In-situ TEM Studies Of The Structure Of Pd-based Nanocatalyst

Catalysts are required for more than 90%of the chemical reactions involved in pro-duction and daily life.The use of the catalyst effectively increases the reaction rate,allevi-ates the reaction conditions and reduces the cost.Noble metal catalysts are widely used in industrial production,exhaust gas treatment,electrochemistry and many other fields be-cause of their excellent catalytic performance.The development of bimetallic nanocatalysts has made great progress for the performance of noble metal nanocatalysts.Previous studies showed that the activity of the catalyst is closely related to its structure(particle size,mor-phology,crystal phase)and composition.In the past few decades,structure controlled syn-thesis of noble metal nanocatalysts has developed rapidly,but the problem of structure un-stablility in working condition has not been solved.Much effort has been made to reveal the relationship between working condition and structure evolution through spectroscopy and electron microscopy methods.However,lacking of observation about dynamic evolu-tion process of nanoparticles remains a maj or obstacle for advancing the research.As one of the best catalyst systems among noble metal nanocatalysts,reseaches about the dynamical evolution in external environment(heating,gas,liquid,electric field,light,etc.)of Pd-based catalyst are important.Here in,with the Cs-corrected TEM,Super-X EDX detector,heating holder system and gas cell holder system,we monitored the transfor-mation process of Pd-based nanocatalyst in working condition at atomic scale from room temperature to 800?,and in gas environment from vacuum to atmosphere.We also re-vealed the potential mechanism of structure evolution,and related it with catalytic property.The detailed results are as follows:1)The decomposition behavior of icosahedron Pd nanoparticles was observed by in-situ TEM at atomic scale for the first time.Cs-corrected TEM and heating holder were used to monitor the dynamical structure evolution under different temperature in vacuum.The results showed that nanoparticles decomposed into single atoms or clusters under e-beam irradiation above certain temperature.Car-bon onions encapsulating nanoparticles formed due to the graphitization of or-ganic compounds adsorbed on the surface under e-beam irradiation.Self-com-pression of carbon onion induced stress was the key factor of decomposition.2)A novel structure with ABB stacking fault was unveiled during the decom-posi-tion of icosahedral FCC Pd nanoparticles.The non-close-packed hexagonal struc-ture(ABB)was characterized in several directions by Cs-corrected electron mi-croscopy and determined by STEM and TEM simulations.Since structure was discovered in 1980 by XRD,it was observed directly in TEM for the first time.Further analysis revealed the encapsulated graphite is essensical for stabilizing the structure and its interaction with nanoparticles might be the reason for for-mation of ABB structure.3)The alloying process of Pd@Au nanoparticle at high temperature was revealed by in-situ TEM at the atomic scale.STEM mode and Super-X detector were used for observing the shape evolution and chemical redistribution simutaneously dur-ing alloying.The results showed that suface Au atoms diffuse under the driving of surface energy before rapid alloying between Pd core and Au shell,causing the surface reconstruction of the nanoparticles and exposing more low-indexed crys-tal planes.Rapid interdiffusion occured between core and shell at high tempera-ture.Alloying degree varied at surface because of the different thicknesses of Au shell,thus decreased the anisotropy of surface energy,and the surface of the par-ticle becomes curved again consiquently.Theoretical calculation showed that the energy barrier of surface atom diffusion is much lower than that of interdiffusion between core and shell,therefore a surface refacetting happened before interdif-fusion.Acccordingly,we propose a step-wised alloying mechanism.4)The structure evolution of spherical Pd@Au nanoparticles in 1 bar CO and O2 was revealed by in-situ TEM and the structure-property relationship was estab-lished.The structure evolution during annealing of Pd@Au nanoparticle under CO or O2 at atmospheric pressure were observed in-situ by TEM and gas cell holder.The structure of nanopaiticles was stable under the protection of Au shell at low temperature.Only after fast alloying started at high temperature could structure evolution be observed.The adsorption of CO on different crystal planes of PdAu alloy changes their surface energy,which in turn transforms the spherical nanoparticles into octahedrons,exposing a large proportion of{111} faces.Pd core segeragation outward was induced by O2 and the formation of PdO at surface was observed.The activity of Pd@Au/TiO2 model nanoparticle pretreated in O2 was decreased due to the appearance of PdO.This ex-situ experiment result was consistent with the in-situ result.Subsequent CO treatment could reduce PdO to Pd and led to an increase in catalytic performance.The structure evolution of Pd-based nanocatalyst under heating and gas environment was observed in-situ at atomic scale,which revealed the significant effect of external field to the structure of noble metal nanocatalyst.Results in this thesis are helpful for further understanding the machenism of heterogenous reaction and designing nanocatalyst with better performance.