J.J. Chludzinski

Filamentous carbon growth on nickel-iron surfaces: The effect of various oxide additives

The growth of carbon filaments on nickel/iron in acetylene was monitored by controlled atmosphere electron microscopy in the absence or presence of oxide additives. The additives were SiO/sub 2/, Al/sub 2/O/sub 3/, TiO/sub 2/, WO/sub 3/, Ta/sub 2/O/sub 5/, and MoO/sub 3/. All oxide additives suppressed carbon filament growth at < 620/sup 0/C. The aluminum and titanium oxides apparently formed a physical barrier towards hydrocarbon adsorption and metal surface decomposition, but spalled off at approx. 640/sup 0/C. Molybdenum, tungsten, and tantalum oxides reduced carbon solubility in the metal but had no effect on carbon diffusion. Silica reduced both the solubility and diffusion of carbon and was the most effective inhibitor of filament growth among the additives tested.

The reversibility of filamentous carbon growth and gasification

Controlled atmosphere electron microscopy observations of the nickel-catalyzed growth and gasification of carbon filaments have shown that these processes can be reversed. This supports the view that growth and gasification in either hydrogen or steam occur by similar mechanisms, where one of the steps involves the diffusion of carbon through the metal. It was observed that the small catalyst particles are the most active in filament formation and steam gasification, while the large particles are the most active for the hydrogenation reaction. This is explained in terms of different rate-controlling steps.

A comparison of the catalytic influence of nickel, iron and nickel-iron on the gasification of graphite in various gaseous environments

Abstract Controlled-atmosphere electron microscopy has been used to study the catalytic influence of nickel, iron and nickel—iron on the graphite-steam and graphite-hydrogen reactions. This study has enabled us to identify some characteristics of the alloy catalyst which were not manifested by either of the pure constituents when reacted under similar conditions. When nickel-iron/graphite specimens were reacted in steam, catalysis occurred exclusively by the channeling mode, in contrast to nickel which exhibited a mixture of both edge recession and channeling modes of attack, while iron appeared to be inactive. Confirmation of the higher catalytic activity of the alloy compared to nickel for the gasification of graphite in steam was obtained from bulk flow reactor measurements. The alloy was also found to be a more active catalyst than either of its pure components for the graphite-hydrogen reaction. In this case, two modes of catalytic action were observed: edge recession below 845°C and channeling above this temperature. This change is believed to arise from a modification in the wetting characteristics of alloy particles along graphite edges. It is significant that the channeling action of the alloy, like that produced by active iron particles, persisted up to temperatures in excess of 1100°C, in contrast to the behavior found with nickel, where the catalyst underwent deactivation.

Potassium-catalyzed gasification of graphite in oxygen and steam

Abstract Controlled atmosphere electron microscopy has been used to study the catalytic influence of potassium salts on the graphite-oxygen and graphite-steam reactions. This investigation showed strong interaction of potassium species with the graphite substrate in both reactant atmospheres. Potassium salt particles in contact with the edges of the graphite layers were observed to wet and spread on the edges at about 500 °C. Gasification then proceeded in both atmospheres by recession of the edges of the layer planes, with little or no attack by discrete catalyst particles. This behavior is consistent with previous indications of high catalyst dispersion associated with the formation of alkali surface complexes as active sites in alkali-catalyzed gasification. The technique also revealed other interesting details regarding the mode of catalyzed attack. In steam the edges show hexagonal facetting with the receding edges parallel to the 〈1120〉 set of crystal directions. This indicates a preferential reactivity in removal of the edge atoms. Occasionally portions of actively receding edges suddenly became inactive. No strong crystallographic preference or deactivation was found in the catalyzed graphite-oxygen reaction.

Further studies of the formation of filamentous carbon from the interaction of supported iron particles with acetylene

Abstract A combination of controlled atmosphere electron microscopy and Mossbauer spectroscopy has been used to investigate the characteristics of supported α-iron and γ-iron particles during the formation of carbon filaments via decomposition of acetylene. γ-iron was found to exhibit a higher intrinsic activity than α-iron for this reaction when the metal was supported on graphite. In both systems, however, catalytic action decreased significantly at temperatures in excess of 700°C. Major changes were observed in the catalytic behavior of the metal particles when they were supported on silica. The rate of formation of carbon filaments from the α-iron/silica system showed a uniform increase up to 900°C. Mossbauer spectroscopy analysis of similarly treated samples revealed that under these conditions α-iron was the only metallic phase present, even though experiments were conducted through a temperature region where the transformation of α-iron to γ-iron can occur, suggesting that silica stabilizes the α-form of iron. In contrast, the catalytic activity displayed by γ-iron particles supported on silica was considerably reduced over that found for the corresponding graphite supported system. The results of this study are discussed in terms of some of the factors controlling the growth characteristics of filamentous carbon.

Filamentous carbon growth on nickel surfaces treated with titanium dioxide: Migration of titania and ramifications of strong metal-support interactions

Abstract Controlled-atmosphere electron microscopy and gravimetric measurements were used to study the effects of depositing powdered TiO2 on nickel surfaces with respect to the formation of filamentous carbon from acetylene and ethane. If the samples were heated directly in a hydrocarbon environment to temperatures near 1000 K, then filamentous carbon formation was initially suppressed on those portions of the nickel surface which contained titania. This inhibition of the formation of filamentous carbon was lost when the temperature was raised to ca. 1120 K, at which temperature the titania broke away from the nickel surface. In contrast, effective passivation of the nickel surface could be achieved at all temperatures by pretreating the TiO 2 Ni samples in hydrogen at ca. 770 K. During this treatment, the titania was observed to wet and spread over the nickel surface. This process could be reversed by heating in oxygen at 845 K, during which the nickel surface was uncovered and particles of TiO2 were observed to form. The passivating effect of titania on nickel was thereby destroyed. These results provide direct evidence that reduced titania species migrate over metal surfaces under reducing conditions and collect into TiO2 particles under oxidizing conditions. As such, these results support the idea that reduced titania species on metal surfaces may be the origin of strong metal-support interactions for titania-supported metal particles. Finally, a means of inhibiting filamentous carbon formation on nickel surfaces has been identified.

Direct observation of wetting and spreading of copper particles on magnesium oxide

Abstract Supported metal catalysts were exposed to various oxidizing and reducing gas atmospheres during their preparation, operation, and regeneration. Interaction with these gases had profound effects on their morphology. In the present investigation a controlled atmosphere electron microscope was used to follow the changes of the morphology of copper particles supported on magnesium oxide in oxygen and hydrogen environments. When the system is heated in oxygen at 200 °C, the metal particles undergo an unusual transformation, discrete solid crystallites changing to toroid shaped oxide structures. Subsequent treatment in hydrogen at 200 °C results in a conversion back to the initial metallic form. These results are discussed in terms of a volcano structure, in which particles consist of a molten core surrounded by a porous solid skin.

Catalytic gasification of graphite by calcium and nickel-calcium

Abstract Controlled atmosphere electron microscopy has been used to study the influence of calcium on the graphite-oxygen and graphite-steam reactions. Direct observation of the specimen surface has enabled us to establish that the high catalytic activity of calcium for graphite oxidation is associated with the ability of the alkaline earth to wet and spread along the graphite edges, which subsequently undergo gasification by the edge recession mode. We have found that in the presence of steam, calcium-coated graphite edges would occasionally deactivate, and this behavior became more pronounced as the temperature was increased. A decrease in rate of gasification was found when nickel was added to calcium, this is attributed to a fundamental difference in the mechanisms by which these two elements catalyze the graphite-steam reaction.

The effect of various bimetallics on the graphite-steam reaction

Abstract This study reports the use of bifunctional catalysts for the carbon-steam reaction, where one component increases the oxygen reactivity and the other component accelerates the supply of carbon from the source material to the catalyst-gas interface. The mixed catalysts selected for this purpose were nickeltitanium, nickelplatinum, nickelruthenium, and platinumtitanium. Using a combination of controlled atmosphere electron microscopy and flow reactor studies, we have found that the bimetallic systems exhibit a rate of catalytic attack of graphite in steam that is higher than either of the respective pure components. In addition to modifying the catalytic activity of a given metal, introduction of a second metal into the system can also produce changes in the wetting characteristics of the catalyst on graphite. In the presence of platinum, ruthenium, and mixtures containing these metals catalytic gasification occurs along the armchair edges of graphite. In all other systems studied here and previously, the receding edges are aligned parallel to the zigzag edges of graphite. A rationale is presented to account for this phenomenon which is based on the nature of the adsorption characteristics of water molecules on the catalyst surface and the chemical state of the prismatic faces of the graphite when reacted in steam.