PhD Thesis Defence Presentations - Sotirios Tsatsos

Mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, solid oxide fuel cell and solid oxide electrolyte cells for hydrogen production) but the basic understanding of the relationship between the structure (both electron and structural - atomic positions, oxygen deficiencies, etc.) and its catalytic activity or selectivity is always a key issue  in the literature. The lack of suitable preparation methods for the selective  synthesis of catalytic surfaces (using different  admixtures  or selective methods for introducing superficial defects) and of surface characterization techniques to determine the number and  type of catalytically active sites (which are further required in order to obtain specific kinetic parameters of the reaction, such as TOF in s^-1) have hampered the development of fundamental concepts and new theories in the field of heterogeneous catalysis. This work uses conventional methods (XPS, UPS,  XRD, TPD, TPSR, etc.) in order  to explain the catalytic activity of mixed oxides, such as the WO₃ / TiO₂ system and introduces a modern perspective on the fundamental relationship of the structure and electron modulation  of the catalytic surface with the corresponding surface reactivity.

A model is proposed in which the mixed oxide (WO₃/TiO₂) is considered as a heterostructure. This description of supported oxide catalysts allows the use of heterojunction physics to predict the electron density and the concomitant potential on the surface of the catalyst through the spectroscopically determined bending of the energy bands. Furthermore, the role of the thickness and coverage of the deposited WO₃  active oxide   phase, in the  possible doping of the support oxide (TiO₂) can be determined in the frame of that model. The estimation of thickness and coverage in each case was carried out by a simulation model of the intensity ratio of selected XPS photoelectron peaks from the support and the active phase, taking into account the composition of the WO₃/ TiO₂ system and its BET specific surface area. From the observed changes in the electronic structure of the WO₃ / TiO₂ system using  XPS, UPS and DRS, quantitative estimates were made for the electronic interactions at the WO₃/TiO₂ interface. It is concluded that modifying the thickness, the coverage and the concentration of the  WO₃ on  the substrate can result in a change of the surface electrons distribution (represented by the surface Fermi level) of the catalyst.

A further scope is to relate the changes in the Fermi level of the oxides with their activity in the reaction of selective catalytic reduction by ammonia (ΝΗ₃) of nitric oxide (NO) to nitrogen (Ν₂) with or without  Ο_2.    The catalytic activity was studied by classical temperature-programmed desorption/reaction methods, such as ΝΗ₃ and ΝΟ TPD, TPSR of adsorbed ΝΗ₃ with ΝΟ and Ο₂ , as well as TPR of reactant mixtures in the  ~40°C  to ~700°C temperature range. Using this information, the quantitative change in the catalytic activity (rate and reaction initiation)  and its dependence on the heterostructure parameters (thickness, coverage,  energy band changes) can be determined for the test reaction   on 1%  and  4%  w/w WO₃/ TiO₂ .