�9 Elsevier Science B. V. All rights reserved Catalyst Deactivation 1997 C.H. Bartholomew and G.A. Fuentes, editors 99

The Rela t ionship Be tween Metal Particle Morpho logy and the Structural Character is t ics of Carbon Deposi ts

R. T. K. Baker a, M. S. Kim b, A. Chambers a, C. Park a and N. M. Rodriguez a

aDept, of Chemistry, Northeastern University, Boston, MA 02115.

bDept, of Chemical Engineering, Myong Ji University, Kyonggi-do, Korea 449-728.

The understanding of the factors controlling the deposition of carbonaceous solids resulting from the decomposition of hydrocarbons over metal particles has a considerable impact on a number of commercial processes including: catalytic steam reforming of methane, catalytic reforming and systems involving carbon monoxide disproportionation reactions. The highest catalytic activity for carbon deposition is exhibited by iron, cobalt and nickel, and alloys containing these metals. During the past few years we have pursued a program designed to achieve conditions where it is possible to control the catalytic properties of a given metal by inducing perturbations to the reactive surfaces of the crystallites. One of the consequences of such an action is to enable one to alter the catalytic reactivity in such a fashion so as to optimize the performance for a desired reaction pathway, while simultaneously suppressing the rate of detrimental side reactions, such as encapsulating forms of carbon deposition. Our strategy has centered around a study of the effect of introducing selected adatoms into the host metal and using the decomposition of ethylene to probe the manner by which the chemistry of the various faces of the crystallites is modified. In this paper a discussion of the information obtained from the use of a combination of controlled atmosphere and high resolution transmission electron microscopy techniques to study the impact of metal particle morphology on the characteristics of the carbon deposit will be given.

1. INTRODUCTION

It should be recognized that the potential for carbon deposition exists in many systems in which hydrocarbons or carbon monoxide undergo decomposition over heated metal surfaces. Several reviews have highlighted the complex nature of carbonaceous deposits (1-6), which can be divided into three main types: amorphous, filamentous and graphitic shell-like structures. During a traditional routine analysis of a spent catalyst, these three forms of carbon would not be necessarily distinguished, but merely referred to collectively as "coke". Available evidence indicates that the amorphous carbon component is formed via condensation and polymerization reactions and this material originates from thermal processes. It is conceivable that a significant amount of hydrogen is retained in the deposit, however, as the temperature is raised dehydrogenation reactions will tend to reduce the hydrogen content. There is now a general consensus that the formation of the filamentous and graphitic forms of carbon require the participation of a catalytic entity.

During the reaction, the metal particles adopt well defined geometries with the carbon-containing gas molecules being adsorbed and decomposed on certain faces of the metal, this process being followed by diffusion of carbon atoms through the catalyst particle

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to precipitate at another set of faces in the form of a fibrous structure. It is well established that carbon diffusion through the particle is the rate determining step in the growth process. Depending on the chemical nature of the catalyst and the conditions at which the reaction is performed, assorted filamentous structures with various morphologies and different degrees of crystallinity can be produced (7,8). While the rudiments of the mechanism for the formation of the graphitic shell-like deposit have not received the same attention as that devoted to the generation of carbon f'llaments it is probable that many of the steps outlined above are also operative in the growth of this form of carbon (9,10). The major difference between the characteristics of the two catalytically formed types of deposit may lie in the number of the faces that perform the dissociative chemisorption of hydrocarbons compared to those that only allow for the carbon precipitation mode.

The modification of the catalytic behavior of a given metal by the addition of controlled amounts of other metals has been used extensively in order to alter both the activity and selectivity as well as to control the deactivation of active surfaces with respect to poisoning and deactivation due to carbon deposition (11-15). It is the understanding of this latter aspect that has eluded researchers in this area and presents challenges to the various mechanisms that have been proposed to account for carbon deposition on metal surfaces. McCarty and coworkers (16,17) used temperature programmed reaction techniques to establish the existence of several forms of carbon on deactivated nickel catalysts. From these studies graphitic oveflayers and filamentous carbon growths were identified as being the most prevalent types associated with metals. The graphitic form of deposit tends to encapsulate the particle surface in contact with the reactant gas and thus normally results in rapid deactivation of the catalyst. In contrast, filamentous carbon is produced by a process in which carbon diffuses through the particle and precipitates at the rear faces, thereby leaving the exposed faces free to undergo continued reaction. The net result of this behavior is that the catalyst system can accumulate large amounts of carbon and maintain activity for prolonged periods of time (18).

It is now being recognized that the incorporation of controlled amounts of certain non-metallic atoms into metals, which have hitherto been regarded as poisons, can actually function as catalyst promoters. This approach is predicated on the assumption that the poison atoms are preferentially chemisorbed on sites that are active for undesirable reactions, while those sites which perform the desired reactions are preserved in an unadulterated state (19,20). Previous work from this laboratory (21-23) has demonstrated that either pretreatment or continuous addition of 5 to 10 ppm H2S to iron, nickel or cobalt undergoing reaction with ethylene had a significant impact on the catalytic activity of these metals, particularly with respect to the enhancement in yields of carbon filaments. We have extended this investigation to cover the influence of chlorine on the same reactions (24). Careful poisoning with chlorine has been found to be a very effective method of altering the activity and selectivity of catalysts and it has been suggested that such behavior could in part be attributable to induced electronic perturbations in the metal (25). In catalytic reforming, the chlorine content derived from the metal precursor salt is thought to be directly related to the amount of carbon that accumulates on the catalyst (26).

2. EXPERIMENTAL

2.1. Materials Bimetallic powders consisting of combinations of iron, nickel, cobalt and copper

were prepared by coprecipitation of the respective metal carbonates from metal nitrate solutions, Fe(NO3)3 9H20 (98%, Fisher Scientific), Co(NO3)2.6H20 (99% Fisher Scientific), Ni(NOa)E.6H20 (99% Fisher Scientific) and Cu(NO3)2 2.5H20 (99.2%, Fisher

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Scientific) using ammonium bicarbonate at room temperature and a pH of about 9.0, followed by oxidation to metal oxides and finally, reduction in 10% hydrogen/helium at 500 ~ for 20 h, as described previously (27). The reduced catalyst was cooled to room temperature under helium, then passivated by a 2% air/helium mixture for 1.0 h and stored in sealed containers. X-ray diffraction analysis was performed on all of the bimetallic powders at room temperature and only peaks corresponding to metallic iron, nickel, cobalt and copper were evident in these samples. It should be recognized, however, that upon heating to 600~ the phase diagrams for these mixed metal systems shows that in all cases alloy formation occurs. Pure iron, nickel, cobalt and copper powders were also prepared according to the above procedure. The gases used in this study, carbon monoxide (99.99%), hydrogen (99.999%), helium (99.99%) and ethylene (99.5%) were obtained from MG Industries and were used without further purification.

Approximately 50 mg of each sample of all these powders were initially given a reduction treatment in 10% H2/He at 500~ and then reacted in a horizontal flow reactor system in the presence of either ethylene/hydrogen (4:1) or carbon monoxide/hydrogen (4:1) mixtures at temperatures over the range 450 to 800~ for a period of 2.0 h. Gas flow rates were maintained at 100 cc/min and regulated with MKS mass flow controllers. At the conclusion of each experiment the solid carbon product was carefully removed from the quartz reactor, weighed and then stored for subsequent structural analysis. In a final series of experiments, iron-nickel powder was reacted in the presence of ethylene/hydrogen (4:1) at temperatures in excess of 800~ in order to investigate the formation of shell-like deposits that are known to form under these conditions.

2.2 Techniques The details of the structural characteristics of individual constituents in the various

carbon deposits were obtained by examination of a number of specimens from each experiment in a JEOL 100 CX transmission electron microscope that was fitted with a high resolution pole piece, capable of 0.18 nm lattice resolution. Suitable transmission specimens were prepared by applying a drop of an ultrasonic dispersion of the deposit in iso-butanol to a carbon support film. In many cases the solid carbon product was found to consist entirely of filamentous structures. Variations in the width of the filaments as a function of both catalyst composition and growth conditions were determined from the measurements of over 300 such structures in each specimen. In certain samples evidence was found for the existence of another type of carbonaceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures. Selected area electron diffraction studies were performed to ascertain the overall crystalline order of the carbon filaments and the shell-like materials produced from the various catalyst systems.

The controlled atmosphere electron microscopy (CAEM) experiments were carried out in a modified JEOL 2000EXII TEM instrument (28). This instrument is equipped with a custom designed environmental cell, which accommodates a heating stage. With this arrangement it is possible to continuously observe changes in the appearance of a specimen as it undergoes reaction in a gas environment at temperatures up to 1000~ The dynamic events occurring during the reaction are captured on video-tape and replayed later for detailed analysis. We estimate that the point-to-point resolution achieved on the TV monitor is of the order of 0.4 nm under sufficiently stable conditions. A recent development with the technique is the capability of performing in-situ electron diffraction analysis of supported small particles to establish the chemical state of the reacting specimen (29).

This technique was used to determine the morphological characteristics of both the pure metal and bimetallic particles supported on single crystal graphite surfaces. The samples used in these experiments were prepared by two methods: (a) single metal/graphite specimens were made by evaporation of the metal in the form of a wire from a tungsten

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filament at 10 -6 Torr onto transmission sections of single crystal graphite; (b) graphite supported bimetallic specimens were produced by introducing the components mixed in the desired ratio onto the substrate medium, as an atomized spray from an aqueous solution of the respective metal nitrates. Prior to reaction in the hydrocarbon environment both these types of systems were pretreated in 1 Torr hydrogen at 400~ for 3 hours.

3. RESULTS AND DISCUSSION

3.1. Dynamic Behavior of Graphite Supported Metal Particles

When samples of either the pure metals or bimetallics dispersed on single crystal graphite were examined in the CAEM following the initial treatment in hydrogen it was evident that the evaporated films had undergone nucleation to form discrete particles that were of the order of 2 nm in diameter and tended to adopt a spherical shape. When such specimens were subsequently heated in either 2.0 Torr ethylene/hydrogen or carbon monoxide/hydrogen mixtures some dramatic changes in both size and morphology were observed. For example, in the presence of an ethylene/hydrogen (4:1) mixture at temperatures approaching 500~ particles of iron-nickel started to undergo reconstruction from a globular to a square form. This aspect is shown in the schematic diagram, Figure 1 a, which taken under reaction conditions. In experiments where the temperature was raised to temperatures in excess of 750~ these particles were observed to gradually transform and adopt a hexagonal morphology. Inspection of copper-iron particles when heated in a CO/H2 (4:1) reactant revealed that in this case, the particles acquired a rectangular morphology as the specimen temperature was increased to 475~ Figure lb. In a further example, copper- cobalt particles exhibited a somewhat fascinating transformation when the system was heated to 525~ in an ethylene/hydrogen (4:1) mixture, the initial spherical-shaped particles transforming into well-defined fiat hexagonal-shaped crystallites, as seen in Figure l c. It should be mentioned that even on continued heating up to 650~ no carbonaceous deposits were formed at the very low pressures at which the CAEM experiments were performed with these particular gas mixtures.

(a) (b) (c) Figure 1. Schematic representation of the observed morphological characteristics of various bimetallic particles during interaction with selected gas environments. (a) an Fe-Ni (5:5) particle undergoing reaction in C2Hn/H2 (4:1) at 500~ (b) a Cu-Fe (2:8) particle heated to 475~ in the presence of CO/H2 (4:1), and (c) a Cu-Co (1:3) particle treated in C2H4/H2 (4:1) at 525~

It was significant to find that in many of these systems the geometric shapes of the particles were not retained when the specimen was slowly cooled to room temperature and the reactant gas flow switched off. Based on these studies it is clear that under reaction

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conditions metal particles can adopt well-defined faceted forms that are dependent not only on the nature of the supporting medium, but also chemical composition of the reactant gas. Experimental evidence indicates that it is highly probable that many of these features would not be revealed in a post-reaction electron microscopic examination, since the strength of the metal/support interaction is not always maintained in the absence of a reactant gas and particles appear to relax to the more energetically favorable form of a sphere under these circumstances.

3.2. Influence of metal particle morphology on the structural characteristics of carbon filaments.

Transmission electron microscopy examinations of the solid carbon produced from the interaction of ethylene/hydrogen or carbon monoxide/hydrogen with a number of pure metals and bimetallics containing copper mixed with iron, cobalt and nickel revealed that the deposit consisted almost entirely of filamentous carbon when such reactions were performed at temperatures up to 650~ The amount of solid material produced from the growth of this type of deposit is generally quite high and the presence of the carbon structure serves as a means of "freezing" the shape of the catalyst particle in its reactive state even when the specimen is cooled to room temperature and the reactant gas mixture removed from the system. This aspect can be seen very clearly in the set of electron micrographs, Figures 2 to 4, where the original shapes of the particles shown in the corresponding set of diagrams, Figures 1 a to lb are preserved during the catalytic growth of carbon filaments.

lOOnm

Figure 2. Appearance of a carbon filament and the associated "diamond" shaped Fe-Ni (5:5) catalyst particle after heating to 600~ in CzH4]H2 (4" 1) at 600~

Inspection of the conformational characteristics of the filaments produced from these three catalyst systems shows that in all three cases, the growth occurs from more than one face of the metal particle. Filaments generated from the interaction of copper-iron with C2H4/H2 (4:1) and iron-nickel particles with CO/H2 (4:1) grow via a hi-directional mode and tend to exhibit a high degree of crystalline order. In contrast, the structures created from the reaction of C2H4/H2 (4:1) with the hexagonal shaped copper-cobalt crystallites are multi- directional with several filaments emanating from a single catalyst particle. As with the previous systems, these carbon structures were also found to be highly crystalline in nature.

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Figure 3. Appearance of a carbon filament generated from the interaction of Cu-Fe (2:8) with a CO/H2 (4:1) mixture at 600~ Note the geometric shape of the catalyst particle.

: ~

. . . . 172 ~. - ' .

� 9 '='~" .~i~"'~ ~ ~ .

. . :~'~,~

lOOnm

Figure 4. Appearance of multi-directional filaments formed at the faces of an hexagonal- shaped Cu-Co (1:3) particle after reaction in C2H4/H2 (4:1) at 575~

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In a further series of experiments the filaments formed from these three catalyst systems were studied by high resolution transmission electron microscopy. From the lattice fringe images it was apparent that the structures consist of graphite platelets that are oriented in various directions with respect to the filament axis. A schematic representation showing the structural relationships between the catalyst particles and the precipitated graphite platelets formed during the growth of the filaments from the three systems is presented in Figure 5. It is apparent from these models that in the case of copper-iron/C2H4/H2 (4:1), Figure 5(a), the graphite platelets constituting the filaments adopt a "herring-bone" arrangement and under these circumstances two faces of the catalyst particle are responsible for the dissociation chemisorption of the hydrocarbon and four faces that participate in the precipitation of graphite. When the filaments are produced from the interaction of iron-nickel particles with CO/H2 (4:1), Figure 5b, the graphite platelets are stacked in the form of a "deck of cards" and examination of the associated catalyst particle suggests that at least four faces are involved in the decomposition of the reactant gases and precipitation of graphite occurs at only two faces. Finally, when C2Hn/H2 (4:1) is allowed to react with copper-cobalt several filaments are generated from each particle and the individual structures exhibit identical characteristics where the graphite platelets are aligned in a direction parallel to that of the face where the carbon precipitation step occurs, Figure 5c. Once again, it is possible that in this system there are two faces of the catalyst particle involved in the hydrocarbon adsorption and decomposition reaction and another set of six faces that are responsible for carbon deposition.

a b c Figure 5. Schematic representation of the different arrangements of graphite platelets generated from the three systems shown in Figures 2 to 4, respectively.

3.3. Formation of graphitic shell-like deposits on iron-nickel particles at >800~

Transmission electron microscope examination of the solid carbon produced on iron- nickel after reaction in C2H4/H2 (4: I) revealed the existence of two types of material; carbon filaments and another type of carbonaceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures, Figure 6. This latter form tended to predominate at 825~ Close inspection of many examples of the shell-like deposit failed to reveal any definite correlation between the width of a given catalyst particle and that of the surrounding graphite structure. Inspection of the metal particles showed that they adopted a faceted outline, which showed a close correspondence to that of the carbon deposit that was generated at the catalyst surfaces. High resolution studies revealed that the platelets were very thin, as evidenced by the fact that it was possible to frequently observe the morphological features of the underlying substrate through the graphitic structures. It was also apparent that as one scanned across any of the platelets the electron density remained

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relatively constant, suggesting that these structures were flat rather than spheroidal in nature. A further feature of note was the finding that the lattice fringes of the deposit were oriented in a direction parallel to the faces of the catalyst particles associated with these platelet structures and had a spacing of 0.34 nm, indicative of a highly graphitic material.

/

200nm ~ - , r " . . . . ~ . . . . . . . . ~ "~

Figure 6. Electron micrograph showing the shell-like appearance of graphite deposits that encapsulate iron-nickel particles when heated at temperatures of about 825~ in a C~Hn/H2 (4:1) mixture.

As mentioned previously, the key steps and factors controlling the growth of the carbon filaments on metal particles are well understood. The range of crystalline order of the carbon deposit generated at the precipitating faces of the particle is controlled to a large degree by the extent of wetting exhibited between the metal and graphite. However, the geometric alignment of the precipitated graphite platelets in the filament structure as well as their degree of crystalline perfection is ultimately determined by the crystallographic orientation of the metal faces in contact with the solid carbon deposit (30-32). While the basics of the formation of the graphitic shell-like deposit have not received the same attention as that devoted to generation of carbon filaments it is probable that many of the steps outlined for the growth of these structures are also operative in the growth of the shell-like form of carbon. In recent years there have been reports of the existence of graphite shell-like structures on supported nickel and cobalt particles following reaction in CO at temperatures over the range 400 to 650~ (9,33). It was claimed that carbon "shells" were the exclusive form of deposit when hydrogen was not present in the gas phase (9). Furthermore, it was stressed that the thickness of the shell surrounding the metal appeared to reach a limiting thickness of about 30 graphite layers at which point the particles were deactivated. This

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observation leads to the conclusion that the growth of graphite layers was not occurring from the surface of the metal, however, no alternative mechanism was suggested (9).

One of the major features to emerge from the present work is the finding that no definite correlation exists between the thickness of the graphitic apron and the size of the associated catalyst particle. A further aspect revealed from this investigation is that the metal particles adopt a well defined faceted structure that appear to function as a template for the growth of graphite platelets. Previous work has indicated that during carbon deposition reactions, catalyst particles acquire well defined crystallographic orientations, where some of the faces will exclusively decompose the hydrocarbon whereas others will only precipitate carbon since they are unable to dissociate carbon-carbon bonds (34).

graphite platelets

(a) (b)

Figure 7. Schematic rendition of a possible growth mechanism of the shell-like graphite deposits, where the catalyst particle shape changes, but the overall volume remains constant.

a b

Figure 8. Schematic rendition of a possible growth mechanism of the shell-like graphite deposits, where the catalyst material undergoes a wetting and spreading action with the platelet structures and the particle is progressively depleted in size as the reaction proceeds.

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Based on the observations of the current investigation we suggest two possible models that should be considered to account for the formation of the graphitic shell-like deposits. The first, which is outlined in the schematic diagram, Figure 7, assumes that during the graphite precipitation step the catalyst particles undergoes a progressive transformation in shape, whilst at the same time maintaining a constant volume. This condition is forced on the system since carbon is being continuously formed on the hydrocarbon decomposing faces, followed by dissolution and diffusion through the particle and eventually precipitating as rigid graphite platelets at other faces. Since the precipitation step imposes a constraint on the particle geometry at any given point in time, the only way for the process to continue in an uninterrupted manner is for the particle to progressively shrink in width in order to accommodate this restriction. A variation on this theme is presented in the model, Figure 8, where the only difference is that in this case, reorganization of the catalyst particle is accompanied by an overall decrease in volume. Indeed, the establishment of a strong interaction of the metal with the edges of the carbon will be manifested by a wetting and spreading action of material along the graphite edge regions. Under these circumstances, it is possible that the volume of the particle is progressively decreased due to the loss of metal as it leaves a monolayer coverage on the graphite edge regions and the catalyst particle can readily conform to the geometric limitations imposed by the platelet structures.

A C K N O W L E D G M E N T S

Financial support for this work was provided by the Department of Energy, Basic energy Sciences Grant No. DE-FG02-93ER14358.

REFERENCES

1. L . J .E . Hofer, in "Catalysis" (P. H. Emmett, Ed.) Reinhold Publ. Co. 4 (1956) 373. 2. H.B. Palmer, and C. F. Cullis, in "Chemistry and Physics of Carbon" (P. L. Walker,

Jr., Ed.) Marcel Dekker, New York, 1 (1965) 265. 3. J. R. Rostrup-Nielsen, "Steam Reforming Catalysts", Tekorisk Forlay A/S (Danish

Technical Press), 1975. 4. D.L. Trimm, Catal. Rev.-Sci. Eng. 16 (1977) 155. 5. R. T. K. Baker, and P. S. Harris, in "Chemistry and Physics of Carbon" (P. L.

Walker, Jr., and P. A. Thrower, Eds.) Marcel Dekker, New York, 14, (1978) 83. 6. C.H. Bartholomew, Catal. Rev.-Sci. Eng. 24 (1982) 67. 7. Baker, R. T. K. in "Carbon Fibers Filaments and Composites" (J. L. Figueiredo et al.

Eds.) NATO ASI Series, Kluwer Academic Publ. Dordrecht, 177 (1990) 405. 8. N.M. Rodriguez, J. Mater. Sci. 8 (1993) 3233. 9. P.E. Nolan, D. C. Lynch, and A. H. Cutler, Carbon 32, (1994) 477. 10. N. M. Rodriguez, M. S. Kim, F. Fortin, I. Mochida, and R. T. K. Baker, Appl. Catal.

in press. 11. J. H. Sinfelt, Accounts of Chem. Res. 10 (1977) 15. 12. W. M. H. Sachtler, and R. A. van Santen, Adv. Catal. 26 (1977). 69 13. V. Ponec, Adv. Catal. 32 (1983) 149. 14. M. J. Kelley, and V. Ponec, Prog. Surf. Sci. 11 (1981) 139. 15. J. H. Sinfelt, "Bimetallic Catalysts", an Exxon Monograph, John Wiley & Sons, New

York, (1983).

109

16. J. G. McCarty, and H. Wise, J. Catal. 57 (1979) 406. 17. J. G. McCarty, P. Y. Hou, D. Sheridan, and H. Wise, in "Coke Formation on Metal

Surfaces" ACS Symposium Series 202 (L. J. Albright and R. T. K. Baker eds.) Washington D.C. (1982) p. 253.

18. N. M. Rodriguez, M. S. Kim, and R. T. K. Baker, J. Phys. Chem 98 (1994) 13108. 19. J. R. Rostrup-Nielsen, in "Catalyst Deactivation" (C. H. Bartholomew and J. B. Butt

Eds.) Elsevier Sci. Publ. Amsterdam, p.85 (1991). 20. J. R. Rostrup-Nielsen, J. Catal. 85 (1984) 31. 21. M.S. Kim, N. M. Rodriguez, and R. T. K. Baker, J. Catal. 143 (1993) 449. 22. W. T. Owens, N. M. Rodriguez, and R. T. K. Baker, Catal. Today, 21 (1994) 3. 23. W. T. Owens, M. S. Kim, N. M. Rodriguez, and R. T. K. Baker, in "Catalyst

Deactivation 1994", (B. Delmon and G. F. Froment, Eds.) Elsevier Sci. Publ. Amsterdam, p.191 (1994).

24. A. Chambers and R. T. K. Baker, J. Phys. Chem. in press 25. X. Wu, B. C. Gerstein, and T. S. King, J. Catal. 135 (1992) 68. 26. R. J. Verderone, C. L. Pieck, M. R. Sad, and J. M. Parera, Appl. Catal. 21 (1986)

239. 27. J. H. Sinfelt, J. L. Carter, and D. J. C. Yates, J. Catal. 24, (1972) 283. 28. N. M. Rodriguez, S. G. Oh, W. B. Downs, P. Pattabiraman, and R. T. K. Baker,

Rev. Sci. Instrum. 61 (1990) 1863. 29. N. M. Rodriguez, S. G. Oh, R. A. Dalla-Betta, R. T. K. and Baker, J. Catal. 157

(1995) 676. 30. M. Audier, A. Oberlin, M. Oberlin, M. Coulon, and L. Bonnetain, Carbon 19 (1981)

217. 31. R. T. Yang, J. P. and Chen, J. Catal. 115 (1989) 52. 32. M. S. Kim, N. M. Rodriguez, and R. T. K. Baker, J. Catal. 134 (1992) 253. 33. M. Audier and M. Coulon, Carbon 23, (1985) 317. 34. N. M. Rodriguez, A. Chambers, and R. T. K. Baker, Langmuir 11 (1995) 3862.