The adsorption of aniline over alumina-supported palladium: an infrared spectroscopic and computational study

Vibrational spectroscopy (predominantly infrared (IR) spectroscopy) is applied to examine the adsorption of aniline over a 5 wt% Pd/Al2O3 catalyst.Diffuse-reflectance Fourier transform infrared spectroscopic measurements at 30C undertaken within a low coverage regime indicate aniline adsorption to occur in a parallel orientation with respect to the metal at the support/Pd interface.At higher coverages, aniline adsorption shifts to the alumina support.Temperature-programmed IR spectroscopic measurements indicate the adsorption strength of aniline partitioned between four adsorption sites to follow the trend: support/Pd interface < type III tri-bridged hydroxyls < type IIa bridged hydroxyls type IIb bridged hydroxyls.Computational (DFT) studies concentrating on the geometry of the Pd-aniline adsorption complex in the low coverage regime discern the parallel orientation to be the preferred structure, in agreement with the infrared spectroscopic studies.The aniline adsorption in a parallel orientation with respect to the metal surface at the support/Pd interface positions the aromatic ring parallel with the active Pd surface, thereby enabling hydrogenation to cyclohexylamine, the first over-hydrogenation product associated with aniline.The predominant re-adsorption of aniline to the -Al2O3 support is proposed to limit product overhydrogenation in nitrobenzene hydrogenation and, thus, contributes to the reported elevated aniline selectivity over Pd/Al2O3 catalysts.