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Carbon from carbon monoxide disproportionation on nickel and iron catalysts: Morphological studies and possible growth mechanisms
1973, Vol. 11, pp. 583-590.
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CARBON FROM CARBON MONOXIDE DISPROPORTIONATION ON NICKEL AND IRON CATALYSTS: MORPHOLOGICAL STUDIES AND POSSIBLE GROWTH MECHANISMS H. P. BOEHM Institut fiir anorganische Chemie der ‘IJniversit%t, Miinchen, Germany (Received 29 May 1973) Abstract-Carbon was prepared by disproportionation of CO; Ni(CO), or Fe(CO)S was fed to the CO stream before it entered the decom~sition furnace. Electron microscopy reveals differences in the carbons obtained: adding Fe(CO), resulted in the usual fibrilar growth whilst Ni(C0)4 gave rise to spherical shells and sausage-like forms which are hollow inside, X-ray powder diffraction shows that the carbon obtained using Ni(C0)4 is not as well organised as the carbon obtained using Fe(CO)S. Possible formation mechanisms are discussed; very likely, the carbon shells obtained using Ni(CO), are precipitated from solution in nickel whereas carbon obtained in the presence of iron is catalytically recrystallised by an iron carbide. A mechanism for the formation of helical fibres is suggested. 1. INTRODUCTION
Carbon formed by catalytic disproportionation of carbon monoxide (“carbon ex CO”) is interesting in many respects. It has already been noticed in the first X-ray diffraction study of this material[l] that it is comparatively well ordered in view of its low formation temperature. This finding was corroborated in later papers@-41. With the advent of electron microscopy, it was noticed that this carbon ex CO has a strong tendency for filamentary growth [4-91. Very frequently, pairs of twisted filaments form strands of double helices. CO is subject to disproportionation according to the Boudouard equilibrium: 2co
= c+co,.
The equilibrium favors the right side at temperatures below 700°C. However, attainment of equilibrium is possible only in the presence of catalysts, mainly iron, cobalt, and nickel. Even then, temperatures in excess of 583
400°C are needed for measurable reaction rates. Maximum rates are observed around 550°C [l, 41. The usual way of preparing carbon ex CO is to pass CO gas over finely divided iron, cobalt or nickel powders. The reaction is accelerated by admixture of water vapor or hydrogen [9, lo]. One may use the oxides as well as the metals since they are reduced by CO under the reaction conditions. However, the active catalyst in the case of catalysis by iron was claimed to be an iron carbide, Fe,C3, [8] or y-FezOB [9]. It was generally observed that the rate of carbon formation decreased after a certain time, and that the quantity of carbon that can be obtained in an experiment is limited. The surface area of carbon ex CO is relatively high, 35 to 170 m2/g, depending on the reaction conditions. It was shown by Nemetschek [7] that in graphitised carbon ex CO the graphite layers are stacked like saucers, with the layer planes more or less perpendicular to the fiber axis as will be explained later on. I
584
H. P. BOEHM
concluded that the layers of the ungraphitised carbon would have similar orientation; the surface should consist mainly of the edges of the carbon layers, i.e. of “prismatic faces”. In view of the relatively perfect structure and the high surface area, carbon ex CO should be a very suitable substrate for studies of surface oxides on carbon provided that sufficient quantities of homogeneous material can be prepared. I hoped to find a continuous process by feeding metal carbonyl into a CO stream before it passed a heated tube. The idea was to obtain fresh metal particles by thermal decomposition of the carbonyl which would catalyse CO disproportionation while passing through the heated zone at a relatively low flow rate. The process was an absolute failure with regard to preparation of fibrilar carbon ex CO in quantity since the yield per experiment was in the range of 10 to 200 mg carbon. However, the carbon obtained when Ni(CO), was used showed an interesting morphology which furthers discussion of the formation mechanism. 2 EXPERIMENTAL
Ni(CO), was partly synthesised by normal procedure [ 111, partly obtained from BASF company, Ludwigshafen a. Rh.; Fe(CO), was also obtained from this source. CO of commercial quality was taken from a steel cylinder. It was either used pure, or diluted with approximately 20% by volume of Hz (control by flow meters). The gas stream was divided so that ca. 20% of it passed through a wash bottle which contained Ni(CO), or Fe(CO), and which was kept at 0°C. The remaining 80% of the gas stream was led directly to the reaction tube. By use of this by-pass, 10 g of Ni(CO), lasted for approximately l-5 hr (vapour pressure at 0°C: 126 Torr). A typical total flow rate was 13 l/hr. In some experiments, the wash bottle with Ni(CO), was replaced by a tube with active nickel prepared in situ[l2]. Thus, Ni(CO), was prepared immediately
before use, and any handling of this highly toxic substance was avoided. Of course, the rate of Ni(C0)4 formation was not known in these experiments. Fe(C0)5 lasted much longer due to its lower vapour pressure (15 Torr at WC). The Ni(CO),-containing gas was then passed to the heated zone, consisting of a vertical 45 mm wide tube of heat-resistent “Supremax” glass within a temperature conPremature decomposition trolled furnace. of the carbonyl was kept back by feeding the gas via a water-cooled tube of 4 mm bore to the wide tube. Nevertheless, considerable quantities-ca. 30% -of Ni(CO), decomposed forming a nickel mirror in the inlet tube and in the colder parts of the wide tube. A large flask was attached to the end of the wide tube for collecting the reaction products. The reaction products were refluxed with acid for semi-concentrated hydrochloric several days until no other lines than those of carbon could be seen in X-ray powder diffractograms. For comparison, CO was passed also over Raney nickel or over iron obtained by reduction of precipitated iron hydroxide with H,. For electron microscopy, the carbons were dispersed in xylene and small drops were placed on Formvar films coated with SiO, by vaporising SiO. A Siemens Elmiskop I was used with 80 kV energy. Powder diffractograms were taken by the Debye-Scherrer method using Cu Ka! radiation, and the films were evaluated densitometrically. L, and L, parameters were estimated in the usual way after appropriate corrections for the diameter of the sample capillaries [2, 131. 3. RESULTS The surprising result was that the carbon obtained from carbon monoxide by feeding nickel carbonyl, Ni(CO),, to the CO stream differed in its morphology from all other preparations of carbon ex CO. Whereas fibrilar and tubular growth has been des-
CARBON
FROM CARBON
MONOXIDE
cribed repeatedly [4-71, this carbon appeared in the form of thin foils reminding one of sausage skins: there were .also spherical skins. In Figs. l-4, such carbons are shown with increasing thickness of the skin. Figures 2 and 3 show tubular growth, too, but these wide, thin-walled tubes are quite different from the fine tubes observed when carbon monoxide is decomposed over fine particlesize nickel, e.g. Raney nickel (Fig. 5). Such fine tubes have alsobeen described earlier [6]. When iron carbonyl, Fe(CO),, was used instead with a CO/H, mixture, the usual fibrilar growth with twisted strands was observed (Figs. 6 and 7). X-ray powder diffraction diagrams of the carbons obtained by feeding Ni(CO), as a catalyst were very similar to those of carbons made in the conventional way by passing carbon monoxide over iron or nickel powder. The (001) reflexes were relatively little broadened, and the d002 values were well below 3.44 8, for material prepared at 550°C or higher temperatures (Table 1). This agrees with earlier observations [1,4] for other carTable 1. X-ray powder diffraction data for carbon ex co Furnace Catalyst temperature (“C) 480” 600” 650” 700” 740” 480” 550” 600” 650’
Ni(CO)* Ni(C0)4 Ni(COk Ni(CO)* Ni(C0)4 Fe(CO), Fe(CO), Fe(C0)5 Fe(CO&
740” 600” 550” 700”
Fe(CO), Raney Ni Fe Fe
dbQ2 (A)
L (A)
Ll (A)
(hkl) reflexes
3*455 3*425 3*410 3~40~ 3~42~ 3*425 3*3S0 3~40~ 3.415* 3.390” 3+39@ 3*39@ 3*4t 3*4t
80 85 150 190 75 95 62t 113t
90
-
70 90 80 80 >80 115t 200t
z + -t +
*Two separate preparations. tData taken from [Z].
DISPROPORTIONATION
585
bons ex CO. Nevertheless, the carbons were turbostratic, no (hkl) reflexes or modulations of the tw~dimensional (hk) bands were noticeable. L, and L, values were determined from the broadening of the reflexes for several samples as a measure of crystalline order that can be compared with earlier observations. The results are listed in Table 1. They are similar to those determined earlier, using iron powder as a catalyst [2-41. For comparison, the earliest values for L, and L, taken from the literature are included in Table 1. It is interesting to note that there was excellent agreement of the L, values for various reaction temperatures found by Hofmann, 1932[2] and found much later with modern equipment by Walker et al., 1959[4]; no L, values were published by the latter authors. Some scatter in the results in Table 1 may be due to differences in experimental conditions, e.g. gas flow rate and residence time in the heated zone. The surface area as determined by lowtemperature nitrogen adsorption was 86 m2/g for the material prepared at 706” using Ni(CO),. This is somewhat higher than the surface areas measured by Walker et al. [4] with carbon ex CO formed over iron catalysts at the same temperature. 4. Discussion of thefo~at~on mechanisms It is obvious that different formation mechanisms must be involved in the heterogeneously catalysed formation of carbon ex CO on nickel and on iron. Ruston et al. [8] concluded that Fe&, is the active species in the latter case. Renshaw et al. [9] showed that the reaction proceeds by diffusion of iron through an iron oxide layer and formation of an iron carbide on the phase boundary to the gas phase. Carbon is formed by breakdown of the carbide, most likely Fe&; this decomposition seems to be favored by the presence of water vapor or hydrogen. Breakdown of Fe& whiskers was thought to be the cause of fiber formation.
586
H. P. BOEHM
Analogously, Renshaw et al. [ 141 concluded that N&C or another nickel carbide is an intermediate product of carbon formation on nickel surfaces. No crystalline nickel carbides could be detected in the carbon fibrils however. The main reaction product seems to be well-crystallised graphite lying with its layer planes flat on the Ni surface. This graphite was very similar to the graphite coating formed by pyrolysis of acetylene on nickel at 1000°C[15} or of methane on nickel at 750”[16]. The mechanism for the formation of graphitic carbon seems to be the same whether CO or organics are used as a starting material. There is strong evidence that this carbon is formed by precipitation from supersaturated solutions in nickel [17]. Very frequently, fibrilar and tubular growth of carbon was observed as well on pyrolysis of organic gases on nickel or nickel-containin alloys, with the fine tubes of a few hundred R diameter usually carrying a catalyst particle on their tips[l4, 16, 18-201. Such tubular filaments are shown in Fig. 5. The morphology of the carbon ex CO shown in Figs. l-4 can be explained by assuming that nickel was formed on pyrolysis of Ni(CO), in globular particles and chains of fused particles analogous to carbon blacks. Such precipitation forms have been observed on pyrolysis of dilute Fe(CO), vapour.” Carbon films precipitated on the nickel surface were left after dissolution of the nickel, giving a replica of the nickel particles. A few such nickel particles survived the acid leaching and can be seen as opaque spheres in Fig. 3. This would explain the occurence of spherical shells as well as the large irregular tubes and “balloons”. Very thin skins collapsed giving the appearance of Fig. 2. Little platelets similar to those described by Renshaw et al.[14] can be distinguished in relatively thick coatings (Fig. 4). It should be remembered that the time for carbon growth was very short in the present work com*Friz, H., Private communication.
pared to the extended times of up to 48 hr used by Renshaw et al A residence time of 2 to 8 min was estimated from the flow rates and the dimensions of the reaction tube. The thickness of the carbon increased with increasing residence time. The growth of fine filaments or tubes seems to be slow in comparison since they appear in quantity only in experiments which allow for long contact of the hot nickel with carbon monoxide. Occasionally, a few tubular filaments can be noticed in the electron microphotographs of the material made by passing CO and Ni(CO)I through a heated tube (Fig. 3). On the other hand, spherical shells could be noticed as well in the carbon obtained by passing CO over Raney nickel (Fig. 5) or over reduced nickel oxide. The X-ray data in Table 1 indicate that the carbon prepared using Ni(CO)* was less perfectly crystallised than the carbon made using Fe(CO)r,. The d,,,, spacing decreased from 3.45 to 3.40 A with increasing temperature. No indication of separate (101) or (112) reflexes could be seen. The carbon prepared with Fe(CO), had somewhat smaller dm2 spacings at the lower temperatures, and (101) and (112) reflexes could be distinguished with the 650” and 740°C samples. This observation is surprising since most earlier authors agree that the graphite platelets formed on nickel by decomposition of carbonaceous gases are well crystallised and give rise to good (hkl) patterns whereas fibrous carbon formed on nickel is distinctly less organised[l4, 161. Perhaps, the less perfect structure can be explained by the short time available for crystal growth. However, rapid growth of graphite from supersaturated solutions in nickel has been described, albeit at a higher temperature of approx. 1000°C [I?]. The carbon obtained by passing CO over Raney nickel at 600” was somewhat better organised and showed distinct (101) reflexes. This might be attributed to the fact that it was several hours at the reaction temperature.
Fig+ 1. Electron microphotograph of a carbon ex CO prepared by feeding Ni(CO), to a CO stream; furnace temperature: ??o”C.
Fig. 2. Electron microphotograph of carbon ex CO prepared by feeding Ni(C0,) to a 80% CO/20% Hz stream: furnace temperature: 550°C. (Facing page 586)
Fig. 3. Electron microphotograph of carbon ex CO prepared by feeding Ni(CO), to a CO stream; furnace temperature: 550°C.
Fig. 4. Electron microphotograph of carbon ex CO prepared by feeding Ni(C0)4 to a 80% CO/20% Hz stream; furnace temperature: 600%.
Fig. 5. Electron microphotograph of carbon ex CO prepared 80% CO/20% Hz over Raney nickel at 600°C.
by passing
Fig. 6. Electron microphotograph of carbon ex CO prepared by feeding Fe(CO)S to a 80% CO/20% Hz stream; furnace temperature: 650°C. Fig. 7. Electron microphoto~aph of carbon ex CO prepared by feeding Fe(CO), to a 80% CO/20% Hz stream; furnace temperature: 650°C.
Fig. 8. Electron mic~photo~aph of carbon ex CO from a blast furnace, heat-treated at 19OO”C, after intercalation of potassium and exfoliation by immersion in water. Courtesy of Prof. Th. Nemetschek, University of Heidelberg.
Fig. 9. Electron microphotograph of carbon ex CO prepared by passing 80% CO/20% H, over reduced iron oxide; reaction temperature: 600°C. The arrows indicate double helices.
CARBON
FROM CARBON
MONOXIDE
The carbon ex CO prepared by feeding iron carbonyl instead of nickel carbonyl to the CO stream was distinctly better organised and gave rise to separate (hkl) reflexes at preparation temperatures at or above 650°C in agreement with the earliest X-ray studies of carbon from iron-catalysed CO disproportionation [ 11. The appearance of broadened but distinct (hkl) reflexes only with samples which had interlayer spacings of 3.39 to 3.40 A or smaller agrees with Franklin’s observations[21]. L, and L, values were calculated from the line broadening of the (002) and (100) or (10) reflexes (Table 1) although it is recognized that these parameters have no real meaning for paracrystalline carbons[22]. These values are however useful indicators of the degree of defects in the layers, particularly if other information is lacking. The carbon ex CO obtained using iron carbonyl appeared in the form of twisted filaments and tubes as well as straight strands (Figs. 6 and 7). There was a small amount of material present, in addition, giving the impression of empty spherical shells of small diameter (approx. O-1 pm). Very likely, this was formed by a similar mechanism as the carbon obtained using nickel carbonyl. It is noteworthy that fibrilar growth was observed to any extent only if CO/H, mixtures were used. With pure CO, empty skins and densely agglomerated material were prevalent. Some of the straight strands in the electron microphotographs (Figs. 6 and 7) show low contrast indicating that these particles are shaped like flat ribbons rather than rods of rectangular or cylindrical cross section. There are dark Bragg diffraction bands at right angles to the fiber axes noticeable in the images of these particles. Since (002) is by far the most intensive reflex of graphite or carbon, it follows that t.he layer planes are oriented either perpendicular to the fiber axis or parallel to the support film. No dark bands across the entire breadth of the fibers would arise if the layers were oriented with
DISPROPORTIONATION
587
the basal planes parallel to the surface of a cylindrical fiber. In this case, the zones of energy loss by diffraction should run along the lengths of the fibers. Also, on the basis of independent observations with twisted carbon filaments, it seems most likely that the c-axes of the carbon particles coincide with the fiber axes. There exists strong evidence, as mentioned in the introduction, that the layers are stacked more or less perpendicular to the fiber axis in the twisted filaments as well. Nemetschek [7] intercalated potassium in carbon ex CO from a blast furnace that had been graphitised or-rather-heat-treated at 1900°C; the intercalation compound was then explosively decomposed by rapid immersion in water. The microphotograph. of the decomposition product (Fig. 8) shows exfoliation of the layers by local heating and the rapid hydrogen evolution. The impression is that the filament is built of shallow domeshaped layers stacked like saucers. This is in contrast to reports in the literature claiming that the layer planes are oriented parallel to the surface of the fibers[9, 16,23,24], although this is very likely true for tubular filaments. Superposition of the image and of the electron diffractogram of a hollow fiber showed strong (002) intensity perpendicular to the fiber axis [23]. This evidence is not conclusive, though, because there is usually rotation of the image in relation to the object in electron microscopes with electromagnetic lenses. The authors did not state which type of instrument was used, but one may conclude that an electromagnetic lense model was used from their prevalence and from the resolution of 20 A. One may wonder why twisting of the filaments is frequent when iron is used as a catalyst. Very often, the twisted filaments form double helices (Fig. 9) or even tripe1 helices [23]. I suggest the following growth mechanism: The primary reaction product of carbon monoxide disproportionation or hydrocarbon decomposition is paracrystalline carbon
588
H. P. BOEHM
of a relatively low degree of organisation corresponding to its low formation temperature. This carbon is catalytically recrystallized by metal carbides, particularly iron carbides, e.g. Fe& or Fe&. Occurence of the Fe& phase during CO disproportionation was demonstrated by selected area electron difraction [9]. However, both carbides, Fe& and Fe&, are not stable above 450°C and decompose giving iron and carbon[25]. Ruston et al. [8] mentioned that carbon filaments grow only from certain crystal faces of the catalyst carbide. If there were specific nucleation sites for graphitic carbon only on particular faces of the carbide particles, the following situation would arise: The carbides are non-stoichiometric compounds with variable composition for a given phase; their composition is a function of the chemical potentials of the constituents at each temperature, and is thus dependent on the lattice perfection of the carbon phase in contact with the carbide. In other words: disorganised or poorly crystalline carbon is more soluble in the carbide phase than graphitic carbon. If well-crystallized carbon is nucleated on a crystal face, the carbide phase would be supersaturated in carbon with respect to this crystalline carbon. In the graphitic carbon would consequence, grow at the expense of the carbon dissolved in the carbide lattice. The growth rate is then controlled by the diffusion of carbon atoms in the carbide, the temperatures allowing sufficient mobility of the carbon atoms. Disorganized or poorly crystalline carbon in contact with other crystal faces of the carbide phase would be transported by diffusion to the thermodynamically more favored better crystallized carbon phase. This is analogous to the mechanism of catalyzed graphitisation by molten metal carbides [26]. If the catalyst particle has rectangular shape, straight fibers would grow from its catalytically active face as shown schematically in Fig. 10a. The height of the fiber perpendicular to the plane of the paper in the
Direction of growth
I
(a)
(b)
Fig. 10. Schematic representation of growth mechanism of filamentary carbon on iron carbide. (a) on rectangular carbide particle, (b) on carbide particle with two active faces at oblique angles. model or of the support film in the electron microscope would be equal to the height of of the catalyst crystal. Thus, the occurence of flat ribbons as well as of thicker rods can easily be explained. If the catalyst particle has an angular shape as depicted in Fig. lob, a curved carbon fiber would grow from the active crystal face due to the different diffusion path lengths in the carbide crystal. Obviously, this must result in helical growth forms if the strands grow to any length, and double helices would result if two adjoining crystal faces are active (Fig. lob). It is not difficult to envisage screw dislocations in the graphitic carbon as the active growth sites in contact with the catalyst. It was shown by Renshaw et al.[14] that nickel carbide platelets form on nickel (111) faces during carbon monoxide disproporand that, ultimately, graphite tionation, crystals are formed with their (001) planes parallel to nickel (111). It seems possible that in this case, too, more poorly crystallized carbon is a precursor to graphite formation. The carbon skins described earlier in this paper are definitely turbostra-
CARBON
FROM CARBON MONOXIDE
tic in nature. The assumption that graphite is formed via a nickel carbide intermediate is not cogent, however, since direct precipitation from the metal phase is possible[l7]. It was also suggested that finely dispersed nickel in the carbon is the active catalyst for carbon formation since diffusion of a carbonaceous species through a layer of well crystallised graphite seems difficult[19]. There was always nickel present in the carbon deposits from hydrocarbons, and there is nickel present also in the carbon ex CO made with the addition of Ni(CO),. About 2.5% of Ni was found in the one sample analysed, but this could easily arise from occlusion of carbony1 nickel by the carbon thus inaccessible to the attack by hydrochloric and dilute nitric acids. The appearance of the shells in Fig. 4 indicates that they are not dense and that CO could diffuse to the central nickel particle. Growth would occur at the phase boundary nickel-carbon. The question remains how the tine tubular filaments are formed in which the carbon c-axes are oriented radially in all likelihood. It should be worth-while to study the various forms of carbon ex CO using high-resolution electron microscopy.
Note Added in Proof
It was suggested above that initially formed disordered carbon may recrystallize by a diffusion mechanism. The formation of disorganised carbon of higher free enthalpy than graphite was recently proven for the disproportionation of CO and for the pyrolysis of CH, on nickel surfaces[27]. Futhermore, it was suggested[28] that in the catalysed decomposition of acetylene thin carbon tubes grow by a mechanism in which carbon diffuses through nickel particles in a temperature gradient. This gradient might be caused by the heat evolved on C2H, decomposition; the disproportionation of CO is exothermic, too. The activation energy for growth of tubular carbon on several catalyst metals agrees very well with the activation energy for diffusion of carbon in these metals[29]. The narrow tubes formed are more reactive towards oxygen, i.e. less organised, in the central part than on the outside [28].
DISPROPORTIONATION
589
Acknowledgements- I thank Miss Heideklang and Mrs. Dehlinger for the electron microscopic work, and Miss Vieweger, Miss Huber and Mr. Sharples for X-ray powder diffractograms and their photometric evaluation. I am indebted to Prof. Nemetschek for his furnishing the electron microphotograph in Fig. 8, and to Dr. Friz for informations regarding the morphology of carbony1 iron and nickel. This work was supported by Deutsche Forschungsgemeinschaft and by Fonds der Chemischen Industrie. I am grateful to BASF company for a gift of Ni(CO), and Fe(CO),.
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