A Nanoreactor Framework of a Au@SiO₂ Yolk/Shell Structure for Catalytic Reduction of p‐Nitrophenol

Abstract

Bifunctional catalysts containing metal nanoparticles embedded on metal oxide supports are basic structures that exhibit high activity and efficiency for significant chemical reactions, such as hydrogenation, oxidation-reduction, and reforming. [1][2][3] In bifunctional structures, high-surface-area supports not only stabilize metal nanoparticles, but also generate new active components on a metal/metal-oxide interface, which facilitates chemical reactions.[4] Accordingly, the interactions between metal nanoparticles and supports should be precisely manipulated in order to maximize their synergetic effects. Metal particles should be dispersed evenly on the supports in order to prevent aggregation between particles in close proximity, and to maintain metal-support contact areas. Numerous processes have been developed for effective dispersion of metal nanoparticles in bifunctional catalysts. [5,6] However, severe particle aggregation during the reaction and surface inhomogeneity of the active particles still pose obstacles to the commercialization of catalysts in terms of high temperature stability and reusability. Recently, Somorjai et al. demonstrated a new concept of a ''nanoreactor'' system employing a Pt@CoO yolk/shell structure.[7] The Pt@CoO structure was generated from the reaction of Pt@Co core/shell particles with O 2 via the Kirkendall effect. The Pt cores in Pt@CoO could act as a catalyst for ethylene hydrogenation, whereas amorphous CoO shells transfer gas-phase reactants and products. [8] This nanoreactor system has many advantages in comparison to conventional catalysts embedded in bulk supports. First, each nanoparticle isolated by a solid shells has a relatively homogeneous environment around the particle surface. The outer shell structure also hinders the aggregation of neighboring particles, even under harsh reaction conditions. Second, the interaction of metal catalysts and supports is more effective than that of the bulk forms, feasibly leading to high catalytic activity. When common metal oxide supports (SiO 2 , TiO 2 , etc.) can substitute in the CoO shell structure, the resulting metal@metal oxide nanoreactor framework is expected to show intriguing catalytic behaviors for many gas and solution phase reactions.Similar yolk/shell-type structures have been known as ''nanorattles''. [9,10] Although such nanorattles are composed of metal particles and either silica or carbon shells, the synthetic procedures are multistep and complex, and achieving control over the metal core structures is challenging. Moreover, these nanorattles have yet to be employed as nanoreactor framework. In the present work, we fabricated a Au@SiO 2 yolk/shell structure through selective etching of metal cores from Au@SiO 2 core/shell particles (Scheme 1). The average size of the metal cores was readily tuned by using different amounts of etchants. This Au@SiO 2 framework was successfully employed in the catalytic reduction of p-nitrophenol as a model reaction. Gold nanoparticles were prepared by a modified polyol proces...

Synthesis of Gold Catalysts Supported on Mesoporous Silica Materials: Recent Developments

“…Lee et al [239] fabricated a nanoreactor framework of a Au/SiO 2 yolk/shell structure through selective etching of metal cores from Au/SiO 2 by treatment with KCN. Although gold core diameters showed high values (43-104 nm) depending on the added amount of KCN, these Au/SiO 2 yolk/shell particles were active for the catalytic reduction of p-nitrophenol by NaBH 4 .…”

Metal@Silica yolk-shell nanostructures as versatile bifunctional nanocatalysts

“…Synthetic methods for yolk-shell nanoparticles can be divided into two types, manipulation of the core and manipulation of the shell. In the case of Au@SiO 2 , we used selective etching of the metal cores in metal@silica core-shell nanoparticles [26]. This method has the advantages of being a simple procedure and has wide applicability, because most metal cores are soluble in weak acids.…”

Silk fibroin fibers supported with high density of gold nanoparticles: fabrication and application as catalyst

“…Pure SF does not catalyze this reaction alone. This reaction is easily monitored by following the absorption decrease of pnitrophenol at 400 nm [21,22]. Figure 5a shows a typical UV-Vis absorption change at 400 nm of the reaction mixture by the addition of Au/SPANI-modified SF composite fibers.…”