Energy & Fuels 1993, 7, 625-631
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Local Structure of Calcium Species Dispersed on Carbon: Influence of the Metal Loading Procedure and Its Evolution during Pyrolysis D. Cazorla-Amor6s, A. Linares-Solano,' and C. Salinas-Martinez de Lecea Departamento de Qulmica Inorgbnica, Universidad de Alicante, Alicante, Spain
M. Nomura Photon Factory, National Laboratory for High Energy Physics, Tsukuba, Japan
H . Yamashita and A. Tomita Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai, Japan Received January 22, 1993. Revised Manuscript Received June 1, 1993
An X-ray absorption fine structure (XAFS) study has been carried out to analyze the characteristics of calcium species dispersed on carbon after the loading step and during subsequent pyrolysis treatments. The results are in full agreement with those previously obtained from a temperatureprogrammed desorption (TPD) study. The combination of both techniques (TPD and XAFS) allows a deeper understanding of the calcium species formed after the loading process and the changes ocurring during the pyrolysis step. The effect of the catalyst loading method on the nature of calcium species and on its dispersion and catalytic activity after pyrolysis have been studied. Three calcium loading procedures have been used: ion exchange, ion exchange at fixed pH (pH = lo), and impregnation. The results show that the largest dispersion results using the ion-exchange procedure without controlling the pH. Moreover, this study confirms a previous one in which it was found that only the ion-exchanged calcium is effective as catalyst and that the use of a high pH during the ion-exchange process should be avoided. In fact, a high pH generates CaC03, which favors the sintering of calcium species during the pyrolysis and, hence, the loss of contact between the catalyst and the carbon.
Introduction In the catalysis of the carbon-gas reactions by calcium, the dispersion and the chemical composition of the catalyst after the loading process and after the pyrolysis step are of paramount importance in understanding its catalytic activity.1J Both chemicalcomposition and dispersion of the calcium species dispersed on carbon will change during the pyrolysisstep due to chemical decomposition and sintering of the catalyst. The extent of these changes are influenced by the method and the experimental conditions of the loading process and by the chemical nature of the catalyst used.3 Calcium dispersion, more precisely the calcium-carbon contact, after the pyrolysis step, is the responsible for its catalytic a ~ t i v i t y Furthermore, .~~~ the behavior of calcium during the gasification process (i.e., sintering of the (1) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983,82, 382-394.
( 2 ) Lmaree-Solano,A.;Salinas-Martfnezde Lecea, C.; Cazorla-Amorb, D.; Joly, J. P.; Charcosset, H. Energy Fuels 1990,4,467-474. (3) Yamashita, H.; Nomura, M.; Tomita, A.Energy Fuels 1992,6656-
661. (4) Cazorla-Amorb, D.; Joly, J. P.; Linarea-Solano,A,;Salii-Martfnez de Lecea, C. Carbon 1991,29, 361-369. (5) Cazorla-Amorb, D.; Linares-Solano, A.; Marcilla-Gomis, A. F.; Salinas-Martlnez de Lecea, C. Energy Fuels 1991,5,796-802.
0887-062419312507-0625$04.00/ 0
catalystl16J) is also dependent on the loading and the pyrolysis steps. From the above comments it is evident that much more attention should be paid to investigate the chemical composition of the catalyst and its changes during the pyrolysis step. The effect of the calcium loading and the experimental conditions used during the preparation process requires special attention. In previous publications218we have investigated the nature and the thermal behavior of calcium species loaded on carbon as a function of the calcium content. These calcium-carbon samples were prepared from calcium acetate solutions by ion exchange and impregnation and were analyzed using two complementary techniques: (1) temperature-programmed desorption (TPDl2 and (2) X-ray absorption fine structure spectroscopy (XAFS).8 The most relevant aspects deduced from the TPD study can be summarized as follows: (i) All the protons from carboxylic groups, present in the carbon surface, are ion(6) Linarea-Solano,A.;Almela-Alarc6n,M.; Salinas-Martinezde Lecea, C.; Cazoral-Amorb, D. R e p . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989,34, 136143. (7) Cazorla-Amorb, D.; Linares-Solano,A.;Salinas-Marthez de Lecea,
C.; Yamashita,H.; Kyotani, T.; Tomita, A.;Nomura, M. Energy Fuels 1993, 7, 139-145. (8)Cazorla-Amorb. D.: Linares-Solano.A,:Salinas-Martfnezde Lecea. C.;Ymashita,H.;Kyo&i,T.;Tomita,A:&ep.Pap.-Am.Chem.Soc.; Diu. Fuel Chem. 1991, 36(3),998-1006.
0 1993 American Chemical Society
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626 Energy &Fuels, Vol. 7, No. 5, 1993
exchanged by Ca2+ ions with a 2/1stoichiometry. (ii) An atomic distribution of the ion-exchanged calcium through the carbon matrix is found for calcium contents lower than or equal to the saturation of the available carboxylicgroups of the carbon surface. The ion-exchanged calcium has its coordination sphere completed with H20 and C02 molecules with a coordination number of six. (iii) When the calcium content exceeds the available carboxylic groups of the carbon surface, two calcium species are present: ion-exchanged calcium and calcium acetate. From TPD results it is deduced that the former acts as a nucleation site where calcium acetate crystals grow. XAFS spectroscopy has gained great utility in the field of catalysis due to the specific information concerning local structure and bonding of the element examinedP16 This technique provides information about atomic distances, coordination number, and disorder degree from the interaction of the emitted electrons with the atomic environment of the absorbing atom (extended X-ray absorption fine structure, EXAFS) as well as information about the electronic structure and the symmetry of the atom from the multiple scattering (shape resonances) of the excited electrons (X-ray absorption near-edge structure, XANES). XAFS has been recently applied to different calcium-coal and calcium-carbon systems with different purposes. 3 7 a 18-16 Considering that the above-mentioned TPD study does not provide direct information about the structure of the analyzed species, an introductory study was carried out using XAFS.a Although the number of experiments of this study was quite limited, two important conclusions were obtained: (i) The calcium of the ion-exchanged sample is linked to carboxylic groups and upon heat treatment (up to 1223 K) it remains highly dispersed and/ or forms amorphous small clusters of CaO. (ii) Upon heat treatment (up to 1223 K), samples with calcium contents larger than the maximum ion-exchange capacity of the carbon show the presence of CaO particles. The present paper deals with the nature of the calcium species after the loading process and their changes upon heat treatment at different temperatures under an inert atmosphere. In this study the effects of the metal content and of the pH used during the ion-exchange process have been analyzed. These two aspects have not been previously studied in spite of being important variables in most of the catalyzed carbon-gas reaction studies. For this reason, the aim of the paper is to get information about the evolution of the local structure and calcium dispersion in 1 1 1
(9)Bart, J. C. J.; Vlaic, G. Adv. Catal. 1987,35,1-138. (10)Sinfelt, J. H., Via, G. H.; Lytle, F. W. Catal. Rev. Sci.-Eng. 1984, 26,81-140. (11)Lytle, F.W.;Greegor, R. B.; Margnes, E. C.; Biebesheimer, V. A.; Sandstrom, D. R.; Horsley, J. A.; Via, G. H.; Sinfelt, J. H. In Catalyst Characterization Science. Surface andSolid State Chemistry;Deviney, M. L., Gland, J. L., Eds.; American Chemical Society: Weshington, DC, 1985;pp 280-293. (12)Claisen, B. S.;Tops@, H.; Candia, R.; Villadsen, R.; Lengeler, B.; Ale-Nielsen, J.; Christensen, F. J. Phys. Chem. 1981,85,386&3872. (13)Chiu, N. S.;Bauer, S. H.; Johnson, M. F. L. J. Catal. 1984,89, 226-243. (14)Chiu, N. S.;Bauer, S. H.; Johnson, M. F. L. J. Catal. 1986,98, 32-50. (15)X-ray absorption; Koningsberger, D. C., Prim, R., Eds.; John Wiley and Sons: New York, 1988. (16)Huggins, F. E.; Huffman, G. P.; Lytle, F. W.; Greegor, R. B. Proceedings of thelnternational Conference on Coal Science;Center for Conference Management: Pittsburgh, PA, 1986,pp 679-682. (17)Huggins, F.E.; Huffman, G. P.; Shah, N.; Jenkins, R. G.; Lytle, F. W.; Greegor, R. B. Fuel 1988,67,938-941. (18)Huggins, F.E.; Shah, N.; Huffman, G. P.; Lytle, F. W.; Greegor, R. B.; Jenkins, R. G. Fuel 1988,67,1662-1667.
samples with calcium contents below and above the saturation of the carboxylic groups of the carbon substrate and to analyze the effect that the pH used during the ion-exchange process has on them.
Experimental Section Sample Preparation. The samples used have been extensively described in previous publi~ations.~J@ In summary, this paper concerns a char from a phenolformaldehyde polymer resin (A) which had been oxidized with HN03 (A2). Calcium was loaded by ion exchange from a calcium acetate solution (1.5 M, 4 h) without controlling the pH (sampleA2-11-2.9)and at pH = 10using NaOH (sampleA2-11-4.1). The calcium-carbon samples are washed until water is free of Ca2+ions. Carbon A2 was also loaded by impregnation with differentcalcium contents using an appropriate calcium acetate solution. In this case the carbon/ calcium acetate solution mixture was heated at 330 K to total elimination of liquid. The impregnated sampleswere not washed. All calcium-carbon samples were dried at 383 K under vacuum. In the nomenclature, 11 stands for ion exchange and I for impregnation and the calcium loading in weight percent is also included. Experimental Procedure. 1. TPD Experiments. About 200 mg of sample was heated to 1225 K under He flow (60 mL/ min) with a heating rate of 20 K/min. A large portion of the evolved gas was evacuatedwith a rotarypump,and a small amount passed the leak valve. The gases ((2.02,CO, and HzO) and the mass 43 (characteristic of calcium acetate decomposition) were analyzed with a quadrupole mass spectrometer. TPD results are expressed as percentvolume per unit weight of carbon sample. 2. XAFS Experiments. XAFS experimentswere carried out in the dried and the heat-treated samples. The heat-treated samples were obtainedby heating in He (60mL/min)at 20 K/min up to different temperatures(603,823,and 1223K)without soak time. Once the fiinal temperature was reached, the sample was cooled down under the same atmosphereand sealed. Wafers for XAFS experiments were prepared under inert atmosphere pressing a homogeneous mixture of calcium-carbon sample and polyethylene in a ratio of 2/1. Finally, the samples were sealed with a polyethylene film to prevent air contact during handling. The X-ray absorption experimentswere performed at BL-7c of the PhotonFactory in the National Laboratory for High Energy Physics (KEK-PF)Tsukuba (Japan). A Si(ll1)double crystal was used to monochromathe the X-ray from the 2.6-GeVelectron storage ring. The Ca K-edge absorption spectra were recorded in the transmission mode at room temperature in a range of photon energy extending from 3940to4720 eV. Specialattention was paid to eliminate the photons from higher order diffraction using a double mirror system. Fourier transformation was performed on ka-weighted EXAFS oscillations, k*x(k),in the range of 4-10 A-1 (FT-EXAFS). The physical basis and numerous applications of XAFS spectroscopy have been discussed elsewhere.%1lP13J5 XAFS Spectra of ReferenceCompounds. Figure1presents XANES spectra and FT-EXAFS of several calcium compounds used as references. The most relevant aspects to consider are the following. 1. XANES Spectra. (a) Calcium oxide exhibits a pre-edge peak (I) at = 4043 eV. (b) Calcium carbonate exhibits a subpeak (11) at = 4059 eV. The pre-edge peak is much less intense than in the calcium oxide. (c) Calcium acetateexhibitsonly one broad main peak without any subpeak. The pre-edge peak evolution is determined by the presence of differentanions in the compounds. Differences in ionic character and symmetry cause differences in the valence orbitals of the calcium. The pre-edge peak intensitydecreases in the order 02, COS2-,CH3COO-. (19)Salinas-MartlnezdeL%cea,C.; hela-Alarcbn, M.;Linares-Solano,
A. Fuel 1990,69,21-27.
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Distance I A Figure 1. XANES spectra (a, b, c) and FT-EXAF'S (A, B, C) of reference compounds: (a, A) calcium oxide; (b, B) calcium carbonate; (c, C) calcium acetate. Energy I eV
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Table I. Characteristics of the Samples ion-exchanged sample Ca (wt %) dispersion reactivity (h-9 A2-11-2.9 A2-1-6.0 A2-1-9.4 A2-11-4.1'
2.9 3.7" 3-30 3.6b
0.55 0.50 0.35 0.42
3.1 3.6 3.5 2.5
0 Quantified from the peak mass 43. b Quantified from the CaCOS decompositionpeak. Sampleprepared by ion exchangeat fixed pH (pH= 10).
and, hence, atomically dispersed throughout the carbon matrix.2 The final characteristics of the calcium catalyst (Le., dispersion and catalytic activity) after the pyrolysis step performed before the gasificationexperiment, are compiled in Table I. The calcium dispersion was measured by the C02 chemisorption method20 and the reactivity was determined in C02 at 1073 K. Table I shows that the dispersion decreases for the three samples prepared without a controlled pH, with increasing the calcium content; however, the reactivity remains almost constant. This behavior has been previously explained2 by considering that, once the saturation level is passed, the calcium in excesss as calcium acetate is not effective for the carbon gasification reaction. Interestingly, the use of a high pH during the ion-exchange process (sample A2-11-4.1) leads to a noticeable decrease in both the dispersion and reactivity. In fact, a sample with a similar weight percent of calcium (3.7 wt %) but prepared without controlling the pH showed a higher reactivity value (3.7 h-'I2 than the sample A2-11-4.1. Moreover, the reactivity and dispersion of sampleA2-11-4.1 is only comparable to that of the sample with a 3.7 wt % calcium after a 30% burn-off in which the reactivity has decreased due to a significant calcium sintering process.5 This indicates that in the sample A211-4.1, as a consequence of the preparation method used (high pH value), the calcium species after the pyrolysis step are more sintered than what should be expected from an ion-exchanged sample. Knowledge of the nature of the calcium species formed in this sample is necessary to understand its lower catalytic activity and dispersion. The reasons of this interesting observation will be discussed later in the paper using the TPD and XAFS experiments performed with this sample.
XAFS and TPD Study 2. FT-EXAFS. (a) Calcium oxide exhibits a Ca-0 peak
corresponding to a distance of -2.0 A (uncorrected for phase shift)and a Ca-Ca peak at -3.0 A due to the nearest 0%and Ca2+ coordination shells, respectively. Peaks appearing at higher distancescorrespondto Ca2+and 0%ionsin more distant neighbor shellsof the calcium oxide crystal. (b)Calciumcarbonate exhibits Ca-0 peaks at -1.9 A and Ca-Ca at =3.7 A. (c) Calcium acetate exhibits only a Ca-0 peak at =2.0 A.
Results and Discussion Characteristics of the Samples. According to the preparation method, three types of samples can be distinguished (Table I): sample A2-11-2.9 prepared by ion exchange under no controlled pH, sample A2-11-4.1 prepared by ion exchange but at a fixed pH value (pH = lo), and samples A2-1-6.0 and A2-1-9.4 prepared by impregnation. The amount of ion-exchanged calcium was determined from TPD experiments using the peak mass 432-resulting from calcium acetate decomposition-for samples A2-1-6.0 and A2-1-9.4 and from the CaC03 decomposition for sample A2-11-4.1 (a deeper explanation of this case is presented later on). TG-DTA and XRD of these samples19 showed that, for calcium contents higher than the saturation of the carboxylic groups of the carbon surface (samples A2-1-6.0 and A2-1-9.41, part of the calcium species is present as calcium acetate. However, for calcium loadings lower than or equal to the saturation level of the carboxylic groups (samplesA2-11-2.9 and A2-11-4.1), calcium is ion-exchanged
Sample A2-11-2.9. The analysis of TPD experiments of calcium-carbon samples has allowed us to investigate how the calcium is bonded to the carbon surface and to get information about its distribution, contact, and growth throughout the carbon matrix.2 The study showed that calcium ions are atomically dispersed throughout the carbon matrix for calcium contents lower than the saturation of the carboxylic groups of the carbon surface (sample A2-11-2.9). Figure 2 shows the XAFS spectra (XANES and FTEXAFS) correspondingto the dried sample A2-11-2.9and the samples heat treated up to 603,823, and 1223 K. The four XAFS experiments performed correspond to (i) the dried sample (Figure 2a,A); (ii) the sample heat treated up to 603 K (Figure 2b,B); (iii) the sample heat treated up to 823 K (Figure 2c,C); and (iv)the sample heat treated up to 1223K (Figure 2d,D) in which the completepyrolysis treatment, similar to that of TPD experiments, is performed. XANES spectrum of the dried sample (Figure 2a) presents a broad main peak without any shoulder. FTEXAFS oscillations (Figure 2A) exhibit only one Ca-0 peak at ~ 2 . A. 0 Comparing calcium acetate spectra (Figure lc,C) with Figure 2a,A, it can be observed that there is a strong similarity. This suggests that calcium is associated with carboxyl groups. ______~~
(20) Linares-Solano, A.; hela-Alarc6nn,M.; SJias-Martlnez Lecea, C. J . Catal. 1990,125,401-410.
de
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Figure 3. TPD spectrum of sample A2-1-9.4 (dotted line, H20; solid line, CO; dashed line, mle = 43; bold solid line, COZ).
Energy I eV
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Figure 2. XANES spectra (a, b, c, d) and FT-EXAFS(A, B, C, D) of sample A2-11-2.9 submitted to different heat treatments. Heat treatmenta are (a, A) dried sample; (b, B) 603 K; (c, C)823 K (d, D)1223 K.
Heating to 603 K (Figure 2b,B) produces almost no differences in XANES and FT-EXAFS results; again, a broad peak in XANES and one Ca-0 peak in FT-EXAFS are observed, indicating that calcium sites do not change greatly with the heat treatment. The only differences between Figure 2a,A and Figure 2b,B are the appearance of a small pre-edge peak at =4040 eV (111) and a little broadening of the main peak in XANES spectrum. The broadening can be due to the increase in calcium concentration in the sample during devolatilization.18 From TG it is observed that when heating to this temperature there is a weight loss of about 15% .19 The presence of the small pre-edge peak in XANES spectrum indicates that oxygen coordination around calcium ions is somewhat distorted from an ideal octahedral ~ y ” e t r y . ~ J ~This J~ is in agreement with TPD results because a heating up to 603 K produces H2O and C02 evolution from the ionexchanged calcium without calcium carboxylate decomposition. This evolution results in a change of the original octahedral coordination sphere of the ion-exchanged calciumS2 The TPD experiment showed that heating to 823 K gives rise to decomposition of calcium carboxylate groupsS2 When the sample is heated to this temperature, XAFS spectrum (Figure 2c,C) shows a broadening in the main peak of XANES spectrum (reflecting the increase in calcium concentration due to a ~ 2 0 % weight loss observed from T G 9 and the appearance of the above-mentioned pre-edge peak (111). FT-EXAFS shows only one Ca-0 peak at -2 A. This indicates that, after heating to this temperature, calcium species are highly dispersed and/or forming noncrystalline clusters having an important distortion in the immediate coordination sphere.
Finally, heating to 1223 K (Figure 2d,D) (complete pyrolysis treatment) produces important modifications in XAFS spectrum. The XANES spectrum (Figure 2d) shows the presence of a new pre-edge peak a t around 4043 eV (I) in addition to a shoulder at around 4065 eV. The intensity of the pre-edge peak at 4040 eV (111)is smaller than in Figure 2, b and c. The FT-EXAFS (Figure 2D) exhibits a Ca-0 peak at =2.0 A and a weak Ca-Ca peak a t ~ 3 . A. 2 These results show that calcium species are highly dispersed and/or forming amorphous small clusters with spectral characteristics similar to those of CaO (Figure la,A). The fact that in the heat-treated sample the CaCa peak appears at larger distance (3.2 A) than the first calcium coordination shell in CaO (3.0 A) reflects the distortion of the CaO clusters. All this interpretation is in good agreement with calcium dispersion measured by C02 chemisorption in this sample after similar heat treatment (dispersion = 0.55). I t must be emphasized that only after the heat treatment to 1223K CaO clusters with some crystallinity are observed. Samples A2-1-6.0 and A2-1-9.4. These samples, prepared by an impregnation method, have calcium contents higher than the maximum ion-exchange capacity of the carbon A2 (3.7 wt % TPD experiments clearly showed that part of the calcium is as calcium acetate. Figure 3 is an example of this type of TPD spectra, where the presence of a peak mass 43 can be observed at -730 K and a CO2 peak (together with a CO peak) at around 1030 K both resulting from calcium acetate decomposition.2 Calcium acetate appears because the following steps occur during the calcium loading process: (i) a fast ion-exchange process in which two protons from carboxylic groups are ion-exchanged by one Ca2+ion and (ii) a process in which the calcium in excess interacts with the calcium already exchanged and growth of calcium acetate takes place (ionexchanged calcium acts as center of nucleation for calcium acetate crystal growth). As a consequence, the constant reactivity found for calcium contents higher than 4.0 wt 5% (near to the ion-exchange capacity (see Table I) can be explained by considering that the calcium in excess as
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Energy I eU Distance 1 A Figure 4. XANES spectra (a, b, c, d) and FT-EXAFS (A, B, C, D) of sample A2-1-6.0 submitted to different heat treatments. Heat treatments are (a,A) dried sample;(b, B)603 K; (c, C)823 K; (d, D)1223 K.
Energy I eU Distance / A Figure 5. XANES spectra (a, b, c, d) and FT-EXAFS (A, B, C, D)of sample A2-1-9.4 submitted to different heat treatments. Heat treatments are (a, A) dried sample; (b, B)603 K (c, C)823 K; (d, D)1223 K.
calcium acetate does not contribute to the catalytic activity because of the lack of contact between the catalyst and the carbon.2 Figures 4 and 5 show the XAFS spectra obtained for samples A2-1-6.0 and A2-1-9.4, respectively. In order to distinguish the different steps of calcium acetate decomposition, the XAFS spectra have been recorded after the samples were subjected to different heat treatments. In this way, the dried sample and samples heated to 603,823, and 1223 K have been studied. The XAFS spectra of samples A2-11-2.9 (Figure 2, a,A and b,B), A2-1-6.0 (Figure 4, a,A and b,B) and A2-1-9.4 (Figure 5, a,A and b,B), obtained after a heat treatment to a temperature lower than 603 K, are similar to each other and similar to the XAFS spectrum of calcium acetate (see Figure lc,C). These results indicate that in all the cases calcium is bonded to carboxylic groups and that from XAFS it is not possible to distinguish between Ca2+ions bonded to the carbon surface and Ca2+ions belonging to calcium acetate crystals, because of the similar coordination sphere in both cases.3 However, TPD experiments allow one to distinguish clearly the presence of calcium acetate. Therefore, a combination of XAFS with TPD results is very effective to identify the nature of the species. A heat treatment up to 603 K produces, in the case of sample A2-11-2.9, COZand HzO from the coordination sphere of the ion-exchanged calcium.2 The elimination of these COZand Hz0 molecules (occupyingfour coordination sites) produces important changes in the coordination of the ion-exchanged calcium. On the contrary, heat treatment up to 603 Kin samples A2-I-6.0and A2-1-9.4 produces mainly HzO, that comes from crystallization water of
calcium acetate.2 Because this crystallization water in calcium acetate corresponds only to 0-1 HzO molecules,21 its evolution is not going to produce a great distortion in the coordination sphere of Ca2+ions. The different change in the calcium coordination sphere produced in both types of samples (sample A2-11-2.9 and samples A2-1-6.0 and A2-1-9.4) is reflected in XANES spectra in the 4040-eV pre-edge peak (111). Sample A2-11-2.9 shows a clearer and more intense pre-edge peak a t 4040 eV than samples A21-6.0 and A2-1-9.4 after heat treatment to 603 K (compare Figure 2b with Figure 4b and Figure 5b). When samples A2-1-6.0 and A2-1-9.4 are heat treated to 823 K, XAFS spectra exhibit characteristics of CaC03 in agreement with previous res~lts.~J9It is interesting to observe the decrease of the pre-edge peak at 4040 eV from XANES spectrum of sample A2-1-6.0 (Figure 4c) to that of A2-1-9.4 (Figure 5c). This is indicative of a higher order at longer distance (i.e less distortion in calcium coordination sphere) in the CaC03 particles of sample A2-1-9.4. This observation is confirmed by the fact that the Ca-Ca peak at ~ 3 . 8, 7 in FT-EXAFS is much more intense for sample A2-1-9.4 than A2-1-6.0. Finally, a heat treatment to 1223 K produces CaC03 decomposition yielding Ca0.2*3This is clearly observed in Figures 4d,D and 5d,D. The spectra exhibit clearly the CaO characteristics. By comparison of XAFS results of samples heat treated up to 1223K as a function of calcium content (from 2.9 to 9.4 wt %), it is clear that as calcium content increases there is a transition from CaO clusters (Figure 2d,D) to crystalline CaO particles (Figures 4d,D (21) Handbook of Chemistry and Physics, 59th ed.;CRC Press: Boca Raton, FL, 1987; p B104.
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VK) Figure 6. TPD spectrumof sample A2-11-4.1(dotted line, HzO; solid line, CO; bold solid line, COz).
and 5d,D). The evolution of the Ca-Ca peak at =3 A in the FT-EXAFS is in agreement with the decrease in calcium dispersion found by C02 chemisorption (from 0.55 (sample A2-11-2.9) to 0.35 (sample A2-1-9.4)). Effect of the pH on the Nature and Dispersion of Calcium. It is well-known that the experimental conditions used to load a catalyst precursor in the support are of great importance on its further activity. In this sense, it can be found in the literature that the pH is a variable of the catalyst preparation step. Several authors have found that an increase in pH produces an increase in the extent of ion For this reason, in the case of calcium usually high pH values are chosen in most of the cases to increase the loading capacity without paying much attention to its effect on catalyst dispersion (and hence on its catalytic a ~ t i v i t y ) . ~ ~ ~ ~ ~ The effect of the pH during calcium introduction was studied in the sample A2-11-4.1. This sample was prepared by ion exchange with a controlled pH of 10 unlike sample A2-11-2.9 which was prepared by the same procedure but with the pH resulting from the contact between the calcium acetate solution and the carbon. Figure 6 presents the TPD spectrum of sample AS-114.1. The most relevant aspect that can be observed is the presence of a CO2 peak at around 1050 K which did not appear in the TPD profile of sample A2-II-2.ga2 From TPD it can be suggested that this CO2 peak comes from CaC03 formed during sample preparation because no peak mass 43 (characteristic of calcium acetate) is present.2The high pH used in the preparation makes possible the carbonation of some of the loaded calcium during the ionexchange process or during the sample drying. The amount of calcium as carbonate can be quantified from (22) Schafer, H. N. S. Fuel 1970,49, 197-213. (23) Schafer, H. N. S. Fuel 1970,49, 271-280. (24) Altekar, V. A.; Shahani, M. T.; Saha, A. K. Fuel 1974,53,29-31. (25) Hippo, E. J.; Jenkins, R. G.; Walker, P. L., Jr. Fuel 1979, 58, 338-344. (26) Takarada, T.; Naba+e, T.; Ohtauka, Y.; Tomita, A. Proc. Int. Conf. Coal Science, Mauatracht, The Netherlands 1987, 547-550. (27) Marcilio, N. R. Ph.D. Thesis, University of Lyon, France, 1990.
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Energy 1 eV Distance I A Figure 7. XANES spectra (a, b, c, d) and FT-EXAFS(A, B, C, D)of sample A2-11-4.1submitted to different heat treatments. Heat treatments are (a, A) dried sample; (b, B)603 K;(c, C)823 K; (d,D)1223 K.
+
the C02 V2CO peak centered at around 1050 K. The amount obtained in this way is 0.5 wt '3%. The remaining 3.6 w t % of calcium should be ion-exchanged with protons of carboxylic groups. This amount is in good agreement with the maximum exchange capacity (3.7 wt %) of the carbon A2.2J9 Figure 7 shows the XANES and FT-EXAFS spectra corresponding to the dried sample A2-11-4.1and the sample heat treated up to 603, 823, and 1223 K. XANES and FT-EXAFS obtained after a heat treatment to a temperature lower than 823 K (Figure 7a,A-c,C), show that some amount of calcium is as crystalline CaC03; in fact, the subpeak I1 in XANES spectra and the Ca-0 and CaCa peaks in FT-EXAFS are similar to those found for CaC03 (see Figure lb,B). The presence of CaCOS is not observed in sample A2-11-2.9 prepared by ion exchange but without controlling the pH. The low intensities of the subpeak I1 in XANES spectra and the Ca-Ca peak in FT-EXAFSindicate that only a small amount of all the calcium present is carbonated. After a further heating to 1223 K, the XAFS spectrum (Figure 7d,D) shows clear spectral characteristics of CaO (see Figure la,A) because, as expected, CaC03 decomposition has occurred. Interestingly, the spectrum resulting for sample A2-11-4.1 (Figure 7d,D) is, in comparison with the other samples studied, the most similar to that for CaO (Figure la,A). This clearly indicates that, in this sample, a higher calcium sintering process has occurred during the pyrolysis step, leading to the lowest reactivity. The evolution of XAF'S spectra with the heat treatment fully confirms the conclusions obtained from the analysis of the TPD experiment.
Calcium Species Dispersed on Carbon The importance of the preparation method on calcium dispersion is clearly observed, not only from the comparison of XAFS spectra of sample A2-11-2.9 (Figure 2) and A2-11-4.1 (Figure 7) and from their dispersion and reactivity (see Table I), but also from the differences found in TPD profiles. In this sense, in the case of sample A211-2.9,heating up to temperatures near to 823 K produces highly dispersed calcium and/or amorphous clusters. Only after a heat treatment to 1223 K are clusters with CaO characteristics observed. These clusters appear because of the agglomeration that can happen as a consequence of the low heating rate used.lJ On the contrary, when high pH is used, as in the case of sample A2-11-4.1, the calcium carbonation occurring during the calcium loading will produce crystalline CaC03. The presence of this CaC03 favors the sintering of calcium because of its higher mobility than that of Ca0.7 For this reason, the heating to 1223K is going to produce larger CaO particles (smaller dispersion) with lower catalytic activity than in the case of sample A2-11-2.9 (see Table I). The results collected in Table I show that calcium dispersion (Le., the external surface of the CaO particles) is not the responsible of the catalytic activity. In fact, from the calcium dispersion it is not possible to calculate the specific activity because the calcium-carbon contact has shown to be the ultimate responsible of the calcium catalytic a c t i ~ i t y . ~Thus, , ~ two samples can have similar calcium dispersion but different reactivity because they may have different calcium-carbon contact. This is observed when comparing the sample A2-11-4.1 with the A2-1-9.4 (see Table I). Though calcium dispersion of the former sample is higher than that of the latter, the opposite order is found for the reactivity. The lower reactivity of sample A2-11-4.1indicates that the calcium-carbon contact is smaller. These differences between the samples appear because there are two points that affect the reactivity (i-e., the extent of calcium-carbon contact). One is the calcium content and the second is the calcium sintering degree. As commented above, sample A2-11-4.1 exhibits a behavior close to that observed for the sample A2-1-3.7 after a 30% burn-off in which the calcium-carbon contact has decreased in relation to the unreacted sample. All these results point out the importance of the experimental method used to load the calcium.
Energy & Fuels, Vol. 7, No. 5, 1993 631
Conclusions XAFS experiments of calcium-carbon samples with different calcium content and prepared by different procedures, in conjunction with previous TPD experiments, have been very useful to analyze the calcium species present after the loading process and the changes produced in the catalyst during subsequent heat treatments. The results obtained from both techniques are in agreement and confirm that TPD is a powerful technique to study the nature of the calcium species in spite of its simplicity. Moreover,the combination of both techniques allows easier identification of the species. In this sense, this study shows the importance of the sample preparation on the nature of the calcium species and on its dispersion after the pyrolysis process. The results show that for calcium contents lower than the saturation of the carboxylic groups of the carbon surface, calcium ions are atomically dispersed and that for higher calcium contents part of the calcium is as calcium acetate. Because in both cases calcium ions are linked to carboxyl groups, XAFS results do not distinguish between calcium acetate and ion-exchanged calcium. It is necessary the use of TPD to differentiate these two species. After the pyrolysis step, calcium oxide clusters are formed and the higher the calcium content, the lower the dispersion and more similarities with calcium oxide are observed. The use of a high pH during the ion-exchange favors the formation of CaC03. The presence of CaC03 enhances the sintering of calcium species during the pyrolysis step and, hence, the loss of contact between the catalyst and the carbon. In this way, only the calcium ions ionexchanged a t a no controlled pH (or at a pH in which no carbonation occurs) give rise, after the pyrolysistreatment, to calcium species with the largest dispersion and contact with carbon. Therefore, there is no advantage in doing the calcium loading at high pH values.
Acknowledgment. The authors thank DGICYT (Project AMB92-1032-C02-02) for financial support. XAFS measurements were performed under the program ( 9 0 2 8 ) of KEK-PF. Program KABO-1written by Drs. Yoshida and Tanaka of Kyoto University was used for the analysis of XAFS spectra.
