Chemisorption and dissociation of O2 on Pd(1 1 1) studied by STM

The adsorption and dissociation of O2 on Pd(1 1 1) at low coverage was studied in a variable temperature scanning tunneling microscope (STM) in the range from 25 to 210 K. Below 60 K the diffusion rate is negligible and individual molecules appear as slightly elongated 5 pm protrusions surrounded by 10 pm depressions at low bias voltage (⪅200 mV). The molecules predominantly appear in three distinct orientations and closely resemble the peroxide species observed on Pt(1 1 1). A small fraction of the molecules occupy a second binding configuration at 25 K, appearing similar to the dominant species but with opposite orientations. No molecules resembling the superoxide precursor state on Pt(1 1 1) were observed. Both thermal activation and interaction with the STM tip induce rotation between the three orientations of each molecule, as well as displacement to adjacent sites and conversion between the two binding geometries. The molecules order in p(2 × 2) clusters, which grow in size upon annealing to 100 K. Rotational disorder is observed within the islands below 60 K, but not at higher temperature due to rapid rotation. Near 120 K, thermal O2 dissociation occurs primarily at the periphery of islands and at active sites associated with impurities beneath the Pd surface.