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Pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of ArKr have been recorded in the wavenumber range 108,000-118,000 cm-1 using a 1 + 1' two-photon resonant excitation scheme from the ground X 0+ state of ArKr via selected intermediate states located just below the Ar(1S0) + Kr([5p[1/2]0) dissociation limit. The positions of ionic vibrational levels with quantum numbers from 1 to 30, from 0 to 8 and from 0 to 6 were determined for the X(1/2), A1(3/2) and A2(1/2) states, respectively. The assignment of absolute vibrational quantum numbers of the ionic states was derived from the isotopic shifts of the spectral lines. Combining these data with literature data on the B(1/2) → X(1/2), C1(3/2) → A1(3/2) and C2(1/2) → A2(1/2) band systems of ArKr+ enabled the derivation of potential energy functions for the lowest six electronic states of ArKr+ using a global potential model that includes the effects of the spin-orbit, charge-exchange and long-range interactions. [ABSTRACT FROM AUTHOR]

Abstract: The gerade autoionizing Rydberg states of Ne2 have been studied in the range 162000–172000cm−1 by 1+1′ resonant two-photon excitation from the Ne2 X ground state via different vibrational levels of the Ne2 C state. A rotationally resolved part of the spectrum allowed the determination of the potential energy functions of two states of 1g and characters in the vicinity of the Ne(2p6 1S0)+Ne (2p54p′) dissociation limit. The presence of maxima in these potential energy functions is interpreted as originating from a repulsive interaction between the Rydberg electron and the neutral atom. [Copyright &y& Elsevier]

Abstract: The surface of vanadium phosphorus oxide (VPO) catalysts was investigated by (in situ) X-ray photoelectron spectroscopy (XPS) under reaction conditions. Two differently prepared VPO samples with similar catalytic activities showed different spectral behaviour while the catalytic conditions were changed. The vanadium surface oxidation state of both catalysts was found to have the same value close to 4 under reaction conditions, while the oxidation state of vanadium in deeper layers differed significantly. The experimental results suggest that in VPO the catalytically active species located in the topmost surface layers (up to 1nm depth) are only weakly related to the structure of deeper layers. Based on our results we suggest that the deeper layers act as a substrate material only and can be different from the surface. [Copyright &y& Elsevier]

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]

In situ analysis of the surface of a working selective oxidation catalyst is an essential yet rarely conducted experiment in attempts to derive structure–function relationships. The case study of n-butane oxidation over vanadyl pyrophosphate (VPP) is used to develop a general working hypothesis and to illustrate that the molecular properties of the substrate sets boundary conditions on the surface chemical properties of the catalyst. Experiments using in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy X-ray absorption spectroscopy are used to derive compositional, electronic, and geometric structural information of the surface of the working VPP. These data allow the conclusion that a surface phase different from VPP must be present covering at least part of the active material. The recent data together with literature observations are used to derive a scenario explaining the function of VPP as a unique catalytic system. [ABSTRACT FROM AUTHOR]

The adsorbate species present during partial oxidation of methanol on a Cu(110) surface have been investigated in the 10-5 mbar range with in situ x-ray photoelectron spectroscopy and rate measurements. Two reaction intermediates were identified, methoxy with a C 1s binding energy (BE) of 285.4 eV and formate with a C 1s BE of 287.7 eV. The c(2×2) overlayer formed under reaction conditions is assigned to formate. Two states of adsorbed oxygen were found characterized by O 1s BE’s of 529.6 and 528.9 eV, respectively. On the inactive surface present at low T around 300–350 K formate dominates while methoxy is almost absent. Ignition of the reaction correlates with a decreasing formate coverage. A large hysteresis of ≈200 K occurs in T-cycling experiments whose correlation with adsorbate species was studied with varying oxygen and methanol partial pressures. The two branches of the hysteresis differ mainly in the amount of adsorbed oxygen, the methoxy species, and a carbonaceous species. Methoxy covers only a minor part of the catalytic surface reaching at most 20%. Above 650 K the surface is largely adsorbate-free. [ABSTRACT FROM AUTHOR]

Abstract: CuX@SWCNT (X=Cl, Br, I) nanostructures were prepared by capillary filling of 1.4–1.6nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X=Cl, Br, I) represents a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI

Abstract: Identifying the composition of the catalytically active state of metal catalysts under dynamic operating conditions is of particular importance for oxidation catalysis. Here we report new insights into the chemical identity of different catalytically active states formed on a Ru(0001) catalyst during CO oxidation at various reaction temperatures. The changes in the surface composition of the Ru catalyst and the CO2 yield under varying reaction conditions in the mbar pressure range were followed in situ by synchrotron-based high-pressure X-ray photoelectron spectroscopy and mass spectroscopy. The results reveal that the catalytic activity of a few layers thick surface oxide without well-defined stoichiometry and structure is comparable with that of the stoichiometric RuO2(110) phase. This surface oxide forms under reaction conditions when RuO2 formation is kinetically hindered and can coexist with RuO2 in wide temperature and pressure ranges. [Copyright &y& Elsevier]

Abstract: The thermal and native oxidation of CuGaSe2 thin films was studied by in situ X-ray photoelectron spectroscopy (XPS). The special design of the XPS chamber allowed to measure XP-spectra under oxidizing gas atmospheres at pressures of up to 5mbar (in situ) or in ultra high vacuum (UHV). During thermal oxidation, the formation of predominantly Ga2O3 and some amount of SeO2 were observed, but no copper oxides could be detected in the near surface region of the thin films. The same oxides were found after native oxidation in air under ambient conditions. Only after long term native oxidation for longer than 4months Cu(OH)2 was detected. An additional sodium oxide compound formed at the thin film surface, Na x O and Na2CO3 after thermal and native oxidation, respectively. The amount of these sodium oxide compounds depends on the Na content on the as prepared surface. The formation of SeO2 under humid conditions at 100°C was found to depend on the surface composition of the thin film. [Copyright &y& Elsevier]

The surface of an activated vanadium phosphorus oxide (VPO) catalyst was investigated by means of in situ X-ray absorption spectroscopy in the total electron yield mode. We observed significant changes of the V L3-near edge X-ray absorption fine structure (NEXAFS) when the material was transferred from room temperature to working conditions at 400 °C in the reaction atmosphere. We studied the same VPO material under different gas compositions comprising the reaction mixture of n-butane and oxygen, pure oxygen and vacuum to elucidate the influence of the gas–surface interaction and the effect of the temperature. The results of this extensive study indicate a dynamic response of the catalyst surface to the applied conditions. [Copyright &y& Elsevier]