Doctor of Philosophy (PhD)
Physics and Astronomy
A model heterogeneous catalyst inspired by a real catalyst is synthesized for the purpose of understanding how it works. To make such model catalysts, we choose to evaporate metal atoms on metal oxide single crystals and measure them under UHV conditions. The study of model catalysts could advance the understandings of fundamental catalytic properties of real catalysts and helps to optimize or redesign industrial catalysts. In our experiments, many ultra-high vacuum (UHV) techniques have been employed to investigate the atomic and electronic structure of the surface as well as the interface of the prepared samples. In particular, the surface-sensitive tools such as electron energy loss spectra (EELS) and low energy ion scattering (LEIS) spectra provide us detailed information of the surface modification. In this dissertation, we confine our attention to three popular catalysts: Cu on ZnO, Au on ZnO and Cu on TiO2, which play primary roles on modern methanol industry. For instance, Au on ZnO and Cu on ZnO are important catalysts for methanol synthesis, water-shift reaction and methanol-steam reforming, while Cu on TiO2 possesses a high photocatalytic activity for photoreduction of CO2 into methanol. It becomes important for us to develop an understanding of which factors determine the functions of the prepared samples. Different metal growth models are observed for the above samples, due to the varied metal-oxide interactions. At the high substrate temperature, full encapsulation of metal nanoparticles takes place to all of the above samples, which dramatically changes the adsorption behavior and catalytic performance. It provides a strong indication that these thin encapsulation layers are very different from their bulk materials in both geometric and electronic sides. The charge transfer in the interface may be responsible for the modification of geometric and electronic structure of surface, and results in high-thermal stability of these ultra-thin films. The above reconstruction leads to an exceptional catalytic activity of CO oxidation through a different reaction kinetics and mechanism. The CO oxidation experiments show a direct relation between encapsulation rate and reaction rate, which indicates the active sites should be localized at these thin oxide films rather than the metal nanoparticles.
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Zhang, Ziyu, "Surface chemistry and growth of oxide supported metal nanoparticles" (2014). LSU Doctoral Dissertations. 1006.
Kurtz, Richard L.