Energy & Fuels 1990,4, 365-371
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Auger Electron Spectroscopy and Electron Probe Microanalysis Observations of Barium and Calcium Loaded on Amorphous Carbon under Gasification Conditions M. Matsukata,**tT. Fujikawa, E. Kikuchi, and Y. Morita Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1Okubo, Shinjuku-ku, Tokyo 169,Japan Received November 9, 1989. Revised Manuscript Received April 30, 1990
In order to investigate the mobility and structural change of barium and calcium compounds loaded on carbon substrate in the courses of heat treatment and of gasification, AES and EPMA were employed to observe the surface structure of amorphous carbon black impregnated by barium or calcium nitrate. We found that barium species migrated into the bulk of carbon in the course of heat treatment up to 1123 K, while calcium species did not. The difference in mobility between barium and calcium species is due to the difference in the reducibility of these cations by the action of carbon in the course of heat treatment. Mineral matter was concentrated at the surface of carbon during gasification and reacted with BaC03 to disperse in the interior of BaC03 particles on the carbon surface. On the other hand, sulfur was found on the BaC03 surface under gasification conditions.
Introduction that alkali-metal and alIt has been widely kaline-earth carbonates are effective catalysts for gasification of carbonaceous materials with HzO and COz. In particular, a strong interaction betwen KzCO3 and the carbon substrate as subgasification temperatures has often been investigated. At these temperatures, KzC03 decomposes to form catalytically active species on the surface via an interaction with carbon. In our recent study on the interaction of K2C03with the carbon surface: we found that potassium migrates into the interior of carbon when KzC03-impregnatedcarbon black is heated at temperatures above 673 K in an inert atmosphere. We believe that such migration of potassium occurs along grain boundaries of the amorphous carbon, because potassium loaded on graphite does not sink into the bulk. It has generally been accepted that the factors governing the rate of gasification are the inherent properties of carbon substrate, such as structure and porosity, and the content of oxygen and mineral matter, whereas we have shown5that the amount of potassium sinking into the bulk is also an essential factor to determine the rate of gasification. A drawback of KzC03 as a catalyst is the loss of catalyst due to evaporation from the carbon surface as potassium metal at gasification temperatures. Evaporated potassium metal may cause the corrosion of the reactor wall. It is k n ~ w n ' - ~that * ~ ' alkaline-earth-metal ~ compounds are considerably active for gasification of carbonaceous materials and that these compounds are hard to evaporte from the carbon surface, since they are different from KzC03catalyst. The gasification rate of carbon substrate strongly depends on the concentration and dispersion of c a t a l y ~ t . ' ~ J ~IfJalkaline-earth ~ elements migrate into the interior of carbon at gasification temperatures like potassium, the catalyst concentration on the carbon surface should decrease on heating. In the present study, we employed electron probe microanalysis (EPMA) and Auger electron spectroscopy (AES) to monitor the interaction of barium or calcium Present address: Department of Industrial Chemistry, Faculty of Engineering, Seikei University, 3-3-1 Kichijojikita-machi, Musashino-shi, Tokyo 180, Japan.
compounds with the surface of carbon. Special attention was paid to determine whether or not barium and calcium sink into the bulk of carbon black. Experimental Section Carbon black, (Tokai Carbon, Seast S, SRF) was used as a representative of the carbon substrate. A portion of the particles in the range of 24-42 mesh was washed with deionizer water, dried a t 383 K, and treated a t 1173 K for 2 h in a stream of nitrogen. Ultimate analysis showed that the resultant carbon black was predominantly composed of carbon with a small amount of mineral matter (0.1 wt %) and without any detectable hydrogen, oxygen, nitrogen, and sulfur. An amount of Ba(NO& or Ca(NO& was dissolved in deionized water. The carbon was permitted to soak in the solution and was then dried a t 383 K in a stream of nitrogen. The amounts of barium and calcium nitrates loaded on carbon black were 3.75 and 1.00 wt % as oxides, respectively. The reaction apparatus was the same as that described in the previous paper.' The carbon sample weighing 0.6 g was heated in a stream of argon from 300 to 1123 K a t a heating rate of 10 K min-'. After treatment at 1123 K for 0.5 h, the sample was soaked in 1.0 N HC1 solution and refluxed for 3 h to extract barium or calcium species. Steam gasification was carried out a t 1073 K after heat treatment. The ~
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(1) Wen, W. H. Catal. Reu.-Sci. Eng. 1980, 22(1), 1-28. (2) Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984, 26, 233-279. (3) Kaptaijin, F.; Moulijin, J. A. NATO ASZ Ser., Ser. E 1986, 105, 291-360. (4) Mataukata, M.; Fujikawa, T.; Kikuchi, E.; Morita, Y. Energy Fuels 1988,2, 750-756. (5) Mataukata, M.; Fujikawa, T.; Kikuchi, E.; Morita, Y. Energy Fuels 1989, 3,336-341. (6) Otto, K.; Bartosiewicz, L.; Shelf, M. Fuel 1979, 58, 565-572. (7) Otto, L.; Bartosiewicz, L; Shelf, M. Carbon 1979, 17, 351-357. (8) McKee, D. W. Fuel 1980,59, 308-314. (9) Lang, R. J.; Neavel, R. C. Fuel 1982,61, 620-626. (IO) Kapteijin, F.; Porre, H.; Moulijin, J. A. AlChE J. 1986, 32, 691-695. (11) Ersolmaz, C.; Falconer, J. L. Fuel 1986,65,400-406. (12) Liu, Z.; Zhu, H. Fuel 1986, 65, 1334-1338. (13) Calahorro, C. V.; Gonzalez, C. F.; Garcia, A. B.; Serrano, V. G. Fuel 1987, 66, 216-221. (14) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983, 82, 382-394. (15) Loye, H. C. L.; Kershaw, R.; Dwight, K.; Pabst, J. K.; Lang, R. J.; Wold, A. Mater. Res. Bull. 1984, 19, 459-463.
0887-0624 f 90f 2504-036WO2.50 f 0 0 1990 American Chemical Society
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366 Energy & Fuels, Vol. 4, No. 4, 1990
a
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Figure 1. Variation in XRD pattern of Ba(NO&loaded carbon (a) before heat treatment, (b) after heat treatment a t 973 K, (c) after heat treatment a t 1123 K, and (d) after partial gasification a t 1073 K (20% carbon converted). partial pressure of steam was adjusted to 91 kPa with a balance of argon. The level of carbon conversion (mol %) was calculated by graphical integration of the gasification rate with time on stream. The chemical change of barium or calcium in the course of heat treatment, extraction with HC1 solution, and steam gasification was studied by means of electron probe microanalysis using an energy-dispersive X-ray spectrometer (HORIBA EPMA-2200) equipped with a scanning electron microscope (SEM; HITACHI S-570). X-rays produced from an area of 22 X 17.5 pm were collected with a beam current of 6 X 10-lo A and a beam energy of 10 keV. The X-ray takeoff angle was 30.10'. Auger electron spectroscopy (AES) was performed in an ion-pumped UHV chamber (PHI MODEL 50) a t a base pressure below 3 X Torr. AES spectra were monitored by using a Physical Electronics spectrometer (MODEL 545-548)with a cylindricalmirror analyzer (CMA), a specimen current of 0.5 A, and a beam energy of 2 keV. All the samples were exposed to air prior to AES measurements. Care was taken that the AES spectrum was not varied by electron-beam exposure. Since barium species were not uniformly distributed on the surface of carbon black as described later, the samples were repeatedly prepared and measured by EPMA and AE!3 to rule out a sampling problem. We collected a lot of EPMA and AES spectra from different samples and show selected representatives in the figures.
Results The structural change of Ba(N03)2loaded on the carbon in the course of heat treatment and of steam gasification was observed by means of X-ray diffraction (XRD) as shown in Figure 1. A broad peak around 28 = 25' (Cu target) is a characteristic reflection of amorphous carbon. The reflection peaks of Ba(N03)2 were observed with Ba(N03)2-loadedcarbon before heat treatment. Figure IC shows that Ba(N03)2 was transformed into BaC03 on heating to 973 K in a stream of argon, whereas these reflection peaks of BaC03 completely disappeared when the carbon sample was treated at 1123 K. The XRD pattern
Figure 2. SEM micrographs of Ba(N03)&aded carbon (a) before heat treatment and (b) after heat treatment a t 1123 K.
indicative of the BaC03 phase appeared on the partially gasified sample (20% carbon converted). Figure 2 shows SEM micrographs of Ba(N03)2-loaded carbon before and after heat treatment at 1123 K. As shown in Figure 2a, mottling contrasts were observed on the sample before heat treatment, possibly indicating the aggregation of Ba(N03)2particles on the carbon surface. Part of the surface exhibiting bright contrasts was analyzed to determine the surface composition by means of EPMA. A typical EPMA spectrum shown in Figure 3a reveals that this part of the surface was composed of carbon, oxygen, and barium. The dark contrasts typically gave different EPMA spectra as shown in Figure 3b. It is clear that a small amount of Ba(N03)2was deposited on this part of the surface. Oxygen was always detected with barium as shown in Figure 3. Although the energy for X-ray production from the K shell of nitrogen is between those from carbon and oxygen, the signal for nitrogen was obscure due
Barium and Calcium Loaded on Amorphous Carbon
Energy & Fuels, Vol. 4, No. 4, 1990 367
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Figure 3. Typical EPMA spectra for Ba(N03),-loadedcarbon before heat treatment. (a) A typical spectrum from the area exhibiting bright contrasts and (b) a typical spectrum from the area exhibiting dark contrasts.
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Figure 5. Typical (a) AES and (b) EPMA spectra for Ba(N03),-loadedcarbon after extraction with HC1 solution.
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Figure 6. Typical (a) AES and (b) EPMA spectra for Ba(N03),-loadedcarbon treated at 1123 K.
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to overlapping with that of carbon. AES was employed for analyzing the composition of the surface of Ba(N03)z-loaded carbon. The AES results shown in Figure 4 reveal that Ba(N03)zwas not as uniformly distributed on the carbon surface as in the cases of EPMA analyses. Ba(N03)2-loadedcarbon was refluxed in HC1 solution for 3 h and then rinsed with deionized water. The results of AES and EPMA measurements shown in Figure 5 show that all of the barium species in Ba(N03),-loaded carbon are extractable in HC1 solution. In addition, the surface of carbon black itself is free of oxygen. The signal of
oxygen appearing at 0.53 keV in the EPMA spectrum is caused by contamination, because EPMA measurements were carried out as a base pressure considerably higher than that for AES. Parts a and b of Figure 6 show typical AES and EPMA spectra for Ba(N03)z-loadedcarbon treated at 1123 K, respectively. Although barium species was highly dispersed on the surface of the carbon on heating to 1123 K in a stream of argon as indicated by XRD, the carbon surface was not wholly covered with barium as shown in Figure 6a. The signals of barium were also observed in the EPMA spectra shown in Figure 6b. The mottling contrasts observed in Figure l a did not appear in a SEM micrograph (Figure lb) of the sample treated at 1123K, in agreement with the disappearance of XRD reflections of barium compounds. Extraction of barium species with HC1 solution was carried out on the sample treated at 1123 K. After measuring a number of AES spectra on this sample, we could fiid no indication of the presence of barium species. In contrast, it should be noted that the signal of barium in the EPMA spectra did not disappear even by extraction with HCl solution as shown in Figure 7b. Additionally, no AES signal of oxygen was formed in Figure 7a, indicating that the surface of the carbon was not appreciably oxidized in the course of heat treatment.
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368 Energy & Fuels, Vol. 4, No. 4,1990
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Figure 9. Typical EPMA spectra for Ca(NO&-loaded carbon (a)before heat treatment, (b) after heat treatment at 1123 K, and (c) after heat treatment followed by extraction with HCl solution.
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Figure 7. Typical (a) AES and (b) EPMA spectra for Ba(NO&loaded carbon treated at 1123 K and then extracted with HCl solution.
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Figure 8. Typical EPMA spectra for BaC12-loadedcarbon (a) after heat treatment at 1123 K and (b)after subsequent extraction with HCl solution. The carbon black was impregnated with BaCl, instead of Ba(N03),. BaC1,-loaded carbon was heated to 1223 K, maintained at that temperature for 0.5 h, then refluxed in HCl solution, and rinsed with deionized water. Figure 8 shows EPMA spectra on the sample before and after extraction with HC1 solution. The spectra show that BaC1, loaded on the carbon is completely extractable with HC1 solution, irrespective of heat treatment at 1223 K. Ca(N03),-loaded carbon was also treated by the same procedure as that used for Ba(N03)P-loaded carbon. EPMA spectra were collected to monitor the change of the surface composition with heat treatment at 1123 K and extraction with HC1 solution. We observed by means of SEM and EPMA that calcium was not uniformly distributed on the carbon surface. Typical spectra are shown in Figure 9. The EPMA spectra reveal that calcium species
contained in the sample was completely washed out in HC1 solution even after the sample was treated at 1123 K. Ba(N03),-loaded carbon was gasified with steam at 1073 K and atmospheric pressure. Figure 10 shows (a) the SEM micrograph of the carbon sample, which was partially gasified up to a 20% level of carbon conversion, and (b) the corresponding elemental mapping of barium. It is obvious that aggregated particles exhibiting bright contrasts contain barium and thus that barium forms massive deposits. Taking into account the XRD pattern shown in Figure Id, these barium compounds appear to be mainly composed of BaC03. Figure 11 presents EPMA spectra from the same particles as that shown in Figure 10. The pronounced feature is that the signals of sodium, aluminum and silicon were manifested with that of sulfur, compared with EPMA spectra from the sample before gasification. Figure 1 2 shows AES spectra from the same sample used in the EPMA measurements shown in Figure 11. The signals of sulfur were observed in AES and EPMA spectra. The difference in the results between AES and EPMA is the lack of signals of mineral matter in the AES spectra. We tried to gather further detailed insight concerning the composition of the barium compound on the surface of partially gasified carbon. As gasification proceeded, a number of hollows were produced as shown in Figure 13a, suggesting that the catalyst possesses a strong interaction with certain areas of the carbon surface having particular morphological faces. We found a tiny particle projecting over a hollow as indicated by an arrow in Figure 13b and analyzed it by EPMA in order to understand qualitatively the chemical composition of the particles showing bright contrasts in secondary electron images. Characteristic X-rays are produced from the interior up to a depth of a few thousand nanometers when a carbon sample is analyzed by EPMA with 10 keV of beam energy.4 Thus, there is a possibility that the EPMA spectra obtained included signals produced from the surface beneath the hollow, if scattered electrons, which were irradiated to the tiny particle, could pass through the particle and reach the surface beneath the hollow. Thus, we measured an EPMA spectrum with an irradiation of electron beams to the point in the vicinity of the particle to ensure the contribution of the EPMA signal produced from the sur-
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Barium and Calcium Loaded on Amorphous Carbon co
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Figure 11. Typical EPMA spectra from the particle that is shown in Figure loa.
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Figure 10. (a) SEM micrograph of Ba(NU3),-loaded carbon partially gasified a t 1073 K (20% carbon converted) and (b) corresponding elemental mapping of barium. 20
face beneath the hollow. Compare the spectra shown in parts a and b of Figure 14. It is clear that the contribution of the EPMA signal from the surface beneath the hollow was negligible. X-rays may be difficult to escape from the hollow toward the detector having 30.10° of takeoff angle. We conclude that the tiny particle was mainly composed of barium, carbon, and oxygen with sodium, aluminum, and silicon. Therefore, BaC03 particles contain a certain amount of mineral matter. Discussion Several important aspects have emerged from the results obtained from XRD, AES, and EPMA experiments. The discovery of the migration of barium species into the bulk of amorphous carbon in the course of heat treatment is the most notable. First, we discuss the migration of barium species into the bulk.
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Figure 12. Typical AES spectra for Ba(NO&-loaded carbon partially gasified a t 1073 K (20% carbon converted).
Before heat treatment, Ba(N03)2loaded on the carbon is extractable with HC1 solution as confirmed by AES and EPMA (Figure 5). In contrast, there is evidence given by EPMA (Figure 7b) that a certain amount of barium species remained in the carbon after the Ba(N03)2-loadedcarbon was treated at 1123 K, followed by extraction with HC1 solution. Barium complex insoluble in HCl solution was not formed on the surface, since the results of AES as shown in Figure 7a reveal that barium species on the carbon surface was completely extracted even after heat treatment. The discrepancy between the two spectroscopies should come from the difference in the depth of signal production. While Auger transitions occur within several
370 Energy & Fuels, VoZ.4, No. 4, 1990
Matsukata et al. .-
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Figure 14. (a) EPMA spectrum from the particle pointed out by an arrow shown in Figure 13b. (b) EPMA spectra from a point in the vicinity of the arrow shown in Figure 13b. I
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Figure 13. SEM micrographs of Eh(NO&loaded carbon partially gasified at 1073 K (20% carbon converted). Micrograph b corresponds to the area pointed out by an arrow in (a).
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layers adjacent the surface, EPMA experiments can yield qualitative information at 10-1OOO nm. Accordingly, we conclude that barium species in the sample treated at 1123 K, followed by extraction with HC1 solution, existed in the bulk of the carbon. In other words, barium species sinks int the bulk of the carbon in the course of heat treatment like potassium species, as previously The results of XRD measurements suggested that the BaC03 phase was predominantly formed on the carbon surface when Ba(N03)2-loadedcarbon was treated at 973 K in a stream of argon and that barium species was well dispersed at temperatures as high as 1123 K. Thus, a barium species, which is probably BaO and is formed by decomposition of Ba(N03)2,strongly interacts with the surface and abstracts carbon to form BaC03. Such a strong interaction between barium species and carbon substrate at subgasifiction temperatures appears to be important for
barium to migrate. Ersolmaz and Falconer" have shown the presence of the interaction between BaC03 and carbon black from the results of a temperature-programmed reaction with isotopically labeled carbonate and carbon. Their conclusions are that carbon accelerates decomposition of carbonate and that a carbonate-carboninteraction occurs well below gasification temperatures. Baker et d.16 have demonstrated the morphological change of BaO on graphite resulting from a strong interaction between BaO and graphite by means of electron microscopy under a gasifiction environment. Their observation has clearly shown that BaO particles located at edges and steps on the surface wet and spread along these sites a t subgasification temperatures, which is consistent with our conclusion that barium species was highly dispersed on the surface of the carbon in the course of heat treatment. We have shown4that potassium migrates along grain boundaries of the amorphous carbon because no potassium migrates into graphite. Likewise, barium species is believed to migrate along the grain boundaries of amorphous carbon. According to the TEM observations by Baker et a1.,16 barium species moved on the surface of carbon and did not diffuse into the bulk of carbon because of the structure of graphite having no grain boundary. Although the nature of barium species sinking into the bulk of the carbon is difficult to characterize directly, possibly part of the Ba2+ions is reduced by carbon to form metallic barium and subsequently migrate into the bulk of the carbon. Other explanations are (1)Ba2+ions migrate into the carbon matrix and are stabilized there or (2) Ba2+ ions are reduced by electron donation from the carbon matrix after migration into the bulk. It is difficult for calcium species to migrate into the bulk of the carbon. Such a difficulty should not be due to the size of Ca2+ion (0.94 A) because larger Ba2+ion (1.35 A) can migrate. Hence, the difference in the mode of mobility (16) Baker, R. T. K.; Lund, C. R. F.; Chludzinski, J. J., Jr. J. Catal. 1984,87,255-264.
Barium and Calcium Loaded on Amorphous Carbon
between the barium and calcium species can be attributed to the different nature of the interaction with the carbon surface. Barium oxide (BaO) has a smaller standard free energy of formation (464 kJ mol-' at lo00 K)" than CaO (548 kJ mol-' at lo00 K),'s suggesting that barium ions are more easily reduced than calcium ions. We, therefore, prefer the explanation that barium ions are reduced by reaction with carbon to form barium metals in the course of heat treatment. They migrate into the bulk of the carbon along grain boundaries by a driving force arising from the difference in the barium concentrations between the surface and the bulk. As K 2 0 has a small standard free energy of formation (322 kJ mol-' at 298 K),l9 this argument also rationalizes that potassium species migrates into the bulk of the carbon in the course of heat treatment. It is noteworthy that when C1- ions are present as a counteranion, the barium species does not sink into the bulk of the carbon. This implies that C1- ions attract Ba2+ ions so strongly that Ba2+ions cannot interact with the carbon surface to migrate. Inhibition of migration by C1ions has also been observed for KC1-loaded carbon black.5 We propose that C1- ions inhibit reduction of Ba2+and K+ ions with carbon because of strong interaction. The agglomeration of BaC03 with carbon conversion was observed by XRD, corresponding to the formation of particles exhibiting bright contrasts in the SEM micrographs. BaC03 particles involved mineral matter such as sodium, aluminum, and silicon (Figure 14). As mineral matter and sulfur cause deactivation of gasification catalysts>'J3 it is valuable to understand the interaction of gasification catalysts with mineral matter and sulfur in the course of gasification. Mineral matter should be concentrated at the carbon surface with gasification, because it is hard to vaporize at temperatures used in this study and it accumulates on the carbon surface with the progress of gasification. However, we found no indication of mineral matter on the carbon surface in the AES spectra shown in Figure 12. These results clearly reveal that the mineral matter tends to react with BaC03 and then to disperse in the interior of BaC03 particles. On the other hand, the (17) Chase, M. W.; Curnutt, J. L.; Prophet, H.; McDonald, R. A,; Syvernd, A. N. J. Phye. Chen. Ref. Data 1975,4, 31. (18) Chase, M. W.; Curnutt, J. L.; Prophet, H.; McDonald, R. A.; Syvernd, A. N. J. Phys. Chem. Ref. Data 1975,4,62. (19) Samsonov, G. V. The Oxide Handbook; IFI/Plenum: New York, Washington, London, 1973; p 38.
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signal of sulfur appeared in the AES spectra as shown in Figures 11 and 14a, so that sulfur is apparently present on the BaC03 surface. Although such results in the present study are caused by a small content of mineral matter, the distribution of mineral matter and sulfur in BaC03 particles allows us to infer the mode of catalyst deactivation caused by reactions with sulfur and mineral matter in the course of gasification. In the initial stage of gasification of carbonaceous materials, barium catalyst may be subjected to poison by sulfur, because BaC03 encapsulates mineral matter if the amount of mineral matter reacting with BaC03 is sufficiently small. Then, barium catalyst may deactivate by reaction with mineral matter as gasifiction proceeds. We believe that migration of barium serves to store catalyst in the bulk of the carbon. Radovic et al.,14Loye et al.,15 and Kaptaijin et al.1° have pointed out that dispersion of alkaline-earth catalysts is an important factor to determine the catalytic efficiency. As described above, barium species on the carbon aggregates to give massive deposits, indicating a serious decrease in dispersion of barium with carbon conversion. However, it seems that the barium species, which has been embedded in the bulk of the carbon, appears on the surface and continuously supplies highly dispersed active sites for gasification. We are not interested in comprehensively grasping which elements have an ability to migrate into the bulk of amorphous carbon, and we intend to report the results in a future paper. We have recently reported the quantitative change of potassium catalyst on the carbon surface mainly by means of extraction with HCl solution. It is likely that the determination of the amount of catalyst by means of extraction with HCl solution likewise facilitates the quantitative discussion on the behavior of barium and calcium species actually existing on the carbon surface in the course of heat treatment and of gasification. Acknowledgment. We express our appreciation to Prof. T. Osaka, the Casting Research Laboratory, Waseda University, for his helpful discussion with the AES work, to M. Suzuki for his skillful assistance with the AES measurements, and to J. Yasui, K. Saga, and M. Miyazawa, School of Science and Engineering, Waseda University, for their technical assistance with the EPMA measurements. Registry No. Ba(N03)2,10022-31-8;Ca(N0J2, 10124-37-5; BaC03, 513-77-9.
