In-situ transmission electron microscopy study of thermal stability and structural transformation of metal nanocatalysts

Abstract

As great importance has been attached to the development of thermal catalysis, some determining factors that always affect metal nanocatalysts from being sustainable and active remain to be explored: thermal stability and structural transformation. Therefore, the real-time insights into their feasible working-temperature range and microstructural reconstruction at thermocatalytic conditions will further greatly extend our knowledge of their physicochemical properties and provide valuable guidance for the designs and applications of metal nanocatalysts. With the aids of in-situ transmission electron microscopy (TEM), the aimed nanocatalysts can be visually revealed for their temperature-dependent effects and microstructural transformation of nanoparticles with high resolution and a wide operating temperature range. In this thesis, the in-situ TEM was conducted to investigate some innovative thermocatalytic nanocatalysts: platinum nanoparticles deposited on the pristine graphene oxide (PtNPs/GO), platinum nanoparticles deposited on the water-etched graphene oxide (PtNPs/GO-H₂O), reduced graphene oxide-based platinum nanoparticle (Pt/rGO) and Pd nanoparticles supported on carbon-coated ZnO nanowires (PdNPs/C&ZnO-NWs) under real-time and real-temperature conditions that perfectly recreated the actual thermal catalysis.

The thesis starts with a brief introduction of the development, essentiality and equipment of in-situ TEM, followed by the literature reviews of some representative in-situ TEM studies under different external stimuli (e.g., thermal treatments, gas-phase, liquid-phase and electrical treatments) in Chapter 1. By operating in-situ TEM technique to examine the aforementioned nanocatalysts, the results were obtained and analyzed in the following chapters:

Chapter 2 revealed that PtNPs/GO-H₂O exhibited significantly higher temperature stability (700 °C) than PtNPs/GO. The time-lapsed statistical data of TEM images verified that the size variation of sintering Pt nanoparticles on GO was in accordance with the Ostwald ripening process. The tendency towards the preferential exposure of high-index Pt nanocrystal facets (higher surface energy sides) at 25–400 °C was discovered to accelerate nanoparticle aggregations, providing microscopic evidence of the catalyst instability at high-temperature ranges.

Chapter 3 exhibited superior thermal stability of PtNPs on the rGO flat surface than on the wrinkle one, indicating that improving the surface flatness of graphene-based supports can increase the thermal stability of deposited metal nanocatalysts. The time-resolved analysis demonstrated that the thermal-induced microstructural transformation (graphitization) of the rGO support caused the discrepancy of PtNPs coarsening from the Ostwald ripening model. Insitu high-resolution TEM images further visualized the process of coalescing microstructures of PtNPs on graphitized rGO.

Chapter 4 disclosed the excellent thermal stability of PdNPs/C&ZnO-NWs in the temperature requirement of Suzuki coupling reactions. The PdNPs aggregations were not observed until reaching 300 °C and carbon-coated layers could functionally prevent PdNPs from sintering even when ZnO-NWs experienced melting at 500 °C. The ZnO encapsulation behaviors (the surface reconstruction of C&ZnO-NWs supports) were subsequently shown to offer the potential microscopic evidence of PdNPs/C&ZnO-NWs deactivation in thermal catalysis.

Chapter 5 reported that deposited PdNPs perform superior thermal stability and catalytic performance on the defective ND@G with core-shell sp³ -sp² structure than on the OLC with pure sp² graphitic structure. In comparison with Pd/OLC, the coarsening and aggregation of PdNPs cannot be detected on ND@G even at 600 °C. The in-situ high-resolution investigations of microstructural transformation of OLC supports at 600–1000 °C elucidated the detailed process of OLC structural transformation to polygonal hollow carbon via hexagonal hollow carbon and the potential deactivation mechanism of PdNPs supported on OLCs.

Chapter 6 focused on electron beam effects in the typical carbon-based nanomaterials being studied in this thesis, and then provided feasible threshold values of electron beam flux for OLC, CNT and GO for the future in-situ TEM studies.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Ning WANG (Supervisor)