ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The adsorption of CO2 on clean silver films, oxygen treated silver films and alkali metal predosed silver films has been studied with surface enhanced Raman spectroscopy (SERS) and work function measurements. At 50 K, CO2 was observed to adsorb on vapor deposited silver films with downward vibrational frequency shifts of (approximate)15 cm−1 from the corresponding gas phase values. On oxygen pretreated films, CO2 reacts to form a surface carbonate. Isotope studies with C18 O2 show that the carbonate is formed from the reaction of CO2 with oxygen, and not from the disproportionation of CO2 . Based on the observed intensities of the vibrational bands, an orientation of the carbonate is suggested in which the molecule's plane is perpendicular to the surface. On silver films dosed with submonolayer amounts of potassium, CO2 reacts to form the C2v isomer of a K+CO−2 surface complex. Bands at 755 and 1220 cm-1 observed with potassium dosed surfaces were assigned to the νs and δ vibrational modes of CO−2. The orientation of the CO−2. ion is likely with its molecular plane perpendicular to the surface and bonded to the alkali metal through the O atoms. Experiments on lithium and cesium dosed silver surfaces led to similar conclusions. The similarity of these vibrational spectra to those reported in our recent study of CO adsorption on potassium predosed silver in which we proposed that a KOC surface complex was formed [K.J. Maynard and M. Moskovits, Chem. Phys. Lett. 142, 298 (1987)] lead us to the conclusion that K+CO−2 was formed, in that case also, from the reaction of CO with residual oxygen. The SERS band intensities associated with CO−2 showed an amonotonic dependence on CO2 exposure. This has been ascribed to depolarization by the induced dipole fields of the adsorbate molecules upon one another. Experiments with varying potassium coverage imply that the K+CO−2 ion pairs are not randomly distributed on the surface but are aggregated into islands. A Monte Carlo simulation confirmed the conclusion that only island formation could adequately account for the observed results.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.456286
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