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.

Vanadia species on aluminas (delta- and gamma-Al2O3) with surface VOx density in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temperature-programmed reduction in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm(-1), observed on dehydrated V/delta-, gamma-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidation reactions catalyzed by supported vanadia. Two types of frequency shifts of the V=O stretching band (1013-1035 cm(-1)) have been observed in the Raman spectra of V/Al2O3: a shift as a function of surface VOx density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VOx density is due to the change of VOx structure. The V=O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymerized VOx species while visible Raman is more sensitive to highly polymerized VOx species and crystalline V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Catalytic hydrogenations are one of the most important processes of the chemical industry and selectivity is a timely issue. The presence of carbon-carbon triple bond compounds in the alkene stream is undesirable in both chemical and polymer-grade propylene and ethylene, due to poisoning of the polymerization catalysts. [1] Starting with a mixture of alkynes and alkenes, Pd-based catalysts are known to be capable to selectively hydrogenate the triple bonds leaving the olefinic function intact. The presence of carbonaceous deposits was assumed to play a key role, influencing the performance of the catalyst. [2] Although the origin of this behavior has been a puzzle so far, very recently a relation between selectivity and subsurface chemistry has been established, showing that the population of subsurface sites of Pd, by either hydrogen or carbon, governs selective alkyne hydrogenation. [3] Hydrogen in the subsurface has been demonstrated to play a crucial role by hydrogenating surface species effectively. [4,5] Herein, we extend our previous studies addressing the fundamental differences of carbon-carbon double or triple bond hydrogenation over Pd. The present results provide solid evidence that the difference in the hydrogenation of alkynes/alkenes stems back to the strongly different state of the near-surface region. We show that the formation of the PdC surface phase is directly related to the nature of the reactive molecule, being a general pattern for alkyne hydrogenation and absent for alkenes. We show in addition that the presence of this PdC phase affects the surface chemistry by decreasing the amount of more active subsurface hydrogen with respect to the more selective onsurface ones, hence controlling the state of the Pd catalyst surface under reactive conditions.In order to unravel the nature of the active Pd surface, we have studied, by in situ X-ray Photoelectron Spectroscopy (XPS), the near-surface region of palladium under various hydrogenation conditions for a set of reacting alkynes and alkenes. Under the reduced pressure conditions of 1 mbar, alkynes are hydrogenated to alkenes while alkene feeds are transformed to alkanes, just as at more realistic conditions. Figure 1 summarizes the state of palladium under such hydrogenation conditions. The palladium 3d core level was found to be the best indicator of the nearsurface state. [3] While the metallic bulk peak is observed at 335 eV, a strong new component (filled peak) appears when alkynes are hydrogenated selectively. This peak at ~ 335.6 eV, when dominating the spectrum, is characteristic of the carbon modified, PdC surface phase. Note that adsorbates give rise to a surface state at similar energy (adsorbate induced surface core level shift); however at the Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac)

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