AVS 46th International Symposium
    Surface Science Division Friday Sessions
       Session SS2-FrM

Paper SS2-FrM7
The Oxidation and Reduction of Pd(111)

Friday, October 29, 1999, 10:20 am, Room 607

Session: Adsorption on Metals and Silicon
Presenter: G. Zheng, Yale University
Authors: G. Zheng, Yale University
E.I. Altman, Yale University
Correspondent: Click to Email
Palladium is a promising catalyst for oxidation reactions. Therefore, the oxidation of Pd(111) was characterized using STM, TPD, and LEED. Exposure of Pd(111) to O@sub 2@ resulted in a (2x2) structure, that saturated after exposure to 30 L. To increase the oxygen coverage, NO@sub 2@ was used. Initial exposure of Pd(111) to NO@sub 2@ also produced the (2x2) structure. Further exposure, however, resulted in additional diffraction spots, which persisted until the oxygen coverage reached approximately 1.0 ML. At oxygen coverages between 1.0 - 2.0 ML, a complicated LEED pattern was observed. This pattern could be explained as the superposition of two surface structures, one with a square surface lattice rotated 15° with respect to the Pd(111) substrate, the other with a rectangular surface lattice rotated 30° with respect to the Pd(111) substrate. In STM images, ad-islands and peninsulas were observed in this oxygen coverage regime. The rectangular structure was found on the original Pd(111) terraces, while the square structure was observed on the islands and peninsulas. A Moiré pattern due to lattice mismatch with underlying layers was also observed on the islands and peninsulas. The lattice constant for the square structure was 0.679 nm; and the two lattice constants for the rectangular structure were 0.394 nm and 0.638 nm, respectively, consistent with LEED observations. After further increasing the oxygen coverage, the complicated LEED patterns became faint. At the same time, a low temperature shoulder associated with bulk PdO developed in TPD traces. These results indicate that oxygen can exist in five different states on the Pd(111) surface. The reactivity of these states towards reduction is being characterized by monitoring the rate of disappearance of the different surface oxygen phases by recording STM movies during reduction by CO, H@sub 2@, and CH@sub 3@OH.