A detailed study of the adsorption of N₂ on clean and chemically modified Ru(001) surfaces suggests that the mechanism of N₂ adsorption is qualitatively different from that of the isoelectronic CO molecule. A multitude of experimental techniques performed on ruthenium preadsorbed with well-characterized coverages of electron donors (potassium) and acceptors (oxygen) have produced the following principal findings: (1) On all surfaces studied, N₂ adsorption produces a negative work-function change which results principally from a transfer of charge from the N₂ molecule to the surface. (2) The charge transferred per N₂ admolecule is reduced in the presence of potassium, increased in the presence of oxygen. (3) N₂ interacts repulsively with potassium adatoms, attractively with oxygen adatoms. (4) The observation that potassium precoverages as low as CTHETAK>0.08 completely suppress the adsorption of N₂ at 85 K enables us to determine a minimum K-N₂ interaction distance of 4.25 A{}. (5) The N₂ adsorption bond is weakened in the presence of potassium, strengthened in the presence of oxygen. (6) N---N bond strengthening (weakening) as observed in the vibrational spectrum is always accompanied by N₂ adsorption bond strengthening (weakening). These experimental results indicate that the adsorption bond of N₂ is formed principally through {σ} donation in contrast to that of CO which is widely believed to be mediated via synergistic charge donation from the CO 5{σ} orbital to the metal and ``back-donation'' from the metal d band to the CO 2{π} orbital. This mechanism contradicts most theoretical models of N₂ adsorption, which predict similar bonding mechanisms for these two diatomic molecules.
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Paola et al. (1987) studied this question.
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