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This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts., (Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The selective hydrogenation of propyne over a Pd-black model catalyst was investigated under operando conditions at 1 bar making use of advanced X-ray diffraction (bulk sensitive) and photo-electron spectroscopy (surface sensitive) techniques. It was found that the population of subsurface species controls the selective catalytic semi-hydrogenation of propyne to propylene due to the formation of surface and near-surface PdCx that inhibits the participation of more reactive bulk hydrogen in the hydrogenation reaction. However, increasing the partial pressure of hydrogen reduces the population of PdCx with the concomitant formation of a β-PdHx phase up to the surface, which is accompanied by a lattice expansion, allowing the participation of more active bulk hydrogen which is responsible for the unselective total alkyne hydrogenation. Therefore, controlling the surface and subsurface catalyst chemistry is crucial to control the selective alkyne semi-hydrogenation. [ABSTRACT FROM AUTHOR]

Abstract: We have performed the first “high-pressure” X-ray photoelectron spectroscopy (XPS) study on the palladium, hydrogen, and olefin (trans-2-pentene) system to gain better insight into the hydrogenation reaction. We report here data collected with the use of a Pd(111) single crystal and a polycrystalline foil. Hydrogenation was observed on polycrystalline foil (RT and 373 K) but not on Pd(111) single crystal, as revealed by on-line mass spectrometry. We observed the reaction in the presence of a huge amount of carbon (up to 73%) in the information depth of XPS. Mainly graphite was present on Pd(111), whereas other components, C–H and C–Pd, were also formed on the foil to a much greater extent. C–Pd characterizes a carbon species in the interaction with palladium, whereas C–H represents hydrogenated carbon, including chemisorbed species. The d-band of the foil showed a remarkable upshift toward compared with Pd(111). We concluded that the differences found in the valence and the C1s region are indicators of different electronic structures that contribute to the variation in activity. The palladium foil lost its activity at an elevated temperature (523 K), most probably because of desorption of hydrogen. From additional UPS measurements, we concluded that trans-2-pentene is hydrogenated in σ-bonded chemisorption modus, at least in UHV conditions. [Copyright &y& Elsevier]

Abstract: In light of accumulating evidence highlighting the major effect of operational conditions (gas composition, pressure, temperature) on the surface/bulk structure of catalytic materials, their characterization should involve more and more in situ methods. We constructed a synchrotron-based high-pressure X-ray photoelectron spectroscopic (XPS) instrument, allowing us to investigate the surface and near-surface state of a catalyst in the mbar pressure range. We discuss here the surface characteristics of palladium samples as a function of gas phase (hydrogen, oxygen) and temperature. We demonstrate that the surface region of catalytic materials behaves dynamically in its composition, always reflecting its environment. For example, surface oxide can be formed on Pd(111) in oxygen, which decomposes rapidly when the gas supply is switched off. The chemical nature of carbonaceous deposits also depends strongly on the operational conditions (gas-phase hydrogen, temperature). This is the first time that an XPS investigation of palladium β-hydride was performed at RT. The possible drawbacks of using a non-UHV setup (e.g., fast carbon accumulation) are also discussed. [Copyright &y& Elsevier]