5230
J . Phys. Chem. 1989, 93, 5230-5237
2
Parameter z = 1 / A , (mole/kJ)
Figure 3. Distribution function G ( z ) , calculated according to eq 13, for the type RKD-4 microporous activated carbon.
tribution function. Equations 14, 15, and 16 permit the calculation of the parameters,,,z, Z, and uz that characterize quantitatively the distribution function G(z); these calculations give zmax= 0.052 mol/kJ, = 0.054 mol/kJ, and u, = 0.012 mol/kJ. Assuming that type RKD-4 activated carbon possesses slitlike micropores Z, and u, with a half-width X, we can convert the quantities,,,,z to xmax,X, and ux. According to experimental studiesI3J6J9of adsorption on microporous activated carbons, z (= 1/2)is proportional to the half-width x with the proportionality constant k = 0.094/@ mol/(kJ-nm)
z = l/2 = kx
(58)
Because the similarity coefficient (3 for benzene is assumed to be k = 0.094 mol/(kJ-nm) was used equal to ~ n i t y ,the ~ . constant ~ to convert,,,,z Z, and uz to x,,, X, and 0,; this conversion gives x,, = 0.55 nm, X = 0.57 nm, and ox = 0.13 nm. These parameters characterize the structural heterogeneity of the micropores of the type RKD-4 activated carbon.
The energetic heterogeneity of these micropores may be characterized by the quantities d, (eq 30) and uA,I (eq 32) that are associated with the adsorption potential distribution function X , ( A ) (viz., eq 33). Calculation of these quantities gives d, = 19.7 kJ/mol and uA,, = 11.8 kJ/mol. In our previous paper,25 we showed that the geometric surface area of micropores is from this proportional relationship, we obtain proportional to 2,; a value for the geometric surface area of the micropores of type RKD-4 carbon equal to 810 m2/g. The addition of this value to the mesopore specific surface area obtained by the as-method (140 m2/g) gives the total specific surface area of the RKD-4 carbon equal to 950 m2/g. This analysis of the benzene adsorption isotherm on type RKD-4 microporous activated carbon illustrates the applicability of our theoretical description for characterizing microporous solids.
Conclusions A complete description of physical adsorption of a vapor on an heterogeneous microporous solid is presented in terms of the distribution functions that characterize the structural and energetic heterogeneities of this solid. It is shown that characterization of the structural heterogeneity of the micropores of a solid by the gamma distribution, viz., eq 13, leads to simple equations for the characteristic adsorption curve, viz., eq 23, the adsorption potential distribution, viz., eq 33, and the thermodynamic functions, viz., eq 38,42, and 43. Another important advantage of the proposed description is its association with the asmethod in order to extract the amounts adsorbed in the micropores and on the mesopore surface. Analysis of these extracted quantities permits evaluating (through eq 57) the adsorption potential distributions that characterize the structural heterogeneity of the micropores and the surface heterogeneity of the mesopores. Acknowledgment. This work was supported in part by the National Science Foundation under Grant No. CBT-872 1495. We thank Dr. J. Choma and Mr. X . Lu for providing figures.
A Study of the Mechanism of the Partial Oxidation of Methane over Rare Earth Oxide Catalysts Using Isotope Transient Techniquest Alfred Ekstrom* and Jacek A. Lapszewicz CSIRO Division of Fuel Technology, Lucas Heights Research Laboratories, Private Mail Bag 7 , Menai, N S W, 2234, Australia (Received: September 26, 1988; I n Final Form: February 8 , 1989)
The mechanism of the partial oxidation reaction of methane over three catalysts (Sm203,Li/Sm2O3,and Pr6OI1)having a range of activities and product selectivities has been studied by using isotope transient techniques. The most important conclusions from this study are ( 1 ) that large amounts of CH4 are adsorbed on all working catalysts, (2) that the reaction takes place on a small number of very active catalyst sites and does not involve the adsorbed CHI, (3) that gas-phase oxygen exchanges rapidly with the lattice oxygen atoms of the working catalysts, (4) that the rate of CH4 conversion is dependent on the rate of lattice oxygen exchange, and ( 5 ) that the carbon oxides are substantially formed by the secondary oxidation of the reaction products. A mechanism based on the formation and reactions of [O-]species is proposed for the C2+products, but a different form of activated oxygen appears to be responsible for the formation of the carbon oxides.
Introduction The catalyzed reaction of methane with oxygen under "fuel rich" (i.e., CH4 >> 0,) conditions appears to have been first studied by Boomer and Thomas,l who found that small amounts of ethane were formed when methane/oxygen mixtures were reacted over a copper catalyst at high pressures. More recently, Keller and Bhasin2 reported that the atmospheric-pressure reaction was ' A partial account of this work was presented at the Symposium on Direct Methane Conversion, held by the Division of Petroleum Chemistry, at the American Chemical Society Los Angeles meeting, Sept 25-30, 1988.
0022-3654/89/2093-5230$01 S O / O
catalyzed by a variety of metal oxides, which gave C2selectivities3 up to 60%. Lunsford et aL4showed that use of a Li2C03-promoted MgO catalyst resulted in high C2 selectivities and relatively high methane conversions. These observations lead to a resurgence ( I ) Boomer, E. H.; Thomas, V. Can. J . Res. Sect. B 1937, 15, 401. (2) Keller, G.E.; Bhasin, M. J . Carol. 1982, 73, 9. (3) The C2selectivity is defined as 2(rate of C2H6 C2H4formation)/(rate of total CHI conversion). (4) Ito. T.: Wang, J.-X.; Lin, C-H.; Lunsford, J. H . J . Am. Chem. SOC. 1985. 107. 5062.
+
6 1989 American Chemical Society
Oxidation of CHI over Rare Earth Oxide Catalysts
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5231 100
L* MASS SPECTROMETER Figure 1. Schematic diagram of the apparatus used for the isotope switching experiments.
of interest in this reaction, and a variety of effective catalysts have now been described.5 Of these, the rare earth oxides present an interesting series. While some (Sm and Eu) are among the most active catalysts found so far, others (Ce, Pr, and Tb) lead almost exclusively to the formation of carbon oxides.6 The mechanism of the catalyzed reaction, which must be controlled by kinetic rather than thermodynamic factors, presents an intriguing problem. Although the evidence for the involvement of methyl radicals in the formation of the C2 products is conclusive,’ details of the mechanism, such as the extent of reactant adsorption on the catalyst, the nature of the reactions leading to the formation of the methyl radicals on the catalyst surface, the factors that control the C2 selectivity, and the extent to which secondary reactions degrade the primary C2 selectivity of the catalyst, are all obscure. A particuIarly interesting aspect of the mechanism is the role of oxygen. It is well-established8that molecular oxygen exchanges rapidly with the lattice oxygen atoms of most metal oxides at high temperatures, a result not considered in recent discussions of the *~ the mechanism of the partial oxidation r e a ~ t i o n . ~However, extent to which oxygen is adsorbed on the catalyst at reaction temperature, and the relative importance of lattice vs adsorbed oxygen, are not known. Work in our laboratory has shownI0 that isotope “switching” techniques” could be used for studying the reactions taking place on a working samarium oxide catalyst. We now report the results of a comprehensive application of this method to three catalysts: S m 2 0 3 , having both a high reactivity and C2 selectivity: (5) Many new catalysts have been reported in the past years. See e.g.: (a) Bytyn, W.; Baerns, M. J . Appl. Catal. 1986, 28, 199. (b) Emesh, I. T. A,; Amenomiya, Y . J . Phys. Chem. 1986, 90, 4785. (c) Aika, K.; Moriyama, T.; Takasaki, N.; Iwamatsu, E. J . Chem. Soc., Chem. Commun. 1986, 1210. (d) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. J . Caral. 1987, 103, 3 1 1. (6) (a) Otsuka, K.; Jinno, K.; Morikawa, A. Chem. Lett. 1985,499. (b) Otsuka, K.; Nakajima, T. Inorg. Chim. Acfa 1986, 120, L27. (c) Otsuka, K.; Komatsu, T Chem. L e t f . 1987, 483. (d) Otsuka, K.; Jinno, K.; Morikawa, A. J . Catal. 1986, 100, 353. (7) (a) Campbell, K. D.; Morales, E.; Lunsford, J. H. J . Am. Chem. SOC. 1987, 109, 7900. (b) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H . J.. Am. Chem. SOC.1985. 107. 58. (8) (a) Winter, E. R. S. Chem. Soc. A 1968, 2889. (b) Winter, E. R. S. J . Chem. SOC.A 1969, 1832. (9) (a) Lin, C.-H.; Campbell, K. D.; Wang, J.-X.; Lunsford, J. H . J . Phys. Chem. 1986,90. 534. (b) Lin, C.-H.; Wang, J.-X.; Lunsford, J. H. J . Catal. 1988. I l l . 302. (c) Hutchinns. G. J.: Scurrell. M. S.;Woodhouse, J. R. J . Chem. Soc., Chem.’CommuL 1987, 1388. (d) Otsuka, K.; Jinno, K.; Morikawa, A. J . C a r d 1986, 100, 353. (e) Cant, N. W.; Lukey, C. A. Nelson, P. F.; Tyler, R. J . J . Chem. Soc., Chem. Commun. 1988, 766. (f) Martin, G.A.; Mirodatos, C. J . Chem. Soc., Chem. Commun. 1987, 1393. (10) (a) Ekstrom, A,; Lapszewicz, J. A. J . Chem. SOC.,Chem. Commun. 1988, 797. (b) Ekstrom, A,; Lapszewicz, J. A. J . Am. Chem. SOC.1988, 110, 5226. (1 I ) Isotope “switching” involves the sudden replacement of one isotopic form of a reactant by a labeled form and following the appearance of the label in the reaction products by a fast analytical method, usually mass spectrometry. Apart from minor isotope effects, no disturbance of the working catalyst takes place. For previous applications of the technique see, for example: Happel, J.; Suzuki, I.; Kokayeff, P.; Fthenakis, V. J . Catal. 1980. 65, 59. ~~
~~
i.
,
100
,
Figure 2. Comparison of the product selectivity (a) and catalyst activity (b) of the three catalysts used in this study. Reaction temperature 750 OC, 10%O2 in CH,, space velocity 700 m L s-I g-’.
Li2C03-dopedSm2O3, having a low activity but high C2 selectivity; and Pr&I, having a high activity, but low c2selectivity.
Experimental Section The apparatus consisted of a simple flow-through reactor holding 30 mg of catalyst in a 4-mm-i.d. quartz tube and a conventional gas feed and isotope switching system based on VALCO multiport valves (Figure 1). Three gas lines were provided with facilities for switching. Rather than the more usual complete switch of the isotopic species, a pulse of approximately 14-s duration was used in most experiments to conserve these expensive reagents. A switching time of only 0.2 s was achieved by the use of high space velocities, the elimination of all possible dead volumes, and a sampling point as close as possible to the end of the catalyst bed. Care was taken to ensure that no change in total flow occurred as a result of these switches. A VG-SX2OO mass spectrometer and computer-controlled data acquisition system were used for the anaiysis of the reaction products. Where necessary, a small dry ice trap was used to prevent water entering the mass spectrometer. A program was written to enable transfer of the data collected by the system to the software package SYMPHONY for further processing. Typically, between 5 0 0 and 2000 data points were taken for each run. The reagents used were all of high-purity grade. The labeled compounds CD4 (99% D), I3CH4(99% 13C), I8O2(97.5% I8O), I 3 C 0 (98% I3c), l 3 C o 2 (99% I3c), I3C2H6(50% l3C), I3C2H4 (99% I3C), and D2 (99% D) were obtained from Novachem Pty. Ltd. The catalysts were purchased from Fluka or B.D.H. and were of 99.9% purity. The Li/Sm203 catalyst contained 3 wt % Li2C03and was prepared as described el~ewhere.~ The catalysts received no special pretreatment but were exposed to reaction conditions for at least 30 min before making any measurements. Because of the high space velocities used, the blank reaction from the empty reactor was negligible. All flow rates given below were measured at STP. Results ( i ) Comparison of Catalyst Activity. A comparison of the reactivities and selectivities for the Sm&, Li/Sm203, and Pr6011 catalysts showed (Figure 2) that the Sm2O3 catalyst is very active, giving a C2 selectivity of 60-70%. The Pr6OI1had a comparable overall activity but functioned essentially as an oxidation catalyst with a C2 selectivity of only 35%. These results are dependent on the precise experimental conditions used but are in accord with previous studies.6 The addition of 3 wt % Li2C03to the S m 2 0 3 resulted in a marked decrease in activity and an improvement in C2 selectivity. This result is in contrast to the addition of Li2C03 to MgO, where the activity of the catalyst was not much affected (even though the surface area is reduced) while the C2 selectivity was also i m p r o ~ e d . ~ ~For , ’ ~Sm203,addition of Li2C03appears to act as an effective catalyst poison. From our work, it appears (12) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. J . Chem. Soc., Chem. Commun. 1987, 1862.
Oxidation of CHI over Rare Earth Oxide Catalysts
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5233
0 0 7 0 0
,
,
,
0 2 13
T i m e (s) Figure 6. Transients observed for I3CH4,13C'*02,and I3C2H6when "CH, replaced CH4. I8O2was used to move the molecular weight of C02 to 48. Sm203catalyst, 700 OC, 10% I8O2in CHI, space velocity 78 mL s-I g-'.
The mixed isotopic forms, H D and CD3CH3(Figure 4,b and d), both rise instantly to a nearly constant rate of formation following the switch to CD4. This rate of mixed product formation during the pulse must reflect the presence of the previous isotopic form on the catalyst surface, but their formation rates are almost 2 orders of magnitude less than that of the normal species. The sharp spike in the formation of these species at the end of the pulse reflects the partial mixing of the isotopic forms at the interface of the labeled and normal reactants. (iu) 13CH4Switches. The C 0 2 and C2H6 formation from Sm203 under synthesis conditions was also studied with 13CH4. Initially, a 13CH4switch was made with normal 02.The results obtained were uninterpretable because of the interference of 13C3H8at m / e 45 used to monitor the formation of I3CO2. Consequently, a CH4/I8O2 reactant mix was used to shift the C 0 2molecular weight to 48. A I3CH4pulse of the normal duration was then applied. As shown in Figure 6 , the I3Cl8O2pulse had the same shape as the labeled methane pulse. The incorporation of I3CH4into carbon dioxide is obviously very fast, and an identical result was obtained for the formation and decay of I3C2H6. This requires that the I3CH4converted to 13C02did not first enter the large pool of CH4 known to be present on the catalyst surface. The 13C02formation from the Pr6011catalyst was identical with that found for Sm2O3. During the replacement of 13CH4by I2CH4in the above experiments using the Sm2O3 catalyst, some mixing of the isotopic forms had occurred in the injection loop at the boundary between the labeled and normal forms at the end of the pulse, and it was possible to obtain 12 data points for the fraction of 13C02and I3C2H6in the products over a 1.9-speriod during which the I3C content of the exit CH4 varied from 100%to 9.2%. Figure 7 shows that the fraction of I3Cin the C2H6 was proportional to the square root of the I3C content of the CH4 but that the I3C content of the C 0 2 was directly proportional to the 13Ccontent of the CH4. This result is expected if the reactants and products are at isotopic equilibrium and shows conclusively that the carbon isotopic composition of the reactant C H 4 is reflected instantly in the products C 0 2 and C2H6. The I3CH4adsorbed on the Sm2O3 catalyst was estimated as 6 X 1020molecules g-I, in excellent agreement with the value determined previously from the CD4 switch. The Pr6011 gave identical results to those described above, but for the Li/Sm203catalyst a long tail in the 13C02evolution was found following the return to the normal isotope. This result is consistent with the incorporation of the 13C02into a carbonate pool whose presence on the catalyst was also suggested by the results described in section ii. ( u ) I8O2Switches. When I8O2replaced I6O2in helium (10% in He) over the Sm20, catalyst in the absence of CHI, a slow loss
, 0 4
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,
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CH, Figure 7. Plot of the I3CH4/I2CH4ratio in the CHI exiting the reactor vs the square root of the l3C/I2Cratio in the C2H6product ( 0 )and the 13C/'2Cratio in the C02product (W). The 12 data points were obtained over 1.9 s following the completion of the "CH, I2CH4pulse shown in Figure 6 . C Fraction
in
-
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130
170
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90
130
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Time ( s ) Figure 8. Comparison of the oxygen isotope transients observed when 1802 replaced I6O2 in helium (a, b) and the labeled CO, transients in methane (c, d). The total flow was 78 mL s-I g-l, with 10% O2 in He
or CH,. of I6O2and a slow rise of the 1802 were observed. Together with the long time required to return the leg2 and 1sO'60signals to their background values (Figure 8a,b), these results clearly indicate the uptake of large amounts of I8O2by an oxygen pool present in the catalyst. A lower limit estimate of 1 X 1021 atoms g-l is made for the number of oxygen atoms exchanged during the pulse. This value is nearly equal to the total number of oxygen atoms (5 X IO2' atoms g-I) present in Sm2O3 and is consistent with data by Winter,8 who found that 1 X 1020 oxygen atoms g-' are available for exchange in SmzO3 at 400 O C . It is also noticeable that as the I6O, is replaced, the l60l8Oformation slows as expected. Immediately after the return to I6O2,a sharp spike in the 160180 formation rate occurs, reflecting the exchange which has taken place in the mixed boundary between the two isotopic forms. As decay of the I80I6Oformation rate continues long after I8O2has been replaced, the exchange must occur, at least partially, on the catalyst surface. The obvious differences in the decay rates of I8O2and l 8 0 I 6 0 following the end of the I8O2pulse reflect the relative abundances of I8O and I6O on the catalyst surface. When the above experiments were carried out in the presence of methane, only small amounts of labeled molecular oxygen appeared in the gas phase, reflecting the high level of O2 consumption in the reaction. However, examination of the labeled carbon dioxide species (Figure 8c,d) showed that their rates of decay after the end of the l 8 0 , pulse were somewhat longer than
5232
Ekstrom and Lapszewicz
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
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(CD4)
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Figure 3. Effects of on/off switches of CHI on the COz and C2H6 formation rate from the three catalysts used in this study: (a) Smt03, (b) Li/Sm203,and (c) Pr60ii. The ion currents have been normalized
and therefore do not represent actual product concentrations. Reaction temperature 700 OC, space velocity 78 mL s-l g-', 10% O2in CH4or He. The C 0 2 was monitored at m / e 44 and the C2H6 at m / e 30. that the activity and selectivity of the Li/MgO and Li/Sm203 catalysts were the same, suggesting that the nature of the support was not important. ( i i ) On/Off Switching of Reactants. To determine their response to the on/off switching of the reactants, the catalysts were heated to reaction temperature in the presence of O2 and a He flow equivalent to that to be used for the CHI. After 2 min at reaction temperature, the H e flow was switched to CHI and the appearance of the products monitored. the C H 4 flow was then switched back to H e after 180 s. For the Sm2O3 catalyst, product formation commenced instantly following a He/CH4 switch (Figure 3a). The end of the C H 4 flow was followed by the very fast termination of the C2H6formation. The C 0 2 formation showed a more pronounced delay, indicating the slow desorption of COz from the catalyst surface. A quite different result was found for the Li/Sm203 catalyst. In this case (Figure 3b), the CzH6 formation reached a broad maximum, before declining slowly to a lower rate. This decline appeared to be matched by a slow rise in the C 0 2 formation rate. It is likely that this effect is related to the formation of the carbonate form of the catalyst, which has also been shown to be present on a working Li/MgO ~ a t a 1 y s t . l ~Termination of the CH, flow again resulted in a very fast end to the C2 formation but a larger and longer delay in the COz formation. Admission of CH, to the Pr6Oll catalyst resulted in the formation of a large pulse of COz and a smaller one for the C2 formation (Figure 3c). As will be shown later, this C 0 2 pulse is a reflection on the ability of CH, to reduce the catalyst and on the ability of part of the lattice oxygen atoms to participate in such a reaction. The end of the C H 4 flow was immediately reflected in the cessation of the C2 and C 0 2 formation rates, only a small retention of COz being observed. After purging the CHI with He for 10 min, O2was admitted, resulting in the formation of a further large amount of CO,. When 0 2 / H e was switched, the CzH6 formation was again instantly terminated for all catalysts. The C 0 2 formation decay was also similar to that following the CHI switch. The preceding results are in contrast to those reported by Lin et al.,9awho found that for Li/MgO product formation continued for some minutes following the termination of the oxygen flow. (iii) CD, Switches. Figure 4a shows the transients for CD, ( m / e 20) following a CH, to CD4 switch at 700 OC on a working (13) (a) Korf, S.J.; Roos, J. A,; de Bruin, N. A.; van Omen, J. G.; Ross, J. R. H.Catal. Today 1988, 2 , 535. (b) Korf, S . J.; Roos, J . A,; van Ommen, J. G.: Ross, J. R.J . Chem. SOC.,Chem. Commun. 1987, 1433.
0
0 0.25
0.10
0
40
80
120
160
200
Time ( s ) Figure 5. Desorption of CD4 from the catalysts at 700 OC, following adsorption at the same temperature from CD4containing 10%02.The data are plotted as first-order kinetics. (a) Sm203,(b) Li/Sm203,(c)
Pr60il. S m 2 0 3catalyst. For these experiments a dry ice trap was used to prevent any water entering the mass spectrometer. In examining these data, it should be remembered that they represent reaction rates. It is obvious that CD4 desorbs from the catalyst for a long time after the return to the normal isotopic form and the almost instantaneous flushing of the CD, from the dead volume of the reactor. A simple calculation gave 6 X IOzo molecules g-' for the number of adsorbed CD4 molecules. This value was found to be independent of temperature in the range 500-700 OC and is considered accurate to within a factor of 2. Similar experiments gave 4 X lozoCD, molecules g-I adsorbed on Li/Sm203 and 7 X IOzo molecules g-' on Pr6Oll. To put these numbers into perspective, the net CH4 conversion rate for the samarium catalyst is in the range of 1019-1020molecules s-l g-' at 750 "C, depending on experimental conditions. A complete CH4/CD4 switch in the absence of oxygen was also made. To increase the fraction of the total CD4 pulse adsorbed, a smaller flow rate (1.3 mL s-I g-l CH4/CD4 in 7 1 mL s-' g-' He) was used. The amount of methane adsorbed was determined as 2 X IOzo molecules g-I, somewhat less than that found in the presence of oxygen. In view of the uncertainty of these measurements, it is not certain that this difference is significant. The desorption of CD4 from these catalysts appeared to obey first-order kinetics (Figure 5 ) . While the CD4 desorption rates from the Sm203and Li/Sm203catalysts were identical, the rate for P r 6 0 iI was significantly slower, the t l / 2values being 13 s for S m 2 0 3and Li/Sm203 and 36 s for P r 6 0 i i . The appearances of the labeled products were essentially instantaneous following the isotopic switches. Figure 4c shows that C2D6 appears with no perceptible delay in response to the isotope switch. This is an unexpected result in view of the slow C H 4 desorption.
The Journal of Physical Chemistry, Vol 93, No 13, 1989
5234
,37 [ l a -
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m/e = 36
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Ekstrom and Lapszewicz
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Time (s) Figure 9. Transients observed when 1802 replaced I6O2 flowing over Sm20, (a-c) and Pr60,, (d-f) catalysts. The switch to I8O2was made at -zero time". 700 OC,flow rate 78 mL s-I g-l, with 10% O2 in helium.
the desorption curves of the 1802 and 160180 species in Figure 8a,b. As the catalyst surface under synthesis conditions is expected to contain additional oxygen forms (e.g., OH-, C032-, H,O), such a result is not unreasonable. These results are in striking contrast to those found for the tracing of I3CH4to C 0 2described in section iv. The above results prompted the following question: what role do the readily exchangeable oxygen atoms in the catalyst lattice play in the formation of carbon dioxide and therefore in the determination of the C 2 selectivity? An attempt to answer this was made as follows, using the Sm@3 and Prf,Oli catalysts, which had similar activities but very different C, selectivities (Figure 2). The catalysts were heated in flowing I6O2at 700 O C for 10 min (10% O2in He, 78 mL s-I g-l), after which the switch to 1802 was 180'60, and I6O2being continually made, the formation of 1802, monitored. When the exchange reaction had equilibrated, the oxygen flow was terminated and the catalyst purged with helium for IO min. Methane was then admitted to the catalyst (71 mL S-I g-I), while the formation of CI6O2,C 1 8 0 1 6 0and , CI8O2was monitored. When this reaction had been completed, I6O2(7.2 mL s-I g-I 0, in 71 mL s-I g-' CH4) was admitted to the reactor, and the formation of the labeled C 0 2 was again determined. Figure 9 shows the traces obtained for the replacement of 1602 for the S m 2 0 3and Pr6Oll catalysts. The data shown in Figure 9 are essentially the same as those shown in Figure 8a,b, except that in this case the I8O2flow was continued until complete exchange was judged to have taken place. Integration of the data showed that 4 X 10,' oxygen atoms g1had been exchanged in S m 2 0 , and 7 X Ioz1atoms g-' in Pr6OII. This represents 76% and 107% of the available oxygen atoms calculated, respectively, for the formula oxides. It is also clear that the exchange rate on Sm203 is significantly slower than on Pr6Oll. This latter result is consistent with work reported by Minachev et aI.l4 The transients obtained when CH4 was admitted to the catalysts after the I8O2was purged from the system are summarized in Figure 10. For the S m 2 0 3catalyst (Figure loa-c), no reaction was observed, indicating that the lattice oxygen atoms played no role in the formation of the reaction products in the absence of molecular oxygen. Addition of I6O2to the C H 4 immediately led to the formation of '80-labeled CO,, consistent with the data presented in Figure 6. However, a very different result was obtained for Pr60Il. As shown in Figure Iod-f, admission of the CH, led to a sharp pulse of '80-labeled C 0 2 . Traces of C2H6 were also found whose relative contribution increased with the time elapsed, the CO2/C,H6 ratio decreasing from approximately 35 immediately after the admission of CH, to 0.3 after 85 s. I t appears that for this catalyst the lattice oxygen can oxidize CH, (14) Minachev, K. M.: Antonshin, G. V . Probl. Kinet. Karol. 1968, 12. 159.
'
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00
-100
100
300
500
Time (s) Figure 10. Transients observed when CHI and CH4/02mixture were admitted to the Sm,O, (a-c) and Pr601,(d-f) catalysts. At a time of -230 s, CH4 was admitted to the catalysts after the remaining O2 had been purged with He. At time zero, an O2flow was added to the CH4, to give the usual 10% mix.
in the absence of molecular oxygen. Takasu et aLi5have reached a similar conclusion with regard to the oxidation of carbon monoxide over Pr6OIl. Admission of I6O2 to the methane flowing over the Pr6011 catalyst led to the formation of the expected 180-labeledCO, (Figure IOd-f). The ratio of the I 8 0 present in C 0 2 formed when methane only was present to that formed in the presence of oxygen was 9.2. This does not necessarily reflect the true extent of the involvement of lattice oxygen atoms in the reaction, because the formation of H 2 0 and CO, whose quantitative analysis by mass spectrometry is not simple, would also consume lattice oxygen atoms. The effect of the Li2C03dopand on the lattice O2 exchange in S m 2 0 3was examined by passing normal O2 (6.5 mL s-' g-I in 65 mL s-* g-' He) over the Li/Sm203 catalyst at 700 "C. A flow of 1% C 0 2 was added to the He to maintain the carbonate form of the catalyst. After the catalyst was on-line for 15 min, a I8O2switch of 14-s duration was carried out in the normal manner. The results showed that only 0.2 X IOzo atoms g-I were exchanged as O2 and a further 0.9 X lozoatoms g-l incorporated into CO,. The above experiments were repeated with the Sm2O3 catalyst. In this case, 1.1 X loz1atoms g-I were found to have exchanged in the presence of C02,in good agreement with the results reported above for Sm,O, in the absence of C 0 2 . It appears from these results that the presence of Li2C03on the Sm203catalyst inhibits the exchange of the lattice oxygen atoms. This result may explain the much greater activity of the Sm2O3 catalyst compared to the lithium-doped form shown by the data in Figure 2. The presence of a rapid lattice oxygen exchange reaction unfortunately made it possible to use the isotope transient technique to determine the degree of molecular oxygen adsorption on the working catalyst. However, an attempt to estimate this important quantity for Sm203was made as by replacing I6O2by Ar and following the decay of the m / e 32 peak. The results showed only a small tailing of this peak, which could be due to the slightly different pumping speeds of oxygen and argon. It is clear, however, that O2 is not significantly adsorbed on the catalyst under thesi: conditions. ( v i ) Introduction of the Reaction Products. The aim of the following experiments was to determine the importance of the secondary reactions which the products CO, CO,, C2H6,and C2H4 might undergo. Of particular interest was the determination of the contribution which the secondary oxidation of C2H6 and C2H4 made to the total carbon oxide yields. The fate of C O was traced by coinjection of a stream of C O (1.7 mL s-I g-l) with the reactants (78 mL s-I g-I, 10% 0 2 in CH4) ( 1 5 ) Takasu, K . ; Matsui, M.; Matsuda, Y . J . Coral. 1982, 76, 61
Oxidation of CH, over Rare Earth Oxide Catalysts
The Journal of Physical Chemistry, Vol. 93, No. 13, I989 5235 h
a
-I
n c)
-09 i
O0.2 . i I
Y
I 1
o . o r , , -6
Time ( s )
-2
,
m/z = 49
'c ,
2
,
,
I
'JC"O*
,
,
10
6
, 14
I
,
18
Time ( s ) Figure 12. 13C'*02and 12C1802 transients observed when I3C2H6replaced the normal form over the Sm203catalyst at 850 OC. The data have been corrected for the 50% 13Ccontent of the I3C2H6.See text for
Figure 11. 13C02transients observed when a pulse of I3COwas incorporated into the reactants: (a) Sm203,450 OC; (b) Sm203,700 OC; (c)
experimental details.
Li/Sm203,700 OC.
TABLE 11: Summary of the Effects of Added He, CzH6, and CzH4 on the Conversion Rates"
TABLE I: Effect of Added CzH6 and CzH4 on the Origin of COz'
catalyst Sm203
Li/Sm203 pr601
I
He
56 CO1 derived from additive 13C2H6 I3C2H4 700°C 850 OC 700 OC 850 OC 70 78 48 46 14 24 9 20 41 46 61 80
CH, conversion rate O2conversion rate 0, consumption, % ' CO formation rate CO, formation rate
344 306 98 58 81
196 305 99
160 307
99 130 99
73 74
Sm203,700 OC
"Conditions: CH4 flow, 70 mL s-I g-l; I8O2flow, 9 mL s-l g-l; additive flow, 9 m L s-I g-]. All data show the percent C 0 2 derived from the additive as determined from the 13C02formed during the additive pulse. onto the catalyst bed and switching the C O to I3CO for 35 s while following the formation of I3CO2. For all three catalysts, approximately 30% of the injected C O was converted to C 0 2 . As shown in Figure 1 I , the C 0 2 formed was adsorbed on S m 2 0 3at 450 OC and strongly adsorbed on the Li/Sm203 catalyst at 700 OC but not adsorbed on Sm2O3 at 700 "c. The Pr6011 catalyst behaved identically to the S m 2 0 3 . In a similar set of experiments, a pulse of l 3 c O 2was incorporated into the feed gas, and the appearance-disappearance of I2CO2and 13C02was followed. The data obtained were virtually identical with those shown in Figure 11, with a significant C 0 2 uptake shown only by the Li/Sm203catalyst. For this catalyst, integration of the area under the trailing part of the Y O 2 release curve after the termination of the pulse gave a value of 1.5 X 1020 molecules of C 0 2 adsorbed during the pulse by 1 g of catalyst, compared to a value of 2 X lozo molecules expected for the complete exchange of a catalyst containing 3% Li2C03. These results provide unequivocal support for the view that at reaction temperature the Li/Sm203catalyst is covered with a carbonate surface, which is absent for the Sm203 and Pr6OIl catalysts. An attempt to estimate the amount of C 0 2 formed by secondary reactions from the products C2H6 and C2H4 under reaction conditions was made as follows. CH, (70 mL s-l g-l), I8O2(9 mL s-l g-l), and He, C2H6, or C2H4(9 mL s-l g-l) were passed over the catalysts. I8O2was again used to avoid interference from C3 products. A switch to 13C2H6or I3C2H4was then made, while and 12C'802 formation was monitored at m / e 48 and the 13C1802 49 (Figure 12). A complete analysis of the reactor effluent was also made using an on-line gas chromatograph. A striking feature of the results (Table I) is that the added C2H6 or C2H4 was substantially incorporated into the C 0 2 for all three catalysts, while the total C 0 2 and CO yields and the oxygen consumption were not significantly affected by this addition, particularly for the addition of C2H6(Table 11). Notably, the net CH, conversion declined on addition of the C2H6 and C2H4. Similar results to those shown in Table I I were found for the other catalysts used
I3C2H6
Sm203,850 OC
CH, conversion rate O2 conversion rate O2 consumption, % CO formation rate CO, formation rate
265 230
139 229
75
74 56 51
46 54
153 25 1 81 95 75
"Conditions: CH, flow, 70 mL s-I g-l; '*02 flow, 9 mL s-' g-I; additive flow, 9 mL s-] g-l. All reaction rates expressed in mol lod g-' s-1.
in this study and have been found by others for Na/CaOgb and Sr016 catalysts. It appears that this unusual result is a general feature of the partial oxidation reaction. Discussion ( i ) The Catalyst Surface during Reaction. The above results show that methane is strongly adsorbed on the catalysts studied in this work. It is not clear from the data whether the presence of oxygen, (i.e., a working catalyst surface) enhances the adsorption. It is interesting that the addition of Li to the S m 2 0 3 has no significant effect on the CHI adsorption, even though it greatly reduced catalyst activity. The adsorption of such an inert molecule on a low surface area oxide at high temperature could be explained in terms of a strongly basic catalyst surface and a weakly acid CH4 and may invdve partially or even wholly dissociated methane (eq 1). Support for this model is provided by h
8.+
CI.'3.--lj 6,
M-0
b
I
+ CHI
M-0
,
(1)
the observationI7 that the reaction is poisoned by the addition of carbon dioxide, a characteristic property of strong base catal y ~ t s . ' ~An ~ J identical ~ mechanism for the adsorption of methane and other alkanes has been suggested in the interpretation of the mechanism of the methane H / D exchange reaction over various (16) Regtop, R., to be published. (17) Clark, K . Presented at the Bicentennial Catalysis Congress, Sydney, Sept 1-2, 1988. ( I 8) Tanabe, K. Catalysis by Solid Bases and Related Topics. In Catalysis by Acids and Bases; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1985.
5236
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
catalyst^.'^ Ito et aI.,O have also observed methane adsorption on specially activated MgO and concluded that the adsorption occurred in a heterolytically dissociated form. A surprising feature of the present results is that, despite the strong methane adsorption, the fast isotope transients found for the reaction products CO, and C2H6 show that the adsorbed methane does not play a significant role in their formation. This aspect will be discussed further below. The major difference between the high-activity catalysts (Sm203, Pr6OIl)and the low-activity Li/Sm203is that the present data, particularly Figure 1 I , show that the Li/Sm203 catalyst surface is in the carbonate form under reaction conditions, while this is not the case for the Sm203 and Pr6OII surfaces. Similar conclusions have been reached with respect to the Li/MgO catalyst previously, and various interpretations of the importance of the presence of the carbonate have been offered.I3 The present results suggest that a major effect of the presence of the carbonate is a reduction in the rate of lattice oxygen exchange. ( i i ) Formation of Carbon Oxides. An understanding of the mechanism of formation of the carbon oxides is of fundamental importance for the development of more selective catalysts. It appears that the C2 selectivity is at least partly a function of the precise conditions of the experiment, possibly including geometric factors,21B22 but the present results also indicate features of this reaction that have not been considered previously. The formation of the CO, is obviously rapid on the time scale of our experiments and, as shown by the data of Figures 6 and 7 , appears not to involve the pool of adsorbed methane present on the catalyst surface. However, examination of Tables I and I 1 shows (a) that a large proportion of the CO, formed during the partial oxidation can be derived from the products, particularly ethylene, (b) that, despite the above, the addition of C2H6 in particular did not lead to a significant increase in the total CO, formation rate and oxygen consumption, even in the presence of excess oxygen, and (c) that significant differences in the proportion of C 0 2 derived from the products were found for the catalysts tested. When C2H4 was added to the reactants, the rates of carbon oxide formation increase slightly. However, at 700 OC, the increase in the C 0 2 O X O rates (40 X IO6 molecules s-' g-I) was greater than the increase in the rate of O2 consumption (21 X IO6 molecules S-I g-l). We conclude, therefore, that some of the C2H4 reacts with excess 0, but that the major part of the C 0 2 derived from the C2H4 is formed by the same mechanism by which the C H 3 and C2H6 are oxidized. The observation that the addition of C2H6 or C2H4 to the reactants did not seriously affect the COz formation rate has been interpretedgbas showing that these were inert to further oxidation. Otsuka et al.9dalso concluded that ethylene was not oxidized over SmzO3. It is obvious from the present data that these conclusions are erroneous. This result has important consequences for reactor design, as product back-mixing must clearly be avoided to obtain have also drawn attention maximum C, selectivity. Roos et to this requirement. An important clue to the mechanism of the formation of the carbon oxides is provided by the independence of their formation rates from added C2H6and C2H4. Such a result is only possible if the formation of the carbon oxides is limited by the availability of the oxidant and not of the reactant. As the effect is observed under only partial oxygen consumption, it is clear that this reaction is not simply a gas-phase oxidation involving molecular oxygen but that it must involve a species formed by the catalyst, which
+
Ekstrom and Lapszewicz reacts either on the surface or in the gas phase. Accordingly, we suggest the following conceptual reaction scheme
///
k l (slow)
02(gasj,
02(surfacej
*R
4 R - R in which two oxygen species, [A] and [B], are formed from O2 adsorbed on the surface, whose concentration is very small. Hydrocarbons (CH4, C2H6, C2H4)react rapidly and competitively with species [A] to give the corresponding alkyl radicals. The competition of the hydrocarbons for species [A] accounts for the decrease in the net CH4 conversion found on addition of C& and C2H4 which, as expected, undergo a much faster oxidative dehydrogenation reaction than CH4. Species [B] reacts rapidly with the alkyl radicals so formed to give carbon oxides in competition to their gas-phase dimerization. The combination of a slow formation of [B] and the rapid reaction with 'R will result in the observed constant CO, yield as C,H, or C2H4 is added. This mechanism is also supported by the data in Figure 7 which can be interpreted as showing that the CO, and C2H6 have a common precursor. Several features of the reaction remain unresolved at present. For example, the mechanism by which the addition of lithium to the catalyst reduces the extent of the oxidation of C& and C2H4 is not obvious. Similarly, although species [A] will be identified as [O-] below, species [B] is more difficult to characterize at present. Possibilities are 0 atoms and O H or H 0 2 radicals. These aspect are under study in our laboratory. (iii) The Role of the Lattice Oxygen Exchange Reaction. For the Sm,03 and Li/Sm203catalysts, the results of Figure I O clearly show that the catalyst lattice oxygen does not participate in the reaction unless gas-phase 0, is simultaneously present. Similarly, the instantaneous termination of the C2 formation following the end of the 0, flow implies that for these materials there is no substantial pool of labile oxygen present on the surface during reaction conditions and that the steady-state concentration of O,(surface) is not detectable by this technique. This confirms the results of the 0 2 / A r switches which also indicated that no significant pool of O2was present on the surface. Following from the discussion above, it is probable that the small size of this pool is crucial to maintaining a low carbon oxide selectivity. Although no 0, adsorption could be detected for the Pr6011 catalyst, the l80,results showed that the lattice oxygen could react with CHI. The reaction probably corresponds to the reduction of P r 6 0 1 1to Pr203, and it is interesting that the product of this reaction is substantially carbon dioxide. This behavior is clearly quite different from the A R C 0 type catalyst^,,^ which give a high C2 selectivity at short reaction times. The present results confirm that a rapid exchange between the gas-phase and lattice oxygen atoms takes place on the working catalyst. According to Winter,8 the mechanism of this reaction (shown schematically in eq 3, in which [-] represents an anion I8O,(gas)
(19) (a) Bird, R.; KembaII, C.; Leach, H. F. J . Card. 1987, 107,424. (b) Utiyama, M.; Hattori, H.; Tanabe, K. J . Card. 1978, 53, 237. (20) [to, T.; Tashiro, T.; Watanabe, T.; Toi, T.; Ikemoto, 1. Chem. Lett. 1987. 1723. (21) I n our experience, >80% C2 selectivity can be achieved with many catalysts by using very low oxygen partial pressures. (22) For a n example of the effects of reactor geometry, see: van Kasteren, H. M . N.; Geerts, J. W. M . H.; van der Wiele, G .Proc. Inr. Congr. Cars/. 9th 1988, 2, 930. (23) Roos, J. A.; Korf, S. J.; Bakker, A. G.; de Bruin, N. A,; van Omen, J. G.; Ross, J. R. H. Srud. Surf. Sci. Catal. 1988, 36, 427.
-
1 8 0,(surface)
'802(surface) + 2[-]
+ {-160-)(lattice)
-
-
2[180-]
(3a) (3b)
{-180-)(Iattice) + [160-]
(3c) vacancy, [O-]an oxygen atom trapped in the vacancy, and 1-0-1 a lattice oxygen) is initiated by the rapid formation of the [O-] centers. The formation of these species requires no activation (24) Sofranko, J. A.; Leonard, J.; Jones, C. A . J . Catal. 1987, 103. 302.
Oxidation of CHI over Rare Earth Oxide Catalysts energy, the rate-determining step being the desorption of molecular oxygen from the surface, i.e.. the reverse of reaction 3a. The importance of the exchange reaction as a means of forming [O-] to the partial oxidation reaction may be judged from the following. First, the [O-] species on oxide surfaces are known to be highly reactive toward C-H bond cleavage, even at temperatures much below those required for the partial oxidation reaction.25 Second, there appears to be a correlation between the rates of the exchange reaction and the overall rates of the partial oxidation. For example, at 750 OC, extrapolation of Winter’s data gives a rate for the exchange reaction of 2 X l O I 9 molecules s-’ g-’ (assuming a surface area of 10 m2 g-I), while the net rate of CHI conversion at this temperature is in the range of 10’9-1020molecules s-I g-’. The correlation observed in this work between the exchange rate and catalyst activity following addition of Li2C03to the Sm2O3 also suggests a link between the rate of exchange and the rate of reaction. Finally, Mehandru et a1.26have recently shown that H abstraction from C H 4 by [O-] in MgO was much more favored than abstraction by [02-]. This important result is discussed further below. ( i o ) The Mechanism of the Partial Oxidation Reaction. The present work has shown that the isotopic composition of the reaction products other than those obviously requiring isotope mixing (e.g., HD, 13CH3’2CH3, CD3CH3,which are always minor components) instantly reflects the isotopic composition of the carbon in the reactant methane. These results present a basic dilemma; viz., how can species such as C&, 13C2H6,and 13C02be formed “instantaneously” following the isotope switch when the catalyst surface appears to be covered with the isotopically normal species that desorb much more slowly than the rate at which the labeled products are formed? One possibility is that the reaction proceeds by a form of interface chemistry recently discussed by Ceyer et al.27 However, a more plausible explanation of the presently available data is that the catalyst contains a small number of active sites having very high turnover frequencies and on which product formation occurs very rapidly and essentially independently of the adsorbed C H 4 . The involvement of only a small number of active sites in the product formation is also consistent with the observation that C2 formation ceases instantly following termination of the CH, flow, even though substantial amounts of CH, are present on the surface. The involvement of only a small number of very active sites was proposed2*to account for the fast H2/D2exchange over MgO at temperatures as low as 77 K and is probably characteristic of reaction paths involving lattice defects and ionic reaction intermediates. The recent results of Mehandru et a1.,26who suggested that the abstraction of H from C H 4 by [O-] involved a linear H3C. -H-.[O-] transition state from which the ‘CH, radical can be released directly into the gas phase where it can undergo dimerization, is important for the interpretation of the present results. Combined with the concept of a small number of active sites having a very high turnover frequency, their suggesttion would easily explain the “instantaneous” appearance of the isotopic form of the reactants in the products. It might be recalled in this connection that Lunsford et al.7ahave estimated that at least 60% of the ethane formed over Li/MgO was the result of the dimerization occurring in the gas phase. (25) (a) Liu, R . 4 . ; Iwamoto, M.; Lunsford, J. H. J . Chem. Soc., Chem. Commun. 1982, 78. (b) Aika, K.; Lunsford, J. H . J . Phys. Chem. 1977, 81, 1393. (c) Aika, K.; Lunsford, J. H. J . Phys. Chem. 1978,82, 1794. (d) Ward, M. B.; Lin, M. J.; Lunsford, J. H. J . Cafal. 1977, 50, 306. (26) Mehandru, S. P.; Anderson, A. B.; Brazdil, J. F. J . Am. Chem. SOC. 1988, 110, 1715. (27) (a) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J . Chem. Phys. 1986, 85, 1693. (b) Ceyer, S. T.; Yang, Q. Y.; Lee, M. B.; Beckerle, J. D.; Johnson, A. D. Stud. Surf. Sci. Catal. 1988, 36, 51. (28) Boudart, M.; Delbouille, A.; Dereouane, E. G.; Indovina, V.; Walters, A. B. J . Am. Chem. SOC.1972, 94, 6622.
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5237
/
0,(gas)
+==.==s
O,(surface)
-L /
\
I
[ 0-1
[-OH]
\
I
H~O
Figure 13. Proposed reaction mechanisms for the partial oxidation of methane.
On the basis of the above, the reaction mechanism shown in Figure 13 is proposed, in which the reactive species is [O-1. The reaction sequence is similar to those suggested by LunsfordgBqband other^^^,^^,^ but includes Winter’s mechanism8 for the exchange of lattice oxygen atoms to regenerate the [O-] species. The inclusion of this reaction readily accounts for the observation that C2 formation ceases instantly following termination of the oxygen flow. The [OH-] formed as a result of the C-H bond scission may disproportionate rapidly to H 2 0 , [O-1, and the anion vacancy [-I as suggested by L u n s f ~ r dfor ~ ~Li/MgO ,~ and Na/CaO. As already discussed in relation to eq 2, in this mechanism the carbon oxides are formed substantially by oxidation of the radicals (‘CH,, ‘C2H5, or ‘C2H3) produced in the primary reaction2s by an oxygen species formed on the catalyst surface which is not molecular 02.This oxidation could occur either on the catalyst surface or in the gas phase. However, some of the C2H4may be directly oxidized by molecular O2present in the reactor. There is now substantial e v i d e n ~ ethat ~ ~ ,the ~ ~geometry of the reactor, and particularly the presence of an “afterburner” effect22when excess O2 is present, is of considerable importance in determining the C2 selectivity. For catalysts such as Pr6OI1,the lattice oxygen can directly oxidize CH, to form C02, in addition to the reactions of the [O-1. This oxidation-reduction cycle is schematically represented by the reaction sequence M-0,
+ CHI M-0,
+
M-O,,
+0 2
+
+ CO, C 0 2 M-0,
(4a) (4b)
The relative contributions to the total carbon oxide formation of the above reaction sequences control the catalyst selectivity. It is obvious, however, that for the Sm2O3 and Li/Sm203 catalysts the carbon oxides are formed by a route that does not involve the reduction of the catalyst surface.
Acknowledgment. We thank Mr. I. Campbell and Drs. R. Regtop and S. Bhargava for assisting in the acquisition of the analyses shown in Table 11. Dr. Regtop also kindly provided the data shown in F i g u r e 2. Registry No. CH4, 74-82-8; S m 2 0 3 ,12060-58-1; Li, 7439-93-2; Pr,12037-29-5.
011,
(29) Lo, M.-Y.; Agarwal, S. K.; Marcelin, G. J . Catal. 1988, 112, 168.
