Chaoxian Xiao

Intermetallic NaAu2 as a Heterogeneous Catalyst for Low-Temperature CO Oxidation

The enhanced stability and modified electronic structure of intermetallic compounds provide discovery of superior catalysts for chemical conversions with high activity, selectivity, and stability. We find that the intermetallic NaAu2 is an active catalyst for CO oxidation at low temperatures. From density functional theory calculations, a reaction mechanism is suggested to explain the observed low reaction barrier of CO oxidation by NaAu2, in which a CO molecule reacts directly with an adsorbed O2 to form an OOCO* intermediate. The presence of surface Na increases the binding energy of O2 and decreases the energy barrier of the transition states.

High‐Temperature‐Stable and Regenerable Catalysts: Platinum Nanoparticles in Aligned Mesoporous Silica Wells

We report the synthesis, structural characterization, thermal stability study, and regeneration of nanostructured catalysts made of 2.9 nm Pt nanoparticles sandwiched between a 180 nm SiO2 core and a mesoporous SiO2 shell. The SiO2 shell consists of 2.5 nm channels that are aligned perpendicular to the surface of the SiO2 core. The nanostructure mimics Pt nanoparticles that sit in mesoporous SiO2 wells (Pt@MSWs). By using synchrotron-based small-angle X-ray scattering, we were able to prove the ordered structure of the aligned mesoporous shell. By using high-temperature cyclohexane dehydrogenation as a model reaction, we found that the Pt@MSWs of different well depths showed stable activity at 500 °C after the induction period. Conversely, a control catalyst, SiO2 -sphere-supported Pt nanoparticles without a mesoporous SiO2 shell (Pt/SiO2 ), was deactivated. We deliberately deactivated the Pt@MSWs catalyst with a 50 nm deep well by using carbon deposition induced by a low H2 /cyclohexane ratio. The deactivated Pt@MSWs catalyst was regenerated by calcination at 500 °C with 20 % O2 balanced with He. After the regeneration treatments, the activity of the Pt@MSWs catalyst was fully restored. Our results suggest that the nanostructured catalysts-Pt nanoparticles confined inside mesoporous SiO2 wells-are stable and regenerable for treatments and reactions that require high temperatures.

Selective Host–Guest Interaction between Metal Ions and Metal–Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid‐State NMR Spectroscopy

The host-guest interaction between metal ions (Pt(2+) and Cu(2+) ) and a zirconium metal-organic framework (UiO-66-NH2 ) was explored using dynamic nuclear polarization-enhanced (15) N{(1) H} CPMAS NMR spectroscopy supported by X-ray absorption spectroscopy and density functional calculations. The combined experimental results conclude that each Pt(2+) coordinates with two NH2 groups from the MOF and two Cl(-) from the metal precursor, whereas Cu(2+) do not form chemical bonds with the NH2 groups of the MOF framework. Density functional calculations reveal that Pt(2+) prefers a square-planar structure with the four ligands and resides in the octahedral cage of the MOF in either cis or trans configurations.

Geometry‐Assisted Three‐Dimensional Superlocalization Imaging of Single‐Molecule Catalysis on Modular Multilayer Nanocatalysts

To establish the structure-catalytic property relationships of heterogeneous catalysts, a detailed characterization of the three-dimensional (3D) distribution of active sites on a single catalyst is essential. Single-particle catalysis of a modular multilayer catalytic platform that consists of a solid silica core, a mesoporous silica shell, and uniformly distributed Pt nanoparticles sandwiched in between these layers is presented. The first 3D high-resolution super-localization imaging of single fluorescent molecules produced at active sites on the core-shell model nanocatalysts is demonstrated. The 3D mapping is aided by the well-defined geometry and a correlation study in scanning electron microscopy and total internal reflection fluorescence and scattering microscopy. This approach can be generalized to study other nano- and mesoscale structures.

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.

Silica‐Encapsulated Pt‐Sn Intermetallic Nanoparticles: A Robust Catalytic Platform for Parahydrogen‐Induced Polarization of Gases and Liquids

Recently, a facile method for the synthesis of size-monodisperse Pt, Pt3 Sn, and PtSn intermetallic nanoparticles (iNPs) that are confined within a thermally robust mesoporous silica (mSiO2 ) shell was introduced. These nanomaterials offer improved selectivity, activity, and stability for large-scale catalytic applications. Here we present the first study of parahydrogen-induced polarization NMR on these Pt-Sn catalysts. A 3000-fold increase in the pairwise selectivity, relative to the monometallic Pt, was observed using the PtSn@mSiO2 catalyst. The results are explained by the elimination of the three-fold Pt sites on the Pt(111) surface. Furthermore, Pt-Sn iNPs are shown to be a robust catalytic platform for parahydrogen-induced polarization for in vivo magnetic resonance imaging.

Palladium–gold bimetallic nanoparticle catalysts prepared by “controlled release” from metal-loaded interfacially cross-linked reverse micelles

Interfacially cross-linked reverse micelles (ICRMs) readily accommodated anionic gold and palladium metal salts in their ammonium-lined hydrophilic cores and allowed facile control of the metallic composition, as well as the metal oxides in the vicinity of the metals. Deposition onto a solid support (P25 TiO2) followed by thermal treatment of Pd–Au-containing ICRMs yielded bimetallic nanoparticle heterogeneous catalysts through the controlled release of metals from the ICRMs. The catalysts allowed efficient oxidation of benzyl alcohol under relatively mild conditions with minimal amounts of oxidant (hydrogen peroxide) in water without any organic solvent.

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.

In situ quantitative single-molecule study of dynamic catalytic processes in nanoconfinement

Understanding the fundamental catalytic principles when the catalytic centre is confined in nanoscale space that is dimensionally comparable to the reactant molecule is crucial for designing high-performance catalysts. Theoretical studies with simplified model systems and ensemble experimental measurements have shown that chemical reactions in nanoconfined environments are largely different from those in bulk solution. Here, we design a well-defined platform with catalytic centres confined in the end of nanopores with controlled lengths to study the in situ dynamic behaviour of catalytic processes under nanoconfinement at the single-molecule and single-particle level. Variable single molecular mass transport behaviour reveals the heterogeneity of the confined environment in the nanopores. With the capability of decoupling mass transport factors from reaction kinetics in the well-defined platform, we quantitatively uncovered a confinement-induced enhancement in the activity of platinum nanoparticles inside the nanopores. The combination of the unique model catalyst and the single-molecule super-localization imaging technique paves the way to understanding nanoconfinement effects in catalysis.Nanoconfinement effects are crucial in any process that involves porous materials. Here, the authors present a nanoporous catalyst platform that enables these effects to be studied in situ at the single-molecule and single-particle level with turnover resolution.