| 1. |
- Ahlstrom Silversand, AF, et al.
(författare)
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Combustion of methane over a Pd-Al2O3/SiO2 catalyst, catalyst activity and stability
- 1997
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 153:1-2, s. 157-175
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Tidskriftsartikel (refereegranskat)abstract
- Palladium supported on Si-stabilised alumina has been demonstrated to be an active and durable catalyst for the combustion of methane. Si is more effective in stabilising alumina than La or Ba when the elements are added through an impregnation technique. Multiple stabilisation with combinations of La, Ba and Si does not increase further the stability against sintering. The stability increases logarithmically with the amount of Si added (0.5-8 atomic%). The rate of sintering is not affected by an increase of the water vapour content of the atmosphere from 1 to 20 vol.%. Doping palladium with rhodium or platinum increases the activity of the catalyst for methane combustion. The high-temperature stability of pure Pd is however superior to the stabilities of the Rh- and Pt-doped catalysts. Addition of La or Ce to the Pd-catalyst increases its stability against thermal deactivation but leads to an overall decrease in activity. The activity of the as-prepared catalysts are affected by the Pd-content below a value corresponding to 5% of the monolayer capacity. Thermally deactivated catalysts show a stronger activity dependence of the Pd-content than as-prepared catalysts. The combustion reaction is first order with respect to methane and zero-order with respect to oxygen (>2 vol.% of oxygen). Carbon dioxide has no inhibitory effects on the combustion. The activity of the Pd-catalyst is decreased by a factor of 5 through deactivation at 1473 K for 768 h. The decrease in activity is linearly correlated to the decrease in specific surface area.
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| 2. |
- Ahlstrom Silversand, AF, et al.
(författare)
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Thermally sprayed wire-mesh catalysts for the purification of flue gases from small-scale combustion of bio-fuel - Catalyst preparation and activity studies
- 1997
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 153:1-2, s. 177-201
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Tidskriftsartikel (refereegranskat)abstract
- Catalytically active wire meshes were used to combust different pollutants present in flue gases from bio-fuel combustion, e.g. CO, propylene, terpenes and tar. Alumina was spray-deposited onto wire meshes of Kanthal AF (8-20 Tayler mesh) with a plasma spray equipment. The specific surface area of the ceramic layer was increased through wash coating, in-situ precipitation or sol-treatment. The catalytically active materials consisted of precious metals (Pd/Pt) or metal oxides (V2O5/CuO) and were added to the ceramic matrix through an impregnation technique. The combustion was in most cases severely limited by external mass transfer. The effects of the specific surface area and the loading of catalytically active material were only significant in the reaction controlled domain. In the mass transfer controlled domain the conversion was affected only by the flow conditions and the external surface area of the wire meshes. CO and most hydrocarbons were effectively combusted at temperatures below 773 on a Pd/Pt-catalyst. The catalyst performance was not affected by the presence of water vapour or carbon dioxide or by low oxygen concentrations. Similar combustion experiments were performed with a commercial monolith and it can be concluded that the performance of the catalytically active wire meshes in the mass-transfer controlled domain was superior. Tar-residues, soot particulates and charcoal particles were effectively combusted over a Pd/Pt-doped oxide-catalyst (V2O5/CuO).
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| 3. |
- Andersson, Arne, et al.
(författare)
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Surface Characterization and Reactivity in Ammoxidation Reactions of Vanadium Antimonate Catalysts
- 1994
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 113:1, s. 43-57
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Tidskriftsartikel (refereegranskat)abstract
- Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidation to benzonitrile and acrylonitrile, respectively, and were characterized by X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Activity data for toluene ammoxidation suggest that excess antimony with respect to the stoichiometric amount required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidation, on the other hand, due to a higher reaction temperature with respect to toluene (500°C vs. 370°C), free vanadia on the surface of the catalyst has a negative influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidation, in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidation mechanisms are interpreted to be primarily related to the different reaction temperatures.
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| 4. |
- Huuhtanen, Jane, et al.
(författare)
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Catalytic and Spectroscopic Studies of Vanadium Oxide Supported on Group IVb and Vb Metal Oxides for Oxidation of Toluene
- 1993
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 97:2, s. 197-221
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Tidskriftsartikel (refereegranskat)abstract
- Vanadia was deposited on TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5 supports using impregnation with either an oxalic acid solution of NH4VO3 or a solution of vanadyl acetylacetonate in ethanol. Prepared samples, with a nominal vanadia content in the range 0.5–2 monolayers, were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and were used in toluene oxidation. XRD did not reveal formation of any vanadia phase. XPS spectra showed deposited vanadium to be present mainly as V5+ on all the supports. A plot of the V:( support metal) ratio determined by XPS showed agglomeration of vanadia on TiO2, ZrO2, HfO2, and Nb2O5 to occur at loadings above 0.5 monolayer, while for Ta2O5 the dispersion seemed independent of the loading. Preparations from vanadyl acetylacetonate gave superior dispersion. Raman spectra showed bands from crystalline V2O5 on all the supports except Nb2O5. Raman and infrared bands from dispersed vanadia were present in the spectra of TiO2 (three species), ZrO2 and HfO2 (both one species) supported catalysts. No bands from vanadium-oxygen vibrational modes were seen in the spectra of Nb2O5 supported samples, but a V-OH band was observed, suggesting an amorphous structure. Some evidence was obtained for formation of amorphous VTa9O25 on Ta205. The activity for toluene oxidation increased with vanadia loading for each support, and the activity varied with respect to support at all loadings in the order TiO2 > ZrO2 > Nb2O5 > HfO2 > Ta2O5. The selectivity for formation of benzaldehyde was the highest using TiO2 and Nb2O5 supports, while for benzoic acid TiO2 was the best support.
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| 5. |
- Lif, Johan, 1964, et al.
(författare)
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Sintering of nickel particles supported on gamma-alumina in ammonia
- 2002
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Ingår i: Applied Catalysis A: General. - 1873-3875 .- 0926-860X. ; 228:1-2, s. 145-154
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Sanati, Mehri, et al.
(författare)
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Zirconia-Supported Vanadium Oxide Catalysts for Ammoxidation and Oxidation of Toluene: A Characterization and Activity Study
- 1993
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 106:1, s. 51-72
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Tidskriftsartikel (refereegranskat)abstract
- A series of samples of vanadia supported on monoclinic zirconia were prepared with nominal loadings from a half up to sixteen theoretical vanadia layers. The samples were characterized with X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray analysis, high-resolution electron microscopy, Raman and diffuse reflectance infrared spectroscopy, and were used in the oxidation and the ammoxidation of toluene. At loadings in the monolayer range, Raman and infrared bands from decavanadate-like and dehydrated tetrahedral vanadia species were at ca. 990 and ca. 1025 cm−1, respectively. Raman bands at 821 and 880 cm−1 were present only at the lowest loading and are characteristic of orthovanadate and pyrovanadate species, respectively. X-ray diffraction, Raman and infrared spectroscopic results revealed formation of some crystalline V2O5 and ZrV2O7 at loadings exceeding a theoretical monolayer. In this case, consideration of Raman intensity variations allowed the conclusion that additional non-crystalline vanadia must be present. According to high-resolution electron micrographs, this vanadia consists of an amorphous overlayer, 4–8 atomic layers thick. In toluene oxidation zirconia-supported vanadia compared with crystalline V2O5 was found less selective for benzaldehyde formation. In toluene ammoxidation, on the other hand, vanadia on zirconia was found to possess good activity and selectivity for benzonitrile formation. Amorphous vanadia was the most active structure on zirconia, while the selectivities for nitrile and aldehyde formations were almost independent of the loading for one theoretical layer and above.
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| 7. |
- Agrell, J., et al.
(författare)
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Production of hydrogen by partial oxidation of methanol over Cu/ZnO catalysts prepared by microemulsion technique
- 2001
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Ingår i: Applied Catalysis A. - 0926-860X .- 1873-3875. ; 211:2, s. 239-250
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Tidskriftsartikel (refereegranskat)abstract
- Production of hydrogen by partial oxidation of methanol, using air as oxidant. has been studied over a series of Cu/ZnO catalysts prepared by microemulsion technique. The catalytic activity was compared to that of a reference catalyst prepared by conventional co-precipitation. The BET surface areas of the microemulsion catalysts (30-70 wt.% Cu) ranged from 22 to 36 m(2)/g and were considerably lower than that of the reference (60 m(2)/g). Nevertheless, the microemulsion catalysts were more active in the partial oxidation reaction and exhibited high hydrogen and carbon dioxide selectivities. At a molar O-2/CH3OH ratio of 0.1, hydrogen production was initiated at about 185 degreesC over the microemulsion catalysts. Over the reference, hydrogen production began at 215 degreesC under the same conditions. The catalytic activity was Found to be strongly dependent on the partial pressure of oxygen, which also plays an important role in determining the product distribution. By increasing the O-2/CH3OH ratio, the methanol conversion and carbon dioxide selectivity increase. while production of water occurs at the expense of hydrogen. By TEM and TPR, it was observed that Cu is less well-dispersed in the microemulsion catalysts than in the reference. The higher catalytic activity is not expected considering the lower number of exposed Cu sites, i.e, the turnover frequencies are substantially higher over the microemulsion catalysts. It is possible that, a strong interaction between a small part of CuO and the ZnO lattice is responsible for the higher turnover frequencies of the microemulsion catalysts, or that particular crystallographic Cu planes or surface imperfections are the active sites of the reaction.
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| 8. |
- Agrell, J., et al.
(författare)
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Production of hydrogen by partial oxidation of methanol over ZnO-supported palladium catalysts prepared by microemulsion technique
- 2003
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Ingår i: Applied Catalysis A. - 0926-860X .- 1873-3875. ; 242:2, s. 233-245
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Tidskriftsartikel (refereegranskat)abstract
- Selective production of hydrogen by partial oxidation of methanol, using air as oxidant, was studied over a series of ZnO-supported Pd catalysts. Microemulsion-assisted synthesis and conventional impregnation techniques were used for preparation of catalysts containing Pd particles of different sizes. Catalyst characterisation included BET, XRD and TEM analyses. The influence of Pd particle size on catalytic activity and product distribution was studied by carrying out activity measurements at temperatures between 230 and 300 degreesC using a stoichiometric feed composition. All catalysts performed well with respect to methanol conversion and hydrogen yield. Both methanol conversion and hydrogen selectivity increased with increasing reaction temperature, the latter at the expense of water formation. Oxygen conversion was complete throughout the examined temperature range. These selectivity trends, with a strong dependence of hydrogen and carbon monoxide selectivities on methanol conversion and reaction temperature, support a reaction scheme consisting of consecutive methanol combustion, steam reforming and decomposition. More importantly, a correlation between Pd particle size and carbon monoxide selectivity was found. When the microemulsion catalysts are compared, carbon monoxide formation increases with increasing particle size. This was not observed over the impregnated reference catalysts, which exhibited high carbon monoxide-levels throughout the examined temperature range. Bimetallic PdZn particles were detected in spent catalysts by means of XRD and it is suggested that the catalytic activity is dependent on the formation of PdZn, the catalytic function being different from that of Pd-0.
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| 9. |
- Agrell, J., et al.
(författare)
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Production of hydrogen from methanol over binary Cu/ZnO catalysts - Part I. Catalyst preparation and characterisation
- 2003
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 253:1, s. 201-211
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Tidskriftsartikel (refereegranskat)abstract
- Mixed copper-zinc oxide catalysts (Cu/ZnO) were prepared by two different techniques, i.e. from hydroxycarbonate precursors formed in aqueous solution and from oxalate precursors formed in water-in-oil microemulsion. Their physicochemical properties were characterised by nitrogen adsorption-desorption, N2O chemisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and oxidation (TPO). The BET surface areas ranged from 34 to 87 m(2)/g, depending on the method of preparation. Cu surface areas between 6.6 and 22 m(2)/g were measured. It was a general observation that catalysts prepared by microemulsion technique had lower Cu dispersions than expected (3.4-5.7%), due to a proposed partial embedding of Cu in ZnO. The catalyst prepared by carbonate co-precipitation exhibited a significantly higher Cu dispersion (10.3%). In addition, this catalyst displayed better resistance to successive TPR/TPO than the microemulsion catalysts, which exhibited significant Cu crystallite growth. However, the microemulsion route provided well-mixed materials with a narrow particle size distribution and the possibility to obtain high BET surface areas (up to 87 m(2)/g) by controlling the water/surfactant ratio in the microemulsion. XPS measurements revealed the existence of Cu+ species on the surface of both types of catalysts after exposure to a O-2/CH3OH mixture. The surface composition of the hydroxycarbonate-derived sample was unaffected by reduction in hydrogen and exposure to O-2/CH3OH, while Zn-enrichment on the surface was observed in the microemulsion catalysts after reduction, indicating sintering of the Cu particles. These observations were consistent with the TPR/TPO measurements.
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| 10. |
- Agrell, J., et al.
(författare)
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Production of hydrogen from methanol over binary Cu/ZnO catalysts - Part II. Catalytic activity and reaction pathways
- 2003
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 253:1, s. 213-223
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Tidskriftsartikel (refereegranskat)abstract
- The activity for conversion of methanol into hydrogen was investigated over binary Cu/ZnO catalysts derived from precursors prepared by two different techniques, viz. oxalates formed in microemulsion and hydroxycarbonates formed in aqueous solution. Some distinct differences in the reaction pathways were observed. During partial oxidation of methanol under a sub-stoichiometric oxygen/methanol ratio, the microemulsion materials exhibited considerably higher combustion activity in the low-temperature region than a catalyst prepared in aqueous solution. Over the former, oxygen was quickly converted by methanol combustion, after which steam reforming was initiated, producing hydrogen at the expense of water and gradually decreasing the net heat of reaction. Hence, a reaction sequence for the partial oxidation reaction over microemulsion catalysts is proposed. consisting of consecutive methanol combustion and steam reforming, followed by decomposition when all oxygen has been consumed. Over the hydroxycarbonate catalyst, the reaction ignited at a higher temperature, directly producing hydrogen by partial oxidation of methanol. When the two types of catalysts were evaluated in the steam reforming reaction, all catalysts displayed the typical S-shaped dependence of methanol conversion on temperature. However, there was a downward shift in the temperature at which methanol reached complete conversion, favouring the hydroxycarbonate, material. Hydrogen was produced selectively over all catalysts, but carbon monoxide formation was more pronounced over the microemulsion materials. The differences in catalytic behaviour are discussed in terms of catalyst morphology and the valence state of Cu in the working catalyst.
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