Activation of carbon dioxide on potassium-modified silver(111) single crystals

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 <0{sub K} <0.47, where the close-packed monolayer corresponds to 0{sub K} + 1/3, exhibit a sharply defined m/e = 44 peak at 796 {plus_minus} 6 K with no evidence for the desorption of CO at any temperature. Similar TPD experiments involving mixtures of natural and {sup 18}O-labeled 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 {approximately} 1480 cm{sup {minus}1}, and XPS spectra of the same interface display a C(1s)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} wasmore » obtained from TPD, which failed to reveal features associated with carbonate (decomposition) expected to be formed via the reaction of CO{sub 2} and adsorbed O. On the basis of these results, it is proposed that CO{sub 2} on K/Ag(111) binds through the carbon to the surface, leading to the {open_quotes}partial{close_quotes} dissociation (or activation) of each CO{sub 2} molecule into adsorbed CO and O. 27 refs., 13 figs., 1 tab.« less