TEM Studies Of The Structure And Performance Of Pd-Based Catalysts Under External Environment

Nanocatalysts are very important for chemical industry and environmental protection.The catalytic performance of nanocatalysts is determined by their morphologies,sizes,and structures,which can easily change during reaction conditions.Studying the structures and dynamic evolution of nanocatalysts under real reaction conditions may help to reveal the catalytic mechanisms and provide fundamental guidances to improve the properties.Here in this thesis,we apply in-situ electron microscopy,online mass spectrometry and theoretic simulation,to study the widely used Pd-based catalysts,including the effect of environment on interactions between bimetallic catalysts and gas molecules using first-principles calculations,the structure evolution of nanoalloy particles during in-situ annealing,the strong metal-support interactions?SMSI?in the complex supported catalysts system under ambient pressure,and the oscillation of methane oxidation over Pd nanoparticles?NPs?at atmospheric pressure.The details are listed below.1.The effect of environment on the interactions between hydrogen and bimetallic surfaces was revealed.We performed the first-principles calculations for the interactions between hydrogen and four low-index surfaces of ordered PdCu alloy.By considering variable chemical potential of hydrogen,we can study the interactions between hydrogen and PdCu surfaces under specific temperatures of reaction and different pressures of hydrogen.It was found that the coverage of hydrogen on PdCu surfaces changed with varying environments,which decreased with increase of the temperature or decrease of the pressure.2.A selective mechanism for Pd-Pt alloy transformation was revealed in situ at atomic scale.Employing in-situ scanning transmission electron microscopy annealing experiments,we monitored the transformation pathway of a single Pd-Pt nanoparticle from a cubic core-shell structure to a round alloyed structure.We found the heating treatments and the shell thicknesses of the core-shell NPs could directly influence the transformation pathways and the intermediates.On the one hand,the transformation pathway can be changed by adjusting the relationship between annealing temperature and the diffusion and migration barriers of Pt atoms;On the other hand,the surface refacetting process and the possible intermediates can be changed by tuning the shell thickness of core-shell NPs.By employing theoretical analysis and kinetic Monte Carlo simulations,we further explained the whole process.3.The facet-dependent strong metal-support interactions were discovered for the first time.Through in situ ambient pressure gas holder and aberration corrected transmission electron microscope,we studied the SMSI behaviors of Pd-TiO₂ model catalysts under oxidative conditions at high temperature.The obvious encapsulation layers were observed on Pd NPs supported on TiO₂?101?and?100?facets,while there was no such encapsulation on TiO₂?001?facet.Moreover,we found the oxidative SMSI states occurring on TiO₂ supports had significant influence on the catalytic reactivity for methane combustion.The encapsulation of Pd NPs decreased the activity for methane combustion and the Pd NPs supported on?100?facets were more affected than?101?facets.4.The force of oscillation during methane oxidation over Pd NPs was unveiled.Using one of the highest temporal and spatial resolution in situ transmission electron microscopy in atmospheric pressure combined with online mass spectroscope,we studied the oscillation of methane oxidation over Pd NPs.The oscillations of temperature,gas pressure and catalyst composition were tracked as functions of time synchronously.It was the first time that we monitored the oscillatory behaviors of individual NPs during the oscillation of methane oxidation.We found the active site of the Pd nanoparticle underwent redox reactions all the time and the oscillation of one individual Pd nanoparticle agreed well with the oscillation of the whole reaction.We proposed that the competition of oxidation of Pd and reduction of PdO was the force of oscillation during methane oxidation over Pd NPs.In this thesis,by employing state of the art electron microscopy and in-situ methods,combining with online mass spectrum,the structures and the dynamic evolutions of nanocatalysts were studied under real reaction conditions,simultaneously with the reaction temperatures and the gas pressures.The structure of catalysts can be directly related to their performance,which clarified the necessary application of this combined techniques in catalytic research.With the theoretic calculation to explain and verify the experimental results,the mechanisms of various catalytic reactions can be further revealed.