Effective Regulation of the Au Spatial Position in a Hierarchically Structured Au/HTS-1 Catalyst: To Boost the Catalytic Performance of Propene Epoxidation with H2 and O2

Abstract

The traditional deposition–precipitation (DP) method is common but not universal in manipulating Au nanoparticles’ spatial locations in propene epoxidation catalysts, especially for hierarchical hydrophilic TS-1 supports. Directional loading of Au nanoparticles to hierarchical hydrophilic TS-1 pores is challenging because the strong hydrophilicity of the hierarchical TS-1 zeolite would make Au nanoparticles loaded by the DP method tend to be deposited on the hydrophilic support surface, thus blocking the pore mouths and leading to deactivation. Therefore, manipulating Au nanoparticles’ spatial locations on hierarchical hydrophilic TS-1 supports is of great significance to enhance the catalytic performance but is a bottleneck problem. In this work, taking hierarchical hydrophilic HTS-1 supports as an example, we propose a new modified isometric impregnation (NIMG) method to manipulate the spatial location of Au nanoparticles. Moreover, the Au spatial location inside HTS-1 pores is quantitatively reflected by introducing the Vna parameter. Combined with the N2 physisorption, high-angle annular dark-field scanning transmission electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy, it is found that the Au nanoparticles loaded by the NIMG method tend to be uniformly distributed inside the pores of HTS-1 due to the capillary effect. As expected, the Au/HTS-1(NIMG) catalyst (with the Au nanoparticles inside the pores) exhibits a much higher propylene oxide (PO) formation rate, PO selectivity, and H2 efficiency than the Au/HTS-1(DP) catalyst (with the Au nanoparticles on the outer surface). This is possibly because loading Au nanoparticles inside the pores is conducive to the transfer of H2O2 from Au to the nearby abundant Ti active sites, thus weakening the decomposition of H2O2 to H2O, avoiding the ring-opening side reactions, and promoting propene epoxidation into PO. This work sheds new light for controlling the spatial location of Au in hierarchically structured hydrophilic catalysts for direct propene epoxidation.