Ind. Eng. Chem. Res. 1993,32, 2835-2840
2835
A New Surface Oxygen Complex on Carbon: Toward a Unified Mechanism for Carbon Gasification Reactions S. G. Chen and R. T. Yang' Department of Chemical Engineering, State University of New York at Buffalo,Buffalo, New York 14260
F. Kapteijn and J. A. Moulijn Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands
Molecular orbital theory calculations and recent high-temperature T P D experiments revealed the feasibility of a new type of active oxygen complex on graphite. This type of complex is formed by bonding of an oxygen atom to a saturated carbon atom (in the caved-in position) adjacent to the unsaturated edge carbon atom that is already bonded t o an oxygen atom. Molecular orbital calculation results also show that this type of complex is substantially more active than the known complexes that have been discussed in the literature, e.g., semiquinones and carbonyls. On the basis of this new type of complex, a unified mechanism is proposed for the gasification reactions of carbon with all oxygen-containing gases. The proposed mechanism can account for all major features of the published experimental results on temperature-programmed desorption, transient kinetic, and steadystate rate studies of the gas-carbon reactions.
Introduction Voluminous literature exists on the mechanisms of the gasification reactions of carbon by oxygen-containing gases such as 0 2 , Con, H20, NO,NzO, and SO2. Despite the large amount of literature, these reactions are not well understood; this is particularly the case for the C-02 reaction. The present understanding of these reactions has been described in reviews (Walker et al., 1959;Ergun and Mentser, 1965; Walker et al., 1968; Yang, 1984; Kapteijn and Moulijn, 1986; Walker et al., 1991). The key to the understanding of the gas-carbon reactions lies in active surface complexes, and the surface oxygen complexes play a dominant role in the reactions involving oxygen-containing gases. Studies on the early work on surface oxygen complexes were reviewed by Boehm (1966) and Puri (1970). However, it is important to note that many of the surface groups are only stable at near-ambient temperatures and, hence, may not play any role in the gasification reactions which take place at much higher temperatures. The temperature for the C-02 reaction is generally higher than 400 OC and that for the C-COz and C-HzO reactions is higher than 700 "C. Only surface groups with intermediate stability at these temperatures contribute to the reactions; highly stable groups may actually be considered as poisons (Walker et al., 1991). More recent research has made significant use of the temperature-programmed desorption (TPD) and transient kinetic (TK) techniques. Followingthe thermal desorption work of Laine, Vastola, and Walker (1963),the TPD studies have been aimed at measuring and characterizingthe active sites on different carbons (Tremblayet al., 1978;Kelemen and Freund, 1985;Su and Perlmutter, 1985;Floess et al., 1988;Marchon et al., 1988a and 1988b;Lizzio et al., 1988; Kyotani et al., 1988;Zhang et al., 1988;Zhu et al., 1989; Hall et al., 1989;Du et al., 1990;Huttinger and Nill, 1990; Lizzio et al., 1990;Radovic et al., 1991;McEnaney, 1991; Walker et al., 1991). Most recently, Pan and Yang (1992) detected a significant amount of oxygen complex on graphitic carbon (but not on nongraphitic carbon) which desorbed as CO at temperatures higher than 1200 "C. On the basis of this result and molecular orbital calculations,
* To whom correspondence should be addressed.
they proposed an oxygen complex which was formed by bonding oxygen atoms on the lattice carbon atoms that were saturated (Pan and Yang, 1992). It was shown that the caved-in carbon atoms on the zigzag edge of graphite could form bonding with oxygen atoms and that such bonding would significantly weaken the carbon-carbon bonds, hence leading to gasification. The TK experiment involves thermal desorption immediately following the termination of the reaction or switching between different labeled reactant molecules. Recently, the TK technique has been used to study the mechanism of carbon gasification reactions (Kapteijn et al., 1991; Meijer, 1992; Kapteijn et al., 1992) and the oxygen-containing complexes on carbon surface and to determine the reactive surface area (or working surface area) for gasification reactions (Lizzioet al., 1990;Radovic et al., 1991;Adschiri et al., 1991). The procedures of TK experimentshave been described in the literature (Meijer, 1992;Kapteijn et al., 1992). By combiningwith the isotopic tracing technique, TK studies from an ideal approach for obtaining direct information on chemical reactivity, pathways of surface species, number of intermediates present on the solid surface, and rate data of elementaryprocesaes. At high reaction temperatures, as in carbon gasification, it is a useful in situ technique and many interesting observations have been obtained (Lizzio et al., 1990, Radovic et al., 1991;Adschiri et al., 1991;Kapteijn et al., 1991;Meijer, 1992;Kapteijn et al., 1992). Although TPD and TK experimental studies have provided important insights into the nature of the active surface complexes as well as the reaction mechanism, some interesting results remain unexplained and more questions have been raised. In this study, the molecular orbital theory is employed to address these unanswered questions, and from the results, a uniform mechanism involving the carbon-oxygen complex is proposed for all reactions with oxygen-containing gases.
Analysis of TK and TPD Results
Zn TK experimental studies, one of the interestingresults was that, upon desorption, the concentration of CO that was desorbed did not decay exponentially (Freund, 1986; Lizzio et al., 1990;Huttinger and Nill, 1990; Radovic et
0888-5885/93/2632-2835$04.00/00 1993 American Chemical Society
2836 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
al., 1991;Kapteijn et al., 1991;Adschiri et al., 1991),which indicated that there were more than one kind of surface complexes. With a carefullydesigned experimental system (yielding a response time of about 1 s), Kapteijn et al. (1991) found that there were mainly two types of CO desorption decay (a similar phenomenon was also found by Bonner and Turkevich in 1951; see Bonner and Turkevich (1951)). One was a fast decay, and it ceased to release CO in several seconds; another was a slow decay which lasted for several minutes. They also found that the fast decay ceased a t the moment when CO2 could not be detected in the gas phase. This result implied that the surface oxygen complex which was responsible for the fast decay was related to the presence of C02 in the gas phase. In order to interpret this result, the following four-step mechanism was proposed: CO,
-
+ Cf co + C(0)
+ + C(0) co + 2C-(CO)
CO2 Cf
(2)
et al., 1988; Zhang et al., 1988; Marchon et al., 1988a and 1988b; Wang and McEnaney, 1989). This result is consistent with the fact that significant carbon gasification for the C-02 system can take place at temperature as low as 400 "C whereas the C-C02 and C-H20 reactions occur a t near 700 OC and above (Walker et al., 1959). It is also consistent with the fact that the activation energy for the C-02 reaction is generally lower than that for the C-C02 and C-H20 reactions, e.g., 58 kcal/mol vs 85 kcal/mol (Walker et al., 1959). Considering these facts, it seems that under gasification conditions,whether in C-02 or C-CO2 and C-H20 systems, there exists on the carbon surface a semiquinone type of complex which is relatively stable, and in addition, there may also exist another type of active complex. This type of complex is more active and decomposes at lower temperatures and hence would make a major contribution toward the gasification in the carbon-02 system. Furthermore, it seems reasonable to postulate that such an active complex is related to the semiquinone structure and may actually be based on it.
Semiempirical Molecular Orbital Calculations
This mechanism could account for the two different decays in CO desorption, but the nature of C-(CO) (a carbonyl type) and C(0) (semiquinone type) was not understood and the model needed further investigation. Using isotope tracing with labeled C02, Kapteijn et al. (1991) were able to separate these two different complexes and, consequently, measured their concentrations and desorption rate constants under various conditions. At 1200 K, k, and k b were about 0.30/s and 0.015/s, respectively, and the concentrations of C-C(0) and C(0) were dependent on the temperature and partial pressure of COz. But in all cases, the concentration of C(0) was greater than that of C-C(O). As mentioned, a large body of information is available in the literature concerning TPD of different carbons preoxidized in different gases (i.e., 0 2 , C02 and H20) under different conditions (Tremblay et al., 1978;Suuberg et al., 1986; Floess et al., 1988; Kyotani et al., 1988; Zhang et al., 1988; Marchon et al., 1988a and 1988b; Wang and McEnaney, 1989; Huttinger and Nill, 1990; Lizzio et al., 1990; Du et al., 1990; Calo and Hall, 1991; Brown et al., 1992; Pan and Yang, 1992). Although the CO desorption patterns differ in shapes and peak positions, all spectra showed a distinct peak in the neighborhood of 950 "C. This was true despite the different oxidizing gases and types of carbon samples that were used, and the carbon samples were oxidized to different burn-off levels. The total amounts of CO released as well as the widths of the desorption peak were also different. An example of the TPD studies was that by Marchon et al. (1988a),who used TPD and XPS to investigate a polycrystalline graphite treated with 0 2 , C02, or H20. All of their TPD spectra exhibited a CO desorption peak in the range 973-1253 K. With the aid of XPS results, this TPD peak was assigned to the desorption of a semiquinone structure on the edge surface of graphite. A major difference has also been observed among the TPD spectra from carbon oxidized by 0 2 and those oxidized by C02 or H20. The desorption of CO from the 02-oxidized samples starta a t substantially lower temperatures, as low as 400 OC, resulting in broad TPD peaks or as a shoulder on the low temperature side of the 950 OC peak (Kyotani
Molecular orbital (MO) theory has been proven to be of great utility in our understanding of chemical bonding. It has been used to study chemisorption on graphite surfaces (Bennettet al., 1971;Hayns, 1975;Chenand Yang, 1989; Pan and Yang, 1990,1992). In this study we used the INDO (intermediate neglect of differential overlap) method, which is modified from the CNDO (complete neglect of differential overlap) metthod (Sadlej, 1985). The computer program was obtained from the Quantum Chemistry Program Exchange, Indiana University (Rinaldi et al., 1990). The INDO program contained geometry optimization for the lowest-energy structures. Five sukstrates involving oxygen complexeson the zigzag edge (loll] face of graphite are chosen in the study as shown in Figure 1. Complexes of similar structures on the armchair edge (1022) face are not energetically feasible (Pan and Yang, 1992) and hence are not considered here. Structure a is the substrate for the zigzag edge of graphite; b is the substrate for semiquinone; c is for carbonyl; and d and e are similar to b and c but have off-plane oxygen atoms chemisorbed on the saturated carbon atoms next to the semiquinone or carbonyl structures. Structure cis chosen because structure d may create structure c after releasing CO molecules during gasification, as shown in Figure 2. Reasons for choosing structures d and e will become clear shortly. Because the unoccupied edge-site carbon has a free sp2 orbital, in order to decrease the edge site effect hydrogen atoms are used to saturate the edge-site carbons (except the edge site carbon atoms of interest in this study), as was done by Hayns (1975). However, the substrates used in this study are much larger than those used by Hayns (1975);hence, the edge effect in our case is not as important as in his case. The results on bond strength are given in Table I. The bond strengths obtained by CNDO or INDO are usually about five times as large as the experimentalvalues(Hayns, 1975), but the relative values are important and reliable for comparison purposes. From Table I it is seen that the semiquinone structure and the carbonyl structure are both stable and that the carbon-carbon bond strengths in these two structures are only slightly weaker than that of the C-C bond in the graphite substrate (see structure a). The diatomic energy of CM and C4-5 in substrate a is 578.42 kcal/mol and that of C ~ and A Cpg in substrate b is 515.89
Ind. Eng. Chem. Res., Vol. 32,No. 11, 1993 2837 Table I. CarbonCarbon Bond Strength in Different Substrates in Terms of Diatomic Energy (kcal/mol)' no. structure a structure b structure c structure d (32-9 569.20 507.31 493.42 416.92 C2-4 578.42 515.89 505.89 360.80 C4-5 578.42 515.89 505.89 360.80 416.92 C6-5 569.20 507.31 493.42 861.03 ('2-0) c3-21 660.49 (C-0) 572.56 861.03 (C-0) CW22 660.49 (0) 572.56 CZl-23 864.81 (C-0) CZZ-24 864.81 (C-0) C3-26
C6Z3
456.61 (C-0)
c423 a
structure e 402.33 386.98 386.98 402133 434.88 434.88 789.82 (C-0) 789.82 (C-0) 480.06 (C-0) 480.06 (C-0)
Empirically, the bond energies are of the order of 1/5of the diatomic energies.
Table 11. Net Charge of Some Carbon Atoms in Different Substrates (Unit = Electron) no. structure a structure b structure c structure d structure e 2 -0.1570 -0.0863 0.1700 -0.1193 0.1508 3 -0.3850 0.2622 -0.4291 0.1468 -0.2226 4 -0.0027 -0.1579 -0.2332 -0.3605 -0.3393 5 -0.3850 0.2622 -0.4291 0.1458 -0.2226 6 -0.0863 0.1507 0.1700 -0.1193 0.1508 8 -0.0790 -0,0296 -0.1865 0.0425 -0.0352 -0.2054 0.2014 -0.1136 9 -0.2552 -0.1151 -0.1858 -0.1743 -0.2386 0.2524 -0.1568 10 -0.2562 -0.1151 -0,2054 0.2014 -0.1136 11 12 -0.0790 -0.0295 -0.1865 0.0425 -0.0352 ~
a 024
230
H*
b
H
* C
H
12
d
e
Figure 1. Structures of chemisorbedoxygen on graphite. All atoms are in-plane except the oxygen atoms numbered 23,25,and 26.
4 CO
d
C
Figure 2. Transformation of structure d into structure e upon CO desorption.
kcal/mol. The corresponding values for C3-21and C5-22 in substrate c, which are the bonds to be broken to release CO, are 572.54 kcal/mol. This result indicates that both semiquinone (substrate b) and carbonyl (substrate c) structures are stable and further bonding with oxygen is necessary to weaken the surface carbon-carbon bonds in order to release CO. Due to the large electronegativity of
~~
oxygen, the carbon atoms with the highest negative charges should be the targets for bonding with oxygen atom. The net charges of carbon atoms in different substrates are given in Table 11. From Table I1it is seen that carbon atom 4 in substrate b has a strong negative net charge (-0.15) and carbon atoms 3 and 5 in substrate c gain a strong negative net charge of -0.43. These sites are most liable to the chemisorption of 0 atoms from the reactant gas molecules and subsequently create structures d and e. Results given in Tables I and I1show that the off-plane oxygen atoms chemisorbed on the saturated carbon atoms give rise to large changes in the local electron densities of the carbon atoms. It is significant to note that when oxygen atoms chemisorb on these saturated sites the neighboring carboncarbon bonds are weakened to a large extent. As shown in Table I, the diatomic energies of (33-4 and (24-5bonds in substrate d and substrate e are only 360.80 and 386.98 kcal/mol, respectively. In substrate e, the diatomic energy of C3-21and C5-22 is 434.88kcal/mol. The bond strengths between the off-plane oxygen atom and the in-plane carbon atoms in substrates d and e are 456.61 and 480.06 kcal/ mol respectively. Thus, it is easier to break the surface C-C bonds rather than the C-O bonds in substrates d and e. The bond strength is different from the activation energy for gasification,but they are directly related. From the results discussed above, one sees that substrates d and e should be much more active than substrates b and c; consequently, substrates d and e will decompose at lower temperatures. For substrate e, there are two possibilities for C-C bond breakage leading to gasification. When C-C bond breakage takes place between carbon numbers 3-21 or carbon numbers 5-22 in substrate e it creates a semiquinonestructure b and release CO. If the C-C bond breakage takes palce between carbon numbers 2-3 and 3-4 or carbon numbers 4-5 and 6-7,a 0-C-C-O molecule is released. Since the dimeric CO is not stable in the gas phase two CO molecules are released. The four oxygen-containing structures discussed above are simplified models of oxygen-containing active complexes on carbon surface. However, it seems reasonable to assume that there are two main groups of active oxygen-
2838 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
containing structures on the edge sites. One is the inplane group which contains oxygen atoms in the plane of the graphite basal plane as shown in substrates b and c; another contains both in-plane and off-plane oxygen atoms as shown in substrates d and e. The former group maintains stronger bonding with the carbon substrate, and they decompose at higher temperatures (near 950 "C), whereas in the latter group the carbon-carbon bonds are substantially weakened, and they decompose a t lower temperatures. This latter group is responsible for the gasification reaction of carbon in oxygen, which occurs at temperatures much below 950 OC. It is worth noting that carbon atoms in the basal plane near edge sites have some heterogeneity; it is easier for oxygen atoms to chemisorb on the sites with larger net charges. The chemisorption will take place near the edges at first, but if the oxygen atoms in the gas phase have sufficiently large activities (for example, in pure oxygen and at a higher partial pressure), oxygen atoms may chemisorb on the basal plane carbon atoms located far from edge sites. Pan and Yang (1992) have discussed this possibility in connection with TPD at temperatures above 1200 "C. From the discussion above, it is reasonable to assume that substrates b and c are associated with the TPD peak of CO desorption around 950 "C and that substrates d and e are responsible for the CO desorption at lower temperatures. The latter structures are apparently abundant in the C-02 reaction. This result leads us to propose a unified mechanism for carbon gasification reactions with oxygencontaining gases.
Unified Gasification Mechanism From the above discussion the following mechanism for gasification of carbon by oxygen-containing gases is proposed. Taking C02 as an example, the mechanism is expressed as follows (for 0 2 and H20, reactions 7 and 8 are the same while reactions 5 and 6 are replaced by the dissociative chemisorption of these gas molecules) CO,
+ Cf
-
Cf(0) + co
K,
C02 + Cf(0) e C(0)Cf(O)+ CO
--coco++
C(O)Cf(O)
Cf(0)
(5)
K,
k,
(6) (7)
Cf(0) Cf k4 (8) where K stand for equilibrium constant and k for rate constant. The symbols Cf(0) and C(O)Cf(O)represent the complexes in substrates b, c and d, e, respectively (or to be more exact, they represent the two main groups of oxygen-containing structures, i.e., in-plane group and offplane group). Cf is the edge carbon site with a free sp2 electron, and C is the saturated carbon atom. This mechanism can account for all results from TK and TPD experiments. As mentioned, Kapteijn et al. (1991) observed two types of decay in CO desorption in their TK experiment (see Figure 3). The first decay is a fast decay that releases a relatively small amount of CO, and the second decay is slower but releases a larger amount of CO. From the mechanism proposed here, the first decay is caused by the complexes with structures shown by substrates d and e and the slow decay is attributed to the complexes containing semiquinone and the carbonyl structures. Because the C(O)Cf(O)structure is less stable than the Cf(0)structure, k~ is greater than k4. The other result on the different amounts of CO released during the TK and TPD experiments can be explained by the proposed mechanism as follows.
-2
i
Total CO
1-
0.75
E
8 0.50 E
8
6 0.25 0.00
'.->--, 0
10
----__~ ,
,
20 30 Time ( 5 )
40
50
Figure 3. Elution rates (pmolls) as a function of time (8) at 1,250 K after a step change of 10% 13C02 in Ar to pure Ar. Sample: 100 mg of Norit RX extra activated carbon (for the experimental details see Kapteijn et al. (1992)).
The equilibrium constant K1 for the carbon402 system has been measured by Ergun (1956) and Strange and Walker (1976). The value of K1 increases with temperature. At 800 "C K1 is in the range 0.03-0.10. Because C(O)Cf(O)is less stable than Cf(O),it is expected that K2 is much less than K1, i.e., KZ
