Relationship between the structure/composition of Co-Mo catalysts and their ability to produce single-walled carbon nanotubes by CO disproportionation

A series of analytical techniques have been employed to fully characterize the structure and chemical state Of Co-Mo/SiO2 catalysts used for the production of single-walled carbon nanotubes (SWNT) by CO disproportionation at 700-850 degreesC. The state of Co and Mo on a series of silica-supported catalysts was investigated using extended X-ray absorption fine structure, X-ray absorption near-edge spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, H-2 temperature-programmed reduction, X-ray photoelectron spectroscopy, and diffuse reflectance Fourier transform infrared spectroscopy of adsorbed NO after two sequential pretreatments and after the production of SWNT under pure CO. It was found that the selectivity of the Co-Mo catalysts toward SWNT strongly depends on the stabilization of Co species in a nonmetallic state before exposure to CO, which results from an interaction with Mo. The extent of this interaction is a function of the Co: Mo ratio and has different forms during the different stages of the catalyst life. From the detailed characterization conducted over the catalyst series it is concluded that after calcination, Mo is in the form of a well-dispersed Mo(6+) oxide while Co is either interacting with Mo in a superficial Co molybdatelike structure (at low Co: Mo ratios) or as a noninteracting Co3O4 phase (at high Co: Mo ratios). After a subsequent treatment in hydrogen, the noninteracting phase is reduced to metallic Co, while the Co molybdate-like species remain as well-dispersed Co2+ ions. During the production of SWNT under pure CO, the Mo oxide species are converted into Mo carbide. This conversion disrupts the interaction between Co and Mo and results in the release of metallic Co in the form of extremely small clusters, which are responsible for the production of SWNT. By contrast, large Co clusters that are formed from the noninteracting Co phase produce the nonselective forms of carbon (multiwalled nanotubes, filaments, graphite, etc.). (C) 2001 Academic Press.