(Financial support: US National Science Foundation)

1. TPD and XPS of Adsorbed Xenon Atoms for the Characterization of Reaction Sites on Oxygen-Modified Ni(110) Surfaces Hansheng Guo, Francisco Zaera Department of Chemistry, University of California Riverside, CA 92521 AVS 51st International Symposium & Exhibition, 11/18/2004, Anaheim, CA and H.-S. Guo and F. Zaera, Nature Materials, 5(6), 489-493 (2006). (Financial support: US National Science Foundation)

2. Techniques and research significance Experiment demands • Samples have to be cooled to  75 K to facilitate Xe adsorption. • The first layer saturates around 75 K on Ni(110) surface • No Xe condensation on the surfaces saturated with CO or NH3. Useful properties of adsorbed Xe • The selective population of specific sites with Xe atoms is governed by the corresponding adsorption energies, and those can be measured by temperature-programmed desorption (TPD) • The electronic propertyof adsorbed Xe is site-sensitive, it depends on the local chemical environment. This can be characterized by X-ray photoelectron spectroscopy(XPS). • A combination of TPD with XPS of adsorbed Xe can therefore be used to characterize local surface heterogeneities such ascatalytic active sites.

3. [1-10] O [001] First row Ni Second row Ni Using Xe to determine unsaturated oxygen Chemisorbed oxygen on the Ni(110) surface O2 exposure: 0.1 – 0.3 L Partially reconst. Ni(110) O2 exposure > 0.4 L Fully reconst. Ni(110) Clean Ni (110)

7. Xe TPD On the Ni(110) surface with these polar molecules ( 0.5 ML), Xe atoms tend to adsorb next to the CO sites but away from the NH3. CH3I adsorbates seem to have a similar but less marked effect on the adsorption of xenon. Xe (3d5/2) XPS Dipolar adsorbates cause shifts on the Xe 3d5/2 level that depend on dipole moment and orientation. This is associated with the local dipole interaction or the local WF change () . Photoelectron ejected from Xe atoms sense the individual neighboring dipoles Xe+CO / Ni(110) Xe+NH3 / Ni(110) Xe+CH3I / Ni(110) sense the negative end: EK slightly increases EB slightly decreases sense the positive end: EK decreases EB increases sense the positive end: EK slightly decreases EB slightly increases + + + C Xe O Xe Xe I C  N   Local work function : increased decreased decreased EB (Xe 3d5/2)  h - 

9. Arrangement of the moleculeson the 0.2 L O-modified surface • 1. NH3 on O/Ni(110) • The terminating atoms of the -Ni-O- added rows provide active sites for the initial population of NH3. • The predosed oxygen shows compensation effects on the NH3-induced heterogeneity for Xe adsorption (Td and Xe 3d5/2). • 2. CO on O/Ni(110) • CO adsorption on the O-modified surface does not show an evident site selectivity between the bare nickel and the end of the -Ni-O- added rows. • Both CO and O modify Xe adsorption in a similar way. • 3. CH3I on O/Ni(110) • CH3I behaves similarly to NH3 on the surface, but shows less site selectivity. • CH3I exerts a lesser effect on Xe adsorption when it is dosed alone or with O2.

10. Summary XPS and TPD of adsorbed Xe can provide valuable molecular-level information about specific sites on heterogeneous surfaces. • Why Xe atoms • Xe exhibits the highest induced dipole moment (). • , adsorption energies (Ead), and core level energies (EB) are all site specific and vary with surface coordination and neighboring environment. • Xe adsorbates tend to form hexagonally close-packed overlayer of known atom density, and that allows for an easy calibration to count active sites. • Advantages • Selective population of specific sites active sites. • Does not affecti surface reactions. • Limitations • The number of different sites should not exceed 2 or 3. • Knowledge of surface geometry desirable. • Data interpretation complicated by coordination numbers, Van der Waals interactions, charge transfers, surface dipoles, etc.