THE EFFECT OF ADSORBATE AND PRETREATMENT ON THE SHAPE AND REACTIVITY OF SILICA-SUPPORTED PLATINUM PARTICLES

Considerable uncertainties still exist concerning the role of adsorbate and support on the shapes of small metal particles. Industrial catalyst supports are very complex and metal particles are often located in a tortuous pore structure that makes it difficult to obtain TEM images which clearly reveal particle shape. In order to unambiguously identify particle shapes, we have used nonporous submicron-sized spheres of silica that allow us to obtain profile views of the metal particles. Pt was deposited by impregnation, and the catalysts were calcined and examined after conventional 673 K reduction in flowing H-2. They were subsequently annealed at 923 and 1173 K in 600 Torr H-2, in air and in a vacuum of 1 X 10(-7) Torr. The role of adsorbate and pretreatment on particle shape was studied. Since the exposed facets are clearly seen only when the particles are imaged along the low-index zone axes, care was taken to tilt the particles before imaging. We found that all of the small particles (< 5 nm) in these catalysts appeared spherical, while well defined facets were seen only on the larger particles. The (111) surfaces are preferentially exposed in the as-prepared catalysts as well as after annealing in H-2 or in vacuum. However, treatment in air at 923 K caused significant rounding of the Pt particles and loss of well defined facets. Catalytic activity of the Pt for n-pentane hydrogenolysis was used to assess the cleanliness of the surface and the role of particle shape. It was found that treatment in air caused a significant drop in activity due to deposition of impurities; however, there was no detectable contamination after the treatment in H-2 or in vacuum. The shapes of the particles had no effect on the selectivity or activity for n-pentane hydrogenolysis.