium sulfate. Using a mixture of TiO 2 and γ-Al 2 O 3 as the support minimized the amount of SO x deposit on a catalyst after the sulfur poisoning test. As to γ-Al 2 O 3 , sulfur desorbed at a lower temperature from the catalyst with lithium doped γ-Al 2 O 3 than the other alkaline or alkali-earth doped γ-Al 2 O 3 after the sulfur poisoning test. It was effective for enhancing the desorption of sulfur from the aged catalyst. To make a uniform catalytic wash-coat thickness on the substrate, a hexagonal cell monolithic substrate was developed. Hydrogen was the most effective gas for enhancing the reduction of barium sulfate in the aged catalyst, and hydrogen generation on catalyst was enhanced by adding Rh/ZrO 2 with high steam reforming reactivity. The catalyst developed by combining these technologies was subjected to on-vehicle durability testing simulating 50,000 km of driving with 30 ppm sulfur fuel to verify the improvement in the NO x purification performance of NSR catalyst.

Hydrogen production via the direct cracking of methane over Ni/SiO2: catalyst deactivation and regeneration

Abstract: The catalytic cracking of methane over supported nickel catalysts is a potential route to the production of CO-free hydrogen and filamentous carbon. Eventually, however, the catalyst deactivates due to the spatial limitations imposed on the filamentous carbon growth by the reactor. In this study we show that a 15 wt.% Ni/SiO 2 catalyst can be fully regenerated at 923 K with steam for up to 10 successive cracking/regeneration cycles without any significant loss of catalytic activity. XRD analyses indicate no increase in the amount of carbon remaining on the catalyst after successive regenerations and no structural changes in the nickel particles as the catalyst is cycled between cracking and steam regeneration. SEM micrographs are in agreement with the XRD results and show that most of the filamentous carbon is removed during steam regeneration, leaving only small pockets of this material which resist this treatment.

Carbon dioxide reforming of methane to synthesis gas over supported rhodium catalysts: the effect of support

Abstract: The carbon dioxide reforming of methane over the reduced supported Rh (0.5 wt.%) catalysts was investigated. Two kinds of oxides, reducible (CeO2, Nb2O5, Ta2O5, TiO2, and ZrO2) and irreducible (γ-Al2O3, La2O3, MgO, SiO2, and Y2O3) were used as supports. Among the irreducible metal oxides, γ-Al2O3, La2O3 and MgO provided stable catalytic activities during the period of study, and the activity increased in the sequence: La2O3

Mechanistic aspects of the dry reforming of methane over ruthenium catalysts

Abstract: Carbon dioxide reforming of methane has been studied over two ruthenium catalysts supported on silica and on γ-alumina. Catalytic activity measurements, infrared spectroscopic analysis and isotopic tracing experiments applied to the study of the surface hydroxyl groups of the supports have allowed different reaction mechanisms to be proposed on the bases of the detected surface species, their mobility, stability and reactivity. Activation of both reactants takes place on the ruthenium surface for Ru/SiO 2 catalyst. The accumulation of carbon adspecies formed from methane decomposition on the metallic particles finally impedes carbon dioxide dissociation and induces rapid deactivation of this catalyst. The alumina support provides an alternate route for CO 2 activation by producing formate intermediates on its surface that subsequently decompose releasing CO. This bifunctional mechanism, in which the hydroxyl groups of the support play a key role, induces greater stability on the Ru/Al 2 O 3 catalyst by significantly decreasing the rate of carbon deposition on the metal. The proposed reaction pathway requires continuous surface mobility of species from the metal to the support and vice versa.

Dehydrogenation of ethane with carbon dioxide over supported chromium oxide catalysts

Abstract: The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over an unsupported Cr2O3 and several supported Cr2O3 catalysts on metal oxides such as Al2O3, SiO2, TiO2, and ZrO2 was investigated and the effect of support on the catalytic activity was studied. The unsupported Cr2O3 shows medium catalytic activity in this reaction; the support will exert a quite different effect on catalytic behavior. The catalytic activity varies with the nature of supports. Cr2O3/SiO2 catalysts exhibit an excellent performance in this reaction. Cr2O3 loading also affects the catalytic activity; 8 wt.% Cr2O3/SiO2 catalysts can produce 55.5% ethylene yield at 61% ethane conversion at 650°C. Characterization indicates that the distribution of chromium oxide on supports and surface chromium species structure are influenced by the nature of supports. The acidity/basicity and redox property of catalysts determines the catalytic activity in the dehydrogenation of ethane by carbon dioxide.

Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane

Abstract: CO2 reforming of CH4 was studied at 1030 K over Ni/ZrO2 catalysts with different preparations. It was shown that catalytic stability depends greatly on the preparation method of the support precursor. The catalyst prepared by impregnation of ultra-fine Zr(OH)4 (6 nm) particles with nickel nitrate showed high and extremely stable activity for syngas production. In contrast, the nickel catalyst prepared using bigger Zr(OH)4 particles deactivated rapidly. Also, the nickel catalyst made from co-precipitation of ultra-fine nickel–zirconium hydroxide (7 nm) was not resistant to coke deposition during the reaction.

Influence of the microscopic properties of the support on the catalytic activity of Au/ZnO, Au/ZrO2, Au/Fe2O3, Au/Fe2O3–ZnO, Au/Fe2O3–ZrO2 catalysts for the WGS reaction

Abstract: It has been established that the gold catalysts on well crystallized supports, Au/Fe2O3 and Au/ZrO2, display higher catalytic activity in the water gas shift (WGS) reaction in comparison with the samples on amorphous and not well crystallized supports — Au/ZnO, Au/ZrO2, Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2. It could be concluded that the catalytic activity of the gold/metal oxide catalysts depends strongly not only on the dispersion of the gold particles but also on the state and the structure of the supports.

Enhanced surface acidity in mixed alumina–silicas: a low-temperature FTIR study

Abstract: The investigation of the surface acidity in commercially available alumina–silicas through FTIR spectroscopy of adsorbed CO probe molecules has been reported. The modification of γ-alumina with silica led to the creation of both highly acidic Lewis and Bronsted acid sites (BASs); the former through isomorphous substitution of Si 4+ ions by Al 3+ ions at tetrahedral lattice sites; and the latter through formation of bridged hydroxy groups, similar to those found in zeolites. The relative strength and quantity of these sites reached a maximum with 40 wt.% silica content, above which the surface of the samples became silica coated and exhibited acidity approximating that of pure silica. This paper highlights (i) the suitability of CO as a probe molecule for the investigation of surface acidity, capable of differentiating between sites of very similar acid strength and/or coordination, and (ii) the enhancement of surface acidity in alumina through silica addition and subsequent formation of a mixed alumina–silica phase.

Carbon dioxide reforming of methane to synthesis gas over supported cobalt catalysts

Abstract: The CO2 reforming of CH4 over Co supported on an alkaline earth metal oxide (MgO, CaO, SrO, or BaO) as well as on γ-Al2O3 and on SiO2 was investigated. Among these supports, only MgO exhibited a high and stable activity. It provided a CO yield of 93% and a H2 yield of 90% at the high space velocity of 60 000 ml g−1 h−1, which remained unchanged during the period of study of 50 h. γ-Al2O3 provided initially a high CO yield, which, however, rapidly decayed. All the other supports exhibited low CO yields and CaO and SiO2 also low stabilities. A solid solution of CoO and MgO was identified by XRD in the calcined MgO supported catalyst, but not in the other supports. Because the oxygens in the solid solution are shared by both Mg and Co and their interactions with Mg are strong, the solid solution is less reducible than the pure CoO and small clusters of metallic Co are generated. Being at least partially embedded in the substrate, these clusters are more stable to sintering than the usual ones; being small they do not favor coke formation. For these reasons, this catalyst exhibits high stability.

Catalyst structure and method of fischer-tropsch synthesis

Abstract: The present invention includes Fischer-Tropsch catalysts, reactions using Fischer-Tropsch catalysts, methods of making Fischer-Tropsch catalysts, processes of hydrogenating carbon monoxide, and fuels made using these processes. The invention provides the ability to hydrogenate carbon monoxide with low contact times, good conversion rates and low methane selectivities. In a preferred method, the catalyst is made using a metal foam support.

Bonding and reactivity at oxide mineral surfaces from model aqueous complexes

Abstract: The kinetic stability of oxide surfaces affects a broad range of physical phenomena, including mineral dissolution1,2,3 and sorption reactions4, stable-isotope fractionation5, and catalyst support degradation6. Our knowledge of the rates of these processes derives mostly from the rates of net mass transfer between the bulk solid and fluid phases. But from such data it is difficult to determine rates of elementary steps that are needed to test theoretical models. Here we determine the rates of oxygen exchange between an aqueous fluid and specific sites on the ‘Al13’ polyoxocation—AlO4Al12(OH)24(H2O)7+12—the structure of which closely resembles the surfaces of some Al-(hydr)oxide minerals in soils and catalyst supports. Extrapolation of these data to 298 K (and near pH 5.3) yields half-lives for oxygen on the complex that range from ∼0.6 milliseconds for bound water to 41 seconds and 13 hours for the two distinct, but structurally similar, bridging hydroxyls. This surprisingly large range of labilities (∼107) indicates that reactivity is very sensitive to molecular structure. Moreover, these results indicate that well chosen aqueous complexes provide important information to relate bonding to reactivity at mineral surfaces.

NO reduction with NH3 over an activated carbon-supported copper oxide catalysts at low temperatures

Abstract: NO reduction with NH3 over activated carbon-supported copper oxide (CuO/AC) catalyst was studied at low temperatures (30-250 degrees C). The attention was focused on the effects of preparation parameters, reaction temperature and SO2 on the structure and activity of the catalyst. The catalysts show high activities for NO reduction with NH3 in the presence of O-2 at temperatures above 180 degrees C and are gradually deactivated at temperatures below 180 degrees C. Cu2O exists in the catalyst and results in low initial activity but it is easily oxidized into active CuO by O-2 during the NO-NH3-O-2 reaction. Calcination temperature and Cu loading of the catalyst strongly influence the activity and structure of the catalyst. The catalyst with 5 wt% Cu loading and calcined at 250 degrees C shows the highest activity. The activities of the catalysts with higher Cu loadings and/or calcined at higher temperatures are relatively low, mainly derived from aggregation of copper species. The CuO/AC catalyst is greatly deactivated by SO2 due to the formation of CuSO4 which is inactive at low temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.

Characteristics of V2O5 supported on sulfated TiO2 for selective catalytic reduction of NO by NH3

Abstract: V2O5 supported on sulfated TiO2 catalyst was investigated by using Raman and infrared spectroscopies to examine the surface structure of vanadia and the hydroxyl groups of titania along with the sulfate species on the catalyst surface. The surface structure of vanadia plays a critical role, particularly for the reduction of NO by NH3. The polymeric vanadate species on the catalyst surface is the active reaction site for this reaction system. The surface sulfate species enhanced the formation of the polymeric vanadate by reducing the available surface area of the catalyst. The formation of the polymeric vanadate species on the catalyst surface also depends on the number of hydroxyl groups on the support. Both the sulfate and the vanadate species strongly interacted with the hydroxyl groups on titania. The fewer the number of the hydroxyl sites on the catalyst surface became by increasing the calcination temperatures, the more the polymeric vanadate species formed. A model was proposed to elucidate the progressive alteration of the surface structure of vanadia by the amounts of V2O5 loadings and the sulfate species on the catalyst surface.

Zeolite/Alumina catalyst support compositions and method of making the same

Abstract: Zeolite/alumina composite, and a method for making, the composite for use as a catalyst substrate or catalyst carrier and comprising zeolite having a silica/alumina ratio of greater than 300 and gamma alumina having a specific surface area of greater than 100 m2/g. The zeolite/alumina composite exhibits a modulus of rupture of at least 750 psi. Additionally, the invention is also directed at a three catalyst (TWC) system for use in the removal of hydrocarbons, carbon monoxide and oxides of nitrogen from waste gas, the TWC system comprising the following components: (1) a zeolite/alumina composite catalyst support exhibiting a modulus of rupture of at least 750 psi and having a zeolite with a silica/zeolite ratio of at least 300 and the alumina comprising a gamma alumina having a specific surface area of greater than 100 m2/g; and, (2) a noble metal catalyst impregnated on the catalyst support, the noble metal selected from the group consisting of platinum, rhodium, iridium and palladium.

Catalytic synthesis of carbon nanotubes over Co, Fe and Ni containing conventional and sol–gel silica–aluminas

Abstract: An attempt has been made to synthesise multiwalled carbon nanotubes using cobalt, iron and nickel supported on different types of silica–aluminas to investigate the rules governing their nanotube producing activity. Acetylene was used as the source of carbon. Decomposition of acetylene has been carried out at atmospheric pressure. The effect of reaction temperature in the 770–970 K range and the flow rate of the hydrocarbon has been investigated. The catalysts were analysed by XRD, UV–VIS, surface area and porosity measurements. Formation of carbon nanotubes was followed by electron microscopy. The amount of deposited carbon increased with increasing reaction temperature and the flow rate of acetylene, but decreased with increasing concentration of alumina in the catalyst support. Each catalyst showed high production of carbon nanotubes at 970 K; however, they were inactive at 770 K. The yield of tube formation was very low at 870 K. The high-resolution transmission electron microscopic (HREM) analysis showed that the outer diameter of the tubes generated varied from 8 to 40 nm, the tubes were multiwalled, and the number of the layers was between 8 and 30. Sol–gel derived samples were also found to be working catalysts, indicating the existence of an optimal metal particle size.

Redox behaviour of ceria–titania mixed oxides

Abstract: The reducibility of the supports is one of the properties influencing the catalytic behaviour of platinum catalysts in the selective hydrogenation of α,β-unsaturated aldehydes. Platinum on reducible supports has shown higher selectivities in carbonyl hydrogenation compared to Pt/Al2O3 and Pt/SiO2 catalysts. We report here a study on the redox properties of mixtures of titania and ceria, precursors of Pt/TiO2–CeO2 catalysts. Two samples, with atomic compositions Ti/Ce 8/2 and 5/5, were prepared using a sol–gel process. They were treated under hydrogen and air in a wide range of temperatures. Surface compositions and the percentage of reduced ceria were determined by X-ray photoelectron spectroscopy. Temperature programmed reduction (TPR) profiles, and X-ray diffraction patterns were interpreted in terms of the formation and stability of several mixed compounds.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts: 3. Catalyst deactivation

Abstract: Extended catalytic tests show that coke deposition is responsible for the deactivation of activated carbon catalysts in the oxidative dehydrogenation of ethylbenzene (ODE). Temperature-programmed desorption (TPD), DRIFTS, textural and elemental analyses of used catalysts have shown that not only the great majority of the micropores become blocked, but also that the amounts of oxygen and hydrogen in the catalyst composition increase with time on stream, leading to a material increasingly more reactive towards oxidation. It was observed that after a few days on stream, the rate of carbon gasification became larger than the rate of coke deposition, leading to a decrease in catalyst weight. Working under milder conditions (lower temperature and oxygen partial pressure) delayed this effect. The increase in the concentration of surface groups with time does not result in a proportional increase in the activity of the catalysts, because the majority of the groups created are not active for the ODE reaction.

Bimetallic Pt–Sn catalysts supported on activated carbon: I. The effects of support modification and impregnation strategy

Abstract: A commercial activated carbon (AC) was used as a support either after washing with HCl or after further oxidation with air or HNO 3 The supports were characterized by N 2 adsorption, TPD and SEM The results indicated the drastic change in physical and surface chemical properties of activated carbons due to the oxidation treatments Three sets of catalysts were prepared on these supports In each set, monometallic Pt/AC and bimetallic Pt–Sn/AC catalysts were prepared Bimetallic catalysts were prepared either by coimpregnation or by sequential impregnation in which the Sn precursor was introduced first In all catalysts, the Pt loading was kept fixed as 1 wt% Two levels of Sn loading, 025 and 050 wt%, were applied in bimetallic samples The catalysts were characterized by H 2 adsorption, TPR, SEM-EDX, XRD, XPS and tested in a structure insensitive reaction (benzene hydrogenation) The results indicated the pronounced changes in catalytic properties depending on the abundance of surface oxide groups of the supports and Pt–Sn interaction Coimpregnation favored Pt–Sn interaction and alloy formation The type of alloy formed was affected by the surface chemistry of the activated carbon; oxidized samples favored the formation of Pt 3 Sn XPS results indicated the migration of Pt from surface to interior sites which is driven by decomposition of oxygen bearing anchoring sites during the reduction The migration was stabilized up to a point by the introduction of Sn as the second metallic phase and, as a consequence, by the Pt–Sn interaction The interaction between metallic phases and alloy formation led to striking decreases in benzene hydrogenation activity, which indicates the lower amount of active Pt sites available Very high H 2 adsorption capacities of bimetallic samples compared to their benzene hydrogenation activities point out to the H 2 adsorption capacities of the Pt–Sn alloys formed

Properties of Pt/C and Pd/C catalysts prepared by reduction with hydrogen of adsorbed metal chlorides: Influence of pore structure of the support

Abstract: A number of catalysts have been prepared by adsorption of H2PdCl4 and H2PtCl6 on activated carbons of different origin followed by drying and reduction in flowing hydrogen. They were characterised by CO chemisorption and by liquid-phase hydrogenation of cyclohexene, cyclooctene and nitrobenzene. A decisive influence of the pore structure of the support on the catalyst properties has been found. By a proper choice of the preparation conditions, it proved possible to obtain a sufficiently high dispersion of the supported metal on any one of the activated carbons used. However, catalytic activity per mass of metal reached a maximum in the range of intermediate metal dispersion, with the limit being dependent on the support used. This is attributed to deposition of metal particles of high dispersion in narrow pores of the supports, which renders part of the metal surface inaccessible to the organic substrates. Such a blocking effect proved especially significant for Pt catalysts on the activated carbons with the smallest micropores.