Nanostructural and Chemical Characterization of Complex Oxide Catalysts by Analytical Electron Microscopy

Complex oxide catalysts are used as heterogeneous catalysts for producing various important organic chemicals. In this thesis, three types of complex oxide catalysts prepared using novel preparation methods have been studied. Each of them has been evaluated for its catalytic performance, namely (i) the selective oxidation of n-butane to maleic anhydride over vanadium phosphate (V-P-O) materials; (ii) the oxidative dehydrogenation (ODH) of ethane to ethylene over niobium phosphate (Nb-P-O) materials, and (iii) the oxidation of methanol to formaldehyde over iron molybdate (Fe-Mo-O) materials. Analytical electron microscopy, X-ray diffraction and other related characterization techniques have been used to provide useful information regarding the morphology, crystallography and chemical composition of these complex oxide catalysts. The underlying aim of this work is to uncover meaningful synthesis-structure-performance relationships for these three complex catalyst systems.;Firstly, a standard methodology for generating V-P-O materials, i.e. the VPD route, has been revisited and modified. A variety of alkanes have been added during the alcohol reduction step of VOPO4·2H2O (dihydrate), which were found to have a remarkable influence on the morphology and structure of the V-P-O materials produced. Either VOHPO4·0.5H2O (hemihydrate) or VO(H2PO4)2 material can be produced depending on the precise alcohol:alkane volume ratio used in the reaction. In addition, the specific order in which the alkane and alcohol are added to VOPO 4·2H2O during the VPD route has a dramatic effect on the morphology of the resultant precursor. Through detailed electron microscopy studies we have been able to unveil the epitaxial relationship between the dihyrate and hemihydrate crystalline phases as being [001]dihydrate // [001]hemihydrate and [100]dihydrate // [110]hemihydrate. A two-step mechanism by which the topotactic transformation from dihydrate to hemihydrate occurs has been proposed.;Secondly, three different novel synthesis routes have been explored for producing V-P-O catalysts. The first route, involving the addition of various V-P-O 'seeds' during the VPD process, was found to have a profound effect on the morphology of the V-P-O precursor and on inducing certain unexpected phase transformations. Specifically, the V-P-O seed was found to induce the transformation of VO(H2PO4)2 to hemihydrate phase in a 3-octanol solution. The second route, namely the use of a di-block copolymer template in the VPO route, was found to generate a more crystalline hemihydrate precursor with a rhomboidal morphology, which could be activated in a much shorter time period as compared to conventional V-P-O precursors. The third route involved encapsulating the fragile V-P-O rosette-type catalysts within a mechanically protective SiO2 shell. When used in a circulating fluidized bed reactor, these core/shell V-P-O catalysts showed a promising initial catalytic performance, but suffered a severe degradation in performance after two years-on-line. We have been able to attribute this degradation to three contributing factors; namely (i) the generation of inactive V 5+ (e.g. beta-VOPO4) phases, (ii) densification of the SiO2 shell and (iii) loss of core V-P-O materials. In addition, through this latter study, the novel X-ray ultramicroscopy (XuM) technique has been shown to have great potential for the non-destructive study of micron-scale catalyst particles.;Thirdly, three different niobium phosphate materials, namely the Nb 2P4O15, NbOPO4 and Nb1.91P 2.82O12 phases, have been synthesized. Each of them was evaluated for the ODH of ethane to ethylene and the oxidation of methanol to formaldehyde, respectively. It was found that the Nb1.91P2.82O 12 phase is the most desirable structure for ethane ODH, whereas the NbOPO4 phase is more effective for methanol oxidation. The morphological and structural changes induced by both reactions on these Nb-P-O catalysts have been monitored, and correlated to the measured changes in their catalytic performance.;Finally, a highly active Fe2(MoO4)3/MoO 3 catalyst for methanol oxidation has been prepared using a novel impregnation method. This catalyst consists of MoO3 nanorods on which epitaxial Fe2(MoO4)3 islands are supported. Based on our microstructural characterization studies, a mechanism has been proposed for the formation of this fascinating nanorod/island morphology which involves a thermally induced solid-state diffusive reaction accompanied by the Kirkendall effect. The excellent catalytic performance of this catalyst is thought to result from the synergistic behaviour between the active surface Fe2 (MoO4)3 islands and the underlying MoO3 reservoir.