Chemoselective Hydrogenation of Nitroaromatics by Supported Gold Catalysts: Mechanistic Reasons of Size- and Support-Dependent Activity and Selectivity

Supported Au nanoparticles (NPs) prepared by colloid deposition method were well characterized, and their catalytic performance was tested for chemoselective reduction of a nitro group of substituted nitroaromatics by H2. Systematic studies on the effects of NPs size and support show small size of Au NPs, and acid-base sites of supports are required for high activity. The Au/Al2O3 catalyst with Au particle size of 2.5 nm selectively hydrogenates a nitro group in the presence of various other reducible functional groups, and it shows higher intrinsic activity than the state-of-the-art catalyst (Au NPs on TiO2). In situ FTIR studies provide a reaction mechanism, which explains fundamental reasons of the observed structure−activity relationship. Cooperation of the acid−base pair site on Al2O3 and the coordinatively unsaturated Au atoms on the Au NPs are responsible for the H2 dissociation to yield a H+/H− pair at the metal/support interface. High chemoselectivity could be attributed to a preferential transfer of the H+/H− pair to the polar bonds in the nitro group as well as a preferential adsorption of nitroaromatics on the catalyst through the nitro group.