Gold Nanoparticles with Enhanced Anti-Sintering Properties and Catalytic

Performance via Multiple-Particle Encapsulation Xiaoqing Yan, Xiaojuan Wang, Jie Fan*

Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University,

Hangzhou, Zhejiang Province 310027, China

jfan@zju.edu.cn

One of the greatest challenges in nanoparticle catalysis is the deactivation of catalysts under realistic process conditions caused by particle sintering.

1 Two mechanisms for metal particle sintering in heterogeneous

catalysts have been identified, Ostwald ripening and particle migration. However, if without a physical or chemical barrier, to suppress these two kinds of sintering pathway in one nanoparticulate catalytic system seems rather difficult. Herein, we report that multiple-particle encapsulation of AuNPs within extra-large spherical cages of ordered mesoporous silica EP-FDU-12 at high-loading amount not only suppresses particle migration, but also atom migration. The mechanisms for controlling the sintering behaviors are well explained based on the unique ordered three-dimensional mesoporous structure and the extra-large spherical cages of the support, EP-FDU-12. Moreover, the improved anti-sintering properties of AuNPs and their inter-particle interaction lead to much better catalytic performance in cyclohexanol selective oxidation.

2

In order to confirm the unusual inverse loading-dependence with particle size, we investigated the thermal

stability of AuNPs loaded within EP-FDU-12 supports with different cage or window sizes. As shown in

Figure 1a, all EP-FDU-12 samples show a similar trend. Importantly, the AuNPs supported EP-FDU-12 with

larger cage sizes (36 nm), show even better anti-sintering properties than nanoparticles encapsulated within

smaller cages at low metal loading. This result differs from previous reports that smaller pore sizes in

mesoporous silica act as better physical barriers to control the rate of particle growth. We believe that the

unique ordered 3-D mesoporous structure and the extra-large spherical cages of EP-FDU-12 are responsible

for the unusual enhanced anti-sintering properties of supported AuNPs at high metal loading. As shown in

Figure 1b, particles-migration is prohibited on the internal surface, because the window size is so narrow that

inhibits the movement of AuNPs from patch to patch; on the other hand, atom-migration through Ostwald

ripening is limited, because each cage is loaded uniformly with AuNPs such that the Au vapor does not have

the driving force to diffuse from one cage to its neighbors. A so-called self-focusing mechanism is proposed

to explain the unusual size evolution of the AuNPs with high loading amount based on inter-particle

influence.2

Catalysis processes benefit from the enhanced thermal ability of AuNPs/EP-FDU-12 and optimal mass diffusion provided by open frameworks of EP-FDU-12. Indeed, in the case of gas-phase cyclohexanol selective oxidation (Figure 1c and d), the 16.7wt%AuNPs/EP-FDU-12 shows the highest specific activity, which is ascribed to the relatively small particle size after the high temperature treatment.

References [1] Cao et al., Phys. Chem. Chem. Phys. 2010, 12, 13499 [2] submitted to JACS

Figure 1. a) The particle size distribution of AuNPs supported on different silica carriers as a function of metal

loadings; b) structural model of EP-FDU-12 and schematic representation of the anti-sintering mechanism of AuNPs,

c) XRD patterns and d) mass specific catalytic activities of the AuNPs supported on EP-FDU-12(36,11).