Energy & Fuels 1991,5, 796-802
796
Calcium Catalytic Active Sites in Carbon-Gas Reactions. Determination of the Specific Activity D. Cazorla-Amorbs, A. Linares-Solpo,* A. F. Marcilla Gomis, and C. Salinas-Martinez de Lecea Departamento de Quimica Inorghnica e Zngenieria Quimica, Universidad de Alicante, Apartado 99, Alicante, Spain Received May 22, 1991. Revised Manuscript Received August 5, 1991 ~~~~~
A new procedure to quantify the catalytic active sites (CAS) in calcium-catalyzed carbon gasification is presented. The method is based on TPD analysis after CO, chemisorption. The second COz composite peak is related to the calcium-carbon contact and the quantification of this peak provides a measure of the CAS. Turnover frequencies based on this CAS for COPand steam gasification are found constant for different calcium loading and for samples with different sintering degrees.
Introduction Carbon-gas reactions are complex heterogeneous reactions with additional complexity as a consequence of the catalytic effects of the inorganic impurities (or added catalysts), which can be dispersed at different levels within the carbonaceous matrix.14 Among the different factors affecting the kinetics of such reactions, the concept of active surface area (ASA) is, undoubtedly, the more significant and the more developed."" As a result of this, and bearing in mind that only a fraction of the ASA is involved in the reaction, concepts such as reactive surface area (RSA)"' or working active site (WAS)12have been deeply investigated. The utility of the ASA concept and particularly that of RSA and WAS terms, in the quantitative analysis of the carbon-gas reactions, justifies the large number of works developed in order to acquire a better understanding of their nature, to quantify them, and to study their variation in the reaction.+l6 (1)Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1968,Vol. 4,pp 287-383. (2)McKee, D.W. In Chemistry and Physics of Carbon; Walker, P. L., ; ,bkker: New York, 1981;Vol. 16,pp 1-118. Thrower, P. A., Ma.Marcel (3)Kapteijn, F.; Moulijn, J. A. In Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J. A,, Eds.; NATO AS1 Series E105;Martinus Nijhoff: Amsterdam, 1986; pp 291-360. (4)Pullen, J. R. 'Catalytic Coal Gasification"; IEA Rep. ICTIOS/ TR26,Coal Research, London, 1984. (5)Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67,2030-2034. (6)Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62, 849-856. (7)Khan, M. R. Fuel 1987,66,1626-1634. (8)McEnaney, B.In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., NATO AS1 Series E192; Kluwer Academic: New York, 1991; pp 175-199. (9)Lizzio, A. A. Ph.D. Thesis, University of Pennsylvania, 1990. (10)Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990,28, 7-19. (11) Radovic, L. R.; Lizzio, A. A.; Jiang, H. In Fundamental Issues in
Control of Carbon Gasification Reactioity; Lahaye, J., Ehrburger, P., NATO AS1 Series E192; Kluwer Academic: New York, 1991;pp 235-251. (12)Zhu, 2.B.;Furusawa, T.; Adschiri, T.;Nozaki, T. Prepr. PapAm. Chem. Soc., Diu. Fuel Chem. 1989,34(l),87-93. (13)Hilttinger, K. J. Carbon 1990,28,453-456. (14)Hottinger, K.J.; Nill, J. S. Carbon 1990,28,457-465. (15)Kapteijn, F.; Meijer, R.; van Eck, B.; Moulijn, J. A. In Fundamental Issues in Control of Carbon Gasification Reactioity; Lahaye, J., Ehrburger, P., NATO/ASI Series E192;Kluwer Academic: New York, 1991;pp 221-230. (16)van Heek, K.H.; Muhlen, H. J. Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye, J., Ehrburger, P., Eds.; NATO AS1 Series E192; Kluwer Academic: New York, 1991;pp 1-30.
The use of the ASA concept (determined by oxygen chemisorption5) assumes that the carbon-gas reactions (except with H,)occur by the formation of superficial carbon-oxygen species at such active sites. Nevertheless, it is not correct (when discussing reaction mechanisms or modeling these reactions) to suppose that all sites, as measured by oxygen chemisorption, are actually active in the carbon-gas reactions. In fact, sites measured by oxygen chemisorption could be unstable at reaction temperatures, and others being stable could not participate in the reaction.8 There is strong experimental evidence that there are, at least, two types of C-0 species,17which makes necessary the distinction between the concept ASA and RSA.8J1 Despite the utility of the ASA concept, there is no doubt that the concepts RSA or WAS are more adequate and relevant, since they relate only the labile carbon-oxygen atoms (and, consequently, reactive bonds), and not the stable atoms to the reactivity of the c a r b ~ n . ' ~ J ~ J ~ The quantification of the concentration of RSA has been recently carried out successfully, by means of techniques based on rapid changes of the experimental conditions, as for instance, changing the reactive gas by an inert gas (in experiments of transient kinetics (TK)) and/or rapid cooling of the reactive gas. Combining both techniques, it has been possible to distinguish between labile and stable oxygen atoms in such a way that interesting correlations have been established between RSA and the reactivity of different carbons and between reactivity and degree of conversion of the carbon? In spite of this success, it should be noted that very few works have quantified the concentration of RSA in the catalyzed reaction. Furthermore, changes in the concentration of RSA during the reaction, as a function of the nature of the catalyst, the nature of the gas, and temperature of the reaction, need to be investigated. In this respect, few articles have been published with regard to the catalysis by iron and potassiu~.1OJ5J8 The results of Lizzio? obtained when studying both the catalyzed and uncatalyzed gasification of different carbons, are of special relevance for the present work. The reactivity and RSA varied in a similar way with the carbon conversion, both in the potassium-catalyzed reaction and in the uncatalyzed reaction. RSA quantification was at-
'
(17)Freund, H. Fuel 1986,65,63-66. (18)Hermann, G.;Hiittinger, K. J. Carbon 1986,24, 429-435.
0887-0624/91/2505-0796$02.50/00 1991 American Chemical Society
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Calcium Catalytic Active Sites
tained by following the evolution of CO when shifting the gas of reaction to an inert gas, whereas the reaction temperature was kept constant. Using such an RSA Lizzio concluded that the ratio between the reactivity and RSA (turn over frequency, TOF) is almost constant for a wide interval of conversion, and that the TOF values for the catalyzed and uncatalyzed reactions are very close to each other. The author, when applying the study to the case of calcium, found that the reactivity could not be normalized by RSA. This fact will be explained later in this article. The present work suggests a new procedure, based on the carbon-catalyst contact, which is able to quantify the number of catalytic active sites (CAS) in the case of the carbon-C02 and carbon-steam reactions catalyzed by calcium. The method (alternative to that used by Lizzios) is based on previous results which show, on the one hand that the dispersion of calcium in a carbonaceous matrix can be quantified by C02 chemisorption at 573 K,ls and on the other hand that the temperature-programmed desorptions (TPD) of calcium-carbon samples after COP chemisorption provide information about the calciumcarbon contact, which is responsible for the catalytic activityS2O The quantification of such contact provides the number of catalytic active sites (reactive sites) of the catalyst. Thus, its meaning is different from that of RSA, since this concept refers to the oxidized sites of the carbon (C(0))which are reactive. The RSA, in the case of calcium (as we will show later), can be very different to that obtained by Lizziog using the TK transient technique. The quantification of the number of CAS, which is carried out by integration of the peaks obtained by deconvolution of the experimental curves after applying a kinetic model, will be used to normalize the C02and steam reactivities of samples with different calcium loading or with different sintering degrees (results showed elseOn the other hand, the deconvolution of the peaks will allow us to obtain kinetic parameters of interest with respect to the mechanism of the carbon-gas reaction catalyzed by calcium.
Experimental Section The experimental data used in the present paper have been previously reported and correspond to reactivities in COz and steam of carbon samples with a different calcium loadingz1and carbon-calcium samples with a different sintering degree.z2 Furthermore, experimental data from TPD of these samples, after a process of COz chemisorption,m are also used. Nevertheless, a brief description of the samples and the experimental techniques used is included. Samples. A high-purity carbon, obtained from a phenolformaldehyde resin oxidized by HN03, was used as the support where the calcium was deposited by means of two different procedures: ionic exchange and impregnation. The nomenclature used is the following: A2 (corresponding to the oxidized carbon) followed by I or 11, in order to distinguish between impregnation and ionic exchange, and by a number, indicating the amount of calcium loaded.z4 (19) Linares-Solano, A.; Almela-Alarcbn,M.; Salinas-Marthez de Lecea, C. J. Catal. 1990, 125,401-410. (20) Cazorla-Amorbs, D.;Linarea-Solano, A.; Salinas-Martinez de Lecea, C.; Joly, J. P. Carbon 1991,29, 361-369. (21) Salinas-Martinez de Lecea, C.; Almela-Alarcbn, M.; Linares-Solano, A. Fuel 1990,69, 21-27. (22) Linarea-Solano,A.; Almela-Alarcbn,M.; Salinas-Martinez de Lecea, C.; Cazorla-Amorb, D. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1989,34 (l),136-143. (23) Linares-Solano, A.; Salinas-Martinez de Lecea, C.; CazorlaAmorba, D.; Joly, J. P. In Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye, J. Ehrburger, P., NATO AS1 Series E192; Kluwer Academic: New York, 1991; pp 409-430.
14001
T(K)
I
t
I
*0°
"
I
0
40
20
60
80
tlmo (mln)
Figure 1. Experimental procedure used showing: T P D in He (20 K/min) of the calcium-carbon sample, the COz treatment, and ita subsequent TPD in He (100 K/min).
0
0
4
2
8
1
0
Cllolwn contont wt 4!
Figure 2. Reactivity versus calcium content: 0,COz at 1073 K; 0,steam a t 1073 K.
Reactivity. Reactivities in C02 (0.1 MPa) and steam (19.7 kPa) have been thermogravimetrically obtained a t 1073 K according to the experimental procedure and equipment described e l s e ~ h e r e In . ~ short, ~~~~ the procedure consists of the following steps: (a) heating of the sample in Nz atmosphere up to 1173 K at 20 K/min with 10 min of soaking time, (b) cooling of the sample in Nz to the reaction temperature (1073 K in this case), (c) switching of Nz to COP The reactivity has been determined from the maximum slope of the weight loss vs time curve and is related to the initial sample mass. TPD Spectra. The equipment used (reactor coupled with a mass spectrometer) has been previously d e s ~ r i b e d . ~ ~The *~ experimental procedure, illustrated in Figure 1, consists of the following steps: (a) heating of the sample in He atmosphere up to 1223 K a t 20 K/min heating rate, (b) cooling of the sample in He atmosphere down to the chemisorption temperature (i.e., 573 K), (c) chemisorption of COz for 5 min, by switching the He to a mixture of 10% COz, 2% Ar, 88% He, (d) T P D in He (100 K/min) up to 1223 K. The flow rate used was 60 cm3/min (25
OC, 1 atm).
Results and Discussion In this paper two series of results-reactivities and TPD after C02chemisorption of samples with different calcium loading and different sintering degrees-are analyzed and ~~
~~~
~
(24) Almela Alarcbn, M. Ph.D. Thesis, University of Alicante, 1988. (25) Joly, J. P.;Cazorla-Amorbs, D.; Charcosset, H.i Linares-Solano, A.; Marcilio, N. R.; Martinez-Alonso, A.; Salinas-Martlnez de Lecea, C. Fuel 1990,69,878-884.
Cazorla- Amorbs et al.
798 Energy & Fuels, Vol. 5, No. 6, 1991 ?bVol/g 20 1
%Vol/g 151
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-
a
15-
il
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#Vol/g 151
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01 700
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Figure 3. (a) TPD-MSspectrum after COPchemisorption at 573 K on sample A2-II-2.9 (bold line, C O ---, C02;-, C02+ '/zCO). (b) Result of balance deconvolution (-, COz + '/&O; bold line,
Figure 4. (a) TPD-MSspectrum after COzchemisorption at 573 K on sample A2-1-7.3 (bold line, CO;---, CO,;-, C02+ 1/2CO). (b) Result of balance deconvolution (--, C02+ '/,eo; bold line, first composite peak; ---, second composite peak).
discussed. It is emphasized that the key factor for understanding the reaction of carbon with C 0 2 or steam, catalyzed by calcium, is the CAS rather than the amount of calcium or the dispersion or the area of the catalyst. This point greatly contributes to a better understanding of the mechanisms of these reactions.23 Figure 2 shows some reactivities in C 0 2 and steam as a function of the calcium loading.21 It can be noticed that the reactivities increase up to a maximum value corresponding to a calcium loading around 4 % . A detailed analysis of the reasons for this feature shows that the ion-exchange capacity of the carbon A2 used is very close to this percentage. A study of the chemical nature of the calcium species at calcium loading lower than 4% (where the reactivity is proportional to the calcium amount) shows that the calcium is atomically d i s t r i b ~ t e d . ~ ~On ~ ~the ' other hand, in samples with amounts of calcium higher than 4%, the calcium coordination is rather different. In all samples, an amount around 3.5% was found to be exchanged, whereas the rest of the calcium was present as calcium acetate.% From this study, it was concluded that
the excess of calcium (over ion-exchange saturation) grew on top of that exchanged and that the contact carboncalcium is only due to the calcium exchanged. This could explain the fact that the reactivities of all samples with calcium amounts higher than 4% are practically equal. In order to get more insight into the carbon-calcium contact, a series of TPD experiments, after COP chemisorption at 573 K for 5 min, was analyzed. In these chemisorption conditions, the C02 (as stated in previous ~ o r k s ' is ~ restricted ~~) to a monolayer and can be used to determine the area (and dispersion) of the particles of the catalyst (CaO). The TPD of carbon-calcium samples are characterized by the presence of two COz peaks and one CO peak.2o The relative intensities of the two C 0 2 peaks depend on the amount of calcium, as can be seen in Figures 3a, 4a, and 5a, corresponding to samples with 2.9, 7.3, and 9.3% of calcium, respectively. From the single peaks, the two composite peaks (COz + l/&O) have been calculated (see Figures 3a, 4a, and 5a) considering the stoichiometry of the gasification reaction. This procedure is introduced to simplify further analysis of TPD spectra, The gasification
first composite peak; - --, second composite peak).
(26) Linaree-Solano, A.; Salinas-Mardnez de Lecea, C.; CazorlaAmorb, D.; Joly, J. P.; Charcomet, H. Energy Fuek 1990,4, 467-474. (27) Cazorla-Amor68, D.; Linares-Solano, A,; Salinas-Martinez de Lecea, C.; Yamashita, H.; Kyotani, T.;Tomita, A. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1991, 36 (31, 998-1006.
(28) Cazorla-Amorb, D.; Joly, J. P.; Linares-Solano, A.; SalinasMartinez de Lecea, C.; Marcilla-Comis, A. J. Phys. Chem. 1991, 95, 6611-6617.
Calcium Catalytic Active Sites
Energy & Fuels, Vol. 5, No.6, 1991 799
KVol/g
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,
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700
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900
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1100
1200
1100
1200
T(K)
b
0 700
aoo
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Figure 5. (a) TPD-MSspectrum after C02 chemisorption at 573 K on sample A2-1-9.4 (bold line, C O ---, CO,;-, COP+ '/&O). (b) Result of balance deconvolution (-, COZ + 1/2C0bold line, first composite peak; - - -, second composite peak).
reaction can be written, as a general form, considering the oxygen-exchange mechanism:'^^ + C(0)+ C(0) According to this, two CO peaks should be expected in TPD spectra; however, under our experimental conditions (COPchemisorption followed by TPD under He at 100 K/min), only one CO peak is observed, which indicates that both CO peaks appear very close. The use of isotopically marked molecules (13C02)has allowed to distinguish the two CO peaks.29 In this sense, for the purpose 2CO) is of this paper, the global reaction (COP+ C reasonable and can be used to simplify TPD analysis. Figures 3b, 4b, and 5b show the results of the deconvolution (as will be shown later) of these peaks. The most remarkable aspect of these results is the fact that the samples with calcium amounts lower than 4% (Figure 3a is one example of the various samples studied) present two COPpeaks of similar intensities. On the other hand, for samples with higher calcium amounts the first COPpeak is observed to increase, whereas the second C o ppeak re-
cop c
-- co
co
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(29) Cazorla-Amorbs, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Meijer, R.; Kapteijn, F. Prep?. Pap.-Am. Chem. Soc.. Diu. Fuel Chem. 1991, 36 (3), 975-981.
800
900
1000
T(K)
Figure 6. Effect of calcium sintering degree on TPD-MS spectra obtained after COz chemisorption at 573 K (a) sample A2-1-3.7; (b) sample A2-1-3.7 after a 45% burnoff (-, CO; ---, COz).
mains almost constant (see Figures 4a and 5a). The CO peak apparently increases with the amount of calcium, but it must be considered that the first CO, peak also increases with the calcium loading and that this C o pmay contribute to the CO evolution by a gasification process. Thus, correcting the CO with this contribution due to the first C02 peak we obtain what we call "net" CO, which is independent of the amount of calcium.20 The evolution of the spectra corresponding to TPD, after C02 chemisorption, of samples with different sintering degrees also shows very interesting features. Figure 6 shows two examples of the effect of the degree of sintering of the catalyst on the CO and C 0 2 peaks, corresponding to the sample A2-1-3.7 (Figure 6a) and to the same sample, but after a 45% burnoff (Figure 6b). A remarkable feature is the increase of the first C 0 2 peak with respect to the second and the decrease of the CO peak when we compare the sample without sintering (Figure 6a) to the sintered sample (Figure 6b). From all these studies it can be concluded that the first CO, peak (and its contribution to the CO peak) corresponds to the nonactive part of the catalyst (that which is not in contact with the carbon), whereas the second C 0 2 peak and the "net" CO would correspond to the active part of the catalyst, i.e., from the carbon-calcium It is worth noting that, for samples with calcium loading
Cazorla-Amords et al.
800 Energy & Fuels, Vol. 5, No. 6,1991
5% Ca
OF, 5%
1.3 2.9 3.7 7.3 9.4
2 2 2 1 2
a Evolved
Table I. Results Obtained from T P D Spectra Deconvolution COP+ 1/2CO: qmol/g peak 1 peak 2 peak 1 peak 2 El, kJ/mol 10”Al, s-l E‘, kJ/mol 10-IoA’, s-l 25 75 200 250 480
115 275 320 360 340
125 130 135 140 140
1.2 1.9 0.9 1.9 10.0
225 230 230 215 220
4.4 1.2 1.9 0.2 0.5
gases per gram of carbon catalyst free.
between 4 and 990, the contact carbon-calcium is constant for all of them, which is in total agreement with the findings presented elsewhere%and previously commented on. Furthermore, all samples show the same reactivity (as can be seen in Figure 2), which confirms and supports the interpretation of the origin of the peaks of CO and COz in TPD experiments after chemisorption.
Deconvolution of the TPD Spectra The balance COP + 1/2C0 calculated from the TPD corresponds to two overlapping peaks. The first peak is related to the external surface of the CaO particles, whereas the second corresponds to the carbon-calcium contact (Figures 3a, 4a, and 5a). The deconvolution of the peaks has been carried out according to the following kinetic model. Kinetic Model. (1) First Composite Peak. Bearing in mind that the COz adsorbed in the external surface of the CaO particles (COz(e))is desorbed in a TPD experiment during the heating of the sample and that the COz evolved into the gas phase (C02(g))can react with the carbon, we consider the following reactions for describing such processes: C02(e) COz(g) Itl (1) COz(g) + C 2CO(g) k2 (2) Step 1represents the desorption of C02from the external surface. Part of this COz may react with the carbon yielding CO according to the gasification reaction (2). Consequently, the kinetic equation corresponding to that desorption, considering fint-order kinetics, may be written as -d[COz(e)l/dt = A1 exp(-E1/RT)[CO2(e)I (3)
--
where the balance of COz(e) at time t is [COz(e)l = [C02(e)lo- AC02(e) - f/,ACO(e) (4) and where [C02(e)lois the initial amount of C02 adsorbed in the external surface and is a fitting parameter representing the area of the first composite peak. AC02(e) is the amount of COz desorbed from the external surface up to a time t and ACO(e) is the amount of CO evolved as a consequence of reaction 2. (2) Second Composite Peak. The COz adsorbed in the carbon-calcium contact (C02(c))can react by means of the direct reaction with carbon through the CAS or can be desorbed yielding C02(g). The following reactions must be considered to describe the second composite peak
-
CO,(C) + c 2CO(g) k, (5) C02(c) COp(g) k, (6) C02k) + C 2CO(g) k7 (7) where step 5 corresponds to the reaction of the C02 adsorbed on the interface carbon-calcium. This step is possible due to the contact between the catalyst and the carbonaceous matrix. Steps 6 and 7 are similar to those used for describing the process of desorption of the C02 -+
adsorbed on the external surface. In this case the rate equation describing the process would be the following -d[CO,(c)l/dt = (k, + k,)[CO2(c)l = kTCOz(c)l which can be written as -d[C02(c)]/dt = A’ exp(-E’/RZ‘)[COz(c)]
(8)
considering that the activation energies for processes 5 and 6 are similar. This can be assumed if process 6 is considered as bulk CaC03 decomposition, since its activation energy is 235 k J / m o P and the activation energy for COz The carbon gasification (process 5) is 240 balance of [C02(c)]at a time t is [CO~(C)] = [CO~(C)]O - AC02(c) - f/,ACO(c) (9) As it is not possible to distinguish CO(g) produced in reaction 5 from that produced in (7), it is assumed that process 7, which corresponds to the uncatalyzed carbon-gas reaction, is negligible. Hence, eq 9 accounts for the CO produced from reaction 5 and the C02 that is directly desorbed from reaction 6. The optimization of the parameters [CO2(e)lo,[COz(c)]o, A,, El, A’, and E’ has been carried out by the simplex method using the following objective function: OF = C((d([C02(e)l+ [ C O 2 ( ~ ) l ) / d t )-~ ~(d([C02(e)l P + [C02(c)l)/dt)c*1)2 Figures 3b, 4b, and 5b show the fittings obtained for the samples A2-11-2.9, A2-1-7.3, and A2-1-9.4, respectively, together with the composite peaks obtained from the deconvolution. Table I shows the OF and the parameters obtained. From Table I it can be observed that the activation energies corresponding to the first composite peak (E,) are in the range of 125-140 kJ/mol. These values are very close to those obtained for the desorption of COz in bulk CaO, after contacting with C02at temperatures lower than 573 K (temperatures where the interaction is restricted to the CaO s u r f a ~ e ’ ~ . ~In~ fact, ) . the values determined in this case are within the range 135-180 kJ/mol. The first value was obtained in a TPD experiment after contact of the CaO with C02 at a temperature (TJ of 323 K, whereas the second value was obtained in a similar way but with a Tt of 573 K. By varying Ttwithin these two limits, we obtained different COz coverages, and consequently it was possible to study the effect of this coverage on the subsequent desorption. In particular, the El values corresponding to the first composite peak are similar to those determined for pure CaO, at the lower C02coverage (i.e., Tt= 323 K), in other words, when the C02 occupies the sites of lower activation energy. This agreement confirms that the first composite peak observed in the TPD, obtained for the samples of calcium-carbon, corresponds to the external surface of the particles of CaO, which does not interact with the carbon.20 (30) Spiro, C. L.; McKee, D. W.; Kosky, P.G.;Lamby, E. J.; Maylotte, 1983,62, 323-330.
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Actividad Especlfica ( h - l )
Actividad Especlfica ( h - l ) a
2001
I
looo----
200 0
A
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10
Contenldo en calcio (# Pero)
Figure 7. Specific activity versus calcium content for the C02-carbonreaction, calculated per superficial calcium atom (0, R,/S,,) and per calcium atom of the contact (A,R,/S,). The values of E’, corresponding to the second composite peak, are also constant and very close to the activation energy for the COPgasification reaction (240 kJ/m~Pa’.~). This fact is very important, since it establishes that the second peak is basically related to a gasification process and not to the decomposition of the superficial calcium carbonate.28 This result is again in accordance with the interpretation of the TPD spectra,20where the second composite peak is explained as follows: during the increase in temperature in a TPD, a redistribution of the COPis produced among the CaO particles. The C02 diffuses from the external surface to the calcium-carbon contact yielding to the formation of CaC03-C species, where the COP is more stabilized than in the surface due to the stabilizing interaction of the ion carbonate with the carbon. The decomposition of these species is produced at a higher temperature and, due to the contact, the main part of the COP reacts with the carbon. This decomposition of the species CaC03-C (*COP)is carried out accordingly to the widely accepted mechanismb3132 *COP + c s *C(O) + co *C(O) co the second composite peak of the TPD being related with the carbon-calcium contact and, consequently, with the number of species *COP(CAS),the decomposition of which produces the gasification of the carbon in two steps. Nevertheless, from the analysis of these TPD, it is not possible to distinguish the two steps of the mechanism (i.e., distinguish between the two types of CO). To do this it would be necessary to carry out experiments of TPD or transient kinetics, with different experimental conditions from those used in this work, or with isotopically marked molecules, as has been done recentlyqZ9
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Specific Activity of the Calcium The reactivities in COP(0.1 MPa) and steam (19.7 kPa) increase linearly with the amount of calcium until a constant loading saturation level (LSL) for calcium amount higher than 4 % (see Figure 2).21 According to the interpretation of the TPD, the catalytic activity of the calcium (31) Raif, A. E.J . Phys. Chem. 1952,56, 785-788. (32) Ergun, S. J . Phys. Chem. 1956,60, 480-485.
2
4
6
8
10
Contenido en calcio ( l b Peso)
Figure 8. Specific activity versus calcium content for the steam-carbonreaction, calculated per superficial calcium atom (0,R,/S,,) and per calcium atom of.the contact (A,R,/S,). Table 11. Determination of Calcium Specific Activity for Sample A2-1-3.7 with Increasing Calcium Sintering Degree
BO,” %
Rt, h-’
dcaO
0 30 45
3.7 2.5 1.5
0.56 0.50 0.35
Con?
rmolfg 520 455 385
pmof/g
COz p t b
RtISc, h-*
320 175 130
960 1190 960
Rtl Sex, h-l 590 460 325
Burn off. *Evolved gases per gram of carbon catalyst free.
is related to the calcium-carbon contact. In this case, the second composite peak should increase until the LSL is reached and remain constant for higher calcium amounts. On the other hand, the first composite peak should increase with the calcium amount in all the range of calcium loading, as a consequence of the increase of the external surface of the CaO particles (see Figure 2). This variation of the two composite peaks is that observed after the deconvolution of the TPD spectra (Table I). These results are again in accordance with the interpretation of the LSL previously mentioned.20*26 Figures 7 and 8 show the variation of the specific activity of calcium (carbon atoms reacted per calcium atom and per hour) in COP (Figure 7 ) and in steam (Figure 8) calculated in two ways: (a) with respect to the calcium atoms of the external surface of the calcium particles (Rt/Se,), determined from the total COzchemisorbed at 573 K, and (b) with respect to the calcium atoms of the carbon-calcium contact which constitute the CAS (Rt/Sc),determined from the COPrelated to the second composite peak from the TPD experiments. On the other hand, Table I1 shows the results of the normalization (R,/S,, and Rt/Sc) corresponding to samples with different sintering degrees. Table I1 also includes other parameters such as reactivity ( R J , catalyst dispersion (d,,,), amount of CaO chemisorbed at 573 K (COP,)and the amount corresponding to the second composite peak (COzPz). Figures 7 and 8 show that the normalization with respect to the superficial calcium atoms decreases with the amount of calcium, whereas it remains constant when it refers to the CAS. This shows that the specific rates normalized with respect to the CAS are a measure of the turnover, since it is constant as can be expected for this type of reactions. Furthermore, results in Table I1 support these arguments, since such normalization is only possible, for samples with different sintering degrees, when it is related to the second composite peak.
802 Energy & Fuels, Vol. 5, No. 6, 1991
All these results prove that the quantification of the CAS by this method seems to be reliable. Furthermore, the observation that this normalization also applies in the case of steam suggests that the CAS determined from the second composite peak are the same for the reactions with both COz and steam. The constancy found for the turnover, independently of the calcium loading, indicates the importance of the carbon-catalyst contact in the gasification reactions. Consequently, it is necessary to investigate the calciumcarbon contact in order to attain a quantitative determination of the specific activity of calcium in carbon-gas reactions. The second composite peak, obtained from the deconvolution of the TF'D spectra after COz chemisorption at 573 K, seems to be related to this contact. Finally, the fact that the rate obtained by using the total COz chemisorbed cannot be normalized for the different calcium loading indicates that not all of the external surface of the CaO is effective for the reaction, since not all the external calcium is in contact with the carbon. In this way, considering that the reaction rate is determined by the disappearance of the C(0) complexes, formed when each CAS (*C(O)) is decomposed, the rate equation would be r = d[*C(O)]/dt = k[*C(O)]
Cazorla-Amor6s et al. toward the interface will gasify the carbon and will produce CO; consequently, the CO measured by Lizzio et al.'OJ1 would not correspond to the concept of RSA. The contribution of the CO proceeding from the decomposition of the catalyst can be important and, thus, the RSA determined in the case of calcium will be overestimated. This is probably the reason why Lizzio et al. were not able to normalize the results of the reaction catalyzed by calcium, whereas they succeeded in the uncatalyzed reaction and in the reaction catalyzed by potassium. Further studies with these samples, but using the method of Lizzio et al. to determine the relation between RSA and CAS, would be necessary in order to confirm the previous comments. On the other hand, our results explain the experiments of F r e u n d " ~who ~ observed an evolution of CO higher than expected in a TK experiment of COz gasification of a calcium-carbon sample. The results of this work show the importance of the catalyst-carbon contact when its catalytic activity is interpreted and present a new method to quantify the number of CAS (suitable for the carbon-gas reaction catalyzed by calcium) based on the TPD after COz chemisorption. The fact that the method allows the determination of the specific activity of calcium is important in order to compare the catalytic activity of different catalysts and test if, as is widely accepted, the reactivity of potassium (by potassium atom) is (or is not) higher than that of calcium. The results from this comparison (not yet carried out) will be very important in order to elucidate the mechanism of reaction of both catalysts.
where [*C(O))] is the number of CAS, corresponding to the calcium-carbon contact, and determined by the second peak from the deconvolution of the TPD. Since r is proportional to the numbe of CAS and not to the total amount of catalyst, it is only possible to normalize r when using Conclusions the second composite peak. The rate constant k is a measure of the specific activity. In this work a new method for quantifying the number According to this interpretation, it is possible to explain of CAS is presented. The method is applicable to the why the experiments of TK by Lizzios do not provide a carbon-gas reaction catalyzed by calcium. The deconvovalue of RSA capable of normalizing the variation of the lution of the TPD spectra after COz chemisorption at 573 reactivity with the conversion in the case of calcium. The K, by using a simple kinetic model, allows the normaliRSA is measured by switching the reaction gas, ((20,)in zation of the variation of the reactivity in COz and in this case, to an inert gas at the reaction temperature. In steam, with the amount of catalyst, and its evolution with this method it is logically assumed that the perturbation the sintering degree. introduced does not modify the system under study and The CAS obtained, which correspond to the calciumthat the decrease in CO with time permits an amount of carbon contact, are responsible for the catalytic activity, CO to be obtained, which Lizzio et al.l0J1have related with proving that not all of the external surface is effective for the number of oxidized sites in the carbon. In the case of the reaction. calcium, and at the temperatures used (953K in the case The fact that the normalization of the reactivity with of Lizzio), the particles of the catalyst are ~ a r b o n a t e d ; ~ ~ C 0 2 and steam is possible indicates that the CAS deterconsequently, when shifting to an inert gas, a decompomined from the TPD experiments participate in both resition of the carbonate will be produced, and consequently actions and that the CAS, determined in this way, are more an additional perturbation of the system under study is relevant for the understanding of the catalyzed reaction also present. The calcium carbonate will decompose, acthan the dispersion or external surface. cording to previous results,m* in part toward the interface Acknowledgment. We thank the DIGICYT (Proyecto and in part toward the exterior. The contribution in each No. PB 88-0295) and MEC for the Thesis Grant of D.C.-A. case will depend on the calcium dispersion and on its Registry No. C, 1440-44-0; COz, 124-38-9;Ca, 7440-10-2. contact with the carbonma In any case, the decomposition (33) Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. Anal. Qdm., in press.
(34) Freund, H. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1986, 30 (l),311-319.
