No description available.

Phase formation studies in the quaternary section Ca_2-<I>y</I>Sr<I>_y</I>Fe_2-<I>x</I>B'<I>_x</I>O<I>_γ</I> : B' = Nb, Ta, 0 ≤ <I>y</I> ≤ 2.0 and 0 ≤ <I>x</I> ≤ 2.0 were carried out. Results are summarised below for B' = Nb; for B' = Ta they were broadly similar. Four solid solution phases were found: i) a cubic solid solution, with a variable compositional extent <I>x</I>, which increased with substitution of Ca by Sr, from 0.45 ≤ <I>x</I> ≤ 0.65 for <I>y</I> = 0, to 0 ≤ <I>x</I> ≤ 1.2 for <I>y</I> = 2.0. ii) A limited orthorhombic solid solution along the Ca₂Fe₂O₅-Ca₂B'₂O₇ join with 0.8 ≤ <I>x</I> ≤ 1.0. iii) A complete range of orthorhombic, brownmillerite solid solution for <I>x</I> = 0 and iv) an orthorhomic, perovskite-related solid solution, isostructural with Ca₂Nb₂O₇, for <I>x</I> = 2.0. A combination of differential thermal analysis, thermogravimetry, Mössbauer spectroscopy, high temperature powder X-ray diffraction, Rietveld refinement, a.c. impedance spectroscopy and magnetic susceptometry has been used to characterise these new phases. Solid solution (i) has variable oxygen content with an oxygen-deficient perovskite structure. As synthesised, for <I>x</I> = 0.6 it contains a mixture of Fe⁴⁺ and Fe³⁺ in the ratio 13/87. The oxygen content could be modified by heat treatment under various atmospheres over the range 5.5. ≤ <I>γ</I> ≤ 5.92 which corresponded to the ratios: 93/7 to Fe³⁺/Fe²⁺ and 42/58 of Fe⁴⁺/Fe³⁺. At higher oxygen contents, <I>γ</I> > 5.7, transformation to an orthorhombic structure was observed. The electrical resistivity was dominated by grain boundary effects. Conduction is electronic and is attributed to hopping between iron in mixed oxidation states. The resistivity increased dramatically on reduction and became <I>n</I>-type, presumably due to a small fraction of Fe²⁺ present.

No description available.

No description available.

The thesis is focused on the study of high temperature water gas shift catalysis, the identification of new improved catalysts and the study of the kinetics and mechanism of reaction over these catalysts. Rh-promoted Fe2O3-Cr2O3 was found to offer best performance which was significantly better than unpromoted catalyst over wide temperatures range. An extensive literature survey is first reported. Guidelines to develop new WGS catalysts are developed. As a result, the activities of precious metals supported on various oxides for high temperature WGS reaction have been tested. Rh(1wt%) doped Fe2O3/Cr2O3, exhibits the highest activity for WGS over a wide temperature range. 5wt%CuO/Fe2O3-Cr2O3, 1wt%Pt/Cr2O3, 1wt%Pt/Fe2O3-Cr2O3, 1wt%Pt/U3O8, 1wt%Pt/10%U3O8-Al2O3 and 1wt%Pt/5%V2O5-TiO2 fall into the second most active catalysts group, with an improved activity compared to commercial Fe2O3-Cr2O3 catalyst. It is clear that both the support/catalyst and the promoter can affect the efficiency of the WGS, leading to the obvious inference that the reaction rate is controlled at the promoter ??? support interface. Further kinetic studies and characterisation (TPR, TPD, pulse-adsorption (reaction)) on Rh/Fe3O4/Cr2O3 have been conducted. The study, conducted under conditions without inhibition from products of both forward and backward reactions, shows that the overall reaction rate expression is described as: 2 22 ??? =0.0041exp(???4042.6 ) 0.64 0.5 ???0.024 exp(???6022.9 ) 0.46 0.73 CO CO H O CO H r PP P P T T . Kinetics studies carried out under fuel reforming gas compositions shows that reaction rate expression changed when the temperature of reaction varied. The reaction rate equations at temperatures of 573K, 623K and 673K are derived as: 573K: 2 2 2 - 2.84 10-6 0.6 0.12 - 9.08 10-7 0.09 0.52 rCO = ?? PCO PH O ?? PCO PH 623K: 2 2 2 - 1.45 10-6 0.99 0.40 - 7.12 10-7 0.11 0.73 rCO = ?? PCO PH O ?? PCO PH 673K: -6 2 2 2 - = 4.37 ?? 10 0.86 0.41 -1.83 ??10-6 0.28 0.66 rCO PCO PH O PCO PH , The apparent activation energy was 61.7??2.5 kJmol-1 . TPR, TPD, TPO characterisation studies and reoxidation of catalysts by CO2 or H2O show that the active site for high temperature WGS reaction on Rh/Fe2O3/Cr2O3 is reduced magnetite Fe3O4 which dissociatively breaks down the H2O to form H* and OH* and adsorbs CO2. The deposited metal, Rh, acts as a promoter by facilitating the uptake of hydrogen (H2) and carbon monoxide (CO), desorption of H2 (at high temperature) and CO2.

The structure of the oxide/aqueous electrolyte interface has been studied. The surface porosity of several oxides to ions is evaluated and the contribution of such porosity to the double layer properties determined by surface charge measurements. The oxides studied are B.D.H. precipitated silica, before and after heat treatment, rutile, goethite, hematite and amorphous ferric oxide. The surface porosity was evaluated using nitrogen adsorption for physical porosity, tritium exchange for surface hydration and dissolution for surface crystallinity. It is found that the surfaces of metal oxides may be divided into two categories; those that are porous to ions and those that are non-porous. Of those studied only the precipitated silica and the amorphous ferric oxide are porous. The porosity is probably due to an easily permeated layer of hydrolysed oxidic material. It does lead to exceptionally high surface charges. However the non-porous oxides also exhibit high surface charges so that while surface porosity may, in some cases, contribute to oxide double layer properties, it cannot be a general explanation of the high differential capacities observed. A site-binding model for non-porous oxide/aqueous electrolyte interfaces is introduced, in which it is proposed that the adsorbed counter ions form interfacial ion pairs with discrete charged surface groups. This model is used to calculate theoretical surface charge densities and potentials at the Outer Helmholtz Plane. The calculated values are consistent with experimental data for oxides provided a high value of the inner zone capacity is accepted. An explanation is provided for the difference between silica and most other oxides in terms of the dissociation constants of the surface groups.

The thesis is focused on the study of high temperature water gas shift catalysis, the identification of new improved catalysts and the study of the kinetics and mechanism of reaction over these catalysts. Rh-promoted Fe2O3-Cr2O3 was found to offer best performance which was significantly better than unpromoted catalyst over wide temperatures range. An extensive literature survey is first reported. Guidelines to develop new WGS catalysts are developed. As a result, the activities of precious metals supported on various oxides for high temperature WGS reaction have been tested. Rh(1wt%) doped Fe2O3/Cr2O3, exhibits the highest activity for WGS over a wide temperature range. 5wt%CuO/Fe2O3-Cr2O3, 1wt%Pt/Cr2O3, 1wt%Pt/Fe2O3-Cr2O3, 1wt%Pt/U3O8, 1wt%Pt/10%U3O8-Al2O3 and 1wt%Pt/5%V2O5-TiO2 fall into the second most active catalysts group, with an improved activity compared to commercial Fe2O3-Cr2O3 catalyst. It is clear that both the support/catalyst and the promoter can affect the efficiency of the WGS, leading to the obvious inference that the reaction rate is controlled at the promoter ??? support interface. Further kinetic studies and characterisation (TPR, TPD, pulse-adsorption (reaction)) on Rh/Fe3O4/Cr2O3 have been conducted. The study, conducted under conditions without inhibition from products of both forward and backward reactions, shows that the overall reaction rate expression is described as: 2 22 ??? =0.0041exp(???4042.6 ) 0.64 0.5 ???0.024 exp(???6022.9 ) 0.46 0.73 CO CO H O CO H r PP P P T T . Kinetics studies carried out under fuel reforming gas compositions shows that reaction rate expression changed when the temperature of reaction varied. The reaction rate equations at temperatures of 573K, 623K and 673K are derived as: 573K: 2 2 2 - 2.84 10-6 0.6 0.12 - 9.08 10-7 0.09 0.52 rCO = ?? PCO PH O ?? PCO PH 623K: 2 2 2 - 1.45 10-6 0.99 0.40 - 7.12 10-7 0.11 0.73 rCO = ?? PCO PH O ?? PCO PH 673K: -6 2 2 2 - = 4.37 ?? 10 0.86 0.41 -1.83 ??10-6 0.28 0.66 rCO PCO PH O PCO PH , The apparent activation energy was 61.7??2.5 kJmol-1 . TPR, TPD, TPO characterisation studies and reoxidation of catalysts by CO2 or H2O show that the active site for high temperature WGS reaction on Rh/Fe2O3/Cr2O3 is reduced magnetite Fe3O4 which dissociatively breaks down the H2O to form H* and OH* and adsorbs CO2. The deposited metal, Rh, acts as a promoter by facilitating the uptake of hydrogen (H2) and carbon monoxide (CO), desorption of H2 (at high temperature) and CO2.

The thesis is focused on the study of high temperature water gas shift catalysis, the identification of new improved catalysts and the study of the kinetics and mechanism of reaction over these catalysts. Rh-promoted Fe2O3-Cr2O3 was found to offer best performance which was significantly better than unpromoted catalyst over wide temperatures range. An extensive literature survey is first reported. Guidelines to develop new WGS catalysts are developed. As a result, the activities of precious metals supported on various oxides for high temperature WGS reaction have been tested. Rh(1wt%) doped Fe2O3/Cr2O3, exhibits the highest activity for WGS over a wide temperature range. 5wt%CuO/Fe2O3-Cr2O3, 1wt%Pt/Cr2O3, 1wt%Pt/Fe2O3-Cr2O3, 1wt%Pt/U3O8, 1wt%Pt/10%U3O8-Al2O3 and 1wt%Pt/5%V2O5-TiO2 fall into the second most active catalysts group, with an improved activity compared to commercial Fe2O3-Cr2O3 catalyst. It is clear that both the support/catalyst and the promoter can affect the efficiency of the WGS, leading to the obvious inference that the reaction rate is controlled at the promoter ??? support interface. Further kinetic studies and characterisation (TPR, TPD, pulse-adsorption (reaction)) on Rh/Fe3O4/Cr2O3 have been conducted. The study, conducted under conditions without inhibition from products of both forward and backward reactions, shows that the overall reaction rate expression is described as: 2 22 ??? =0.0041exp(???4042.6 ) 0.64 0.5 ???0.024 exp(???6022.9 ) 0.46 0.73 CO CO H O CO H r PP P P T T . Kinetics studies carried out under fuel reforming gas compositions shows that reaction rate expression changed when the temperature of reaction varied. The reaction rate equations at temperatures of 573K, 623K and 673K are derived as: 573K: 2 2 2 - 2.84 10-6 0.6 0.12 - 9.08 10-7 0.09 0.52 rCO = ?? PCO PH O ?? PCO PH 623K: 2 2 2 - 1.45 10-6 0.99 0.40 - 7.12 10-7 0.11 0.73 rCO = ?? PCO PH O ?? PCO PH 673K: -6 2 2 2 - = 4.37 ?? 10 0.86 0.41 -1.83 ??10-6 0.28 0.66 rCO PCO PH O PCO PH , The apparent activation energy was 61.7??2.5 kJmol-1 . TPR, TPD, TPO characterisation studies and reoxidation of catalysts by CO2 or H2O show that the active site for high temperature WGS reaction on Rh/Fe2O3/Cr2O3 is reduced magnetite Fe3O4 which dissociatively breaks down the H2O to form H* and OH* and adsorbs CO2. The deposited metal, Rh, acts as a promoter by facilitating the uptake of hydrogen (H2) and carbon monoxide (CO), desorption of H2 (at high temperature) and CO2.

Thesis (Ph.D.)--McMaster University (Canada), 1994. / Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4338. Adviser: J. E. Greedan.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.