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(2)) and PtSn (PtSn@mSiO(2)) 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, Pt@mSiO(2 )intermetallic NPs are derived from Pt@mSiO(2) seeds where the mSiO(2) 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(2) seeds is accommodated by chemical etching of the mSiO(2) shell. The effect of this etching on the atomic-scale structure of mSiO(2) has not been previously examined, nor has the extent of the structural similarity to MCM-41. Here, the quaternary Q(2), Q(3), and Q(4) sites corresponding to formulas Si(O-1/2)(2)(OH)(2), Si(O-1/2)(3)(OH)(1), and Si(O-1/2)(4) in MCM-41 and the mesoporous silica of Pt@mSiO(2 )and Pt@mSiO(2), NPs were identified and quantified by conventional and dynamic nuclear polarization (DNP)-enhanced Si-29 magic angle spinning nuclear magnetic resonance. The connectivity of the -Si-O-Si- network was revealed by DNP-enhanced two-dimensional Si-29-Si-29 correlation spectroscopy.