BINAP adsorption on palladium: A combined infrared spectroscopy and theoretical study

The adsorption of (rac)-BINAP on palladium nanoparticles as well as on a Pd film has been investigated with the aim to unravel the adsorption geometry of BINAP applied for the chiral modification of Pd metal surfaces in catalysis. The Pd nanoparticles with a narrow size distribution were characterized by attenuated total reflection (ATR)-IR spectroscopy. The adsorption geometry of BINAP was determined from the analysis of the experimental IR spectra and their comparison with the spectrum calculated by DFT using a model where BINAP is adsorbed on the (111) surface of a Pd cluster. The studies revealed that BINAP adsorbs with its C2 symmetry axis perpendicular to the Pd surface, and due to the steric hindrance by the phenyl rings connected to the two P atoms, the main interaction with the surface occurs via the two aromatic rings that are oriented parallel to the surface and not directly via the two P atoms. The remaining two phenyl rings are orientated in a tilted position toward the surface. To examine the generality of the proposed adsorption geometry, we also studied the adsorption of (rac)-BINAP from an organic liquid phase on a Pd model film by in situ ATR-IR spectroscopy combined with modulation excitation spectroscopy (MES). Two methods for enhancing the transient sorption of BINAP were applied: (a) temperature increase, and (b) introduction of coadsorbates which could compete with BINAP for adsorption sites on the Pd surface. The latter method proved to be powerful for adsorption studies at moderate temperature when the substrate strongly interacts with the surface and signals from surface adsorbates are very weak.