Role of Hydrogen Species in Palladium-Catalyzed Alkyne Hydrogenation

Selective alkyne hydrogenation in the presence of carbon-carbon double bond compounds, for which Pd is an excellent catalyst, is a strategically important large-scale industrial process. Although in palladium, functionality and structure are closely interrelated, knowledge of the structure of Pd is insufficient as the interaction with the chemical environment causes drastic compositional changes near the subsurface region: while unselective hydrogenation proceeds in the presence of a beta-hydride phase, selective hydrogenation can be achieved only in the presence of a near-surface Pd-C phase. Here, we show from a combination of in situ prompt gamma activation analysis and ab initio simulations based on density functional theory that (i) the presence of the Pd-C phase created under selective conditions implies a strong change in the surface and subsurface stability of hydrogen, (ii) there is still a slower exchange of bulk and surface hydrogen, and (iii) the reaction rate for selective hydrogenation is independent of the bulk H/Pd ratio.