The Role of Hydrogen in the Hydrogenation and Hydrogenolysis of Aniline on the Nickel Single Crystal Surfaces: Its Implication on the Mechanisms of HDN Reactions

Abstract

The selectivity of hydrogenation and hydrogenolysis reactions for organonitrogen compounds on transition metal surfaces depends heavily on the availability of surface hydrogen surface under reaction conditions. The surface hydrogen produced during dehydrogenation of adsorbed aniline upon thermal activation does not significantly modify hydrogenolysis reactions because it desorbs below the reaction temperatures. A series of experiments which use external hydrogen to control the concentration of surface hydrogen at reaction temperatures are reported here. In situ kinetic measurements in the presence of reactive hydrogen environments have been used to probe the details of the adsorbed species and reaction mechanisms. Nickel single crystals have been used as well defined model catalysts for hydrodenitrogenation (HDN) reactions. Previously, the effect of external hydrogen on aniline hydrogenolysis on the Pt(111) surface has been reported. On Pt(111), C-N bond activation is substantially enhanced in the presence of hydrogen. The increased C-N bond cleavage is facilitated by hydrogen which maintains a parallel adsorption of the aromatic derivative of aniline. In the absence of surface hydrogen, the adsorbed intermediate tilts away from surface because of partial dehydrogenation with increasing temperature at about 400 K. This paper will discuss a recent study of aniline reactions on the Ni(100) and Ni(111)more » surfaces both in the presence and absence of hydrogen. Reactivity comparisons will also be made for these two nickel surfaces towards adsorbed aniline.« less