In situ dynamic observation of carbon nanotube growth on Co/SBA-15 by acetylene decomposition in an environmental TEM Francisco J. Cadete Santos Aires* a , Thierry Epicier b , Jakob B. Wagner c , Thomas W. Hansen c , Mimoun Aouine a , Rafal E. Dunin-Borkowski c,d , Miriam González Pedrero a , Alain Tuel a a Institut de Recherches sur la Catalyse et l’Environnement de Lyon – IRCELYON (UMR 5256 CNRS/Univ. Lyon I), 2 Avenue Albert Einstein, 69626 – Villeurbanne cedex, France. b Laboratoire Matériaux : Ingénierie et Science – MATEIS (UMR 5510 CNRS/INSA de Lyon), Bât. B. Pascal, 7, Avenue Jean Capelle, 69621 – Villeurbanne cedex, France. c Center for Electron Nanoscopy – Cen. Technical University of Denmark (DTU), Blds 307/314, 2800 – Kongens Lyngby, Denmark. d Ernst Ruska-Centre (ER-C) and Peter Grünberg Institute (PGI), Research Centre Jülich, 52425 – Jülich, Germany. ABSTRACT The formation of carbon nanotubes by acetylene decomposition on Co catalysts inserted in mesoporous silicas (SBA-15) has been studied in the transmission electron microscope (TEM). Two types of tubular carbon structures are observed: smaller nanotubes with internal diameters seemingly regulated by the size the Co nanoparticles formed within the SBA- 15 pores and larger nanotubes. In situ observation of carbon nanotube growth in an environmental TEM allows both the dynamics of the formation of the nanotubes and the evolution of the morphology of the metal nanoparticles during growth to be followed under realistic conditions in order ton better understand the reaction mechanisms. Keywords: Carbon nanotube growth, ETEM, Co/SBA-15, acetylene decomposition 1. INTRODUCTION In situ studies of micro- and nano-objects in their characteristic environment have been performed ever since the early days of electron microscopy 1 . Over several decades, the in situ observation of the synthesis of filamentous carbon (nanotubes/nanofilaments) during hydrocarbon decomposition has been one of the most popular topics 2-14 for investigation in the environmental transmission electron microscope (ETEM). Here we study the growth of carbon nanotubes by the decomposition of acetylene on Co nanoparticles inserted into mesoporous silicas (SBA-15) using both conventional post mortem TEM measurements and real-time in situ ETEM observations. 2. RESULTS AND DISCUSSION 2.1 Ex situ study Co/SBA-15 catalysts were prepared using several methods, including the so-called "two solvent" method, in which Co particles are preformed before catalytic reaction and located almost exclusively inside the mesopores 15 . TEM examination of the as-synthesised catalyst revealed the presence of internal Co oxide particles of similar size to the mesopore dimension, i.e. ~ 6 nm (Fig. 1a). After reduction of Co oxide nanoparticles in a hydrogen flow, the decomposition of acetylene takes place at 700°C, in the presence of hydrogen, to produce filaments (Fig. 1b) that are essentially multi-walled nanotubes (MWNTs). The total amount of carbon formed on the catalyst (6 wt.%) was found to be significantly lower than that produced over a catalyst prepared using a conventional impregnation method (11.9 wt.%), for which insertion of the Co nanoparticles into the mesopores of SBA-15 was less efficient 16 . This difference suggests that carbon nanotube formation takes place when particles are located or arrive at the pore mouths. The TEM images suggest that the internal diameter of the MWNTs is regulated by the Co nanoparticles formed within the pores. *francisco.aires@ircelyon.univ-lyon1.fr; phone 33 4 72 44 53 03; fax 33 4 72 44 53 99; www.ircelyon.univ-lyon1.fr