Photocatalytic reduction of nitrobenzene to aniline over CdS nanorods: the impacts of reaction conditions and the hydrogenation mechanism

Abstract

Photocatalysis provides a safe and green protocol for the reduction of nitroaromatics to anilines. However, a systematic exploration of the conversion under different reaction conditions is still highly required. To enrich the understanding of the conversion, noble metal (Au, Pd, Pt, and Ru) modified CdS nanorods were fabricated as photocatalyst prototypes and then used for the reduction of the typical nitrobenzene (NB) to aniline (AN). The effects of various reaction conditions on the conversion of NB and the formation of nitrosobenzene (NSB), N-phenylhydroxylamine (PHA), azoxybenzene (AOB), azobenzene (AB), hydrazobenzene (HAB), and AN were investigated systematically. The hydrogenation mechanism was finally proposed based on the formation of reactive species and the evolution of the intermediates and AN. The results indicate that an anaerobic atmosphere with ethanol as solvent and AF as the hydrogen source is crucial for the conversion. Noble metals can promote the reduction except for Pt and Pd shows the highest performance. The conversion is triggered by CO₂˙⁻ and photoinduced e⁻via a stepwise hydrogenation route (NB → NSB → PHA → AN) accounting for 10% AN yield and an indirect route (AOB → AB → AN) for the remaining 90% AN. The reductions of NSB and AOB are the rate-determining steps. Although NB can be fully converted, the maximum yield of AN is only ca. 77% as an equilibrium state is finally reached between AN, NSB, and AB. We believe that this work will enrich the fundamental understanding of the photocatalytic reduction of nitroaromatics to anilines and provide a valuable reference for their scaling-up implementation and the optimization of the reaction conditions.