CARBON MONOXIDE OXIDATIONON PALLADIUM It should be noted that the family of calculated curves for the limiting currents and half-wave potentials predicts a shift to the right as the concentration of depolarizer increases (see Figure 1, for example). The experimental curves taken a t three different concentrations of anthracene follow exactly the predicted behavior in both cases. This means, of course, that measurements taken under aprotic conditions will be less affected by any acid impurities when the concentration of the depolarizer is high. One must, however, compromise when choosing the concentration of the depolarizer, because high concentrations and thus high currents may result in the maxima as discussed above. It is also important to note that a proton donor
3621
present at the concentration less than M (when the concentration of R is millimolar) would not affect the polarographic results regardless of the magnitude of the rate constant (see Figures 1 and 2). One can also conclude from Figure 1 that plots of the dependence of the limiting current on the concentration of the depolarizer will not be linear when taken within the kinetic-controlled region of the ilim-log CHXplots (ascending part). Thus this commonly employed method of analysis of polarographic data must be made with care.
Acknowledgment. We are indebted to Mr. R. La Budde for ,aid in preparing the computer program.
Simultaneous Kinetic and Infrared Spectral Studies of Carbon Monoxide Oxidation on Palladium under Steady-State Conditions by Raymond F. Baddour, Michael Modell, and Ulrich K. Heusser Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02189 (Received April 12, 1968)
An apparatus is described for obtaining simultaneously infrared spectra of chemisorbed species and kinetic data for pressed disks of supported metal catalysts. Obtaining these data under steady-state conditions facilitates determination of diffusional limitations and eliminates complications of changing mechanisms which may occur under batch-reaction conditions. Considerably more insight into the reaction mechanism is obtained by simultaneous kinetic and spectroscopic studies than by both of these techniques used separately. The method was used to study CO oxidation on silica-supported palladium. Carbon monoxide inhibits the reaction even under conditions of moderate CO surface coverage. Sites for oxygen adsorption must be distinguished from sites on which carbon monoxide adsorbs.
Introduction I n the past decade, infrared spectroscopy has been applied extensively to the study of species adsorbed on so1ids.l Much of the previous work has been directed at studying chemisorption on catalysts, and the results we often used to infer mechanisms of catalyzed reactions. Such a procedure is open to criticism, since surface species observed during chemisorption studies may not be kinetically significant. Furthermore, intermediates which may be present under reaction conditions may not be present under chemisorption conditions in which the catalyst is not exposed to all reactants simultaneously; the converse is also true. I n some prior studies, spectra of adsorbed species have been obtained for gas mixtures under conditions of catalytic reaction.2-11 These investigations were conducted under batch conditions either by measuring
spectra before and after the reaction or by obtaining spectra during the course of the reaction. I n the for(1) L. H. Little, “Infrared Spectra of Adsorbed Species,” Academic Press, Inc., New York, N. Y., 1966. (2) R. P. Eischens and W. A. Pliskin, Advan. Catal., 9, 662 (1957). (3) N. N. Kavtaradze, E. G. Boreskova, and V. J. Lygin, Kinet. Catal., 2, 349 (1961). (4) A. C. Yang and C. W. Garland, J . P h y s . Chem., 61, 1504 (1957). (5) A. W. Smith, J . Catal., 4, 172 (1965). (6) G. Blyholder and L. D. Neff, J . Phys. Chem., 66, 1464 (1962). (7) G. Blyholder, Proc. I n t . Congr. Catal., 3rd, Amsterdam, 1964, 1, 657 (1965). (8) W. M. H. Sachtler and J. Fahrenfort, Proc. I n t . Congr. Catal., Pnd, Paris, 1860, 1, 831 (1961). (9) H. Heyne and F. C. Tompkins, Proc. Roy. Soc., A292, 460 (1966). (10) J. F. Harrod, R. W. Roberts, and E. F. Rissmann, J . Phys. Chem., 71, 343 (1967). (11) R. P. Young and N. Sheppard, J . Catal., 7, 223 (1967).
Volume ‘72, Number 10 October 1968
3622 mer case, the relevance of infrared observations to kinetically important species is questionable, as discussed above for chemisorption studies. Observation of spectra during the course of a batch reaction can contribute significant information on the mechanism. However, interpretation of data from batch reactions is complicated by the fact that some mechanisms change significantly as a function of surface coverage. The object of this work was to develop a method for measuring simultaneously infrared spectra and kinetics under steady-state reaction conditions. A recirculating-loop flow reactor was used. The system studied was carbon monoxide oxidation on silica-supported palladium. This system is one of a few for which separate kinetic (batch and steady state) and infrared spectral studies have been reported previously. Experimental Section
Gas Feed Sgstem. Gas mixtures at about 780 torr of total pressure were prepared by mixing CO (Matheson CP, 99.5% minimum purity) and 0 2 (99.5% minimum purity) with He (99.99% minimum purity). The gases were metered separately, mixed, and passed over Drierite and Ascarite. The total feed rate was approximately 150 cm3/min, and the concentrations of CO and 0 2 were each below 1 vol %. Recirculation Loop. The gas mixture was fed to the recirculation loop ahead of the blower assembly (see Figure 1). Recirculation was provided by a miniature vane-axial blower (Globe Industries, R!todel 19A1389) modified with dry Teflon bearings (Barden Corp.). The blower was totally enclosed in a glass housing. The only potential source of system contamination from the blower was the insulating coating on the motor windings. This insulation was a silicone compound made for high temperature use (ZOO"). The recirculating gas mixture was cooled to 20" prior to entering the blower assembly, and the assembly itself was water-cooled by external copper coils. The temperature of the gases leaving the blower assembly was approximately 20". The lower half of the recirculation loop was isolated by valves so that the blower was not exposed to the high vacuum necessary for catalyst pretreatment. The recirculation rate was approximately 55 l./min. The volume of the entire recirculation loop was estimated to be 0.9 1. Consequently, the conversion per pass was low, and the loop was treated mathematically as equivalent to a well-stirred reactor. Reactor. The catalyst was housed in a quartz tube (2.8 cm i.d. by 24.5 cm). A calcium fluoride window was permanently sealed to one end of the tube with an epoxy-silicone rubber potting compound (Emerson and Cuming, Eccosil4712), which is 100% solid after curing and is stable up to 235". This material has enough flexibility to compensate for the difference in thermal expansion of quartz and CaF2. The other CaFz winThe Journal of Physical Chemistry
R. F. BADDOUR, M. MODELL, AND U. K. HEUSSER
rl DETECTOR
Figure 1. Experimental apparatus: A, water-cooled blower assembly; B, high-vacuum valves; C, preheater; D, reactor; E, catalyst disk; F, CaFt window; G, removable catalyst mount; H, water-cooled condenser; I, mass flow meter.
Figure 2. Catalyst and removable mount: A, brass cap; B, palladium rods; C, epoxy cement; D, CaFz wirdupr; E, catalyst disk; F, quartz ring; G, palladium wire.
dow was attached to the removable catalyst mount (see Figure 2). The mount consisted of a brass ring through which two palladium rods (a/16-in. diameter) were fastened. The rods were electrically insulated from the brass ring by Eccosil epoxy. The catalyst disk was mounted at the ends of the palladium rods. The face of the disk was perpendicular to the direction of gas flow. Catalyst. The catalyst was prepared by the addition of Cab-0-Si1 M5 (Cabot Cor .), a nonporous silica with a mean particle size of 120 and surface area of 175225 m2/g, to an aqueous solution of PdClz (Fisher Chemical Co.). The proportions were adjusted to yield a final catalyst composition of 10 wt % palladium metal. The slurry was dried at 70" and was ground into a fine powder. For the preparation of a catalyst disk, approximately 0.1 g of the powder was pressed under 3000 psig into a thin self-supporting disk enclosed in a quartz ring (see Figure 2). The quartz ring (22 mm i.d., 25 mm o.d., and 1.5 mm thick) facilitated handling and provided a means of supporting the disk. The die used to press the disks was similar to those previously described for pressing KBr pel1ets.lz
H
(12) D. H. Anderson and R. G. Smith, Anal. Chem., 26, 1674 (1964).
3623
CARBON MONOXIDE OXIDATION ON PALLADIUM The catalyst temperature was followed by monitoring the resistance of a 2 mil in diameter palladium wire (99.9% purity, Engelhsrd Industries), which was embedded in the disk. The wire was placed in the powder prior to compression. The ends of the wire, which extended beyond the quartz ring, were spot-welded to the palladium rods of the catalyst mount. The resistance of the embedded wire was large compared with the resistance of the rods, so that the measured resistance was directly related to the disk temperature. -4fter the catalyst mount as sealed in place, the sample was dried overnight at 260" while pumping at 4 X torr. The PdClz was reduced to the metal by heating for 2 hr under 20 torr of Hz at 160°, evacuating, heating for 2 hr under 15 torr of Hz at 220°, evacuating, heating for 2 hr under 15 torr of Hz a t 260", and degassing at 280" for 3 hr and at 200" for 13 hr. Recording of Spectra. The reactor was placed in the beam of a single-beam infrared spectrometer (PerkinElmer Model 12C, Model 81 amplifier, Globar source, LiF prism), and the entire assembly was placed in a closed Plexiglas box which was flushed with dry nitrogen. The path of the beam was altered with two extraplanar and one extraspherical mirrors SO that the catalyst disk was situated at a focal point of the beam. The detector was a thermocouple (C. H. Reeder and Co.) with a sensitivity of 14.7 pV/pW. All spectra reported here were observed in the presence of a gas phase of CO and He or CO, 0 2 , He, and sometimes COz. The spectral region from 2150 to 1800 cm-l was scanned at a constant slit width of 0.33 mm. The background of a reduced sample was measured at room temperature prior to admitting gases. Background transmission a t 2000 cm-' was approximately 8%. The spectra of adsorbed CO were obtained by comparison with the background spectrum. A single disk was used for all the data reported here. Recording of Kinetic Data. For a well-stirred reactor with a feed free of reaction products, the rate of reaction is equal to the product of the exit flow rate and the product concentration in the exit stream. The exit flow rate was measured with a mass flow meter (Hastings-Raydist, Inc.). Samples of the exit stream were analyzed for CO, 02, and COz with an on-line FisherHamilton gas partitioner (Fisher Scientific Co., Model 29). The gas partitioner was calibrated frequently with a standard mixture of He, CO, 02,and C02 (Matheson Chemical Co.). I n any given run, the exit gas stream was analyzed periodically. Steady-state operation was assumed when there was less than 2% variation in outlet composition over a 30-min period.
Results R o o m Temperature Xpectra. Since most spectra reported previously for CO on silica-supported palladium were obtained at room temperature, spectra were recorded a t this temperature for the purpose of compari-
50
v
t
40
2150
2100
a 5 0
1x0
ax0
1950 FRECCEtKY(cm-I)
1850
Figure 3. Room temperature spectrum (0.04 torr of GO, 0.21 torr of 02, 780 torr of He).
son. It was observed that the spectrum and catalytic activity obtained with a freshly prepared catalyst w re irreversibly altered by exposure of the catalyst to reaction mixtures at elevated temperatures. This breakin occurred over a period of 2-6 days, after which changes in spectra and catalytic activity with time were undetectable. The data reported here were obtained after the catalyst had been broken-in. The spectrum shown in Figure 3 was obtained a t 20.5" with a mixture of 0.04 torr of CO and 0.21 torr of 02. The spectrum was unchanged by cutting off the 0 2 flow while maintaining the CO partial pressure constant. No CO oxidation was detected at the conditions of this run. The room temperature spectrum contained an intense absorption band at 2095 and at 1990 cm-l, a band of moderate intensity at 1960 cm-l, and a weak broad band in the region of 1885 cm-l. E f e c t of Temperature. The temperature was increased from 20.5 to 143.0" in four steps, while the inlet concentrations of CO and O2 were kept approximately constant. The steady-state reaction rates and gasphase concentrations are given in Table I, and the corresponding spectra are illustrated in Figure 4. It should be noted that the ordinate in Figure 4 is only approximate, since each spectrum was corrected for background transmission of the catalyst disk at room temperature. Spectrum A, observed at 20.5", was discussed in the preceding section. No measurable reaction was obTable I : Effect of Temperature on Reaction Rate" A
T, "C PO,, torr PCO,torr Pcoz, torr Reaction rate (lo7),mol of COa/min a
20.5 0.206 0.044
> BL, and OL becomes appreciable only
For small 8, OB as e approaches unity.
The Journal of Physical Chemiatry
If BL and OB are eliminated from eq 18 and 19
As 0 approaches unity so that
Since 0 2 adsorption is assumed to occur only on bare surface sites, the definition of 0 given in the preceding discussion is identical with that used in eq 5 and in the derivation of eq 10. Thus, if eq 5 and 21 are combined for the case of OZ-adsorptionrate limiting
For the case of surface-reaction rate limiting, the observed rate expression can be derived if it is assumed that Oco in eq 9 is proportional to 0 as used in eq 16-21. I n this case
The interpretation of eq 22 and 23 is that under the conditions of interest, the bridged species of the type shown in Figure Ga are well developed (Le., they have approached their maximum coverage). On the remaining sites, oxygen and carbon monoxide compete for adsorption as OZand CO(L), respectively. The kinetics of the competition are such that most of these remaining sites are filled by linear CO (i.e., CO(L) inhibits the adsorption of 02).Because the two ratelimiting processes considered are quite similar phenomenologically, a distinction between them cannot be made on the basis of the data presently available. Comparison with Batch Reactor Results. Several studies have been made for CO oxidation on palladium using batch-reactor conditions. In one case,'* kinetic measurements were made and reaction orders were determined (see Table 111). I n other the reaction was studied by preadsorbing one reactant, evacuating, and then dosing with the other reactant. Stephensz4 determined the extent of reaction and adsorption by measuring pressure before and after dosing with and without a cold trap for condensing COD The reaction of CO with preadsorbed oxygen was found to be rapid. For O2with preadsorbed carbon monoxide, surface reaction was preceded by an induction period during which no measurable COz formation or 02 adsorption occurred. Once the reaction began, how(24) S. J. Stephens, J . Phys. Chem., 63, 188 (1959). (25) E.G.Alexander and W. W. Russell, ibid., 68, 1614 (1964). (26) K. Kawasaki, T.Sugita, and S. Ebisawa, J . Chem. Phys., 44, 2313 (1966).
CARBONMONOXIDE OXIDATION ON PALLADIUM ever, it proceeded readily with complete uptake of 0 2 and formation of CO2. Stephens attributed the induction period to the inability of O2 molecules to find sites on which dissociation can occur. The first 0 2 dose, during which the induction period was observed, required the order of 1000 min for conversion to C02. Approximately 10% of the initial preadsorbed CO reacted during the first dose. Successive doses of 0 2 required less than 2 min for complete uptake and reaction. To determine which portion of Stephens' results correspond to the steady-state results reported here, the following calculation was made. The steady-state rate that would have been observed with a pressed disk catalyst under Stephens' initial conditions (O", lou2 torr of CO before evacuation, and lom2torr of 02) was calculated using the rate observed at 143" (4 X lo-' mol of C02/min) and corrected to 0" using an apparent activation energy of 28 kcal/mol. Under these conditions, the rate which would have been obmol of C02/min. If it is assumed served is 9 X that the initial CO coverage would have corresponded to 10l6 molecules/cm2 and that the palladium surface area of the pressed disk was 0.5 m2 (a typical value for this type of catalyst), the time required to form an amount of COz equal to 10% of the initial CO coverage was calculated. Thus if it is assumed that the reaction rate would have remained constant during conversion of 10% of the adsorbed CO, it would have taken9 X lo7 min to react that amount of CO.
3629 The interpretation of this calculation is that the kinetics observed during steady-state operation correspond to the phenomenon occurring during the induction period observed by Stephens. Thus during the batch-reaction induction period, bare surface sites are created by the reaction of adsorbed molecular oxygen and adsorbed CO to form gaseous COZ. When a sufficient number of sites have been freed, oxygen will increasingly adsorb dissociatively, and the reaction rate will increase rapidly. The kinetic expression reported by Daglish and Eleyl8 (see Table 111)would clearly be weighted heavily by the phenomena occurring after the induction period. Thus the inverse second order in CO would be consistent with surface reaction between adsorbed carbon monoxide and atomic oxygen. From the above discussion, it should be clear that in a batch system, the reaction mechanism may change during the course of a run. If two or more different mechanisms of approximately equal rates occur during a run, the interpretation of kinetic data can be complex and/or misleading.
Acknowledgment. This work was supported in part by the National Science Foundation (Grant GP-607). The authors are grateful to the Atlantic Refining Co. for the use of the spectrophotometer. The authors are indebted to Drs. R. P. Eischens, C. W. Garland, and R. C. Lord for discussions and advice during the course of the investigation.
Volume 78, Number 10 October 1068
