Improved Low Temperature CH4 Oxidation over Pd/H-LTA with Si/Al > 8
Tala Mon, University at Buffalo
Tala Mon1, Jingzhi Liu2, Viktor J. Cybulskis2, Eleni A. Kyriakidou1*
1Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA
2Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, NY 13244, USA
Natural gas, composed mainly of methane (CH4), is a cleaner fuel than gasoline or diesel as its combustion products are carbon dioxide (CO2) and water (H2O) . In practice, the majority of CH4 emissions from natural gas vehicles (NGVs) occur at the exhaust due to unburnt CH4 leaving the tailpipe . The resulting CH4 emissions have a 25 times greater impact to global warming than CO2 . Oxidation catalysts are used for CH4abatement with Pd/Al2O3 being the most widely used one. However, Pd/Al2O3 catalysts suffer from low CH4 conversion due to Pd sintering in the presence of water (5-10%) . To address this challenge, small-pore zeolites such as LTA with Si/Al > 8 were considered as an alternative support due to their tuned hydrophobicity and hydrothermal stability. Herein, LTA was synthesized with Si/Al molar ratios of 31, 39, 52, 54, and 80 and they were subsequently ion-exchanged with 1 wt.% Pd. The Pd/H-LTA catalysts were evaluated for their CH4 oxidation performance and stability before and after exposure to 650 oC for 1 h in a feed containing 1500 ppm CH4, 5% O2, 5% H2O, and Ar balance. The temperatures required to achieve 90% CH4 conversion (T90) for Pd/H-LTA with Si/Al molar ratios of 31 and 80 before and after aging increased from 419 and 433 oC to 427 and 440 oC, respectively. On the other hand, the T90’s for Pd/H-LTA with Si/Al molar ratios of 39, 52, and 54 decreased after aging from 407, 407, and 403 oC to 384, 372, and 381 oC, respectively. Pd/H-LTA with moderate Si/Al molar ratios (39, 52, 54) outperformed all studied samples towards the CH4oxidation reaction with similar T90’s before aging at 407, 407, and 403 oC, respectively. However, after aging, Pd/H-LTA (52) achieved the lowest T90 at 372 oC compared to Pd/H-LTA (39, 54) that had T90’s of 384 and 381 oC, respectively. For optimum CH4 oxidation performance at low temperatures, Si/Al molar ratio of 52 should be targeted for Pd/H-LTA.
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