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|Title: ||Tailored nanoarchitectures based on transition metals for heterogeneous catalysis|
|Authors: ||Cargnello, Matteo|
|Supervisor/Tutor: ||Fornasiero, Paolo|
|Co-supervisor: ||Gorte, Raymond|
|Issue Date: ||29-Mar-2012|
|Publisher: ||Università degli studi di Trieste|
|Abstract: ||Presented in this thesis are experimental results obtained on the preparation of tailored architectures based on transition metals for applications in heterogeneous catalysis. The major focus of these studies was the preparation of active and thermally stable materials for several catalytic reactions and the accurate study of size-activity relationships in ceria-based systems. An array of preparation procedures was described starting with relatively simple approaches to more advanced methods based on self-assembly and the preparation of artificial atoms. Detailed catalytic and characterization studies unambiguously demonstrated the properties and functions of the prepared materials as well as their different and improved characteristics with respect to state-of-the-art conventional systems.
Initially, relatively simple procedures for the encapsulation of Pd and Au inside ceria layers demonstrated the promise of the embedding procedure and its limitations (Chapter 3). In the case of Pd catalysts, improvements in stability for the water-gas-shift reaction of Pd@CeO2 over a Pd/CeO2 catalyst formed by impregnation suggested that the formation of Pd-ceria, core-shell catalysts have great promise for this application. However, the need to develop core-shell structures that effectively prevented Pd sintering but left the Pd accessible to gas-phase reactants was imperative. In the case of Au catalysts, very good catalytic activity under real PROX conditions was obtained with low metal loading (1 wt %). Moreover, the small deactivation observed for the embedded Au@CeO2 catalysts after aging at 250 °C was fully reversible upon mild oxidation treatment. In both cases, it was clear that a correct balance between encapsulation of the particles and manipulation of the pore structure around them was fundamental to achieve materials with the desired performance and thermal stability.
In order to better manipulate the metal-support interface and tailor the core-shell structure to a fine extent, we then prepared functionalized Pd and Pt nanoparticles with rather small average diameter and narrow size distribution (Chapter 4). The main focus of the chapter was to prepare particles that exposed on the edge of the monolayer carboxyl groups by using a functionalized protecting thiol (11-mercaptoundecanoic acid, MUA), useful for the subsequent reaction with the metal oxide precursors. A systematic study of the synthetic conditions applied to the production of the nanoparticles (reaction temperature, thiol/Pd molar ratio and reductant addition rate) as well as their morphological outcome has been properly addressed. The procedure was then extended, with minor modifications, to the preparation of MUA-Pt nanoparticles. Finally, the MUA-Pd nanoparticles have been scrutinized as highly efficient, easily handled and re-usable catalysts for Suzuki cross-coupling reaction with different aryl halides.
The functionalized Pd and Pt nanoparticles were then employed for the successful preparation of dispersible metal@oxide core-shell nanostructures (Chapter 5). We showed that the method is versatile allowing the preparation of several combinations using Pd or Pt as metal core and titania, zirconia or ceria as oxide shell. The method is based on the self-assembly between the MUA-Pd or MUA-Pt nanoparticles and titanium, zirconium or cerium alkoxides. The core-shell nanostructures were effectively dispersible in a range of organic solvents without any sign of agglomeration. We demonstrated that the dimension of the metal core and the thickness of the oxide layer could be tuned and that the metal phase was accessible.
The applicability of these structures for the preparation of heterogeneous catalysts was then demonstrated in several areas. In particular, the results showed that the ceria in the core-shell catalysts deposited onto alumina exhibited different redox properties from those found for the ceria in conventional catalysts, possibly due to structural changes associated with the Pd-core template (Chapter 6). Even when the Pd in the oxidized catalysts was accessible to gas-phase reactants, reduction of the ceria could cause encapsulation of the Pd, which can in turn lead to deactivation of the catalyst for reactions like water-gas shift. However, these same properties can be advantageous for other applications, such as the case of SOFC electrodes. In addition to the high thermal stability of the core-shell catalysts, it was demonstrated that the synergic interactions between Pd and ceria increased the oxidation activity of Pd/CeO2 catalysts with the consequence that high power densities and thermal stabilities have been obtained for their use as fuel cell materials under high-temperature reaction conditions.
The method for the preparation of dispersible core-shell structures was then adapted for their deposition around Multi Wall Carbon Nanotubes (MWNTs) (Chapter 7). In particular, a modular procedure for the preparation of nanocomposites where MWNTs were embedded inside mesoporous oxide layers containing metal particles dispersed within was reported. We showed that the different materials have very promising catalytic activity for different heterogeneous reactions. In particular, in stark contrast with what observed with the Pd@CeO2 structures dispersed on alumina (Chapter 6), it was shown that the nanocomposites with MWNTs were much more stable under WGS conditions, demonstrating the importance of the support properties to tune the final catalytic performances.
A further application of the dispersible Pd@CeO2 structures consisted in their deposition as single entities onto a modified alumina (Chapter 8). The materials showed outstanding catalytic properties for methane oxidation. The synergic effect between the core and the shell components and the self-assembly of the building blocks in solution helped to form stable and active materials that demonstrate unusual, promoting catalytic properties.
In a final stage, we developed a unique capability of prepared d8 metal particles (Ni, Pd and Pt) to study the size-activity relationships of ceria-based materials (Chapter 9). The extremely precise particle size was advantageously employed to prepare catalysts with finely tuned metal-support interface. We definitely proved that CO oxidation on the d8 metals deposited on CeO2 is size-dependent, with a direct participation in the reaction of metal atoms at the perimeter and ceria oxygen lattice. This study is of guidance for the preparation of embedded systems with tailored interface to further enhance the performance of the core-shell-type catalysts.
In conclusion, this thesis provided numerous methods for the preparation of tailored nanoarchitectures and their application in several areas of heterogeneous catalysis. The work was based on the precise tailoring of the building blocks in order to organize them trough self-assembly and supramolecular chemistry principles in hierarchical, final structures with defined metal-support interfaces. It is expected that these principles will be of guidance for the development of this concept and its extension to other areas, associated with the increasingly better ability to further tune the special properties of the materials presented in this thesis.|
|PhD cycle: ||XXIV Ciclo|
|PhD programme: ||SCUOLA DI DOTTORATO DI RICERCA IN NANOTECNOLOGIE|
|Main language of document: ||en|
|Type: ||Tesi di dottorato|
|Scientific-educational field: ||CHIM/03 CHIMICA GENERALE E INORGANICA|
|Appears in Collections:||Scienze chimiche|
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