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Langmuir 1999, 15, 3798-3806
Adsorption Studies of Gases on Pt-Rh Bimetallic Catalysts by Reversed-Flow Gas Chromatography D. Gavril, A. Koliadima, and G. Karaiskakis* Department of Chemistry, University of Patras, GR-26500 Patras, Greece Received November 30, 1998. In Final Form: March 1, 1999 In the present work, the relatively new technique of reversed-flow gas chromatography was applied for the study of adsorption of carbon monoxide, oxygen, and carbon dioxide on Pt-Rh bimetallic catalysts. Using suitable mathematical analysis, equations were derived by means of which rate constants for adsorption, desorption, and disproportionation reaction were calculated. From the variation of these rate constants with temperature and the nature of the catalyst (Pt content), as well as from the finding that the CO adsorption is a dissociative process, useful conclusions concerning the mechanism for the CO oxidation reaction over Pt-Rh bimetallic catalysts were extracted. The catalytic fractional conversions for the CO disproportionation reaction were found to be higher for the Pt-Rh bimetallic catalysts than those for the pure Pt catalyst, indicating the presence of beneficial Pt-Rh synergism.
Introduction In heterogeneous catalysis the reaction process can be resolved into the following steps: (i) transport of the reactants from the reaction mixture to the external surface of the catalyst particles; (ii) transport of the reactants from the external surface of the catalyst particles to the internal surface; (iii) adsorption of the reactants on the active centres of the catalyst; (iv) reaction between adsorbed reactants; (v) desorption of the reaction products; (vi) transport of the reaction products from the internal surface of the catalyst particles to the external surface; and (vii) Transport of the reaction products from the external surface of the catalyst to the reaction mixture. It is clear that in studies regarding the mechanism of catalytic reactions considerable attention must be paid to the processes of adsorption of the reactants on the catalyst active sites and desorption of the reaction products. In the present work, we shall study in detail the adsorption of carbon monoxide, oxygen, and carbon dioxide on Pt-Rh bimetallic catalysts and the influence of such adsorption on the kinetics of the CO oxidation reaction in order to extract useful conclusions about the mechanism of the CO oxidation reaction over the bimetallic Pt-Rh catalysts. The CO oxidation was chosen because of the well-known interest in this reaction in connection with the control of automotive emissions. For the study of adsorption and desorption of carbon monoxide, oxygen, and carbon dioxide over the Pt-Rh bimetallic catalysts supported on SiO2, the relatively new technique of reversed-flow gas chromatography (RFGC) was applied.1-4 This method permits the determination of adsorption/desorption rate and equilibrium constants and of catalytic rate constants, all simultaneously in a single gas chromatographic experiment. It consists in reversing the direction of the carrier gas flow, usually for a short time interval (5-100 s). This is shown schematically in Figure 1. If pure carrier gas is passing through * To whom correspondence should be addressed. (1) Katsanos, N. A.; Karaiskakis, G. In Advances in Chromatography; Giddings, J. C., Ed.; Marcel Dekker, Inc.: New York, 1984; Vol. 24, p 125. (2) Karaiskakis, G.; Katsanos, N. A. J. Phys. Chem. 1984, 88, 3674. (3) Gavril, D.; Karaiskakis, G. Instrum. Sci. Technol. 1997, 25(3), 217. (4) Gavril, D.; Karaiskakis, G. Chromatographia 1998, 47, 63.
the sampling column, nothing happens on reversing the flow. But if a solute comes out of the diffusion column at z ) 0 (cf. Figure 1) as the result of its diffusion into the carrier gas, filling the column z and also running along the sampling column l′ + l, the flow reversal records the concentration of the solute at the junction x ) l′, at the moment of the reversal. This concentration recording has the form of extra chromatographic peaks “sample peaks” superimposed on the otherwise continuous detector signal. An example is given in Figure 2. The peaks can be made as narrow as one wants, since the width at their halfheight is equal to the duration of the backward flow of the carrier gas through the empty sampling column. If the concentration of a constituent in the flowing gas depends on a rate process taking place within the gas diffusion column L containing the catalyst bed, then, by repeatedly reversing the flow, one performs a repeated sampling of this rate process. Using suitable mathematical analysis, the so-called chromatographic sampling equation, describing the concentration-time curve of the sample peaks created by the flow reversals, is derived by means of which the rate coefficients of the slow processes responsible for the sample peaks are determined. Reversed-flow gas chromatography has been successfully used to determine gas diffusion coefficients in binary and ternary mixtures,5,6 adsorption equilibrium constants,7 mass transfer coefficients,2-4,8-10 molecular diameters and critical volumes of gases,11 Lennard-Jones parameters,12 activity coefficients,13 Flory-Huggins interaction parameters and solubility parameters in polymer-solvent systems,14 rates of drying of catalysts,15 rate (5) Katsanos, N. A.; Karaiskakis, G. J. Chromatogr. 1982, 237, 1. (6) Karaiskakis, G.; Katsanos, N. A.; Niotis, A. Chromatographia 1983, 17, 310. (7) Karaiskakis, G.; Katsanos, N. A.; Niotis, A. J. Chromatogr. 1982, 235, 21. (8) Katsanos, N. A.; Dalas, E. J. Phys. Chem. 1987, 91, 3103. (9) Katsanos, N. A.; Agathonos, P.; Niotis, A. J. Phys. Chem. 1988, 92, 1645. (10) Agathonos, P.; Karaiskakis, G. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1357. (11) Karaiskakis, G.; Niotis, A.; Katsanos, N. A. J. Chromatogr. Sci. 1984, 22, 554. (12) Karaiskakis, G. J. Chromatogr. Sci. 1985, 23, 360. (13) Katsanos, N. A.; Karaiskakis, G.; Agathonos, P. J.Chromatogr. 1986, 349, 369. (14) Agathonos, P.; Karaiskakis, G. J. Appl. Polym. Sci. 1989, 37, 2237.
10.1021/la981653k CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999
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Figure 1. Schematic representation of the reversed-flow gas chromatographic technique for studying the adsorption of gases on Pt-Rh bimetallic catalysts.
Figure 2. Reversed-flow chromatogram for the adsorption of CO on the catalyst 25% Pt + 75% Rh. The first peak belongs to the adsorbate CO, while the second one belongs to the CO2 resulting from the dissociative adsorption of CO.
constants, activation parameter, as well as catalytic conversions of the reactants into products for various important surface-catalyzed reactions,16-19 etc. Experimental Section Materials. The catalysts used, which were supplied by Dr. Nieuwenhuys at Leiden University (The Netherlands), were pure Pt supported on SiO2 (3% w/w) and the bimetallic catalysts Pt0.25 (15) Karaiskakis, G.; Lycourghiotis, A.; Katsanos, N. A. Chromatographia 1982, 15, 351. (16) Katsanos, N. A.; Karaiskakis, G.; Niotis, A. J. Catal. 1985, 94, 376. (17) Karaiskakis, G.; Katsanos, N. A.; Lycourghiotis, A. Can. J. Chem. 1983, 61, 1853. (18) Kapolos, J.; Katsanos, N. A.; Niotis, A. Chromatographia 1989, 27, 333. (19) Gavril, D.; Koliadima, A.; Karaiskakis, G. Chromatographia 1999, 49, 285.
+ Rh0.75 and Pt0.75 + Rh0.25 supported also on SiO2 (3% w/w). The method of preparation and the surface characterization of the catalysts using TDS and XPS have been presented previously.20,21 Before use, the catalysts were reduced at 628 K for 10 h in flowing hydrogen at a flow rate of 1.0 cm3 s-1. Carbon monoxide from Linde A.G. (99.97% pure), carbon dioxide from Matheson Gas Products (99.97% pure), and oxygen from BOC Gases (99.999%) were used as model gas adsorbates. The carrier gas was helium (99.999% pure) from BOC Gases, while the hydrogen for the reduction of the catalysts was from Linde A.G. (99.999% pure). The chromatographic material was silica gel 80-100 mesh from Supelco A.G. Apparatus and Procedure. The experimental setup of the reversed-flow gas chromatography technique for the adsorption studies of gases on the Pt-Rh bimetallic catalysts, which has been described in detail elsewhere1-4 and in the Introduction, is shown diagrammatically in Figure 1. The lengths l′ and l of the stainless-steel sampling cell were 38 cm each, while the length L of the diffusion column, which was connected perpendicularly at its lower end to the middle of the column l′ + l, was 117.6 cm. Both columns, which were accommodated inside the oven 1 of a usual gas chromatograph (Shimadzu 8A), were empty of any material, except for a short length (∼1 cm) at the upper end of column L, which contained the catalyst (∼0.1 g). The end D1 of the sampling column l′ + l was connected, via a six-port valve, to the carrier gas helium supply, while the other end D2 was connected to the separation column L′, which was placed in oven 2 of another gas chromatograph (Pye-Unicam, series 104). The end of this column, which has been filled with 7.6 g of silica gel 80-100 mesh for the separation of the adsorbate gases with possible products (e.g., separation of CO from CO2 resulted from the CO disproportionation reaction), was connected to the thermal conductivity detector (TCD), as shown. Before measurements, the whole system was conditioned by heating in situ the catalyst bed at 743 K and the silica gel at 423 K both for 20 h, under carrier gas helium flow of 1 cm3 s-1. Following this, the temperatures of the two ovens were regulated to those chosen for a kinetic experiment. At these working temperatures preliminary injections of 1 cm3 of the adsorbates CO, O2, and CO2 (at atmospheric pressure) were made with a gas-tight syringe to establish constant catalytic activity. After the chromatographic trace on the recorder had subsided to a negligible height above the baseline, new 1 cm3 injections of CO, O2, and CO2 were made at the top of the diffusion column L. In about 10 min a continuous concentration-time curve, increasing initially and then decreasing after a maxiumum, was recorded (20) Wolf, R. M.; Siera, J.; van Delft, F. C. M. J. M.; Nieuwenhuys, B. E. Faraday Discuss. Chem. Soc. 1989, 87, 275. (21) van Delft, F. C. M. J. M.; Nieuwenhuys, B. E.; Siera, J.; Wolf, R. M. ISIJ Int. 1989, 29, 550.
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due to both the adsorbate and the possible products. During this period, flow reversals for 5 s, which is a shorter time period than the gas holdup time in both column sections l and l′, were effected by means of the six-port valve. When the gas is restored to its original direction, one or two (in case the adsorbate reacts on the catalyst surface and gives a product) symmetrical “sample peaks”, like those in Figure 2, were recorded corresponding to various times from the beginning. The first peak in Figure 2 belongs to the adsorbate CO and the second to the product CO2, which results from the CO disproportionation reaction. The above flow reversal procedure was repeated many times at each temperature, giving rise to one (for the adsorbates O2 and CO2) or two (for the adsorbate CO) series of sample peaks, each peak or pair of peaks corresponding to a different time from the adsorbate injection. The pressure drop along the whole system was 0.33 atm, and the catalyst bed was under a pressure of 1.33 atm during all experiments.
Theory The sample peaks of Figure 2 are predicted theoretically by the “chromatographic sampling equation” describing the concentration-time curve of the sample peaks created by the flow reversals. The area under the curve or the height H from the continuous signal of the sample peaks, measured as a function of time t when the flow reversals were made, is proportional to the concentration of the substance under study at the junction x ) l′ of the sampling cell (cf. Figure 1), at time t
H1/M ) gc(l′,t)
(1)
where M is the response factor of the detector and g a proportionality constant (usually assumed unity for convenience) pertaining to the detector calibration. Measuring H experimentally as a function of t, one can construct the diffusion band, which in the case of adsorption of the gases CO, O2, and CO2 on the Pt-Rh bimetallic catalysts is described by the equation22
H
1/M
Z X+Y )N1+ exp t + Y 2 Z X-Y t (2) N1exp Y 2
(
) (
)
(
) (
gma1 2(1 + V1)V4
(5)
Z ) X - 2(k-1 + k2)
(6)
L21
(9)
In the above equations, m is the injected amount of the adsorbate gases in mol, V˙ is the volumetric flow-rate of the carrier gas, k1 and k-1 are the rate constants for adsorption and desorption, respectively, k2 is the firstorder rate constant for a possible surface reaction, is the external porosity of the catalyst bed, VG and V′G are the gaseous volumes of empty sections L1 and L2, respectively (cf. Figure 1), and D1 and D2 are the diffusion coefficients of the adsorbate gases in the carrier gas helium at temperatures T1 (in the column L1) and T2 (in the column L2 filled with the catalyst bed), respectively. Equation 2 consists of the sum of two exponential functions of time. Quantitative results leading to the determination of the kinetic parameters k1, k-1, and k2 for the adsorption of CO, O2, and CO2 on the Pt-Rh bimetallic catalysts used are obtained by first calculating the preexponential factors A1 ) N(1 - (Z/Y)) and A2 ) N(1 + (Z/Y)) and the exponential coefficients of time B1 ) - (X - Y)/2 and B2 ) - (X + Y/2) of eq 2. By adding the two exponential coefficients B1 and B2 of eq 2, which can be calculated by a nonlinear regression analysis computer program, we find the value of X:
(
B 1 + B2 ) -
X-Y X+Y + ) -X 2 2
) (
)
(10)
Subracting B1 from B2, we obtain the value of Y:
(
B 2 - B1 ) -
X+Y X-Y - ) -Y 2 2
) (
)
(11)
The value of Z is found from the ratio of the two preexponential factors A1 and A2 (F ) A1/A2):
Z)
k1 )
X2 - Y2 a1(k-1 + k2) V1k1k2 ) + 4 1 + V1 1 + V1
2D1
V′G a1 + VG a2
(3)
(4)
a1 )
(8)
L22
1-F Y 1+F
(12)
The rate constants k1, k-1, and k2 can be calculated from the found values of X, Y, and Z in conjuction with eqs 4, 5, and 6, by means of the following relations:
V1k1 a1 + + k-1 + k2 1 + V1 1 + V1
X)
2D2
V1 ) 2
)
where
N)
a2 )
(7)
(22) Topalova, I.; Niotis, A.; Katsanos, N. A.; Sotiropoulou, V. Chromatographia 1995, 41, 227.
(X + Z)(1 + V1) - 2a1 2V1
(13)
X-Z - k2 2
(14)
k-1 )
k2 )
(X2 - Y2)(1 + V1) - 2a1(X - Z) 2(X + Z)(1 + V1) - 4a1
(15)
The previous mathematical analysis can be applied to the adsorption of carbon monoxide on the catalysts, which react on the catalyst surface. When the adsorbate does not react on the catalyst surface (k2 ) 0), as is the case for the adsorption of oxygen and carbon dioxide, the rate constants k1 and k-1 are calculated by the relations
k1 )
(X + Z)(1 + V1) - 2a1 2V1
(16)
Studies of Gases on Pt-Rh Bimetallic Catalysts
k-1 )
X-Z 2
Langmuir, Vol. 15, No. 11, 1999 3801
(17)
where
X)
a1 + V1k1 + (1 + V1)k-1 1 + V1
(18)
Table 1. Rate Constants for Adsorption (k1), Desorption (k-1), and Disproportionation Reaction (k2) of Carbon Monoxide, as Well as Rate Constants for Adsorption and Desorption of Oxygen and Carbon Dioxide over the Pure Pt Catalyst Determined by Reversed-Flow Gas Chromatography (RFGC) at Various Temperatures gas substance T/°C k1 (10-1 s-1) k-1 (10-4 s-1) k2 (10-4 s-1) CO
a1k-1 X 2 - Y2 ) 4 1 + V1
(19)
Z ) X - 2k-1
(20)
The last equations show that for the calculation of the rate constants k1, k-1, and k2 by the reversed-flow gas chromatography technique, the following parameters are required: (i) The auxiliary parameters X, Y, and Z calculated by means of eqs 10, 11, 12, 18, 19, and 20. (ii) The diffusion parameters a1 and a2, which are found from the diffusion coefficients D1 and D2, respectively. The determination of diffusion coefficients in binary or ternary gas mixtures is described in detail elsewhere.5,6 (iii) The parameter V1 defined by eq 9. (iv) The external porosity of the catalyst bed, which can be measured by a method described elsewhere.23 Results and Discussion The experimental data show that the adsorption of carbon monoxide on the catalysts used is a dissociative process, as each flow reversal of the RFGC technique gives two sample peaks (cf. Figure 2). Identification of the two sample peaks shows that the first peak belongs to the adsorbate CO, while the second one to the product CO2 resulted from the disproportionation reaction of CO. XRD analysis of the catalysts at the end of the experiments has shown the deposition of carbon on the catalytic surface and verified the dissociative adsorption of CO. On the other hand, the adsorption of oxygen and carbon dioxide on the same catalysts is a nondissociative process, as each flow reversal in RFGC gives only one peak (the first peak of Figure 2), which belongs to the adsorbate oxygen or carbon dioxide. The finding that the adsorption of CO on the Pt-Rh bimetallic catalysts is a dissociative process, leads to the conclusion that the oxidation of CO on noble metals follows the mechanism:
CO(g) + * T CO(ads) CO(ads) + * f C(ads) + O(ads) O2(g) + 2* T 2O(ads) CO(ads) + O(ads) f CO2(g) + 2* where * denotes the concentration of vacant active sites. In the above mechanistic scheme it was taken into consideration that the primary reaction pathway for the oxidation of carbon monoxide on noble metals is a surface reaction between adsorbed CO molecules and O adatoms.20,21,24 From the above it is concluded that the adsorption of CO leads to the determination of rate constants for adsorption (k1), desorption (k-1), and disproportionation (23) Katsanos, N. A.; Thede, R.; Roubani-Kalantzopoulou, F. J. Chromatogr. A 1998, 795, 133. (24) Oh, S. H.; Carpenter, J. E. J. Catal. 1986, 98, 178.
O2
CO2
282 301 323 343 361 370 385 400 419 435 451 279 300 330 350 371 400 430 280 301 325 350 371 375 400 425 450
1.33 1.41 1.59 1.63 1.86 1.84 1.83 1.98 2.13 2.27 2.68 1.49 1.56 1.80 1.81 1.78 1.99 2.31 1.17 1.50 1.58 1.53 1.53 1.56 1.76
6.09 6.48 6.76 6.85 7.35 7.63 7.82 8.23 8.74 9.29 9.62 9.20 9.90 10.24 11.22 11.39 12.55 12.69 7.75 7.61 8.69 9.36 9.55 10.61 10.86 11.83 11.37
2.80 2.90 3.63 4.04 4.13 3.78 3.98 4.00 4.15 4.58 4.53
reaction (k2), while the adsorption of O2 and CO2 leads to the determination of the rate constants k1 and k-1, as the adsorbate gases do not react on the catalyst bed and k2 ) 0. The rate constants for adsorption (k1) of carbon monoxide, oxygen, and carbon dioxide on the pure Pt catalyst, as well as on the bimetallic Pt-Rh (Pt0.25 + Rh0.75 and Pt0.75 + Rh0.25) catalysts supported on SiO2 are listed in Tables 1-3. From these values and their variation with temperature shown in Figure 3 the following conclusions can be drawn: (i) All the k1 values for the adsorption of CO, O2, and CO2 on the pure Pt catalyst increase with increasing temperature (cf. Figure 3a). (ii) The k1 values for the adsorption of CO2 (k1CO2) on the pure Pt catalyst are lower than those for the adsorption of O2 (k1O2) and CO (k1CO) in all the working temperature range (279 < T < 451 °C). (iii) At temperatures lower than about 360 °C, which is close to the characteristic temperature with the maximum conversion of the CO disproportionation reaction over the pure Pt catalyst (cf. Table 4 and Figure 8), the k1 values follow the order: k1O2 > k1CO > k1CO2. At temperatures higher than about 360 °C the k1O2 value approaches that of k1CO, and the k1 values follow the order k1O2 ≈ k1CO > k1CO2. (iv) The k1 values for the adsorption of CO, O2, and CO2 on the catalyst Pt0.25 + Rh0.75 increase with increasing temperature (cf. Figure 3b). (v) At temperatures lower than about 380 °C, which is the temperature in which the maximum conversion, xmax, for the CO disproportionation reaction is observed (cf. Table 4 and Figure 8), the k1 values for the adsorption of CO, O2, and CO2 on the bimetallic catalyst Pt0.25 + Rh0.75 follow the order k1CO2 > k1O2 > k1CO. On the other hand, at temperatures higher than 380 °C the k1 values vary as follows: k1CO > k1O2 > k1CO2. (vi) The k1 values for the adsorption of CO and CO2 on the bimetallic catalyst Pt0.75 + Rh0.25 increase with increasing temperature, while those for the adsorption of O2 on the same catalyst decrease with increasing temperature (cf. Figure 3c). The latter,
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Table 2. Rate Constants for Adsorption (k1) and Desorption (k-1) of Carbon Monoxide, Oxygen, and Carbon Dioxide, as Well as Rate Constants for the Disproportionation Reaction of CO (k2) over the Bimetallic Pt0.75 + Rh0.25 Catalyst Determined by RFGC at Various Temperatures gas substance T/°C k1 (10-1 s-1) k-1 (10-4 s-1) k2 (10-4 s-1) CO
O2
CO2
279 302 321 340 361 380 401 436 279 301 321 341 361 381 401 436 280 301 321 361 381 401 435
0.91 1.04 1.54 1.38 1.53 1.16 1.36 1.52 2.12 1.77 1.89 1.39 1.33 1.17 1.35 1.01 0.97 0.91 1.01 1.21 1.34 1.67
6.33 6.57 6.86 7.34 7.97 8.18 8.88 9.50 6.09 8.41 8.84 10.02 9.24 10.90 10.93 10.77 7.35 7.71 7.79 8.51 9.05 9.73 10.22
1.86 2.03 2.37 2.28 2.37 2.18 2.31 2.45
Table 3. Rate Constants for Adsorption (k1) and Desorption (k-1) of Carbon Monoxide, Oxygen, and Carbon Dioxide over the Bimetallic Pt0.25 + Rh0.75 Catalyst, as Well as Rate Constants for the Disproportionation Reaction of CO (k2) over the Same Catalyst Determined by RFGC at Various Temperatures gas substance T/°C k1 (10-1 s-1) k-1 (10-4 s-1) k2 (10-4 s-1) CO
O2
CO2
280 301 326 351 376 405 420 464 280 301 325 350 375 400 425 450 472 280 301 325 350 375 400 425 435
0.76 0.73 0.94 1.21 1.39 1.83 2.35 2.64 0.91 1.08 1.09 1.27 1.33 1.73 2.01 2.45 2.78 1.32 1.59 1.52 1.59 1.63 1.83 1.87
2.59 2.29 3.70 4.80 6.19 7.26 8.44 13.21 8.52 8.95 9.50 10.26 10.88 12.06 12.70 14.18
5.21 5.76 5.34 5.36 4.72 3.84 3.90
6.84 7.74 8.28 8.50 8.99 9.66 10.87 11.48
which has been also presented previously,22,24 can be attributed to the following reason: The Pt surface concentration in the Pt-rich catalyst (Pt0.75 + Rh0.25), compared to the Rh surface concentration, according to the surface phonon softening model,20,21 becomes larger with increasing temperature. Considering that the Pt surface in the Pt-Rh alloy catalysts is predominantly covered with CO, while the Rh surface strongly adsorbs oxygen, the adsorption of oxygen on the Pt-rich catalyst decreases with increasing temperature.
Figure 3. Variation of rate constant for the adsorption (k1) of carbon monoxide, oxygen, and carbon dioxide on the catalysts: (a) 100% Pt, (b) 25% Pt + 75% Rh, and (c) 75% Pt + 25% Rh.
As far as the rate constants for the desorption (k-1) are considered (cf. Tables 1-3), the k-1 values for the desorption of CO, O2, and CO2 from the catalysts pure Pt, Pt0.25 + Rh0.75, and Pt0.75 + Rh0.25 increase with increasing temperature (cf. Figure 4). The k-1 values follow the same order for all catalysts in the working temperature range k-1O2 > k-1CO2 > k-1CO. From the variation of the k1CO values with temperature for the three catalysts used, which is shown in Figure 5a, it is concluded that in all cases k1CO increases with increasing temperature. Another important conclusion that can be extracted from the same figure is that at temperatures lower than 370 °C the k1CO values follow the order k1CO (100% Pt) > k1CO (75% Pt) > k1CO (25% Pt),
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Table 4. Catalytic Fractional Conversions, x, of Carbon Monoxide to Carbon Dioxide for the CO Disproportionation Reaction over Pt-Rh Catalysts Calculated by RFGC at Various Temperatures Catalyst 100% Pt
75% Pt + 25% Rh
25% Pt + 75% Rh
T/°C
x
xmax
282 301 343 361 385 419 456 279 302 321 340 361 380 401 435 280 301 326 351 376 390 405 420 435 464
0.108 0.161 0.229 0.230 0.232 0.160 0.120 0.292 0.306 0.315 0.318 0.319 0.302 0.282 0.258 0.337 0.370 0.392 0.429 0.446 0.441 0.408 0.392 0.366 0.265
0.232
0.319
0.446
while at temperatures higher than 370 °C the order of the k1CO values changes: k1CO (25% Pt) > k1CO (100%) Pt) > k1CO (75% Pt). The same holds for the variation of the k-1CO values with temperature shown in Figure 5b. The k-1CO values for the three catalysts used increase with increasing temperature, and their variation with the catalyst Pt content follows the order k-1CO (75% Pt) ≈ k-1CO (100% Pt) > k-1CO (25% Pt). The rate constants for the disproportionation reaction of CO, k2CO (cf. Tables 1-3) increase with increasing temperature for the catalysts 100% Pt and 75%Pt + 25% Rh and decrease with increasing temperature for the catalyst 25% Pt + 75% Rh (cf. Figure 5c). This anomalous observation can be explained as follows: The calculated rate constants for the disproportionation reaction of CO, k2CO, are the apparent ones. These are related to the true k2CO(true) values via the equation: k2CO(true) ) k2CO/K, where K is the equilibrium constant for the Boudouard (disproportionation) reaction 2CO T C + CO2. It is well known in the literature25,26 that the equilibrium constant K for the Boudouard reaction over noble metals decreases drastically with temperature due to the fact that at low temperatures the CO disproportionation is kinetically controlled, while at higher temperatures equilibrium controls the product composition. Thus, a small decrease of the k2CO value with temperature, accompanied by a higher decrease of the equilibrium constant K, leads to an increase in the k2CO(true) value with temperature, as the theory predicts. The experimental observation of the k2CO decrease with temperature only in the Rh-rich catalyst (Pt0.25 + Rh0.75) can be explained by the fact that this catalyst is the most active for the CO disproportionation reaction, as the present k2CO values indicate (cf. Tables 1-3 and Figure 5c) and the literature data24-28 confirm. The experimental observation of the k2CO decrease with temperature in the presence of the Rh-rich catalyst should (25) Maciejewski, M.; Baiker, A. J. Phys. Chem. 1994, 98, 285. (26) Low, G. G.; Bell, A. T. J. Catal. 1979, 57, 397. (27) Beck, D. D.; Carr, C. J. J. Catal. 1993, 144, 296. (28) Hu, Z.; Allen, F. M.; Wan, C. Z.; Heck, R. M.; Steger, J. J.; Lakis, R. E.; Lyman, C. E. J. Catal. 1998, 174, 13.
Figure 4. Temperature dependence of rate constant for the desorption (k-1) of CO, O2, and CO2 from the catalysts: (a) 100% Pt, (b) 25% Pt + 75% Rh, (c) 75% Pt + 25% Rh.
be also explained in terms of thermodynamics, taking into account the enthalpy (∆H°) of the CO disproportionation reaction. The variation of K with temperature is described by the well-known van’t Hoff equation. The disproportionation reaction may be an exothermic (∆H° < 0) or an endothermic (∆H° > 0) process. In the presence of the Rh-rich active catalyst, the disproportionation reaction is an exothermic process, which leads to the decrease of K with temperature. As was pointed out previously, a drastic decrease of K with temperarture explains the anomalous behavior of the decrease of the apparent rate constant k2CO with temperature.
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Figure 6. Variation of the rate constants k1 (a) and k-1 (b) for the oxygen adsorption on the catalysts shown in the scheme with temperature.
temperatures it follows the order k1O2 (100% Pt) > k1O2 (25% Pt) > k1O2 (75% Pt). From Figure 6b, in which the variation of rate constant for the desorption of oxygen, k-1O2, with temperature is shown, it is concluded that the k-1O2 values vary with the catalyst Pt content as follows: k-1O2 (100% Pt) > k-1O2 (25% Pt) > k-1O2 (75% Pt). The k1 and k-1 values for the adsorption of carbon dioxide on all the catalysts increase with increasing temperature (cf. Figure 7) and vary with the catalyst Pt content as follows: Figure 5. Variation with temperature of the rate constants for the CO adsorption (a), desorption (b), and disproportionation reaction (c) over the catalysts shown in the scheme.
k1CO2 (25% Pt + 75% Rh) > k1CO2 (100% Pt) >
The variation of the k2CO with the catalyst Pt content, as Figure 5c shows, follows the order
k-1CO2 (25% Pt + 75% Rh) > k-1CO2 (100% Pt) >
k2CO (25% Pt) > k2CO (100% Pt) > k2CO (75% Pt) T < 410 °C k2CO (100% Pt) > k2CO (25% Pt) > k2CO (75% Pt) T > 410 °C The k1O2 values for the adsorption of oxygen increase with increasing temperature for the catalysts 100% Pt and 25% Pt + 75% Rh, while those on the catalyst 75% Pt + 25% Rh decrease with increasing temperature (cf. Figure 6a). The variation of the k1O2 values with the catalyst Pt content at low temperatures follows the order k1O2 (75% Pt) > k1O2 (100% Pt) > k1O2 (25% Pt), while at high
k1CO2 (75% Pt + 25% Rh) k-1CO2 (75% Pt + 25% Rh) The catalytic fractional conversion x for the disproportionation reaction of CO over the bimetallic Pt-Rh alloy catalysts was calculated, via eq 21, from the heights h of the CO and CO2 sample peaks (cf. Figure 2) detected after each flow reversal. The heights of the two sample peaks were used, instead of their areas, because the two peaks had approximately the same widths at their half-heights.
x)
RhCO2 RhCO2 + hCO
)
0.966hCO2 0.966hCO2 + hCO
(21)
where R ) 0.966 is the relative molar response of the thermal conductivity detector for CO2 to that for CO under
Studies of Gases on Pt-Rh Bimetallic Catalysts
Langmuir, Vol. 15, No. 11, 1999 3805
Figure 8. Variation of the fractional conversion, x, of carbon monoxide to carbon dioxide with temperature over the catalysts shown. (a) CO oxidation in the presence of excess oxygen, (b) CO disproportionation in the absence of oxygen.
Figure 7. Temperature dependence of the rate constants k1 (a) and k-1 (b) for the adsorption of carbon dioxide on the catalysts shown in the figure.
the experimental conditions used. All conversions given in this paper (cf. Table 4) for a particular CO disproportionation kinetic run are average values of the conversions calculated from eq 21 for each pair of sample peaks. The catalytic fractional conversions for the disproportionation of CO appear a maximum value, xmax, at a characteristic temperature depending on the catalyst nature (cf. Figure 8). For comparison purposes in Figure 8 the variation of conversions for the CO catalytic oxidation with temperature is also shown. The conversions for the CO catalytic oxidation, which were presented in a previous work,19 were also calculated from eq 21 by performing experiments under the same experimental conditions, as those for the CO disproportionation reaction, but with oxygen (7% v/v) in the carrier gas helium (93% v/v). From Figure 8 it is concluded that at each temperatue the x values for both the disproportionation (CO-D) and oxidation (CO-O) reactions of CO, over the Pt-Rh bimetallic catalysts used, vary with the catalyst Pt content as follows:
CO-D: x(25% Pt + 75% Ph) > x(75% Pt + 25% Rh) > x(100% Pt) CO-O: x(75% Pt + 25% Rh) > x(25% Pt + 75% Rh) > x(100% Pt) The same variation was also observed for both the xmax values (cf. Figure 9).
Figure 9. Variation of the maximum conversion, xmax, for the CO oxidation (1) and disproportionation (2) reactions over PtRh alloy catalysts with the atomic percentage of Pt.
The most active bimetallic catalyst for the disproportionation reaction of CO is the Rh-rich (25% Pt + 75% Rh) catalyst, which is the less active catalyst for the CO oxidation reaction, in accordance with previous results.20,21,24-28 The xmax values for the disproportionation (oxidation in the absence of oxygen) reaction of CO are lower than those for the CO oxidation reaction in the presence of excess oxygen (cf. Figures 8 and 9). The variation of xmax with the catalyst Pt content (cf. Figure 9) follows a different order in the two processes mentioned above, due to the fact that the Rh-rich catalyst is more active for the CO dissociative adsorption, while the CO oxidation reaction is more favorable over the Pt-rich catalyst.19-21,24-28 Due to Pt-Rh synergism the x and xmax values are higher in the bimetallic catalysts than those in the pure Pt catalyst for both the disproportionation and oxidation reactions of CO. This can be attributed to the fact that Pt(CO source) and Rh(oxygen source) are randomly distributed on the surface, increasing the probability of finding surface CO and oxygen in the vicinity of each other. The increased catalytic activity with increasing Pt suggests that the Pt in the bimetallic catalyst directly participates in the oxidation reaction. On the other hand, the high activity of the Rh-rich catalyst (25% Pt + 75% Rh) in the CO disproportionation reaction (dissociative adsorption), which has been also observed previously,23,27,28 can be attributed to creation of new sites
3806 Langmuir, Vol. 15, No. 11, 1999
at the Rh-SiO2 interface, which provides another pathway toward dissociation, assuming that CO can be titled with the oxygen atom toward the catalyst support SiO2. Conclusions The present work resulted in the following conclusions: (i) The reversed-flow gas chromatography technique can be used with simplicity and accuracy for the study of adsorption of carbon monoxide, oxygen, and carbon dioxide on Pt-Rh bimetallic catalysts. Using suitable mathematical analysis, equations were derived by means of which rate constants for adsorption, desorption, and disproportionation reaction of CO on the catalysts were calculated. (ii) The experimental results show that the interaction of CO with the Pt-Rh catalysts supported on SiO2 leads to its disproportionation and subsequent incorporation of carbon into the Pt-Rh lattice. Thus, useful conclusions concerning the mechanism for the CO oxidation reaction can be extracted. (iii) All rate constants for adsorption (k1), desorption (k-1), and CO disproportionation reaction (k2) increase with increasing temperature, except of the rate constants k1 for the adsorption of oxygen on the Pt-rich catalyst and k2 for the CO disproportionation reaction over the Rhrich catalyst, which decrease with increasing temperature. The reasons for these anomalous observations were explained in the Results and Discussion.
Gavril et al.
(iv) The Rh-rich catalyst (Pt0.25 + Rh0.75) is more active for the CO dissociative adsorption, while the CO oxidation reaction is more favorable over the Pt-rich catalyst (Pt0.75 + Rh0.25). (v) The k1CO (100% Pt) values are higher than the k1CO (25% Pt + 75% Rh) values, incidating that carbon monoxide is adsorbed more strongly on the pure-Pt catalyst than on the Pt-Rh bimetallic catalysts, in accordancce with previous results.20,21-24 (vi) In favor of the conclusion that the Rh-rich catalyst is the most active for the CO disproportionation reaction is the finding that k-1CO (25% Pt + 75% Rh) < k-1CO (75% Pt + 25% Rh) (cf. Figure 5b) and k2CO (25% Pt + 75% Rh) > k2CO (75% Pt + 25% Rh) (cf. Figure 5c). (vi) The catalytic fractional conversions for both the CO disproportionation (disssociative adsorption) and oxidation reactions were found to be higher for the bimetallic catalysts than those for the pure Pt catalyst, indicating the presence of beneficial Pt-Rh synergism over the wide catalyst composition range considered. Acknowledgment. The authors thank Dr. Nieuwenhuys at the Leiden University (The Netherlands) for supplying the catalysts and Mrs. M. Barkoula for technical assistance. LA981653K
