Preparation and characterization of catalysts
The preparation of a supported metal catalyst is a complex process where some steps are often not completely understood. The work of this thesis is focused on understanding and improving the processes involved in the preparation of a catalyst, and in particular aims to find a route for the preparation of homogeneous supported alloy catalysts and to establish how particle size can be controlled during the preparation. One of the difficulties preparing homogeneous alloys with traditional methods is the formation of single metal particles. To avoid this, a method based on the selective reduction of a second metal on the surface of a single metal supported catalyst (parent) was chosen. This method has the advantage of producing bimetallic particles where the two metals are in close proximity and can be subsequently alloyed by thermal treatment. Due to the difficulty in characterising supported alloy nano-particles, a combination of different techniques (in situ XRD, XPS, temperature programmed reduction, pulse chemisorption, temperature programmed hydrogen desorption etc.) were used to investigate the prepared catalysts and to study the degree of alloying as a function of the reduction temperature. The catalysts were also tested in a fixed bed gas phase reactor for the hydrogenation of crotonaldehyde. To achieve a background of knowledge on the single metal parents used for the selective deposition method, Pd-Al2O3, Ni-Al2O3, and Co-Al2O3 were prepared and characterised. Some important aspects developed in this preliminary work were the solubility of hydrogen in Pd (and its use in characterizing Pd particle size and degree of alloying) and the interaction between Co and Ni with the alumna support. After the single metal study the deposition of Ni over Pd and of Pd over Ni were attempted. The latter system led to the successfully preparation of core-shell particles where the Pd covers the Ni. The Pd-Ni-Al 2O3 bimetallic catalyst was then reduced at different temperatures in order to alloy the particles. The results revealed that alloys could be formed at relatively low temperature (300 °C). The last part of the work diverged from the main path of my project. It was a work of pure characterisation using in situ techniques. An interesting series of XRD and Raman in situ experiments on vanadium phosphorus oxide (VPO) catalysts revealed the formation of a metastable phase (co-VOPO4). The study of the transformation of this phase induced by various reactants under reaction conditions provided useful information in understanding the structure of the active catalyst and cast new light on the on role of the reactant and product gas mixtures in forging the structure of the catalyst.
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