Thermally stable encapsulated intermetallic and bimetallic nanoparticles for heterogeneous catalysis

Intermetallic compounds have shown promise as efficient heterogeneous catalysts, displaying improved performance over their monometallic parent metals for chemical transformations. Furthermore, colloidal intermetallic nanoparticles (iNPs) afford additional improvements via increased catalyst surface area and enhanced catalytic performance. However, these iNPs are not efficiently protected against sintering during thermal treatment procedures such as catalyst regeneration, resulting in a loss of their active surface area. Herein, we have successfully developed porous encapsulation strategies using inert silica that afford monodisperse iNPs that address this concern. These iNPs are highly stable thermally and the inert silica-based mesoporous encapsulation doesn\u27t present mass transfer obstacles during catalysis. Their efficiency as heterogeneous catalysts are also extensively explored

An inorganic capping strategy for the seeded growth of versatile bimetallic nanostructures

Metal nanostructures have attracted great attention in various fields due to their tunable properties through precisely tailored sizes, compositions and structures. Using mesoporous silica (mSiO2) as the inorganic capping agent and encapsulated Pt nanoparticles as the seeds, we developed a robust seeded growth method to prepare uniform bimetallic nanoparticles encapsulated in mesoporous silica shells (PtM@mSiO2, M = Pd, Rh, Ni and Cu). Unexpectedly, we found that the inorganic silica shell is able to accommodate an eight-fold volume increase in the metallic core by reducing its thickness. The bimetallic nanoparticles encapsulated in mesoporous silica shells showed enhanced catalytic properties and thermal stabilities compared with those prepared with organic capping agents. This inorganic capping strategy could find a broad application in the synthesis of versatile bimetallic nanostructures with exceptional structural control and enhanced catalytic properties

Morphology inherence from hollow MOFs to hollow carbon polyhedrons in preparing carbon-based electrocatalysts

Hollow carbon nanostructures are emerging as advanced electrocatalysts for the oxygen reduction reaction (ORR) due to the effective usage of active sites and the reduced dependence on expensive noble metals. Conventional preparation of these hollow structures is achieved through templates (e.g. SiO2, CdS, and Ni3C), which serve to retain the void interiors during carbonization, leading to an essential template-removal procedure using hazardous chemical etchants. Herein, we demonstrate the direct carbonization of unique hollow zeolitic imidazolate frameworks (ZIFs) for the synthesis of hollow carbon polyhedrons (HCPs) with well-defined morphologies. The hollow ZIF particles behave bi-functionally as a carbon source and a morphology directing agent. This method evidences the strong morphology inherence from the hollow ZIFs during the carbonization, advancing the significant simplicity and environmental friendliness of this synthesis strategy. The as-prepared HCPs show a uniform polyhedral morphology and large void interiors, which enable their superior ORR activity. Iron can be doped into the HCPs (Fe/HCPs), providing the Fe/HCPs with enhanced ORR properties (E1/2 = 0.850 V) in comparison with those of HCPs. We highlight the efficient structural engineering to transform ZIFs into advanced carbon nanostructures accomplishing morphological control and high electrocatalytic activity

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

Mesoporous silica encapsulated Pt (Pt@mSiO2) and PtSn (PtSn@mSiO2) 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@mSiO2intermetallic NPs are derived from Pt@mSiO2 seeds where the mSiO2 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@mSiO2 seeds is accommodated by chemical etching of the mSiO2 shell. The effect of this etching on the atomic-scale structure of the mSiO2 has not been previously examined, nor has the extent of the structural similarity to MCM-41. Here, the quaternary Q2, Q3 and Q4 sites corresponding to formulas Si(O1/2)2(OH)2, Si(O1/2)3(OH)1 and Si(O1/2)4, in MCM-41 and the mesoporous silica of Pt@mSiO2 and PtSn@mSiO2 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 two-dimensional 29Si-29Si correlation spectroscopy

Pt Nanoclusters Confined within Metal–Organic Framework Cavities for Chemoselective Cinnamaldehyde Hydrogenation

A highly selective and robust catalyst based on Pt nanoclusters (NCs) confined inside the cavities of an amino-functionalized Zr-terephthalate metal–organic framework (MOF), UiO-66-NH2 was developed. The Pt NCs are monodisperse and confined in the cavities of UiO-66-NH2 even at 10.7 wt % Pt loading. This confinement was further confirmed by comparing the catalytic performance of Pt NCs confined inside and supported on the external surface of the MOF in the hydrogenation of ethylene, 1-hexene, and 1,3-cyclooctadiene. The benefit of confining Pt NCs inside UiO-66-NH2 was also demonstrated by evaluating their performance in the chemoselective hydrogenation of cinnamaldehyde. We found that both high selectivity to cinnamyl alcohol and high conversion of cinnamaldehyde can be achieved using the MOF-confined Pt nanocluster catalyst, while we could not achieve high cinnamyl alcohol selectivity on Pt NCs supported on the external surface of the MOF. The catalyst can be recycled ten times without any loss in its activity and selectivity. To confirm the stability of the recycled catalysts, we conducted kinetic studies for the first 20 h of reaction during four recycle runs on the catalyst. Both the conversion and selectivity are almost overlapping for the four runs, which indicates the catalyst is very stable under the employed reaction conditions

Surface-Mediated Hyperpolarization of Liquid Water from Parahydrogen

This article reports the discovery of surface-mediated parahydrogen-induced alignment of the proton magnetic moments in liquid water as well as methanol and ethanol. In this SWAMP (surface waters are magnetized by parahydrogen) effect, the spin polarization of the solvent protons is enhanced simply by the bubbling of parahydrogen through a suspension of Pt3Sn intermetallic nanoparticles (iNPs) encapsulated within a protectective mesoporous silica shell (Pt3Sn@mSiO2). The conversion of singlet spin order into magnetization is mediated by symmetry-breaking interactions on the surface of the iNPs. Stimulated emission NMR signals of the exchangeable hydroxy protons are observed. Non-exchangeable methyl or methylene protons are also hyperpolarized, an observation that provides insight into the molecular mechanism. SWAMP has a myriad of potential applications, ranging from low-field MRI to drug discovery

Toward Phase and Catalysis Control: Tracking the Formation of Intermetallic Nanoparticles at Atomic Scale

Intermetallic nanoparticles (iNPs) have yielded enormous successes in catalytic applications by the formation of ordered phases. However, atomic level understanding of the alloying mechanism, which plays a pivotal role for controlling intermetallic phases and tailoring their catalytic properties, is still elusive. In this study, we discovered a consecutive formation of ordered Pt3Sn and PtSn phases during the growth of Pt–Sn iNP inside a well-defined nano-reactor at elevated temperature using in-situ scanning transmission electron microscopy. We found that the surface-mediated diffusion of Sn controls overall dynamics of the reaction, while the unique coherent interfacial structure is determinative for the PtSn transformation. We then further controlled the phase selection of Pt–Sn iNPs and demonstrated their distinguishable catalytic behaviors. Our findings not only provide detailed experimental evidence on the alloying mechanism in intermetallic nanoscale systems, but also pave the way for mechanistic control of synthesis and catalytic properties of iNPs

Kinetics, Energetics, and Size Dependence of the Transformation from Pt to Ordered PtSn Intermetallic Nanoparticles

The outstanding catalytic activity and chemical selectivity of intermetallic compounds make them excellent candidates for heterogeneous catalysis. However, the kinetics of their formation at the nanoscale is poorly understood or characterized, and precise control of their size, shape as well as composition during synthesis remains challenging. Here, using well-defined Pt nanoparticles (5 nm and 14 nm) encapsulated in mesoporous silica, we study the transformation kinetics from monometallic Pt to intermetallic PtSn at different temperatures by a series of time-evolution X-ray diffraction studies. Observations indicate an initial transformation stage mediated by Pt surface-controlled intermixing kinetics, followed by a second stage with distinct transformation kinetics corresponding to a Ginstling-Brounstein (G-B) type bulk diffusion mode. Moreover, the activation barrier for both surface intermixing and diffusion stages are obtained through the development of appropriate kinetic models for analysis of experimental data. Our density-functional-theory (DFT) calculations provide further insights into the atomistic-level processes and associated energetics underlying surface-controlled intermixing