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.
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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
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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
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-
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-
L
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-
-
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0.300
I
I
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-17‘
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.200t
’
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2.4
I
o a b
0.
0 0
2.003 3.265 5.002
0.io0
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0
O
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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.
