Surface diffusion of carbon monoxide and potassium coadsorbed on Ru(001): Confirmation of a 1:1 CO:K trapping interaction

Abstract

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO≳ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO<4×10−10 cm2/s for ΘCO<0.10 ML and increased to DCO=2.5×10−7 cm2/s for ΘCO≳0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO<4×10−10 cm2/s for ΘCO<0.25 ML and increased to DCO=5×10−9 cm2/s for ΘCO≳0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.