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.
The CO content of hydrogen feed to proton exchange membrane fuel cells (PEMFC) must be kept under 1-100 ppm for their proper operation. This can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen (PROX). The present study reports on the mechanism of PROX reaction on Pt/CeO 2 catalyst, by using catalytic tests, in-situ DRIFTS, high-pressure XPS, HRTEM and TDS techniques. Bulk metallic, pronounced adsorbate-induced surface Pt and a small amount of oxidized Pt sites were detected by in-situ, high pressure XPS under PROX conditions. The pre-oxidized ceria surface was strongly reduced in pure H 2 but significantly re-oxidized under PROX conditions (i.e. O 2 +CO in excess hydrogen) at T=358 K. The remaining small amount of Ce 3+ decreased with increasing temperature. HRTEM found well-crystallized CeO 2 particles (8-10 nm) in the case of activated (pre-oxidized) sample that transformed in a large extent to an oxygen deficient ceria super-cell structure after PROX reaction. Metallic Pt particles (2-3 nm) and small (0.5-0.6 nm) Pt clusters were found by HRTEM. These findings were in accordance with the variations in relative intensity of the corresponding Pt-CO bands (DRIFTS). Different types of carbonate and formate species were detected (XPS and DRIFTS). Their possible role in the reaction mechanism is discussed. Resolved OH bands could not be found by DRIFT in the PROX reaction mixture indicating significant amount of adsorbed water in a hydrogen-bonded structure. Its presence seems to suppress hydrogen oxidation while CO oxidation still takes place, as the metallic particles are covered by CO (DRIFTS). The direct contribution of surface water in a low-temperature water-gas-shift (LTWGS) type reaction in the PROX mixture is proposed.
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