Preparation Of Micro-fibrous Nitrogen-doped Carbon Nanotubes Coated Paper-like Sintered Stainless Steel Fibers Composite Catalyst For Catalytic Degradation Of Phenolic Wastewater

Phenolic wastewater comes from a wide range of sources,which has high toxicity and poor biodegradability,and has become one of the focal points of current environmental protection.Therefore,developing highly efficient and cost-effective degradation technology for phenolic wastewater treatment has important guiding significance.In recent years,Advanced Oxidation Processes（AOPs）based on peroxides activation have become a hot topic for organic wastewater treatment,due to its high degradation efficiency and easy operation,while the emphasis is on the development of efficient catalysts.Nano-carbon materials are promising catalysts in AOPs,which can be equally effective to that of metal catalysts.However,generally used powdered nano-carbon catalysts are not suitable for the industrial flow system,and are prone to raising larger pressure drop and causing catalyst loss.Microfiber-based composite materials have high voidage,can effectively reduce the bed resistance and enhance mass transfer.Therefore,combining nano-carbon and micro-fibrous composite materials to design structured catalysts with high porosity and mechanical strength,so as to effectively enhance mass transfer and improve contact efficiency,is a cutting-edge topic with application value.Firstly,the N-CNTs/PSSF composite catalyst was synthesized using melamine as sole precursor through chemical vapor deposition method,then tested for catalytic wet peroxide oxidation（CWPO）of phenol in a fixed-bed reactor.A monolithic paper-like sintered stainless steel fibers（PSSF）was synthesized by wet papermaking and sintering processes,then nitrogen-doped carbon nanotube films（N-CNTs）were grown by CVD on the surface of PSSF to fabricate N-CNTs/PSSF composite catalyst.Effects of synthesis temperature,melamine dosage and carrier gas flow rate on the growth of N-CNTs films were studied by using SEM,TEM,XRD,XPS,N₂adsorption-desorption,and other characterization techniques.The specific surface area of the composite catalyst reached 14.5 m²/g,which was 50 times higher than that of the bare support,and the surface nitrogen content was 5.18 at.%under optimal synthesis conditions.Typically,N-CNTs exhibited unique Fe₃C nanoparticles encapsulated structure.The catalytic results showed that the encapsulated Fe₃C nanoparticles promoted the CWPO of phenol,and the reaction temperature significantly improved the degradation efficiency.However,the change of fixed bed height and the feed flow rate showed an unusual fluctuation trend due to the unique polar surface of N-CNTs,which resulted in the change of the contact efficiency between pollutants and oxidizing active radicals.The repeated use stability tests showed that the degradation efficiency of phenol increased with the number of cycles,and 90%phenol conversion and 41%TOC conversion can be achieved after fourth use.Long-term stability tests showed that the N-CNTs/PSSF has higher catalytic stability than CNTs/PSSF and Fe-CNTs/PSSF composite catalyst,maintaining 90%phenol conversion and 45%TOC conversion after 100 h reaction.Studies on the mechanism of deactivation indicated that the strong polar surface of N-CNTs reduced the blocking of the active sites by the intermediate product due to the preferential adsorption of H₂O₂,thus improved its catalytic stability.Secondly,the nitrogen doping content of N-CNTs/PSSF composite catalyst was adjusted by using melamine and acetylene as double precursors.The composite catalyst was used to activate peroxymonosulfate（PMS）for phenol degradation.Firstly,fully doped and semi-doped composite catalysts were synthesized by changing the order and the amounts of precursors used.The surface characteristics of catalysts before and after acid treatment were characterized.The characterization results showed that the full doping process mainly resulted in the formation of N-CNTs,in which increasing the amount of acetylene would reduce the nitrogen doping content.While both the N-CNTs and CNTs structures were formed by the semi-doping process.The acid treatment could remove impurities such as amorphous carbon and encapsulated catalyst particles formed during the growth of carbon nanotubes,improving the specific surface area of the catalysts,and the surface nitrogen doped content did not decrease significantly.In the fully doped process,the maximum nitrogen doping content was 4.94 at.%.Catalytic results showed that the increase of nitrogen doping content of composite catalysts improved the activating efficiency of PMS for phenol degradation.While higher nitrogen doped content lead to higher Fe leaching,relating to the loss of encapsulated catalyst particles of N-CNTs.HPLC analysis showed that the degradation products were mainly small-molecule carboxylic acids with low toxicity,and the aromatic intermediates were almost completely converted.