In Situ X-ray Absorption Spectroscopy Studies of Kinetic Interaction between Platinum(II) Ions and UiO-66 Series Metal–Organic Frameworks

The interaction of guest Pt(II) ions with UiO-66–X (X = NH2, H, NO2, OMe, F) series metal–organic frameworks (MOFs) in aqueous solution was investigated using in situ X-ray absorption spectroscopy. All of these MOFs were found to be able to coordinate with Pt(II) ions. The Pt(II) ions in UiO-66–X MOFs generally coordinate with 1.6–2.4 Cl and 1.4–2.4 N or O atoms. We also studied the time evolution of the coordination structure and found that Pt(II) maintained a coordination number of 4 throughout the whole process. Furthermore, the kinetic parameters of the interaction of Pt(II) ions with UiO-66–X series MOFs (X = NH2, H, NO2, OMe, F) were determined by combinational linear fitting of extended X-ray absorption fine structure (EXAFS) spectra of the samples. The Pt(II) adsorption rate constants were found to be 0.063 h–1 for UiO-66–NH2 and 0.011–0.017 h–1 for other UiO-66–X (X = H, NO2, OMe, F) MOFs, which means that Pt(II) adsorption in UiO-66–NH2 is 4–6 times faster than that in other UiO-66 series MOFs. FTIR studies suggested that the carboxyl groups could be the major host ligands binding with Pt(II) ions in UiO-66 series MOFs, except for UiO-66–NH2, in which amino groups coordinate with Pt(II) ions

Tandem Catalysis by Palladium Nanoclusters Encapsulated in Metal–Organic Frameworks

A bifunctional Zr-MOF catalyst containing palladium nanoclusters (NCs) has been developed. The formation of Pd NCs was confirmed by transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS). Combining the oxidation activity of Pd NCs and the acetalization activity of the Lewis acid sites in UiO-66-NH2, this catalyst (Pd@UiO-66-NH2) exhibits excellent catalytic activity and selectivity in a one-pot tandem oxidation-acetalization reaction. This catalyst shows 99.9% selectivity to benzaldehyde ethylene acetal in the tandem reaction of benzyl alcohol and ethylene glycol at 99.9% conversion of benzyl alcohol. We also examined various substituted benzyl alcohols and found that alcohols with electron-donating groups showed better conversion and selectivity compared to those with electron-withdrawing groups. We further proved that there was no leaching of active catalytic species during the reaction and the catalyst can be recycled at least five times without significant deactivation

Conversion of Levulinic Acid to γ-Valerolactone over Few-Layer Graphene-Supported Ruthenium Catalysts

Few-layer graphene (FLG) supported ruthenium nanoparticle catalysts were synthesized and used for the hydrogenation of levulinic acid (LA), one of the “top 10” biomass platform molecules derived from carbohydrates. FLG-supported ruthenium catalyst showed 99.7% conversion and 100% selectivity toward γ-valerolactone (GVL) at room temperature in a batch reactor under high-pressure hydrogen. This catalyst showed 4 times higher activity and exceptional stability in comparison with traditional activated carbon supported ruthenium catalysts (Ru/C). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies suggest that the superior catalytic properties of Ru nanoparticles supported on FLG in LA hydrogenation could be attributed to the greater metallic Ru content present in the Ru/FLG in comparison to that in Ru/C

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

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

A Ship-in-a-Bottle Strategy To Synthesize Encapsulated Intermetallic Nanoparticle Catalysts: Exemplified for Furfural Hydrogenation

Intermetallic compounds are garnering increasing attention as efficient catalysts for improved selectivity in chemical processes. Here, using a ship-in-a-bottle strategy, we synthesize single-phase platinum-based intermetallic nanoparticles (NPs) protected by a mesoporous silica (mSiO2) shell by heterogeneous reduction and nucleation of Sn, Pb, or Zn in mSiO2-encapsulated Pt NPs. For selective hydrogenation of furfural to furfuryl alcohol, a dramatic increase in activity and selectivity is observed when intermetallic NPs catalysts are used in comparison to Pt@mSiO2. Among the intermetallic NPs, PtSn@mSiO2 exhibits the best performance, requiring only one-tenth of the quantity of Pt used in Pt@mSiO2 for similar activity and near 100% selectivity to furfuryl alcohol. A high-temperature oxidation–reduction treatment easily reverses any carbon deposition-induced catalyst deactivation. X-ray photoelectron spectroscopy shows the importance of surface composition to the activity, whereas density functional theory calculations reveal that the enhanced selectivity on PtSn compared to Pt is due to the different furfural adsorption configurations on the two surfaces

Intermetallic structures with atomic precision for selective hydrogenation of nitroarenes

Bridging the structure-properties relationship of bimetallic catalysts is essential for the rational design of heterogeneous catalysts. Different from random alloys, intermetallic compounds (IMCs) present atomically-ordered structures, which is advantageous for catalytic mechanism studies. We used Pt-based intermetallic nanoparticles (iNPs), individually encapsulated in mesoporous silica shells, as catalysts for the hydrogenation of nitroarenes to functionalized anilines. With the capping-free nature and ordered atomic structure, PtSn iNPs show \u3e99% selectivity to hydrogenate the nitro group of 3-nitrostyrene albeit with a lower activity, in contrast to Pt3Sn iNPs and Pt NPs. The geometric structure of PtSn iNPs in eliminating Pt threefold sites hampers the adsorption/dissociation of molecular H2 and leads to a non-Horiuti-Polanyi hydrogenation pathway, while Pt3Sn and Pt surfaces are saturated by atomic H. Calculations using density functional theory (DFT) suggest a preferential adsorption of the nitro group on the intermetallic PtSn surface contributing to its high selectivity

Tuning surface properties of amino-functionalized silica for metal nanoparticle loading: The vital role of an annealing process

Metal nanoparticles (NPs) loaded on oxides have been widely used as multifunctional nanomaterials in various fields such as optical imaging, sensors, and heterogeneous catalysis. However, the deposition of metal NPs on oxide supports with high efficiency and homogeneous dispersion still remains elusive, especially when silica is used as the support. Amino-functionalization of silica can improve loading efficiency, but metal NPs often aggregate on the surface. Herein, we report that a facial annealing of amino-functionalized silica can significantly improve the dispersion and enhance the loading efficiency of various metal NPs, such as Pt, Rh, and Ru, on the silica surface. A series of characterization techniques, such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Zeta potential analysis, UV–Vis spectroscopy, thermogravimetric analysis coupled with infrared analysis (TGA–IR), and nitrogen physisorption, were employed to study the changes of surface properties of the amino-functionalized silica before and after annealing. We found that the annealed amino-functionalized silica surface has more cross-linked silanol groups and relatively lesser amount of amino groups, and less positively charges, which could be the key to the uniform deposition of metal NPs during the loading process. These results could contribute to the preparation of metal/oxide hybrid NPs for the applications that require uniform dispersion