A multi-technique study of CO adsorption on FeO magnetite.

The adsorption of CO on the FeO(001)-(2 × 2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO binds most strongly at defects related to Fe, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO adsorbs at fivefold-coordinated Fe sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (2 × 2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.