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

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


Mon, T. - Presenter, University at Buffalo
Kyriakidou, E., SUNY at Buffalo
Chen, J., University At Buffalo
Liu, C. H., University At Buffalo
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 (H2­O). 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% H2­O 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 (T­50,90) of 379 and 419 oC, respectively over 1 wt.% Pd/Na0.98-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.