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Preparation of nano-structured catalysts

Alhumaimess, Mosaed 2012. Preparation of nano-structured catalysts. PhD Thesis, Cardiff University.
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gold catalysts have been found to be effective for many oxidation reactions and it is known that the performance of these catalysts depends strongly on the particle size of Au nanoparticles. However, other factors have strong influence on the catalytic activity such as the preparation methods, choice of support, the structure and morphology of supports. The effect of support morphology and structure on the activity of Au catalysts was investigated using two hydrothermally prepared supports, CeO2 and MnO2. Ceria foams as a support for nano-clusters of gold were synthesised hydrothermally at 160 °C by the reaction of L-Asparagine and CeCl3.7H2O at different crystallisation times. The effect of the reaction time on the morphology of prepared CeO2 was investigated. The morphology varied remarkably and it was found to change from spherical particles to foam and eventually to a collapsed foam as the crystallisation time increased. Gold catalysts were prepared by sol-immobilisation, supported on the foam ceria and examined for solvent free oxidation of benzyl alcohol using molecular O2 as an oxidant and the effect of the support was compared with commercial ceria. Au/CeO2 foam catalysts were more active than the Au/commercialCeO2 although the Au nanoparticles were larger in ceria foam supports. This was due to the greater lability of surface oxygen in the foam support compared with commercial CeO2 materials. The Au/CeO2 foam catalyst was found to be reusable over three experiments. The effect of catalyst loading, oxygen pressure and reaction time-online were also studied. It was found that there was no mass transfer limitation when the mass of catalyst varied from 5 to 40 mg under the reaction conditions. The conversion of benzyl alcohol decreased as the oxygen pressure decreased which shows that oxygen was involved in the oxidation process. For time-online study, the conversion increased as the reaction time increased with slightly increase. MnO2 supports were synthesised by reacting MnSO4.H2O with (NH4)2S2O8 hydrothermally at 160 °C. Two different phases and morphologies of MnO2 were formed and as the reaction time increased the morphology changed from microspheres to nanowires and the MnO2 phase changed from α- to β-. Gold was deposited on all prepared MnO2 materials and the catalysts were examined for solvent free benzyl alcohol and CO oxidation. The influence of the preparation method on the catalytic activity was studied and sol-immobilisation was found to be the best for benzyl alcohol VII oxidation whereas the deposition-precipitation was found to be the best for CO oxidation. Impregnation method exhibited poor activity for both reactions. The effect of the morphology and phase on the catalyst activity for both reactions was researched and Au/α-MnO2 microspheres catalysts were best for benzyl alcohol oxidation while Au/β- MnO2 nanowires catalysts exhibited better performance for CO oxidation due to their smaller Au nanoparticles and easier surface reduction. The catalysts reusability, timeonline and the effect of catalyst loading were also studied of an Au/MnO2 microsphere catalyst for benzyl alcohol oxidation reaction. Vanadium phosphate catalysts have been extensively studied for the selective oxidation of butane to maleic anhydride. The catalytic activity of vanadium phosphates is greatly dependant on the preparation method of the catalyst precursor VOHPO4·0.5H2O. Poly (acrylic acid-co-maleic acid) copolymer, PAAMA, was employed as a structure directing agent in the preparation of VOHPO4.0.5H2O via two routes. The effect of PAAMA concentration on the structure morphology of VOHPO4·0.5H2O was studied in both preparation routes. As the concentration of PAAMA increased the morphology changed from rosette like for the standard precursors to rosette-like agglomerates with isolated rhomboidal platelets and eventually to isolated rhomboidal platelets at highest concentration of PAAMA. The XRD confirmed that all precursors were VOHPO4·0.5H2O but as the concentration of PAAMA increased the (001) reflection increased and the (220) reflection decreased. When these precursors were tested for butane selective oxidation, the standard precursors that contain rosettes VPO0 and VPD0 activated to the active phase ((VO)2P2O7) over typically observed time (> 100 h). While the precursors that had rosette-like agglomerates with isolated rhomboidal platelets and a relative intensity ratio of the (001)/(220) reflections of around 1.4, VPO5 and VPD15, activated much faster, less than 20 h with a comparable conversion and selectivity. The isolated rhomboidal platelets precursors which had a high relative intensity ratio of the (001)/(220) reflections, VPO15, VPO25 and VPD25, displayed very poor activity because the thin platelets were rapidly oxidised to αII-VOPO4 phase as confirmed by the XRD and Laser Raman Spectroscopy.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:10

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