Surface science and catalysis studies of the structure and reactivity of model catalysts

In this thesis, the structure and reactivity of palladium on titania has been studied on high surface area powdered materials and on low surface area models. Mathematical models have been established which relate the particle radius to various parameters such as particle surface area, density of particles, interparticle distance and coverage. The sintering process is studied through STM imaging and particle size distribution (PSD) calculation on two models. The technique resulting in PSD consists in the calculation of the particle density as a function of 4 parameters: perimeter, area, height and volume of the particle. This is done through the analysing of STM images using a build-in-house software. It is found that Cu followed the Ostwald ripening process while Pd followed the coalescence sintering. From the analysis of the Auger spectra, the growth mode of Cu and Pd were found to follow the Volmer -Weber mode consisting of 3D particle growth. The structure of the Pd/Ti02 system is investigated via STM and LEED analysis. It is shown that Pd nanoparticles supported TiC>2 reconstructed upon annealing into hexagonal, wagonwheel, star shape and zigzag superstructures. These various structures have unit cell dimensions varying from 9.5A to 25A and consist of mixed layer of titanium and palladium in their ground state. The possible structures are modelled and it is concluded that the Ti or Pd adatoms can be situated in atop, 2- or 3-fold sites. TPR results shows that CO uptake capacity is lost when the catalyst is reduced to high temperature. However, the CO oxidation reaction is enhanced. It is shown that encapsulation of Pd by TiOx species is responsible for the loss of CO adsorption but also for the improvement of the CO oxidation. Weakly bound forms of CO and new active centres, as a result of the reconstruction, can improve the activity of the CO oxidation reaction