Ammonia oxidation on Pt(410)

Article abstract of DOI:10.1016/j.jcat.2006.06.011

The adsorption of both O₂ and NH₃ on Pt(410) was studied using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Molecular NH₃ desorbed from Pt(410) between 100 and 450 K, and dissociation was not observed. Radiation (X-rays, electrons) induced NH_3ad dissociation, and as a result several dissociation products (NH_2ad, NH_ad, and N_ad) were observed in the N 1s core-level spectrum. NH_ad is a rather stable dissociation product that starts to dehydrogenate above 350 K. The N_ad and H_ad formed in this process desorbed on formation (as N₂ and H₂). Both molecular and dissociative O₂ adsorption were observed after the surface was exposed to O₂(g) at 100 K. Molecularly adsorbed O₂ desorbed below 200 K, whereas atomic oxygen desorbed (as O₂) between 600 and 900 K, in two distinct desorption peaks. In the O 1s core-level spectrum, both molecular O₂ and two different types of O_ad were distinguished. NH_3ad dissociation was observed on an oxygen-presaturated surface. The NH_3ad oxy-dehydrogenation started at 150 K. NO_ad and NO(g) were also observed, but only during experiments in which an excess of O_ad was available. NO_ad desorbed/decomposed between 400 and 500 K. For the steady-state ammonia oxidation reaction, N₂ and H₂O were the major products at low temperatures, whereas the selectivity toward NO and H₂O changed at higher temperatures. This selectivity change can be attributed to changes in surface composition.

Full text of DOI:10.1016/j.jcat.2006.06.011

Journal of Catalysis 242 (2006) 184–194
www.elsevier.com/locate/jcat
Ammonia oxidation on Pt(410)
C.J. Weststrate a,∗, J.W. Bakker a, E.D.L. Rienks a, C.P. Vinod a,b, A.V. Matveev c, V.V. Gorodetskii c,
B.E. Nieuwenhuys a,b
a
Leids Instituut voor Chemisch Onderzoek, Universiteit Leiden, P.O. Box 9502, Einsteinweg 55, 2333 CC Leiden, The Netherlands
b
Technische Universiteit Eindhoven, Schuit Institute of Catalysis, P.O. Box 5600 MB, Eindhoven, The Netherlands
c
Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russia
Received 19 April 2006; revised 12 June 2006; accepted 13 June 2006
Available online 12 July 2006
Abstract
The adsorption of both O and NH on Pt(410) was studied using temperature-programmed desorption (TPD) and X-ray photoelectron spec-
2
3
troscopy (XPS). Molecular NH desorbed from Pt(410) between 100 and 450 K, and dissociation was not observed. Radiation (X-rays, electrons)
3
induced NH
dissociation, and as a result several dissociation products (NH , NH , and N ) were observed in the N 1s core-level spec-
3ad
2ad ad ad
trum. NH is a rather stable dissociation product that starts to dehydrogenate above 350 K. The N and H formed in this process desorbed
ad
ad
ad
on formation (as N and H ). Both molecular and dissociative O adsorption were observed after the surface was exposed to O (g) at 100 K.
2
2
2
2
Molecularly adsorbed O desorbed below 200 K, whereas atomic oxygen desorbed (as O ) between 600 and 900 K, in two distinct desorption
2
2
peaks. In the O 1s core-level spectrum, both molecular O and two different types of O were distinguished. NH dissociation was observed
2
ad
3ad
on an oxygen-presaturated surface. The NH
oxy-dehydrogenation started at 150 K. NO and NO(g) were also observed, but only during ex-
3ad
ad
periments in which an excess of O was available. NO desorbed/decomposed between 400 and 500 K. For the steady-state ammonia oxidation
ad
ad
reaction, N and H O were the major products at low temperatures, whereas the selectivity toward NO and H O changed at higher temperatures.
2
2
2
This selectivity change can be attributed to changes in surface composition.
© 2006 Elsevier Inc. All rights reserved.
Keywords: O adsorption; NH ; Pt(410); Ammonia oxidation; NO formation; X-ray photoelectron spectroscopy; Synchrotron radiation; Temperature-programmed
2
3
desorption
1. Introduction
presence of oxygen, several other species can are involved in
the surface chemistry, including Oad, OHad, H2Oad, and NOad.
The complexity of the surface chemistry is reflected in the gas-
phase product selectivity during catalytic NH3 oxidation. Dif-
ferent gaseous products, including N2, NO, and N2O, can be
formed. The selectivity depends on the type of catalyst, its sur-
face structure, and the experimental conditions, such as reactant
pressures, NH3/O2 ratio, and catalyst temperature.
It has been found that ammonia decomposition does not take
place on Pt surfaces at temperatures below 400 K. Some NH3
decomposition has been observed above the NH3 desorption
temperature; in that case, stepped surfaces were found to be
more active than “smooth” surfaces [1,4,5]. On several metals,
including Pt, Ag, Ir, and Cu, the presence of oxygen has been
found to enhance NH3 dissociation [3,6–11].
Catalytic ammonia oxidation is an important process in the
chemical industry. It is used on a large scale to produce nitric
oxide, a precursor for nitric acid. The catalyst of choice for this
process is an alloy of Pt and Rh. A number of surface science
studies have been performed to unravel the surface chemistry
of NH3 on Pt surfaces in both the absence and presence of co-
adsorbed oxygen [1–3].
The surface chemistry of ammonia on a metal surface is
complex, involving several different surface species. Adsorp-
tion and decomposition of NH3 can in principle lead to five
different species: NH3ad, NH2ad, NHad, Nad, and Had. In the
*
In this paper we present the results obtained for ammonia ox-
idation on a Pt(410) surface, which consists of {100} terraces
Corresponding author.
E-mail address: c.westrate@chem.leidenuniv.nl (C.J. Weststrate).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.06.011

C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
185
and {110} steps. Faceting of the surface was observed, resulting
in the formation of larger {100} terraces on which a (5 × 20)
reconstruction occurred. A more detailed discussion concern-
ing the actual surface structure of our Pt(410) sample has been
provided previously [12].
XPS was used to study the nature and concentration of
surface species, and the gas-phase reaction products were de-
tected by quadrupole mass spectrometry (QMS). The aim of
our experiments was to obtain fundamental insights into sur-
face processes, which can help elucidate the reaction pathways
involved in catalytic ammonia oxidation. Based on our previ-
ous studies, we also can compare the NH3 chemistry on Pt(410)
with that on (previously studied) Ir surfaces [9,13–16].
sorption experiments, radiation damage due to electrons from
the QMS filaments (∼70 eV) can be excluded, because the
QMS was situated in a separate compartment, with only a
small hole connecting it to the main chamber (where the crystal
was located). In this setup, the likelihood of electrons gener-
ated by the QMS filaments reaching the sample surface is very
low. During the XPS measurements, NH3ad dissociation was
induced by the intense radiation (either X-rays or ∼100 eV pho-
toelectrons). This is described in more detail in Section 3.1.
3. Results
3.1. NH3 surface chemistry
2. Experimental
Thermal desorption spectra obtained after exposure to NH3
at 100 K are shown in Fig. 1a. Ammonia bonded directly to
the surface (through the nitrogen atom) desorbed molecularly
between 200 and 450 K. After an NH3 exposure of ∼3 L,
a new desorption feature appeared, with a desorption maximum
at around 175 K. In line with literature assignments, this peak
can be attributed to desorption of a second NH3 layer that was
hydrogen-bonded to the first, chemisorbed layer [5,13,23–26].
Even higher exposures led to the formation of NH3 multilayers
that desorbed at around 120 K [5,27]. The observed peaks are
very similar to the desorption spectra observed for other Pt sur-
faces, and the presence of steps seemed to have little effect on
the desorption spectra. N2 or H2 desorption was not observed,
indicating that NH3 dissociation did not take place on the clean
surface.
The thermal desorption studies were performed using the ex-
perimental setup in Leiden, which has been described in more
detail elsewhere [12]. High-energy resolution fast XPS mea-
surements [17,18] were performed on the SuperESCA beam-
line of ELETTRA, the synchrotron radiation facility in Tri-
este, Italy. The vacuum system, with a base pressure of ∼1 ×
10−10 mbar, is equipped with a sputter ion gun for sample
cleaning, a mass spectrometer, and LEED optics. Details about
the sample used in this study and the cleaning procedure can be
found elsewhere [12].
The N 1s spectra were measured with a photon energy of
496 eV, and the O 1s spectra were measured with a photon en-
ergy of 650 eV. Temperature-programmed XPS (TP-XPS) mea-
surements [19] were performed at a heating rate of 0.25 K s−1
.
Fig. 2a shows one of the N 1s core-level spectra obtained
during XPS experiments of an adsorbed NH3/NHxad layer. It
also shows the fitting components used to deconvolute the spec-
tra. The BEs of all of the fitting components used to evaluate the
XPS data are summarized in Table 1. Fig. 2b shows a series of
spectra taken during heating of an adsorbed NH3/NHxad layer
in vacuum. Ammonia was dosed at 175 K, to avoid formation of
the NH3 double layer. During the uptake (not shown), initially
only molecular ammonia was observed (at 400 eV). However,
the intense radiation (either X-rays or the resulting ∼100-eV
photoelectrons) induced dissociation of the adsorbed NH3, and
another peak developed at 398.3 eV, assigned to NHad. We sug-
gest that another small peak was present at 399.1 eV, which is
assigned to NH2ad on the basis of its BE. This peak was never
clearly resolved, and the concentration of this species was low.
Heating of the dissociated NHxad layer (Figs. 1c and 2b) re-
sulted in the formation of an additional (small) peak at 397.4 eV,
assigned to Nad. This assignment is in line with literature data
[8,13,21] in which the ꢀBE among various NHxad species was
reported to be ∼1 eV on several different metal surfaces. The
same BE component was observed after NO dissociation on this
surface [12]. Fig. 1c shows the results obtained after evaluation
of the data shown in Fig. 2b using the components shown in
panel (a). NH3ad desorbed between 175 and 400 K, and NH2ad
decomposed (into NHad and Had) between 175 and 250 K.
NHad decomposition was observed above 350 K. As a result,
a small amount of Nad that desorbed almost immediately on
formation (as N2) was observed.
The different core level regions were measured in a separate ex-
periment; that is, either O 1s or N 1s could be measured during
an experiment. The XPS spectra were evaluated after a linear
background was subtracted by fitting the peaks with a Doniach–
˘
Sunjic´ function convoluted with a Gaussian function [20].
Core-level binding energies of the different species were
measured with respect to the Fermi level. The position of the
Fermi edge was determined using an photon energy of 110 eV.
In recent experiments, we found that the position of the Fermi
level should be measured using the photon energy used for the
actual measurements as well, because the calibration of the pho-
ton energy was not very accurate (e.g., when the excitation
energy was set to 650 eV, the actual energy was ∼647 eV).
As a result of the approach used here (i.e., measuring the Fermi
edge with a different photon energy than for the actual mea-
surements), the reported absolute binding energies (BEs) can
exhibit a (constant) shift with respect to literature values. There-
fore, the ꢀBEs between different species instead of absolute
BEs should be used for comparison with literature data. This
issue affected only the comparison with literature data; the po-
sition of the different species remained constant throughout the
experiments. The N 1s and O 1s signals were normalized us-
ing the intensities obtained for a saturated NO layer at 100 K
(which was arbitrarily set to 1).
Both Sun et al. [21] and Schwaner et al. [22] found that ad-
sorbed NH3 decomposed when exposed to a beam of (∼50 eV)
electrons. In the experimental setup used for the thermal de-

186
C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
Fig. 2. (a) Components used to fit the N 1s spectra obtained during experiments
with ammonia (average of three spectra). (1) Datapoints, (—) fit. (b) The ac-
−1
tual spectra observed during heating (0.25 K s ) of an irradiated NH
layer.
3ad
Radiation damage causes the formation of NH
(x = 0, 1, 2).
xad
Table 1
a
Binding energies found for NH , NO and O adsorption on Pt(410)
3
ad
Assignment
N 1s BE (eV)
O 1s BE (eV)
NH
NH
NH
400.0
399.1
398.3
397.4
400.7
–
–
–
–
–
–
–
–
–
3ad
2ad
ad
N
ad
−1
Fig. 1. (a) NH desorption after exposure to ammonia at 100 K (3 K s ). (b)
3
NO
533.0
530.8
531.7
532.4
534.2
534.1
ad
−1
TP-XPS results obtained during heating of an adsorbed NH layer (0.25 K s
,
3
O
O
O
I
exposed to NH at 175 K). The exposure to the X-rays was kept as low as pos-
3
II
2
sible. (c) Thermal behavior of NH dissociation products (formed by radiation
3
−1
damage) on Pt(410) (0.25 K s ).
H O
2
ad
CO
ad
a
See Section 2 for more details about the calibration.
The data presented in Fig. 1b differ drastically from that
shown in Fig. 1c. In the experiment presented in panel (c),
the surface was already exposed to radiation during exposure
to NH3, and it was also continuously exposed to the X-rays
during heating. As a result, the concentration of dissociated
species was high. For the experiment presented in Fig. 1b,
the NH3 uptake was done in the absence of radiation, and
during heating the number of data points was kept low, to
minimize the radiation dose (each marker represents one data
point, ∼30 s). When the experiment was done it this way,
NH3 dissociation was almost absent, in line with the TPD data.
This shows that radiation has a profound effect on the sur-
face chemistry of NH3ad, and confirms that NH3 dissociation
did not occur on the Pt(410) surface (in the absence of radia-
tion).
3.2. Characterization of the oxygen-covered surface
Because no literature reports on O2 adsorption on Pt(410)
are available, here we report a brief investigation of oxygen ad-
sorption on Pt(410). Fig. 3 shows several O 1s core-level spec-
tra of an oxygen-saturated (O2ad/Oad) Pt(410) surface (10 L,
100 K), during both heating in vacuum [panels (b–d)] and heat-
ing in the presence of H2 [panel (a)]. Fig. 4 shows the thermal

C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
187
Fig. 4. (a) Oxygen desorption from Pt(410) after exposure to O at 100 K
2
−1
(3 K s ). (b) Thermal behavior of different oxygen species during heating
−1
of an adsorbed O /O layer in vacuum (0.25 K s ) and (c) heating of an
adsorbed O /O layer in H (0.25 K s , 1 × 10 mbar H ).
2ad ad
−1
−7
2ad ad
2
2
O2ad was observed at 530.7 eV, whereas Oad was observed at
529.9 eV. Their BEs do not exactly match the values that we
obtained,1 but the ꢀBE between O2ad and OII was ∼0.8 eV in
both cases.
Fig. 3. XP-spectra taken during (a) heating of an adsorbed O /O layer in
2ad ad
−1
−7
the presence of H (0.25 K s , 1×10 mbar H ) and (b, c, d) during heating
2
2
−1
(0.25 K s ) of a adsorbed O /O layer in vacuum.
2ad ad
Above 200 K, two peaks can be distinguished, at 530.8 eV
(OI) and 531.7 eV (OII), both of which are assigned to Oad.
The OI species already formed at 100 K during exposure to O2,
whereas OII was formed during O2ad decomposition at around
150 K. During the slow heating (0.25 K s−1) used in the TP-
XPS experiment, OII was consumed via reaction with gases
from the background (H2 and CO), whereas OI began to re-
act when OII was removed. This indicates that OII is slightly
more reactive than OI.
behavior of the different O 1s components in more detail. Panel
(a) shows the thermal desorption of O2 during heating of an
oxygen-saturated surface in vacuum, and panel (b) shows the
corresponding TP-XPS results. The results shown in panel (c)
were obtained during TP-XPS of an O2-saturated surface in the
presence of 1 × 10−7 mbar H2.
Molecularly adsorbed O2 was stable only at low temper-
atures and was observed as a broad, asymmetric peak at
532.4 eV. During O2 uptake (not shown) at 100 K, both Oad and
O2 grew simultaneously. O2ad desorbed/dissociated at around
150 K and was not observed above 200 K. Freyer et al. [28]
reported the BEs for both O2ad and Oad on Pt(110)-(1×2).
The O2 thermal desorption spectra above 200 K (i.e., via
Oad combination) show two distinct high-temperature desorp-
1
See Section 2 for details.

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C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
tion features, the first at around 670 K and the second at
around 800 K. For thermal desorption from other stepped Pt
surfaces, like (533) [29,30] two high-temperature O2 desorp-
tion peaks were observed as well, whereas only a single high-
temperature O2 desorption peak was found for the low-index
planes [31–33]. This suggests that the origin of the two desorp-
tion peaks is related to the presence of steps. It may be that these
two distinct O2 desorption peaks correspond to the two Oad
species observed by XPS. The XPS measurements were lim-
ited to 600 K, and thus this assignment could not be validated.
During heating of an adsorbed O2ad/Oad layer in the pres-
ence of H2(g), an additional species was observed in the O 1s
spectrum, between 100 and 200 K, with a BE of 534.2 eV. The
fact that it appeared in the presence of H2 (/Had) suggests that
it was either OHad or H2Oad. The ꢀBE of 2.5 eV between this
species and OII corresponded to the shift observed between Oad
and H2Oad on Pt(111) [34,35]. Kiskinova et al. [36] reported a
1-eV shift between Oad and OHad on Pt(111). In contrast, EELS
experiments on Pt(111) [3] indicated that OHad formed during
NH3ad dehydrogenation and that was stable up to 285 K, so this
is another possibility. We tentatively assign this peak to H2Oad
on the basis of the BE, but we cannot exclude the possibility
that it was OHad rather than H2Oad.
3.3. The effect of Oad on the NH3 surface chemistry
Fig. 5 shows the results obtained during heating of an
oxygen-saturated surface (10 L at 200 K), postdosed with NH3
(3 L). Panel (a) shows the gas-phase products observed dur-
ing heating (3 K s−1). Molecular NH3 desorption was found
between 110 and 450 K, with a peak at around 150 K (sec-
ond NH3 layer) and a broad desorption region between 150
and 450 K. H2O desorption occurred between 200 and 350 K.
A small amount of N2 formation (observed for both m/e = 28
and 14) was already found at around 200 K, but the largest part
of the N2 desorption was observed between 350 and 500 K. NO
formation was not observed in this particular experiment. The
conditions under which it forms are discussed in Section 3.4.
Similar experiments have been performed by Mieher and Ho [3]
for Pt(111), who found N2 desorption between 400 and 600 K
on Pt(111), somewhat higher than in our experiment, in which
a part of the N2 desorption occurs already at around 200 K, and
above 500 K N2 desorption was no longer observed; that is, N2
desorption (via Nad combination) occurred at a lower tempera-
ture on Pt(410).
Figs. 5c and 5d show the TP-XPS results obtained during
a similar experiment. We took great care to avoid beam dam-
age during the experiments and checked the influence of the
radiation on the outcome of the experiments. The effect of the
radiation was negligible in the results presented here. For the
N 1s spectra, we dosed NH3 at 175 K, to avoid the formation
of either a second NH3 layer or NH3 ice, which can obscure
the N 1s spectra [13]. For the O 1s spectra, we dosed NH3
at 100 K, and in this experiment the second NH3 layer was
formed. Desorption of this layer, which partly blocks the sig-
nal from the Oad layer underneath, explains the increase in the
Oad signal between 100 and 150 K. Starting at 100 K allowed
Fig. 5. (a) NH , H O and N desorption observed during heating of a co-ad-
3
2
2
−1
sorbed NH /O layer (3 K s ). (b) The derivative of the XPS data (similar
3ad ad
to thermal desorption spectra). (c) Intensities of the different N 1s compo-
−1
nents obtained during heating of a co-adsorbed NH /O layer (0.25 K s ).
3ad ad
(d) Intensities of the different O 1s components obtained during heating of a
−1
co-adsorbed layer (0.25 K s ).
us to observe the onset of H2Oad formation and thus the onset
of NH3ad dehydrogenation, which was found to occur already
around 150 K. About 40% of the Oad initially present formed
H2Oad below 200 K, whereas the remaining 60% reacted be-
tween 200 and 300 K, forming H2O(g). We did not observe
OHad formation, but this is not surprising, because calcula-
tions performed by Offermans et al. [Pt(111)] showed [37], that
OHad species are more reactive than Oad and immediately react
with NHxad species to form H2Oad. In the N 1s spectra several
NHxad species were already present at the start of the experi-

C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
189
ment at 175 K (because the reaction between Oad and NH3ad
already occurred at around 150 K).
The NH3ad concentration was found to decrease between
175 and 400 K, due to both decomposition and desorption. The
NH2ad concentration also decreased between 150 and 250 K,
and its concentration was very low above 300 K. Both the Nad
and NHad concentrations increased between 175 and 300 K
(due to NH3ad and NH2ad decomposition) and decreased again
above 300 K (NHad dehydrogenated to Nad, which desorbed as
N2). Because both Nad and NHad were present at the N2 de-
sorption temperature, we cannot exclude the possibility that N2
formation involves N2Hx species.
The presence of NHad above 300 K, after all of the Oad was
consumed, seems to contradict the thermal desorption data. De-
composition of this NHad (in the absence of Oad) should result
in the formation of H2, but this was not observed in the TPD ex-
periment. However, during heating of an Oad layer in vacuum,
we found (see Section 3.2) that Oad was slowly removed by re-
action with H2 and CO from the residual gas. Due to the low
heating rate (0.25 K s−1) used in the TP-XPS, much Oad will
be removed in this way, leaving less Oad available to react with
the remaining NHad. In the TPD experiment, the heating rate
was about 10 times higher (i.e., 3 K s−1), and thus Oad removal
by reaction with residual gases played only a minor role.
The XPS results can be used to calculate the thermal des-
orption spectra. The TPD signal corresponds to −dθ/dT , and
by differentiating the θ(T ) signals found with XPS, the thermal
desorption spectra can be “calculated.” This is a useful method
for comparing the TP-XPS and TPD results. The differenti-
ated θO+H O signal [Fig. 5b] was found to be very similar to
2
the H2O formation observed in the gas phase. This shows that
the results found for TPD and TP-XPS are very similar, even
though the experimental conditions were different (i.e., differ-
ent heating rates). The differentiated (θNH + θN) signal below
300 K shows that NHad and Nad formed at the same temper-
ature as H2O(g), whereas the differentiated (θNH + θN) signal
above 300 K matches the N2 desorption observed in the gas
phase; that is, the N2 desorption peak was a result of desorp-
tion of Nad (and decomposition of NHad) already present on the
surface at 300 K.
Fig. 6. (a) N formation during heating of an O saturated surface (10 L) after
2
ad
−1
different NH post-exposures (3 K s ). (b) NO formation during heating of
3
−1
an O saturated surface (10 L) after different NH post-exposures (3 K s ).
ad
3
(c) N 1s and O 1s components during heating of a co-adsorbed NH /O
3ad ad
−1
−8
layer (0.25 K s ) in the presence of 5 ×10 mbar O . The O signal shown
2
ad
is the sum of O and O .
3.4. N2 vs. NO formation
I
II
A recent publication [12] discussed the surface chemistry of
NO on Pt(410). In the N 1s spectra, two types of NO, with
N 1s BEs of 400.7 and 401.3 eV, were observed. In the O 1s
spectra, only one species was observed, at 533.0 eV. In some
of the experiments with ammonia and oxygen, the formation
of NOad was also observed. The peak due to NOad appeared at
400.7 eV in the N 1s region and at 533 eV in the O 1s region.
Mieher and Ho [3] showed that NO formation on Pt(111)
depends strongly on the amount of available Oad compared
with the amount of NH3ad present. They reported the forma-
tion of NO(g), but only when the surface was precovered with a
large amount of Oad and subsequently exposed to a relatively
small amount of NH3. This observation was rationalized by
the following mechanism: Most Oad is consumed during NH3ad
dehydrogenation (forming H2O), and NO can be formed only
when Oad remains after NHxad dehydrogenation, that is, when
the amount of NH3ad is small compared with the amount of Oad.
A similar trend was observed on Pt(410). Figs. 6a and 6b
show both N2 and NO desorption after exposure of an oxygen-
saturated surface (10 L at 200 K) to different amounts of NH3.
Similar to Pt(111), NO desorption occurred only after small
doses of ammonia (0.1–0.2 L). N2 formation observed in a
TPD experiment after these low NH3 doses shows two max-
ima at around 350 and 450 K. We suggest that these different
peaks originated from two different mechanisms. The oxygen
remaining on the surface after NH3ad dehydrogenation reacted
with Nad to form NOad between 200 and 400 K. The Nad that
did not react with Oad desorbed as N2 at around 350 K. This ex-

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C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
plains the first N2 desorption peak. NOad decomposition started
at above 400 K [12], and the Nad formed in this process de-
sorbed as N2 at around 450 K, the second N2 desorption peak.
A similar reaction scheme was proposed by Bradley et al. [11]
for Pt(100).
This specific experiment was difficult to reproduce during
the TP-XPS measurements (due to reaction of Oad with back-
ground gases); thus, a different approach was adopted. An O2
uptake experiment (not shown) on a NH3ad-saturated surface (at
175 K) revealed that O2 could access the surface and dissoci-
ate, even in the presence of a saturated NH3ad layer. We drew on
this finding in an attempt to make NOad. In this particular ex-
periment, a mixed Oad/NH3ad layer was heated in the presence
of 5 × 10−8 mbar O2. Here O2 could replenish the Oad con-
sumed during NHxad dehydrogenation, and as a result Oad was
present throughout the experiment. Fig. 6c shows the results of
this experiment, as observed with XPS.
Comparing these results with those of a similar experiment
in the absence of O2(g) but in the presence of Oad (see Fig. 5)
reveals several differences. The NHad concentration decreased
at a lower temperature in the presence of O2(g), due to the fact
that in this experiment more Oad was available and NHad could
be completely dehydrogenated via reaction with Oad. The for-
mation of NOad was observed only in the presence of O2(g).
This finding is in line with the model presented in the previ-
ous paragraphs; that is, NOad could form only when Oad was
available after all NHxad was dehydrogenated.
3.5. Steady-state NH3 oxidation on Pt(410)
Fig. 7 shows the results obtained during steady-state NH3
oxidation (ratio 1:1). Panels (a) and (b) show the gas-phase
products, and panels (c) and (d) and (e) and (f) show the N 1s
and O 1s components, respectively. Both the results obtained
during heating [(a), (c), and (e)] and during subsequent cooling
[(b), (d), and (f)] are shown. The surface was saturated with Oad
(10 L O2, 200 K) before being exposed to the reaction mixture.
At 250 K, the surface was covered with a mixture of NH3ad
(+NH2ad), NHad, Nad, and NOad. The concentrations of NH3ad
(+NH2ad), NHad, and Nad decreased with increasing temper-
ature. The adsorption of O2 from the gas phase seemed to be
hindered by the NHxad species present on the surface, and all
O
ad that formed on the surface immediately reacted with NHxad
species. This explains why the Oad concentration remained low.
NOad formation was already observed around 250 K. In
the N 1s spectrum, the peak due to NOad was not very well
resolved, due to the presence of several other N-containing
species (especially NH3ad), but it was clearly observed in the
O 1s spectra, being the only peak present at this temperature.
The amount of NOad did not change between 200 and 400 K,
but above 400 K, NO decomposition started (because no Oad
was present, which would otherwise inhibit NOad decomposi-
tion). As a result, the NOad concentration dropped. It is impor-
tant to note that only a part of the Nad formed NOad, whereas
an equal part did not react and was present as Nad.
In the gas phase, both H2O and N2 formation were ob-
served between 400 and 600 K. The N-selectivity of the reac-
tion changed above 600 K from N2(g) to NO(g). The surface
was almost empty at this temperature; that is, the NHxad, NOad,
and Oad concentrations were almost zero. Due to experimental
limitations, we were not able to observe the surface coverage
above this temperature.
The change in N-selectivity was accompanied by a decrease
in H2O(g) formation. According to the overall reaction equa-
tions (1) and (2), more oxygen is needed for the formation of
NO and H2O than for formation of N2 and H2O. The ratio of
1:1 used in this experiment means that only a relatively small
amount of oxygen was available, too little to oxidize all of the
NH3 to NO. As a result, the NH3 conversion dropped above
600 K. In other words, the reaction rate during NO + H2O for-
mation decreased due to a lack of oxygen:
The differentiated total N 1s signal (θNH + θN + θNO, not
x
shown) corresponds rather well to the TPD results obtained
using a low NH3 postexposure; the decreased N 1s intensity
between 300 and 350 K corresponds to the first N2 desorp-
tion peak. A part of the Nad formed NOad, which desorbed/
decomposed between 400 and 500 K. NOad can be clearly ob-
served in both the N 1s and O 1s region above 400 K, where
it was the only species (next to Oad) present. Below 400 K,
determining the exact intensity of the NOad peak is more diffi-
cult, due to the fact that other species (especially NH3ad) over-
lapped with the NOad peak. Therefore, the exact temperature
at which NOad formation started cannot be determined. From
spectra obtained during steady-state NH3 oxidation (see Sec-
tion 3.5), we tentatively conclude that NOad formation started
already at around 200 K. Reflection absorption infrared spec-
troscopy (RAIRS) experiments done by Kim et al. [38] showed
that NOad formation on Pt(100) started at around 275 K.
4NH3 + 3O2 → 2N2 + 6H2O,
4NH3 + 5O2 → 4NO + 6H2O.
(1)
(2)
From the XPS data, we cannot distinguish NOad decomposi-
tion from NOad desorption, because Nad formed during dissoci-
ation immediately desorbed as N2 and was not observed on the
surface. The fact that the NOad concentration dropped at around
450 K and not at around 400 K, the temperature at which NOad
decomposition started (see Ref. [12]), indicates that NOad de-
sorbed rather than decomposed. Experiments done by Bradley
et al. on Pt(100) indicated that a high Oad concentration in-
hibited NOad dissociation [11]. In our experiment, we found
a rather high Oad concentration at around 450 K, so it is very
well possible that it blocked NOad dissociation and that the de-
creased NOad was due to desorption rather than to dissociation.
The cooling branch provides a better source of information
about the steady-state reaction, because non-steady-state ef-
fects, caused by desorption of adsorbates present at low tem-
perature, are absent. Above 400 K, the reaction, as observed
by the gas-phase products, proceeded in the same manner, and
all essential features, including reaction rate, selectivity change
(around 600 K), and starting/stopping of the reaction (around
400 K), were similar for the heating and cooling branches.
The surface composition as a function of temperature was
slightly different in the cooling branch, specifically below

C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
191
Fig. 7. Gas phase products (a, b) and surface coverage (c–f) during NH oxidation on Pt(410), using a NH /O ratio of 1:1. The heating and cooling branch are both
3
3
2
−7
−8
−1
−1
shown (pNH = 1 × 10 mbar (TPR)/5 × 10 mbar (XPS), NH :O = 1:1, heating rate 0.5 K s (TPR)/0.25 K s (XPS)), 1 data point per ∼10 K.
3
3
2
∼500 K. The surface began to be covered by Nad in the cool-
ing branch, followed by NHad; at even lower temperatures, both
NH3ad (+NH2ad) and NOad were observed. The amount of Nad
present in the cooling branch was larger than that in the heating
branch, whereas the NHad and NH3ad coverages were lower.
the amount of H2O formation (i.e., the reaction rate) was about
1.5 times higher for the 1:5 ratio than for the 1:1 ratio. The
greatest difference between the two ratios was the temperature
at which the change in the N-selectivity occurred. For a ratio of
1:1, the selectivity changed at around 600 K, whereas for a ra-
tio of 1:5, the selectivity changed at around 450 K. Because of
the large amount of O2 present, the H2O formation for a ratio
of 1:5 was not influenced by the selectivity change, in contrast
to what we observed for a ratio of 1:1. Above 800 K, the reac-
tion rate dropped. This is explained by the fact that Oad starts
to desorb (as O2) (see Section 3.2), resulting in a lower Oad
concentration, and thus a lower reaction rate. A small peak was
also observed for m/e = 44 between 400 and 500 K, somewhat
higher than the m/e = 44 desorption peak found for a ratio of
1:1. We tentatively assign this peak to N2O formed via reaction
between Nad and NOad.
3.5.1. The influence of the reactant ratio
We also studied the effect of O2 partial pressure on the re-
action rate and selectivity. The results presented in Fig. 8 were
obtained using an NH3/O2 ratio of 1:5, i.e., an excess of O2.
The NH3 pressure was similar to the pressure used for the ratio
1:1, and the O2 pressure was adjusted according to the desired
ratio. The surface was precovered with Oad before the reaction
mixture was admitted.
The nature and abundance of the gas-phase products ob-
served below ∼400 K in the first heating branch were the re-
sult of a complex combination of desorption of the initially
present surface species and the products of steady-state re-
action. N2 formation showed two distinct peaks, one around
380 K and another one around 450 K. H2O formation started
at around 350 K, a similar temperature as for a reactant ratio
of 1:1. The amount of water formed provides a good measure
of the reaction rates for the two different ratios. For both ratios,
the maximum H2O formation occurred at around 500 K, and
In the cooling branch, only the products of the steady-state
reaction were observed. During cooling, N2 formation showed
only a single peak, at around 420 K. The N2 desorption peak
observed at around 380 K in the heating branch is therefore
assigned to a transient desorption peak, due to desorption of
N
ad already present on the surface at lower temperature.
The results of the XPS measurements are shown in Figs. 8c–
8f. The thermal behavior of the different NHxad species was

192
C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
Fig. 8. Gas phase products (a, b) and surface coverage (c–f) during NH oxidation on Pt(410), using a NH /O ratio of 1:5. The heating and cooling branch are both
3
3
2
−1
−7
−1
−8
shown (TPR: 0.5 K s , 1 × 10 mbar NH , TP-XPS: 0.25 K s , 5 × 10 mbar NH , 1 data point per ∼10 K).
3
3
very similar for both NH3/O2 ratios (1:1 and 1:5), whereas
the NOad concentration was rather different for both ratios.
For the ratio 1:5, NOad was present between 200 and 450 K;
at 450 K, its concentration increased, reaching a maximum at
around 480 K. For higher temperatures, the NOad concentra-
tion decreased again. NOad dissociation did not occur when a
ratio of 1:5 was used, because Oad was present on the surface
above ∼450 K, and it inhibited NOad dissociation.
A hysteresis was observed in the Oad concentration; that is,
Oad was observed on the surface down to 400 K, whereas in
the heating branch Oad was observed only above 500 K. This
also influenced the NHxad chemistry; more Nad than NHad was
present on the surface in the cooling branch.
4. General discussion
The O 1s spectral region shows a large difference between
the two ratios. For the 1:5 ratio, the surface became Oad-covered
at around 450 K, in contrast to what was found for a 1:1 ratio.
The change in surface coverage (from NHxad-to Oad-covered)
found for the 1:5 ratio coincides with the N-selectivity change
in the gas phase. This shows that the selectivity of the NH3 ox-
idation reaction on Pt(410) was determined by the surface cov-
erage; when the surface was NHxad/Nad-covered, the reaction
product was mainly N2, but when the surface was Oad-covered,
NO was the major product. The small amount of N2O observed
was formed only when both Nad and NOad coexisted on the
surface. This suggests that it formed via a reaction between Nad
and NOad.
Thermal desorption spectra of NH3 co-adsorbed with Oad
showed some N2(g) formation already around 200 K, simi-
lar to the low-temperature N2 desorption peak observed for
Pt(100) [11] but significantly less than N2 desorption from
Pt(111) (>400 K), as observed by Mieher and Ho [3]. A sim-
ilar trend was observed for NOad formation. Kim et al. found
that NOad formation started at around 300 K on Pt(100). We
found that NOad formation started at between 200 and 300 K on
Pt(410). On Pt(111), on the other hand, the formation of NOad
was not detected at all by EELS, whereas NO(g) was observed
above 400 K [3]. These results indicate a special reactivity
of the {100} facets of Pt, for several elementary steps in the
NH3 oxidation mechanism. Recent density functional theory
(DFT) calculations [39,40] indicated that the activation ener-
gies for Nad combination and NOad formation were much lower
The nature and concentration of the surface species in the
cooling branch also differed from that in the heating branch.

C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
193
on (bulk-terminated) Pt(100) and Pt(410) (∼90 and ∼70 meV
per molecule, respectively) than on other Pt surfaces. There-
fore, we explain the low-temperature N2 desorption and the
low-temperature NOad formation by the fact that the activa-
tion barriers associated with these processes are much lower
on Pt(410) than on, for example, Pt(111).
Pt(111) [14,45–47], whereas NO desorption occurs at around
the same temperature on both surfaces [45,48]. On Pt surfaces,
NOad formation from Nad and Oad was found to be exothermic
[39,40], which is the driving force for NOad formation on Pt.
Ir(111) interacts more strongly with the atomic adsorbates (as
shown by the higher N2 and O2 desorption temperatures), im-
plying that NOad formation is less exothermic on Ir(111) than
on Pt(111). As a result Ir(111) is less active for NO formation
(and more active for NO dissociation) than Pt(111).
Several authors have investigated steady-state NH3 oxida-
tion on stepped Pt single crystal surfaces, including Pt(533)
[4-atom-wide {111} terraces, {100} steps], Pt(443) [7-atom-
wide {111} terraces, {111} steps] and Pt(S)-12(111)×(111)
[2,41,42]. The general trend found for all these surfaces is that
N2 + H2O forms at low temperatures, whereas NO + H2O
forms at high temperatures. The exact temperature at which the
N-selectivity changes varies for the different surfaces. Compar-
ison between the different surfaces is complicated by the fact
that the experimental conditions vary significantly. Scheibe et
al. studied both Pt(533) and Pt(443), using similar reaction con-
ditions for both surfaces and concluded that Pt(533) is more
active than Pt(443). They offered two explanations for this dif-
ference: (i) the step density is higher for Pt(533) or (ii) the na-
ture of the steps is different; that is, the {100} steps on Pt(533)
are particularly reactive. In view of the preceding discussion,
we support the second model.
In 2003, Günther et al. [43] published a high-pressure
(∼ mbar) XPS study of ammonia oxidation on Pt(533) [{111}
terraces, {100} steps]. They reported several peaks in the N 1s
region, very similar to the peaks that we found on Pt(410). They
also found that the NHxad chemistry on the surface was similar
at both low and high reactant pressures. These results show that
low-pressure studies like ours are also relevant for understand-
ing catalysis at industrially relevant pressures.
In a recent publication, we reported our results on NH3
oxidation on Ir(110) and Ir(111) [9,13,14]. On Ir(110), we
found that the change of surface composition from Nad-covered
to Oad-covered occurred at significantly lower temperatures
(ꢀT ∼ 200 K) than the change in gas-phase selectivity. This
was explained by a difference between the activation ener-
gies between N2 formation and NO formation on Ir(110). On
Pt(410), we showed that the change in surface coverage corre-
lates with the change in gas-phase selectivity. Therefore, in the
case of Pt(410), the gas-phase selectivity is governed by the sur-
face coverage rather than by any difference in activation energy.
These kinetic considerations cannot be extended directly to
other Ir and Pt facets, because the surfaces used in this case
both demonstrate some special behavior. The activation barri-
ers associated with N2 and NO formation are much lower on
Pt(410) than on other Pt surfaces, which should be kept in mind
when applying insights obtained on surfaces containing {100}
facets [like Pt(100), Pt(410), and Pt(533)] to other surfaces. On
Ir(110), the presence of Oad has a large effect on the N2 forma-
tion rate [44].
5. Conclusion
This study has investigated several elementary steps occur-
ring during NH3 oxidation on Pt(410). Ammonia adsorbs mole-
cularly on Pt(410) and desorbs between 200 and 450 K. Radia-
tion can induce dissociation, and several NHxad surface species
have been observed. NHad was found to be the most abundant
(and probably most stable) surface species; it dissociates above
350 K. The Nad (and Had) formed immediately desorb as N2
(and H2).
O2 adsorbs both molecularly and dissociatively on Pt(410).
Heating of an adsorbed O2/Oad layer results in O2 desorption at
around 160 K. The atomic oxygen desorbs (as O2) in two steps
at around 650 and 800 K. The O 1s core level spectra show both
O2ad and two Oad species. In the presence of H2, the formation
of H2Oad occurs at around 150 K. H2Oad desorbs above 200 K.
The presence of Oad enhances NH3ad dissociation. Hydrogen
abstraction starts at around 150 K, and H2Oad formed in this
process desorbs above 200 K. N2 desorption occurs between
200 and 500 K. NOad and NO(g) formation are also observed,
but only during experiments where a large amount of Oad is
available. NOad desorbs/decomposes between 400 and 500 K,
and both NO(g) and N2(g) are observed in this temperature re-
gion.
The steady-state reaction between NH3 and O2 leads to N2
and H2O formation, but at higher temperature the selectivity
changes toward NO and H2O. The exact temperature at which
the selectivity change occurs depends strongly on the reactant
ratio. The selectivity change is accompanied by a change in
surface coverage; N2 is formed when the surface is NHxad
-
covered, and NO is formed when the surface is Oad-covered.
We have compared our results with literature data for other Pt
surfaces as well as results obtained on Ir surfaces. We propose
that the main difference between Pt and Ir catalysts lies in the
lower NO formation rate on Ir, rather than in a higher NO de-
composition rate.
Acknowledgments
The authors acknowledge ELETTRA and the European
Union for financial support for measurements at the Super-
ESCA beamline of ELETTRA. They thank S. Lizzit, L. Petac-
cia, and A. Baraldi for their help during the beam time, and
R.C.V. van Schie for his invaluable technical support. C.J.W.,
A.V.M., and V.V.G. acknowledge the Netherlands Technology
foundation STW, the applied science division of NOW, and the
technology program of the Ministry of Economic Affairs for
Thermodynamic considerations offer a different, more gen-
eral explanation for the different selectivities for Pt and Ir cata-
lysts. These considerations are based on literature data regard-
ing the (111) surfaces of Pt and Ir. Both Oad desorption (as
O2) and Nad desorption (as N2) occur at a higher temperature
(+300 K for Oad and +50 K for Nad) from Ir(111) than from

194
C.J. Weststrate et al. / Journal of Catalysis 242 (2006) 184–194
financial support under project UPC.5037. A.V.M. and V.V.G.
also acknowledge the NWO Russia program for financial sup-
port under project number 047-015-002.
[19] A. Baraldi, G. Comelli, S. Lizzit, D. Cocco, G. Paolucci, R. Rosei, Surf.
Sci. 367 (3) (1996) L67–L72.
[20] J.J. Joyce, M. del Guidice, J.H. Weaver, J. Electron Spectrosc. Relat. Phe-
nom. 49 (1) (1989) 31–45.
[21] Y.-M. Sun, D. Sloan, H. Ihm, J.M. White, J. Vac. Sci. Technol. A 14 (3)
(1996) 1516–1521.
References
[22] A.L. Schwaner, E.D. Pylant, J.M. White, J. Vac. Sci. Technol. A 14 (3)
(1996) 1453–1456.
[1] J.L. Gland, E.B. Kollin, Surf. Sci. 104 (1981) 478–490.
[2] J.L. Gland, V.N. Korchak, J. Catal. 53 (1978) 9–23.
[3] W.D. Mieher, W. Ho, Surf. Sci. 322 (1995) 151–167.
[4] D.G. Löffler, L.D. Schmidt, Surf. Sci. 59 (1976) 195–204.
[5] J.M. Gohndrone, C.W. Olsen, A.L. Backman, T.R. Gow, E. Yagasaki, R.I.
Masel, J. Vac. Sci. Technol. A 7 (3) (1989) 1986–1990.
[6] D.M. Thornburg, R.J. Madix, Surf. Sci. 220 (1990) 268–294.
[7] A. Fahmi, R.A. van Santen, Z. Phys. Chem. 197 (1996) 203–217.
[8] B. Afsin, P.R. Davies, A. Paskusky, M.W. Roberts, D. Vincent, Surf.
Sci. 284 (1993) 109–120.
[9] C.J. Weststrate, J.W. Bakker, E.D.L. Rienks, S. Lizzit, L. Petaccia, A. Ba-
raldi, C.P. Vinod, B.E. Nieuwenhuys, Phys. Chem. Chem. Phys. 7 (13)
(2005) 2629–2634.
[10] M. Baerns, R. Imbihl, V.A. Kondratenko, R. Kraehnert, W.K. Offermans,
R.A. van Santen, A. Scheibe, J. Catal. 232 (2005) 226–238.
[11] J.M. Bradley, A. Hopkinson, D.A. King, J. Phys. Chem. 99 (1995) 17032–
17042.
[12] C.J. Weststrate, J.W. Bakker, E.D.L. Rienks, C. Vinod, S. Lizzit, L. Petac-
cia, A. Baraldi, B.E. Nieuwenhuys, Surf. Sci. 600 (10) (2006) 1991–2001.
[13] C.J. Weststrate, J.W. Bakker, E.D.L. Rienks, S. Lizzit, L. Petaccia, A. Ba-
raldi, C.P. Vinod, B.E. Nieuwenhuys, J. Chem. Phys. 122 (18) (2005)
184705.
[14] C.J. Weststrate, J.W. Bakker, E.D.L. Rienks, J.R. Martinez, S. Lizzit,
L. Petaccia, A. Baraldi, C.P. Vinod, B.E. Nieuwenhuys, J. Catal. 235
(2005) 92–102.
[15] S.A.C. Carabineiro, B.E. Nieuwenhuys, Surf. Sci. 505 (2002) 163–170.
[16] S.A.C. Carabineiro, B.E. Nieuwenhuys, Surf. Sci. 532 (2003) 87–95.
[17] A. Baraldi, G. Comelli, S. Lizzit, M. Kiskinova, G. Paolucci, Surf. Sci.
Rep. 49 (6–8) (2003) 169–224.
[23] G.B. Fisher, Chem. Phys. Lett. 79 (3) (1981) 452–458.
[24] C. Benndorf, T.E. Madey, Surf. Sci. 135 (1–3) (1983) 164–183.
[25] D.R. Jennison, P.A. Schultz, M.P. Sears, Surf. Sci. 368 (1996) 253–257.
[26] B.A. Sexton, G.E. Mitchell, Surf. Sci. 99 (1980) 523–538.
[27] G.J. Szulczewski, J.M. White, Surf. Sci. 406 (1998) 194–205.
[28] N. Freyer, M. Kiskinova, G. Pirug, H.P. Bonzel, Surf. Sci. 166 (1986)
206–220.
[29] A. Rar, T. Matsushima, Surf. Sci. 318 (1994) 89–96.
[30] K. Schwaha, E. Bechtold, Surf. Sci. 65 (1977) 277–286.
[31] N.R. Avery, Chem. Phys. Lett. 96 (3) (1983) 371–373.
[32] J.L. Gland, B.A. Sexton, G.B. Fisher, Surf. Sci. 95 (1980) 587–602.
[33] Y. Ohno, T. Matsushima, Surf. Sci. 241 (1–2) (1991) 47–53.
[34] G.B. Fisher, J.L. Gland, Surf. Sci. 94 (1980) 446–455.
[35] M.A. Henderson, Surf. Sci. Rep. 46 (2002) 1–308.
[36] M. Kiskinova, G. Perug, H.P. Bonzel, Surf. Sci. 150 (1985) 319–338.
[37] W.K. Offermans, A.P.J. Jansen, R.A. van Santen, Surf. Sci. 600 (2006)
1714–1734.
[38] M. Kim, S.J. Pratt, D.A. King, J. Am. Chem. Soc. 122 (2000) 2409–2410.
[39] A. Eichler, J. Hafner, J. Catal. 204 (2001) 118–128.
[40] Q. Ge, M. Neurock, J. Am. Chem. Soc. 126 (2004) 1551–1559.
[41] A. Scheibe, U. Linz, R. Imbihl, Surf. Sci. 577 (2005) 1–14.
[42] A. Scheibe, U. Linz, R. Imbihl, Surf. Sci. 577 (2005) 131–144.
[43] S. Günther, A. Scheibe, D. Albrecht, R. Imbihl, H. Bluhm, M. Mävecker,
A. Knop-Gericke, E. Kleimenov, BESSY Annu. Rep. (2003) 274–276.
[44] C.A. de Wolf, J.W. Bakker, P.T. Wouda, B.E. Nieuwenhuys, A. Baraldi,
S. Lizzit, M. Kiskinova, J. Chem. Phys. 113 (2000) 10717–10722.
[45] J.C.L. Cornish, N.R. Avery, Surf. Sci. 235 (1990) 209–216.
[46] D.M. Collins, J.B. Lee, W.E. Spicer, Surf. Sci. 55 (1976) 389–402.
[47] K. Schwaha, E. Bechtold, Surf. Sci. 66 (1977) 383–393.
[48] R.J. Gorte, L.D. Schmidt, J.L. Gland, Surf. Sci. 109 (1981) 367–380.
[18] A. Baraldi, M. Barnaba, B. Brena, D. Cocco, G. Comelli, S. Lizzit, G. Pao-
lucci, R. Rosei, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 145–149.