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 AVS 51st International Symposium & Exhibition, 11/18/2004, Anaheim, CA and H.-S. Guo and F. Zaera, Nature Materials, 5(6), (2006). (Financial support: US National Science Foundation)

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 NH 3. Useful properties of adsorbed Xe –The s elective 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 property of 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 as catalytic active sites.

Using Xe to determine unsaturated oxygen First row NiSecond row NiO [001] [1-10] Clean Ni (110) O 2 exposure: 0.1 – 0.3 L Partially reconst. Ni(110) O 2 exposure > 0.4 L Fully reconst. Ni(110) Chemisorbed oxygen on the Ni(110) surface

O C N Xe Local work function  : increased decreased decreased I Xe C 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 NH 3. CH 3 I adsorbates seem to have a similar but less marked effect on the adsorption of xenon. Xe (3d 5/2 ) XPS Dipolar adsorbates cause shifts on the Xe 3d 5/2 level that depend on dipole moment and orientation. This is associated with the local dipole interaction or the local WF change (  ). +  +   + E B (Xe 3d 5/2 )  h -  Photoelectron ejected from Xe atoms sense the individual neighboring dipoles sense the negative end: E K slightly increases E B slightly decreases Xe+CO / Ni(110) sense the positive end: E K decreases E B increases Xe+NH 3 / Ni(110) sense the positive end: E K slightly decreases E B slightly increases Xe+CH 3 I / Ni(110)

1.NH 3 on O/Ni(110) The terminating atoms of the -Ni-O- added rows provide active sites for the initial population of NH 3. The predosed oxygen shows compensation effects on the NH 3 -induced heterogeneity for Xe adsorption (T d and Xe 3d 5/2 ). 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. CH 3 I on O/Ni(110) CH 3 I behaves similarly to NH 3 on the surface, but shows less site selectivity. CH 3 I exerts a lesser effect on Xe adsorption when it is dosed alone or with O 2. Arrangement of the molecules on the 0.2 L O-modified surface

Summary Why Xe atoms 1.Xe exhibits the highest induced dipole moment (  ). 2. , adsorption energies (E ad ), and core level energies (E B ) are all site specific and vary with surface coordination and neighboring environment. 3.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 1.Selective population of specific sites active sites. 2.Does not affecti surface reactions. Limitations 1.The number of different sites should not exceed 2 or Knowledge of surface geometry desirable. 3.Data interpretation complicated by coordination numbers, Van der Waals interactions, charge transfers, surface dipoles, etc. XPS and TPD of adsorbed Xe can provide valuable molecular-level information about specific sites on heterogeneous surfaces.