A series of Pd catalysts with different nanoparticle sizes encapsulated in Silicalite-1 (S-1) zeolite were synthesized using the one-step hydrothermal method to investigate the size dependence on methane catalytic oxidation. The obtained Pd nanoparticle (NP) sizes ranging from 1.8 to 3.2 nm were modulated by changing the addition of the ethylenediamine (EN) ligand during the synthesis process, and Pd/S-1-in-6EN (2.1 nm, the molar composition of 1 Pd:6 EN) displayed the remarkable catalytic performance. Meanwhile, long-term stability tests indicate the outstanding water resistance of Pd/S-1-in-6EN with a smaller particle size. According to the results of in situ CO-DRIFTS, a volcano-shaped curve between the fraction of the step site by size control of Pd nanoparticles and the catalytic performance was presented, and Pd/S-1-in-6EN possessed the optimum one. These allow the further study of the particle size effect on zeolite-confined noble metal catalysts for methane combustion.
Catalytic complete oxidation is an efficient approach to reducing methane emissions, a significant contributor to global warming. This approach requires active catalysts that are highly resistant to sintering and water vapor. In this work, we demonstrate that Pd nanoparticles confined within silicalite-1 zeolites (Pd@S-1), fabricated using a facile in situ encapsulation strategy, are highly active and stable in catalyzing methane oxidation and are superior to those supported on the S-1 surface due to a confinement effect. The activity of the confined Pd catalysts was further improved by co-confining a suitable amount of Ce within the S-1 zeolite (PdCe0.4@S-1), which is attributed to confinement-reinforced Pd–Ce interactions that promote the formation of oxygen vacancies and highly reactive oxygen species. Furthermore, the introduction of Ce improves the hydrophobicity of the S-1 zeolite and, by forming Pd–Ce mixed oxides, inhibits the transformation of the active PdO phase to inactive Pd(OH)2 species. Overall, the bimetallic PdCe0.4@S-1 catalyst delivers exceptional outstanding activity and durability in complete methane oxidation, even in the presence of water vapor. This study may provide new prospects for the rational design of high-performance and durable Pd catalysts for complete methane oxidation.
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