Rare-earth-platinum alloy nanoparticles in mesoporous zeolite for catalysis.

Platinum is a much used catalyst that, in petrochemical processes, is often alloyed with other metals to improve catalytic activity, selectivity and longevity1-5. Such catalysts are usually prepared in the form of metallic nanoparticles supported on porous solids, and their production involves reducing metal precursor compounds under a H2 flow at high temperatures6. The method works well when using easily reducible late transition metals, but Pt alloy formation with rare-earth elements through the H2 reduction route is almost impossible owing to the low chemical potential of rare-earth element oxides6. Here we use as support a mesoporous zeolite that has pore walls with surface framework defects (called 'silanol nests') and show that the zeolite enables alloy formation between Pt and rare-earth elements. We find that the silanol nests enable the rare-earth elements to exist as single atomic species with a substantially higher chemical potential compared with that of the bulk oxide, making it possible for them to diffuse onto Pt. High-resolution transmission electron microscopy and hydrogen chemisorption measurements indicate that the resultant bimetallic nanoparticles supported on the mesoporous zeolite are intermetallic compounds, which we find to be stable, highly active and selective catalysts for the propane dehydrogenation reaction. When used with late transition metals, the same preparation strategy produces Pt alloy catalysts that incorporate an unusually large amount of the second metal and, in the case of the PtCo alloy, show high catalytic activity and selectivity in the preferential oxidation of carbon monoxide in H2.

Morphology-Dependent Interaction of Silica Nanoparticles with Intestinal Cells: Connecting Shape to Barrier Function.

The intestinal compartment ensures nutrient absorption and barrier function against pathogens. Despite decades of research on the complexity of the gut, the adaptive potential to physical cues, such as those derived from interaction with particles of different shapes, remains less understood. Taking advantage of the technological versatility of silica nanoparticles, spherical, rod-shaped, and virus-like materials were synthesized. Morphology-dependent interactions were studied on differentiated Caco-2/HT29-MTX-E12 cells. Contributions of shape, aspect ratio, surface roughness, and size were evaluated considering the influence of the mucus layer and intracellular uptake pathways. Small particle size and surface roughness favored the highest penetration through the mucus but limited interaction with the cell monolayer and efficient internalization. Particles of a larger aspect ratio (rod-shaped) seemed to privilege paracellular permeation and increased cell-cell distances, albeit without hampering barrier integrity. Inhibition of clathrin-mediated endocytosis and chemical modulation of cell junctions effectively tuned these responses, confirming morphology-specific interactions elicited by bioinspired silica nanomaterials.

Lanthanum-catalysed synthesis of microporous 3D graphene-like carbons in a zeolite template.

Three-dimensional graphene architectures with periodic nanopores­reminiscent of zeolite frameworks­are of topical interest because of the possibility of combining the characteristics of graphene with a three-dimensional porous structure. Lately, the synthesis of such carbons has been approached by using zeolites as templates and small hydrocarbon molecules that can enter the narrow pore apertures. However, pyrolytic carbonization of the hydrocarbons (a necessary step in generating pure carbon) requires high temperatures and results in non-selective carbon deposition outside the pores. Here, we demonstrate that lanthanum ions embedded in zeolite pores can lower the temperature required for the carbonization of ethylene or acetylene. In this way, a graphene-like carbon structure can be selectively formed inside the zeolite template, without carbon being deposited at the external surfaces. X-ray diffraction data from zeolite single crystals after carbonization indicate that electron densities corresponding to carbon atoms are generated along the walls of the zeolite pores. After the zeolite template is removed, the carbon framework exhibits an electrical conductivity that is two orders of magnitude higher than that of amorphous mesoporous carbon. Lanthanum catalysis allows a carbon framework to form in zeolite pores with diameters of less than 1 nanometre; as such, microporous carbon nanostructures can be reproduced with various topologies corresponding to different zeolite pore sizes and shapes. We demonstrate carbon synthesis for large-pore zeolites (FAU, EMT and beta), a one-dimensional medium-pore zeolite (LTL), and even small-pore zeolites (MFI and LTA). The catalytic effect is a common feature of lanthanum, yttrium and calcium, which are all carbide-forming metal elements. We also show that the synthesis can be readily scaled up, which will be important for practical applications such as the production of lithium-ion batteries and zeolite-like catalyst supports.

Direct observation of bond formation in solution with femtosecond X-ray scattering.

The making and breaking of atomic bonds are essential processes in chemical reactions. Although the ultrafast dynamics of bond breaking have been studied intensively using time-resolved techniques, it is very difficult to study the structural dynamics of bond making, mainly because of its bimolecular nature. It is especially difficult to initiate and follow diffusion-limited bond formation in solution with ultrahigh time resolution. Here we use femtosecond time-resolved X-ray solution scattering to visualize the formation of a gold trimer complex, [Au(CN)2(-)]3 in real time without the limitation imposed by slow diffusion. This photoexcited gold trimer, which has weakly bound gold atoms in the ground state, undergoes a sequence of structural changes, and our experiments probe the dynamics of individual reaction steps, including covalent bond formation, the bent-to-linear transition, bond contraction and tetramer formation with a time resolution of ∼500 femtoseconds. We also determined the three-dimensional structures of reaction intermediates with sub-ångström spatial resolution. This work demonstrates that it is possible to track in detail and in real time the structural changes that occur during a chemical reaction in solution using X-ray free-electron lasers and advanced analysis of time-resolved solution scattering data.

Oxygen activation on the interface between Pt nanoparticles and mesoporous defective TiO2 during CO oxidation.

Platinum-based heterogeneous catalysts are mostly used in various commercial chemical processes because of their high catalytic activity, influenced by the metal/oxide interaction. To design rational catalysts with high performance, it is crucial to understand the relationship between the metal-oxide interface and the reaction pathway. Here, we investigate the role of oxygen defect sites in the reaction mechanism for CO oxidation using Pt nanoparticles supported on mesoporous TiO2 catalysts with oxygen defects. We show an intrinsic correlation between the catalytic reactivity and the local properties of titania with oxygen defects (i.e., Ti3+ sites). In situ infrared spectroscopy observations of the Pt/mesoporous TiO2-x catalyst indicate that an oxygen molecule bond can be activated at the perimeter between the Pt and an oxygen vacancy in TiO2 by neighboring CO molecules on the Pt surface before CO oxidation begins. The proposed reaction pathways for O2 activation at the Pt/TiO2-x interface based on density functional theory confirm our experimental findings. We suggest that this provides valuable insight into the intrinsic origin of the metal/support interaction influenced by the presence of oxygen vacancies, which clarifies the pivotal role played by the support.

Nanocage-Confined Synthesis of Fluorescent Polycyclic Aromatic Hydrocarbons in Zeolite.

Polycyclic aromatic hydrocarbons (PAHs) attract much attention for applications to organic light-emitting diodes, field-effect transistors, and photovoltaic cells. The current synthetic approaches to PAHs involve high-temperature flash pyrolysis or complicated step-by-step organic reactions, which lead to low yields of PAHs. Herein, we report a facile and scalable synthesis of PAHs, which is carried out simply by flowing acetylene gas into zeolite under mild heating, typically at 400 °C and generates the products of 0.30 g g-1 zeolite. PAHs are synthesized via acetylene polymerization inside Ca2+-ion-exchanged Linde type A (LTA) zeolite, of which the α-cage puts a limit on the product molecular size as a confined-space nanoreactor. The resultant product after the removal of the zeolite framework exhibits brilliant white fluorescence emission in N-methylpyrrolidone solution. The product is separated into four different color emitters (violet, blue, green, and orange) by column chromatography. Detailed characterizations of the products by means of various spectroscopic methods and mainly mass spectrometric analyses indicate that coronene (C24H12) is the main component of the blue emitter, while the green emitter is a mixture of planar and curved PAHs. The orange can be attributed to curved PAHs larger than ovalene, and the violet to smaller molecules than coronene. The PAH growth mechanism inside Ca2+-exchanged LTA zeolite is proposed on the basis of mass spectral analyses and density functional theory calculations.

Non-Topotactic Transformation of Silicate Nanolayers into Mesostructured MFI Zeolite Frameworks During Crystallization.

Mesostructured MFI zeolite nanosheets are established to crystallize non-topotactically through a nanolayered silicate intermediate during hydrothermal synthesis. Solid-state 2D NMR analyses, with sensitivity enhanced by dynamic nuclear polarization (DNP), provide direct evidence of shared covalent 29 Si-O-29 Si bonds between intermediate nanolayered silicate moieties and the crystallizing MFI zeolite nanosheet framework.

Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts.

Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion. This has been realized by synthesizing zeolite nanocrystals, by exfoliating layered zeolites, and by introducing mesopores in the microporous material through templating strategies or demetallation processes. But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.

Two-minute assembly of pristine large-area graphene based films.

We report a remarkably rapid method for assembling pristine graphene platelets into a large area transparent film at a liquid surface. Some 2-3 layer pristine graphene platelets temporally solvated with N-methyl-2-pyrrolidone (NMP) are assembled at the surface of a dilute aqueous suspension using an evaporation-driven Rayleigh-Taylor instability and then are driven together by Marangoni forces. The platelets are fixed through physical binding of their edges. Typically, 8-cm-diameter circular graphene films are generated within two minutes. Once formed, the films can be transferred onto various substrates with flat or textured topologies. This interfacial assembly protocol is generally applicable to other nanomaterials, including 0D fullerene and 1D carbon nanotubes, which commonly suffer from limited solution compatibility.

Synthesis of Silicate Zeolite Analogues Using Organic Sulfonium Compounds as Structure-Directing Agents.

A microporous crystalline silica zeolite of the MEL structure type and three other zeolite analogues composed of germanosilicate frameworks were synthesized using tributylsulfonium, triphenylsulfonium, or tri(para-tolyl)sulfonium as the structure-directing agent. The germanosilicates thus obtained had ISV, ITT, or a new zeolite structure depending on the synthesis conditions. The structure of the new germanosilicate was solved using X-ray powder diffraction data with the aid of a charge-flipping method. The solution indicated a crystal structure belonging to the P63/mmc space group with cell parameters of a=16.2003 Šand c=21.8579 Å. After calcination, the new germanosilicate material exhibited two types of accessible micropores with diameters of 0.61 and 0.78 nm.

Co-development of crystalline and mesoscopic order in mesostructured zeolite nanosheets.

Mesoporous zeolites are a new and technologically important class of materials that exhibit improved diffusion and catalytic reaction properties compared to conventional zeolites with sub-nanometer pore dimensions. During their syntheses, the transient developments of crystalline and mesoscopic order are closely coupled and challenging to control. Correlated solid-state NMR, X-ray, and electron microscopy analyses yield new molecular-level insights on the interactions and distributions of complicated organic structure-directing agents with respect to crystallizing zeolite frameworks. The analyses reveal the formation of an intermediate layered silicate phase, which subsequently transforms into zeolite nanosheets with uniform nano- and mesoscale porosities. Such materials result from coupled surfactant self-assembly and inorganic crystallization processes, the interplay between which governs the onset and development of framework structural order on different length and time scales.

Conversion of kraft lignin over hierarchical MFI zeolite.

Catalytic pyrolysis of kraft lignin was carried out using pyrolysis gas chromatography/mass spectrometry. Hierarchical mesoporous MFI was used as the catalyst and another mesoporous material Al-SBA-15 was also used for comparison. The characteristics of mesoporous MFI were analyzed by X-ray diffraction patterns, N2 adsorption-desorption isotherms, and temperature programmed desorption of NH3. Two catalyst/lignin mass ratios were tested: 5/1 and 10/1. Aromatics and alkyl phenolics were the main products of the catalytic pyrolysis of lignin over mesoporous MFI. In particular, the yields of mono-aromatics such as benzene, toluene, ethylbenzene, and xylene were increased substantially by catalytic upgrading. Increase in the catalyst dose enhanced the production of aromatics further, which is attributed to decarboxylation, decarbonlyation, and aromatization reactions occurring over the acid sites of mesoporous MFI.

Random-graft polymer-directed synthesis of inorganic mesostructures with ultrathin frameworks.

A widely employed route for synthesizing mesostructured materials is the use of surfactant micelles or amphiphilic block copolymers as structure-directing agents. A versatile synthesis method is described for mesostructured materials composed of ultrathin inorganic frameworks using amorphous linear-chain polymers functionalized with a random distribution of side groups that can participate in inorganic crystallization. Tight binding of the side groups with inorganic species enforces strain in the polymer backbones, limiting the crystallization to the ultrathin micellar scale. This method is demonstrated for a variety of materials, such as hierarchically nanoporous zeolites, their aluminophosphate analogue, TiO2 nanosheets of sub-nanometer thickness, and mesoporous TiO2, SnO2, and ZrO2. This polymer-directed synthesis is expected to widen our accessibility to unexplored mesostructured materials in a simple and mass-producible manner.

Assessment of acid catalytic properties of ferrosilicate MFI zeolite by methanol-to-hydrocarbon conversion.

Four representative synthetic methods were employed to prepare Fe-containing siliceous MFI zeolites. The obtained Fe-MFI zeolites exhibited markedly different catalytic performances in the methanol-to-hydrocarbon (MTH) conversion reaction depending on the type of Fe incorporation within the siliceous framework. The catalytically active Brønsted acid sites were analyzed using pyridine adsorption experiments combined with Fourier transform infrared spectroscopy, providing characteristic signal intensities according to the acid-base interactions. Based on the MTH conversion results and acidity analyses, a suitable synthetic method was identified for the incorporation of Fe within the MFI zeolite framework. However, compared to other catalytic reactions, structural analyses by transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray absorption spectroscopy were much less conclusive.

Low-Temperature controlled synthesis of nanocast mixed metal oxide spinels for enhanced OER activity.

The controlled cation substitution is an effective strategy for optimizing the density of states and enhancing the electrocatalytic activity of transition metal oxide catalysts for water splitting. However, achieving tailored mesoporosity while maintaining elemental homogeneity and phase purity remains a significant challenge, especially when aiming for complex multi-metal oxides. In this study, we utilized a one-step impregnation nanocasting method for synthesizing mesoporous Mn-, Fe-, and Ni-substituted cobalt spinel oxide (Mn0.1Fe0.1Ni0.3Co2.5O4, MFNCO) and demonstrate the benefits of low-temperature calcination within a semi-sealed container at 150-200 °C. The comprehensive discussion of calcination temperature effects on porosity, particle size, surface chemistry and catalytic performance for the alkaline oxygen evolution reaction (OER) highlights the importance of humidity, which was modulated by a pre-drying step. The catalyst calcined at 170 °C exhibited the lowest overpotential (335 mV at 10 mA cm-2), highest current density (433 mA cm-2 at 1.7 V vs. RHE, reversible hydrogen electrode) and further displayed excellent stability over 22 h (at 10 mA cm-2). Furthermore, we successfully adapted this method to utilize cheap, commercially available silica gel as a hard template, yielding comparable OER performance. Our results represent a significant progress in the cost-efficient large-scale preparation of complex multi-metal oxides for catalytic applications.

Microporous aluminophosphate nanosheets and their nanomorphic zeolite analogues tailored by hierarchical structure-directing amines.

Multiamines with amphiphilic structures have been synthesized to serve as simultaneous structure-directing agents in micro- and meso-structural levels for aluminophosphate materials (AlPOs) and their analogues, such as silicoaluminophosphate, cobalt aluminophosphate, and gallium phosphate. The amine molecules are assembled into a micelle with a specific morphology to function as a meso-level structure director. Individual amine groups in the micelle are able to direct the formation of microporous crystalline AlPO structure. The resultant meso-level morphologies of the AlPOs are typically nanosheets of uniform thickness, which can be tailored in the range of 2-5 nm by the number of amine groups. Sponge-like disordered mesoporous morphologies can be generated, depending on the amine structures. Using such multiamines provides a versatile route to various phosphate materials with a structural hierarchy for enhanced porous functionalities.

Capping with multivalent surfactants for zeolite nanocrystal synthesis.

Multiammonium surfactants exhibited a remarkable capping effect for zeolite synthesis in the forms of nanoparticles, nanorods, and nanosponges in cases where common monovalent surfactants failed. A nanorod-shaped mordenite zeolite synthesized in this manner showed significantly enhanced catalytic lifetimes in acid-catalyzed cumene synthesis reactions.

Tuning CO2 Hydrogenation Selectivity through Reaction-Driven Restructuring on Cu-Ni Bimetal Catalysts.

Tuning the selectivity of CO2 hydrogenation is of significant scientific interest, especially using nickel-based catalysts. Fundamental insights into CO2 hydrogenation on Ni-based catalysts demonstrate that CO is a primary intermediate, and product selectivity is strongly dependent on the oxidation state of Ni. Therefore, modifying the electronic structure of the nickel surface is a compelling strategy for tuning product selectivity. Herein, we synthesized well dispersed Cu-Ni bimetallic nanoparticles (NPs) using a simple hydrothermal method for CO selective CO2 hydrogenation. A detailed study on the monometallic (Ni and Cu) and bimetallic (CuxNi1-x) catalysts supported on γ-Al2O3 was performed to increase CO selectivity while maintaining the high reaction rate. The Cu0.5Ni0.5/γ-Al2O3 catalyst shows a high CO2 conversion and more CO product selectivity than its monometallic counterparts. The surface electronic and geometric structure of Cu0.5Ni0.5 bimetallic NPs was studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier-transform spectroscopy under reaction conditions. The Cu core atoms migrate toward the surface, resulting in the restructuring of the Cu@Ni core-shell structure to a Cu-Ni alloy during the reaction and functioning as the active site by enhancing CO desorption. A systematic correlation is obtained between catalytic activity from a continuous fixed-bed flow reactor and the surface electronic structural details derived from AP-XPS results, establishing the structure-activity relationship. This investigation contributes to providing a strategy for controlling CO2 hydrogenation selectivity by modifying the surface structure of bimetallic NP catalysts.

Efficient functional delivery of siRNA using mesoporous silica nanoparticles with ultralarge pores.

Among various nanoparticles, mesoporous silica nanoparticles (MSNs) have attracted extensive attention for developing efficient drug-delivery systems, mostly due to their high porosity and biocompatibility. However, due to the small pore size, generally below 5 nm in diameter, potential drugs that are loaded into the pore have been limited to small molecules. Herein, a small interfering RNA (siRNA) delivery strategy based on MSNs possessing pores with an average diameter of 23 nm is presented. The siRNA is regarded as a powerful gene therapeutic agent for treatment of a wide range of diseases by enabling post-transcriptional gene silencing, so-called RNA interference. Highly efficient, sequence-specific, and technically very simple target gene knockdown is demonstrated using MSNs with ultralarge pores of size 23 nm in vitro and in vivo without notable cytotoxicity.