EFFECTS OF K-ADATOMS, O-ADATOMS, AND H-ADATOMS ON THE ADSORPTION-KINETICS OF CO ON PT(111)

The sticking coefficient, S, of CO on clean and K-, O- and H-modified Pt(111) surfaces has been measured directly using a simple kinetic uptake method, as a function of CO and modifier coverage and surface temperature. A simple method is also introduced to determine the coverage-dependent desorption energy from these measurements without the need to know the pre-exponential factor or desorption order for those systems where S is only a function of coverage or only weakly dependent on temperature. The initial sticking coefficient of CO on clean Pt(111) is 0.9 and is independent of substrate temperature. The major influence of adsorbed K on the initial sticking coefficient of CO on Pt(111) at 320 K is site blocking, and the magnitude of this effect correlates well with the size of the adsorbed K ion or atom. The shape of S as a function of CO coverage for low K coverages in the ionic regime is similar to that of CO on clean Pt(111). A dramatic change occurs for K coverages near or greater than 0.24 ML, where S passes through a maximum at about 1/3 of the saturation CO coverage and coincident CO and K desorption is observed. This behavior of S, which has not been seen before, can be fit quite adequately by a simple model that incorporates the reduced site blocking effects of K due to the CO-induced transition of K from a metallic neutral to an ionic state and the accompanying size change of K atoms. These results are best described by a coverage-dependent alkali-CO interaction. In additional studies, we find that adsorption of 0.25 ML O on Pt(111) has no effect on S for low CO coverages at 100 K, and only the saturation CO coverage is reduced from 0.59 to 0.31 ML. In contrast, adsorbed H reduces not only the CO saturation coverage but also S even at low CO coverages.