Comparative study of CO2 formation in CO oxidation by O2, NO and N2O on Pd(1 1 0) surface using infrared chemiluminescence

Article abstract of DOI:10.1016/j.susc.2006.01.148

The infrared (IR) chemiluminescence spectra of CO₂ were measured during steady-state CO oxidation by O₂, NO and N₂O over Pd(1 1 0) surface. Kinetics of these reactions were studied using a molecular-beam reaction system, a

Full text of DOI:10.1016/j.susc.2006.01.148

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Surface Science 600 (2006) 4221–4227
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Comparative study of CO2 formation in CO oxidation by O2, NO
and N2O on Pd(110) surface using infrared chemiluminescence
*
Kenji Nakao, Shin-ichi Ito, Keiichi Tomishige, Kimio Kunimori
Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
Received 11 August 2005; accepted for publication 18 January 2006
Available online 19 May 2006
Abstract
The infrared (IR) chemiluminescence spectra of CO2 were measured during steady-state CO oxidation by O2, NO and N2O over
Pd(110) surface. Kinetics of these reactions were studied using a molecular-beam reaction system, and the order of the activity was
as follows: CO + O2 > CO + NO > CO + N2O. From the analysis of the IR emission spectra, it is found that the antisymmetric vibra-
tional mode of CO2 was more excited in the case of the CO + O2 and CO + N2O reactions, in contrast, the bending vibrational mode was
more excited in the case of the CO + NO reaction. These results suggest that the activated complex of CO2 in the CO + NO reaction has
more bent structure than those in the CO + O2 and CO + N2O reactions. The reason of this different behavior of the CO + NO reaction
is discussed based on higher structure-sensitivity of the CO + NO reaction than the CO + O2 and CO + N2O reactions.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: IR chemiluminescence; Carbon dioxide; Carbon monoxide; Nitric oxide; Nitrous oxide; Palladium
1. Introduction
and the structure of the activated complex can be discussed
[7–13].
In order to elucidate heterogeneous catalysis, various
physicochemical methods have been attempted, and one
of the conventional methods is spectroscopic observation
of reaction intermediates [1–3]. Another approach is an
analysis of the products desorbed from the catalyst surface
in terms of translational and internal energies [4–15]. The
investigation of the internal energies, i.e., vibrational and
rotational states, can be useful tools [4–13]. This is because
the energy states of desorbed molecules can be related to a
transition state, where the surface species interacts with the
surface and the activated complex is formed. When des-
orbed molecules are excited vibrationally, infrared chemi-
luminescence (IR emission) can be observed during the
relaxation [4–13]. Based on the IR emission spectra, the
vibrational state of the desorbed molecules can be given,
Our group has reported IR chemiluminescence of CO2
from steady-state CO + O2 and CO + NO reactions on sin-
gle crystal Pd surfaces combined with kinetic results [8–13].
It has been suggested that the activated complex of CO2
formation (i.e., the transition state of CO2 formation from
CO(a) + O(a)) had more bent structure on Pd(111) and
relatively linear structure on Pd(110), since the bending
mode of CO2 from Pd(111) was more vibrationally excited
than that of CO2 from Pd(110) [8,9,12], and this can be
interpreted by the plane surface structure of Pd(111).
The results also indicate that the IR chemiluminescence
method can provide a direct energetic evidence of the
reaction mechanism and the activated complex of CO2
formation.
In this article, the vibrational states of CO2 from
CO + NO and CO + N2O reactions on Pd(110) were
investigated, and they are compared to those from
CO + O2 reaction. It is found that the vibrationally excited
states of CO2 from the CO + NO reaction were signifi-
*
Corresponding author. Tel.: +81 29 853 5026; fax: +81 29 855 7440.
E-mail address: kunimori@ims.tsukuba.ac.jp (K. Kunimori).
0039-6028/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2006.01.148

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K. Nakao et al. / Surface Science 600 (2006) 4221–4227
cantly different form those from the CO + O2 and
CO + N2O reactions at a high surface temperature
(750 K). The detailed analysis was carried out here, and
the interpretation in terms of the structure of the activated
complex and the reaction mechanism is discussed.
reaction. The mass signal intensity was calibrated. When
the 13CO pulse was introduced at 0 s, the signal of
m/e = 29 due to 13CO increased, and at the same time,
the signal of m/e = 45 due to 13CO2 increased. Assuming
that the reaction rate of 13CO + N2O is the same as that
of 12CO + N2O, the formation rate of CO2 was estimated.
In addition, under these reaction conditions, the signal of
m/e = 32 due to O2 was comparable to the background
level, and this indicates that O2 was not formed in the
CO + N2O reaction. Steady-state CO oxidation by O2,
NO and N2O (pressure range at the flux conditions =
10ꢀ3–10ꢀ2 Torr) was performed at temperatures of 400–
850 K. Another UHV chamber (base pressure < 2.0 ·
10ꢀ10 Torr) was used to prepare and characterize the clean
Pd(110) surface. It was equipped with a molecular-
beam reaction system, an Ar+ ion gun, low energy electron
diffraction (LEED), and a QMS. Before the molecular-
beam reaction, the Pd(110) surface was cleaned using a
standard procedure (O2 treatment, Ar+ bombardment
and annealing) [8–13]. After cleaning, the sharp (1 · 1)
LEED pattern was observed, and the reaction occurs
on the (1 · 1) structure under steady-state condition
[14].
The IR emission spectra of the CO2 molecules desorbed
from the surface were measured with 4 cmꢀ1 resolution.
At that low resolution (4 cmꢀ1 resolution), no individual
vibration–rotation lines were resolved. The IR emission
spectra were analyzed based on simulation of model spec-
tra [5,9,11]. The average vibrational Boltzmann tempera-
ture (TAVV: an average temperature of the antisymmetric
stretch, symmetric stretch and bending modes), which
was calculated from the degree of the red-shift from the
fundamental band (2349 cmꢀ1) [5,7–13]. The emission
intensity is related to the extent of excitation in the anti-
symmetric stretch of CO2 [10–13]. Therefore, the antisym-
metric vibrational temperature (T AVS) can be estimated
from the normalized emission intensity [10–13]. Based on
TAVS and TAVV, it is possible to deduce the bending vibra-
tional temperature (T BV). The relation between T AVV and
respective vibrational temperature is represented as
2. Experimental
A molecular-beam reaction system, in combination with
a FT-IR spectrometer (InSb detector Nexus670; Thermo
Electron Corp.), was used to measure IR emissions of
product CO2 molecules just desorbed from metal surfaces
during catalytic reactions [8–13]. A UHV chamber (base
pressure < 1.0 · 10ꢀ9 Torr) was equipped with a CaF2 lens,
which collected IR emission, an Ar+ ion gun for sample
cleaning, and a quadrupole mass spectrometer (QMS,
QME200; Pfeiffer Vacuum Technology AG) with a differ-
ential pumping system. Two free-jet molecular-beam noz-
zles (0.1-mm-diameter orifice) supplied the reactant gases
[7]. The reactant fluxes were controlled using mass flow
controllers. The CO flux was fixed at 4.1 · 1018 cmꢀ2 sꢀ1
,
and the O2, NO and N2O fluxes were 0.41–12 · 1018,
2.0–12 · 1018 and 2.0–12 · 1018 cmꢀ2 sꢀ1, respectively. The
reactant gases (CO/O2 = 0.3–10, CO/NO = 0.3–2, or CO/
N2O = 0.3–1) were exposed to Pd(110) surface. In the
CO + N2O reaction, the formation rate of CO2 cannot be
measured by the QMS because of the overlap of mass frag-
ments at m/e = 44 of N2O and CO2. Therefore, we used la-
beled 13CO, which obtained from ISOTEC INC. Pulses of
13CO gas (2.5 · 1020 molecules, 10 ml at atmospheric pres-
sure) were introduced to 12CO reactant gas under steady-
state reaction conditions. The ratio of (13CO + 12CO)/
N2O was constant during the 13CO pulse. Fig. 1 shows
an example of the mass signal profiles in the case of
13CO pulse introduction to the steady-state 12CO + N2O
N2O and 12CO2
(m/e=44)
5× 10-10
A
TVAV ¼ ðT AVS þ TSVS þ 2TVBÞ=4;
ð1Þ
where 2TBV corresponds to the degeneration of the bending
vibration. Assuming that TBV is equal to T VSS because
of the Fermi resonance [4,6], TBV is expected to be
ð4TAVV ꢀ TAVSÞ=3. This assumption is plausible on the basis
13CO (
m
/e=29)
13CO2 (
m
/
e
=45)
× 100
× 100
of previous reports [4,5]. It should be added that TVAV
,
TAVS and TBV were used here as parameters to characterize
the extent of the vibrational excitation of the product
CO2. It took about 15–90 min for the measurement of
the IR emission spectra with 1000–6000 scans. The stable
steady-state activity was obtained during the measurement.
O2 (m/e
=32)
0
100
200
300
400
Time /sec
3. Results and discussion
Fig. 1. Mass signal intensity as a function of time during steady-state
CO + N2O reaction with 13CO pulse. The surface temperature (TS) was
750 K. The total flux of reactants (CO + N2O) was 6.2 · 1018 cmꢀ2 sꢀ1 at
the CO/N2O = 0.5.
Fig. 2 shows the rate of CO2 formation in the steady-
state CO oxidation by O2, NO and N2O on Pd(110) as a

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K. Nakao et al. / Surface Science 600 (2006) 4221–4227
4223
50
40
30
20
10
0
80
60
40
20
0
CO = 4.1× 1018 cm-2 s-1
: O2 (× 0.5)
: NO
: N2O
TS=750 K
: O2
: NO
: N2O
400 500 600 700 800 900
Surface temperature [ S] /K
0
5
10
15
Oxidant flux /× 1018 cm-2 sec-1
T
(a)
Fig. 2. The formation rate of CO2 during CO oxidation by O2, NO and
N2O on Pd(110). The total flux of reactants of (CO + O2) and (CO + NO)
was 4.1 · 1018 cmꢀ2 sꢀ1 at the CO/O2 and CO/NO = 1, and that of
(CO + N2O) was 6.2 · 1018 cmꢀ2 sꢀ1 at the CO/N2O = 0.5.
CO=4.1× 1018 cm-2 s-1
TS=750 K
10-2
10-3
10-4
: O2
: NO
: N2O
θ
function of surface temperature (TS). The CO + O2 reac-
tion proceeded at temperatures greater than 450 K. The
surface temperature dependence of the formation rate
showed a maximum on the Pd(110) surface. The behavior
agrees well with the general Langmuir–Hinshelwood (LH)
kinetics of CO + O2 reaction on Pd surfaces [2,3,8,12,16].
The temperature at which the highest activity was obtained
is denoted as TSmax. At temperatures lower than TmS ax, the
surface coverage of CO is known to be high. The rate-
determining step is O2 adsorption on the vacant site, which
is formed by the desorption of CO(a). At temperatures
higher than TmS ax, the formation rate of CO2 decreased
gradually with increasing surface temperature. This behav-
ior is attributable to the decreased CO coverage. The
CO + NO reaction proceeded above 550 K, and the tem-
perature dependence of the formation rate had also a max-
imum. The CO + N2O reaction also proceeded above
550 K and the temperature dependence of the formation
rate also had a maximum. It is shown that the reaction rate
was much dependent on the oxidizing agents, and the order
is as follows: O2 > NO > N2O.
Fig. 3(a) shows the rate of CO2 formation in the steady-
state CO oxidation by O2, NO and N2O on Pd(110) as a
function of the oxidant flux under the fixed CO flux at
TS = 750 K. In the CO + O2 reaction, the rate of CO2
formation increased with increasing O2 flux in the low
flux range (<2.0 · 1018 cmꢀ2 sꢀ1), in contrast, the rate
decreased with increasing O2 flux in the high flux range
(>2.0 · 1018 cmꢀ2 sꢀ1). This suggests that the kinetics
of CO + O2 reaction changes around O2 flux = 2.0 ·
1018 cmꢀ2 sꢀ1. The rate of both CO + NO and CO + N2O
reactions increased almost proportionally with increasing
the oxidant flux. In order to understand why the behavior
of each reaction over Pd(110) surface is strongly depen-
dent on the kind of oxidizing agents, we calculated the
0
5
10
15
Oxidant flux /× 1018 cm-2 sec-1
(b)
100
CO=4.1× 1018 cm-2 s-1
TS=750 K
: O2
: NO
: N2O
θ
10-1
10-2
10-3
0
5
10
15
Oxidant flux /× 1018 cm-2 sec-1
(c)
Fig. 3. (a) The formation rate of CO2 during CO oxidation by O2, NO
and N2O on Pd(110) as a function of the oxidant flux under the fixed
CO flux at TS = 750 K. (b,c) Oxidant flux dependence of (b) CO and (c)
oxygen coverages.
coverages of adsorbate species during the CO + O2 and
CO + NO reactions on the basis of a reaction model as
shown in the reference [13].

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K. Nakao et al. / Surface Science 600 (2006) 4221–4227
The estimated coverages (hCO and hO) are plotted in
sticking probability at high temperatures in the previous re-
port [19]. Furthermore, the elementary steps (2) and (3) are
considered to be more rapid than the reaction rate, and the
adsorption of CO and N2O reaches equilibrium. In order
to estimate the coverages of CO and oxygen, kinetic
parameters with respect to N2O(a) dissociation are neces-
sary. However, these kinetic parameters have not been re-
ported, and they are not available at present. Therefore,
we estimated the coverages in a manner different from
the case of CO + O2 and CO + NO reactions. Since the
Fig. 3(b) and (c). For the CO oxidation by O2, the hO in-
creased sharply in the low flux range, and it was almost
constant in the high flux range. On the other hand, the
hCO gradually decreased in all the flux range. This can ex-
plain the mountain-type dependence of CO2 formation rate
in the CO + O2 reaction. The hO is low at the low O2 flux,
therefore, the rate increased with increasing the O2 flux be-
cause of increasing hO. On the other hand, at the high O2
flux, hO is high and approaches the saturation level
(hO ꢁ 0.5) [17]. Therefore, the rate decreased with increas-
ing the O2 flux because of decreasing hCO. The hO in
the CO + NO reaction was much lower than that in the
CO + O2 reaction. The hO increased gradually and the
hCO was almost constant in all the flux range. The hCO in
the CO + NO reaction was higher than that in the
CO + O2 reaction, and this is reflected by the low hO and
the low CO2 formation rate in the CO + NO reaction.
Based on the estimated coverages in the CO + O2 and
CO + NO reactions, it is possible to estimate the rate
constant (kr) in the CO(a) and O(a) reaction by using
h
CO in the CO + N2O reaction is expected to be very small
based on the low CO2 formation rate, it is supposed that
the hCO in the CO + N2O reaction is equal to that in the
CO + NO reaction. In this case, the hO can be estimated
from the equation, rCO ¼ krhCOhO. Here, rCO is the CO2
2
2
formation rate in the CO + N2O reaction, which was ob-
tained from the experimental results shown in Fig. 3(a).
The kr value for the CO(a) + O(a) reaction can also be used
in the step (5). The result showed that the hO was much
lower than those of the CO + O2 and CO + NO reactions,
and it increased gradually as shown in Fig. 3(c).
kr ¼ rCO =hCOhO [13]. The kr value obtained here is
Fig. 4 shows IR emission spectra of CO2 molecules pro-
duced by the CO + O2, CO + NO and CO + N2O reactions
on Pd(110) surface at TS = 750 K for various fluxes of each
oxidant at the fixed CO flux. The CO2 emission spectra were
observed in the region of 2400–2200 cmꢀ1, while the emis-
sion spectra centered at 2143 cmꢀ1 are due to the IR emis-
sion of the non-reacted CO molecules, which are scattered
from the surface. In addition, the peaks centered at
2223 cmꢀ1 in the CO + N2O reaction are IR emission from
the non-reacted N2O molecules. The CO2 emission spectra
are considerably red-shifted from 2349 cmꢀ1 (the funda-
mental band of antisymmetric stretch). The degree of the
2
kr = 9.5 · 106 sꢀ1
.
In the CO + N2O reaction, one of the most probable
model mechanisms is as follows [18]:
COðgÞ þ V $ COðaÞ;
ð2Þ
ð3Þ
ð4Þ
ð5Þ
N2OðgÞ þ V $ N2OðaÞ;
N2OðaÞ ! N2ðgÞ þ OðaÞ;
COðaÞ þ OðaÞ ! CO2ðgÞ þ 2V:
In this model, it is assumed that the rate-determining step is
N2O(a) dissociation [Eq. (4)], and this is based on the low
CO=4.1× 1018 cm-2 s-1
CO=4.1× 1018 cm-2 s-1
CO=4.1× 1018 cm-2 s-1
NO / × 1018 cm-2 s-1
O2 /× 1018 cm-2 s-1
N2O /× 1018 cm-2 s-1
TS=750 K
TS=750 K
TS=750 K
2305
2298
× 0.5
12.3
8.2
4.1
2.0
0.4
12.3
2310
12.3
8.2
8.2
4.1
2.0
2400
2300
Wavenumber /cm-1
2200
2100
2400
2300
2200
2100
2400
2300
Wavenumber /cm-1
2200
2100
Wavenumber /cm-1
(a)
(b)
(c)
Fig. 4. IR emission spectra of CO2 desorbed by (a) CO + O2, (b) CO + NO and (c) CO + N2O reactions on Pd(110). The surface temperature (TS) was
750 K. The oxidants flux was changed at the fixed CO flux. The emission intensity was normalized per unit of CO2 yield.