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 and reactions of cyclohexylamine (CHA) on the Ni(111) surface in the presence and absence of hydrogen atmospheres. The surface reactions are characterized 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 to 0.01 Torr. The adsorbed configurations of CHA and CHA-derived species are characterized by ultrasoft X-ray spectroscopy. Dehydrogenation is the dominant surface reaction for chemisorbed CHA, resulting in the formation of stable polymeric species. Hydrogen addition results in simultaneous 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 activation products increases with H-2 pressure. Single deuteration of both benzene and ammonia is observed at 380 K during experiments in external deuterium. On the basis of these experiments and comparisons with reactivity patterns on the more open Ni(100) surface, we propose that C-N bond activation on the flat Ni(111) surface occurs through hydrogen addition to the C-N bond in an aromatic intermediate adsorbed parallel to surface. The primary polymeric overlayer is stable up to 600 K and decomposes to yield N-2 at 820 K. Above 500 K, carbon in the dehydrogenated intermediate diffuses into the nickel subsurface. External hydrogen decreases the stability of the high-temperature CHA derived polymeric species, yielding predominantly HCN and C2H4 beginning near 450 K.