The coadsorption of nitrogen with carbon monoxide and oxygen on the Ru(001) surface: Local chemical interactions in mixed overlayers

Abstract

High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. D i s o r d e r e d oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of o r d e r e d oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.