The oxidation of Pd(111)

Abstract

The oxidation of Pd(111) was characterized using scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). Initial exposure of Pd(111) to O 2 at temperatures between 300 and 575 K resulted in a (2×2) structure observable by both LEED and STM. The maximum coverage achieved by O 2 exposure at 300 K was 0.25 ML. By increasing the temperature above 500 K, the oxygen coverage could be increased to 0.37 ML. To increase the oxygen coverage further, NO 2 was used as the oxidant. On Pd(111), NO 2 dissociatively adsorbs, with NO going to the gas phase below 500 K, leaving oxygen on the surface. Above 500 K, initial exposure of NO 2 to Pd(111) also produced the (2×2) structure. Increasing the oxygen coverage to between 1.0 and 2.2 ML resulted in a complicated LEED pattern. This pattern could be explained as the superposition of three equivalent domains of two surface structures: one with a square surface lattice rotated 15° with respect to Pd[110], the other with a rectangular surface lattice with the short sides of the rectangles parallel to Pd[110]. In STM movies, ad-islands and peninsulas were observed to nucleate and grow as the oxygen coverage reached this regime. The rectangular structure was observed on the original Pd(111) terraces, and the square structure on the islands and peninsulas. This suggested that when the oxygen coverage exceeds 0.25 ML, oxygen atoms penetrate the surface creating a rectangular structure with a lower Pd atom density than the clean surface; the liberated Pd atoms, along with oxygen, then form the islands and peninsulas. The lattice constants obtained from the STM images were 0.679±0.012 nm for the square structure and 0.394±0.008 nm and 0.638±0.022 nm for the rectangular structure, both consistent with LEED observations. Neither of these structures can be simply related to any crystal orientation of Pd or PdO, indicating that there are states intermediate between Pd and PdO. After further increasing the oxygen coverage, the complicated LEED patterns became faint and a low temperature shoulder attributed to PdO decomposition developed in TPD traces. The results indicate that Pd(111) oxidation proceeds through three stages involving four distinct surface states.