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.

CeO2-encapsulated noble metal nanocatalysts: enhanced activity and stability for catalytic application

“…Figure 14 shows the TEM images of encapsulated Au NPs on the inner walls of hollow CeO 2 nanospheres, which were reported by Zhang and coworkers. 97 The 'hard-templating' method used is similar to that for the aforementioned Pt@CeO 2 hollow nanofiber and Pd@CeO 2 hollow nanospheres. The difference is that Zhang's group used a SiO 2 layer as the hard template.…”

Clean synthesis of Cu2O@CeO2 core@shell nanocubes with highly active interface

“…The main reason for the lack of successful hybridization is that the preformed Cu 2 O cores cannot withstand the conditions during the coating of CeO 2 in the following step (Cu 2 O could be severely etched by ammonium, which is indispensable during the CeO 2 coating process). However, recently, an important one-step synthetic strategy called 'clean synthesis' [17][18][19][20][21][22][23] inspired us to solve this problem. Herein, 'clean' means not only free of toxic raw materials, especially organic compounds, but also low-cost and low-temperature aqueous synthesis, a surface free of residues and a greatly simplified post-treatment process, which are extremely valuable for practical applications.…”

“…In this strategy, Ce(OH) 3 and unsaturated oxides, such as WO 2.72 , are the most reported reductant precursors due to their strong reducing capability, whereas noble metal ions, such as Ag(NH 3 ) 2 + and PtCl 4 2 − , serve as the oxidant precursors. 18,[21][22][23] These clean synthesis routes are mainly based on the redox principle: an auto-catalytic redox reaction followed by an spontaneous self-assembly process. During the reaction, mutual inhibition between different components generates final products with a uniform structure in the nanometer scale, uniform metal dispersion, tunable metal particle size and narrow metal particle size distribution.…”

In situ redox strategy for large-scale fabrication of surfactant-free M-Fe2O3 (M = Pt, Pd, Au) hybrid nanospheres

“…Not far from recently, an interesting clean synthesis of surfactant-free noble metal catalysts has become a hot spot owing to its fast and facile synthetic process and especially the avoidable post-treatment [45][46][47][48][49]. The synthesis was conducted by utilizing the redox relationship either between noble metal salts and reductive supports or between noble metal salts and low-valence transition metal precursors.…”

“…For instance, through an in situ redox between the reductive WO 2.72 substrate and the oxidative noble metal salt precursors [45], a series of noble metal/WO 3 nanocomposites with uniform metal dispersion, tunable metal particle size, and narrow metal particle size distribution were obtained in the absence of reducing agent or surfactants. This clean synthesis route has also been developed to the Ag-CeO 2 system [48,49] by triggering the autocatalyzed reaction between Ce(OH) 3 and Ag(NH 3 ) 2 + to form the final rice-ball like hybrid nanostructures. Besides, we focused on this subject and successfully developed a new non-organic synthetic method to fabricate pomegranate-like Pt@CeO 2 multi-cor e@shell high-temperature-stable catalysts [50].…”