Hydrogen induced carbon-nitrogen bond activation on metallic single crystal surfaces.

Abstract

Hydrogen induced carbon-nitrogen bond activation is a surface process of great importance in catalytic hydrodenitrogenation (HDN) reactions, as well as in developing fundamental understanding of the mechanisms of elementary hydrogen addition steps. This thesis research is aimed toward developing correlations between the reactivity and the structure of adsorbed organonitrogen model compounds on nickel and platinum single crystal model catalysts. The reactivities of adsorbed aniline and cyclohexylamine are studied using temperature programmed desorption, and in situ fluorescence detection of soft X-ray absorption in up to 0.05 Torr H$\sb2$ pressures. The bonding and structure of reaction intermediates are characterized using a combination of surface analytical spectroscopies. External hydrogen significantly modifies the selectivity among hydrogenation, hydrogenolysis and dehydrogenation for both adsorbed aniline and cyclohexylamine, favoring hydrogenolysis to produce benzene and ammonia. The C-N bond cleavage near 380 K is substantially enhanced by external hydrogen via: (1) increasing the availability of surface hydrogen, (2) preventing C-N bonds from reorientation at the hydrogenolysis temperature through retaining the amino hydrogens, and (3) hydrogenating aromatic rings adjacent to the C-N bonds to break the resonant structure that stabilizes the C-N bonds. The surface reactions of aniline and cyclohexylamine are structure sensitive reactions. The chemical reactivity and surface symmetry of the metal substrates have substantial effects on the hydrogenolysis activity. The relatively open (100) surfaces exhibit considerably higher hydrogenolysis activity than the closed-packed (111) surfaces. The hydrocarbon group adjacent to the C-N bond determines the ease and extent of hydrogenolysis. Adsorbed cyclohexylamine exhibits a stronger tendency to undergo hydrogenolysis than adsorbed aniline. The C-N bond cleavage is facilitated by the saturation of the 6-member carbon ring. Aniline hydrogenolysis in the presence of external hydrogen is facilitated by a hydrogenated precursor, which is structurally similar to cyclohexylamine. C-N bond activation is facilitated by an adsorbed configuration in which both the $\alpha$-carbon and nitrogen are positioned near the substrate atom(s). At the hydrogenolysis temperature, this configuration can only be observed in the presence of external hydrogen, which keeps the C-N bond from reorienting. However, parallel adsorption on the (111) surfaces appears to enhance dehydrogenation and polymerization.