Ex-situ and in-situ spectroscopic studies of the passive film on alkali and alkaline earth metals in nonaqueous solvents

Abstract

The adsorption of carbon dioxide on potassium-dosed Ag(111) has been investigated with temperature-programmed desorption (TPD), work function measurements, and Auger electron (AES), X-ray photoelectron (XPS), and high resolution electron energy loss (HREELS), spectroscopies. Unlike the behavior observed for other K-modified single-crystal metal surfaces, the TPD spectra of near-saturation coverages of CO{sub 2} on K/Ag(111) for K coverages in the range 0.13 < {Theta}{sub K} < 0.47 exhibit a sharply defined m/e = 44 peak at 796 +/{minus} 6 K with no evidence for the desorption of CO at any temperature. Similar TPD experiments involving mixtures of natural and {sup 18}0-labelled CO{sub 2} indicate that the oxygen atoms undergo partial scrambling, suggesting that the overall process cannot be represented in terms of a simple adsorption/desorption of CO{sub 2}. The HREELS spectra of CO{sub 2}-saturated K/Ag(111) show, in addition to very minor features, a sharp peak at about 1480 cm{sup {minus}1}, and XPS spectra of the same interface display a C(ls) peak with a binding energy characteristic of an electron-rich carbon species. This information is consistent with the presence of a carbon-bound CO species on the surface. Evidence against the complete dissociation of CO{sub 2} was obtained from TPD which failed tomore » reveal features associated with carbonate (decomposition) expected to be formed via the reaction of CO{sub 2} and adsorbed O. It is proposed that CO{sub 2} on K/Ag(111) binds through the carbon to the surface leading to the partial'' dissociation (or activation) of each CO{sub 2} molecule into adsorbed CO and O. Within this model, such adsorbed O would serve as a bridge between the carbon atoms of neighboring activated'' CO{sub 2} molecules, and therefore undergo exchange prior to or during thermal desorption.« less