Download CO Oxidation on Palladium. 2. A Combined

2978
J. Phys. Chem. 1994,98, 2978-2981
CO Oxidation on Palladium. 2. A Combined Kinetic-Infrared Reflection Absorption
Spectroscopic Study of Pd( 11 1)
Janos Szanyi,? W. Kevin Kuhn,* and D. Wayne Goodman'
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received: September 9, 1993; In Final Form: December 9, 1993"
The oxidation of carbon monoxide was studied on a single-crystal Pd( 111) catalyst using a n elevated pressure
I R cell/ultrahigh vacuum surface analysis system. The reaction kinetics were followed a t elevated pressures
by measuring the total pressure change during the course of reaction. In the temperature range of 500-570
K an apparent activation energy of 28.1 f 0.3 kcal/mol was determined, which agrees well with the 25 f 2
kcal/mol obtained for the heat of adsorption of CO on Pd( 11 1). Below 500 K the apparent activation energy
decreases to 21 kcal/mol in concert with the decrease in the CO heat of adsorption.
Introduction
The transition metal catalyzed oxidation of carbon monoxide
has been investigated extensively on both supported14 and singlecrystals-14 metal catalysts. The oxidation of carbon monoxide is
a particularly interesting reaction for two reasons: (a) it is a key
reaction in automotive and industrial pollution control, and (b)
its relative simplicity makes it an ideal reaction to investigate
basic phenomena of heterogeneous catalytic processes. The
platinum group metals of Pt, Ir, Ru, Rh, and Pd are the most
frequently studied and used C O oxidation catalysts.
Pd has been extensively studied with respect to C O adat ultrahigh vacuum
sorptionls-19 and CO 0xidation5~~~11~13~1~~20
(UHV) and at elevated pressure conditions. Using molecular
beam methods, Engel and Ertl showed conclusively that CO
oxidation on Pd( 111) proceeds via a Longmuir-Hinshelwood
mechanism21. At low C O coverages (high temperature or low
C 0 / 0 2 ratios), these authors measured a reaction activation
energy of 25 kcal/mol, whereas at high COcoverages an activation
energy of 14 kcal/mol was found.
From these and other extensive studies of C O oxidation, some
general conclusions can be drawn about the mechanism of this
reaction on Pd. The reaction follows a Langmuir-Hinshelwood
mechanisms.* in that both reactants, CO and 02, adsorb on the
catalyst surface prior to reaction between the adsorbed species
(CO, and Oa). Because the surfaces of these metal catalysts are
predominantly covered by adsorbed CO, the rate-limiting step is
believed to be the desorption of CO, which, in turn, determines
the rate of oxygen adsorption. The reaction between CO, and
0, is very fast and the COz desorbs rapidly upon formation.
The oxidation of CO by 0 2 is considered to be a structureinsensitive reaction; however, recent comparative studies of this
reaction on single crystals of Pd indicate subtle differences in the
kinetics among the ( l l l ) , (IIO), and (100) planes13J4. It is
noteworthy in this context that the heats of CO adsorption as a
function of CO coverage on the low-index planes [(loo), (1 lo),
and (1 1l)] of Pd are significantly different, particularly at the
high CO coverage limit's. Since the apparent activation energy
of CO oxidation generally follows the CO heat of adsorption,
then, to the extent that the adsorption of CO is structure sensitive,
the oxidation of CO should be structure sensitive13J4.
The present studies including those described in the preceding
paper were undertaken to establish unequivocally a correlation
Present address: Los Alamos National Laboratory, CLS-1, MS 5565,
Los Alamos, NM 87545.
Present address: P.O. Box 233, USAF Academy, CO 80840-0233.
* To whom correspondence should be addressed.
*Abstract published in Aduance ACS Absrracts, February 15, 1994.
0022-3654/94/2Q98-2918$04.50/0
between the apparent activation energies observed for C O
oxidation on specific crystal planes of Pd and the corresponding
activation energies for CO desorption on that plane measured at
comparable conditions using infrared reflection absorption spectroscopy (IRAS). Here we discuss specifically the results for
Pd(ll1).
Experimental Section
The experiments were carried out in a combined elevated
pressure reactor-UHV surface analysis system described in detail
e l s e ~ h e r e The
~ ~ ~UHV
~ ~ .chamber, with a base pressure of 1.O
X 1 W 0 Torr, is equipped with the basic surface analytical
techniques of Auger electron spectroscopy (AES), low energy
electron diffraction (LEED), and temperature-programmed
desorption (TPD). The elevated pressure reactor cell is interfaced
to the UHV chamber via a doubly differentially pumped Teflon
sliding seal, which allows experiments to be performed a t pressures
up to 1000 Torr. The reaction cell is equipped with two C a F
windows which allows in situ infrared reflection absorption
spectroscopic studies. The IRAS results of C O adsorption on
Pd( 111) reported here were acquired under identical experimental
conditions as described in ref 17.
The Pd( 111) crystal was heated resistively by tantalum leads
attached to the back face of the crystal, and the sample
temperature was monitored by a W-5% Re/W-26% Re thermocouple spot-welded to the edge of the crystal. The manipulator
also allowed the sample to be cooled to 90 K. The sample was
cleaned using the procedures described in ref 24, followed by
annealing to 1200 K. The cleanliness and the long-range order
were verified with AES and LEED, respectively.
Catalytic experimentswere carried out in the elevated pressure
reaction cell in the batch mode. Since in the CO + l / 2 0 2 C02
reaction three molecules of reactants produce two molecules of
products, and no product other than C02 is formed, the kinetics
of C O oxidation can be followed conveniently by monitoring the
total pressure change during the course of reaction. By measuring
the total pressure as a function of reaction time, initial rates of
C02 formation can be estimated.
Research purity (>99.999%, Matheson) CO and 02were used
in this study. CO was further purified by storing the glass bulb
containing the CO under liquid nitrogen throughout the study to
ensure the removal of transition metal (Ni, Fe) carbonyl
impurities. Oxygen was used as received, without any further
purification. CO/Oz gas mixtures with the required ratios were
prepared in the reaction cell before the kinetic measurements.
-
0 1994 American Chemical Society
CO Oxidation on Palladium
A O P :TT
0.400
0.300 +=
.
CO+YB, 4 0 ,
.
,
A
The Journal of Physical Chemistry, Vot. 98, No. I I, I994 2979
(CO+02)/Pd(l19)
V
1.00Torr ; P(02) = 0.50Torr
P(C0)
0
V
Pd(ll1)
V
V
V
V
T
P o = 1.OOTorr
P,,=O.BOTorr
'
T
I
b
b
0.000
0
200
o
400
600
Time, sec
e
b
o
b
o
800
b
o
1000
o
1200
Figure 1. Total pressure change as a function of reaction time in the
(20-02reaction on Pd(ll1). PCO = 1-00Torr; Pot = 0.50 Torr; T =
475, 500,525, 550, 575, and 600 K.
Results and Discussion
The temperature dependence of the COz formation rate on a
Pd( 111) catalyst was studied in the temperature range of 470600Kusing a C 0 / 0 2 = 2/ 1reactant gas mixture a t a total pressure
of 1.5 Torr. The total pressure change (APT)during the course
of reaction is a very convenient and reliable way to determine
initial reaction rates. This is particularly true at high reaction
temperatures where the reaction rate is relatively high. The total
pressure change as a function of reaction time for several reaction
temperatures are displayed in Figure 1. In the temperature range
of 475-525 K the total pressure decreased linearly over the entire
course of the reaction. For temperatures >525 K and at certain
CO conversions (at specific CO partial pressures), the change in
total pressure deviates from linearity toward a lower reaction
rate. The higher the reaction temperature, the lower the C O
conversion (the higher the CO partial pressure) at which the
break in linearity occurs.
As discussed in the preceding paper, the breaks in the data of
the total pressure change versus reaction time represent points
a t which the conversion of CO is sufficient to reduce its partial
pressure below that required to maintain a C O coverage above
the critical CO-limiting coverage a t that particular temperature.
Further reaction leads to a further reduction in the CO partial
pressure (and coverage), yielding a steadily decreasing reaction
rate. At sufficiently high CO conversion, the reaction rate is
essentially independent of temperature reflecting the compensating effect of the reduced CO coverage on the overall reaction
rate. IRAS confirms this explanation in that subsequent to the
breaks in the data of Figure 1, no C O was detected on the surface.
From the initial, linear portion of these total pressure change
versus reaction time plots, initial specific COZproduction rates
can be estimated. The turnover frequency (TOF), or molecules
produced per atom site per second, as a function of reaction time
is shown in Figure 2 in Arrhenius form. This plot is linear in the
temperature range of 500-575 K; however, below 500 K some
change in the slope of the plot is apparent. From the hightemperature part of this plot, an apparent activation energy of
28.1 f 0.3 kcal/mol can be estimated. This number is in good
agreement with the value of 25 kcal/mol obtained by Engel and
Ertl21 in UHV studies.
In order to estblish a correlation between the CO heat of
adsorption and the apparent activation energy for the CO + 0 2
reaction, the adsorption of C O on Pd( 111) was investigated in
the pressure and temperature ranges of 1 X 10-7-10.0 Torr and
9&1000 K, re~pectivelyl~.A representative series of IRAS
spectra of CO on Pd( 111) can be found in Figures 1 and 2 of ref
17 for CO pressures of 10.0 and 1 X 10-6 Torr, respectively. In
Figure 3 the integrated I R intensities are plotted versus the
0.1
-
1.75
1.85
1.95
2.05
. 2.15
1000/(T,K)
Figure 2. Specific COz formation rate as a function of reaction
temperature for the CO-02 reaction on Pd(ll1). P a = 1.00 Torr; PO,
= 0.50Torr.
I
100
COIPd(l1 I)
300
500
700
Temperature, K
900
Figure 3. Representative adsorption isobars of CO on Pd(111) at pressures
(a) 1 X 1od Torr, (b) 1 X lk3Torr, and (c) 10.0 Torr.
adsorption temperature for three selected CO pressures of 1 X
1 V , 1 X le3,
and 10.0 Torr. In thecoverage regionof 0.0-0.33
monolayer, CO adsorbs only in the threefold hollow configuration," and its integrated I R intensity increases linearly with
increasing coverage. At f?co = 0.33 the C O overlayer undergoes
a phase transition, resulting in the formation of an adlayer in
which CO is in the bridging position. At the CO coverage of this
phase transition, a break is evident in the I R intensity versus
temperature plot. The integrated I R intensity drops sharply and
then increases with increasing CO coverage. This behavior can
be attributed to the varying I R absorption cross sections for CO
molecules adsorbed in different bonding configurations. CO is
adsorbed more strongly in the threefold hollow positions than in
the bridging sites, consistent with the C-0 bond having different
dynamic dipole moments in the two positions, and thus different
IR cross sections. Because of the uncertainties in determining
the integrated I R intensities at 6 ~ >
0 0.33, the CO desorption
energy was followed as function of CO coverage only in the
coverage rangeof 0.0-0.3monolayer. From the adsorption isobars
of Figure 3, isosteric plots can be constructed by plotting the
logarithm of the CO pressure versus the reciprocal temperature
at a constant C O coverage. In constructing isosteric plots of this
Szanyi et al.
2980 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
-11
1
v
-12 n-13
0
0
0
-
-
t
I
0.05 M L
0.12 M L
0.24 ML
0.32 ML
,,o’
-3
co,
Pd(li1)
bbb
-
ICp”
P,=i.OOTorr
n
-
L
n
-
-
--14-
lo-’
S
8
t0
T=500K
0.300
I
I
0
n
-15 -
-17‘
CO+’/Zo,
0.40
.200t
’
1.4
1.6
1.8
2.0
2.2
2.4
I
o
a
b
0.
0
0
2.003
3.265
5.002
0.io0
in x io3 (K“)
0
O
D
0
Figure 4. Representative isosteric plots of In Pm versus 1/T for the
CO/Pd(l 11) system.
) 0 O 0
0.ooo
2
4
6
Time,min.
8
*
N p e 6. Total presure change as a function of reaction time for the
CO-02 reaction on Pd( 1 11) at selected 02 partial pressures. Pm = 1-00
Torr; T = 500 K.
c
reaction conditions is estimated to be approximately 0.4 f 0.05
monolayer. This coverage corresponds to a C O heat of adsorption
of 25 f 2 kcal/mol over the entire temperature range of 470-600
K. This value is in good agreement with both our experimental
activation energy of 28.1 i 0.3 kcal/mol obtained at elevated
pressures and the 25 kcal/mol determined by Engel and Ert121
under UHV conditions. The Arrhenius plot of Figure 2 shows
that the activation energy for the CO 0 2 reaction below 500
K is lower than that found above 500 K. Under UHV conditions,
Engel and Ertl2I also observed a significantly lower activation
energy at reaction temperaturesbelow 500 K. Fromour IRdata,”
a CO coverage of approximately 0.5 monolayer is estimated at
500 K, corresponding to a C O desorptionenergy of approximately
17 kcal/mol. For the low-temperature region under UHV
conditions an activation energy of 14 kcal/mol is found.21 This
reduced value was interpreted to arise due to the compression
and thus the decreased stability of oxygen islands. The results
of the present study suggest that the reduction in theCO desorption
energy alone is adequate to explain the observed decrease in the
apparent activation energy.
The effect of oxygen partial pressure on the C02 formation
rate was studied at a reaction temperature of 500 K in the oxygen
pressure range of 0.5-5.0Torr at a constant CO pressure of 1.0
Torr. The total pressure change during the CO + 0
2 reaction
on Pd( 11 1) as a function of reaction time for different oxygen
pressures is shown in Figure 6. The total pressure changeslinearly
over the entire course of the reaction for a stoichiometric C 0 / 0 2
gas mixture (Pol = 0.5 Torr). As the oxygen partial pressure is
increased, the initial rate of COz formation increases, reflecting
the positive order of the reaction rate in oxygen pressure. The
steadily increasing rates observed for the oxygen-rich mixtures
are a consequence of the increasing Oz/CO ratio with reaction
time. At a particular CO conversion in Figure 6, a break in the
total pressure change versus time plot is apparent and is due to
departure of the CO coverage from the critical CO-limitingvalue.
IRAS inspectionof the surfacesubsequent to the breaks in Figure
7, as noted for the breaks in the data of Figure 1, shows no
detectable CO on the surface.
From the initial slopes of the total pressure change versus
reaction time plots, the kinetic order with respect to the oxygen
pressure can be determined. Specific C02 formation rates as a
function of oxygen pressure are shown in Figure 7. From the
slope of this plot the order of oxygen pressure is found to be 0.97
+
I
0.0
0.1
0.2
0.3
0.4
1
CO COVERAGE (ML)
Figure 5. Isosteric heat of adsorption of CO on Pd( 111) as a function
of CO coverage. For comparison literature data of Ertl et al.z5and Yates
et a1.16 are also displayed.
kind, identical integrated IR intensities obtained at different CO
pressures are assumed to represent identical CO coverages.
Representative isosteric plots are shown in Figure 4 for CO
coverages of 0.005, 0.12, 0.24, and 0.31 monolayer. Applying
the Clausius-Clapeyron equation, d(ln P)/d(l/T) = -E/R,
desorption activation energies can be determined for various CO
coverages. The isosteric heat of adsorption of CO on Pd( 111)
as a function of CO coverage is displayed in Figure 5. For
comparison, literature data16f5 from two additional studies are
also shown. The initial CO heat of adsorption of 34.9 kcal/mol
is in good agreement with the previous literature values. As the
CO coverage increases from 0.005 to 0.3 monolayer, the CO
desorption activationenergy decreasesmonotonically and reaches
a value of 24.7 kcal/mol at 8co = 0.3. The change in desorption
activation energy with coverage is similar to that measured by
Guo et a1.,I6 but significantly lower than that determined by
Conrad et al.25 The desorption energy of CO on Pd( 11 1) in
GUO’Swork’6wasdeterminedfromTPDdata, while work function
measurements were used in Conrad’s study.25
Adsorbed oxygen does not affect significantly the adsorption
of CO on Pd(lll).2l On the other hand, a strong inhibition of
oxygen adsorption on Pd( 11 1) was observed for a CO precovered
surface. On the basis of these results and data for the CO + 02
reaction on Pd( 100),14 the adsorption of CO apparently is not
significantly affected by the presence of gas-phase oxygen.
Therefore, the CO adsorption IR data obtained at 1.O Torr of CO
pressure can be used to estimate the C O coverages on Pd( 11 1)
under CO oxidation conditions. From the CO stretching
frequency versus C O coverage plot, the CO coverage under our
CO Oxidation on Palladium
CO+’hO, -GO,
t
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2981
Pd(il1)
/
P,,=l .OOTorr
T=500K
agreement with the 25 f 2 kcal/mol determined for the CO heat
of adsorption on Pd(ll1). At temperatures below 500 K the
apparent activation energy decreases in concert with the decrease
in the CO heat of adsorption.
Acknowledgment. We acknowledge with pleasure the support
of this work by the Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences.
References and Notes
(1) Oh,S.H. J . Curd 1990, 124, 477.
0.0
0.5
Log(P,,Torr)
I.o
F¶gure 7. Specific C02 formation rate on Pd( 111) as a function of 02
partial paressure. Pm * 1.00 Torr;T = 500 K.
f 0.1, in good agreement with the generally found positive first
order oxygen partial pressure dependence of COz formation.
Summary
(1) Following the total pressure change during the oxidation
of carbon monoxide is shown to be a convenient way to define
the kinetics and to investigate the reaction dynamics over a wide
range of reaction parameters.
(2) The apparent activation energy of 28.1 f 0.3 kcal/mol
found over the temperature range of 470-570 K is in good
(2) Cant, N. W.; Hicb, P. C.; Lennon, B. S. J . Cutul. 1978, 54, 372.
(3) Oh, S. H.; Carpenter, J. E. J. Cutul. 1983, 80, 472.
(4) Cant, N. W.; Angove, D. E. J . Curd. 1986, 97, 36.
(5) Engcl, T.; E d , G. Adu. Cutal. 1978, 28, 1.
(6) Pcden, C. H. F.; Houston, J. E. J. Carol. 1991, 128,405.
(7) Logan, A. D.; Paffett, M. T. J. Cutal. 1992, 133, 179.
(8) Oh,S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J. Curd
1986, 100, 360.
(9) Goodman, D. W.; Pcden, C. H. F. J. Phys. Chem. 1986,90,4839.
(10) Pcden. C. H. F.: Goodman. D. W.: Blair. B. S.:Berlowitz., P. J.:.
Fisher; G. B.; Oh, S. H. Phys. Chem. l d , 92,’1563.‘
(11) Berlowitz, P. J.; Peden, C. H. F.; Goodman, D. W. J. Phys. Chem.
1988, 92, 5213.
(12) Pcden, C. H. F.; Goodman, D. W.; Weisel, M.D.; Hoffmann, F. M.
Surf.Sci. 1991, 253, 4.
(13) Xu, X.; Szanyi,J.; Goodman, D. W.Cutal. Toduy, in press.
(14) Szanyi,J.; Goodman, D. W. J. Phys. Chem., preceding paper in this
issue.
(15) Conrad, H.; Ertl, G.; Koch, J.; Latta, E. E. Surf. Sei. 1974,13,46.
(16) Guo, X.; Yates, J. T. J. Chem. Phys. 1989,90 ( l l ) , 6761.
(17) Kuhn, W. K.; Szanyi, J.; Goodman, D.W., Surf.Sci. 1992, 274,
L611.
(18) Szanyi, J.; Kuhn, W. K.; Goodman, D. W. J . VUC.Sci. Techno/.A
1993, 11, 1969.
(19) Bradshaw, A. M.; Hoffmann, F. M. Surf.Sei. 1978, 72, 513.
(20) Stuve, E. M.; Madix, R. J.; Brundk, C. R.Surf. Sci. 1984,146,155.
(21) Engel, T.; Ertl, G. J . Chem. Phys. 1978,69 (3), 1267.
(22) Leung, L-W. H.; He, J-W.; Goodman, D. W. J . Chem. Phys. 1990,
93, 8378.
(23) Campbell, R. A,; Goodman, D. W. Reo. Sci. Instrum. 1992,63,172.
(24) Grunze, M.; Ruppender, H.; Elshazly, 0. J. Vuc. Sci. Techno/. A
1988, 6, 1266.
(25) Conrad, H.; Ertl, G.; Kuppers, J. Surf.Sci. 1978, 76, 323.