On the Orientation of Quinoline on Pd{111}:  Implications for Heterogeneous Enantioselective Hydrogenation

The adsorption geometry of quinoid species at platinum and palladium surfaces is relevant to an understanding of enantioselective catalytic hydrogenation. Accordingly, quinoline multilayers and monolayers on Pd{111} have been characterized by XPS and the adsorption geometry within the monolayer determined by NEXAFS at 295 K and 360 K. The molecule lies approximately flat (α < 18° ± 5°), bound to the Pd{111} surface predominantly via the aromatic π-system. This adsorption geometry remains unaffected upon heating to 360 K, and the adsorbed layer does not exhibit long range order under any conditions. Comparison with the corresponding results for Pt{111} indicates that the different intrinsic rates and the reversal in the sense of enantioselectivity observed for pyruvate hydrogenation on palladium catalysts relative to platinum is unlikely to originate in either significantly different modifier adsorption geometries or spatial distributions in the two cases. In addition, it now seems less likely that the collapse in enantioselectivity observed beyond ∼320 K on quinoid-modified palladium catalysts originates in a change in the adsorption geometry of the anchoring moiety of the chiral modifier.