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 (FT-IR) spectroscopic measurements at 30°C, 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 (TP-IR) spectroscopic measurements indicate that 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 (density functional theory (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 IR 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 γ-Al2O3 support is proposed to limit product over-hydrogenation in nitrobenzene hydrogenation and, thus, contributes to the reported elevated aniline selectivity over Pd/Al2O3 catalysts. This article is part of the theme issue 'Surfaces, interfaces and heterogeneous catalysis'.