Adsorption of gas-phase oxygen atoms on Pt(1 0 0)-hex-R0.7°: Evidence of a metastable chemisorbed phase

We utilized temperature programmed desorption (TPD) and low energy electron diffraction (LEED) to study the chemisorption of gas-phase oxygen atoms on Pt(1 0 0)-hex-R0.7° at 450 K and 573 K, and find that the types and relative populations of oxygen phases that develop are highly dependent on the surface temperature during adsorption. At both temperatures, oxygen atoms initially adsorb on defects associated with the surface reconstruction. Increasing the coverage to about 0.32 ML (monolayers) at 573 K causes deconstruction and population of a phase with apparent (3 × 1) symmetry that desorbs in a single feature centered at about 672 K. Saturating at 0.63 ML leads to the formation of an additional “complex” ordered phase that desorbs in a sharp feature exhibiting autocatalytic behavior as it shifts from approximately 631 K to 642 K. Uptake at 450 K also initiates deconstruction, but in this case two desorption maxima at about 652 K and 672 K grow simultaneously with increasing coverage to about 0.32 ML. The feature at 672 K is associated with the disordered (3 × 1) phase, while the feature at 652 K has not been previously reported. We attribute this new feature to desorption from disordered arrangements of high oxygen concentrations on (1 × 1) surface regions. As the coverage increases to about 0.51 ML, small amounts of the complex phase grow, while this “high-concentration” (1 × 1) and the (3 × 1) phases continue to develop. We conclude that the complex phase is energetically preferred over the high-concentration (1 × 1) phase, but kinetic barriers hinder its formation at 450 K, causing oxygen to become trapped in the high-concentration (1 × 1) phase. Therefore, the high-concentration (1 × 1) phase is metastable relative to the complex phase. Lastly, above about 0.51 ML, further adsorption at 450 K promotes the growth of Pt oxide islands as detailed in a future investigation.