(521bz) Co-Cation Promoted CH4 Oxidation over Pd/SSZ-13 Catalysts

Methane (CH₄) is a valuable fuel for power generation and natural gas-powered vehicles, owing to its abundance, and low CO₂ emissions attributed to the low H/C (4) ratio of CH₄. However, CH₄ emissions have exacerbated concerns about its global warming potential, which is 25 times greater than that of CO₂. Thus, there is an immediate need for catalytic materials capable of reducing CH₄ [1]. Although Pd-supported zeolite catalysts have been reported to reduce CH₄ emissions effectively in the presence of water [2], the effect of co-cation incorporation in Pd/zeolites is still under-investigated. Herein, the Na/Al molar ratio (x) of 1 wt.% Pd/Na_x-SSZ-13(Si/Al=15) was varied from 0 to 1.22. The catalysts were pretreated with 20% O₂/Ar (500 ⁰C/20 min) followed by two consecutive CH₄ oxidation cycles under 1500 ppm CH₄, 5% O₂, 5% H₂O/Ar (200-650^oC (5^oC/min)). Pd/Na₁-SSZ-13(15) achieved 90% CH4 conversion (T₉₀) at 420⁰C and outperformed all other Pd/Nax-SSZ-13(15) catalysts and 1 wt.% Pd/Al₂O₃ for CH₄ oxidation (Fig. 1(A)). Increases in the Na/Al molar ratio from 0 to 1 over Pd/Na_x-SSZ-13(15) led to a decrease in Brønsted acidity and an increase in the amount PdO (31.4 to 106.7 μmol/g_cat) (Fig. 1(B)), resulting in improved CH₄ oxidation activity. Increasing the Na/Al ratio >1 (Pd/Na_1.22-SSZ-13(15)) led to a decrease in CH₄ oxidation activity compared to Pd/Na₁-SSZ-13(15) that can be attributed to Na masking PdO sites. Improvements in CH₄ oxidation performance by other co-cations (e.g., M = Sr, Mg) and the co-cation effect on the CH₄ oxidation performance of high silica Pd/SSZ-13(60) will also be reported. This work illustrates the role of co-cations in decreasing the Brønsted acidity and promoting PdO formation over Pd/SSZ-13 catalysts with a goal to improve the CH₄ oxidation performance.