Atomic-Scale Structure of Mesoporous Silica-Encapsulated Pt and PtSn Nanoparticles Revealed by Dynamic Nuclear Polarization- Enhanced 29Si MAS NMR Spectroscopy.

Mesoporous silica encapsulated Pt (Pt@mSiO) and PtSn (PtSn@mSiO) nanoparticles (NPs) are representatives of a novel class of heterogeneous catalysts with uniform particle size, enhanced catalytic properties, and superior thermal stability. In the ship-in-a-bottle synthesis, PtSn@mSiO intermetallic NPs are derived from Pt@mSiO seeds where the mSiO shell is formed by polymerization of tetraethyl orthosilicate around a tetradecyltrimethylammonium bromide template, a surfactant used to template MCM-41. Incorporation of Sn into the Pt@mSiO seeds is accommodated by chemical etching of the mSiO shell. The effect of this etching on the atomic-scale structure of the mSiO has not been previously examined, nor has the extent of the structural similarity to MCM-41. Here, the quaternary Q, Q and Q sites corresponding to formulas Si(O)(OH), Si(O)(OH) and Si(O), in MCM-41 and the mesoporous silica of Pt@mSiO and PtSn@mSiO NPs were identified and quantified by conventional and dynamic nuclear polarization enhanced Si-29 Magic Angle Spinning Nuclear Magnetic Resonance (DNP MAS NMR). The connectivity of the -Si-O-Sinetwork was revealed by DNP enhanced two-dimensional Si-Si correlation spectroscopy.