Browsing by Author "VOLKMANN, UG"

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    MODIFICATION OF THE ELECTRICAL-RESISTANCE OF THIN COBALT FILMS UPON THE ADSORPTION OF CARBON-MONOXIDE
    (1995) CABRERA, AL; GARRIDOMOLINA, WH; VOLKMANN, UG
    Thin films of pure Co with thicknesses ranging between 10 and 50 nm were produced in UHV conditions. The films were deposited onto mica substrates with dimensions of 1 cm x 1 cm x 0.1 cm. Thin gold wires were attached to the ends of the films in order to monitor their electrical resistivity. The films were introduced into a UHV chamber which consists of a quadrupole mass spectrometer and an Ar-sputtering gun for cleaning. The films were exposed to 10(-6) torr of high-purity CO and the resistivity of the films was monitored during exposure. The resistivity changed in a ''sawtooth'' fashion similar to the changes observed by Pick in the Nb/H-2 system. The aim of these studies is to correlate changes in resistivity of the film with well-characterized adsorption states of CO on the Co surface. Carbon monoxide desorption from the cobalt films was studied (using a mass spectrometric method in an ultrahigh vacuum system) and two carbon monoxide desorption peaks were observed identical to prior work using Co foils. These two states correspond to molecular CO and presumably dissociated CO. Changes in resistivity of the thin Co films appear to be related only to the molecular CO adsorbed on the Co surface.
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    STUDIES OF CARBON-MONOXIDE AND HYDROGEN ADSORPTION ON NICKEL AND COBALT FOILS AIMED AT GAINING A BETTER INSIGHT INTO THE MECHANISM OF HYDROCARBON FORMATION
    (1994) CABRERA, AL; GARRIDO, WH; VOLKMANN, UG
    Systematic studies of adsorption of hydrogen and carbon monoxide on polycrystalline surfaces of nickel and cobalt have been carried out. The aim of these studies was to pin a better insight into the catalyzed formation of hydrocarbons from H-2-CO mixtures. We have studied the adsorption of these gases by means of thermal desorption spectroscopy (TDS) on nickel foils and powders. More recently, we have obtained desorption spectra of hydrogen adsorbed on cobalt foils and powders. In this work we described desorption spectra of carbon monoxide on cobalt foils. Carbon monoxide desorption from cobalt foils was studied in a similar way as prior work, using a mass spectrometric method in an ultra high vacuum system. Two carbon monoxide desorption peaks were observed. These two states correspond to molecular CO and presumably dissociated CO, as it is observed in the case of stepped surfaces of Ni and Co single crystals. An activation energy of around 4.0 kcal/mol is obtained for the molecular state while for the dissociated state the energy is coverage-dependent with a value between 8.0 and 15.0 kcal/mol. The carbon monoxide desorption peaks were fitted with near Gaussian curves which facilitates the analysis of the data to obtain activation energies for desorption. Kinetic parameters for carbon monoxide and hydrogen desorption from nickel and cobalt foils are provided and compared with already published data involving single crystals.
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