A new molecular pathway allows the chemoselective reduction of nitroaromatics on non-noble metal catalysts

At difference with noble metals, the oxophylic character of non-noble metals strongly facilitates the rupture of the NO bonds in nitrobenzene, yielding nitrosobenzene as primary reaction intermediate. By combining periodic DFT calculations and kinetic studies, a direct pathway involving successive dissociation of NO bonds followed by two hydrogenation steps, Ph-NO2 → Ph-NO → Ph-N → Ph-NH → Ph-NH2, has been found as most favorable on Ni catalysts. The rate determining step of the global process is the hydrogen transfer to adsorbed Ph-N intermediate. The catalyst surface becomes partly oxidized during reaction, which favors the vertical adsorption of the nitroaromatic compounds and enhances selectivity, while total surface oxidation leads to catalyst deactivation. It is proposed that both catalytic activity and selectivity of Ni and, possibly, other non-noble metals can be tuned by controlling the degree of oxidation of the metal surface.