Catalytic activity and stability of multiwalled carbon nanotubes in catalytic wet air oxidation of oxalic acid: The role of the basic nature induced by the surface chemistry

Different chemical and thermal treatments (nitric acid at boiling temperature, liquid-phase urea treatment at 200 °C, and gas-phase thermal treatment with nitrogen at 600 °C) were applied to multiwalled carbon nanotubes (MWCNTs) in order to produce materials with different textural and chemical properties. Nitrogen adsorption isotherms, temperature programmed desorption, pHpzc (point of zero charge), elemental analysis and X-ray photoelectron spectroscopy were used to characterize these materials. The chemical and thermal treatments have influence on the materials pHpzc, which decreases upon nitric acid treatment and increases with urea or gas-phase thermal treatments. The original and modified MWCNTs, without any impregnated metal, were investigated as catalysts in the catalytic wet air oxidation (CWAO) process, using oxalic acid as model compound at 140 °C and 40 bar of total pressure. Oxalic acid is very stable under non-catalytic conditions but can be totally degraded in less than 30 min in the presence of MWCNTs. The rate of oxidation of oxalic acid depends on the chemical properties of MWCNTs, the apparent initial first-order rate constants being lower for the MWCNTs with a marked acid character. The textural properties of MWCNTs are stable in cyclic CWAO experiments, but a decrease of their basic character leads to the reduction of their catalytic activity, even if this activity is still high with reused catalysts (nearly total oxalic acid degradation in 45 min and complete mineralization in 120 min). Therefore, MWCNTs without any impregnated metal, especially those of more basic character, are very active catalysts for CWAO, their activity strongly depending on the stability of their surface chemistry.

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► MWCNTs without any impregnated metal are good catalysts for CWAO.
► Oxalic acid can be totally degraded at 140 °C in less than 30 min.
► The catalytic activity of MWCNTs can be enhanced by increasing their basic sites.