Adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium at high gas pressures

Article abstract of DOI:10.1021/jp963429e

Surface-enhanced Raman spectroscopy (SERS) in conjunction with mass spectroscopy (MS) has been utilized to investigate the adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium surfaces. The SERS-active Rh substrates were prepared by electrodeposition of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400 °C) and pressures (here up to 1 atm). The SER spectra reveal that CO adsorbed primarily on atop sites (v_Rh-C = 470 cm^-1) and desorbed by about 250-300°C under all gas-phase conditions examined. Partial dissociation of the CO adlayer, however, was obtained at temperatures as low as 100°C, most likely facilitated by the large number of steps and kinks present on these roughened surfaces. This was evidenced by a partial removal of CO at temperatures (ca 100°C) well below those expected for thermal desorption (200-250°C) and supported by the observed formation of surface carbonate (665 cm^-1) under these conditions. The CO dissociation, however, is hampered at lower temperatures (<200°C) when gas-phase H₂ and/or CO are present, most likely due to blocking of site ensembles necessary for decomposition to proceed. Interestingly, heating a CO adlayer in pure H₂ resulted in the formation of an adsorbed oxygen species (v_Rh-O = 295 cm^-1) at temperatures above 250°C. The CO hydrogenation reaction was examined over a wide range of gas-phase conditions (H₂:CO = 99:1 to 4:1 at 1 atm), with methane being the only hydrocarbon product detectable with MS. In addition to the presence of adsorbed CO observed up to 250°C under all H₂/CO reaction ratios, the adsorbed oxygen species noted above was detected at higher temperatures (>250°C) when a low percentage of CO (≤1%) in the feed reactant stream was used. The influence of the adsorbed species on the overall methanation rates is discussed in light of these findings. Utilizing the seconds time-scale resolution of SERS, the exchange between gas-phase and adsorbed CO was also studied. The results of such transient ¹³CO/¹²CO exchange experiments reveal that this desorption pathway is weakly activated (≈1 kcal mol^-1), first order with respect to CO coverage, yet independent of CO partial pressure in the regime studied (8-760 Torr). This contrasts the first-order pressure dependence for much lower CO partial pressures (≤10^-5 Torr) reported earlier in the literature. A rate law and mechanism are proposed which account for these differences and rationalize the observed behavior.

Full text of DOI:10.1021/jp963429e

2874
J. Phys. Chem. B 1997, 101, 2874-2883
Adsorption and Hydrogenation of Carbon Monoxide on Polycrystalline Rhodium at High
Gas Pressures
†
†
,‡
,†,§
Christopher T. Williams, Carol A. Black, Michael J. Weaver,* and Christos G. Takoudis*
School of Chemical Engineering and Department of Chemistry, Purdue UniVersity,
West Lafayette, Indiana 47907
X
ReceiVed: October 31, 1996; In Final Form: February 6, 1997
Surface-enhanced Raman spectroscopy (SERS) in conjunction with mass spectroscopy (MS) has been utilized
to investigate the adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium surfaces.
The SERS-active Rh substrates were prepared by electrodeposition of ultrathin films on electrochemically
roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400
°
C) and pressures (here up to 1 atm). The SER spectra reveal that CO adsorbed primarily on atop sites
-
1
(νRh-C ) 470 cm ) and desorbed by about 250-300 °C under all gas-phase conditions examined. Partial
dissociation of the CO adlayer, however, was obtained at temperatures as low as 100 °C, most likely facilitated
by the large number of steps and kinks present on these roughened surfaces. This was evidenced by a partial
removal of CO at temperatures (ca 100 °C) well below those expected for thermal desorption (200-250 °C)
-
1
and supported by the observed formation of surface carbonate (665 cm ) under these conditions. The CO
dissociation, however, is hampered at lower temperatures (<200 °C) when gas-phase H and/or CO are present,
most likely due to blocking of site ensembles necessary for decomposition to proceed. Interestingly, heating
2
-
1
a CO adlayer in pure H
temperatures above 250 °C. The CO hydrogenation reaction was examined over a wide range of gas-phase
conditions (H :CO ) 99:1 to 4:1 at 1 atm), with methane being the only hydrocarbon product detectable with
MS. In addition to the presence of adsorbed CO observed up to 250 °C under all H /CO reaction ratios, the
2
resulted in the formation of an adsorbed oxygen species (νRh-O ) 295 cm ) at
2
2
adsorbed oxygen species noted above was detected at higher temperatures (>250 °C) when a low percentage
of CO (e1%) in the feed reactant stream was used. The influence of the adsorbed species on the overall
methanation rates is discussed in light of these findings. Utilizing the seconds time-scale resolution of SERS,
13
12
the exchange between gas-phase and adsorbed CO was also studied. The results of such transient CO/ CO
-
1
exchange experiments reveal that this desorption pathway is weakly activated (≈1 kcal mol ), first order
with respect to CO coverage, yet independent of CO partial pressure in the regime studied (8-760 Torr).
-5
This contrasts the first-order pressure dependence for much lower CO partial pressures (e10 Torr) reported
earlier in the literature. A rate law and mechanism are proposed which account for these differences and
rationalize the observed behavior.
Introduction
ethoxy,10,11,17 formate,4 and carbonate,4,9 although in many
instances these species are present only on the catalyst sup-
The interaction of carbon monoxide with rhodium surfaces
has been well scrutinized over the past 20 years. Additionally,
CO hydrogenation is a surface process that has received much
attention. As well as its technological significance regarding
the formation of hydrocarbons and/or oxygenates, the chemical
richness of the CO + H2 reaction makes it interesting from a
fundamental standpoint. A major focus has been placed on the
factors that determine selectivity toward any one of the
numerous possible reaction products. On polycrystalline,
single-crystal, and supported Rh surfaces,
predominantly formed along with much smaller amounts of
other hydrocarbons. However, oxygenate formation has been
observed over some types of supported Rh
4,10,11,17
port.
As they are central to understanding the mechanism
of this reaction, the rate and extent of CO dissociation on
1
,13,18-23
rhodium surfaces have also been investigated.
It is
generally accepted that the probability for CO decomposition
1
,13,20-23
is greatest on stepped and kinked surfaces.
However, while much has been discovered about the interac-
tion of CO with H2 on single-crystal and supported Rh, there
are very few studies involving polycrystalline surfaces.1 More
importantly, neither of these involves the monitoring of reaction
intermediates in situ at high gas pressures of more direct
technological significance. While many of the intermediates
mentioned above for supported catalysts may be of mechanistic
significance, it is still important to determine which species
would form independently if there were no influence from the
support material. We have demonstrated in previous papers2
how surface-enhanced Raman spectroscopy (SERS) can be used
as an interfacial probe of heterogeneous catalytic reaction
systems. One process that has been of particular interest to us
,2
1
,2
2
3-8
methane is
9-11
and on rhodium
These investigations reveal that selectivity
2
,12
oxide surfaces.
is highly dependent on a number of experimental parameters,
including support material, temperature, pressure, and the
reactant ratio utilized. Numerous surface reaction intermediates
4-32
1
,4,6,13
1
have been reported, including carbon,
subsurface oxygen,
1
4
8,15,16
CHx,5 methoxy,
10,11,17
ethylidene, carbonyl hydride,
24,26,27,29-31
32
32
is NO reduction on Rh,
Pt, and Pd. The surfaces
†
School of Chemical Engineering.
Department of Chemistry.
utilized for these studies were electrochemically deposited
transition-metal thin films on roughened SERS-active gold
substrates. These surfaces exhibit stable as well as intense SERS
activity, even at elevated temperatures, and largely mimic the
‡
§
Presently with the Department of Chemical Engineering, University
of Illinois at Chicago.
X
Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5647(96)03429-3 CCC: $14.00 © 1997 American Chemical Society

Carbon Monoxide on Polycrystalline Rhodium
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2875
adsorptive and catalytic behavior of their transition-metal
analogues. One advantage of this approach is the ability to
monitor adsorbed oxygen and other species, X, from their
characteristic surface-X vibrations, since SERS is sensitive to
were used without further purification. The isotopes utilized
in this study were obtained from Cambridge Isotope Laboratory
(99% pure CO, 98% pure O2) and Matheson (99% pure D2)
and were used without further purification.
1
3
18
-
1
modes in this low-frequency (200-1000 cm ) region. The
advent of charge coupled device (CCD) technology along with
the intrinsically intense SERS signals also allows the acquisition
of spectra on a seconds time scale, thereby enabling us to probe
the temporal evolution of surface species by perturbing the
system parameters (e.g., temperature, reactant ratio) and moni-
toring the transient spectroscopic response. Kinetic measure-
ments obtained simultaneously during real-time SERS mea-
surements by means of mass spectrometry (MS) allow for direct
correlations to be made between overall rates of reaction and
surface speciation.2
As the surface area of the substrate utilized for SERS was
2
too small (0.07 cm ) to yield reliable kinetic data, a higher
surface area version was prepared. This consisted of a 5 m
long, 0.05 mm diameter gold wire (99.998% pure, Johnson
Matthey) that was subjected to the same oxidation-reduction
cycling and Rh deposition described above and provided a
2
catalyst with ca. 7 cm surface area. The reactions were carried
3
out in a small-volume (4 cm ) stainless steel reactor (Spectra
-
6
Physics) that could be evacuated to 10 Torr via a turbopump.
Gas-phase composition was measured by leaking the gas outlet
into a Lybold Inficon IPC-50 quadrupole mass spectrometer.
9,31,32
In this paper we report the results of a detailed SERS study
involving the adsorption, desorption, and dissociation of carbon
monoxide on a polycrystalline rhodium surface in the absence
and presence of gas-phase CO. Furthermore, we present
complementary SERS and MS results regarding the reaction of
CO with H2 at several reaction conditions with a view toward
determining which surface species may have a direct influence
upon the reaction mechanism.
Results and Discussion
The electrochemically deposited Rh thin films were pretreated
by heating to ca. 150 °C in 100 cm min of H2 for about 5
3
-1
min. This procedure resulted in a clean SERS background in
-
1
the 200-1000 cm range but retained features in the 1300-
-
1
1
600 cm
region due to surface carbonaceous residues.
Complete removal of these bands required prior high-temper-
ature oxidation, which resulted in severe disruption of the film
and substantial loss of SERS activity following reduction; this
was therefore not used as part of the pretreatment procedure in
the following experiments.
Experimental Section
The SERS-active rhodium films were prepared utilizing 6
mm diameter disks cut out of 0.1 mm thick gold foil (99.95%
pure, Johnson Matthey), and polished using 0.3 µm alumina
powder. They were then placed into an electrode holder that
Before proceeding, it is worthwhile to review our previous
efforts with regard to adsorption of CO on rhodium surfaces.
2
24,26,27
exposed ca. 7 mm of the surface and subjected to 25 oxidation-
In earlier SERS studies
it was found that CO adsorbed
-
1
reduction cycles from -0.3 to 1.2 V vs SCE at 0.5 V s in 0.1
primarily in a terminal configuration on thin Rh films, as
M KCl to produce the surface roughness necessary for Raman
discerned by the observed frequencies of the Rh-CO stretching
33
-1
-1
enhancement. The surface was rinsed thoroughly with deion-
ized water and subsequently transferred to another cell for
electrochemical deposition of the Rh overlayers. The procedure
for Rh deposition utilized a 0.3 mM solution of RhCl3 in 0.1
M HClO4 at 0.1 V vs SCE. The cathodic charge was monitored
and controlled in order to limit the deposition to the desired
film thickness, usually between 2 and 5 equivalent monolay-
(νRh-C ) 465 cm ) and C-O stretching (νC-O ) 2040 cm )
vibrations. Similar observations were made more recently
during a combined SERS and MS investigation of the NO-
2
9
CO reaction on rhodium. However, several aspects of CO
adsorption on these rough polycrystalline Rh surfaces have not
yet been examined, including the desorption behavior and extent
of dissociation. With regard to the former, the time resolution
afforded by SERS (Vide supra) enables the monitoring of CO
desorption in both the presence and absence of gas-phase CO.
In the case of the latter, the sensitivity of SERS to low-frequency
3
4,35
ers.
This choice of rhodium film conditions has been found
28,34,35
to optimize the SER spectral response.
The reactor utilized for the SERS experiments consists of a
3
-1
1
00 cm volume six-way cross, equipped with a turbomolecular
(<800 cm ) Rh-adsorbate vibrations is useful for detecting
-
6
pump (Balzers TPH 060) yielding a base pressure of 2 × 10
dissociation products such as adsorbed oxygen.
Torr. A combination of needle, gate and butterfly valves allows
Adsorption and Dissociation of CO. The desorptive and
dissociative behavior of adsorbed CO was first investigated. A
reduced Rh surface was exposed to 1 atm of CO at 25 °C,
-
3
operation of the reactor at pressures ranging from 10 to 760
Torr. The pressure over the entire range is measured using a
combination of Pirani and cold-cathode gauges, controlled by
a Balzers TPG 252 pressure controller. One of the reactor arms
contains an optical viewport, over which the specimen is
mounted on a stainless steel cylindrical sample holder which
can be heated to 500 °C. Inside the sample holder, a solid piece
of stainless steel is surrounded by a coil of thin Nichrome wire
to provide resistive heating. Through the center of this heater
is a K-type thermocouple which leads to a programmable
temperature controller (Omega CN2041). Laser excitation is
-
5
followed by evacuation of the reactor to ca. 10 Torr. Nitrogen
3
-1
was then introduced at a flow rate of 100 cm min , with typical
low-frequency SER spectra as presented in Figure 1A. Unless
otherwise noted, a spectral acquisition time of 10 s was
employed for all SER spectra reported. The 25 °C spectrum
-
1
(bottom) reveals the presence of the expected 470 cm band
assigned to the νRh-C stretch of terminally adsorbed CO (Vide
supra). Furthermore, examination of the higher frequency
-
1
region (1800-2300 cm ) revealed the corresponding νC-O
+
-1
provided by a Kr laser at 647.1 nm (Spectra Physics), and
stretch at 2040 cm (not shown). The sample was subsequently
-
1
scattered light is collected by a three-stage spectrometer (SPEX
Triplemate). The spectrometer is equipped with a liquid
nitrogen-cooled Photometrics CCD, allowing acquisition of real-
time spectral sequences typically even on a seconds time scale
with a good signal-to-noise ratio. The reactants are introduced
into the reactor through a manifold that allows mixing of up to
four inlet gases and flow rates are measured with a bubble flow
meter. Gases were obtained from Airco (>99.9% purity) and
heated at a rate of ca. 0.5 K s in a stepwise periodic fashion,
remaining at each temperature until the spectra became time
independent (ca. 1-2 min). This same heating protocol was
utilized for all further experiments unless otherwise noted. The
intensity of the νRh-C band was seen to decrease as the
temperature was raised, with complete removal occurring by
300 °C. As expected, a similar temperature-dependent behavior
-
1
was observed for the corresponding 2040 cm νC-O stretch

2876 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Williams et al.
A
Figure 2. Temperature-dependent SER spectra of a Rh surface exposed
3
-1
B
to 100 cm min of pure CO at 1 atm. The temperature was raised in
a stepwise manner, starting at 25 °C, to the values indicated. Integration
time for each spectrum was 10 s.
adsorption of CO32 on silver electrodes36a,b and is consistent
-
3
6c
with the δa asymmetric normal mode of adsorbed carbonate.
-1
Furthermore, we would expect at least a 10-15 cm downshift
1
8
in the frequency of this vibration with O substitution due to
the participation of all three oxygen atoms in the vibration; this
-
1
is consistent with the observed behavior. Thus, the 655 cm
feature observed here is tentatively assigned to surface carbon-
ate.
Heating a CO adlayer in the presence of gas-phase CO yielded
somewhat different results, as shown in Figure 2. The procedure
utilized was identical to that described above for Figure 1A,
3
-1
except that the sample was heated in 100 cm min of CO at
atm rather than N2. In contrast to the behavior observed
during CO desorption in the absence of gas-phase CO, the
1
-
1
intensity of the 470 cm νRh-C band remained unchanged as
the temperature was raised to 150 °C. However, further heating
resulted in an intensity decrease starting at 200 °C, with eventual
-
1
removal of adsorbed CO by 350 °C. The 655 cm feature
became evident above 150 °C (Figure 2), and its intensity
increased markedly as the temperature was elevated further.
Figure 1. (A) Temperature-dependent SER spectra of a CO-saturated
Rh surface exposed to 100 cm min of pure N at 1 atm. The
2
temperature was raised in a stepwise manner, starting at 25 °C, to the
3
-1
values indicated. Integration time for each spectrum was 10 s. (B) SER
The significant apparent decrease in the surface concentration
of CO by 100 °C upon heating in N2 (Figure 1A) differs from
thermal desorption studies of CO from polycrystalline rhodium
surfaces into vacuum, which report desorption temperatures in
3
-1
2
spectra of a Rh surface exposed to 100 cm min of 80% O /20% CO
1
6
18
at 350 °C and 1 atm utilizing
O
2
(bottom spectrum) and
2
O (top
spectrum). Integration time for each spectrum was 10 s.
1,18
not shown). Additionally, a broad feature at 655 cm-1 became
excess of 150 °C.
Thus, another explanation for this marked
(
-
1
intensity decrease of the 470 cm νRh-C feature is suggested.
A plausible cause for this finding is that a portion of the CO
adlayer has dissociated by 100-150 °C. Indeed, several
investigations have shown that CO dissociates on rhodium at
evident by 100 °C and increased slightly with further heating.
In our recent SERS investigation of the CO-NO reaction on
Pt and Pd, an apparently similar band was observed under
reaction conditions and was tentatively assigned to a surface
carbonate species formed by the reaction between surface
carbonaceous impurities and oxygen.32 Interestingly, we have
1
,13,20-23
elevated temperatures,
albeit to a small extent. For
instance, CO dissociation has been reported in temperature-
programmed desorption (TPD) studies by Somorjai and co-
-
1
also noted the formation of an apparently similar 655 cm
1
20
feature during reaction between CO and excess O2 on Rh at
high pressures and elevated temperatures (Figure 1B). The
workers on polycrystalline and highly stepped single-crystal
Rh surfaces. A high-temperature (ca. 700 °C) CO desorption
18
substitution of O2 under those conditions resulted in a ca 20
peak was attributed to the recombination of carbon and oxygen
-1
1,20
cm downshift (Figure 1B), confirming the presence of oxygen
in the species responsible for the vibration. The frequency of
produced from dissociation at lower temperatures.
The
apparent observation of CO dissociation on the roughened
substrates examined here is consistent with the previous findings
-1
this 655 cm band is similar to SERS features observed during

Carbon Monoxide on Polycrystalline Rhodium
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2877
that indicate that this process is enhanced on stepped and kinked
surfaces.1
,13,20-23
The apparent formation of surface carbonate under these
conditions (Vide supra) may also be a consequence of CO
dissociation. The two formation pathways involve the reaction
of adsorbed CO and/or surface carbon with adsorbed oxygen
formed by CO dissociation, as indicated in eqs 2a and 2b,
respectively (S denotes a free site):
CO(ads) + S w C(ads) + O(ads)
(1)
(2a)
(2b)
CO(ads) + 2O(ads) w CO3(ads) + 2S
C(ads) + 3O(ads) w CO3(ads) + 3S
The carbonate suggested by the 655 cm- band in the present
study is stable up to at least 400 °C, possibly suggesting that
the carbon and oxygen exist in this form before eventually
breaking apart and desorbing as CO at higher temperatures.
However, another explanation of the formation of carbonate may
be that CO is reacting with oxygen impurities that may be
present in the system. Thus, the presence of this moiety is not
alone sufficient to imply that CO dissociation has occurred.
Interestingly, the presence of gas-phase CO appears to hamper
dissociation at lower temperatures. While the marked intensity
1
Figure 3. Temperature-dependent SER spectra of a CO-saturated Rh
surface exposed to 100 cm min of pure H at 1 atm. The temperature
2
was raised in a stepwise manner, starting at 25 °C, to the values
indicated. Integration time for each spectrum was 10 s.
3
-1
-
1
decrease of the 470 cm νRh-C band by 100 °C during heating
in pure N2 indicated that some CO was dissociating (Figure
1
A), the intensity of this feature remained essentially unchanged
remained unchanged until higher temperatures (ca. 150-200
°C). It therefore appears that the presence of H2 can also inhibit
CO decomposition at lower temperatures, most likely by a site-
blocking effect similar to that mentioned above. Notwithstand-
ing this, however, there does appear to be some CO dissociation
at elevated temperatures in the presence of H2. As heating a
Rh surface in pure H2 resulted in no discernible SERS features
at elevated temperatures (Vide supra), the emergence of the 295
until 200 °C in the presence of gas-phase CO (Figure 2). The
most likely explanation for this latter result is that CO blocks
sites necessary for decomposition at lower temperatures. Such
a site-blocking mechanism is consistent with the experimental
results of Sachtler and Ichikawa, who showed that blocking a
small fraction of Rh sites with Zn (θZn ≈ 0.2) resulted in a
marked decrease in CO dissociation activity.37 Furthermore, de
Koster et al. showed via ab-initio quantum chemical calculations
that CO dissociation requires an ensemble of 5-7 free surface
-
1
cm band at ca. 300 °C indicates that the interaction of
adsorbed hydrogen and CO has resulted in the production of a
new species. The low vibrational frequency of this band
indicates that it is arising from a Rh-adsorbate stretch or bend.
Experiments similar to that shown in Figure 3 were performed
3
8
atoms.
Interaction of Adsorbed CO with H2. Prior to probing the
state of a Rh surface during exposure to various reactive
mixtures, the behavior of adsorbed CO was examined in the
presence of pure hydrogen. This was prompted by the desire
to isolate any effects that hydrogen may exert on CO desorption
and dissociation. After exposing a reduced Rh surface to 1 atm
of CO, the reactor was evacuated and the sample was exposed
to 100 cm min of H2. This was followed by heating the
sample in a stepwise fashion, with typical SER spectra shown
in Figure 3. At 25 °C, the 470 cm- band associated with
terminal carbon monoxide is clearly seen. This band remained
unaltered until about 150-200 °C, where its intensity began to
1
3
involving CO and D2 in order to determine the origin of this
vibration. By employing a simple harmonic oscillator ap-
4
1
proximation for Rh-adsorbate vibrations, a downward shift
-
1
13 12
of at least 10 cm is predicted upon C/ C or D/H isotope
substitution for numerous carbon or hydrogen-containing species
3
-1
-1
(e.g., CHx, OH). As no frequency downshift for the 295 cm
1
3
peak was detected when either CO or D2 was employed,
however, it is more likely that this vibration is arising instead
from a Rh-O stretch of some type of adsorbed oxygen.
In order to determine the likelihood that such an oxygen
species could be stable at high temperatures in the presence of
H2, that is, provide a plausible origin of the present 295 cm
feature, the following experiment was performed. A reduced
Rh sample was first oxidized in 100 cm min O2 at 200 °C
for 5 min and cooled to 25 °C. This produced a surface covered
1
-
1
decrease and a weak feature becomes discernible at 390 cm .
A similar band has been observed previously during high-
resolution electron energy loss spectroscopic (HREELS) studies
of CO-H2 interactions with Rh(100) and was assigned to the
-
1
3
-1
3
9,40
νRh-C stretch of CO adsorbed at 2-fold bridging sites.
In
-
1
the same investigation, H2 saturation of dilute CO adlayers
resulted in conversion from a terminal to bridging configuration,
qualitatively similar to the behavior observed here. This effect
apparently results from strong repulsive forces, coupled with
segregation of CO and atomic hydrogen within the adlayer.
Further heating resulted in complete removal of adsorbed CO
by 300 °C, accompanied by the appearance of a lower frequency
by Rh2O3, as evidenced by the Rh-O stretch at 540 cm , and
-
1
another distinct form of adsorbed oxygen (νRh-O ) 290 cm ),
consistent with previously recorded SER spectra of oxidized
2
8
Rh films. Following evacuation of the reactor, the sample
3
-1
was exposed to 100 cm min of H2 at 1 atm and heated in a
stepwise fashion, with typical results shown in Figure 4. Both
features were attenuated as the temperature was raised, with
Rh2O3 and the other oxygen species being removed by 150 and
250 °C, respectively. Interestingly, however, a new feature at
-
1
peak at 295 cm (Figure 3), which survived up to at least 350
°
C. The assignment of this feature will now be considered.
In contrast with the significant dissociation observed during
-
1
290 cm appeared at 300 °C and increased in intensity as the
temperature was raised to 350 °C. Substituting O2 for O2
1
8
16
heating in N2 (Figure 1A), the concentration of terminal CO

2878 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Williams et al.
Figure 4. Temperature-dependent SER spectra of a previously oxidized
Figure 5. Temperature-dependent SER spectra of a Rh surface exposed
3
-1
to 100 cm3 min of a 99% H
-1
Rh surface exposed to 100 cm min of pure H
2
at 1 atm. The
2
/1% CO mixture at 1 atm. The
temperature was raised in a stepwise manner, starting at 25 °C, to the
values indicated. Integration time for each spectrum was 10 s. The top
spectrum was obtained at 350 °C for the same experimental conditions,
temperature was raised in a stepwise manner, starting at 25 °C, to the
values indicated. Integration time for each spectrum was 10 s.
1
8
but utilizing an
2
O -oxidized surface.
surface heating in the presence of various mixtures of CO and
H2. The first ratio examined was a 99:1 H2:CO mixture, and
the behavior was found to be very similar to the case of heating
a CO adlayer in pure H2 (Figure 3). A rhodium surface was
in the above experiment (Figure 4, top spectrum) resulted in a
-
1
-1
ca. 15 cm frequency downshift in this 290 cm band,
confirming that oxygen is present in the species responsible for
this vibration, probably as the lead-in (i.e., surface-bound) atom.
As Rh2O3 extends at least 3 monolayers into the rhodium thin
film,28 a significant amount of subsurface oxygen must exist
after the reduction of this species at 150 °C. In addition to
numerous reports of subsurface oxygen on rhodium sur-
3
-1
exposed to 100 cm min of 99% H2/1% CO at 1 atm total
pressure and heated in a stepwise fashion. Typical SER spectra
obtained during such a procedure are presented in Figure 5,
-1
with the 470 cm band associated with terminal CO observed
at 25 °C (bottom spectrum). The concentration of this species
remains relatively unchanged until ca. 200 °C, where a fraction
of the CO adlayer switched to a bridging geometry, as evidenced
1
,43,44
faces,
XPS measurements of oxidized Rh thin films have
indicated the presence of such a species at temperatures higher
-1
by a slight appearance of the 390 cm feature. Further heating
28
-1
than 200 °C. Therefore, the high-temperature 290 cm feature
observed in Figure 4 can reasonably be attributed to adsorbed
oxygen, which results from the diffusion of lattice oxygen closer
to the surface. Most notable is the stability of this moiety at
elevated temperatures in the presence of H2 in the present case.
resulted in the removal of the CO bands by 350 °C, which was
about 100 °C higher than the removal temperature of CO in
-1
the presence of pure H2 (Figure 3). Additionally, the 295 cm
oxygen feature appeared at 300 °C and grew notably stronger
upon heating to 350 °C.
On the basis of these observations and the evidence garnered
Increasing the amount of gas-phase CO in the reactant flow
1
3
-1
from the CO and D2 isotopic labeling experiments (Vide
results in the disappearance of the 295 cm oxygen feature.
-1
supra), we assign the 295 cm feature observed during heating
a CO-saturated Rh surface in H2 (Figure 3) to adsorbed atomic
oxygen formed following CO dissociation. This oxygen may
reside just below the first layer of Rh, which would explain its
stability in the presence of H2. Based on this interpretation,
the differences between heating a CO-covered Rh surface in
N2 (Figure 1A) and H2 (Figure 3) can be understood. In the
former case, oxygen and carbon formed from CO dissociation
at 100 °C may remain on the surface (possibly in the form of
a carbonate species) or react to form CO2. However, in the
latter case the dissociation occurs at higher temperatures (ca.
This effect is best illustrated via a transient experiment involving
switching from one reactant ratio to another. A Rh surface was
initially heated to 300 °C in 1000 cm3 min-1 of 99% H /1%
2
CO at 1 atm total pressure in a manner identical to that described
for Figure 5. At time t ) 0 s the flow was changed to 1000
cm3 min-1 of 90% H /10% CO, with subsequent SER spectra
2
acquired every 20 s. Typical results obtained by means of such
a procedure are shown in Figure 6. Before the change in
reactant ratio (bottom spectra), the 295 cm-1 ν
Rh-O
band was
clearly evident along with a trace of terminally adsorbed CO
(480 cm-1). However, the intensity of the 295 cm-1 feature
2
00-250 °C), with any carbon reacting with hydrogen and
was significantly diminished within 2 min after the reactant
switch, with complete removal occurring by ca. 4-5 min. This
result indicates that the removal of this oxygen species is
facilitated by greater amounts of carbon monoxide.
leaving oxygen on the surface. This latter result appears to be
1
,2
consistent with the results of Somorjai and co-workers, who
observed the formation of subsurface oxygen and carbon during
the reaction of CO and H2 on polycrystalline Rh.
Interestingly, the behavior of adsorbed CO was essentially
unaffected over the range of H2:CO ratios (99:1 to 80:20)
examined here. As an example of this, typical SER spectra for
Steady-State Reaction of CO + H2. In order to establish
whether any of the above species and their related processes
may be significant in influencing pathways for the CO-H2
reaction, SERS measurements were obtained during stepwise
3
-1
a Rh surface heated in 100 cm min of 80% H2/20% CO are
presented in Figure 7. Of note is that the behavior of adsorbed

Carbon Monoxide on Polycrystalline Rhodium
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2879
Figure 8. Line a: turnover numbers for methane formation using a
Rh-coated gold wire pretreated at 150 °C in H
in 99% H /1% CO at 1 atm. Line b: same as (a) but with 80% H
0% CO at 1 atm. Line c: same as (a) but with 400 °C pretreatment.
2
for 5 min and heated
2
2
/
2
Line d: Same as (c) but using a polycrystalline Rh catalyst. The
assumption was made that the site density was 1015 sites cm for all
rate calculations. The error bars represent the standard deviation of
each point.
Figure 6. Real-time SER spectra of a Rh surface that was initially
-2
3
-1
held at 300 °C in 1000 cm min of a 99% H
atm. At t ) 0 s the flow was switched to 1000 cm min of a 90%
/10% CO at 1 atm, with SER spectra recorded at 20 s intervals and
2
/1% CO mixture at 1
3
-1
H
2
a spectral acquisition time of 20 s.
speciation. After reducing samples in H2 at 150 or 400 °C for
ca. 5 min, catalysts were subsequently exposed to either 99%
H2/1% CO or 80% H2/20% CO reaction mixtures at 1 atm and
heated in a stepwise periodic fashion identical to that utilized
in Figures 5 and 7. Gas-phase compositions were measured
utilizing a mass spectrometer (Vide supra), and several m/e
values were recorded to account for all possible products, which
included CH4 (16, 15), H2O (18), CO2 (44), CH3OH (31, 32),
CH2O (30), C2H6 (28, 27), and C3H8 (44, 29). A series of
kinetic results obtained using the above-mentioned pretreatment
procedures and reaction conditions are presented in Figure 8.
15
-2
Assuming, as conventional, the presence of 10 sites cm , the
reaction rates are presented as turnover numbers with units of
-
1 -1
molecules site s . Methane was the only detectable hydro-
carbon product under all conditions studied. This is similar to
the literature mentioned above for polycrystalline Rh,1 where
methane selectivity was typically 90-95%. While the high
background level of water in the MS chamber made it
impossible to monitor its formation, the absence of any other
oxygen-containing species (e.g., CO2, CH3OH, CH2O) suggests
that H2O was indeed being produced.
,2
It is interesting to correlate the threshold temperature at which
measurable reaction occurs for the catalysts pretreated at 150
Figure 7. Temperature-dependent SER spectra of a Rh surface exposed
to 100 cm min of a 80% H /20% CO mixture at 1 atm. The
2
temperature was raised in a stepwise manner, starting at 25 °C, to the
values indicated. Integration time for each spectrum was 10 s.
°
C in H2 with the relative amount of CO on the surface, as
3
-1
deduced roughly from the νRh-C band intensity. The amount
of adsorbed CO decreases significantly from 250 to 300 °C for
both reactant ratios (Figures 5 and 7), which coincides roughly
with the first detectable levels of methane formation during the
corresponding kinetic experiments (Figure 8, symbols a and b).
Thus, it is plausible that the high coverage of CO is hindering
methane formation at lower temperatures (<250 °C). This
phenomenon has been observed during studies of CO hydro-
CO is identical to that observed in the former 99:1 H2:CO case
(
Figure 5). Furthermore, the 295 cm-1 oxygen feature is not
observed under these conditions, consistent with the results of
the transient switching experiment (Figure 6).
In order to elucidate the possible influence of these adsorbed
species upon the overall CO/H2 reaction mechanism, comple-
mentary kinetic measurements were made by utilizing the small-
volume reactor and higher-surface-area Rh thin-film samples
described above (as well as actual polycrystalline Rh). Employ-
ing the same pretreatment procedure and reaction conditions
as during the Raman experiments (Figures 5 and 7) allows direct
correlations to be made between catalyst activity and surface
4
,6,8
genation on supported Rh catalysts,
with several explanations
6
proffered. Koertz et al. show that the activation energy for
carbon monoxide dissociation is lowered as CO surface coverage
decreases. This causes greater levels of CO dissociation at
higher temperatures where the CO coverage begins to decrease,
thus producing more of the active surface carbon that is
presumably an intermediate for methane formation. Somewhat

2880 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Williams et al.
related to this effect is the hypothesis that adsorbed CO blocks
H2 from adsorbing on active sites that are necessary for
hydrogenation of the active carbon intermediate.4 Therefore,
the reaction rate increases when CO desorbs, and the coverage
of hydrogen becomes significant. Comparison of the kinetic
results for these two reactant ratios also suggests that the 295
-
1
cm oxygen species (Figure 5) has little or no effect on the
overall methanation rate. This is an indication that it is a
“
spectator” with regard to CO hydrogenation, perhaps residing
at inactive surface sites. It is also interesting that the metha-
nation rate is relatively unaffected by the change in reactant
ratio (Figure 8, symbols a and b)
This high selectivity toward methane and the observed
turnover numbers are partly consistent with one of the most
comprehensive studies of CO hydrogenation on polycrystalline
1
Rh. However, the measured activation energy (Eact ) 9.0 (
-
1
3
.0 kcal mol ) is a factor of 2 smaller than that measured by
1
Sexton and Somorjai. To determine whether the low-temper-
ature H2 pretreatment of the surface (Figure 8, symbols a and
b) might be causing this effect, several experiments were
performed that involved pretreating both thin-film and bulk
polycrystalline Rh surfaces at 400 °C in H2 for 5 min. Figure
8
(symbols c and d) shows typical data obtained during reaction
Figure 9. Real-time SER spectra of a Rh surface at 200 °C during
13
12
2
CO/ CO isotope switching in an 80% H /20% CO mixture at 1 atm.
in 99% H2/1% CO at 1 atm after this more rigorous pretreatment
procedure was employed. While the turnover numbers are about
1
1
3
3
-1
After holding the sample in a mixture containing CO, 1400 cm min
of the mixture containing 12CO was flowed through though the reactor
at t ) 0 s. Spectra were recorded at 10 s intervals with an acquisition
time of 10 s. The spectra were smoothed using the Savitsky-Golay
approach, and only selected spectra are shown here.
order of magnitude higher than those obtained with the 150
°
C pretreatment, the activation energy remained essentially
unchanged. However, methane formation could not be detected
upon subsequent reaction at lower temperatures over these
catalysts unless the surface was rigorously pretreated again in
pure H2, indicating that surface deactivation was occurring.
Therefore, a more plausible explanation for the low activation
energy noted above may be that the surface becomes progres-
sively deactivated through site blocking by contaminants as the
sample is heated to higher temperatures, partly nullifying the
temperature-induced rate increases as prescribed by the Arrhe-
nius law. While Sexton and Somorjai observed no catalyst
the authors postulated some other mechanism must be respon-
sible for the CO exchange at lower temperatures. Lombardo
49
and Bell have succeeded in modeling CO exchange phenom-
5
0,51
52
ena measured for polycrystalline Pd
and Ni(100) using
Monte Carlo simulations and propose that the observed behavior
results from the effects that the repulsive lateral interactions
have on the activation energy for desorption.
While a few recent studies of the CO-H2 reaction have
1
deactivation during their experiments under similar pressures,
directly measured the rate at which CO adsorbs/desorbs on the
this is probably a result of the much more rigorous pretreatment
cleaning procedure (including annealing at 1000 °C and ion
bombardment in ultrahigh vacuum) than we had available in
the present study.
3,53
catalyst surface at higher pressures,
there has been no
systematic effort made to elucidate the mechanism by which
this exchange occurs. One obstacle to monitoring this process
at elevated pressures has been the challenge of acquiring high-
quality vibrational spectra with a rapidity sufficient to follow
the switching in real time. Thus, the ability to record SER
spectra on a seconds time scale provides a novel method of
probing this process directly. As this study was conducted at
pressures which were 9-10 orders of magnitude higher than
CO Adsorption/Desorption Mechanism. A less-studied
aspect of CO adsorption and reaction with hydrogen on Rh is
the rate and mechanism by which gas-phase CO exchanges with
adsorbed CO on the catalyst surface. While CO desorption from
rhodium surfaces into Vacuum has been well characterized for
1
,18
46
polycrystalline Rh, it has long been known that the adsorp-
those used by Song et al., several differences were expected
tion/desorption kinetics are significantly enhanced by the
to be found between the observed kinetics, most notably in the
pressure dependencies. Also of particular interest was discern-
ing the effect (if any) that hydrogen has on this process.
presence of gas-phase CO. Yates et al.4
4,45
observed that
adsorbed 13CO was completely replaced in 11 min by 12CO at
1
3
12
2
5
2 °C when a surface saturated with the former was exposed to
0 Torr of the latter. In order to explain the facilitation of this
Transient CO/ CO Exchange Experiments. Transient
response experiments involved switching from a flow containing
1
3
12
exchange at temperatures well below those reported for thermal
desorption (150-200 °C),
pressure of CO produced a transient surface intermediate (e.g.,
Rh-(CO)x) that was responsible for this behavior. More
recently, Zhai and co-workers have reported extensive investiga-
tions of this assisted desorption process on Rh(111) in the
presence of low CO pressures.46-48 The most recent of these
CO to one with CO at a total pressure of 1 atm and
1,18
it was postulated that the high
monitoring the time-dependent spectral response. As an il-
lustration of this protocol, typical SER spectra for such an
isotopic substitution in the presence of H are presented in Figure
2
9. The spectra for increasing times are presented right to left,
with only selected spectra presented for clarity. In this case,
the spectra have also been smoothed using the Savitsky-Golay
-
8
13
investigations was performed in the presence of (2-9) × 10
approach. A reduced sample was exposed to a 20% CO/80%
Torr of CO.46 The apparent activation energy for the exchange
H2 mixture at 1 atm of total pressure and 25 °C. After heating
to 200 °C and allowing the spectra to become time-independent,
-
1
of CO on the surface was about 8.5 kcal mol , significantly
-1
3
-1
12
lower than the activation energies of 30-32 kcal mol1 reported
1400 cm min of 20% CO/80% H2 at 1 atm was flowed
through the reactor, with SER spectra being acquired every 10
,18,46
for desorption into vacuum from various Rh surfaces.
Thus,

Carbon Monoxide on Polycrystalline Rhodium
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2881
s. This high flow rate was sufficient to essentially completely
flush out the reactor of the previous mixture within 10 s (i.e.,
-
1
3
residence times). Before the switch, the 10 cm downshifted
13
-1
νRh-C band of terminal CO is evident at 460 cm . It is
1
3
evident upon careful examination that the adsorbed CO is
replaced by 12CO within ca. 100 s, with the 460 cm-1 feature
-
1
shifting upward in frequency to a final value of 470 cm .
Essentially identical transient response experiments were
performed under three different sets of conditions (100% CO,
20% CO/80% H2, and 1% CO/99% H2) at 1 atm total pressure
and varying temperature. The results indicate that, at least up
to 200 °C, the presence of H2 has no significant effect on the
CO exchange rates. Furthermore, varying the partial pressure
12
of CO by 2 orders of magnitude, 8-760 Torr, produced no
detectable change in the CO turnover rate, indicating a zero-
order dependence on CO partial pressure in this range.
The observed lack of dependence on gas-phase CO concen-
tration at our elevated pressures contrasts the results of Zhai
Figure 10. Relative 13CO coverage as a function of time during 13CO/
CO switching for the conditions described in Figure 9. Data points
presented here include those from the experiment described by Figure
9. Least-square fits to first- and second-order kinetic models are
represented by the solid and dashed lines, respectively. The error bars
represent the standard deviation of each point.
4
6-48
and co-workers,
who have observed a first-order depen-
12
-
5
dence on CO partial pressure below 10 Torr. While Yates et
44,45
46
al.
and Song et al. suggest that the CO exchange proceeds
through some intermediate species (possibly a gem dicarbonyl),
this explanation cannot adequately explain the zero-order
dependence of the exchange on gas-phase CO concentration at
higher pressures. The experiments in the literature (and here)
have been performed under conditions where almost all possible
chemisorption sites at a given temperature and CO partial
pressure are occupied. However, surface sites that are energeti-
cally less favorable, yet feasible, for CO chemisorption should
still be available under these conditions. Specifically, these sites
may facilitate the formation of metastable Rh-CO intermediates
capable of exchanging with other adsorbed CO molecules. One
can propose a mechanistic scheme for this process along the
following lines,
The predicted θ13 reaction order may be verified and the rate
constant for the exchange process (ke) estimated by analysis of
1
3
12
the CO/ CO exchange results (Vide supra). Assuming that
there is no dipole-dipole coupling and that large overlap exists
1
3
12
between the νRh- C and νRh- C bands, the observed νRh-C
1
3
frequency will essentially vary linearly with CO coverage.
Using these assumptions, a plot showing the decrease in
fractional CO coverage (θ13/θ13,o) as a function of time may
be produced for each set of transient experiments. This is
achieved by carefully measuring the change in the band
frequency as a function of time and normalizing to the overall
1
3
1
2
12
-1
shift, 10 cm . An example of such an analysis applied to data
CO(g) + [ ] S CO-[ ]
(3a)
*
*
from isotope exchange conducted at 20% CO/80% H2 and 200
°C is presented in Figure 10. Note that the data points utilized
here include those obtained during the experiment presented in
Figure 9. Performing linear regression on the data with a kinetic
model that was first order in θ13 produced notably better fits
than for higher-order cases. (A second-order fit is shown in
Figure 10 for comparison.) Similarly, the exchange process was
most effectively modeled by first-order kinetics under all other
conditions studied (Vide supra), consistent with the derived rate
law above (5). We note again that the presence of H2 in the
gas phase had no observable effect on the exchange process
under these conditions.
1
2
13
12
13
CO-[ ] + CO
S CO
+ CO-[ ] (3b)
*
(ads)
(ads)
*
1
3
13
CO-[ ] S CO + [ ]
(3c)
*
(g)
*
where [ ] denotes an energetically unfavorable site. Assuming
*
13
that the surface concentration of CO-[ ] is negligible (which
*
13
would be expected due to the absence of gaseous CO during
the switch) and using the pseudo-steady-state approximation on
1
2
CO-[ ], a rate equation with the following form is derived
*
dθ13
dt
KP12
In order to gain insight into the mechanism for CO turnover
under reaction conditions, an Arrhenius plot was constructed
)
- keθ13
(4)
[
]
1 + KP12
(Figure 11) consisting of the first-order rate constants obtained
where θ13 is the 13CO coverage, ke is the rate constant for the
at different temperatures under 20% CO/80% H at 1 atm. It is
2
1
2
13
exchange between CO-[ ] and CO (eq 3b, forward), K is
immediately evident that the rate at which CO is exchanging
*
-
1
the equilibrium constant from eq 3a, and P12 is the partial
(kd,tot) is only weakly activated (Ed,tot ) 1 ( 0.1 kcal mol ) at
temperatures from 25 to 100 °C, with a small preexponential
pressure of 12CO. This mechanism and accompanying rate
54
-1
equation are well-known in heterogeneous catalysis and are
factor (kdo,tot ) 0.035 ( 0.003 s ). As no thermal desorption
closely analogous to the well-known Michaelis-Menten kinetics
is expected to occur by these temperatures, Ed,tot and kdo,tot are
essentially equal to E and k , respectively. This activation
for enzymatic systems.55 As the pressure increases, the [ ] sites
e
eo
*
-
1
are filled until saturation is reached, at which point the exchange
energy is much lower than the ca. 8.5 kcal mol observed at
-
8
46
process undergoes a transition from a first-order dependence
lower pressures (10 Torr) by Song et al., again suggesting
a change in the exchange mechanism with increasing CO
pressure (Vide supra). The observed 5-fold increase in the
activation energy between 150 and 200 °C marks the emergence
of competing pathways for CO removal, including thermal
desorption and/or reaction with hydrogen. For instance, based
on CO partial pressure4
6-48
to a pressure-independent behavior,
the rate now being controlled solely by the kinetics of the
exchange reaction. Thus, for the present high-pressure condi-
tions,
dθ /dt ) -k θ
(5)
1,18
1
3
e
13
on reported TPD spectra of CO from polycrystalline Rh, both

2882 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Williams et al.
Figure 12. Calculated fractions of overall CO desorption accounted
for by the exchange (solid line) and thermal (dashed line) pathways as
a function of temperature. These data were produced by using the kinetic
parameters and generating a series of points over the wide temperature
range indicated.
Figure 11. Arrhenius plot consisting of overall desorption first-order
1
3
12
rate constants (kd,tot) obtained during CO/ CO isotope switching at
various temperatures in 80% H /20% CO at 1 atm.
2
-
1
of activation (∆Gact ) 7.5 ( 0.6 kcal mol ) were obtained.
CO exchange (3b) and thermal desorption/adsorption would be
taking place to a significant extent at temperatures greater than
-
8
By varying the pressure from 1 × 10 to 760 Torr and
examining the magnitude of P12 in eq 4, it was determined that
the exchange is an equilibrium-controlled process with a first-
1
50 °C. Thus, the overall exchange first-order rate constant
(kd,tot) measured at these (and actually all) temperatures can be
-
4
order pressure dependence below ca. 10 Torr. In contrast,
represented as a sum of the rate constants for each of the
competing first-order processes:
-
2
higher than ca. 10 Torr the process exhibits no pressure
dependence and is solely controlled by kinetics of the exchange
mechanism (3b).
-
RT
Ee
-Ed
RT
kd,tot ) k + k ) k exp
+ k exp
(6)
e
d
eo
(
)
do
(
)
Concluding Remarks
where ke and kd are first-order rate constants, keo and kdo are
preexponential factors, Ee and Ed are activation energies, R is
the gas constant, and T is temperature. The subscript e refers
to the exchange reaction while d refers to thermal desorption.
By employing the kinetic parameters obtained for the exchange
In this paper we have presented in-situ monitoring of several
processes occurring on a polycrystalline rhodium catalyst in the
presence of ambient pressures of CO and H2. This has been
accomplished through the use of surface-enhanced Raman
spectroscopy (SERS) and mass spectrometry (MS). Several
pieces of insight have been gained:
1. The dominant desorption pathway for adsorbed CO from
Rh in the presence of high CO partial pressures appears to
change from a weakly activated first-order exchange process at
lower temperatures (e150 °C) to first-order thermal desorption
at higher temperatures (g200 °C). The exchange process likely
proceeds via adsorption of CO molecules on unstable, energeti-
cally unfavorable sites, followed by exchange with chemisorbed
CO. Interestingly, the presence of hydrogen had no noticeable
effect on the exchange rates. The exchange mechanism itself
also falls into two regimes, one of which is dependent on
1
3
-1 17
reaction at 25 and 100 °C (ke), assuming that kdo ) 10 s ,
and using the kd,tot values obtained for 150 and 200 °C (Figure
1
0), the activation energy for thermal desorption under these
high-pressure conditions was estimated. A value of Ed ) 31.3
-
1
(
3 kcal mol is obtained, which is consistent with previously
1
,18
reported values for CO desorption from polycrystalline Rh.
Using these kinetic parameters, both the exchange and thermal
desorption rate constants may be calculated for a wide range of
temperatures. From these a plot was constructed that illustrates
the relative importance of exchange and thermal desorption to
the overall CO turnover rate as a function of temperature (Figure
-
4
12). As can be seen, the assisted pathway is dominant for
pressure (<10 Torr) and another that is controlled by the
-
2
temperatures up to 150 °C. However, at higher temperatures
the rate of the thermal process eclipses that of the assisted route,
accounting for almost all of the CO desorption by 250 °C.
Perhaps not surprisingly, thermal desorption begins to dominate
at the temperature for which maxima in TPD curves have been
exchange kinetics (>10 Torr).
2. Methane was observed to be the major product, with
kinetic measurements indicating that the removal of CO from
the surface between 250 and 300 °C causes the reaction rate to
increase dramatically, possibly by freeing active sites for
hydrogen adsorption. Catalyst deactivation was observed after
methanation at 400 °C under all reaction conditions, most likely
as a result of impurities in the reaction system and the lack of
rigorous oxidation/reduction pretreatments of the catalyst at high
temperatures.
1
,18
observed during CO adsorption on polycrystalline Rh.
This
observed change in the dominant CO desorption pathway
underscores the major differences that can exist between
adsorption/desorption processes in higher-pressure and ultrahigh-
vacuum environments.
Having examined the kinetics for assisted CO desorption at
elevated pressures, it is interesting to analyze the lower pressure
3. While only CO was observed on the catalyst surface when
utilizing reactive mixtures with greater than 1% CO, a form of
adsorbed oxygen was also present at higher temperatures when
smaller amounts of CO were reacted. As the presence of this
adsorbed oxygen did not affect overall rates for or selectivity
toward CH4 production, it is probable that this species resides
at sites not involved in the CO hydrogenation reaction.
4. Some CO dissociation was detected at temperatures as
low as 100 °C on these polycrystalline surfaces, as discerned
46
regime investigated by Song et al. on Rh(111). Utilizing their
-
8
pressure-dependent ((2-9) × 10 Torr) CO coverage versus
4
6
time data with the kinetic parameters for ke (keo ) 0.035 (
-
1
-1
0
.003 s , Ee ) 1 ( 0.1 kcal mol ) in eq 4, the equilibrium
constant, K, may be calculated. Using the reported apparent
activation energy of 8.5 ( 0.5 kcal mol , both the preexpo-
nential factor (Ko ) (3.0 ( 0.4) × 10 Torr ) and a free energy
-
1
11
-1

Carbon Monoxide on Polycrystalline Rhodium
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2883
by the temperature-dependent behavior of the 470 cm-1 νRh-C
feature. The apparent concurrent formation of surface carbonate
may also result from this dissociation process. Moreover, the
presence of gas-phase CO or H2 appears to hamper CO
dissociation at lower temperatures, most likely due to blocking
of the sites necessary for the process to proceed.
(24) Wilke, T.; Gao, X; Takoudis, C. G.; Weaver, M. J. Langmuir 1991,
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, 714.
25) Wilke, T.; Gao, X; Takoudis, C. G.; Weaver, M. J. J. Catal. 1991,
(
1
30, 62.
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28) Tolia, A. A.; Smiley, R. J.; Delgass, W. N.; Takoudis, C. G.;
(
5
. The presence of hydrogen causes some adsorbed CO to
(
-
1
switch from a terminal configuration (νRh-C ) 470 cm ) to
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(29) Tolia, A. A.; Williams, C. T.; Takoudis, C. G.; Weaver, M. J. J.
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-
1
2
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(
30) Tolia, A. A.; Williams, C. T.; Weaver, M. J.; Takoudis, C. G.
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(
Acknowledgment. This work was supported by a grant to
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Foundation.
(
(
Electroanal. Chem. 1987, 233, 211.
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(
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