(42b) High-Silica Pd/H-LTA Catalysts for Low Temperature CH4 Oxidation

Natural gas has garnered attention as a cleaner alternative fuel for vehicles compared to gasoline or diesel. The main component of natural gas is methane (CH₄) and is 25 times more potent than CO₂ in impact to global warming over a 100-year period. The conventional solution for CH₄ remediation is its catalytic oxidation using Pd/Al₂O₃ catalysts. However, Pd/Al₂O₃ catalysts suffer from low conversions and deactivation through Pd sintering in typical exhausts of natural gas vehicles containing high amounts of water vapor (5 – 10%) at temperatures < 400 °C. A promising support alternative is small-pore zeolites that are known for their hydrothermal stability and tunable hydrophobicity by varying the Si content to prevent water inhibition. Herein, small-pore H-LTA zeolites were synthesized with Si/Al molar ratios of 1 and ∞, loaded with 1 wt.% Pd, and evaluated for low temperature CH₄ oxidation performance (1500 ppm CH_4­, 5% O₂, 5% H₂O, Ar balance). Pd/H-LTA (Si/Al = ∞) outperformed Pd/H-LTA (Si/Al = 1) and Pd/Al₂O₃ (Fig. 1). Pd/H-LTA (1) did not achieve temperatures required for 90% (T₉₀) CH₄ conversion up to 650 ^oC while Pd/H-LTA (∞) converted CH₄ at lower temperatures than Pd/Al₂O₃ with T₉₀’s of 381 and 500 ^oC, respectively. Intermediate Si/Al molar ratios (15 and 34) of Pd/H-LTA are investigated to determine the full extent of the relationship between Si/Al and CH_4­ oxidation performance along with the apparent activation energies, CH₄, O₂, and H₂O orders. Stability testing is performed at <10% CH₄ conversion to determine the ability of the catalysts to maintain the CH₄ oxidation performance and regain activity with the addition and removal of 5% H₂O. Overall, the current trend suggests Pd/H-LTA (∞) is optimum catalyst due to the hydrophobicity of the high-silica H-LTA support.