Support effects studied on model catalysts

In order to obtain a more thorough understanding of the interactions between catalytic materials and oxide supports, the structure and adsorption properties of a series of model catalysts were studied under conditions of varying support structure and composition, catalyst loading, and pretreatment conditions. These catalysts were prepared by evaporation of Pt onto a series of non-porous, single crystal oxide surfaces in UHV. The oxides studied included $\alpha$-Al$\sb2$O$\sb3$(0001), ZnO(0001)Zn, ZnO(0001)O, and ZrO$\sb2$(100). Model catalysts composed of TiO$\sb{\rm x}$ evaporated onto polycrystalline Rh foil were also investigated. The growth modes of the supported metal and oxide phases were monitored using Auger electron spectroscopy (AES) in conjunction with a calibrated film thickness monitor and transmission electron microscopy (TEM). The structure of the supported material was determined using transmission electron diffraction (TED). Temperature programmed desorption (TPD) was used to monitor the adsorption properties of the catalysts. The results of this work indicated that the composition and structure of the oxide can affect both the growth and adsorption properties of supported catalytic metals. Pt was found to grow as oriented, 2-dimensional layers on both ZnO and ZrO$\sb2$ and as randomly oriented 3-dimensional clusters on $\alpha$-Al$\sb2$O$\sb3$. CO TPD peak temperatures were shifted downward for low coverages of Pt when supported on oxide surfaces containing metal cations compared to surfaces containing only oxygen anions. These results are interpreted as indicating a chemical interaction between Pt and the metal cations on these surfaces. These effects appeared to be very local and were only observed for Pt atoms in direct contact with the oxide surface. Studies of TiO$\sb{\rm x}$/Rh model catalysts revealed that high coverages of TiO$\sb{\rm x}$ were able to migrate into Rh at temperatures above $\sim$850K in vacuum. These results imply that the unique chemisorption and reaction properties observed for titania supported catalysts are related to decoration of the metal particles by TiO$\sb{\rm x}$. The generation of acid sites on oxide catalysts was also investigated using both model and high-surface-area niobia/alumina catalysts. The results of this work indicated that the reported cracking activity of these catalysts may not be related to the acidity of these materials