Adsorbate island dimensions and interaction energies from vibrational spectra: Co on Pt{001} and Pt{111}

Abstract

Abstract Reflection-absorption infrared spectra, sticking probabilities, absolute surface coverages and desorption spectra are reported for CO chemisorption on Pt{001} and Pt{111}. Isotopes (12CO and 13CO) are used to determine both true singleton (isolated species) CO stretching frequencies and the extent of the dipole-coupling induced frequency shift as the coverage is increased. Vibrational spectra obtained at temperatures over the range 160 to 440 K provide clear evidence for the equilibrium formation of islands at low coverages on Pt {001}, and enable an estimate to be made of island sizes. The halfwidth of the CO absorption band indicates a surprising uniformity in island dimensions at a given coverage and temperature; at a coverage of 1 × 1014 molecules cm−2 and at 160 K, the average island contains ∼70 CO molecules, decreasing to ∼10 molecules at 400 K. A simple statistical mechanical model produces an attractive pairwise interaction energy between adsorbed species of 6.3 kJ mol−1 from the data. On Pt{111} at 300 K adsorption initially occurs into isolated singletons; at low coverages (∼0.4 × 1014 molecules cm−2) islands begin to form, until at ∼1.6 × 1014 molecules cm−2 the island phase is dominant.