Study On The Surface Morphology Of Metal And Metal Oxide Nanoparticle Catalysts Under Gas Ambience

As important parts of catalysts, metals and metals oxide have attracted wide range of attentions. Especially noble metals, such as Pt, Pd, and transition metal oxides, like TiO2, have broad prospects in the fields of environment and energy. Pt-based bimetallic catalysts are widely utilized for both anodic and cathodic reactions in low-temperature fuel cells, in particular PEMFCs. Palladium virtually plays a significant role in every aspect of hydrogen-involved industries including hydrogen purification, hydrogenation and dehydrogenation. TiO2-based materials are widely used in photocatalytic hydrogen production and pollutants degradation. Nevertheless, all the properties are greatly associated with surface structures of these materials.In this work, in-situ electron microscopy and first-principle theory were applied to investigate surface behaviors of Pt, PdCu, and TiO2 nanoparticles in gas environment. The main contents are listed below.(1) New reconstruction Pt-{100}-(2×2)-O has been observed, and an orderly moving atoms model was proposed in this work. A new reconstruction formed on the {100} surface of Pt nanoparticles in 400℃ and 0.1 Pa oxygen has been observed. Series of HRTEM images with different defocus demonstrated that reconstruction was primitive (2X2). An atomic reconstruction model (orderly moving atoms model) were established and used for HRTEM simulations. The simulated data and the experimental results are well matched.(2) The most stable atomic structure model of TiO2-{001}-(1×4) reconstruction has been determined. With the aid of first principle theory calculations and introducing oxygen chemical potential, surface energy of the five most widely known models have been analyzed systematically. Therefore, the most stable structure was determined successfully.(3) Mechanism of morphology transformation of PdCu nanoparticle under different hydrogen pressures was revealed. In this work, heating gas holder and transmission electron microscope were used to record the morphology evolution of PdCu nanoparticles. It was found that PdCu can maintain spherical morphology in 0.016 bar H2, but transformed into polyhedron with distinct facets in 1 bar H2. Combining first-principle calculations with Langmuir adsorption isotherm, corrected surface energy of three low index facets (001), (110), (111) under different hydrogen pressures were calculated, which perfectly demonstrated that the transformation was driven by great reduce of surface energy.