Structural Modification of Platinum Model Systems under High Pressure CO Annealing

Abstract

Using temperature-programmed desorption experiments, we have studied the coordination dependent adsorption of CO on a platinum (Pt) single crystal, and mass-selected Pt nanoparticles in the size range of 3 to 11 nm, for CO dosing pressures in 10–7 mbar and mbar ranges. From low pressure CO adsorption experiments on the Pt(111) crystal, we establish a clear link between the degree of presputtering of the surface prior to CO adsorption, and the amount of CO bound at high temperature. It was found that for rougher surfaces, i.e., with more undercoordinated surface atoms, a feature appears in the CO desorption spectra at high temperature. The result is consistent with literature results from stepped single crystals that have found high temperature CO desorption features due to the presence of undercoordinated step and kink sites on the crystal facets. For the nanoparticles, a study of the dependence of the CO desorption profile with particle size found more prominent high temperature CO desorption features as the nanoparticle size was decreased, consistent with the expectation for a higher proportion of undercoordinated sites at smaller particle sizes. Thus, for both systems there is a clear relation between surface atom coordination, and the desorption temperature of CO. Investigation of these structural features was then made for CO dosing pressures in the mbar range. Intriguingly, from the mbar pressure experiments it was observed that elevated CO pressures enhanced the annealing of the Pt(111) surface, but on the otherhand, caused an apparent roughening of the nanoparticles.