Lithium promoted adsorption of CO on Ag/Ru(001)

Abstract

The Li promoted adsorption of CO on annealed multilayer Ag on Ru(001) was studied, using metastable quenching spectroscopy (MQS) and thermal desorption spectroscopy (TDS). CO interacts weakly with Ag, and does not adsorb at 70 K in UHV. It is found that preadsorbing a small amount of Li ( θ Li = 0.05) on the Ag leads to a CO saturation coverage of θ CO = 0.32. The MQS spectrum indicates that the CO orientation is vertical, with the C atom toward the Ag surface. No isotopic scrambling of labeled CO was observed. The CO/Li ratio of 6:1 indicates a long range interaction. A surface structure is proposed which fits this data and gives a LiCO interaction distance of 5 Å. The CO is physisorbed, and desorbs around 100 K. Potassium did not show any promotion effect for the adsorption of CO on Ag. These results are discussed in terms of models for alkali promotion of CO adsorption, and work function measurements which show that the charge transfer from Li to Ag is stronger than that from K to Ag at low alkali coverages.