Insights into the mechanism of nitrobenzene reduction to aniline over Pt catalyst and the significance of the adsorption of phenyl group on kinetics

Aniline (C₆H₅NH₂) plays a significant role in both industry and daily life, and can be synthesized via catalytic hydrogenation of nitrobenzene (C₆H₅NO₂) over transition metals; however fundamental investigations on reaction mechanisms in the heterogeneous catalysis are still lacking. In this work, the nitrobenzene reduction reaction over the Pt(111) model catalyst was studied using density functional theory (DFT) with the inclusion of van der Waals interaction, for fundamentally understanding the mechanisms at atomic and molecular levels. It was found that the double H-induced dissociation of N-O bond was the preferential path for the activation of nitro group, having a much lower reaction barrier than that of the direct dissociation and single H-induced dissociation paths. The overall mechanisms have been identified as: C₆H₅NO₂* → C₆H₅NOOH* → C₆H₅N(OH)₂* → C₆H₅NOH* → C₆H₅NHOH* → C₆H₅NH* → C₆H₅NH₂*. The overall barrier of the nitro group reduction was calculated to be 0.75 eV, which is much lower than that of the benzene reduction (1.08 eV). Our DFT data elucidates clearly the reason why the major product of nitrobenzene reduction reaction was aniline. Furthermore, the adsorption/desorption of phenyl group was found to have significant impacts on kinetic barriers. Generally, in the hydrogenation process (N-H or O-H bond association), the phenyl group preferred to adsorb on the surface; but in the dissociation process (N-O bond dissociation) it preferred to desorb transiently at the transition state and to adsorb again when the dissociation was completed. This study also provides a solid theoretical insight into the selective catalysis of the large aromatic compounds.