Palladium is of interest as an oxidation catalyst for applications such as methane combustion. Under the reaction conditions the catalyst may change state from Pd metal to Pd oxide. These changes are accompanied by vast changes in reactivity though it is still debated in the literature as to whether the metal or the oxide is the more reactive phase. Therefore we have been studying the oxidation and reduction of Pd to address this question; the work described below focuses on the oxidation

When oxygen initially adsorbs on Pd(111) it forms a simple (2x2) structure. At higher oxygen coverages, however, the extremely complex diffraction pattern shown above appears.

As the complex diffraction pattern forms, islands and peninsulas are observed to grow on the terraces. This growth is attributed to oxygen penetration into the surface to form a surface with a lower Pd atom density. The released Pd atoms either nucleate islands or attach to step edges.

When we zoom in on the wide range image, we find that the original terraces display a different structure than the islands and peninsulas (A). The peninsulas and islands (B) display a square structure, while the original terrace (C) displays a rectangular structure. Three equivalent domains of the three different structures accounts for the complexity of the diffraction pattern. Note that the three domains of the rectangular structure can be seen in (A). The lattice constants of both the square and rectangular structures cannot be related to either the Pd substrate structure or the PdO structure thus these can be considered intermediate phases. Thermal desorption results also indicate a Pd-O bond strength intermediate between chemisorbed O and PdO.

Under certain tunneling conditions an additional periodicity can be seen on the islands and peninsulas. This results in the superposition of a striped structured on top of the square structure. This pattern can be understood in terms of a Moiré pattern due to the lattice mismatch between the surface layer and the underlying Pd. This pattern indicates that the Pd beneath the square structure is unreconstructed.