IN-SITU STUDIES OF CYCLOHEXYLAMINE DEHYDROGENATION AND HYDROGENATION ON THE NI(111) SURFACE

As part of our program to characterize the primary factors influencing C-N bond activation on Ni surfaces, we have studied the adsorption an d reactions of cyclohexylamine (CHA) on the Ni(111) surface in the pre sence and absence of hydrogen atmospheres. The surface reactions are c haracterized with temperature-programmed reaction spectroscopy (TPRS) under ultrahigh-vacuum (UHV) conditions and with in situ fluorescence yield (FY) studies on ultrasoft X-ray absorption in H-2 pressures up t o 0.01 Torr. The adsorbed configurations of CHA and CHA-derived specie s are characterized by ultrasoft X-ray spectroscopy. Dehydrogenation i s the dominant surface reaction for chemisorbed CHA, resulting in the formation of stable polymeric species. Hydrogen addition results in si multaneous reaction-limited desorption of benzene and ammonia at 380 K as the C-N bond is being cleaved, The yield of the C-N bond activatio n products increases with H-2 pressure. Single deuteration of both ben zene and ammonia is observed at 380 K during experiments in external d euterium. On the basis of these experiments and comparisons with react ivity patterns on the more open Ni(100) surface, we propose that C-N b ond activation on the flat Ni(111) surface occurs through hydrogen add ition to the C-N bond in an aromatic intermediate adsorbed parallel to surface. The primary polymeric overlayer is stable up to 600 K and de composes to yield N-2 at 820 K. Above 500 K, carbon in the dehydrogena ted intermediate diffuses into the nickel subsurface. External hydroge n decreases the stability of the high-temperature CHA derived polymeri c species, yielding predominantly HCN and C2H4 beginning near 450 K.