(674f) Improved Low-Temperature Catalytic Combustion of Methane over Pd in High-Silica Chabazite Zeolites

Catalytic combustion of CH₄ to CO₂ can reduce the environmental impact of CH_4­ emissions from point sources in the fossil energy sector by more than 96% while generating less NO_­x compared to thermal combustion processes.¹ Pd/Al₂O₃ catalysts can oxidize CH₄ under stoichiometric (𝜆 = 1) conditions, but under CH₄-lean (𝜆 > 1), low temperature conditions, particularly in the presence of H₂O, it cannot achieve sufficient CH₄ conversion and remain stable. Pd-containing high-silica zeolites, such as Pd/SSZ-13 (CHA), outperform Pd/Al₂O₃ for complete CH₄ oxidation in the presence of H₂O and remain hydrothermally stable due to their hydrophobicity and superior durability.²

CHA zeolites (Si/Al = 15-137) were synthesized in both hydroxide (OH) and fluoride (F) media,³ ion-exchanged to 1 wt.% Pd, and then evaluated for CH₄ oxidation activity before and after simulated aging for 1 h at 650 ºC under wet-lean conditions (0.15% CH₄, 5% O₂, 5% H₂O, bal. Ar). A comparison of temperatures required to achieve 50% and 90% CH₄ conversion (i.e., T₅₀, T₉₀) revealed that Pd/CHA (Si/Al > 33) demonstrates improved light-off activity and stability under dry and wet conditions compared to Pd/Al₂O₃. Decreasing T₅₀ and T₉₀ temperatures for Pd/CHA with increasing Si/Al molar ratios and results for Pd/CHA‑OH (137) and Pd/CHA-F (80) of similar Si/Al ratio suggest that more hydrophobic zeolites improve low temperature performance and hydrothermal stability. Steady state CH₄ oxidation rates measured on 1 wt.% Pd/CHA-F-80 at 250 ºC (0.15 % CH₄, 5% O₂, N₂ bal.) were ~3.5 × 10^-7 moles CH₄ (g_cat)^-1 s^-1, or ~2× greater than Pd/Al₂O₃ under similar conditions (Figure 1). Higher CH₄ oxidation rates, higher apparent CH₄ orders (~1), and lower activation barriers for Pd/CHA-F (Si/Al > 33) suggest that the nature of the active Pd is different on these hydrophobic zeolites with sites that can more easily activate CH₄ compared to Pd/Al₂O₃.