| Nano-carbons show great promise as high-performance materials because of their unique electronic structure and abundant surface functional groups. Their applications as metal-free catalysts have attracted extensive attentions in recent years. The catalytic wet air oxidation(CWAO) is an efficient technique for the treatment of waste water containing refractory organics, such as phenols, cyanides and pesticides. Searching for appropriate environmental catalysts without secondary pollution during reaction is one of the key issues for the application of CWAO technique. In this work, carbon nanotubes(CNTs) were studied as a metal-free catalyst in CWAO of the typical phenol-containing wastewater. The impacts of oxidative modification and nitrogen doping of CNTs on CAWO activity, as well as the underlying reaction mechanism were discussed in detail.The liquid-phase oxidative modification with nitric acid was used to introduce oxygen groups on the surfaces of CNTs. It was found that oxygen groups, especially carboxyl groups, played an important role in CWAO of phenol. The higher the carboxyl group content is, the higher the reaction activity will be.It was also found that the catalytic function of carboxyl groups strongly depended on the structure of CNTs. CNTs with cylindricalgraphitic walls displayed the highest catalytic activity after nitric acid oxidation, due to the carboxylated carbonaceous fragments coating on its surface.For herringbone CNTs, carboxyl groups are mostly linked to its sidewalls, where abundant edge carbons are exposed. In the case of modified herringbone CNTs, the synergistic effect of carboxyl groups and adjacent edge carbons is responsible for the activation of O2, leading to the higher CWAO activity. N-doping can improve the activity of CNTs. Moreover, the modification with nitric acid further enhanced the activity of N-CNTs. It was proposed that the CWAO activity of oxidized N-CNTs can be attributed to the carboxyl groups and the graphitic nitrogen sites, which may activate O2.Despite the highest activity of HNO3 oxidized CNTs, the complex oxidation procedures with strong acid result in environmental risks caused by the secondary acid pollution. To overcome this, N-CNTs were explored as catalysts for CWAO because of their high activity without any post-treatment. Five N-CNTs were manufactured in a pilot-scale tube furnacewith aniline, ethylenediamineand cyclohexane as precursors and in NH3 or N2 atmospheres. It was discovered that thecontent of graphitic nitrogen is relevant to the CWAO activity, indicating the catalytic role of the graphitic sites. A parametric study on the CWAO reaction was carried out over the selected N-CNTs prepared from aniline in NH3. It was determined that phenol can be completely degraded and 75.6% TOC removal can be reached after 3.5h reactionunder the conditions: the oxygen pressure of 4MPa, reaction temperature of 180 ℃ and with 80 mg catalyst loading. |
