Noble Metal-Coated Cu Nanoparticle Catalysts for Selective Hydrogenation of Nitrobenzene: Modifying Properties of Noble Metal

Abstract

The development of a sustainable catalytic system for optimizing the use of noble metals (NMs) plays a crucial role in expanding the application of NMs. In this paper, we synthesized a series of core–shell nanoparticles (NPs) with metal Cu as a core and ultrathin NM M (M = Pt, Pd, Rh, Ru, Ir, and Au) as a shell. Cu–M NPs were fabricated via a one-pot method, which involved a two-step process of Cu nucleating first at a low temperature and then NMs nucleating and growing on the Cu core at a high temperature later. For Pd precursor salt with a lower decomposition temperature, introducing trioctylphosphine (TOP) to enable the in situ formation of Pd-TOP complex was pivotal to obtaining Cu–Pd. X-ray diffraction, high-angle annular dark-field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy-mapping characterizations highlight the core–shell NPs with an ultrathin shell. The catalytic performance of the Cu–M NPs was investigated by using selective hydrogenation of nitrobenzene as a probe reaction. Among them, Cu–Rh demonstrated enhanced activity toward nitrobenzene hydrogenation to aniline with a TOF value as high as 2847 h–1. While a part of aniline would undergo excessive hydrogenation to cyclohexylamine over a pure Rh NP catalyst. Valence band spectra measurement and density functional theory calculations both prove that the activity modulation is principally governed by the charge transfer. The introduction of Cu core downshifts the d-band center of the Rh shell, weakens the adsorption of aniline, and thereby terminates the further reduction of aniline. The synthesis of Cu–M NPs can not only reduce the dosage of NMs but also present the potential for activity control by adjusting the properties of the shell metals. These core–shell NPs provide considerable promise for terminating the reaction to the intermediate product in consecutive hydrogenation reactions.