THERMAL CHEMISTRY OF DIHALOPROPANES ON NI(100) SINGLE-CRYSTAL SURFACES - FORMATION OF CYCLOPROPANE, PROPENE, AND PROPANE

The thermal chemistry of 1,3-diiodopropane, 1-chloro-3-iodopropane, 1- iodopropane, 1-chloropropane, 3-chloropropene, propene, and cyclopropa ne on Ni(100) surfaces has been studied under ultrahigh vacuum conditi ons by using temperature-programmed desorption (TPD) and X-ray photoel ectron spectroscopy (XPS). Cyclopropane, propene, propane, and hydroge n are all produced by thermal activation of the diiodopropane at low c overages, and iodopropane and molecular desorption are also seen at hi gher coverages. In contrast with this, only cyclopropane, propene, and chloropropane-but no propane-were observed after thermal activation o f 1-chloro-3-iodopropane. Both I 3d and Cl 2p XPS spectra suggest that the adsorption of the dihaloalkanes is molecular below 100 K and that at low coverages both halogen atoms interact directly with the surfac e. They also point to the fact that the C-I (C-CI) bonds break between 140 and 180 K in steps that most likely lead to the generation of a t hree-carbon metallacycle on the surface. These metallacycles may then undergo either intramolecular coupling to form cyclopropane or a dehyd rogenation step to generate propenyl groups on the surface, and propen yl moieties, produced either by thermal activation of the metallacycle mentioned above or directly via the surface decomposition of 3-chloro propene, dehydrogenate to propene. Finally, some iodopropyl species ar e generated at high 1,3-diiodopropane coverages, and those incorporate a surface hydrogen to produce iodopropane (which either desorbs or un dergoes a second C-I bond scission to give propyl species, the same as when starting directly with 1-iodopropane), 1-Chloro-3-iodopropane lo ses an iodine atom first, and some of the resulting 3-chloropropyl yie lds 1-chloropropane and 3-chloropropene, while the rest reacts further to produce cyclopropane and propene, as in the diiodopropane case. Ne vertheless, very little cyclopropane is detected at high 1-chloro-3-io dopropane coverages or in the presence of coadsorbed hydrogen, presuma bly because the formation of metallacycle species is not favorable in that case. Finally, 1-chloropropane mostly desorbs molecularly and doe s not yield any hydrocarbon products.