Cu₂O Cubes Decorated with Azine-Based Covalent Organic Framework Spheres and Pd Nanoparticles as Tandem Photocatalyst for Light-Driven Degradation of Chlorinated Biphenyls

Covalent organic frameworks (COFs) are promising candidates for heterogeneous photocatalytic reactions, though highly efficacious semiconductor–metal assemblies are often required to foster their photocatalytic performance. Herein, we report an efficient photocatalytic hybrid material that involves loading azine-based COF spheres onto Cu2O cubes and decorating them with palladium nanoparticles. The photocatalytic performance of the material was studied via the light-driven degradation of chlorinated biphenyls. The Cu2O-ACOF-1@Pd system demonstrated an outstanding performance over the bare Cu2O or ACOF-1, which can be attributed to the synergistic effect induced by the multicomponent tandem photocatalyst. It is shown that for monochlorinated biphenyls, the congener with a chlorine atom in the para position is more vulnerable to degradation than its meta and ortho counterparts because of electronic effects and being less sterically hindered. Moreover, the presence of a chlorine atom in the para position as an electron donor increases the conjugation between the phenyl rings, which in turn increases the driving force for planarity that facilitates the removal of the chlorine atom. This trend could be attributed to the reactivity of superoxide radicals toward the different congeners of monochlorinated biphenyls. The data revealed that nucleophilic substitution occurring at the para position is characterized by the lowest Gibbs free energy, while that occurring at the ortho position is characterized by the highest Gibbs free energy.