Size-selective Catalysis by Encapsulated Metal Nanoparticles

Supported metal nanoparticles (NPs) represent a large category of heterogeneous catalysts that are important for oil refinery, chemical production, and many emerging applications such as biomass conversion, hydrogen fuel cell, artificial photosynthesis, and electrocatalysis. A considerable amount of work has been devoted to the synthesis and characterization of supported metal NPs. However, supported NPs prepared by traditional methods, e.g. deposition-precipitation, ion exchange, electrostatic adsorption, hardly exhibit size-selectivity. Herein, we have developed a general protocol to encapsulate metal NPs into zeolites by atomic layer deposition (ALD) to force reactants diffuse through channels of zeolitic supports before they access metal NPs. Three pathways were investigated: (1) Induced ALD coating of metal NPs/support; (2) Area-selective ALD (AS-ALD) coating of metal NPs/support; (3) Using the intrinsic properties of supports. We have found AS-ALD coating of metal NP/zeolite can be used to attain the target nanostructures. The synthesized nanostructures with metal NPs fully coated and zeolite support (SAPO-34, ZSM-35, ZSM-5 and Beta) partially-coated exhibit high size-selective properties in hydrogenation reactions of olefins. It has appreciable activities for ethylene and almost no activity for the bulkier isobutylene. H2-D2 exchange reactions also proved the synthesized structure, where the coated silane/alcohol-Pt/ZSM-35 showed obvious lower activities than coated silane/alcohol-Pt/Beta because of hindered diffusion. Various techniques such as energy-dispersive X-ray spectroscopy (EDS), N2 physisorption, Transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR) have been used to characterize the target nanostructures. Compared to the existing methods, our proposed strategy is applicable to many types of metal NPs and zeolites