J . Phys. Chem. 1986,90, 4819-4824 deactivation was observed for the KCo catalyst after approximately 20 h on stream, as shown in Figure 4. Figure 5 shows that the hydrocarbon activity went through a minimum, while a monotonic decrease in the rate of C02 formation was noted throughout the run. At the end of the run, the COZ/HC ratio was near unity, indicating that the rate of carbon deposition had decreased while the hydrogenation increased. It is reasonable to expect that good initial contact between K and Co was achieved through the use of MCCs as catalyst precursors, and it may be that the strong effect of the promoter begins to diminish as carbon deposition proceeds. This is supported by the continuously declining chain growth probability for C3-Cs hydrocarbons in Figure 6. Some decrease in K-Co interaction could result from the physical removal of K from the catalyst surface; however, the continued high selectivity to olefins would not be expected had all the K been removed. Also, atomic absorption analyses on the fresh and used samples did not show any loss of metal or potassium for this catalyst. The work of Bonzel et al.93 showed that when carbon was deposited on K-promoted Fe foils the location of the K was always at the top of the carbon layer. The carbon buildup observed during the initial stages of the activity maintenance run on KCo could have, therefore, removed a fraction of the promoter from the cobalt surface. This possibility is consistent with the work of Audier et a1.I8 Some cleaning of the catalyst surface may be occurring (93) Bonzel, H. P.; Broden, G.; Krebs, H. J. Appl. SurJ Sci. 1983,16,373. (94) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. the d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
4819
after 20 h on-stream because of the increased C O to hydrocarbon activity observed. Although water production was not explicitly quantified in this study, an inspection of the chromatograms taken near the end of the activity maintenance run showed a sharply reduced water peak. This implied that at these longer times, the fraction of the total COz formed by the water-gas shift reaction had increased, while the C 0 2 resulting from carbon deposition was reduced. The suppression of the carbon deposition rate after longer times on-stream indicated that no reassociation of the K with the partially cleaned Co surface took place. In summary, we have shown that well-dispersed, carbon-supported Fe, Co, Fe-Co, and K-promoted Fe-Co catalysts can be prepared from stoichiometric mixed-metal carbonyl clusters. These catalysts can be very active for C O hydrogenation and do not require a high-temperature reduction step because of the initial zero-valent state of the metal atoms and the absence of oxygen functional groups on the carbon surface. The Co/C catalyst derived from C O ~ ( C Owas ) ~ especially active, produced only paraffins, and exhibited very good activity maintenance. The Fe/C catalyst had much lower turnover frequencies but a much higher olefin/paraffin ratio. The mixed-metal clusters showed catalytic behavior intermediate between these two extremes, and the unpromoted samples displayed more Fe-like behavior after a hightemperature treatment. Addition of a potassium atom to the MCCs markedly decreased activity but greatly enhanced olefin selectivity. Further control of the K/metal ratio by coimpregnating MCCs and K-containing MCCs could provide a method to optimize olefin yields.
Acknowledgment. This study was supported by the National Science Foundation through Grant No. CPE-8205937. Registry No. CO, 630-08-0; C O ~ ( C O )10210-68-1; ~, Fe3(C0),*, 17685-52-8; Fe, 7439-89-6; Co, 7440-48-4; K, 7440-09-7.
A Kinetic Study of the Catalytic Oxidation of CO over Nafion-Supported Rhodium, Ruthenium, and Platinum Vincent D. Mattera, Jr.? Denise M. Barnes,*Sanwat Noor Chaudhuri,s William M. Risen, Jr.,* Department of Chemistry, Brown University, Providence, Rhode island 0291 2
and Richard D. Gonzalezl Department of Chemistry, University of Rhode island, Kingston, Rhode island 02881 (Received: December 20, 1985)
The Rh, Ru, and Pt ionomers of perfluorocarbonsulfonic acid (Nafion or PFSA) polymers have been formed and have been reduced to investigate the formation of metal particles within the ionic domains of the materials. The particle size distributions are peaked in the 25-40-A range postulated for these domains and are consistent with exertion by the ionomer of morphological control over the growth of transition-metal particles. The reduced ionomers catalyze the CO oxidation with the activity sequence Ru > Rh > Pt. Diffusion limitations occur over the Rh and Ru, but not the Pt, ionomer catalysts. Turnover frequencies for the formation of COz by oxidation of CO are reported as a function of temperature.
Functionalized organic polymers have been used as supports for heterogenized homogeneous catalytic processes.' Typically, transition-metal-containing complexes which are either active catalysts or potential catalytic precursors are bound to the support
by covalent attachmenk2 Although there have been reports of catalytic reactions involving transition-metal complexes incorporated into ion-exchange (polyelectrolyte) resins?" those materials are not thought to form ionic domains or to have morphological properties of ionomers. Ionomers are polymers that are func-
+AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, N J 07974. 'AT&T Bell Laboratories, 5 5 5 Union Boulevard, Allentown, PA 18103. $Department of Fuels and Engineering, University of Utah, Salt Lake City, UT 84112. Department of Chemical Engineering, University of Illinois at Chicago, P.O.Box 4348, Chicago, IL 60680.
(1) Chauvin, Y.; Commereuc, D.; Dawans, F. Prog. Polym. Sci. 1977, 5 , 95. (2) Smith, T. W.; Wychick, D. J . Phys. Chem. 1980, 84, 1621. (3) Drago, R. S.;Nyberg, E. D.; A'amma, E. A.; Zombeck, A. Inorg. Chem. 1981, 26, 641. (4) Smith, R. T.;Ungar, R. K.; Baird, M. C. Tramition Met. Chem. ( N Y ) 1982, 7, 288.
Introduction
0022-3654/86/2090-4819$01.50/0
0 1986 American Chemical Society
4820
The Journal of Physical Chemistry, Vol, 90, No, 20, 1986
IONIC DOMAIN
IONIC CHANNEL
IONIC DOMAIN
Figure 1. A schematic representing the cluster morphology of PFSA
(Nafion). after Gierke (see ref 10 and references therein). tionalized with ionic groups attached at various points along the polymeric Much of the interest in ionomers is associated with their tendency to form ionic domains in which ionic groups are microphase separated from the hydrophobic portions of the polymer. The microscopic morphology of perfluorocarbonsulfonic acid (PFSA), known commercially as Nafion, has been studied by a variety of diffraction,8 spectrosc~pic,~ and other techniques, and it has been described as a material in which the sulfonate groups form ionic domains that are embedded randomly in a fluorocarbon matrix. Although the size of the ionic domains is sensitive to the degree of hydration and the ionic site concentration, they are typically 40 8, in diameter and are connected by hydrophilic channels of about 10 8, in diameter as depicted in Figure 1. Theoretical studies on these materials are consistent with the experimental results and suggest that the formation of the siteconnecting channels is slightly exothermic so that the ionic domains can become isolated at elevated temperatures.I0 A potential advantage to using Nafion materials as a polymeric support for catalytic and gas separation applications involving transition metals is that the fluorocarbon backbone imparts a thermal stability to the Nafion materials which far exceeds that of the widely studied functionalized polystyrene-supported catalysts. Thus, transition-metal-containing Nafions can be thought of, conceptually, as an assembly of small (40 A) isolated chemical reactors contained within a matrix which doubles as a support material. The physical structure of the ionic domains might contribute to their potential to function as chemical reactors by enabling them to confine a reactant molecule in the proximity of an active site and other reactants. Any reactant molecule that diffuses into the domains could be confined for a residence time that would permit the sites and reactants to interact in the domain much as they would if they were surface sites and gaseous reactants from a gas at much higher pressures. Thus, the possibility exists that reactions can be catalyzed in Nafion ionomers under much milder conditions than would be required otherwise. There is another conceptual advantage to using Nafion as a catalytic support. Because the transition metals are constrained within the domain morphology during the reduction process, a narrow distribution of metal particle sizes may result. Furthermore, the results of spectroscopic investigations"-i4 demonstrate ( 5 ) Ionic Polymers; Holiday, L., Ed.; Applied Science: London, 1975. (6) Ion Containing Polymers: Physical Properties and Structures; Eisenberg, A.; King, M.; Eds.; Academic: New York, 1977. (7) Ions in Polymers; Eisenberg, A,; Ed.; American Chemical Society: Washington, DC, 1980. (8) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J . Polym. Sci., Polym. Phys. Ed. 1981, 19, 1687. (9) Mattera, Jr., V. D.; Peluso, S. L.; Tsatsas, A,; Risen, Jr., W. M.. to be submitted for publication. (IO) Hsu, W. Y . ;Gierke, T. D. Macromolecules 1982, 15, 101. ( 1 I ) Peluso, S. L. Ph.D. Thesis, Brown University, 1980. (12) Barnes, D. M. Ph.D. Thesis, Brown University, 1981. (13) Chaudhuri, S. N. Ph.D. Thesis, Brown University, 1983. (14) Chryssikos, G. D.; Mattera, Jr.. V. D.; Tsatsas. A. T.: Risen, Jr., W. M. J . Catal. 1985. 93. 430.
Mattera et al. that the ionic domains present in Nafion are readily accessible to transition-metal cations and metal-containing species by ion exchange and that reactions of small gaseous molecules, including CO, 02,NO, H2. C2H4,and C2HZ,with Nafion-supported Ag, Pd, Ru, Rh, Pt, and Ir occur to form known intermediates of important heterogeneous catalytic reactions and in some cases novel transition-metal carbonyls, nitrosyls, and alkyl-containing species. There are a number of interesting and important reactions that are catalyzed by conventional heterogeneous catalysts at temperatures below 500 K whose kinetics and mechanisms could be investigated by using Nafion-supported transition metals. The overall objectives of this research were to ( I ) investigate the formation of transition-metal particles within the ionic domains of Nafion, (2) explore the possibility that the unique morphological features of Nafion impart an intrinsic control over the metal particle size, and (3) establish the physical and chemical limitations of Nafion-supported transition metals as heterogeneous catalysts. The results of the studies reported here will lead to a comparison of the growth of transition-metal particles and of the kinetic data for the CO oxidation reaction catalyzed by PFSA-Rh, PFSA-Pt, and PFSA-Ru. Aside from its practical importance, this reaction was chosen because it has been investigated over platinum group metals on other supports since it was first reported by LangmuirI5 and because it is considered to proceed through a relatively simple Langmuir-Hinshelwood mechanism. Furthermore, the reaction has been found to be structure insensitive, so that, in general, the results of single-crystal studies reflect the behavior of real catalysts.16 In their comparative study of the C O oxidation reaction over silica-supported Ru, Rh, Pd, Ir, and Pt, Cant et al." determined the activation energies, kinetic orders, and relative activities of the metal in order to bridge the gap between high-pressure studies on highly dispersed metals and low-pressure studies on well-defined single-crystal surfaces. In that study it was found that ruthenium undergoes changes in reaction rates for many hours following changes in the temperature or pressure even though the initial behavior was qualitatively similar to that observed for rhodium and palladium. The extensive deactivation of ruthenium observed under oxidative conditions was attributed by Cant to the formation of a subsurface oxide which inhibits the reaction rate. The reactions over Ir, Rh, and Pd were found to have activation energies of about 100 kJ/mol and were characterized by kinetic orders of ca. -1 and + 1 in C O and 02,respectively. In contrast, the platinum-catalyzed reaction had an activation energy of 56 kJ/mol and the kinetic order was only slightly negative in CO. It was also found that the reaction rate over supported ruthenium was almost an order of magnitude greater than that of rhodium. Kiss and Gonzalez have used a combined infrared/gas chromatographic technique to study the C O oxidation over silicasupported ruthenium18 and r h ~ d i u m . ' ~The results for the ruthenium system indicate that the deactivation is due to the formation of a subsurface oxide or lattice oxygen which produces coordinatively unsaturated Ru surface atoms. This subsurface oxygen is thought to react more slowly with chemisorbed CO than does chemisorbed oxygen. The catalytic deactivation as well as the reaction rate hysteresis was found to be more extensive in an 0,-rich reactant gas mixture. In addition, multiple steady-state oscillations were observed after reaching steady-state reaction rates. Finally, it was inferred that heat-transfer limitations were important since temperature equilibration between the metal particles and the support was observed to be slow following a decrease in the reaction rate. The results for the rhodium system indicate that both heattransfer effects and diffusion limitations occur at high C O concentrations. Some modification of the reaction rate, leading to slight deactivation was observed in oxygen-rich reactant gas ( I 5 ) Langmuir, I. Trans. Faraday SOC.1922, 17, 672. (16) Engle, T.; Ertl, G. Adu. Catal. 1979, 28, 2. (17) Cant, N. W.; Hicks, P. C.; Lemon, B. S . J . Catal. 1978, 54, 372. ( 1 8 ) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem. 1984, 88, 892. (19) Kiss, .I. T.: Gonzalez, R. D. J . Phys. Chem. 1984, 88, 898.
Oxidation of C O over Nafion-Supported Rh, Ru, and Pt [(CF CF 1 -CF-CF21n
22m/l \
\I oCF2CF2 /z
S03H
Figure 2. The chemical composition of perfluorocarbonsulfonicacid (PFSA or Nafion).
mixtures. This was attributed to the formation of a Rh(1) species. A comparison of the reaction rates as a function of the rhodium oxidation state indicates that the rates are faster on Rh(D) than Rh(1). In choosing PFSA-Ru, PFSA-Rh, and PFSA-Pt for study, it was clear that the chemical phenomena characteristic of the C O oxidation reaction over each metal could be largely separated from physical phenomena, such as the metallic dispersions and gas diffusion, that are associated with the polymer support.
Experimental Section Materials. The Nafion materials used for catalytic supports in this study were kindly provided by the Plastics Department of the E.I. dePont de Nemours Co. The chemical composition of the materials is given in Figure 2. The materials were supplied in the form of powdered granules which were useful for studying cataltyic reaction rates by using gas chromatography. Granules of white PFSA-K+ (Nafion 501) with an average diameter of ca. 1 mm and an equivalent weight of 1200 were used. After ion exchange they were used to study the catalytic reaction rate by using gas chromatography. PFSA-Rh catalysts were prepared by first ion exchanging PFSA-Kf in 1 X lo-, M aqueous solutions of Rh(N03)3.2H20 (Alfa Products). After stirring for 1 week in such solutions these PFSA-Rh powders had acquired a yellow color. The elemental analysis of the dehydrated PFSA-Rh powders was performed by Schwartzkopf Microanalytical Laboratory, Woodside, NY. They were found to contain 21.2 wt % C, 3.26 wt % S, and 0.42 wt % Rh. The PFSA-Ru catalysts were prepared analogously by aqueous ion exchange in 1 X lo-’ M aqueous solutions of RuCI3.3H2O (Alfa Products). The powders turned dark brown after being stirred for 1 week in such solutions. Dehydrated PFSA-Ru powders were also analyzed by Schwartzkopf and found to contain 20.65 wt % C, 65.67 wt % F, and 0.19 wt % Ru. The PFSA-Pt catalysts were prepared by aqueous ion exchange M solutions of Pt(NH3)&12-H20(Alfa Products). in 1 X After stirring for 1 week in such solutions, the powders retained their white color. Dehydrated PFSA-Pt(NH3),, powders were analyzed by Schwartzkopf and were found to contain 21.26 wt % C and 1.24 wt % Pt. The gases used in these studies were subjected to the following purification treatments: C O (99.995%) was purified by passing it through an Analabs 13-X molecular sieve trap; H z (ultrahigh purity, New England Oxygen) was purified by first passing it through a deoxo unit to convert 0, impurities to H 2 0 , which was then removed by passing the gas through a molecular sieve maintained at 77 K. The He carrier gas (99.999%) was passed through a molecular sieve also maintained at 77 K. The O2 concentration in the carrier gas at the catalyst sample was checked by placing activated MnO in the microreactor. The absence of a color change is consistent with the presence of no more than 1 ppm of 0,. Reactor. The reactor-flow system which enables the use of an infrared cell or a Pyrex microreactor as either a pulse microreactor or a single-pass differential reactor was designed and constructed by Miura and co-workers and has been described previously.20 (20) Miura, H.; Gonzalez, R. D. J. Phys. E 1982, 15, 373.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4821 Conventional Pyrex microreactors which had volumes ranging from 4.4to 4.7 mL constructed from 12-mm Pyrex tubing were used. Product analyses were performed using a Perkin-Elmer Model Sigma 3-B gas chromatograph located downstream from the reactor. The carbosieve S (100-200 mesh) which was used to perform the separation was packed in a stainless steel column 1 m in length and 3.17-mm 0.d. Temperature was controlled by using an oven connected to a Valley Forge (Model 6000) linear temperature programmer. Procedure and Measurement of Reaction Rate. The flow rates of the reactant gases were fixed by using rotometer controllers. The total flow rate of the CO/02 feedstream was usually set to 20-25 mL/min. During this procedure, the flow system was maintained in the pulse mode so as to bypass the catalyst until the desired stoichiometries and flow rates were obtained. In a typical run the following procedure was used: approximately 500 mg of the catalyst was heated in flowing He (25 mL/min) from 300 to 443 K at 10 K/min and was held at 443 K for 1 h. The temperature was then increased at 10 K/min to 478 K and heated at 478 K for 0.5 h. It was then cooled to 300 K in flowing He (25 mL/min). Next, following a temperatureprogrammed increase at 10 K/min, the catalyst was heated in flowing Hz (25 mL/min) at 473 K for 2.5 h and then cooled to 300 K in flowing H2. Finally, the catalyst was exposed to flowing He (30 mL/min) while the temperature was increased to 448 K. Treatment time in H e at 448 K was 1 h. The catalyst was then exposed to the CO/O2 (2:l mole ratio) feedstream at 300 K for 10-15 min before the first measurements of the product gas composition were carried out. The C O / 0 2 feedstream was then allowed to react over the catalyst at the indicated temperature until the reaction rate had stabilized. A 15-20-min induction time was allowed for temperature equilibration when the temperature was changed. Transmission Electron Microscopy. Catalyst samples were prepared for transmission electron microscopy (TEM) by embedding the material in Spurr’s low-viscosity embedding medium (Electron Microscopy Sciences, Fort Washington, PA) and microtoming the composite into thin cross sections. The samples were supported on Cu grids and examined with a Philips Model 201 transmission electron microscope equipped with a liquid nitrogen cooled stage. When operated at 100 kV, in the high-field mode a t a calibrated magnification of 200 OOOX the instrument is capable of resolving features that are 5 A in diameter. Observation of highly dispersed transition-metal particles allowed for a determination of particle size distributions. Particle size distributions were obtained by measuring the diameter of no fewer than 200 particles from enlarged photographs of the original micrographs. Calculations of metal dispersions also were made on the basis of the particle sizes obtained from the TEM data.,’
Results PFSA-Rh. A transmission electron micrograph of the actual PFSA-Rh C O oxidation catalyst is shown in Figure 3. The presence of dispersed rhodium particles is apparent. The particle size distribution for the catalyst given in Figure 3 is quite narrow and only somewhat skewed as shown in Figure 4. The average diameter, d R h , of the particles is 28 A. This particle size distribution supports the proposition that the microscopic morphology of PFSA promotes a high degree of metal dispersion and exerts for control over particle size. With this knowledge of the dRRh PFSA-Rh, the actual rhodium dispersion DRh,for PFSA-Rh was calculated to be 0.39. The rhodium dispersion calculated from the TEM data can be compared to that determined by CO chemisorption measurements over a PFSA-Rh catalyst reduced under similar conditions. In these experiments, 100-pL pulses of 99.95% C O were passed consecutively over a known amount of catalyst at 300 K. The rhodium dispersion calculated from these experiments assuming a CO/Rh ratio of 1 was found to be 0.048. This value is almost (21) Structure of Metallic Curalysts; Anderson, J. R., Ed.; Academic: New York, 1975.
4822
,.
The Journal ojPhysica1 Chemisfry. Vol. 90. No. ?O. 1986
p. ...-
.-. .:.
. . '. .
.
. . . .. . . ... ..,. . .. . *
.
_.~.
.
5
.
Mattera et al.
TABLE I: Summary of Turnover Frequencies lor C02 Formation during the Oxidation of CO on P E A - R Y TOF. molecules/ (sites) x IO' T. K this work ref 19
. ,
:..---.:.;a, ..'.>*. . . . . I '
.
- .
I
0.48
373 383 393 398 423 433 448 473 493
.
I
0.50 1.12 5.0
1.66 9.46 15.38 22.56 3 I .44 27.85
'Metal loading, 0.42 wt %,dispersion. 0.39; weight of catalyst. 528 mg; flow rate. 20.4 rnL/min. Rcactant gases were 5.25% CO in He and 5.2% O2 in He; CO/O, = 2.
I-
P F S A - A h CO OXIDATION C A T A L Y S T
PFSA-Rh I
300
350
I 400
I 4 SO
5 0
T IKI
Figure 5. The temperature dependence of the effluent CO/O, ratio for the PFSA-Rh CO oxidation catalyst.
0.2
0.4
2s
SO DIAMETER
75
i
Figure 4. The particle size distribution of the PFSA-Rh CO oxidation catalyst. 1 order of magnitude lower than that calculated from the T E M data. This apparent low value of the dispersion measured by the CO uotake reflects limitations of the diffusion of the eas " within the catalyst at 300 K. Turnover freauencies (TOF's) for the rate of CO, formation at a CO/O, stoichiometric ratio of 2 were obtained ai a function of temperature. Turnover frequencies were based on dispersion measurements calculated from TEM results. The results of these rate measurements are shown in Table 1. Because the T O F for CO, formation at 373 K is in excellent agreement with that ~ infer that the obtained on a silica-supported Rh ~ a t a l y s t . 'we interconnecting channels of the Nafion support are not blocked and that the domains are accessible to the reactant gas molecules. The results listed in Table I show that the reaction experiences a sharp increase in rate from 400 to 472 K but decrease in rate at temperatures greater than 475 K. The catalyst did not experience any apparent deactivation with time during the rcaction. The TOF's shown in Table I were obtaincd during a n increasing temperature sequence. However.
similar trends were observed during a decreasing temperature sequence. A plot of the In T O F vs. T'for the data shown in Table I was performed lo obtain an activation energy for the reaction. assuming Arrhenius behavior. Using a linear least-squares lit of the data, we obtained an activation energy of 66 kJ/mol. This can be compared to the results of Kiss and Gonzalez, who found activation energies of about 100 kJ/mol for the stoichiometric CO oxidation reaction on silica-supported r h ~ d i u m . ' The ~ low value for the activation energy on PFSA-Rh suggests at least in part. that for PFSA-Rh the reaction is diffusion controlled. During the course of the reaction over PFSA-Rh the CO/O, was observed to increase after flowing over the catalyst. The apparent increase in the CO/O, ratio with increasing temperature is shown in Figure 5 . PFSA-Ru. The particle size distribution for the PFSA-Ru catalyst was obtained by using transmission electron microswpy in a manner which was identical with that used for the PEA-Rh catalysts. Its particle size distribution is shown in Figure 6 . The average diameter, d,,, is 33 A. and the actual Ru dispersion, D,,, was calculated lo be 0.40. Turnover frequencies based on a dispersion of 0.40 for the rate of C 0 2formation as a function of temperature are listed in Table 11. At temperatures less than 400 K, the T O F increased over the first 6 0 min, after which it remained constant for the next hour. Above 400 K, the catalyst underwent different degrees of deactivation which depended on time. For example, at 423 K, the catalyst experienced a deactivation after 30 min which amounted to 26% of its initial activity. When the catalyst was operated at 473 K, for about 1000 min. the deactivation resulted in a decrease in activity to about 60% of its initial value. Turnover frequencies listed in Tzble I1 were obtained following the attainment of the steady state. As in the case of PFSA-Ru, turnover frequencies were in excellent agreement with rate measurements
Oxidation of C O over Nafion-Supported Rh, Ru, and Pt
-'-I
PFSA-RU
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4823
I 0.2
0.2
0.1
0.4
25 25
50
75
1
50 DIAMETER
75
H
Figure 7. The particle size distribution of the PFSA-Pt CO oxidation
DIAMETER
catalyst.
Figure 6. The particle size distribution of the PFSA-Ru CO oxidation
catalyst. TABLE II: Summary of Turnover Frequencies for C02 Formation during the Oxidation of CO on PFSA-Ru"
T, K 323 343 373 383 393 398 403 438 448 473
TOF, molecules/ (site s) x 10' this work ref 18 5.7 10.65 15.05 15.83 16.30 17.08 19.95 21.25 22.10 31.30
17 30.4 63.5
"Metal loading, 0.19 wt %; dispersion, 0.40; weight of catalyst, 813 mg; flow rate, 25 mL/min. Reactant gases were 5.25% CO in He and 5.2% O2 in He; CO/O, = 2. TABLE 111: Summary of Turnover Frequencies for C02 Formation during the Oxidation of CO on PFSA-Pt" TOF, molecules/(site s) X IO3 T, K 403 433 443
0.05 0.16 0.21
"Metal loading, 1.24 wt %; dispersion, 0.33; weight of catalyst, 550 mg; flow rate, 23 mL/min. Reactant gases were 5.25% CO in He and 5.2% O2 in He; CO/O2 = 2. performed on silica-supported Ru. A linear least-squares fit of the TOF vs T I data listed in Table I1 leads to a calculated activation energy of 12 kJ/mol. This can be compared to the results of Kiss and Gonzalez, who found activation energies of 106 kJ/mol for the stoichiometric C O oxidation reaction over silica-supported R u . ' ~This suggests that the reaction is strongly diffusion limited. PFSA-Pt. The particle size distribution for the PFSA-Pt catalyst was obtained by using transmission electron microscopy which was similar to that used for PFSA-Ru and PFSA-Ru. Its particle size distribution is shown in Figure 7. The average &, is 34 A, and the actual dispersion Dptwas calculated to be 0.33. Turnover frequencies for CO, formation based on a dispersion of 0.33 are listed in Table 111. By comparison to the TOF's obtained for PFSA-Rh and PFSA-Ru, the C O oxidation rate on PFSA-Pt is much slower. Because diffusion would be less of a limitation for slower reactions, the calculated activation energy for the reaction over PFSA-Pt should be similar to that calculated for other platinum catalysts. A least-squares fit of the In T O F vs. T I data listed in Table I11 yields an activation energy of 56
kJ/mol. This is in excellent agreement with those reported by Cant et al." (56 kJ/mol) and Sarkany and Gonzalez22(46 kJ/ mol) for the CO oxidation over silica-supported Pt.
Discussion The excellent agreement between the turnover frequencies for C 0 2 formation obtained in this study and those reported over conventional silica-supported noble-metal catlaysts suggests that the polymeric network of interconnecting channels and domains is readily accessible to the reacting gas-phase molecules. However, activation energies for C O oxidation over the Nafion-supported noble-metal catalysts were considerably lower than those obtained over the conventional silica-supported noble-metal catalysts. The PFSA-Ru was the most active catalyst, and the PFSA-Pt was the least active. This reactivity sequence was identical with that observed on the silica-supported noble-metal catalysts. Because of the high reactivity in the C O oxidation rate over PFSA-Ru, diffusional limitations were severe. On the other hand, over PFSA-Pt, activation energies were identical with those obtained over Pt/SiO,, suggesting that diffusion effects were small. The results of the transmission electron microscopy experiments were interesting in that particle size distributions for PFSA-Ru, PFSA-Rh, and PFSA-Pt were very similar. Evidently, the morphology of the ionic domains has a strong influence on the metal particle size distribution. In addition, the particle size distribution was observed to peak sharply at about 25-30 8, for all three catalysts and did not depend on the metal precursor used to prepare the catalyst. In particular, the method by which the PFSA-Pt catalyst was prepared was somewhat different from that used to prepare the PFSA-Rh and PFSA-Ru catalysts. The platinum-tetraammine complex was introduced into the Nafion by the following ion-exchange reaction: 2PFSA-H
+ Pt(NH3)dClZ
W20)
PFSA-Pt(NH,),
+ 2HCl(aq)
Therefore, the complex is contained within the ionic domains of the Nafion support. The thermal decomposition of other catalysts prepared by ion exchanging the platinum-tetraamine complex into zeolites has been studied by several investigators. Boudart found23 that if the thermal decomposition of the tetraammine complex takes place in the presence of H,, the reduction is complete at about 570 K. However, the platinum dispersion is found to be 0.08. When the catalyst is decomposed in air, the tetraammine complex begins to decompose at about 520 K. When the resulting material is calcined in air at 620 K and reduced in H, at 670 K, the resulting platinum dispersion is found to be 1.OO. These results suggest that a highly mobile species is formed before the reduction (22) Sarkany, J.; Gonzalez, R. D. Appl. Catal. 1983,5 , 8 5 . (23) Dalla Betta, R. A.; Boudart, M. Proc. Int. Congr. Catal. 5th, 1972 1973,2, 1329.
4824
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
is complete during decomposition in H,. The mobile species is thought to be a neutral platinum complex such that the following reactions would occur.
+
[Pt(NH3)4]2f 2H2
-
[Pt(NH3),H2]
+ 2NH3 + 2H' Pt + 2NH3 + 2H2
[Pt(NH3),H,]
-
These reactions are thought to compete with the process involving direct loss of ammonia [Pt(NH3)4]2f followed by H 2 reduction: Pt2'
+ H2
-
Pt2'
Pt
+ 4NH3
+ 2H'
Unfortunately, this study does not allow for direct comparison of these results to those of PFSA-Pt because the Nafion-supported materials were thermally decomposed in flowing He. However, Exner et al.24have recently reported a more detailed investigation in which the decomposition of platinum- and palladium-tetraammine faujasite X zeolites was camed out under conditions which were identical with those in which the PFSA-Pt was prepared. Their results show that when the platinum complexes are decomposed in a vacuum or in flowing argon, the following reactions occur: T < 475 K
[Pt("3)4I2' [Pt(NH3),I2'
T > 475 K
Pto-(NH)
[Pt(NH3),I2'
Pto-(NH)
T>475K ___+
+ (4 - x)NH3
+ 2Hf + (x - l ) N H 3
PtO + '/2N2
+ '/2H2
It is reasonable to assume that the same reactions occur on PFSA-Pt(NH,), and result in the formation of a dispersed platinum cataIyst.I3 The observation that the CO/Oz ratio increases after the reactant gas mixture is passed over the PFSA-Rh catalyst at high temperatures is interesting. There are a number of possible explanations for this observation. It is possible that the Boudouard reaction 2CO
Rh
Rh-C
+ CO2
occurs to some extent, depositing carbon on the surface of the catalyst. The presence of this surface-active carbon would lead to an increased consumption of 0, by the following reaction: Rh-C(surface)
+ O2
-
Rh
+ C02(g)
This would introduce a new source of C 0 2 into the overall stoichiometry of the reaction and would decrease the 0, concentration. However, it would be expected that, at temperatures below 500 K, this reaction should proceed more rapidly over PFSA-Ru since Ru is more effective than Rh in dissociating CO. Because no similar changes in the CO/O2 ratio were observed over PFSA-Ru, it is reasonable to conclude that the contribution of the Boudouard reaction is not substantial. On the other hand, the decrease in the amount of oxygen could be the result of a higher diffusion coefficient for O2than CO over the indicated temperature range. Since the results for the same (24) Exner, D.; Jaeger, N.; Moller, K.; Schulz-Ekloff, G. J . Chem. Soc., Faraday Trans. I 1982, 78, 3537.
Mattera et al. reaction over PFSA-Ru and PFSA-Rh do not indicate a similar trend, the possibility that a morphological feature of the Nafion support allows for a higher diffusion coefficient of 0, can be neglected. These observations suggest that the feature is characteristic of the rhodium particles. In this regard it seems reasonable to assume that a phenomenon, which may be related to an increase in the sticking probability of 0, onto the rhodium particles, occurs at higher temperatures. This is an interesting possibility in light of a report by White et al.,25who found that CO adsorption on polycrystalline rhodium was inhibited by chemisorbed oxygen at temperatures between 440 and 530 K. They concluded that oxygen was trapped at lattice oxygen or as a subsurface oxide, since it was found that oxygen adsorbed at higher temperatures was catalytically less reactive than oxygen adsorbed at room temperature. Furthermore, there is evidence from low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) s t ~ d i e s , ~that , ~ ' a structural change in oxygen chemisorbed on Rh( 111) occurs at about 400 K and is associated with the penetration of oxygen into the outer layer of the Rh lattice. These results are thought to reflect the formation of a rhodium oxide species. This is consistent with the results of the present study since the CO/O2 ratio begins to increase at about 400 K. Even though 0, continues to be selectively adsorbed from 473 to 493 K, the extent of C O conversion decreases somewhat over the same temperature range. These results on NAFION-supported Rh, Ru, and Pt together with related work on ionomer-supported transition metal^^*^^^ and on the application of the superacid catalytic functionality of the H form of Nafion30 demonstrate some of the potential utility of ionomer-based catalysts.
Conclusions Several important conclusions emerge froin this study: 1. The ionic domains of Nafion are readily accessible to transition-metal ions and can exert control over the growth of transition-metal particles during vaiious reductive treatments. 2. Turnover frequencies for the formation of CO, over Nafion-supported noble-metal catalysts are consistent with those observed over conventional silica-supported noble-metal catalysts; i.e., the activity sequence Ru > Rh > Pt does not depend on the support material. 3. Diffusion limitations occur over PFSA-Ru and PFSA-Rh, but not over PFSA-Pt. 4. When a stoichiometric CO/O, feedstream is passed over PFSA-Rh, an increase in the oxygen consumption occurs at temperatures in excess of 400 K. This leads to the formation of a rhodium oxide species (presumably Rh203). Acknowledgment. Acknowledgment is made to the National Science Foundation, which provided funds for this research under Grants No. DMR 78-18917 and CPE-79 20155. We are also grateful for the financial support of the Office of Naval Research and the Brown University Materials Research Laboratory. Helpful discussions with George Chryssikos of Brown University are appreciated. Registry No. CO, 630-08-0; Pt, 7440-06-4; Ru, 7440-18-8, Rh, 7440- 16-6; Nafion, 39464-59-0. (25) Kim, Y.; Shi, S. K.; White, J. M. J . Catal. 1980, 61, 374. (26) Thiel. P A,; Yates, Jr., J. T.; Weinberg, W. H. Surf. Sci. 1979, 82,
22. (27) Yates, Jr., J. T.; Thiel, P. A,; Weinberg, W. H. Surf. Sci. 1979, 82, 45.
(28) Mattera, Jr., V. D.; Risen, Jr., W. M . Inorg. Chem. 1984, 23, 3597. (29) Shim, I. W.; Mattera, Jr., V. D.; Risen, Jr., W. M. J . Catal. 1985, 94, 531. (30) Olah, G. A,; Husain, A,; Singh, B. P. Synthesis 1983, 892.
