(349t) Effect of Na in Low Temperature CH4 Oxidation over Pd/H-SSZ-13

Compared to gasoline or diesel, natural gas is a cleaner and less expensive alternative fuel due to its large domestic abundance and relatively low emissions of greenhouse gases.^1,2 Diesel and gasoline vehicles can burn natural gas through drop-in conversions, reducing emissions of existing vehicles. Natural gas is composed of methane (CH₄) which combusts to produce carbon dioxide (CO₂) and water (H_2­O). Unfortunately, incomplete CH₄ combustion leads to 25 times greater impact to global warming than CO₂.³ The conventional solution is through catalytic oxidation of CH₄ using palladium supported on γ-alumina (Pd/γ-Al₂O₃). However, Pd/γ-Al₂O₃ deactivates with CH₄ conversion reaching only 20% when exposed to 5% H_2­O at 500 ^oC.⁴ Zeolites like SSZ-13 are a promising support alternative known for their hydrothermal stability. Pd/H-SSZ-13 catalysts can maintain 80% CH­₄ conversion in the presence of steam at 415 ^oC; however, when exposed to a short aging condition (650 ^oC for 1h), sintering occurs through Pd migrating on Brønsted acid sites. Pd migration may be prevented using Na to convert Brønsted to Lewis acid sites. ⁵ Herein, the Na/Al molar ratio of Pd/H-SSZ-13 with Si/Al of 15 was varied from 0-1.22, followed by evaluation of the CH­₄ conversion before and after aging in 1500 ppm CH­₄, 5% O₂, and 5% H­₂O. The optimum Na/Al molar ratio was determined to be 0.98 with temperatures required for 50% and 90% CH₄ conversion (T­_50,90) of 379 and 419 ^oC, respectively over 1 wt.% Pd/Na_0.98-SSZ-13 (Fig. 1a). O₂-TPD showed an increase in PdO decomposition with the addition of Na indicating the formation of more active PdO (Fig. 1b). NH₃-TPD revealed that Na reduces Brønsted acidity and occupies the ion-exchange site instead of Pd, limiting migration. This work illustrates that Na improves CH₄ oxidation performance through increased formation of active PdO and prevention of sintering by blocking Pd migration.