J. Phys. Chem. 1986, 90, 4734-4738
4734
apparent from these results that ruthenium is considerably more active than platinum over the range 600-800 OC and is superior to nickel at 800 O C . ( c ) Catalyzed Graphite-Oxygen Reaction. Very little catalytic action was observed during CAEM studies of the effect of nickelI3 and iron15 on the graphite-oxygen reaction. Although some evidence of short channels and pits were apparent with both additives, these forms of attack quickly subsided as the particles lost their activity. Simple thermodynamic calculations indicate that the ferromagnetic metals are unusual in that under the prevailing experimental conditions their oxides are readily converted to carbides. One reason for the observed inactivity may be that carbides do not catalyze oxidation of carbon.21 In contrast, the noble metals are all very active catalysts for this reaction. The mode of attack in the respective reactive temperature regions is governed by the chemical state of the catalyst. With iridium and rhodium, the oxides IrOz and Rh203 are formed at temperatures below 1000 "C and they exhibit a strong interaction with the oxygenated graphite edge regions, and as a consequence, catalytic gasification occurs by the edge recession mode. At temperatures in excess of 1000 "C the stable solid phases are the respective metals and they have a weaker interaction with the support and catalyze the removal of carbon by creating deep channels across the surface.l2 This pattern of behavior is different than that found for platinum and palladium, which are (21) Baker, R. T. K., submitted to Carbon.
present in the metallic state throughout the gasification sequence and display a single continuous catalytic channeling action on the graphite-oxygen reaction.' From the present study it is evident that ruthenium behaves in a similar fashion to iridium and rhodium in that it also possesses two distinct activity regimes. At temperatures below 1000 OC, ruthenium will be present on the specimen as Ru02, and this oxide is expected to readily wet the graphite and catalyze carbon removal by the edge recession mode. At high temperatures the metal becomes the stable phasez2,23and this transition correlates with a major change in both the mode of catalytic attack and the kinetics of the gasification process. Figure 5 is a collective plot of the catalytic influence of various noble metals on the graphite-oxygen reaction for conditions where the metallic state is the active entity in all cases. From this dependence it appears that the order of activity follows the sequence Pd > Pt > Ir Rh N Ru. Registry No. Ru, 7440-18-8; graphite, 7782-42-5.
'
-
(22) Chaston, J. G.; Platinum Met. Reu. 1965, 9, 51. (23) Samsonov, G. V., Ed. The Oxide Handbook, 2nd ed, Johnson, R. K., Trans].; Plenum: New York. (24) In this paper the periodic group notation (in parentheses) is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 and 13.)
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In-Situ Electron Microscopy Studies of the-Behavior of Supported Ruthenium Particles. 2. Carbon Deposition from Catalyzed Decomposition of Acetylene R. T. K. Baker* and J. J. Chludzinski, Jr. Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: March 13, 1986)
Controlled atmosphere electron microscopy has been used to directly follow the formation of carbonaceousdeposits on ruthenium supported on graphite, silica, and titanium oxide when reacted in a hydrocarbon environment. This study has enabled us to identify the role of the metal-support interaction in determining the mode by which filamentous carbon is produced. The observations indicate that ruthenium exhibits a stronger interaction with graphite and silica than with titanium oxide. However, when the latter system was preheated in hydrogen at temperatures in excess of 500 OC, complete suppression of catalytic carbon formation was achieved during subsequent reaction in acetylene. This effect is believed to originate from blocking of the metal surface by Ti-0 species generated by reduction of the titanium oxide support.
Introduction In recent years there has been considerable interest in the study of the catlaytic decomposition of carbon-containing gases in the presence of various metals. M a t of these investigations have been performed with nickel, iron, or cobalt, which are known to be very active catalysts for carbon deposition, and the influence of other metals has tended to be neglected.] In the present work we have examined the formation of carbon on supported ruthenium particles from the pyrolysis of acetylene. Since ruthenium does not form a bulk carbide under normal reaction conditions, it is an ideal candidate to test the hypothesis that carbide formation is a necessary prerequisite for the formation of certain types of carbon deposits on metal surfaces.z Interest in the behavior of supported ruthenium systems has been stimulated because of the finding that ruthenium appears (1) Baker, R. T. K.; Harris, P. S . In Chemisrry and Physics of Carbon, Vol. 14, Walker, P. L., Jr., Thrower, P. A,, Eds.; Dekker: New York, 1978;
p 83. (2) Sacco, A., Jr.; Thacker, P.; Chang, T. N.; Chiang, A. T. S. J . Catal. 1984, 85, 224.
0022-3654/86/2090-4734$01.50/0
to be the most active element, based on exposed surface atoms, for carbon monoxide hydr~genation.~Furthermore, ruthenium supported on titanium oxide appears to exhibit a higher reactivity than when the metal is dispersed on other supports and reacted under similar condition^.^ In a recent investigation, Wise and McCartyS determined the thermodynamic properties of the surface carbon formed on ruthenium particles supported on alumina. They concluded that the carbon was present as isolated atoms coordinated to ruthenium without formation of carbon islands and graphite overlayers. This arrangement was consistent with the finding that the carbon could be removed in hydrogen at temperatures as low as 0 "C. In the present work we have used controlled atmosphere electron microscopy to directly follow the formation of carbonaceous deposits on ruthenium supported on graphite, silica, and titanium oxide when reacted in acetvlene. This studv has enabled us to identify the role of the metal-support interadtion in determining (3) Vannice, M. A. J . Catal. 1978, 50, 228. (4) Vannice, M. A. J . Catal. 1982, 74, 199. (5) Wise, H.; McCarty, J. G. Surf Sci. 1983, 133, 311.
0 1986 American Chemical Society
Supported Ruthenium-Gas Interactions the mode by which catalytic carbon is formed. Finally, the catalytic activity of ruthenium/graphite and ruthenium/silica for carbon deposition is compared with that of iron, cobalt, and nickel when supported on the same materials and reacted under similar conditions.
Experimental Section All the data presented here were obtained from experiments performed in the controlled atmosphere electron microscope, the details of which have been given previously.6 Transmission specimens of single-crystal graphite, silica, and titanium oxide were prepared according to standard p r o c e d ~ r e s . ' ~Ruthenium was applied to these supports, either as (a) an atomized spray from a 0.01% aqueous solution of ruthenium nitrate or (b) by ultrasonically dispersing ruthenium black powder (Alfa Chemicals 99.9% purity) in n-butyl alcohol and then introducing a fraction of the supernate to the support medium as an atomized spray. The gases used in this study, acetylene, hydrogen, oxygen, and argon, were obtained from Scientific Gas Products, Inc., with stated purities of >99.95% and were used without further purification. Experiments where the effects of steam were examined were accomplished by allowing the carrier gas to flow through a bubbler containing deionized water maintained at 25 O C , a procedure which produced a gas/water ratio of about 40/1. Results ( a ) RutheniumlGraphite-Acetylene. When ruthenium/graphite samples were pretreated in 1 Torr of hydrogen or heated directly in 5 Torr of acetylene, particles which collected on the graphite edge sites were observed to firstly wet and then spread along these features as a thin film as the temperature was gradually raised from 500 to 700 OC. In this morphological condition the ruthenium appeared to be unable to catalyze the formation of filamentous carbon, and, as a consequence, these regions of the specimen maintained a smooth profile throughout the hydrocarbon decomposition reaction . In contrast, metal particles located on the graphite basal plane did not undergo this spreading action, suggesting that their interaction with the support was not as strong. Nevertheless, this interaction was of sufficient strength to prevent particles from being lifted off the support when carbon filaments started to form at 725 O C . As a result, filaments grew exclusively by the extrusion mode, a condition which persisted even when the temperature was raised to 900 OC. In this conformation, the catalyst particle remains affixed to the support, and the filament is normally produced by a very erratic "explosive" process. The average length of the filaments produced in this system was 0.5 pm, and the widths of the filaments were of the diameter of the particles from which they were produced. As the temperature was raised, so the number of filaments being produced increased. At 900 OC, a survey of a given specimen showed that approximately 75% of the ruthenium particles had filaments associated with them. Unfortunately, because of the irregular mode of formation of these filaments, it was not possible to obtain any meaningful kinetic data from the various growth sequences. Continuous observation of the deposition reaction showed that, as the reaction proceeded, amorphous carbon also accumulated on the surface. This type of deposit is believed to originate from condensation and polymerization reactions occurring in the gas phase and was formed over a much longer time period and at a slower rate than the filamentous carbon. More information was obtained about the characteristics of the carbon deposits from experiments where the samples were subsequently heated in 5 Torr of oxygen. At 525 OC metal particles located on the support which were not associated with filaments were observed to catalyze the gasification of the amorphous carbon (6) Baker, R.T.K.; Harris, P. S. J . Phys. E . 1972, 5 , 795. (7) Hennig, G. R. In Chemistry nnd Physics of Carbon, Vol. 2, Walker, P.L., Jr., Ed.; Dekker: New York, 1966; p 1. (8) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J . Catal. 1979, 56, 390. (9) Tatarchuk, B. J.; Dumesic, J. A. J . Catal. 1981, 70,308.
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4135
component. This action was seen as the development of channels through the deposit and was responsible for an increased rate of sintering resulting from catalyst particle mobility and collisions between particles. At the same time, the filaments started to gasify in a very irregular manner. The initial point of attack appeared to take place at preferred sites along the filament skin. As this process continued, it became evident that this reaction was also being catalyzed by small particles, 2.5-5 nm in size, which had probably been dispersed within the filament structure during the growth sequence. Complete removal of all the deposited carbon was achieved by raising the temperature to 575 O C . Inspection of the graphite edge regions showed that particles had re-formed at these locations, suggesting that a weakening of the particle-support interaction had occurred by substituting oxygen for the reducing environment. When the temperature was increased to 630 OC,many of these particles proceeded to create channels across the graphite support. On continued heating to 700 OC, some of the previously inactive particles located on perfect areas of the graphite basal plane became mobile, and this action resulted in more particles collecting at edge sites on the graphite. (b) RutheniumlSilica-Acetylene. When ruthenium/silica specimens were reacted directly in 5 Torr of acetylene, a pattern of behavior very similar to that described for the previous system was observed. The carbon filaments that started to form at 675 O C were produced by the extrusion mode and were preceded by the accumulation of amorphous carbon on the surface at 600 O C , which tended to collect around the ruthenium particles and surface imperfections of the support. Because of the difference in textural qualities of the two support media, the formation of amorphous carbon was more easily detected on silica than on graphite. The oxidation characteristics of the carbonaceous deposits produced in this system were identical with those produced on the graphite support. In a second series of reactions the ruthenium/silica specimens were pretreated in 1 Torr of hydrogen in order to achieve improved metal particle formation. Provided that the temperature of this cycle was kept below 650 OC,then the particles grew to about 15 nm, and on subsequent reaction with acetylene prolific filament formation occurred, and the growth pattern followed that observed with direct acetylene exposure. However, if samples were heated to 800 O C in hydrogen, then the ruthenium particles appeared to undergo an interaction with the silica support. Particles were observed to transform into very dense needlelike shapes up to 100 nm in length. When the specimen was cooled and the hydrogen replaced by acetylene, then on reheating, amorphous carbon was observed to collect on the surface at around 600 O C , but no evidence of filamentous carbon growth was seen even when the temperature was increased to 1000 O C . It was significant that when these specimens were subsequently heated in 5 Torr of oxygen a temperature of 650 O C was required to gasify the amorphous carbon, indicating that the process was only occurring by the uncatalyzed route. (c) RutheniumlTitanium Oxide-Acetylene. Experiments performed with ruthenium/titanium oxide in 5 Torr of acetylene showed some dramatic differences in behavior compared to the situations where the metal was supported on either graphite or silica. When the system was reacted directly in acetylene, there was no evidence of spreading by particles located at edges, and at 475 OC many of these particles and ones located on the basal planes started to catalyze the formation of filamentous carbon. In this case the filaments were produced by the conventional mode with the catalyst particle being carried away from the support surface by the growing filament. It was noticeable that at any given temperature smaller particles were producing filaments at a faster rate than larger ones. This type of filament growth continued in a very uniform manner across the specimen surface until the temperature was increased to 750 OC. At this point, the majority of filaments ceased to grow and particles that still exhibited activity were producing filaments by the extrusion mode. This change in deposition characteristics coincided with a massive structural transformation of the support from a relatively smooth structureless condition to large hexagonal platelets. If heating
4736 The Journal of Physical Chemistry, Vol. 90, No. 20, I986
Baker and Chludzinski acetylene amorphous carbon was observed to collect on the surface at 470 OC, but even though there was an abundance of ruthenium particles present on the support, no evidence of filament formation was seen. This condition persisted even when the temperature was increased to 800 "C. When these specimens were cooled and the hydrocarbon replaced with 5.0 Torr of oxygen, then on reheating it was necessary to raise the temperature to 850 OC and hold for 10 min in order to remove the amorphous carbon deposit. If following the gasification step the oxygen was removed and 5.0 Torr of acetylene reintroduced, then on reheating sparse filament formation was observed at 470 OC. As the temperture was slowly raised to 800 "C the number of filaments being produced increased, but it was significantly less than the amount formed on ruthenium/titanium oxide samples which were treated directly in the hydrocarbon.
T I
(K) x 103
Figure 1. Arrhenius plot for filamentous carbon growth from titanium
oxide supported ruthenium-acetylene interaction. was continued to 800 OC all filament growth ceased. It was significant that this sequence of events had no effect on the deposition of amorphous carbon, which had started to collect on the surface at 475 O C , and was still being produced when experiments were terminated at 900 "C. Detailed quantitative measurements were made from the filament growth sequences and the data are expressed in the form of an Arrhenius relationship (Figure 1). In an effort to overcome variations in filament growth rates with catalyst particle size, all measurements are based on 30-nm-diameter particles. From the slope of this line it has been possible to evaluate an apparent activation energy of 26.2 f 3 kcal mol-' which, based on analogous data from other systems,' is probably the value for the activation energy for diffusion of carbon through ruthenium. Treatment in oxygen of ruthenium/titanium oxide specimens containing both growth forms of carbon showed some superb examples of how the metal particles enhanced the gasification of the carbonaceous deposit. At 525-575 "C in 5.0 Torr of oxygen, ruthenium particles located at the tips of filaments were observed to catalyze the oxidation of the filament they had produced during the acetylene cycle and, as a consequence, retraced a pathway back to their original location on the support. Having accomplished this feat, the particles then proceeded to catalyze the removal of the amorphous carbon on the support surface. Figure 2, A-D, is a sequence showing the gasification of a filament at 600 "C in 5.0 Torr of oxygen; the location of the particle is indicated by the arrow in each frame. In situations where the catalyst particle became detached from the tip of the filament, gasification did not take place until the temperature was increased to 650 OC. However, in the majority of experiments complete removal of the carbonaceous deposit occurred after heating the sample at 675 OC for 10 min. When the gasification step was performed in 2 Torr of wet argon catalytic gasification of the filamentous structures took place at 610 OC by the same mode as observed in oxygen. In this case. a temperature of 71 5 OC was required to achieve complete removal of the carbon residue. In a second series of carbon deposition experiments ruthenium/titanium oxide specimens were pretreated in 1 Torr of hydrogen at 550 OC for 1 h. During this step it was apparent that the support underwent a major restructuring into large, flat hexagonal islands. On subsequent treatment in 5.0 Torr of
Discussion It is clear from the observations reported here that the behavior of supported ruthenium particles is extremely complex and not what one might have predicted from studies of other metal/ support-gas systems. The carbon deposit growth characteristics are dependent on the type of support, specimen pretreatment conditions, and the reaction temperature. A major finding to emerge from this investigation is that, even though ruthenium does not form a bulk carbide under the present conditions, it is nevertheless a very active catalyst for carbon filament formation. It would therefore appear that carbide formation is not a necessary prerequisite for filament formation. Furthermore, the ability of ruthenium to catalyze the growth of this form of carbon indicates that carbon can dissolve and diffuse through the metal. We believe that the simplest way to discuss the various aspects of this work is in terms of the individual metal/support systems. ( a ) Ruthenium-Graphite Interaction. The observation that ruthenium particles undergo a wetting and spreading action along graphite edges at 700 OC in hydrogen or acetylene suggests that a strong metal-support interaction exists under these conditions. This postualte is supported by the finding that during continued reaction in acetylene highly dispersed ruthenium was incapable of catalyzing the growth of filamentous carbon from the coated edge regions of the graphite. Furthermore, filaments that were produced from ruthenium particles situated on the graphite basal plans grew exclusively by the extrusion mode, a growth process which is indicative of a strong interaction between the catalyst particle and the support surface.I0 This conclusion is also supported by the thermodynamic determinations reported by Wise and McCarty, which indicated that the binding energy of carbon to ruthenium was unusually high compared to other metals such as nickel.' The morphological change from a discrete particle form to that of a thin film is generally associated with the existence of a significant degree of atomic mobility within the material. This condition is achieved when the material is heated to its Tammann temperature, 0.52[bulk melting point (K)], the value of which is 1070 "C for ruthenium. The fact that this transformation is observed for ruthenium on graphite at 700 "C implies that either the temperature of the metal particles is considerably higher than that of the support or the transformation can occur at a lower level than the Tammann temperature. A major difference in behavior to the present system was observed when graphite supported iron, cobalt, and nickel were reacted under the same In these cases, there was no tendency for metal to spread along the graphite edges, and when filaments were formed they grew by the conventional mode, i.e., during the growth process the catalyst particle was carried away from the support surface and remained at the tip of the filament. All these characteristics are indicative of a relatively weak in(10) Baker, R. T. K.; Chludzinski, J. J., Jr.; Dudash, N. s.;Simoens, A. J. Carbon 1983, 21, 463. ( 1 1 ) Baker, R. T. K.; Harris, P. S.; Thomas, R. B.; Waite, R. J. J . Card. 1973, 30, 86. ( 12) Baker, R. T. K.; Barber, M. A,; Feates, F. S.; Harris, P. S.; Waite, R. J. J . Catal. 1972, 26, 5 1 .
Supported Ruthenium-Gas Interactions
The Journal of Physical Chemisfry, Vol. 90. No. 20, 1986 4737
Figure 2. (A-D) catalyzed oxidation of a carbon filament by n ruthenium particle at 600 Y m 5.0 .Torr of oxygen. Thc location n l the x t i w particle
is indicated i n each framc.
teraction between the ferromagnetic metals and graphite under the prevailing conditions. (b)Ruthenium-Silico Interortion. Despite the observation that carbon filaments were formed by the extrusion mode when ruthenium/silica specimens were reacted in acetylene, one must be cautious about drawing conclusions on the strength - of the ruthenium-silica interaction. It is not possible to discern whether the metal particles exhibit a strong interaction with silica or with the carbon overlayer which is produced from the gas-phase decomposition of acetylene. The finding that the catalytic activity of ruthenium for filamentous carbon formation can be suppressed by pretreating the ruthenium/silica specimens in hydrogen at 800 OC prior to reaction with acetylene is consistent with the behavior of other metals treated in a similar manner.'o~" It was claimed that following treatment of silica-supported metals a t elevated temperatures silicon species become incorporated within the catalyst particles. It was suggested that dissolved silicon was effective as an inhibitor because of its ability to reduce the solubility of carbon and also decrease its rate of diffusion through the catalyst particle, the two controlling parameters in carbon filament formation. Although we have not demonstrated here that silicon becomes incorporated in the ruthenium particles, the dramatic change in morphology of the ruthenium indicates the existence of reaction between the metal and the support with the formation of an inactive material for carbon filament formation. (c) Ruthenium-Tifonium Oxide Interaction. The behavior of this system represents a striking example of how the strength of
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(13) Baker, R. T. K.; Chludrinski, J. J. J. Carol. 1980, 64. 464.
the metalkupport interaction can be selectively altered by merely reacting the system in different gas environments. Consider first the information relating to this aspect which can be deduced from the filament growth characteristics. When the ruthenium/titanium oxide system is heated directly in acetylene, filaments are formed initially by the conventional growth mode, indicating that under these Anditions the metal pafticles are only weakly hound to the support surface, as depicted in Figure 3a. However, as the temperature was raised, the support underwent a structural transformation and there is a concomitant change in filament growth characteristics from conventional to extrusion formation, and eventually complete inhibition of filament formation occurs. This sequence of events is best understood from a consideration of the situations presented in Figure 3, b and c. It is probable that at 750 O C sufficient hydrogen is generated from the acetylene decomposition reaction to induce the conversion of TiO, to Ti,O,? A further, but as yet unexplored, reason could be that carbon could reduce TiO, to a lower oxide. This nonstoichiometric oxide will have an extremely high interfacial energy, and, as a consequence, wetting and spreading of the metal on the support will be facilitated. Under these circumstances filamentous carbon formation will be restricted to growth via the extrusion mode. Furthermore, it has been recently demonstrated that at these temperatures reduced titanium oxide exhibits a significant degree of mobility." Under these circumstances, there will be a tendency for Ti-0 species to migrate onto the surfaces of the ruthenium particles, and the fraction of metal surface available for adsorption and (14) Baker. R.T. K.;Chludzinski, J. J.; Dumesic,J. A. J. Colol. 1985.93. 312.
4738
The Journal of Physical Chemistry, Vol. 90,No. 20, 1986
Baker and Chludzinski
n
Uncatalyzed Gasification e.g. Fe, Co & Ni
600 "C
600 "C
I Tio2
-c
M
3
Catalyzed Gasification e.g. Ru
Ti.0,
Figure 4. Schematic representation of the fates of catalyst particles during gasification of filamentous carbon in oxygen.
CzH2-800"C
Oz-65O"C
Figure 3. Stages in the behavior of Ru/TiO, sample during reaction in acetylene (a-c) and oxygen (d).
decomposition of acetylene slowly decreases. Ultimately, the situation shown in Figure 3c is reached, where the particles are completely encapsulated by reduced titanium oxide and filament growth ceases. This model has been presented previously to account for the loss in hydrogen chemisorption capacity of titanium oxide supported metals following reduction in hydrogen at temperatures of about 500 0C.15-19There is little doubt that this condition would also have been attained when ruthenium/titanium oxide samples were pretreated in hydrogen a t 550 OC,and it is therefore expected that the system would be catalytically inactive when subsequently reacted in acetylene. Treatment of the deactivated metal particles in oxygen would be expected to result in the fragmentation of the Ti4O7overlayer into discrete globular particles of T i 0 2 (Figure 3d). During this process a fraction of the surface of the metal particles will be exposed and available for subsequent catalytic interaction with acetylene. Finally, it is interesting to note that when a coked supported ruthenium catalyst is heated in an oxygen environment the catalytic action of the metal facilitates removal of the deposited (15) Resasco, D. E.; Haller, G. L. J . Cutuf. 1983, 82, 279. (16) Jiang, X-Z.; Hayden, T. F.; Dumesic, J. A. J. Cutul. 1983, 83, 168. (17) Cairns, J. A,; Baglin, J. E. B.; Clark, G . J.; Ziegler, J. F. J . Cutul. 1983, 83, 301. (18) Chung, Y . W.; Xiang, G.; Kao, C. C. J . Cutul. 1984, 85, 237. (19) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J. Cutul. 1984, 86, 359.
carbon, whereas under the same circumstances the ferromagnetic metals iron, cobalt, and nickel have little influence on the decoking reaction.20 The ramifications of this difference can be seen from the schematic representation (Figure 4) depicting the fate of catalyst particles during gasification of filamentous carbon. During this process the majority of ruthenium particles return to the support, whereas there is a high probability that the ferromagnetic metal particles will be lost from the support and swept out of the reactor by the gas stream.
Summary Ruthenium appears to exhibit a stronger interaction with graphite and silica than with titanium oxide (Ti02). Pretreatment of ruthenium/silica with hydrogen at 800 O C results in inhibition of filamentous carbon growth when the system is subsequently heated in acetylene. This is possibly caused by ruthenium silicide formation. Pretreatment of ruthenium/titanium oxide with hydrogen at 550 OC results in inhibition of filamentous carbon growth when the system is subsequently heated in acetylene. This effect is believed to originate from blocking of the metal surface by Ti-0 species generated by reduction of the titanium oxide support. The finding that ruthenium is an active catalyst for filament growth and under the experimental conditions used in this work ioes not form a carbide-indicates that carbide formation is not a necessary requirement for filament formation. Registry No. Ch=CH, 74-86-2; Ru, 7440-18-8; TiO,, 13463-67-7. (20) Baker, R. T. K.; Chludzinski, J. J.; Sherwood, R. D. Carbon 1985,
23, 245.
