[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Two oxygen species, which are constituents of the active centers for ethylene epoxidation over silver, have been characterized by a number of physical methods sensitive to adsorbate electronic structure such as x-ray photoelectron, ultraviolet photoelectron, Auger, and x-ray-absorption near-edge structure spectroscopy. One of the species denoted as nucleophilic oxygen due to its activity in total oxidation only exhibits spectroscopic characteristics close to those of bulk Ag2O. This allows us to describe this species as atomically adsorbed oxygen in the structure of surface silver(I) oxide. The considerable extent of the covalency in bonding of this oxidelike oxygen with the silver surface due to hybridization of O2p levels with Ag4d and Ag5sp orbitals should be also emphasized. Contrary to this, only 5sp orbitals of silver hybridize with 2p orbitals of oxygen as the other oxygen species forms. As a consequence, this species being also atomic oxygen is characterized by a lower oxygen-silver bonding interaction and a lower charge on the oxygen. The latter causes the activity of this electrophilic species in epoxidation. Possible models of adsorption centers for these oxygen species are discussed.
Physical Review B 06/2003; 67(23). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Results of a spectroscopic study of two forms of adsorbed atomic oxygen on a silver surface, which participate in ethylene epoxidation reaction, are presented. The possibility of the combined use of the methods of photoelectron spectroscopy and X-ray absorption for a detailed analysis of adsorbate electron structure on solid surfaces is demonstrated. It is found that a significant difference in the position of O 1s lines for nucleophilic (528.3 eV) and electrophilic (530.4 eV) oxygen is determined by the effects of the initial state, that is, by the difference in the charge state of oxygen anions. The use of the well-know correlation of the Auger line splitting with a Pauling charge at an oxygen atom showed a substantial difference (1 electron charge unit) in charge transfer from metal to the nucleophilic or electrophilic adsorbed oxygen atom. Based on the X-ray absorption data of the oxygen K-edge, it is found that there is a substantial overlap of the 4d- and 5sp orbitals of silver with oxygen 2p orbitals in the nucleophilic state in the formation of an Ag–O bond and there is only an overlap of 5sp orbitals of silver with oxygen 2p orbitals in the electrophilic state. Structural models of the adsorption site are presented for both states. The conclusion is drawn that the charge state of oxygen in oxide systems may depend substantially on its binding to metal atoms.
Kinetics and Catalysis 01/2003; 44(3):432-440. · 0.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The oxidation of ammonia to nitrogen or nitric oxide was investigated using on the one hand a polycrystalline copper foil and on the other hand deposited copper clusters prepared with the inert gas aggregation technique. The behavior in the oxidation of ammonia of both model catalysts was studied using in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the soft X-ray range and mass spectrometry. It is shown that the copper foil reacts in a similar way to the copper clusters. Differences appear only with respect to the reaction temperature required, which is lower for the cluster sample. It can be concluded that the results obtained in experiments with polycrystalline copper foil are exemplary for and can be transferred to a supported copper catalyst consisting of small copper particles.
[Show abstract][Hide abstract] ABSTRACT: In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.
Journal of Physical Chemistry B, v.107, 4587-4596 (2003). 01/2003;
[Show abstract][Hide abstract] ABSTRACT: In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in overexchanged Fe/ZSM5. The Fe L2,3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed
understanding of the L2,3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains FeIII ions in an octahedral surrounding, with a total crystal field splitting of ~1.0 eV. This value is significantly smaller than that for Fe2O3, which is indicative of a much weaker Fe-O bonding. The reduced form of Fe/ZSM5 has
FeII ions in a tetrahedral oxygen surrounding. The Fe L2,3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350 °C under the same pressure further reduces the iron valence to 2.15. The oxygen spectra show that the autoreduction is accompanied by a loss of molecular oxygen and water. Reoxidation with 5% O2 in helium yields a valence of >2.90 after 10 min.
Journal of Physical Chemistry B, v.107, 13069-13075 (2003). 01/2003;
[Show abstract][Hide abstract] ABSTRACT: We present the V L3 near edge X-ray absorption fine structure (NEXAFS) of a vanadium phosphorus oxide (VPO) catalyst. The spectrum is related to the V3d–O2p hybridised unoccupied states. The overall peak position at the V L3-absorption edge is determined by the formal oxidation state of the absorbing vanadium atom. Details of the absorption fine structure are influenced by the geometric structure of the compound. Empirically we found a linear relationship between the energy position of several absorption resonances and the V–O bond length of the participating atoms. This allows identification of the contribution of specific V–O bonds to the near edge X-ray absorption fine structure. The bond length/resonance position relationship will be discussed under consideration of relations between geometric structure and NEXAFS features observed in X-ray absorption experiments and theory.
Journal of Electron Spectroscopy and Related Phenomena 01/2002; 125(2):79-87. · 1.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Activation of clean polycrystalline silver by a C2H4 + O2 reaction mixture has been studied by XANES, XPS, and UPS. In situ monitoring of the O K-edge XAS spectrum of the pre-treated silver surface revealed a broad signal at 10–20 eV above the threshold. The comparison of the X-ray absorption spectra with O 1s and valence band photoemission data allowed us to attribute this XAS signal to electrophilic oxygen (E
b(O 1s) = 530.4 eV) which is known to be active in ethylene epoxidation. The complete absence of XAS features in the photon energy range typical for * and * transitions of molecular oxygen (530–535 eV) indicates both the atomic origin of the electrophilic oxygen and the absence of molecular species on the catalyst surface under the present reaction conditions.
[Show abstract][Hide abstract] ABSTRACT: The oxidation of ammonia over polycrystalline copper was investigated by means of in situ NEXAFS (near-edge X-ray absorption fine structure) spectroscopy in the soft X-ray range. The reaction, carried out in a 1:12 excess of oxygen, was observed by mass spectrometry. The simultaneous detection of the surface electronic structure and its catalytic performance allows correlation of different reaction products to the current surface structure of the catalyst. It is shown that a change in total pressure from 0.4 to 1.2 mbar severely affects the reaction path. Copper(I) nitride was identified as poison and a copper oxide was found to be the active phase for the selective oxidation of ammonia to nitrogen.
[Show abstract][Hide abstract] ABSTRACT: Adsorbed oxygen species realized in the course of ethylene epoxidation over polycrystalline silver have been characterized by X-ray absorption near the edge structure and X-ray photoelectron spectroscopy. Namely, the combined application of XANES and XPS in similar UHV conditions using the same sample allowed us to assign an XAS feature to the nucleophilic and electrophilic oxygen. This is of great significance, since these species are suggested to be included into the active center for ethylene epoxidation. The differences in the oxygen–silver bonding of these oxygen species are discussed.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2001; · 1.14 Impact Factor