Coadsorption of K and CO on Cu(111) surfaces

Abstract

It is well established that for submonolayer coverages the strong interaction between CO + K is not only influenced by the adsorptives but also by the substrate itself. We report about investigations of CO + K on the Cu(111) surface, where the CO/Cu(111) interaction is rather weak. The analytical techniques employed include thermal desorption spectroscopy, LEED and work function change measurements. We find the interaction between K + CO on Cu(111) to be very complex. Three different regimes of CO/K interaction can be distinguished. (1) At very low ΘK (0.0–0.02) TDS measurements show a shift of the CO desorption maximum (for low CO coverages) towards lower temperature. ΔΦ indicates a decrease and finally a vanishing of the effective negative CO dipole moment. (2) At ΘK = 0.02−0.155 a new CO desorption maximum appears, shifting continuously from 165 to 270 K with increasing K coverage. ΔΦ indicates a positive effective dipole moment (increasing 2π∗ backdonation). Between ΘK = 0.13 and 0.18 around 300 K a very sharp and highly asymmetric CO desorption peak can be detected with no coincident K desorption. (3) At ΘK> 0.2, where the transition from “ionic-” to “metal-like” K occurs, the strong interaction between CO/K is evidenced by a thermal stabilization of the adlayer to temperatures> 700 K. Under these conditions several additional desorption peaks are detected between 400 and 800 K. Desorption of CO2 was detected under these ΘK conditions, probably due to a partial dissociation and recombination reaction.