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Abstract

Alkynes can be selectively hydrogenated to alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of Pd, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ x-ray photoelectron spectroscopic measurements under reaction conditions indicate that much less carbon is dissolved in Pd during unselective, total hydrogenation. Additional studies of hydrogen content with in situ prompt gamma activation analysis, which allows us to follow the hydrogen content of Pd during catalysis, indicate that unselective hydrogenation proceeds on hydrogen-saturated -hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus the population of subsurface sites of Pd, either by hydrogen or by carbon, governs the hydrogenation events on the surface.