Article Cite This: Chem. Mater. 2018, 30, 8070−8078
pubs.acs.org/cm
Conversion Chemistry of Nanoscopically Confined Manganese Silicate: Solid-State Route toward Porous Metal Oxide Catalyst− Support Taewan Kwon, Ki-Wan Jeon, Soumen Dutta, and In Su Lee* National Creative Research Initiative Center for Nanospace-Confined Chemical Reactions (NCCRs) and Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
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S Supporting Information *
ABSTRACT: By implementation of the nanospace-confinement strategy using silica nanosphere as reaction medium, the interesting transformation behavior of the nanosized Mn− silicate phase could be perceived in rarely explored hightemperature environment. The investigation of MnO nanocrystal (NC) within a silica nanosphere with increasing the annealing temperature showed the stepwise transformation from solid to hollow and back to solid interior structures. This conversion could be elucidated by the multistep process, including the formation of hollow Mn−silicate layer with lowered glass transition temperature (Tg) and its subsequent void-filling diffusion, which are attributed to the space-confinement effect within a nanoscale environment. In addition, the thermal oxidation of the resultant low-density Mn−silicate phase led to an important distinctive phase-segregation phenomenon, credited to the nanoscopic reaction medium circumscribed by a tight silica shell, which creates a highly porous nanostructure of the phase-segregated manganese(III) oxide (p-Mn2O3). The p-Mn2O3, isolated from a silica medium without affecting the overall morphology and porosity, was employed as catalyst−support which inhibits the problematic thermal sintering process for tiny Pt NCs (∼3 nm) even at high temperature of 400 °C. The p-Mn2O3− supported Pt NCs have demonstrated a superior long-term stability in catalyzing oxygen reduction reaction to those of commercial Mn oxide based analogue and Pt/C catalyst.
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INTRODUCTION Despite significant advances in the synthesis technique of colloidal nanocrystals (NCs) over the past decades, the understanding and control of the crystalline and morphological evolution of NCs under various environmental conditions and the consequent changes in their properties remains a challenging but imperative subject that needs to be resolved for reliable and durable operation in NC applications.1 Recently, the concept of NC conversion chemistry has been adopted to study the transformation of preformed NCs during various chemical reactions including Kirkendall voiding, galvanic replacement, ion exchange, and Oswald ripening,2−4 and the formation of high-complexity nanostructures, especially with hollow interiors, which cannot be obtained using traditional methods,5−9 whereas, given that most of the reactions were performed in a solution-like phase, the application range of the NC conversion approach is limited only to solvent-dispersible reactants and low-temperature (
