S. Abbet et al., Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties, J AM CHEM S, 123(25), 2001, pp. 6172-6178
CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied
by means of thermal desorption (TDS) and Fourier transform infrared (FTIR)
spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of
about 250 K, which corresponds to a binding energy, E-b, of about 0.7 +/- 0
.1 eV. FTIR spectra suggest that at saturation two different sites for CO a
dsorption exist on a single Pd atom. The vibrational frequency of the most
stable, singly adsorbed CO molecule is 2055 cm(-1). Density functional clus
ter model calculations have been used to model possible defect sites at the
MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes
located at regular or low-coordinated O anions of the surface exhibit consi
derably stronger binding energies, E-b = 2-2.5 eV, and larger vibrational s
hifts than were observed in the experiment. CO/Pd complexes located at oxyg
en vacancies (F or F+ centers) are characterized by much smaller binding en
ergies, E-b = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with th
e experimental value. CO/Pd complexes located at the paramagnetic F+ center
s show vibrational frequencies in closest agreement with the experimental d
ata. These comparisons therefore suggest that the Pd atoms are mainly adsor
bed at oxygen vacancies.