Room-temperature 129Xe NMR is a convenient means of studying heterogeneous distributions of HMB in N a y , providing the heterogeneity length scales are larger than about 10 pm. Such situations may lend themselves to NMR chemical shift imaging methods in which a linear magnetic field gradient might be used to image xenon profiles in samples possessing macroscopic adsorbate heterogeneities. The suitability of multiple-quantum NMR spectroscopy for probing adsorbate distributions quantitatively is due primarily to the sensitivity of the technique to the number of dipole-dipole coupled spins in a collection of isolated molecules. Counting the number of proton spins in clusters of chemisorbed organic species yields information on their spatial distributions and, thus, about microstructural features of the adsorption sites themselves. Because of the central importance of these sites to the reaction process, multiple-quantum NMR represents a potentially valuable means by which a catalyst's microscopic adsorbate structure can be correlated with its chemical reaction properties. Such information, used in conjunction with the macroscopic adsorbate distributions measured by '29Xe N M R spectroscopy, is key to characterizing intracrystalline mass transport and adsorption of reactant species within zeolitic catalysts. Acknowledgment. We thank D. N. Shykind and M. Trecoske for assistance with the multiple-quantum experiments and with zeolite sample preparation. M. G.Physicochemical characterization of calcined and sulfided CoO/SiO, catalysts were carried out to reveal the interaction modes between cobalt and SiOz using XPS, TPR, TEM, DRS-VIS, and XRD techniques. The CoO/Si02 catalysts were prepared by an impregnation method using cobalt acetate as well as cobalt nitrate and by an ion-exchange technique. It was found that several kinds of cobalt species are formed on CoO/SiO,. These species are assigned to Co304, Co-Si4 mixed oxide, surface Co3+ species, surface silicate, surface Co2+ species in the order of the TPR reduction temperature. Their proportions strongly depended on the starting salt, cobalt content, and preparation method. Cobalt acetate was found to provide highly dispersed CoO/Si02 catalysts with a uniform distribution of cobalt species throughout the catalyst particles as compared to conventionally employed cobalt nitrate. The proportion of Co3+ greatly decreased when cobalt acetate was used instead of cobalt nitrate. All the cobalt species interacting with SiO, were found to be sulfided at 673 K. It was demonstrated that sulfided CoO/Si02 catalysts prepared from cobalt acetate show several times higher hydrogenation activity than the catalysts from cobalt nitrate. On the basis of the XPS characterization of uncalcined precursors, the effects of starting salt on the cobalt-Si0, interaction modes and cobalt dispersion and distribution are discussed.
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Sensitive and rapid methods have been proposed for the determination of pert per billion levels of elements in polycarbonate(PC) used as optical materials. The contents of Cr, Mn, Fe, Co, Ni, Cu and Na were determined directly by graphite furnace atomic absorption spectrometry(GFAAS) with a solid sampling method. Na+ and Cl" were analyzed by ultrapure water extraction-GFAAS and ion chromatography, respectively. The accuracy of these methods was confirmed by comparative analyses. The proposed methods were applied to optical fiber and disk of PC. The analysis time required a few hours.
It was found that the catalytic activities and selectivities of cobalt sulfide catalysts are significantly affected by the calcination temperature of precursor cobalt oxide for the hydrogenation of butadiene and the isomerization of 1-butene. When cobalt sulfide was prepared from cobalt oxide calcined at 1473–1573 K, the specific activities per surface area of the sulfide were enhanced by 30–40 times those of cobalt sulfide prepared from cobalt oxide calcined at a conventional temperature, 573–973 K. The reaction selectivities were also strongly modified by the calcination temperature of precursor oxide. It is suggested that two different types of active sites are formed on cobalt sulfide, depending on the calcination temperature of the precursor. On the basis of XPS, XRD, SEM, and NO adsorption techniques, it is concluded that the particle size of the precursor cobalt oxide and sulfiding conditions determine the surface sulfur content and hence the catalytic properties of the sulfide catalysts. The sulfidation mechanism of cobalt oxide and active site models are proposed.
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