This work is a result of project “AIProcMat@N2020 - Advanced
Industrial Processes and Materials for a Sustainable Northern Region of
Portugal 2020”, with the reference NORTE-01-0145-FEDER-000006,
supported by NORTE 2020, under the Portugal 2020 Partnership
Agreement, through the ERDF and of Project Associate Laboratory
LSRE-LCM - UID/EQU/50020/2019 - funded by national funds through
FCT/MCTES (PIDDAC). M. Martín Martínez acknowledges the postdoc
grant 2017-T2/AMB-5668 from Comunidad de Madrid, Programme “Atracción de talento investigador”. B. Machado acknowledges the
exploratory project under FCT Investigator Programme (ref. IF/00301/
2015) with the financial support of FCT/MCTES through national funds
(PIDDAC). S. Morales Torres acknowledges the financial support from
University of Granada (Reincorporación Plan Propio).
Three magnetic carbon nanotube (CNT) samples, named A30 (N-doped), E30 (undoped) and E10A20 (selectively
N-doped), synthesized by catalytic chemical vapor deposition, were modified by introducing oxygenated
surface groups (oxidation with HNO3, samples CNT-N), and by heat treatment at 800 °C for the removal of
surface functionalities (samples CNT-HT). Both treatments lead to higher specific surface areas. The acid
treatment results in more acidic surfaces, with higher amounts of oxygenated species being introduced on Ndoped
surfaces. Heat-treated samples are less hydrophilic than those treated with nitric acid, heat treatment
leading to neutral or basic surfaces, only N-quaternary and N-pyridinic species being found by XPS on N-doped
surfaces. These materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated
4-nitrophenol solutions (4-NP, 5 g L−1) at atmospheric pressure, T=50 °C and 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. The high
temperature treatment enhanced significantly the activity of the CNTs towards CWPO, evaluated in terms of
4-NP and total organic carbon conversion, due to the increased hydrophobicity of their surface. In particular,
E30HT and E10A20HT were able to remove ca. 100% of 4-NP after 8 h of operation. On the other hand, by
treating the CNTs with HNO3, the activity of the less hydrophilic samples decreased upon increasing the
concentration of surface oxygen-containing functionalities, whilst the reactivity generated inside the opened
nanotubes improved the activity of the highly hydrophilic A30 N.
