Conversion Chemistry of Nanoscopically Confined Manganese Silicate: Solid-State Route toward Porous Metal Oxide Catalyst-Support

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 high-temperature 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 (T-g) 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 (similar to 3 nm) even at high temperature of 400 degrees 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.