Unexpected hydrogen induced displacement of chemisorbed CO from the Ni(100) surface

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Abstract
We report the first observation of hydrogen induced displacement of chemisorbed CO from the Ni (100) surface. This displacement is unexpected since the heat of adsorption for CO is 30 kcal/mol, about 7 kcal/mol larger than the 23 kcal/mol heat of adsorption for hydrogen. These displacement studies were performed in a UHV system equipped with Auger electron spectroscopy and facilties for temperature programmed desorption. Rates of displacement were measured by integrating CO temperature programmed desorption spectra for a series of displacement times. Hydrogen pressures in the 10−3 to 10−4 Torr range cause displacement of chemisorbed CO in the 290 to 330 K temperature range in a matter of minutes. After displacement, the surface contained only undisplaced CO and adsorbed hydrogen. No surface contamination was detected following CO temperature programmed desorption. The displacement reaction is clearly positive order in CO coverage. The CO coverage data suggests two first order reaction regions with a decrease in rate with decreasing coverage. Displacement is about half‐order in hydrogen pressure. For low CO coverages (below 0.3 to 0.4 monolayer) and 309 K the displacement probability is around 5×10−7 per incident H2 molecule and has an activation energy of about 12±1 kcal/mol. At high coverages the displacement probability is about 1×10−6 per incident H2 molecule and the activation energy decreases to about 8±2 kcal/mol. No CO displacement was observed in the 270 to 330 K temperature range for pressures up to 10−3 Torr of He, Ne, CH4 or N2. The CO displacement rate is also insensitive to deuterium substitution. The thermal activation energies measured indicate that the metal surface is furnishing 8 and 12 kcal/mol for the displacement of high and low coverages of adsorbed CO, respectively. In order for adsorbed CO to be removed from the surface e i t h e r the heat of desorption for a portion of the adsorbed CO must be decreased to match the thermal activation energieso r direct energy transfer must be occurring during the displacement process. The current results do not allow us to distinguish between these two types of molecular mechanisms.

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