ACTIVATION OF CARBON-DIOXIDE ON POTASSIUM-MODIFIED AG(111) SINGLE-CRYSTALS

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 CO2 on K/Ag(111) for K coverages in the range 0.13 < theta(K) < 0.47, where the close-packed monolayer corresponds to theta(K) = 1/3, exhibit a sharply defined m/e = 44 peak at 796 +/- 6 K with no evidence for the desorption of CO at any temperature. Similar TPD experiments involving mixtures of natural and O-18-labeled CO2 indicate that the oxygen atoms undergo partial scrambling, suggesting that the overall process cannot be represented in terms of a simple adsorption/desorption of CO2. The HREELS spectra of CO2-saturated K/Ag(111) show, in addition to very minor features, a sharp peak at approximately 1480 cm-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 CO2 was obtained from TPD, which failed to reveal features associated with carbonate (decomposition) expected to be formed via the reaction of CO2 and adsorbed O. On the basis of these results, it is proposed that CO2 on K/Ag(111) binds through the carbon to the surface, leading to the "partial" dissociation (or activation) of each CO2 molecule into adsorbed CO and O. Within this model, such adsorbed O would serve as a bridge between the carbon atoms of neighboring "activated" CO2 molecules and therefore undergo exchange prior or during thermal desorption. Adventitious water or oxygen in the system and/or defect sites on the surface give rise to an additional m/e = 44 TPD peak at a much higher temperature. The height of this new feature is increased significantly by predosing the K/Ag(111) surface with O2 or H2O at coverages as low as 0.05 L. The XPS spectra for these purposely contaminated surfaces reveal features very different from those observed in the absence of such impurities, but consistent with the presence of an ordinary form of carbonate. Ag(111) surfaces which had been damaged prior to K deposition and subsequent CO2 adsorption were found to yield significant amounts of CO in the TPD spectra at lower temperatures.