Zeolite supported Pd for complete methane oxidation: the effect of silica to alumina ratio
Ida Friberg, Competence Centre for Catalysis, Chalmers University of Technology
Ida Friberga, Nadezda Sadokhinaa, Louise Olssona*
a Competence Centre for Catalysis, Chalmers University of Technology, Gothenburg, 412 96, Sweden *louise.olsson@chalmers.se
Catalysts for complete CH4 oxidation serve as important tools to decrease emissions of unburnt CH4 residuals in the exhaust gases from combustion of for instance natural gas and biogas. Palladium based catalysts show the highest activity for CH4 oxidation and PdO, or the combination PdO-Pd, has been identified as the active catalytic phase [1]. The Pd is typically supported on metal oxides with high surface area, such as γ-Al2O3. The major drawback for Pd/Al2O3 is sever deactivation in the presence of water vapor and SO2. However, Pd supported on zeolites with high Si/Al ratio has been reported to have high tolerance to water vapor, due to high hydrophobicity and the formation of active PdO agglomerates [2]. In the present study we have fundamentally investigated the effect of Si/Al ratio on CH4 oxidation for zeolite supported Pd catalysts in the presence of vapor and SO2 by using various characterization techniques in combination with activity measurements under different gas and temperature conditions.
Samples with Pd supported on three different support materials (H-beta zeolite with high SAR, H-beta with low SAR and γ-Al2O3) were prepared. Dealumination of commercial zeolite beta was performed at elevated temperature in oxalic acid solution. The SAR (molar ratio Si/Al2) was determined with ICP to 556 and 38 for the zeolites. Furthermore, the incipient wetness impregnation method was used to add 1 wt.% Pd to all the support materials. Cordierite monoliths with 300 mg washcoat were prepared for each sample and were subsequently tested in flow reactor.
During exposure to wet reaction mixture (0.05 vol.% CH4, 8 vol.% O2 and 5 vol.% H2O) at 450˚C, the CH4 conversion over Pd/Al2O3 dropped linearly from 100% to 72% during the first 3 hours. In the same time interval, the CH4 conversion over Pd/Beta38 dramatically decreased to only 14%. In contrast to Pd/Beta38 and Pd/Al2O3, the catalytic activity of Pd/Beta556 remained remarkably stable at 93% conversion. This result clearly indicates that the SAR of the zeolite support has a major impact on the water deactivation of the catalyst. The Pd supported on highly siliceous zeolites provides both significantly higher and more stable CH4 oxidation activity in the presence of water vapor. We suggest that this a result of the high hydrophobicity of highly siliceous zeolites [2]. Furthermore, the resistance to sulfur was tested by addition of 10 ppm SO2 to the reaction feed. The CH4 oxidation activity dropped rapidly for all zeolite samples whereas the deactivation progressed more slowly for Pd/Al2O3. However, it was observed during the subsequent regenration steps that a large part of the catalytic activity could be recovered more easily for the zeolite samples compared to the Pd/Al2O3. We suggest that this is because of higher sulfur adsorption on the Al2O3 support than on the zeolite supports. The Al2O3 will therefore act as a sulfur sink which prevents formation of PdSOx species and results in slow deactivation [3]. However, the low SO2 adsorption on the zeolite supports resutls in low sulfur spill-over from the support to the Pd during the regeneration and the activity can therefore be recovered more easily. However, it should be noted the Pd/Al2O3 sample could almost be completely regenrated which was not possible for the zeolite samples.
In conclusion, we have shown that zeolites are promising substitutions to the more traditionally used Al2O3 support for Pd based catalyst aimed for complete CH4 oxidation. Pd supported on highly siliceous zeolites provides high and stable activity in the presence of water vapor. Furthermore, it was also observed that zeolite supported Pd is sensitive to SO2 but a large part of the activity can be regenrated more easily compared to Pd/Al2O3.
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[2] K. Okumura, E. Shinohara, M. Niwa, Catal. Today 117 (2006) 577-583.
[3] J.K. Lampert, M.S. Kazi, R.J. Farrauto, Appl. Catal. B: Environ. 14 (1997) 211-223.