Noble Metal-Coated\nCu Nanoparticle Catalysts for Selective\nHydrogenation of Nitrobenzene: Modifying Properties of Noble Metal

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