Unusual Adsorption Site Occupation Sequence: NO Adsorption on Stepped Pd(112)

Abstract

The adsorption of NO on a stepped Pd(l 12) surface has been investigated at 300-373 K with high resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption ion angular distribution (ESDIAD) techniques. At exposures below 5.7 X 1014 NO/cm2, NO molecules preferentially adsorb on the terrace sites, with an NO stretching mode observed by HREELS at 1535-1550 cm-1, and an 0+ ESDIAD beam directed near the (111) terrace-normal direction. For NO exposures above ~6 X 1014 molecules/cm2, step sites are occupied in addition to the terrace sites, with an NO stretching mode observed in HREELS at -1655-1670 cm-1 and an additional 0+ ESDIAD beam oriented in the "downstairs" direction. This is an unusual reversal of the site occupation sequence since one usually expects stronger adsorbate bonding on the step sites causing the steps to fill first. The N-0 bond stretching frequencies observed in HREELS are indicative of weaker bonding of NO molecules on the step sites than on the terrace sites.

Adsorbate Reorganization at Steps: NO on Pd(211)

“…At first, the exceptional experimental finding [6,7] of preferential adsorption onto the terraces for the NO͞Pd(211) system mentioned above seems to be at variance with the present Letter. However, this comes about only when the experimental data is interpreted in terms of a traditional sequential site population model.…”

“…However, this comes about only when the experimental data is interpreted in terms of a traditional sequential site population model. In the light of our finding above that such a model is not always applicable we now revisit the experimental data [6]. The experiment was performed at a range of NO exposures.…”

“…In this linear model the adsorption sites with the largest bond energy are filled first, and as the chemical potential of the adsorbate is increased, sites that are less and less stable are populated [3][4][5][6][7]. In the present Letter we provide theoretical evidence from selfconsistent density functional theory calculations that such a simple view is not always applicable.…”

“…Moreover, the experimental results for this surface are very interesting. Combined ES-DIAD and HREELS experiments have shown [6,7] that at low coverages NO is adsorbed in a state with the NO axis perpendicular to the terraces and as the coverage is increased a new state, which is tilted with respect to the terrace plane, is observed. In the sequential adsorption model this is interpreted as NO adsorbed first at the terraces and then-with a smaller adsorption energy-at the steps.…”

Bridging the structure gap: Chemistry of nanostructured surfaces at well-defined defects

“…Such surfaces were in some way considered to be fundamental, being flat and more compact, although these epithets apply only loosely, e.g. to bcc (111) surfaces. The attention to more complex surfaces, obtained by cutting the crystal at a slant to LMI planes and thus exposing vicinal faces, is more recent although some papers date back to the very early days of surface science [1][2][3].…”