Thermal Conversion of 2-Propyl Iodide on O/Ni(100): Changes in Product Distribution with Varying Oxygen Coverages

Abstract

The oxidation of 2-propyl iodide on oxygen-covered Ni(100) surfaces has been studied with temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ion-scattering spectroscopy (ISS). It was found that the product distribution is strongly dependent on oxygen preexposure, with partial oxidation being favored at low oxygen coverages and total oxidation dominating on thin oxide films. XP I 3dcore-level spectra indicate that the adsorption of 2-propyl iodide below 100 K is molecular, and ISS data strongly suggest preferential bonding to Ni sites, Annealing the 2-propyl iodide adsorbed on O/Ni(100) surfaces between 120 to 200 K generates 2-propyl groups on the nickel sites 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–dehydrogenation 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 ∼325 K, at which point some undergo β-hydride elimination to yield acetone. The fact that the rate of desorption of acetone from the reaction of 2-propyl iodide with oxygen is reaction limited is supported by the observation that the desorption of molecular acetone from Ni(100) occurs below 300 K. Also, other TPD experiments indicate that propene does not react to yield acetone on O/Ni(100) surfaces, and that the peak shapes and temperatures for acetone desorption from the reaction of 2-propyl iodide and 2-propanol on O/Ni(100) are nearly identical, suggesting that they form from a common intermediate, 2-propoxide.