Abstract
The N, S-co-doping of commercial carbon nanotubes (CNTs) was performed by a solvent-free mechanothermal approach using thiourea. CNTs were mixed with the N, S-dual precursor in a ball-milling apparatus, and further thermally treated under inert atmosphere between 600 and 1000 °C. The influence of the temperature applied during the thermal procedure was investigated. Textural properties of the materials were not significantly affected either by the mechanical step or by the heating phase. Concerning surface chemistry, the developed methodology allowed the incorporation of N (up to 1.43%) and S (up to 1.3%), distributed by pyridinic (N6), pyrrolic (N5), and quaternary N (NQ) groups, and C–S–, C–S–O, and sulphate functionalities. Catalytic activities of the N, S-doped CNTs were evaluated for the catalytic wet air oxidation (CWAO) of phenol in a batch mode. Although the samples revealed a similar catalytic activity for phenol degradation, a higher total organic carbon removal (60%) was observed using the sample thermally treated at 900 °C. The improved catalytic activity of this sample was attributed to the presence of N6, NQ, and thiophenic groups. This sample was further tested in the oxidation of phenol under a continuous mode, at around 30% of conversion being achieved in the steady-state.
Highlights
Wet air oxidation (WAO) is an alternative to conventional technologies for the treatment of organic pollutants in wastewaters, especially from highly pollutant industries such as pulp and paper industries, petrochemicals, and wine distilleries [1,2]
The process is suitable to treat organic compounds that show some resistance to conventional treatment technologies or to treat effluents presenting concentrations too high to be treated by biological processes or too low for incineration. It is classified as an advanced oxidation process (AOP), since it is based on the production and use of strongly reactive radicals to mineralise the organic compounds into CO2 and H2 O or into biodegradable by-products
The slight increase may be due to the opening of the carbon nanotubes (CNTs) closed tips during ball milling, commonly observed when CNTs and powder precursors are mixed under ball milling [37]
Summary
Wet air oxidation (WAO) is an alternative to conventional technologies for the treatment of organic pollutants in wastewaters, especially from highly pollutant industries such as pulp and paper industries, petrochemicals, and wine distilleries [1,2]. The process is suitable to treat organic compounds that show some resistance to conventional treatment technologies or to treat effluents presenting concentrations too high to be treated by biological processes or too low for incineration It is classified as an advanced oxidation process (AOP), since it is based on the production and use of strongly reactive radicals to mineralise the organic compounds into CO2 and H2 O or into biodegradable by-products. The scarcity of these materials, and the frequent leaching and deactivation phenomena, have prevented the design of a clean catalytic wet air oxidation (CWAO) process [7,14] During some of these studies, carbon materials were tested as supports for these metals and oxides, and it was found that the carbon material could act as a catalyst on its own, and as a support. The outstanding ability to tune the textural and chemical properties of carbon materials make them potential catalyst candidates for a sustainable technology
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