Pd nanoparticle concentration dependent self-assembly of Pd@SiO2 nanoparticles into leaching resistant microcubes

Abstract

Pd NP concentration guided the self-assembly of core–shell Pd@SiO2 nanoparticles (NPs) into microcubes. The Pd NPs were stacked by molten dodecyltrimethylammonium bromide (DTAB) to create the SiO2 envelope. The microcubes demonstrated improved leaching resistance in heterogeneous catalysis over a conventional porous support.Although discrete NPs could be easily synthesized by surfactant assisted wet-chemical routes, easy handling of these NPs requires well-defined superstructures for practical applications.1,2 In the past, the self-assembled superstructures or microstructures were obtained by solvent evaporation,3 surfactant templates,4,5 or molecular cross-linking.6 Molecular simulations have shown that NPs coated with surfactants can self-assemble into larger structures based on solvent selectivity of the NPs and the surfactants.7 NPs tend to remain aggregated in order to minimize their free energy and the self-assembled shapes are dependent on a particular solvent medium. The self-assembly processes have been so far shown to be dependent on specific and non-specific interactions of the surfactants with the NPs. Also, the shape and size of nanocomposites are known to be engineered by the NPs and surfactants.8–11 However, keeping the synthesis parameters constant, the self-assembly process was never observed to be guided by the NP concentration. In most of the metal NP and SiO2 composites, the shape of the SiO2 support was governed by the organic linkers and the metal NPs were added post-fabrication of the support.12,13 Quite a few studies are available on in situ synthesis where the metal NPs occupy site selective locations.14–16 The Pd NP–SiO2 composites are excellent heterogeneous catalysts in the formation of C–C and C–X (X = Cl, Br, I) bonds.12,17,18 Self-assembly of an ensemble of Pd NPs into 3-dimensional (3D) micron-sized architectures can improve the robustness of the system, still maintaining the novel properties of the NPs. A precise knowledge of the metal NP distribution is necessary such as if the NPs are immobilized on the outer surface or embedded inside the support to understand the transport of reactant molecules to the active sites.18,19 The designed 3D composites should offer improved control over the surface chemistry and NP size, which are key factors in the construction of next generation catalysts. Herein we demonstrate the morphology evolution of porous silica to non-porous Pd–SiO2 microcubes aided by <0.5 wt% Pd NPs and DTAB. Acid hydrolysis of TEOS in the presence of DTAB generated porous SiO2 and with the optimal presence of Pd NPs in the solution, micron sized cubes of SiO2 was formed. Acid hydrolysis was carried out with different concentrations of Pd NPs separately in air and N2 to synthesize the as-synthesized (AS) products which were hydrogenated at 550 °C. ICP-MS analyses of the hydrogenated (H-550) Pd–SiO2 composites provided the final weight% of Pd NPs. Two Pd–SiO2 composites were synthesized in N2 with Pd wt% of 0.09 and 0.34, whereas those synthesized in air had Pd wt% of 0.14 and 0.41. Also a separate sample was synthesized where 0.37 wt% Pd NPs were exclusively immobilized on the surface of mesoporous SiO2 by a PVP binder. Fig. 1a shows the transmission electron micrograph (TEM) of the pristine citrate ligand stabilized spherical Pd NPs of diameter 4.3 ± 0.4 nm. The inter-planar spacing of 0.23 nm corresponds to the (111) reflection. The X-ray diffraction (XRD) patterns of the H-550 composites correspond to metallic Pd0 (JCPDS No. 87-0639) over a background of amorphous SiO2 (Fig. S2, ESI†).