Adsorption-Dependent H₂O Effect for Nitrobenzene Hydrogenation to Aniline over Pt and Ni Catalysts

Liquid-phase hydrogenation of nitrobenzene (NB) over supported metal catalysts has been identified as one of the most effective ways to produce aniline as an important building block. However, the influence of H2O, either from the byproduct or the solvent, has not been well addressed. Herein, we demonstrate that the effect of H2O on NB hydrogenation depends on the metal type and the H2O state. On Pt/SiO2, vapor H2O accelerates the reaction rate, while liquid H2O restrains the reaction. By contrast, H2O promotes the reaction on Ni/SiO2 at any H2O state. Through kinetics analysis and probe reaction of benzene hydrogenation, it reveals that although H2O lowers the hydrogenation barrier on both Pt and Ni catalysts at any H2O state, the different adsorption preference of NB on the two metals and the competitive adsorption between H2O and NB would determine the activity trend at liquid H2O conditions. The Pt surface favors parallel adsorption configuration with phenyl ring interaction while the nitro group tilts away from the surface. The liquid H2O limits the hydrophobic phenyl ring access to the Pt surface, which should be one key reason for the detrimental effect of H2O on NB hydrogenation on Pt catalyst. On the other hand, the oxophilic Ni prefers vertical adsorption geometry with nitro bonding to Ni atoms. The competitive adsorption between nitro and H2O is less significant; as a result, no inhibition in reactivity is observed on Ni catalyst.