No 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.

Hydrotalcite-supported gold nanoparticles (Au/HT) were found to be a highly efficient heterogeneous catalyst for the aerobic oxidation of alcohols under mild reaction conditions (40 8C, in air). This catalyst system does not require any additives and is applicable to a wide range of alcohols, including less reactive cyclohexanol derivatives. This Au/HT catalyst could also function in the oxidation of 1-phenylethanol under neat conditions; the turnover number (TON) and turnover frequency (TOF) reached 200,000 and 8,300 h À1 , respectively. These values are among the highest values compared to those of other reported catalyst systems at high conversion. Moreover, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity.

Hydrotalcite-supported Pt nanoparticles catalyze the direct transformation of furfural to 1,2-pentanediol in a high yield of 73% under additive-free conditions. The Pt nanoparticle catalyst is easily recoverable and reusable while maintaining high activity and selectivity.

We report a facile synthesis of new core-Au/shell-CeO2 nanoparticles (Au@CeO2) using a redox-coprecipitation method, where the Au nanoparticles and the nanoporous shell of CeO2 are simultaneously formed in one step. The Au@CeO2 catalyst enables the highly selective semihydrogenation of various alkynes at ambient temperature under additive-free conditions. The core-shell structure plays a crucial role in providing the excellent selectivity for alkenes through the selective dissociation of H2 in a heterolytic manner by maximizing interfacial sites between the core-Au and the shell-CeO2.