Energy & Fuels 1992,6,343-351
of 12C02in the steady state. The results of the TPR in l2C0 flow (Figure 7) indicate that 'TO2 can be formed with the oxidation of l2C0 in the gas phase by the sodium oxide. However, the peak temperature of 12C02desorption in the TPD was always slightly lower than that of l2C0. In the pulsed reaction, the position of 12C02satellite peak always appeared before the l2C0 satellite peak. These results indicate that 12C02 can be formed not only by the oxidation of gas-phase l2C0 by sodium oxide but by the oxidation of the 12C(0)-like surface complex by sodium oxide.
Conclusion Pulsed reaction and TPD techniques using 13C02suggested the existence of many different reaction paths in sodium-catalyzed C02gasification of carbon. In the catalyst oxidation step, two reaction paths proceed on two types of sites. One is a fast reduction of C02 on the dis-
343
persed sodium metal, and another is a slow oxidation of the sodium cluster through the cluster-C02 complexes. In the catalyst reduction step, two paths proceed on the same two kinds of site as catalyst oxidation. One is the fast oxidation of carbon with active oxidant, and the other is the slow oxidation of carbon by the sodium cluster oxide through two-step processes. The catalyst oxidation step affects the catalyst reduction step greatly. Under steady-state reaction conditions, the reaction proceeds with the combination of these fast and slow processes in which the fast paths proceed predominantly. The characteristics of carbon affect the reaction mechanisms, but the same reaction paths proceed in any type of carbon. Considerable I2CO2formation would be expected in the gasification of carbon in C02. Registry No. Na2C03,497-19-8;NaOH, 1310-73-2;graphite, 7782-42-5.
Alkali Metal Catalyzed C02 Gasification of Carbon Toshimitsu Suzuki,*vt Hiroyuki Ohme, and Yoshihisa Watanabe Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan, and Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka 564, Japan Received September 9, 1991. Revised Manuscript Received April 2, 1992
The mechanisms for C02gasification of carbon with alkali metal catalysts were investigated with pulse reaction and temperature-programmed desorption techniques. Details of oxygen-transfer reactions between oxidant and alkali metal, and between alkali metal oxygen complex and carbon, were discussed. It is suggested that two processes are involved in both catalyst oxidation and catalyst reduction steps. The two active sites, alkali metal cluster (or aggregated metal) and dispersed alkali metal which is a precursor for the cluster, exist for all alkali metals. On these sites, fast and slow oxidation and reduction cycles proceed. However, the characteristics of these sites are quite different among the alkali metals. The stability of cluster oxide on carbon decreased in the following order: Li > Na > K > Rb, and the reverse order afforded the decrease in the degree of interaction between metal and carbon. The reactivity of catalyst oxidation is the same order as that of the stability of cluster oxide, and the reactivity of catalyst reduction is essentially the same order as that in the degree of interaction with carbon. The stability of cluster oxide and the degree of interaction between the alkali metal and carbon were reflected to the mechanism of the steady-state gasification. Both the fast and the slow paths proceeded in Na- or K-loaded carbon. However, the slow path was not seen in Li-loaded carbon. Introduction Alkali metals are effective catalysts for H 2 0 or C02 gasification of carbon. However, the mechanism is still uncertain in spite of intensive research activities.'-" Until now, the following simplified oxygen-transfer processes have been proposed. MxOy+ COZ 4 MxOy+,+ CO (1)
-
MxOy+l+ Cf
MxOy+ CO
(2)
We have reported on the mechanism of sodium-catalyzed C02gasification on various carbons by using pulse and temperature-programmed desorption (TPD) techniques using labeled COPu Reactions involving oxidation t Kaneai University.
(eq 1)and reduction (eq 2) proceed via several different oxygen-containing sodium species. Two reaction paths in (1) Mckee. D. W. Fuel 1983.62. 170. (2) Yokoyba, S.;Miyahara, K.'; Tanaka, K.; Tashiro, J.; Takakura, F . J. Chem. SOC.Jpn. 1980, 6 , 974. (3) Kapteijin, F.; Abbel, G.; Moulijn, J. A. Fuel 1984, 63, 1036. (4) Wood, B. J.; Brittain, R. D.; Lau, K. H. Carbon 1986, 23, 73. (5) Mms, C.A.; Rose, K. D.; Melchior, M. D. J. Am. Chem. SOC.1982, 104, 6886. (6) Sams,D. A.; Shadman, F. AIChE J . 1986,32, 1132.
(7) Freriks. I. L. C.: van Wechem. M. H.: Stuiver. J. M.: Bouwman. R. Fuel 1980,60,463. (8) Cerfontain, M. B.; Moulijn, J. A. Fuel 1986, 65, 1349. (9) Huggins, F. E.; Shah, N.; Huffman, G. P.; Lytle, F. W.; Greegor, R. B.; Jenkins, R. G. Fuel 1988,67, 1662. (IO) Huhn, F.; Klein, J.; Juntgen, H. Fuel 1983, 62, 196. (11) Ratcliffe, C. T. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. '
1986, 30, 330. (12) Saber, J. M.; Falconer, J. L.; Brown, L. F. J. Catal. 1985,90,65.
0887-0624/92/2506-0343$03.00/00 1992 American Chemical Society
Suzuki et al.
344 Energy & Fuels, Vol. 6,No. 4, 1992
the oxidation step are as follows: (I) the adsorption of C02 on the dispersed metal having strong interaction with carbon (C,M) and its desorption resulting in CO formation (eq 3), and (11)the formation of sodium cluster complexes (MJO)) containing COz and CO, and their decomposition (eqs 4-6). Although the nature of the alkali metal cluster
(I)
C,M
+ COZ
(11)
-
[C,M-COz]
-
C,MO
+ CO
(4)
[Mx(0)-COzl+ [M,(O)O-COI
(5)
+ CO
(11’)
+ Cf
M,(O)O + Cf [M,(O)O-C,]
-
-
C,M
+ CO (or COz) (7)
[Mx(0)O-Cfl M,(O)
+ CO
(a)
40 -
20
(6)
was not elucidated,16it is considered to be aggregated alkali metal species on carbon, but not bulk alkali metal itself. The cluster complex in path I1 exhibited similar characteristics to that of sodium carbonate loaded on carbon. The reaction paths in (I) and (11) did not occur independently but they were closely related to each other. Part or all of C,MO in path I are very active for the oxidation of carbon and this active species are tentatively called “active oxidant”. Two paths in the reduction step are as follows: (1’) the fast oxidation of the carbon to give CO or C02by active oxidant on the dispersed metal (eq 7), and (11’) the slow oxidation by the metal cluster oxide (eqs 8 and 9). Path
(1’) C,MO (active oxidant)
-
~
(3)
+ COz + [MAO)-COzl [M,(O)O-CO] + M,(O)O
1-
-
20
-
. E .-c
-
i
10 -
(8) (9)
I’ had close relation to metal-oxygen complexes formed in path I and the reaction proceeded almost simultaneously with (I). Path 11’ showed a so-called satellite peak in the pulsed reactionz0and it seemed to consist of a two-step reaction. Little attention has been paid to the differences in the detailed mechanism of carbon gasification catalyzed with different alkali metal species.12J3 In this study, we apply pulae and TPD techniques using l3CO2to alkali metal catalysts (lithium,sodium, potassium, rubidium) loaded on various carbon species, in order to discuss the mechanism of alkali metal catalyzed carbon gasification. Experimental Section Materials. The alkali metal (lithium, sodium, potassium, and rubidium) loaded carbon samples were prepared by impregnation onto carbon from aqueous solutions of alkali metal carbonates or alkali metal hydroxides. Three coal chars (Yallourn = YL, Wandoan = WD,Hongay = HG), carbon black (Mitsubishi Kasei No. 30B = CB), and Graphite (Nacalai Tesque = GR) were used. Coals were demineralized using 6 M hydrochloric acid and 48% (13)Saber, J. M.; Kester, K. B.; Falconer, J. L.; Brown, L. F. J.Catal. 1988,109, 329.
(14)Chang,J.;Lauderback, L. L.; Falconer, J. L. J.Catal. 1989,122,
. A
LU.
(15)Cerfontain, M. B.; Agalianoe, D.; Moulijn, J. A. Carbon 1987,25, 351. (16) Cerfontain, M. B.; Kapteiin, F.; Moulijn, J. A. Carbon 1988,26, 41. (17) Mataukata, M.; Fujikawa, T.; Kikuchi, E.; Morita, Y. Energy Fuels 1989,3, 336. (18)Suzuki,T.;Ohme, H.; Watanabe, Y. Energy Fueb 1992, preceding paper in this issue. (19)MaKee, D.W.; Chatterji, D. Carbon 1975, 13, 381. (20)Suzuki, T.;Inoue, K.; Watanabe, Y. Energy Fuels 1988,2, 673.
200
400 600 Temperature
800 (OC)
1000
Figure 1. Temperature-programmed decomposition of alkali metal carbonate on carbon black carbon, 50 mg; Catalyst, 2.4 mmol/g of carbon; heating rate, 50 OC/min. (a) Li; (b) Ne; (c) K; (d) Rb. hydrofluoric acid. Coal chars were prepared by a heat treatment at 800 O C for 30 min under argon. The properties of carbon samples used in this experiment were described in a previous paper.18 The amount of catalyt on carbon was determined with atomic absorption spectroscopy (Jarrel Ash AA780) after extraction with hydrochloric acid. Isotope-labeled 13C02was prepared from Ba13C03(99% isotope purity) by acidifying it with perchloric acid. Experimental Procedures. Details of the apparatus and procedure of experiment have been published elsewhere.18p20p21 The apparatus was assembled by modifying a commercial gas chromatograph. A 50-mg sample of carbon was placed in a quartz reactor tube which was placed in a furnace between the sample injection port and d y z e r s (a quadrupole mass spectrometer and a gas chromatograph equipped with an activated carbon column). Prior to pulsed reaction, the sample was heat treated up to loo0 OC at a heating rate of 50 OC/min under helium (30 mL/min). A t a certain temperature, 13C02(1.0 mL, 41 pmol) WBB injected and the effluent gasea were analyzed by the computer-aidedmass spectrometer and the gas chromatograph. Mass spectra were obtained in every second from m/z = 20 to 50. The successive pulsed reaction was done by injecting 10 0.25-mL pulses every 5 s. A temperature-programmed desorption (TPD) or a temperature-programmed reaction (TPR) was carried out at a heating rate of 50 OC/min to lo00 OC and the sample was quenched (21)Suzuki,T.;Inoue, K.; Watanabe, Y. Fuel 1989,68,626. (22)Mims, C. H.; Pabst, J. K. Fuel 1983, 62, 176. (23)Marchon, B.; Carrazza, J.; Heinemann, H.; Somorjai,G. A. Carbon 1988, 26, 507.
-
Alkali Metal Catalyzed COP Gasification of Carbon
0
Energy & Fuels, VoE. 6, No. 4, 1992 345 4
2
I--.----
a
. -
A
v)
Ea
- 2 P)
I
m
K
0
2
0 3
Time 0
100 Time
(S)
Figure 2. Responses to W02pulsed reaction of alkali metal loaded carbon black at 800 OC: W02,41 wmol; carbon, 50 mg; catalyst; 2.4 mmol/g of carbon. (a) Li; (b) Na; (c) K; (d) Rb. immediately. The TPR was carried out in the flow of He + CO or He + COP
Results Decomposition of Alkali Metal Carbonates on Carbon Black. Figure 1 shows the temperature-programmed decomposition patterns of alkali metal carbonates on the carbon black. A broad desorption pattern of COP(2O(t-900 "C) and distinct desorption of CO above 700 OC with a shoulder peak at 950 "C were observed in all cases. In addition, a small amount of CO formation was seen at 300 "C except lithium. The amount of COPformed increased in the following order: Li < Na < K < Rb. The temperature of the peak top (temperature showing maximum desorption rate) decreased in the reverse order. Responses to 13C02Pulsed Reaction with Alkali Metal Catalysts. Figure 2 shows the responses to the 13C02pulsed reaction of the alkali metal loaded carbon black at 800 "C. When 13C02was pulsed, a large peak of 13C0and a small peak of l2COappeared on W02passing through the carbon bed and responses of 13C02and 12C02 were followed. In Li- and Na-loaded cases,a broad second peak of l2C0 (satellite peak)la was observed with tailing production of WO. The l2C0satellite peak of Na-loaded carbon black appeared earlier than that of Li-loaded case. In K- and Rb-loaded cases, the satellite peak of l2C0 was not observed, although remarkable tailing of l2C0 was observed.
(SI
Figure 3. Responses to 13C02 pulsed reaction of alkali metal loaded demineralized Wandoan char at 775 OC: (a) Li; (b) Na; (c) K. Conditions and keys are the same as in Figure 2. Figure 3 shows the responses to the 13C02pulsed reaction of alkali metal loaded Wandoan char at 775 "C. In Li- and Na-loaded cases, the l2C0 satellite peak was observed. In addition, the 12C02appeared as double peaks in the Na-loaded case. Hereafter, we also call the latter 12C02as a satellite peak. The amount of 12C02was small with Li but significant for K. The satellite peak of l2C0 was observed on all the carbon species loaded with Li and on all the carbons with Na except Yallourn char. The satellite peak of 12C02was observed only on Na-loaded Wandoan and Hongay chars. In K and Rb, no satellite peaks were observed irrespective of carbon species. Desorption Profiles in the T P D of Various Alkali Metal Catalysts. Figure 4 shows the TPD patterns of alkali metal loaded carbon black after pulsing 100 rmol of 13C02at 250 "C. TPD pattern was divided into four temperature regions in order to facilitate further discussion: region A (450"C), region B (450-550 "C), region C (550-900 "C), and region D (>900 OC), Sodium, potassium, and rubidium showed similar patterns. Two 13C02,13C0, and l2C0peaks and one small 12C02peak were observed. In region B, large sharp peaks of W02and ' T O were observed at the same temperature. In region C, the peak top temperatures of 13C02,13C0,12C02,and l2C0desorption increased in this order. In region D, one l2C0 desorption peak was observed. Lithium showed different TPD pattern. Small 13C02 and 13C0peaks were observed in region A but no clear peak was observed in region B. In region C, the desorption pattern was the same as those of other alkali metals but
Suzuki et al.
346 Energy & Fuels, Vol. 6, No. 4, 1992
10-
.........
/
0 , 200
Figure 4. W D spectra of alkali metal loaded carbon black after
13C02pulsed reaction at 250 OC: (a) Li; (b) Na; (c) K; (d) Rb. Conditions are the same as in Figures 1 and 2. Li
Na K
Rb 500
600
700
800 900 1000 I100
Temperature (OC) Figure. 5. Temperatures of peak top of TPD after '%02pulsed reaction a t 500 OC. Heating to 1050 OC. Other conditione are the same as in Figures 1 and 2. ( 0 )13C02,(A)13C0,(+) 12C02, (m, 0 ) W O .
the amount of C02 desorbed was very small. The temperatures of the peak top of carbon oxides desorption in various alkali metal loaded carbon black in regions C and D are shown in Figure 5. The peak top temperatures of all the carbon oxides in region C decreased in the following order: Li > Na > K > Rb. The peak top temperatures of desorbed game increased in the following order: W02C ' T O < 12C02< l2C0. The l2C0 peak in region C seemed to consist of two overlapped peaks. Thw two peaks were undoubtedly resolved in the case of Li but not in the case of Rb. The peak top temperature of l2C0 in region D did not depend on the nature of the alkali
-6'3
,*,...
....
..........c--,
400
600
..... (
800
I
I
1000
Temperature (OC ) Figure 6. TPR spectra of alkali metal loaded carbon black in 1.7% CO/He flow after 13C02pulsed reaction at 300 OC: (a) Li; (b) Na; (c) K. Conditions and keys are the same as in Figure 4. metal except in the case of Li. Similar TPD patterns were observed in alkali metal loaded different carbon species. However, in Wandoan and Hongay char, the peaks in regions A and B were not observed and the amount of CO formed in region C was emall. In graphite, no W-containing product was observed except a small broad desorption of 13C02under the reaction at 500 "C. The peak top temperatures of all the gaees in region C did not depend on carbon species. TPR Experiment in CO/He or C02/He Flow. Figure 6 shows the TPR spectra in 1.7% CO/He flow of alkali metal loaded carbon black after pulsing W02at 300 OC in pure helium. Desorption pattem was different from that in pure He and effects of catalyst were significant. In the Li-loaded case (Figure 6a), 13C0 desorbed at a lower temperature (750"C) than in a pure He flow, corresponding to the decrease in the concentration of l2C0 in the atmosphere. The desorption pattern is similar to that in the He flow. In the Na-loaded case (Figure 6b), both W02and I3CO peaks in region C shifted to lower temperatures (720and 780 "C). The amount of W O desorbed in region C decreased and the amount of desorbed 'W02 increased. The concentration of 'WOin the gas phase decreased in region C accompanied by the desorption of W O and W02. However, 'TO in the atmosphere seemed not to affect the pattern in region B. The TPR pattern of K-loaded carbon black was similar to that of the Na-loaded one, but the effect of the CO atmosphere was more significant (much lower desorption temperature of W O ) than that of Na (Figure 6c). Figure 7 shows the TPR spectra in C02/He flow after pulsing 13C02at 300 "C in pure He. Desorption profiles
Alkali Metal Catalyzed CO, Gasification of Carbon
Energy & Fuels, Vol. 6, No. 4, 1992 347
1
----
...--./.~.............-..--. o-
10
--*' I
I
n
I
200
400 600 Temperature
800 (OC)
1000
Figure 7. TPR spectra of alkali metal loaded carbon black in a C02/He flow after I3CO2pulsed reaction at 300 "C: (a) Li in a 1.6% C02/He flow; (b) Na in a 0.3% C02/Heflow; (c) K in a 0.3% C02/Heflow. Conditionsand keys are the same as in Figure 4.
were greatly affected with the presence of C02as compared to the CO/He flow. In the Li-loaded case, the desorption pattern was not affected by a 0.3% C02/He flow. However, in a 1.6% C02/He flow (Figure 7a), a large amount of 13C02desorbed at a lower temperature region showing the peak tops at 340 and 450 "C and these peak top temperatures were the same as thoee in the He flow. In region C, the amount of 13C0 desorbed was small but the peak top temperature did not shift as compared to that in the He flow. In region C, 13C02and 12C02disappeared simultaneously a t 720 "C and appeared again with an increase in the temperature. In the Na- or K-loaded case (Figure 7b,c), 13C02desorbed from 150 "C and ita peak in region C shifted to much lower temperature as compared to that in the He flow. However, the 13C02peak top temperature in region B was not affected by C02in the atmosphere. A very small amount of 13C0 desorbed but the peak top temperature was not affected. In the K-loaded case (Figure 7 4 , the TPR pattern in the C02 atmosphere was similar to that of Na-loaded case but the effect of COPin the atmosphere was much larger than that of the Na-loaded case similar to the effect of CO in the atmosphere. Successive Pulsed Reaction. It is obvious that several reaction paths could be involved in all the alkali metals loaded carbon, similar to Na-loaded case (eqs 3-9).lS However, it is difficult to determine which path proceeds in the steady-state gasification. In order to shed light on this, successive pulsed reaction was carried out to simulate quasi-steady-state reaction (Figure 8). The amount of Li loaded was reduced to half the amount of Na or K because
Time (S) Reaponses to 13C02succesaive pulsed reaction of alkt metal loaded carbon black at 800 " C (a) Li 1.2 mmol/g of carbon; (b)Na 2.4 mmol/g of carbon; (c) K 2.4 mmol g of carbon. 13C02, 0.25 mL (4.1 pmol) X 10 times. Other con itions are the same as in Figure 2 and keys as in Figure 4.
Figure
I
d:
0 ;
80
Y'
-
60
B O
:5
E.
40
-E, 20 Li
Na K CB
Li Na K YL
Li
Na K WD
Li
Na K HG
Figure 9. Amount of 13C0 formed per 100 pmol of catal st in the 'WO2 pulsed reaction at 800 "C. 66% equivalent of b o 2 to the amount of the catalyst on carbon surface was pulsed.
of the high reactivity of Li catalyst. At the initial stage of the reaction, only 13C0 and l2C0 were formed. After several 13C02pulses were injected, unreacted 13C02appeared together with distinct peaks of 12C02. The peak top positions of 13C0, l2C0, 13C02,and 12C02coincided with each other in every pulse injection. After 10 pulses of 13C02,a broad satellite peak or tailing production of l2C0 was observed. Effect of Catalyst and Carbon Species on the Reactivities of Alkali Metals. The results described above suggest differences in the reaction mechanism among Li, Na, K, and Rb. In the previous paper,18 we have shown the effect of carbon species on the reaction mechanisms. The differences in the nature of the carbon species are expected to affect the reactivities. The activities among
Suzuki et al.
348 Energy & Fuels, Vol. 6, No. 4, 1992
-D-
80
e
60
B
-
n
e
40 P)
I
5
20
0
Li
NE
CB
K
Li
NE
YL
K
Li
NE
WD
K
LI
NE
K
HG
Figure 10. The oxygen recovery in the l3COZ dsed reaction at 800 "C. Oxygen recovery was calculated by ([P2COI + [12COz1X 2)/[l3C0]. White area shows oxygen of 'VO and black area shows oxygen of 12C02.
catalysts and carbon species are compared in Figures 9 and 10 by using pulse technique. The comparison was carried out in the following way. Since Matsukata et al. showed that the catalyst only on the carbon surface was responsible for the reaction,l' the catalyst content on the carbon surface was measured by atomic absorption spectroscopy after extraction with 6 M hydrochloric acid. 13C02 amounting to 66% equivalent of alkali metal on the carbon surface was pulsed at 800 OC. The amount of 13C0 liberated per 100 pmol of catalyst was taken as a measure of the oxidizability of the respective alkali metal loaded on the carbon. The oxygen recovery as l2C0 and 12C02followed by the pulsed reaction was compared as a measure of catalyst reduction. The higher reactivity of the catalyst reduction would give a higher oxygen recovery within a limited reaction time. However, the value of oxygen recovery was varied depending on the amount of 13C0 formed if a constant reaction time is taken. Therefore, in this case, the time scale was standardized in proportion to the amount of 13C0produced. That is, the ratio of the time scale to the amount of 13C0was constant in any case. The oxidizability of catalyst increased in the following order: K C Na C Li among the alkali metals and HG C WD 5 YL = CB among carbon species. The ability of catalyst reduction increased in the reverse order for the alkali metals and was not affected with carbon species. However, the proportion of 12C02in the oxygen recovery increased in the following order: CB I YL < WD C HG. Summary of Results. Information obtained from pulsed reaction, TPD, TPR, and successive pulsed reaction are summarized in Table I.
Discussion Mechanism and Reactivity of Catalyst Oxidation. In the previous paper,18 we proposed the following mechanisms for the oxidation step of the sodium catalyst. The first path proceeds through the adsorption of COPon the dispersed sodium metal associated with carbon and its decomposition to give CO and metal oxide (path I). C,Na + COz == [C,Na-C02](a) C,NaO + CO (3) The second path proceeds via metal cluster-COzcomplex and metal cluster oxide-CO complex (path 111, based on the finding that these reactions are only observed at higher sodium metal loading.18 M,(O) + COz [M,(o)-Co2l(b) (4) [Mx(o)-co21(b) + [M,(O)O-CO](C) (5) [M,(O)O-CO](C) M,(O)O + CO (6) where M,(O) designates the metal cluster containing ox-
-
ygen.
Table I. Summary of Results methods catalyst results pulsed common first peaks and followed broad reaction formations of all the products satellite peak of l2CO Li Na satellite peaks of l2CO and W02 no satellite peak K, Rb TPD common peak top temperatures in region C W02 l3cO c Wo2 12co Rb < K < Na < Li Li no peak in region B Na, K, Rb large sharp peaks in region B TPR common shift to lower temperature of peaks in region region C desorption of W02 from low temperature in COz atmosphere suppression of the formation of WO in C Li smaller effect of atmosphere than Na and K Na, K small effect of atmosphere to the peaks in region B and large effect to the peaks in region C successive common exact responses of l2C0 and 12COzpeaks pulsed to the I3CO peak and broad formation reaction after injection
Path I corresponds to the first 13C0 peak in the 13C02 pulsed reaction and the desorption peaks in region B in the TPD. Path I1 corresponds to the tailing production in the pulsed reaction and the desorption peaks in region C in the TPD. Path I is a similar phenomenon as the CO overshoot in the step response experiment observed by Cerfontain et al.15 The analogous reaction patterns involving 13Ccontaining products were observed in cases of Li, K, and Rb. The mechanism for catalyst oxidation is considered to be similar to that of Na. However, several different aspects were also observed from the Na-loaded case. In the TPD of the Na-loaded case, the amount of 13C0 formed in region B was smaller than that of K or Rb. In the Li-loaded case, even the peak in region B was not observed. In region C, the pattern, where 13C0showed a peak at a higher temperature than 13C02peak, did not change among catalyta. However, the ratio of 13C0to '%02was much affected with the type of catalyst. The ratio of 13C0to 13C02desorbed in region C increased in the following order: Rb C K < Na C Li (Figure 4). The ratio of CO to C02 desorbed in alkali metal carbonate decomposition (Figure 1)and the peak top temperatures in region C (Figure 5 ) also incread in the same order. The order of Rb C K C Na < Li is the same order as that of increasing ionization potential or melting point of the alkali metals. Interaction between the alkali metals and the carbon seems to vary according to the type of alkali metal and to affect the oxidation and reduction paths. In the cases of K and Rb, the patterns of TPD and TPR were similar to those of Na-loaded case and the interaction between the metals and carbon is considered to be strong to form C,M complex. In the Li-loaded case, the interaction between the metal and carbon seems to be weak and the C,M complex in path I would not be formed. Although the interaction with carbon is weak, the dispersed metals for path I exist as a precursor to the metal cluster. In the TPR in a COZ/He flow, 13C02showed a considerable peak at 350 OC (region A) but very small I3CO2peak was observed in the TPD in a pure He flow. Deeorption of '%02in C02-He flow would have shown a peak in region C as the desorption of 13C0 in a pure He flow. The carbon-metal interaction also affects path 11. As described below, the metal cluster-COz complex decom-
Alkali Metal Catalyzed CO, Gasification of Carbon
Energy 6 Fuels, Vol. 6, No. 4, 1992 349
posed with the interaction with carbon. The degree of interaction between the metals and carbon affects the mechanism in different alkali metals. In the Li-loaded case, the [Mx(0)0-13CO]complex is stable and reaction 5 shifta right. By contrast, this complex is unstable in the rest of the alkali metals. Differences in alkali metals were clearly observed in TPR, where Na and K complexes were much affected by the atmosphere (CO/He or C02/He) but that of Li was not. From the results shown in Figure 9, reactivity toward oxidation of the alkali metals on carbon by C02 is in the following order: Li > Na > K. It corresponds to the stability of metal cluster complexes. The nature of carbon species also affects the mechanisms and reactivities of the oxidation of the alkali metals. As described previously, a higher temperature was required for the catalyst oxidation in Wandoan char, Hongay char, and graphite.18 Mechanism and Reactivity of Catalyst Reduction. l2C0 Formation. Reduction of sodium-oxygen species by carbon was reported to involve two reaction paths.18 One was the fast oxidation of carbon by active oxidant (path 1', eq 7) and the other was the two-step slow oxidation of carbon with the metal cluster oxide (path 11', eqs 8 and 9). Path I' corresponded to the first peak and path (1') C,MO(active oxidant) + Cf C,M + CO (or C02) (7)
(11')
M,(O)O
+ Cf
-
-
[M,(o)O-Cfl
[M,(O)O-C~l- MJO)
+ CO
(8) (9)
11' corresponded to the second satellite peak in the pulsed reaction. Path I' seems to be correlated to the path I in the catalyst oxidation over well-dispersed sodium metal on carbon. Path 11' would be correlated to the path I1 in the catalyst oxidation on the metal cluster on carbon. The response to 13C02pulsed reaction of Li-loaded carbon suggests the possibilities of the same paths as in the Na-loaded case. The pulsed reaction patterns of Kand Rb-loaded carbon had no satellite peak of l2C0, but the remarkable and long tailing production of l2C0 was observed. However, in the case of Na-loaded Yallourn char, only l2C0 tailing production was observed without a satellite peak.18 The TPD patterns in region C of K and Rb loaded cases were similar to that of Na. From these results, the same paths as in the Na-loaded case seem to be applicable to K- and Rb-loaded cases. The reason for the differences in the pulsed reaction pattern among various catalysts or among various carbon species is considered as follows: one is the difference in the reaction rates between eqs 8 and 9 and another is the difference in the stability of the metal cluster oxides. In order to understand the details of reactions 8 and 9, simplified kinetic studies were performed by a curvefitting method for the production of l2C0satellite peak. Firstorder kinetic equations (eqs 10 and 11)based on eqs 8 and 9 were used. In the numerical calculation, the following d[Mx(0)O-'2C]/dt = ki[M,(O)O]
(10)
d[12CO]/dt = k2[M,(0)O-'2C]
(11)
assumptions were made. The total amount of l2C0 produced in the satellite peak was taken for the initial concentration of a metal cluster oxide [M,(0)O]o which is estimated from the amount of 13C0produced minus that of l2C0 formed in the first peak. The first-order rate constant k2 was estimated from the logarithmic plot of l2C0 elution at the later stage of the l2C0 satellite peak against time. Li-loaded Yallourn char and Li- or Na-
130- 1
0.6
s
0 Time
(s)
Figure 11. Observed and simulated curve for '2CO satellite peak in the 13C02pulsed reaction of Li-loaded Yallourn char a t 800 OC. Li, 2.4 mmol/g of carbon; '%02, 41 pmol. (0) Observed total amount of l2C0in the satellite peak; ( 0 )observed rate of l2C0 in the satellite peak. Table 11. Simulated Rate Constants in the Alkali Metal Loaded Carbon Materials rate constant, s-l carbon metal 775 OC 800 "C 825 OC Yallourn char Li kla 0.025 0.046 0.075 k t 0.014 0.019 0.038 kc 0.022 0.036 0.060 Carbon Black Li klo 0.012 0.020 0.045 kzb 0.010 0.014 0.020 k' 0.010 0.021 0.032 Carbon Black Na kla 0.10 0.25 0.34 kZb 0.013 0.019 0.036 'k 0.035 0.063 0.090
kl,simulated rate constant of eq 7. kz,simulated rate constant of eq 8. 'k, experimental rate constant of 13C0tailing production.
loaded carbon black were subjected to calculation. Because 12C02production was very small in these cases and production of 12C02can be ignored without fatal error. Figure 11illustrates a typical example of the curve fib ting and Table 11shows the calculated rate constants from the simulation described above. Experimentally obtained fmt-order rate constants k of 13C0tailing production are also shown in Table 11. In the Li-loaded cases, rate constants kl and k2 were of the same order of magnitude but in the Na-loaded carbon black much larger kl than k2 was obtained. In both cases k2 was the same order of magnitude between Li and Na and much smaller kl was obtained in the Li-loaded case. These results indicate that the rate of eq 8 is much affected by the nature of the alkali metal. It can be easily understood that the l2C0 satellite peak was not observed due to the fast reaction of (8) for the K- and Rb-loaded cases. The carbon species seem to affect the rate of reaction 7 because Na-loaded Yallourn char did not give any satellite peak. The results in Table I1 show that the rate constants k of 13C0tailing production are smaller than kl. The 13C0tailing production is presumed to be due to the decomposition of the metal cluster [M,(0)0-13COl complex and this step is slower than the transfer of oxygen in the cluster to carbon. The fact that TPD peak top temperatures in region C increased in the order 13C02< 13C0 < l2C0 strongly supports above mechanisms. The details of path I' giving the first peak of l2C0 in the pulsed reaction are not clear. In the TPD, any distinct peak correspondingto path I' was not observed. The '%o first peak in the pulsed reaction always appeared simultaneously with the peak of 13C0 and did not show the
350 Energy & Fuels, Vol. 6, No. 4, 1992
Table 111. Carbon Sources and Oxygen Sources for 12C02 Formation carbon source oxygen source product W O formed in path I' dispersed metal oxide [C,MO] product %O formed in path 11' metal cluster oxide [M,(O)O] surface W=O complex formed by the C,MO in path I'
tailing. The 13C0 peak was ascribed to the oxidation of the dispersed metal. McKee proposed the possibility of formation of peroxide (M202)or higher alkali metal oxide in a flowing 02,from thermodynamic consideration.lg The fast oxidation of carbon may be caused either by the active oxidant like oxygen atom or by strong interaction of alkali metal with carbon to weaken C=C bond giving the M-0-C bond.22 In the TPD, l2C0 desorption around 950 "C was observed. This probably corresponds to the long tailing production of l2C0 followed by the satellite peak in the pulsed reaction. Most of oxygen in the metal cluster oxide transformed to carbon to give the l2C0 satellite peak or tailing production in the pulsed reaction. In the TPD, metal cluster oxide only gave desorption of W O in region C. Therefore, the oxygen for l2C0 peak in region D seems to originate in the path I in the catalyst oxidation. The peak temperature of l2C0in region D was not affected by the alkali metal species, in contrast to that in region C. Therefore, for this l2C0 desorption, the alkali metal would not be associated with the oxygen on carbon. Marchon et al. investigated the TPD of pure graphite after adsorption of 02,COP,and H20. They observed three CO desorption peaks at 700,820, and 980 "C.= Therefore, it is reasonable to consider that l2C0in region D corresponds to the peak at 980 "C reported by Marchon. This is considered to be due to the decomposition of the complex like C = O which was formed incidental to path I or path 1'. 12C02Formation. Formation of 12C02has been considered to be due to the oxidation of product CO with metal oxide.13 However, the results of the pulsed reaction suggest more complicated paths. Two 12C02production profiles were detected in the pulsed reaction. One was the fast formation showing the peak in accordance with the first l2C0 peak and the other was the slow formation showing a satellite peak or tailing production. In the TPD, the 12C02peak in region C is considered to correspond to the slow 12C02formation. The fast path showing the fmt peak in the pulsed reaction is considered to be formed by the active oxidant or weakened C 4 bond similar to the l2C0 formation described above. For the slower path of 12C02formation, three carbon sources and two oxygen sources can be considered and these sources are listed in Table 111. 12C02seems to be formed by the combination of them. In a TPR of Na- or K-loaded carbon black with 5% CO in He, a large amount of 12C02was formed (see Figure 6). However, the formation of 13C0 in region C which is the evidence of the formation of metal cluster oxide was very small in this experiment. These results show that the oxygen source is the dispersed metal oxide and carbon source is l2C0 from the gas phase in the TPR. In the pulsed reaction of Na-loaded Wandoan or Hongay char, the 12C02satellite peak eluted before l2C0satellite peak. The first-order rate constant of 12C02formation at the later stage of satellite peak formation was much larger than that of l2C0. These results show that l2C0 formed in path I' or path 11' is not a carbon source of 12C02satellite peak. For the 12C02satellite peak in the pulsed reaction,
Suzuki et al.
Table IV. Oxygen Balance in the Successive Pulsed Reaction Estimated from the Peak, the Peak Maximum, and Peak Minimum" Lib av of ratiosd Lic NaC KC D12CO/D13C0 0.40 0.30 0.28 0.28 T12CO/T13C0 0.34 0.29 0.37 0.35 B12CO/B13C0 0.28 0.29 0.45 0.38 0.39 0.13 D12C02/DW0 0.14 T'2COz/T13C0 0.33 0.21 0.22 B12C02/B'3C0 0.29 0.29 0.33 Conditions are the same aa in Figure 8. 1.2 mmol/g of carbon. 2.4 mmol/g of carbon. dThe ratios of 12CO/13C0and 12COz/13C0 were determined as in Chart I.
the oxygen source seems to be a metal cluster oxide and the carbon source seems to be a surface C=O complex. The following order of increasing oxygen recovery, Li < Na < K, reflects the decreasing order of stability of metal cluster complexes (Figure 10). By contrast, the selectivity for the l2C0formation showed the opposite order to the oxygen recovery within alkali metals. In every combination of carbon and oxygen sources described above, the role of the metal cluster is important. In the Li-loaded case, the small 12C02formation seems to be caused by the higher stability of the metal cluster. The order of oxygen recovery did not change in different carbon species but the selectivity for CO was much affected with the carbon species. In the TPD, the peak top temperatures were not affected by the characteristics of the carbon species. Therefore, the differences in the selectivity among carbons are considered to be caused by the different combinations of the carbon and oxygen sources. Proposed Gasification Mechanism in the Steady State. It was made clear by the pulse and the TPD techniques that two paths, fast and slow, exist in both the catalyst oxidation and catalyst reduction steps. However, it is important to know which path proceeds predominantly in the steady-state gasification. The synchronized responses of the 13C0and l2C0peaks to the W02peaks in the successive pulsed reaction (Figure 8) show that the fast paths certainly occur on all the catalysts. The slow path was observed clearly after 10 successive l3Co2pulses as evidenced from the formation of large amounts of tailing productions or satellite peaks of 'TO. Table IV depicts the average ratios of l2C0 and W02 production rates against 13C0production rate calculated from the peak shape in the successive pulsea (from the fifth pulse to the tenth pulse). In the single-pulsed reaction at about 700 "C, only fast oxidation proceeded as shown in Figure 12. In these reactions, the ratios of (l2C0 production rate) / (l3CO production rate) measured every second were the same at any point from the appearance of the l2C0 peak going through the peak top to the peak bottom. If only the fast paths occurs, the three ratios, "delta, top, bottom" (see Table IV), in the successive pulsed reaction measured at three different stages in changing concentration with W02 pulse should be the same. If slow paths proceed in the quasi-steady-state condition, the ratios would vary with the progress of the reaction. In the Li-loaded case, the ratios of d[12CO]/d[13CO]and d[12C02]/d[13CO]decreased from "delta" to "bottom" or
Alkali Metal Catalyzed CO, Gasification of Carbon
Energy & Fuels, Vol. 6, No. 4, 1992 351
13C02waa swept out. The results of TPR in CO/He or C02/He flow also suggest above discussion. The variation in the ratio of ‘2C02/’3C0from “delta” to “bottom” corresponded to those of 12CO/13C0in all cases. This indicates that 12C02formed via a similar combination of path I’ and 11’ to those of l2C0 in the steady state. As described above, the fast path through dispersed metal and active oxidant is important for the gasification of carbon. In this path, l2C0 and 12C02formations, which had been considered to be the rate-determiniig step of the gasification, are not the rate-determining steps.18 The key point to promote the reaction seems to be the formation of active oxidant.
Time
(0)
Figure 12. Responses to 13C02pulsed reaction of Li-loaded carbon black. Reaction temperature: (a) 675 OC, (b) 700 OC, (c) 725 O C . Conditions and keys are the same as in Figure 2.
did not change. This indicates that path I1 of the catalyst oxidation proceeded to some extent but path 11’ of the catalyst reduction did not proceed in the steady state. In the Na- or K-loaded cases,the ratio increased from “delta” to “bottom”. This indicates that path I1 proceeded to a larger extent and path 11’proceeded to some extent. Path I1 is faster than path 11’ as shown in Table I but the suppression of the slow paths is considered to occur with the effect of C02and product CO in the gas phase, because the l2C0 satellite peak always appeared after unreacted
Conclusions The mechanisms of C02gasification of carbon catalyzed by alkali metals were investigated with pulse and TPD techniques using 13C02. The catalytic mechanism is quite similar for all alkali metals. The catalyst oxidation involves two processes on two sites. One is the dispersed alkali metal and the other is alkali metal cluster. The dispersed alkali metal is rapidly oxidized to give 13C0and the alkali metal cluster could be oxidized slowly. The catalyst reduction involves two processes corresponding to the two processes in the catalyst oxidation. However, the details of the catalyst oxidation and catalyst reduction depend on the alkali metal species. The degree of interaction of the metal or the metal cluster oxide with carbon increased in the following order; Li < Na < K < Rb. The stability of the metal cluster oxides increased in the reverse order. The activity of the catalyst oxidation decreases in the order Li > Na > K Rb and the activity of the catalyst reduction decreased in the reverse order. The carbon species also affect the reaction paths and reactivity. In the steady state, the faster paths on the dispersed metal through active oxidant proceed predominantly and the slower paths on the metal cluster partly contributes except for Li-loaded carbon. Registry No. C,7440-44-0;Li, 7439-93-2;Na, 7440-23-5;K, 7440-09-7; Rb, 7440-17-7; COZ, 124-38-9;CO,630-08-0.
