Abstract
Four different types of structured nitrogen-doped carbon nanotube membranes (C50, NC25, NC50, N25C25) were used as metal-free catalysts for phenol degradation. Firstly, nitrogen-doped carbon nanotubes were prepared on paper-like sintered stainless steel fibers (PSSF) by chemical vapor deposition using melamine and acetylene as dual precursors, then the nitric acid was used to remove amorphous carbon and catalyst particles in the tubes. Subsequently, TEM, XPS, N2 adsorption and desorption and other characterization techniques were used to investigate the structure and morphology of the catalysts. Characterization results showed that the use of dual precursor could well adjust the nitrogen content in carbon nanotubes, and nitrogen doping could change the hollow structure of carbon nanotubes into rough bamboo-like and corrugated structure. Meanwhile, acid treatment significantly increased the specific surface area, among which the value of HN25C25 reached 38.15 m2/g. Pristine and acid-treated catalysts were then used to activate peroxymonosulfonate (PMS) in a structured fixed bed reactor. NC25 and N25C25 were able to mineralize 46% and 17.5% of phenol at 1 h, but the TOC conversion rate only maintained at about 10% in the subsequent reaction time. After acid treatment, HNC25 and HN25C25 were able to mineralize up to 76% and 84% of phenol, and the TOC conversion rate maintained at about 70% within 7 h. the high leaching of Fe in the acid-treated catalysts could be attributed to the formation of small carboxylic acids and the destruction of the PSSF support during the reaction. At last, radical inhibition experiments and ESR spectra showed that sulfate radicals and hydroxyl radicals were the main active species in the reaction. The increase in the proportion of active nitrogen species and the improvement in contact efficiency could explain the higher mineralization efficiency of phenol catalyst after acid treatment.
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