Carboxyl groups trigger the activity of carbon nanotube catalysts for the oxygen reduction reaction and agar conversion

Ozone treatment is a common way to functionalize commercial multi-walled carbon nanotubes （CNTs） with various oxygen functionalities like carboxyl, phenol and lactone groups, in order to enhance their textural properties and chemical activity. In order to detail the effect of each functional group, we correlated the activity with the surface density of each group, and found that the carboxyl groups play a pivotal role in two important catalytic reactions, namely the electrochemical oxygen reduction reaction （ORR） and agar conversion to 5-hydroxymethylfurfural （HMF）. During the processes, the hydrophilic surface provides a strong affinity for reaction substrates while the improved porosity allows the efficient diffusion of reactants and products. Furthermore, the activity of functionalized CNTs for agar conversion remained almost unchanged during nine cycles of reaction. This work highlights a strategy for improving the activity of CNTs for electrochemical ORR and agar conversion reactions, as well a promising application of carboxyl-rich CNTs as a solid acid catalyst to produce high-purity HMF--an important chemical intermediate.