Preparation of Multiwalled Carbon Nanotube-Supported Nickel Catalysts Using Incipient Wetness Method † Pooya Azadi, ‡ Ramin Farnood,* ,‡ and Emanuel Meier § Department of Chemical Engineering and Applied Chemistry, UniVersity of Toronto, Toronto, Canada, Department of Chemistry, ETH, Zurich, Switzerland ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: August 29, 2009 In this paper, a systematic study on preparation of multiwalled carbon nanotube (MWCNT)-supported nickel catalyst is pursued. Functional groups are introduced on the surface of MWCNTs using nitric acid, sulfuric acid, and partial oxidation in air. Nickel oxide nanoparticles are formed on the surface of functionalized multiwalled carbon nanotubes by incipient wetness impregnation of nickel nitrate, followed by calcination in air. The effects of acid type and concentration, acid treatment time, partial oxidation, nickel loading, precursor solvent, and calcination temperature on the size of the nickel nanoparticles and homogeneity of the composite material are evaluated. Characteristics of the Ni/MWCNT catalysts were examined using BET, scanning transmission electron microscopy, X-ray diffraction, thermogravimetric analysis in air and nitrogen, temperature- programmed reduction, X-ray photoelectron spectroscopy, acid-base titration, and -potential analyzer. Results of this work are useful for formulating CNT-supported nickel catalysts for a wide range of different applications, such as reforming of hydrocarbons, catalytic hydrothermal gasification of biomass, and energy storage. Introduction Since their discovery in 1991, many applications have been suggested for carbon nanotubes (CNTs). Carbon nanotubes offer excellent properties as a catalyst support, such as proper pore sizes, moderate to high specific area, great thermal stability and stability in acidic or basic environments. Due to the novel properties of these cylindrical carbon molecules, they can act as a catalyst support. However, due to lack of oxygen functional groups on their outer surface and their hydrophobicity, formation of bonds between a metal precursor and a CNT is not an easy task as compared to the metal oxide-supported catalysts, such as alumina. Once synthesized, CNTs contain some impurities. Hence, some pretreatment steps are usually implemented in preparation of CNT-supported catalysts (such as mild acid treatment) for removal of amorphous carbon and the metal nanoparticles that are used to catalyze the CNT synthesis. Then a controlled number of oxygen functional groups are added to the outer surface by either partial oxidation in air or by means of a liquid oxidizing agent, such as hydrogen peroxide, nitric acid, etc. 1 A few techniques 2 have been developed for decoration of carbon nanotubes. Among them are electrochemical methods, 3–5 wet impregnation, 6 and incipient wetness. 7 Each method leads to a certain degree of control over nanoparticle size and metal dispersion. The functionalized CNTs readily interact with the metal precursors, which either can be reduced directly to metal in a reducing atmosphere or it can be calcined to create metal oxides, followed by a reduction in hydrogen flow. Regarding the oxidation of CNT, it is found that acid concentration and treatment time play a significant role in functionalization of the nanotubes. In addition, it has been reported that the carboxylic groups are the dominant groups added on the CNT during the oxidative treatment by nitric acid. Furthermore, acid treatment may increase the specific area of the CNT, particularly for multiwalled carbon nanotubes by oxidizing and dissolving the outer walls of CNTs, as well as breaking the CNT particles. Many researchers have investigated the activity of CNT- supported catalysts for variety of applications, including syn- thesis of more carbon nanotubes, 1 reforming of hydrocarbons, 7 hydrogen storage, 8 and hydrogenation, 9 but there is a lack of systematic study on different aspects of particle formation on the surface of carbon nanotubes. Experimental methods Multiwalled carbon nanotubes with average length and diameter of 7 µm and 120 nm were obtained from Sigma- Aldrich, Oakville, Canada. An oxidative pretreatment in boiling nitric or sulfuric acid is conducted to introduce oxygenated functional groups onto the surface of the nanotubes. The following conditions were used throughout catalyst preparation unless mentioned otherwise. The nitric acid concentration and treatment time were 10 M and 5 h, respectively. In all experiments, 0.2 g of MWCNT was dispersed in 50 mL of acid, and the mixture was boiled using a hot plate and reflux system. The functionalized CNTs were washed with water, centrifuged twice, and dried at 110 °C overnight. Following this step, the MWCNTs were impregnated with a nickel precursor, followed by calcination at 350 °C for 3 h. The nickel loadings were controlled by changing the concentration of the nickel nitrate. A Quantachrome catalyst characterization unit † Part of the special issue “Green Chemistry in Energy Production Symposium”. * Corresponding author. E-mail: ramin.farnood@utoronto.ca. ‡ University of Toronto. § ETH. TABLE 1: Elemental Composition of Nickel Decorated MWCNT Obtained by XPS Nitric Acid Concentration preparation step carbon oxygen nickel nitrogen no treatment 98.5 1.5 0 0 acid treated 89 11 0 0 impregnated 75 19 2.8 3.2 calcined 82 11.6 6.4 0 J. Phys. Chem. A 2010, 114, 3962–3968 3962 10.1021/jp907403b  2010 American Chemical Society Published on Web 10/12/2009