Degradation and mineralization of oxalic acid using catalytic wet oxidation over carbon coated ceramic monoliths

Abstract

Carbon coated ceramic monoliths were prepared by two strategies: (i) washcoating with N-doped carbon nanotubes (CNTs) using an active binder and (ii) chemical vapour deposition (CVD) growing carbon nanofibers (CNFs). In both cases the material was well adhered to the monoliths. These materials were tested in the catalytic wet oxidation of oxalic acid (1000 mg L−1) under continuous operation.For the washcoating method, the oxalic acid removal was not improved by adding the N-doped CNTs, most likely due to the CNTs coverage by the binder. However, a high temperature of the final thermal treatment (900 °C), was found to be important not only to achieve high oxalic acid conversions (over 55%), but also to improve the stability (less than 10% reduction in conversion in 24 h). Regarding the materials prepared by CVD, the dimensions of the CNFs were dependent on the presence of an alumina coating before their growth. The presence of NH3 during the carbon growth resulted in a steep decline in the amount of carbon formed, decreasing the activity of the monolith. However, the removal of oxalic acid per mass of carbon was much higher in this case, around 22 mg L−1 mgcarbon−1, due to the nitrogen groups.The highest removal was reached with the monolith produced by washcoating without the N-doped CNTs, thermally treated at 900 °C, achieving over 55% conversion of oxalic acid for 48 h on stream at 140 °C and 8 bar of oxygen partial pressure.