Unified Mechanism of Alkali and Alkaline Earth Catalyzed Gasification

Energy & Fuels 1997, 11, 421-427

421

Unified Mechanism of Alkali and Alkaline Earth Catalyzed Gasification Reactions of Carbon by CO2 and H2O S. G. Chen Illinois State Geological Survey, Natural Resources Building, Champaign, Illinois 61820

R. T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received June 24, 1996X

From molecular orbital calculations, a unified mechanism is proposed for the gasification reactions of graphite by CO2 and H2O, both uncatalyzed and catalyzed by alkali and alkaline earth catalysts. In this mechanism, there are two types of oxygen intermediates that are bonded to the active edge carbon atoms: an in-plane semiquinone type, Cf(O), and an off-plane oxygen bonded to two saturated carbon atoms that are adjacent to the semiquinone species, C(O)Cf(O). The rate-limiting step is the decomposition of these intermediates by breaking the C-C bonds that are connected to Cf(O). A new rate equation is derived for the uncatalyzed reactions, and that for the catalyzed reactions is readily available from the proposed mechanism. The proposed mechanism can account for several unresolved experimental observations: TPD and TK (transient kinetics) desorption results of the catalyzed systems, the similar activation energies for the uncatalyzed and catalyzed reactions, and the relative activities of the alkali and alkaline earth elements. The net charge of the edge carbon active site is substantially changed by gaining electron density from the alkali or alkaline earth element (by forming C-O-M, where M stands for metal). The relative catalytic activities of these elements can be correlated with their abilities of donating electrons and changing the net charge of the edge carbon atom. As shown previously (Chen, S. G.; Yang, R. T. J. Catal. 1993, 141, 102), only clusters of the alkali compounds are active. This derives from the ability of the clusters to dissociate CO2 and H2O to form O atoms and the mobility of the dissociated O atoms facilitated by the clusters.

Introduction Alkali and alkaline earth oxides and salts are the best catalysts for the gasification reactions of carbon by CO2 (to form CO) and H2O (to form H2 and CO), which are the bases of coal gasification processes. Because of their importance, voluminous literature1-6 has been devoted to this area, and much progress has been made during the past two decades toward the understanding of the mechanisms of these catalyzed reactions, possible active intermediates, and phenomena involved in catalyst behaviors. Many types of intermediates have been proposed and intensively investigated for different catalysts. For example, the proposed active intermediates for potassium catalyst include metallic K,7 K2O,7-9 K2O2,10 K2CO3,7,8 K-O-C,11-16 and clusters which are nonstoichi* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. In Advances in Catalysis; Acadmic Press: New York, 1959; Vol. 11, p 133. (2) Wen, W. Y. Catal. Rev. Sci. Eng., 1980, 22, 1. (3) McKee, D. W. In Chemistry and Physics of Carbon; Walker, P. L., Jr.; Thrower, P. A., Eds.; Dekker, New York; 1981; Vol. 16. (4) Wood, B. J.; Sancier, E. M. Catal. Rev. Sci. Eng. 1984, 26, 233. (5) Yang, R. T. In Chemistry and Physics of Carbon; Thrower, P. A. Ed.; Dekker, New York, 1984; Vol. 21. (6) Baker, R. T. K. In Carbon and Coal Gasification-Science and Technology; Figuereido, J. L., Moulijn, J. A., Eds.; NATO ASI Series E. No. 105, Martinus Nijhoff: Amsterdam, 1986; p 231.

S0887-0624(96)00099-0 CCC: $14.00

ometric compounds with excess metal.13,17,18 For calciumcatalyzed gasification reaction, the proposed active intermediates include CaCO3,19,20 CaO,21 CaO2,21 and CaxOy.22 Among all these proposed intermediates, clusters (or particles) are the most active species. In our previous studies,23,24 we have proven that the C-O-K group has only little catalytic activity as compared to catalyst (7) McKee, D. W. Fuel 1983, 62, 170. (8) Veraa, M. J.; Bell, A. T. Fuel 1978, 57, 194. (9) McKee, D. W.; Chatterji, S. Carbon 1982, 20, 59. (10) Saber, J. M.; Falconer, J. L.; Brown, L. F. Fuel 1986, 65, 1356. (11) Delannay, F.; Tysoe, W. T.; Heinemann, H.; Somorjai, G. A. Carbon 1984, 22, 401. (12) Mims, C. A.; Pabst, J. K. Prepr. Pab.sAm. Chem. Soc. Div. Fuel Chem. 1980, 25, 258. (13) Mims, C. A.; Rose, K. D.; Memchior, M. T.; Pabst, J. K. J. Am. Chem. Soc. 1982, 104, 6887. (14) Mims, C. A.; Pabst, J. K. Fuel 1982, 62, 176. (15) Yuh, S. J.; Wolf, E. E. Fuel 1983, 62, 252. (16) Freriks, I. L. C.; van Wechem, H. M. H.; Stuiver, J. C. M. Fuel 1981, 60, 463. (17) Saber, J. M.; Falconer, J. L.; Brown, L. F. J. Catal. 1984, 90, 65. (18) Mims, C. A.; Pabst, J. K. In Proceedings of the International Conference on Coal Science, Verlag Gluckauf GmbH: Essen, 1981; p 730. (19) Cazorla-Amoros, D.; Linares-Solano, A.; Dekker, F. H. M.; Kapteijn, F. Carbon 1995, 33, 1147. (20) Cazorla-Amoros, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Joly, P. J. Carbon 1991, 29, 361. (21) Lizzio, A. A.; Radovic, L. R. Ind. Eng. Chem. Res. 1991, 30, 1735. (22) Kapteijn, F.; Porre, H.; Moulijn, J. A. AIChE J. 1986, 32, 691. (23) Chen, S. G.; Yang, R. T. J. Catal. 1992, 138, 12. (24) Chen, S. G.; Yang, R. T. J. Catal. 1993, 141, 102.

© 1997 American Chemical Society

422 Energy & Fuels, Vol. 11, No. 2, 1997

particles and clusters. In the gasification reaction of carbon with 21 Torr of H2O at 700 °C, the turnover frequencies are 0.08 s-1 (uncatalyzed), 0.15 s-1 (catalyzed by the C-O-K group), and 7.8 s-1 (catalyzed by KOH particles or clusters). The reaction mechanism of the catalyzed gasification has been another widely studied subject. A generally postulated mechanism25-28 consists of an oxidationreduction cycle in which oxygen is transferred to the carbon active sites through the catalytically active alkali and alkaline earth species, followed by the liberation of CO from the active carbon-oxygen complexes (by breaking the neighboring C-C bonds). The last step is considered as rate-limiting. This mechanism accounts for many experimental observations, e.g., the activation energies of the catalyzed and uncatalyzed reactions are nearly the same;29-32 the oxygen atoms in the gas-phase molecules (CO2 or H2O) can exchange rapidly with the oxygen atoms in the alkali and alkaline earth catalysts.10,17,29,33-36 But the proposed redox-type of mechanisms, so far, can only explain some of the experimental facts. There are other proposed mechanisms, e.g., for calcium catalyzed gasification reaction,19,20 and they can also explain only some of the experimental facts. For example, both transient kinetic (TK) and TPD experiments showed that the desorbed CO amount (from active oxygen complexes) is much larger in the catalyzed system than in the uncatalyzed system. In TK experiments, desorption is conducted by switching the reactant gas to an inert gas at the reaction temperature. According to the redox-type mechanism, uncatalyzed and catalyzed gasification reactions have the same ratelimiting step (implying the same active oxygen complexes are at work) and these oxygen complexes are in equilibrium with their gas phase reactant (CO2 or H2O). Then the question remains, how could the catalysts change the concentrations of the oxygen complexes in catalyzed systems without changing the activation energy? Moreover, TK experiments19,20,30,37-39 showed that the decays of CO desorption in the catalyzed systems (both catalyzed by alkali and alkaline earth) are not exponential, which are similar to the uncatalyzed system. The nonexponential decay of CO desorption indicates that there is more than one kind of active oxygen complex under gasification reaction conditions. (25) Holstein, W. L.; Boudart, M. J. Catal. 1982, 75, 337. (26) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983, 82, 382. (27) Mims, C. A.; Pabst, J. K. J. Catal. 1987, 107, 209. (28) Chang, J.; Lauderback, L. L.; Falconer, J. L. Carbon 1991, 29, 645. (29) Kelemen, S. R.; Freund, H. J. Catal. 1986, 102, 80. (30) Freund, H. Fuel 1986, 65, 63. (31) Pereira, P.; Csencsits, T.; Somorjai, G. A.; Heinemann, J. J. Catal. 1990, 123, 463. (32) Freund, H. Fuel 1985, 64, 657. (33) Saber, J. M.; Kester, K. B.; Falconer, J. L.; Brown, L. F. J. Catal. 1988, 109, 329. (34) Chang, J.; Lauderback, L. L.; Falconer, J. L. J. Catal. 1990, 122, 10. (35) Chang, J.; Adcock, J. P.; Lauderback, L. L.; Falconer, J. L. Carbon 1989, 27, 593. (36) Cerfontain, M. B.; Meijer, R.; Kapteijn, F.; Moulijn, J. A. J. Catal. 1987, 107, 173. (37) Radovic, L. R.; Jiang, H.; Lizzo, A. A. Energy Fuels 1991, 5, 68. (38) Carzola-Amoros, D.; Linares-Solano, A.; Salinas-Martinez de Lacea, C.; Joly, J. P. Energy Fuels, 1992, 6, 287. (39) Meijer, R. Ph.D. Dissertation, University of Amsterdam, The Netherlands, 1992.

Chen and Yang

The two important observations described above have not been fully explained in the literature. One of the objectives of this work is to answer these two questions. In addition to the active intermediates and mechanism of the catalyzed reactions, the rank order of the catalytic activities of alkali and alkaline earth elements is also an important subject of investigation. Most results showed that the catalytic activities of the alkali elements are in the following rank order: Li < Na < K < Rb < Cs. The same rank order (for Li, Na, and K) was also obtained by the authors of this article.23 In our work, the etch-decoration TEM technique was used to study the monolayer channeling rates of the alkali and alkaline earth catalysts. By comparing the monolayer channeling rates, the above rank order of activities was also obtained.23 In this work, we will use MOPAC (molecular orbital package), one of the most accurate semiempirical molecular orbital packages, to investigate the oxygen complexes on graphite. On the basis of the molecular orbital calculations, we propose a mechanism which can account for all of the major features of alkali and alkaline earth catalyzed gasification reactions and it can also answer the two questions described above as well as explain the rank order of the catalytic activities of the alkali elements. Results of Molecular Orbital Calculations In our previous study,24 a semiempirical CNDO/2 (complete neglect differential overlap approximation) method was used. The program package named GEOMOS, QCPE No. 584, was obtained from Quantum Chemistry Program Exchange Center of Indiana University.40 In the present study, we used MOPAC, a more accurate semiempirical molecular orbital package, to calculate the localized electron density, net charge, heat of formation, and bond order (energy) of the oxygen complexes of interest. MOPAC uses MNDO type of approximation methods and is more accurate in calculating the molecular properties, especially the heat of formation of molecules. In fact, the parameters in MOPAC are obtained by using experimental data of heats of formation as the target function. The program used in this work was MOPAC version 6 with QCPE No. 465.45 In MOPAC version 6, only Li among the alkali elements has built-in parameters. Instead of parametrizing the real alkali elements, the author of the program introduced sparkles which represent pure ionic charges. For example, a unipositive sparkle with the symbol + has the following properties:45 (1) It consists of a unipositive nucleus and an electron. The electron is donated to the substrate during the quantum mechanics calculation. (2) It has an ionic radius of 0.7 Å. (3) It has zero heat of atomization, no orbitals and no ionization potential. Sparkle + can be regarded as an unpolarizable ion of diameter 1.4 Å and chemically it is equivalent to the cesium or potassium atom. (40) Zhang, Z.; Kyotani, T.; Tomita, A. Energy Fuels 1988, 2, 679. (41) Suzuki, T.; Ohme, H.; Watanabe, Y. Energy Fuels 1992, 6, 336. (42) Suzuki, T.; Ohme, H.; Watanabe, Y. Energy Fuels 1992, 6, 343. (43) Suzuki, T.; Ohme, H.; Watanabe, Y. Energy Fuels 1994, 8, 649. (44) Rinaldi, D.; Hoggan, P. E.; Cartier, A. QCPE Program No. 584; Quantum Chemistry Program Exchange, Indiana University, Department of Chemistry, 1990. (45) Steward, J. J.; QCPE Program No. 584; Quantum Chemistry Program Exchange, Indiana University, Department of Chemistry, 1990.

Gasification Reactions of Carbon by CO2 and H2O

Figure 1. Model structures with zigzag edges of graphite for MNDO(PM3) molecular orbital calculations. Two surface oxygen species (one in-plane and one off-plane are shown; both are after geometry optimization. The off-plane oxygen is equidistant to two C atoms.

Considering cesium or potassium is a simplification of a real element and may not be feasible for some chemical reaction processes. However, we are only concerned with the changes of the local electron density in the substrates in this study. The local electron density is strongly affected by the alkali element and the magnitude of charge is determined by the ability of donating electrons by the alkali element. Therefore, for our purposes, sparkle is a good model for the cesium or potassium atom. Only two alkali elements, Li and + (equivalent to Ce or K) have built-in parameters in MOPAC. In order to compare the activity rank order of all alkali elements, we resort to the CNDO method for sodium. Fortunately, according to our reaction mechanism, the activity rank order of alkali elements is mainly determined by the local electron density and the net charge of certain carbon atoms in the graphite structure (see below). For these properties, the CNDO approximation method usually can yield relatively accurate results.46,47 Thus, we will use CNDO (in GEMOS) to calculate graphite substrates with Na and MNDO (in MOPAC) to calculate the substrates with Li or sparkle + (which is close to K or Ce). Meanwhile, for a cross reference, we also use CNDO to calculate the substrates with K atoms. The substrates used in this study are shown in Figure 1. These substrates are the same as the substrates used in our previous study.24 The only difference is that in the present study, we also optimize the position of the oxygen atom that is chemisorbed on the basal plane between C27 and C28. Only zigzag edges of graphite are considered here because we have shown previously that (46) Pople, J. A.; Beveridge, P. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. (47) Sadlej, J. Semiempirical Methods of Quantum Chemistry; Ellis Horwood: Chichester, UK, 1985.

Energy & Fuels, Vol. 11, No. 2, 1997 423

Figure 2. The same graphite model structures as in Figure 1 except the nonsurface edge sites are saturated by hydrogen atoms.

the C-C bonds are stronger on zigzag edges, and hence breaking these bonds is rate-limiting.23,24,48-50 In order to investigate the edge effects, another system with edge carbon atoms saturated by H atoms is also used in this study. The corresponding substrates are shown in Figure 2. Bond energy is not available from molecular orbital theory. In order to compare the stability of the different oxygen complexes of interest in this study, we calculated the bond dissociation energy51,52 by the following procedure (take substrate 1 as an example): Step 1. Optimize the geometry of substrate 1 (in Figure 1 or 2) and obtain a total energy (or heat of formation). This optimized geometry is then used for further calculations, i.e., in further calculations, we do not change the geometry of the substrate and only break bonds between atoms by removing atoms. Step 2. Break (by break, we mean to take two atoms of fragments far apart) bond O31-C29 and calculate the total energy (heat of formation) of the system. The difference of the total energies between step 1 and this step is the dissociation energy of bond O31-C29. Step 3. Break substrate 1 into three fragments: C29, O31, and the remainder of substrate 1. In the same manner, we calculate the total energy (or heat of formation) of the system. The difference between the total energies or heats of formation between step 1 and this step is the total dissociation energy of bond O31C29, bond C29-C28, and bond C29-C12. Consequently, the difference between the total energies (or heats of formation) between step 2 and step 3 is the total bond dissociation energy of C29-C28 and C29-C12 corresponding to the procedure above. As this (48) Pan, Z. J.; Yang, R. T. J. Catal. 1990, 123, 206. (49) Pan, Z. J.; Yang, R. T. Ind. Eng. Chem. Res. 1992, 31, 2675. (50) Chen, S. G.; Yang, R. T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1993, 32, 2835. (51) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (52) Sanderson, R. T. Chemical Bond and Bond Energy, 2nd ed.; Academic Press: New York, 1976.


Chen and Yang

Table 1. Net Charge of Carbon Atoms in Structure 2 of Figure 1 with Different Alkali Atoms Attached no.

no catalyst (CNDO)

Li(PM3)

Na(CNDO)

K(CNDO)

+(PM3)

12 13 14 24 25 26 27 28

0.1579 0.2401 0.1359 0.1579 0.2401 0.1359 0.3211 -0.0128

-0.0978 0.0101 0.0378 -0.1086 -0.0978 0.0378 0.0681 -0.3224

-0.0879 -0.0098 -0.1405 -0.0879 -0.0098 -0.1405 -0.1623 -0.3424

-0.1548 -0.0626 -0.1605 -0.1548 -0.0626 -0.1605 -0.1867 -0.4863

-0.2080 -0.0094 -0.1178 -0.2080 -0.0094 -0.1551 0.0737 -0.4374

Table 2. Carbon-Carbon Bond Energies (kcal/mol) in Different Substrates (in Figures 1 and 2) struct 2 struct 4 bonds struct 1 struct 2 struct 3 struct 4 (M ) Li) (M ) Li) Edge Carbon Atoms Are Not Saturated by H Atoms C28-29 289.25

290.26

242.05

242.45

271.30

235.63

C12-29 Edge Carbon Atoms Are Saturated by H Atoms C28-29 274.80

272.65

238.07

252.22

273.08

248.75

C12-29

method cannot differentiate between the bond energies of the C29-C28 and the C29-C12 bonds, we simply take this total value for comparison. The total bond dissociation energy of C29-C28 and C29-C12 in other substrates can be obtained in the same manner. The only difference is that for some other substrates we need to break the substrates into more pieces at a time. Only for diatomic molecules is the bond energy equal to the bond dissociation energy,51 if only one chemical bond in the molecule is broken in a chemical reaction. For a multiatomic molecule, bond dissociation energy is usually different from bond energy. This is because when a molecule is broken into several fragments, the multiatomic fragments will reorganize themselves which will result in a reorganization energy.52 This reorganization energy is responsible for the difference between the bond dissociation energy and the bond energy. In this study, one of the fragments (the remainder of substrate 1 by removing O31 or by removing O31-C29) is multiatomic. This multiatomic fragment will have an electron reorganization energy52 and as a result the bond energy will be different from the bond dissociation energy. However, the multiatomic pieces in step two and step three are very large and are similar. A reasonable approximation is to assume that these two electron reorganization energies are about the same. Consequently, the bond dissociation energy calculated by the procedure outlined above will be a good approximation of bond energy. Table 1 shows the results of net charges of some carbon atoms in substrate 2 of Figure 1. Columns 2 (no catalyst) and 5 (with K) are the results obtained in our previous study.24 Comparing column 5 (CNDO) and column 6 (PM3 in MOPAC), one can find that for all carbon atoms (except atom number 27), the net charges calculated by CNDO and MNDO(PM3) are close. This suggests that CNDO is satisfactory for calculating local electron densities. Table 2 shows the results of bond energies calculated by MOPAC. There are several MNDO-type of approximation methods available in MOPAC version 6. We chose PM3 for all substrates. From Table 2 it is clear

Table 3. Carbon-Carbon Bond Order in Model Substrates (in Figure 1) struct 2 struct 4 bonds struct 1 struct 2 struct 3 struct 4 (M ) Li) (M ) Li) C28-29 C28-30

1.0050 1.0050

1.0590 1.0590

0.9106 0.9106

0.8866 0.8866

1.0001 1.0001

0.8640 0.8640

that the total bond energy of C29-C28 and C29-C12 is substantially decreased when an O atom is chemisorbed on the basal plane between C28 and C27 atoms in both catalyzed and uncatalyzed systems. The decreases in the total bond energy (C29-C28 and C29-C12) are of the order of 20% for the uncatalyzed systems and of the order of 10% for the catalyzed systems. The final bond energies of structures 3 and 4 are, however, not different. This explains why catalyzed and uncatalyzed gasification reactions have similar activation energies. The last two columns (with K replaced by Li) of Table 2 are the results for structures 2 and 4 with the sparkle atoms substituted by Li atoms. Compared with columns 3 and 5 (corresponding to sparkle atoms), one can see that the bond energies are very close, indicating that different alkali elements do not have a large influence on the total bond energies of the C29-C28 and C29-C12 bonds. This result is in agreement with the experimental fact that the gasification reactions catalyzed by all alkali elements showed very close activation energies.29,32 The net charges of substrates in Figure 2 (i.e., substrates in Figure 1 with edge carbon atoms saturated by hydrogen atoms) are not given here. Our results show that the net charges of atoms in these two different systems are very close. The bond energies of these corresponding substrates in Figure 2 are listed in Table 2. In comparing the results of these two systems, we see that the bond energies are slightly different in value but the two systems show the same features and trend. Hence both systems are considered satisfactory. In addition to using bond energy to investigate the model substrate, we can also use bond order index to see the changes of the C-C bond strengths caused by chemisorption of O atoms on the basal plane between the C27 and C28 atoms, i.e., the off-plane O complex. Bond order is available from MOPAC directly. Table 3 shows the results of bond order for the two C-C bonds that are directly influenced by the off-plane O complex. Comparing the bond orders (Table 3) and the bond energies (Table 2), we find that these two values are correlated very well. In fact, bond order index is often used for estimating bond energy. In the present study, we optimized the position of the off-plane O atom that is chemisorbed on the caturated C (C28). The optimized geometry indicates that this O atom is not vertically on top of C28 (as predicted in our previous results, refs 19, 20, and 44-46) but is directly above C27 and C29 and is equidistant to these two carbon atoms (as also shown in ref 49). Our results also show that the total bond energies of C29-C28 and C29-C12 in substrates 1 and 2 are very close. This is different from our previous study (Chen and Yang23,50), where the results showed that the existence of alkali element increased the diatomic energy (i.e., the diatomic energy is larger in substrate 2 than in substrate 1). This discrepancy arises either because CNDO is not accurate enough for calculating bond energies or because the diatomic energy is not a good representation for bond energy, or both.



The result of this study is, however, in agreement with the experimental data obtained in our previous study,23 where we have shown that the C-O-K group has only a very small activity (the turnover frequencies of uncatalyzed and C-O-K catalyzed active sites are 0.08 and 0.15 s-1, respectively). Unified Mechanism Based on the Off-Plane Oxygen Intermediate Our previous molecular orbital results have shown that chemisorption of oxygen on the saturated edge carbon atom (i.e., C28 in Figure 1) can substantially weaken the C-C bonds on the edge surfaces of graphite.24,49 This is true for both zigzag and armchair edges. On the basis of this off-plane oxygen species, we have proposed a unified mechanism24 for all carbon gasification reactions by oxygen-containing reactants (e.g. O2, H2O, CO2, and NO). The unified mechanism of Chen and Yang is also capable of explaining the kinetics of the C + H2O and C + CO2 reactions catalyzed by alkali and alkaline earth catalysts.24 The unified mechanism proposed by Chen and Yang24 was based on CNDO molecular orbital calculations and experimental results on alkali-catalyzed gasification reactions.19,20 We have further extended this unified mechanism based on INDO (intermediate neglect of differential overlap) calculations.50 The discussion in all of our previous work was based on diatomic energy, which is not a good representation for bond energy, as shown by the foregoing discussion. The results obtained in this study are based on bond dissociation energy, which is a good approximation of the bond energy. With the results of this study, we are able to address the previously unresolved questions on the catalyzed C + H2O and C + CO2 reactions. The unified mechanism based on the off-plane oxygen intermediate for the uncatalyzed reactions that was proposed by Chen and Yang23,24,50 can be represented as follows (using CO2 as the reactant; CO2 can be replaced in steps 1 and 2 by another oxygen-containing reactant where the reactant undergoes dissociation to form the oxygen intermediates): i1

CO2 + Cf S Cf(O) + CO j1

K1

(1)

Figure 3. Schematic showing two surface oxygen intermediates. Cf(O) is the semiquinone type species. C(O)Cf(O) is the species with an adjacent off-plane oxygen which weakens the Cf-C bonds.

4 is slower than step 3, since it involves breakage of the strong (unweakened) C-C bonds. They both are rate-limiting, but they are parallel reactions. Their relative importance to the gasification reaction depends on the rate constant of each step and concentrations of each complex. The rate constants i3 and i4 are related by i3 . i4. But the concentrations of Cf(O) and C(O)Cf(O) are related by Cf(O) . C(O)Cf(O) when the reactant is CO2 or H2O. It is hard to say which reaction contributes more to the overall gasification reaction rate. The equilibrium constants K1 and K2 could be measured experimentally if Cf(O) and C(O)Cf(O) could be distinguished. Ergun53 and Strange and Walker54 measured the equilibrium constants with respect to the total oxygen complexes (i.e., including Cf(O) and C(O)Cf(O)). The combined equilibrium constant (K1 + K2(C(O)/Cf) at 800 °C is in the range 0.03-0.10. Both K1 and K2 increase with temperature. Since C(O)Cf(O) is less stable than Cf(O), it is expected that the concentration of C(O)Cf(O) is less than that of Cf(O). The features of catalyzed and uncatalyzed gasification reactions of carbon by the same gases are as follows: (1) they have the same activation energies, (2) the amounts of CO released are different from different reactants, and (3) the amount of CO released is greater for catalyzed reactions. Items 2 and 3 have been observed by both TK and TPD experiments. On the basis of the off-plane oxygen species, we propose a more comprehensive mechanism for alkali and alkaline earth catalyzed gasification reactions of carbon by H2O and CO2. This mechanism can account for all features described above. The mechanism is given by using CO2 as the reactant:

CO2 + * S CO + O*

i2

CO2 + Cf(O) S C(O)Cf(O) + CO j2

i3

C(O)Cf(O) 98 CO + Cf(O) i4

Cf(O) 98 CO + Cf

K2

(2)

K′

diffusion of O* O* + Cf S Cf(O*)

(6) K1′′

(3)

where K stands for equilibrium constant and i and j are rate constants. The symbols Cf(O) and C(O)Cf(O) represent two different types of intermediate, and are shown in Figure 3. The O in C(O) represents the offplane oxygen. The rate-limiting step is C-C bond breakage, step 3. In the mechanism above, step 1 is the formation of the semiquinone intermediate. Step 2 is for the formation of the off-plane oxygen species that weakens the C-C bonds, leading to step 3. Step 4 is the decomposition of the semiquinone species that does not have an off-plane oxygen species adjacent to it. Step

(7) K2′′

O* + Cf(O*) S C(O*)Cf(O*) (4)

(5)

C(O*)Cf(O*) f CO + CCf(O*) + * Cf(O*) f CO + Cf*

(8) i3

i4

(9) (10)

where K stands for equilibrium constant and i for rate constant. * is the catalyst cluster which is a nonstoichiometric compound and could be described as MxOy (where M is the alkali or alkaline earth metal atom and x and y are changeable during the gasification reaction). The MxOy is a nonstoichiometric cluster (compound). At (53) Ergun, S. J. Phys. Chem. 1956, 60, 480. (54) Strange, J. F.; Walker, P. L., Jr. Carbon 1976, 14, 345.


Chen and Yang

the surface of a cluster in contact with the reactant gas phase, this compound could be in the state of alkali or alkaline earth carbonate19,20 (in case of CO2 as the reaction gas) as reaction 5 is not an instantaneous reaction. At the surface in contact with a carbon substrate it could be in the state of metal oxide with an excess of metal as the oxygen is consumed by gasification reaction. The symbols Cf(O*) and C(O*)Cf(O*) represent the two types of oxygen complexes which are substrates 2 and 4 covered by catalyst clusters or particles (or to be more exact, they represent two main groups of oxygen-containing structures, i.e., the in-plane group and in- and off-plane group). The two oxygen intermediates are shown in Figure 3. Cf is the edge carbon site with a free sp2 electron, and C is the saturated carbon atom. In this mechanism the first step takes place at the catalyst surface. The second step, diffusion, may be accomplished by electron transfer between oxygen atoms in the cluster. So the migration of O* is very fast and the diffusion step may be omitted. Like the uncatalyzed reactions, the carbon-carbon bond breakage is the rate-limiting step. It should be pointed out, however, that in the gasification reaction system (both uncatalyzed and catalyzed) other gas-solid reactions could also take place to produce product other than CO (e.g. CO2). But they are not the dominant, primary reaction. In this paper, we focus only on the pathway of oxygen atom in oxygencontaining gases (such as H2O, CO2, NO). Other intermediates not related to oxygen species, even though they may affect gasification reaction, are not considered here. The rate equations of catalyzed and uncatalyzed reactions are

catalyzed:

catalyzed:

K1′′ )

[CO][Cf(O*)] [CO2]

r ) i3[C(O*)Cf(O*)] + i4[Cf(O*)] (11)

Each of the two rate equations consists of two terms: one due to the breakage of the weakened C-C bonds that are caused by the off-plane oxygen species (i.e., the i3 term), and one due to the semiquinone type oxygen complex. From eqs 1-4, one can derive a rate equation by using the stationary state theory, i.e.

d[Cf(O)]/dt ) 0

(13)

d[C(O)Cf(O)]/dt ) 0

(14)

Thus, the i3 term for the uncatalyzed rate equation is r) i1i2 [CO2] i1 + i2 i1j2 i1i2 i3j1 + j1j2[CO][CO2] [CO] + [CO2] + (i1 + i2)i3 (i1 + i2)i3 (i1 + i2)i3 [CO2]

(15) The i4 term remains to be the familiar LangmuirHinshelwood (LH) rate equation:1,5

r)

i1[CO2] j1 i1 1 + [CO] + [CO2] i4 i4

(16)

)

fCOf1′′ fCO2

exp(-∆1/kT) (17)

r ) i3[C(O)Cf(O)] + i4[Cf(O)] (12)

uncatalyzed:

1+

For the catalyzed reaction (eqs 5-10), similar rate equations can be derived. Equation 16 is the traditional Langmuir-Hinshelwood rate equation for C + CO2 and C + H2O reactions.1,5 With the new off-plane oxygen intermediate, Equation 15 is the resulting rate equation when the dissociation of the new complex, C(O)Cf(O), is the ratelimiting step. Equation 15 yields a constant rate at high CO2 concentrations, and thus it has the correct limit. The limiting rate constants for eqs 15 and 16 are i3 and i4, respectively. The rates given by the two equations differ, however, when the CO2 concentration is very small, although both approach zero at zero concentration. When [CO2] is small, eq 16 is a linear function of [CO2], while eq 15 is a function that is proportional to [CO2]2. That is, eq 15 gives an S-shaped curve. The literature results on the low [CO2] data are not adequate to assess if eq 15 is valid or the new mechanism dominates. However, it is expected that eq 16 should dominate (i.e., the i4 term dominates the i3 term in eq 11) when [CO2] is low, because of two reasons: (1) the amount of the off-plane oxygen species is small, and (2) it requires two neighboring off-plane oxygen species for one CfdO to break off (see Figure 3). This may be the reason why the traditional LH rate equation has worked well. Nonetheless, the limiting rates (at high [CO2]) should have a different meaning in light of the new mechanism that we have proposed. An additional function of the catalysts that may also contribute to the catalyzed rates is based on a thermodynamics or transport reasoning. According to chemical reaction theory,49 equilibrium constants K1′′, K2′′, and K1 and K2 can be expressed as

uncatalyzed: K2′′ )

K1 )

K2 )

[CO][C(O*)Cf(O*)] ) [Cf(O*)][CO2] fCOf2′′ exp(-∆2/kT) (18) f1′′fCO2 fCOf1 exp(-∆1/kT) fCO2

(19)

fCOf2 exp(-∆2/kT) f1fCO2

(20)

[CO][Cf(O)] [CO2]

)

[CO][C(O)Cf(O)] [Cf(O)][CO2]

)

Here f stands for partition function of the species indicated by subscripts except subscripts 1 and 2, which correspond to two types of oxygen complexes. ∆1 and ∆2 are the energies of the complexes Cf(O*) and C(O*)Cf(O*) (or the Cf(O) and C(O)Cf(O)) with reference to the reactants, at zero absolute temperature. Energetically, the complexes Cf(O*) and C(O*)Cf(O*) should be the same as Cf(O) and C(O)Cf(O)sin other words, they have the same heat of formation (or enthalpy). However, the states of the complexes C(O*)Cf(O*) and Cf(O*) are very different from C(O)Cf(O) and Cf(O). When there are no catalysts on the carbon surface or when the active sites are covered only by isolated C-O-M groups (where M stands for alkali metal), the oxygen atoms in these complexes can only


Figure 4. Schematic of different states of oxygen complexes in uncatalyzed and alkali catalyzed systems, showing the increase in the off-plane oxygen intermediate by a cluster of catalyst.

exchange with the oxygen atoms in the gas molecules. When the catalyst loading is high, most of the catalyst will exist in the form of clusters or particles. Meanwhile, if the gasification temperature is high (e.g., 700 °C) and the cluster size is small, the atoms (or ions) in these (liquidlike) clusters will gain great mobility. The oxygen atoms in these clusters can migrate from one carbon active site to another with a much smaller energy barrier as compared to the uncatalyzed reactions. This will be especially true for O* on the basal plane. Because this O* atom has a lower diffusion activation energy as compared with the O* atom on the edge surface. Nevertheless, when all the active sites on the edge and basal plane surfaces are covered by catalysts particles, the oxygen atom in the active complexes will be able to move from one active site to another without exchanging with oxygen atoms in the gas reactant molecules. These extra degrees of freedom will result in a larger partition function of the active species in clusters as shown by the difference of localized adsorption and completely mobile adsorption processes.55 If the oxygen atoms in catalyst particles are completely mobile as in a two-dimensional fluid, the difference in partition functions between mobile and localized situations can be as large as a factor of 104.55 This means that the equilibrium constants K1′′ and K2′′ in the catalyzed environment will be much greater than K1 and K2 in the uncatalyzed environment. Thus, if the surface coverages of C(O*)Cf(O*) ad Cf(O*) are very small, the concentrations of C(O*)Cf(O*) and Cf(O*) should increase by the same order of magnitude by the catalysts. In the C/CO2 (or C/H2O) system, the coverages of C(O*)Cf(O*) and Cf(O*) are indeed very low, but the oxygen atoms are not as mobile as in a twodimensional fluid. Hence the increase in gasification rate by the catalyst will be much less than 104. Figure 4 is a schematic picture of different states of oxygen atoms on the uncatalyzed, C-O-K group covered and the particle (or cluster) covered carbon surface. This mechanism can account for all major experimental features. For example, many authors have observed32,36 that alkali and alkaline earth catalysts only increase the gasification rates but the activation energy remains unchanged. Freund32 observed that potassium can increase the gasification rates by 2 orders of magnitude and attributed it to increases in the active sites. It is well-known that the active sites for the carbon-CO2 and carbon-H2O reactions are the edge sites;5 the basal plane carbon atoms are usually not gasified by the CO2 and H2O. Therefore, the catalytic effect can only be attributed to an increase in the active intermediate but not the active sites. The mechanism proposed in the present study not only provides an (55) Dumesic, J. A.; Rudd, D. F.; Aparicia, L. M.; Rekoske, J. E.; Trevino, A. A. Microkinetics of Heterogeneous Catalysis; American Chemical Society: Washington, DC, 1993.


explanation of why catalysts can change gasification rates without changing the activation energy but also gives an explanation of how the catalysts change the concentration of active complexes. The higher concentration of the oxygen complexes (i.e., off-plane oxygen) on the carbon surface will decrease the C-C bond energy and thus decrease the activation energy of the reaction. This effect would be strong if more than two neighboring bridge atoms (like C28) were occupied by O* atoms. The probability of this is low because in carbon-CO2 and carbon-H2O reactions (catalyzed or uncatalyzed), the total concentration [C(O*)Cf(O*)] + [Cf(O*)] is only a small fraction of the total edge sites. But in the carbon-O2 reaction, this could be the case and the C-C bond energy will be significantly weakened, and hence the activation energy should be decreased. This remains to be proven by experiments. Relative Activities of Different Alkalis According to our proposed mechanism, the concentration of the oxygen atom chemisorbed on the bridge C atom (C28) plays an important role in the gasification reaction. The affinity between the bridge C atom and the O atom is strongly affected by the local electron density of the bridge C atom. By attaching an alkali atom to the edge, the net charge of the bridge atom is substantially increased because of the conjugate effect. Since H2O and CO2 prefer to chemisorb on the atoms with larger densities, the existence of alkali metal will facilitate the chemisorption of H2O and CO2 on the bridge C atom, and as a result, the C-O-M group shows the catalytic activity. In much the same manner, when alkali atoms exist on the edge, the neighboring edge C atoms with the free sp2 orbital also increase their net charge. This means that alkali atoms can increase the concentration of the in-plane type of complex and, consequently, increase the gasification rate. This effect is important especially for the carbon-H2O and carbon-CO2 reactions. In these reactions the complex concentration is very small and most edge sites are free; hence any favorable factors would increase the concentration of the complexes. On the other hand, it may not be important for the carbon-O2 reaction, because most of the edge sites are already occupied by O atoms in this reaction, so the concentration of the complexes will not increase. The discussion above can also account for the rank order of activities of different alkali metals. According to our molecular orbital calculations using both MNDO(PM3) and CNDO, the net charge of the bridge atom C28 with different alkali metals has the following rank order: Li < Na < K ) + (see Table 1), because the larger the net charge, the larger the affinity between C28 and O. Hence the rank order of activity follows that of the net charge of the carbon atom (C28) with different alkalis. In addition to the effect of the net charge, two other factors may also be important: the ability of the catalyst clusters to dissociate the reactant molecule (CO2 or H2O), and the mobility of dissociated O atoms that is facilitated by the catalyst. Acknowledgment. This work was supported by the National Science Foundation under Grant CTS 9523801. EF960099O

Unified Mechanism of Alkali and Alkaline Earth Catalyzed Gasification

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