Adlayer effects on the surface reactivity of W(100) and Mo(100) single crystals

The surface reactivity of W(100) and Mo(100) single crystals with adsorbed carbon and oxygen was studied using flash desorption spectroscopy and temperature programmed reaction spectroscopy. Carbon and oxygen adatoms were deposited by the thermal cracking of ethylene and the chemisorption of molecular oxygen, respectively. For either adatom several ordered overlayers were observed as a function of coverage and temperature. The presence of either carbon or oxygen severely hindered the ability of W(100) and Mo(100) to dissociatively chemisorb hydrogen or carbon monoxide. In the case of carbon, a correlation was found between the available fourfold sites on the surface and the amount of carbon monoxide dissociated. The ability of the carbide surface to adsorb molecular carbon monoxide was not significantly affected. On the other hand, oxygen was found to suppress the chemisorption of molecular carbon monoxide at room temperature as it reduced the carbon monoxide binding energy considerably more than carbon.