Room-Temperature Reaction of Oxygen with Gold: An In situ
Ambient-Pressure X-ray Photoelectron Spectroscopy Investigation
Peng Jiang,†,‡ Soeren Porsgaard,†,§ Ferenc Borondics,| Mariana Ko¨ber,†,⊥ Alfonso Caballero,†,#
Hendrik Bluhm,| Flemming Besenbacher,§ and Miquel Salmeron*,†,‡
Materials Sciences DiVision, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California
94720, Department of Materials Science and Engineering, UniVersity of California, Berkeley, California 94720,
Interdisciplinary Nanoscience Center (iNANO), Aarhus UniVersity, DK 8000 Aarhus C, Denmark, Chemical
Sciences DiVision, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, Instituto
de Microelectro´nica de Madrid, c/ Isaac Newton 8, 28760 Tres Cantos, Spain, and Instituto de Ciencia de
Materiales and Departamento Quı´mica Inorga´nica, CSIC-UniVersidad of SeVilla, SeVilla, Spain
Received December 3, 2009; E-mail: MBSalmeron@lbl.gov
Gold is commonly regarded as the most inert element.1 However,
the discovery of the exceptional catalytic properties of gold
nanoparticles (NPs) for low temperature CO oxidation2 initiated
great interest due to its promising applications and spawned a large
number of studies devoted to the understanding of the reaction
mechanism.3-6 Nevertheless, no consistent and conclusive picture
has arisen.7-13
One of the most important and controversial issues is the
activation mechanism of O2 on Au.9,14-19 For instance, on the basis
of the experimental observation that a bilayer Au structure, which
completely wets the oxide support, exhibits extraordinary catalytic
activity, Goodman et al. proposed that Au can be directly involved
in the activation of O2.9 In contrast, more recently, Behm et al.
found that the amount of active oxygen species on the Au/TiO2
surface has a linear relationship with the number of perimeter sites
at the interface between the oxide support and the Au NPs,
indicating that the support-Au interface plays a dominating role in
the oxygen activation.19 Furthermore, on the basis of theoretical
calculations, under-coordinated Au atoms were proposed to have
the capability to adsorb and even dissociate O2.14 Using high-
intensity in situ X-ray absorption near-edge structure (XANES),
van Bokhoven and co-workers observed an increased white line
intensity at the Au L3 edge for Au NPs supported on Al2O3 and
TiO2 substrates during treatment in O2, indicating that Au NPs can
be oxidized by molecular oxygen.17,18 However, Liu et al. calculated
that the dissociation barrier is larger than 2 eV on nonsupported
Au and even at the Au/TiO2 interface the dissociation barrier is
still 0.52 eV,15 which means that O2 has a very weak interaction
with Au, and thus spontaneous dissociation of molecular oxygen
on the Au surface is not energetically favorable.
To reconcile these controversial observations and proposals we
investigated the reactivity of O2 with bulk Au foil and Au NPs
supported on TiO2(110) surface using in situ ambient pressure X-ray
photoelectron spectroscopy (AP-XPS) at O2 pressures of up to 1
Torr.20,21 Our data demonstrate that molecular oxygen does not
have a strong interaction with Au surfaces at room temperature.
However, we observed that molecular oxygen can be activated on
both types of samples by X-ray irradiation to produce oxidic Au.
We further investigated and compared the stability of oxidized Au
on both model systems.
The experiments were performed at beamline 11.0.2 of the
Advanced Light Source (ALS) at Lawrence Berkeley National
Laboratory.20,21 This system consists of a preparation chamber and
an AP-XPS analysis chamber with base pressure of 3 × 10-10 Torr.
The Au foil and the TiO2(110) single crystal samples were cleaned
by several cycles of Ar+ sputtering and annealing. Au NPs were
obtained by thermal evaporation of Au onto the TiO2(110) surface
at 320 K, following a procedure described in the literature.22 The
amount of deposited Au was calibrated by the ratio between Au 4f
and Ti 2p peaks. O2 gas was dosed through a leak valve. A photon
energy of 690 eV was used to acquire all the spectra. The binding
energy scale was calibrated by using the metallic Au 4f7/2 (binding
energy 84.0 eV) peak and the Fermi edge of Au foil as references.
The photon flux density was approximately 4 × 1012 photons mm-2
s-1.
The insets in Figure 1a,b show the Au 4f spectra obtained from
the gold foil and from a 0.3 ML (monolayer) of Au on TiO2 under
UHV and under 1 Torr of O2, respectively. In comparison with
† Materials Sciences Division, Lawrence Berkeley National Laboratory.
‡ University of California at Berkeley.
§ Aarhus University.
| Chemical Sciences Division, Lawrence Berkeley National Laboratory.
⊥ Instituto de Microelectro´nica de Madrid.
# CSIC-Univsidad of Sevilla.
Figure 1. Time evolution of the XPS spectra of Au 4f under X-ray
irradiation at room temperature. (a) Au foil and (b) Au NPs on TiO2 support
in the presence of 1 Torr O2. (c) Oxidized Au foil and (d) oxidized Au NPs
on TiO2 support in UHV. The peaks on the higher energy binding side of
the metallic 4f7/2 and 4f5/2 peaks are due to oxidized gold. The bold black
and red curves correspond to the first and the last spectrum, respectively.
The arrows indicate the direction of the intensity changes. The insets show
the Au 4f spectra from the test experiments under similar condition, but
without X-ray irradiation for the specified time.
Published on Web 02/10/2010
10.1021/ja909987j  2010 American Chemical Society2858 9 J. AM. CHEM. SOC. 2010, 132, 2858–2859

bulk Au foil (84.0 eV), the Au 4f7/2 peak of the submonolayer
deposit shifts to a higher binding energy (84.2 eV), due to a
combination of initial and final state effects.23,24 After exposure to
O2 at 1 Torr for several hours in the absence of X-rays, both peak
position and width remain the same. These results demonstrate that
O2 does not have a strong interaction with Au, both in bulk form
and in the form of NPs, under the above conditions. This
observation is consistent with the known inertness of Au, which
originates from the filled d-band structure.1
However, we observed that under X-ray irradiation in the
presence of oxygen gas, the Au surfaces could be oxidized. Figure
1 panels a and b show the time evolution of the Au 4f region under
irradiation of X-rays in the presence of 1 Torr of O2 over 15 min.
On the foil the intensity of the metallic Au 4f peaks (Au 4f7/2 84.0
eV and Au 4f5/2 87.7 eV) decreases with time, while at the same
time two additional peaks appear, which are shifted by 1.3 eV to
higher binding energies relative to the metallic Au 4f peaks.
Previous studies have reported similar peak shifts for oxidized Au
surfaces prepared using different oxidation methods.25-28 On the
Au NPs formed by evaporation on the TiO2 crystal, two additional
peaks develop also at the higher binding energy side that can be
attributed to oxide as well. It is noteworthy to mention that both
the oxidation rate and the chemical shift of the oxide peaks depend
strongly on the size of NPs,28-30 which is still under further
investigation.
X-ray induced formation of chemisorbed oxygen species have
been reported previously.31,32 However, these measurements were
carried out under ultrahigh vacuum at temperatures below 30 K,
where chemisorbed oxygen is produced from physisorbed molecular
oxygen. Therefore, both the experiment conditions and the activation
process are dramatically different from the ambient conditions in
this communication.
The stability of the oxidic Au species formed in this way was
subsequently investigated also under X-ray irradiation. We found
that the two samples show different behaviors. While the oxidic
Au in the NPs on TiO2(110) could be readily reduced under X-ray
irradiation, the oxidic Au film formed at the surface of the foil
was resistant to X-ray irradiation. This is shown in Figure 1c,d,
where the spectra were recorded sequentially during 10 min. The
difference in reduction behavior lends support to the oxygen
spillover model proposed by Ono and Cuenya,33 based on the well-
known facile reducibility of the TiO2 support. Under X-ray
irradiation, oxygen vacancies can be generated on the TiO2
surface.34 For the NP sample oxygen can spill from the oxidized
Au NP to the reducible TiO2 substrate, while this reduction channel
is not available for bulk gold. The different reduction behavior
makes it difficult to compare the oxidation rate of the two samples
during X-ray induced oxidation because for Au NPs the oxidation
and reduction processes induced by X-rays take place simulta-
neously. To exclude any effect related to the UHV condition alone,
we have investigated the stability of the oxidic species under UHV
without X-ray irradiation. As shown in the insets of Figure 1c,d,
both samples are stable after 10 min under UHV in the absence of
the X-ray beam.
In summary, using AP-XPS we have demonstrated that molecular
oxygen does not oxidize Au at room temperature, either in the form
of supported NPs on TiO2(110) or in bulk (foil) form at pressures
of up to 1 Torr. These observations indicate that the proposed gold-
only activation mechanism of O2 is unlikely under these reaction
conditions.8,13,15,16,19 With the help of X-ray irradiation, however,
both surfaces can be effectively oxidized under 1 Torr of O2.
Therefore, our observations demonstrate that X-rays play a critical
dual role during in situ measurements and that extreme care must
be taken to carry out experiments and interpret spectra, especially
when using intense synchrotron radiation.
Acknowledgment. This work was supported by the Director,
Office of Science, Office of Basic Energy Sciences, Chemical
Sciences, Geosciences, and Biosciences Division, under the Depart-
ment of Energy Contract No. DE-AC02-05CH11231. S.P. acknowl-
edges the Carlsbergs Mindelegat foundation for financial support.
M.K. was supported by the Spanish Council for Scientific Research
through an I3P scholarship. We also thank Jinghua Guo and Zhi
Liu for their helpful discussions.
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