Energy & Fuels 1995,9, 630-634
630
Dynamics of Surface Oxygen Complexes during Carbon Gasification with Oxygen Qianlin Zhuang, Takashi Kyotani,* and Akira Tomita Institute for Chemical Reaction Science, Tohoku University, Sendai, 980-77 Japan Received October 12, 1994@
The dynamic behavior of surface oxygen complexes formed during the gasification of carbon with isotopically labeled oxygen was studied by transient kinetics (TK). The surface oxygen complexes after the gasification were analyzed with temperature-programmed desorption (TPD). From the results of the TK and TPD, a detailed mass balance of oxygen during the gasification was established and the fate of each surface oxygen complex such as acid anhydride, lactone, carbonyl, or ether during the gasification was clarified. Furthermore, it was verified that carbonyl and ether produce COZand CO through the reaction with 0 2 , while acid anhydride and lactone desorb as C02 (and CO) without direct interaction with 0 2 . As a whole, the formation of carbonyl and ether by 0 2 chemisorption and their reaction with 0 2 were found t o be the main route for the carbon gasification with 0 2 under the present conditions.
Introduction The mechanism of carbon gasification with oxygen has been widely studied. It is generally accepted that molecular oxygen dissociatively chemisorbs on carbon free sites to form surface oxygen complexes, which then decompose to form C 0 2 and/or CO. As a result of the removal of carbon atoms, new free sites are exposed for subsequent chemisorption. Thus the number of surface oxygen complexes is a key factor affecting the gasification reactivity of carbon. In the past several decades, much effort has been made t o correlate the carbon reactivity with the amounts of surface oxygen complexes by using the temperature-programmed desorption (TPD) or the transient kinetic (TK) te~hniques.l-~Some success in predicting carbon reactivity has been attained by this approach, which, however, does not give a clear view on the mechanism of carbon gasification. The full understanding of the chemical form of surface oxygen complexes and their dynamic behavior during the gasification is essential for the elucidation of the gasification mechanism a t a molecular level. The TPD technique, which has been often used to characterize surface complexes, does not allow the exact identification of the chemical form of the complexes. In order to clarify the chemical form, spectroscopic techniques such as Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy have been utilized in many recent r e ~ e a r c h e s . ~In- ~our previous paper, by combining diffuse reflectance infrared Fourier trans* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, May 1, 1995. (1)Laine, N. R.; Vastola, F. J.; Walker, Jr. P. L. J . Phys. Chem. 1963,67,2030-2034. (2) Radovic, L. R.; Walker, Jr. P. L.; Jenkins, R. G. Fuel 1983,62, 849-856. (3) Adschiri, T.; Nozaki, T.; Furusawa, T.; Zhu, Z-B.AZChE J . 1991, 37,897-904. (4) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990,28,7-19. (5) Starsinic, M.; Taylor, R.; Walker, Jr. P. L.; Painter, P. C. Carbon 1983 21,69-74. (6) Kelemen, S. R.; Freund, H. Energy Fuels 1988,2,111-118. (7) Marchon, B.: Carrazza. J.; Heinamann. H.: Somoriai. G. A. Carbon 1988,26,507-514. ( 8 ) Fanning, P. E.; Vannice, M. A. Carbon 1993,31, 721-730.
form (DRIFT) spectroscopy with TPD technique, we studied surface oxygen complexes formed during gasification with ~ x y g e n ,and ~ found that lactone, acid anhydride, carbonyl, and ether type complexes are formed on the carbon surface during the gasification. Furthermore, we clarified which surface complex corresponds to the COS or CO desorption in the TPD. For the clarification of the behavior of the surface complexes during carbon gasification, isotope-labeling technique is undoubtedly useful. By using l 8 0 z and 1 6 0 2 as gaseous reactants, Walker et al. investigated the gasification behavior of carbon black with oxygen.1° They demonstrated for the first time that COZ is a primary product from very labile complexes on carbon surface. By combining labeling technique with TPD, we attempted to clarify the mechanism of Ca catalysis in carbon gasification with 0xygen.l' This work confirmed that the surface complexes as a reaction intermediate are formed mainly on the area in contact with the catalyst particle. Recently, Crick et al. applied a pulse technique using l a 0 2 isotope to the analysis of carbon gasification.12 A 1 8 0 2 step-response experiment in the lSOz gasification was also performed by the same g r 0 ~ p . lThey ~ experimentally showed that the importance of the interaction between gaseous oxygen and the surface complexes. For the COz gasification, Kapteijn et al. investigated its mechanism with TK technique using labeled C O Z . They ~ ~ proposed the presence of two types of the unstable complexes that desorb at different decay rates in the TK run and concluded that the complexes with higher decay rate contribute to the (9) Zhuang, Q. L.; Kyotani, T.; Tomita, A. Energy Fuels 1994,8 , 714-718. (10) Walker, Jr. P. L.; Vastola, F. J.; Hart, P. J . In Fundamentals of Gas-Surface Interactions, Academic Press Inc., New York, 1967; pp 307-317. (11)Kyotani, T.; Hayashi, S.; Tomita, A.Energy Fuels 1991,5,683688. (12) Crick, T. M.; Silveston, P. L.; Miura, K.; Hashimoto, K. Energy Fuels 1993,7, 1054-1061. (13) Miura, K.; Zha, H.; Hashimoto, K. Proc. 30th Conf: Coal Sci., Tokyo, Jpn, Oct. 25-26, 1993,63-66. (14) Kapteijn, F.; Meijer, R.; Moulijn, J . A,; Energy Fuels 1992,6, 494-497.
0887-0624/95/2509-0630$09.00/0 0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 4, 1995 631
Carbon Gasification with Oxygen gasification much larger even though they are present in smaller amount.15 In spite of much effort to identify the chemical form of the surface complexes and their role in the gasification, the mechanism of the carbon gasification with oxygen is, at best, poorly understood as yet. One of the reasons for this is that each useful technique has not been effectively combined in most of the past works. In this study, we investigated the transient response during the carbon gasification where 1 8 0 2 was quickly switched to 1 6 0 2 . Only this TK experiment using isotopes allowed us to get the i n situ information on the dynamics of surface oxygen complexes during the 0 2 gasification with oxygen. In addition, the TPD technique was employed to analyze surface complexes remaining in the l80-and then 160-gasifiedcarbon. The gas analyses during the 1 6 0 2 gasification and during the subsequent TPD made it possible to establish a detailed mass balance of lSO during the 1 6 0 2 gasification. Furthermore, the chemical form of the surface complexes was estimated from the results obtained in the previous DRIFT/TPD study. From this combined utilization of various techniques, we attempted to quantitatively elucidate the dynamic behavior of surface complexes during the gasification and thereby discussed the mechanism of carbon gasification with oxygen.
Experimental Section Materials. The carbon sample used in this work was prepared from phenol-formaldehyde resin (PF). The details of the preparation method was described in the previous paper.s The PF char (100 x 200 mesh) was subjected to 0 2 gasification a t 773 K up to the conversion of 52% in a vertical quartz reactor (i.d., 10 mm). The resultant gasified char was employed throughout most of this study. The surface area of the gasified char was determined to be 720 m2/g by N2 adsorption at 77 K using BET equation. Helium (minimum purity, 99.995%) was used as a n inert gas and a diluent. It was further purified by flowing through a deoxygenator (GC-RP, Nikka Seiko Co., Ltd.). Labeled oxygen, l 8 0 2 , which was purchased from Isotec Inc., has isotopic purity of 98.1 atom %. Oxygen Gasification, "K and TPD. The gasified PF char (about 210 mg) was placed in the same reactor as above. The sample was first heat-treated in He at 1223 K for 30 min t o remove most of the surface oxygen complexes formed in the previous gasification stage. Then the sample was cooled to the gasification temperature of 773 K , followed by the introduction of ' 8 0 2 (5% in He) into the reactor at a total flow rate of 200 cm3 (STP)/min. After the 1 8 0 2 gasification for 3 min, the following TK experiment was carried out. The flow of 1 8 0 2 was switched to that of 1 6 0 2 as quickly as possible with the temperature unchanged, and the l6O2 gasification was continued for 3,10, 30, or 60 min. After the 1 6 0 2 gasification, the atmosphere was switched to He. Then the sample was subjected t o TPD experiment from the gasification temperature to 1223 K at a heating rate of 10 K under a He flow. The gas analysis during the gasification and the TPD was made with a quadrupole mass spectrometer (AQA-100R,Nichiden Anelva). Experiment on Isotope-Exchange and Secondary Reactions. In order to check the possibility of isotope-exchange and other secondary reactions, the following two experiments were carried out: (1)without the sample, the mixture of I S 0 2 (5%)and Cl6O (0.5%)was introduced t o the reactor under the gasification conditions, and (2) when the PF char was being (15) Kapteijn, F.; Meijer, R.; Moulijn, J. A.; Cazorla-Amoros, D. Carbon 1994,32,1223-1231.
gasified with 1 8 0 2 (5%)at 773 K, Cl602 (0.5%), or C l 6 0 (0.5%) was introduced into the gasification system. The gas analysis was carried out in the same manner as above.
Results Possibility of Secondary and Isotope-Exchange Reactions. Since there exist secondary and isotopeexchange reactions in addition to the main gasification reaction, it is necessary to first know whether the observed C02 and CO gases are primary products or secondary ones. One of the possible secondary reactions is CO oxidation with 0 2 . The contribution of this reaction was checked by how much Cl60l8Owas formed when the mixture of 1 8 0 2 and Cl60 was introduced to the reaction system. This experiment was carried out at the gasification temperature without char. As a result, no detectable C160180was observed. Therefore, the gas-phase oxidation did not occur a t the present conditions. The reaction of CO with surface oxygen complex can be another secondary reaction. The possibility of this reaction was discussed by Hall et aZ.16 In order to check this reaction, Cl60 was introduced to the 1802-gasification system. If the secondary reaction (Cl60 180-containingcomplex C160180)took place, we would have found a remarkable increase in C160180 evolution. The introduction of Cl60, however, gave no remarkable effect on the evolution rates of C160180and other gases. Therefore, we can neglect the contribution of this secondary reaction to the present gasification system. Furthermore, the above experiment supported the absence of the following CO-related exchange reactions.
-
+
+ P o 2= Cl8O + c1601so Cl6O + c160180 = Cl8O + P o , c160
(3)
If these reactions took place to a great extent, we should have observed the increase in l60l8O,Cl80, and C1602 and the decrease in 1 8 0 2 and C1802 evolutions upon the introduction of Cl60. In order to check C02related exchange reactions, a similar experiment was carried out by introducing Cl6O2to the 1802-gasification system. The gas profiles for 1 8 0 2 and C02 are illustrated in Figure l. The formation rate of Cl8O2 decreased and C160180 started to evolve when Cl6O2 was introduced. The formation of C160180 can be explained by the following isotope-exchange reaction.
P o , + Cl8O2= 2c160180
(4)
By assuming a random distribution of l80among the three kinds of COz, the equilibrium rate of C160180 evolution was determined. It was found that the actual Cl60l8O formation rate is always about 70% of the calculated equilibrium rate. This finding indicates that, although the isotope-exchange reaction 4 took place to some extent, its reaction rate was not fast enough t o reach the equilibrium state. The present result is very similar to that reported by Walker et aZ.lo They gasified graphitized carbon black with isotopically labeled 0 2 at (16) Hall, P. J.; Calo, J. M. Energy Fuels 1989,3,370-376.
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632 Energy & Fuels, Vol. 9, No. 4, 1995
Introduction of C W , T
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Time (min) Figure 1. Effect of Cl602 introduction on the COz evolution profile during the lB0zgasification at 773 K. h
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Time (mid Figure 2. COz and CO evolution profiles of the '*OZgasification and the subsequent l6O2gasification at 773 K.
a very low pressure in a closed reactor system and found that the isotope-exchange reaction 4 was slower than the COS formation from the gasification. Gasification and TK profiles. The char was gasified with lSO2 for 3 min and then the flow of lSO2was switched to that of 1 6 0 2 . The details of the gas profile upon the switching are given in Figure 2, which indicates that the replacement of l S 0 2 with lSO2 was completed in the period of less than 10 s. This quick and smooth switching from one gas to another is essential for the detailed analysis of the transient response. Even during the 1 8 0 2 gasification, a slight formation of C160180was observed together with the Cl8O2 and ClS0formation. This is due to the presence of the small amount of l60in the starting char and in the 180z gas cylinder. Upon switching to 1602, the formation rates of Cl8O2 and ClS0promptly dropped to a certain level and then gradually decreased. In other words, the decay of Cl802 and ClS0 took place in two steps: the rapid decay and the gradual one. At the same time, the rapid formation of C160180,Cl6O2, and Cl60 was observed due to the gasification with 1 6 0 2 . Despite this switching, the rates of the total C02 (ClSO2, C160180,and Cl6O2)formation as well as CO (Cl80and Cl60)formation did not show any discontinuous change at this point. This ensures that the 0 2 gasification itself was not disturbed by the switching. As the 1 6 0 2 gasification proceeded, the formation rate of Cl8O2 decreased and approached almost zero after the 1 6 0 2 gasification of 30 min. However, the evolution of Cl80 lasted even after 30 min. It is noteworthy that the formation rate of Cl8O was almost equal to that of
Temperature (K) Figure 3. TPD patterns of lBOz-gasifiedcarbon after l6O2 gasification: (a) COZevolution, (b) CO evolution.
C160180in the whole 1 6 0 2 gasification range. The Cl6O2 and Cl60 formation rates were kept almost constant except for the very initial stage of the 1 6 0 2 gasification. TPD Analysis. After the l 8 0 2 gasification and the subsequent lSO2 gasification, the surface oxygen complexes remained in the char were analyzed by TPD. The COZ-and CO-TPD patterns with different 1 6 0 2 gasification periods are given in Figure 3, a and b. It should be first noted that in all the cases the amount of total COZevolved in the TPD was much smaller than that of total CO. In other words, the amount of CO-yielding complexes was much larger than that of CO2-yielding ones. The COZand CO desorption patterns were very broad and had peaks a t 900 and 970 K, respectively. The sample just after the 1 8 0 2 gasification without 1 6 0 2 gasification only showed substantial peaks of Cl8O2 and Cl80. With proceeding of the 1 6 0 2 gasification, the amount of Cl8O2 in the TPD rapidly decreased and its desorption could be scarcely observed when the gasification lasted for 30 min. On the other hand, the Cl6O2 desorption increased with the gasification time. The change of C160180evolution was not remarkable from 3 to 10 min and then it decreased slowly. In the case of CO desorption, the amount of Cl80 desorption decreased with the lSO2 gasification. However, in contrast with the case of Cl8O2, ClS0 desorption was still observed even after 60 min. The amount of Cl60 increased with the 1 6 0 2 gasification.
Discussion
TK Profile upon the Switchingfrom 1602 to l6O2. As described before, the decay of C1802 and ClS0 upon the switching can be divided into the fast and the slow steps. In the former step, the evolution of the Cl8O2
Carbon Gasification with Oxygen
Energy & Fuels, Vol. 9, No. 4,1995 633
and Cl80 suddenly dropped as soon as 1 8 0 2 was flushed out. Therefore, this step would be relevant to the reactions where 1 8 0 2 functions as a reactant with CO2 (48)and Cl80 as products. One of the possible reactions for this step is the CO2 and CO formation by a reaction of surface oxygen complex with gaseous oxygen, like reaction 5, where Cf and C(l80) are free site and l80Cf
+ ~ ( " 0+) 1802- ~ ' ~+0 ,
+ l8O2- C(l80) + P o
Cl8O2 and Cl80 formation in the slow decay region continued even after 1 8 0 2 was completely flushed out. This means that such Cl8O2 and Cl80 evolution during the 1 6 0 2 gasification can be attributed to the reactions where 1 8 0 2 is not directly involved as a reactant. There are two probable reactions. One is the desorption of lSOlabeled surface complex.
-
~ ( ' ~ 0C"O, ) and/or
(7)
The other is the Cl80 formation through the reaction of 180-labeledsurface complex with 1 6 0 2 . This can be expressed as follows. Cf
+ C(l80)+ l6O2- P o , + Cl8O
(8)
The possibility for these reactions will be discussed in the following sections. Oxygen Balance of \the Surface Oxygen Complexes and the Gaseous Products during the Gasification. Figure 4 illustrates the fraction of l80 atoms in the gases evolved in each stage of the 1 6 0 2 gasification together with those in the 180-containing surface complexes remained on carbon after the 1 6 0 2 gasification. The latter was determined by the subsequent TPD run. The first bar corresponds to the TPD just after the 1 8 0 2 gasification without the 1 6 0 2 gasification (the uppermost TPD patterns in Figure 3). According to the previous TPDDRIFT study, the CO2 desorption at around 900 K can be ascribed to the decomposition of acid anhydride and lactone, and the CO desorption mainly originates from carbonyl and ether type ~omplexes.~ Thereby, most of the complexes formed during the 1 8 0 2 gasification are carbonyl (17) Su, J.-L.; Perlmutter, D. D. AlChE J . 1985,31, 1725-1727. (18) Ahmed, S.; Back, M. H. Carbon 1985,23, 513-524. (19)Tucker, B. G.; Mulcahy, M. F. R. Trans. Faraday SOC.1969, 65,274-285.
1 8 0 evolved during 1 6 0 2 gasification:
80
C1802
&C160180 I
0 C180
or c(l80) (5)
labeled surface complex on carbon, respectively. The presence of such reaction between surface oxygen complex and 0 2 was first discussed by Walker et aZ. in the gasification of graphitized carbon black with 02.1° They thought, however, that this reaction is not a major route for the C02 formation. Su et aZ.17and Ahmed et aZ.18pointed out the importance of this reaction in the case of carbon with high coverage of oxygen. Recently, Miura et aZ.12,13demonstrated in the pulse gasification with 1 8 0 2 that the complex, which is stable even at 1173 K, participates in the 0 2 gasification at 773 K through the reaction with 0 2 . Another possible reaction for the Cl80 formation which suddenly stops upon the switching t o 1 6 0 2 (the fast decay) may be the following one proposed by Tucker and M~1cahy.l~ Cf
100
1 8 0 desorbed in TPD after 1 6 0 2 gasification:
20
C1802
R C160180
0
CIS0 0
3
10
30
60
1602-gasificationtime (min)
Figure 4. Fate of l80in 1 8 0 2 gasification and subsequent 1 6 0 2 gasification. Upper part in each bar: l80evolved during l S 0 2 gasification;lower part: l80remaining on carbon and desorbed in the subsequent TPD.
and ether, and the amounts of acid anhydride and lactone are relatively small. During the 1 6 0 2 gasification, some of the l80-containingcomplexes were evolved as Cl8O2, Cl60l8O,and ClsO. The composition for such lsO-containing gases is illustrated in the upper half of each stack column in Figure 4 and l80remaining on the char (l80evolved in the TPD after the 1 6 0 2 gasification) is indicated in the lower half. It should be noted that the sum of l80in the Cl8O2 evolved in the TPD and l80 in the C1802 during the 1 6 0 2 gasification appears almost constant whatever the gasification time is. Since almost half of the 180-gasified carbon was consumed during the subsequent l602 gasification for 60 min, we can say that the observed constancy in the sum of l80is held over a relatively wide conversion range. This constancy suggests that the ClsO2-yielding complexes, that is, l80-containing acid anhydride and lactone, desorb without direct interaction of gaseous oxygen during the 1 6 0 2 gasification. As deduced in the last section, this desorption is thought to be the origin of the slow decay of Cl8O2 in the TK profile (Figure 2). On the other hand, the sum of Cl80 in the TPD and Cl80 evolved during the gasification decreases with gasification time. This result cannot be interpreted only from the desorption of lsO-containing carbonyl and ether type complexes during the gasification. The above unbalance is due to the formation of Cl60l8O. This finding confirms the presence of the direct reaction of 180-containingcarbonyl and ether with 1 6 0 2 during the 1 6 0 2 gasification. Figure 4 gives a clear view how the ls0-containing surface complexes changed with the subsequent 1 6 0 2 gasification. To our knowledge, this is the first example that shows the fate of several kinds of surface oxygen complexes during the gasification. Mechanism of Carbon Gasification with 0 2 . On the basis of the above discussion, we propose the followingreaction scheme of carbon gasification with 0 2 .
+ 0, - carbonyl, ether (+ CO) C, + 0, - lactone, acid anhydride Cf
Cf
+
(9)
(10)
+ carbonyl, ether + 0, C02
CO or carbonyl, ether (11)
Zhuang et al.
634 Energy & Fuels, Vol. 9, No. 4, 1995
C,
+ carbonyl, ether + 0, -
lactone, acid anhydride (12)
lactone, acid anhydride
-
carbonyl, ether
C 0 2 (+ CO)
-
CO
+ C,
+ 2Cf
(13) (14)
As described before, the fast and the slow decays of C1802 in the TK profile upon the switching from 1 8 0 2 to ' 6 0 2 (Figure 2) can be ascribed to reaction 11 and the desorption reaction 13, respectively. Since CO is produced via the reactions 9 and 11, these reactions are responsible for the fast decay of Cl80. Furthermore, the reaction 11 can produce Cl80 even during the l S 0 2 gasification (the reaction 8). Thus, this reaction, as well as the reactions 13 and 14, is the origin of Cl80 formation in the slow decay. Now, the contribution of these reactions to the gasification will be discussed. The reactions 9 and 10 depict the chemisorption process of 0 2 . The former reaction must predominate between the two reactions, because the amount of CO desorption was much larger than that of CO2 in the TPD run for the gasified char. The contribution of the reactions 11and 13 to CO2 formation can be estimated from the TK profile of C1802 upon the switching from 1 8 0 2 t o l S 0 2 (Figure 21, because the two reactions correspond to the fast and the slow decays of Cl8O2, respectively. Since the ordinate in this figure expresses the formation rate of C02, the extents of the decrease of ClSO2in the fast and slow decays upon the switching correspond the rates of reactions 11 and 13, respectively. Thus, it can be estimated that reaction 11contributed to C02 formation 4 or 5 times as largely as that through reaction 13. The presence of the isotope-exchange reaction 4, however, makes it difficult to do further quantitative analysis. The above result indicates that reaction 13 is not important for CO formation. Although the contribution of reaction 12 cannot be evaluated, the necessity of this reaction can be justified from the fact that a considerable amount of C02 (46)was evolved in the TPD after the 1 6 0 2 gasification. We can conclude that under the present conditions the gasification reaction proceeds mainly via reactions 9 and 11. In other words, 0 2 chemisorbs on free site of
carbon to form stable carbonyl and ether type complexes (with production of CO), and then some of them react with 0 2 to produce C02 and CO (or carbonyl, ether). We believe that a part of surface complexes appears to be very stable, judging from the fact that there still remained the 180-containing carbonyl and ether type complexes even under the 1 6 0 2 exposure for 60 min. The mechanism proposed in this study is similar to that reported by Back20 and Crick et a1.12 in terms of the presence of the reaction between surface complex and 0 2 . The key differences are as follows. First, we could specify the type of surface complex in the reaction mechanism. Second, we could separate each elementary process for C02 formation from the TK profile. Furthermore, the elementary steps in the mechanism were reasonably proposed on the basis of the detailed oxygen balance in gaseous product and surface oxygen complex. The establishment of such balance is possible only when the results of the gas analysis during the gasification with labeled oxygen are combined with the information on the chemical form of the surface complexes and their amounts determined in the subsequent TPD.
Conclusions The in situ information on the dynamics of surface oxygen complexes during the 0 2 gasification was successfully obtained by the TK experiment where lSO2flow was switched to 1 6 0 2 flow. The transient response in C02 and CO decay was divided into two steps, that is, the fast and the slow decays. It was found that the fast C02 decay corresponds to the reaction of carbonyl and ether with 0 2 and the slow C02 decay to the desorption of acid anhydride and lactone. From the gas analysis during the 1 6 0 2 gasification of l8O2-gasifiedcarbon together with the subsequent TPD, the detailed mass balance of oxygen was established. Based on the result of the TK and the mass balance, a main route of the 0 2 gasification was proposed as follows. Oxygen molecule chemisorbs on free site of carbon to form stable carbonyl and ether type complexes (with producing CO), and then some of them react with 0 2 to produce C02 and CO (or surface oxygen complexes). EF940195U (20) Back, M.
H.Carbon 1991,29, 1290-1291.
