The internal energy of CO2 produced from catalytic oxidation of CO by NO

Article abstract of DOI:10.1063/1.476613

Internal energy distributions for CO₂ produced in the catalytic oxidation of CO by NO on polycrystalline platinum have been directly measured using a high-resolution tunable diode laser spectrometer. Absorption spectra have been collected for CO₂ produced in a flow cell reactor as a function of surface temperature and reactant ratio. Vibrational excitation of the product CO₂ is observed for all reaction conditions. The vibrational energy of the asymmetric stretching mode is higher than the bending and symmetric stretching modes. The level of vibrational excitation for all normal modes is sensitive to the NO:CO ratio. The observed internal energy distribution of the product CO₂ is compared to that of CO₂ produced from CO oxidation by O₂, using the same method. The similarities in the vibrational excitation and its dependence upon oxygen coverage suggest that the transition state for the CO₂ formation step is the same for both the CO+NO and CO+O₂ catalytic oxidation reactions. This similarity of likely transition states suggests that the CO+NO reaction proceeds by dissociative adsorption of NO, producing adsorbed oxygen atoms which oxidize CO to form CO₂. This is the same route to the formation of CO₂ which is observed for the CO+O₂ reaction on Pt.

Full text of DOI:10.1063/1.476613

The internal energy of CO 2 produced from catalytic oxidation of CO by NO
Daniel J. Bald and Steven L. Bernasek
Citation: The Journal of Chemical Physics 109, 746 (1998); doi: 10.1063/1.476613
View online: http://dx.doi.org/10.1063/1.476613
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 2
8 JULY 1998
The internal energy of CO2 produced from catalytic oxidation of CO by NO
Daniel J. Bald and Steven L. Bernasek
Department of Chemistry, Princeton University, Princeton, New Jersey 08544
͑Received 23 February 1998; accepted 1 April 1998͒
Internal energy distributions for CO2 produced in the catalytic oxidation of CO by NO on
polycrystalline platinum have been directly measured using a high-resolution tunable diode laser
spectrometer. Absorption spectra have been collected for CO2 produced in a flow cell reactor as a
function of surface temperature and reactant ratio. Vibrational excitation of the product CO2 is
observed for all reaction conditions. The vibrational energy of the asymmetric stretching mode is
higher than the bending and symmetric stretching modes. The level of vibrational excitation for all
normal modes is sensitive to the NO:CO ratio. The observed internal energy distribution of the
product CO2 is compared to that of CO2 produced from CO oxidation by O2, using the same
method. The similarities in the vibrational excitation and its dependence upon oxygen coverage
suggest that the transition state for the CO2 formation step is the same for both the COϩNO and
COϩO2 catalytic oxidation reactions. This similarity of likely transition states suggests that the
COϩNO reaction proceeds by dissociative adsorption of NO, producing adsorbed oxygen atoms
which oxidize CO to form CO2. This is the same route to the formation of CO2 which is observed
for the COϩO2 reaction on Pt. © 1998 American Institute of Physics. ͓S0021-9606͑98͒70226-0͔
INTRODUCTION
Dissociation rates for NO have been shown to be dependent
upon several factors; surface coverages of oxygen and CO,8,9
availability of adjacent open surface sites, and coverage of
NO.10
The reaction between nitric oxide and carbon monoxide
on platinum has practical importance in the removal of pol-
lutants from combustion exhaust. The overall reaction is
shown below in Eq. ͑1͒. Automobile emissions of nitrogen
oxides (NOx)
The reaction between CO and NO is considered to pro-
ceed by a Langmuir–Hinshelwood mechanism between two
adsorbed species. However, there has not been complete
agreement in the literature as to which of the surface species
are involved in the CO2 formation step. Most studies have
suggested that NO dissociates to provide atomic oxygen
which is scavenged by CO, similar to the COϩO2
reaction.11–14 However, several studies have proposed a bi-
molecular surface reaction between adsorbed NO and CO
species to directly yield CO2 as product.9,15–17 The respective
CO2 formation steps for the two mechanisms are shown be-
low.
1
NO͑g͒ϩCO͑g͒→2 N2͑g͒ϩCO2͑g͒
͑1͒
are controlled by the after treatment of engine exhaust with
catalytic converters that contain a mixture of three noble
metals; Pt, Rh, and Pd. The removal of NO is also important
from a kinetic point of view since the presence of NO is
known to inhibit the catalytic oxidation of CO and hydrocar-
bons by oxygen.1 All of these reactions are important in the
removal of harmful components in automobile exhaust.
This reaction is also fundamentally important since it is
slightly more complicated than the classical oxidation reac-
tion of CO by oxygen. This system can provide the first test
for the extension of any models developed, using the
COϩO2 reaction, to understand catalytic oxidation. The
added complexity is due to both the production of molecules
other than the oxidation product and the presence of addi-
tional surface species. These additional factors have the po-
tential to either promote oxidation by opening up additional
surface sites for reaction or inhibit oxidation by altering the
surface energetics and relative surface concentrations by re-
pulsive interactions.2
Dissociative
CO͑ad͒ϩO͑ad͒→CO2͑g͒.
Bimolecular
CO͑ad͒ϩNO͑ad͒→CO2͑g͒ϩN͑ad͒.
͑2͒
͑3͒
The internal energy of CO2 produced from CO oxidation
by NO has not been probed previously. However, the dy-
namics of CO2 produced from CO oxidation by O2 has been
investigated. The CO2 product is found to desorb with excess
translational18–21 and vibrational22–29 energy. For the system,
changes in oxygen coverages have been found to affect the
energy of the desorbing CO2. In previous work,22–24 the vi-
brational energy distribution of CO2 produced from CO oxi-
dation by O2 was measured. An increase in vibrational en-
ergy for all modes was observed with increasing oxygen
coverage. Ertl and co-workers20,30 have seen sharpening of
the desorption angular distribution with increasing oxygen
coverage on Pt͑111͒ surfaces. Mantell et al.,26 using low-
Unlike the catalytic oxidation of CO by O2 on platinum
which is insensitive to surface structure,3 the COϩNO reac-
tion is very sensitive to the surface structure of the metal.
Studies using Pt͑100͒ have shown high activity for
reaction,4–6 while Pt͑111͒ has been shown to have very little
activity.7,8 The origin of this surface structure sensitivity is
generally attributed to differences in NO dissociation rates.8
0021-9606/98/109(2)/746/7/$15.00
746
© 1998 American Institute of Physics
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J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
747
resolution time resolved IR spectroscopy to track the vibra-
tional spectrum of CO2 produced from polycrystalline Pt foil,
observed that the vibrational temperature increased with in-
creasing oxygen coverage. In high-resolution IR emission
studies for the reaction on Pd foil28 an increase in the O2:CO
reactant ratio from 1:1 to 2:1 resulted in a significant in-
crease in vibrational energy for all vibrational modes.
In the present study a tunable diode laser ͑TDL͒ is used
to obtain the absorption spectrum of CO2 produced in a flow
cell reactor. Due to the high resolution of the TDL spectrom-
eter (0.001 cmϪ1), single rotational transitions can easily be
resolved and populations of individual states can accurately
be determined. Flynn and co-workers31–36 have demonstrated
in collisional energy-transfer experiments that diode laser ab-
sorption spectroscopy is well suited for measuring vibra-
tional energy distributions of CO2. The relative population of
any state can be determined, including the ground state
͑which cannot be probed with emission techniques͒ and ‘‘IR
inactive’’ symmetric stretching states, by tuning the laser to
a transition of the following type.
center of the flow tube. A 60 l/s mechanical pump provides a
linear flow of 1800 cm/s for a 1.7 Torr stream of high-purity
argon. Electronic mass flow meters are used in each gas line
to allow accurate measurement and control of reactant flow
rates. The high-purity CO ͑Matheson 99.99%͒ and NO
͑Matheson 99.0%͒ were premixed behind a piezoelectric
valve in order to insure consistent composition of the gases
as they entered the established carrier gas flow. The platinum
surface is a 3.5ϫ2.0 cm piece of 100 mesh high-purity poly-
crystalline gauze.
The striped TDL is a standard Laser Photonics element
tunable over the spectral region of the CO2 asymmetric
stretch, 2275–2340 cmϪ1, in a series of modes ranging from
0.8 to 2.5 cmϪ1 in width. The resolution of the spectrometer
is better than 0.001 cmϪ1. The infrared signal is detected by
a 3 mm diameter 77 K InSb photovoltaic detector and pre-
amplifier. The signal is amplified, then digitized and sent to a
PC for data collection and manipulation. The same com-
puter, through a Laser Photonics control module model
L5820, controls the output of the laser during each sweep in
a typical 1000 sweep data set. Frequency calibration was
performed using a solid Ge etalon ͑free spectral range of
0.0482 cmϪ1) as described previously.22
Prior to all reaction studies the platinum gauze was
heated to 1100 K in a flow of 1.0 Torr of argon and 200
SCCM ͑standard cubic centimeters per minute͒ O2 for 45
min in an effort to maintain consistent surface conditions.
Checks of the steady-state reaction for a given gauze tem-
perature and reactant ratio using the mass spectrometer
showed constant levels of CO2 production over a period
greater than 5 h. This insured all data sets taken in a series
probed the same density of product CO2 molecules. Note that
it was not imperative in these experiments to know abso-
lutely how much CO2 was being detected by the laser, it was
only necessary to know that the effective pressure was con-
stant.
l
2
␯ ␯ ␯ ;Jϩh␷ 4.3 ␮m͒→␯ ␯ l ␯ ϩ1͒;JϮ1,
͑4͒
͑
͑
1
3
1
2
3
␯ is the quantum number for the symmetric stretch mode,
1
␯ is the bending mode, ␯ is the asymmetric mode, and J is
2
3
the rotational quantum number. l is the angular momentum
associated with the doubly degenerate bending motion. Due
to the large oscillator strength of the asymmetric stretch, all
such transitions are allowed. The relative populations for
states in each of the vibrational modes are used to determine
individual Boltzmann temperatures which allows for energy
deposition comparisons between the modes.
The ␯1 ,␯2 ,␯3 ,(r) notation used to denote a given vibra-
tional band follows that of Rothman.37 The symbol r repre-
sents the level in the various Fermi-resonating groups. In this
scheme all bands in a Fermi-resonating group have the same
quantum numbers and the value of r decreases with increas-
ing band energy. The angular momentum associated with the
bending motion is always equal to ␯ in this notation and
2
will be omitted.
The presence of contaminants in the NO reactant gas
was taken into consideration during the absorption measure-
ments. It was determined, initially from mass spectrometer
measurements, that there was a significant amount of gas
with a mass of 44 amu in the NO cylinder. This mass corre-
sponds to CO2 and N2O, both of which are products of the
COϩNO reaction under certain conditions and are unavoid-
able trace contaminants in the industrial purification of NO.
The presence of both CO2 and N2O were confirmed by their
characteristic IR absorptions using the diode laser spectrom-
eter. In order to avoid erroneous determination of relative
rovibrational populations due to the absorption of CO2 in the
flow, the NO flow was maintained during the collection of
the background spectrum. The background spectrum was
taken in the absence of reaction ͑CO removed from flow͒ and
subsequently divided by the spectrum taken during the reac-
tion. This was done in order to account for fluctuations in the
diode laser power with injection current. By using this pro-
cedure, no additional correction for the trace CO2 needed to
be performed.
In the work reported here, high-resolution absorption
spectroscopy is used to probe the internal energy of CO2
produced by CO oxidation by NO on polycrystalline Pt in a
flow reactor. To date almost all of the work in this area has
used emission techniques to probe the vibrational and rota-
tional energy of the desorbing CO2. By using diode laser
spectroscopy, the ground-state levels can be probed and a
much higher resolution spectrum obtained. The vibrational
energy distribution ͑and its dependence on reaction condi-
tions͒ is compared to that observed for oxidation of CO by
oxygen.22 A significant increase in vibrational excitation as
the NO/CO reactant ratio increases is also reported here. The
similarity in vibrational excitation of the product CO2 pro-
vides evidence that the same CO2 formation step occurs for
both oxidation reactions.
EXPERIMENT
The flow reactor/diode laser spectrometer has been de-
scribed in detail previously.22 The flow cell is connected to a
mass spectrometer chamber through a capillary placed in the
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748
J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
FIG. 2. A portion of the absorption spectrum collected for CO oxidation by
NO at 800 K and a 5:1 NO:CO ratio.
temperature range in Fig. 1. At the lower gauze temperatures,
TGϽTmax , the reaction is negative order in CO pressure and
positive order in NO pressure. At TGϾTmax the reaction is
seen to be positive order in CO pressure and slightly positive
in NO pressure. The observed steady-state kinetics will be
discussed in the context of the dissociation mechanism. This
mechanism is the most widely accepted and allows for a
straightforward interpretation of the observed results.
At the lower temperatures, the observed kinetics are a
result of the surface being covered predominantly by CO3.
Adsorbed CO has been found to inhibit dissociation and ad-
sorption of NO8, so the reaction is initially limited by CO
desorption. As the CO molecules desorb, sites are open for
NO adsorption and dissociation into N͑ad͒ and O͑ad͒ spe-
cies. Once an oxygen atom is on the surface it immediately
combines with an adsorbed CO and is converted to the prod-
uct. As the temperature is increased, more CO can desorb
and the CO2 production rate increases. In addition there is
likely a significant amount of nitrogen atom recombination
and desorption even at the lowest reaction temperatures. This
will open up additional sites for NO adsorption and dissocia-
tion. Above Tmax , CO coverage on the surface has decreased
and O coverage has increased, so that the reaction is limited
by CO that adsorbs and reacts before desorbing. In addition,
NO also may desorb from the surface before it is able to
dissociate which would cause a decrease in the reaction rate.
FIG. 1. CO conversion percentage as a function of surface temperature and
reactant ratio. The three NO:CO reactant ratios ͑SCCM͒ are 7:1 ͑350:50͒,
3:1 ͑150:50͒, and 1:1 ͑50:50͒.
RESULTS AND DISCUSSION
Steady state kinetics
The production of CO2 during the COϩNO reaction on
polycrystalline Pt was followed as a function of temperature
and reactant ratio using mass spectrometry and absorption
spectroscopy. The results of the mass spectrometry study are
compiled in Fig. 1 for three different NO:CO reactant ratios;
1:1, 3:1, and 7:1. The CO conversion percentage was ob-
tained by simultaneously monitoring the CO and CO2 signals
and normalizing the CO:CO2 ratio to the active cross section
of the Pt gauze. A curve similar to Fig. 1 was obtained using
the diode laser tuned to a single transition of the ground state
͑R18͒, monitoring the intensity as a function of temperature,
and correcting for changes in rotational temperature.
ABSORPTION MEASUREMENTS
The CO2 production rate for the NOϩCO reaction in-
creases with increasing temperature beginning at 550–600
K. The reaction rate increases until a maximum is observed
at a temperature, Tmax . The values of Tmax are roughly 780,
800, and 820 for the 1:1, 3:1, and 7:1 reactant ratios, respec-
tively. The actual reactant fluxes in standard cubic centime-
ters per minute ͑SCCM͒ for the three ratios were 50:50,
150:50, and 350:50. The total CO2 production, and therefore
CO conversion percentage, increases with increasing NO:CO
ratio. The reaction rate decreases with increasing temperature
above Tmax until the reaction levels off at 1050 K. The CO2
production rate determined from the IR absorption technique
͑which is more sensitive͒ goes to zero near 995 K.
Absorption spectra were collected for CO2 produced
from CO oxidation by NO on platinum under various reac-
tion conditions. In some cases the surface temperature was
changed while the reactant ratio remained constant and vice
versa. A portion of a typical spectrum is shown in Fig. 2.
This scan was taken at a 5:1 NO:CO ratio and 800 K surface
temperature. It was obtained by sweep integration for 1000
iterations and subsequent division by the background laser
intensity in the absence of CO in the flow. As mentioned
above, there was a detectable amount of CO2 as a trace gas in
the NO source, so the background scan was taken with the
NO flow maintained. Therefore any laser absorption from
this CO2 is divided out and does not affect the rovibrational
population measurements. There is also a detectable amount
The kinetics of the reaction vary substantially over the
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J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
749
of N2O in the NO flow. N2O has been observed previously
for CO oxidation by NO, but under the reaction conditions
studied here, no product N2O was observed.
Normalized intensities from the absorption spectra were
used to determine relative rovibration state populations. The
procedure used to assign rotational and vibrational tempera-
tures is identical to that used for the COϩO2 reaction and is
described in detail in Ref. 22. A rotational temperature for
each vibrational band is calculated from the relative popula-
tions of different rotational states within a given vibrational
state. The populations are fit to a Boltzmann distribution
with the variable parameter being temperature. All of the
bands exhibited the same rotational temperature for a given
set of reaction conditions. This rotational equilibrium is due
to the significant number of collisions with the Ar carrier gas
as discussed previously.22
FIG. 3. A plot of relative vibrational population versus vibrational energy
for all the CO2 levels detected for a 10:1 NO:CO ratio. The straight line is
The vibrational Boltzmann temperatures for the CO2
produced from the COϩNO reaction are determined by com-
paring the relative populations of vibrational bands, YV , to
the vibrational energy of those bands. This population rela-
tive to CO2 at 296 K is given by Eq. ͑5͒ below and is dis-
cussed in greater detail in Ref. 22.
drawn through all of the levels which correspond to the
i.e., all except the 001 and 011 levels.
ϩ
manifold,
v1 v2
brate relatively quickly with each other and any subsequent
vibrational relaxation could affect both equally.40 In order to
mix with the Fermi-resonant modes, the asymmetric stretch-
ing mode requires nonresonant energy transfer which is a
IN␯
S0vj
BJ Jϩ1͒
1
1
͑
YVϭln
0Ϫ
Ϫ
ͩ
ͪ
k
T0 TJ
much slower process. In the data presented here the ␯ and
1
␯ modes are combined to one Fermi-resonant ␯1ϩ␯ mani-
2
2
EV
1
1
ϭ
Ϫ
ϩC.
͑5͒
ͩ
ͪ
fold, but the ␯ mode can be described by a distinct tempera-
3
k
T0 TV
ture.
In Eq. ͑5͒, IN is the normalized intensity, S0vj is the line
Figure 3 shows a plot of YV versus EV for the COϩNO
reaction at 800 K for a 10:1 NO:CO ratio. The straight line is
drawn through all of the vibrational levels which correspond
to either the bending or symmetric stretching mode. For this
reaction system YV for the bending and symmetric stretching
modes are combined to obtain a single vibrational tempera-
ture. The temperature of the bending and symmetric stretch-
ing modes, TB and TS , is determined from the relative popu-
lations of the 000, 010, 100͑2͒, 020, 100͑1͒, 110͑2͒, 030, and
110͑1͒ levels. The temperature of the asymmetric mode,
TAS , is determined from the relative populations of both the
000 and 001 levels and the 010 and 011 levels.
Figure 4 shows the vibrational temperature for the nor-
mal modes as a function of reactant ratio for the COϩNO
reaction at 800 K. All of the vibrational temperatures in-
crease with increasing NO:CO ratios. TAS is consistently
greater than TB and TS , and all of the temperatures are
above the surface temperature. The rotational temperatures
calculated at each of the reactant ratios are 680 K for 2:1,
690 K for 5:1, and 705 K for 10:1. All of the rotational
temperatures are below surface temperature. As in the case
of the COϩO2 reaction, there is significant rotational relax-
ation and equilibration, which is a result of the CO2 colli-
sions with the argon carrier gas.
strength at 296 K from the HITRAN database,38 ␯ is the line
0
position, EV and EJ are the vibrational and rotational ener-
gies, T0 is 296 K, and TV and TJ are the vibrational and
rotational Boltzmann temperatures to be determined. C rep-
resents terms which do not change during the course of the
experiment. These include vibrational and rotational partition
functions, path length, and molecular density. An average
value of YV is calculated for all rotational transitions de-
tected within each band. This average valueϮone standard
deviation is plotted versus EV for bands within a given vi-
brational mode and the best straight line through these points
has a slope that is proportional to the vibrational Boltzmann
temperature TV .
The product CO2 in these experiments undergoes a sig-
nificant number of collisions with the three other gases in the
flow ͑Ar, CO, and O2). Most of the collisions occur with the
Ar carrier gas. It was estimated that under experimental con-
ditions ͑1.7 Torr͒ a CO2 molecule has 1 100 Ar collisions
during the time it takes to travel 2.5 mm downstream from
the Pt gauze. The distance, 2.5 mm, conservatively estimates
the average distance a molecule travels into the laser beam
before it is detected. Using similar analysis it was estimated
that product CO2 would have 35–40 reactant gas collisions
for a flux of 100 SCCM in the flow. The total number of
collisions should ensure that the rotational state distributions
are equilibrated. Based on collisional energy transfer rates,39
each vibrational mode should also be well described by a
Boltzmann distribution. However, the various modes should
not necessarily be equilibrated with one another. The Fermi-
resonant bending and symmetric stretching modes equili-
The relative vibrational population of CO2 was obtained
for the COϩNO reaction at a reactant ratio of 5:1 for two
different reaction temperatures, 900 and 800 K. A rise in
vibrational energy is observed that is commensurate with the
increase in reaction temperature. This dependence of vibra-
tional energy on reaction temperature is similar to that ob-
served for the COϩO2 reaction.22 However, a more complete
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750
J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
sticking coefficient for nitric oxide which was taken to be 0.1
at 800 K,8 and R is the NO:CO ratio. The ␪O values obtained
for the ratios in Fig. 1 are 0.12 at 2:1, 0.25 at 5:1, and 0.38
for 10:1. Assumptions 2–5 are all reasonable at the high
temperature used for these measurements. Any deviation of
these calculated coverages from the actual coverages will
most likely be generated from a breakdown of the first as-
sumption. As has been discussed earlier, NO dissociation
does not readily occur under some reaction conditions and is
inhibited by O͑ad͒ and CO͑ad͒. If the reaction proceeds by
the bimolecular mechanism, altering the reaction ratio should
vary the surface coverage of NO in a similar way. The maxi-
mum coverage of NO on the surface is 0.75 ML10 so the only
difference in the NO and O coverages determined from Eq.
͑6͒ would be a factor of 1.5.
The coverages determined from Eq. ͑6͒ are only rough
estimates of the surface coverages for oxygen ͑or nitric ox-
ide͒. At first approximation, the correlation between reactant
ratio and oxygen coverage is half that for the CO/O2 system.
Essentially, there are two factors which dictate the relative
oxygen coverage for a given set of reaction conditions, oxy-
gen atom content of the oxidant gas and efficiency of CO
conversion. A reaction with a 10:1 NO:CO ratio has the
same oxygen coverage as a reaction with a 5:1 O2:CO ratio
at the same CO conversion percentage. However, the conver-
sion percentage of the NOϩCO reaction is generally smaller
than that of the COϩO2 reaction. This means the oxygen
coverage for the former reaction is probably more than half
of the oxygen coverage of the latter reaction at the same
reactant ratios.
FIG. 4. Vibrational temperature of normal modes of CO2 as a function of
reactant ratio for the COϩNO reaction at 800 K.
survey of reaction temperatures is limited by signal levels.
The CO2 signal levels for the higher ͑and lower͒ temperature
reaction were much lower because of the drop in reaction
rate at 900 K ͑see Fig. 1͒.
The vibrational excitation of CO2 produced from the
NOϩCO reaction is observed to increase with increasing
NO:CO ratio. As in the case of the COϩO2 reaction, an
increased reactant ratio translates into higher coverages of
that reactant. However, in the NOϩCO reaction the correla-
tion between reactant gas flux and surface coverage is more
complicated than that for the COϩO2 reaction. The compli-
cation arises from the additional process of NO dissociation
͑or nondissociation͒. In order to get an idea of the surface
coverages during the reaction, two cases will be discussed;
total dissociative adsorption of NO and no dissociation of
adsorbed NO ͑bimolecular mechanism͒.
If all of the NO that adsorbs on the surface dissociates,
one can correlate the reactant ratio to oxygen coverage in a
manner similar to that discussed for the CO/O2 system.22,24
Several assumptions have to be made; ͑1͒ all of the NO that
adsorbs immediately dissociates into atomic oxygen and ni-
trogen, ͑2͒ the sticking coefficient for NO is surface structure
insensitive, ͑3͒ NO adsorption is first order with respect to
vacant sites, ͑4͒ all of the atomic nitrogen immediately re-
combines and desorbs as N2 thereby opening surface sites for
additional NO adsorption, and ͑5͒ the CO surface coverage is
relatively low. If all of these conditions are met, then the
following equation can be used to estimate oxygen coverage
on the Pt surface,41
COLLISIONAL DEACTIVATION
One of the factors that must be considered when inter-
preting the internal energies observed for CO2 in the flow
reactor experiment is collisional cooling by the other gases in
the cell ͑Ar, NO, and CO͒. A complete discussion of the
effect of CO2–Ar and CO2–CO collisions on the nascent
internal energy distribution of carbon dioxide is provided in
Ref. 22. Most of the CO2 collisions that occur in the flow
reactor are with Ar. These collisions cause complete rota-
tional equilibration with the flow and vibrational equilibra-
tion within each normal mode, but little vibrational deactiva-
tion occurs. The CO2–CO collisions are more efficient at
vibrational deactivation of the CO2, but literature values for
deactivation rates43,44 suggest any vibrational cooling would
disproportionately affect the ␯ mode as opposed to the ␯
3
1
ϩ␯ manifold. An additional factor in the COϩNO reaction
2
is the deactivation efficiency of CO2–NO collisions.
In order to track energy leakage from the normal modes
due to CO2–NO interactions, one can consider the deactiva-
tion rates of the ͑010͒ level and ͑001͒ level of CO2. In the
case of CO2 ͑001͒ deactivation by NO, the processes in-
volved are shown in the following equations.
CO2͑001͒ϩNO͑0͒→CO2͑110͒ϩNO͑0͒ϩ272 cmϪ1, ͑7͒
CO2͑001͒ϩNO͑0͒→CO2͑000͒ϩNO͑1͒ϩ474 cmϪ1. ͑8͒
␪Oϭ␪
1ϪX / S R͒ ,
͑6͒
͑
͕
max
͖
CO
O
where ␪
͑the maximum coverage of oxygen͒ corresponds
The V-R,T process in Eq. ͑7͒ most likely dominates because
of the lower ⌬E, but both processes contribute to the ͑001͒
max
to 0.5 ML,42 XCO is the fractional CO conversion, SO is the
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J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
751
described here as the difference between the surface tem-
perature and the vibrational Boltzmann temperature. In addi-
tion, the vibrational temperatures of the ␯ and ␯ modes for
1
2
the COϩO2 reaction were combined to obtain a single vibra-
tional temperature for these Fermi-resonant modes. Both the
vibrational excitation and its dependence upon reactant ratios
are similar for the two reaction systems. This provides strong
evidence that the CO2 formation steps for the two systems
are the same, which suggests the NOϩCO reaction proceeds
by the dissociation mechanism.
The internal energy distribution of CO2 produced in the
catalytic oxidation of CO has been linked to the geometry
and energetics of the surface transition state.28 This was dis-
cussed in the context of the COϩO2 reaction previously.22
Although exact transition state geometries and energies are
difficult to extract directly from the CO2 internal energies
measured under the conditions in this study, conclusions can
be drawn from comparisons of different reactions. The CO2
produced in the COϩNO reaction exhibits the same prefer-
ential channeling of energy into the asymmetric mode as was
observed for CO2 produced in the COϩO2 reaction. Also,
the vibrational energies are directly proportional to oxygen
coverage in both systems, which indicates a similar
coverage-dependent shift in potential energy of the transition
state. Vibrational excitation of the ␯1ϩ␯ manifold for the
2
CO2 produced from the COϩNO reaction is marginally
lower than that for oxidation by O2. This may indicate that
the transition state has a slightly different geometry, possibly
a larger O–C–O angle. The observed similarities in vibra-
tional excitation trends suggest that the transition state for
the CO2 formation step is very similar for each of the reac-
tion systems. Therefore the CO2 formation step involves a
reaction between adsorbed oxygen and CO ͓Eq. ͑2͔͒ which is
consistent with the dissociation mechanism for the COϩNO
reaction.
FIG. 5. A plot of vibrational excitation (Tvibration-Treaction) versus O atom:
CO ratio for both the COϩNO ͑open data points͒ and COϩO2 ͑from Ref.
22, filled data points͒ reactions.
deactivation. The deactivation probability of the ͑001͒ state
by NO is approximately 1ϫ10Ϫ4 at 700 K.45 This is roughly
five times greater than the deactivation probability in a
CO2–O2 collision.45
The CO2 ͑010͒ level relaxes to the ground state by the
V-R,T process shown in Eq. ͑9͒
However, there is a small difference in vibrational exci-
tation of the Fermi-resonant modes of the product CO2 as
shown in Fig. 5. The TSS and TB are lower for the NOϩCO
reaction than for the COϩO2 reaction for the same reactant
ratio. This suggests that the transition state for the NOϩCO
reaction is less bent than the transition state for the COϩO2
reaction. Alternatively, the difference in vibrational excita-
tion of the Fermi-resonant modes could result from the dif-
ferent collisional deactivation properties of NO and O2. It is
difficult to draw any strong conclusions since the equilibra-
tion of the two normal modes clouds the issue somewhat.
In contrast, if the NOϩCO reaction occurs by the bimo-
lecular mechanism ͓Eq. ͑3͔͒ one would expect to observe a
drastically different internal energy distribution in the prod-
uct CO2. The transition state energetics for the CO2 produc-
tion step would most likely be less sensitive to adsorbate
coverage and any similarities in vibrational excitation for the
two oxidation reactions would be fortuitous. The transition
state geometry, potential energy, and the energy available to
the nascent CO2 should all be different for the two reactions.
Presumably, the transition state for reaction 3 would involve
OC–O bond formation in conjunction with N–O bond cleav-
age. The transition state for the COϩO2 reaction involves
OC–O bond formation in conjunction with Pt–O bond cleav-
CO2͑010͒ϩNO͑0͒→CO2͑000͒ϩNO͑0͒ϩ667 cmϪ1. ͑9͒
Published results indicate46 that the vibrational deactivation
probability for process ͑9͒ is 7ϫ10Ϫ5. Collisional relaxation
of CO2 by NO should not be significant under the flow con-
ditions of this study. We observe an increase in vibrational
temperature for each of the modes as the amount of NO ͑and
correspondingly CO2–NO collisions͒ increases, as was the
case for CO2–O2 collisions in the COϩO2 reaction. There-
fore any additional deactivation of the nascent CO2 by NO is
either negligible or small compared to the increased excita-
tion.
Comparison to CO؉O2 reaction
By comparing both the internal energies of the CO2 pro-
duced from the COϩNO and COϩO2 reactions, and the cov-
erage dependence of those internal energies, we provide evi-
dence which supports one of the proposed reaction
mechanisms for CO oxidation by NO. Figure 5 shows the
change in vibrational excitation for CO2 produced from both
of the reaction systems as a function of oxygen atom to CO
molecule ratio. One should keep in mind that the reaction
temperature is 800 K for the COϩNO reaction and 900 K for
the COϩO2 reaction. Therefore the vibrational excitation is
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752
J. Chem. Phys., Vol. 109, No. 2, 8 July 1998
D. J. Bald and S. L. Bernasek
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the nascent CO2 desorbs directly from the Pt surface. This
would be expected to drastically alter any energy transfer
between desorbing CO2 and the surface, and thus the ob-
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ACKNOWLEDGMENT
This research was supported by the Chemistry Division
of the National Science Foundation.
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