The oxidation of Pd(111)

(T)he oxidation of Pd( 111) was characterized using scanning tunneling micr oscopy (STM), temperature-programmed desorption (TPD), and low energy elect ron diffraction (LEED). Initial exposure of Pd(lll) to O-2 at temperatures between 300 and 575 K resulted in a (2 x 2) structure observable by both LE ED 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 b e increased to 0.37 ML. To increase the oxygen coverage further, NO2 was us ed as the oxidant. On Pd(111), NO2 dissociatively adsorbs, with NO going to the gas phase below 500 K, leaving oxygen on the surface. Above 500 K, ini tial exposure of NO2 to Pd(111) also produced the (2 x 2) structure. Increa sing the oxygen coverage to between 1.0 and 2.2 ML resulted in a complicate d LEED pattern. This pattern could be explained as the superposition of thr ee equivalent domains of two surface structures: one with a square surface lattice rotated 15 degrees with respect to Pd[110], the other with a rectan gular surface lattice with the short sides of the rectangles parallel to Pd [110]. In STM movies, ad-islands and peninsulas were observed to nucleate a nd grow as the oxygen coverage reached this regime. The rectangular structu re was observed on the original Pd(lll) 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 liberat ed Pd atoms, along with oxygen, then form the islands and peninsulas. The l attice 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 rectang ular structure, both consistent with LEED observations. Neither of these st ructures can be simply related to any crystal orientation of Pd or PdO, ind icating that there are states intermediate between Pd and PdO. After furthe r increasing the oxygen coverage, the complicated LEED patterns became fain t and a low temperature shoulder attributed to PdO decomposition developed in TPD traces. The results indicate that Pd(lll) oxidation proceeds through three stages involving four distinct surface states. (C) 2000 Elsevier Sci ence B.V. All rights reserved.