Confining Au nanoparticles (NPs) in a restricted space
(e.g., zeolite micropores) is a promising way of
overcoming
their inherent thermal instability and susceptibility to aggregation,
which limit catalytic applications. However, such approaches involve
complex, multistep encapsulation processes. Here, we describe a successful
strategy and its guiding principles for confining small (<2 nm)
and monodisperse Au NPs within commercially available beta and MFI
zeolites, which can oxidize CO at 40 °C and show size-selective
catalysis. This protocol involves post-synthetic modification of the
zeolite internal surface with thiol groups, which confines AuCl
x
species inside microporous frameworks during
the activation process whereby Au precursors are converted into Au
nanoparticles. The resulting beta and MFI zeolites contain uniformly
dispersed Au NPs throughout the void space, indicating that the intrinsic
stability of the framework promotes resistance to sintering. By contrast, in situ scanning transmission electron microscopy (STEM)
studies evidenced that Au precursors in bare zeolites migrate from
the matrix to the external surface during activation, thereby forming
large and poorly dispersed agglomerates. Furthermore, the resistance
of confined Au NPs against sintering is likely relevant to the intrinsic
stability of the framework, supported by extended X-ray absorption
fine structure (EXAFS), H2 chemisorption, and CO Fourier
transform infrared (FT-IR) studies. The Au NPs supported on commercial
MFI maintain their uniform dispersity to a large extent after treatment
at 700 °C that sinters Au clusters on mesoporous silicas or beta
zeolites. Low-temperature CO oxidation and size-selective reactions
highlight that most gold NPs are present inside the zeolite matrix
with a diameter smaller than 2 nm. These findings illustrate how confinement
favors small, uniquely stable, and monodisperse NPs, even for metals
such as Au susceptible to cluster growth under conditions often required
for catalytic use. Moreover, this strategy may be readily adapted
to other zeolite frameworks that can be functionalized by thiol groups.