Role of Co-W Interaction in the Selective Growth of Single-Walled Carbon Nanotubes from CO Disproportionation

A combination of characterization techniques has been employed to obtain a full description of the structure and chemical state of a series of Co-W/SiO2 catalysts used for the production of single-walled carbon nanotubes (SWNT) by CO disproportionation at 750-950 °C. The state of Co and W on a series of silica-supported catalysts has been investigated using extended X-ray absorption fine structure, X-ray absorption near-edge spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, H2 temperature-programmed reduction, and Raman spectroscopy. The results are compared to those obtained in a previous study on a highly selective CoMo catalyst. Strong similarities and interesting differences between the two catalytic systems are revealed. First, it has been found that, similar to the Co-Mo system, the selectivity of the Co-W catalysts toward SWNT strongly depends on the stabilization of Co species in a nonmetallic state before exposure to CO. This stabilization is a consequence of the interaction of Co with tungsten oxide. From the detailed characterization conducted over the catalyst series it has been concluded that after calcination, W is in the form of a welldispersed W(VI) oxide while Co is either interacting with W in a superficial Co tungstate like structure (at low Co:W ratios) or as a noninteracting Co3O4 phase (at high Co:W ratios). From the SWNT growth studies and the TPR data it is shown that a carefully chosen reduction pretreatment is critical for the good performance of the selective Co-W catalyst. The comparison of SWNT selectivity and TPR profiles between two selective Co-Mo and Co-W catalysts suggests that both activity and selectivity toward SWNT production strongly depend on the degree of the interaction between Co and W (or Co and Mo) and a specific partial reduction pretreatment.