Co-adsorption of aniline and H2 over Pd/Al2O3: An infrared spectroscopic study

Abstract

In order to rationalise high aniline selectivity in nitrobenzene hydrogenation over alumina-supported Pd catalysts, the adsorption complex geometry of aniline during co-adsorption of H2 and aniline on a 5 wt % Pd/Al2O3 catalyst is investigated via infrared spectroscopy. With respect to molecular symmetry and the metal surface selection rule, observation of solely out-of-plane aniline modes at low aniline exposures at 30 oC indicate a parallel orientation of aniline with respect to the Pd surface. The simultaneous emergence of negative hydroxyl features indicate adsorption occurs at the metal/support interface. Increasing exposure reveals additional adsorption of aniline to a range of hydroxyls of the alumina support. The full range of adsorption spectra reveal that the presence of H2 in the co-feed accelerates aniline adsorption to the catalyst and permits reagent hydrogenation to cyclohexylamine. Temperature-programmed infrared measurements show no change in the adsorption complex geometry as a function of surface coverage. A previous reaction scheme is modified in which nitrobenzene and aniline adsorption geometry are intrinsically related to the high aniline selectivity reported for Pd/Al2O3 catalysts.