Scanning Probe Microscopy (spm) Studies of Palladium Thin Film Model Catalysts.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM), jointly known as scanning probe microscopy (SPM), have been used to image the surface morphology of Pd supported on various thin film substrates. The objective of this study is to correlate the morphology of Pd thin film catalysts with its activity and selectivity in various Pd thin film model catalyst systems. The effect of gas pretreatment on catalyst morphology was studied. AFM results show the study of activation concluded that O_2 followed by H _2 treatments prior to reaction wet and redisperse the Pd film and hence activate the catalyst. Such observations were accompanied by lower Pd binding energy and activation energy for the 1,3 butadiene hydrogenation reaction compared to those only treated in reduced atmosphere alone. The effect of sulfur on catalyst deactivation was also studied and a correlation between activity and S/Pd surface ratio was developed. This study demonstrated that a correlation between the physico-chemical information from SPM, XPS of a model catalyst and kinetic data is possible with its activity, both quantitatively and qualitatively. To examine the structural effect, various catalyst structures using electropolishing techniques were created. Structure with the most protrusion was found to have the highest activity with highest activation energy and pre-exponential factor. It was also found that the combination effect of quantity and quality of the active sites dictates the overall reactivity of the catalyst. A UHV STM system was developed in this study to examine the effect of annealing temperature and catalyst substrates on catalyst microstructure and activity. The STM images of a Pd/C film catalyst indicated that, at the resolution used, the surface microstructure of the Pd thin film is similar at ambient and UHV conditions with the resolution employed. By increasing the annealing temperature, surface SiO_2 enrichment of Pd/SiO _2/C film catalysts by diffusion and the decline of activity were observed. It was also found that Pd film sinters more easily on W(110) than W(211). On Pd/W single crystal films, the anchoring effect of the saw-tooth W(211) on Pd appears to reduce Pd sintering on W(211). Also, in both Pd/SiO_2/C and Pd/W single crystals the reaction pathways of 1,3 butadiene hydrogenation remains unchanged.