Design of a Silver–Cerium Dioxide Core–Shell Nanocomposite Catalyst for Chemoselective Reduction Reactions

Abstract

The interaction of metals with ligands is the key factor in the design of catalysts and much effort has been devoted to the rational control of metal-ligand interactions in order to exploit catalytic properties. Quite sophisticated heterogeneous catalysts have been produced by controlling the size and shape of active metal species, and by screening and altering the composition of the supports.[1] The supports can be considered as "macro ligands" for supported active metals, and the fine-tuning of the interactions between active metal species and supports is the most important factor through which high catalytic performance can be attained. Despite many intrinsic advantages of heterogeneous catalysts over homogeneous ones, such as their durability at high temperatures and reusability, the fine-tuning of metal-ligand interactions in heterogeneous catalysts is more difficult than in homogeneous catalysts, and remains a challenging objective.Our research group has recently reported that silver nanoparticles (AgNPs) on a basic support of hydrotalcite (Ag/HT) catalyzed the chemoselective reductions of nitrostyrenes [2] and epoxides [3,4] to the corresponding anilines and alkenes when using alcohols or CO/H 2 O as a reducing reagent while retaining the reducible C=C bonds. During the reductions, polar species of hydrides and protons were formed in situ at the interface of AgNPs/HT through a cooperative effect between the AgNPs and basic sites (BS) of HT, which were then exclusively active for the reduction of the polar functional groups (Figure 1). However, the use of H 2 instead of alcohols or CO/H 2 O in our Ag catalyst system caused reductions of both the polar groups (nitro and epoxide) and the nonpolar C=C bonds. This nonselective reduction was due to the formation of nonpolar hydrogen species through the homolytic cleavage of H 2 at the AgNPs surface, which is active for C = C bond reduction (Figure 2 a).We envisioned that AgNPs covered with a basic material (BM), namely, the core-shell nanocomposite AgNPs@BM, would be a reasonable structure for performing the above complete chemoselective reductions (Figure 2 b). The AgNPs@BM structure can maximize the interface area of the AgNPs-BM, while minimizing the area of the bare AgNPs. This property would enable the exclusive formation of the heterolytically cleaved hydrogen species through a concerted effect between AgNPs and basic sites of BM that suppresses the unfavorable formation of homolytically cleaved hydrogen species on the bare AgNPs. The resulting Ag hydride and proton species would lead to complete chemoselective reduction of polar functionalities while retaining the C=C bonds.

Chemoselective Hydrogenation of Nitrostyrene to Aminostyrene over Pd- and Rh-Based Intermetallic Compounds

“…Mitsudome et al designed a core−shell-structured Ag− CeO 2 nanocomposite with high chemoselectivity because of its maximized fraction of interfacial sites. 9 In this context, an ideal structure for chemoselective hydrogenation would be an arrangement of interfacial sites that exhibit high order at the atomic level. One of the promising candidates for such a material is an intermetallic compound.…”

Pt–Zn nanoparticles supported on porous polymeric matrix for selective 3-nitrostyrene hydrogenation

“…nitrostyrenes, NS), where the crucial issue is to avoid the preferential attack of the key (C@C) functionalities [2,3]. Highly selective catalysts (VA yields up to 100%) based on Au [4,5], Ag [6,7], Mo [8] were developed for NS hydrogenation. However, they exhibit low activity compared to other metals [4] and therefore demand more harsh reaction conditions [2].…”

Co3O4@CeO2 Core@Shell Cubes: Designed Synthesis and Optimization of Catalytic Properties

“…One way to optimize the fraction of the interfacial area of the two components is to prepare core@ shell structures. [25] To the best of our knowledge, there has been no successful work on Co 3 O 4 @CeO 2 core@shell nanostructures yet reported.…”