| 1. |
- Persson, Katarina, et al.
(author)
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Characterisation and microstructure of Pd and bimetallic Pd-Pt catalysts duirng methane oxidation
- 2007
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 245:2, s. 401-414
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Journal article (peer-reviewed)abstract
- The catalytic oxidation of methane was studied over Pd/Al2O3 and Pd-Pt/Al2O3. It was found that the activity of Pd/Al2O3 gradually decreases with time at temperatures well below that of PdO decomposition. The opposite was observed for Pd-Pt/Al2O3, of which the activity decreases slightly with time. Morphological studies of the two catalysts showed major changes during operation. The palladium particles in Pd/Al2O3 are initially composed of smaller, randomly oriented crystals of both PdO and Pd. In oxidising atmospheres, the crystals become more oxidised and form larger crystals. The activity increase of Pd-PuAl2O3 is probably related to more PdO being formed during operation. The particles in Pd-Pt/Al2O3 are split into two different domains: one with PdO and the other likely consisting of an alloy between Pd and Pt. The alloy is initially rich in palladium, but the composition changes to a more equalmolar Pd-Pt structure during operation. The ejected Pd is oxidised into PdO, which is more active than its metallic phase. The amount of PdO formed depends on the oxidation time and temperature.
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| 2. |
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| 3. |
- Persson, Katarina, et al.
(author)
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Stability of palladium-based catalysts during catalytic combustion of methane: The influence of water
- 2007
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In: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 74:3-4, s. 242-250
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Journal article (peer-reviewed)abstract
- The stability of methane conversion was studied over a Pd/Al2O3 catalyst and bimetallic Pd-Pt/Al2O3 catalysts. The activity of methane combustion over Pd/Al2O3 gradually decreased with time, whereas the methane conversion over bimetallic Pd-Pt catalysts was significantly more stable. The differences in combustion behavior were further investigated by activity tests where additional water vapor was periodically added to the feed stream. From these tests it was concluded that water speeds up the degradation process of the Pd/Al2O3 catalyst, whereas the catalyst containing Pt was not affected to the same extent. DRIFTS studies in a mixture of oxygen and methane revealed that both catalysts produce surface hydroxyls during combustion, although the steady state concentration on the pure Pd catalyst is higher for a fixed temperature and water partial pressure. The structure of the bimetallic catalyst grains with a PdO domain and a Pd-Pt alloy domain may be the reason for the higher stability, as the PdO domain appears to be more affected by the water generated in the combustion reaction than the alloy. Not all fuels that produce water during combustion will have stability issues. It appears that less strong binding in the fuel molecule will compensate for the degradation.
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| 4. |
- Ersson, Anders, et al.
(author)
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A comparison between hexaaluminates and perovskites for catalytic combustion applications
- 2006
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In: Catalysis Today. - : Elsevier BV. - 0920-5861 .- 1873-4308. ; 112:04-jan, s. 157-160
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Journal article (peer-reviewed)abstract
- Hexaaluminates and perovskites are two promising candidates for use in catalytic combustion applications. In the present study two hexaaluminates, LaMnAl11O19 and LaCoAl11O19, were compared with two perovskites, LaMnO3 and LaCoO3, with respect to their thermal stability and catalytic activity for combustion of methane and gasified biomass. The results showed that the hexaaluminates retained a much higher surface area even after calcination at 1200 degrees C compared to the perovskites. LaMnAl11O19 showed the highest catalytic activity of all catalysts. LaCoAl11O19 generally showed low activity. Of the two perovskites, LaCoO3 was the most active, and the initial test run the activity for biomass combustion were close to that one of LaMnAl11O19 even though its surface area was only one tenth of the hexaaluminate's. However, it was severely deactivated in the second test run. Similar deactivation but less severe was also found for the other catalyst.
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| 5. |
- Persson, Katarina, et al.
(author)
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Catalytic combustion of methane over bimetallic Pd-Pt catalysts: The influence of support materials
- 2006
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In: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373. ; 66, s. 175-185
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Journal article (peer-reviewed)abstract
- The effect of support material on the catalytic performance for methane combustion has been studied for bimetallic palladium-platinum catalysts and compared with a monometallic palladium catalyst on alumina. The catalytic activities of the various catalysts were measured in a tubular reactor, in which both the activity and stability of methane conversion were monitored. In addition, all catalysts were analysed by temperature-programmed oxidation and in situ XRD operating at high temperatures in order to study the oxidation/reduction properties.The activity of the monometallic palladium catalyst decreases under steady-state conditions, even at a temperature as low as 470 degrees C. In situ XRD results showed that no decomposition of bulk PdO into metallic palladium occurred at temperatures below 800 degrees C. Hence, the reason for the drop in activity is probably not connected to the bulk PdO decomposition.All Pd-Pt catalysts, independently of the support, have considerably more stable methane conversion than the monometallic palladium catalyst. However. dissimilanties in activity and ability to reoxidise PdO were observed for the various support materials. Pd-Pt supported on Al2O3 was the most active catalyst in the low-temperature region, Pd-Pt supported on ceria-stabilised ZrO2 was the most active between 620 and 800 degrees C, whereas Pd-Pt supported on LaMnAl11O19 was superior for temperatures above 800 degrees C. The ability to reoxidise metallic Pd into PdO was observed to vary between the supports. The alumina sample showed a very slow reoxidation, whereas ceria-stabilised ZrO2 was clearly faster
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| 6. |
- Persson, Katarina, et al.
(author)
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Influence of molar ratio on Pd-Pt catalysts for methane combustion
- 2006
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517. ; 243:1, s. 14-24
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Journal article (peer-reviewed)abstract
- The catalytic oxidation of methane was investigated over six catalysts with different palladium and platinum molar ratios. The catalysts were characterised by TEM, EDS, XPS, PXRD and temperature-programmed oxidation. The results suggest that in the bimetallic catalysts, an alloy between Pd and Pt was formed in close contact with the PdO phase, with an exception for the Pt-rich catalyst, where no PdO was observed. It was found that the molar ratio between palladium and platinum clearly influences both the activity and the stability of methane conversion. By adding small amounts of platinum into the palladium catalyst, improved activity was obtained in comparison with the monometallic palladium catalyst. However, higher amounts of platinum are required for stabilising the methane conversion. The most promising catalysts with respect to both activity and stability were Pd67Pt33 and Pd50Pt50. The platinum-rich catalyst showed very poor activity for methane conversion.
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