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Large-scale and low-cost controlled production of single-walled carbon nanotubes by catalytic methods
We have developed a controlled production method of SWNT. This implies the ability to control the selectivity towards SWNT by changing catalyst formulations and operating conditions, combined with an effective purification strategy and a quantitative determination of the SWNT obtained.
The use of heterogeneous catalysts allows us to tailor the material in such a way that selectivity and yield are maximized. The method we employ involves the disproportionation of CO at moderate pressure and temperature, which results in a scalable and cost-effective process.
Understanding the mechanism of catalytic production, catalyst deactivation and relationship between catalyst structure and performance
Among the many catalyst formulations investigated, we have found a bi-metallic Co-Mo catalyst to be the most effective. A synergistic effect between Co and Mo has been observed. When both metals are simultaneously present, particularly when Mo is in excess, the catalyst is very effective.
To understand this synergistic effect we have used a battery of characterization techniques to study the state of Co and Mo in the catalyst before and after the production of SWNT. The selectivity of the Co-Mo catalyst towards SWNT production by CO disproportionation (CoMoCat process) strongly depends on the stabilization of the Co2+ species before the formation of SWNT. We have proposed a reaction mechanism based on this observations.
TEM images of carbon deposits obtained after reaction with CO at 700°C. Left: monometallic Co catalyst - Center: bimetallic CoMo catalyst - Right: monometallic Mo catalyst
Optimization of reaction conditions, carbon yield and selectivity
We have studied the reversibility of the reaction that leads to the formation of single-wall carbon nanotubes (SWNT) from the CO decomposition on solid catalysts.
Under the temperature range that is necessary to produce SWNT with high selectivity (700-1200C), the reaction is reversible and can be limited by thermodynamic equilibrium. We have confirmed that high pressure and high concentration of CO are needed in order to counteract the effect of the temperature and shift the reaction in the forward direction. High flow rates are also needed to avoid external diffusional effects in the catalyst particle.
Development of separation and purification methods for SWNT obtained by the CoMoCat process
We are in the process of developing a purification protocol for the raw material obtained in the CoMoCat process. So far this purification method involves attack of the catalyst support and metals with a combination of caustic and acid solutions. This process results in a different degree of functionalization of the tubes by oxygen-containing groups.
We have characterized the chemical nature of the resulting functional groups by XPS and FTIR. To quantify the density of functional groups a combination of TPD/TPO and XPS was also employed. To characterize the structural properties of the SWNT obtained after these series of treatments a combination of techniques involving Raman Spectroscopy, AFM and TEM are also being used.
Growth of Vertically-Aligned SWNT
We found a simple method to grow SWNT on a silicon wafer. With a Co-Mo solution formulation developed in our group, the catalyst is easy to prepare by simply dropping the solution on silicon wafers.
Through a so called DSD (drop-spread-dry) process, a uniform catalyst film composed of well-distributed nanoparticles is formed. By changing the concentration of catalyst solution, various forms of SWNT including random network, vertically aligned arrays, and scattered orderly arrays were synthesized through atmospheric CVD, which in the future will lead to many potential applications based on flat substrate.