The oxidation mechanism of Pd(1 0 0)

The oxidation of Pd(1 0 0) was characterized using temperature programmed desorption (TPD), low-energy electron diffraction (LEED), and in situ variable-temperature scanning tunneling microscopy (STM). The results indicate that Pd(1 0 0) oxidation proceeds through four stages involving up to five surface phases. In the first stage, oxygen chemisorbs atop the Pd surface resulting in p(2×2) and c(2×2) overlayers that desorb at 800 and 700 K, respectively. As the overlayers saturated, island and peninsula growth was observed in STM movies. The islands are one Pd atom high and so the growth is attributed to Pd atoms ejected onto the terraces which then either nucleated islands or attached to preexisting steps. During this second stage, no change in the surface periodicity was observed. After growth stops, the surface reconstructs in the third stage. Above 475 K, STM images revealed the formation of a ( 5 × 5 ) R27° reconstruction. The images of the ( 5 × 5 ) R27° structure are twofold symmetric, consistent with the formation an epitaxial PdO(0 0 1)-like layer. In addition, a (5×5) reconstruction was observed with LEED prior to the ( 5 × 5 ) R27° reconstruction. As the oxygen coverage entered the regime where these reconstructions were observed, a new desorption peak appeared at 650 K. This temperature is higher than that expected for PdO dissociation but lower than that observed for chemisorption suggesting that the Pd–O bond strength in the reconstructed surfaces is intermediate between the bulk oxide and chemisorbed oxygen. In the fourth stage, the LEED patterns began to fade, the STM images showed three-dimensional clusters, and a low-temperature shoulder consistent with PdO decomposition developed in TPD traces. Thus the surface roughens as the bulk oxide forms. The mechanism outlined above is similar to that recently observed for Pd(1 1 1) oxidation.