To realize the remarkable properties of single-wall nanotubes, synthesis must be precisely controlled. CoMoCAT® is SouthWest NanoTechnologies Inc.’s unique catalytic method that produces single-wall nanotubes of high quality at very high selectivity and with a remarkably narrow distribution of tube diameters.
The CoMoCAT® process can grow significant amounts of single-wall nanotubes in less than one hour, maintaining a selectivity rate of better than 90 percent. The ready scalability and intrinsic high selectivity that is preserved as the reactor size is scaled up provide the dual benefit of low cost and high product quality. Based on original work conducted by the research group of Prof. Daniel Resasco at the University of Oklahoma, we have developed a unique catalytic method (CoMoCAT®) that produces SWCNT of high quality at very high selectivity, and with a remarkably narrow distribution of tube diameters.
In this method, SWCNT are grown by CO disproportionation (decomposition into C and CO2) at 700-950ºC in flow of pure CO at a total pressure that typically ranges from 1 to 10 atm. In a three-year research program of catalyst and reactor development, which included detailed characterization and testing of a large number of catalyst formulations and operating conditions, we developed a process that is able to grow significant amounts of SWCNT in less than one hour, keeping a selectivity towards SWCNT better than 90 percent.
Two of the unique characteristics of the CoMoCAT® process are that it is readily scalable and its intrinsic high selectivity is preserved as the reactor size is scaled up. These characteristics impart the SWCNT product of the CoMoCAT® process the dual benefit of low cost and high product quality.
We discovered a synergistic effect between Co and Mo that is critical for the performance of the catalyst. The catalyst is effective when both metals are simultaneously present on a silica support with a low Co:Mo. Separated, they are unselective. Through a detailed characterization that involved a variety of characterization techniques such as EXAFS, XANES, UV/Vis-DRS, XPS and DRIFTS, we were able to explain the reasons for the high selectivity of these catalysts.
We established that the selectivity of the Co-Mo catalysts towards SWCNT production by CO disproportionation strongly depended on the stabilization of Co2+ species by Mo oxide species. The extent of the Co-Mo interaction is a function of the Co:Mo ratio in the catalyst and has different forms during the different stages of the catalyst life. We found that in the calcined state, Mo is in the form of a well dispersed Mo(6+) oxide. The state of Co strongly depends on the Co:Mo ratio. At low Co:Mo ratios, it interacts with Mo in a superficial Co molybdate-like structure. At high Co:Mo ratios, it forms a non-interacting Co3O4 phase.
During the subsequent reduction treatment in hydrogen, the non-interacting Co phase is reduced to metallic Co, while the Co molybdate-like species remain as well-dispersed Co2+ ions. Two different forms of Co can be present in a catalyst, the selective Co species (interacting with Mo) and the unselective one (non-interacting). The Co-Mo interaction inhibits the Co sintering that typically occurs at the high temperatures required for the formation of carbon nanotubes. When large Co particles are present less desirable forms of carbon (MWCNT, fibers, and graphite) are produced. By contrast, when the Co clusters are small enough (i.e. <2 nm) only SWCNT are formed.