Nano-Cu Derived from a Copper Nitride Precatalyst for Reductive Coupling of Nitroaromatics to Azo Compounds.

The study of structural reconstruction is vital for the understanding of the real active sites in heterogeneous catalysis and guiding the improved catalyst design. Herein, we applied a copper nitride precatalyst in the nitroarene reductive coupling reaction and made a systematic investigation on the dynamic structural evolution behaviors and catalytic performance. This Cu3N precatalyst undergoes a rapid phase transition to nanostructured Cu with rich defective sites, which act as the actual catalytic sites for the coupling process. The nitride-derived defective Cu is very active and selective for azo formation, with 99.6% conversion of nitrobenzene and 97.1% selectivity to azobenzene obtained under mild reaction conditions. Density functional theory calculations suggest that the defective Cu sites play a role for the preferential adsorption of nitrosobenzene intermediates and significantly lowered the activation energy of the key coupling step. This work not only proposes a highly efficient noble-metal-free catalyst for nitroarenes coupling to valuable azo products but also may inspire more scientific interest in the study of the dynamic evolution of metal nitrides in different catalytic reactions.