THERMAL-CONVERSION OF 2-PROPYL IODIDE ON O NI(100) - CHANGES IN PRODUCT DISTRIBUTION WITH VARYING OXYGEN COVERAGES/

The oxidation of 2-propyl iodide on oxygen-covered Ni(100) surfaces ha s been studied with temperature-programmed desorption (TPD), X-ray pho toelectron spectroscopy (XPS), and ion-scattering spectroscopy (ISS). It was found that the product distribution is strongly dependent on ox ygen preexposure, with partial oxidation being favored at low oxygen c overages and total oxidation dominating on thin oxide films. XP I 3d c ore-level spectra indicate that the adsorption of 2-propyl iodide belo w 100 K is molecular and ISS data strongly suggest preferential bondin g to Ni sites, Annealing the 2-propyl iodide adsorbed on O/Ni(100) sur faces between 120 to 200 K generates 2-propyl groups on the nickel sit es via dissociation of the C-I bond, the same as on the clean surface. For submonolayer oxygen coverages the 2-propyl groups then follow one of two reaction pathways: they either undergo hydrogenation-dehydroge nation on the nickel sites to form propane, propene, and hydrogen, or, in the case of those formed near the oxygen sites, migrate and react to form 2-propoxide groups. The 2-propoxide moieties are stable on the surface up to similar to 325 K, at which point some undergo beta-hydr ide elimination to yield acetone. The fact that the rate of desorption of acetone from the reaction of 2-propyl iodide with oxygen is reacti on limited is supported by the observation that the desorption of mole cular acetone from Ni(100) occurs below 300 K. Also, other TPD experim ents indicate that propene does not react to yield acetone on O/Ni(100 ) surfaces, and that the peak shapes and temperatures for acetone deso rption from the reaction of 2-propyl iodide and 2-propanol on O/Ni(100 ) are nearly identical, suggesting that they form from a common interm ediate, 2-propoxide. (C) 1997 Academic Press.