OBSERVATION OF A STABLE METHOXY INTERMEDIATE ON CR(110)

Ultraviolet photoelectron spectroscopy (UPS) and work function changes are used to identify a stable methoxy intermediate, CH3O(ads), on Cr(110) at 90 K following low exposures of methanol, CH3OH(g). Higher methanol exposures at 90 K add subsequent layers of physisorbed methanol which desorb above 120 K. Methoxy dissociation into atomic fragments occurs between 250 and 400 K. The absence of a stable CO(ads) dissociation intermediate indicates that methyl group dehydrogenation is rate limiting on Cr(110). As was previously found on Fe(100) and Mo(100), the stability of CH3O(ads) over CO(ads) on Cr(110) is opposite of that proposed by Sexton based upon relative heats of adsorption. Consideration of the CH3O: metal and CO: metal bonding mechanisms leads to the proposition that substrate electronic properties, rather than steric or substrate morphological factors, are responsible for the anomalous methoxy stability on these surfaces.