Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

TLDR

Graphitic carbon nitride (g‑C3N4) is synthesized by polymerizing cyanamide, dicyandiamide, or melamine, and varying reaction conditions yields materials with different condensation levels, properties, and nanostructures such as nanoparticles or mesoporous powders that can be tuned for heterogeneous reactions. Model calculations are presented to explain this unusual case of heterogeneous, metal‑free catalysis. High‑resolution TEM confirms the extended two‑dimensional structure of g‑C3N4, which exhibits unexpected metal‑free catalytic activity for benzene activation, trimerization, and CO₂ activation, and can also serve as a heterogeneous reactant to produce new metal‑nitride nanostructures from oxides.

Abstract

Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.