Increase of Pd surface area by treatment in dioxygen

The surface area of Pd(111) and Pd(110) single crystals increased after oxidation in O2 transformed them to PdO. The surface area of the oxide decreased after complete reduction in H2. The techniques of STM, TPD, XPS, AES, and LEED were employed to study the Pd(111) and Pd(110) single-crystal surfaces after they were subjected to O2 oxidation, methane combustion, and H2 reduction. The surface area of the treated palladium single crystals was measured by 18O isotope exchange and by direct measurements using the STM image. These two methods showed agreement within 20%. After oxidation in O2 (100 and 150 Torr) at 600 K, the surface area for both Pd(111) and Pd(110) single crystals increased by a factor of approximately two. The effect was more pronounced on the Pd(111) surface. The oxidized surfaces were covered with 3- to 4-nm semispherical oxide agglomerates that formed a “cauliflower-like” structure 10–20 nm in size. Similar surface structures were observed after exposure of the Pd single crystals to a lean O2 and CH4 reaction mixture (O2:CH4=10:1). Thus, the oxidized single crystal becomes amorphous. Reduction in H2 decreased the surface area of the preoxidized Pd(111) and Pd(110) crystals. An amorphous metallic surface was produced after H2 reduction at 373 K, whereas a smooth surface with characteristic single-crystal features was observed after reduction at 673 K. These experiments suggest that oxidizing a Pd metal catalyst or reducing an oxidized Pd catalyst, for example, before palladium metal surface area measurement, will affect the surface area of the sample. It also shows that the increase in surface area on Pd catalysts after oxidation treatment is caused by surface roughening.