Infrared characterization of Rh surface states and their adsorbates during the NO-CO reaction

Article abstract of DOI:10.1021/jp9922155

In situ IR coupled with TPR technique was used to characterize Rh surface states and their adsorbates during the NO-CO reaction and the reaction of a gas stream simulated to automobile exhaust. The TPR profiles of adsorbates and CO₂ showed that Rh surface states and their adsorbates are governed by the redox reaction cycle of NO-CO. Variation in the intensities of NO and CO adsorbates with temperature and reactant partial pressures indicated the dependence of the redox reaction cycle on temperature and reactant partial pressure. The oxidation rate increased faster with temperature than the reduction rate below light-off and vice versa above light-off. The extent and rate of oxidation increased with the partial pressure of NO and CO. The shift in the TPR curves of NO conversion, CO₂ formation, and Rh⁺(CO)₂ to low temperatures at NO/CO = 1/9 suggested that excess CO accelerated the rate of the reduction step. In contrast, a high NO/CO ratio shifted the TPR curves of NO conversion and CO₂ formation to high temperatures and slowed the reduction step. The presence of oxygen in the reactant stream oxidized the Rh surface and slowed the NO-CO reaction. Adsorbed oxygen from the oxygen molecule depleted adsorbed CO and oxidized Rh⁰ to Rh⁺. O₂, H₂, and C₃H₈ present in simulated gas competed with NO and CO for the Rh site, lowering NO reduction activity.

Full text of DOI:10.1021/jp9922155

J. Phys. Chem. B 2000, 104, 2265-2272
2265
Infrared Characterization of Rh Surface States and Their Adsorbates during the NO-CO
Reaction
Khalid A. Almusaiteer and Steven S. C. Chuang*
Department of Chemical Engineering, The UniVersity of Akron, Akron, Ohio 44325-3906
ReceiVed: June 29, 1999; In Final Form: January 4, 2000
Rh surface states and their adsorbates during the NO-CO reaction have been characterized by the in situ
infrared (IR) coupled with temperature-programmed reaction (TPR) technique. The TPR profiles of adsorbates
and CO2 show that Rh surface states and their adsorbates are governed by the redox reaction cycle of NO-
CO. Adsorbed oxygen from dissociated NO oxidizes Rh0 to Rh+; adsorbed CO reduces Rh+ to Rh0. The
extent of oxidation and reduction of Rh0/Rh+ is in part reflected in the intensity of the adsorbates residing on
these sites (i.e., Rh+(CO)2, Rh0-CO, Rh-NO+, and Rh0-NO-). An increasing NO/CO ratio shifts the TPR
profiles of Rh+(CO)2, NO conversion, and light-off to higher temperatures. The results reveal that a high
NO/CO ratio or high concentration of oxidant enhances the extent of oxidation of Rh0 to Rh+, resulting in
low catalyst activity for NO reduction. Keeping the Rh surface in the Rh0 state by a low NO/CO ratio decreases
the Rh+(CO)2 intensity and shifts the light-off to a lower temperature. O2, H2, and C3H8 present in simulated
gas compete with NO and CO for the Rh site, lowering NO reduction activity.
Introduction
dispersion.10,24 They also showed that decreases in dispersion
cause a decrease in catalyst activity for the NO-CO reaction.10,24
Hecker and Bell found that preoxidation of Rh/SiO2 increases
its activity for the NO-CO reaction, whereas prereduction of
Rh/SiO2 decreases its activity for the NO-CO reaction.12
Changes in the catalyst state during reaction have long been
recognized in catalysis.1-8 In homogeneous catalysis, the catalyst
precursor transforms to the active catalyst form through ligand
addition and dissociation.9 In heterogeneous catalysis, the
catalyst surface state could undergo various forms of transfor-
mation (e.g., oxidative disruption, reductive agglomerization,
redispersion, sintering, etc.), depending on the reaction
environment.1-8,10 Despite the recognition of the possible
changes in the catalyst surface state, understanding of catalyst
surface states under reaction conditions has been limited. This
is due to the difficulties in in situ characterization of catalyst
surfaces under practical reaction conditions.
The most investigated catalysts for surface reconstruction
during adsorption and reaction processes are Rh catalysts1,3,4,10-28
because of their important role in the automobile catalytic
converter. Yang and Garland were the first to observe that
adsorption of CO over Rh0 crystallite on SiO2 or Al2O3 caused
disruption of the Rh0 crystallites.1 Solymosi and co-workers have
found that CO adsorption on Rh led to oxidative disruption of
the Rh0 crystallites to isolated Rh+ at 300 K, while CO
adsorption on isolated Rh+ sites at temperatures above 448 K
resulted in the formation of Rh0 crystallites.15,19,27 The former
was termed as CO-induced oxidative disruption and the latter
as CO-induced reductive agglomerization.15 The CO-induced
oxidative disruption process was further shown to be involved
with surface OH and was facilitated on small Rh crystallites.16
The process can be assisted by NO, which oxidizes Rh0 to
Rh+.15
Although the role of CO and NO in modifying the Rh surface
state and morphology has been clearly elucidated, little is known
about the effect of NO and CO partial pressures on the Rh
surface states and their adsorbates/activities during the NO-
CO reaction. The present study is aimed at determining the
oxidation state of Rh and its adsorbates as a function of
temperature and NO/CO partial pressure during the NO-CO
reaction. Infrared spectroscopy (IR) coupled with temperature-
programmed reaction (TPR) is used to determine the adsorbate
structure for elucidation of the Rh surface states and product
formation during the NO-CO reaction and the reaction of a
gas stream simulated to automobile exhaust. To determine the
role of Rh+ in the reaction, oxygen was added to the NO/CO
stream to increase the number of Rh+ sites. This study is
expected to contribute to a better understanding of Rh surface
states during the NO-CO reaction in automobile exhaust
catalysis.
Experimental Section
The 2 wt % Rh/Al2O3 catalyst was prepared by incipient
wetness impregnation of RhCl3‚2H2O (Alfa Chemicals) onto
γ-alumina support (Alfa Chemicals, 100 m2/g). The catalyst was
dried overnight in air at room temperature, calcined by flowing
air at 723 K for 6 h, and then reduced by flowing H2 at 723 K
for 6 h. The average Rh crystallite size was determined to be
52 Å by H2 chemisorption.
The experimental apparatus including an in situ infrared (IR)
reactor cell with CaF2 windows has been reported in detail
elsewhere.29 The catalyst powder was pressed into three self-
supporting disks. One (25 mg) was placed in the path of the IR
beam in the center of the IR reactor cell; others (98 mg) were
Changes in catalyst surface states undoubtedly have a great
influence on the catalyst activity. Schmidt and co-workers have
shown that in the pretreatment of Al2O3- and SiO2-supported
Rh catalysts (i) NO increases the Rh dispersion, (ii) H2 causes
sintering of Rh particles, and (iii) CO has no effect on the Rh
* To whom correspondence should be addressed. Phone: (330)972-6993.
E-mail: schuang@uakron.edu.
10.1021/jp9922155 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/19/2000

2266 J. Phys. Chem. B, Vol. 104, No. 10, 2000
Almusaiteer and Chuang
Figure 1. (a) IR spectra of adsorbates during the temperature-programmed reaction of the 0.5% NO/0.5% CO (60 cm3/min) flow at 10 K/min. (b)
Adsorbate intensity vs temperature. (c) Rate of NO conversion and N2O and CO2 formation vs temperature.
broken into flakes and placed in the immediate vicinity of the
self-supporting disk to increase NO conversion and product
formation.
cm-1, nitrato species [NO3-] on Al2O3 at 1617 cm-1, and Rh-
NCO at 2145 cm-1 12,15
occurrence of N-O dissociation and combination of adsorbed
N and CO as follows:
.
Formation of Rh0-NCO indicates the
Flows of He, H2, NO, CO, and simulated gas were controlled
by mass flow controllers. The simulated gas consists of 0.105%
NO, 0.7765% CO, 0.0865% C3H8, 0.2673% H2, 0.5392% O2,
and 15.3% CO2 balanced with He. The simulated gas composi-
tion is of the same order of magnitude as automobile exhaust
composition,30 except that N2 and H2O are not included. Prior
to each TPR experiment, the catalyst was further reduced by
H2 at 673 K for 2 h and IR background spectra were collected
while the catalyst was cooled in He flow from 673 to 323 K. A
step switch from He to NO/CO or simulated gas flow was made
at 323 K. Upon the MS intensities of gaseous species reaching
a steady-state level, the temperature was increased from 323 to
673 K at 10 K/min. The background spectra were subtracted
from IR spectra collected during the TPR to obtain the spectra
of adsorbates.
Rh0-NO + Rh0 f Rh0-N + Rh0-O
Rh0-CO + Rh-N f Rh0-NCO + Rh0
(1)
(2)
Following prolonged exposure of the catalyst to 0.5% NO/0.5%
CO flow at 323 K, Rh+(CO)2, Rh0-CO, Rh0-NO-, Rh0-NCO,
-
and NO3 approached a steady-state level.
Absence of CO2 during the formation of various adsorbates
such as Rh+(CO)2 and NO3 suggests that Rh0-O produced
-
from NO dissociation, i.e., reaction 1, provides adsorbed oxygen
-
for oxidizing Rh0 to Rh+ and forming NO3 species.
An increase in the reaction temperature led to an increase in
NO conversion and N2O/CO2 formation as well as variation in
the intensities of Rh0-NCO, Rh+(CO)2, Rh0-CO, Rh0-NO-,
and NO3-. To correlate adsorbate intensity and NO conversion/
product formation, variation of adsorbate intensity with tem-
perature was plotted along with NO conversion and N2O/CO2
formation rates in Figure 1b,c.
To increase the number of Rh+ sites, 10 cm3 of O2 was pulsed
into He flow which was further mixed with 0.5% NO/0.5% CO
flow in the in situ IR cell. It is necessary to separate the O2
pulse and the NO flow to avoid the reaction of NO with O2 in
the transportation lines at room temperature.
Increasing the reaction temperature to 398 K decreases the
Rh0-CO intensity and increases that of Rh+(CO)2. Increases
in the Rh+(CO)2 intensity reflect the increase in the number of
Rh+ sites. The light-off temperature occurs at 427 K as shown
in Figure 1c, where near 50% NO conversion was achieved.
Increasing the temperature above light-off causes (i) a decrease
in the intensities of Rh+(CO)2 and Rh0-NO-, (ii) an increase
Results
Effects of NO and CO Concentrations on the Adsorbates
and Reaction Rate. Parts a-c of Figure 1 show the IR spectra
of adsorbates, variation in adsorbate intensity with temperature,
and variation in the rates of NO conversion and N2O and CO2
formation with temperature during the TPR of 0.5% NO/0.5%
CO in He at a total flow rate of 60 cm3/min. Flowing NO/CO
at 323 K over the prereduced catalyst produced linear CO [Rh0-
CO] at 2060 cm-1 and bent NO [Rh0-NO-] at 1706 cm-1
during the first 0.96 min. An increase in the NO/CO exposure
time increased the Rh0-CO intensity and resulted in the
emergence of gem-dicarbonyl [Rh+(CO)2] at 2085 and 2020
in the intensities of NO3- and carbonato (CO32-) at 1570 cm-1
(iii) an emergence of Rh+-CO at 2087 cm-1 and Rh0-CO at
2038 cm-1 31,32
and (iv) an increase in the NO conversion and
,
,
N2O/CO2 formation. At 673 K, nearly complete NO conversion
was achieved and Rh+-CO at 2087 cm-1 and Al-NCO at 2035
cm-1 12,15,25,32 became the dominant adsorbates on the catalyst
surface.

IR Characterization of Rh Surface States
J. Phys. Chem. B, Vol. 104, No. 10, 2000 2267
Figure 2. (a) IR spectra of adsorbates during the temperature-programmed reaction of 5% NO/5% CO (60 cm3/min) flow at 10 K/min. (b) Adsorbate
intensity vs temperature. (c) Rate of NO conversion and N2O and CO2 formation vs temperature.
According to the above observations, the reaction may be
divided into three stages: (I) Below the light-off temperature
(i.e., 427 K), low NO conversion with Rh+(CO)2 and Rh0-
NO- was observed. (II) Between light-off and 548 K, Al-NCO,
the intensities of Rh+-CO at 2104 cm-1 and Rh-NO+ at 1904
cm-1. The Rh-NO+ intensity increases at the expense of Rh+-
CO. Interestingly, Al-NCO, Rh-NCO, NO3-, and CO32- were
not formed in the entire reaction process. Although NO was in
great excess of CO, complete NO conversion was achieved
probably due to the stoichiometric NO decomposition process
(i.e., 2NO f N2 + 2Oad). The NO conversion began decreasing
as Oad accumulated on the Rh surface faster than its removal
by CO to form CO2. The NO-CO reaction eventually ap-
proached the steady-state NO conversion at 11%.
Effect of Simulated Gas on the Adsorbates and Reaction
Rate. Parts a-c of Figure 6 show the IR spectra of adsorbates,
variation in adsorbate intensity with temperature, and variation
in the MS intensity with temperature of the IR cell effluent
during the simulated gas TPR at a total flow rate of 60 cm3/
min. The concentration of NO in the simulated gas flow is lower
than that of 0.5% NO flow, whereas the CO concentration is
higher than that of 0.5% CO flow in Figure 1. Both simulated
gas and NO-CO reactions produced the same type of adsorbates
and products, but differed in intensity. The notable differences
are that (i) the Rh0-CO was present in the entire temperature
range during the simulated gas reaction and (ii) the NO
conversion in the simulated gas reaction was considerably lower
than that in the NO-CO reaction. Significant conversion of H2,
O2, NO, and possibly C3H8 as well as the formation of H2O,
CO2, and Al-NCO began at 480 K. The highest NO conversion
achieved was 30% at 560 K.
2-
Rh0-CO, NO3-, and CO3 were the major adsorbates. (III)
Above 548 K, nearly complete NO conversion with Al-NCO
and Rh0-CO as the main adsorbates was observed. Further
increases in the temperature produced Rh+-CO.
Parts a-c of Figure 2 show the IR spectra of adsorbates,
variation in adsorbate intensity with temperature, and variation
in the rates of NO conversion and N2O/CO2 formation with
temperature during the TPR of 5% NO/5% CO in He at a total
flow rate of 60 cm3/min. Comparison of the results in Figure 2
and those in Figure 1 shows that increasing the NO and CO
concentrations (i) increases the light-off temperature and the
Rh+(CO)2 intensity, (ii) promotes the formation of Al-NCO
and Rh-NO+ below light-off, (iii) increases the intensities of
Al-NCO and Rh+-CO above light-off, and (iv) decreases the
NO conversion.
Parts a-c of Figure 3 show the IR spectra of adsorbates,
variation in adsorbate intensity with temperature, and variation
in the rates of NO conversion and N2O/CO2 formation with
temperature during the TPR of 17% NO/17% CO in He at a
total flow rate of 60 cm3/min. The results in Figure 3 show that
further increases in the NO and CO concentrations increase the
light-off temperature as well as the intensity and wavenumber
of Rh+(CO)2, promote the formation of Rh+-CO at 2107 cm-1
above light-off, and decrease the NO conversion.
Effect of the NO/CO Ratio on the Adsorbates and
Reaction Rate. Comparison of results of 0.1% NO/0.9% CO
(Figure 4) with those of 0.5% NO/0.5% CO (Figure 1) shows
that excess CO in the reactant stream decreases the light-off
temperature and the intensities of Al-NCO and Rh+(CO)2. No
Rh+-CO was observed during the TPR. In contrast, comparison
of results of 0.9% NO/0.1% CO (Figure 5) with those of 0.5%
NO/0.5% CO (Figure 1) shows that excess NO in the reactant
stream results in an increase in the light-off temperature and in
Effect of an O2 Pulse on the Adsorbates and Reaction
Rate. To determine the role of Rh+ in the NO-CO reaction,
oxygen was added to the NO/CO stream to increase the number
of Rh+ sites. Parts a-c of Figure 7 show the IR spectra of
adsorbates, variation in adsorbate intensity with time, and
variation in the MS intensity with time of the IR cell effluent
while 10 cm3 of O2 was pulsed into the 0.5% NO/0.5% CO (60
cm3/min) flow at 473 K. Argon (Ar) was added to the O2 stream
as a tracer to signify the lead-lag relationship (e.g., formation/
disappearance) between reactants, adsorbates, and products.

2268 J. Phys. Chem. B, Vol. 104, No. 10, 2000
Almusaiteer and Chuang
Figure 3. (a) IR spectra of adsorbates during the temperature-programmed reaction of 17% NO/17% CO (60 cm3/min) flow at 10 K/min. (b)
Adsorbate intensity vs temperature. (c) Rate of NO conversion and N2O and CO2 formation vs temperature.
Figure 4. (a) IR spectra of adsorbates during the temperature-programmed reaction of 0.1% NO/0.9% CO (60 cm3/min) flow at 10 K/min. (b)
Adsorbate intensity vs temperature. (c) Rate of NO conversion and N2O and CO2 formation vs temperature.
Flowing NO/CO over prereduced Rh/Al2O3 produced Al-NCO
at 2253 cm-1, Rh0-CO at 2031 cm-1, and Rh0-NO- at 1745
cm-1. The difference between this spectrum and that in Figure
1 at 473 K is due to the difference in temperature history and
exposure time of the catalyst to the NO/CO flow. Pulse injection
of oxygen causes the following: (i) disappearance of Rh0-CO
and Rh0-NO-, (ii) reduction in the intensity of Al-NCO, (iii)
emergence of Rh-NO+ at 1893 cm-1, and (iv) an increase in
the MS intensity of NO and CO2. The increase in the NO MS
intensity, corresponding to an increase in NO concentration,
reflects a decrease in the rate of NO conversion, while the
increase in the CO2 MS intensity indicates an increase in the
rate of CO2 formation during the O2 pulse.
Parts b and c of Figure 7 show that the appearance of the
gaseous NO profile lagged behind changes in the Rh-NO- and
Rh-NO+ profiles, indicating the transfer of Rh-NO- to Rh-
NO+ prior to desorption as gaseous NO. The significant increase
in the CO2 formation rate during the O2 pulse can be understood

IR Characterization of Rh Surface States
J. Phys. Chem. B, Vol. 104, No. 10, 2000 2269
Figure 5. (a) IR spectra of adsorbates during the temperature-programmed reaction of 0.9% NO/0.1% CO (60 cm3/min) flow at 10 K/min. (b)
Adsorbate intensity vs temperature. (c) Rate of NO conversion and N2O and CO2 formation vs temperature.
Figure 6. (a) IR spectra of adsorbates during the temperature-programmed reaction of simulated gas (60 cm3/min) flow at 10 K/min. (b) Adsorbate
intensity vs temperature. (c) MS intensity vs temperature.
by the high O2 concentration, i.e., 50 000 ppm, at the maximum
point of the pulse, which is 1 order of magnitude greater than
that of NO and CO. High O2 concentration appeared to cause
rapid CO oxidation, depleting Rh0-CO for NO reduction.
As the O2 pulse left the IR reactor cell, CO began to reduce
the Rh+ site to Rh0 as evidenced by gradual reappearance of
Rh0-CO. In the absence of O2, the reduction process of Rh+
to Rh0 is significantly faster than the oxidation process. As a
result, the catalyst surface did not return to its initial state, as
shown by the significant difference in the IR spectra before and
after the O2 pulse.
Since the first O2 pulse resulted in an increase of the steady-
state NO conversion (i.e., lowered the NO MS intensity) and
CO2 formation (i.e., intensified the CO2 MS intensity) as well
as modified the catalyst surface, the second O2 pulse was carried
out to further examine the effects of O2.
Comparison of the results of the first pulse (Figure 7) and
second pulse (Figure 8) shows that they differed in the rates of

2270 J. Phys. Chem. B, Vol. 104, No. 10, 2000
Almusaiteer and Chuang
Figure 7. (a) IR spectra of adsorbates during the first O2 pulse (10 cm3) into the 0.5% NO/0.5% CO (60 cm3/min) flow at 473 K. (b) Adsorbate
intensity vs time. (c) MS intensity vs time.
Figure 8. (a) IR spectra of adsorbates during the second O2 pulse (10 cm3) into the 0.5% NO/0.5% CO (60 cm3/min) flow at 473 K. (b) Adsorbate
intensity vs time. (c) MS intensity vs time.
disappearance and formation of adsorbates and gaseous products.
The second O2 pulse results in (i) a short lead-lag relationship
among reactants, adsorbates, and products, (ii) a slow rate of
disappearance for Rh0-CO and Al-NCO, and (iii) formation
of NO2. An initial high rate of NO2 formation during the second
O2 pulse suggests that the reduced Rh0 site plays a significant
role in the NO2 formation. The low rate of NO2 formation during
the trailing portion of O2 could be due to oxidation of Rh0 to
Rh+ by O2. In contrast to the first O2 pulse, all the reaction
rates returned to the same steady level prior to the second O2
pulse 150 s after the pulse.
Discussion
The generally accepted mechanism for the NO-CO reaction
is listed in Table 1.12,33 This mechanism may describe the
reaction process on the Rh single-crystal surfaces which contain

IR Characterization of Rh Surface States
J. Phys. Chem. B, Vol. 104, No. 10, 2000 2271
TABLE 1: General Proposed NO-CO Reaction
attributed to the dependence of the redox reaction cycle of NO-
CO on temperature (i.e., activation energy for reduction and
oxidation steps). Figures 1-4 show that the Rh+(CO)2 intensity
increased with temperature at low NO conversion. An increase
in the Rh+(CO)2 intensity suggests that the oxidation rate (step
10) increases faster with temperature than the reduction rate
(step 11). As temperature approaches the point where ap-
preciable NO conversion occurs, further increasing the temper-
ature leads to a decrease in the Rh+(CO)2 intensity with
increasing CO2 formation. Formation of CO2 (steps 11-13) is
a result of removal of both Rh0-O and (Rh+)2O2-. In other
words, part of the CO2 formation is a result of reduction of
Rh+ to Rh0.
Effects of the NO/CO Concentration and Ratio on the
Surface State and Reaction Rate. Absence of CO2 formation
and a gradual increase in the Rh+(CO)2 intensity at 323 K in
Figure 1 suggest that oxygen from dissociated NO oxidized Rh0
to Rh+ instead of reaction with CO to form CO2. The extent
and rate of oxidation increased with the partial pressure of NO
and CO (i.e., concentration). An increase in the intensities of
Rh+(CO)2 and Rh-NO+ indicates an increase in their number
and sites for adsorption.
Mechanisma
S + NO(g)
S + CO(g)
S-NO + S
S S-NO
S S-CO
f S-N + S-O
S-CO + S-O f 2S + CO2(g)
S-NO + S-N f 2S + N2O(g)
S-N + S-N f 2S + N2(g)
a S ) active site.
TABLE 2: Proposed Mechanism for the NO-CO Reactiona
I. Adsorption
step 1
step 2
step 3
step 4
step 5
step 6
step 7
step 8
Rh0 + CO(g)
a Rh0-CO (linear CO)
a (Rh0)2-CO (bridged CO)
a Rh+(CO)2 (gem-dicarbonyl)
a Rh0-NO- (bent NO)
a Rh+-CO (linear CO)
a Rh-NO+ (cationic NO)
a Rh0-NO- + CO(g)
2Rh0 + CO(g)
Rh+ + 2CO(g)
Rh0 + NO(g)
Rh+ + CO(g)
Rh+ + NO(g)
Rh0-CO + NO(g)
Rh+(CO)2 + NO(g)
a Rh-NO+ + 2CO(g)
II. Oxidation
step 9
Rh0-NO- + Rh0
a Rh0-N + Rh0-O
a (Rh+)2O2-
step 10 Rh0-O + Rh0
III. Reduction
step 11 2Rh+(CO)2 + O2-
step 12 Rh0-CO + Rh0-O
a 2Rh0-CO + CO2(g) + CO(g)
a 2Rh0 + CO2(g)
In the absence of gaseous product, the number of adsorbates
and corresponding sites may be related to the adsorbate
intensities by their extinction coefficient. Extinction coefficients
have been determined to be 10.8 cm/µmol for Rh+(CO)2 and
9.9 cm/µmol for Rh0-NO-.35 The Rh-NO+ extinction coef-
ficient has not yet been determined. Thus, the relative abundance
of Rh+ and Rh0 sites may be estimated by the intensity of the
adsorbates residing on them.
step 13 (Rh+)2O2- + Rh0-CO a 3Rh0 + CO2(g)
IV. Formation of N2, N2O, and -NCO
step 14 Rh-NO + Rh0-N
step 15 Rh0-N + Rh0-N
step 16 Rh0-N + CO
a 2Rh0 + N2O(g)
a 2Rh0 + N2(g)
a Rh0-NCO
step 17 Rh0-NCO + Al
a Al-NCO + Rh0
a Rh0-NO- as a whole should be considered a neutral species.
A shift in the TPR curves of NO conversion, CO2 formation,
and Rh+(CO)2 to low temperatures at NO/CO ) 1/9 (Figure 4)
suggests that excess CO accelerates the rate of the reduction
step. In contrast, a high NO/CO ratio (Figure 5) shifts the TPR
curves of NO conversion and CO2 formation to high temper-
atures and slows the reduction step. The NO/CO ratio affects
not only the surface state but also the activation energy of the
redox steps. The dependence of the activation energy of the
redox steps on the NO/CO ratio is reflected by the slope of the
Rh+(CO)2 and CO2 formation curves. The sharper change in
these curves indicates the greater difference in activation energy
between the reduction and oxidation steps.
Effects of Oxygen on the Surface State and Reaction Rate.
Disappearance of Rh0-CO and emergence of Rh-NO+ in
Figures 7 and 8 indicate that adsorbed oxygen from the oxygen
molecule depletes Rh0-CO and oxidizes Rh0 to Rh+. The
spectra of adsorbates during oxygen addition resemble that of
NO/CO ) 9/1 at 473 K, indicating that O2 and NO play similar
roles in the redox cycle of the Rh catalyst surface. The presence
of oxygen in the reactant stream oxidizes the Rh surface and
slows the NO-CO reaction.
The most obvious effect of the oxygen pulse is to increase
the NO conversion and CO2 formation. The enhancement of
the catalyst activity brought about by the oxygen pulse is
consistent with the observation that preoxidation increases the
activity for the NO-CO reaction over Rh/SiO2.12 The observed
increase in the intensities of Rh0-CO and (Rh0)2-CO and
decrease in the Rh-NO+ intensity suggest that the oxygen pulse
modifies the Rh surface, increasing the number of reduced Rh0
sites and promoting the NO dissociation.
An interesting observation during the first and second oxygen
pulses is the lack of correlation between the recovery of the
NO conversion/CO2 formation rates and that of Rh0-CO IR
intensity. For example, at 240 s in both Figures 7a,c and 8a,c,
only the reduced Rh sites. For supported Rh catalyst containing
both Rh0 and Rh+ sites, a more comprehensive mechanism is
needed to explain the formation and disappearance of adsorbates
and reactants/products. Table 2 lists both observed adsorbates
and postulated steps involved in the NO-CO reaction on Rh0
and Rh+ sites of supported Rh catalysts.34
Adsorption of NO and CO and Surface Redox Reaction.
Rh0 chemisorbs CO as linear CO [Rh0-CO] (step 1) and bridged
CO [(Rh0)2-CO] (step 2) and NO as bent NO [Rh0-NO-] (step
3); Rh+ chemisorbs CO as gem-dicarbonyl [Rh+(CO)2] (step
4) and linear CO [Rh+-CO] (step 5) and NO as cationic NO
[Rh-NO+] (step 6).1,3,4,11,12,15-17,23,25,27 Competitive adsorption
studies at 300 and 373 K25 show that NO competes over CO
for Rh0 sites (step 7); NO can replace Rh+(CO)2 to form Rh-
NO+, and CO can replace Rh-NO+ to produce Rh+(CO)2 (step
8), depending on NO and CO partial pressures. Adsorbates on
Rh+ sites shift from Rh+(CO)2 to Rh+-CO and then to Rh-
NO+ as the NO/CO ratio increases from 1/9 to 9/1 (Figures
1-5).
Since the formation of these adsorbates is governed by the
availability of Rh0 and Rh+ sites, the formation of these
adsorbates may be explained by redox of Rh0/Rh+. The reduced
Rh0 site can be oxidized to a Rh+ site via adsorbed oxygen
(i.e., Rh0-O) from NO dissociation (step 9). Conversion of
adsorbed oxygen into an oxygen anion in the metal oxides (step
10) generally takes place at high temperature and prolonged
O2 exposure. Nevertheless, Rh appears to have a propensity to
be oxidized at low temperature due to oxidative disruption of
Rh crystallites brought about by both NO and CO. Rh+ can be
reduced to Rh0 via either the reaction of adsorbed oxygen with
Rh+(CO)2 (step 11) or the reaction of O2- with Rh0-CO (step
13).27,34
Effects of Temperature on the Surface State and Reaction
Rate. Variation of Rh+(CO)2 intensity with temperature can be

2272 J. Phys. Chem. B, Vol. 104, No. 10, 2000
Almusaiteer and Chuang
NO conversion and CO2 formation rates reached the steady state,
whereas the intensities of Rh0-CO and Rh+-CO continued to
change with time. Accordingly, the reaction step, which
consumed the IR-observable adsorbates, is not the rate-
determining step.
surface and slows the NO-CO reaction. Adsorbed oxygen from
the oxygen molecule depletes adsorbed CO and oxidizes Rh0
to Rh+.
Although the surface state of Rh in the simulated gas
environment resembles that of Rh in the NO/CO ) 1/9 (i.e.,
reducing) environment, the NO reduction activity under the
simulated gas environment is significantly lower than under the
NO/CO ) 1/9 environment. Low NO conversion under simu-
lated gas reaction conditions can be attributed to competition
of reductants for the same Rh0 sites. The significant difference
in the NO reduction rate between the NO-CO reaction and
simulated gas reaction points to (i) the importance of studying
the NO-CO reaction under practical conditions and (ii) the need
to keep Rh in the reduced state and to ensure the proper balance
of competitive adsorption for NO, CO, and C3H8.
Effects of Simulated Gas on the Oxidation State and
Reaction Rate. The species in the simulated gas can be grouped
into reductant (i.e., CO, H2, C3H8) and oxidant (i.e., NO, O2).
The Rh surface state is expected to be influenced by the relative
rate of the reduction and oxidation with reductant/oxidant. It is
interesting to note that the surface state of Rh in the simulated
gas environment (Figure 6) resembles that of Rh in the NO/
CO ) 1/9 (Figure 4) environment as evidenced by similarity
in the Rh0-CO and Rh+(CO)2 intensities. The decrease in the
Rh+(CO)2 intensity accompanied with the increase in NO
conversion and CO2 formation in Figures 1-5 was also observed
for the simulated gas reaction in Figure 6. However, the catalyst
in the simulated gas environment exhibits a significantly lower
activity for the NO-CO reaction than that in the NO/CO )
1/9 environment. Low NO conversion could be due to competi-
tion of reductants for the same Rh0 sites since all the reactions
between the oxidant and reductant have the same light-off
temperature. Results of this study will serve as an excellent basis
to estimate the potential loss in catalyst activity as the conditions
move from the ideal NO-CO setting to the simulated gas
reaction environment.
Acknowledgment. The research described in this paper has
been funded wholly by the U.S. Environmental Protection
Agency under assistance agreement R823529-01-0 to the
University of Akron.
References and Notes
(1) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504.
(2) Wang, T.; Schmidt, L. D. J. Catal. 1980, 66, 301.
(3) Cavanagh, R. R.; Yates, J. T., Jr. J. Phys. Chem. 1981, 74, 4150.
(4) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789.
(5) Chakraborti, S.; Datye, A. K.; Long, N. J. J. Catal. 1987, 108,
444.
Reaction of Al-NCO. The decrease in the Al-NCO
intensity during the O2 pulse and simulated gas reaction could
be due to the reaction of adsorbed oxygen with Al-NCO to
produce N2 and CO2 as follows:
(6) Shen, J. Y.; Sayari, A.; Kaliaguine, S. Appl. Spectrosc. 1992, 46,
127.
(7) Goodman, D. W. Chem. ReV. 1995, 95, 523.
(8) Almusaiteer, A. K.; Chuang, S. S. C. J. Catal. 1999, 184, 189.
(9) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992; p 77.
(10) Schwartz, J. M.; Schmidt, L. D. J. Catal. 1994, 148, 22.
(11) Solymosi, F.; Rasko, J. J. Catal. 1980, 63, 217.
(12) Hecker, W. C.; Bell, A. T. J. Catal. 1983, 84, 200.
(13) Oh, S. H., and Carpenter, J. E., J. Catal. 1986, 101, 114.
(14) Wong, C.; McCabe, R. W. J. Catal. 1987, 107, 535.
(15) Solymosi, F.; Bansagi, T.; Novak, E. J. Catal. 1988, 112, 183.
(16) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988,
110, 2074.
2Al-NCO + 2Oad f N2 + 2CO2
(3)
Reaction 3 requires either (i) migration of adsorbed oxygen from
the Rh site to the Al2O3 site for reaction with Al-NCO or (ii)
migration of adsorbed Al-NCO from Al2O3 to Rh to react with
adsorbed oxygen on Rh sites. The latter appears to be more
likely than the former because of facile migration of Al-NCO.
(17) Dictor, R. J. Catal. 1988, 109, 89.
Conclusions
(18) Cho, B. K.; Shanks, B. H.; Bailey, J. E. J. Catal. 1989, 109, 89.
(19) Buchanan, D. A.; Hemandez, M. E.; Solymosi, F.; Whit, J. M. J.
Catal. 1990, 125, 456.
(20) Oh, S. H.; Eickel, C. C. J. Catal. 1991, 128, 526.
(21) Paul, D. K.; Yates, J. T., Jr. J. Phys. Chem. 1991, 95, 1699.
(22) Logan, A. D.; Sharoudi, K.; Datye, A. K. J. Phys. Chem. 1991,
95, 5568.
(23) Anderson, J. A. J. Chem. Soc., Faraday Trans. 1991, 87, 3907.
(24) Krause, K. R.; Schmidt, L. D. J. Catal. 1993, 140, 424.
(25) Srinivas, G.; Chuang, S. S. C.; Debnath, S. J. Catal. 1994, 148,
748.
(26) Berko, A.; Menesi, G.; Solymosi, F. J. Phys. Chem. B 1996, 100,
17732.
In situ IR coupled with TPR studies allow observation of
various forms of NO and CO adsorbates as a function of
temperature and reactant partial pressures. Below light-off, Rh+-
(CO)2, Rh-NO+, Rh0-CO, and Rh0-NO- are the major
adsorbates; above light-off, Rh+-CO, Rh0-CO, and Al-NCO
are the major adsorbates. Al-NCO formation is enhanced and
Rh-NO+ is suppressed in the reducing (i.e., low NO/CO ratio)
environment and vice versa in the oxidizing environment (i.e.,
high NO/CO ratio or in the presence of O2).
Variation in the intensities of NO and CO adsorbates with
temperature and reactant partial pressures indicates the depen-
dence of the redox reaction cycle on temperature and reactant
partial pressures. The oxidation rate increases faster with
temperature than the reduction rate below light-off and vice
versa above light-off. The extent and rate of oxidation increase
with the partial pressure of NO and CO. The shift in the TPR
curves of NO conversion, CO2 formation, and Rh+(CO)2 to low
temperatures at NO/CO ) 1/9 suggests that excess CO
accelerates the rate of the reduction step. In contrast, a high
NO/CO ratio shifts the TPR curves of NO conversion and CO2
formation to high temperatures and slows the reduction step.
The presence of oxygen in the reactant stream oxidizes the Rh
(27) Novak, E.; Sprinceana, D.; Solymosi, F. Appl. Catal. A 1997, 149,
89.
(28) Conners, L.; Hollis, T.; Johnson, D. A.; Blyholder, G. J. Phys.
Chem. B 1998, 102, 10112.
(29) Chuang, S. S. C.; Brundage, M. A.; Balakos, M. W.; Srinivas, G.
Appl. Spectrosc. 1995, 49, 1151.
(30) Taylor, K. C. Catal. ReV.-Sci. Eng. 1993, 35, 457.
(31) Li, Y. E.; Gonzalez, R. D. J. Phys. Chem. 1988, 92, 1589.
(32) Chuang, S. S. C.; Pien, S. I. J. Catal. 1992, 135, 618.
(33) Simon Ng, K. Y.; Belton, D. N.; Schmieg, S. J.; Fisher, G. B. J.
Catal. 1994, 146, 394.
(34) Krishnamurthy, R.; Chuang, S. S. C. J. Phys. Chem. 1995, 99,
16727.
(35) Krishnamurthy, R.; Chuang, S. S. C.; Balakos, M. W. J. Catal.
1995, 157, 512.