Pseudosolid, shear-thinning gel formation in binary dispersions of metal oxide nanoparticles at low volume fractions.

Hydroxyl groups on the surface of metal oxide nanoparticles (nps) can be protonated or deprotonated depending on solution pH, changing both the magnitude and sign of the nps' surface charge. Binary mixtures of fumed metal oxide nps, dispersed in water at a solution pH where one species is positively charged and the other is negatively charged, form pseudosolid gels at volume fractions as low as 1.5 vol %. This work maps out regions of gel formation for binary mixtures of silica and alumina nps, silica and titania nps, and alumina and titania nps. The microscopic structure of these gels is investigated using transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), acoustic spectroscopy, and light microscopy.

Kinetic modeling of the SWNT growth by CO disproportionation on CoMo catalysts.

A kinetic model has been developed to describe the growth of single-walled carbon nanotubes (SWNT) in the CoMoCAT method, which is based on the disproportionation of CO on supported CoMo catalysts. The model attempts to capture mathematically the different stages involved in this method: (i) catalyst activation or in-situ creation of active sites, i.e., reduced Co clusters by transformation of CoMoOx precursor species, or oxidized sites; (ii) CO decomposition over active sites, which increases the surface fugacity of carbon until reaching a certain threshold; (iii) nucleation of ordered forms of carbon; (iv) C diffusion (both across the surface and into the metal particle); (v) SWNT growth; (vi) termination, by either deactivation of the catalyst active sites or by increase in the carbon concentration at the metal/SWNT interface, approaching that of the metal/gas interface and eliminating the driving force for diffusion. Previous investigations have only explained the growth termination by the former. Here, we emphasize the possible contribution of the later and propose a novel "hindrance factor" to quantify the effect of nanotube interaction with its surroundings on the growth termination. To test the kinetic model and obtain typical values of the physical parameters, experiments have been conducted on a CoMo/SiO2 catalyst in a laboratory flow reactor, in which the rate of carbon deposition was continuously evaluated by the direct measurement of the CO2 evolution as a function of time. The experimental data are fitted very well with model.

Raman characterization of single-walled nanotubes of various diameters obtained by catalytic disproportionation of CO.

Single-walled carbon nanotubes prepared by disproportionation of CO over Co-Mo/SiO2 catalysts have been characterized by Raman spectroscopy, using several excitation energies. By varying the reaction temperature, different ranges of nanotube diameter were obtained. The average diameter of a single-walled nanotube produced at 750 degrees C was 0.9 nm, while it increased up to about 1.5 nm when the synthesis was conducted at 950 degrees C. The analysis of the Raman spectra obtained with a range of laser excitation energies not only gives a definite description of the single-walled nanotubes diameters but also helps differentiate the metallic or semiconducting character of the samples. This analysis can be done by comparing the experimental data with calculated gap energies as a function of nanotube diameter as well as comparing the relative intensity of bands centered at 50-60 cm-1 lower than the tangential G mode. The analysis of this feature, which can be fitted with a Breit-Wigner-Fano line, offers a method for distinguishing between metallic and semiconducting single-walled carbon nanotubes.