Graphene- and CNTs-based carbocatalysts in persulfates activation: Material design and catalytic mechanisms

This article aims to present a state-of-art critical review on the recent practical and theoretical development in graphene and carbon nanotubes (CNTs)-based materials as carbocatalysts for the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS). The introduction of heteroatoms can modify the catalytic activity of carbocatalysts. The possible doping elements, the various doping and characterization methods, along with the corresponding doping level and effective bonding configurations are discussed. Radical and non-radical pathways are evaluated in terms of 1) comparison between the characteristics of radical and non-radical processes; and 2) the decisive factors to induce radical and non-radical pathway, namely, the intrinsically structural and chemical complexity of carbocatalysts, the intrinsic difference between PDS and PMS, and substrate-specificity. The possibly involved reaction mechanisms in the carbocatalysts/persulfates system are discussed with respect to the adsorption performance and persulfates activation by graphene- and CNTs-based carbocatalysts. The employment of density functional theory (DFT) for the investigation and prediction of the structural and electronic properties of 2D carbocatalysts, active sites and possible reaction mechanisms in the persulfates/carbocatalyst system are discussed. Identification of reactive oxygen species (ROS) generated in the radical and non-radical processes is addressed based on electrochemical methods (e.g., chronoamperometry, linear sweep voltammetry (LSV)) aside from the use of electron paramagnetic resonance (EPR), chemical probes and radical scavengers. Challenges with respect to poor catalyst stability, environmental concerns, and high cost of production are identified. Prospects of the application of graphene- and CNTs-based carbocatalysts for persulfates application are also proposed.