Electrochemical Hydrogenative Coupling of Nitrobenzene into Azobenzene over Mesoporous Palladium-Sulfur Cathode

Abstract

Azobenzene (AZO) and its derivatives are of great importance in dyestuff and pharmaceutical industry, but their sustainable synthesis is seriously slower than expected due to the lack of high-performance catalysts. In this work, we reported a robust yet highly efficient catalyst of PdS mesoporous nanospheres (MNSs) with confined mesostructure and binary elemental composition that achieved sustainable electrosynthesis of value-added AZO by selective hydrogenative coupling of nitrobenzene (NB) feedstocks in H2O under ambient conditions. With renewable electricity and H2O, binary PdS MNSs disclosed remarkable NB conversion of 95.4 %, impressive AZO selectivity of 93.4 %, and good cycling stability in selective NB hydrogenation reaction (NBHR) electrocatalysis. Detailed mechanism studies revealed that confined mesoporous microenvironment of PdS MNSs facilitated the hydrogenative coupling of key intermediates (nitrosobenzene and phenylhydroxylamine) into AZO and/or azoxybenzene (AOB), while its electron-deficient S sites stabilized the Pd-spillovered active H* and inhibited the over-hydrogenation of AZO/AOB into AN. By coupling with anodic methanol oxidation reaction (MOR), the (-)NBHR || MOR(+) two-electrode system hold much better NB-to-AZO performance in a sustainable and energy-saving manner. This work thus paved the way for designing functional mesoporous metal alloy electrocatalysts applied in the sustainable electrosynthesis of industrially value-added chemicals.