Understanding Ligand-Directed Heterogeneous Catalysis: When the Dynamically Changing Nature of the Ligand Layer Controls the Hydrogenation Selectivity.

We present a mechanistic study on the formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β-unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and close to 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real-space microscopic and in-operando spectroscopic surface-sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable N-butylimine species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.