The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase).

The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam significantly increases both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V(5+) species to the extent of Mo(6+), a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. Furthermore we could prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure are induced by the reaction mixture, which is however not dependent on the steam content.

The electronic factor in alkane oxidation catalysis.

This article addresses the fundamental question of whether concepts from semiconductor physics can be applied to describe the working mode of heterogeneous oxidation catalysts and whether they can be even used to discriminate between selective and unselective reaction pathways. Near-ambient-pressure X-ray photoelectron spectroscopy was applied to the oxidation of n-butane to maleic anhydride on the highly selective catalyst vanadyl pyrophosphate and the moderately selective MoVTeNbO(x) M1 phase. The catalysts were found to act like semiconducting gas sensors with a dynamic charge transfer between the bulk and the surface, as indicated by the gas-phase-dependent response of the work function, electron affinity, and the surface potential barrier. In contrast, only a minor influence of the gas phase on the semiconducting properties and hence no dynamic surface potential barrier was monitored for the total oxidation catalyst V2O5. The surface potential barrier is hence suggested as descriptor for selective catalysts.

Direct imaging of octahedral distortion in a complex molybdenum vanadium mixed oxide.

Complex Mo,V-based mixed oxides that crystallize in the orthorhombic M1-type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X-ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively.

Scanning electrochemical microscopy for the differentiation of radical-induced degradation mechanisms in polymer electrolyte membranes.

In this work, a spatially resolved analytical method based on scanning electrochemical microscopy (SECM) to distinguish different degradation phenomena in polymer electrolyte membranes was developed. SECM was combined with a Franz diffusion cell to distinguish between radical-induced aging of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer due to deactivation of the sulfonic acid groups followed by a decreased proton conductivity, and the radical-induced formation of cracks and holes in the polymer. The experiments were performed with ferrocyanide as redox mediator to detect holes and cracks, and protons (sulfuric acid) to determine the through-plane proton conductivity, respectively. A pristine Nafion™ membrane, a pristine Nafion™ membrane with an artificial pinhole and a Nafion™ membrane aged with Fenton's reagent were investigated to prove the measurement principle. It could be shown that holes and cracks can be reliably detected with this approach and discriminated from a change in proton conductivity. The presence of holes in the investigated aged membranes was confirmed by scanning electron microscopy, whereas the loss of sulfonate groups could be supported by infrared spectroscopy measurements.

Dynamic accelerated stress test and coupled on-line analysis program to elucidate aging processes in proton exchange membrane fuel cells.

The application of hydrogen proton exchange membrane fuel cells (PEMFC) in greenhouse gas emission free heavy-duty vehicles requires extremely durable PEMFC components with service lives in the range of 30,000 h. Hence suitable test and analysis methods are required that reflect realistic operation scenarios, but significantly accelerate aging. For this purpose, a dynamic accelerated stress test was developed, which is coupled with a comprehensive in-depth in-situ and ex-situ analysis program to determine the aging processes of a PEMFC membrane electrode assembly (MEA). The test comprehends dynamic cycling between low, moderate and high load, different temperature and humidity conditions as well as recovery sequences to distinguish between reversible and irreversible failure modes. All phases of the PEMFC system (i.e. solid, liquid and gaseous) are monitored on-line during aging by sophisticated electrochemical, mass spectrometric and ion chromatographic analytical methods. The structural and elemental composition of the MEA before and after the aging program (post-mortem) are investigated by X-ray fluorescence, scanning and transmission electron microscopy. This program was able to age a commercial PEMFC to end-of-life in 1000 h, while providing an accurate picture of the aging processes involved.

The microwave cavity perturbation technique for contact-free and in situ electrical conductivity measurements in catalysis and materials science.

We have developed a noncontact method to probe the electrical conductivity and complex permittivity of single and polycrystalline samples in a flow-through reactor in the temperature range of 20-500 °C and in various gas atmospheres. The method is based on the microwave cavity perturbation technique and allows the simultaneous measurement of microwave conductivity, permittivity and of the catalytic performance of heterogeneous catalysts without any need for contacting the sample with electrodes. The sensitivity of the method towards changes in bulk properties was proven by the investigation of characteristic first-order phase transitions of the ionic conductor rubidium nitrate in the temperature range between 20 and 320 °C, and by studying the temperature dependence of the complex permittivity and conductivity of a niobium(V)-doped vanadium-phosphorous-oxide catalyst for the selective oxidation of n-butane to maleic anhydride. Simultaneously, the catalytic performance was probed by on line GC analysis of evolving product gases making the technique a real in situ method enabling the noninvasive investigation of electronic structure-function relationships.

Investigating and correlating photoelectrochemical, photocatalytic, and antimicrobial properties of [Formula: see text] nanolayers.

Semiconducting transition metal oxides such as [Formula: see text] are promising photo(electro)catalysts for solar water splitting and photoreduction of [Formula: see text] as well as for antibacterial, self-, water and air-cleaning coatings and admixtures in paints, building materials, on window glass or medical devices. In photoelectrocatalytic applications [Formula: see text] is usually used as photoanode only catalyzing the oxidation reaction. In coatings and admixtures [Formula: see text] works as heterogeneous catalyst and has to catalyze a complete redox cycle. While photoelectrochemical charge transport parameters are usually quite well accessible by electrochemical measurements, the quantitative description of photocatalytic properties is more challenging. Here, we present a systematic structural, photoelectrocatalytic, photocatalytic and antimicrobial study to understand if and how photoelectrochemical parameters can be used to predict the photocatalytic activity of [Formula: see text]. For this purpose [Formula: see text] thin films on flourine-doped tin oxide substrates were prepared and annealed at temperatures between 200 and 600 [Formula: see text]. The film morphologies and thicknesses were studied by GIXRD, FESEM, and EDX. Photoelectrochemical properties were measured by linear sweep voltammetry, photoelectrochemical impedance spectroscopy, chopped light chronoamperometry, and intensity modulated photocurrent/ photovoltage spectroscopy. For comparison, photocatalytic rate constants were determined by methylene blue degradation and Escherichea coli inactivation and correlated with the deduced photoelectrocatalytic parameters. We found that the respective photoactivities of amorphous and crystalline [Formula: see text] nanolayers can be best correlated, if the extracted photoelectrochemical parameters such as charge transfer and recombination rates, charge transfer efficiencies and resistances are measured close to the open circuit potential (OCP). Hence, the interfacial charge transport parameters at the OCP can be indeed used as descriptors for predicting and understanding the photocatalytic activity of [Formula: see text] coatings.

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses.

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.