Influence of the microstructure of carbon nanotubes on the oxidative dehydrogenation of ethylbenzene to styrene J.J. Delgado a,1 , X. Chen b , J.P. Tessonnier a , M.E. Schuster a , E. Del Rio b , R. Schlo ¨ gl a , D.S. Su a, * a Fritz-Haber-Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany b Departamento de Ciencia de los Materiales e Ingenierı´a Metalu ´rgica y Quı´mica Inorga ´nica, Facultad de Ciencias, Universidad de Ca ´diz, E-11510 Puerto Real (Ca ´diz), Spain 1. Introduction Nanostructured carbon materials, especially carbon nanotube (CNT), have attracted the fancy of many scientists worldwide concerning their synthesis, properties, characterization and technological applications [1]. Among some of the most promising applications we can mention field emitter devices, drugs delivery and catalysis [1–7]. Carbon materials have been extensively used in catalysis not only as catalyst supports but also as highly active catalysts [8,9]. The new synthesis routes of nanostructured carbon materials allow to modulate their physical/chemical properties, opening new opportunities in the design of suitable catalysts for different reactions [10–13]. The oxidative dehydrogenation (ODH) of ethylbenzene to produce styrene has been proposed as a promising alternative to avoid the thermodynamic limitations, waste of energy and catalyst deactiva- tion of the current industrial process [14]. Different carbon materials have been reported as active catalysts for the ODH reaction [15]. Activated carbon was proposed as a suitable catalyst for this reaction, but it exhibits a low and unstable activity in an oxidative atmosphere, hindering its use as industrial catalyst [16–18]. Carbon nanofila- ments, tubes and onion-like carbon have also been reported to be active in the ODH of ethylbenzene to styrene and show high catalytic activity and stability [19]. The graphitic structure of CNTs and their low porosity compared with activated carbons are the main reason for their high stability under harsh operation condition. The nature and amount of basic groups, the crystallinity, textural properties and the ratio between the prismatic area and the basal plane area have been intensively studied in the literature to rationalise the catalytic performances of different carbon materials [20–24]. A redox mechanism involving quinone/hydroquinone groups presented on the carbon surface is suggested in the literature when activated carbon is used for the reaction [17]. A similar mechanism has been proposed for the CNTs where the basal planes exhibit metallic properties for oxygen activation, and nucleophilic Brønsted basic –C 55 O groups located at the edge/kink are the active sites for ethylbenzene dehydrogenation [19]. While the kinetics of the reaction and the influence of the pre-treatment of the nanocarbons on the reaction have been investigated [25], there is a lack of investigation on the effect of the graphitization degree on the intrinsic activity of CNTs, as well as on the active sites, their nature and stability during the reaction. Here we report on the use of commercial CNTs with different graphitization degree as a model catalyst for the understanding of the structure–activity correlation in the ODH of ethylbenzene. The catalytic performance of the CNTs after different annealing pre- treatments gives us the role of the surface defects in the catalytic activity. The effect of the graphitization degree on the reaction rate and selectivity of ODH of ethylbenzene to styrene was studied. The surface and structural characterization of the fresh and used samples were investigated by TG–MS, XPS, TEM and EELS. Catalysis Today 150 (2010) 49–54 ARTICLE INFO Article history: Available online 25 August 2009 Keywords: Ethylbenzene Styrene Oxidative dehydrogenation Carbon nanotube ABSTRACT The effect of graphitization of carbon nanotubes (CNTs) on the oxidative dehydrogenation of ethylbenzene to styrene was studied. An elimination of functional groups was observed by treating the catalyst under inert gas at temperatures below 1100 8C, while its microstructure and reactivity in TPO experiments did not change significantly. A decrease in the initial catalytic performances was observed, but the functional groups can be regenerated during the catalytic reaction, thus leading to a significant improvement in the catalytic activity. Annealing CNTs above 1500 8C leads to a well graphitisized wall structure of CNTs with a nearly oxygen-free surface with a low number of defects. The obtained samples show low but stable catalytic performances, indicating that the oxygenated active sites cannot be regenerated on this well-organized and low defective surface. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 30 8413 5406; fax: +49 30 8413 4401. E-mail addresses: juanjose.delgado@uca.es (J.J. Delgado), dangsheng@fhi-berlin.mpg.de (D.S. Su). 1 Actual address: Departamento de Ciencia de los Materiales e Ingenierı ´a Metalu ´ rgica y Quı ´mica Inorga ´ nica, Facultad de Ciencias, Universidad de Ca ´ diz, Spain. Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2009.07.103