Partial oxidation of hydrocarbons on nickel: from surface science mechanistic studies to catalysis

Abstract

The mechanistic details of hydrocarbon partial oxidation reactions were studied with both ultra-high vacuum (UHV) modern surface-sensitive techniques and a micro-batch reactor. The focus of this work was to characterize the reactivity of alkyl species and of alcohols on nickel substrates. In the UHV experiments the chemistry of oxides was mimicked via the oxidation of metal surfaces under controlled conditions, and alkyl surface moieties were prepared by thermal excitation of adsorbed alkyl halides. The oxidation of 2-iodopropane on Ni(100) in particular was found to yield several products in a distribution dependent on oxygen pre-coverage, with partial oxidation being favored at low oxygen coverages and total oxidation dominating on thin oxide films. X-ray photoelectron (XPS) I 3d core level spectra indicate that the adsorption of 2-iodopropane below 100 K is always molecular, and ion-scattering spectra (ISS) data strongly suggest preferential bonding to Ni (not O) sites. Annealing the alkyl iodide adsorbed on O/Ni(100) surfaces below 200 K leads to the dissociation of the C–I bond, and generates 2-propyl groups bonded to nickel atoms, the same as on the clean nickel metal. 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 the moieties adsorbed near the oxygen sites, they incorporate an oxygen atom to form 2-propoxide groups. 2-Propoxide moieties, which can also be prepared by decomposition of 2-propanol, are stable on the surface up to ∼325 K, at which point they follow a β-hydride elimination step to yield acetone. Additional temperature-programmed desorption (TPD) experiments indicated that propene does not convert directly to acetone on these O/Ni(100) surfaces, and that the presence of hydroxo groups greatly enhances the yield for ketone production. Lastly, preliminary atmospheric pressure experiments proved that alcohols can be oxidized catalytically to aldehydes or ketones on nickel surfaces under controlled conditions, a result that validates the knowledge developed by the mechanistic surface-science studies.