In-Situ TEM Studies Of Structure Evolution Of Pd Nanocrystals Under Gas Environment

The morphology and structure of the catalystic nanoparticles are key factors for their extraordinary catalytic activities.Nevertheless,during the reaction,changes in the external environment including gas composition,temperature and pressure will induce a large evolution in the surface structure of the nanoparticles,thereby affecting the reaction path and the performance of the catalysts.Therefore,exploring the surface structure of nanocatalysts in an external field environment is of great significance for revealing the catalytic mechanism and designing higher performance catalytic materials.In the past few decades,many advances have been made in the study of the structure of metal nanocatalysts in a gaseous environment.However,the gas pressure of the reaction still has a huge gap with the actual environment in which the catalyst works.In order to truly understand the influence of the external environment on the structure of metal nanocrystals,it is necessary to break through the low temperature and low pressure operating conditions of traditional characterization methods to obtain the atomic scale information of the nanocrystal structure evolution in atmospheric gas environment.In this work,through the state-of-the-art spherical aberration-corrected transmission electron microscope(TEM)and the Gas Cell TEM holder system in the Center of Electron Microscopy of Zhejiang University,we developed an in situ characterization method to study the morphology and structure evolution of materials in atmospheric external gas environment at atomic scale.Making palladium nanocrystals with excellent catalytic reactivity as research object,we studied their dynamic behavior in different gas environments.Combined with theoretical calculations,the response relationship between the surface structure of Pd nanocrystals and the external environment such as gas and temperature was revealed.The main work and achievements have been listed as follows:(1)The shape evolution mechanism of Pd nanocrystals in oxygen and hydrogen at amospheric pressure was revealed.In 1 bar oxygen and hydrogen at atmospheric pressure,the dynamic changes like sintering and surface structure evolution of cubic Pd nanocrystals were observed in situ by TEM and Gas Cell holder.According to F-G adsorption isotherm,DFT calculations and Wulff construction,multi-scale structure reconstruction model was carried out to simulate the equilibrium structure of the reactive nanocrystals.It was pointed out that under the external environment of gas and heating,O2 and H2 gas molecules will change the surface energy of Pd facets through adsorption,which will in turn tailor the morphology of the nanoparticles.We found that the cubic Pd nanocrystals became truncated at the edges under 1 bar O2 at 200 °C with extended {110} facet,and the particles mainly consist of {100} and {110} facets.Under 1 bar H2 at 300 °C,Pd nanocrystals were transformed into truncated shape enclosed by{100},{110}and{111}facets.The theoretical calculations are in great agreement with the in situ TEM observations,further clarifying the reshaping mechanism of metal nanocrystals.(2)The mechanism of the surface refacetting of Pd nanocrystal caused by inert gas component nitrogen was investgated.By using the Gas Cell TEM holder system and TEM,the surface structure evolution of Pd nanocrystals under atmospheric pressure N2 condition was characterized in situ.The results indicated that the rounded surface of the Pd particles would become flat at high temperature,with exposure of{110}facets,and this phenomenon did not occur in Ar envrionment.The gas adsorption on surface at different temperature was further analyzed.Combined with multi-scale structure reconstruction model calculations,it was reavealed that N2 is not always an"inert"gas,but has the ability to stabilize the Pd{110}facet under certain conditions.(3)The complete oxidation process of Pd nanocrystals under atmospheric pressure envimment was demonstrated at atomic scale.Combined with the in situ TEM and scanning transmission electron microscopy(STEM)at atmospheric pressure,the oxidation process of Pd nanocrystals was observed,and the atomic-scale structure information about the oxidation interface and defects during Pd oxidation was obtained.The results demonstrated that the oxidation of cubic Pd nanoparticles began preferentially at the apex angle and then occured simultaneously on each {100} surface,gradually forming a Pd/PdO core-shell structure.At high temperatures,the Pd particles could be completely oxidized into distinct oxide domains and formed polycrystalline PdO.Through the detailed analysis of the oxidation interface atomic structure,the epitaxial orientation relationship between Pd and PdO was clarified,and the misfit dislocations due to their lattice mismatch were found to distribute on the Pd/PdO oxidation interface periodically.It was predicted that the existence of these dislocations was an important reason for enhancing the diffusion of oxygen ions,which led to the oxidation of Pd under experimental conditions without generating Kirikendall voids.In this thesis,the atomic-scale structure evolution of Pd nanocrystals in atmospheric gas environment reveals the great significance of the external field environment on the metal nanocatalysts.It provides important experimental and theoretical support for further understanding of gas-solid reaction and paves the way to explore the catalytic mechanism and design new catalysts with higher performance.