Platinum Nanoparticles Supported on Exfoliated Layered Double Hydroxides Nanosheet as a Reusable Catalyst for the Reduction of 4-Nitrophenol

Abstract

Supported transition metal nanoparticles (NPs) have attracted much interest in the field of heterogeneous catalysts, because the nature of supporting materials offers a strong possibility of controlling metal particle size and preventing the aggregation of metal NPs, which can consequently affect the catalytic performance of the supported NPs. Mesoporous silica, graphitic carbons, polymer nanobeads and layered double hydroxides (LDHs) have been potentially used as the supports to immobilize the metal NPs. Recently, two-dimensional nanosheets of exfoliated LDHs have emerged as a new type of supports to immobilize the metal NPs due to the unique two dimensional structure and large reactive surface of the LDH host layers. However, there have been only a few papers reported on the synthesis of metal NPs in the exfoliated LDH nanosheets. The LDHs, also known as hydrotalcite-like clays, consist of positively charged metal hydroxides and charge balancing anions, expressed by the general formula [M(1-x)Mx(OH)2]Ax/n ·mH2O, wherein M and M can be any divalent and trivalent metal cation occupied in the octahedral holes of a brucite-like layer and A is any hydrated exchangeable anion positioned in the gallery between the layers through a strong electrostatic and intermolecular interaction. The LDH nanosheets with a high-level of positive charge density can be produced by the exfoliation of LDHs into single hydroxide layers, which leads to electrostatic interactions between the LDH layers and metal precursors and/or metal NPs. Recently, Haraguchi et al. reported Au NPs modified LDH nanosheets, prepared by in situ reduction of metal precursor in aqueous solution. However, the aqueous media usually cause not only the LDH nanosheets to be restacked but also the metal NPs to be aggregated into large forms. We chose formamide as a reaction medium in order to exfoliate the LDHs and form the metal NPs. In this study, we present a very simple but successful synthesis of welldefined spherical Pt NPs on the LDH nanosheets without any stabilizing agent for metal NPs. We have also evaluated the catalytic performance of the Pt NP-LDH nanocomposites, prepared in this study, in the reduction of 4-nitrophenol. The structure and synthetic procedure for the nanocomposite of the LDH nanosheet and Pt NPs in formamide are schematically described in Scheme 1. For obtaining the well-defined LDH nanosheet, as-prepared carbonate form of LDHs was hydrothermally treated and further reacted by anion-exchange with nitrate. The carbonate form of Mg2AlLDH crystal, which has a well-crystallized rhombohedral phase with a basal spacing of 7.56 A, was converted into a nitrate form with an 8.96 A basal spacing (Figure S1a) using a salt/acetate buffer treatment. A characteristic band for the N-O stretching mode in the FT-IR spectra (Figure S1b) supports the intercalation of the nitrate into interlayers of the LDH crystals. The LDH crystals was exfoliated in formamide by continuous stirring for 5 days, and a transparent solution was then obtained as shown in Figure 1(a), indicating a successful exfoliation of the LDH crystals. The solution of the exfoliated LDH nanosheets has an excellent stability for 2 months. The transmission electron microscope (TEM) image shown in Figure 1(b) clearly indicates the exfoliated LDH nanosheets maintaining their plate-like structure, with a diameter of approximately 300-400 nm. The TEM also confirms the formation of very thin nanosheets compared to the pristine LDH particle (inset in Figure 1(b)). The zeta potential measurement clearly demonstrates the positively charged state of the Mg2Al-LDH nanosheet Scheme 1. Schematic representation of synthesis of Pt NPs on the exfoliated Mg2Al-LDH nanosheet.