Energy & Fuels 1993, 7, 139-145
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XAFS and Thermogravimetry Study of the Sintering of Calcium Supported on Carbon D. Cazorla-Amorb, A. Linares-Solano,' and C. Salinas-Martinez de Lecea Departamento de Qufmica Znorgknica, Universidad de Alicante, Alicante, Spain
H. Yamashita, T. Kyotani, and A. Tomita Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai, Japan
M. Nomura Photon Factory, National Laboratory for High Energy Physics, Tsukuba, Japan Received May 19, 1992. Revised Manuscript Received September 16, 1992
An X-ray absorption fine structure (XAFS) and thermogravimetry (TG) study has been carried out to analyze the sintering of calcium supported on carbon during pyrolysis treatment and during COz-carbon gasification. The following variables have been studied: (a) the heating rate effect during pyrolysis on calcium dispersion as a function of calcium content; (b) calcium sintering and deactivation during CO2-carbon gasification as a function of the reaction temperature and of the initial calcium dispersion, and (c) the effect of the chemical state of calcium (CaC03 or CaO) on the calcium sintering rate. The results confirm the well-known effect of the heating rate during a pyrolysis treatment on calcium dispersion: the higher the heating rate, the higher the calcium dispersion. The study of calcium dispersion evolution (measured by C02 chemisorption)during CO2-carbon gasification allows us to understand the mechanism of calcium deactivation. XAFS results show the great effect of the chemical state of calcium on the sintering rate. In this sense, it is observed that CaC03 hlfs a much higher mobility on the surface of the carbon than CaO due to the great difference in their Tammann temperatures.
Introduction The factors influencing the catalytic activity of calcium in carbon-gas reactions are numerous.' For a given carbon many of these factors (Le., surface chemistry, surface area, porosity, inorganic constituents, etc.) are kept constant. However, the experimental conditions used during the calcium loading, the pyrolysis step, and the gasification process may influence its catalytic activity. Although the effect of pyrolysis conditions on reactivity is well-known, little attention has been paid to analyze the heating rate effect.2 An interesting correlation between the heating rate and the dispersion was found? unfortunately, the dispersion was only estimated from XRD. More recently, calcium dispersion determined by C02 chemisorption4has been applied to analyze calcium sintering behavior5 and X-ray absorption near-edge structure (XANES) and Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) results have been used to investigate calcium structure.6-10 (1) Linares-Solano,A.; Salinas-Martfnezde Lecea, C.;Cazorla-Amorb, D.; Joly, J. P. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; NATO/ASI Series E192; Kluwer Academic: New York, 1991; pp 409-430. (2) van Heek, K. H.; Mtkhlen, H. J. Fuel 1985,64,1405-1414. (3) Radovic, R. L.; Walker, P. L.; Jenkins, R. G. J. Catol. 1983, 82, 382-394. (4) Linarea-Solano, A.; A l m e l a - h c 6 n ,M.;Salinas-Martfnezde Lecea, C. J . Catal. 1990,125,401-410. (5) Linarea-Solano,A.; Almela-Alarc6n,M.; Salinas-Martfnezde Lecea, C.; Cazorla-Amorb, D. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989,34, 136-143. (6) Huggins, F. E.; Huffman, G.P.; Lytle, F. W.; Greegor, R. B. Proc. Int. Conf. Coal Scrence, Rttsburgh, PA 1983, 679-682.
Huggins et al. have successfully applied XAFS spectroscopy to examine the state of calcium in chars prepared by rapid and slow pyrolysis treatment."t8 They clearly observed that, in rapidly pyrolyzed chars, the calcium state remained highly dispersed. No spectral evidence was found for the formation of CaO or other discrete compound in these chars. On the other hand, for chars prepared by slow pyrolysis, significant fractions of the calcium had been transformed to bulk CaO. In this paper, the effect of the heating rate on reactivity is analyzed paying attention to dispersion and structural changes caused during the pyrolysis step of carbon samples with different calcium contents. Calcium dispersion is determined from C02 chemisorption data and calcium structure is analyzed from XAFS spectra. The second objective of this paper is to analyze the mechanism of calcium sintering during the gasification process and the effect of the chemical state of calcium on the sintering rate, about which there is an important lack of knowledge. To analyze calcium agglomeration, classical theories of particle sintering have to be applied." These (7) Huggins, F. E.; Huffman, G. P.; Shah, N.; Jenkins, R. G.;Lytle, F. W.; Greegor, R. G. Fuel 1988,67, 938-941. ( 8 ) Huggins, F. E.; Shah, N.; Huffman, G. P.; Lytle, F. W.; Greegor, R. G.; Jenkins, R. G. Fuel 1988,67, 1662-1667. (9) Cazorla-Amorb, D.; Linares-Solano,A.; Salinas-Martlnezde Lecea, C.; Yamashita, H.; Kyotani, T.; Tomita, A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991,36 (3), 998-1006. (10) Yamashita, H.; Nomura, M.; Tomita, A. Energy Fuels 1992, 6, 656-661. (11) Ruckenstein, E. In Metal-Support Interactions in Catalysis, Sintering and Redisperson; Stevenson, S . A., Dumesic, J. A., Baker, R. T. K., Ruckenstein, E., Eds.; Van Nostrand Reinhold Catalysis Series; Van Nostrand Reinhold New York, 1987; pp 141-229.
0887-0624/93/2507-0139$04.00/00 1993 American Chemical Society
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140 Energy &Fuels, Vol. 7, No. 1, 1993
theories describe the decay of the surface area of the catalyst as a function of time; in this sense, a measure of the catalyst surface area is necessary to use the corresponding equations. In the case of calcium-catalyzed carbon gasification, in which CaO particles can be highly dispersed, the surface area evolution can be followed by applying the above-mentioned C02 chemisorption technique. In this paper, the CO2 chemisorption method is used to analyze catalytic activity loss and changes in the dispersion of calcium as a function of the initial calcium dispersion and the gasification temperature, allowing the sintering mechanism of calcium to be understood. Experimental Section Sample Preparation. A high-purity carbon, obtained from a phenol-formaldehyde resin oxidized by HN03,was used as the substrate where calcium was deposited by means of two different procedures: ionic exchangeand impregnation. The nomenclature used is the following: A2 (corresponding to the oxidized carbon) followed by I or I1 in order to distinguish between impregnation and ionic exchange, and by a number, indicating the amount of calcium loaded.12 Char Reactivity and Calcium Dispersion Measurements. Reactivity of the calcium-carbon samples (previouslypyrolyzed in situ) was determined by isothermal thermogravimetric analysis (TGA) in 0.1 MPa of C02. The experimental procedure is as follows: (a) pyrolysis treatment up to 1173K under slow or fast heating rate (20 and -300 K/min, respectively); (b) cooling of the sample in Nz to the reaction temperature (between 973 and 1073K); (c)switchingof NZto C02. The reactivity was determined from the maximum slope of the weight loss vs time curve referred to the initial sample weight. Calcium dispersion was measured by the COn chemisorption method4. The procedure consists of the followingsteps: (a) slow or fast heat treatment up to 1173 K (b) cooling of the sample in N2to the chemisorption temperature (573 K) with a soak time of 10 min; (c) switching of N2 to COz with a soak time of 30 min; (d) switching of COz to Nz at 573 K, 10 min soaking time. The C 0 2 uptake was followed in a TG system. The same procedure was used to measure calcium dispersion in partially gasified samples and in samples submitted to additional heat treatments. XAFS Experiments. Samples were heat treated in Nz using both slow or fast heating rates up to 1223 K to study the effect of this parameter on calcium dispersion. In order to analyze the effect of the chemical state of the catalyst on calcium sintering, after a fast heat treatment different XAFS experiments were conducted after (i) COn contact at 673 K, 30 min; (ii) slow heat treatment of the sample resulting in (i); and (iii) N1 contact at 673 K, 30 min and slow heat treatment. For the XAFS measurements, wafers of the samples were prepared under inert atmosphere by pressing a homogeneous mixture of calcium-carbon sample and polyethylene powder in a ratio 2/1. Finally, samples were sealed with a polyethylene film to prevent contact with air during handling. X-ray absorption experiments were performed at the Photon Factory in the National Laboratory for High Energy Physics (KEK-PF) in Tsukuba (Japan). A Si(ll1) double crystal was used to monochromatize the X-ray from the 2.5-GeV electron storage ring, and the Ca K-edge absorption spectra were recorded in the transmission mode at room temperature over a range of photon energy extending from 3940 to 4720 eV. Photons from higher-order diffraction were eliminated using a double mirror system. Fourier transformation was performed on k3-weighted EXAFS oscillation, k 3 x ( k ) ,in the range 4-10 A-l (FT-EXAFS). The physical basis and numerous applications of XAFS spectroscopy have been discussed elsewhere.13-15 (12) Almela-Alarc6n, M. Ph.D. Thesis University of Alicante, 1988. (13) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. Catal. Reu. Sci.-Eng. 1984, 26 (l),81-140.
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Energy i eV Distance 1 A Figure 1. XANES spectra (a, b) and FT-EXAFS (A, B) of referencecompounds: (a,A) calcium oxide; (b, B)calcium acetate. Table I. Characteristics of Sample A2-11-3.0 sample
R (h-l)
FH SH
5.6 3.2
d 0.80 0.55
Rld (h-1)
7.0 5.8
Results and Discussion Heating Rate Effect on Calcium Dispersion and Catalytic Activity. Table I presents the results of the calcium dispersions measured by COz chemisorption ( d ) , the reactivities determined at 1073 K (R), and the Rld ratio obtained for the two heating rates used for sample A2-11-3.0. The calcium dispersion of both samples is quite high. However, the rapidly pyrolyzed sample has a much higher value in agreement with its higher reactivity. It is important to recall that the dispersion of calcium in these samples cannot be analyzed with the more classical XRD technique because of their small particle size.5 The almost constant Rld ratio obtained with both slow and fast heat treatments confirms that calcium dispersion measured by C02 chemisorptionis useful to interpret calcium catalytic activity in CO2-carbon gasification. This is because the calcium in these two samples is highly dispersed; however, as the dispersion decreases, this is not a valid because calcium dispersion, and hence external surface area of calcium particles, is not the key factor which determines calcium catalytic activity.'J6 The calcium-carbon contact and the number of catalytic active sites (determined by TPD after COzchemisorption) are much more importantl6 The effect of the pyrolysis heating rate is also analyzed from XAFS measurements on calcium-carbon samples with different metal contents. Figure 1shows the XAFS spectra corresponding to calcium oxide and calcium acetate (standard compounds), whereas Figures 2 and 3 show XAFS results of calcium-carbon samples obtained using slow and fast pyrolysis treatment respectively. The moat relevant characteristics of CaO and Ca(COOCH3)2spectra are the following: the XANES shows a pre-edge peak at 4043 eV for CaO, and only one broad main peak for Ca(COOCH3)2; the CaO FT-EXAFS exhibits a Ca-0 peak corresponding to distances of about 2.0 A (uncorrected for phase shift) and a Ca-Ca peak at about 3.0 A; only one Ca-0 peak at about 2.0 A is observed for Ca(COOCH&. (14) Lytle, F.W.;Greegor, R. B.; Marques, E. C.; Biebesheimer, V. A.; Sandstrom, D. R.; Horeley, J. A.; Via, G. H.; Sinfelt, J. H. In Catalyst Characterization Science Surface and Solid State Chemistry; Deviney M. L., Gland,J. L., Ede.; ACS Symposium Series 288; American Chemical Society: Waehington, DC 1986; pp 280-293. (15) Bart, J. C.; Vlaic, G. Adu. Catal. 1987, 35, 1-138. (16) Cnzorla-Amorb, D.;Linares-Solano, A,; Mnrcilla-Gomis, A. F.; Salinas-Mnrtfnez de Lecea, C. Energy Fuels 1991, 5, 796-802.
Sintering of Calcium Supported on Carbon
Energy I eV Distance I A Figure 2. XANES spectra (a-d) and FT-EXAFS (A-D) of calcium-carbon samples after heat treatment in Nz at 20 K/min up to 1223 K: (a, A) A2-11-3.0;(b, B)A2-1-4.3; (c, C)A2-1-6.0; (d, D)A2-1-9.4.
Energy 1 eV
Distance 1
A
Figure 3. XANES spectra (a-d) and FT-EXAFS (A-D) of calcium-carbon samplesafter heat treatment in NBat 300 K/min up to 1223 K: (a, A) A2-11-3.0;(b,B)A2-1-4.3; (c, C)A2-1-6.0;(d, D) A2-1-9.4.
First, let us analyze the heating rate effect on the XAFS results of the ion-exchanged calcium sample (A2-11-3.0). After the ion-exchange process the calcium was shown to be atomically dispersed through the carbon matrix, all
Energy & Fuels, Vol. 7, No. 1, 1993 141 the protons from the carboxylic groups of the carbon are ion-exchanged by Ca2+ions with a 2/ 1stoichiometry.' This is in agreement with XAFS results in which the spectral characteristics of this ion-exchanged sample are quite similar to those of calcium a~etate.6.~ Upon heat treatment during the pyrolysis step, the initial dispersion of the calcium will change depending on the heating rate and the maximum temperature used. As commented above, the reactivity and the dispersion of the fastly pyrolyzed sample are significantly higher than those obtained using a slow heat treatment. This clearly indicates that the slow heat treatment favors the sintering process of the atomically dispersed calcium. The XAFS results of the slow and rapidly pyrolyzed samples (Figures 2a,A and 3a,A) and their comparison with the standard compounds (Figure 11, also allow us to show that the slow heat treatment produces a higher sintering effect. In fact, the XAFS spectra of the rapidly pyrolyzed sample (Figure 3a,A) are similar to those of Ca(COOCH3)Zand therefore also similar to the untreated sample with atomically dispersed Ca2+ions. However, the XAFS spectra of the sampleprepared by slow pyrolysis (Figure 2a,A) have some characteristics of CaO, i.e., the presence of a Ca-Ca peak at about 3.0 A. The weakness of the CaO spectral features of the slowly pyrolyzed sample indicates that Ca species are highly dispersed forming small clusters. XAFS results are in good agreement with calcium dispersion measured by COZ chemisorption. Calcium dispersion is 80 75 after a fast pyrolysis treatment (almost atomic dispersion), while 55% is obtained after a slow one. CaO particles with 80% dispersion can not form crystalline CaO particles (possibly only several CaO molecules participate in each cluster). However, for a 55% dispersion the mean particle size estimated (considering that3 D (A) = 12.4/d, where D (in A) is the particle size and d is the dispersion) is about 20 A. From this value, the number of Ca atoms forming each CaO cluster is calculated to be about 280. In this case, the clusters can have some structural order, as is observed in Figure 2a,A. The effect of the calcium content in the XANES and FT-EXAFS results is shown in Figures 2 and 3. The calcium content of these samples (except for the A2-113.0) is larger than the maximum ion-exchange capacity of the carbon. TPD,TG, XRD,' and XAFSgof the untreated sample showed that calcium in "excess" is present as calcium acetate. As the pyrolysis temperature increases, calcium acetate decompositionto calcium carbonate and calcium oxide takes place. At a given heating rate (Figure 2 or Figure 31, it is observed that an increase in calcium content produces an increase of CaO spectral characteristics (Figure 11, indicating that larger crystalline CaO particles are formed independently of the heating rate used. In the case of a slow pyrolysis treatment, the calcium dispersion decreases with the increase in calcium content from 0.55 for the A2-11-3.0 sample to 0.35 for the A2-11-9.4 sample. Therefore, the spectra of the latter, after the heat treatment, should be more similar to bulk calcium oxide as observed (Figure 2d,D). The effect of the pyrolysis heating rate on the samples with different calcium content can be analyzed from comparison of the XAFS results of Figures 2 and 3. It is important to note that a fast pyrolysis produces, in all the samples studied, weaker CaO spectral characteristics than a slow pyrolysis, confirming that the sintering process is much lower when fast pyrolysis is used. Furthermore, the
142 Energy &Fuels, Vol. 7, No. 1, 1993
Cazorla-Amorbs et al.
RIwtlvlty (h-')
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Figure 4. COz reactivity versus B O (X) rapidlypyrolyzed sample (1073 K); ( 0 ) slowly pyrolyzed sample (1073 K);(+) slowly pyrolyzed sample (1023 K); (*) sample A2-11-3.2-TT(1023 K). dlrporr)on
Figure 4 also includes reactivity at 1023K for sample A211-3.2-TT and at 1073 K for the sample prepared by fast pyrolysis. In the case of the rapidly pyrolyzed sample, calcium dispersion evolution with BO is not included; for this sample the application of the COZchemisorption method gives unreliable results for BO > 0 because of ita high calcium dispersion. As will be shown later, after stopping the COZreaction at a given BO level at 1073 K, calcium (present as CaC03) has a high mobility. This mobility produces a fast calcium agglomeration (and hence a decrease in calcium dispersion) which is not due to carbon gasification (even from a BO = 0%). The difference in dispersion observed for the sample prepared by slow pyrolysis at gasification temperatures of 1073 and 1023 K (Figure 5 ) could also be explained considering both the high mobility of CaC03 and the very different reaction rates for carbon gasification at both reaction temperatures (see Figure 4). Later on, when the importance of the chemical state of calcium on sintering rate is analyzed, a more detailed explanation for this observation will be presented. As already commented, the heating rate used in the pyrolysis treatment produces different initial dispersions and hence different reactivities; the additional heat treatment (at 1223 K during 2 h) gives rise to the sample A2-11-3.2-TT with the lowest dispersion (Figure 5 ) and reactivity (Figure 4). All the samples show a more or less marked decrease in reactivitywith the extent of gasification which is quite parallel to that found for the dispersion of the calcium during the reaction (see Figures 4 and 5). This similar behavior corroborates the relationship existing between reactivity and di~persion.~J~ Except for the rapidly pyrolyzed sample, a continuous decrease in reactivity is observed in all the cases. The fastly pyrolyzed sample presents a zone corresponding to a burn-off level between 0 and 30 % in which reactivity is constant. Considering the above-commented relationship between reactivity and dispersion, it can be deduced that the calcium dispersion of the sample prepared by fast pyrolysis is constant for BO = 0-3075. For BO > 30% calcium deactivation starts. In the following, an analysis of the sintering behavior of calcium species during the COz-carbon gasification, as a function of the initial sample dispersion and of the heating rate during the pyrolysis, will be obtained from the results of Figures 4 and 5. As will be shown later, the chemical form of the calcium is an important parameter in the sintering rate. Therefore, it is important to note that the chemical stateof the calcium species during the COz carbon gasification, presented in Figure 4, is assumed to be the same in all the cases. This assumption is based on the results of a previous study's in which the state of calcium species present during the COZcarbon gasification was analyzed in detail from TPR experiments. The results showed that, using a 10% COZ gas mixture, the calcium species are completely carbonated in a temperature range from 800 to 1020 K and that, in this temperature range, the catalyst was active for the gasification reaction. As temperature was further increased during the TPR experiment, in addition to an increase in the CO evolution (increase in reactivity), the
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(n) Figure 5. Calcium dispersion versus BO: (0) slowly pyrolyzed sample (1073K);(+) slowlypyrolyzedsample(1023K); (*)sample A2-11-3.2-TT (1023 K). LO.
sintering process, which is favored with increasing calcium content, is partially counterbalanced by the fast heating rate used. Thus, after a fast heat treatment, the sample with 6.0 wt % (Figure 3c,C) presents spectral characteristics similar to those of the 3.0 wt % sample prepared by slow pyrolysis (Figure 2a,A). Calcium Sintering and Deactivation during COzCarbon Gasification. Both catalytic activity loss during the gasification process and changes in the dispersion of calcium have been analyzed as a function of the initial calcium dispersion and the gasification temperatures. From these studies, the mechanism of the sintering of the calcium can be fully understood. Three calcium samples with different initial dispersion but with the same calcium content (3.2 wt 5%) have been used. The different initial dispersion of the calcium is obtained by submitting the calcium-carbon sample to three different heat treatments: (i) slow and (ii) fast heat treatments, and (iii) heat treatment at 1223 K for 2 h of a previously slowly pyrolyzed sample, in order to modify ita initial calcium dispersion (sample A2-11-3.2-TT). Figures 4 and 5, respectively, show the evolution of the COP reactivity (at 1023 and 1073 K)and the CaO dispersion (obtained from COz chemisorption) as a function of the burn-off degree (BO) for the slowly pyrolyzed sample.
(17) Salinas-MartlnezdeLecea, C.;Almela-Alar&n,M.;Linaree-Solano, A. Fuel 1990,69, 21-27. (18) Cazorla-Amorb, D.; Linaree-Solano, A.; Joly, J. P.; SalinasMartInez de Lecea, C. Energy Fuel8, 1992,6, 287-293.
Energy & Fuels, Vol. 7, No. 1, 1993 143
Sintering of Calcium Supported on Carbon
/
1- 0 0
20
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60 00 XM t h o (mid
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Figure 6. Decay of dispersion with time: (0) slowly pyrolyzed sample (1073K); (+) slowlypyrolyzedsample (1023K);(*) sample A2-11-3.2-TT(1023 K).
CaC03 decomposition started. At temperatures higher than 1200 K most of the CaC03 was converted to CaO. Considering the well-known effect of the COZ partial pressure on CaC03 decomposition,*gthe use of 100% C02 in this study instead of 10?% makes it reasonable to assume that the calcium species are as CaCO3in all the experiments shown in Figure 4. A sintering study of a catalyst which participates in a carbon gasification reaction has to be made considering that two processes take place: (i) particle diffusion on the surface of the substrate and (ii) particle movement as a consequence of substrate gasification (pore development). Classical theories of particle Sintering" were developed considering that (1) migration (surface diffusion) of crystallites and their coalescence (mechanismvalid in our study) and/or (2) emission of single atoms by the small crystallites and capture of single atoms by the large ones are the phenomena responsible for particle sintering. In all cases modifications of the support are not considered. In the case of catalyst sintering and deactivation during carbon gasification, important support modification occure. This makes the overall process of catalyst sintering much more complex. The theories developed to explain catalyst sintering by particle migration and coalescence result in the following power-law relation for the decay of the surface area (5') of the catalyst -dS/dt = C'Sp (1) where the exponent p and the constant C' depend on the mechanism involved. For surface diffusion control, the exponent p varies between 6 and 4. When coalescence constitutes the rate-determining step of the process, an equation of the same form as (1)is obtained with p = 3-2." The application of integrated eq 1 to the dispersion resulta of the slbwly pyrolyzed sample subjected to C02 gasification at 1023 and 1073 K and of the A2-11-3.2-TT sample partially gasified at 1023 K is shown in Figure 6. It can be observed that the three dispersion curves are well linearized. However, in the case of the slowly pyrolyzed sample gasified at 1073 K, two zones can be differentiated. The first one correspondsto times
