Displacement of CO by more weakly adsorbed hydrogen on the Ni(100) surface

We report here hydrogen induced displacement of chemisorbed CO from the Ni(100) surface. The displacement reaction is unexpected since CO is adsorbed more strongly than atomic hydrogen on the Ni(100) surface. Hydrogen pressures of 10−5 to 10−3 Torr cause displacement of chemisorbed CO in the 309–330 K temperature range in a matter of minutes. After displacement, the surface contained only the undisplaced CO and hydrogen. No surface contamination was observed either by Auger spectroscopy or temperature programmed desorption after evacuation. The rates were measured by integration of the thermal desorption spectra taken for a series of displacement times. The displacement reaction is first order in CO surface coverage with a break in rate occurring with decreasing coverage. The activation energy for the high‐coverage displacement rate is ∼8±2 kcal/mol while the activation energy for the low‐coverage displacement rate is ∼12±1 kcal/mol. No CO displacement is observed with either He or Ne. Adsorbed hydrogen is displaced by CO. The energetics of the displacement reaction coupled with the reaction probabilities observed suggest either direct energy transfer between the dissociating hydrogen and coadsorbed CO or substantial modification of adsorbed CO by high coverages of coadsorbed hydrogen. Using the current results we cannot distinguish between these mechanistic possibilities at present.