J . Phys. Chem. 1988, 92, 5213-5221 results in the stabilization of ethyl thiolate with respect to C-S bond cleavage. Low-temperature C-S bond cleavage at low exposure ultimately results in nonselective decomposition to surface carbon and sulfur. At high coverage, the activation energy for decomposition becomes sufficiently high so that hydrocarbon formation competes effectively with nonselective decomposition.
5213
Acknowledgment. This work was Supported by the Department of Energy, Basic Energy Sciences (Grant DE-FG02-84ER13289). C.M.F. and J.T.R. thank R. J. Madix for allowing us the use of his high-resolution electron energy loss spectrometer. Registry No. Mo, 7439-98-7; C2H5SH,75-08-1.
Kinetics of CO Oxidation on Single-Crystal Pd, Pt, and I r Paul J. Berlowitz,+ Charles H. F. Peden,* and D. Wayne Goodman Sandia National Laboratories, Albuquerque, New Mexico 871 85 (Received: January 12, 1988)
The activity of Pt( loo), Pd( 1 lo), Ir( 1 1 l), and Ir( 110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O2 and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O2 partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed by deliberate oxidation and could be detected as C 0 2 desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable to 475 K on Pd(ll0) in an 11:l 02:C0 mixture and to 500 K on Ir(ll1) in an 8O:l O2:COmixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker. Negative-order dependence in O2 pressure was not observed for Pt, which could not be oxidized under reaction conditions.
Introduction The oxidation of C O by O2 over group VI11 metal catalysts has been the subject of a large body of ultra-high-vacuum (UHV) surface science and high-pressure catalysis work due to its importance in pollution control.' Currently, the removal of C O as C 0 2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. This has led to numerous recent studies of the kinetics of this reaction on supported metal catalyst^^-^ and transient kinetic studies on polycrystalline foils,*-13which have sought to identify and quantify the parameters of the elementary mechanistic steps in C O oxidation. The relative simplicity of this reaction makes CO oxidation an ideal model system of a heterogeneous catalytic reaction. Each of the mechanistic steps, adsorption and desorption of the reactants, surface reaction, and desorption of products, has been probed extensively with surface science techniques, as has the interaction between adsorbed 0 atoms and CO molecule^.'^^^ These studies have provided essential information necessary for understanding the elementary processes which occur in C O oxidation. Recent reviews by Engel and Ertl have summarized most of the chemisorption and low-pressure catalytic findings2* In general, the reaction proceeds through a Langmuir-Hinshelwood mechanism involving adsorbed CO and 0 atoms. Under reaction conditions typical in most high-pressure supported catalyst studies, and most low-pressure (UHV) studies on model catalysts, the surface is almost entirely covered by CO, and the reaction rate is determined by the rate of desorption of CO. As first determined by Langmuir for Pt wire catalysts,29the observed activation energy is close to the binding energy of adsorbed CO. Oxygen can only adsorb at sites where CO has desorbed, leading to first-order *Author to whom correspondence should be addressed. 'Present address: Exxon Research and Engineering Co., Linden, NJ 07036.
0022-3654/88/2092-5213$01.50/0
dependence in oxygen pressure, negative-first-order dependence in CO partial pressure, and zero-order total pressure dependence. (1) Kummer, J. T. J . Phys. Chem. 1986, 90, 4747. (2) Cant, N. W.; Hicks, P. C.; Lennon, B. S. J . Cuful. 1978, 54, 372. (3) (a) Kiss, J. T.; Gonzalez, R. D. J. Phys. Chem. 1984,84892. (b) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem. 1984, 88, 898. (4) Cant, N. W.; Angove, D. E. J . Cutal. 1986, 97, 36. (5) Shishu, R. C.; Kowalczyk, L. S. Platinum Met. Rev. 1974, 18, 58. (6) Voltz, S. E.; Morgan, C. R.; Liederman, D.; Jacob, S. M. Ind. Eng. Chem. Prod. Res. Deu. 1973, 12, 294. (7) Nicholas, D. M.; Shah, Y. T. Ind. Eng. Chem. Prod. Res. Dev. 1976, 15, 35. (8) Matsushima, T. J . Phys. Chem. 1984, 88, 202. (9) Kawai, M.; Onishi, T.; Tamaru, K. Appl. Surf. Sci. 1981, 8, 361. (10) McCarthy, E.; Zahradnik, J.; Kuczynski, G. C.; Carberry, J. J. J . Catal. 1975, 39, 29. (11) (a) Matsushima, T.; White, J. M. J . Catal. 1975, 39, 265. (b) Matsushima, T.; Mussett, C. J.; White, J. M. J . Catal. 1976, 41, 397. (c) White, J. M.; Golchet, A. J . Chem. Phys. 1977, 66, 5744. (12) (a) Matsushima, T. J . Catal. 1978,55,337. (b) Matsushima, T. Surf: Sci. 1979, 79, 63. (c) Matsushima, T. Surf.Sci. 1979, 87, 665. (13) Hori, G. K.; Schmidt, L. D. J . Catal. 1975, 38, 335. (14) Taylor, J. L.; Ibbotson, D. E.; Weinberg, W. H. Surf. Sci. 1979, 90, 37. (15) Hagen, D. I.; Nieuwenhuys, B. E.; Rovida, G.; Somorjai, G. A. Surf. Sci. 1976, 57, 632. (16) Kuppers, J.; Plagge, A. J . Vac. Sci. Technol. 1976, 13, 259. (17) (a) Zhdan, P. A,; Boreskov, G. D.; Boronin, A. I.; Schepelin, A. P.; Withrow, S.P.; Weinberg, W. H. Appl. Surf. Sci. 1979, 3, 145. (b) Zhdan, P. A,; Boreskov, G. K.; Egelhoff, W. F., Jr.; Weinberg, W. H. Surf:Sci. 1976, 61, 377. (18) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf.Sci. 1984, 146, 155. (19) Gland, J. L.; Kollin, E. B. J . Chem. Phys. 1983, 78, 963. (20) Conrad, H.; Ertl, G.; Kuppers, J. Surf:Sci. 1978, 76, 323. (21) Mantell, D. A,; Ryali, S. B.; Haller, G. L. Chem. Phys. Lett. 1983, 102, 37. (22) Akhter, S.; White, J. M. Surf:Sci. 1986, 171, 527. (23) (a) Engel, T.; Ertl, G. J . Chem. Phys. 1978,69, 1267. (b) Engel, T. J. Chem. Phys. 1978, 69, 373. (24) Matolin, V.; Gillet, E.; Gillet, M. Surf.Sci. 1985, 162, 354.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
Berlowitz et al.
1000 These features have allowed many of the reaction parameters determined in UHV to be applied directly to the kinetics at higher pressures.30 Models based on chemisorption and kinetic parameters de100 termined in surface science studies have been successful at predicting most of the observed high-pressure behavior. Recently, Oh et have developed a mathematical model for CO oxidation by O2or NO on Rh which correctly predicts the absolute rates, activation energy, and partial pressure dependence. Similarly, Z studies by Schmidt and co-workers of the C O + O2 reaction on II-! Rh( 11 1)31 and C O + NO on polycrystalline Pt32have demonstrated the applicability of steady-state measurements in UHV and relatively high (1 Torr) pressures in determining the reaction mechanisms and kinetic parameters. Recently, work in this laboratory on the steady-state reaction 0" 0.1 kinetics at high pressure over R u ( O O O ~ and ) ~ ~ Rh( 111) and Rh0 (1 00) single crystals34and field emitter tips35has been performed. The single crystals exhibited rates and activation energies which 0.0 1 were almost identical with measurements over supported metal catalyst^.**^^^^ In addition, discrepancies between UHV and high-pressure measurements of the activity of Ru surfaces were successfully explained by studying the oxidation behavior of the 0.001 I I I I I I I Ru surface. These studies have convincingly demonstrated the 1.4 1.6 1.8 2.0 2.2 2.4 2.8 applicability and advantages of model single-crystal studies, which 1000/T (K-l) combine UHV surface analysis techniques with high-pressure Figure 1. Specific rates of reaction (turnover frequencies) as a function kinetic measurements, in the elucidation of reaction mechanisms of inverse temperature for single-crystal and supported* catalysts. over supported catalysts. The present study examines the oxidation of C O by O2 in a temperature was measured by a 0.08-mm chromel-alumel thercombined high-pressure reaction-UHV surface analysis system. mocouple spot-welded to the sample edge. A wide range of temperatures and 0 2 : C 0ratios are employed, AES was used to characterize the cleanliness of the samples. with emphasis on comparisons between previous work on high In addition, the cleanliness of the Pt( 100) sample was checked surface area catalysts and the single crystals in this study. The for carbon impurities by O2adsorption/desorption and for Si and effects of catalyst oxidation and implications for rate oscillations previously observed on Pt, Pd, and Ir polycrystalline c a t a l y s t ~ ~ ~ , ~Ca~ impurities by high-temperature oxidation (1 123 K, 1 X Torr of 02).39 The Pd crystal was cleaned by placing it in the are discussed. reactor with 8 Torr of CO and 8 Torr of O2 and heating to 600 K for 1-2 min. One to three cycles of this treatment produced Experimental Section a clean surface. Large carbon impurities were cleaned from both Ir samples by oxidation in 1 X Torr of O2at 1000 K for 5-10 The apparatus used in these experiments has been described min, followed by a 3-min anneal in vacuo to 1600 K. Small p r e v i o u ~ l y . ~Briefly, ~ the system consists of a UHV surface quantities of carbon were removed from the Ir samples by reaction analysis chamber equipped with Auger electron spectroscopy in 8 Torr of O2and 4 Torr of CO for 2 min at 600-625 K, followed (AES), a quadrupole mass spectrometer for temperature-proby a brief flash to 1600 K. The Pt sample was cleaned by grammed desorption (TPD), and an ion sputter gun, contiguous sputtering at 1100 K in 5 X loT5Torr of Ar for 30 min (1.0-kV to a high-pressure reaction chamber. The single-crystal samples beam energy) to remove Si and Ca impurities; this was followed were mounted on the arm of a retractable bellows, allowing the by heating in 0.1 Torr of O2 at 1100 K for 30 min and then sample to be transferred in vacuo between the two chambers. The annealing at 1300 K in vacuo. Several cycles of this procedure Pt(100), Ir(llO), and P d ( l l 0 ) crystals measured 0.92 cm in produced a clean Pt surface which could not be oxidized under diameter by 0.1 1 cm thick. The Ir( 11 1) sample was elliptically UHV conditions at high temperature. No carbon was detectable shaped, measuring 0.75 cm by 0.55 cm by 0.3 mm thick. The either in AES or as C 0 2 desorbing from the surface in TPD samples were heated resistively by two high-purity, 0.5 1-mm experiments performed in an O2 ambient (lo-' Torr). tungsten leads spot-welded to the back of the crystal; sample Gas chromatography with flame ionization detection was used to analyze the reaction products. CO and C 0 2were catalytically converted to methane before analysis. Rates of reaction are (25) Matsushima, T.; Asada, H. J . Chem. Phys. 1986, 85, 1658. expressed as turnover frequencies (TOF), defined as the number (26) Palmer, R. L.; Smith, J. N., Jr. J . Chem. Phys. 1974, 60, 1453. (27) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. J . Chem. Phys. of C 0 2 molecules produced per active metal site per second. For 1980, 73, 5862. Pd and Ir, the entire crystal (front, back, and edge) was counted (28) (a) Engel, T.; Ertl, G. Adu. Catal. 1979, 28, 1. (b) Engel, T.; Ertl, in the determination of the total number of sites; for Pt, only the G. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysts; front face was included, as t h e back and edge of the crystal were King, D. A,, Woodruff, D. P., Eds.; Elsevier: Holland, 1982; Vol. 4. (29) Langmuir, I. Trans. Faraday SOC.1921-22, 17, 621, not subjected to the sputter cleaning procedure. Research grade (30) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J . Catal. CO (99.99%) and O2 (99.995%) were supplied by Matheson. CO 1986, 100, 360. was further purified before use by slowly passing it through a (31) Schwartz, S. B.; Schmidt, L. D.; Fisher, G. B. J . Phys. Chem. 1986, molecular sieve trap at 77 K. No metal carbonyls (e.g., Ni(CO),) 00, 6194. (32) Klein, R. L.; Schwartz, S.; Schmidt, L. D. J . Phys. Chem. 1985.89, were detected in any experiment in postreaction AES analysis. 4908. The experimental procedure has been described in detail pre(33) Peden, C. H. F.; Goodman, D. W. J. Phys. Chem. 1986, 90, 1360. v i o u ~ l y . ~Briefly, ~" after cleaning and characterization by AES, (34) (a) Peden, C. H. F.; Goodman, D. W.; Blair, D. S.; Berlowitz, P. J.; the sample is removed to the reactor, which is charged with Fisher, G. B.; Oh, S. H. J . Phys. Chem. 1988, 92, 1563. (b) Goodman, D. W.: Peden. C. H. F. J . Phvs. Chem. 1986. 90. 4839. reactants. Most experiments were performed with 16 Torr of CO (35) (a) Kellogg, G. L.3. Catal. 1985, 92, 167. (b) Kellogg, G. L. Surf. and 8 Torr of 02.The sample is heated to the desired temperature Sci. 1986. 171. 359. for a specified time, the products are then allowed to mix for 15 (36) Turner, J. E.; Sales, B. C.; Maple, M. B. Surf.Sci. 1981, 103, 54.
(37) Turner, J. E.; Sales, B. C . ; Maple, M. B. Surf.Sci. 1981, 109, 591. (38) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J. T., Jr. J . Catal. 1980, 63, 226.
( 3 9 ) Mundschau, M.; Vanselow, R. Surf S r i . 1985, 157, 87
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5215
Kinetics of C O Oxidation
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Figure 3. CO partial pressure dependence at constant oxygen pressure and 525 K on Pd( 110). The maximum rate occurs at an O2:CO ratio of 12:l for both oxygen pressures.
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Figure 2. Effect of particle size on the C 0 2 formation rate on Ir/SiOZ2 and Ir single-crystal catalysts.
TABLE I: Activation Energies catalyst temp, K Ir(ll0) 425-625 Ir(ll1) 425-625 Ir/Si02 425-475 Pd(ll0) 475-625 Pd(ll0) 0.2 V) more negative (VG = Vc2)or positive (V, = VG3) of E“, only the reduced or oxidized sites are present, respectively, and the device is off. V-Me2+) and pOly(P-V-H2+)allows demonstration of the behavior of microelectrochemical transistors4 based on the viologen redox 0 1988 American Chemical Society
