Catalytic wet oxidation of organic compounds over N-doped carbon nanotubes in batch and continuous operation

Abstract

Multi-walled carbon nanotubes (MWCNTs) were treated by ball milling with and without a nitrogen precursor (melamine) to introduce nitrogen functionalities. These materials were tested as catalysts in batch and continuous catalytic wet oxidation (CWO) experiments for the degradation of phenol in aqueous solution. The influence of several reaction parameters (temperature, dissolved oxygen concentration and initial phenol concentration) were studied in both operating modes. Temperature had a more significant effect on the reaction rate than the dissolved oxygen concentration, an optimal temperature of 160°C being determined. The experiments with different initial phenol concentrations showed that higher amounts of phenol and carbon contents were removed when the initial concentration of phenol was increased. MWCNTs with N-groups showed high catalytic activity for phenol removal in both operation modes, as also observed when performing some experiments with oxalic acid. For instance, complete phenol degradation and 50% of total organic carbon (TOC) removal were achieved after 2h in batch operation at 160°C and 6bar of oxygen partial pressure with the N-doped catalyst (against 68% and 50%, respectively, with the undoped sample). In the continuous mode reactor, 80% and 50% of phenol and TOC removals were observed at the steady state with the N-doped catalyst, in this case at 160°C and 12bar of oxygen partial pressure with an initial phenol concentration of 500mgL−1. A significant regeneration of the N-doped catalyst was achieved by thermal treatment at 600°C under nitrogen atmosphere.