Desorption kinetics on an alkali metal-precovered surface: CO and K on Pt(111)

Abstract

The desorption kinetics of CO initially coadsorbed with K on Pt(111) at 100 K has been measured as a function of both CO and K coverage (θ) using temperature programmed desorption (TPD). CO and K desorb separately when the initial θCO is low, supporting the theory that CO and K interact via a strong Coulombic attraction and do not form an oxocarbon (salt-like) compound. Although precoverage with ≥0.15 ML K does induce a few percent of the desorbing CO to undergo atomic exchange, the effect is very small in comparison to that observed on other surfaces. The desorption is found to be first order in θCO on both clean and K-precovered surfaces. With low initial θCO the activation energy (E) and preexponential factor (ν) for desorption from the K-free surface are found to be 28.8±0.7 kcal mol−1 and 1013.3±0.2 s−1, respectively, in excellent agreement with previously reported results. As observed with most alkali metal-precovered surfaces, the low coverage CO desorption peak temperature increases monotonically with K coverage. We find that the higher desorption temperatures result from the combined effects of increasing E and simultaneously decreasing ν, with the change in ν causing most of the shift; when low θCO are initially coadsorbed with 0.15 ML K E=31.3±0.6 kcal mol−1 and ν=1011.4±0.2 s−1. For all surfaces studied both E and ν decreased approximately linearly with increasing θCO>0.1 ML. The θCO and θK dependence of the kinetics is generally consistent with the proposed structural model for this surface based on the formation of CO+K islands. Possible mechanisms for the K-induced change in the kinetics are discussed and further investigations are proposed.