SITE BLOCKING BY HYDROGEN - CO ON CLEAN AND H-PRESATURATED FE(100)

Because CO has been observed to facilitate transfer of adsorbed hydrogen to surface-bound ethyl groups on Fe(100) and to weaken the Fe-H bond, we have studied CO binding on hydrogen-presaturated Fe(100) surfaces with electron energy-loss spectroscopy (EELS) to identify differences in CO binding on the clean and hydrogen-saturated surface. The dominant binding site is shown to be quite different for the hydrogen-presaturated surface. We reproduce the CO(alpha-3) desorption state on the clean surface reported by others, representing CO bound in four-fold hollows, which exhibits an unusually low stretching frequency of 1140 cm-1. On the clean surface this state dominates at low CO coverage and remains as bridging and atop CO states are populated. The hydrogen-presaturated surface clearly blocks binding of CO in this state; the CO(gamma) stretching frequency observed is characteristic of bridging and atop binding. Conversion to the CO(alpha-3) state occurs after heating to 343 K to desorb the hydrogen. These results show (i) that coadsorption of CO onto the hydrogen-saturated Fe(100) surface does not lead to phase separation into the phase expected for CO alone on Fe(100) and (ii) that site conversion of CO with hydrogen removal may contribute to the driving force for hydrogen transfer reactions.