Confining Gold Nanoparticles in Preformed Zeolites by Post-Synthetic Modification Enhances Stability and Catalytic Reactivity and Selectivity

Abstract

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.

Scheme 1. Comparative Thermal Stabilities of Au Nanoparticles Supported on Surface-Modified (Top) and Unmodified (Bottom) Commercially Available Beta Zeolites

Figure 1

XRD patterns (a), solid-state ¹³C{¹H} CP MAS NMR spectra (b), TGA profiles (c), and Ar isotherms (d) of f-beta and u-beta samples.

Figure 2

XRD patterns of Au/u-beta (a) and Au/f-beta (b) samples taken after each successive step in the Au loading process: O₂ calcination (at 350 °C for Au/u-beta, at 550 °C for Au/f-beta) and H₂ reduction at 350 °C. In each case, the XRD pattern of u-beta is displayed in black for comparison.

Figure 3

Representative STEM images of Au/u-lab-beta (top row) and Au/f-lab-beta (bottom row) samples after AuCl₄^– impregnation (a, d), O₂ calcination at 350 °C (b, e), O₂ calcination at 550 °C (f), and H₂ reduction at 350 °C (c, g).

Figure 4

XRD patterns of Au/f-MFI (blue) and Au/u-MFI (red); the XRD pattern of the commercial MFI is shown in black for comparison.

Figure 5

XRD patterns of calcined Au/f-beta (a) and Au/f-MFI (b) after heat treatment at 500, 600, and 700 °C. Both samples contained 0.48 wt % Au.

Figure 6

Au mass-specific activities (mol_CO mol_Au^–1 min^–1) calculated from experiments at 10 min in CO oxidation reaction under humid conditions: (a) 1.3 Au/f-beta, 0.48 Au/f-beta, 0.28 Au/f-beta, and 1.1 Au/u-beta and (b) 0.48 Au/f-MFI, 0.28 Au/f-MFI, 0.12 Au/f-MFI, and 0.48 Au/u-MFI. Reaction conditions: weight hour space velocity (WHSV) = 0.0619 h^–1, temperature = 40 °C, CO/H₂O/O₂/Ar = 0.5:5:10:84.5.