The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation

Abstract

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

Giant carbon solubility in Au nanoparticles

“…Recently, there has been increased awareness of the role of subsurface solutes in bulk-like model catalysts (Pd, Cu, etc.) by recognizing, for example, that carbon in the bulk and the subsurface, usually considered ''frozen,'' actually changes the structure of the surface and even participates in heterogeneous catalysis [55][56][57][58]. Our findings suggest that similar effects may be even more pronounced in nanoparticles, capable of forming mixed metal-carbon structures that have no bulk equivalent and give rise to extremely high solute solubilities.…”

Models in Catalysis

“…The traditional opinion that only surface hydrogen species participate in the hydrogenation process was also questioned by Ceyer and co-workers studying ethylene hydrogenation on Ni(111) [79]. More recently, other experimental and theoretical evidences for the important role of subsurface hydrogen were reported and it has been suggested that the carbonaceous species control the hydrogen distribution on and in the particle [80][81][82]. The existence of different hydrogen species in and on the nanoparticles may potentially result in some important implications for the overall activity and selectivity of the hydrogenation catalyst: if one of the key reaction stepsthe formation of subsurface hydrogen, which is known to be a structure sensitive process-is slow under reaction conditions, the hydrogen permeability of the metal surface may be decisive for hydrogenation activity.…”

Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

“…Although we have not observed sensible modifications on C 1s binding energy by XPS, it cannot be discarded that a modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites could be also responsible for the high partial alkyne hydrogenation as observed in more classical Pd heterogeneous catalysts. 61 Moreover, the partial hydrogenation of the selected alkynes provides TOFs up to 1282 h À1 . The TOF values were calculated considering (I) all the metal amounts employed in the reaction and (II) only the metallic atoms exposed on the NP surface.…”