PowerPoint Presentation Oxygen Environmental Auger Electron Spectroscopy

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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)
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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 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.

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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 gt 0.4 L Fully reconst. Ni(110)
Clean Ni (110)
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Clean and partially and fully reconstructed
Ni(110) surfaces display distinctively different
Xe adsorption states. For low Xe exposures, Xe
atoms populate strong bonding sites selectively.
Clean surface
Part. reconst.
Xe TPD 0.6LXe/ xLO2/Ni(110) T(dose
O2)400K T(dose Xe)75K When the clean and fully
reconstructed Ni(110) surfaces are regarded as
homogenous for Xe adsorption, the partially
reconstructed one shows distinctive heterogeneity
for Xe adsorption, providing active sites for
its initial population.
Fully reconst.
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Xe (3d5/2) XPS EB0 BE of Xe atom on clean
Ni(110). ?EB(?) 0.17 eV BE change due to
polarization. ?EB(?O?) - 0.3eV BE change
caused by an
additional neighboring O?. ?? 0.38 eV
macroscopic WF change. J.T. Yates, et al,
Surf. Sci. 44 (1974) 489.
Clean Ni (110)
Partially reconst. Ni(110)
Fully reconst. Ni(110)
EB EB0 ?EB(?O?) ? 669.63 eV
EB EB0- ??- ?EB(?) ? 669.40 eV
EB0 669.94 eV


Xe
Xe
Xe
-
O
?
?
O
-Ni-O-Ni-O-Ni-O-Ni-O-Ni-
?
?
?
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Using Xe to determine local surface dipoles
? 0.5 ML of molecules are predosed, leaving space
for Xe adsorption.
Xe TPD 0.6LXe/ Mol./Ni(110) T(dose
Mol.)90K T(dose Xe)75K Surface CO has similar
effects as oxygen on Xe adsorption, but more
effective. Both CH3I and NH3 destabilize Xe
adsorbates, NH3 is more effective.
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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
XeCO / Ni(110)
XeNH3 / Ni(110)
XeCH3I / 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? - ?
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Using Xe to probe coadsorption effects
NH3, CO, and CH3I molecules (? 0.5 ML) on 0.2 L
O-modified surface
O-modified Ni as reference
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Arrangement of the molecules on 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.
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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.