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. Kyriakidou^1*

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

^*elenikyr@buffalo.edu

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) [1].  In practice, the majority of CH₄ emissions from natural gas vehicles (NGVs) occur at the exhaust due to unburnt CH₄ leaving the tailpipe [2].  The resulting CH₄ emissions have a 25 times greater impact to global warming than CO₂ [3].  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%) [4].  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.

References:
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[2] N.N. Clark, D.R. Johnson, D.L. McKain, W.S. Wayne, H. Li, J. Rudek, R.A. Mongold, C. Sandoval, A.N. Covington, J.T. Hailer, J Air Waste Manage Assoc, 67 (2017) 1328-1341.
[3] S. Solomon, M. Manning, M. Marquis, D. Qin, Climate change 2007-the physical science basis: Working group I contribution to the fourth assessment report of the IPCC, Cambridge university press 2007.
[4] R. Gholami, M. Alyani, K.J. Smith, Catalysts, 5 (2015) 561-594.

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