J . Phys. Chem. 1986, 90, 622-628
622
Catalytic Oxidation of Carbon Monoxide over Ir/Si02. An in Situ Infrared and Kinetic Study Ryadh A. Saymeh Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881
and Richard D. Gonzalez* Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60680 (Received: February 4, 1985)
The oxidation of CO on a highly dispersed Ir/Si02 catalyst has been studied both in a Pyrex microreactor and by using an in situ infrared cell-reactor. Multiple steady states obtained under conditions of increasing and decreasing CO partial pressure were observed to lead to reaction rate hysteresis. The area under the hysteresis loop is strongly dependent on reaction temperature. This is explained by invoking competitive adsorption between CO and O2as a function of temperature. Self-sustained oscillations were obtained when certain conditions of partial pressure and temperature were met. Quantitative estimates of fluctuations in surface coverage and temperature during these oscillations were 10% and 3 K, respectively. CO islands of reactivity were not observed under the conditions of this study. The absence of CO islands is explained by considering shifts in the position of the infrared stretching frequency for CO during the diffusion-controlled and the kinetically controlled regimes. Reaction rate hysteresis following exposure of the catalyst to the reactant gas mixture at high temperatures was observed. Rates obtained under conditions of increasing temperature were higher than those obtained under conditions of decreasing temperature by a factor of between 2 and 3. The presence of higher oxidation states of IR could not be unequivocally determined by using infrared spectroscopy.
Introduction The catalytic oxidation of C O over noble metal catalysts has been the subject of considerable study over supported metals and those who study catalytic reactions over well defined crystallographic surfaces. Most of the studies that have appeared in the catalytic literature have been performed on either supported Pt or Pd.’” CO oxidation studies over other supported noble metals are less numerous. Cant et aL7 have performed a comparative study of five noble metals in an attempt to bridge the gap between high-pressure studies on highly dispersed metal powders and low-pressure studies on well-defined single crystal surfaces. Kiss and Gonzalez have recently performed CO oxidation studies over Rh/Si028 and R u / S i 0 2 catalysts. This paper, therefore, represents the third in a continuing series of studies in which in situ infrared spectroscopy coupled with kinetic rate measurements is being used to obtain information regarding the catalytic oxidation of CO over supported noble metal catalysts. Variables which may have an important effect on the catalytic activity of CO oxidation include (1) the oxidation state of the metal;8-’0 (2) the occurrence of islands of reactivity;’q1’*’2(3) dispersion effects;13(4)the composition of the reactant gas mixture initially contacted with the catalyst;I4 ( 5 ) the appearance and disappearance of new surface phases during the course of self(6) surface temperature fluctuations which sustained ~scillations;~~ may lead to self-sustained oscillations;6 (7) mass-transfer effects ( I ) Haaland, D. M.; Williams, F. L. J . Catal. 1982, 76, 450. (2) Varghese, P.; Carberry, J. J.; Wolf, E. E J . Cutal. 1978, 55, 76. (3) Sarkany, J.; Gonzalez, R. D. J . Appl. Caral. 1983, 85. (4) Cant, N. W.; Donaldson, R. A. J . Catal. 1981, 71, 320. (5) Baddour, R. F.; Modell, M.; Goldsmith, R. L. J . Phys. Chem. 1970, 74, 1787. (6) Kaul, D. J.; Wolf, E. E. J. Catol. 1984, 89, 348. (7) Cant, N. W.; Hicks, P. C.; Lemon, B. S. J . Carol. 1978, 54, 372. (8) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem. 1984, 88, 898. (9) Kiss, J. T.; Gonzalez, R. D. J. Phys. Chem. 1984, 88, 892. (10) Oh, S . H.; Carpenter, J. E. J . Caral. 1983, 80, 472. (1 1) Barshad, J.; Gulari, E., presented in part at the AIChE Meeting, San Francisco, CA, 1984; Paper N. 89e. (12) Wicke, P., personal communication. (13) Boudart, M. “Proceedings of the Ibero American Conference on Catalysis, Lisbon, Portugal, 1984”. Vol. I , p 3. (14) Kiss, J. T.; Gonzalez, R. D. Znd. Eng. Chem. Prod. Res. Deu., in press. (15) Saranteas, C.; Stoukides, M., presented in part at the AIChE Meeting, San Francisco, CA, 1984; Paper No. 9b.
0022-3654/86/2090-0622$01.50/0
either in the gas phase or as a result of intraparticle diffusion into the pore network of the catalyst;16 (8) reaction rate hysteresis which may occur as a result of either mass-transfer effects or nonequilibrium absorption;” and (9) the role of oxygen as a modifier of the active sites responsible for C O oxidation.*s9 The Occurrence of self-sustained oscillations during oxidation reactions has recently been the subject of intense debate. In particular, Kaul and Wolf6 have recently observed large temperature excursions during the oxidation of C O over both supported Pt and Pd. These sharp temperature excursions occurred either during the period of steady-state oscillations or during forced concentration programming. Several investigator^^^^^^'^ have attributed these self-sustained oscillations to fluctuations in the surface phases during the oscillatory period. However, it is difficult to distinguish beween a new surface phase and changes which occur on the surface as a result of strongly chemisorbed oxygen. The occurrence of C O islands of reactivity has been experimentally observed by Engel and Ertl, who have published an excellent review on the subject.ls According to their model, islands of CO occur on the catalyst surface during the reaction. These CO islands are surrounded by chemisorbed oxygen in such a way that the reaction occurs at the CO-oxygen island boundary. The existence of these C O islands of reactivity has also been implicated in studies on supported noble metal Haaland and Williams’ suggest that the invariance in the infrared frequency of the C O absorption band with the surface coverage is due to the existence of CO islands in which dipole-dipole coupling occurs. Similar infrared in situ C O oxidation studies over Pd/Si02, by Barshad and Gulari” and by Wicke,12 support the results of Haaland and Williams.’ However, it is noteworthy that these studies have been performed on either supported Pt or Pd catalysts having very low dispersions. It is reasonable to expect that studies on supported metals would agree with those reported on single crystals when the crystallites are sufficiently large. However, it would be harder to rationalize C O island formation on highly dispersed crystallites. The purpose of this study is, therefore, to investigate the CO oxidation reaction under conditions which would reduce the (16) Hegedus, L. L.; Oh, S . H.; Baron, K. K. AZChE J . 1977, 23, 632. (17) Herz, R. K.; Marin, S . P. J . Catul. 1980, 65, 281. (18) Engel, T.; Ertl, G. Adu. Coral. 1979, 28, 2.
0 1986 American Chemical Society
Catalytic Oxidation of Carbon Monoxide
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 623
possibility of C O island formation and which would minimize intraparticle mass-transfer effects which might lead to reaction rate hysteresis. For this reason we have performed a C O oxidation study on a highly dispersed Ir/SiOz catalyst. It is reasonably easy to prepare a highly dispersed Ir catalyst having a relative high weight loading. The choice of Cab-0-Si1 as a support should also minimize mass-transfer effects due to porous silica networks.
Experimental Section The flow system which enables use of the infrared cell as either a pulse microreactor or a single-pass differential reactor has been described previo~sly.'~The infrared cell-reactor was constructed by the Byron-Lambert Co. of Franklin Park, IL, and has the important feature that the reactant gases are forced through the sample disk with little or no leakage occurring around the edges of the sample. For this reason, the infrared cell-reactor can effectively be used as a differential single-pass reactor. In several experiments, a conventional Pyrex microreactor constructed from 12-mm Pyrex tubing having a volume of 4.4 mL was used. It had a length of 70 mm and was connected to the flow system by using 3.1-mm tubing and swagelock fittings. Product analysis was performed by using a gas chromatograph (Perkin-Elmer Model Sigma 3-B) located downstream of the reactor. Adequate chromatographic separation of the reaction products could be obtained by using a 1-m stainless steel column packed with Carbosieve (100-200 mesh). Infrared spectra were recorded on a Perkin-Elmer Model 28 1 infrared spectrophotometer interfaced to a Perkin-Elmer data station to facilitate processing of the data. The adsorption bands due to the support and the gas phase were subtracted from the resulting transmittance spectra and the result was either replotted as absorbance or reconverted to transmittance. Materials. The silica-supported Ir catalysts were pepared from solutions containing the appropriate weight IrC13.3H20 (Strem Chemical). The solutions were mixed with Cab-0-Si1 (grade M-5, average pore diameter 14 nm, Cabot Corp., Boston, MA). Metal loading were 0.3 mmol of metal/g of catalyst. The subsequent handling of the resultant slurry has been previously d e ~ c r i b e d . ~ For use in the spectroscopic reactor, the sample disks were prepared by pressing the ground catalysts into self-supporting disks 25 mm in diameter with an optical density of approximately 25 mg/cm2. The gases used in this study were subjected to purification treatments as previously d e ~ c r i b e d .It~ was found convenient to use O2 and C O premixed with H e to give the following compositions: 5.25% C O in He and 5.20% 0, in He. The 0, concentration in the carrier gas was reduced to the ppb range through the use of a molecular sieve maintained at 77 K and an oxypurifier (Supelco Co.). These were backed by an MnO trap activated in hydrogen at 673 K. Gas flow rates were controlled by using Tylan (Model FC260) electronic flow controllers. The flow controllers were calibrated by the manufacturer using gas mixtures which were identical with those used in this study. The calibration was rechecked in our laboratory by means of a bubble flow meter and a gas chromatograph. Procedure. Fresh, silica-supported Ir catalysts were treated according to the following pretreatment schedule: the catalyst was heated in flowing H2at 403 K for 0.5 h, the temperature was then increased at a rate of 10 K/min from 403 to 673 K, and the catalyst was reduced at this temperature for 3 h. The catalyst was then outgassed in flowing H e at 723 K for 1 h followed by cooling to room temperature in flowing He. Treatment of the sample disks used in the infrared cell-reactor was identical with that used in the Pyrex microreactor except that the final reduction was performed at 650 K in order to minimize damage to the CaF, optical lenses. Chemisorption measurements were performed by using the dynamic pulse method.20 Metal dispersions were calculated on (19) Miura, H.;Gonzalez, R.D. J . Phys. E. 1982, 15, 373. (20) Sarkany, J.; Gonzalez, R. D. J . Catal. 1982, 76, 75.
.TEMPERATURE(K)
Figure 1. Reaction of a C O / 0 2reactant gas mixture having a ratio of 4 (A) and 2 (B) on an Ir/Si02 catalyst as a function of temperature. Metal loading was 0.3 mmol of Ir/g of catalyst.
the basis of a CO/Ir(s) adsorption ratio of Ir dispersions were cross-checked by using O2chemisorption. An 02/Ir(s) adsorption stoichiometry of 0.33 was used.21 These adsorption stoichiometries have been moderately well established. Extinction coefficient measurements, used to make semiquantitative estimates of the surface coverage under reaction conditions, were determined by a procedure which was identical with that reported by Sarkany and Gonzalez.,,
Results Reaction Rate Hysteresis as a Function of Temperature and Reactant Partial Pressures. Turnover frequencies for the rate of CO, formation were obtained as a function of both temperature and reactant gas composition. In these studies the reactant gas mixture was contacted with about 300 mg of the catalyst which was placed in the Pyrex microreactor. The reactant gas flow rate was maintained constant at 25 mL/min. However, reactant gas compositions could be accurately controlled by means of the electronic flow controllers. Turnover frequencies for the rate of CO, formation as a function of temperature are shown in Figure 1 under conditions of both increasing and decreasing temperature. In these experiments steady-state turnover frequencies were measured at each temperature. The attainment of a steady-state reaction rate was usually observed to occur at each temperature following reaction for 30 min. Turnover frequencies obtained under conditions of decreasing temperature were lower than those obtained under conditions of increasing temperatures by a factor of between 2 and 3. However, the resulting temperature-dependent rate hysteresis was considerably smaller than that observed over Ru/Si02? The turnover frequency-temperature plots for CO/O2 reactant gas ratios of 4 (excess CO) and 2 (corresponding to a stoichiometric mixture) were similar. In both studies, the temperature was increased until full ignition was obtained. Ignition here is taken to mean 100% conversion of the limiting reactant. Activation energies were obtained under conditions of increasing temperature by using low conversion data points. At conversions in excess of 70%, gas-phase diffusion limitations to the surface were observed to occur. These diffusion limitations resulted in a nonlinear Arrhenius behavior of the rate vs. 1 / T plots. The activation energy was 38 kJ/mol and did not depend on the CO/O2 reactant gas ratio. However, it was somewhat lower than the value reported by Cant et al.' When the catalyst was contacted with a reactant gas mixture which had excess O2 ( C O / O 2= O S ) , ignition occurred at very low temperatures, making it difficult to obtain turnover frequency-temperature plots similar to those shown in Figure 1 for CO/O2reactant gas ratios of 4 and 2. However, the activation energy based on two low-temperature data points yielded a value of 40 kJ/mol, which agreed well with the value of 38 kJ/mol obtained at higher C O / O 2ratios. A complete tabulation of the (21) Falconer, J. L.; Wentrecek, P. R.; Wise, H. J . Catal. 1976, 45, 248. (22) Sarkany, J.; Gonzalez, R. D. J . Appl. Speclrosc. 1982, 36, 320.
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The Journal of Physical Chemistry, Vol. 90, No. 4, 1986
Saymeh and Gonzalez
TABLE I: Summary of Kinetic Data for the Oxidation of CO on Ir/Si02 Catalysts" Pcn, Pa
Po,,Pa
4256
1053 1756
3546 1773
3513
4256 3546
1053 1756
1773
35 1 3d
323 K
0.063
363 K
0.38
molecule/(site s) 383 K 403 K 423 K Increasing Temperature 2.8 1 4.84 4.96 9.01 0.85 4.2OC Decreasing Temperature 0.46 1.19 1.07 2.51 4.2OC
Ea9
443 K
463 K
473 K
kJ/mol
7.16 18.32
