Helmut Schmelz

Characterization of mixed copper-manganese oxides supported on titania catalysts for selective oxidation of ammonia

Titania-supported copper-manganese mixed oxides were investigated in view of the selective oxidation of ammonia to dinitrogen and characterized by X-ray diffraction, photoelectron spectroscopy and temperature programmed reduction. The mixed oxides proved to be more active than the corresponding individual oxides. The TPR-results show that the reduction occurs at lower temperatures than that of the unsupported oxides. It is concluded that titania facilitates the change of oxidation states. Most active and selective for N2-formation is a catalyst with a molar ratio of Cu: Mn = 20:80. It contains a crystalline Cu1+xMn2−xO4 spinel phase (determined by XRD) and an additional X-ray amorphous Mn2+ -containing species (determined by XPS), possibly Mn3O4 or MnO. Copper is present in the mono- and divalent oxidation state (shown by XPS). All catalysts hardly loose activity with time on stream, above all the copper-rich catalysts are very stable.

Effect of the method of preparation on the properties of titania-supported vanadia catalysts

The influence of the method of preparation and of calcination temperature and atmosphere on the nature of V2O5/TiO2 catalysts has been investigated by IR, Raman and UV - VIS spectroscopy, X-ray diffraction and surface-area measurements. After calcination at moderate temperatures (673 and 773 K), catalysts prepared in the presence of water by different methods show the formation of polyvanadate on the titania anatase surface. Calcination at 873 K causes rutilization and a dramatic decrease in surface area, the polyvanadate being transformed into V2O5. Furthermore, a VxTi1−x,O2 phase is formed. Calcination of catalysts prepared by grinding of V2O5 or NH4VO3 with TiO2 in a mortar causes spreading of V2O5 over the carrier without a strong interaction. The extent of spreading is higher if V2O5 is used and seems to depend on the crystal orientation between V2O5 and TiO2, crystal size and morphology.

Vanadia supported on titania—silica: Physical characterization and activity for the selective reduction on nitric oxide

Abstract Silica-supported titania samples containing between 8 and 34% (w/w) TiO2 were prepared and used as supports to prepare supported vanadia [7% (w/w) V2O5] catalysts for the selective catalytic reduction of nitric oxide. All materials were physically characterized by surface area measurements, UV-VIS diffuse reflectance and Raman spectroscopy and temperature-programmed reduction. The vanadia-containing materials were further characterized by their catalytic activity for the NO + NH3 reaction. The titania-silica preparations contain highly dispersed titania with a tendency to form aggregates with an anatase structure as the titania loading increases. It appears that two types of surface vanadates are being formed, namely a polyvanadate species anchored onto the uncovered SiO2 surface and a second species which is bonded to titania. The catalytic activity of vanadia supported on titania-silica was significantly greater than that of vanadia on silica and increased with titania loading until it reached the theoretical monolayer capacity. A correlation between catalytic activity and concentration of dioxovanadium species was not observed.

An X-ray photoelectron spectroscopy study of oxides of arsenic supported on TiO2

Abstract XPS binding energies for the 3d levels of As5+ and As3+ have been determined to be 46.0 ± 0.17 eV and 44.6 ± 0.13 eV respectively, for oxides of arsenic being adsorbed on titania. When As2O3 is deposited on titania from the vapour phase it is oxidized to form As5+ species, probably orthoarsenate (V) species which replace OH groups on the titania surface. An X-ray induced reduction of these pentavalent As surface species during XPS data acquisition was observed.

Extended defects in hexagonal BaTiO3

Abstract At 1460°C in air, BaTiO3 shows a phase transition from the cubic to the hexagonal phase. Under reducing conditions, however, the phase transition temperature can be lowered to 1330°C. After cooling, the hexagonal phase is preserved at room temperature. Hexagonal BaTiO3 ceramics prepared in a reducing atmosphere were analysed by transmission electron microscopy. Stacking faults in the basal plane extending over several hundred micrometres were found with a density of 106 m−1. The density of dislocations was approximately 108 cm−2 and was much higher than in cubic BaTiO3 ceramics processed by the same heat treatment. Perfect and partial dislocations lying in the basal plane were analysed by their diffraction contrast. Perfect dislocations have Burgers vectors b of〈100〉 which is the shortest lattice vector of this structure. Perfect dislocations dissociate into Shockley partial dislocations. Diffraction contrast analysis revealed that stacking faults have shear vectors R of ⅓〈111〉. Such defects are ...

Preparation of selective catalytic reduction catalysts via milling and thermal spreading

Abstract New catalyst preparation methods which avoid the formation of waste water are desirable for environmental reasons, especially in the case of catalysts produced in amounts as large as those in the DeNOx sector. Thermal spreading has recently been applied in the preparation of titania-supported tungsten and molybdenum monolayer catalysts. In this paper we compare catalysts which are prepared via ball milling of mixtures of the individual precursor oxides with subsequent thermal spreading to catalysts which are prepared by a conventional coprecipitation process. Titania (anatase) was milled in the presence of either tungsten trioxide or molybdenum trioxide and these mixtures were subsequently calcined in a stream of oxygen which contained water vapor. After calcination, no crystalline WO3 or MoO3 was detected by XRD or DRIFTS, indicating a good dispersion of the active oxides. The overtones of tungsten and molybdenum oxygen double bond vibrations, which are representative for the formation of surface polyoxo compounds, were detected with DRIFT spectroscopy for samples prepared either by coprecipitation or via ball milling. The intensity distribution of the hydroxyl groups for ball mill prepared samples was different from the distribution for coprecipitated samples. When the physical mixtures were milled for several hours, the anatase was transformed into rutile as shown by X-ray diffraction. Furthermore, a decrease in surface area and changes in the pore size distribution were observed in correlation with the milling.

Infrared-spectroscopic investigations of selective catalytic reduction catalysts poisoned with arsenic oxide

Abstract Titania-supported tungsten and molybdenum SCR catalysts were investigated before and after poisoning with arsenic(III) oxide. Diffuse reflectance and transmission infrared spectroscopy, including the adsorption of carbon monoxide and ammonia as probe molecules, were applied. The surface of the unpoisoned catalysts is characterized by hydroxyl groups of the support and by surface polytungstate and polymolybdate species carrying terminal metal oxygen double bonds. Carbon monoxide adsorbs on coordinatively unsaturated (cus) metal cations, representing Lewis acid sites, and via H-bonding on hydroxyl groups, representing Bronsted acid sites. From the carbon monoxide adsorption data it is concluded, that the cus cations on tungsten containing samples are stronger Lewis acid sites than those on molybdenum containing samples. Ammonia is adsorbed on Lewis acid sites on both catalysts. Poisoning of the samples by arsenic(III) oxide leads to replacement of the hydroxyl groups by new species that are assigned to arsenic-hydroxyls of a well distributed surface arsenate species. Interaction of the arsenate with the polyoxostructures is shown by the perturbation of the metal-oxygen stretching band of the active phase after poisoning. Important with regard to catalysis may be the fact that neither carbon monoxide nor ammonia can be adsorbed on Lewis surface sites on catalysts poisoned with sufficient amounts of arsenic(III) oxide.

Interaction of arsenious oxide with DeNox-catalysts: An X-ray absorption and diffuse reflectance infrared spectroscopy study

The interaction of arsenious oxide with deNO{sub x}-catalysts such as titania-supported oxides of molybdenum and tungsten has been studied by X-ray absorption spectroscopy (XANES and EXAFS) and by diffuse reflectance FT infrared spectroscopy (DRIFT). It is shown by XANES at the As K edge that the majority oxidation state of the arsenic surface species is As{sup 5+}. EXAFS beyond the As K edge indicates a (3+1) oxygen coordination shell around the arsenic with As-O distances of 1.67 and 1.94 {angstrom}, respectively. A characteristic new infrared hydroxyl stretching band at 3610 cm{sup -1} is formed and can be assigned as an AsO-H stretching mode. These experimental results can best be associated with an orthoarsenate(V) surface species which forms when arsenious oxide is deposited on the catalyst surfaces. XANES at the W L{sub 1} and W L{sub 3} edges suggest that the orthoarsenate interacts directly with tungsten sites, a condition which is supported by the strong perturbation of the new W{double bond}O stretching overtone on formation of the arsenate. It is tentatively suggested that the poisoning of titania-supported tungsten oxides deNO{sub x}-catalysts is due to blocking of active sites which involve coordinatively unsaturated tungsten centers by the orthoarsenate surface species.

Low temperature infrared study of carbon monoxide adsorption on sulfated titania

Acidic properties of a TiO2 sample containing approximately 1 wt% S were examined by low-temperature IR spectroscopy of CO and by coadsorption of CO and NH3. Due to the almost negligible OH content of the sample, Brønsted acidity could not be detected. However, Lewis acid sites with enhanced acid strengths as compared to sulfate-free titania could clearly be detected. It is inferred that these sites (namely cus Ti4+ sites) are located in close proximity to sulfate groups and that CO (and NH3) coordinated to the Ti4+ sites interact with the sulfate groups presumably via electronic inductive effects.

Preparation of AlVO4-films for sensor application via a sol-gel/spin-coating technique

Abstract The preparation of thin AlVO4 films on corundum substrates for possible sensor application is described. Thermally stable, crack-free thin films having thicknesses typically below 0.1 μm were obtained by a sol-gel process using alkoxide reagents combined with a spin-coating technique. The films were structurally characterized by XRD, Raman and FTIR spectroscopy and their morphology was analyzed by SEM. XRD showed that the films consisted almost exclusively (> 90%) of AlVO4 after calcination at 923 K.