We demonstrate a molecular-level observation of driving CO2 molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO2 accumulation in the interface between a metal–organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO2 is observed at the interface. The high surface concentration leads CO2 molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO2 molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO2 is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>105 bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO2 conversion and ambient solid–gas operation.

To promote the effect of halide ions (Cl-, Br-, and I-) in facet-selective growth of {111} and {100} of shaped metal nanocrystals, we utilize PDADMAC, a hybrid cationic, polymeric stabilizer. SERS and synthesis experiments provide evidence supporting that the higher amount of PDADMA+ at surfaces promotes the local adsorption of halides, allowing the creation of Pd cubes, octahedra, and cuboctopods.

To promote the performance of nanoparticle electrocatalysts, a facile hydrothermal method is developed to generate core–shell nanocomposites with a metal catalyst core and a carbon shell. In this method, glucose serves as the reducing agent and carbon precursor while cetyltrimethylammonium surfactants serve as both pore structure-directing agents and nanoparticle capping agent. The accessibility of the metal catalyst core was examined by gas-phase ethylene hydrogenation and the electrocatalytic activity was tested by formic acid oxidation (FOR). The selection of halide counter ions used during the synthesis was found to be critical. The optimized sample of Pd-carbon nanocomposite exhibits a FOR current density of 2.55 mA/cm2, which is higher than that of un-coated Pd nanoparticles with no support (0.89 mA/cm2) and with carbon nanotube support (1.08 mA/cm2).

Complete −CH2CH2– dehydrogenation of 1,2-dimethyl-1,2-BN-cyclohexene (1) was achieved using a Pd/C catalyst in a gas-phase microreactor. Arrhenius analysis yielded an activation energy (Ea) of 10.3 ± 0.3 kcal mol–1 and a pre-exponential factor (A) of 2.2 ± 0.2 (log A), respectively. These terms reflect a lesser kinetic favorability in comparison to those determined for all-carbon dimethylcyclohexene (Ea = 8.6 ± 0.3 kcal mol–1, log A = 3.6 ± 0.1). Despite being isostructural and isoelectronic with a C═C bond, the B–N bond of 1 thus appears to confer a different measure of activity with respect to Pd-catalyzed −CH2CH2– dehydrogenation.

We develop a new concept to impart new functions to biocatalysts by combining enzymes and metal–organic frameworks (MOFs). The proof-of-concept design is demonstrated by embedding catalase molecules into uniformly sized ZIF-90 crystals via a de novo approach. We have carried out electron microscopy, X-ray diffraction, nitrogen sorption, electrophoresis, thermogravimetric analysis, and confocal microscopy to confirm that the ∼10 nm catalase molecules are embedded in 2 μm single-crystalline ZIF-90 crystals with ∼5 wt % loading. Because catalase is immobilized and sheltered by the ZIF-90 crystals, the composites show activity in hydrogen peroxide degradation even in the presence of protease proteinase K.

The focus on surface lattice strain in nanostructures as a fundamental research topic has gained momentum in recent years as scientists investigated its significant impact on the surface electronic structure and catalytic properties of nanomaterials. Researchers have begun to tell a more complete story of catalysis from a perspective which brings this concept to the forefront of the discussion. The nano-‘realm’ makes the effects of surface lattice strain, which acts on the same spatial scales, more pronounced due to a higher ratio of surface to bulk atoms. This is especially evident in the field of metal nanoparticle catalysis, where displacement of atoms on surfaces can significantly alter the sorption properties of molecules. In part, the concept of strain-engineering for catalysis opened up due to the achievements that were made in the synthesis of a more sophisticated nanoparticle library from an ever-expanding set of methodologies. Developing synthesis methods for metal nanoparticles with well-defined and strained architectures is a worthy goal that, if reached, will have considerable impact in the search for catalysts. In this review, we summarize the recent accomplishments in the area of surface lattice-strained metal nanoparticle synthesis, framing the discussion from the important perspective of surface lattice strain effects in catalysis.

The creation of hierarchical porosity in metal–organic frameworks (MOFs) could benefit various applications of MOFs such as gas storage and separation. Having single-crystalline microcrystals instead of poly-crystalline composites is critical for these potential applications of MOFs with hierarchical porosity. We developed a room temperature synthetic method to generate uniform hollow and mesoporous zeolitic imidazolate framework-8 (ZIF-8) microcrystals with a single-crystalline structure via overgrowing a ZIF-8 shell in methanol solution on a ZIF-8 core with water adsorbed in the pores. The cavities formed as a result of the different solvent micro-environment. This double-solvent mediated overgrowth method could be applied to prepare other MOFs with hierarchical porosity.

Under linker exchange conditions, large guests with molecular diameters 3–4 times the framework aperture size have been encapsulated into preformed nanocrystals of the metal–organic framework ZIF-8. Guest encapsulation is facilitated by the formation of short-lived “open” states of the pores upon linker dissociation. Kinetic studies suggested that linker exchange reactions in ZIF-8 proceed via a competition between dissociative and associative exchange mechanisms, and guest encapsulation was enhanced under conditions where the dissociative pathway predominates.

Composite nanomaterials are attractive for a diverse range of applications in catalysis, plasmonics, sensing, imaging, and biology. In such composite nanomaterials, it is desired, yet still challenging to create a controlled alignment between components with lattices in disparate scales. To address this challenge, we report a new concept of colloidal synthesis, in which self-assembled molecular layers control the alignment between materials during the synthesis. To illustrate this concept, self-assembled cetyltrimethylammonium bromide (CTAB) molecules are used to control interfaces in a core–shell nanocomposite with a well-defined metal nanocrystal core and a metal–organic-framework (MOF) shell, which differ in structural dimensions by orders of magnitude. We show that single metal nanocrystals are captured individually in single-crystalline MOFs, and an alignment between the {100} planes of the metal and {110} planes of the MOFs is observed. By utilizing the same concept, a layer of mesostructured silica is formed over MOF crystals. These multilayered core–shell structures demonstrate a controlled alignment across a wide range of materials, from the metal nanocrystals, extending to nanoporous MOFs and mesostructured silica.

Encapsulating well-defined nanoparticle catalysts into porous materials to form a core–shell nanostructure can enhance the durability, selectivity, or reactivity of the catalysts and even provide additional functionalities to the catalysts. Using metal–organic frameworks (MOFs) as the encapsulating porous materials has drawn great interest recently because MOFs, as a class of crystalline nanoporous materials, have well-defined pore structures and unique chemical properties. Also, the structures and properties of MOFs are tunable. In this perspective review, we examine recent progress in the development of synthetic methods for metal@MOF core–shell nanostructures as catalysts. Potential directions in the field are also discussed.Keywords: catalysis; core−shell structure; metal nanocrystal; metal−organic frameworks; nanoporous material

Submicron-sized mesoporous nickel ferrite (NiFe2O4) spheres were prepared by an aerosol spray pyrolysis method using Pluronic F127 as a structure-directing agent, and their photocatalytic performance for hydrogen (H2) evolution was examined in an aqueous MeOH solution by visible light irradiation (λ > 420 nm). The structure of the spherical mesoporous nickel ferrites was studied by transmission electron microscopy, powder X-ray diffraction, and N2 adsorption–desorption isotherm measurements. Mesoporous NiFe2O4 spheres of high specific surface area (278 m2 g–1) with a highly crystalline framework were prepared by adjusting the amount of structure-directing agent and the calcining condition. High photocatalytic activity of mesoporous NiFe2O4 for H2 evolution from water with methanol was achieved due to the combination of high surface area and high crystallinity of the nickel ferrites.Keywords: Crystallinilty; Hydrogen evolution; Metal oxide sphere; Nickel ferrite; Photocatalysis; Ramp rate; Surface area

We have developed a general synthetic route to encapsulate small molecules in monodisperse zeolitic imid-azolate framework-8 (ZIF-8) nanospheres for drug delivery. Electron microscopy, powder X-ray diffraction, and elemental analysis show that the small-molecule-encapsulated ZIF-8 nanospheres are uniform 70 nm particles with single-crystalline structure. Several small molecules, including fluorescein and the anticancer drug camptothecin, were encapsulated inside of the ZIF-8 framework. Evaluation of fluorescein-encapsulated ZIF-8 nanospheres in the MCF-7 breast cancer cell line demonstrated cell internalization and minimal cytotoxicity. The 70 nm particle size facilitates cellular uptake, and the pH-responsive dissociation of the ZIF-8 framework likely results in endosomal release of the small-molecule cargo, thereby rendering the ZIF-8 scaffold an ideal drug delivery vehicle. To confirm this, we demonstrate that camptothecin encapsulated ZIF-8 particles show enhanced cell death, indicative of internalization and intracellular release of the drug. To demonstrate the versatility of this ZIF-8 system, iron oxide nanoparticles were also encapsulated into the ZIF-8 nanospheres, thereby endowing magnetic features to these nanospheres.Keywords: cellular uptake; drug delivery; nanoparticles; ZIF-8

Bimetallic nanoparticle catalysts provide enhanced activity, as combining metals allows tuning of electronic and geometric structure, but the enhancement may vary during the reaction because the nanoparticles can undergo metal migration under catalytic reaction conditions. Using cyclic voltammetry to track the surface composition over time, we carried out a detailed study of metal migration in a well-defined model Au–Pd core–shell nanocatalyst. When subjected to electrochemical conditions, Au migration from the core to the shell was observed. The effect of Pd shell thickness and electrolyte identity on the extent of migration was studied. Migration of metals during catalytic ethanol oxidation was found to alter the particle’s surface composition and electronic structure, enhancing the core–shell particles’ activity. We show that metal migration in core–shell nanoparticles is a phenomenon common to numerous electrochemical systems and must be considered when studying electrochemical catalysis.Keywords: Au; electrocatalysis; ethanol oxidation; lattice strain; metal migration; Pd;

Shape-controlled metal nanoparticles (NPs) interfacing Pt and nonprecious metals (M) are highly active energy conversion electrocatalysts; however, there are still few routes to shaped M–Pt core–shell NPs and fewer studies on the geometric effects of shape and strain on catalysis by such structures. Here, well-defined cubic multilayered Pd–Ni–Pt sandwich NPs are synthesized as a model platform to study the effects of the nonprecious metal below the shaped Pt surface. The combination of shaped Pd substrates and mild reduction conditions directs the Ni and Pt overgrowth in an oriented, layer-by-layer fashion. Exposing a majority of Pt(100) facets, the catalytic performance in formic acid and methanol electro-oxidations (FOR and MOR) is assessed for two different Ni layer thicknesses and two different particle sizes of the ternary sandwich NPs. The strain imparted to the Pt shell layer by the introduction of the Ni sandwich layer (Ni–Pt lattice mismatch of ∼11%) results in higher specific initial activities compared to core–shell Pd–Pt bimetallic NPs in alkaline MOR. The trends in activity are the same for FOR and MOR electrocatalysis in acidic electrolyte. However, restructuring in acidic conditions suggests a more complex catalytic behavior from changes in composition. Notably, we also show that cubic quaternary Au–Pd–Ni–Pt multishelled NPs, and Pd–Ni–Pt nanooctahedra can be generated by the method, the latter of which hold promise as potentially highly active oxygen reduction catalysts.Keywords: core−shell; electrocatalysis; lattice strain; multilayered; nanoparticle synthesis; Ni; Pd; Pt; sandwich; shape

A heterogeneous catalyst in industry consists of nanoparticles with variable crystallite sizes, shapes, and compositions. Its catalytic performance (activity, selectivity, and durability) derives from surface chemistry of catalyst nanoparticles during catalysis. However, the surface chemistry of the catalyst particles during catalysis, termed in-situ information, is a “black box” because of the challenges in characterizing the catalysts during catalysis. The lack of such in-situ information about catalysts has limited the understanding of catalytic mechanisms and the development of catalysts with high selectivity and activity.The challenges in understanding heterogeneous catalysis include measurement of reaction kinetics, identification of reaction intermediates, bridging pressure gap and materials gap. The pressure gap is the difference in surface structure and chemistry between a catalyst during catalysis and under an ultrahigh vacuum (UHV) condition. The materials gap represents the difference between the structural and compositional complexity of industrial catalysts and the well-defined surface of model catalysts of metals or oxides.Development of in-situ characterization using electron spectroscopy and electron microscopy in recent decades has made possible studies of surface chemistry and structure of nanocatalysts under reaction conditions or during catalysis at near ambient pressure. In this Account, we review the new chemistries and structures of nanocatalysts during reactions revealed with in-situ analytical techniques. We discuss changes observed during catalysis including the evolution of composition, oxidation state, phase, and geometric structure of the catalyst surface, and the sintering of catalysts. These surface chemistries and structures have allowed researchers to build a correlation between surface chemistry and structure of active nanocatalysts and their corresponding catalytic performances. Such a correlation provides critical insights for understanding catalysis, optimization of existing nanocatalysts, and development of new nanocatalysts with high activity and selectivity.

Developing syntheses of more sophisticated nanostructures comprising late transition metals broadens the tools to rationally design suitable heterogeneous catalysts for chemical transformations. Herein, we report a synthesis of Pd–Rh nanoboxes by controlling the migration of metals in a core–shell nanoparticle. The Pd–Rh nanobox structure is a grid-like arrangement of two distinct metal phases, and the surfaces of these boxes are {100} dominant Pd and Rh. The catalytic behaviors of the particles were examined in electrochemistry to investigate strain effects arising from this structure. It was found that the trends in activity of model fuel cell reactions cannot be explained solely by the surface composition. The lattice strain emerging from the nanoscale separation of metal phases at the surface also plays an important role.

A general synthetic strategy for yolk–shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk–shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu2O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu2O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk–shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis.

Metal nanocrystals (NCs) comprising rhodium are heterogeneous catalysts for CO oxidation, NO reduction, hydrogenations, electro-oxidations, and hydroformylation reactions. It has been demonstrated that control of structure at the nanoscale can enhance the performance of a heterogeneous metal catalyst, such as Rh, but molecular-level control of NCs comprising this metal is less studied compared to gold, silver, platinum, and palladium. We report an iodide-mediated epitaxial overgrowth of Rh by using the surfaces of well-defined foreign metal crystals as substrates to direct the Rh surface structures. The epigrowth can be accomplished on different sizes, morphologies, and identities of metal substrates. The surface structures of the resulting bimetallic NCs were studied using electron microscopy, and their distinct catalytic behaviors were examined in CO stripping and the electro-oxidation of formic acid. Iodide was found to play a crucial role in the overgrowth mechanism. With the addition of iodide, the Rh epigrowth can even be achieved on gold substrates despite the rather large lattice mismatch of ∼7%. Hollow Rh nanostructures have also been generated by selective etching of the core substrates. The new role of iodide in the overgrowth and the high level of control for Rh could hold the key to future nanoscale control of this important metal’s architecture for use in heterogeneous catalysis.