Langmuir 1989,5, 758-766
Microcalorimetric Study of the Progressive Oxidation of the Surface of Graphite-Supported Iron Microcrystals Robert R. Gatte and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802 Received July 25,1988. In Final Form: January 13, 1989 The adsorption of oxygen at 303 K on iron microcrystals supported on high surface area graphite was studied by using a novel Calvet-type differential microcalorimeter. Both the rate and the heat of adsorption were determined for incremental oxygen doses. Chemisorption was found to proceed through two distinct stages. Initially, the adsorption was rapid and accompanied by the evolution of large amounts of heat. This stage is attributed to the formation of a nonequilibrium film of precursory oxide with an FeO stoichiometry. In the eecond stage, chemical adsorption occurred with progressively lower heats and at progreasively slower rates, apparently until the formation of a passivating oxide film with Fe304stoichiometry was achieved. In total, two atomic layers of iron in the original particles were fully oxidized. All of these results agree with previous stttdiea of the oxidation of iron single crystals and polycrystalline films. Mossbauer spectroscopy, transmission electron microscopy, and X-ray diffraction were used to determine the particle morphology and the particle size distribution.
Introduction There are relatively few past studies in which adsorption calorimetry has been utilized for the analysis of supported metal particles.l+ However, the measurement of the heat of chemisorption of gases on supported-metal (catalyst) particles as a function of surface coverage can provide a potentially valuable characterization tool for the analysis of the chemical nature and distribution of active sites on the metal surface. The following is a report on the use of a true differential adsorption microcalorimeter of novel design to study the heat and kinetics of oxygen adsorption on graphite-supported iron microcrystals at 303 K. The low-temperature oxidation of iron surfaces and small iron particles has received considerable attention in the both because it has direct application to the study of corrosion processes (of electrodes, magnetic films, and tapes) and because it is a good model for understanding the general phenomenon of metal oxidation. Most studies have focused on the structure of the oxide layer that forms during exposure of iron surfaces to oxygen. Studies of single crystals and polycrystalline films have been conducted using virtually the entire arsenal of ultra-high-vacuum techniques.e22 Studies of the rate of formation and thickness of oxide films formed on small iron particles have been conducted as As a result of all of this work, the surface "oxide" structure is fairly predictable as a function of both the rate and the pressure of oxygen exposure. This has been well summarized by Langell and Somorjai.lo However, few direct thermodynamic measurements have ever been made.*31 The most comprehensive of these studies was performed by Brennan, Hayward, and Trapnell.2B These workers measured the differential heat of adsorption of oxygen on evaporated iron films using an isoperibol ( B e e ~ k - t y p e calorimeter. )~~ The results of the present study of the heat and rate of oxidation of supported iron particles correlate extremely well with these previous thermodynamic studies, as well as the structural studies mentioned above. This correlation of results indicates the usefulness of the new calorimeter for the study of supported metal (catalyst) particles. Experimental Section Catalyst Preparation. The support material was GTA grade Grafoil (Union Carbide Corp.), which is a high surface area (ca.
* Author to whom correspondence should be addressed. 0743-7463/89/2405-0758$01.50/0
22 mz/g),highly graphitic, very pure carbon. It has been shown in previous work*% that this support material does not interact
(1) Herrmann, J. M.; Gravelle-Rumeau-Maillot, M.; Gravelle, P. C. J. Catal. 1987, 104, 136. (2) Prinsloo, J. J.; Gravelle, P. C. J. Chem. SOC.,Faraday Trans. 1 1980, 76, 2221. (3) Prinsloo, J. J.; Gravelle, P. C. J. Chem. Soc., Faraday Trans. 1 1980, 76, 512. (4) Topsoe, H.; Topsoe, N.; Bohlbro, H.; Dumesic, J. A. Proc. Int. Congr. Catal., 7th 1981, 247. (5) El Shabaky, G.; Gravelle, P. C.; Teichner, S. J. J. Catal. 1969,14, 4. (6) Pignocco, A. 3.; Pellissier, G. E. J. Electrochem. SOC.1965, 112, 1188. (7) Pignocco, A. J.; Pellissier, G. E. Surf. Sci. 1967, 7, 261. (8) Legg, K. 0.;Jona, F. P.; Jepsen, D. W.; Marcus, P. M. J.Phys. C: Solid State Phys. 1975,8, L492. (9) Simmons, G. W.; Dwyer, D. J. Surf. Sci. 1976, 48,373. (10) Langell, M.; Somorjai, G. A. J. Vac. Sci. Technol. 1982,21,858. (11) Bruker, C. F.; Rhodin, T. N. Surf. Sci. 1976,57, 523. (12) Pirug, G.; Broden, G.; Bonzel, H. P. Surf. Sci. 1980, 94, 323. (13) Miyano, T.; Sakisaka, Y.; Komeda, T.; Onchi, M. Surf. Sci. 1986, 169, 197. (14) Erley, W.; Ibach, H. Solid State Commun. 1981,37,937. (15) Papagno, L.; Caputi, L. S.; Chiarello, G.; Delogu, P. Surf. Sci. 1986, 175, L767. (16) Leygraf, C.; Ekelund, S. Surf. Sci. 1973, 40, 609. (17) Ertl, G.; Wandelt, K. Surf. Sci. 1975, 50, 479. (18) Yu, K. Y.; Spicer, W. E.; Lindau, I.; Pianetta, P.; Lin, S.-F.Surf. Sci. 1976, 57, 157. (19) Gimzewski, J. K.; Padalia, B. D.; Affrossman, S.; Watson, L. M.; Fabian, D. J. Surf. Sci. 1977, 62, 386. (20) Brundle, C. R. Surf. Sci. 1977, 66, 581. (21) Brundle, C. R.; Chuang, T. J.; Wandelt, K. Surf. Sei. 1977,6%,459. (22) Lee, Y. C.; Montano, P. A. Surf. Sci. 1985, 149, 471. (23) Haneda, K.; Morrish, A. H. Surf. Sci. 1978, 77, 584. (24) Haneda, K.; Morrish, A. H. Nature 1979, 282, 186. (25) Hayashi, M.; Tamura, I.; Fukano, Y.; Kanemaki, S.; Fujio, Y. J. Phys. C : Solid State Phys. 1980,13,681. (26) Morrish, A. H.; Haneda, K. J.Magn. Magn. Mater. 1980,15-18, 1089. (27) Tamura, I.; Hayashi, M. Surf. Sci. 1984, 146, 501. (28) Du, Y.-W.; Wu, J.; Lu, H.-X.; Wang, T.-X.; Qiu, 2.-Q.; Tang, H.; Walker, J. C. J. Appl. Phys. 1987, 61, 3314. (29) Brennan, D.; Hayward, D. 0.;Trapnell, B. M. W. Proc. R. SOC. London 1960, A256,81. (30) Wedler, G. Physik 2.Chemie N.F. 1961, 27, 388. (31) Alshorachi, G.; Wedler, G. Appl. Surf. Sci. 1985,20, 279. (32) O'Grady, W. E. J. Electrochem. SOC.1980, 127, 555.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 3, 1989 759
Microcalorimetric Study of 0,Adsorption on Fe a.
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elemental analysis (Galbraith Laboratories). The sample was vacuum dried at room temperature for 6 h and further dried in air at room temperature for several days prior to use. Approximately 250 mg of the catalyst was loaded into the calorimeter pretreatment reactor (see Figure la) and treated by using a standard in flowing hydrogen at 400 "C for 12 h to fully reduce the metal to the zero valent state and then under vacuum (1 X 10" Torr) at 400 OC for 4 h to remove the surface hydrogen. The sample was cooled to room temperature, transferred into the calorimeter cell, and allowed to equilibrate to 300 K overnight, all the while maintaining a dynamicvacuum (1X lo4 Torr). UHP oxygen (Matheson, 99.999%)was further purified by flowing through a trap containing 13X molecular sieve and introduced in differential doses to oxidize the surface of the reduced metal particles. The system was held at 303 K for the duration of the experiment. The oxidationwas continued stepwise until it was apparent that the metal particles had been saturated with oxygen, and only physical adsorption w a taking ~ place. The sample was then evacuated overnight at 303 K, and a subsequent physical adsorption experiment was conducted. Two such experiments were conducted to test the repeatability of the procedure. A separate control study was conducted on a sample of blank Grafoil, which was identically pretreated. No oxygen chemisorption occurred on the Grafoil, indicating all irreversible oxygen uptake could be attributed to the iron particles. Massbauer Spectroscopy. Constant-accelerationMhbauer spectra were recorded by using an Austin Science Associates, Inc., 5-600Mossbauer spectrometer. The source, 50 mCi of 61C0 diffused into a palladium matrix, was obtained from New England Nuclear, Inc. Details of the data acquisition system and the in situ sample cell are given elsewhere.%*%Liquid helium data were collected in an Air Products and Chemicals Helitran cell. The data are presented relative to an NBS standard iron foil (I& = 330.0 kOe, AEQ = 0.0 mm/s, yk = 0.0 mm/s, r = 0.28 mm/s). X-ray Diffraction. XRD patterns were collected by using a Rigaku powder diffractometer (Model GeigerflexD/Max-IIIA), which uses a copper target X-ray source, curved crystal monochromator,and TT-driftedNaI scintillation detectors. The sample was pressed into a wafer at 10000 psi pressure, and diffraction patterns were collected versus a blank Grafoil sample, which was similarlypretreated and pressed.wl Particle sizes were estimated on the basis of conventional methods, summarized by Matyi et a1.40
reactor connection
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valve
Figure 1. Calorimeter design. (a) Overhead view of the calo-
rimeter. The dosing volume (12.05 cm3),including the pressure heads, is located between the two valves. The sample volume (37.5 cm3) includes the pretreatment reactor. (b) Side view of the sample cell, showing the thermopile "sandwich" and heat sinks. appreciably with supported metal particles and therefore does not influence the chemistry of the metal. The support was finely ground to a powder and pretreated in hydrogen at lo00 K for 24 h to remove any tracea of residual sulfur and to anneal the graphite crystals. Impregnationwas achieved by using the incipient wetness technique with an aqueous solution of Fe(N03)3.9H20,and the final catalyst composition was calculated to be approximately 5.15% Fe (by weight). This composition was later confirmedby (33) Cohen, M.; Mitchell, D.; Hashimoto, K. J.Electrochem. SOC.1979, 126, 442. (34) Beeck, 0. Adu. Catal. 1950,2, 151. (35) Lin, S.4.; Phillips, J. J. Appl. Phys. 1985, 58, 1943. (36) Gatte, R. R.; Phillips, J. J. Phys. Chem. 1987,91, 5961. Phillips, J. Surf. Sci. 1987, 184, 463. (37) Wu, N.-L.; (38) Phillips, J.; Clausen, B.; Dumesic, J. A. J.Phys. Chem. 1980,84, 1814.
Transmission Electron Microscopy. TEM micrographs were obtained in a Phillips Model 420 equipped with EDX. Small flakes of the sample were supported on a copper grid and examined for particle size, morphology, and composition. Differential Adsorption Microcalorimetry. The principles of Calvet heat flow calorimeters have been described in detail elsewhere.414 A schematic diagram of the system used in the present study is shown in Figure 1. In many ways the instrument is similar in design to one described recently.43 The present instrument has several modifications that result in improved performance. The most important change is in the pressure measurement system,which has been incorporated into the dosing volume such that dynamic pressure changes can be followed during each dose. This gives the instrument unique capabilities. The pressure measurement is achieved by using Baratron (MKSInstruments, Inc.) capacitancemanometers. Two Baratrons were used, one with a 0.1-10oO-T0~range and the other with a 0.0001-1.0-Torr range, which provides accurate pressure measurement over the span of typical experimental pressures. These pressure gauges are connectedto a laboratory IBM-PC/AT via a 12-bitLabmaster A/D converter (Scientific Solutions, Inc.), which simultaneously monitors the voltage output from the thermopilesthat surround the catalyst sample. In this way, heat (39) Boudart, M.; Delbouille, A.; Dumesic, J. A,; Khammouma, S.; Topsoe, H. J. Catal. 1975,37,486. (40) Matyi, R. J.; Schwartz, L. H.; Butt, J. B. Catal. Rev.-Sci. Eng. 1987, 29, 41. (41) Gravelle, P. C. Adu. Catal. 1972, 22, 191. (42) Gravelle, P. C. Catal. Reu.-Sci. Eng. 1977,113, 37. (43) O'Neil, M. K.; Lovrien, R.; Phillips, J. Rev. Sci. Instrum. 1985, 56, 2312. (44) O'Neil, M. K.; Phillips, J. J.Phys. Chem. 1987, 91, 2867.
760 Langmuir, Vol. 5, No. 3, 1989
Gatte and Phillips Table I. Heats of Formation of Iron Oxide Phases at 300 K vs Experimental (Integrated) Heats of Adsorption/Oxidation heat of formation, oxide phase kcal/mol of O2 FeO 130.0 Feo.MTO (wustite) 127.3 FeaOl (magnetite) 133.7 Fe203(hematite) 131.3 integrated heat
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assumed oxide stoichiometry
FeO Fe0.M70
20
40
60
-
I
0
0
i
20
40
60
80
total metal oxidized, % 14.0 13.2 10.5 9.3
“Based on 62.5 pmol of O2adsorbed/g of sample with 5.15w t %
V (micromoles 0 2 adsorbedlg sample)
801
Fe30,
80
Figure 2. Differentialheats of oxygen adsorption. Two identical 303 K adsorption experiments were conducted on the sample following (i) 400 O C H2reduction, (ii) 400 O C evacuation (1x 1O-a Torr), and (iii) cool down, transfer, and thermal equilibration to 303 K under dynamic vacuum (1 X 10“ Torr). The three adsorption regions were determined on the basis of the changes in the heats and dynamics of adsorption (see text).
P
3.0
* 3.0
Table 11. Calculated Fractional Oxidation of Total Irona
0
0
131.0 128.0
100
P (equilibrium pressure, torr)
Figure 3. Adsorption isotherms. The chemical adsorption isotherms ( 0 , O ) were determined from the same two data seta used to plot Figure 2. The physical adsorption isotherms (A,A) were also measured in the calorimeter, after evacuation of the sample at 303 K overnight to 1 X 10” Torr. The gas uptake during the second (physical adsorption) isotherms corresponds roughly to the amount adsorbed in region I11 of the first (chemical adsorption) isotherms. Data marked by a + are those measured during oxygen adsorption on a blank Grafoil sample. evolution and pressure dynamics can be directly compared to obtain both qualitative and quantitative kinetic information from the adsorption process under study. The entire calorimeter system is enclosed in a ‘tent”. This greatly reduced baseline fluctuationsdue to temperature changes in the laboratory. Calibration of the instrument was achieved by using heat pulses of known magnitude generakd with an electricalresistance heater. This is a standard technique of instrument calibration and has been described in detail elsewhere.“ A thorough calibration procedure showed that the mea of the measured thermogram did not depend upon the rate of heat generation, the sample configuration, or even the residual gas pressure within the sample cell.
Fe.
Results Heat of Adsorption. Two adsorption runs were conducted to assure data repeatability. The heat data are presented in Figure 2, while Figure 3 shows the corresponding adsorption isotherms. The reproducibility is excellent for the two different runs, demonstrating the precision of the instrument. The data can be separated into three adsorption regions, as indicated in Figure 2. In region I, the differential heats start at a high value near 140 kcal/(gmol) O2adsorbed and drop very slowly to values near 135 kcal/(gmol) O2 adsorbed. During this stage of the process, virtually all of the oxygen admitted into the sample cell with each dose is adsorbed by the metal. In region 11,there is a rapid and nearly linear decrease in the heat values. However, Figure 3 shows that the oxygen uptake re& high, nearly 100%. Finally, region I11 represents the end of the oxidation, as physical adsorption becomes the primary process. This is verified by the very low heats (
