Preparation, Structure And Gas Phasecatalytic Performance Of ZnO Nanowires Supported Noble Metal Catalysts

Catalysis occupies an extremely important position in the field of chemical industry. Nanomaterials have drawn attention in heterogeneous catalysis research due to the superior catalytic properties compared to conventional materials. The design and fabrication of nanomaterials are the key issues in heterogeneous catalysis to achieve desired performance. Preparing novel cat alyst with special shape and structure can not only improve the reaction efficiency, but also promote the study of heterogeneous catalytic mechanism.We have developed a process of large-scale fabrication of Zn O nanowires which primarily consist of the {10-10} nanoscale facets. By using a modified wet chemistry method and apt post-synthesis treatments we have been able to grow a variety of noble metals epitaxially onto the {10-10} nanofacets of the Zn O nanowires. The catalytic performance and stability of t hese novel supported metal catalysts have been evaluated.Zn O nanowires, which primarily consist of flat and clean Zn O{10-10} nanoscale facets, were used to prepare the Au/Zn O catalysts. For comparison, commercially available Zn O powders were also used as support to grow Au nanoparticles. The preparation conditions, such as preparation method, gold loading, the p H value and the temperature of reaction solution all have great influence on their catalytic activities. The synthesized Au/Zn O nanowires catalysts showed high activity for CO oxidation at moderate temperatures and resistant to sintering for calcination temperatures as high as 600°C.Pt/Zn O catalysts prepared with different supports, Zn O nanowires and Zn O powders, after reduced at different temperatures were tested in water gas shift reaction. The Pt/Zn O powders catalysts with irregular spherical Zn O morphology exhibited a higher activity than Pt/Zn O nanowires catalysts after same reduction treatment. Both catalysts exhibited higher activity after reduced at 300°C which is originated from the formation of Pt Zn alloy. A further increase in reduction temperatures led to a decrease in catalytic activity. Pt Zn alloy epitaxial growth on well-defined Zn O nanowires support demonstrated good stability during high temperature treatments and catalytic reactions.The catalytic performance over M/Zn O(M=Pt, Pd, Au, etc.) and Pd/MOx(M=Fe, Ce, Ti, etc.) for methanol steam reforming were evaluated. It was found that Pd/Zn O nanowires and Pd/Zn O powders catalyst are the most active and selective catalysts. The effect of Pd loading and reduction temperature on the formation of Pd Zn were investigated. The results showed that the high CO selectivity of 2wt%Pd/Zn O nanowires catalysts after reduced at 400°C is due to the formation of Pdx Zny(x>y) alloy. Increasing of Pd loading or reduction temperature is favorable for the transformation of Pdx Zny(x>y) to Pdx Zny(x=y) alloy which can inhibit CO formation. The better stability of Pd/Zn O nanowires catalysts is due to the highly stable Zn O nanowires primarily enclosed by the low-energy {10-10} surfaces and the epitaxial relationships between Pd Zn nanoparticles and Zn O nanowires support.The composition, structure and the interfacial structure between supported metal/alloy nanoparticles and Zn O nanowires have been characterized by X-ray diffraction(XRD), N2 adsorption-desorption, thermogravimetric analysis(TGA), X-ray photoelectron spectroscopy(XPS), scanning electron microscope(SEM) and aberration corrected scanning transmission electron microscope(AC-STEM). Especially, in order to understand the metal-Zn O interactions during the post-synthesis process, the sub-angstrom resolution HAADF imaging technique was used to characterize the metal/Zn O nanowires system. It was concluded that all metal/alloy nanoparticles grew with a fixed crystallographic orientation relationship with respect to the Zn O{10-10} nanoscale facets. The well-controlled structure of Zn O nanowires and strong interfacial interactions are responsible for stabilizin g metal/alloy nanoparticles at high temperatures. The excellent thermal stability property of these catalysts may provide a new opportunity in the development of stable supported noble catalysts for applications in high-temperature environment. The fundamental understanding of the active sites in morphology-tunable oxides that are enclosed by reactive crystal facets is expected to direct the development of highly efficient nanocatalysts.