Framework iron (Fe³⁺) species in silicalite-1 stabilize PdO_x for efficient methane combustion at low temperatures

Supported palladium (Pd) catalysts are known for their exceptional catalytic activity in methane combustion; however, they often undergo rapid deactivation due to sintering under practical reaction conditions. This study aims to develop highly active and stable catalysts for CH₄ combustion by investigating the effect of guest elements within silicalite-1 (S-1) zeolite as supports for Pd. Various transition metals (TMs) were introduced into the S-1 gel to modify its structural properties. The TMs were incorporated into both framework and extra-framework positions, as demonstrated for iron (Fe) and titanium (Ti), resulting in the formation of FeS-1 and TS-1, respectively. The structures were confirmed by Fourier transform infrared (FTIR) and ultraviolet-visible diffused reflectance (UV-vis DR) spectroscopy. Following Pd impregnation, the resulting Pd/FeS-1 and Pd/TS-1 exhibited high activity in CH₄ combustion. Compared to 1Pd/S-1, both 1Pd/FeS-1 and 1Pd/TS-1 achieved excellent low-temperature catalytic performance (T₉₀ < 400 °C), along with high reaction rates, turnover frequency (TOF) and stability. Characterization of the catalysts confirms the stabilizing effects of Fe and Ti on Pd species, primarily attributed to the formation of PdO_x nanoclusters. Fe³⁺ maintained PdO_x in a more oxidized state, facilitated the activation of gas-phase oxygen, and provided anchoring sites for CH₄ activation into CH₃^∗, which combined with activated oxygen species. The reaction follows the Langmuir-Hinshelwood (L-H) mechanism, in which both CH₄ and oxygen are first adsorbed on the catalyst surface. Adsorbed and activated CH₄ species are sequentially oxidized by active oxygen species, leading to the formation of chemisorbed CO species, which are transformed into CO₂ and H₂O as the final products.