Coordination Chemistry of Metal Surfaces. 2. Chemistry of CH3CN and CH3NC on Nickel Surfaces

Article abstract of DOI:10.1021/ja00394a007

The coordination chemistry of the isomeric molecules CH3CN and CH3NC on a nickel has been studied as a function of surface crystallography and surface composition by electron spectroscopic and diffraction techniques, by chemical displacement reactions, by thermal desorption spectrometry, and by isotopic labeling experiments.The surfaces studied were the low Miller index planes (111), (110), and (100) and also the stepped and stepped-kinked planes 9(111)x(111) and 7(111)x(310), respectively.Effects of surface carbon, sulfur, and oxygen contaminant atoms on the chemistry were examined for these surfaces.Acetonitrile was weakly and reversibly bound on the (111), stepped, and stepped-kinked surface and was probably oriented with the CN vector normal to the flat surface sections.The thermal desorption temperature was about 90 deg C.Some irreversible decomposition, probably focused at step or kink sites, was observed in the thermal desorption experiments.Consistently then, acetonitrile on the super-stepped (110) surface largely underwent decomposition on heating; very little reversible chemisorption was observed.Binding of the nitrile on the (100) surface was a significantly different than for the more closely packed (111) surface; the thermal desorption temperature maximum was about 20 deg C higher than for Ni(111)-NCCH3.The Ni(100)-NCCH3 state was ordered, c(2x2); here the nitrile nitrogen atom may lie in the fourfold sites.Labeling studies of the acetonitrile chemisorption showed there was no reversible bond breaking process from 20 to ca. 100 deg C.Neither carbon nor sulfur contaminant atoms quantitatively altered this nickel surface-acetonitrile chemistry.In sharp contrast to acetonitrile, methyl isocyanide was strongly bound to all the nickel surfaces and essentially could not be thermally desorbed from the clean surfaces; decomposition to H2(g) and N2(g) prevailed.Sulfur contaminant atoms on the nickel surfaces did not alter this chemistry but carbon did.Methyl isocyanide chemisorbed on carbon containing Ni(111), Ni<9(111)x(111)>, and Ni<7(111)x(310)> surfaces rearranged on heating and desorbed as acetonitrile at ca. 90 deg C.The chemistry was similar on the carbon-containing (110) and (100) surfaces to that for acetonitrile on the respective Ni(110)-C and Ni(100)-C surfaces.Thus, isomerization of the isocynide also appeared to prevail on these surfaces.Structural aspects of this surface chemistry are discussed.Experiments with "real" nickel surfaces on which isomerization of CH3NC to CH3CN was demonstrated are also described.Oxygen (oxide) contamination of all the nickel surfaces drastically reduced the sticking coefficient of the two isomeric molecules; essentially no chemisorption was observed at 10^-8-10^-10 torr and 20 deg C.