In Situ Transmission Electron Microscopy Study Of Nanocatalyst Under Gas Environment

It is well-known that catalyst is the heart of the modern chemical industry,because more than 90%of chemical processes and more than 60%of products are related with catalytic technology.With the development of nanomaterial preparation methods,nanocatalysts are playing a more and more important role.In order to further understand the catalytic mechanism and design more efficient catalysts,it is of great importance to study the working state of the catalyst in situ.The rapid development of gas involved in situ electron microscopy technology provides an opportunity to achieve this goal.In this thesis,the environmental transmission electron microscopy and gas cell holder system are used to monitor the evolutions of morphology,structure and composition of nanocatalyst under gas environment.First,we studied the structure of supported metal nanoparticles under different gas enviroments.At the same time,we developed a theoretical model to predict it.Secondly,we in situ investigated the oxidation of Ni nanoparticles at atmospheric pressure,and discussed the effects of temperature on the oxidation behavior.Further,using in situ electron diffraction,we successfully obtained the ultrafast oxidation and reduction kinetics of Ni nanoparticles,and investigated the corresponding kinetic models.At last,we studied the oxidation of Mo grid,focusing on the growth of its oxide(MoO2),whose surface structure was reconstructed.The detailed research contents and achievements are listed below:1.The structure evolution mechanism of oxide supported metal nanoparticles under different gas environments was revealed in situ at atomic scale.Based on the Wulff-Kaischew theory,Langmuir isotherm adsorption and density functional theory,we developed a multiscale structural reconstruction model for the variation of supported metal nanoparticles in catalytic reaction gases.We explored the three-dimensional structure of Cu/ZnO catalysts,especially the perimeter interface structure,in H2O vapour at different temperatures.The result was in good agreement with the previous experimental research.Further,we studied the structure of the Pt/SrTiO3 system in 300 ffound,and the interface area was clearly reduced.Combined with the results of the theoretical model,a comprehensive understanding of the structure of Pt/SrTiO3 system in H2 was achieved.2.The oxidation mechanism of Ni nanoparticles at different temperatures was uncovered.The dynamic behaviors of Ni nanoparticles oxidized at 600℃ and 800 ℃at ambient pressure were in situ observed using transmission electron microscopy equipped with a gas cell holder.The NiO formed at 600℃ had a core-shell structure and exhibited a typical Kirkendall effect.On the contrary,NiO formed at 800 ℃ had no voids.In situ observation revealed that at 600 ℃ the surface of Ni nanoparticles was fully oxidized first and the bulk followed afterwards.But the oxidation of Ni nanoparticles at 800 ℃ proceeded from one side to the other.The different oxidation behaviors are determined by the diffusion rates of the O active species along the metal surface and the metal-oxide interface.By changing the temperature,the oxidation behavior of nanoparticles can be altered.In order to verify the universality of the theory,the oxidation behaviors of Cu and Co nanoparticles at different temperatures were also studied,and similar phenomenons were captured.3.The ultrafast oxidation kinetic of Ni naoparticles at ambient pressure was unveiled,which is different from the traditional ones.With the help of the gas cell holder system and OneView camera,the ultrafast oxidation kinetics of Ni nanoparticles in oxygen was vividly obtained.In contrast to the well accepted Wagner and Mott-Cabrera models(diffusion-dominated),the oxidation of Ni nanoparticles is linear at the initial stage(<0.5 s),and follows the Avrami-Erofeev model(n= 1.12)at the following stage,which indicates the oxidation of Ni nanoparticles is a nucleation and growth dominated process.We also studied the reduction kinetic of NiO at atmospheric pressure and verified that it conformed to the nucleation and growth model.4.A new layer-by-layer growth mode of the MoO2(011)facet with surface reconstruction involved was discovered.The environmental transmission electron microscopy was applied to study the oxidation of Mo grid,especially the growth process of MoO2 with surface reconstruction.With the HRTEM simulation,the reconstruction structure of the MoO2(011)facet is conformed.In situ study of the layer-by-layer growth and decomposition process of MoO2 reveals that the reconstruction structure acts as an intermediate phase in both processes.Moreover,due to the modulation of the surface energy by surface reconstruction,an oscillation mode was observed in the growth and decomposition processes.