Nitrogen-doped graphene-based materials for advanced oxidation processes

Abstract

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.

Tuning CNT Properties for Metal-Free Environmental Catalytic Applications

“…The obtained results were very similar to those obtained in the absence of the radical scavenger, suggesting that hydroxyl radicals in solution were not the relevant species involved in the oxidation mechanism, and that oxalic acid oxidation should be mainly promoted by active oxygen species on the CNT surface produced from the decomposition of oxygen. When other N-doped carbon materials were tested in CWAO (carbon xerogels [85] and graphene based materials [51]) similar evidences were observed, suggesting again that hydroxyl radicals in solution are not in fact the main species involved in the oxidation mechanism, and that oxalic acid conversion by CWAO can occur by means of surface active species produced from the decomposition of oxygen. The enhanced catalytic activity observed in the presence of N-functionalities seems to result from the interaction of oxygen with the carbon surface [88].…”

“…Such active oxygen species can react with adsorbed organic compounds leading to the oxidation of the organic pollutants. Therefore, in the presence of N-functionalities on the carbon surface, hydroxyl radicals in the liquid phase may not be required, suggesting that the oxidation reaction can occur by an alternative surface reaction mechanism, possibly as described above [51].…”

“…Although this is well accepted in literature for COZ, in the case of CWAO there is a controversy regarding the active sites on carbon nanotubes (i.e., sites of basic or acidic nature) required to achieve the best performance. The catalytic activity of nanocarbons successfully functionalized with different heteroatoms has been investigated in CWAO [31,38,52,56,[70][71][72][73] and in COZ [26,51,56,[67][68][69][74][75][76][77][78] by several authors. O-, N-and S-doped CNTs have been studied as catalysts in both processes.…”

“…The modification of metal-free carbon materials by tailoring textural and surface chemical properties can play an important role in their catalytic performance; in particular, N-doping was demonstrated to increase the activity of carbon catalysts in oxidation reactions [11,31,51,52,56,85,86]. Regarding the use of N-doped CNTs in oxalic acid oxidation by CWAO, CNTs modified with nitric acid, urea and gas-phase thermal treatment showed the highest basic character and also the highest catalytic activity, but the performance is similar to that observed with the original CNTs after cyclic runs [31].…”

“…Recently, an easy to handle method to prepare N-doped carbon nanotubes [50] and also N-doped graphene oxide (GO) was developed [51], involving ball milling, followed by thermal treatment under N 2 up to 600˝C, avoiding the use of solvents and production of wastes. Incorporation of large amounts of N-groups (pyridine, N-6; pyrrole, N-5, and quaternary nitrogen, N-Q) is especially obtained using melamine as N-precursor by this method (see Figure 5).…”