CO oxidation on Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces studied by infrared chemiluminescence spectroscopy

Article abstract of DOI:10.1016/j.susc.2007.04.015

The infrared (IR) chemiluminescence studies of CO₂ formed during steady-state CO oxidation over Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces were carried out. Analysis of their emission spectra indicates that the order of the average vibrational temperature (T_V^AV) values of CO₂ formed during CO oxidation was as follows: Pd(1 1 1) > Pt(1 1 1) > Rh(1 1 1), and the order is coincident with the potential energy in the transition state expected by the theoretical calculations. Furthermore, it is suggested that the bending vibrational temperature (T_V^B) can also be influenced by the angle of O-C-O (∠_OCO) of the activated complex in the transition state, which has also been proposed by the theoretical calculations.

Full text of DOI:10.1016/j.susc.2007.04.015

Surface Science 601 (2007) 3796–3800
www.elsevier.com/locate/susc
CO oxidation on Pd(111), Pt(111), and Rh(111) surfaces
studied by infrared chemiluminescence spectroscopy
Kenji Nakao, Osamu Watanabe, Toshiaki Sasaki, Shin-ichi Ito,
*
Keiichi Tomishige, Kimio Kunimori
Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
Available online 7 April 2007
Abstract
The infrared (IR) chemiluminescence studies of CO2 formed during steady-state CO oxidation over Pd(111), Pt(111), and Rh(111)
surfaces were carried out. Analysis of their emission spectra indicates that the order of the average vibrational temperature (TAVV) values
of CO2 formed during CO oxidation was as follows: Pd(111) > Pt(111) > Rh(111), and the order is coincident with the potential energy
in the transition state expected by the theoretical calculations. Furthermore, it is suggested that the bending vibrational temperature (TBV)
can also be influenced by the angle of O–C–O (\OCO) of the activated complex in the transition state, which has also been proposed by
the theoretical calculations.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: IR chemiluminescence; Carbon dioxide; Carbon monoxide; CO oxidation; Noble metal
1. Introduction
dynamics of CO oxidation) from which the gas-phase mol-
ecules desorbed.
The catalytic oxidation of carbon monoxide (CO) on
platinum group metal surfaces has been one of the most
widely studied surface-catalyzed reactions [1–17]. The reac-
tion is of practical importance for the environmental pollu-
tion control. On the other hand, the reaction is also of
scientific interest because the total reaction can be divided
into a few elementary steps and the theoretical approaches
can be applied to the reaction to describe the kinetics of the
surface-catalyzed reaction. In contrast, in order to eluci-
date the dynamics of the surface-catalyzed reaction, it is
effective to investigate internal (vibrational and rotational)
[6–14] and translational [15] energy of product molecules.
Measurements of the vibrational and rotational states of
the product CO2 molecules have been performed by infra-
red chemiluminescence (IR emission) technique [6–14].
Analysis of the vibrational states can give the information
on the structure of the activated CO2 complex (i.e., the
Coulston and Haller [6] studied the dynamics of CO oxi-
dation on polycrystalline Pd, Pt, and Rh surfaces by mea-
suring high-resolution IR emission spectra and reported
that the order of the apparent vibrational temperatures
are as follows: Pd > Pt > Rh. Our group has reported IR
emission of CO2 from steady-state CO oxidation on sin-
gle-crystal Pd and Pt surfaces combined with kinetic results
[9–14]. These suggest 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
less bent structure on Pt(111), since the bending mode of
CO2 from Pd(111) was more vibrationally excited than
that of CO2 from Pt(111) [12,13]. Furthermore, we have
confirmed that the product CO2 molecules on Pd(111)
and Pd(110) was also rotationally excited [12]. These re-
sults indicate that the IR chemiluminescence method can
provide a direct energetic evidence of the reaction mecha-
nism and the activated complex of CO2 formation.
In this article, the dynamics of CO2 formation during
the CO oxidation over single-crystal Rh(111) surface is
*
Corresponding author. Tel.: +81 29 853 5026; fax: +81 29 853 4490.
E-mail address: kunimori@ims.tsukuba.ac.jp (K. Kunimori).
0039-6028/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2007.04.015

K. Nakao et al. / Surface Science 601 (2007) 3796–3800
3797
reported, and this corresponds to the first presentation of
the IR emission studies on a single-crystal Rh surface.
The obtained results were compared with those on
Pd(111) and Pt(111), and the tendency was interpreted
from the comparison between the experimental results
and the reported theoretical studies on the transition state
of CO oxidation.
This assumption is plausible on the basis of previous re-
ports [6,8]. It should be added that T AVV, TVAS and TVB were
used here as parameters to characterize the extent of the
vibrational excitation of the product CO2. It took about
30–90 min for the measurement of the IR emission spectra
with 2000–6000 scans. The stable steady-state activity was
obtained during the measurement.
2. Experimental
3. Results and discussion
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 prod-
uct CO2 molecules just desorbed from metal surfaces dur-
ing catalytic reactions [10–14]. 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.
The reactant fluxes were controlled using mass flow
controllers. The CO and O2 gases (total flux of
8.2 · 1018 cmꢀ2 sꢀ1; CO/O2 = 1) were exposed to single-
crystal surfaces (Pd(111), Pt(111), and Rh(111)). Stea-
dy-state CO oxidation was performed at temperatures of
400–900 K. Another UHV chamber (base pressure
<2.0 · 10ꢀ10 Torr) was used to prepare and characterize
the clean surfaces. It was equipped with a molecular-beam
reaction system, an Ar+ ion gun, low energy electron dif-
fraction (LEED), and a QMS. Before the molecular-beam
reaction, the single-crystal surfaces were cleaned using a
standard procedure (O2 treatment, Ar+ bombardment
and annealing) [9–14]. After cleaning, the sharp (1 · 1)
LEED pattern was observed.
Fig. 1a shows the rate of CO2 formation in the steady-
state CO oxidation on Pd(111), Pt(111), and Rh(111) as
a function of surface temperature (TS) (CO/O2 = 1). The
CO oxidation proceeded above 500 K on Pd(111) and
50
a
CO/O2=1
Pd(111)
40
30
20
10
0
Rh(111)
Pt(111)
400
500
600
700
800
900
Surface temperature [TS] / K
42
40
38
36
34
32
The IR emission spectra of the CO2 molecules desorbed
from the surface were measured with 4 cmꢀ1 resolution. At
that low resolution, no individual vibration–rotation lines
were resolved. The IR emission spectra were analyzed
based on simulation of model spectra [7,12,13]. The aver-
age vibrational Boltzmann temperature (TAVV: an average
temperature of the antisymmetric stretching, symmetric
stretching and bending modes) was calculated from the
degree of the red-shift from the fundamental band
(2349 cmꢀ1) [7,9–13]. The emission intensity is related to
the extent of excitation in the antisymmetric stretching of
CO2 [11–14]. Therefore, the antisymmetric vibrational tem-
perature (TAVS) can be estimated from the normalized emis-
sion intensity [11–14]. Based on TAVV and TVAS, it is possible
to deduce the bending vibrational temperature (T BV). The
relation between TAVV and respective vibrational tempera-
ture is represented as
b
: Pd(111)
: Pt(111)
: Rh(111)
1
1.5
2
1000/TS / K-1
TVAV ¼ ðTAVS þ TSVS þ 2T VBÞ=4;
ð1Þ
Fig. 1. (a) The formation rate of CO2 during CO oxidation on Pd(111),
Pt(111), and Rh(111), and (b) the Arrhenius plot obtained from (a). The
total flux of reactants of (CO + O2) was 8.2 · 1018 cmꢀ2 sꢀ1 at the CO/
O2 = 1. The values of Pd(111) and Pt(111) are taken from Refs. [12,13].
where 2TBV corresponds to the degeneration of the bending
vibration. Assuming that T BV is equal to TVSS because of the
Fermi resonance [6,8], T BV is expected to be ð4TVAVꢀ T AVSÞ=3.

3798
K. Nakao et al. / Surface Science 601 (2007) 3796–3800
Rh(111), and 550 K on Pt(111). The surface temperature
dependence of the formation rate showed a maximum on
all the surfaces. The behavior agrees well with the general
Langmuir–Hinshelwood (LH) kinetics of CO oxidation
on noble metal (Pd [1,4,6,12], Pt [2,6,13], and Rh [3,5,6])
surfaces. The temperature at which the highest activity
was obtained is denoted as TmS ax. 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 tem-
peratures higher than TmS ax, the formation rate of CO2 de-
creased gradually with increasing surface temperature.
This behavior is attributable to the decreased CO coverage.
The starting temperatures of reaction and T mS ax on Pd(111),
Pt(111) and Rh(111), i.e., 650, 775 and 650 K, respec-
tively, are similar to those on polycrystalline surfaces re-
ported by Coulston and Haller [6]. In contrast, the order
of production rate of our results (Pd(111) > Rh(111) >
Pt(111)) at TmS ax is different from results of polycrystalline
surfaces (Pd > Pt > Rh [6]). Generally, a polycrystalline
surface consists of low-index planes such as (111), (100)
and (110) [17]. It has been reported that the maximum
production rate was strongly dependent on surface struc-
ture. In the case of Pd and Rh, the order is Pd(100) >
Pd(110) > Pd(111) [9], and Rh(100) ꢁ Rh(111) [3].
Therefore, it is thought that polycrystalline Pd surface
can give higher catalytic activity than Pd(111), and poly-
crystalline Rh surface can be comparable to Rh(111).
Unfortunately, there is no report on the comparison in cat-
alytic activity on Pt low-index surfaces, however, the differ-
ent order between the polycrystalline and single-crystal
surfaces presented here suggests Pt(100) > Pt(111) and
Pt(110) > Pt(111). The CO2 formation rate is plotted as
a function of inverse surface temperature in the Arrhenius
form as shown Fig. 1(b). From the low temperature range
TS = 475–600 K of this plot, the apparent activation en-
ergy (Eapp) of Pd(111) is estimated as 27.8 kcal/mol, from
TS = 550–625 K, that of Pt(111) is estimated as 19.0 kcal/
mol, and from TS = 550–575 K, that of Rh(111) is esti-
mated as 22.2 kcal/mol. These values agree with the value
of 28.1 kcal/mol on Pd(111) [4], which was obtained by
Goodman et al., that of 24.1 kcal/mol on Pt(111) [2],
which was obtained by Ertl et al., and that of 19.9 kcal/
mol on Rh(111) [5], which was obtained by Schmidt
et al.. It has been reported that the surface is oxidized dur-
ing the CO oxidation in high reaction pressure (about
10 Torr) or low CO/O2 ratio conditions (CO/O2 = 1/30),
especially in the case of Rh surface [3]. However, we think
that the surface keeps metallic state, because our reaction
pressure (ꢂ10ꢀ2 Torr) is three orders of magnitude lower
than their condition, and the pressure ratio is CO/
O2 = 1/1.
Rh(111)
CO=O2=4.1 1018 cm-2 s-1
X
TS / K
800
750
700
650
600
575
2400
2300
2200
2100
Wavenumber / cm-1
Fig. 2. IR emission spectra of CO2 desorbed by CO oxidation on
Rh(111). The surface temperature (TS) was 575–800 K. The flux condi-
tions are as descried in Fig. 1. The emission intensity was normalized per
unit of CO2 yield.
the non-reacted CO molecules, which are scattered from
the surface. The CO2 emission spectra are considerably
red-shifted from 2349 cmꢀ1 (the fundamental band of anti-
symmetric stretch). The degree of the red-shift from the
fundamental band, which reflects the average vibrational
state of the excited CO2 molecules, is not strongly influ-
enced by the surface temperatures. The emission intensity
is also almost constant under various surface temperatures.
Fig. 3 shows the average vibrational temperature (TVAV
)
derived from IR emission spectra of CO2 on Pd(111),
Pt(111) and Rh(111) surfaces as a function of surface tem-
perature. The TAVV values are much greater than TS, which
indicates that the product CO2 is vibrationally excited. It is
shown that the order of T AVV is as follows: Pd(111) >
Pt(111) > Rh(111). This tendency agrees well with the re-
sults of polycrystalline Pd, Pt, and Rh surfaces studied by
Coulston and Haller [6]. In addition, the TAVV values on
Pd(111) is much more dependent on the TS than those
on Pt(111) and Rh(111). The reason for different surface
temperature dependence on these surfaces is not clear at
present.
Eichler [16] has studied CO oxidation on transition metal
surfaces using density functional theory (DFT) calcula-
tions. He reported that the potential energies of transition
state (ETS) and the activation energies (Ea) in CO oxidation
on Pd(111), Pt(111) and Rh(111) as listed in Table 1, and
the potential energy diagram is illustrated in Fig. 4. He
exhibited that the ETS values in CO oxidation on
Pd(111), Pt(111) and Rh(111) were ꢀ0.98, ꢀ1.38 and
Fig. 2 shows IR emission spectra of CO2 molecules pro-
duced by the CO oxidation on Rh(111) surface at various
surface temperatures. The CO2 emission spectra were ob-
served in the region of 2400–2220 cmꢀ1, while the emission
spectra centered at 2143 cmꢀ1 are due to the IR emission of

K. Nakao et al. / Surface Science 601 (2007) 3796–3800
3799
2500
2000
1500
1000
CO/O2=1
Pd(111)
Pt(111)
Rh(111)
400
500
600
700
800
900
Fig. 4. Potential energy diagram for the CO oxidation on noble metal
surfaces (Pd, Pt, Rh). The Eini and ETS are the potential energies in the
initial and transition states, respectively. The Ea and Eexcited are the
activation energy in CO oxidation and the excited energy of the product
CO2, respectively (see Table 1).
Surface temperature [TS] / K
Fig. 3. Surface temperature dependence of average vibrational tempera-
ture (TAVV) of CO2 formed in CO oxidation on Pd(111), Pt(111) and
Rh(111). The flux conditions are as described in Fig. 1. The values of
Pd(111) and Pt(111) are taken from Refs. [12,13].
er vibrational temperatures on Pd(111), compared to
Pt(111) or Rh(111), are in good agreement with the poly-
crystalline results of Coulston and Haller [6]. They argued
that the excess bending excitation in the case of Pd might
be due to the higher density of states at Fermi level com-
pared to Pt or Rh. In fact, the antisymmetric vibrational
temperature on Pd(111) was also rather high, and this
can be corresponded to higher potential energy of the tran-
sition state (Table 1). Regarding Pt(111) and Rh(111), the
bending vibrational temperature of Pt(111) was higher than
that of Rh(111) and this can be related to the angle of O–C–
O (\OCO) in the transition state. Eichler [16] has also re-
ported that the \OCO in the transition states on Pt(111)
and Rh(111) were 109ꢁ and 112ꢁ, respectively, as shown
in Table 1. In addition, the smaller the \OCO at the transi-
tion state, the larger the energy amount in the bending
vibrational mode of desorbed CO2 molecules. This can be
explain the difference in the bending vibrational tempera-
ture on Pt(111) and Rh(111). On the other hand, in the
case of Pd(111), the angle is relatively large like Rh(111).
However, the product CO2 molecules on Pd(111) are much
more excited in both bending and antisymmetric vibrations
than those on Pt(111) and Rh(111). At present, it is inter-
preted that the vibrational excited states on Pd(111) can be
controlled mainly by the large excited energy (Eexcited) than
by the angle of activated complex in the transition state.
Table 1
Potential energies in the initial (Eini) and the transition (ETS) states, the
activation energies (Ea) and O–C–O angle (\OCO) at the transition states
for the CO oxidation over Pd(111), Pt(111) and Rh(111) surfaces taken
from Ref. [16]a
Pd(111)
Pt(111)
Rh(111)
Einib/eV
ETS/eV
Eac/eV
ꢀ2.38
ꢀ0.98
1.40
ꢀ2.12
ꢀ1.38
0.74
ꢀ2.91
ꢀ1.88
1.03
\
112ꢁ
109ꢁ
112ꢁ
OCO
a
The zero of potential energies based on the free molecules (CO +
1/2O2).
b
The initial states means the states of CO and O adsorbed on surfaces.
Ea = ETS ꢀ Eini
c
.
ꢀ1.88 eV, respectively [16], that is, the transition state on
Pd(111) has the highest potential energy and that on
Rh(111) is the lowest one. IR emission measurements are
reflected by the excited energy (Eexcited) of the product
CO2, which can be distributed to internal (vibrational and
rotational) and translational energies of desorbed CO2 mol-
ecules. Therefore, it is suggested that the excitation level of
desorbed CO2 can be originated from the height of potential
energy of the transition state as shown in Fig. 4.
Fig. 5 shows the bending vibrational temperature (TBV)
and the antisymmetric vibrational temperature (TAVS) ob-
tained from the IR emission intensity of CO2 as a function
of surface temperature. The T BV values are higher than those
of TAVS on each surface, which means that the bending vibra-
tional mode is more excited than the antisymmetric
vibrational mode on each surface. However, the bending
vibrational temperature at TS = 800 K is much higher on
Pd(111) (TBV ¼ 2200 K) than on Pt(111) (TBV ¼ 1750 K)
and Rh(111) (TBV ¼ 1400 K) as shown in Fig. 5a. The high-
4. Conclusions
We measured the steady-state activity of CO oxidation
over Pd(111), Pt(111), and Rh(111) surfaces in the tem-
perature range of 400–900 K. Measurements and analyses
of IR chemiluminescence of CO2 formed during the stea-
dy-state CO oxidation supplied the vibrational energy

3800
K. Nakao et al. / Surface Science 601 (2007) 3796–3800
TAVV values of CO2 formed during CO oxidation was as fol-
2500
2000
1500
1000
lows: Pd(111) > Pt(111) > Rh(111). It is suggested that
the order corresponds to the potential energy of the transi-
tion state expected from the theoretical studies. The TBV val-
ues are higher than those of TAVS on each surface, which
means that the bending vibrational mode is more excited
than the antisymmetric vibrational mode. The order of
the T BV was as follows: Pd(111) > Pt(111) > Rh(111),
and this can be influenced by both the angle of the acti-
Pd(111)
Pt(111)
vated complex (\OCO) and Eexcited
.
Acknowledgements
Rh(111)
This work was supported by the 21st Century Center of
Excellence (COE) Program of the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
K. Nakao is grateful for his Research Fellowship from
the Japan Society for Promotion of Science (JSPS) for
Young Scientists.
400
500
600
700
800
900
Surface temperature [TS] / K
2500
2000
1500
1000
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states of CO2, as the average vibrational temperature
(TAVV), antisymmetric vibrational temperature (T AVS), and
bending vibrational temperature (T BV). The order of the