Adsorption and decomposition of NO on O-covered planar and faceted Ir(2 1 0)

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Adsorption and decomposition of NO on O-covered planar and faceted Ir(2 1 0)
Wenhua Chena,*, Alan L. Stottlemyerb, Jingguang G. Chenb, Payam Kaghazchic, Timo Jacobc,d,
Theodore E. Madeya,1, Robert A. Bartynskia
aDepartment of Physics and Astronomy, and Laboratory for Surface Modification, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, United States
bCenter for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, United States
cFritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14 195 Berlin-Dahlem, Germany
dTheoretische Elektrochemie, Universität Ulm, Albert-Einstein-Allee 47, D-89 069 Ulm, Germany
a r t i c l e i n f o
Article history:
Received 14 July 2009
Accepted for publication 27 August 2009
Available online 2 September 2009
Keywords:
Iridium
Faceting
Nitric oxide
Oxygen
Adsorption
Decomposition
a b s t r a c t
We report on the adsorption and decomposition of NO on O-covered planar Ir(2 1 0) and nanofaceted
Ir(2 1 0) with variable facet sizes investigated using temperature programmed desorption (TPD),
high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT). When
pre-covered with up to 0.5 ML O, both planar and faceted Ir(2 1 0) exhibit unexpectedly high reactivity
for NO decomposition. Upon increasing the oxygen coverage to 0.7 ML O, planar Ir(2 1 0) has little activity
while faceted Ir(2 1 0) still remains active toward NO decomposition, although NO decomposition is com-
pletely inhibited when both surfaces are pre-covered by 1 ML O. NO molecularly adsorbs on O-covered Ir
at 300 K. At low NO and oxygen coverage, NO adsorbs on the atop sites of planar Ir(2 1 0) while on the
bridge and atop sites of faceted Ir(2 1 0) composed of (1 1 0) and {3 1 1} faces. No evidence for size effects
in the decomposition of NO on O-covered faceted Ir(2 1 0) is observed for average facet size in the range
5–14 nm. Our findings should be of importance for development of Ir-based catalysts for NO decompo-
sition under oxygen-rich conditions.
? 2009 Elsevier B.V. All rights reserved.
1. Introduction
Catalytic conversion of NO to N2under oxygen-rich conditions
has received increasing attention due to the challenge for future
new exhaust catalysts needed for current diesel and lean-burn en-
gines [1–6], which operate under high oxygen concentrations that
poison most platinum group metal catalysts. Of particular interest
is the selective catalytic reduction (SCR) of NO by hydrocarbons
(HCs) in the presence of excess oxygen, which is believed to be
the most promising approach for the removal of NO [7]. It has been
shown that Pt- and Ir-based catalysts are the most active among
noble metals for SCR of NO by HCs [4] and Ir is more active and
selective than Pt at higher temperatures [3]. However, the SCR of
NO by HCs leads to the formation of secondary pollutants such
as oxygenated HCs, CO and N2O. The direct decomposition of NO
without using any reductant can avoid these secondary pollutants
with the exception of N2O, which is probably the most attractive
solution in pollution control [7].
The adsorption and decomposition of NO in the presence of pre-
adsorbed oxygen have been studied both experimentally and the-
oretically on many metal surfaces such as Pd [8], Pt [9,10], Rh
[11–14], Ru [15], Ir [16,17], and Ni [18,19]. In general, with pre-ad-
sorbed oxygen the surface becomes less active for NO dissociation.
The reduced reactivity of the surface is most likely governed by the
reduced degree of backbonding between the local d-band of the
metal and the 2p* antibonding orbital of NO due to electrons with-
drawn from the metal by pre-adsorbed oxygen [14,18]. As the pre-
adsorbed oxygen coverage increases, the degree of backbonding
and thus electron density in the 2p* antibonding orbital of NO re-
duces, leading to a higher activation barrier for NO dissociation
[14,18]. Nevertheless, when comparing the activation barrier for
NO dissociation on Rh with different surface structures pre-cov-
ered with oxygen, it has been found that steric effects exceed elec-
tronic effects [14], i.e. the local geometrical structure of the surface
plays a key role in determining the activation barrier for NO
dissociation.
An industrial catalyst is typically in the form of small metal par-
ticles on the nanometer scale highly dispersed over an oxide sup-
port, which usually has wide distribution of the metal particle
sizes and shapes. In studies of supported metal catalysts, it has
been shown that metallic particle size, shape and structure as well
as support all influence their catalytic performance. In order to
achieve a fundamental understanding of the reaction mechanism,
planar surfaces of metal single crystals or metallic nanoclusters
grown on such surfaces are often used as model systems. Following
0039-6028/$ - see front matter ? 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2009.08.027
* Corresponding author.
E-mail address: wchen@physics.rutgers.edu (W. Chen).
1In memory of Prof. Theodore E. Madey who passed away on July 27, 2008 at the
age of 70.
Surface Science 603 (2009) 3136–3144
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this approach, we use metallic single crystals to reproducibly pre-
pare in situ model surfaces with different surface morphologies,
ranging from planar surfaces to nanometer-scale faceted surfaces
with well-defined structure and controlled size, free of any support
material. The faceted surfaces serve as model catalysts to bridge
the gap between the planar surfaces of metal single crystals and
supported metal nanoparticles.
In this work, an Ir(2 1 0) crystal was used to prepare planar or
faceted Ir(2 1 0) with tunable facet size between 5 and 14 nm,
which then were used to investigate the effects of pre-adsorbed
oxygen on adsorption and decomposition of NO. Planar Ir(2 1 0)
is an atomically rough surface with four layers exposed [20], while
faceted Ir(2 1 0) is composed of three-sided nanoscale pyramids
exposing one (1 1 0) face and two {3 1 1} faces on each pyramid
[21,22]. The fabrication of clean planar and faceted Ir(2 1 0) en-
ables exploration of structure sensitivity and size effects in NO
decomposition on O-covered unsupported Ir. The well-defined
crystallographic orientation of the facets allows for detailed exper-
imental and theoretical characterization of not only planar Ir(2 1 0)
but also faceted Ir(2 1 0). A combination of temperature pro-
grammed desorption (TPD), high resolution electron loss spectros-
copy (HREELS) and density functional theory (DFT) was used in the
present study.
We have recently demonstrated that clean planar and faceted
Ir(2 1 0) are very active and selective to N2formation from NO
decomposition, with faceted Ir(2 1 0) being more active and selec-
tive at higher NO coverage [23]. Here, we report our findings that
planar and faceted Ir(2 1 0) show unexpectedly high reactivity for
NO decomposition in the presence of pre-adsorbed oxygen. In par-
ticular, faceted Ir(2 1 0) exhibits unusually high reactivity for NO
decomposition at high fractional oxygen coverage. In addition,
we provide evidence for structure sensitivity in adsorption sites
and thermal decomposition of NO on O-covered faceted Ir(2 1 0)
versus O-covered planar Ir(2 1 0).
2. Experimental and theoretical procedures
TPD and HREELS measurements were conducted in two sepa-
rate ultra-high vacuum (UHV) systems located at Rutgers Univer-
sity and University of Delaware, respectively [23]. Both UHV
systems contain Auger electron spectroscopy (AES), low-energy
electron diffraction (LEED), and a quadruple mass spectrometer
(QMS). All TPD spectra were acquired at a sample heating rate of
?5 K/s. The HREELS spectrometer (LK 3000) was operated at elec-
tron energy of 6.0 eV with a typical resolution between 40 and
60 cm?1. All HREELS spectra were measured in the specular direc-
tion at an angle of 60? with the sample held at 130–150 K.
In both TPD and HREELS experiments, the same Ir(2 1 0) crystal
was used but different cleaning procedures were applied due to
different sample mounting, which give the same planar and fac-
eted Ir(2 1 0) surfaces [23]. At Rutgers University, clean planar
Ir(2 1 0) was prepared by cycles of flashing the sample to 1700 K
in O2(5 ? 10?8Torr) followed by flashing to 1700 K in UHV. Clean
faceted Ir(2 1 0) was generated through two steps. In the first step,
oxygen-covered faceted Ir(2 1 0) was prepared by annealing clean
planar Ir(2 1 0) in O2(5 ? 10?8Torr) at 600–1700 K and subse-
quent cooling in O2to 300 K; the average facet size from 5 nm to
14 nm was controlled by the annealing temperature [22]. In the
second step, clean faceted Ir(2 1 0) was generated via a reaction
with H2(5 ? 10?9Torr) at 400 K to remove surface oxygen while
facets retained their original structure and size. At University of
Delaware, clean planar Ir(2 1 0) was prepared by cycles of Ne+
sputtering (3 KV and 8–10 lA) and annealing at 700 K in O2
(5 ? 10?8Torr) to remove surface carbon contamination followed
by annealing at 400 K in H2(1 ? 10?8Torr) and heating to 700 K
in UHV to remove surface oxygen and relax faceted Ir(2 1 0) to pla-
nar Ir(2 1 0). Clean faceted Ir(2 1 0) was generated by annealing
clean planar Ir(2 1 0) in O2(5 ? 10?8Torr) at 600 K for 2 min and
subsequent cooling in O2to 300 K to form oxygen-covered faceted
Ir(2 1 0), which was followed by a reaction with H2(1 ? 10?8Torr)
at 400 K to remove surface oxygen. This gave rise to clean faceted
Ir(2 1 0) with an average facet size of 5 nm [22]. Surface cleanliness
was checked using AES and TPD while surface structure was mon-
itored by LEED.
In all experiments,15NO was used to distinguish15N2from CO
and
(15NO), hydrogen (H2) and oxygen (O2) are of research purity and
were used without further purification; all gases were dosed onto
the Ir surfaces at 300 K by backfilling the chambers. NO and oxygen
exposures were reported in Langmuir (1 L = 10?6Torr s) and
uncorrected for ion gauge sensitivity. Oxygen coverage expressed
in monolayer (ML) for TPD spectra was determined from a oxygen
uptake curve obtained on the basis of the integrated area under the
TPD spectra of O2on Ir(2 1 0) following adsorption at 300 K [21,24],
where 1 ML O is defined to be the saturation coverage at ?80 L O2
dosed on Ir(2 1 0) at 300 K.
To determine energetically favorable binding sites of NO and O
on the Ir surfaces for coadsorbed NO and O adlayers, the binding
energies of (NO + O) on Ir(2 1 0), Ir(3 1 1) and Ir(1 1 0) with varying
adsorption sites of NO and O were calculated by DFT using the CA-
STEP code [25]. Vanderbilt-type ultrasoft pseudopotentials [26]
were applied together with the generalized gradient approxima-
tion (GGA) exchange-correlation functional proposed by Perdew,
Burke, and Ernzerhof (PBE) [27]. The Ir(2 1 0), Ir(3 1 1) and
Ir(1 1 0) surfaces were represented by 16-layer, 11-layer and 12-
layer slabs separated by ?12 Å of vacuum, respectively. The bot-
tom three layers for Ir(2 1 0) and Ir(3 1 1) and the bottom four lay-
ers for Ir(1 1 0) were fixed at the calculated bulk structures, while
the geometries of the remaining layers and the adsorbates were al-
lowed to fully optimize. For all coadsorbed systems of NO and O, a
cutoff energy of 340 eV was used and the Brillouin zones of the
(1 ? 1) unit cells of Ir(2 1 0), Ir(3 1 1) and Ir(1 1 0) were sampled
with 10 ? 8, 14 ? 8, and 14 ? 10 Monkhorst-Pack k-point meshes
[28], respectively.
15N2O from CO2 in the TPD measurements. Nitric oxide
3. Results
3.1. TPD Study
Before presenting our TPD spectra from NO on O-covered planar
and faceted Ir(2 1 0), we briefly summarize our results of TPD mea-
surements for NO on clean Ir surfaces that are relevant to this
work. The detailed description of adsorption and decomposition
of NO on clean planar and faceted Ir(2 1 0) have been published
elsewhere [23].
At low NO exposure, adsorbed NO undergoes complete
decomposition yielding only N2as N-containing species on clean
planar (<3 L) and faceted (63 L) surfaces of Ir(2 1 0). When both
surfaces are saturated by 40 L NO, the dominant desorbing prod-
ucts are large amounts of N2 and small amounts of NO with
traces of N2O from planar Ir(2 1 0) but no formation of N2O from
faceted Ir(2 1 0). For all NO exposures studied, the formation of
NO2 is not observed and both surfaces are covered by oxygen
after TPD which completely desorbs from the surfaces at
?1600 K. Although there are notable differences in the N2 TPD
spectral profiles between the two surfaces, the N2peak temper-
atures from the two surfaces are almost the same. Only a single
N2peak is observed on the two surfaces with peak temperatures
below 560 K.
In the current work, the products monitored during TPD for NO
on O-covered planar and faceted Ir(2 1 0) are N2, NO, N2O and NO2,
where clean Ir surfaces are first pre-dosed with oxygen and then
W. Chen et al./Surface Science 603 (2009) 3136–3144
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exposed to NO prior to heating. For all NO and oxygen exposures
studied, no desorption of NO2is detected and both surfaces are
covered with oxygen after TPD which desorbs totally from the sur-
faces at ?1600 K. The formation of N2signifies direct decomposi-
tion of NO on O-covered planar and faceted Ir(2 1 0).
3.1.1. NO on O-covered planar Ir(2 1 0)
Fig. 1 shows a series of TPD spectra of N2, NO and N2O from var-
ious exposures of NO on planar Ir(2 1 0) pre-dosed to 1 L O2
(0.5 ML O). No desorption of N2O is observed, although traces of
N2O are seen for NO on clean planar Ir(2 1 0) at P3 L NO [23], indi-
cating that the formation of N2O is suppressed in the presence of
pre-adsorbed oxygen. For 0.5 L NO, all adsorbed NO decomposes
to produce only N2with peak temperature below 550 K. For 1 L
NO, a new N2feature appears as a small shoulder above 600 K to-
gether with traces of NO desorption, in addition to the original N2
peak below 550 K. For NO exposures P2 L, the original N2peak be-
low 550 K is dramatically attenuated and the new N2feature above
600 K becomes a dominant peak. Simultaneously, a large amount
of NO desorbs from the surface, which scales up with NO exposure.
Comparison with the TPD spectra from NO on clean planar Ir(2 1 0)
[23] leads to the conclusion that the ratio of NO desorption to N2
desorption here is higher than that on the clean Ir surface [23]
for the same NO exposure, which implies that the presence of
pre-adsorbed oxygen reduces the surface reactivity for NO decom-
position. Notably, the N2peak above 600 K in Fig. 1 is not present
on clean planar Ir(2 1 0) where N2desorbs in a single peak with
peak temperature below 560 K [23].
To further examine the effects of pre-adsorbed oxygen on NO
decomposition, we have varied oxygen exposure while keeping
NO exposure constant. Fig. 2a shows TPD spectra of N2and NO
from 1 L NO on planar Ir(2 1 0) pre-covered with various amounts
of oxygen. We note that no desorption of N2O is seen. In the ab-
sence of pre-adsorbed oxygen, adsorbed NO decomposes totally
to evolve only N2with peak temperature below 550 K. When the
surface is pre-covered by 0.3 ML O, NO also totally decomposes
to produce only N2and the position of the N2peak (P1) remains
the same as that on the clean surface. As oxygen coverage in-
creases, the intensity of the original N2peak (P1) is attenuated
gradually and a new N2feature develops on the high temperature
side (initially a shoulder and finally a peak ‘‘P2” with peak temper-
ature located above 600 K), which is accompanied by NO desorp-
tion that increases with oxygen coverage. When the surface is
pre-covered with 1 ML O, no formation of N2is detectable and all
adsorbed NO desorbs without dissociation; NO decomposition is
completely suppressed. As NO exposure increases to 2 L (Fig. 2b),
the intensity of the N2peak on the clean surface increases com-
pared to 1 L NO: the integrated N2area from 2 L NO is about twice
of that from 1 L NO. With pre-adsorbed oxygen, a similar new N2
feature on the high temperature side (initially a shoulder and final-
ly a peak) is also observed, which is visible at 0.3 ML O together
with a reduction of the original N2peak and emergence of NO
desorption. It is noted that the shoulder of the new N2feature here
appears at lower oxygen coverage compared to that for 1 L NO in
Fig. 2a. At 0.7 ML O, only a small amount of adsorbed NO decom-
poses and adsorbed NO mainly desorbs molecularly.
3.1.2. NO on O-covered faceted Ir(2 1 0)
TPD spectra of N2and NO from different exposures of NO on fac-
eted Ir(2 1 0) (with average facet size ?14 nm) pre-dosed to 1 L O2
(0.5 ML O) are presented in Fig. 3. No evidence for the formation of
N2O is found. For NO exposures 62 L, adsorbed NO entirely decom-
poses to form N2. For NO exposure P3 L, NO desorption is visible
which increases with NO exposure while the desorption of N2pro-
gressively broadens but no peak above 600 K develops, in contrast
300 400 500 600 700 800 900
Temperature (K)
0.5ML O
N2
1.0
0.5
2.0
3.0
10.0
NO exposure (L)
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
0.5ML O
NO
2.0
1.0
0.5
3.0
10.0
NO exposure (L)
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
0.5ML O
NO x 10
0.5
1.0
NO exposure (L)
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
0.5ML O
0.5
1.0
2.0
3.0
10.0
NO exposure (L)
N2O x10
TPD Signal (a. u.)
Fig. 1. TPD spectra of N2, NO and N2O from different exposures of NO on planar Ir(2 1 0) pre-dosed to 1 L O2(0.5 ML O).
3138
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to NO on 0.5 ML O-covered planar Ir(2 1 0) shown in Fig. 2a where
a new peak grows above 600 K.
Fig. 4 displays TPD spectra of N2and NO from 2 L NO on faceted
Ir(2 1 0) as a function of pre-adsorbed oxygen coverage. No evi-
dence for the formation of N2O is found. In the absence of pre-ad-
sorbed oxygen, only a narrow intense N2peak is seen, indicative of
complete decomposition of NO. When the surface is pre-covered
by 0.5 ML O, NO also completely decomposes to give rise to only
N2 and the position of the N2 peak (F1) remains the same. At
0.65 ML O, a new N2feature develops as a shoulder above 600 K to-
gether with the reduction of the original N2peak (F1), which is
accompanied by NO desorption. Further increasing oxygen cover-
age to 0.7 ML O, the original N2 peak (F1) diminishes and the
new N2feature becomes an evident peak (F2) and NO desorption
further increases. When the surface is pre-covered with 1 ML O,
adsorbed NO desorbs without dissociation; NO decomposition is
entirely inhibited.
Fig. 5 compares the TPD spectra of N2and NO from 2 L NO on O-
covered faceted Ir(2 1 0) (with average facet size ?14 nm) with
those on O-covered planar Ir(2 1 0). At 0.3 ML O, both surfaces ex-
hibit high activity in NO decomposition. N2desorbs in a single peak
(F1) from faceted Ir whereas N2appears in two peaks (P1 and P2)
from planar Ir. No desorption of NO from faceted Ir is observed,
indicating complete decomposition of NO although small amount
of NO desorbs from planar Ir. At 0.5 ML O, both surfaces are still ac-
tive for NO decomposition, which is in contrast to Ir(1 0 0) on
which 0.5 ML O completely suppresses NO decomposition [17].
Moreover, the intensity of the N2peak ‘‘P1” drastically reduces
and the intensity of the N2peak ‘‘P2” increases while the peak tem-
perature of the N2feature from the faceted Ir (F1) does not change,
compared to 0.3 ML O. Although NO desorption from planar Ir
increases, there is still no desorption of NO from faceted Ir, i.e.
NO dissociates entirely on faceted Ir. At 0.7 ML O, planar Ir has little
activity whereas faceted Ir still remains active in NO decomposi-
tion. This is evidenced by dominant NO desorption with traces of
N2desorption from planar Ir but much more N2desorption (com-
parable to NO desorption) from faceted Ir. The unusually high reac-
tivity of faceted Ir for NO decomposition at such high fractional
oxygen coverage is a surprising result. Notably, the original N2
peak (F1) disappears and a new N2peak (F2) on the high temper-
ature side with peak temperature above 600 K develops together
with NO desorption from faceted Ir. The striking differences in
desorption of N2and NO between O-covered planar and faceted
Ir(2 1 0) provide evidence for strong structure sensitivity in NO
decomposition with faceted Ir(2 1 0) being more active than planar
Ir(2 1 0). It is worth mentioning that previous TPD measurements
from CO on O-covered planar and faceted Ir(2 1 0) have indicated
that planar Ir(2 1 0) exhibits higher reactivity for oxidation of CO
than faceted Ir(2 1 0) [24].
To search for facet size effects in NO decomposition on O-cov-
ered faceted Ir(2 1 0), we have measured TPD spectra from NO on
O-covered faceted Ir(2 1 0) with different facet sizes (5 and
14 nm). No formation of N2O is observed from the two faceted sur-
faces, indicating no change in selectivity to N2formation for NO
decomposition. The TPD spectra of N2and NO from the two faceted
surfaces appear to be very similar in terms of spectra profile, peak
position and peak intensity (spectra not shown), implying no
change in reactivity for NO decomposition either. All these similar-
ities indicate that on the O-covered faceted Ir(2 1 0) NO decompo-
sition is insensitive to the facet size in NO decomposition, similar
to oxidation of CO on O-covered faceted Ir(2 1 0) [24]. The absence
of facet size effects in NO decomposition over O-covered faceted
Ir(2 1 0) containing (1 1 0) and {3 1 1} facets suggests that the edge
and corner atoms between the boundaries of the facets, which
increase in concentration as facet size decreases, are not as signif-
icant for this process as are the planar microfacets on the facet
300 400 500 600 700 800 900
Temperature (K)
a
P2
P1
N2
1L NO
0.0
0.3
0.5
0.6
0.65
1.0
oxygen coverage (ML)
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
1L NO
NO
0.0
0.3
0.5
0.6
0.65
1.0
oxygen coverage (ML)
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
2L NO
0.7ML O
b
N2
0.5ML O
0.3ML O
0.0ML O
TPD Signal (a. u.)
300 400 500 600 700 800 900
Temperature (K)
0.7
2L NO
NO
0.3
0.0
0.5
oxygen
coverage (ML)
TPD Signal (a. u.)
Fig. 2. TPD spectra of N2and NO from NO on planar Ir(2 1 0) pre-covered with various amount of oxygen. (a) 1 L NO; (b) 2 L NO.
W. Chen et al./Surface Science 603 (2009) 3136–3144
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planes, where {3 1 1} consist of both (1 1 1) and (1 0 0) microfacets
[29].
3.2. HREELS Study
Fig. 6a shows the HREEL spectrum from adsorption of 2 L NO on
planar Ir(2 1 0) pre-dosed to 1 L O2at 300 K. Only a single N–O
stretching loss feature, m(N–O), is observed at 1790 cm?1. This N–
O stretching frequency is assigned to the N–O stretching vibration
for NO on the atop sites of Ir(2 1 0) based on a comparison with the
characteristic N–O stretching feature of atop bound NO on clean Ir
and other meal surfaces [23]. The assignment is also supported by
our DFT calculations, which will be described in Section 3.3. The
frequency of m(N–O) is slightly higher than that on the clean sur-
face [23], indicative of slight N–O bond strengthening induced by
the presence of pre-adsorbed oxygen. Fig. 6b shows the HREEL
spectrum from adsorption of 2 L NO on faceted Ir(2 1 0) pre-dosed
to 1 L O2at 300 K. Two N–O stretching loss features,m1(N–O) andm2-
(N–O), at 1600 cm?1and 1770 cm?1, respectively, are observed in
contrast to that on the planar surface, indicating structure sensitiv-
ity in adsorption sites of NO. These two N–O stretching
modes,m1(N–O) and m2(N–O), on faceted Ir(2 1 0) are assigned to
the stretching vibration of NO on bridge and atop sites of
Ir(1 1 0) and {3 1 1} faces on faceted Ir(2 1 0), respectively. The
assignments are based on a comparison with the characteristic
N–O stretching features of bridge and atop bound NO on clean Ir
and other meal surfaces [23]. The assignments are also supported
by our DFT calculations, which will be described in Section 3.3.
The frequencies of m1(N–O) and m2(N–O) are slightly higher than
those on the clean surface [23], indicating slight N–O bond
strengthening induced by pre-adsorbed oxygen. The vibrational
feature for CO (?2000 cm?1) is due to background gas adsorption.
3.3. DFT study
First, we briefly summarize our DFT investigations on adsorp-
tion sites of O and NO on Ir, respectively, which have been pub-
lished elsewhere [23,30]. Fig. 7 shows the possible adsorption
sites for adsorbate on Ir(2 1 0), Ir(1 1 0) and Ir(3 1 1), respectively.
In Fig. 8a and b, we have labeled the energetically favorable bind-
ing sites of O (red2circles) and NO (blue hexagons) on the different
Ir surfaces based on an adsorbate coverage of 1 GML O and 1 GML
NO, respectively, where 1 GML refers to one geometrical monolayer
and is defined as one O or NO per (1x1) surface unit cell. The corre-
sponding binding energies (BEs) are summarized in Table 1. In the
case of O on Ir (Fig. 8a), O prefers to bind on atop (T), bridge (D
and C) and hollow (B) sites of Ir(2 1 0) with similar binding energies
(1.74–1.78 eV) but only on bridge sites of Ir(1 1 0) (D) and Ir(3 1 1)
(A) [30]. As for NO on Ir (Fig. 8b), NO binds most strongly on the atop
(T) sites of Ir(2 1 0) while on both atop and bridge sites of Ir(1 1 0)
(T and D) and Ir(3 1 1) (A and T) [23].
DFT calculations on coadsorption of (NO + O) on Ir(2 1 0),
Ir(1 1 0) and Ir(3 1 1) have been performed for one NO molecule
-9
0.5ML O
0.5
900
1.0
2.0
3.0
6.0
10.0
NO exposure (L)
N2
TPD Signal (a. u.)
Temperature (K)
-9
-8
3.0
2.0
0.5
900
1.0
6.0
10.0
NO exposure (L)
0.5ML O
NO
TPD Signal (a. u.)
Temperature (K)
300400500 600700800
300400500600700800
Fig. 3. TPD spectra of N2, NO and N2O from different exposures of NO on faceted
Ir(2 1 0) pre-dosed to 1 L O2(0.5 ML O).
300400500
Temperature (K)
600700800900
-9
-8
1.0
2L NO
N2
F2
F1
0.0
0.5
0.65
0.7
oxygen coverage (ML)
TPD Signal (a. u.)
300400500
Temperature (K)
600700800900
1.0
2L NO
0.0
0.5
0.65
0.7
oxygen
coverage (ML)
NO
TPD Signal (a. u.)
Fig. 4. TPD spectra of N2and NO from 2 L NO on faceted Ir(2 1 0) pre-covered with
different amount of oxygen.
2For interpretation of color in Fig. 8, the reader is referred to the web version of
this article.
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W. Chen et al./Surface Science 603 (2009) 3136–3144