Structure of activated complex of CO2 formation in a CO + O 2 reaction on Pd(110) and Pd(111)

Article abstract of DOI:10.1021/jp0518456

Examinations of CO₂2 formed during steady-state CO oxidation reactions were performed using infrared (IR) chemiluminescence. The CO ₂ was formed using a molecular-beam method over Pd(110) and Pd(111). The CO₂ formation rate is temperature dependent under various partial pressure conditions. The temperature of the maximum formation rate is denoted as 7_s^max. Analyses of IR emission spectra at surface temperatures higher than 7_s^max showed that the average vibrational temperature (T_V^AV) is higher for Pd(111) than for Pd-(110). The antisymmetric vibrational temperature (T_V ^AV) is almost equal on both surfaces. These results suggest that the activated CO₂ complex is more bent on Pd(111) and straighter on Pd(110). Furthermore, the difference in the T_V^AV value was small for surface temperatures less than T_s^max. The T_V^AV value was much higher than T_V^AV on both surfaces. These phenomena were observed only when the surface temperature was lower than T_s^max: they became more pronounced at lower temperatures, suggesting that the activated complex of CO₂ formation is much straighter on both Pd surfaces than that observed at higher surface temperatures. Combined with kinetic results, the higher CO coverage at the lower surface temperatures is inferred to be related to the linear activated complex of CO₂ formation. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0518456

J. Phys. Chem. B 2005, 109, 17553-17559
17553
Structure of Activated Complex of CO2 Formation in a CO + O2 Reaction on Pd(110) and
Pd(111)
Kenji Nakao, Shin-ichi Ito, Keiichi Tomishige,* and Kimio Kunimori*
Institute of Materials Science, UniVersity of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
ReceiVed: April 9, 2005; In Final Form: July 25, 2005
Examinations of CO2 formed during steady-state CO oxidation reactions were performed using infrared (IR)
chemiluminescence. The CO2 was formed using a molecular-beam method over Pd(110) and Pd(111). The
CO2 formation rate is temperature dependent under various partial pressure conditions. The temperature of
the maximum formation rate is denoted as TSmax. Analyses of IR emission spectra at surface temperatures
max
higher than TS showed that the average vibrational temperature (TVAV) is higher for Pd(111) than for Pd-
(110). The antisymmetric vibrational temperature (TVAS) is almost equal on both surfaces. These results suggest
that the activated CO2 complex is more bent on Pd(111) and straighter on Pd(110). Furthermore, the difference
AV
AS
in the TV value was small for surface temperatures less than TSmax. The TV value was much higher than
AV
TV on both surfaces. These phenomena were observed only when the surface temperature was lower than
TSmax: they became more pronounced at lower temperatures, suggesting that the activated complex of CO2
formation is much straighter on both Pd surfaces than that observed at higher surface temperatures. Combined
with kinetic results, the higher CO coverage at the lower surface temperatures is inferred to be related to the
linear activated complex of CO2 formation.
Introduction
structure on Pd(110) at higher surface temperatures (TS > 650
K) because CO2 from Pd(111) is more excited vibrationally than
Elucidation of reaction mechanisms is important for catalyst
design and control of reaction routes. Single-crystal surfaces
can be used to elucidate reaction mechanisms over heteroge-
neous catalysts.1,2 An effective method is spectroscopic observa-
tion of reaction intermediates, which are surface species in a
metastable state, to obtain information about reaction mecha-
nisms. Another method is the investigation of internal (vibra-
tional and rotational) energy and translational energy of desorbed
molecules, which are the products of catalytic reactions, from
the catalyst surface.3-19 The energy states of desorbed molecules
can reflect the catalytic reaction dynamics, which can correspond
to a transition state.3-19 Numerous investigations of CO oxida-
tion on Pt and Pd surfaces have revealed the elemental steps of
surface catalyzed reactions.1-24 Furthermore, CO oxidation is
a prototype reaction for the study of dynamics; measurements
of internal energy states of the produced CO2 molecules have
been performed using the infrared chemiluminescence (IR
emission) method under steady-state catalytic reactions using
molecular-beam techniques.3-15 Analyses of vibrational and
rotational states can yield information about the structure of
the activated CO2 complex (i.e., the dynamics of CO oxidation)
from which the gas-phase molecules desorbed. Furthermore, the
vibrational energy state of product CO2 depends on its surface
structure.8-16 Consequently, information about the active sites
is obtainable from the IR emission spectra of CO2 under a
steady-state catalytic reaction.
CO2 from Pd(110).14,15 Results of kinetic investigations show
that the CO2 formation rate in the CO + O2 reaction increases
with increasing surface temperature in the low-temperature
range. The temperature at which the catalytic activity of CO2
formation is maximal is denoted herein and in previous studies
by our group as TS .
max 14,15 We have specifically examined higher
temperature conditions than TSmax and find that as the catalytic
activity in the CO + O2 reaction is greatly dependent on the
CO coverage in this range, it decreases with increasing surface
temperatures. However, catalytic reactions using real and
practical catalysts are usually carried out at lower reaction
temperatures. For that reason, chemiluminescence investigation
at a lower temperature range is of greater practical value. This
study specifically examines such reaction conditions.
Experimental Procedures
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 that desorbed on metal surfaces during catalytic
reactions.14-16 A UHV chamber (base pressure <1.0 × 10-9
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 Technol-
ogy AG) with a differential pumping system. Two free-jet
molecular-beam nozzles (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 2.1-20 ×
1018 cm-2 s-1; CO/O2 ) 0.3-7) were exposed to single-crystal
Pd surfaces (Pd(110) and Pd(111)). Steady-state CO + O2
reactions were performed at temperatures of 400-900 K.
Another UHV chamber (base pressure <2.0 × 10-10 Torr) was
used to prepare the samples and to characterize Pd(110) and
Our group has reported IR chemiluminescence of CO2 from
the steady-state CO + O2 reaction on single-crystal Pd surfaces
combined with kinetic results.11-15 The activated complex of
CO2 formation (the transition state of CO2 formation from CO-
(a) + O(a)) has a more bent structure on Pd(111) and a straighter
* Corresponding authors. Tel: +81-29-853-5026. Fax: +81-29-855-
7440. E-mail: (K.K.) kunimori@ims.tsukuba.ac.jp; (K.T.) tomi@
tulip.sannet.ne.jp.
10.1021/jp0518456 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/25/2005

17554 J. Phys. Chem. B, Vol. 109, No. 37, 2005
Nakao et al.
previous results (TVAS ) 1600 K).4 We estimated the emission
AS
AS
intensity at TV ) 1600 K. This emission intensity and TV
on the polycrystalline Pt surface were used as standards for
various conditions on Pd surfaces (Figure 2c). On the basis of
TVAS and TVAV, it is possible to deduce the bending vibrational
temperature (TVB). The relation between TVAV and the respective
vibrational temperatures is represented as
TVAV ) (TVAS + TVSS + 2 TVB)/4
(2)
Figure 1. IR emission spectra of CO2 desorbed by the CO + O2
reaction on Pd(111). The surface temperature (TS) was 850 K. The total
flux of reactant CO and O2 was 8.2 × 1018 molecules cm-2 s-1 at CO/
B
where 2TV corresponds to the degeneration of two bending
vibrational modes. Assuming that TVB is equal to TVSS because
O2 ) 1. The spectral resolution was 1 cm-1
.
of the Fermi resonance,5,23 TV is expected to be (4TV
-
B
AV
TVAS)/3. This assumption is plausible on the basis of previous
reports.4,23 It should be added that TVAV, TVAS, TVB, and TR were
used here as parameters to characterize the extent of the
vibrational and rotational excitation of the product CO2. About
900-1800 s were required to measure the IR spectra with
1000-2000 scans. The activity was stable during measurement.
Therefore, we infer that the results reflected the CO2 states under
steady-state conditions.
Pd(111) surfaces. It was equipped with an identical molecular-
beam reaction system, an Ar+ ion gun, low energy electron
diffraction (LEED), and a QMS. Before the molecular-beam
reaction, Pd(110) and Pd(111) were cleaned using a standard
procedure (O2 treatment, Ar+ bombardment, and annealing).14-16
The IR emission spectra of the CO2 molecules desorbed from
the surface were measured with 4 cm-1 resolution unless stated
otherwise. At that low resolution, no individual vibration-
rotation lines were resolved. Figure 1 shows an IR emission
spectrum of CO2 produced by the CO + O2 reaction on Pd-
(111) at surface temperature (TS) ) 850 K. It was taken with 1
cm-1 resolution. The CO2 emission spectrum was observed in
the region of 2400-2200 cm-1, whereas the emission spectrum
of the nonreacted CO, which was scattered from the surface,
was centered at 2143 cm-1. In the spectrum with a 1 cm-1
resolution, the rotational lines (P and R branches) of CO
molecules are clearly visible; notwithstanding, those of the CO2
molecules were not resolved.
Results and Discussion
Figure 5 shows the rate of CO2 formation in the steady-state
CO + O2 reaction on Pd(110) as a function of surface
temperature (TS) for various total fluxes of reactants (CO/O2 )
1). The CO oxidation proceeded at temperatures greater than
500 K. The formation rate profiles showed maxima on the Pd-
(110) surface. The temperature at which the highest activity was
obtained is denoted as TSmax. It is shifted to a higher temperature,
and the formation rate of CO2 increases with the increasing total
flux of reactants. These behaviors agree well with the general
Langmuir-Hinshelwood (LH) kinetics of CO oxidation on Pd
surfaces.1,2 At temperatures lower than TSmax, 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 greater than TSmax, the
formation rate of CO2 decreases gradually with increasing
surface temperature. This behavior is attributable to the de-
creased CO coverage. Generally, in the CO + O2 reaction on
Pd surfaces at lower surface temperatures, the reaction order of
CO is negative first-order and that of O2 is positive first-order.
Consequently, the formation rate of CO2 is zero-order with
respect to total pressure. Goodman et al. reported that the
reaction rate of CO oxidation on Pd(110) at temperatures TS )
450-600 and at a total pressure of 16 Torr is zero-order with
respect to total pressure.22 On the other hand, at higher surface
The IR emission spectra of product CO2 were analyzed based
on simulations of model spectra. They yielded an average
vibrational Boltzmann temperature (TVAV: an average temper-
ature of antisymmetric stretch, symmetric stretch, and bending
modes) and a rotational Boltzmann temperature (TR).4,6 TV
AV
was calculated from the degree of red-shift from the fundamental
band (2349 cm-1), and then TR was derived from the emission
bandwidth (Figure 2a,b). Although the IR emission observed
l
here is in the antisymmetric stretch vibrational regions(nSS, nB ,
l
n
AS) f (nSS, nB , nAS - 1)sthe vibrational excitation levels of
symmetric stretch (nSS) and bending (nB) also affect this
region.4,15 Here, nSS, nB, and nAS are the vibrational quantum
numbers of respective modes. The quantum number of vibra-
tional angular momentum in linear molecules is denoted by l.
Note that the emission intensity is normalized using the rate of
CO2 production. Consequently, the emission intensity is related
to the extent of excitation in the antisymmetric stretch of CO2,
which is given by the following equation:
temperatures, the CO coverage (θCO) can be minute (θCO
,
0.01), and oxygen coverage (θO) approaches the saturation level
(θO ≈ 0.5).25 Therefore, the reaction orders of CO and O2 are,
respectively, inferred to be positive first-order and zero-order.
The total formation rate of CO2 included the reaction order of
total pressure as positive first-order with a constant CO/O2 ratio.
These explanations are true for our cases, in which the gas
pressure range is lower than those reported previously.22
Figure 6 shows IR emission spectra of CO2 molecules
produced by CO + O2 reactions on Pd(110) at TS ) 650 K for
the various total fluxes (CO/O2 ) 1). The CO2 emission spectra
observed in the region of 2400-2200 cm-1 are considerably
red-shifted from 2349 cm-1 (fundamental band of antisymmetric
stretch). The degree of red-shift from the fundamental band,
which reflects the vibrational state of excited molecules, is nearly
equal for all total flux conditions. On the other hand, the
emission intensity increases with increasing total flux, indicating
AS
f
exp(-∆EV/kBTV
)
(1)
where f is the emission intensity normalized per unit of CO2
yield, ∆EV is the energy spacing, kB is the Boltzmann constant,
AS
and TV is the antisymmetric temperature. High-resolution
steady-state results (0.06 cm-1)4 show the energy distribution
in respective vibrational modes, TVSS, TVB, and TVAS, where the
superscripts, respectively, indicate symmetric stretch, bending,
and antisymmetric stretch. Here, the steady-state CO + O2
reaction on polycrystalline Pt foil was performed with the
previous conditions as (CO ) O2 ) 4.1 × 1018 cm-2 s-1 4
;
Figure 3). The IR emission spectra of CO2 molecules were
measured with 4 cm-1 resolution at the surface temperature (TS)
of 900 K (Figure 4). The obtained spectrum was compared with

Activated Complex of CO2 Formation in a CO + O2 Reaction
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17555
Figure 2. Analyses of average vibrational temperature (TVAV), rotational temperature (TR), and antisymmetric vibrational temperature (TVAS): (a)
TVAV as a function of degree of red-shift from fundamental bands; (b) TR as a function of full-width half-maximum (TVAV ) 1500, 1700, and 2100
AS
K); and (c) TV as a function of relative emission intensity normalized per unit of CO2 yield.
Figure 3. Formation rate of CO2 during the CO + O2 reaction (CO/
O2 ) 1) on polycrystalline Pt foil as a function of surface temperature.
The total flux of reactant CO and O2 was 8.2 × 1018 molecules cm-2
s-1 at CO/O2 ) 1.
Figure 5. Formation rate of CO2 during the CO + O2 reaction (CO/
O2 ) 1) on Pd(110). The total flux of reactants (CO + O2) was 2.1-
20 × 1018 molecules cm-2 s-1
.
AV
TV value is shown as much higher than TS, indicating that
the product CO2 is vibrationally excited. However, it is almost
constant under all flux conditions. The TR value is also shown
as higher than TS, indicating that the product CO2 is rotationally
excited. The TR value is shown as nearly constant between 2.1
and 8.2 × 1018 cm-2 s-1. However, the value decreases
gradually beyond the total flux ) 1.0 × 1019 cm-2 s-1. The
decrease of the TR value is attributable to rotational relaxation
by the gas-phase collision between departing (CO2) and arriving
(CO + O2) molecules.7 Therefore, we measured the IR emission
spectra of CO2 molecules with no rotational relaxation condi-
Figure 4. IR emission spectra of CO2 desorbed by the CO + O2
reaction on polycrystalline Pt foil. The surface temperature (TS) was
900 K. The total flux of reactant CO and O2 was 8.2 × 1018 molecules
cm-2 s-1 at CO/O2 ) 1.
tions where the total flux of reactants is 8.2 × 1018 cm-2 s-1
.
Figure 7b shows the antisymmetric vibrational temperature
(TVAS) and the bending vibrational temperature (TVB) derived
from IR emission intensity of CO2 as a function of the total
changes in the antisymmetric vibrational excited state (TVAS),
as explained later.
AS
flux. The TV value increases gradually with increasing total
Figure 7a shows the vibrational temperature (TVAV) and
rotational temperature (TR) derived from IR emission spectra
of CO2 at TS ) 650 K as a function of the total flux. Each
flux.
Figure 8 shows the CO2 formation rate in the steady-state
CO + O2 reaction on Pd(110) as a function of surface

17556 J. Phys. Chem. B, Vol. 109, No. 37, 2005
Nakao et al.
Figure 8. Formation rate of CO2 during the CO + O2 reaction (CO/
O2 ) 0.3-7.0) on Pd(110). The total flux of reactants (CO + O2) was
8.2 × 1018 molecules cm-2 s-1
.
Figure 6. IR emission spectra of CO2 desorbed by the CO + O2
reaction on Pd(110). The surface temperature (TS) was 650 K. The total
flux of reactants was 2.1-20 × 1018 molecules cm-2 s-1 at CO/O2 )
1. The emission intensity was normalized per unit of CO2 yield.
Figure 9. IR emission spectra of CO2 desorbed by the CO + O2
reaction on Pd(110). The surface temperature (TS) was 600 K. The total
flux of reactants was 8.2 × 1018 molecules cm-2 s-1 at CO/O2 ) 0.3-
7.0. The emission intensity was normalized per unit of CO2 yield.
of 0.3-2.1, the CO2 formation rate increases with increasing
CO/O2 ratio. This behavior is implied by the reaction orders.
max
Furthermore, the CO2 formation rate at TS under each CO/
O2 ratio increases concomitant with the increasing CO/O2 ratio
of 0.3-2.1 but decreases for ratios of 3.0-7.0. Especially at
very high CO/O2 ratios, the oxygen coverage decreases, thereby
lowering the CO2 formation rate. This study measured the IR
emission spectra at TS ) 600 K because this temperature is lower
than TSmax under CO/O2 ) 1.4-7.0. It is higher than TSmax under
CO/O2 ) 0.3-1.0. Observations at this temperature reveal the
max
relation between the vibrational state of CO2 and TS
.
AV
Figure 7. (a) Total flux dependence of vibrational temperature (TV
)
Figure 9 depicts the IR emission spectra of CO2 molecules
produced by the CO + O2 reaction on Pd(110) at TS ) 600 K
for various CO/O2 ratios. The red-shift from the fundamental
band is shown to be almost equal for various CO/O2 ratios.
However, the emission intensity increases concomitant with the
increasing CO/O2 ratio. Figure 10a shows TVAV and TR, obtained
from IR emission spectra of CO2, as a function of the CO/O2
ratio. The TR value is almost identical (1000 K), indicating that
the rotationally excited state is independent of the CO/O2 ratio.
and rotational temperature (TR) of CO2 formed in CO + O2 reaction
on Pd(110). (b) Total flux dependence of antisymmetric vibrational
temperature (TVAS) and bending vibrational temperature (TVB). The
surface temperature (TS) was 650 K. The total flux of reactants was
2.1-20 × 1018 molecules cm-2 s-1 at CO/O2 ) 1.
temperature (TS) for various CO/O2 ratios. The total flux of
reactants is shown as constant at 8.2 × 1018 cm-2 s-1. The
temperature dependence of the curves shows the maximum
value: the observed behavior resembles that shown in Figure
5. In Figure 8, TSmax is shown to increase considerably with the
AV
The TV value increases slightly with the increasing CO/O2
ratio, whereas the value remains nearly unchanged at a low value
max
AS
B
increasing CO/O2 ratio. At surface temperatures lower than TS
of CO/O2 (CO/O2 < 0.6). Figure 10b shows TV and TV
(e.g., 525 K) under all pressure conditions, the CO2 formation
rate decreases sharply with the increasing CO/O2 ratio, which
is explainable by the reaction orders with respect to CO and
O2, as depicted in Figure 5. In stark contrast, at surface
obtained from IR emission intensity of CO2 as a function of
CO/O2. In the low CO/O2 region, the TV value is nearly
constant, but the TV increases sharply in the higher CO/O2
AS
AS
region. This result indicates that the antisymmetric vibrational
mode of product CO2 is much more excited with higher CO/
max
temperatures higher than TS (e.g., 700 K) for CO/O2 ratios

Activated Complex of CO2 Formation in a CO + O2 Reaction
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17557
as in Figure 11. From the temperature range TS ) 475-600 K
of this plot, the apparent activation energies of Pd(111) and
Pd(110) are estimated, respectively, as 27.8 and 34.3 kcal/mol.
These values agree with the value of 28.1 kcal/mol on Pd(111)21
and that of 33.1 kcal/mol on Pd(110),20 which were obtained
by Goodman et al. in high-pressure conditions, or that of 25
kcal/mol on Pd(111), which was obtained by Engel and Ertl1
in UHV studies. The agreement of activation energies of CO
oxidation over Pd surfaces suggests that the surface structure
was maintained during catalytic and steady-state reactions.
Generally speaking, the planar surface prepared by treatments
such as annealing can be changed to a rough surface during
reactions. However, Pd(110) and Pd(111) have considerably
stable surface structures even under reaction conditions. On the
basis of this inference, we can estimate the TOF of CO2
formation over Pd(110) and Pd(111). At TS ) 575 K, TOFs
over Pd(110) and Pd(111) are calculated, respectively, as 440
and 74 s-1
.
Figure 12a,b shows IR emission spectra of CO2 molecules
produced by the CO + O2 reaction on Pd(110) and Pd(111)
surfaces (CO/O2 ) 1) for various surface temperatures. The
peaks centered at 2143 cm-1 are IR emissions from the
nonreacted CO molecules, which are fully accommodated to
the surface. The higher the surface temperature, the greater was
the red-shift from the antisymmetric stretch fundamental (2349
cm-1) observed in the CO2 emission spectra. In contrast, at low
surface temperatures (TS ) 550-600 K), the degree of red-
shift from 2349 cm-1 is almost constant, even though the
emission intensity is extremely high in this temperature range.
AV
Figure 10. (a) CO/O2 ratio dependence of vibrational temperature
(TVAV) and rotational temperature (TR) of CO2 formed in CO + O2
reaction on Pd(110). (b) CO/O2 ratio dependence of antisymmetric
vibrational temperature (TVAS) and bending vibrational temperature
(TVB). The total flux of reactants was 8.2 × 1018 molecules cm-2 s-1
at CO/O2 ) 0.3-7.0.
Figure 13a,c shows the average vibrational temperature (TV
)
and the rotational temperature (TR) on Pd(110) and Pd(111)
surfaces derived from IR emission spectra of CO2 as a function
AV
of TS. The TV and TR values are much greater than that of
TS, which also indicates that the product CO2 is excited both
vibrationally and rotationally. The TR value is almost constant,
thereby indicating that the rotationally excited state is indepen-
dent of the surface temperature. The TVAV value increases with
increasing TS at temperatures greater than 600 K. In particular,
the TVAV values on Pd(111) are much higher than those of Pd-
(110), whereas it was almost constant at low surface tempera-
AS
tures (TS < 600 K) on both surfaces. Figure 13b,d shows TV
B
and TV derived from IR emission intensities of CO2 as a
AS
function of TS. For surface temperatures above 600 K, the TV
value increases gradually with increasing TS. In contrast, the
AS
TV value increases drastically with decreasing TS at low
Figure 11. CO2 formation rate during the CO + O2 reaction (CO/O2
) 1) on (a) Pd(110) and (b) Pd(111). The total flux of reactants (CO
temperatures of 550-600 K, indicating that the vibrational states
of desorbed CO2 change at about 600 K on both surfaces. The
TVAS values of Pd(111) are similar to those of Pd(110) in Figure
+ O2) was 8.2 × 1018 molecules cm-2 s-1
.
B
O2. Particularly, much higher TVAS is apparent at CO/O2 ) 7.0,
where TS < TSmax. These results suggest that the vibrational
13b,d at the higher surface temperatures. However, the TV
values of Pd(111) are much higher than those of Pd(110),
meaning that the bending vibrational mode is more highly
excited than the antisymmetric vibrational mode on Pd(111) at
the higher TS. This fact is reflected in the structure of the
activated complex of CO2 formation. Particularly, in the bending
vibrational mode, CO2 on Pd(111) is excited. For that reason,
the structure of the activated complex is more bent. In contrast,
CO2 on Pd(110) is slightly more excited in the antisymmetric
stretching vibration than in the bending one. Therefore, the
activated complex has a straighter form.
state of CO2 at TS lower than TSmax differs from that at TS higher
max
than TS
.
Figure 11 shows the rate of CO2 formation in the steady-
state CO + O2 reaction on Pd(110) and Pd(111) as a function
of surface temperature. At temperatures greater than 500 K, the
CO oxidation proceeds, and the temperature dependence of the
formation rate is maximal on both Pd surfaces. Descriptions
similar to those of Figures 5 and 8 are applicable to these
behaviors. The TSmax and the formation rate of CO2 also differed
for each surface. Comparison of the two surfaces revealed that
the CO oxidation on Pd surfaces is structure-sensitive under
the steady-state reaction condition with the total pressure of ca.
10-3 to 10-2 Torr. The CO2 formation rate can be plotted as a
function of inverse surface temperature in the Arrhenius form,
On the basis of those behaviors, Figure 14a,b presents the
max
structure of the activated complex at 800 K, higher than TS
.
Along with them, an appropriate model is presented and
described. At higher temperatures, adsorbed oxygen and CO
are diffused on the surface. However, it is known that a 3-fold

17558 J. Phys. Chem. B, Vol. 109, No. 37, 2005
Nakao et al.
Figure 12. IR emission spectra of CO2 desorbed by the CO + O2 reaction on (a) Pd(110) and (b) Pd(111). The surface temperature (TS) was
550-850 K. The total flux of reactants was 8.2 × 1018 molecules cm-2 s-1 at CO/O2 ) 1. The emission intensity was normalized per unit of CO2
yield.
Figure 13. (a and c) Surface temperature dependence of vibrational temperature (TVAV) and rotational temperature (TR) of CO2 formed in CO +
O2 reaction on (a) Pd(110) and (c) Pd(111). (b and d) Surface temperature dependence of antisymmetric vibrational temperature (TVAS) and bending
vibrational temperature (TVB) on (b) Pd(110) and (d) Pd(111). The total flux of reactants was 8.2 × 1018 molecules cm-2 s-1 at CO/O2 ) 1.
hollow site is more stable as an adsorption site of oxygen.26,27
A stable adsorption site of CO is a 3-fold hollow site on Pd-
(111)2,28 and a bridge site on Pd(110).2,29 Therefore, the model
describes that CO2 is formed by the reaction between two
adsorbed species at each stable site. For Pd(111), adsorbed
oxygen and carbon atoms in CO are located at a similar distance
from the surface. The activated complex of CO2 formation can
be bent. On the other hand, in the case of Pd(110) because the
adsorbed oxygen is located at the lower level, it is expected
that the activated complex has a straighter structure than that
on Pd(111). In contrast to the profile at higher temperatures, a
different tendency is apparent on both surfaces at lower
temperatures (e.g., 550 K; Figure 14c,d). Under such conditions,
it is characteristic that TVAS is much higher than TVB. This result
indicates that desorbed CO2 is highly excited in the antisym-
metric vibrational mode and that the excitation level of the
bending vibrational mode is low, which is explainable by the
linear structure of the activated complex. At low temperatures
such as 550 K, adsorbed CO has some mobility, but the adsorbed
oxygen’s mobility is much lower because its adsorption energy
is much higher than that of CO2. Therefore, we infer that CO,
mounted on adsorbed oxygen, can form a linear activated
complex. The coverage of adsorbed CO is high at lower surface
temperatures. Therefore, the structure of the activated complex
can be influenced by the CO coverage.
Regarding angular and velocity distribution measurements for
CO2 molecules produced in CO oxidation on Pd(110), Mat-
sushima et al.18 showed that the translational energy of CO2
increases with increasing CO pressure and increases with
decreasing surface temperature (TS < 500 K). They concluded

Activated Complex of CO2 Formation in a CO + O2 Reaction
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17559
AS
temperature was less than TSmax. Furthermore, the TV value
of CO2 increased drastically with decreasing TS (<TSmax),
thereby indicating that antisymmetric vibration is much more
highly excited than other vibrational modes. In turn, that high
excitation suggests that the structure of the activated complex
of CO2 formation is straighter.
AS
(5) The high TV value was always observed at TS lower
than TSmax, a fact that can be related to high CO coverage. The
structure of an activated complex of CO2 formation with the
interaction of the surrounding CO(a) molecules is in a more
linear form.
Acknowledgment. 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.
References and Notes
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(2) Engel, T.; Ertl, G. AdV. Catal. 1979, 28, 1.
(3) Mantell, D. A. Ph.D. Thesis, Department of Physics, Yale
University, New Haven, CT, 1983.
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Figure 14. Structure of the activated complex of CO2 formation and
vibrationally excited state of desorbed CO2 molecules: (a) Pd(111) at
the higher temperature region; (b) Pd(110) at the higher temperature
region; (c) Pd(111) at the lower temperature region; and (d) Pd(110)
at the lower temperature region. The temperatures described in each
model are based on the reaction under the total flux of reactants, which
was 8.2 × 1018 molecules cm-2 s-1 at CO/O2 ) 1.
that these phenomena are closely related to CO coverage because
the amount of CO(a) increases at low surface temperature and
AS
at high CO pressure. Our study showed that the TV value
increases at the low surface temperature and at the high CO/O2
flux ratio in a high CO coverage region. High CO coverage
can render the activated complex of CO2 formation as linear.
Conclusions
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400-900 K under various reactant pressure conditions. The CO2
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formation rate profile with regard to the surface temperature
max
was maximal. This surface temperature was denoted as TS
.
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CO2 formed during the steady-state CO oxidation obtained
vibrational and rotational energy states of CO2 as the average
vibrational temperature (TVAV), antisymmetric vibrational tem-
perature (TVAS), and rotational temperature (TR). Results showed
that TVAV, TVAS, and TR values were much higher than surface
temperature (TS) under all conditions, indicating that the product
CO2 was excited both vibrationally and rotationally.
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(3) The average vibrational temperature (TVAV) of CO2 formed
CO oxidation on Pd(111) was much higher than those of CO2
on Pd(110) at surface temperatures higher than TSmax. This fact
suggests that the activated CO2 complex is more bent on Pd-
(111) and straighter on Pd(110) at the higher surface temper-
ature, where the CO coverage was rather low.
AV
(4) The TV values of CO2 formed in CO oxidation were
similar on Pd(110) and Pd(111) surfaces when the surface