HRTEM Characterization Of Low Dimensional Novel Catalysts

The field of catalysis is of paramount importance.They allow chemical reactions to be carried out at lower energy. They play a significant role in the reduction of harmful emissions from vehicles and are crucial for designing alternative energy sources for the future.Many catalysts take the form of nano-sized particles or low dimensional nanocatalysts which bring about challenges for their design and characterization.Due to the small size of nanocatalysts, the ratio of surface area and volume is quite large.Chemical state of the surface and the inner particle is quite different. Surface smoothness deteriorates as a result of the formation of uneven atomic steps, leading to an increased ratio of surface active position. In consequences, nanocatalysts have quite a high catalytic activity. However, due to the resolution of the optical microscope constraints,observation is a hard task in this subject.other techniques such as X-ray diffraction,scanning tunneling microscopy, atomic force microscopy are all indirect observation, a powerful tool which can be utilized to directly observe the nanocatalysts is strongly needed.With the continuous development of the electron microscope with a resolution breakthrough from nm to today’s sub-angstrom level,it has become one of the most powerful characterization techniques for observing small size nanomaterials.In this thesis,we utilize aberration corrected transmission electron microscopy, scanning transmission electron microscopy, diffraction pattern, off-axis electron holography, Energy Dispersion Spectroscopy for atomically resolved investigations of the structure of catalytically active nanoparticles as freshly produced and to provide insightful information of deactivation in treated and used catalysts.Our finding are summarized below:1. Ag nanoparticles is quite often used as catalysts for oxygen adsorption, and the properties have always been concerned. In this thesis, we use aberration corrected electron microscopy to observe the coalescence process of Ag particles below 10 nm and accordingly conclude the aging mechanism of such catalysts. On another hand, by recording phase information, we obtained the mean inner potential, thus get the three-dimensional morphology information and surface tension variation with the size. These properties are all quite related to the catalytic activities.2.Bimetallic particles is an ideal catalyst for the oxygen reduction reaction of the fuel cell. Using a microscopic dark field image, we obtained the distribution of various orientations. Atomic resolution EDS mapping confirmed that the distribution of chemical elements-Cu, Au in a single particle distribution.3. In order to explore new catalysts to reduce the cost of the Pt in the use of automobile exhaust catalysts, we used hydrothermal together with ion exchange and successfully synthesized NixZn1-xGa2O4 (x=0.05-0.9),a novel catalyst. The catalyst not only exhibited a good catalytic performance, but also expanded the reaction temperature window. We used electron microscopy to carefully characterized the particles and found that the particles are single crystals and has a uniform size distribution of about 6-8nm. With the increase of Ni concentration, there is a lattice contraction and an increase in the particle size. Such catalyst is solid solution with cubic spinel structure, and most exposed surfaces are{111}. There is more corners and edges exposed to the surface on such surfaces with high catalytic activity.4. Two-dimensional graphene has long been a focus. It not only has a high catalytic performance, but also a suitable support for catalysts. Our study found that the edge of the graphene has a high catalytic activity towards oxygen reduction reaction in fuel cells, thereby characterizing the morphology and the folded edges is of significance. With the use of spherical aberration correction electron microscope, we identified layers, orientation, electron diffraction spots and confirmed the produced graphene samples were of a high purity. And they are stacks of differently oriented monolayer or multilayer graphene.