Early transition metal oxides, used as catalysts or catalyst supports, play a significant role in acid/base and redox reactions, which are among the most common reaction types in upgrading biomass-derived molecules and intermediates to fuels/chemicals . A key piece of knowledge for designing high-performance metal oxide catalysts is a molecular-level understanding of structure-activity correlations for desired reactions. However, the chemical and structural complexity of metal oxides makes it challenging to unambiguously describe these materials at an atomic-level, even more so to identify and tailor active surface ensembles. Simplified, well-defined model catalysts are highly desirable in this regard. Recent studies using VOx/CeO2 catalysts for methanol oxidative dehydrogenation (ODH) reveal that the catalytic performance is highly dependent on the exposed facet of the CeO2 support. This strong dependence is attributed to oxygen defect density difference for different facets, which impart great influences on redox properties of the VOx active centers and activation energies for methoxyl group decomposition, the rate limiting step for this particular reaction . However, studies on the roles for different oxide facets on catalytic systems are still at a very early stage.
Research described in this work investigates the facet-dependent properties of two widely used metal oxides, TiO2 and ZrO2. Single-facet-dominated anatase titania were synthesized using hydrothermal methods. These TiO2 nanocrystals exhibited well-defined shapes of truncated tetragonal bipyramid, rod, and nano-sheet, with the exposed dominated facets of (101), (010), and (001), respectively. Using a similar hydrothermal method, monoclinic ZrO2 nanocrystals were also prepared in shapes of spheres and rods. In contrast to TiO2, these ZrO2 nanocrystals possessed more complex facets. The spheres were composed of (111), (1̅11), and (110) facets, while the rod appeared to be dominated by (011) and (001) facets on the longitudinal side and small portions of other facets on the ends. The synthesized TiO2 and ZrO2 model catalysts were tested for the dehydration of isopropanol, bio-diols, and triols to probe their acid activities. They were also employed as supports of VOX for the methanol ODH to evaluate the redox activities of the catalysts. A series of microscopic and spectroscopic techniques, such as scanning/transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), Raman and solid state nuclear magnetic resonance (NMR) spectroscopies were utilized to identify the active domains and correlate the structural properties of different facets to the catalytic performance. The key findings from the characterization studies will also be discussed in this talk.
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