Staffan Hansen

On the Non-Stoichiometry in Rutile-Type »SbVO4

Heating equimolar mixtures of Sb{sub 2}O{sub 3} and V{sub 2}O{sub 5} at 800{degrees}C in flowing gas with varying O{sub 2}/N{sub 2} ratios produces a continuous nonstoichiometric series of rutile type, i.e., Sb{sub 0.9}V{sub 0.9+x}{open_square}{sub 0.2-x}O{sub 4}, Sb{sub 0.9}{sub 0.9}{open_square}{sub 0.2}O{sub 4}, a = 4.63, c = 3.03 {angstrom} (X-ray powder data, XRD), is formed in pure oxygen and exhibits a modulated structure with an approximate supercell: 2{radical}2a, 2{radical}2b, 4c (electron diffraction, (ED)). In pure nitrogen, reduced Sb{sub 0.9}V{sub 1.1}O{sub 4}, a = 4.60, c = 3.08 {angstrom} (XRD), with the supercell {radical}2a, {radical}2b, 2c (ED), is produced. Heating at intermediate partial pressures of oxygen give phases with the basic rutile cell a = b, c (XRD, ED). The formulation of this series is supported by data obtained by Fourier transform infrared spectroscopy. Under reducing conditions (in pure nitrogen), a solid solution series of Sb{sub 0.9}V{sub 1.1}O{sub 4} and VO{sub 2} is observed, i.e., Sb{sub 0.9-y}V{sub 1.1+y}O{sub 4}, 0 < y < 0.7. Vanadium-rich Sb{sub 0.2}V{sub 1.8}O{sub 4}, with a = 4.56, c = 2.99 {angstrom} (XRD), exhibits a basic rutile lattice with diffuse intensity between Bragg spots (ED).

On the nonstoichiometry in rutile-type {approximately}SbVO{sub 4}

Heating equimolar mixtures of Sb{sub 2}O{sub 3} and V{sub 2}O{sub 5} at 800{degrees}C in flowing gas with varying O{sub 2}/N{sub 2} ratios produces a continuous nonstoichiometric series of rutile type, i.e., Sb{sub 0.9}V{sub 0.9+x}{open_square}{sub 0.2-x}O{sub 4}, Sb{sub 0.9}{sub 0.9}{open_square}{sub 0.2}O{sub 4}, a = 4.63, c = 3.03 {angstrom} (X-ray powder data, XRD), is formed in pure oxygen and exhibits a modulated structure with an approximate supercell: 2{radical}2a, 2{radical}2b, 4c (electron diffraction, (ED)). In pure nitrogen, reduced Sb{sub 0.9}V{sub 1.1}O{sub 4}, a = 4.60, c = 3.08 {angstrom} (XRD), with the supercell {radical}2a, {radical}2b, 2c (ED), is produced. Heating at intermediate partial pressures of oxygen give phases with the basic rutile cell a = b, c (XRD, ED). The formulation of this series is supported by data obtained by Fourier transform infrared spectroscopy. Under reducing conditions (in pure nitrogen), a solid solution series of Sb{sub 0.9}V{sub 1.1}O{sub 4} and VO{sub 2} is observed, i.e., Sb{sub 0.9-y}V{sub 1.1+y}O{sub 4}, 0 < y < 0.7. Vanadium-rich Sb{sub 0.2}V{sub 1.8}O{sub 4}, with a = 4.56, c = 2.99 {angstrom} (XRD), exhibits a basic rutile lattice with diffuse intensity between Bragg spots (ED).

Catalysis and structure of the SbVO4/Sb2O4 system for propane ammoxidation

Pure Sb0.9V0.9O4 and various preparations with excess of either vanadia or antimony oxide, including mechanical mixtures, have been investigated for propane ammoxidation to acrylonitrile. The catalysts were characterized before and after use in catalysis by various methods, including electron microscopy, infrared spectroscopy and XPS. The catalytic data show that preparations with approximate to SbVO4 and alpha-Sb2O4, compared with the single phases, are more selective to acrylonitrile formation on the condition that the excess antimony oxide is present while synthesising the approximate to SbVO, phase. Considering the catalytic data together with the results from the characterisations, various possibilities are discussed to explain the role of excess alpha-Sb2O4 in propane ammoxidation. Possibilities that can be excluded on rational grounds are catalysis on two phases, or, at grain boundaries, an influence on the morphology of approximate to SbVO4 from alpha-Sb2O4, the formation of alpha-Sb2O4 containing vanadium, defect formation, creation of active sites by the spillover of oxygen, and formation of VSb2O5. Instead, the observed synergy effect is due to the formation of approximate to SbVO4 enriched with antimony at the surface, creating isolation to a suitable level of the V-centres. The explanation is consistent with several observations including catalytic data for a series of vanadium compounds with different vanadium content, showing that structural isolation of the vanadium is necessary for obtaining high selectivity to acrylonitrile. (Less)

Expansive properties of ettringite in a mixture of calcium aluminate cement, Portland cement and β-calcium sulfate hemihydrate

Abstract The hydration of a paste consisting of 25 wt.% calcium aluminate cement, 12.5 wt.% Portland cement, 12.5 wt.% β-calcium sulfate hemihydrate and 50 wt.% water was studied at 20°C and 100% relative humidity, using in-situ synchrotron X-ray powder diffraction, isothermal conduction calorimetry and dilatometric measurements. Initially, gypsum and ettringite form, while hemihydrate is consumed (0–45 min). Ettringite then continues forming at the expense of gypsum. When gypsum is depleted after 2 h and 45 min, aluminate-AFm starts forming, while the amount of ettringite stays constant up to 7 h. The first peak in the heat rate curve includes contributions from mechanical mixing, initial wetting and dissolution plus the formation of ettringite and gypsum, the second maximum involves the replacement of gypsum by ettringite, and the third corresponds to the formation of aluminate-AFm. The replacement of gypsum by ettringite is accompanied by an average linear expansion of 0.7%.

X-ray study of the nepheline hydrate I structure

The structure of nepheline hydrate I, Na3Al3Si3O12·2H2O, synthesized hydrothermally at 473 K, has been determined. It is orthorhombic, space group Pna21 with a = 16.426(1), b = 15.014(1), c = 5.2235(5) A, Z = 4 and Dc = 2.38 g cm−3. The final R = 0.051. The tetrahedral framework consists of a set of parallel two-repeat chains, with single and double chains alternating. The largest channels are bound by eight-ring apertures. Two of the Na ions are coordinated by seven framework O atoms, while the coordination of the third Na ion is three framework and three water O atoms. The structure constitutes a link between those of anhydrous tektosilicates and the zeolites. A typical, butterfly-shaped, twinned crystal was examined, and a structural interpretation proposed.

The importance of site isolation and phase cooperation in propane ammoxidation on rutile-type vanadia catalysts

Propane ammoxidation to acrylonitrile over rutile-type vanadia catalysts is discussed regarding phase cooperation and site isolation effects. Compared with the pure phases, biphasic catalysts with both ∼SbVO4and α-Sb2O4are considerably more selective to the formation of acrylonitrile. It is demonstrated that cooperation between the phases during the calcination of the catalyst and the use in propane ammoxidation results in spreading of antimony species from free antimony oxide to the surface of ∼SbVO4, forming Sb5+–V3+/V4+supra-surface sites being involved in the formation of acrylonitrile. Dilution and isolation of the vanadium centers in ∼SbVO4through the partial replacement with, e.g., Al, Ti and W improves the catalytic properties. Structure–reactivity correlations using data for a nominal Sb0.9V0.9-xTixOyseries indicate that the activation of propane occurs on a V3+site and the activation of ammonia requires an Sb5+site.

Catalytic Anisotropy of MoO3 in the Oxidative Ammonolysis of Toluene

The oxidative ammonolysis of toluene, i.e., ammoxidation without the presence of molecular oxygen, was studied over a series of samples of MoO3 crystals. The specific surface areas of the various faces were determined from SEM micrographs. Correlations between activities and surface planes were found. For the formation of nitrile the specific activity decreased in the order {001} and {h01} > {100} > {010}. Also, for the formation of carbon oxides the terminations in the [001] direction were found to be especially active. These results are discussed in relation to surface structures and bond strength values of various oxygen species. It is concluded that the presence of both oxygen vacancies and nucleophilic oxygen species is a prerequisite for selective reaction to occur and that electrophilic oxygen species are the source for formation of carbon oxides. The characteristics of the various faces, as they emerge from the results on oxidative ammonolysis of toluene, seem to be of general significance for reactions occurring at the same types of active sites. They are shown to be applicable to results published in the literature on the oxidation of both propene and isobutene.

Ammoxidation of propane over vanadium-antimony-oxide catalysts. Role of phase cooperation effects

V-Sb-O catalysts with different Sb:V ratios were prepared and used for the ammoxidations of propane and propylene. XRD and Raman data show the presence of SbVO4/V2O5 when Sb:V 1. For Sb:V = 1, SbVO4 was the predominant phase. The activity data show that a Sb:V ratio above unity is needed to have a catalyst selective for acrylonitrile formation, an effect that primarily is related to the catalyst function for transformation of propylene, an intermediate in propane ammoxidation, to acrylonitrile. XPS data reveal the superior phase to be SbVO4 with supra-surface Sb-sites formed as a result of migration of antimony from a-Sb2O4 during the catalytic reaction. According to Raman results, pure SbVO4 without the copresence of α-Sb2O4 has a low capability for the conversion of formed propylene to acrylonitrile due to slow reoxidation of active [Sb-O-Sb] sites.

Dissolution of CaSO4·1/2H2O and precipitation of CaSO4·2H2O: A kinetic study by synchrotron X-ray powder diffraction

Abstract Time-resolved X-ray powder diffraction has been performed on hydrating samples of calcium sulfate hemihydrate with 0.50 wt.% dihydrate seeds added. Data were recorded in transmission mode using a position sensitive detector and synchrotron X-ray radiation of optimised intensity (wavelength 1.4 A). The dissolution of the hemihydrate and the formation of the dihydrate were both monitored. Varying the water/solid weight ratio (w/s) from 0.50 to 1.50 did not affect the reaction rate significantly. Addition of 0.50 wt.% potassium sulfate to the water accelerates the reaction and 0.25 wt.% citric acid monohydrate causes retardation; times for complete reaction: 17 and 42 min, respectively, compared to 28–32 min without additive (w/s=1.00). A comparison of the reaction curves shows (i) that hemihydrate dissolves and dihydrate precipitates at the same rate, and (ii) that preferred orientation effects are absent in the diffraction experiments.

Distribution of iron among ferrite hydrates

The hydration products of Ca2AlFeO5 pastes at 20 °C were characterized by X-ray powder diffraction, secondary electron imaging and X-ray microanalysis. In pure water, two X-ray amorphous AFm phases, one Al-rich and one Fe-rich, are initially formed. AFm is then replaced by hydrogarnet (Al/Al + Fe ≈ 34). In the presence of gypsum, AFt needles are first formed, followed by platy sulphate-AFm (both with Al/Al + Fe ≈ 34). With Ca(OH)2 and gypsum, the following was observed (cf. gypsum only): (i) higher Fe content in AFt and AFm, i.e., Al/Al + Fe ≈ 12; (ii) decreased amount of S in AFt; and (iii) a general decrease in the crystal size of AFt and AFm. X-ray amorphous pseudomorphs after ferrite grains were present in all runs. Despite the formation of these CaFe4O7 ·residues, much Fe enters the other products, especially when a base is added. It is proposed that the passivity of the ferrite surface is caused by a compact, X-ray amorphous layer desiccated by crystallizing AFt.