Electron microscopy cobalt catalysts

A combination of controlled atmosphere electron microscopy and flow reactor techniques have been used to investigate the influence of hydrogen sulfide on the catalytic activity of cobalt. Changes in the behavior of cobalt were monitored by the use of probe reactions which are sensitive to the chemical nature of the catalyst surface. These included graphite gasification in oxygen and hydrogen, and carbon deposition from decomposition of hydrocarbons. 

Even more spectacular results in terms of the increasing importance of nanocatalysis for bulk industrial processes have recently been reported by Kuipers and de Jong [32, 33]. By dispersing metallic cobalt nanoparticles of specific sizes on inert carbon nanofibers the authors were able to prepare a new nano-type Fischer-Tropsch catalyst. A combination of X-ray absorption spectroscopy, electron microscopy, and other methods has revealed that zerovalent cobalt particles are the true active centers which convert CO and H2 into hydrocarbons and water. Further, a profound size effect on activity, selectivity, and durability was observed. Via careful pressure-size correlations, Kuipers and de Jong have found that or cobalt particles of 6 or 8nm are the optimum size for Fischer-Tropsch catalysis. The Fischer-Tropsch process (invented in 1925 at the Kaiser-Wilhelm-Institute for... 

Temporal changes of structure and composition of the iron catalyst have been studied with XRD-, Moesssbauer- and XPS-techniques [11]. Temporal changes of the surface stmcture of cobalt during FT-s5mthesis have been observed by tunneling electron microscopy [12]. 

Single shell carbon nanotubes were produced in over 70 percent yields by condensation of a laser-vaporized carbon-nickel-cobalt mixture at 1200°C [12] [81]. No multishell nanotubes were detected in the VLS process. X-ray diffraction and electron microscopy showed that the single shell nanotubes have uniform diameters and self-organize into metallic ropes (mats or arrays) of 100-500 nanotubes having a single-rope resistivity of <10 ohm-cm at 300 K. The particulate mixed-metal Ni-Co catalyst exists at the live end of the growing nanotube and leaves the end by evaporation. 

In this paper a study is presented on the preparation of a series of supported catalysts by precipitation of metal cyanide complexes in the presence of suspended supports. As supports alumina, titania, and silica, have been used. The metals studied comprise iron, cobalt, nickel, copper, manganese, palladium, and molybdenum. Both monometallic, bimetallic and even trimetallic cyanides were precipitated. The stoichiometry of the precipitated complexes was controlled by the valency of the metal ions and by using both nitroprusside and cyanide complexes. Electron microscopy was used to evaluate the distribution of the deposited complex cyanides on the supports. 57Fe-M6ssbauer spectra were measured on the dried precipitated complexes to gain information on the chemical composition of the iron containing complexes. 

Pyrolysis of acetylene on the surface of MCM-41 silicas containing metaUic catalysts has been studied. Application of matrices with the supported metals as catalysts enabled us to attain rather high yields of carbon nanostructures (20-30% of the process product). The electron microscopy data provided evidence for formation of nanotubes with an external diameter of 42-84 nm in the case of the cobalt catalyst and 14-200 nm in the case of the iron catalyst Formation of carbon fibers with diameters in the interval 80-110 nm for the nickel catalyst was also observed. In the absence of a catalyst on such matrices small yields (up to 2%) of carbon nanotubes were attained. 

For better appreciation of the performances, the new catalysts were tested in the hydrogenation of carbon monoxide. In view of the results presented above, and taking into account the size and distribution of the metallic particles as determined by X-ray diffraction and Transmission Electron Microscopy (ref. 10), only the Co products prepared according to reaction (2) were studied. In fact, the best results were obtained with cobalt using the impregnation method (ref. 11). 

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