No description available.

Vanadium magnesium oxide (VMgO) catalysts with different vanadium loadings were synthesized and tested for catalytic activity using pure «-hexane and «-octane as feeds. High surface area catalysts were obtained by the wet impregnation of magnesium oxide with an aqueous ammonium metavanadate solution. The optimum loading of vanadium was shown to be 19 % (calculated as weight % of V205). Catalysts were characterized by x-ray diffraction (XRD), inductively coupled plasma - atomic emission spectroscopy (ICP-AES), Brunauer-Emmet-Teller (BET) surface area, differential scanning calorimetry - thermogravimetric analysis (DSC-TGA), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), x-ray induced photoelectron spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Magnesium oxide (MgO) and magnesium orthovanadate (Mg3(V04)2 were the only phases observed in each catalyst. VMgO catalysts were tested under both oxygen-rich and oxygen-lean conditions. «-Hexane as feed yielded benzene, 1-hexene, 2-hexene, propane, propene, carbon oxides and water as products, n- Octane as feed yielded styrene, ethylbenzene, xylene, benzene, octenes, carbon oxides and water. 19VMgO was promoted with different loadings of molybdenum oxide (M0O3), cesium oxide (Cs20), antimony oxide (Sb20s), niobium oxide (Nb205), bismuth oxide (Bi203) and tellurium oxide (Te02). The promoted catalysts were tested in specially designed and constructed parallel fixed bed continuous flow reactors. / Thesis (Ph.D.)-University of KwaZulu-Natal, 2006.

The influence of the form of the ZSM-5 zeolite, vanadium content and the elimination of the exterior surface acidity on the activity and selectivity of n-hexane oxidation was studied using a fixed bed reactor. Blank reactor studies (carborundum packed reactor) showed no conversion below 450°C with the highest conversion (8%) at 500°C. The dominant products were found to be carbon oxides (Sel./% = 90) with minor selectivities to the hexene isomers (7%) and the remainder being cracked products, THF and benzene. H-ZSM-5 with different SiO2/Al2O3 ratios (100 and 320) and Na-ZSM-5 (SiO2/Al2O3 ratio of 100) were tested under non-oxidative and oxidative conditions. Under oxidative conditions as the ratio of the SiO2/Al2O3 increased, the aluminium content decreased and so too did the cracking ability of the zeolite (i.e. yield of cracked products dropped from 36% to 8%). However, the use of the Na- form of ZSM-5 completely eliminated acid cracking. Under oxidative conditions H-ZSM-5 (100) was found to be more active and resulted in higher formation of cyclic and aromatic compounds. With increasing time on-stream and higher temperatures the catalyst was found to deactivate. Evidence of this was seen by a decrease in surface area and pore volumes of the spent catalyst. The Na-ZSM-5 (100) showed lower activity, but deactivation was shown to be lower. These findings led to the investigation of vanadium ion-exchanged Na-ZSM-5 catalyst for n-hexane activation. Catalysts with different vanadium loadings were prepared using the solid state ion-exchange method. Catalysts were characterised using various methods. These techniques showed that vanadium was successful loaded onto the catalyst and that the highest vanadium loading that could be achieved was 2.5%. The lower loadings were not found to alter the catalyst structure while the highest loading of 2.5% was found to show some pore blockage and to possibly alter the structural environment of the zeolite. Time on stream experiments were conducted and temperature (350, 400 and 450°C), contact time (0.5, 0.8, 1.1 and 1.5 s) and fuel-air ratios (0.7, 1.3 and 2) were varied. The optimum conditions (Conv./% = 39) for terminally activated products were found using the Na-V-ZSM-5 (0.9%) at a temperature of 400°C, a contact time of 1.1 s and fuel-air ratio of 1.3. With the lower fuel-air ratio of 0.7 (oxygen rich conditions), hexanal formation was favoured. The Na-V-ZSM-5 catalyst could be regenerated with initial activity and selectivity being regained. Silanisation was found to be possible, however, the extent and degree of silanisation was difficult to control. Pore blockage was possibly responsible for the lower activity and selectivity obtained using the silanised Na-V-ZSM-5. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2010.

Bibliography: leaves 230-242 / 242 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1986

Proefschrift Universiteit van Amsterdam. / Met lit. opg. - Met samenvatting in het Nederlands.

Frankfurt (Main), Univ., Diss., 2004.

Th.--Sci. phys.--Toulouse--I.N.P., 1983. N°: 69.

Th. doct.-ing.--Sci. des mater.--Nancy--I.N.P.L., 1982.

No description available.

An investigation of the low temperature tensile properties of vanadium single crystals was carried out, using zone-refined metal. Single crystals of predetermined axial [110] orientation were grown by a melt solidification technique, using an electron beam, floating-zone refiner. Tensile specimens were prepared from these single crystals. A plot of yield stress versus test temperature was found to be discontinuous and consisted of two curves which intersected at -125° C. The plot of log yield stress versus reciprocal temperature yielded two straight lines which also intersected at -125° C. The slip system was identified as <111>{112}, which is different from that found for iron single crystals. X-ray, metallographic and electrical resistance data indicate that the phenomenon is primarily a yield point effect. The results of tensile tests performed on single crystals at a very low strain rate, and on polycrystalline specimens indicated that the temperature dependence of yield stress is itself orientation dependent. Three possible explanations of the unusual temperature dependence of yield stress are given: (1) A change in deformation mechanism occurs, for example, from slip to twinning. (2) A minor ordering reaction occurs. (3) A change in the mechanism by which dislocations are unlocked from their atmospheres occurs; for example, two impurity atmospheres surrounding different dislocations, each impurity showing a separate temperature dependence of yield stress. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate