Energy & Fuels 1994,8, 649-658
649
Mechanisms of Alkaline-Earth Metals Catalyzed CO2 Gasification of Carbon Toshimitu Suzuki,*p+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, 564, Japan Received September 2,1993. Revised Manuscript Received January 24, 1994"
The mechanism of COZgasification of carbon catalyzed with alkaline earth catalyst (Ca, Sr, Ba) was investigated with pulsed reaction and temperature programmed desorption techniques using labeled C02. Several reaction paths similar to those of alkali metals were observed in both the oxidation of the catalyst and the reduction of the catalyst. Dispersed alkaline earth metal oxide and metal oxide cluster or alkaline earth metal cluster containing a large amount of oxygen were the main surface species. The catalysts oxidation by COZand reduction with carbon proceed on these sites but characteristic features of these sites were quite different between Ca and Ba or Sr. The reaction patterns of Ba- and Sr-loaded carbon were very similar to those of Li-loaded one. A weak interaction between the metals and carbon and strong interaction between the metals and oxygen in the cluster are suggested for Ba and Sr. The reaction patterns of Ca were quite different from those of Ba and Sr and strong interaction between the cluster and carbon is suggested.
Introduction
It is well-knownthat alkaline-earth metals are effective catalysts for gasification of carbon. Many intensive researches have been carried out to investigate the catalytic mechanism.l-17 In these studies, two types of oxidationreduction mechanisms were proposed. One proceeds via alkaline earth c a r b ~ n a t e . ' - ~ J ~
-
MO + C02
MCO,
(1)
+ Cf
MO + 2CO
(2)
MCO,
The other proceeds via peroxide type metal oxide.2~5~9J1J2
MO + CO,
+
MOO + CO
(3)
MOO + Cf -,MO + CO (4) In a recent series of studies, Solano and co-workers proposed more detailed two-step mechanisms involving surface oxide and carbonate species.15J6 t
Kansai University.
* Abstract published in Advance ACS Abstracts, March 1, 1994.
(1) McKee, D. W. Carbon 1979,17,419-425. (2) Seare,C. L.;Muraldihara,H. S.; Wen,C. Y.Znd.Eng.Chem.Process. Des. Dev. 1980, 19,358-364 . (3) McKee, D. W.; Yates, J. T., Jr. J. Catal. 1981, 71, 308-315. (4) Rao, M. B.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1983,21,401407. (5) Radovic, L. R.; Walker, P. L., Jr. Jenkins, R. G. J. Catal. 1983,82, RRP-RQA ---. (6) Casanova, R.; Cabrera, A. L.; Heinemann, H.; Somorjai, G. A. Fuel 1985,62, 1138-1144. (7) Baker,R. T. K.; Lund, C. R. F.; Chludzinski, J. J., Jr. J.Catal. 1984, 87,255-264. (8) Freund, H. Fuel 1986,65,63-66. (9) Kapteijn, F.; Porre, H.; Moulijn, J. A. AIChE J. 1986,32,691-695. (10) Ersolmaz, C.; Falconer, J. L. Fuel 1986,65,400-406. (11) Chang, J.-S.; Adcock, J. P.; Lauderback, L. L.; Falconer, J. L. Carbon 1989,27, 593-602. (12) Zhang, 2.-G.; Kyotani, T.; Tomita, A. Energy Fuels 1988,2,679684. (13) Huggins,F. E.; Shah, N. E.; Huffman, G. P.;Lytle,F. W.; Greegor, R. B.; Jenkins, R. G. Fuel 1988,67, 1662-1667.
---
An oxygen-transfer mechanism has also been proposed for gasification of carbon with alkali-metal catalyst. However, our previous studies have demonstrated that the gasification process with alkali metal involved more complicated reaction paths.20121 In those studies, the pulsed reaction technique using W02afforded useful information in combination with temperatureprogrammed desorption (TPD) technique. In this paper, we have extended pulsed and TPD techniques to understand mechanisms of alkaline-earth metal catalyzed COz gasification of various carbon species.
Experimental Section Materials. Activated carbon (Norit A) (AC), carbon black (Mitaubishi Kasei No. 30B) (CB),and coal char (frompulverized acid washed Yallourn coal) (YL)were used as carbon sources. For activated carbon and Yallourn coal (100-200 mesh), demineralization was carried out using 6 M hydrochloric acid and 48% hydrofluoricacid. The surface areas (BETmeasured in N2 at 77 K)of pure carbon were 920 m2/g (activated carbon), 260 m2/g (Yallourn char),and 90 m2/g(carbon black). Catalysts (Ca, Sr, Ba) were impregnated onto activated carbon, Yallourn coal, and carbon black fromaqueous solutionsof alkaline-earthacetate (Sr,Ba) or nitrate (Ca). Coalchar was prepared by heat treatment (14) Matukata, M.; Fujikawa,T.; Kikuchi, E.; Morita, Y.EnergyFuels 1990,4, 365-371. (15) (a) Joly, J. P.; Cazorla-Amoros,D.; Charcosset, H.; Linares-Solano,
A.; Marcilio, N. R.; Martinez-Alonso, A,; Salinas-Martinezde de Lecea, C. Fuel 1990,69, 878-884. (b) Cazorla-Amoros,D; Linares-Solano, A,; Salinas-Martinezde de Lecea, C. Carbon 1991,29, 361-369. (16) (a) Linares-Solano, A.; Almela-Alarcon, M.; Salinas-Martinez de Leacea C. J . Catal. 1990,125,401-410. (b)Cazorla-Amoros,D.; LinaresSolano, A.; Salinas-Martnez de Lecea, C.; Joly, J. P. Energy __ Fuels 1992.
6, 287-293. (17) Yamashita, H.; Nomura, M.; Tomita, A. Energy Fuels 1992, 6, 656-661 (18) Suzuki, T.; Inoue, K.; Watanabe, Y . Energy Fuels 1988,2,673679. (19) Suzuki, T.; Inoue, K.; Watanabe, Y . Fuel 1989,68,626-630. (20) Suzuki, T.; Ohme, H.; Watanabe, Y . Energy Fuels 1992,6,336343. (21) Suzuki, T.; Ohme, H.; Watanabe, Y . Energy Fuels 1992,6,343351.
0887-0624/94/2508-0649$04.50/00 1994 American Chemical Society
Suzuki et al.
650 Energy & Fuels, Vol. 8, No. 3, 1994
37
1
4
'i
-------- 1
3
'
7
\
co
l2
4
Q)
.L
m
U
,
1i
U
If4
1
I I
a
2
0
:~, A -
12co x 2
..........
--------
I \
1.: 1
.-.
.
(c)
'%A
/LI
13co '2c02 x 2 1 3 ~ 0 2
----200
'
400 ' 600 ' Temperature
800
'
1000
'
(OC )
Figure 2. TPD spectra of alkaline-earth-metalloaded Yallourn chars in He flow after pulsing 13C02at 100 "C: (a) Ca, (b) Sr, (c) Ba. Heating rate: 50 "C/min. Other conditions are the same as in Figure 1.
Results Effect of Catalyst in Pulsed Reaction and TPD. The responses to the pulsed reaction (at 800 "C) and TPD spectra (after pulsing W02at 100 "C) of the Yallourn char loaded with Ca, Sr, and Ba are shown in Figures 1 and 2. In Sr- and Ba-loaded chars, similar responses and TPD patterns were observed but in the Ca-loaded char different response and TPD pattern were observed from those of Sr or Ba. In the pulsed reaction of Ca-loaded case (Figure 1a), 13C02and 13C0showed peaks with shoulder, respectively, and 12C02 exhibited a broad peak. 12CO appeared as a sharp peak followed by a gradual decrease. In addition, a so-calledsatellite peak appeared.la21 (Clear features of shoulder peaks and satellite peak are shown in Figure 11.) By contrast, in the cases of Sr and Ba (Figure 1, b and c), 13C0showed a sharp peak with a long tailing and l2C0 showed a sharp peak in accordance with the 13C0peak and a very broad satellite peak. Such patterns of Sr and Ba were similar to those of Na- or Li-loaded
carbon.21 In these experiments, 41 pmol of 13C02,which was 66% of catalyst metal content, was pulsed. The amount of 13C0 formed in the pulsed reaction at 800 "C in the Sr- or Ba-loaded case was 38.3 and 32.5 pmol, respectively, but that in the Ca-loaded case was only 18.0 pmol. In the TPD of Ca-loaded char (Figure 2a), peaks were observed above 500 "C except a small 13C02peak at 280 "C. W02showed a peak at about 600 "C and W O showed at 680 "C. "4202 showed a small peak at 690 "C. '2CO showed peaks a t 710 "C and at lo00 "C. The order of peak top temperatures (13C02 < 13C0 = l2CO2 < l2C0) was the same as those of alkali metal loaded case.20r21The peak of l2C0 a t 710 "C was sharp (peak width at halfheight was 60 "C) as compared with those of I3CO and W02(peak width was 150 "C). In the TPD of Sr and Ba (Figure 2, b and c), two small peaks of 13C0were observed in the low-temperature region and one large broad peak was observed at about 750 "C. l2C0showed peaks at 820 "C and 1000 "C. In these cases, C02 formation was not observed although the amount of I3CO2introduced before TPD was sufficient. In the case of Sr, W O continued to desorb until higher temperature region. TPD patterns of Sr and Ba were similar to that of Li-loaded carbon.21 Effect of Carbon Species. The pulse responses and TPD patterns of Ca-loaded three different carbon species are shown in Figure 3 (pulsed reaction at 800 "C)and Figure 4 (TPD after pulsing at 500 "C). The pulse responses of AC (Figure 3a) was similar to that of Caloaded YL (Figure 3b). A small l2C0 sattelite peak was not observed clearly owing to the broad l2C0peak. l2CO2 satellite peak was slightly observed at 35 s. In CB (Figure 3c), the patterns were similar to those of AC and YL but reactivity was very small (large unreacted 13C02peak).
COz Gasification of Carbon
0
20 Time
Energy & Fuels, Vol. 8,No. 3, 1994 651
40
60
Figure 3. Responses to 13C02 pulsed reaction of Ca-loaded various carbons at 800 "C: (a) activated carbon, (b) Yallourn char, (c) carbon black. Other conditions are the same as in Figure 1
L. "1
-
(a)
4-
2-
E
(b)
-
O
1-
m
R
-
4-
200
400 600 Temperature
SO0
0
100
Time
(S)
1000
(OC)
Figure 4. TPD spectra of Ca-loadedvariouscarbonsafter pulsing WO2at500 O C : (a)activated carbon,(b)Yallourn char, (c)carbon black. Other conditions and legends are the same as in Figure 2. The amount of W O formed in the pulsed reaction increased in the following order: CB < YL < AC. TPD patterns were also affected by carbon sources. In the TPD of YL (Figure 4b), which was recorded after l3C02 pulsing at 500 "C, the amounts of 13C02and l2CO2 desorbed were larger than those measured after pulsing
200
(s)
Figure 5. Responses to 13C02pulsed reaction of various Baloaded carbons at 800 "C: (a) activated carbon, (b) Yallourn char, (c) carbon black. Other conditionsare the same as in Figure 1and legends are as in Figure 3. at 100 "C. In this case, l2CO2showed two peaks and the temperature of the first I2C02peak agreed with the 13C02 peak and the second l2CO2 peak appeared at the same temperature as l2C0peak. In the TPD of AC (Figure 4a), 13C0and 13C02showed desorption peaks a t about 700 "C with a shoulder peak. Desorption patterns of W02 and W O were similar to those of YL. In the TPD of AC after pulsing 13C02at 100 "C (not indicated here), no peak was observed below 500 "C and '3CO showed clearly two peaks at 640 "C and 700 "C. In the TPD of CB (Figure 4c), a large 13C02peak and avery small 13C0peak were observed. l2C02showed very small peak at the same temperature as 13C02and continued to desorb up to 880 "C. No l2C0 peak was observed except the W O intrinsic to the carbon black. In the TPD of CB after pulsing 13C02at 100 "C (not indicated here), one more 13C02peak was observed at 550 "C, and 13C0, l2C02, and l2C0 desorption peaks were not observed. The pulse responses and TPD patterns of Ba-loaded three different carbon species are shown in Figure 5 (pulsed reaction at 800 "C) and Figure 6 (TPD after pulsing W02 at 100 "C). The pulse responses of AC and CB (Figure 5, a and c) were similar to that of Ba-loaded YL (Figure 5b). In AC, W O did not show a satellite peak although broad l2C0 formation was observed. The TPD of AC (Figure 6a) was similar to that of YL (Figure 6b). In the TPD of CB (Figure 6c), the desorption of W O in the low-temperature region was not observed and the peaks of l3CO and W O at about 800 "C were very small. However, the TPD of CB after pulsing at 500 "C (not indicated here) showed the same pattern as those of AC and YL. Effect of Catalyst Loading Level. The pulse responses and TPD spectra of Yallourn char at various Ca loading levels are shown in Figure 7 (pulsed reaction at 800 "C) and Figure 8 (TPD after pulsing at 500 "C). Since the reactivity of Ca-loaded char toward 13C02 is low as
Suzuki et al.
652 Energy &Fuels, Vol. 8,No. 3, 1994
1
i
-200
Temperature
(OC
I
50
Time
t
8
8
100 (5)
Figure 7. Effect of Ca loading level in the WOz pulsed reaction of Yallourn char at 800 OC. Catalyst level: (a) 2.4, (b) 1.2,(c) 0.6 mmol/g of carbon. Other conditions are the same as in Figure 1 and legends are as in Figure 3. compared to Ba or Sr-loaded char, W02was pulsed at 500 "C instead 100 "C before TPD. The patterns of pulse response and TPD spectra were not affected with loading levels. Three peaks (at 10,22,and -40 s) are observed
-
'
400
'
600
Temperature
)
Figure 6. TPD spectra of Ba-loaded various carbons pulsing after W02at 100 OC: (a) activated carbon, (b) Yallourn char, (c) carbon black. Other conditions and legends are the same as in Figure 2.
0
n I
(c)
1
2
800
'
7
1000
(OC)
Figure.8. Effect of Ca loading levelon TPD spectra of Yallourn char after pulsing W02at 500 "C. Catalyst level: (a) 2.4, (b)1.2, (c) O.Gmmol/g of carbon. Other conditions and legends are the same as in Figure2. for l2C0 production, although the third peak was not distinct in the reaction with the Ca loading level of 0.6 and 2.4mmol/g of carbon. In the TPD, with increasing catalyst content, the amount of desorption products below 800 "C increased. At a loading level of 0.6 mmollg of carbon, only one l2CO2peak was observed at the same temperature as that of l2C0. Effect of loading level of Ba-loaded carbon black on the pulsed reaction and TPD profiles was small similar to the cases of Ca (Figures 9 and 10). In the pulsed reaction of CB with a high loading level of Ba, the first peaks of 13C0 and l2C0 were very small (Figure 9a, note the expanded scale of the ordinate), and tailing production of 13C0 and satellite peak of l2C0 were predominant. In TPD the peaks of 13C0and l2C0at about 870 "C became large with increasing catalyst content. However, the amount of 12CO formed above lo00 "C did not change. Effect of Reaction Temperature. Effect of reaction temperature on the pulse response was shown in Figure 11 (Ca-loadedYL) and Figure 12 (Ba-loadedYL). In case of Ca, the first peaks appeared at the same position irrespective of the pulsed reaction temperature. However, the broad 13C0,13C02,l2C0 (flat part in the pattern), and "4202 responses approached to the first peak with an increase in the reaction temperature. The l2C0 satellite peak (750"C a t 130 s) also shifted to the front end (800 "C 40 s). Similar changes in the patterns with the variations in the pulsed reaction temperature were observed in the Ba-loaded case. Figure 13 shows the TPD of Ca-loaded Yallourn char after pulsing 13C02at 775 "C followed by immediate quenching. In addition to the peaks a t around 700 "C, a sharp W O peak was observed a t 810 "C. This W O peak was considered to correspond to the ' T O satellite peak in the pulsed reaction.
COPGasification of Carbon
Energy & Fuels, Vol. 8, No. 3, 1994. 653
'*co
0.3
x 2
50
0 Time 0
4
200
Time
(5)
Figure 9. Effect of Ba loadinglevel in the 13C02pulsed reaction of carbon black at 800 O C . Catalyst content: (a) 2.4, (b) 1.2, (c) 0.6 mmol/g of carbon. Other conditions and legends are the same as in Figure 1.
Figure 11. Effect of reaction temperature in the W02pulsed reaction of Ca-loadedYallourn char: (a) 800, (b) 775, (c)750 "C. Other conditions and legends are the same as in Figure 1.
O
Time
T
200
,
,
,
400
Temperature
(S)
Figure 12. Effect of reaction temperature in the l*COz pulsed reaction of Ba-loadedcarbon black (a) 825, (b) 800, (c) 775 OC. Other conditions and legends are the same as in Figure 1.
, 600
160
80
"0 6-
100 (s)
800
1000
(OC)
Figure 10. Effect of Ba loading level on TPDspectra of carbon black after pulsing W02at 100 "C. Catalyst level: (a) 2.4, (b) 1.2, (c) 0.6 mmol/g of carbon. Other conditions and legends are the same as in Figure 2.
TPR Experiment in CO/He and C02/He Flow. Figure 14 shows the T P R spectra of Ca- or Ba-loaded activated carbon in 6.3% CO in an He flow. When Caloaded carbon was used after pulsing W02at 500 OC
(Figure 14a), broad desorptions of 13C0 and 13C02were observed a t around 650 OC. This temperature was 100 "C lower than that in the T P D in pure He (Figure 4a). l2CO2 also desorbed at a lower temperature and the amount of l2C02 desorbed at 670 OC (the first peak) increased significantly as compared with that in pure He (compare with Figure 4a). The decrease in the l2C0 concentration at around 600 "C corresponded to the desorption of W 0 2 , 13C0, and l2CO2. Judging from a slight increase in the
654
Suzuki et al.
Energy & Fuels, Vol. 8, No. 3, 1994
-
10-
-----
..........
.--------
*......**..............
. 200
400 600 Temperature
800 )
1000
(OC
Figure 13. TPD spectraof Ca-loaded Yallourn char after pulsing W O 2 at 775 O C followed by immediate quenching. Other conditions and legends are the same as in Figure 2.
68
200
64
400 Temperature
(OC)
Figure 15. TPR spectraof alkalime-earth-metalloaded activated carbon in 1.9% COdHe flow after pulsing W O 2 at 500 "C: (a) Ca, (b) Ba. Other conditions are the same as in Figure 2.
0 200
-
e-#--*\. I
400
600
Temperature
800
1000
1
(OC)
Figure 14. TPR spectraof alkaline-earth-metalloaded activated carbon in 6.3% CO/He flow: (a) Ca-loaded carbon after W02 pulsing at 500 "C, (b) Ba-loaded carbon after pulsing W02at 200 O C . Other conditions and legends are the same as in Figure 2.
"WO concentration at about 800 "C, the amount of l2C0 from carbon seemed to be very small. In the Ba-loaded case (Figure 14b), after pulsing 13C02 at 200 "C, small desorption peaks of 13C0 were observed at 280,400,550, and 830 "C. Peaks of 13C0at 280 "C and 400 "C were the same temperatures as those in the TPD in pure He (Figure 6a). The 13C0 desorption at 550 "C was considered to correspond to the l3CO desorption at about 850 "C in pure He. The reason for this is that the desorption of 13C0 shifted to higher temperature region when the TPR was carried out under lower l2C0 concentration. A decrease in l2C0 concentration at about 500 "C corresponded to the desorption of W O (CO exchange from solid surface and gas phase). The increase in the ' T O concentration above 850 "C is the desorption of 12CO from the carbon. This was observed at the same temperature in a pure He flow and desorption of l2CO2 was not observed. During cooling down after heating to 1000 "C, a large amount of l2C0 in the gas phase was trapped on the catalyst judging from the decrease in the concentration of 'WO. This phenomenon was not observed in Ca-loaded case.
Figure 15 shows the TPR spectra of Ca- or Ba-loaded activated carbon in a 1.9% COZin He flow after pulsing l3COZat 500 "C. In the Ca-loaded case (Figure 15a), a large amount of l3COZdesorbed at a lower temperature region, although the l3CO2was reacted at 500 "C. Five 13C02desorption peaks were observed at 140, 340, 450, 600, and 760 "C, respectively. In the TPD in a pure He flow after 13C02 pulsing at 500 "C (Figure 4a), only two overlapped peaks were observed at 710 and 770 "C. l3CO showed a peak at 780 "C although two overlapped peaks were observed in a pure He at 710 and 770 "C. The concentration of W O Z in the gas phase decreased corresponding to the l3CO2desorption. In addition, l2C0z showed two peaks at 750 and 810 "C. Desorption of W O at 800 "C and at 1000 "C was similar to those in a pure He but the amount of l2C0desorbed at 800 "C was smaller than that at 1000 "C. In the Ba-loaded case (Figure 15b), l3CO2desorbed at 160,300,420,and 600 "C, though the 13C02treatment was carried out at 500 "C. The 13C0peak was observed at 840 "C and this peak top temperature was the same as that in a pure He. The concentration of WOZ in the flowing gas decreased corresponding to the l3COZdesorptions, and 12CO~ was consumed for gasification of carbon above 700 "C. l2C0 showed overlapped peaks at 870 and 910 "C. Successive Pulsed Reaction. To understand the reaction mechanism at steady-state, successive pulse reaction was carried out for Ca or Ba loaded Yallourn char (Figure 16). In Ca-loaded case (Figure 16a), the so called overshoot20,21of 13C0 and 12CO was observed until the third pulse injection and then quasi steady-state was achieved. The responses of 12CO~ peaks corresponded to the 13C0 peak position but the peak position of the responses of l2C0against those of 13C0were not distinct. After the 10 pulses of l3COZ,the concentrations of CO and COZ decreased sharply except 'WO. l2C0 decreased gradually and exhibited a satellite peak. In the Ba-loaded case (Figure 16b),CO overshoots were observed but they were not significant as compared to the case of Ca. A t the quasi steady-state conditions, the
Energy & Fuels, Vol. 8, No. 3, 1994 655
CO, Gasification of Carbon
the pulsed reaction and TPD (Figures 1 and 2). This indicates the similarity in the reaction mechanisms for Ba- and Sr-loaded carbon. The pulsed reaction, TPD, and TPR patterns of Ba and Sr-loaded carbon were quite similar to those of the Li-loaded case. A reaction mechanism similar to that of the lithium loaded case is considered for Ba and Sr.20121 We proposed the following catalytic path ways for C02 gasification with alkali metals consisting of two types of paths:
fast path
Q,
.c.
(II
a
C,M
+ *COz-
[C,M-*C02]
1
C,MO
+
[C,M-*C02] C,MO
(5)
+ *CO
(6)
+ Cf---+C,M + CO (or CO,) + C(0)
(7)
+ *COz F! [M,(O)-*COZ]
(8)
slow path M,(O)
0 0
50 Time
100
(s)
[M,(O)-*COJ
F i g u r e 16. Responses to W02successive pulse reaction at 800 O C : (a) Ca-loaded Yallourn char, (b) Ba-loaded Yallourn char. W02:0.25 mL X 10. Other conditions are the same as in Figure 1 and legends as in Figure 3.
responses of "4202 against 13C0peaks were clear but those of "CO were not clear. In addition, the shapes of '3CO peak was not steep as compared with those of Ca. The pattern after 10 W02pulses was similar to that of the single-pulse reaction. Summary of Results. The results of pulsed reaction, TPD, TPR, and successive pulsed reactions are summarized in Table 1. For comparison, the results for alkalimetal-loaded cases are also included.
Discussion Catalytic Mechanism with Barium and Strontium Catalysts. Ba and Sr exhibited quite similar patterns in
[M,(O)O-*CO]
F!
+ Cf
--*
[M,(O)O-*COl
[M,(O)O-Cf]
+ *CO
(9)
(11)
where *C indicates the carbon introduced from reaction gas. In the fast paths, CnM designates well-dispersed metal speciesstrongly interacting with carbon and CnMO denotes the active oxidant. The active oxidant oxidizes carbon very fast and produces CO, COZ,and C(0) surface complex. In the slow paths, M,(O) shows a metal cluster containing a small amount of oxygen and M,(O)O shows a metal cluster oxide. C02 interact with metal cluster and cluster oxide complex forms. The decomposition of cluster
Table 1. Summary of Results and Comparison with Alkali Metal in t h e Pulsed Reaction with 'FOz, T P D after Pulsing WOz, and T P R typical responses of carbon oxides" Ca
alkali metals (Li, Na, K,Rb) first sharp peaks followed pulsed reaction with WO2 first sharp peaks (all carbon oxides) first sharp peaks followed broad formation and l2C0 satellite peak by broad formation and followed by broad formation 12CO satellite peak for Li and Na with W O broad satellite peak and W 0 2 satellite peak for Na smaller W02formation than Ca large sharp peaks of W O and no peaks below 500 "C and TPD after pulsing 13C02 small l3CO peaks below 500 "C and WO2 at around 300 OC and large peaks of W02,W O , large W O and W O peaks 500 OC, and peaks of WO2, 12C02 and W O above 500 OC above 500 OC W O , 12CO2,12COover 500 "C no W02and l2C02peaks large effect of carbon species large effect of carbon species and small effect of carbon species and and catalyst content small effect of catalyst content catalyst content large amount of W02desorption desorption of W02from low desorption of 13C02from low TPR in W O or W 0 2 in low-temperature region and temperature and suppression temperature and suppression of suppression of 13CO formation of 13CO formation in W O formation in in COZatmosphere COz atmosphere CO2 atmosphere no effectfor the-peaks below formation of W O in low-temperature large amount of W 0 2 500 oc formation in CO atmosphere region and no W 0 2 formation in CO atmosphere exact responses of 12CO and exact responses of W 0 2 peak and Successive pulsed reaction exact responses of l2C02peak and delayed responses of W O peak 1 2 C 0 2 peaks in W O peaks delayed responses of '2CO and to W O peaks W O peaks broad formation after the no broad formation after the continued broad formation after last W02injection last W02injection the last 13C02injection Ba and Sr
~~
a
When no response is specified, it corresponds to several carbon oxides.
Suzuki et a1.
656 Energy & Fuels, Vol. 8, No. 3, 1994
complexes indicated in reactions 10 and 11 occurs with slow interaction of the cluster with carbon.20121 However, the nature of surface species of the Ba- or Sr-loaded case was slightly different from those of alkali metals. (i) The amount of oxygen in the cluster was small for alkali metals. (ii) The alkaline earth metal oxides can not be reduced to metals with carbon by heating up to 1000 "C. Decomposition of BaC03 on carbon to BaO was proposed by McKee et al.3 and Ersolmaz et al.lo Therefore, for interpreting alkaline-earth catalyzed carbon gasification, M in eqs 5-7 should be oxides of alkali earth metals. The cluster of Sr or Ba is considered to contain a large amount of oxygen. The adsorption of CO after the end of heat-treatment in the TPR in a CO/He flow (Figure 14b) implies the existence of a large amount of oxygen in the cluster [M,(O) + CO M,(O)-COI. In a CO atmosphere, the large I3CO peak was observed at 540 "C. This temperature was about 300 "C lower than that observed for the TPD in a pure He flow (Figures 10b and 14b). An exchange reaction between I3CO in the [M,(0)0-13C01 complex and I2COin an atmosphereseems to occur rapidly. In the cases of Na and K, l2C0 in the gas phase rapidly exchanged with not only 13C0 in the [M,(O)O-WOI complex but also I3CO2in the [M,(O)WO21 complex. Consequently, the formation of metal cluster oxide was restricted due to the limited oxidation of M,(O) with C O Z . ~A~large , ~ ~amount of l2CO2desorbed in a 12CO atmosphere for the Na- and K-loaded cases, but such effects of CO atmosphere were not observed in Baloaded case. The interaction between the metals and oxygen in the cluster of Ba or Sr seems to be strong as compared with Na- and K-loaded carbon. For the strong interaction between the metals and oxygen in the cluster, it is considered that the main products were 13C0 and I2COin both pulsed reaction and TPD. Peak temperatures of W O and W O desorption above 600 "C in the TPD were higher than those of Na and K catalysts due to the weak interaction between cluster and carbon (less reactive). TPR in a l2CO2atmosphere liberated 13COzat a lower temperature region (Figure 15b). This seems to indicate that weaker interaction between cluster oxide and CO2 in surface complex shown in eq 9 did occur. For Na and K, the reactions of the cluster were also affected by the C02 atmosphere and reactions 9 and 10 did not proceed. By contrast, for Na and K, reactions of [C,M-C021 complex were not much affected with COZ atmosphere. The differences between the TPR in CO2 and the TPD in He with the Ba-loaded case suggest that the interaction between carbon and dispersed metal or metal in the dispersed metal oxide in the fast paths is weak for Ba and Sr. The weak interaction between carbon and dispersed metal was also considered in the case of Lia21 As reported previously,20#21 the reaction mechanism with the alkali metals was much affected by the catalyst loading level and the nature of carbon materials. By contrast, in the Ba-loaded case, the catalyst loading level did not affect the reaction pattern (Figures 9 and 10). The nature of carbon materials slightly affected the reaction pattern. In the pulsed reaction of the Ba-loaded AC, the satellite peak of I2COwas not observed and only a long tailing of I2CO was observed (Figure 5) different from that of YL or CB. In the TPD of CB, the desorption of 13C0 below 500 "C was not observed and the desorption of 13C0 above 500 "C wasvery small (Figure6). These findings are considered
-
to be caused by the better contact of the cluster with carbon for AC than YL and CB and by the lower dispersion for CB than AC and YL. McKee et al. proposed the following mechanism for Ba catalyst.3
-
BaO + CO, BaCO,
+C
BaCO,
(13)
BaO + 2CO
(14)
Kapteijn et aL9and Ersolmaz et al.1° also proposed the similar mechanism for Ba-catalyzed carbon gasification. However, it was proposed only by the results of TPD or TPR. These reaction paths seem to correspond only to slow paths on the cluster. With the advent of pulsed reaction technique, possibility of another reaction path, the fast path, was suggested. Under steady-state conditions, the fast path seems to proceed, as it is suggested from the results of the successive pulsed reaction (Figure 16b). In our pulsed reaction experiment, the slow paths proceeded after 13C02passed through the carbon bed. The 13C02in the atmosphere and products ( W O , l2C0, and 12C02) seems to interact with the cluster and restrain the advance of the slow path as the result of TPR. The remarkable point in the successivepulsed reaction is that the harmonized responses of 12C02 against 13C0 peaks were observed but those against W O were less clear. In the successive pulsed reaction for alkali metals including Li, the responses of W O and l2C02against those of 13C0were distinct. These results indicate that the nature of active oxidant of Ba or Sr in the fast path different from that of alkali metals. The active oxidants of Ba- and Sr-loaded on carbon are considered to be 02 type oxidant and they oxidize carbon to produce C02. Catalytic Mechanism with Calcium. The mechanisms of catalyst oxidation and reduction of Ca showed different patterns in the pulsed reaction and the TPD from those of Sr- or Ba-loaded case. Kapteijn et al. suggested the difference in the mechanism between Ca and Ba, Sr from TPD s t ~ d i e s .In ~ our study, the results of pulsed reaction and TPD using 13C02 suggested the difference in the mechanism between Ca and Ba or Sr in both oxidation and reduction processes. For the Ca catalyst, oxygen transfer mechanisms were proposed15J6 similar to the cases with alkali-metal loaded carbon (eqs 15 and 16).2&&W2
+ + + + -
CaO + CO, CaOO CaO(suf) CaCO,(suf) CaCO,
CaOO + CO
C,
CO,
CaO
-
CaO-C(O)
CaCO,(suf)
CaO(su0 + CaCO, CaO + CaC0,-C
CaO-C
CaC0,-C
CaO + CO
CaO-C(O)
-
CaO-C
+ CO
+ CO
COz Gasification of Carbon Cazola-Amores et al. proposed formation of surface carbonates species and oxygen transfer from carbonate to carbon as indicated in reactions 17-21.15J6 However, our results suggested a more complicated mechanism €or Ca. In the Ca-loaded case, two paths in both catalyst oxidation and reduction can be considered from the results of the pulsed reaction. The fast paths correspond to the first peaks in the pulsed reaction and the slow paths correspond to the tailing productions of carbon oxides in the pulsed reaction. As described above, in the alkali-metal-loaded cases, the first peaks in the pulsed reaction are ascribed to the oxygen-transfer reactions through active oxidant on the well-dispersed metals strongly interacting with carbon. The peak position of the first peaks in the pulsed reaction did not shift when the reaction temperature was changed. A similar finding observed for Ca indicates the formation of active oxidant. Huggins et al.13and Yamashita et al.17 investigated the surface Ca species with EXAFS. In their studies, only the EXAFS peak due to neighboring oxygen atom (Ca-0) was observed and the peak due to calcium metal (Ca-Ca) was not observed under the same pyrolysis conditions as our case. These results seem to show that Ca exists as a highly dispersed state and is only surrounded with oxygen. The first peaks in the pulsed reaction did not give any corresponding desorption peaks in the TPD. Two overlapping peaks of 13C02and 13C0 of Ca-loaded AC (see Figure 4a) observed at about 700 "C seem to correspond to the tailing production in the pulsed reaction. In the alkali-metal-loaded cases, the peaks corresponding to the first peaks in the pulsed reaction were not observed in the TPD.20g21Such findings indicate that only by means of the pulse reaction technique can the fast path be detected. In the TPR under a 12C02/Heflow (Figure E a ) , several W02peaks which were not observed in a pure He were observed. The desorption of 13C02seems to be due to the exchange with l2CO2in the gas phase. The desorption of 13C02occurred only above at certain energy levels as evidenced by clear peaks. Therefore, 13C02is considered to desorb from the dispersed metals which are precursor of the cluster. From these results, the first peaks in the puked reaction seemed to be arised from the well dispersed metals and via active oxidant same as the case of alkali metal loaded case (eqs Carbon oxides formation in the tailing region in the pulsed reaction were observed in alkali-metal-loaded cases. They are ascribed to the metal cluster species. In the Ca-loaded case, carbon oxides in the tailing region in the pulsed reaction are considered to correspond to the desorption peaks above 500 "C in the TPD. The desorption patterns and desorption temperatures of carbon oxides from Ca-loaded carbons in the TPD were similar to those of the alkali metals. The fast path seemed to proceed on highly dispersed metal oxide as described above. By contrast, the slow path appeared to proceed on metals or metal oxides with lower dispersion. Because interaction between surface metal oxide and carbon species would be weak, the reaction proceeds slowly, and it corresponded to high TPD desorption peaks and tailing production in the pulsed reaction. However, Huggins et al. and Yamshita et al. did not specify dispersion state of Ca.13J7 Therefore, the metal oxide cluster model seems to be appropriate to the Ca catalyst. This would be schematically shown below:
Energy & Fuels, Vol. 8,No. 3, 1994 657 CaO*CaO CaO*Cao*CaO //// c C
+ 13cq
-
13c0
CaO-CaO 0 CaO*CaO*CaO //// c C
13co
In the TPR under a CO/He flow (Figure 14a),the peaks of 13C02 and '3CO shifted to lower temperatures as compared to the TPD in He, but the shift was smaller than that observed €orthe Ba-loaded case. A large amount of l2C02 formed in the Ca-loaded case but l2CO2was not observed in Ba-loaded case. The effects of CO atmosphere on the TPR suggest strong interaction between the cluster and carbon and weak interaction between metals and oxygen in the cluster. Tailing production of l2C0in the pulsed reaction seems to be formed via Ca cluster oxide-carbon complex after the decomposition of the cluster oxide-13C0 complex, shown above. The small satellite peak of lZCOin the pulsed reaction which corresponds to the TPD peak at around 800 "C seen in Figure 13 seems to be formed from a poorly dispersed metal oxide rather than the cluster which slowly contacts with carbon as the Fe catal~st.~8J9 In the TPP, one more l2C0 peak was observed around lo00 "C similar to Ba- and Sr-loaded cases. This W O peak is considered to be due to the decomposition of surface C(0)complex or to the oxidation of carbon by oxygen contained in the cluster. Mechanism of l2CO2 Formation for Ca-Loaded Carbon. The different production patterns were observed between l2C0 and 12C02. In the pulsed reaction, the response of I2CO2was similar to the production of l3CO (see Figure llc). By contrast, l2C0 continued to form at a certain level after the decrease in I3CO formation. The differences between production patterns of l2C0 and l2COz was seen in the TPD and TPR. The most characteristic feature in Ca-loaded case was the large amount of l2CO2 formation both in the pulsed reaction and in the TPD. In the fast path, the active oxidant seems to give l2CO2 similar to the Ba- and Sr-loaded cases. In the slow path, several paths for l2CO2formation are considered similar to the cases in alkali metals. The result that l2CO2 showed the peaks in the same temperature region as 13C0and 12CQin the TPD showsthe participation of Ca cluster in the l2CO2 formation. In the TPD and TPR, two I2CO2peaks were observed. Two paths, direct oxidation of carbon with Ca cluster oxide and the oxidation of surface C(0) complex with cluster oxide, are considered for l2C02formation. The first peak of l2CO2in the TPD and TPR (Figure 14a 680 "C, Figure 15a 750 "C) is considered to correspond to the direct oxidation and the second peaks to the oxidation of C(0) complex, respectively. However, the difference between formation patterns of "CO and l2CO2in the tailing region in the pulsed reaction shows the existence of two kinds of cluster oxide
658 Energy & Fuels, Vol. 8,No.3, 1994
Suzuki et al.
to give W 0 2 or l2C0. Possible processes for this step would be illustrated as follows:
oxidatim of C(0):
CaO*CaO
cao*cao*cao(o) 5; //// c //// C
-
'2C02
(24)
The oxidation of produced CO with cluster oxide (eq 25) is also a possible path from the result of TPR in CO/
He flow but the result in the pulsed reaction reveals minor contribution of this path.
l2c0+ Ca,(O)O
-
12C02+ Ca,(O)
(25)
The isotope exchange reaction between 13C02 and produced l2C0 (eq 26) would not contribute to the l2CO2 formation, because the result of TPR in the CO/He flow in which the peak top temperature of l2CO2 appeared at different temperature from those of 13C0 and l3CO2.
-
I2co+ 13c02 Wo, + 13c0
(26)
Effect of Carbon Materials and Catalyst Content on the Mechanism of Ca-Loaded Carbon. The effect of the type of carbon materials is significant for the Ca catalyst. In the TPD of AC, two peaks of 13C0and W02 were observed at about 700 "C (Figure 4a). This implies that two kinds of Ca cluster exist on the carbon surface. By contrast, in the TPD of CB, only a large 13C02peak was observed (Figure 4c). The dispersion of Ca seems to be lower than those of AC and YLdue to the smaller surface area of CB. However, the peak top temperature of I3CO2 desorption was much lower than the decomposition temperature of calcium carbonate on the carbon surface l ~ dispersion of Ca or the reported by Chang et ~ 1 . High good contact between Ca cluster and carbon bIack could be provided in our case, as evidenced by the lower desorption temperatures of carbon oxides. In the pulsed reaction of Ca-loaded CB, the oxygen recovery is high in spite of a small amount of 13C0 formation (Figure 3c). This indicates that the low dispersion of Ca on CB affects the catalyst oxidation step and does not affect the catalyst reduction step. The effect of catalyst content on the pulsed reaction and TPD patterns was insignificant and the reaction mechanisms seem not to be changed by the catalyst content. The reason for the smaller effects of catalyst content as compared to alkali-metal-loaded cases seemed to be the lower dispersion state of Ca catalyst even at a lower loading.
Proposed Mechanism under the Steady-State for Ca-Loaded Carbon. In the successive pulsed reaction, a quasi-steady-state reaction could be observed after providing the overshoots of 13C0 and l2C0. After 10 13COZpulses were injected, the tailing productions of 13COZ,13C0,and lzC02were not observed (see Figure 16a). The tailing productions of 13C02, 13C0 and l2CO2 were observed in the successive pulsed reaction for all alkali metals and Ba-loaded cases (Figure 16b). The slow path seems to have proceeded simultaneously in the steadystate. The small W O tailing pattern after the last 13C02 peak clearly suggests this. Good agreement between the responses of l2CO2against 13C0peak were observed but the responses of l2C0against 13C0 were not clear as observed in the Ba-loaded case. This indicates that catalyst reduction proceeds to produce WOZ mainly. In previous studies by other investigators l2CO2formation were not treated. However, our results showed the importance of C02 formation as described above. Further studies are required for the understanding of the C02 formation. Conclusion The C02 gasification of carbon catalyzed with alkali earth metals (Ca, Sr, Ba) was investigated with pulse, TPD, and TPR techniques by using isotope labeled 13C02. The existence of several paths in both catalyst oxidation step and catalyst reduction step was shown with pulse experiment and detailed information of these paths were obtained with TPD and TPR experiments. Similar reaction patterns to those of alkali metals were obtained and the catalytic mechanisms were discussed based on those of alkali metal loaded cases previously reported. The reaction patterns of Ba- or Sr-loaded carbon were almost identical and were similar to those of Li-loaded carbon in the pulsed reaction, TPD, and TPR experiments. The reaction patterns of Ca-loaded carbon showed different aspect from those of Ba- and Sr-loaded cases and essentially the same as those of Na- or K-loaded carbon. From these findings, two paths, fast paths and slow paths, are proposed in both catalyst oxidation and catalyst reduction steps as those of alkali metals. The fast path proceeds via the dispersed metal and active oxidant which produces COZ. The slow path proceeds via the metal cluster containing large amount of oxygen in which slow oxygen transfer from C02 to carbon occurs. However, the nature of these two surface species is different between Ca and Ba or Sr. For Ba and Sr, the interaction between metal and carbon is weak and the interaction between metal and oxygen is strong. By contrast, the interaction between metal and carbon is strong for Ca. Due to these differences Ba and Sr are active for catalyst oxidation and Ca is active for catalyst reduction. The importance of fast paths in the steady-state gasification was clarified by using pulsed reaction techniques. In the steady state,the fast paths only proceed for Ba and Sr but both the fast and the slow paths proceed for Ca. The formation of C02 is very important because almost the same amount of C02 as CO is produced during quasi steady-state gasification.
