Preparation And Catalytic Properties Of SnO₂with Different Morphologies

SnO2is an important semiconductor with unique gas sensing, electrical, optical,catalytic and electrochemical properties. Due to these specific properties, SnO2hasbeen widely used in gas sensors, solar cells and Li-ion batteries, has been investigatedextensively and intensively. However, its catalytic property still lacks systematicstudy. In this thesis, SnO2with various morphologies were synthesized throughtraditional precipitation, hydrothermal and molten salt methods. The as-synthesizedsamples were characterized by X-ray Diffraction (XRD), Field Emission ScanningElectron Microscopy (FESEM), Transmission Electron Microscopy (TEM) andhigh-resolution transmission electron microscopy (HRTEM), N2adsorption-desorption isotherm and X-ray Photoelectron Spectroscopy (XPS). The formationmechanism of nanostructures, their redox properties and CO oxidation performancesare also investigated. The main contents and results are listed as follows:Part I: SnO2nanorods were prepared by molten salts method. The products havea uniform size and a smooth surface, with a diameter of40~60nm and a lengthranged from0.6~10um. It is found that they have exposed (110) facets preferably,which can provide more low-coordnated active sites, thus leading to an enhancementof CO oxidation performance, on which CO reached100%conversion at260°C. Themolten salt medium plays a key role in the growth of nanoparticles to formone-dimensional nanorod. The structure and performance of SnO2nanorods is verystable because of the high temperature molten salt disposition process. After runningat260°C for100h, the conversion of CO over the SnO2nanorods was almostmaintained.This indicates that the redox properties of the SnO2nanorods is quitestable under the CO oxidation atmosphere, and SnO2nanorods may have potentialapplication in industrial processes.Part II: SnO2solid microspheres were obtained by hydrothermal method. Thediameter of microspheres is relatively larger, and its mean particle size is up to2um.The samples are well dispersed though a slight agglomeration is also observed. Thesurface of SnO2solid microspheres is formed by nanosheet while its bulk phase is assembled by small grains, therefore the crystallinity of SnO2solid microspheres isquite low. Due to its large particle size, the specific surface area of SnO2solidmicrospheres does not increases, furthermore only a few (110) plane was observedfrom its surface, hence leading to a poor catalytic performance though there exist ofsome mesopores on the surface. This indicates that SnO2solid microspheres may notbe an optimal catalyst for CO oxidation, which need further improvement.Part III: SnO2nanoparticles were prepared by hydrothermal and precipitationmethod, respectively. The shape of SnO2nanoparticles prepared by hydrothermalmethod (SnO2-NPh) is irregular with an average grain size around30nm. SnO2-NPhuniformly distributed and has a clean and defined surface whose lattice fringes can beclearly seen, with mainly (221) and (110) facets preferably exposed. Compared withSnO2nanoparticles prepared by precipitation method (SnO2-NPp), SnO2-NPh has alarger crystalline size, a lower specific surface area which is not favourable for thecollision between CO and active sites on the catalyst’s surface. In addition, as abovementioned (110) facet is the active facet for CO oxidation. As a result, the catalyticactivity of SnO2-NPh for CO oxidation is much worse than that of SnO2-NPp.Part IV: SnO2hollow spheres were synthesized by hydrothermal method, with adiameter around0.2~1.5um and shell thickness about20nm, whose surface iscomposed of micropores and mesopores. Most of samples were refined spherical,while some of them were cracked during calcination. The prepared SnO2hollowspheres were assembled by nanoparticles around5nm, which has a larger specificsurface area and pore volume than SnO2nanoparticles. Subsequently, the catalyticactivity and stability for CO oxidation is significantly improved. The excellentperformance can be ascribed to the existence of micropores and mesopores whichfacilitate the diffusion of reactants. Theses features could enlarge its applicationsunder some extreme condition.