Ni-Fe Competition in the Catalytic Growth of Carbon Nanotubes
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0963-Q02-06
Ni-Fe Competition in the Catalytic Growth of Carbon Nanotubes V. Z. Mordkovich, D. N. Kharitonov, I. A. Maslov, and E. B. Mitberg Advanced Materials, YRD Center, Leninski pr., 55/1, str.2, Moscow, 119333, Russian Federation
ABSTRACT
The paper presents the results of competitive catalysis investigation of the carbon nanotube growth in situ of the partial oxidation process of methane. The competition between Ni and Fe results in suppression of Ni catalytic activity and the growth of Fe-capped carbon nanotubes. The discrimination is so strong that iron is segregated from Ni-Fe based stainless steel alloy leaving characteristic Ni-enriched corrosion caverns. The process strongly depends on temperature. Depending on particular catalyst bed composition, the nanotubes of various morphology may occur. In particular, the use of perovskite-type catalyst leads to formation of “olive-branch”-like peculiar carbon nanostructures.
INTRODUCTION
Despite a great variety of known catalysts for carbon nanotube growth, there is no clear understanding of how the nucleation/growth mechanism changes with the change of the catalyst formulation. The problem of competition of two catalysts placed in the same reaction bed brings even more intrigue and uncertainty to the reaction outcome. Meanwhile, the problems of intercatalyst competition are of great practical importance because the competitive catalytic growth of nanotube is a core of carbonization and corrosion phenomena in many industrial processes such as methane steam conversion, partial oxidation, pyrolysis, etc. Ni and Fe may be considered as model competetitor catalysts, though the impact of support often overwhelms and proves more important than the difference between Ni and Fe [1-3]. The growth of carbon deposits in situ of the partial oxidation process of methane is less sensitive to support and hence more suitable for catalyst comparison [4]. Ni catalysts on oxide supports invariably form multiwall carbon nanotubes in the conditions of methane oxidation process [4].
Unsupported bulk metals are less inclined to form nanotube deposits and rather show graphite or amorphous carbon formation [5-8]. One should note, however, that the authors of the works [5-7] did not study the process of partial oxidation in situ, but rather subjected their catalysts to the flow of H2-CH4-CO-CO2-H2O gas mixture. Zhang et al. [5] observed formation of either filamentous carbon (not clear if it was nanotubes) or graphite as a result of bulk iron carburization in the flow of (1-95)% CO-(98.8-4.8)% H2-0.2% H2O gas mixture. The precondition for the filamentous carbon formation is very low CO concentration below 1 %. Millward et al. [6] received similar results on nickel and nickel-rich stainless steels, where amorphous carbon filaments with cap-embedded Ni nanocrystals grew in more oxidative (methane and hydrogen free) atmosphere. The most comprehensive study by Natesan at Argonne National Lab [7] did not reveal any nanotube or carbon filament growth during carburization e.g. metal dusti
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