Characteristics of Cobalt Nanoneedles in 10% Co/Aerogel Fischer−Tropsch Catalyst

The 10% Co/aerogel catalyst used in Fischer−Tropsch (FT) synthesis recently by Dunn et al. (Dunn, B. C.; Cole, P.; Covington, D.; Webster, M. C.; Pugmire, R. J.; Ernst, R. D.; Eyring, E. M.; Shah, N.; Huffman, G. P. Appl. Catal., A 2005, 278, 233) is investigated using the complimentary techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and detailed magnetic measurements. TEM studies show that the catalyst contains Co nanoneedles with diameter d ≈ 1 nm, whereas the XRD studies show the presence of CoO and cobalt silicate hydroxide Co3(Si2O5)(OH)2, in addition to Co. Analysis of the temperature and magnetic field variation of the magnetization from 5 to 350 K shows that the measured magnetic properties are primarily due to the Co nanoneedles oriented randomly, with concentration ≈8%. The presence of the unreduced CoO and the Co silicate explains why the FT activity of this catalyst is about equal to that of the 6% Co/aerogel catalyst. Some interesting magnetic properties of the Co nanoneedles are also presented.