SURFACE-DIFFUSION OF CARBON-MONOXIDE AND POTASSIUM COADSORBED ON RU(001) - CONFIRMATION OF A 1 1 CO/K TRAPPING INTERACTION/

The surface mobilities of both CO and K coadsorbed on Ru(001) were stu died using laser-induced thermal desorption (LITD) techniques. The LIT D measurements revealed that CO was essentially immobile on the potass ium-promoted Ru(001) surface when the CO surface coverage, theta(CO), was less than the potassium coverage, Theta(K). The CO surface mobilit y increased dramatically when Theta(CO)>Theta(K). At Theta(K)=0.10 ML and 315 K, the CO diffusion coefficient was D-CO<4x10(-10 cm?2)/s for Theta(CO)<0.10 ML and increased to D-CO=2.5x10(-7) cm(2)/s for Theta(C O)>0.10 ML. At Theta(K)=0.25 ML and 315 K, the CO surface diffusion co efficient was D-CO<4x10(-10) cm(2)/s for Theta(CO)<0.25 ML and increas ed to D-CO=5X10(-9) cm(2)/s for Theta(CO)>0.25 ML. The potassium surfa ce mobility also decreased dramatically as a function of CO coverage a t both Theta(K)=0.10 ML and Theta(K)=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction be tween coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal d esorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) technique. At Theta(K)=0.10 ML and CO cov erages ranging from Theta(CO)=0.06 ML to Theta(CO)=0.42 ML, CO TPD pea k temperatures were consistently higher than CO peak temperatures obta ined on clean Ru(001). These TPD results indicate that CO is stabilize d by K on Ru(001). In addition, the potassium TPD spectra at various C O coverages with Theta(K)=0.10 ML and Theta(K)=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 con sistent with the coadsorbate surface diffusion results. (C) 1996 Ameri can Institute of Physics.