Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

Article abstract of DOI:10.1016/j.susc.2006.07.025

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ～ 4 cm^-1) peak at 3322 cm^-1. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.

Full text of DOI:10.1016/j.susc.2006.07.025

Surface Science 600 (2006) 4563–4571
www.elsevier.com/locate/susc
Formation and hydrogenation of p(2 · 2)-N on Pt(111)
*
Eldad Herceg, James Jones, Kumudu Mudiyanselage, Michael Trenary
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7061, United States
Received 14 March 2006; accepted for publication 14 July 2006
Available online 8 August 2006
Abstract
The formation of a well-ordered p(2 · 2) overlayer of atomic nitrogen on the Pt(111) surface and its reaction with hydrogen were
characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy elec-
tron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 · 2)-N overlayer is
formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to
400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed
above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is
confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic,
rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indi-
cated by the N2 TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with
RAIRS by an intense and narrow (FWHM ꢀ 4 cmꢁ1) peak at 3322 cmꢁ1. The areas of the H2 TPD peak associated with NH dissoci-
ation and the XPS N1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the
methods used here.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Ammonia; Oxygen; Nitrogen; Hydrogen; Platinum; Infrared absorption spectroscopy; Low energy electron diffraction; Thermal desorption
spectroscopy
1. Introduction
first step for subsequent studies aimed at establishing the
surface chemical properties of this important species, such
as its hydrogenation activity. Second, the work also pro-
vides new insights into the important ammonia oxydehy-
drogenation reaction on platinum surfaces.
The properties of atomic nitrogen on platinum surfaces
are relevant to several important catalytic processes. These
include the reduction of NOx by automotive catalytic con-
verters and the industrial synthesis of nitric oxide through
the catalytic oxidation of ammonia [1]. While the forma-
tion of molecular nitrogen through oxydehydrogenation
of ammonia is an unwanted reaction in nitric acid synthe-
sis, it is the desired product in the catalytic reduction of
nitrogen oxides. Two primary objectives motivated the
study reported here. First, we wished to establish a simple
procedure to produce a clean, well-ordered overlayer of
atomic nitrogen on Pt(111). Such a procedure provides a
The reaction between oxygen and ammonia has been
investigated on many metallic surfaces including ruthenium
[2,3], rhodium [4,5], iridium [6–8], nickel [9,10], copper [11–
13], and other metals [5,14–16]. Platinum surfaces have
been also widely explored and studies have been carried
out on supported Pt [17–19], polycrystalline Pt [5,20–26],
and single-crystal Pt [21,27–35]. In the mid-1960s, Fogel
et al. [36] proposed a reaction scheme in which N2 is
formed in a bimolecular surface reaction between NO
and NH3, whereas NO is formed by a one-step mechanism
from the NH3 + O2 and/or NH3 + O reactions. Although
an imide radical mechanism was inferred, Fogel et al. failed
to detect any intermediates using secondary ion mass
*
Corresponding author. Tel.: +1 312 996 0777; fax: +1 312 996 0431.
E-mail address: mtrenary@uic.edu (M. Trenary).
0039-6028/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2006.07.025

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E. Herceg et al. / Surface Science 600 (2006) 4563–4571
spectrometry. Kinetic studies [5,22] over the pressure range
0.1–1.0 Torr showed that the reaction rates observed can
be described by several Langmuir–Hinshelwood rate
expressions based on the chemical reactions given above.
This was further supported by studies performed by Nutt
and Kapur [20]. The models explaining the overall reaction
mechanism [5,20,27,28] have combined the findings by Fo-
gel et al. with steps involving the competition for active
sites between NH3, O2, and the NO intermediate. However,
these models do not provide any details of the elementary
reaction steps.
ously [31,32], but not its reactivity towards hydrogen.
However, our study differs from this earlier study in several
ways as described below. The effect of different ammonia
exposures on the overall surface chemistry for Pt(111) sur-
faces saturated with molecular O2 and atomic O has also
been explored. In addition, we find that higher coverages
of both N and NH can be formed through the ammonia
oxydehydrogenation reactions described here than can be
formed through a method we recently described [40]
involving electron-induced NH3 dissociation on Pt(111).
Ultra high vacuum (UHV) steady-state reaction
studies by Gland and Korchak [27] on stepped Pt(S)-
12(111) · (111) have shown that the product content is
strongly influenced by substrate temperature and reactant
composition. Excess O2 and temperatures above ꢀ650 K
favor production of NO, while excess NH3 and tempera-
tures between 400 and 650 K result in N2 production
[27]. It was concluded that step edges are the most active
catalytic sites, which was further confirmed by comparison
of results obtained on Pt(111) [28]. Interestingly, they used
AES to show that below ꢀ650 K a nitrogenic surface spe-
cies is the only adsorbate, although this species was not
identified. This supports the idea of formation of N2 with-
out invoking NO as an intermediate, which contradicts
the models referred to above. The insignificance of the
NH3 + NO reaction in NH3 oxidation can also be seen
from the absence of any reaction between these two species
coadsorbed on Pt(111) under UHV [37,38]. To account for
the production of NH and OH radicals over polycrystalline
Pt as detected by laser induced fluorescence, Selwyn and
Lin [23] proposed that atomic and radical species should
be considered. Recent work on Pt(111) [29,32] also
has led to consideration of different mechanisms of NO
formation, such as N + O ! NO recombination and
NH + O ! NO + H exchange. In these schemes, molecu-
lar nitrogen is formed via recombination of N atoms. Mie-
her and Ho identified NH, NH2, and OH as intermediate
species on Pt(111) using electron energy loss spectroscopy
(EELS) [32]. They concluded that NH3 is activated through
the stripping of one or several H atoms by O or OH fol-
lowed by the N + O reaction to form NO and the N + N
reaction to form N2. This reaction picture was further sup-
ported by theoretical calculations [39].
2. Experimental
The results were obtained in two separate UHV cham-
bers using two different Pt(111) crystals. The X-ray
photoelectron spectroscopy (XPS) results were obtained
in a chamber (chamber 1) with a base pressure of ꢀ2 ·
10ꢁ10 Torr. The system has been described in detail else-
where [41]. In brief, the UHV chamber is equipped with
low energy electron diffraction (LEED), an XPS system,
a quadrupole mass spectrometer (QMS) for TPD, and with
a Fourier transform infrared (FTIR) spectrometer for
reflection absorption infrared spectroscopy (RAIRS) stud-
ies. The XPS system consists of a VG CLAM2 hemispher-
ical analyzer and dual anode X-ray source. Mg Ka
radiation was used for the XPS study and the spectrometer
was calibrated with the Pt4f7/2 peak at a binding energy of
71.2 eV. All TPD, RAIRS, and AES experiments were per-
formed in a second chamber (chamber 2) with a base pres-
sure of ꢀ1 · 10ꢁ10 Torr. A detailed description of this
system can be found elsewhere [42]. In brief, it consists of
a stainless steel UHV chamber equipped for AES, LEED,
and TPD experiments with a QMS. The chamber is cou-
pled to a commercial Fourier transform infrared (FTIR)
spectrometer, a Bruker IFS 66 v/S. The IR beam enters
and exits the UHV chamber through differentially pumped
O-ring sealed KBr windows and passes through a polarizer
before reaching the infrared detector. To achieve maximum
sensitivity, an InSb detector was used with a tungsten
source for the spectral region above 2500 cmꢁ1, while an
MCT (HgCdTe) detector was used with a SiC source for
the lower wavenumber region. In cases where the sample
was annealed to a temperature above 85 K, the sample
was then cooled back to 85 K before the spectrum was ac-
quired. The background reference spectrum was also taken
at 85 K. A resolution of 2 cmꢁ1 was used for the spectra
above 2500 cmꢁ1 and 4 cmꢁ1 was used for the lower wave-
number region. For the TPD results, signal from the QMS
was recorded for each mass at a temperature interval of
approximately every 2 K. The data was smoothed using
the Savitzky–Golay method with 1.5% of the data points
used near each point. The smoothed TPD peak positions
can be determined with a precision of 1 K, which is also
the accuracy of the temperature measurement. The Pt(111)
surfaces were cleaned and judged free of impurities by a
standard procedure described earlier [43]. Ammonia
(99.9992%), oxygen (99.998%), and hydrogen (99.9999%)
The work presented here offers new information on the
oxydehydrogenation of NH3 under UHV conditions. We
confirm many of the previous findings from Ref. [32] and
find that N2 is definitely formed from the combination of
surface N atoms. In addition, we present spectroscopic evi-
dence for the reaction of NH3 with adsorbed molecular O2
at 85 K. The reaction with molecular oxygen provides a
simple route to the preparation of
a well-ordered
p(2 · 2)-N layer that is free of oxygen. We find that this
atomic nitrogen layer readily reacts with hydrogen to form
the NH species, although it appears that only a fraction of
the N layer can be converted to NH. Formation of a
p(2 · 2)-N structure on Pt(111) has been reported previ-

E. Herceg et al. / Surface Science 600 (2006) 4563–4571
4565
were purchased from Matheson Tri-gas, Inc. and used
without further purification. The exposure of 2.0 L of O2
at 85 K was sufficient to saturate the molecular state of
oxygen (0.44 ML) on the clean Pt(111) surface as con-
firmed by comparison of our O2 TPD spectra with that
of others [44,45]. Saturated atomic oxygen on Pt(111)
was prepared by exposing the surface to 1.0 L of O2 at
85 K and then annealing to 300 K for 60 s. Only the high
temperature O2 desorption peak at ꢀ750 K due to recomb-
inative oxygen desorption was observed in the TPD exper-
iment. An intense and sharp p(2 · 2) (0.25 ML) LEED
pattern due to the atomic oxygen layer was observed.
Fig. 1a than in Fig. 1b simply because less nitrogen and
hydrogen are available for the lower NH3 exposure. If
all of the NH3 is completely dehydrogenated through
the reaction with O2, then the maximum surface N atom
coverage for 0.1 L NH3 would be ꢀ0.13 ML. However,
some NH3 desorbs (Fig. 1a trace m/e = 17) instead of
reacting, even though there is enough surface oxygen to
consume all the hydrogen from the NH3. Some of the sur-
face nitrogen also desorbs in the form of NO as shown in
Fig. 1a. In contrast, there is no NO desorption for the
higher NH3 exposure, Fig. 1b. The area under the N2
TPD spectrum in Fig. 1b is approximately 3.3 times larger
than in Fig. 1a and the N2 peak position shifts from
510 K for 0.1 L of NH3 to 453 K for 0.4 L of NH3, which
is in agreement with the second-order kinetics expected
for recombinative desorption. The fact that more surface
nitrogen is produced for 0.4 L of NH3 can be seen also
through the increase in the amount of H2O desorption.
It is also interesting to note the differences in O2 desorp-
tion for the two cases. First, the amount of molecular O2
desorption is significantly lower in the case of the higher
NH3 coverage (Fig. 1b). Comparison with the desorption
of 2.0 L of O2 from clean Pt(111) indicates that the
m/e = 32 trace in Fig. 1b represents ꢀ22% of the initial
amount of oxygen adsorbed on Pt(111). Second, the O2
desorption peak shifts from 150 to 170 K when the NH3
exposure is increased from 0.1 to 0.4 L. This suggests that
molecular oxygen is stabilized by the presence of ammo-
nia, in agreement with previous studies [32]. Finally, the
high temperature peak due to recombinative oxygen
desorption is absent in Fig. 1b, implying that the surface
oxygen is completely consumed in the reaction with
ammonia.
3. Results
3.1. Temperature programmed desorption of
NH3/O2/Pt(111)
Before obtaining the TPD results shown in Fig. 1, the
Pt(111) crystal was exposed to 2.0 L of O2 at 85 K fol-
lowed by NH3 exposures of 0.1 L (a) and 0.4 L (b). On
clean Pt(111), a 0.1 L NH3 exposure corresponds to a
coverage of ꢀ0.13 ML, whereas 0.4 L of NH3 forms a bi-
layer [40,46]. The following desorption products were
monitored: H2 (m/e = 2), NH3 (m/e = 17), H2O (m/e =
18), N2 (m/e = 28), NO (m/e = 30), and O2 (m/e = 32).
Nitric oxide (NO) was the only oxide of nitrogen ob-
served. It is immediately clear that the desorption prod-
ucts observed depend on the ammonia exposure for the
same nominal O2 coverage. Hydrogen desorption is ab-
sent for both NH3 exposures. It is completely consumed
by the surface oxygen to form H2O as shown in the
m/e = 18 traces. Much less H2O and N2 are observed in
Fig. 1. TPD spectra for H2 (m/e = 2), NH3 (m/e = 17), H2O (m/e = 18), N2 (m/e = 28), NO (m/e = 30), and O2 (m/e = 32) obtained after exposing
Pt(111) at 85 K to 2.0 L of O2 and (a) 0.1 L of NH3, (b) 0.4 L of NH3.

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E. Herceg et al. / Surface Science 600 (2006) 4563–4571
3.2. Temperature programmed desorption of
NH3/O/Pt(111)
Fig. 2 shows TPD results analogous to those of Fig. 1
for the case of 0.4 L of NH3 deposited onto a p(2 · 2)
atomic oxygen layer. In order to make the comparison be-
tween Figs. 1 and 2 easier, the same intensity scale is used
for both. As before, hydrogen is completely consumed by
the H2O formation reaction. A flat line is observed in
Fig. 2 for m/e = 32 showing that all atomic oxygen desorbs
from the surface in the form of H2O or NO. Two distinc-
tive H2O desorption peaks are present at 216 and 330 K.
The stoichiometry of the reaction implies that the
maximum coverage of atomic nitrogen is limited to
ꢀ0.17 ML, or 2/3 of the initial O atom coverage of
0.25 ML. As expected, the N2 TPD peak area is signifi-
cantly smaller than what was observed in Fig. 1b, indicat-
ing incomplete dehydrogenation of the ammonia as also
indicated by the higher amount of NH3 desorption in
Fig. 2 than in Fig. 1b. Thus, Figs. 1 and 2 reveal that a sat-
urated O2 layer yields more complete ammonia dehydroge-
nation for a given NH3 exposure. The optimum NH3
exposure is revealed in Fig. 3, which is a plot of N2 TPD
peak area as a function of ammonia exposure to an O2 sat-
urated surface. Initially the N2 desorption increases with
ammonia coverage as expected and then decreases after
0.4 L of NH3 in part due to displacement of O2 from the
surface [32] and in part due to blocking of active sites by
the excess ammonia.
Fig. 3. N2 TPD peak area as a function of initial NH3 exposure on O2
saturated Pt(111). Saturated O2/Pt(111) was prepared by exposing 2.0 L
of O2 at 85 K.
3.3. Reflection absorption infrared spectroscopy of
NH3/O2/Pt(111)
Fig. 4 shows RAIR spectra of the NH3 + O2 reaction.
Upon exposure of 2.0 L of O2 onto the clean Pt(111) sur-
face, a peak is observed at 874 cmꢁ1, which is readily as-
signed to the internal vibration of the superoxo species
[45,47,48]. New features develop in the spectrum following
exposure to 0.4 L of NH3 at 85 K. At the same time, the
peak at 874 cmꢁ1 disappears implying that the O2 coverage
has been reduced below the RAIRS sensitivity limit. The
TPD data in Fig. 1b establishes that there is still some small
amount of molecular oxygen present on the surface,
although the fact that it desorbs at a higher temperature
Fig. 2. TPD spectra for H2 (m/e = 2), NH3 (m/e = 17), H2O (m/e = 18),
N2 (m/e = 28), NO (m/e = 30), and O2 (m/e = 32) obtained after exposing
Pt(111) pre-covered with p(2 · 2) atomic oxygen to 0.4 L of NH3. The
p(2 · 2)-O was prepared by annealing Pt(111) to 300 K for 60 s following
1.0 L of O2 exposure at 85 K.
Fig. 4. RAIR spectra representing thermal evolution of Pt(111) exposed
to 2.0 L of O2 and 0.4 L of NH3 at 85 K (two upper most spectra). Before
acquiring each spectrum at 85 K, the surface was annealed to the indicated
temperatures. The intensity of the spectra in the 2500–3800 cmꢁ1 region
has been multiplied by a factor of 2.

E. Herceg et al. / Surface Science 600 (2006) 4563–4571
4567
than on the clean surface indicates that it is stabilized by
the presence of ammonia. Formation of an NH3–O2 com-
plex has been suggested on Ag(111) and Zn(0001) [14,49]
and has been discussed as an intermediate in a theoretical
study of ammonia oxidation on a Cu(111) surface [50].
For a 0.4 L NH3 exposure on the clean surface, we observe
peaks centered at 1617 cmꢁ1 and 1211 cmꢁ1 with a shoul-
der at 1174 cmꢁ1 [40]. Because of the variety of structural
arrangements assumed by the ammonia molecules even
on the clean surface, a wide range of frequencies are asso-
ciated with the various modes of ammonia [51]. Under
slightly different conditions, Villegas and Weaver [51] as-
signed peaks at 1076 and 1240 cmꢁ1 to ds(NH3) of first
layer NH3 molecules adsorbed at threefold hollow sites
on the surface and to NH3 molecules hydrogen-bonded
to first layer molecules, respectively. They also tabulated
literature values for the vibrational frequencies of NH3 in
various environments, including the solid phase where
ds(NH3) occurs at 1058–1063 cmꢁ1 and das(NH3) occurs
H2O on the surface implies that even at 85 K ammonia
oxydehydrogenation is feasible, in agreement with previous
studies [32]. This is confirmed by the presence of m(NH) of
the NH species, an ammonia dehydrogenation product, at
3299 cmꢁ1. The peak at 1451 cmꢁ1 (1433 cmꢁ1 for 18O2) is
at a similar position to one observed at 1480 cmꢁ1 on
Ag(111) that was assigned to m(O–O) of O2 in an NH3–
O2 complex [14]. The implications of Fig. 5 for spectral
assignments associated with a NH3–O2 complex are consid-
ered in Section 4.
Annealing the surface decreases the intensity of all the
peaks associated with NH3 except for the most intense ones
due to ds(NH3) in the range of 1134–1265 cmꢁ1. The disap-
pearance of the NH3 peaks is a result of both desorption
and reaction with coadsorbed O2. The ds(NH3) peaks lose
their intensity dramatically above ꢀ380 K as shown in
Fig. 4. At 150 K a new peak appears at 1062 cmꢁ1, which
shifts to 1078 cmꢁ1 upon annealing to 200 K. Although
this peak is at a reasonable value for the bending mode
of an adsorbed hydroxyl species [52], it does not shift upon
18O2 substitution and is therefore assigned to ds(NH3) of
ammonia. Oxygen also reacts with the NH species as
shown by a significant decrease in intensity of the peak at
3299 cmꢁ1 upon annealing to 150 and 200 K. This peak
shifts to 3317 cmꢁ1 above 250 K and increases slightly in
intensity upon annealing to 380 K due to the N + H ! NH
reaction. Further details on the spectroscopic characteriza-
tion of the NH species on Pt(111) are given in Ref. [40].
Comparison of the RAIRS results in Fig. 4 with the TPD
results shown later in Fig. 9 reveals that there is a small
range of temperatures between about 400 and 450 K where
NH decomposes and where the resulting N atoms recom-
bine to desorb as N2.
at 1640–1646 cmꢁ1
. Therefore, we assign peaks at
1236 cmꢁ1 and 1641 cmꢁ1 to the symmetric and asymmet-
ric NH3 deformation modes. The 1134 cmꢁ1 peak corre-
sponds to the shoulder seen on the clean surface at
1174 cmꢁ1 and is likewise assigned to ds(NH3) of NH3.
The 3242 cmꢁ1 peak is the NH stretch of intact NH3
and is at essentially the same position as this peak
(3245 cmꢁ1) for NH3 on clean Pt(111) [40]. The peak at
1622 cmꢁ1 can be attributed either to H2O or NO,
although the latter is highly unlikely since its desorption
is not observed in the TPD experiment (Fig. 1b) for the
conditions used in Fig. 4. In an effort to make more defin-
itive spectral assignments of peaks that might be associated
with oxygen-containing species, spectra obtained for
approximately the same coverages of ammonia deposited
on surfaces at 85 K covered with 16O2 and 18O2 are com-
pared in Fig. 5. We assign the peak at 1641 cmꢁ1 to
das(NH3) of NH3 and the one at 1622 cmꢁ1 to d(HOH)
of H2O because no shift in the frequency of the former,
but a redshift of 6 cmꢁ1 for the latter, is observed upon
substitution with 18O2. This redshift is a reasonable value
for the HOH bend of a water molecule. Identification of
3.4. Auger electron spectroscopy of NH3/O2/Pt(111)
The AES spectra shown in Fig. 6 confirm that a nitrogen
overlayer on Pt(111) can be prepared free of oxygen and
other contaminants. The spectra were obtained with an
incident beam current of 10 lA and an energy of 3 kV.
Upon adsorption of O2 at 85 K (Fig. 6b) on the clean
Pt(111) surface (Fig. 6a), the oxygen KLL peak at
516 eV is observed. Although the Auger peak correspond-
ing to surface nitrogen overlaps with one of the platinum
transitions at 390 eV, the changes in this peak upon ammo-
nia exposure clearly reveal the presence of surface nitrogen.
The intensity of this peak remains essentially unchanged
after annealing to 400 K, whereas the peak corresponding
to the surface oxygen completely disappears (Fig. 6d).
The small peak at around 273 eV is due to carbon contam-
ination to the extent of about 2% of a monolayer. Other-
wise, the surface is covered with a pure N overlayer.
3.5. Low energy electron diffraction of NH3/O2/Pt(111)
The LEED pattern shown in Fig. 7 was obtained after
the Pt(111) surface was exposed to 2.0 L of O2 at 85 K,
Fig. 5. RAIR spectra of Pt(111) acquired after exposing (a) 2.0 L of O2
and 0.4 L NH3 at 85 K; and (b) 2.0 L 18O2 and 0.4 L NH3 at 85 K.

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E. Herceg et al. / Surface Science 600 (2006) 4563–4571
additional LEED spots originate either from a mixture of
surface species present below 400 K, or from N atoms in
p
p
a ( 3 · 3)R30ꢁ structure. For instance, both (2 · 2) and
p
p
( 3 · 3)R30ꢁ-N phases have been observed on
Ru(0001), with the latter being formed at lower tempera-
tures than the (2 · 2)-N phase [53].
3.6. Hydrogenation of surface N atoms to form NH:
RAIRS, TPD, and XPS
Fig. 8 shows RAIR spectra of the hydrogenation of the
p(2 · 2)-N structure formed by the NH3 + O2 reaction. The
surface was first exposed to 2.0 L of O2 and 0.4 L of NH3 at
85 K followed by a 400 K anneal for 60 s and then cooled
back to 85 K where spectrum (a) was taken. No peaks are
observed in Fig. 8a nor were any peaks observed in the
800–2000 cmꢁ1 region. This is consistent with a surface
covered only with N atoms, where the only surface IR ac-
tive mode would be well below the cutoff of our detector.
For spectra (b)–(f), the surface from (a) was exposed to
the indicated amount of H2 at 300 K, annealed to 380 K,
then cooled to 85 K where the spectra were obtained. For
a 10 L H2 exposure, a peak appears at 3320 cmꢁ1, which
is readily assigned to m(NH) of the NH species [40]. The
NH stretch peak area continues to increase for exposures
of 20, 40, and 80 L, but does not show a further increase
for 160 L. The FWHM reaches its minimum value of
4.4 cmꢁ1 for a 40 L exposure, but then increases for higher
H2 exposures. The fact that the NH peak area appears to
Fig. 6. AES spectra of (a) the clean Pt(111) surface, (b) after exposing the
surface in (a) to 2.0 L of O2, (c) after exposing the surface in (b) to 0.4 L of
NH3, (d) after annealing the surface in (c) to 400 K for 60 s.
Fig. 7. LEED photograph of the p(2 · 2)-N layer on Pt(111). The surface
was exposed to 2.0 L of O2 and 0.4 L of NH3 at 85 K followed by a 400 K
anneal for 60 s before the photograph was taken.
exposed to 0.4 L of NH3, and then annealed to 400 K for
60 s. An intense and sharp p(2 · 2) pattern is observed in
agreement with previous studies [31,32]. The p(2 · 2) pat-
tern implies an absolute N coverage of 0.25 ML. Annealing
to temperatures above 400 K led to a decrease in inten-
sity of the p(2 · 2) spots due to desorption of nitrogen.
In contrast, if the surface was annealed to only 380 K,
in addition to the p(2 · 2)-N pattern weak spots due
Fig. 8. RAIR spectra showing formation of the NH species. (a) Spectrum
acquired at 85 K after the Pt(111) surface was exposed to 2.0 L of O2 and
0.4 L of NH3 at 85 K and then annealed to 400 K for 60 s. (b–f) Spectra
obtained after the surface in (a) was exposed to the indicated amounts of
H2 at 300 K, annealed to 380 K for 30 s, and cooled back to 85 K.
p
p
to a ( 3 · 3)R30ꢁ superstructure were observed. These

E. Herceg et al. / Surface Science 600 (2006) 4563–4571
4569
reach a limiting value for the spectra of Fig. 8, suggests that
either all of the N is converted to NH, or that the reaction
has reached a limiting value for some other reason. The ob-
served FWHM in Fig. 8d of 4.4 cmꢁ1 corresponds to an
intrinsic FWHM of ꢀ3.9 cmꢁ1, after correcting for the
instrumental contribution. Such unusually narrow line
widths are generally indicative of a highly homogeneous
environment for the adsorbate. Although the intensity of
the NH stretch peak is high, this does not necessarily imply
a high NH coverage. However, independent estimates of
the NH coverage can be obtained from H2 TPD and from
XPS of the N1s region.
The TPD spectra in Fig. 9 were collected for the system
represented in Fig. 8f. The only desorption products ob-
served were H2 (m/e = 2) and N2 (m/e = 28). Nitrogen
desorption starts at ꢀ400 K as shown by the m/e = 14
channel, which is due to N atoms from N2 dissociation in
the ionizer of the mass spectrometer. Two peaks are pres-
ent in the m/e = 28 desorption trace. The one centered at
481 K is due to N2 desorption as confirmed by comparison
with the m/e = 14 signal. The 408 K m/e = 28 peak is due
to CO adsorbed from the background. The m/e = 12 signal
due to C atoms from CO dissociation supports this inter-
pretation. The H2 TPD peak observed at 356 K consists
of multiple components, and the overall H2 desorption pro-
file was fit with four separate peaks. These four peaks were
a convolution of Lorentzian and Guassian line shapes and
the peak area, width, and peak temperature of each compo-
nent was varied to give the best fit to the overall shape.
The areas of these peaks can be compared to the area
corresponding to a saturation coverage of hydrogen on
Fig. 10. The N1s XPS peak obtained (a) after exposing 2.0 L of O2 and
0.4 L of NH3 to the sample at 85 K, followed by annealing to 400 K; (b)
after exposing the surface in (a) to 300 L of H2 at 300 K and subsequent
annealing to 380 K for 30 s.
Pt(111) (not shown), which is known to be 1.0 ML [54].
The lowest temperature peak is due to desorption from
the tungsten heating wires, rather than from the Pt(111)
surface. The next two components correspond to H2 rec-
ombinative desorption of hydrogen adsorbed from the
background. The combined area of these two peaks implies
a hydrogen coverage of 0.17 ML from background adsorp-
tion. The highest temperature of the four peaks is centered
at 405 K and is assumed to be associated with NH dissoci-
ation. The area of this peak yields an NH coverage of
0.15 ML, indicating that ꢀ60% of the surface N atoms
are converted to NH. However, the uncertainty in fitting
the H2 TPD area and separating the part of the H2 desorp-
tion due to NH dissociation from the part due to back-
ground adsorption makes the uncertainty in NH
coverage determined by this method rather high.
In Fig. 10 we compare XPS N1s binding energies for the
p(2 · 2)-N covered Pt(111) surface (a) before and (b) after
hydrogenation. In (a) the N1s peak is well described by a
single peak at 399.0 eV. Upon hydrogenation, the peak
shifts to 399.7 eV and contains a low-binding-energy shoul-
der, indicative of incomplete hydrogenation. The two peaks
fitted to reproduce spectrum (b) in Fig. 10 indicate that
ꢀ68% of the surface N atoms are converted to NH, in good
agreement with the TPD results.
4. Discussion
The importance of the ammonia oxidation reaction over
platinum catalysts has stimulated several previous surface
science studies of this reaction on platinum single crystal
surfaces [21,27–35]. On many points these studies are in
agreement with each other and with the results presented
here. Thus, in every case it is found that NH3 is readily oxi-
dized to H2O, which desorbs below 400 K. In addition, NO
can also be formed but is favored under oxygen rich condi-
tions [27]. The nitrogen is removed from the surface
primarily in the recombinative desorption of N2, which
Fig. 9. TPD spectra for H2O (m/e = 18), NO (m/e = 30), O2 (m/e = 32),
H2 (m/e = 2), C (m/e = 12), N (m/e = 14), and N2 (m/e = 28) obtained for
the surface in Fig. 8f.

4570
E. Herceg et al. / Surface Science 600 (2006) 4563–4571
peaks at ꢀ450 K [29,32,39]. One previous RAIRS study of
the reaction of NH3 with atomic oxygen on Pt{100}
showed spectra of only one surface species, NO, but also
referred to spectra not shown indicating the presence of
NH2 on the surface [35]. Our results do not offer any signif-
icant new insights into the conditions that favor NO forma-
tion, which have been a primary focus of the previous
studies. Instead, we focus on the spectra of the mixed O2/
NH3 layer and on the high reactivity of the surface nitrogen
atoms towards H to form the NH species on Pt(111),
which has not been reported by other research groups. In
the present study we are able to explore the NH formation
reaction starting with a well-ordered p(2 · 2)-N layer.
Similar vibrational features that we observe for the
NH3/O2 layer on Pt(111) are also apparent in the HREEL
spectra of Mieher and Ho [32]. They assigned a 1097 cmꢁ1
peak to ds(NH3) of second and multilayer NH3. Peaks at
1452 and 3371 cmꢁ1 in their spectra were not specifically
assigned but were also associated with an increase in
NH3 coverage. Regardless of whether an actual NH3–O2
complex forms from the interaction of O2 and NH3 on
Pt(111), as proposed by Carley et al. [14] in a study on
Ag(111), there are three reasons to associate the peaks
we observe at 1134, 1451, and 3375 cmꢁ1 to an interaction
between O2 and NH3. First, on the clean Pt(111) surface at
85 K ammonia coverages ranging from submonolayer to
multilayer gave no peaks at these three frequencies. Sec-
ond, the peaks appeared when O2, but not O, was present
on the surface. Third, they disappear simultaneously upon
annealing the surface to 150 K (Fig. 4). The peaks at 1134
and 3375 cmꢁ1 would then correspond to ds(NH3) and
mas(NH) of an NH3 molecule perturbed through its interac-
tion with O2. However, it is not plausible to assign the peak
at 1451 cmꢁ1 to a perturbed ammonia peak, nor does it
seem to correspond to likely reaction products such as
NH2 or OH. The d(NH2) mode of the NH2 species has been
ruled out as it has been reported to be at 1555 cmꢁ1 on
Pt(111) [55] and the PtOH bending mode of OH on
Pt(111) has been reported to be in the range of 863–
919 cmꢁ1 [32]. The d(OOH) mode occurs at 1389 and
1380 cmꢁ1 for isolated 16O16OH and 18O18OH molecules,
respectively [56]. The same frequency ratio applied to the
1451 cmꢁ1 peak would imply a shift to 1441 cmꢁ1, rather
than to 1433 cmꢁ1as observed in Fig. 5 upon replacement
of 16O2 with 18O2. Carely et al. [14] assigned peaks at
1480 and 1640 cmꢁ1 to m(O–O) to two different forms of
molecular oxygen. The assignment is strongly supported
by their observation that the two peaks are in exactly the
same position when NH3 is replaced with ND3. A physi-
sorbed form of molecular oxygen observed at 30 K on
Pt(111) ꢁh1as a m(O–O) value of 1543 cmꢁ1 for 16O2 and
1457 cm for 18O2 [47]. If the peak that we observe at
1451 cmꢁ1 were also due to m(O–O) of an O2 molecule,
we would expect it to shift to approximately 1370 cmꢁ1
upon replacement of 16O2 with 18O2. Thus the shift we ob-
serve upon 18O2 substitution is more consistent with a
d(OOH) mode rather than a m(O–O) mode of a NH3–O2
complex. Since the exact positions of such a mode for
either an OOH surface species, or for a mode involving
an H atom shared between O2 and NH3 molecules are un-
known, this assignment must be tentative.
Mieher and Ho [32] identified NH2 based on the weak
peaks at 1613, 1750 and 3306 cmꢁ1 that they observed at
temperatures between 285 and 380 K. They also observed
a strong ds(NH3) peak of ammonia at 285 K, which atten-
uates after annealing to 380 K, in agreement with our
observations (Fig. 4). An HREELS study by Sun et al.
[55] assigned losses observed at 488, 830, 1392, 1555, and
3250 cmꢁ1 to NH2, which was produced when NH3 on
Pt(111) was bombarded with 50 eV electrons. They [55]
concluded that the NH2 species is stable only to 300 K.
However, even if the electron bombardment of NH3 does
produce NH2, the latter species is not stable at tempera-
tures that are high enough to desorb undissociated NH3;
consequently, this method cannot lead to a pure NH2 layer
on Pt(111). This makes definitive spectroscopic identifica-
tion of NH2 in the course of ammonia oxydehydrogenation
difficult. Moreover, we have shown clearly that the peak
at 3317 cmꢁ1 from Fig. 4 corresponds to m(NH) of the
NH species. This peak presumably corresponds to the
3306 cmꢁ1peak of Mieher and Ho [32].
One focus of this study was the formation of an N layer
on Pt(111) and its reactivity towards hydrogen. From
Fig. 7 we know that under the conditions used here the
nitrogen atoms will form a p(2 · 2) structure implying an
absolute coverage of 0.25 ML. Figs. 6d and 8a confirm
the absence of any other surface species and hence the high
purity of the N layer on Pt(111). Introduction of H2 to the
N covered surface leads to the appearance of a RAIRS
peak (Fig. 8) centered at 3320 cmꢁ1, which shifts slightly
to 3322 cmꢁ1 for higher H2 exposures. Comparison with
our previous work on electron stimulated ammonia disso-
ciation [40] and the absence of other vibrational modes in
the spectra in Figs. 8b–f supports assignment of this peak
to the NH species. The NH coverage for the conditions
of Fig. 8f can be obtained from a comparison of the frac-
tion of the H2 desorption that occurs above 380 K to the
area of the H2 peak for a saturation coverage of H on
Pt(111), which is known to be 1.0 ML [54]. The compari-
son yields an NH coverage of 0.15 ML, indicating that only
a fraction (ꢀ60%) of the N atoms on Pt(111) can react
with hydrogen to form NH. This observation is further
supported by the XPS results in Fig. 10, which imply an
NH coverage of 0.17 ML.
Finally, we have compared the results presented here
with the results described recently involving electron-in-
duced NH3 dissociation on Pt(111) [40]. We find that the
oxydehydrogenation of ammonia through the NH3–O2
complex is a more efficient method for producing surface
N atoms on Pt(111). This route gives not only a well-
ordered p(2 · 2)-N structure but also yields ꢀ25% more
atomic nitrogen on the surface. If we assume that the
NH stretch peak area measured with RAIRS is propor-
tional to the NH coverage, we find that the maximum

E. Herceg et al. / Surface Science 600 (2006) 4563–4571
4571
[12] B. Afsin, P.R. Davies, A. Pashusky, M.W. Roberts, D. Vincent, Surf.
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NH coverage achievable from hydrogenation of the
p(2 · 2)-N layer is 70% higher than can be achieved from
electron-induced dissociation of NH3. This also indicates
that a higher fraction of the surface N atoms in the
p(2 · 2) structure can be hydrogenated. However, it re-
mains puzzling as to why the p(2 · 2)-N cannot be com-
pletely converted to a p(2 · 2)-NH overlayer. This would
be expected since all of the N atoms are equivalent in the
well-ordered structure and if any of the N atoms can be
converted to NH, then all should be capable of undergoing
the same reaction. This line of reasoning, however, ignores
the influence of adsorbate–adsorbate interactions. In this
case, repulsive interactions between NH molecules may
make a complete (2 · 2) NH layer less energetically favor-
able than a mixed N and NH layer.
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5. Summary
A clean well-ordered p(2 · 2) layer of N atoms on the
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ammonia with molecular oxygen. The use of molecular
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NH species. However, the hydrogenation reaction does
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