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

Abstract

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, PtSn@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 atomicscale structure of the 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 PtSn@mSiO 2 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-Si-network was revealed by DNP enhanced twodimensional 29 Si-29 Si correlation spectroscopy.

Recent developments in MAS DNP-NMR of materials

“…DNP has been used to characterize mesoporous silica functionalized by organic species (see Figure 1c) [36,102,122,208,228,232,252,253] or metal complexes [134,236,[254][255][256][257][258][259][260] as well as encapsulating metal nanoparticles [261]. The sensitivity provided by DNP is particularly useful for the detection of surface sites, notably when they are occupied by insensitive nuclei, such as 15 [222,262].…”

Synthesis of a Rationally Designed Multi-Component Photocatalyst Pt:SiO2:TiO2(P25) with Improved Activity for Dye Degradation by Atomic Layer Deposition

“…Higher loadings decreased the photocatalytic activity even to lower values than intrinsic TiO 2 (P25) (>2 wt % Pt). In earlier research, SiO 2 layers often have been used to encapsulate Pt nanoclusters on the surface of TiO 2 to improve the Pt nanoparticle dispersion by preventing noble metal cluster agglomeration, improving the catalytic performance [31,32,39]. According to our previous results, it would be more advantageous to keep Pt clusters exposed on the surface by the deposition of SiO 2 followed by Pt, since both materials, Pt and SiO 2 , are crucial for the generation of radicals on the surface.…”

“…Another approach is to modify the surface of an already photocatalytic material with a coating to change the deposition properties of the subsequently added co-catalyst i.e., increasing control of the particle size by modifying the surface or preventing the agglomeration of deposited nanoclusters of the surface by a cover layer [27][28][29][30]. However, the manipulation of physical properties such as increased surface area, the addition of magnetic properties, or a beneficial layer to improve the dispersion of co-catalyst particles [31,32] is more straightforward than modifying the photocatalytic mechanisms themselves. The question arises whether multiple components, each enhancing the photocatalytic process, can be combined to reach higher enhancement than each component individually.…”

Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuĊ6H6) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

“…Heterogeneous catalysis has been an important topic in the chemical sciences for many years, 9,26,27 in which oxide environments in general 28,29 and silicious environments in particular, including mesoporous silica 9,10,30,31 and zeolites, 32−34 have often been used as supports for catalytically active species. The acidic properties of such environments 32,34−36 are also important, via proton or H atom transfer reactions to guest molecules 7,12,27,32−34,37−39 that can also form free-radical intermediates, important to hydrogenation reactions in general 6,39 and for benzene in particular.…”

“…Heterogeneous catalysis has been an important topic in the chemical sciences for many years, ,, in which oxide environments in general , and silicious environments in particular, including mesoporous silica ,,, and zeolites, − have often been used as supports for catalytically active species. The acidic properties of such environments ,− are also important, via proton or H atom transfer reactions to guest molecules ,,,− ,− that can also form free-radical intermediates, important to hydrogenation reactions in general , and for benzene in particular. ,,,, Other than from muon science ,,,− though, exemplified by the present paper as well, there are very few examples of the direct observation of such H-adduct free radicals by spectroscopic techniques, notable exceptions relevant here being the HĊ 6 H 6 cyclohexadienyl radical seen in ZSM-5 zeolite and the ethyl CH 3 ĊH 2 and HĊ 6 H 6 radicals interacting with PdNPs investigated by ESR in silica environments .…”