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The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.

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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.

Nitrogen-doped graphene-based materials were prepared by the modified Hummers method using natural graphite as primary precursor, followed by chemical and thermal reduction processes, and finally ball milled with urea or melamine. The graphene-based materials were characterized at different stages of their synthesis by different techniques (including temperature programmed desorption and X-ray photoelectron spectroscopy) and then tested as metal-free catalysts in the degradation of oxalic acid and phenol by two different oxidation processes: catalytic wet air oxidation (temperature between 413 and 433 K, and 7 bar of O 2 ) and catalytic ozonation (room temperature and atmospheric pressure). The melamine treated sample was always found to be more active due to the presence of nitrogen groups and adequate surface area available.

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