Synthesis of carbon nanotubes by catalytic conversion of methane: Competition between active components of catalyst V.Z. Mordkovich a, * , E.A. Dolgova a , A.R. Karaeva a , D.N. Kharitonov a , I.A. Maslov a , A.A. Kamenev a , V.F. Tretjakov b a YUKOS R&D Centre, Leninsky pr. 55/1, build. 2, Moscow 119333, Russia b M.V. Lomonosov Moscow State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow 119571, Russia Received 9 January 2006; accepted 11 August 2006 Available online 6 October 2006 Abstract Catalytic growth of carbon nanotubes from methane was studied as competitive catalytic process in situ of both oxidative (partial oxidation) and non-oxidative (pyrolysis) conversion. Ni and Fe may act as either competing or cooperative catalyst components in the process of carbon nanotubes growth. The competition between Ni and Fe in the process of carbon nanotube growth on Ni–Fe based stainless steel alloy during partial oxidation of methane results in suppression of Ni catalytic activity in favor of Fe. The discrimination is so strong that iron is segregated from Ni–Fe based stainless steel alloy leaving characteristic Ni-enriched corrosion caverns. Interaction between Ni and Fe during non-oxidative conversion, in contrast to the oxidative conversion, leads to cooperative effect; the activity of bimetallic catalyst increases as compared with monometallic one. Depending on particular catalyst bed composition, the nanotubes of various morphologies may occur. In particular, the use of perovskite-type catalyst for partial oxidation of methane leads to formation of ‘‘olive-branch’’-like peculiar carbon nanostructures. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanotube-containing deposits often occur as by- products of the catalytic conversion of methane [1,2]. On the other hand, the carbon nanotubes or nanofibres (CNT/CNF) may also be a target product of oxidative and non-oxidative conversion of hydrocarbons. A great variety of catalysts for CNT synthesis has been discovered and described recently [3–7]. Despite the great number of publications, the mechanism of nucleation and growth of carbon deposits is not yet quite clear. The role and compe- tition of various active components of bimetallic and poly- metallic catalysts was insufficiently discussed in literature. Meanwhile, the problems of inter-catalyst competition are of great practical importance because the competitive catalytic growth of nanotubes is a core of carbonization and corrosion phenomena in many industrial processes such as methane steam reforming, partial oxidation, pyro- lysis, etc. Processes of catalytic pyrolysis (non-oxidative conver- sion) and partial oxidation (oxidative conversion) of meth- ane proceed at the temperatures 500–800 °C in the presence of the catalysts containing iron and nickel. At these condi- tions growth of CNT/CNF on metal particles of the cata- lyst is possible in both non-oxidative and oxidative process. In the former case growth of CNT/CNF is desir- able since the nano-structured carbon deposit is a target product – high activity, selectivity and life-time of catalyst are required. As for the partial oxidation, the carbon for- mation on the catalyst is usually considered as undesirable co-process, which leads to the decrease of catalysts activity and reactor clogging. The reactor materials, which usually contain both iron and nickel, can serve as catalysts of car- bon growth from either methane or carbon monoxide CO 0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.08.006 * Corresponding author. Tel.: +7 495 730 61 01; fax: +7 495 730 61 02. E-mail address: mordkovich@yandex.ru (V.Z. Mordkovich). www.elsevier.com/locate/carbon Carbon 45 (2007) 62–69