Haifeng Xiong

Autoreduction and Catalytic Performance of a Cobalt Fischer–Tropsch Synthesis Catalyst Supported on Nitrogen-Doped Carbon Spheres

Carbon materials have been investigated in a wide variety of applications due to their good mechanical stability and electrical conductivity. They have also been used as a catalyst support but in order to establish a uniform coverage of metal particles on the surface of pure carbon materials, it is necessary to activate the chemically inert surface. Generally, different acid or oxidizing treatments have been used to functionalize the carbon surface and to create carboxylic, carbonyl and hydroxy groups that are able to bind the carbon surface to metal clusters. Unfortunately, these treatments can considerably reduce the mechanical and electronic performance of the carbon due to the introduction of a large number of defects. Recently, the doping of heteroatoms into carbon materials has been used as an alternative procedure to successfully bind metals to carbon materials. Nitrogen-doped carbon materials contain sites that are chemically active and allow for the attachment of metal precursors onto the surface of carbon materials without functionalization by strong acid treatments. Following on from the discovery of fullerenes and later the seminal studies on carbon nanotubes by Iijima, the role of curved sp-hybridized carbon atoms was seen to play a key role in the new graphitic carbon structures. 6] This should also apply to carbon materials with other shapes, such as carbon spheres (CSs). Indeed, although carbon spheres have been known for decades, recent studies have paved the way for a reinvestigation of the synthesis, chemistry, and properties of spherical carbons. In the past few years, new synthesis methods have been reported to make a variety of carbon spheres (hollow, solid, core/shell) and these new carbon spheres are expected to exhibit excellent physical and chemical properties. Their use as a catalyst support has, however, hardly been studied. Co and Fe catalysts have been used in Fischer–Tropsch synthesis (FTS) 13] and NH3 decomposition studies. [14] The reduction of the metal oxide to the metal is an indispensable step in activating the catalyst, and is closely related to catalytic performance. 15] However, reduction is affected by the strong metal–support interaction (SMSI). This process typically inhibits the metal reduction process, leading to a lower catalytic activity. Herein, we report for the first time that cobalt oxide supported on nitrogen-doped carbon spheres (N-CSs) can be autoreduced completely by the carbon support. The autoreduced cobalt catalyst pretreated in Ar showed superior FTS catalytic performance to a Co catalyst reduced in H2. Nitrogen-doped carbon spheres (N-CSs) were prepared by chemical vapor deposition (CVD) through the pyrolysis of acetylene and NH3 at 900 8C. [9] This synthesis gave smooth, round carbon spheres with a uniform diameter (ca. 700 nm, BET surface area = 3.4 m g ; see the Supporting Information, Figure S1). Elemental analysis revealed that the nitrogen content of the as-prepared carbon spheres was approximately 2 wt % (see the Supporting Information, Table S1). The N-CSs-supported cobalt catalyst (Co/N-CSs) was prepared by a homogeneous deposition precipitation method using urea as the deposition agent at 90 8C. After filtration, the material was dried in an oven at 100 8C for 12 h and was found to contain 2.3 wt % Co (measured by ICP-AES). The Co/N-CSs had an average cobalt oxide particle size of approximately 13 nm and transmission electron microscope (TEM) images show that the cobalt species were retained on the surface of the carbon spheres even after sonication for 4–5 min (Figure 1 a). Addition of cobalt to CSs that did not contain nitrogen led to large Co particles, even at loadings below 1.5 wt % Co (Figure 1 b). The catalyst was characterized by thermogravimetric analysis (TGA, Perkin–Elmer STA 6000) under N2 using a heating rate of 10 8C min . Figure 2 displays the TGA curves of the nitrogendoped carbon spheres and the 2.3 wt % Co/N-CSs catalyst under N2. A weight loss of approximately 6 % was detected when the N-CSs were heated to 900 8C (Figure 2), owing to the loss of the nongraphitic carbon. The weight loss of greater than 10 % detected for 2.3 wt % Co/N-CSs in the temperature range 400–900 8C can be attributed to CO2 formation as the cobalt oxide is reduced. A possible cobalt-catalyzed loss of carbon from the matrix may also have contributed to the weight loss. The effect of the pretreatment temperature (prior to catalyst testing) on the reduction behavior of the resulting 2.3 wt % Co/N-CSs was monitored by hydrogen temperatureprogrammed reduction (TPR, Micromeritics Auto Chem II) under 5 % H2/Ar. Figure 3 presents the TPR profiles of 2.3 wt % Co/N-CSs pretreated in a flow of high purity Ar at different temperatures. As can be seen, the TPR profile of 2.3 wt % Co/N-CSs after pretreatment at 250 8C (Figure 3 a) has two reduction peaks, corresponding to the reduction of Co3O4 and a mixture of Co3O4 and CoO, respectively. [17] The first peak, [a] Dr. H. Xiong, Prof. D. G. Billing, Prof. N. J. Coville DST/NRF Centre of Excellence in Strong Materials University of Witwatersrand, Johannesburg 2050 (South Africa) Fax:(+27) 11-7176749 E-mail : neil.coville@wits.ac.za [b] Dr. H. Xiong, M. Moyo, M. K. Rayner, Prof. D. G. Billing, Prof. N. J. Coville School of Chemistry, University of the Witwatersrand Johannesburg 2050 (South Africa) [c] M. Moyo, Dr. L. L. Jewell School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg 2050 (South Africa) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.200900309.