Increase in the effective viscosity of polyethylene under extreme nanoconfinement.

Understanding polymer transport in nanopores is crucial for optimizing heterogeneously catalyzed processes in polymer upcycling and fabricating high-performance nanocomposite films and membranes. Although confined polymer dynamics have been extensively studied, the behavior of polyethylene (PE)-the most widely used commodity polymer-in pores smaller than 20 nm remains largely unexplored. We investigate the effects of extreme nanoconfinement on PE transport using capillary rise infiltration in silica nanoparticle packings with average pore radii ranging from ∼1 to ∼9 nm. Using in situ ellipsometry and the Lucas-Washburn model, we discover a previously unknown inverse relationship between effective viscosity (ηeff) and average pore radius (Rpore). Additonally, we determine that PE transport under these extreme conditions is primarily governed by physical confinement, rather than pore surface chemistry. We refine an existing theory to provide a generalized formalism to describe the polymer transport dynamics over a wide range of pore radii (from 1 nm and larger). Our results offer valuable insights for optimizing catalyst supports in polymer upcycling and improving infiltration processes for nanocomposite fabrication.

Synthesis and Characterization of Stable Cu-Pt Nanoparticles under Reductive and Oxidative Conditions.

We report a synthesis method for highly monodisperse Cu-Pt alloy nanoparticles. Small and large Cu-Pt particles with a Cu/Pt ratio of 1:1 can be obtained through colloidal synthesis at 300 °C. The fresh particles have a Pt-rich surface and a Cu-rich core and can be converted into an intermetallic phase after annealing at 800 °C under H2. First, we demonstrated the stability of fresh particles under redox conditions at 400 °C, as the Pt-rich surface prevents substantial oxidation of Cu. Then, a combination of in situ scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, and CO oxidation measurements of the intermetallic CuPt phase before and after redox treatments at 800 °C showed promising activity and stability for CO oxidation. Full oxidation of Cu was prevented after exposure to O2 at 800 °C. The activity and structure of the particles were only slightly changed after exposure to O2 at 800 °C and were recovered after re-reduction at 800 °C. Additionally, the intermetallic CuPt phase showed enhanced catalytic properties compared to the fresh particles with a Pt-rich surface or pure Pt particles of the same size. Thus, the incorporation of Pt with Cu does not lead to a rapid deactivation and degradation of the material, as seen with other bimetallic systems. This work provides a synthesis route to control the design of Cu-Pt nanostructures and underlines the promising properties of these alloys (intermetallic and non-intermetallic) for heterogeneous catalysis.

Synthesis and Characterization of Core-Shell Cu-Ru, Cu-Rh, and Cu-Ir Nanoparticles.

Optimizing the use of expensive precious metals is critical to developing sustainable and low-cost processes for heterogeneous catalysis or electrochemistry. Here, we report a synthesis method that yields core-shell Cu-Ru, Cu-Rh, and Cu-Ir nanoparticles with the platinum-group metals segregated on the surface. The synthesis of Cu-Ru, Cu-Rh, and Cu-Ir particles allows maximization of the surface area of these metals and improves catalytic performance. Furthermore, the Cu core can be selectively etched to obtain nanoshells of the platinum-group metal components, leading to a further increase in the active surface area. Characterization of the samples was performed with X-ray absorption spectroscopy, X-ray powder diffraction, and ex situ and in situ transmission electron microscopy. CO oxidation was used as a reference reaction: the three core-shell particles and derivatives exhibited promising catalyst performance and stability after redox cycling. These results suggest that this synthesis approach may optimize the use of platinum-group metals in catalytic applications.

Modulating Interactions between Molten Polystyrene and Porous Solids Using Atomic Layer Deposition.

Understanding and modulating the interactions between molten polymers and porous solids is important for numerous processes and phenomena including catalytic conversion of polymers and fabrication of nanocomposites and nanostructured materials. Although changing the surface composition of pores would enable modulation of interactions between polymers and nanoporous solids, it is challenging to achieve such a control without inducing significant changes to the size and structure of nanopores. In this work, we demonstrate that the interactions between molten polystyrene (PS) and disordered packings of SiO2 nanoparticles (NPs) can be modulated by changing the surface composition of the NPs using atomic layer deposition (ALD). A disordered packing of silica NPs is modified with varying surface coverages of TiO2, WO3, and CaCO3, with coverages estimated by the mass gain and the refractive index change of NP packings. Based on the time required to fully infiltrate these ALD-modified NP packings via capillarity, the contact angles for PS on different surfaces prepared via ALD are determined. The contact angle gradually changes from that of pure SiO2 to that of the fully covered surfaces. The contact angles for PS on SiO2, TiO2, WO3, and CaCO3 are found to be 20, 62, 70, and 10°, respectively. Interestingly, the contact angles and interfacial energies between PS and the ALD-modified surfaces do not correlate strongly with the water contact angle of these surfaces; thus, caution must be exercised in predicting how a polymer would wet or interact with porous solids solely based on their hydrophilicity. The method presented in this work can be extended to study the interactions between a wide range of polymers and surfaces in porous media, which will have important implications for designing new catalytic materials for polymer upcycling reactions and novel NP-polymer composite films and membranes with enhanced mechanical and transport properties.

Two-Dimensional Perovskite Crystals Formed by Atomic Layer Deposition of CaTiO3 on γ-Al2O3.

CaTiO3 films with an average thickness of 0.5 nm were deposited onto γ-Al2O3 by Atomic Layer Deposition (ALD) and then characterized by a range of techniques, including X-ray Diffraction (XRD) and High-Resolution, Transmission Electron Microscopy (HRTEM). The results demonstrate that the films form two-dimensional crystallites over the entire surface. Lattice fringes from HRTEM indicate that the crystallites range in size from 5 to 20 nm and are oriented in various directions. Films of the same thickness on SiO2 remained amorphous, indicating that the support played a role in forming the crystallites.

Metal Exsolution to Enhance the Catalytic Activity of Electrodes in Solid Oxide Fuel Cells.

Exsolution is a novel technology for attaching metal catalyst particles onto ceramic anodes in the solid oxide fuel cells (SOFCs). The exsolved metal particles in the anode exhibit unique properties for reaction and have demonstrated remarkable stabilities under conditions that normally lead to coking. Despite extensive investigations, the underlying principles behind exsolution are still under investigation. In this review, the present status of exsolution materials for SOFC applications is reported, including a description of the fundamental concepts behind metal incorporation in oxide lattices, a listing of proposed mechanisms and thermodynamics of the exsolution process and a discussion on the catalytic properties of the resulting materials. Prospects and opportunities to use materials produced by exsolution for SOFC are discussed.

Highly active dry methane reforming catalysts with boosted in situ grown Ni-Fe nanoparticles on perovskite via atomic layer deposition.

With the need for more stable and active metal catalysts for dry reforming of methane, in situ grown nanoparticles using exsolution are a promising approach. However, in conventional exsolution, most nanoparticles remain underneath the surface because of the sluggish diffusion rate of cations. Here, we report the atomic layer deposition (ALD)-combined topotactic exsolution on La0.6Sr0.2Ti0.85Ni0.15O3-δ toward developing active and durable catalysts. The uniform and quantitatively controlled layer of Fe via ALD facilitates the topotactic exsolution, increasing finely dispersed nanoparticles. The introduction of Fe2O3 yields the formation of Ni-Fe alloy owing to the spontaneous alloy formation energy of -0.43 eV, leading to an enhancement of the catalytic activity for dry methane reforming with a prolonged stability of 410 hours. Overall, the abundant alloy nanocatalysts via ALD mark an important step forward in the evolution of exsolution and its application to the field of energy utilization.

Epitaxial and Strong Support Interactions between Pt and LaFeO3 Films Stabilize Pt Dispersion.

The ability to stabilize very small Pt crystallites in supported-metal catalysts following harsh treatments is an important industrial problem. Here, we demonstrate that Pt particles can be maintained in the 1- to 2-nm range following multiple oxidation and reduction cycles at 1073 K when the particles are supported on 0.5-nm LaFeO3 films that have been deposited onto MgAl2O4 using atomic layer deposition. Characterization by scanning transmission electron microscopy suggests that when the catalyst is oxidized at 1073 K, the Pt crystallites are oriented with respect to the underlying LaFeO3. X-ray absorption spectroscopy also shows evidence of changes in the Pt environment. CO-oxidation rates for the reduced catalyst remain unchanged after five redox cycles at 1073 K. Epitaxial growth of Pt clusters and the consequent strong metal-support interaction between Pt and LaFeO3 are suggested to be the main reasons for the enhanced catalytic performances.

Atomic Layer Deposition for Preparing Isolated Co Sites on SiO2 for Ethane Dehydrogenation Catalysis.

Unlike Co clusters, isolated Co atoms have been shown to be selective for catalytic dehydrogenation of ethane to ethylene; however, preparation of isolated Co sites requires special preparation procedures. Here, we demonstrate that Atomic Layer Deposition (ALD) of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) (Co(TMHD)3) on silica and other supports is effective in producing these isolated species. Silica-supported catalysts prepared with one ALD cycle showed ethylene selectivities greater than 96% at 923 K and were stable when CO2 was co-fed with the ethane. Co catalysts prepared by impregnation formed clusters that were significantly less active, selective, and stable. Rates and selectivities also decreased for catalysts with multiple ALD cycles. Isolated Co catalysts prepared on Al2O3 and MgAl2O4 showed reasonable selectivity for ethane dehydrogenation but were not as effective as their silica counterpart.

Enhancing Oxygen Exchange Activity by Tailoring Perovskite Surfaces.

A detailed understanding of the effects of surface chemical and geometric composition is essential for understanding the electrochemical performance of the perovskite (ABO3) oxides commonly used as electrocatalysts in the cathodes of ceramic fuel cells. Herein, we report how the addition of submonolayer quantities of A- and B-site cations affects the rate of the oxygen reduction reaction (ORR) of Sr-doped LaFeO3 (LSF), LaMnO3 (LSM), and LaCoO3 (LSCo). Density functional theory calculations were performed to determine the stability of different active sites on a collection of surfaces. With LSF and LSM, rates for the ORR are significantly higher on the A-site terminated surface, while surface termination is less important for LSCo. Our findings highlight the importance of tailoring the surface termination of the perovskite to obtain its ultimate ORR performance.

Smart Pd Catalyst with Improved Thermal Stability Supported on High-Surface-Area LaFeO3 Prepared by Atomic Layer Deposition.

The concept of self-regenerating or "smart" catalysts, developed to mitigate the problem of supported metal particle coarsening in high-temperature applications, involves redispersing large metal particles by incorporating them into a perovskite-structured support under oxidizing conditions and then exsolving them as small metal particles under reducing conditions. Unfortunately, the redispersion process does not appear to work in practice because the surface areas of the perovskite supports are too low and the diffusion lengths for the metal ions within the bulk perovskite too short. Here, we demonstrate reversible activation upon redox cycling for CH4 oxidation and CO oxidation on Pd supported on high-surface-area LaFeO3, prepared as a thin conformal coating on a porous MgAl2O4 support using atomic layer deposition. The LaFeO3 film, less than 1.5 nm thick, was shown to be initially stable to at least 900 °C. The activated catalysts exhibit stable catalytic performance for methane oxidation after high-temperature treatment.

Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution.

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes. Efficient hydrogen evolution from water is one promising route towards a renewable energy economy and sustainable development. Here we show a tridimensional electrocatalytic interface, featuring a hierarchical, co-axial arrangement of a palladium/titanium dioxide layer on functionalized multi-walled carbon nanotubes. The resulting morphology leads to a merging of the conductive nanocarbon core with the active inorganic phase. A mechanistic synergy is envisioned by a cascade of catalytic events promoting water dissociation, hydride formation and hydrogen evolution. The nanohybrid exhibits a performance exceeding that of state-of-the-art electrocatalysts (turnover frequency of 15000 H2 per hour at 50 mV overpotential). The Tafel slope of ∼130 mV per decade points to a rate-determining step comprised of water dissociation and formation of hydride. Comparative activities of the isolated components or their physical mixtures demonstrate that the good performance evolves from the synergistic hierarchical structure.

Evidence and Model for Strain-Driven Release of Metal Nanocatalysts from Perovskites during Exsolution.

The evolution of the surface morphology during exsolution of Ni from the perovskite La0.4Sr0.4Ti0.97Ni0.03O3-δ under reducing conditions was determined using atomic force microscopy. The exsolution process was found to initially induce the formation of a 20-30 nm deep pit on the oxide surface followed by the emergence of a Ni particle at the bottom of the pit. Continued emergence of the particle results in it nearly filling the pit, producing a unique structure in which the Ni particle is socketed into the oxide surface. We also show that this morphological evolution can be explained using a simple energy-based model that accounts for the interplay between the surface free energy and the strain energy induced by the included metal nucleate. The unique socketed structure results in strong anchorage between the exsolved particles and the oxide host lattice, which imparts both high thermal stability and unique catalytic activity.

Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution.

Metal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic applications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal-oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimension for tailoring particle-substrate interactions in the context of increasing interest for emergent interfacial phenomena.

Dynamic structural evolution of supported palladium-ceria core-shell catalysts revealed by in situ electron microscopy.

The exceptional activity for methane combustion of modular palladium-ceria core-shell subunits on silicon-functionalized alumina that was recently reported has created renewed interest in the potential of core-shell structures as catalysts. Here we report on our use of advanced ex situ and in situ electron microscopy with atomic resolution to show that the modular palladium-ceria core-shell subunits undergo structural evolution over a wide temperature range. In situ observations performed in an atmospheric gas cell within this temperature range provide real-time evidence that the palladium and ceria nanoparticle constituents of the palladium-ceria core-shell participate in a dynamical process that leads to the formation of an unanticipated structure comprised of an intimate mixture of palladium, cerium, silicon and oxygen, with very high dispersion. This finding may open new perspectives about the origin of the activity of this catalyst.

Surface modification of La(0.8)Sr(0.2)CrO(3-δ)-YSZ dual-phase membranes for syngas production.

Oxygen permeation fluxes were studied in Mixed Ionic and Electronic Conducting (MIEC) membranes based on composites of 40 vol% La(0.8)Sr(0.2)CrO3 (LSCr) and 60 vol% yttria-stabilized zirconia (YSZ), using ambient air and flowing CO to establish a P(O2) gradient. The ambipolar conductivity of the dense LSCr-YSZ composite was determined for membranes with dense layers that were 115 µm and 650 µm thick. Other parts of the investigation focused on how modifications to the surface on the CO side affected the fluxes. Using a porous LSCr-YSZ composite on the surface as the base case, oxygen fluxes were shown to increase dramatically upon addition of 5 wt% CeO2 as a catalyst and an additional increase was observed with 1 wt% Pt. Changes in the structure of the porous composite LSCr-YSZ surface to improve connectivity of the YSZ phase also led to large increases in the oxygen fluxes.

Efficient removal of organic ligands from supported nanocrystals by fast thermal annealing enables catalytic studies on well-defined active phases.

A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely applicable to nanostructures of different sizes, shapes, and compositions. Being rapid and effective, this procedure allows the production of monodisperse heterogeneous catalysts for studying a variety of structure-activity relationships. We show here results on methane steam reforming, where the particle size controls the CO/CO2 ratio on alumina-supported Pd, demonstrating the potential applications of the method in catalysis.

Playing with structures at the nanoscale: designing catalysts by manipulation of clusters and nanocrystals as building blocks.

The purpose of this Concept is to highlight some of the most recent and promising methods for the preparation of tailored catalysts by designing and preparing the component building blocks and by assembling them in a controlled fashion. We want to emphasize how rational design and synthesis of catalysts must be coupled to precise catalytic and structural characterization of the systems in an ideal feedback loop. New catalyst design and preparation techniques, dictated by information about the active sites that the specific application requires, are frequently available. The building blocks for developing these novel catalysts include colloidal methods for the preparation of uniform nanostructures, physical methods for rational assembly of the building blocks (Langmuir-Blodgett, liquid-air self-assembly), and development of rational interactions between the building blocks for enhanced activity of the assemblies. These methods, which apply techniques normally used in other fields of nanotechnology to catalysis, offer exciting opportunities to help improve currently available catalytic systems in terms of activity, stability and selectivity.

Control of metal nanocrystal size reveals metal-support interface role for ceria catalysts.

Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts. We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals. We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support.