(264h) Improving Low-Temperature CH4 Oxidation Performance with High-Silica Pd/CHA Zeolite Catalysts

Methane (CH₄) is a vital energy carrier and chemical feedstock but has more than 80 times the global warming potential of CO₂ over a 20‑year span [1]. Thus, complete catalytic oxidation of CH₄ to CO₂ is often desirable to reduce the environmental impact of CH_4­ emissions by more than 96% while generating far less NO_­x compared to thermal combustion. Pd/Al₂O₃ catalysts are state-of-the-art for CH₄ oxidation around 500 ºC under CH₄-rich conditions but require high precious metal loadings (> 5%) and cannot maintain sufficient CH₄ conversion (> 90%) at lower temperatures (< 400 ºC) or under lean/ultra‑lean conditions [2]. Additionally, H₂O and sulfur poison Pd sites and metal dispersion is lost through sintering upon regeneration at high-temperatures, resulting in decreased CH₄ oxidation performance and shortened catalyst life. Recently, Pd-containing high-silica zeolites, such as Pd/SSZ-13 (CHA topology), have emerged as promising catalysts for low‑temperature CH₄ oxidation owing to their hydrophobicity and superior hydrothermal durability [3].

High-silica CHA zeolites (Si/Al molar ratios = 15-137) were prepared in both hydroxide (OH) and fluoride (F) media, 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 (T₅₀, T₉₀), revealed that Pd/CHA (Si/Al > 33) outperforms Pd/Al₂O₃ (Figure 1) with steady-state CH₄ oxidation rates on Pd/CHA‑F (80) were ~4× higher than Pd/Al₂O₃ at 250 ºC (0.4 % CH₄, 5% O₂, N₂ bal.). 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 result in improved lower temperature performance and durability in the presence of H₂O.

References

1. Solomon, S., Manning, M., Marquis, M., Qin, D. Cambridge University Press (2007).
2. Raj, A., Matthey Technol. Rev. 2016, 60, 228.
3. Dusselier, M.; Davis, M. E., Rev. 2018, 118, 5265.