CO adsorption on Co(0001) revisited: High-coverage CO superstructures on the close-packed surface of cobalt

Abstract

The adsorbate overlayer structures formed by CO on highly covered Co(0001) were investigated using temperature programmed desorption, low energy electron diffraction, reflection absorption infrared spectroscopy and synchrotron-based x-ray photoemission spectroscopy, with the purpose to clarify some recent confusion on CO adsorption at high coverages. TPD shows that the coverage of the (2√3 × 2√3)R30o structure (abbreviated as 2√3 structure) is 7/12 (0.583) ML. Its unit cell contains one COtop and six CObridge species that form three ‘bridge dicarbonyls’ in which two CObridge adsorbates share the same cobalt surface atom. Between 0.58 and 0.63 ML the 2√3 structure breaks up into small 2D domains separated by antiphase domain boundaries that consist of ‘double bridge dicarbonyls’ in which three CObridge adsorbates share two cobalt surface atoms. A phase transition that occurs around 0.63 ML creates previously unknown c(8 × 2) and c(12 × 2) domain boundary structures which consist of high density strips with a (2 × 2)–3CO structure and top/hollow site occupation which are separated by antiphase domain boundaries with a lower local coverage. Up to 0.63 ML the structures found at low temperature in UHV are very similar to those observed by others using in-situ STM at 300 K under CO pressure. This changes above 0.63 ML, where STM shows that moiré structures form instead of the c(n × 2) domain boundary structures. We propose that the moiré and c(n × 2) structures have a very similar enthalpy of formation so that small variations of temperature and CO chemical potential can lead to different structures for the same CO coverage. We show that these high-density phases that exist above 0.5 ML do not form at the typical pressures and temperatures used in applied FTS, but they do need to be considered when CO adsorption at room temperature is used as a diagnostic tool to characterize the catalyst surface.