28 research outputs found

    Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by in Situ Environmental TEM

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    Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the particles' nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600\degreeC). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. In situ mapping of the valence state and the corresponding nanoporous structures allows quantitatively analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms.Comment: ACS Nano, accepte

    Adsorption and reaction of CO and H2 on K-promoted Rh/SiO2 catalysts

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    The adsorption of CO and H2 on a series of alkali-promoted Rh/SiO2 catalysts was investigated by IR spectroscopy and volumetric chemisorption. The characteristics of the support as well as the method of addition of the alkali species were found to influence the adsorptive properties of the catalysts. Alkali species on wide-pore Rh/SiO2 tended to partition to the support and did not interact strongly with the Rh crystallites. When alkali and metal salts were coimpregnated onto a nonporous SiO2 support, intimate alkali-metal contact resulted in significant electronic interactions between the alkali species and the metal. When alkali species were added to a prereduced Rh/SiO2 (nonporous) catalyst, a chemical interaction between a tilted adsorbed CO and the alkali species was suggested. The nature and location of the alkali species were suggested to be important parameters in determining the effect of alkali promoters on Rh/SiO2 catalysts. The rate of CO conversion decreased substantially with promotion for all of the promoted catalysts. An unusually low apparent activation energy was found for the sequentially impregnated (nonporous SiO2) promoted catalyst, and it was suggested that this might be related to the unusually low frequency peak seen in the IR spectrum of adsorbed CO on this catalyst

    Adsorption and reaction of CO and H2 on K-promoted Rh/SiO2 catalysts

    No full text
    The adsorption of CO and H2 on a series of alkali-promoted Rh/SiO2 catalysts was investigated by IR spectroscopy and volumetric chemisorption. The characteristics of the support as well as the method of addition of the alkali species were found to influence the adsorptive properties of the catalysts. Alkali species on wide-pore Rh/SiO2 tended to partition to the support and did not interact strongly with the Rh crystallites. When alkali and metal salts were coimpregnated onto a nonporous SiO2 support, intimate alkali-metal contact resulted in significant electronic interactions between the alkali species and the metal. When alkali species were added to a prereduced Rh/SiO2 (nonporous) catalyst, a chemical interaction between a tilted adsorbed CO and the alkali species was suggested. The nature and location of the alkali species were suggested to be important parameters in determining the effect of alkali promoters on Rh/SiO2 catalysts. The rate of CO conversion decreased substantially with promotion for all of the promoted catalysts. An unusually low apparent activation energy was found for the sequentially impregnated (nonporous SiO2) promoted catalyst, and it was suggested that this might be related to the unusually low frequency peak seen in the IR spectrum of adsorbed CO on this catalyst
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