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Abstract

Alkynes can be selectively hydrogenated on palladium catalysts. This process requires however a strong modification of the near-surface region of Pd, in which carbon (autocatalytically fragmented feed molecules) occupies interstitial lattice sites. We have developed a new, in situ Prompt Gamma Activation Analysis (PGAA) technique, which allows us to perform nondestructive elemental analysis (and in particular the quantification of absorbed hydrogen in palladium) during catalytic reaction. By the help of PGAA, we conclude that unselective hydrogenation of alkynes proceeds on hydrogen saturated beta-hydride, while selective hydrogenation was only possible after decoupling bulk properties from the surface events. This latter was possible after carbon dissolved in the top few palladium layers and ceased the equilibrium between surface, subsurface and bulk-dissolved hydrogen. The modified electronic structure of Pd and the inhibition of subsurface hydrogen participating in alkyne hydrogenation rendered the reaction selective.