No abstract

The adsorption and desulfurization of thiophene on clean and sulfided Ni(lll) surfaces were studied with LEED, AES, TPR, and RAIS. The RAIS data indicate that thiophene adsorbs with its ring parallel or nearly parallel to the surface below room temperature. On clean Ni(lll), thiophene polymerizes slightly above room temperature, as evidenced by paraffinic and aromatic C-H stretches in the reflection infrared spectrum and TPR product yields. Decomposition of the polymer aggregates produces a wide variety of hydrocarbons including C6 fragments at 470 K. On a more acidic sulfided Ni(lll) surface (0S = 0.19), thiophene appears to undergo electrophilic attack at an -carbon as evidenced by TPR products and by methane evolution from the reaction of 2,5-dimethylthiophene. An electrophilic attack by a surface metal atom at an -carbon of thiophene is discussed in terms of molecular orbital theory.

No abstract

The adsorption of benzene on the Ni(100) and the Ni(lll) crystal faces was compared in order to investigate the effect of crystallographic orientation on the interaction of benzene with nickel. Temperature programmed reaction (TPR) was used to characterize adsorption bond strengths and determine product distributions. Benzene was found to adsorb 44 kj/mol less strongly on the Ni(lll) plane than on the Ni(100) surface. Di-hydrogen evolution formed after decomposition of benzene was similar for both surfaces. Benzene chemisorption was modeled by using extended Hückel theory (EHT), a semiempirical molecular orbital method. The calculations predict bonding of benzene over a threefold hollow site on Ni(l 11). Multicenter

The adsorption and reaction of a variety of organonitrogen compounds on a Ni(100) surface have been examined with temperature-programmed reaction, Auger electron spectroscopy, and infrared spectroscopy. Monomethylamine adsorbs via the nitrogen lone pair of electrons and then undergoes C-N bond scission yielding adsorbed carbon, dihydrogen, and ammonia. Aniline ir-bonds to the surface and polymerizes to form a thermally stable poly(aniline) surface film. Pyridine undergoes a temperature-induced orientational transformation. At low temperatures pyridine adsorbs with its ring parallel to the surface.At higher temperatures it appears to form an -pyridyl species with an activation barrier of 85 kJ/mol. Methyl groups on 2,6-lutidine sterically hinder this reaction. Methyl groups on 3,5-lutidine stabilize bonding via the nitrogen lone pair of electrons. The methyl groups on 3,5-lutidine increase electrophilic addition activity relative to pyridine and lead to polymerization of 3,5-lutidine, forming a thermally stable polymer film. Pyrimidine reacted in almost identical fashion to pyridine, suggesting that increased nucleophilic activity had little effect on the reaction behavior of heterocyclic compounds and that electrophilic reactions predominate.