Relationships between composition, structure and performance of nanostructured metal / metal oxide catalyst systems

This thesis is aimed at the development of an improved understanding of the selection criteria for heterogeneous catalysts associated with energy production and utilization. Fundamental questions regarding the structure/composition variations in nanostructure metal/metal oxide catalysts and how the catalytic performance varies have been explored to develop a rational for further improvement of such catalyst.;During the energy production and utilization, the important reactions could be divided into two groups, the electrochemical reactions or non-electrochemical reactions. The first group of reactions is driven by the electric field, which was widely used in the fuel cell and electrochemical energy storage. However, those applications are still limited by the efficiency, low stablity and high cost of electrocatalyst. In this thesis work, the electrochemical methanol oxidation and oxygen reduction reaction (ORR) were investigated with the non-platinum or less platinum catalyst. Au is a stable and relatively abundant electrocatalyst, and catalytically active in small size. In my investigation of using Au in ORR, the kinetic current was found to be 2.5 times higher for the gold nanoparticles of 3 nm compared to the gold nanoparticles of 7 nm, since the 3 nm particles were found to facilitate four electron electro-reduction whereas a two-electron electro-reduction was inferred on the 7 nm particles. Besides the electronic effect of AuPt alloying was also discussed in this thesis. For the methanol oxidation reaction in an acidic environment, the addition of Au to Pt improved the stability while decreasing the overall Pt loading at equal activity. In alkaline environment, alloying Pt with Au showed a significant increase in oxidation activity of up to 14 times compared to pure Pt. For the oxygen reduction reaction, no change in activity was observed in acidic electrolytes while a 2-fold improvement and lower overpotential to drive the reduction reaction improving the overall cell voltage was observed in basic electrolytes. The possible mechanisms for the catalytic improvement of Au-Pt catalyst for methanol oxidation and oxygen reduction were due to the modification of the d-band center of Pt by Au.;The other group of reactions can only be driven by the thermodynamic factor, such as temperature and pressure. In this thesis, the efforts were put on developing doped metal oxide catalyst to tune the oxidation activity. Using a combination of theory and experimental analysis, it was demonstrated that the introduction of Ti dopants into the inert ZnO host activates a non-traditional Mars-van Krevelen (MVK) mechanism. Doping ZnO with Ti promotes the adsorption and activation of dioxygen on the Ti sites for reaction with CO. The proposed mechanisms are supported by density functional theory calculations and by experiments involving isotopically labeled dioxygen. Understanding the impact of specific dopants and the vacancy energetics allowed us to extend the approach used for C-O oxidation to CH bond activation. The choice of dopant strikes a fine balance and relationship between the ability to dissociate the C-H bond on the dopant and oxidize the reactant, with the ability to form the vacancies. Electrophilic Pt atoms were used as atomic dopants in ceria hosts to regulate and utilize the mobile oxygen vacancies. Whereas complete combustion of methane in oxygen occurs preferentially on traditionally supported Pt catalysts, high selectivity for the partial oxidation of methane to CO and H2 was achieved on Pt-doped CeO2.