Concave Silica Nanosphere with a Functionalized Open-Mouthed Cavity as Highly Active and Durable Catalytic Nanoreactor

Abstract

Despite increasingly intensive research into catalytic hollow nanoreactors, most of the work has focused on the enclosed cavity structure, and attempts to use the open-mouthed cavity have not been made so far, most likely due to the lack of methodologies for producing and functionalizing such a structure. This paper reports a synthetic strategy toward open-mouthed cavity-based nanoreactors, which renders the SiO2 nanosphere with a concave surface and also immobilizes catalytic nanocrystals (NCs) specifically inside the concave region. By putting the Janus silica-encapsulated manganese oxide (MnO) nanoparticle, with its highly off-centered core@shell structure, through the thermal hollow-conversion process, the edge-touching MnO nanoparticle was transformed into a hollow hemispherical manganese silicate layer with an opening to the outside, thus producing the bitten apple-like structure, conc-(Ni/HMS)@SiO2, with an open-mouthed cavity on the SiO2 nanosphere. The galvanic replacement reaction occurring on the manganese-silicate layer of the conc-(Ni/HMS)@SiO2 afforded the site-specific immobilization of catalytic Pt NCs on the preformed concave interior surface, signifying the possible postsynthetic functionalization of an open-mouthed cavity which could be adapted for the development of a nanoreactor system. The newly developed nanoreactor, Pt@conc-SiO2, carrying tiny catalytic Pt NCs inside the semiexposed and also semiprotected pocket-like space, exhibited an increased reaction rate and a more extended range of applicable substrates in catalyzing the reduction of nitroarene compounds, compared with the enclosed cavity-based analogue, while preserving the high immobilization stability of Pt nanocatalysts during the recycling process.