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J . Phys. Chem. 1986, 90, 4130-4734
In-Situ Electron Microscopy Studies of the Behavior of Supported Ruthenium Particles. 1. The Catalytic Influence on Graphite Gasification Reactions 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)
We have used controlled atmosphere electron microscopy in conjunction with flow reactor studies to obtain both qualitative and quantitative information on the influence of ruthenium on the gasification of graphite in hydrogen, oxygen, and steam. The observations indicate that ruthenium is one of the most catalytically active group VI11 (groups 8-10) metals for the hydrogenation of graphite. Ruthenium exhibits an unusual activity pattern for the graphite-oxygen reaction which is associated with the existence of an oxide state at temperatures below 1000 OC and the metal at higher temperatures. In this regard it resembles iridium and rhodium, but differs from platinum and palladium, which remain in the metallic state throughout the gasification sequence. Ruthenium is found to be far superior to other group VIII (groups 8-10) metals as a steam gasification catalyst and does not show a tendency to deactivate at high temperatures like nickel or platinum.
Introduction Carbon deposition produced during hydrocarbon conversion processes is one of the major routes whereby supported metal catalysts lose their activity. As a consequence, periodic decoking (carbon gasification) cycles are required to regenerate a spent catalyst, and the metal particles which are present may, under certain conditions, facilitate the removal of carbon. Because of its high activity and desired selectivity pattern, ruthenium has become a metal of considerable interest for a number of catalytic applications.' It is, therefore, of the utmost importance to establish the influence of this metal on the gasification of carbon in various gaseous environments. Rewick and co-workers2 studied the catalytic effect of a number of group VI11 (group 8-10)24 metals, including ruthenium on the carbon-hydrogen reaction, and concluded that the function of the catalyst was that of a dissociation center for molecular hydrogen and that the reaction proceeded via diffusion of atomic hydrogen across the metaljcarbon interface and attack of the carbon to form methane. According to these workers, ruthenium was more active than rhodium or platinum for this reaction; however, all three metals exhibited deactivation at high temperatures. Tomita and Tamai3q4studied the reaction of several carbons, including graphite, with hydrogen in the presence of group VI11 metals. They found that the noble metals, particularly rhodium and ruthenium, were more active catalysts for this reaction than the ferromagnetic metals iron, cobalt, and nickel. Although there have been numerous investigations of the effects of various noble metals on the graphite-oxygen reaction, surprisingly little information is available on the influence of ruthenium on this reaction. Based on controlled atmosphere optical microscopy observations, McKee' claimed that finely dispersed noble metal particles, including ruthenium, catalyzed the oxidation of graphite by creating pits in the basal plane surfaces. From bulk measurements, Heintz and Parker6 derived a value of 23.9 kcal mol-' for the activation energy of the ruthenium-catalyzed oxidation of graphite in air. Several studies have been made on the influence of the group VI11 metals on the carbonsteam reaction; however, there is some inconsistency in the relative activities of the different metals used in these investigations. According to Watanabe' at 800 OC and 2-atm steam pressure, the order of activity is Ru > Rh > Ir > Os > Pd > Co > Ni > Fe. Otto and Shelea report that at 850 OC and 20 torr of steam, the activity of the noble metals falls in (1) Vannice, M. A. J. Caral. 1977, 50, 228. (2) Rewick, R. T.; Wentrcek, P. R.; Wise, H. Fuel 1974, 53, 274. (3) Tomita, A.; Tamai, Y. J. Catal. 1972, 27, 293. (4) Tamai, Y.; Watanabe, H.; Tomita, A. Carbon 1977, I S , 103. (5) McKee, D. W. Carbon 1970, 8, 623. (6) Heintz, E. A.; Parker, W. E. Carbon 1966, 4 , 473. (7) Watanabe, H. M. S. Thesis, Tohaku University, Japan. (8) Otto, K.; Shelef, M. Carbon 1977, 15. 317.
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the sequence Rh > Ru > Pd > Pt. A similar reactivity trend was found by Tamai and c o - ~ o r k e r s .In ~ contrast, Rewick and coworkers* claimed that platinum was the most active catalyst and that the activity of the group VI11 metals followed the ranking, Pt > Ru > Ni > Co > Fe. In this work the emphasis is placed on information obtained from controlled atmosphere electron microscopy studies concerning the physical factors which determine the likelihood of ruthenium particles acting as catalysts for graphite gasification and the modes by which such particles operate in the various gas environments. Catalytic steam gasification studies have also been performed in a flow reactor at higher pressures than those used in the microscopy investigations. Finally, the influence of ruthenium on the graphite gasification reactions is discussed in terms of a comparison with that exhibited by other group VI11 metals.
Experimental Section Transmission specimens of single-crystal graphite were prepared by a standard p r o c e d ~ r e . ~Ruthenium was introduced onto the graphite surface as an atomized spray from an ultrasonic dispersion of ruthenium black powder (Alfa Chemicals 99.9% purity) in n-butyl alcohol. The gases used in this work, hydrogen, oxygen, and argon, were purchased from Scientific Gas Products, Inc. with stated purities of >99.95% and were used directly. Experiments where the effects of steam were examined were accomplished by flowing argon through deionized water at 25 "C; this procedure produced a steam/argon ratio of about 1/40. SP-1 graphite (Union Carbide) was used as the carbonaceous medium in the macroscale experiments. Ruthenium nitrate (research grade) was obtained from Engelhard Chemicals; nickel nitrate and chloroplatinic acid, both research grade, were obtained from Alfa Chemicals. The 5% metal/graphite samples were prepared by aqueous impregnation of the graphite with the appropriate amounts of the respective salts. The impregnates were heated in a 10% hydrogen/argon mixture at 500 OC for 4 h for conditioning before gasification runs. The apparatus used for these experiments has been described previously.I0 The reactant gas was generated by bubbling an argon/0.5% hydrogen mixture through water at 25 "C at a flow rate of 50 sccm. Experiments were performed at 600, 700, and 800 OC, and estimates were made of the amount of carbon gasified after 2.0 h by weight loss measurements. For this purpose, the starting weight was taken as the initial sample weight, less the amount lost during the reduction treatment as found from blank experiments. (9) Hennig, G. R. In Chemistry and Physics of Carbon Vol. 2, Walker, P. L.. Jr.. Ed.: Dekker: New York. 1966: D 1.
(10) Baker, R. T. K.; Dudash, N. S . ; L i n d , C. R. F.; Chludzinski, J . J , Jr. Fuel 1985, 64, 1151.
0 1986 American Chemical Society
Supported Ruthenium-Gas Interactions
Results Controlled Atmosphere Electron Microscopy (CAEM) Studies. (a) RutheniumlGraphite-Hydrogen. In this experiment it was necessary to initialy heat the freshly prepared specimens in 5 Torr of oxygen at 366 O C to achieve good particle nucleation. When these specimens were subsequently heated in 1.O Torr of hydrogen, catalytic attack of the graphite by ruthenium commenced at 535 "C. Initially, the catalytic action consisted of both edge recession and channeling, but as the temperatue was gradually raised, the latter mode became the exclusive form of attack. The channels possessed all the characteristics normally associated with those formed during catalytic hydrogenation of graphite; they were predominantly straight, remained parallel-sided throughout their propagation period and exhibited a preferred orientation, being parallel to the ( 1120) crystallographic directions. The channeling activity was followed up to 1200 O C when it was evident that the rate of reaction was tending to slow down. At this stage, if the temperature was reduced, then channeling activity came to a complete halt a t 605 O C . It was interesting to find that, during this cooling period and subsequent heating-cooling cycles, the initial edge recession mode of attack was not observed. In other experiments this hydrogenation procedure was modified by including various reaction steps designed to probe the nature of the catalyst deactivation phenomenon observed after treatment at temperatures in excess of 1000 "C. Following an initial treatment in hydrogen up to 1100 "C, specimens were then reacted in 5 Torr of oxygen at 635 O C , and finally reheated in 1 Torr of hydrogen. Under these circumstances, the catalytic reactivity realized during the second hydrogen cycle was similar to that observed during the first hydrogen treatment. If the ruthenium/graphite specimens were preheated in either argon at 800 OC for 2 h, or acetylene at 500 O C for 10 min, then on reaction in hydrogen only the channeling attack occurred and the rate of this process was considerably slower than that observed with specimens which were heated directly in hydrogen. From analysis of the video recordings of these experiments, it has been possible to obtain quantitative kinetic information on the channeling sequences under various conditions. The data measured from 25-nm-diameter particles producing channels of similar depth are presented in the form of an Arrhenius plot (Figure 1). Inspection of this data shows a number of interesting features. There is indeed an inversion in the intrinsic channeling rate, which, under the conditions employed here, occurs at about 950 "C. If this temperature is exceeded then at subsequent cooling and reheating in hydrogen, the particles exhibit a lower level of activity. This condition is also achieved with specimens which were preheated in either argon or acetylene. If an intermediate oxidation step is performed on a partially deactivated sample, then on subsequent reaction in hydrogen the high activity pattern is restored. From the slopes of the lines in the high and low activity regimes it has been possible to derive apparent activation energies of 14.4 f 2 and 16.6 f 2 kcal mol-', respectively, for the rutheniumcatalyzed hydrogenation of graphite. ( b ) RutheniumlGraphite-Steam. When ruthenium/graphite specimens were heated in 2.0 Torr wet argon particle nucleation took place at 400 O C . When the temperature was raised to 555 "C, particles which had formed along the graphite edge regions were observed to undergo a morphological transformation. Particles which had initially been in a nonwetting condition reorganized to a wetting configuration and over a period of about 2 min disappeared as they continued to spread and form a thin film along the graphite edges. Immediately following this action, regions of the graphite coated with ruthenium started to gasify by the edge recession mode. The catalytic attack took place in a highly ordered fashion causing edges to acquire a hexagonally faceted profile. The receding edges moved in directions parallel to the (1070) crystallographic orientations, as determined by reference to the ( 1010) orientation of twin bands present in the graphite crystals. Edge recession remained the exclusive mode of gasification up to 850 OC, at which point some edges stopped moving and at the same
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4731
1.5
I I
\
- 0.5 o - 0.5
0.7
.
0.8
0.9 l/T(K) x
o 1.o
1.1
lo3
Figure 1. Arrhenius plots of ruthenium-catalyzed hydrogenation of graphite: 0 , initial heating in hydrogen; 0 , cooling, after heating in hydrogen >lo00 O C ; 0, after heating in argon at 800 "C, and then reacting in hydrogen; A, after heating in hydrogen >lo00 OC, and then reacting in oxygen at 635 O C , and finally in hydrogen; A, after heating in acetylene at 500 OC and finally reaction in hydrogen.
time the dispersed ruthenium re-formed into discrete particles. This transformation in catalyst morphology did not result in complete disruption of gasification activity, as once formed, the particles proceeded to create channels across the graphite surface. This behavior became more widespread as the temperature was increased, and at 950 O C channeling action was the only mode of gasification. The channels were predominantly straight, being preferentially orientated to the ( lOT0) crystallographic directions. Furthermore, the widths of the channels remained constant throughout their propagation period, indicating that in the higher temperature regime there was a reduction in the wettability factor of the ruthenium on graphite. The rate of progress of channeling particles was followed up to a maximum temperature of 1120 "C, a condition where there was no evidence for a loss of catalytic activity. In some experiments the temperature was gradually decreased, and in these cases the channeling action persisted down to about 600 OC, there being no tendency for the particles to spread and enhance gasification by the edge recession mode. Quantitative kinetic analysis was performed on reaction sequences where the catalyst was operating by the edge recession mode, and from the data presented in Figure 2 it has been possible to derive an apparent activation energy of 28.1 f 3 kcal mol-] for the ruthenium-catalyzed steam gasification of graphite. ( c ) RutheniurnlGraphite-Oxygen. The deposited ruthenium film nucleated to form small discrete particles (2.5-4.0 nm diameter) when specimens were treated in 5.0 Torr of oxygen at 366 O C . On continued heating to 633 O C particles which had collected along graphite edges were observed to undergo spreading which resulted in these regions acquiring a very smooth profile. Following this transformation, edges started to gasify by the edge recession mode, while in other areas of the specimens pits were being produced by catalyst particles interacting with surface defects. The rate of edge recession increased in a systematic fashion as the temperature was gradually raised. At 7 7 2 O C particles which had remained inactive on the basal plane started to exhibit mobility and many of them tended to migrate to the graphite edges. As a result of this action, previously "clean" edge regions became coated with ruthenium, and this behavior caused a sharp increase in the overall gasification rate of a given specimen.
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The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
Baker and Chludzinski I
2.5
I
I
0
2 .o
1.5 v)
-EJ 2
1.0
' c
0.5
0
-1.01 0.85
I
0.90
I
Figure 2. Arrhenius plot of ruthenium-catalyzed steam gasification of
graphite. TABLE I: Percent Burn-Off of SP1 Graphite When Reacted in a Flowing Mixture of 0.5%Hz/2.65% H,O/Ar for 2.0 h in the Presence of Various Metal Catalysts % wt loss catalyst 5% Ni/graphite 5% Ru/graphite 5% Pt/graphite
600
700 OC
800 O C
17.12 f 1.16 12.87 f 0.43 0.07 f 0.03
13.10 f 0.40 24.88 f 1.62 0.00 f 0.01
O C
16.13 f 0.60 5.27 f 0.22 0.88 f 0.06
A dramatic change in catalytic action occurred at 980 O C . The rate of edge recession dropped markedly as the catalyst film ruptured leading to the re-formation of particles on edges. These particles then proceeded to cut deep channels across the graphite basal plane. Continuous observation of the process showed that the channels had characteristic remarkably similar to those formed by platinum'' and iridium12 under the same reaction conditions. It was evident that the channels did not follow any preferred orientation and that they maintained their initial width for considerable distances, indicating that at high temperature there was a reduction in the wetting properties of the catalyst on graphite, Le., material no longer showed a tendency to spread along edges. Detailed kinetic analysis of several experiments confirmed that there were two separate catalytic activity regions. Arrhenius plots of the results obtained from the low temperature edge recession mode yielded an apparent activation energy of 25.9 f 3 kcal mol-' and from the high temperature channeling action a value of 58.5 f 6 kcal mol-' (Figure 3). This latter figure was based on the rates of reaction of 25-nm particles cutting channels of similar depth. ( d ) Flow Reactor Studies. The results presented in Table I clearly show major differences in the reactivity trends of graphite samples impregnated with ruthenium, nickel, and platinum when heated in steam over the temperature range 600-800 OC. The activity of ruthenium increases uniformly as the reaction temperature rises, whereas nickel tends to deactivate at 800 OC and platinum, which exhibits very low activity at 600 OC, decays rapidly with increasing temperature. It is interesting to note that these general trends show a good correlation with the experiments performed on model systems at lower gas pressures in the CAEM study. ~~~~
- 0.5
I 1 .oo
0.95 l/T(K) x 103
~~
~
( 1 1) Baker. R. T. K.; France, J. A.; Rouse, L.; Waite, R. J. J . Cutal. 1976, 41, 22. (12) Baker, R. T.K.; Sherwood, R. D. J . Carul. 1980, 61, 3 7 8 .
0.65
0.7
0.8 0.9 l/T(K) x lOJ
1.o
Figure 3. Arrhenius plots of ruthenium-catalyzed graphite-oxygen reaction at low (770-910 "C) and high (980-1230 " C ) temperatures.
Discussion The most logical method of dealing with the results obtained in the present study is by comparison of the behavior of ruthenium in graphite gasification reactions with that exhibited by other group VI11 metals. ( a ) Catalyzed Hydrogenation of Graphite. CAEM studies of the influence of a number of group VI11 metals on the graphite-hydrogen reaction show that in all cases the catalysts operate by the channeling mode.I3-I5 Furthermore, the channels created in these systems have many common characteristics. They generally remain parallel-sided throughout their propagation period and follow straight tracks which are aligned along the (1 120) crystallographic orientations of the graphite, indicating that the catalyst particles preferentially wet the zig-zag faces of the substrate. There are, however, significant differences in the behavior of the active metal particles at temperatures in excess of 950 OC, which are probably related to differences in the metal-carbon chemistry of these systems. The formation of metal-carbon bonds with subsequent reaction of carbidic carbon with atomic hydrogen to form methane are key steps in the proposed mechanism of catalytic hydrogenation of carbon.I6 Surface carbides are readily formed when metals are heated in the presence of a source of carbon. If the carbidic species are not removed fast enough, then a reduction in catalytic activity will be observed, and under extreme circumstances where total blocking of the particle surface occurs, then the catalyst will completely lose its activity. This latter condition was postulated to account for the loss in activity of nickel particles which resulted from a transformation of active metal particles into a thin inert film along edges of the catalytic ~hanne1s.I~In contrast, iron maintained its channeling activity even at 11 50 OC, although there was a definite slowing down in rate, and occasionally channels were observed to come to a halt.ls However, unlike the case of nickel, with iron the integrity of the catalyst particles was preserved throughout the reaction. In contrast, no reduction in catalytic activity was apparent when either platinum/graphite or iridium/graphite were heated in the (13) Baker, R. T. K.; Sherwood, R. D. J . Catal. 1981, 70, 198. (14) Baker, R. T. K.; Sherwood, R. D.; Dumesic, J. A. J . Cutal. 1980,66,
56.
(15) Baker, R T.K.; Chludzinski, J. J., Jr.; Shenvood, R. D. Carbon 1985, 23, 245. (16) Holstein, W. L.: Boudart, M. J . Cafal. 1981, 72, 328
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4133
Supported Ruthenium-Gas Interactions
1000°C 900°C
I
4.0
800°C
700°C
I
"C
I
I
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i-,
r
..2
2 2.0
E
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0.9 0.6
0.7
0.8
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Figure 4. Comparison of the catalytic effect of various metals on the
graphite-hydrogen reaction. presence of 1.0 Torr of hydrogen to 1250 OC.14 It should be mentioned, however, that the bulk studies reported by Rewick et aL2did indicate a tailing off in rate for the hydrogenation of various carbons impregnated with platinum. The current results indicate that the high temperature behavior of ruthenium/graphite is very similar to that displayed by iron/graphite when reacted in hydrogen. From a consideration of the rate data presented in Figure 1, it is possible to speculate as to the causes of the deactivation of ruthenium particles, which is observed when specimens are reacted in hydrogen to temperatures in excess of -950 O C . We suggest that under these conditions the surface of the ruthenium particles becomes partially blocked by carbon (graphite) overlays via a solution/precipitation mechanism previously proposed by Lund." Since the intrinsic rate of catalytic hydrogenation of graphite is proportional to the surface area of the catalyst p a r t i ~ l e ,this ' ~ form of deactivation would be expected to produce a uniform drop in the rate of channeling of a given sized particle during a subsequent cooling cycle, i.e., a partially deactivated large particle would exhibit the same activity as that displayed by a "fresh" smaller particle. Consequently, one would not expect to find large differences in the values of the apparent activation energies during the heating and cooling cycles. Treatment of a deactivated catalyst in oxygen a t 635 O C would tend to remove the carbon overlayers and thus restore the initial high activity of the ruthenium particles. Conversely, prolonged reaction in argon at 800 OC or exposure to acetylene at 500 OC could lead to premature deactivation as a result of a net supply of carbon, from the support and gas phase, respectively. The plots presented in Figure 4 indicate the catalytic influence, based on intrinsic channeling rates, of several metals on the graphite-hydrogen reaction. To allow for the effects of particle size and channel depth on rate, we obtained these measurements from 2.5-nm-diameter particles cutting channels of similar depth. The data presented in this figure are for the behavior of "fresh" catalyst particles, and at 800 OC the order of activity follows the (17) Lund,
C. R. F. J . Curd.1985, 95, 71.
I 7
0.8
I 0.9 i / t (K) x 103
I
I
1 .o
1.1
Figure 5. Comparison of the catalytic effect of noble metals on the graphite-oxygen reaction.
sequence Ru > Re > Cu > Ir > Pt > Ni > Fe. Comparison of this trend with that reported by Tomita and Tamai3s4from bulk studies shows excellent agreement: Rh 5 Ru > Ir > Pt > Ni > Fe . (b) Catalyzed Steam Gasification of Graphite. The observed behavior of the group VI11 metals on the steam gasification of graphite can best be understood from a consideration of the proposed mechanism of this reaction.I8 The first step in this mechanism is the formation of carbidic carbon on the metal, which depends upon the rupture of C-C bonds in the bulk carbon source, followed by diffusion of carbon species through or over the metal particle surface. The second step involves dissociative adsorption of water and the final step the formation of CO by reaction of carbidic carbon with adsorbed oxygen species. Previous CAEM studies showed that nickel was capable of performing these functions very effectively, operating exclusively by the edge recession mode, whereas iron was converted to an unreactive oxide under the prevailing c o n d i t i ~ n . ' Platinum ~ was found to be significantly less active than nickel and operated predominantly by the channeling mode.I9 This lack of activity is probably related to the fact that platinum does not readily dissociate steam.20 Furthermore, like nickel, platinum was susceptible to deactivation at high temperatures. Ruthenium exhibits behavior which is intermediate between that of nickel and platinum, functioning by the edge recession mode at lower temperatures and switching over to a channeling mode at about 900 "C. This change in mode of catalytic attack of the graphite is caused by a decrease in the wettability factor of the catalyst on the substrate and might result from carbon dissolving in the metal or accumulating at the particle surface. This notion is supported by the finding that, once the channeling action was initiated, it persisted down into the lower temperature regime when the specimen was cooled. It is interesting that in this case the catalyst showed no signs of deactivation, a feature which was confirmed by the bulk studies (Table I). It is also (18) Holstein, W. L.; Boudart, M. J. Card. 1982, 75, 337. (19) Lund, C. R. F.; Chludzinski, J. J., Jr.; Baker, R. T. K. Fuel 1985, 64, 789. (20) Fisher, G . B.; Gland, J. L. Surf. Sci. 1980, 94, 446.
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.
'
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(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.)
-
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
