MICROSTRUCTURE AND REACTIVITY OF PT-CE AND RH-CE PARTICLES ON SILICA

The interactions of Ce with 20 to 200-Å-diameter particles of Pt and Rh on planar Si02 are examined using transmission electron microscopy (TEM) and reactivity measurements following treatment in H2 and O2 atmospheres. Samples are transferred repeatedly between furnace and TEM so that the time evolution of the microstructures of individual particles can be observed. Upon heating Pt-Ce in HZ at 650°C, Ce forms a thin amorphous film on the SiO2, and the only crystalline phase observed is Pt metal for low Ce loadings. Pt particle sizes are approximately the same as without Ce, but all Pt particles are round and contain many internal defects, while all particles form single crystal cubes without Ce. For high Ce loadings a phase is detected with a diffraction pattern of cerium hydride, and dark field imaging shows that this forms as cubes near the Pt particles. Heating in O2 causes the Ce to convert to Ce02 particles which form around the Pt particles. When Rh-Ce is heated in H2, a thin film of cerium forms on the SiO2. At high Ce loadings, diffraction patterns which appear to be CeH2.53 are observed. The Rh particles contain more defects and show less faceting than without Ce. Heating in O2 at high Ce loadings causes Ce from the film to coalesce and form large, flat (>1000 Å diameter and ∼100 Å thick) Ce02 particles. Upon reduction in H2 at 600-750°C, these particles disperse very slowly and may form compounds with the SiO2. It is also observed that Rh catalyzes reduction of adjacent CeO2 back to Ce because holes form in the Ce02 near Rh particles. The oxidation-reduction cycle results in a redispersion of the Rh with Ce present. Reactivities of Rh-Ce catalysts in CO hydrogenation and in ethane hydrogenolysis were also measured. For CO hydrogenation, Ce increases activity by a factor of ∼30, and the selectivity shifts toward higher alkanes and alkenes. For ethane hydrogenolysis, it was found that activities increase upon addition of Ce, but activities are affected more by pretreatment in H2 or O2 than by Ce addition. © 1991.