A Dual Catalyst with SERS Activity for Probing Stepwise Reduction and Oxidation Reactions

Aromatic azo compounds are high-value chemicals extensively used as pigments, drugs, and food additives, but their production typically requires stoichiometric amounts of environmentally unfriendly metals or nitrites. There is an urgent need to develop a dual catalytic system capable of reducing nitroaromatics to aromatic amines, followed by their oxidation to azo compounds. Here we report such a dual catalyst based on Ag@Pd-Ag core-frame nanocubes for the stepwise conversion of 4-nitrothiophenol to trans-4,4′-dimercaptoazobenzene under ambient conditions. Our in situ surface-enhanced Raman spectroscopy study reveals three sequential processes that include the Pd-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol by hydrogen, a period during which the 4-aminothiophenol remain unchanged until all hydrogen has depleted, and the Ag-catalyzed oxidation of 4-aminothiophenol to trans-4,4′-dimercaptoazobenzene by the O₂ from air. This work will lead to an environmentally friendly and sustainable approach to the production of aromatic azo compounds.