Changes in the adsorption and desorption behavior of cyclohexane and benzene on Ni(111) induced by a monoatomic potassium layer

Abstract

The adsorption and desorption behavior of cyclohexane and benzene on a clean, a potassium precovered, and an oxygen precovered Ni(111) surface have been studied by TPD and LEED. Below 120 K, cyclohexane is molecularly adsorbed on all substrates. On the bare Ni(111) surface cyclohexane shows Stranski-Krastanov growth as concluded from the subsequent development of 3 peaks α,β and γ in the TPD spectra that are assigned to desorption from the first layer, the second layer and bulk cyclohexane, respectively. From the zeroth order behavior of the a-peak, desorption from two-dimensional islands via a two-dimensional gas is concluded. Growth in two-dimensional islands is also suggested by the observation of a (√7 × √7)R19.1 ° LEED structure starting at a coverage as low as 0.04 ML up to the saturation coverage of 0.143 ML. On the potassium precovered surface (θ k = 0.34 ML) cyclohexane exhibits Volmer-Weber growth as indicated by the observation of one single peak in the TPD spectra. This change in growth mode is attributed to a strongly reduced van der Waals interaction of cyclohexane to the K-precovered surface. The results are compared to those obtained for a Ni(111) surface precovered with a p(2 × 2) layer of oxygen (θ o = 0.25 ML). Dramatic changes are also induced in the adsorption behavior of benzene by preadsorbed potassium. Benzene is no longer chemisorbed as on the clean Ni(111) surface, but is only weakly bonded to the substrate. Whereas layer-by-layer desorption is observed in the TPD spectra on the clean Ni(111) surface, on the K-precovered surface desorption of benzene occurs in one single peak, again indicating a change from Stranski-Krastanov growth on the clean surface to Volmer-Weber growth on the K-precovered surface.