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On the mechanism of catalysis of the Boudouard reaction by alkali-metal compounds
W~8-5223/82/030207-0650).0010 Pergamon Press Ltd.
Carbon Vol. 20, No. 3, pp. 207-212. 1982 Printed in Great Britain.
ON THE MECHANISM OF CATALYSIS OF THE BOUDOUARD REACTION BY ALKALI-METAL COMPOUNDS Y. K. RAO, A.
ADJORLOLOand J. H. HABERMAN
Metallurgical Engineering, FB-10,University of Washington,Seattle,WA 98195,U.S.A. (Received 31 August 1981) Abstract-The mechanism of catalysis of the Boudouard (C-C02) reaction by the alkali-metal compounds appears to involve gaseous intermediates M(g), CO(g) and CO2(g) where M(g) may be Li, K, Na, etc. The cyclical mechanism M(g)+M2CO3+M(g) withstood the rigorous thermodynamic analysis presented here and may be regarded as the most likely mechanism. Detailed calculations showed that formation of intercalation compounds of the type C2,M is not very likely except at very high temperatures (1400K and above) and under nearly pure CO environments. Thus the likelihood of intercalation compound mechanism being valid is not substantial except under stringent conditions as noted. The electrochemical mechanism probably operates parallel to the cyclic mechanism; and its relative contribution to the overall reaction is not known at this time.
1. GENERAL
be noted, is characterized by the C-CO? reaction as the rate-controlling step. Wen[l6] extended the cyclical mechanism (of McKee and Chatterji)[ll] by suggesting that the alkali metal liberated by carbothermic reduction of MXOs forms intercalates, or alkali metal-carbon compounds, which then partake in the catalysis. Wen[16] calls these compounds CT (charge-transfer) complexes or EDA (electron donor-acceptor) complexes. Jalan and Rao[l2] proposed an electrochemical mechanism for the catalysis of C-CO, reaction by L&CO3 (or LiaO). The main features of this mechanism are that the catalyst serves as a conductor of 02- ions and that the underlying carbon particle becomes corroded progressively as carbon combines with 02- ions to form CO gas. This mechanism is likely to be more important when the catalyst is in molten state as it would greatly facilitate the transfer of 02- ions. The purpose of this note is to present a detailed thermodynamic analysis of the different mechanistic schemes and draw conclusions as to the possible ranking of alkali catalysts.
The catalysis of the Boudouard (C-COZ) reaction has been investigated by numerous researchers [ l-141. Recognition of the catalytic influence of alkali-metal compounds dates back to the early work of Taylor and Neville[l]. They studied the catalytic activities of a number of different substances and concluded that potassium and sodium carbonates are efficient catalysts for the Boudouard reaction. Fox and White[3] found that sodium carbonate enhances the rate of the Boudouard reaction and thereby produces gases richer in carbon monoxide and hydrogen in a gas producer. Recently, the subject of catalysis of carbon-gas reactions has been receiving greater attention mainly due to the revival of interest in coal gasification and other coal conversion processes. Despite the wealth of factual information that exists on the catalysis of C-CO2 reaction, the mechanism of catalysis by alkali-metal salts remains imperfectly understood. The oxygen-transfer mechanism [lo] which adequately accounts for the observed catalytic behaviour of transition metals and their compounds, has not been found satisfactory in interpreting the marked catalytic activity of alkali-metal salts. Harker[6] proposed that the alkali salts tend to neutralize the acidic surface oxide on carbon and thereby favor the formation of alkali metal-carbon complexes. McKee and Chatterji[ll] favor a catalytic mechanism which involves a cyclical sequence M2COs+ M + MXO,; a similar mechanism was proposed earlier by Fox and White[3] for explaining the catalytic action of sodium carbonate. McKee and Chatterji[ 111found that the catalytic activity diminishes progressively from LiXO, to Na$ZOs and proposed the following ranking:
2. CYCLICAL. MECHANISM
This mechanism consists of the following three steps which together constitute a cyclic sequence: Reduction: MzCO& 1)t 2C(s) = 2M(g) t 3CO(g).
(cl)
Oxidation: 2M(g) + CO&) = M2O(s, 1)+ CO(g).
LXO, > C&O3 > RbXO3 > KzCOs > Na2C03.
(c2)
Carbonation:
This ranking, however, diverges from the results of Taylor and Rao[lS] who found, in connection with the carbothermic reduction of iron oxides, that potassium carbonate is a much better catalyst than lithium carbonate. The carbothermic reduction of iron oxides, it will
MzO(s, 1)+ CO*(g) = MzCOs(s, 1).
(c3)
Reactions (~2) and (~3) are expected to proceed rapidly and in all likelihood reaction (cl) is the rate-controlling step. It will be noted that the summation of these three 207
Y. K. RAOet al.
208
the JANAF Tables [17] . At 800 K:
steps yields the Boudouard reaction, i.e. 2C(s) t 2COz(g) = 4CO(g).
K,, = 2.3922 x lo-”
(B)
PK = 3.7163x 10e6atm.
Reactions (~2) and (~3) when combined yield 2M(g) t 2CO&) = MXOds, 1)+ CO(g).
(c4)
Thus the cyclic sequence may also be viewed as consisting of steps (cl) and (~4). In the foregoing M is any alkali metal. The present analysis is based on the following premises: (1) Assume that reaction (cl) is at equilibrium in an isolated system; this yields P&(P&) values which correspond to the equilibrium of reaction (cl). (2) Assume that reactions (cl) and (~4) are simultaneously at equilibrium. This implies that the Boudouard reaction is also at equilibrium. The corresponding values of PM are found. (3) The difference (P&-P,,), labeled “V, signifies the “excess” or “available” alkali vapor which partakes in the catalysis. (4) Larger values of “8” indicate greater catalytic activities. A detailed picture of how this mechanism might operate is shown in Fig. 1. Reaction (cl) produces M(g) which is then oxidized by CO*(g) to carbonate, as per reaction (c4), which then deposits near the original carbonate melt (or solid) on the carbon surface. The driving force for this mechanism, as indicated above, is measured by the quantity S. At equilibrium of reactions (cl), (~4) and (B), clearly M(g) will have the lowest concentration. At equilibrium of reaction (cl), in contrast, M(g) will have the highest concentration. Let us find the values of P;X and PM for the case of potassium. KzCO&, 1)t 2C(s) = 2K(g) + 30(g).
Similar calculations were made at other temperatures. The data are presented in Fig. 2 on a semi-log plot (log Pff vs T). In order to calculate PK, the partial pressure of potassium when the three reactions are at equilibrium, we need to consider, in addition to eqn (kl), the following: 2K(g) f 2CO&) = KXOs(s, 1)+ CO(g)
(k2)
C(s) t co2 = 2CO(g).
(W
Only two of these three reactions are truly independent. We have for this system: Species: KXOs, K, C, CO, COZ; N = 5. Phases: KXO3, C, gas; p = 3. Independent Equilibria: r = 2. Number of stoichiometric constraints, s = 0. Number of special constraints, t = 0. Application of the phase rule yields F=N-pt2-r-s-t=5-3+2-2-O-0=2.
The system possesses 2 degrees of freedom. Thus, both temperature and pressure must be defined. PKtPcotP
(kl)
Furthermore
coz=P=
latm
(a)
K., = PK*P&
(b)
Ke>= Pco/PK*P&.
(c)
These three equations are solved simultaneously. For instance, at 800 K:
PC0 = 1.5PK(stoichiometry).
The equilibrium constant is given by 5 K,, = P,‘P& = 3.375PK .
PK = 1.675X lo-‘* atm, PCO= 9.482 x lo-* atm, PC@ = 0.9052 atm
The requisite thermodynamic data were obtained from
Thus, for this temperature, we find that SK is equal to
M$03(1) 2td(g)
+2cCs) + 2CO2(9)
carbon Fig.
= 2Mt.J)
+ 3CO(g)
= M2C03(_e)+Co(g)
0 .
@
1100 1000 Temperature
surface
1. An expanded view of the carbon/catalyst junction. Reaction paths are indicated.
Fig.
2.
1200 (K)
1300
1400
Equilibrium vapor pressures of potassium at different temperatures (see text for explanationof Pi and PK).
209
Mechanism of catalysis of the boudouard reaction 3.7163 x 10m6atm. Similar calculations of PK for the three-reaction simultaneous equilibrium were performed at other temperatures. The results are shown in Fig. 2. The action of NazC03 may be analyzed similarly. For example at 800 K, we find
PA, = 2.3491 x 10m6atm PNa = 5.322 x lo-” atm SNa= 2.3491 x 10m6atm. Likewise for lithium carbonate, at 800 K, we find P& = 1.3069X lo-‘atm
d / 800
~_~A.
900
IO00
1100
Temperature (K)
Fig. 4. Equilibrium vapor pressures of lithium at different temperatures (see text for explanation of P& and PL,).
PLi = 1.229X 1O-‘8atm SLi= 1.3069X lo-’ atm. The results for NazC03 and LX03 are shown in Fig. 3 and 4, respectively. If our premises, stated earlier, are accepted, then it follows that potassium carbonate, being associated with the largest S-values should prove to be the best catalyst; and the lithium carbonate the least effective of these three carbonates. Needless to say that sodium carbonate lies in between but decidedly closer to KXO3. As stated earlier, in the carbothermic reduction of iron oxides the catalytic activities were found to rank as K2C03 > NaXO, > LiXO3.
3. INTERCALATION COMPOUND MECHANISM
Wen[ 161modifies the cyclical mechanism by proposing the formation of intercalation compounds (carbon-alkali metal) which partake in the reaction mechanism. Thus we have: MXO3(s, 1)f 2C(s) = 2M(g) t 3CO(g) 2M(g) t 2nC(s) = 2C,M(s)
(w2)
2&M(s) t COdg)= (2nC). MlO(s) t CO(g) (~3) (2nC)~MzO(s)+COz(g)=2nC(s)tMzCO~(s,
1). (w4)
Reactions (~3) and (w4), when added, yield 2&M(s) t 2COz(g) = 2nC(s) + MXO&
One of the factors which might upset this ranking is the slowness of reaction (kl). For instance at a temperature of, say, llOOK, K2COs and NazCOj are both in solid form whereas Li2C03 is in liquid form. This might facilitate the reduction of LiZCOj while that of K2C03 and NaX03 remains sluggish at 1100K. This might lead to the conclusion, justifiably so, that L&CO3 is more powerful than the other carbonates at temperatures below the melting points of Na2C03 (and K2COX)but at temperatures at which LX03 is molten. At temperatures like 12OOK,where all these carbonates are in molten state, the present analysis predicts that K&O3 is the most potent catalyst among the three carbonates considered here.
(WI)
1)f CO(g) (w5)
Here M is alkali metal Li, Na, K, etc. and n = 6, 12, or 18 for Li; 64 for Na; and 8,24,36,48 or 60 for K, Rb or Cs. Underlying this mechanism is the assumption that the intercalation compounds of the type &M(s) are stable at reaction temperatures and atmospheres normally used in the study of the catalysis of the Boudouard reaction. There is a tremendous lack of thermochemical data pertaining to the intercalation compounds. The data of Aronson et a/.[181 and Berger et a/.[191 do not extend to high temperatures. The possibility of intercalation compound formation is first investigated for KXOs catalyst. The analysis is based on the following premises: (1) K(s) is the standard (or reference) state for &K(s). (2) The solid solution C,.K(s) is ideal, i.e. it obeys Raoult’s law. Hence a~ = xK = mole fraction of K = 1/2n t 1, i.e. 2n = (l/aK)- 1. (3) The gas-phase composition corresponds to the Boudouard equilibrium. The equations to be considered are as follows: KzCO3(s, 1)t (4n t 2)C(s) = 2Cz.K(s) t 3CO(g)
(il) Temperature
(K)
Fig. 3. Equilibrium vapor pressures of sodium at different temperatures (see text for explanation of PA, and P&.
KzCOs(s, 1)t 2C(s) = 2K(s) + 3CO(g)
(i2)
4nC(s)+ 2K(s) = 2Cz,K(s); ideal
(i3)
Y. K. RAOet al. Table 1. Thermodynamics of intercalation compound formation in K&O& mixtures
1300
2.2760~10-~
1.5097x10y
185.3688
0.99466
186.36350
4.8O93xlO-3
207
1400
3.2077~10-~
1.0189~10~
571.6822
0.99826
572.68010
5.6784x10-*
16.6
2(&K(s) t 2CO&) = KEOs(s, 1)+ CO(g) +4&(s).
From eqns (1) and (6) we have (i4) Kir =
aK*q?&.
(8)
The equilibrium constants are defined by Upon combining eqns (7) and (8) and simplifying Kir = ~aK2Pg0 = eK*p3 co aKzCO#k
Ki4=
(1) (9)
2
aK PC@’
(2)
Additionally PC0 t Pcoz = 1 atm.
(3)
Thus, from the knowledge of q (r and Pco), Kir and Kid we can calculate the value of OK, hence xK (and 2n), at any given temperature. The following example will illustrate the method of calculation. Consider a temperature of 800 K:
Let r = P~o/Pcoz be the equilibrium constant for the Boudouard reaction. It can be shown that r = (KitKid)“*
(6)
Upon combining eqns (2) and (6) we obtain
Kid =
qPc02 2 2 = ,,*z, aK PC02
PC02 = aKflK.4
r =
(7)
9.9311X low3
q = 0.10474. Hence
(5)
Using thermochemical data from standard sources [ 171, the values of T and PC0 were calculated at a number of temperatures. These data are given in Table 1. Furthermore, let (KirKid)“* q = PcolPco2 = r/P00 = p. P co
AG!d= -314,884 J; Kid= 3.6287X 1020
(4)
The equilibrium PC0 value, for the Boudouard reaction is given by Pco=-0.5r+0.5(r2t4r)“*.
AGPl= 376,238J; Kir = 2.7180 X 1O-25
a, = 1.7858x 10-l’ and 2n = 5.5992x 10”. Similar calculations were performed at other temperatures. Only at a temperature of MOOK does the value of 2n appear reasonable from the view-point of intercalation compound formation. At low temperatures, in the presence of even a small concentration of CO2, the intercalation compound readily breaks down. The results of these calculations are given in Table 1. Similar analysis was carried out for sodium and lithium carbonates for which the results are summarized in Tables 2 and 3, respectively. Here too the evidence is unmistakable that the intercalation compound is not predisposed to form except at very high temperatures, i.e. MOOK and higher.
211
Mechanismof catalysis of the boudouardreaction Table 2. Thermodynamicsof intercalationcompoundformationin Na,CO,/C mixtures P2
TK
K
Kil
i4
r.=co
%02
9.9311x1O-3
2n
pco atm
q
9.4813x1O-2
0.10474
4.7902x10-"
2.0876~10~~
a Na
800
1.8121~10-~~
5.4427x10"
900
1.0182~10-~~
3.0391x1016
0.1759
0.34058
0.51649
5.0766x10-'
1.9698~10~
1000
3.9799x10-l4
7.5260~10'~
1.7307
0.70930
2.43999
3.3396x1O-7
2.9944x106
1100
2.2441x10-lo
5.5139x1O11
11.1238
0.92336
12.04720
1.6885~10-~
5.9225x104
1200
2.4321x10-'
1.1070x10'"
51.8878
0.98144
52.86922
5.0723~10-~
1971
1300
8.4806~10-~
4.0518~10~
185.3688
0.99466
186.36350
9.2832x1O-3
107
1400
1.2823~10-~
2.5487~10~
571.6822
0.99826
572.68010
0.11353
7.8
It should however, be pointed out that in a COz-free environment, the chances of intercalation compound formation are excellent as was discovered by Hawkins et al.[20]. But in the gasification systems there always is some COZ and it would surely render the intercalation compound unstable. It is reasonable to conclude that the formation of intercalation compounds is not very likely under the oxidizing (Cot) environment that is present in the carbon solid reacting with CO2 gas. The one possibility that may invalidate this conclusion is that the “dissolved” potassium in the intercalation compound may exhibit a strong negative deviation from Raoult’s law. But even then, in the face of 2n values running into 2 x 10” and greater (at 800 K), the logical conclusion is that GM must be close to an infinitely dilute solid solution. Whether the alkali (M) in such low concentrations can act through the electron-transfer mechanism[21] is a matter for conjecture. 4. CONCLUSIONS It
appears that catalysis of the Boudouard reaction by alkali metal compounds is more likely to occur through a cyclical mechanism involving gaseous intermediates M(g), CO(g) and COz(g). At temperatures higher than the melting point of K&Z03 the following ranking of
catalytic activities is suggested: KXOj > Na2C03 > Li&Os. The possibility of intercalation compound (C,,M) formation is slight for all the three carbonates at low and medium temperatures. At very high temperatures, i.e. 1400K and above, there is some likelihood of Cz,M formation. The importance of the electrochemical mechanism[l2] is difficult to predict except that such a mechanism will not make a substantial contribution to the overall reaction should the catalyst be in a solid form. Acknowledgement-The authors gratefully acknowledge the financial support of the National Science Foundation. REFERENCES
1. H. S. Taylor and H. A. Neville, J. Am. Chem. Sot. 43, 2055, (1921). 2. J. G. King and J. H. Jones, J. Inst. Fuel 5, 39 (1931). 3. D. A. Fox and A. H. White, Ind. Engng Chem. 23,259 (1931). 4. G. E. Goring, G. P. Curran, R. P. Tarbox and E. Gorin, Ind. Engng Chem. 44, 105I (1952). 5. E. A. Gulbransen and K. F. Andrew, Ind. Engng
1048(1952). 6. H. Harker, Proc. 4th Conf. on Carbon. Press, New York (1960).
Chem. 44,
p. 125. Pergamon
Table 3. Thermodynamics of intercalation compound formation in Li2CO,/C mixtures
T"K
Kil
K.
14
800
8.2468~10-~~
1.1959x1O25
900
1.7467~10-~~
1.7715~10~'
r = $,
9.9311x1O-3 0.1759
9.4813x1O-2
0.10474
9.8366x1O-14
0.51649
0.34058 I
1.0166~10'~ 4.7557x1010
2.1027x10-" I
I
1000
1.7858~10-~~
1.6773~10'~
1.7307
0.70930
2.43999
2.2370x10-9
4.4702~10~
1100
1.3597x10-l4
9.1oo4x1o'5
11.1238
0.92336
12.04720
1.3142~10-~
7.6090~10~
1200
2.1872x10-'l
1.23O9x1O14
51.8878
0.98144
52.86922
4.8101x10-6
2.0789x105 -
1300
1.o757x1o-8
3.1943x1012
185.3688
0.99466
186.36350
1.0455x10-4
9564
1400
2.0836~10-~
1.5685~10"
571.6822
0.99826
572.68010
1.4473x10-3
690
212
Y. K. RAOet al.
7. J. F. Rakszawski, F. Rusinko, Jr. and P. L. Walker, Jr., Proc. 5th Conf. on Carbon, Vol. 2, p. 243, Pergamon Press, New York (1962). 8. J. M. Thomas, In Chemistry and Physics of Carbon (Edited by P. L. Walker Jr.) Vol. 1, p. 121. Marcel Dekker, New York (1966). 9. G. R. Hennig, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. )Vol. 2, p. 1. Marcel Dekker, New York (1%6). 10. P. L. Walker, Jr., M. Shelef and R. A. Anderson, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.) Vol. 4, p. 287, Marcel Dekker, New York (1%8). 11. D. W. McKee and D. Chatterji, Carbon 13, 381 (1975). 12. B. P. Jalan and Y. K. Rao, Carbon 16, 175(1978). 13. M. J. Veraa and A. T. Bell, Fuel 57, p. 194(1978). 14. D. W. McKee, Mechanisms of Catalyzed Gasification of Carbon. Report No. 80 CRD 143, General Electric Co.,
Schenectady, New York (1980). 15. W. C. Taylor, D. T. Williams and Y. K. Rao, Catalysis of the Carbothermic Reduction of Iron Oxides. Paper presented at the 109th Annual Meeting of AIME, Las Vegas, Nevada (1980). 16. W. Y. Wen, Catal. Rev. Sci. Engng 22, l(1980). 17. JANAF Thermochemical Tables, 2nd Edn. National Bureau of Stds., U.S. Dept. of Commerce, Washington, D. C. (1971). 18. S. Aronson, F. J. Salzano and D. Bellafiore, J. Chem. Phys. 49,434 (1968). 19. D. Berger, B. Carton, A. Metrot and A. Herold, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr), Vol. 12, p. 1. Marcel Dekker, New York (1976). 20. R. J. Hawkins, L. Monte and J. J. Waters, Ironmaking and Steelmaking3, 151 (1974).
21. F. .I. Long and K. W. Sykes, Proc. Roy. Sot. Series A, A215, 100(1952).
(London),
Carbon Vol. 20, No. 3, pp. 207-212. 1982 Printed in Great Britain.
ON THE MECHANISM OF CATALYSIS OF THE BOUDOUARD REACTION BY ALKALI-METAL COMPOUNDS Y. K. RAO, A.
ADJORLOLOand J. H. HABERMAN
Metallurgical Engineering, FB-10,University of Washington,Seattle,WA 98195,U.S.A. (Received 31 August 1981) Abstract-The mechanism of catalysis of the Boudouard (C-C02) reaction by the alkali-metal compounds appears to involve gaseous intermediates M(g), CO(g) and CO2(g) where M(g) may be Li, K, Na, etc. The cyclical mechanism M(g)+M2CO3+M(g) withstood the rigorous thermodynamic analysis presented here and may be regarded as the most likely mechanism. Detailed calculations showed that formation of intercalation compounds of the type C2,M is not very likely except at very high temperatures (1400K and above) and under nearly pure CO environments. Thus the likelihood of intercalation compound mechanism being valid is not substantial except under stringent conditions as noted. The electrochemical mechanism probably operates parallel to the cyclic mechanism; and its relative contribution to the overall reaction is not known at this time.
1. GENERAL
be noted, is characterized by the C-CO? reaction as the rate-controlling step. Wen[l6] extended the cyclical mechanism (of McKee and Chatterji)[ll] by suggesting that the alkali metal liberated by carbothermic reduction of MXOs forms intercalates, or alkali metal-carbon compounds, which then partake in the catalysis. Wen[16] calls these compounds CT (charge-transfer) complexes or EDA (electron donor-acceptor) complexes. Jalan and Rao[l2] proposed an electrochemical mechanism for the catalysis of C-CO, reaction by L&CO3 (or LiaO). The main features of this mechanism are that the catalyst serves as a conductor of 02- ions and that the underlying carbon particle becomes corroded progressively as carbon combines with 02- ions to form CO gas. This mechanism is likely to be more important when the catalyst is in molten state as it would greatly facilitate the transfer of 02- ions. The purpose of this note is to present a detailed thermodynamic analysis of the different mechanistic schemes and draw conclusions as to the possible ranking of alkali catalysts.
The catalysis of the Boudouard (C-COZ) reaction has been investigated by numerous researchers [ l-141. Recognition of the catalytic influence of alkali-metal compounds dates back to the early work of Taylor and Neville[l]. They studied the catalytic activities of a number of different substances and concluded that potassium and sodium carbonates are efficient catalysts for the Boudouard reaction. Fox and White[3] found that sodium carbonate enhances the rate of the Boudouard reaction and thereby produces gases richer in carbon monoxide and hydrogen in a gas producer. Recently, the subject of catalysis of carbon-gas reactions has been receiving greater attention mainly due to the revival of interest in coal gasification and other coal conversion processes. Despite the wealth of factual information that exists on the catalysis of C-CO2 reaction, the mechanism of catalysis by alkali-metal salts remains imperfectly understood. The oxygen-transfer mechanism [lo] which adequately accounts for the observed catalytic behaviour of transition metals and their compounds, has not been found satisfactory in interpreting the marked catalytic activity of alkali-metal salts. Harker[6] proposed that the alkali salts tend to neutralize the acidic surface oxide on carbon and thereby favor the formation of alkali metal-carbon complexes. McKee and Chatterji[ll] favor a catalytic mechanism which involves a cyclical sequence M2COs+ M + MXO,; a similar mechanism was proposed earlier by Fox and White[3] for explaining the catalytic action of sodium carbonate. McKee and Chatterji[ 111found that the catalytic activity diminishes progressively from LiXO, to Na$ZOs and proposed the following ranking:
2. CYCLICAL. MECHANISM
This mechanism consists of the following three steps which together constitute a cyclic sequence: Reduction: MzCO& 1)t 2C(s) = 2M(g) t 3CO(g).
(cl)
Oxidation: 2M(g) + CO&) = M2O(s, 1)+ CO(g).
LXO, > C&O3 > RbXO3 > KzCOs > Na2C03.
(c2)
Carbonation:
This ranking, however, diverges from the results of Taylor and Rao[lS] who found, in connection with the carbothermic reduction of iron oxides, that potassium carbonate is a much better catalyst than lithium carbonate. The carbothermic reduction of iron oxides, it will
MzO(s, 1)+ CO*(g) = MzCOs(s, 1).
(c3)
Reactions (~2) and (~3) are expected to proceed rapidly and in all likelihood reaction (cl) is the rate-controlling step. It will be noted that the summation of these three 207
Y. K. RAOet al.
208
the JANAF Tables [17] . At 800 K:
steps yields the Boudouard reaction, i.e. 2C(s) t 2COz(g) = 4CO(g).
K,, = 2.3922 x lo-”
(B)
PK = 3.7163x 10e6atm.
Reactions (~2) and (~3) when combined yield 2M(g) t 2CO&) = MXOds, 1)+ CO(g).
(c4)
Thus the cyclic sequence may also be viewed as consisting of steps (cl) and (~4). In the foregoing M is any alkali metal. The present analysis is based on the following premises: (1) Assume that reaction (cl) is at equilibrium in an isolated system; this yields P&(P&) values which correspond to the equilibrium of reaction (cl). (2) Assume that reactions (cl) and (~4) are simultaneously at equilibrium. This implies that the Boudouard reaction is also at equilibrium. The corresponding values of PM are found. (3) The difference (P&-P,,), labeled “V, signifies the “excess” or “available” alkali vapor which partakes in the catalysis. (4) Larger values of “8” indicate greater catalytic activities. A detailed picture of how this mechanism might operate is shown in Fig. 1. Reaction (cl) produces M(g) which is then oxidized by CO*(g) to carbonate, as per reaction (c4), which then deposits near the original carbonate melt (or solid) on the carbon surface. The driving force for this mechanism, as indicated above, is measured by the quantity S. At equilibrium of reactions (cl), (~4) and (B), clearly M(g) will have the lowest concentration. At equilibrium of reaction (cl), in contrast, M(g) will have the highest concentration. Let us find the values of P;X and PM for the case of potassium. KzCO&, 1)t 2C(s) = 2K(g) + 30(g).
Similar calculations were made at other temperatures. The data are presented in Fig. 2 on a semi-log plot (log Pff vs T). In order to calculate PK, the partial pressure of potassium when the three reactions are at equilibrium, we need to consider, in addition to eqn (kl), the following: 2K(g) f 2CO&) = KXOs(s, 1)+ CO(g)
(k2)
C(s) t co2 = 2CO(g).
(W
Only two of these three reactions are truly independent. We have for this system: Species: KXOs, K, C, CO, COZ; N = 5. Phases: KXO3, C, gas; p = 3. Independent Equilibria: r = 2. Number of stoichiometric constraints, s = 0. Number of special constraints, t = 0. Application of the phase rule yields F=N-pt2-r-s-t=5-3+2-2-O-0=2.
The system possesses 2 degrees of freedom. Thus, both temperature and pressure must be defined. PKtPcotP
(kl)
Furthermore
coz=P=
latm
(a)
K., = PK*P&
(b)
Ke>= Pco/PK*P&.
(c)
These three equations are solved simultaneously. For instance, at 800 K:
PC0 = 1.5PK(stoichiometry).
The equilibrium constant is given by 5 K,, = P,‘P& = 3.375PK .
PK = 1.675X lo-‘* atm, PCO= 9.482 x lo-* atm, PC@ = 0.9052 atm
The requisite thermodynamic data were obtained from
Thus, for this temperature, we find that SK is equal to
M$03(1) 2td(g)
+2cCs) + 2CO2(9)
carbon Fig.
= 2Mt.J)
+ 3CO(g)
= M2C03(_e)+Co(g)
0 .
@
1100 1000 Temperature
surface
1. An expanded view of the carbon/catalyst junction. Reaction paths are indicated.
Fig.
2.
1200 (K)
1300
1400
Equilibrium vapor pressures of potassium at different temperatures (see text for explanationof Pi and PK).
209
Mechanism of catalysis of the boudouard reaction 3.7163 x 10m6atm. Similar calculations of PK for the three-reaction simultaneous equilibrium were performed at other temperatures. The results are shown in Fig. 2. The action of NazC03 may be analyzed similarly. For example at 800 K, we find
PA, = 2.3491 x 10m6atm PNa = 5.322 x lo-” atm SNa= 2.3491 x 10m6atm. Likewise for lithium carbonate, at 800 K, we find P& = 1.3069X lo-‘atm
d / 800
~_~A.
900
IO00
1100
Temperature (K)
Fig. 4. Equilibrium vapor pressures of lithium at different temperatures (see text for explanation of P& and PL,).
PLi = 1.229X 1O-‘8atm SLi= 1.3069X lo-’ atm. The results for NazC03 and LX03 are shown in Fig. 3 and 4, respectively. If our premises, stated earlier, are accepted, then it follows that potassium carbonate, being associated with the largest S-values should prove to be the best catalyst; and the lithium carbonate the least effective of these three carbonates. Needless to say that sodium carbonate lies in between but decidedly closer to KXO3. As stated earlier, in the carbothermic reduction of iron oxides the catalytic activities were found to rank as K2C03 > NaXO, > LiXO3.
3. INTERCALATION COMPOUND MECHANISM
Wen[ 161modifies the cyclical mechanism by proposing the formation of intercalation compounds (carbon-alkali metal) which partake in the reaction mechanism. Thus we have: MXO3(s, 1)f 2C(s) = 2M(g) t 3CO(g) 2M(g) t 2nC(s) = 2C,M(s)
(w2)
2&M(s) t COdg)= (2nC). MlO(s) t CO(g) (~3) (2nC)~MzO(s)+COz(g)=2nC(s)tMzCO~(s,
1). (w4)
Reactions (~3) and (w4), when added, yield 2&M(s) t 2COz(g) = 2nC(s) + MXO&
One of the factors which might upset this ranking is the slowness of reaction (kl). For instance at a temperature of, say, llOOK, K2COs and NazCOj are both in solid form whereas Li2C03 is in liquid form. This might facilitate the reduction of LiZCOj while that of K2C03 and NaX03 remains sluggish at 1100K. This might lead to the conclusion, justifiably so, that L&CO3 is more powerful than the other carbonates at temperatures below the melting points of Na2C03 (and K2COX)but at temperatures at which LX03 is molten. At temperatures like 12OOK,where all these carbonates are in molten state, the present analysis predicts that K&O3 is the most potent catalyst among the three carbonates considered here.
(WI)
1)f CO(g) (w5)
Here M is alkali metal Li, Na, K, etc. and n = 6, 12, or 18 for Li; 64 for Na; and 8,24,36,48 or 60 for K, Rb or Cs. Underlying this mechanism is the assumption that the intercalation compounds of the type &M(s) are stable at reaction temperatures and atmospheres normally used in the study of the catalysis of the Boudouard reaction. There is a tremendous lack of thermochemical data pertaining to the intercalation compounds. The data of Aronson et a/.[181 and Berger et a/.[191 do not extend to high temperatures. The possibility of intercalation compound formation is first investigated for KXOs catalyst. The analysis is based on the following premises: (1) K(s) is the standard (or reference) state for &K(s). (2) The solid solution C,.K(s) is ideal, i.e. it obeys Raoult’s law. Hence a~ = xK = mole fraction of K = 1/2n t 1, i.e. 2n = (l/aK)- 1. (3) The gas-phase composition corresponds to the Boudouard equilibrium. The equations to be considered are as follows: KzCO3(s, 1)t (4n t 2)C(s) = 2Cz.K(s) t 3CO(g)
(il) Temperature
(K)
Fig. 3. Equilibrium vapor pressures of sodium at different temperatures (see text for explanation of PA, and P&.
KzCOs(s, 1)t 2C(s) = 2K(s) + 3CO(g)
(i2)
4nC(s)+ 2K(s) = 2Cz,K(s); ideal
(i3)
Y. K. RAOet al. Table 1. Thermodynamics of intercalation compound formation in K&O& mixtures
1300
2.2760~10-~
1.5097x10y
185.3688
0.99466
186.36350
4.8O93xlO-3
207
1400
3.2077~10-~
1.0189~10~
571.6822
0.99826
572.68010
5.6784x10-*
16.6
2(&K(s) t 2CO&) = KEOs(s, 1)+ CO(g) +4&(s).
From eqns (1) and (6) we have (i4) Kir =
aK*q?&.
(8)
The equilibrium constants are defined by Upon combining eqns (7) and (8) and simplifying Kir = ~aK2Pg0 = eK*p3 co aKzCO#k
Ki4=
(1) (9)
2
aK PC@’
(2)
Additionally PC0 t Pcoz = 1 atm.
(3)
Thus, from the knowledge of q (r and Pco), Kir and Kid we can calculate the value of OK, hence xK (and 2n), at any given temperature. The following example will illustrate the method of calculation. Consider a temperature of 800 K:
Let r = P~o/Pcoz be the equilibrium constant for the Boudouard reaction. It can be shown that r = (KitKid)“*
(6)
Upon combining eqns (2) and (6) we obtain
Kid =
qPc02 2 2 = ,,*z, aK PC02
PC02 = aKflK.4
r =
(7)
9.9311X low3
q = 0.10474. Hence
(5)
Using thermochemical data from standard sources [ 171, the values of T and PC0 were calculated at a number of temperatures. These data are given in Table 1. Furthermore, let (KirKid)“* q = PcolPco2 = r/P00 = p. P co
AG!d= -314,884 J; Kid= 3.6287X 1020
(4)
The equilibrium PC0 value, for the Boudouard reaction is given by Pco=-0.5r+0.5(r2t4r)“*.
AGPl= 376,238J; Kir = 2.7180 X 1O-25
a, = 1.7858x 10-l’ and 2n = 5.5992x 10”. Similar calculations were performed at other temperatures. Only at a temperature of MOOK does the value of 2n appear reasonable from the view-point of intercalation compound formation. At low temperatures, in the presence of even a small concentration of CO2, the intercalation compound readily breaks down. The results of these calculations are given in Table 1. Similar analysis was carried out for sodium and lithium carbonates for which the results are summarized in Tables 2 and 3, respectively. Here too the evidence is unmistakable that the intercalation compound is not predisposed to form except at very high temperatures, i.e. MOOK and higher.
211
Mechanismof catalysis of the boudouardreaction Table 2. Thermodynamicsof intercalationcompoundformationin Na,CO,/C mixtures P2
TK
K
Kil
i4
r.=co
%02
9.9311x1O-3
2n
pco atm
q
9.4813x1O-2
0.10474
4.7902x10-"
2.0876~10~~
a Na
800
1.8121~10-~~
5.4427x10"
900
1.0182~10-~~
3.0391x1016
0.1759
0.34058
0.51649
5.0766x10-'
1.9698~10~
1000
3.9799x10-l4
7.5260~10'~
1.7307
0.70930
2.43999
3.3396x1O-7
2.9944x106
1100
2.2441x10-lo
5.5139x1O11
11.1238
0.92336
12.04720
1.6885~10-~
5.9225x104
1200
2.4321x10-'
1.1070x10'"
51.8878
0.98144
52.86922
5.0723~10-~
1971
1300
8.4806~10-~
4.0518~10~
185.3688
0.99466
186.36350
9.2832x1O-3
107
1400
1.2823~10-~
2.5487~10~
571.6822
0.99826
572.68010
0.11353
7.8
It should however, be pointed out that in a COz-free environment, the chances of intercalation compound formation are excellent as was discovered by Hawkins et al.[20]. But in the gasification systems there always is some COZ and it would surely render the intercalation compound unstable. It is reasonable to conclude that the formation of intercalation compounds is not very likely under the oxidizing (Cot) environment that is present in the carbon solid reacting with CO2 gas. The one possibility that may invalidate this conclusion is that the “dissolved” potassium in the intercalation compound may exhibit a strong negative deviation from Raoult’s law. But even then, in the face of 2n values running into 2 x 10” and greater (at 800 K), the logical conclusion is that GM must be close to an infinitely dilute solid solution. Whether the alkali (M) in such low concentrations can act through the electron-transfer mechanism[21] is a matter for conjecture. 4. CONCLUSIONS It
appears that catalysis of the Boudouard reaction by alkali metal compounds is more likely to occur through a cyclical mechanism involving gaseous intermediates M(g), CO(g) and COz(g). At temperatures higher than the melting point of K&Z03 the following ranking of
catalytic activities is suggested: KXOj > Na2C03 > Li&Os. The possibility of intercalation compound (C,,M) formation is slight for all the three carbonates at low and medium temperatures. At very high temperatures, i.e. 1400K and above, there is some likelihood of Cz,M formation. The importance of the electrochemical mechanism[l2] is difficult to predict except that such a mechanism will not make a substantial contribution to the overall reaction should the catalyst be in a solid form. Acknowledgement-The authors gratefully acknowledge the financial support of the National Science Foundation. REFERENCES
1. H. S. Taylor and H. A. Neville, J. Am. Chem. Sot. 43, 2055, (1921). 2. J. G. King and J. H. Jones, J. Inst. Fuel 5, 39 (1931). 3. D. A. Fox and A. H. White, Ind. Engng Chem. 23,259 (1931). 4. G. E. Goring, G. P. Curran, R. P. Tarbox and E. Gorin, Ind. Engng Chem. 44, 105I (1952). 5. E. A. Gulbransen and K. F. Andrew, Ind. Engng
1048(1952). 6. H. Harker, Proc. 4th Conf. on Carbon. Press, New York (1960).
Chem. 44,
p. 125. Pergamon
Table 3. Thermodynamics of intercalation compound formation in Li2CO,/C mixtures
T"K
Kil
K.
14
800
8.2468~10-~~
1.1959x1O25
900
1.7467~10-~~
1.7715~10~'
r = $,
9.9311x1O-3 0.1759
9.4813x1O-2
0.10474
9.8366x1O-14
0.51649
0.34058 I
1.0166~10'~ 4.7557x1010
2.1027x10-" I
I
1000
1.7858~10-~~
1.6773~10'~
1.7307
0.70930
2.43999
2.2370x10-9
4.4702~10~
1100
1.3597x10-l4
9.1oo4x1o'5
11.1238
0.92336
12.04720
1.3142~10-~
7.6090~10~
1200
2.1872x10-'l
1.23O9x1O14
51.8878
0.98144
52.86922
4.8101x10-6
2.0789x105 -
1300
1.o757x1o-8
3.1943x1012
185.3688
0.99466
186.36350
1.0455x10-4
9564
1400
2.0836~10-~
1.5685~10"
571.6822
0.99826
572.68010
1.4473x10-3
690
212
Y. K. RAOet al.
7. J. F. Rakszawski, F. Rusinko, Jr. and P. L. Walker, Jr., Proc. 5th Conf. on Carbon, Vol. 2, p. 243, Pergamon Press, New York (1962). 8. J. M. Thomas, In Chemistry and Physics of Carbon (Edited by P. L. Walker Jr.) Vol. 1, p. 121. Marcel Dekker, New York (1966). 9. G. R. Hennig, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. )Vol. 2, p. 1. Marcel Dekker, New York (1%6). 10. P. L. Walker, Jr., M. Shelef and R. A. Anderson, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.) Vol. 4, p. 287, Marcel Dekker, New York (1%8). 11. D. W. McKee and D. Chatterji, Carbon 13, 381 (1975). 12. B. P. Jalan and Y. K. Rao, Carbon 16, 175(1978). 13. M. J. Veraa and A. T. Bell, Fuel 57, p. 194(1978). 14. D. W. McKee, Mechanisms of Catalyzed Gasification of Carbon. Report No. 80 CRD 143, General Electric Co.,
Schenectady, New York (1980). 15. W. C. Taylor, D. T. Williams and Y. K. Rao, Catalysis of the Carbothermic Reduction of Iron Oxides. Paper presented at the 109th Annual Meeting of AIME, Las Vegas, Nevada (1980). 16. W. Y. Wen, Catal. Rev. Sci. Engng 22, l(1980). 17. JANAF Thermochemical Tables, 2nd Edn. National Bureau of Stds., U.S. Dept. of Commerce, Washington, D. C. (1971). 18. S. Aronson, F. J. Salzano and D. Bellafiore, J. Chem. Phys. 49,434 (1968). 19. D. Berger, B. Carton, A. Metrot and A. Herold, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr), Vol. 12, p. 1. Marcel Dekker, New York (1976). 20. R. J. Hawkins, L. Monte and J. J. Waters, Ironmaking and Steelmaking3, 151 (1974).
21. F. .I. Long and K. W. Sykes, Proc. Roy. Sot. Series A, A215, 100(1952).
(London),