Role of Nitrogen Doping on the Performance of Carbon Nanotube Catalysts: A Catalytic Wet Peroxide Oxidation Application

• Published in: ChemCatChem Vol. 8; no. 12; pp. 2068 - 2078
• Main Authors: Martin-Martinez, Maria, Ribeiro, Rui S., Machado, Bruno F., Serp, Philippe, Morales-Torres, Sergio, Silva, Adrián M. T., Figueiredo, José L., Faria, Joaquim L., Gomes, Helder T.
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 21.06.2016; Wiley Subscription Services, Inc; Wiley
• Subjects: Chemical Sciences; Coordination chemistry; doping; environmental chemistry; heterogeneous catalysis; nanotubes; nitrogen; oxidation
• ISSN: 1867-3880; 1867-3899; 1867-3899
• DOI: 10.1002/cctc.201600123

Four magnetic carbon nanotube (CNT) samples (undoped, completely N‐doped, and two selectively N‐doped) were synthesized by chemical vapor deposition. The materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4‐nitrophenol solutions (4‐NP, 5 g L−1). Relatively mild operating conditions were considered (atmospheric pressure, T=50 °C, pH 3), using a catalyst load of 2.5 g L−1 and the stoichiometric amount of H2O2 needed for the complete mineralization of 4‐NP. N‐doping was identified to influence considerably the CWPO performance of the materials. In particular, undoped CNTs, with a moderate hydrophobicity, favor the controllable and efficient decomposition of H2O2 into highly reactive hydroxyl radicals (HO.), thus showing high catalytic activity for 4‐NP degradation. On the other hand, the completely N‐doped catalyst, fully hydrophilic, favors a quick decomposition of H2O2 into nonreactive O2 and H2O species. The selectively N‐doped amphiphilic catalysts, that is, hybrid structures containing undoped sections followed by N‐doped ones, provided intermediate results, namely, a higher N content favored H2O2 decomposition towards nonreactive H2O and O2 species, whereas a lower N content resulted in the formation of HO., increasing 4‐NP mineralization. Catalyst stability and reusability were also investigated by consecutive CWPO runs. Watch out for nitrogen: N groups enhance the catalyst activity for H2O2 decomposition into HO. radicals, but completely N‐doped carbon nanotubes promote HO. recombination into nonreactive O2 and H2O species. Less selectively N‐doped nanotubes take advantage of a rapid H2O2 decomposition achieving high 4‐nitrophenol (4‐NP) removal.