Single metal atoms and metal clusters have attracted much attention thanks to their possibility as heterogeneous catalysts.[1,2] However, the generation of stable single atoms and clusters on a solid support is still challenging. Recently, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional precursor of the zeolite into three-dimensional structure. These subnanometric Pt species are stabilized in the zeolite, even after treatment in air up to 540 oC. Furthermore, we have studied the dynamic structural transformation of those subnanometric Pt species during reduction-oxidation treatments and under reaction conditions (CO+O2, water-gas shift, NO+CO and NO+H2) by environmental transmission electron microscopy (ETEM).
The Pt@MCM-22 was prepared by the transformation of 2D into 3D zeolite, using purely siliceous ITQ-1 as precursor. In situ electron microscopy experiments were performed using a Titan 80-300 Environmental Transmission Electron Microscope at the Centre for Functional Nanomaterials (CFN), Brookhaven National Laboratory.
Subnanometric Pt species are finely dispersed in MCM-22 crystallites. With the help of aberration-corrected electron microscopy, we are able to directly measure the size of those Pt species and figure out their position in the zeolite. Some of the Pt atoms and clusters are located in the surface “cups” of MCM-22 and some of them are anchored to the zeolite framework. Nevertheless, a large part of subnanometric Pt species are located in the internal space of the structure, which is confirmed by the size-selective hydrogenation of propene and isobutene. The exceptional high stability of the encapsulated subnanometric Pt species has also been reflected in the propane dehydrogenation reaction to propylene, showing higher activity and stability than Pt nanoparticles prepared by conventional impregnation method.
Furthermore, using Pt@MCM-22 material as a model system, we have studied the evolution of Pt single atoms and clusters during reduction-oxidation treatments and under reaction conditions by ETEM. Pt nanoparticles and clusters will disintegrate into smaller clusters or even single atoms after calcination in O2. And single atoms and clusters will agglomerate into small particles after reduction by H2. Besides, it can be a general phenomenon that subnanometric metal species will undergo dynamic structural evolution under reaction conditions. For the same reaction, the states of metal species are dependent on the temperature and atmosphere.
Our synthesis strategy can also be applied for preparation of Au@MCM-22 and Pd@MCM-22 materials, with subnanometric Au and Pd species encapsulated in MCM-22 zeolite. Those subnanometric metal clusters show unique catalytic behavior for selective aerobic oxidation of cyclohexane to KA-oil and low-temperature combustion of CH4, respectively.
Subnanometric metal species (single atoms and clusters with a few atoms) with exceptional high stability can be generated and stabilized in MCM-22 zeolite. Those subnanometric metal species show dynamic structural transformation during reduction-oxidation treatments and under reaction conditions and also show unique catalytic properties for various reactions.
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