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The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high-resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High-resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

The activation of the dissociative chemisorption of CO on Ni(111) by translational and vibrational energy is probed. Molecular beam techniques produce CO molecules with high kinetic energies and with some vibrational excitation. Thermal desorption and high resolution electron energy loss spectroscopy detect the product of the chemisorption event. The maximum translational and vibrational energies attainable in these experiments, 45 and 18 kcal/mol, respectively, are observed not to activate the dissociative chemisorption of CO. These experiments are sensitive to dissociation probabilities as small as 2×10-6 and 9×10-4 at the maximum values of translational and vibrational energy, respectively. It is concluded that translational energies greater than 45 kcal/mol do not contribute to the CO dissociation rate at high pressures. Rather, the potential energy surface of the CO–Ni(111) interaction likely requires vibrational excitation greater than the amount that can be achieved in this experiment for activation of the C 3/4 O bond. [ABSTRACT FROM AUTHOR]

A rapid site exchange process is observed in the equilibrated chemisorbed layer of CO on Ni(111). Following adsorption at 298 K, the relative populations of CO adsorbed on atop sites and twofold bridge sites are monitored by the high resolution electron energy loss intensities of the respective CO vibrational modes as a function of surface temperature. Since equilibrium is established, the binding energy difference between the terminal and bridge adsorption sites is determined. The bridge site is more stable than the atop site by 0.94±0.15 kcal/mol at a coverage of 0.13. As the coverage is increased to 0.42, the difference in binding energies decreases to 0.44±0.07 kcal/mol. At saturation coverage, 0.5, the binding energy difference effectively becomes very large, resulting in CO occupation of the twofold bridge sites exclusively. [ABSTRACT FROM AUTHOR]

The effect of translational energy on the molecular chemisorption of CO on a Ni(111) surface is used as a probe of the dynamics of the adsorption process. Initial adsorption probabilities, apparent saturation coverages, spatially resolved Auger coverage profiles, and high resolution electron energy loss spectra of CO deposited on the Ni surface from a supersonic molecular beam are measured as a function of translational energy. It is found that the initial adsorption process for CO molecules incident with energies less than 4 kcal/mol differs from that for molecules incident with higher energies. Molecules with kinetic energies below 4 kcal/mol adsorb with an initial adsorption probability of 0.85±0.04 and a high apparent saturation coverage. Molecules with translational energies between 7 to 30 kcal/mol have an initial adsorption probability of 0.46±0.03, and an apparent saturation coverage approximately half that of the low energy molecules. Since the CO packing density and the final chemisorption states are shown to be independent of incident energy, the two apparent saturation coverages are the result of a difference in the surface area over which the CO molecules are spread. This is verified by spatially resolved Auger coverage profile measurements. Molecules at low energies are initially adsorbed with higher mobility than those incident with larger translational energies. High resolution electron energy loss spectra and thermal desorption spectra show no translational energy-induced dissociation. The frequency shift of the bridge-bonded CO stretching mode measured at the periphery of the molecular beam image shows that the energy-induced difference in the CO mobility during the chemisorption process is qualitatively similar on both the clean and partially CO-covered surface. These results are interpreted as evidence for two adsorption pathways into the molecular chemisorption state. Molecules incident on the surface with low energies are... [ABSTRACT FROM AUTHOR]

Studies the effect of copper and niobium addition on the crystallization processes and soft magnetic properties for iron-silicon-boron alloys. Experimental procedures; Information on the permeability values of alloys; Thermograms of the amorphous alloys.