The adsorption and reaction of aniline, phenol, methoxybenzene, benzonitrile, chlorobenzene, nitrosobenzene, nitrobenzene, and benzaldehyde were studied on Ni(ll1) groups to elucidate the role of substituent groups in adsorption and reaction of substituted benzenes. Temperature-programmed reaction (TPR) and reflection absorption infrared spectroscopy were used to characterize modes of bonding and reaction paths. The absorption bonds were also modeled by using the semiempirical intermediate neglect of differential overlap technique (INDO) with a 19 nickel atom cluster having the symmetry of the (111) surface. The nickel surface acted as an electron acceptor in electrophilic reactions with adsorbates. Chlorobenzene was found to adsorb flat, with the chlorine atom participating in the bond to the surface. With electron-withdrawing groups, the substituents were the site of chemical activity. The CN side group in benzonitrile rehybridized upon adsorption on Ni(lll), resulting in a bonding configuration in which the phenyl ring was tilted away from the surface. Benzaldehyde also apparently interacts mainly through the C 4 bond of the CHO side group. NO bonds were particularly reactive. Nitrosobenzene and nitrobenzene bonded dissociatively through their side group, and NO bond scission was very facile. Electron-donating groups activated the benzene for electrophilic addition reactions. It is postulated that the amine substituent in aniline was activated by electrophilic attack by the surface leading to polymerization. The polyaniline was thermally very stable and did not decompose to above 700 K. Evidence for phenol polymerization was also found. In methoxybenzene, the bulky methyl side group hindered the polymerization, and the molecule decomposed. INDO calculations were found to be useful in identifying charge transfer during adsorption and in elucidating the reaction pathways.
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 of benzene, toluene, o-, m-, and p-xylene, and mesitylene on the Ni(100) crystal face was studied in order to elucidate the role of methyl substitution on the interaction of the aromatic ring with the surface. Temperature-programmed reaction (TPR) and reflection absorption infrared spectroscopy (RAIS) were used to characterize adsorption bond strengths and modes of bonding to the surface. All molecules appear to initially adsorb with the ring parallel to the surface. Methyl substituents were found to decrease the binding energy of the ring to the surface by about 65 kJ/mol independent of the number of substituents. Whereas benzene and mesitylene desorption occurred in single peaks, toluene and xylenes exhibited several additional lesser peaks or shoulders, suggesting the possibilty of more than one type of bonding configuration. The placement of the methyl groups on the ring was found to influence reactivity with the Ni(100) surface: m-and p-xylene decomposed to give only H2 and adsorbed carbon, whereas o-xylene evolved various hydrocarbon fragments as well. Studies with partially deuteriated toluene showed that the methyl group hydrogens are lost before most of the ring hydrogens. The chemisorption of benzene and the methyl-substituted benzenes was also modeled by using a semiempirical molecular orbital method, extended Hückel theory (EHT). EH calculations show that the bonding configuration of the molecules is with the ring nearly parallel to the surface. Methyl substituents were found to weaken the chemisorption bond by destabilizing the -bond to the surface; the repulsive interaction of the methyl group with the nickel atoms was much less significant. Although EHT was in qualitative agreement with the experiments, it overestimated the effect of the methyl groups on binding energy for xylenes and mesitylene. EHT was incorrect in predicting the direction of charge transfer, a consequence of the underestimation of the benzene -* energy gap by EHT.
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