The influence of ball-milling in the texture and surface chemistry of multi-walled carbon nanotubes (MWCNT) was studied in this work. Treatment times up to 360 min at constant frequency (15 vibrations/s) and frequencies from 10 to 20 vibrations/s during 30 min were used for the preparation of the modified samples. These were characterized by nitrogen adsorption at-196 ºC, temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The milled samples were used as catalysts for the ozonation of oxalic acid. The surface area of the MWCNT increases, whereas the particle size decreases with the ball-milling time until 240 min at 15 vibrations/s. The functionalization of MWCNT surface is not achieved by ball-milling under the conditions used. The catalytic performance of the ball-milled samples for oxalic acid mineralization increased significantly when compared to the unmilled MWCNT. Therefore, ball-milling is an effective and simple method to increase the surface area of commercial carbon nanotubes without significant changes of their structural properties, and, consequently, this method allows increasing their catalytic performance in ozonation processes.

Low-temperature (<200 °C) hydrocarbon selective catalytic reduction of NO x has been achieved for the first time in the absence of hydrogen using a solvent-free mechanochemically prepared Ag/Al2O3 catalyst. Catalysts prepared by this ball-milling method show a remarkable increase in activity for the reduction of nitrogen oxides with octane by lowering the light-off temperature by up to 150 °C compared with a state-of-the-art 2 wt % Ag/Al2O3 catalyst prepared by wet impregnation. The best catalyst prepared from silver oxide showed 50% NO x conversion at 240 °C and 99% at 302 °C. The increased activity is not due to an increased surface area of the support, but may be associated with a change in the defect structure of the alumina surface, leading to the formation of the small silver clusters necessary for the activation of the octane without leading to total combustion. On the other hand, since one possible role of hydrogen is to remove inhibiting species from the silver, we cannot exclude some change in the chemical properties of the silver as a result of the ball-milling treatment.

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