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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