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Catalytic gasification of graphite by calcium and nickel-calcium
Carbon Vol. 23, No. 6. pp. 635444. Prmted in the U.S.A.
OOOS-622311985 $3.00~ 00 0 1985 Pergamon Press Ltd.
1985
CATALYTIC
GASIFICATION OF GRAPHITE AND NICKEL-CALCIUM R. T. K.
Corporate
BAKER and J. J. CHLUDZINSKI,
BY CALCIUM
JR.
Research Science Laboratories, Exxon Research and Engineering Clinton Township, Route 22E, Annandale, NJ 08801, U.S.A. (Received 7 December
Company,
1984)
Abstract-Controlled atmosphere electron microscopy has been used to study the influence of calcium on the graphite-oxygen and graphite-steam reactions. Direct observation of the specimen surface has enabled us to establish that the high catalytic activity of calcium for graphite oxidation is associated with the ability of the alkaline earth to wet and spread along the graphite edges, which subsequently undergo gasification by the edge recession mode. We have found that in the presence of steam, calcium-coated graphite edges would occasionally deactivate, and this behavior became more pronounced as the temperature was increased. A decrease in rate of gasification was found when nickel was added to calcium, this is attributed to a fundamental difference in the mechanisms by which these two elements catalyze the graphite-steam reaction. Key Words-Catalysis,
1.
gasification
calcium,
nickel-calcium,
INTRODUCTION
electron microscopy
of the same reaction systems. Using thermodynamic arguments, he interpreted the catalytic effect in terms of a sequential series of reaction steps involving reduction of a carbonate to oxide at the catalyst-graphite interface and followed by reformation of carbonate by reaction of the oxide with ambient CO and H,O. He suggested that the rate-determining step was the conversion of carbonate to oxide. In contrast, Coates and coworkers[lO], using in situ electron microscopy, were unable to observe any reaction when graphite samples containing added calcium oxide were exposed to water vapor at 500°C. The purported ability of calcium to achieve and maintain by itself intimate contact with graphite is a necessary but not sufficient condition to produce good initial catalytic activity on chars and less-ordered carbons. In these cases dispersion is important. It has been shown by several workers[ 1 l-131 that significant gasification activity is obtained on these materials when the calcium is highly dispersed by ion exchange onto the carbon or coal precursor. A wide variation in total reactivity results from different catalyst impregnation techniques and from different subsequent thermal treatment prior to gasification. A literature search reveals very little information on the catalyzed steam gasification of carbons by mixtures containing either calcium or nickel. From their study of the influence of nickel-potassium on the carbon-steam reaction, Wigmans and Moulijn[l4] concluded that the two elements operated by fundamentally different reaction mechanisms, and that the effect of the mixed catalyst could be simply described as the mathematical sum of the separate catalyst actions. In the present investigation, direct observations of specimen surfaces undergoing reaction have enabled us to determine the mode by which the catalyst functions during gasification of graphite. Furthermore, from detailed quantitative analysis of recorded sequences, we have been able to obtain kinetic data on these processes.
As part of a continuing effort to understand the manner by which various additives catalyze the gasification of carbonaceous solids, we have used CAEM to study the influences of calcium on the graphite-oxygen reaction and of calcium and nickel-calcium on the graphite-steam reaction. Even though calcium is known to be an active catalyst for the graphite-oxygen reaction1 I], very few reports have appeared in the literature discussing the mechanism. The most revealing study was that described by Cairns et a1.[2], who, by means of CAEM, identified CaO as the impurity in single-crystal graphite, which was responsible for enhancing its oxidation. Although they did not perform a detailed investigation of the system, they were able to establish that the catalytic mode of attack took place by pitting and channeling at 6oo”C, possibly aided by other impurities. Amariglio and Duval[3] compared the reactivity of a number of additives, including calcium, for graphite oxidation, and, according to them, the order of catalytic activity was Na > Ba > Sr > Mg > Ca. Using data obtained by Amariglio[4], Heintz and Parker[S] estimated a value of 25.0 kcal mole-’ for the activation energy of the calcium-catalyzed graphite-oxygen reaction. Several workers[6-IO] have reported that calcium is an effective catalyst for steam gasification of carbonaceous materials. Otto and co-workers[8], using a simple weight loss technique, measured steam gasification rates of graphite impregnated with various alkaline earth salts as a function of temperature. From a combination of this kinetic data and electron microscopy observations, they concluded that the increase in gasification rate was associated with the ability of the alkaline earths to maintain extensive and intimate contact with the graphite surface. They also reported that the catalytic effect increased in the order Ca < Sr < Ba. A similar reactivity trend was found by McKee[B] from his thermogravimetric studies 635
R. T. K.
636
BAKER and I.
2. EXPERKMENTAL
A detailed description of the CAEM technique can be found elsewherc[ 151.Calcium was introduced onto transmission specimens of single crystal graphite (Ticonderoga, New York State) as an atomized spray from a 0.1 o/c aqueous solution of calcium nitrate, and nickel-calcium was added in a similar manner from an aqueous solution of the respective nitrates mixed in a I : 1 ratio. In some experiments the oxidation characteristics of graphite specimens, which did not contain any added catalysts, were also examined as controls. The reactant gases used in this investigation-oxygen and argon-were obtained from Scientific Gas Products. Inc. with stated purities of 99.999% and were used without further purification. Steam was introduced by allowing argon to flow through a bubbler containing deionized water maintained at 25% prior to entering the gas reaction cell conditions, which produced an argon/water ratio of about 40: 1.
2.1 Calciumlgraphite-oxygen When the calcium/graphite system was heated in 2.0 torr oxygen, particles were observed to form on the surface at 475°C. At a slightly higher temperature of 535°C. some particles were observed to penetrate the graphite basal plane to produce pits, while particles which had accumulated at edges and steps started to propagate channels. During this stage of the reaction, the channels followed random pathways and tended to remain parallelsided for relatively long periods of time. There was a sharp transformation in the mode of catalytic attack at 610°C. The catalyst particles at the head of channels were gradually reduced in size as a result of material adhering to the channel walls. This action caused channels to decrease in width and channel walls to gasify at a faster rate than uncontaminated graphite edges, which resulted in channels becoming fluted in outline. As a given catalyst particle became smaller. so the rate of the channel formation increased to the point where the particle was completely dissipated and further forward development of the channel ceased. Also, at this temperature, particles, which encountered edges and steps, no longer showed a tendency to create channels but instead underwent a change in morphology due to a spreading action to form a thin film along these features. Eventually the coated edges and steps were observed to recede along directions parallel to the (1010) crystallographic orientations. As the temperature was increased, the propensity for this behavior to occur grew, and edge recession quickly became the exclusive mode of gasification. At temperatures in excess of 750°C. the receding edges lost their uniform profile and became more ragged in appearance. Whether the reason for this effect is associated with a modification in the influence of calcium on the gasification reaction or is due to the interference of the uncatalyzed attack is not possible to resolve. However, it was interesting to find that, if the specimen was cooled to 65O”C, the receding edges once again assumed their ordered outline. Furthermore, dropping the
J.
CWJDZINSKI, JR.
temperature to 575°C resulted in the termination of edge recession as particles reformed, and gasification by the channeling mode was once again observed. Finally, continuous observations showed that the particles located on the perfect regions of the basal plane, which did not come into contact with receding edges or steps, remained motionless throughout the majority of reaction sequences and only exhibited significant movement in the few experiments where the temperature was increased to 1220°C. Quantitative analysis of the edge recession process showed that, unlike the corresponding action observed in the calcium catalyzed steam gasification of graphite (see following section), once initiated, all receding edges maintained their activity for the duration of the experiment, provided that the temperature was kept above about 610°C. The dependence of edge recession rate on temperature is expressed in terms of an Arrhenius relationship (Fig. 1). From the slope of this line, a value of 24.5 f 3 kcal mole“ was derived for the apparent activation energy of the calcium-catalyzed oxidation of graphite 2.2 Calciumlgraphite-steam Particle nucleation was achieved by heating the graphite specimens coated with the calcium salt at 400°C in 2 torr wet argon. Particles were in the size range of 1.5 to 3.0 nm, and those which had collected at graphite edges and steps were observed to bc in a nonwetting conftguration. This condition changed at 475°C when these particles transformed into a wetting state and eventually continued to spread along the edges, so that all indications of their previous existence was erased. In contrast, particles which had formed on the graphite basal plane continued to exhibit normal growth characteristics, reaching an average size of 10 nm. At 555”C, edges which had been “coated” with calcium started to undergo gasification. As the temperature was raised to 650°C. the rate of edge recession increased significantly. At this temperature the reaction had progressed to a stage where it was possible to determine that edges were receding in a very uniform manner and along directions parallel to the (1120) crystallographic orientations. Figure 2 is a photograph taken from the video playback showing the appearance of edges created during treatment in 2 tot-r wet argon at 700°C. From this picture it is possible to see the 30” and 90” angles produced by the edges with respect to the twin band At?. This specimen had previously been reacted in 2.0 torr oxygen at 600°C. a condition where channels had been propagated, and during subsequent treatment in wet argon the catalyst particles underwent a spreading action which eventually resulted in edge recession. Although the reaction appeared to follow the expected trend of an increase in rate with increasing temperature, there were some isolated edge regions which showed anomalous behavior; recession came to an abrupt halt and activity could not be restored by either raising or lowering the temperature. This behavior has also been observed in the K,CO,-catalyzed steam gasification reaction1 161. This phenomenon became more widespread as the temperature reached 900°C. Figure 3A-F shows the sequence
Catalytic
gasification
of graphite
637
l.! j-
1.tD\ \
0
\
.!5-
O
P
0 \
0
: 5 5 al 5 -0 $
0-
- O.!5-
0
\ - 1.tDO.’9
Fig. 1. Arrhenius
Fig. 2. Appearance
1
1 .o
I
I
1.1 I/T (K) x 10’
plot of calcium-catalyzed
graphite-oxygen
1.2
reaction.
of edges created during treatment in 2 torr wet argon at 700°C in presence of calcium; AB indicates the direction of a twin band on the graphite.
R. T. K. BAKER and J. J. CHLUDZINSKI.JR.
638
Fig. 3. (A)-(F)
Sequence showing the loss of edge activity as a calcium/graphite specimen 715OC to 920°C in 2 torr wet argon. Direction of edge recession is indicated.
is heated
Catalytic gasification of graphite
Fig. 3. (Continued).
639
640
R. T. K.
BAKER and J.
of events leading to the loss of edge activity as a specimen is heated from 715°C up to 920°C. The direction of motion of receding edges is indicated by the arrows. At 715°C edge recession is seen to occur in a very smooth, well-oriented manner. At 87X, edges start to undergo “eruption”, with the formation of a myriad of tiny particles which rapidly lose their catalytic activity, and this behavior results in edges acquiring a feathery outline. Furthermore, even when fresh particles came into contact with those “dead” edges, gasification activity was not regenerated. Particles located on the basal plane did not exhibit the same degree of wettability with the graphite as those which came into contact with edges. Nevertheless, the interaction with the support was sufficiently strong to cause complete suppression of particle motion and inhibit particle growth via the atomic migration mode. A further feature of the reaction was that, unlike the barium/graphite-steam system[ 171, there was no tendency for the catalytic action to change to a channeling mode at temperatures >9OO”C. Detailed quantitative analysis of many edge recession sequences revealed some interesting aspects regarding I.!
I
3.
CHLUDZINSKI,JR.
this form of catalytic attack of graphite. At any given temperature all edges which were undergoing gasification appeared to recede at the same rate within experimental error, regardless of their thicknesses or lengths. The variation in edge recession rate was determined as a function of temperature, and this data is presented in the form of an Arrhenius plot in Fig. 4. From the slope of this line it has been possible to evaluate an apparent activation energy of 51.3 +- 6 kcal mole-’ for the calcium-catalyzed steam gasification of graphite. 2.3 Nickel-calciumlgraphite-steam When nickel-calcium/graphite specimens were heated in 2.0 tot-r wet argon, nucleation of small particles, 35-nm diameter, was observed between 4OO’Cto 475°C. Inspection of particles which had collected at the graphite edge regions showed that they were in a nonwetting state. Over the temperature range 555°C to 610°C these particles underwent a transformation in morphology, first wetting the substrate and then proceeding to spread along the edge and step regions in the form of a thin film. It was necessary to raise the temperature to 795°C in order to achieve catalytic attack of the graphite, which ,
I
b 1.C
.E
0
- i.a
I
9
0.95
I
1.00
I.059
I/T(K)xlO'
Fig. 4. Arrhenius plot of calcium-catalyzed graphite-steam reaction.
Catalytic gasificationof graphite took place by the edge recession mode. At first the edges acquired a rippled appearance, which gradually changed to a well-de~ned faceted outline as gasification proceeded. Although the rate of recession was extremely slow, it was possible to determine that edges undergoing attack were aligned in directions parallel to the (1120) crystallographic orientations of the graphite. This form of attack continued up to 112O”C,there being no evidence of channel propagation or reformation of particles at the edge regions. Fu~he~ore, although the edges tended to lose their sharp profiles at the highest temperatures, there was no indication of a “tailing off” in the recession rate. A survey of the basal plane regions of the graphite showed that the particles which had formed in these areas remained static throughout the reaction and only took part in the reaction if they were undefined by a receding edge. When such a situation occurred, a particle would rapidly undergo spreading along the receding edge and disappear. The variation in the edge recession rate was measured as a function of temperature, and this data is presented in the form of an Arrhenius plot in Fig. 5. From the slope of this line it has been possible to evaluate an apparent activation energy of 5 1.1 2 6 kcal mole-’ for the nickelcalcium-catalyzed steam gasification of graphite. Also included on this plot are comparable data obtained for the single calcium and nickel[ 181 systems. 2.4 Graphite-steam In a final series of experiments we have examined the effect of heating graphite specimens which did not contain any added catalyst in the presence of 2.0 ton: wet
1.5
\ \ \ \
1.0
g
0.5
; 5 B 2! 2 z k -I
0
‘\ *
l
Y--Ca’cium \\ \\ \\ \\ \\ \\ \\ \\ \ \
\\ ‘1 ‘\ .yNickel ‘1 0
-0.5
\
l
l
*
* \ -1.0 0.7
I 0.0
\ \ \ \\ \ t
'\ '\ '\
0.9
0
'\ ‘\
,‘1.. 1.0
l/T (K)x 103
Fig. 5. Axrhenius plot of nickel~alcium-ca~lyzed graphitesteam reaction. Also included for comparisonpurposes are the plots for calcium and nickel alone for the same reaction.
641
argon. Very little change in the appearance of the surface was observed until the temperature was raised to 990°C. At this tem~rature, edges were seen to undergo recession, but, unlike the equivalent catalytic process, the movement did not appear to follow any particular orientation, and the profile of the receding edges tended to be ragged. Figure 6A-D is a sequence showing edge recession due to uncatafyzed attack as a graphite specimen is heated in steam from 950°C to 1040°C. The overall rates of edge recession at 900°C for the uncatalyzed reaction are over 1000 times slower than those of the calcium-catalyzed edges at the same conditions. 3. DISCUSSION
In comparing the catalytic carbon gasi~cation activity of calcium and barium, the most feasible approach to this task is by reference to the factors which govern the overall activity-the intrinsic catalytic activity and the degree of wetting of the graphite by the catalyst. For the graphite-oxygen reaction both additives show a similar trend in cataiytic behavior. At tem~ratures of about 575°C the active species are present as discrete particles and attack the graphite predominantly by the channeling mode. On continued heating, the catalyst particles undergo a transformation in shape from globules to a thin fifm, which is caused by speading of material along the graphite edges and steps. This action is also manifested by a change in mode of catalytic attack from channeling to edge recession. The fact that this modification in catalytic attack is observed at a lower temperature for calcium (610°C) than for barium (750°C) implies that the former has a somewhat higher wetting tendency on graphite edges. Unfo~unate~y, from our studies it is not possible to make a valid comp~ison between the intrinsic activities of these catalysts, since the kinetic data for calcium is based on edge recession attack and that of barium is for the channeling mode. In spite of this lack of information, we would not expect a large difference in the respective overall activities of these catalysts for the graphite-oxygen reaction. For both catalysts, edge recession catalyzed by a thin film of catalyst is a major mode of attack in steam. This agrees with conjectures in the literature]81 that good catalyst-carbon contact is maintained by a spreading of the catalyst. Closer comparison of the influence of the two alkaline ear&rmetals on the ~aphite-stew reaction shows that, although similarities do exist, in this case them are some major differences in the behavioral patterns. Although both materials show an affinity towards wetting and spreading along the graphite steps and edges, calcium seems to possess the stronger capability for attaining this state. This is deduced from the fact that edge recession is the only form of catalytic attack observed in the calcium/graphite system, whereas barium exhibits a mixture of edge recession and channeling actions, although the contribution of the latter mode to the overall activities is of secondary importance at temperatures befow 900°C. Both catalysts show a stronger tendency to wet and spread in steam than in oxygen. Edge recession ceases in both systems at 9OO”C,and
642
R. T. K. BAKERand I. J. CHLUDZINSKI, JR.
Catalytic
gasification
particles reform in these regions; however, the subsequent fate of these particles differs for the two catalysts. Those formed in the barium/graphite system continue to exhibit catalytic action by propagating channels. In contrast, the particles generated from the calcium species rapidly lose all activity. At the present time we do not understand why the calcium film, presumably in the form of oxide, suddenly becomes deactivated in steam at lower temperatures or as particles at high temperatures. Calcium catalyst on char-like carbons can also lose its reactivity in steam following exposure to inert or reducing gases at 650SOO”C[131. This phenomenon may also have taken place to a lesser degree in the barium/graphite-steam system, but, because the greater majority of particles retained their activity, its existence would have been overlooked. The deactivation may arise from chemical changes in the catalyst or the graphite surface or loss of bonding contact between the catalyst and the graphite. One possibility is that the catalyst becomes coated with a layer of carbon or even reacts to form a carbide. However, the reactivity of calcium carbide in water at room temperature argues against the carbide as a refractory repository of calcium in high-temperature steam. It is believed that the catalyst-carbon interfacial bonding which is responsible for the wetting interaction of calcium at lower temperatures (600-700°C) involves surface oxide groups. It is possible that the graphite edge cannot support a sufficient inventory of oxygen groups at high temperature, and the interfacial bonding is broken, thus allowing the catalyst to sinter. Other workers[8, 91 find that barium is a more active steam gasification catalyst than calcium. This is true despite the fact that calcium appears to wet and spread along the graphite edges more proficiently than does barium. Actual catalyst usage or quantitative dispersion on the graphite cannot be determined in these systems which wet and spread because the thickness of the catalyst film cannot be measured. The edge recession rates obtained here are of the same order as those found for barium[ 171. Therefore, the intrinsic activities of the two catalysts are similar. The difference in total reactivity probably reflects higher dispersion of barium as well as a higher incidence of deactivation events with calcium. The agreement between the apparent activation energies obtained here for the calcium-catalyzed graphiteoxygen reaction of 24.5 kcal mole-’ and that quoted by Heintz and Parker[S] of 25.0 kcal mole-l from bulk experiments is excellent. The corresponding values for the calcium-catalyzed steam gasification of graphite of 5 1.3 kcal mole-’ (present work) and bulk values of 53.5 kcal mole-‘[9] and 70.0-70.8 kcal mole-‘[8] are also in satisfactory agreement. Although it was not possible to determine the surface composition of the mixed catalyst during reaction, it is possible, using simple thermodynamic considerations, to predict which component would be expected to preferentially segregate to the catalyst surface in a steam environment. Thermodynamic calculations show that at the steam pressures used in this study nickel will remain in the metallic state and calcium, which has a high affinity
643
of graphite
for oxygenated species, will tend to segregate to the surface and probably be present in the oxidized state. The decrease in the gasification rate found when nickel is added to calcium is at first sight very perplexing, since both elements on their own are very active catalysts for steam gasification of graphite. The answer to this problem lies in the fundamental differences in the mechanisms by which these two elements operate in the carbon-steam reaction. Kapteijn and Moulijn[ 191 proposed the following sequence of reaction steps to account for the influence of calcium on the graphite-steam reaction: CaO + H,O + CaO . 0 + H,,
(I)
CaO . 0 + C +
(2)
CaO + C[O],
cc01 -+ co.
(3)
In this mechanism the rate-determining to be the release of CO from the carbon A somewhat different reaction scheme by Holstein and Boudart[20] to explain lyzed carbon-steam interaction. C-C-C
+ M%C-C
M + HzO+M--O M-C
+ M-O--+2M
step is claimed structure. was presented the metal-cata-
+ M-C,
(4)
+ Hz,
(5)
+ CO.
(6)
The first step in the mechanism is the formation of carbidic carbon on the metal, which depends upon the rupture of a C-C bond in the bulk carbon source, followed by diffusion of carbon species through or over the metal surface. The second step involves the dissociative adsorption of water, and the final step the formation of CO by reaction of carbidic carbon with adsorbed oxygen species. The rate-determining step in the nickel-catalyzed steam gasification of graphite is believed to be carbon diffusion through the metal[21,22], i.e. one of the factors in step (4). When only the calcium salt is present, the catalytic species are in close contact with the graphite steps and edges, and there is direct access to the carbon source. However, as pointed out above, in the mixed system calcium loses its intimacy with the graphite since nickel tends to concentrate at these regions. Consequently, the carbon supply to calcium is attenuated by the presence of a nickel diffusion barrier. It is interesting to find that the apparent activation energy for the nickel-calcium/ graphite-steam reaction is almost the same as that found for the influence of calcium alone on the same reaction, suggesting that the same rate-controlling step is operative in both systems. It would appear that in the mixed catalyst nickel merely serves as an obstacle to the smooth supply of carbon necessary for the efficient conversion of the calcium peroxide species to calcium oxide, and this aspect results in a lowering of the catalytic activity of’ the calcium component. Finally, it is significant to note that, under most of the conditions employed in our CAEM studies, the rate of the uncatalyzed steam gasification of graphite reaction is so slow that it can be legitimately neglected. Furthermore, the finding that edge recession due to uncatalyzed
R. T. K. BAKER and J. J. CHLUDZINSKI,JR.
644 attack does not follow
any preferred orientation is a property which enables one to distinguish this reaction from one where the same topographical change in the graphite is being produced by a catalytic event.
activity compared to that observed with nickel or calcium alone.
REFERENCES 4. SUMMARY
Direct observation of the specimen surface has enabled us to establish that the high catalytic activity of calcium for graphite oxidation is associated with the ability of the alkaline earth to wet and spread along the graphite edges, which subsequently undergo gasification by a recession mode. A comparison of the influence of calcium with barium[ 171 shows that these materials share many common characteristics in the graphite oxidation systems. For the graphite-oxygen reaction both additives show a similar trend in catalytic behavior; with increasing temperature the mode of attack switches from channeling to edge recession. Although both materials show an affinity towards wetting and spreading on graphite edges in the presence of steam, calcium possesses the stronger capacity for performing this function. In the presence of steam, calcium-coated graphite edges would occasionally deactivate, and this behavior became more pronounced as the temperature increased. Some tentative explanations for this phenomenon are presented. It is significant that barium did not appear to be as susceptible to this deactivation effect. Finally, when nickel-calcium/graphite specimens are reacted in steam, preferential segregation of the calcium to the catalyst/gas interface is believed to take place, and this results in a corresponding enrichment of nickel at the graphite interface. During reaciion it is probable that nickel hinders the supply of carbon necessary for the efficient conversion of calcium peroxide species to calcium oxide, and the net result is lowering of the catalytic
1.
D. W. McKee, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. and P. A. Thrower), Vol. 16, p. 1. Marcel Dekker, New York (1981).
2. J. A. Cairns, C. W. Keep, H. E. Bishop and S. Terry, J. Catal. 46, 120 (1977).
3. H. Amariglio and X. Duval, Carbon 4, 323 (1966). 4. H. Amariglio, Ph.D. Thesis, University of Nancy, France (1961).
5. E. A. Heintz and W. E. Parker, Carbon 4, 473 (1966). 6. F. J. Long and K. W. Sykes, Proc. Roy. Sot. London A215, 100 (1952).
7. W. P. Haynes, S. J. Gasior and A. J. Fomey, Advances in 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Chemistry Series, No. 131, p. 179. Am. Chem. Sot., Washington, D.C. (1974). K. Otto, L. Bartosiewicz and M. Shelef, Carbon 17, 351 (1979). D. W. McKee, Carbon 17, 419 (1979). D. J. Coates, J. W. Evans and H.. Heinemann, Applied Catal. 7, 233 (1983). R. J. Lang and R. C. Neavel, Fuel 61, 620 (1982). E. J. Hippo, R. G. Jenkins and P. L. Walker, Jr., Fuel 58, 338 (1979). L. R. Radovic, P. L. Walker, Jr. and R. G. Jenkins, J. Caral. 82, 382 (1983). T. Wigmans and J. A. Moulijn, Scud. Surf. Sci. 7, Part A, 501 (1981). R. T. K. Baker, Catal. Rev. Sci. Engng 19 (2), 161 (1979). C. A. Mims, J. J. Chludzinski, Jr., J. K. Pabst and R. T. K. Baker, J. Catal. 88, 97 (1984). R. T. K. Baker, C. R. F. Lund and J. J. Chludzinski, Jr., J. Catal. 87, 255 (1984). R. T. K. Baker, J. J. Chludzinski, Jr. and R. D. Sherwood, Carbon (in press). F. Kapteijn and J. A. Moulijn, private communication. W. L. Holstein and M. Boudart, J. Cafal. 75, 337 (1982). J. L. Figueriedo and D. L. Trimm, J. Catal. 40, 154 (1975). R. T. K. Baker and R. D. Sherwood, J. Caral. 70, 198 (1981).
OOOS-622311985 $3.00~ 00 0 1985 Pergamon Press Ltd.
1985
CATALYTIC
GASIFICATION OF GRAPHITE AND NICKEL-CALCIUM R. T. K.
Corporate
BAKER and J. J. CHLUDZINSKI,
BY CALCIUM
JR.
Research Science Laboratories, Exxon Research and Engineering Clinton Township, Route 22E, Annandale, NJ 08801, U.S.A. (Received 7 December
Company,
1984)
Abstract-Controlled atmosphere electron microscopy has been used to study the influence of calcium on the graphite-oxygen and graphite-steam reactions. Direct observation of the specimen surface has enabled us to establish that the high catalytic activity of calcium for graphite oxidation is associated with the ability of the alkaline earth to wet and spread along the graphite edges, which subsequently undergo gasification by the edge recession mode. We have found that in the presence of steam, calcium-coated graphite edges would occasionally deactivate, and this behavior became more pronounced as the temperature was increased. A decrease in rate of gasification was found when nickel was added to calcium, this is attributed to a fundamental difference in the mechanisms by which these two elements catalyze the graphite-steam reaction. Key Words-Catalysis,
1.
gasification
calcium,
nickel-calcium,
INTRODUCTION
electron microscopy
of the same reaction systems. Using thermodynamic arguments, he interpreted the catalytic effect in terms of a sequential series of reaction steps involving reduction of a carbonate to oxide at the catalyst-graphite interface and followed by reformation of carbonate by reaction of the oxide with ambient CO and H,O. He suggested that the rate-determining step was the conversion of carbonate to oxide. In contrast, Coates and coworkers[lO], using in situ electron microscopy, were unable to observe any reaction when graphite samples containing added calcium oxide were exposed to water vapor at 500°C. The purported ability of calcium to achieve and maintain by itself intimate contact with graphite is a necessary but not sufficient condition to produce good initial catalytic activity on chars and less-ordered carbons. In these cases dispersion is important. It has been shown by several workers[ 1 l-131 that significant gasification activity is obtained on these materials when the calcium is highly dispersed by ion exchange onto the carbon or coal precursor. A wide variation in total reactivity results from different catalyst impregnation techniques and from different subsequent thermal treatment prior to gasification. A literature search reveals very little information on the catalyzed steam gasification of carbons by mixtures containing either calcium or nickel. From their study of the influence of nickel-potassium on the carbon-steam reaction, Wigmans and Moulijn[l4] concluded that the two elements operated by fundamentally different reaction mechanisms, and that the effect of the mixed catalyst could be simply described as the mathematical sum of the separate catalyst actions. In the present investigation, direct observations of specimen surfaces undergoing reaction have enabled us to determine the mode by which the catalyst functions during gasification of graphite. Furthermore, from detailed quantitative analysis of recorded sequences, we have been able to obtain kinetic data on these processes.
As part of a continuing effort to understand the manner by which various additives catalyze the gasification of carbonaceous solids, we have used CAEM to study the influences of calcium on the graphite-oxygen reaction and of calcium and nickel-calcium on the graphite-steam reaction. Even though calcium is known to be an active catalyst for the graphite-oxygen reaction1 I], very few reports have appeared in the literature discussing the mechanism. The most revealing study was that described by Cairns et a1.[2], who, by means of CAEM, identified CaO as the impurity in single-crystal graphite, which was responsible for enhancing its oxidation. Although they did not perform a detailed investigation of the system, they were able to establish that the catalytic mode of attack took place by pitting and channeling at 6oo”C, possibly aided by other impurities. Amariglio and Duval[3] compared the reactivity of a number of additives, including calcium, for graphite oxidation, and, according to them, the order of catalytic activity was Na > Ba > Sr > Mg > Ca. Using data obtained by Amariglio[4], Heintz and Parker[S] estimated a value of 25.0 kcal mole-’ for the activation energy of the calcium-catalyzed graphite-oxygen reaction. Several workers[6-IO] have reported that calcium is an effective catalyst for steam gasification of carbonaceous materials. Otto and co-workers[8], using a simple weight loss technique, measured steam gasification rates of graphite impregnated with various alkaline earth salts as a function of temperature. From a combination of this kinetic data and electron microscopy observations, they concluded that the increase in gasification rate was associated with the ability of the alkaline earths to maintain extensive and intimate contact with the graphite surface. They also reported that the catalytic effect increased in the order Ca < Sr < Ba. A similar reactivity trend was found by McKee[B] from his thermogravimetric studies 635
R. T. K.
636
BAKER and I.
2. EXPERKMENTAL
A detailed description of the CAEM technique can be found elsewherc[ 151.Calcium was introduced onto transmission specimens of single crystal graphite (Ticonderoga, New York State) as an atomized spray from a 0.1 o/c aqueous solution of calcium nitrate, and nickel-calcium was added in a similar manner from an aqueous solution of the respective nitrates mixed in a I : 1 ratio. In some experiments the oxidation characteristics of graphite specimens, which did not contain any added catalysts, were also examined as controls. The reactant gases used in this investigation-oxygen and argon-were obtained from Scientific Gas Products. Inc. with stated purities of 99.999% and were used without further purification. Steam was introduced by allowing argon to flow through a bubbler containing deionized water maintained at 25% prior to entering the gas reaction cell conditions, which produced an argon/water ratio of about 40: 1.
2.1 Calciumlgraphite-oxygen When the calcium/graphite system was heated in 2.0 torr oxygen, particles were observed to form on the surface at 475°C. At a slightly higher temperature of 535°C. some particles were observed to penetrate the graphite basal plane to produce pits, while particles which had accumulated at edges and steps started to propagate channels. During this stage of the reaction, the channels followed random pathways and tended to remain parallelsided for relatively long periods of time. There was a sharp transformation in the mode of catalytic attack at 610°C. The catalyst particles at the head of channels were gradually reduced in size as a result of material adhering to the channel walls. This action caused channels to decrease in width and channel walls to gasify at a faster rate than uncontaminated graphite edges, which resulted in channels becoming fluted in outline. As a given catalyst particle became smaller. so the rate of the channel formation increased to the point where the particle was completely dissipated and further forward development of the channel ceased. Also, at this temperature, particles, which encountered edges and steps, no longer showed a tendency to create channels but instead underwent a change in morphology due to a spreading action to form a thin film along these features. Eventually the coated edges and steps were observed to recede along directions parallel to the (1010) crystallographic orientations. As the temperature was increased, the propensity for this behavior to occur grew, and edge recession quickly became the exclusive mode of gasification. At temperatures in excess of 750°C. the receding edges lost their uniform profile and became more ragged in appearance. Whether the reason for this effect is associated with a modification in the influence of calcium on the gasification reaction or is due to the interference of the uncatalyzed attack is not possible to resolve. However, it was interesting to find that, if the specimen was cooled to 65O”C, the receding edges once again assumed their ordered outline. Furthermore, dropping the
J.
CWJDZINSKI, JR.
temperature to 575°C resulted in the termination of edge recession as particles reformed, and gasification by the channeling mode was once again observed. Finally, continuous observations showed that the particles located on the perfect regions of the basal plane, which did not come into contact with receding edges or steps, remained motionless throughout the majority of reaction sequences and only exhibited significant movement in the few experiments where the temperature was increased to 1220°C. Quantitative analysis of the edge recession process showed that, unlike the corresponding action observed in the calcium catalyzed steam gasification of graphite (see following section), once initiated, all receding edges maintained their activity for the duration of the experiment, provided that the temperature was kept above about 610°C. The dependence of edge recession rate on temperature is expressed in terms of an Arrhenius relationship (Fig. 1). From the slope of this line, a value of 24.5 f 3 kcal mole“ was derived for the apparent activation energy of the calcium-catalyzed oxidation of graphite 2.2 Calciumlgraphite-steam Particle nucleation was achieved by heating the graphite specimens coated with the calcium salt at 400°C in 2 torr wet argon. Particles were in the size range of 1.5 to 3.0 nm, and those which had collected at graphite edges and steps were observed to bc in a nonwetting conftguration. This condition changed at 475°C when these particles transformed into a wetting state and eventually continued to spread along the edges, so that all indications of their previous existence was erased. In contrast, particles which had formed on the graphite basal plane continued to exhibit normal growth characteristics, reaching an average size of 10 nm. At 555”C, edges which had been “coated” with calcium started to undergo gasification. As the temperature was raised to 650°C. the rate of edge recession increased significantly. At this temperature the reaction had progressed to a stage where it was possible to determine that edges were receding in a very uniform manner and along directions parallel to the (1120) crystallographic orientations. Figure 2 is a photograph taken from the video playback showing the appearance of edges created during treatment in 2 tot-r wet argon at 700°C. From this picture it is possible to see the 30” and 90” angles produced by the edges with respect to the twin band At?. This specimen had previously been reacted in 2.0 torr oxygen at 600°C. a condition where channels had been propagated, and during subsequent treatment in wet argon the catalyst particles underwent a spreading action which eventually resulted in edge recession. Although the reaction appeared to follow the expected trend of an increase in rate with increasing temperature, there were some isolated edge regions which showed anomalous behavior; recession came to an abrupt halt and activity could not be restored by either raising or lowering the temperature. This behavior has also been observed in the K,CO,-catalyzed steam gasification reaction1 161. This phenomenon became more widespread as the temperature reached 900°C. Figure 3A-F shows the sequence
Catalytic
gasification
of graphite
637
l.! j-
1.tD\ \
0
\
.!5-
O
P
0 \
0
: 5 5 al 5 -0 $
0-
- O.!5-
0
\ - 1.tDO.’9
Fig. 1. Arrhenius
Fig. 2. Appearance
1
1 .o
I
I
1.1 I/T (K) x 10’
plot of calcium-catalyzed
graphite-oxygen
1.2
reaction.
of edges created during treatment in 2 torr wet argon at 700°C in presence of calcium; AB indicates the direction of a twin band on the graphite.
R. T. K. BAKER and J. J. CHLUDZINSKI.JR.
638
Fig. 3. (A)-(F)
Sequence showing the loss of edge activity as a calcium/graphite specimen 715OC to 920°C in 2 torr wet argon. Direction of edge recession is indicated.
is heated
Catalytic gasification of graphite
Fig. 3. (Continued).
639
640
R. T. K.
BAKER and J.
of events leading to the loss of edge activity as a specimen is heated from 715°C up to 920°C. The direction of motion of receding edges is indicated by the arrows. At 715°C edge recession is seen to occur in a very smooth, well-oriented manner. At 87X, edges start to undergo “eruption”, with the formation of a myriad of tiny particles which rapidly lose their catalytic activity, and this behavior results in edges acquiring a feathery outline. Furthermore, even when fresh particles came into contact with those “dead” edges, gasification activity was not regenerated. Particles located on the basal plane did not exhibit the same degree of wettability with the graphite as those which came into contact with edges. Nevertheless, the interaction with the support was sufficiently strong to cause complete suppression of particle motion and inhibit particle growth via the atomic migration mode. A further feature of the reaction was that, unlike the barium/graphite-steam system[ 171, there was no tendency for the catalytic action to change to a channeling mode at temperatures >9OO”C. Detailed quantitative analysis of many edge recession sequences revealed some interesting aspects regarding I.!
I
3.
CHLUDZINSKI,JR.
this form of catalytic attack of graphite. At any given temperature all edges which were undergoing gasification appeared to recede at the same rate within experimental error, regardless of their thicknesses or lengths. The variation in edge recession rate was determined as a function of temperature, and this data is presented in the form of an Arrhenius plot in Fig. 4. From the slope of this line it has been possible to evaluate an apparent activation energy of 51.3 +- 6 kcal mole-’ for the calcium-catalyzed steam gasification of graphite. 2.3 Nickel-calciumlgraphite-steam When nickel-calcium/graphite specimens were heated in 2.0 tot-r wet argon, nucleation of small particles, 35-nm diameter, was observed between 4OO’Cto 475°C. Inspection of particles which had collected at the graphite edge regions showed that they were in a nonwetting state. Over the temperature range 555°C to 610°C these particles underwent a transformation in morphology, first wetting the substrate and then proceeding to spread along the edge and step regions in the form of a thin film. It was necessary to raise the temperature to 795°C in order to achieve catalytic attack of the graphite, which ,
I
b 1.C
.E
0
- i.a
I
9
0.95
I
1.00
I.059
I/T(K)xlO'
Fig. 4. Arrhenius plot of calcium-catalyzed graphite-steam reaction.
Catalytic gasificationof graphite took place by the edge recession mode. At first the edges acquired a rippled appearance, which gradually changed to a well-de~ned faceted outline as gasification proceeded. Although the rate of recession was extremely slow, it was possible to determine that edges undergoing attack were aligned in directions parallel to the (1120) crystallographic orientations of the graphite. This form of attack continued up to 112O”C,there being no evidence of channel propagation or reformation of particles at the edge regions. Fu~he~ore, although the edges tended to lose their sharp profiles at the highest temperatures, there was no indication of a “tailing off” in the recession rate. A survey of the basal plane regions of the graphite showed that the particles which had formed in these areas remained static throughout the reaction and only took part in the reaction if they were undefined by a receding edge. When such a situation occurred, a particle would rapidly undergo spreading along the receding edge and disappear. The variation in the edge recession rate was measured as a function of temperature, and this data is presented in the form of an Arrhenius plot in Fig. 5. From the slope of this line it has been possible to evaluate an apparent activation energy of 5 1.1 2 6 kcal mole-’ for the nickelcalcium-catalyzed steam gasification of graphite. Also included on this plot are comparable data obtained for the single calcium and nickel[ 181 systems. 2.4 Graphite-steam In a final series of experiments we have examined the effect of heating graphite specimens which did not contain any added catalyst in the presence of 2.0 ton: wet
1.5
\ \ \ \
1.0
g
0.5
; 5 B 2! 2 z k -I
0
‘\ *
l
Y--Ca’cium \\ \\ \\ \\ \\ \\ \\ \\ \ \
\\ ‘1 ‘\ .yNickel ‘1 0
-0.5
\
l
l
*
* \ -1.0 0.7
I 0.0
\ \ \ \\ \ t
'\ '\ '\
0.9
0
'\ ‘\
,‘1.. 1.0
l/T (K)x 103
Fig. 5. Axrhenius plot of nickel~alcium-ca~lyzed graphitesteam reaction. Also included for comparisonpurposes are the plots for calcium and nickel alone for the same reaction.
641
argon. Very little change in the appearance of the surface was observed until the temperature was raised to 990°C. At this tem~rature, edges were seen to undergo recession, but, unlike the equivalent catalytic process, the movement did not appear to follow any particular orientation, and the profile of the receding edges tended to be ragged. Figure 6A-D is a sequence showing edge recession due to uncatafyzed attack as a graphite specimen is heated in steam from 950°C to 1040°C. The overall rates of edge recession at 900°C for the uncatalyzed reaction are over 1000 times slower than those of the calcium-catalyzed edges at the same conditions. 3. DISCUSSION
In comparing the catalytic carbon gasi~cation activity of calcium and barium, the most feasible approach to this task is by reference to the factors which govern the overall activity-the intrinsic catalytic activity and the degree of wetting of the graphite by the catalyst. For the graphite-oxygen reaction both additives show a similar trend in cataiytic behavior. At tem~ratures of about 575°C the active species are present as discrete particles and attack the graphite predominantly by the channeling mode. On continued heating, the catalyst particles undergo a transformation in shape from globules to a thin fifm, which is caused by speading of material along the graphite edges and steps. This action is also manifested by a change in mode of catalytic attack from channeling to edge recession. The fact that this modification in catalytic attack is observed at a lower temperature for calcium (610°C) than for barium (750°C) implies that the former has a somewhat higher wetting tendency on graphite edges. Unfo~unate~y, from our studies it is not possible to make a valid comp~ison between the intrinsic activities of these catalysts, since the kinetic data for calcium is based on edge recession attack and that of barium is for the channeling mode. In spite of this lack of information, we would not expect a large difference in the respective overall activities of these catalysts for the graphite-oxygen reaction. For both catalysts, edge recession catalyzed by a thin film of catalyst is a major mode of attack in steam. This agrees with conjectures in the literature]81 that good catalyst-carbon contact is maintained by a spreading of the catalyst. Closer comparison of the influence of the two alkaline ear&rmetals on the ~aphite-stew reaction shows that, although similarities do exist, in this case them are some major differences in the behavioral patterns. Although both materials show an affinity towards wetting and spreading along the graphite steps and edges, calcium seems to possess the stronger capability for attaining this state. This is deduced from the fact that edge recession is the only form of catalytic attack observed in the calcium/graphite system, whereas barium exhibits a mixture of edge recession and channeling actions, although the contribution of the latter mode to the overall activities is of secondary importance at temperatures befow 900°C. Both catalysts show a stronger tendency to wet and spread in steam than in oxygen. Edge recession ceases in both systems at 9OO”C,and
642
R. T. K. BAKERand I. J. CHLUDZINSKI, JR.
Catalytic
gasification
particles reform in these regions; however, the subsequent fate of these particles differs for the two catalysts. Those formed in the barium/graphite system continue to exhibit catalytic action by propagating channels. In contrast, the particles generated from the calcium species rapidly lose all activity. At the present time we do not understand why the calcium film, presumably in the form of oxide, suddenly becomes deactivated in steam at lower temperatures or as particles at high temperatures. Calcium catalyst on char-like carbons can also lose its reactivity in steam following exposure to inert or reducing gases at 650SOO”C[131. This phenomenon may also have taken place to a lesser degree in the barium/graphite-steam system, but, because the greater majority of particles retained their activity, its existence would have been overlooked. The deactivation may arise from chemical changes in the catalyst or the graphite surface or loss of bonding contact between the catalyst and the graphite. One possibility is that the catalyst becomes coated with a layer of carbon or even reacts to form a carbide. However, the reactivity of calcium carbide in water at room temperature argues against the carbide as a refractory repository of calcium in high-temperature steam. It is believed that the catalyst-carbon interfacial bonding which is responsible for the wetting interaction of calcium at lower temperatures (600-700°C) involves surface oxide groups. It is possible that the graphite edge cannot support a sufficient inventory of oxygen groups at high temperature, and the interfacial bonding is broken, thus allowing the catalyst to sinter. Other workers[8, 91 find that barium is a more active steam gasification catalyst than calcium. This is true despite the fact that calcium appears to wet and spread along the graphite edges more proficiently than does barium. Actual catalyst usage or quantitative dispersion on the graphite cannot be determined in these systems which wet and spread because the thickness of the catalyst film cannot be measured. The edge recession rates obtained here are of the same order as those found for barium[ 171. Therefore, the intrinsic activities of the two catalysts are similar. The difference in total reactivity probably reflects higher dispersion of barium as well as a higher incidence of deactivation events with calcium. The agreement between the apparent activation energies obtained here for the calcium-catalyzed graphiteoxygen reaction of 24.5 kcal mole-’ and that quoted by Heintz and Parker[S] of 25.0 kcal mole-l from bulk experiments is excellent. The corresponding values for the calcium-catalyzed steam gasification of graphite of 5 1.3 kcal mole-’ (present work) and bulk values of 53.5 kcal mole-‘[9] and 70.0-70.8 kcal mole-‘[8] are also in satisfactory agreement. Although it was not possible to determine the surface composition of the mixed catalyst during reaction, it is possible, using simple thermodynamic considerations, to predict which component would be expected to preferentially segregate to the catalyst surface in a steam environment. Thermodynamic calculations show that at the steam pressures used in this study nickel will remain in the metallic state and calcium, which has a high affinity
643
of graphite
for oxygenated species, will tend to segregate to the surface and probably be present in the oxidized state. The decrease in the gasification rate found when nickel is added to calcium is at first sight very perplexing, since both elements on their own are very active catalysts for steam gasification of graphite. The answer to this problem lies in the fundamental differences in the mechanisms by which these two elements operate in the carbon-steam reaction. Kapteijn and Moulijn[ 191 proposed the following sequence of reaction steps to account for the influence of calcium on the graphite-steam reaction: CaO + H,O + CaO . 0 + H,,
(I)
CaO . 0 + C +
(2)
CaO + C[O],
cc01 -+ co.
(3)
In this mechanism the rate-determining to be the release of CO from the carbon A somewhat different reaction scheme by Holstein and Boudart[20] to explain lyzed carbon-steam interaction. C-C-C
+ M%C-C
M + HzO+M--O M-C
+ M-O--+2M
step is claimed structure. was presented the metal-cata-
+ M-C,
(4)
+ Hz,
(5)
+ CO.
(6)
The first step in the mechanism is the formation of carbidic carbon on the metal, which depends upon the rupture of a C-C bond in the bulk carbon source, followed by diffusion of carbon species through or over the metal surface. The second step involves the dissociative adsorption of water, and the final step the formation of CO by reaction of carbidic carbon with adsorbed oxygen species. The rate-determining step in the nickel-catalyzed steam gasification of graphite is believed to be carbon diffusion through the metal[21,22], i.e. one of the factors in step (4). When only the calcium salt is present, the catalytic species are in close contact with the graphite steps and edges, and there is direct access to the carbon source. However, as pointed out above, in the mixed system calcium loses its intimacy with the graphite since nickel tends to concentrate at these regions. Consequently, the carbon supply to calcium is attenuated by the presence of a nickel diffusion barrier. It is interesting to find that the apparent activation energy for the nickel-calcium/ graphite-steam reaction is almost the same as that found for the influence of calcium alone on the same reaction, suggesting that the same rate-controlling step is operative in both systems. It would appear that in the mixed catalyst nickel merely serves as an obstacle to the smooth supply of carbon necessary for the efficient conversion of the calcium peroxide species to calcium oxide, and this aspect results in a lowering of the catalytic activity of’ the calcium component. Finally, it is significant to note that, under most of the conditions employed in our CAEM studies, the rate of the uncatalyzed steam gasification of graphite reaction is so slow that it can be legitimately neglected. Furthermore, the finding that edge recession due to uncatalyzed
R. T. K. BAKER and J. J. CHLUDZINSKI,JR.
644 attack does not follow
any preferred orientation is a property which enables one to distinguish this reaction from one where the same topographical change in the graphite is being produced by a catalytic event.
activity compared to that observed with nickel or calcium alone.
REFERENCES 4. SUMMARY
Direct observation of the specimen surface has enabled us to establish that the high catalytic activity of calcium for graphite oxidation is associated with the ability of the alkaline earth to wet and spread along the graphite edges, which subsequently undergo gasification by a recession mode. A comparison of the influence of calcium with barium[ 171 shows that these materials share many common characteristics in the graphite oxidation systems. For the graphite-oxygen reaction both additives show a similar trend in catalytic behavior; with increasing temperature the mode of attack switches from channeling to edge recession. Although both materials show an affinity towards wetting and spreading on graphite edges in the presence of steam, calcium possesses the stronger capacity for performing this function. In the presence of steam, calcium-coated graphite edges would occasionally deactivate, and this behavior became more pronounced as the temperature increased. Some tentative explanations for this phenomenon are presented. It is significant that barium did not appear to be as susceptible to this deactivation effect. Finally, when nickel-calcium/graphite specimens are reacted in steam, preferential segregation of the calcium to the catalyst/gas interface is believed to take place, and this results in a corresponding enrichment of nickel at the graphite interface. During reaciion it is probable that nickel hinders the supply of carbon necessary for the efficient conversion of calcium peroxide species to calcium oxide, and the net result is lowering of the catalytic
1.
D. W. McKee, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. and P. A. Thrower), Vol. 16, p. 1. Marcel Dekker, New York (1981).
2. J. A. Cairns, C. W. Keep, H. E. Bishop and S. Terry, J. Catal. 46, 120 (1977).
3. H. Amariglio and X. Duval, Carbon 4, 323 (1966). 4. H. Amariglio, Ph.D. Thesis, University of Nancy, France (1961).
5. E. A. Heintz and W. E. Parker, Carbon 4, 473 (1966). 6. F. J. Long and K. W. Sykes, Proc. Roy. Sot. London A215, 100 (1952).
7. W. P. Haynes, S. J. Gasior and A. J. Fomey, Advances in 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Chemistry Series, No. 131, p. 179. Am. Chem. Sot., Washington, D.C. (1974). K. Otto, L. Bartosiewicz and M. Shelef, Carbon 17, 351 (1979). D. W. McKee, Carbon 17, 419 (1979). D. J. Coates, J. W. Evans and H.. Heinemann, Applied Catal. 7, 233 (1983). R. J. Lang and R. C. Neavel, Fuel 61, 620 (1982). E. J. Hippo, R. G. Jenkins and P. L. Walker, Jr., Fuel 58, 338 (1979). L. R. Radovic, P. L. Walker, Jr. and R. G. Jenkins, J. Caral. 82, 382 (1983). T. Wigmans and J. A. Moulijn, Scud. Surf. Sci. 7, Part A, 501 (1981). R. T. K. Baker, Catal. Rev. Sci. Engng 19 (2), 161 (1979). C. A. Mims, J. J. Chludzinski, Jr., J. K. Pabst and R. T. K. Baker, J. Catal. 88, 97 (1984). R. T. K. Baker, C. R. F. Lund and J. J. Chludzinski, Jr., J. Catal. 87, 255 (1984). R. T. K. Baker, J. J. Chludzinski, Jr. and R. D. Sherwood, Carbon (in press). F. Kapteijn and J. A. Moulijn, private communication. W. L. Holstein and M. Boudart, J. Cafal. 75, 337 (1982). J. L. Figueriedo and D. L. Trimm, J. Catal. 40, 154 (1975). R. T. K. Baker and R. D. Sherwood, J. Caral. 70, 198 (1981).