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- All Subjects: Catalysis
- Creators: Liu, Jingyue
Description
Operando transmission electron microscopy (TEM) is an extension of in-situ TEM in which the performance of the material being observed is measured simultaneously. This is of great value, since structure-performance relationships lie at the heart of materials science. For catalyst materials, like the SiO2-supported Ru nanoparticles studied, the important performance metric, catalyst activity, is measured inside the microscope by determining the gas composition during imaging. This is accomplished by acquisition of electron energy loss spectra (EELS) of the gas in the environmental TEM while catalysis is taking place. In this work, automated methods for rapidly quantifying low-loss and core-loss EELS of gases were developed. A new sample preparation method was also established to increase catalytic conversion inside a differentially-pumped environmental TEM, and the maximum CO conversion observed was about 80%. A system for mixing gases and delivering them to the environmental TEM was designed and built, and a method for locating and imaging nanoparticles in zone axis orientations while minimizing electron dose rate was determined.
After atomic resolution images of Ru nanoparticles observed during CO oxidation were obtained, the shape and surface structures of these particles was investigated. A Wulff model structure for Ru particles was compared to experimental images both by manually rotating the model, and by automatically determining a matching orientation using cross-correlation of shape signatures. From this analysis, it was determined that most Ru particles are close to Wulff-shaped during CO oxidation. While thick oxide layers were not observed to form on Ru during CO oxidation, thin RuO2 layers on the surface of Ru nanoparticles were imaged with atomic resolution for the first time. The activity of these layers is discussed in the context of the literature on the subject, which has thus far been inconclusive. We conclude that disordered oxidized ruthenium, rather than crystalline RuO2 is the most active species.
After atomic resolution images of Ru nanoparticles observed during CO oxidation were obtained, the shape and surface structures of these particles was investigated. A Wulff model structure for Ru particles was compared to experimental images both by manually rotating the model, and by automatically determining a matching orientation using cross-correlation of shape signatures. From this analysis, it was determined that most Ru particles are close to Wulff-shaped during CO oxidation. While thick oxide layers were not observed to form on Ru during CO oxidation, thin RuO2 layers on the surface of Ru nanoparticles were imaged with atomic resolution for the first time. The activity of these layers is discussed in the context of the literature on the subject, which has thus far been inconclusive. We conclude that disordered oxidized ruthenium, rather than crystalline RuO2 is the most active species.
ContributorsMiller, Benjamin (Author) / Crozier, Peter (Thesis advisor) / Liu, Jingyue (Committee member) / McCartney, Martha (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2016
Description
Non-stoichiometric oxides play a critical role in many catalytic, energy, and sensing technologies, providing the ability to reversibly exchange oxygen with the ambient environment through the creation and annihilation of surface oxygen vacancies. Oxygen exchange at the surfaces of these materials is strongly influenced by atomic structure, which varies significantly across nanoparticle surfaces. The studies presented herein elucidate the relationship between surface structure behaviors and oxygen exchange reactions on ceria (CeO2) catalyst materials. In situ aberration-corrected transmission electron microscopy (AC-TEM) techniques were developed and employed to correlate dynamic atomic-level structural heterogeneities to local oxygen vacancy activity.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
ContributorsLawrence, Ethan Lee (Author) / Crozier, Peter A. (Thesis advisor) / Lin, Jerry (Committee member) / Liu, Jingyue (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2019