Changes in the bonding of NO on Pt(111) induced by the coadsorption of high coverages of oxygen atoms have been studied with temperature programmed desorption (TPD), vibrational spectroscopy using high resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). Modification of the electronic structure of surface Pt atoms by the strongly electron-withdrawing adsorbed oxygen atoms alters the relative stabilities of NO adsorption sites and the nature of the Pt-NO bond. Coadsorption of 0.25 ML of O(a) destabilizes the two-fold bridge site for NO adsorption that is energetically preferred on clean Pt(111) and causes preferential NO adsorption in the atop site initially. For this oxygen coverage, some population of the bridge site occurs at the highest NO coverages, but occupation of this site can be eliminated completely by preadsorption of 0.75 ML of oxygen. This high coverage of coadsorbed oxygen now induces a further change in the nature of the NO chemisorption bond for NO adsorbed in atop sites, forming bent NO rather than the linear NO species formed on clean Pt(111). The saturation coverage of bent NO is 0.15 ML on this 0.75 ML oxygen-precovered surface and the heat of adsorption is only 1-2 kcal/mol less than linear NO adsorbed in atop sites on clean Pt(111). By using the HREELS and UPS data to identify these three chemically distinct forms of NO(a), we are able to rationalize their formation (and subsequent properties) in different electronic environments by correlating bonding configurations with the charge-transfer capabilities of the Pt substrate. Finally, we note that despite the presence of large excesses of O(a), NO is never oxidized to form NO2, unlike the analogous facile oxidation of CO on Pt. This contrast in oxidation energetics is readily explained in terms of the measured relative barrier heights for oxidation versus desorption.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry