Combined theoretical and experimental studies of the VO(O2)Hheida− anion dissolved in water that may serve as a functional model for vanadium haloperoxidase enzymes have been performed. The ...geometrical structure and absorption and vibrational spectra of this system have been evaluated within the framework of density functional theory (DFT). The obtained theoretical results on the equilibrium structure and optical spectra are in quite good agreement with the experimental data. With the aid of the combination of UV–visible spectroscopy and electronic structure calculations, it has been revealed that, in the apparent absorption spectra of the VO(O2)Hheida− anion, the highest in energy band corresponds to a ligand to metal electron excitation, while the band with a maximum at 430 nm arises from the peroxo group. The calculations also reproduce quite well the positions, intensities and the grouping of frequencies in the near-infrared (NIR) spectra. The visualization of the calculated vibrations in the energy range of 400–1100 cm–1 has been presented.
In this account the application of synchrotron radiation based X-ray photoelectron spectroscopy (XPS) for the investigation of electrochemically active gas-solid and liquid-solid interfaces will be ...discussed. The potential of Near Ambient Pressure XPS (NAP-XPS) for the estimation of the surface electronic structure of electrochemically active interfaces will be described by two examples. Thereto the oxygen evolution reaction (OER) over Pt and IrOx anodes will be introduced. In particular the analysis of XP core level spectra of IrOx requires the development of an appropriate fit model. Furthermore the design of reaction cells based on proton exchange membranes (PEM) and on electron transparent graphene membranes, which enables the investigation of liquid-gas and liquid-solid interfaces under electrochemical relevant conditions will be discussed. In the last part of this article a perspective to the EMIL project at the synchrotron radiation facility BESSY will be given. The purpose of this project is the implementation of two new beamlines enabling X-ray photoelectron spectroscopy in the X-ray regime from 80eV–8keV under reaction conditions. The extension to the so called tender X-ray regime will allow the release of higher kinetic energy photoelectrons which have a higher inelastic mean free path compared to photoelectrons excited by soft X-ray radiation and therefore will enable the investigation of solid-liquid interfaces under electrochemical reaction conditions.
Progress in science often follows or parallels the development of new techniques. The optical microscope helped convert medicine and biology from a speculative activity in old times to today's ...sophisticated scientific disciplines. The telescope changed the study and interpretation of heavens from mythology to science. X-ray diffraction enabled the flourishing of solid state physics and materials science. The technique object of this review, Ambient Pressure Photoelectron Spectroscopy or APPES for short, has also the potential of producing dramatic changes in the study of liquid and solid surfaces, particularly in areas such as atmospheric, environment and catalysis sciences. APPES adds an important missing element to the host of techniques that give fundamental information, i.e., spectroscopy and microscopy, about surfaces in the presence of gases and vapors, as encountered in industrial catalysis and atmospheric environments. APPES brings electron spectroscopy into the realm of techniques that can be used in practical environments. Decades of surface science in ultra high vacuum (UHV) has shown the power of electron spectroscopy in its various manifestations. Their unique property is the extremely short elastic mean free path of electrons as they travel through condensed matter, of the order of a few atomic distances in the energy range from a few eV to a few thousand eV. As a consequence of this the information obtained by analyzing electrons emitted or scattered from a surface refers to the top first few atomic layers, which is what surface science is all about. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), and other such techniques have been used for decades and provided some of the most fundamental knowledge about surface crystallography, composition and electronic structure available today. Unfortunately the high interaction cross section of electrons with matter also prevents them from traveling long distances unscattered in gas environments. Above the millibar pressure range this distance is reduced to less that a millimeter, effectively preventing its use in the most relevant environments, usually between millibars and atmospheric pressures. There is therefore a large gap of several orders of magnitude where information about surfaces is scarce because these powerful electron spectroscopies cannot operate. One characteristic of surfaces in ambient pressure environments is that they are covered by dense layers of molecules, even when their binding energy is weak. Water for example is known to form layers several molecules thick at room temperature in humid environments. Metals readily form oxide films several layers thick in oxygen atmospheres. Dense layers of adsorbed molecules can also be produced in ultra high vacuum, often by the simple and expedient method of cooling the sample to cryogenic temperatures. A large amount of data has been obtained in the past in UHV by surface scientists using this method. While this has provided valuable information it begs the question of whether the structures formed in this manner represent equilibrium structures or metastable ones, kinetically trapped due to high activation energies that cannot be overcome at low temperature. From a thermodynamic point of view is interesting to consider the entropic contribution to the Gibbs free energy, which we can call 'the pressure factor', equal to kT.logP. This factor amounts to a sizeable 0.3 eV difference at room temperature between UHV (<10{sup -8} Pascal) and atmospheric pressures. Such change if free energy can definitely result in changes in surface structure and stability. Entire areas of the phase diagram are out of reach due to the pressure gap. Even when cooling is not necessary, many surface treatments and most chemical reactions necessitate the presence of gases at pressures ranging from millibar to bars. What is the structure and chemical nature of the species formed on the surface in equilibrium with such gases? As we shall illustrate in this review, APPES provides a much needed electron spectroscopy to analyze surface electronic structure and composition in equilibrium with gases.
Steam reforming of methanol (SRM) was investigated over copper-containing catalysts supported on four different oxides and mixed oxides: Cu/ZnO/Al
2O
3, Cu/ZrO
2/CeO
2, Cu/SiO
2, and Cu/Cr
2O
3/Fe
2O
...3. After observing slight differences in the way of catalyst aging and experimental exclusion of mass transport limitation effects, a detailed kinetic study was carried out at 493 K. The dependence of the reaction rate on the molar ratio of methanol and water was determined, as was the influence of addition of inert nitrogen and the main reaction products hydrogen and carbon dioxide to the reactant mixture. Although there were remarkable differences in the catalytic activity of the samples, the main mechanistic steps reflected in the rate law were similar for all catalysts. The reaction rate is determined mainly by the methanol partial pressure, whereas water is not involved in the rate-determining step, except over Cu/Cr
2O
3/Fe
2O
3, where several differences in the chemistry were observed. Hydrogen and carbon dioxide were found to inhibit the reaction. These results were confirmed by a DRIFT study at 493 K using an equimolar reactant mixture and an excess of 4:1 of water and methanol, respectively. The same surface species could be identified on each catalyst, but neither kinetic modeling nor the DRIFT spectra could clearly indicate whether the reaction pathway occurs via a dioxomethylene species or a methyl formate species as intermediate. Similar activation energies of SRM confirm the assumption that the surface chemistry of SRM over copper-based systems is independent of the catalyst support material.
The effective spin of paramagnetic centers generated during the reaction of a mixture of CH4 and O2 on activated MgO was determined with transient nutation EPR (TN-EPR). For the first time it could ...be shown that apart from the generation of surface-trapped superoxide radicals O2 –, a significant number of paramagnetic centers are formed with an effective spin larger than 1/2. Results from HYSCORE and other pulsed EPR experiments further give evidence that these centers are localized at the surface of the MgO catalyst. These centers are proposed to be exchange coupled clusters of holes and electrons, respectively, some of them formed at specific exposed sites of the highly structured MgO surface. Judging from the relative high concentration of the centers ascribed to hole clusters, it is tempting to assume that they are involved in the catalytic reaction network of the oxidative coupling of methane.
The structurally well-defined intermetallic compound PdGa – a highly selective catalyst for the semi-hydrogenation of acetylene – was characterized by Fourier transform infrared spectroscopy (FTIR)
...in situ X-ray photoelectron spectroscopy and
in situ prompt gamma activation analysis. A strong modification of the electronic states in PdGa compared to elemental Pd was revealed as well as the complete isolation of the Pd atoms on the surface of PdGa.
In situ investigations proved the high stability of the surface, thus excluding segregation phenomena (common for alloys) or sub-surface chemistry involving C and/or H atoms (known for elemental Pd). By suppressing the sub-surface chemistry, the electronic modification as well as the site isolation lead to the high selectivity and long-term stability of PdGa in the semi-hydrogenation of acetylene.
The incorporation of heteroatoms like N in activated carbons is of interest to modify the surface chemistry of the materials and, then, to improve their behavior as catalyst or catalyst support. In ...this work, N-containing activated carbon fibers have been prepared using a petroleum pitch with a low softening point and an N-containing resin. The novelty of the preparation method is that it involves the steps used in the synthesis of activated carbon fibers, i.e. spinning, stabilization, carbonization and activation. The materials have been characterized with techniques such as XPS and UPS, which allows us to follow the changes in both the chemical state of N species and the valence band structure of the carbon samples during the preparation steps.
•The oxidation of methanol over a monolayer V2O5/TiO2 catalyst was examined.•At low temperatures, dimethoxymethane and methyl formate are the main products.•Under mild conditions, the production of ...formaldehyde is greatly inhibited.•According to in situ X-ray photoelectron spectroscopy, the reaction involves reversible reduction of V5+ cations.•The oxidation of methanol proceeds through the classical Mars–van Krevelen mechanism.
The oxidation of methanol over highly dispersed vanadia supported on TiO2 (anatase) has been investigated using in situ Fourier transform infrared spectroscopy (FTIR), near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), X-ray absorption near-edge structure (XANES), and a temperature-programmed reaction technique. The data were complemented by kinetic measurements collected in a flow reactor. It was found that dimethoxymethane competes with methyl formate at low temperatures, while the production of formaldehyde is greatly inhibited. Under the reaction conditions, the FTIR spectra show the presence of non-dissociatively adsorbed molecules of methanol, in addition to adsorbed methoxy, dioxymethylene, and formate species. According to the NAP XPS and XANES data, the reaction involves a reversible reduction of V5+ cations, indicating that the vanadia lattice oxygen participates in the oxidation of methanol via the classical Mars–van Krevelen mechanism. A detailed mechanism for the oxidation of methanol on vanadia catalysts is discussed.
The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron ...microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.
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