Metals on Titania/HOPG as Models for Heterogeneous Catalysts
The work done in this thesis is mainly focused on understanding the nanoscale morphology and activity of model heterogeneous catalysts. In order to do this we design and prepare model catalytic surfaces in ultrahigh vacuum (UHV) and study these surfaces using electron and ion based microscopies and spectroscopies. The nucleation, growth and chemical activity of mono and bimetallic clusters on TiO2(110) were investigated in order to understand how activity is influenced by cluster sizes, interactions with the oxide support and surface compositions of the bimetallic clusters. For example, Au-based and Pt-based bimetallic clusters such as Co-Au, Co-Pt, and Pt-Re were grown on titania and their activity for CO adsorption as well as methanol reaction were investigated. The nucleation and growth of these metals were found to be dependent upon the mobility of each metal on the titania surface. Furthermore, bimetallic clusters can be grown on the surface by taking advantage of the relative surface mobilities; when clusters of the less mobile metal are deposited first, the subsequent deposition of the second, more mobile metal results in nucleation exclusively at the existing seed clusters. Furthermore, bimetallic Pt-Ru clusters on highly oriented pyrolytic graphite (HOPG) have been studied as model systems for understanding direct methanol fuel cell catalysts. For Pt-Ru clusters deposited on powdered carbon supports, the novel electroless deposition (ED) method produces exclusively bimetallic clusters in contrast to the existing wet impregnation methods. The ED grown clusters were compared with clusters grown via vapor deposition by sequential deposition of Pt on Ru seed clusters. For both ED and vapor deposition, exclusively bimetallic clusters were produced since the deposition of Pt did not result in the formation of new clusters. In addition to bimetallic clusters, we have also successfully synthesized MoS2 clusters on titania to understand the structural changes that occurs in the presence of different reactant molecules since the activity of MoS2 towards hydrodesulphurization type reactions is very sensitive to the structure of the clusters. What we have discovered was that these structures are very stable towards adsorption of MeOH, CO, H2O and H2 in UHV.