General Method to Synthesize Highly Stable Nanoclusters via Pickering-Stabilized Microemulsions.

The ability to not only control but also maintain the well-defined size of nanoclusters is key to a scientific understanding as well as their practical application. Here, we report a synthetic protocol to prepare and stabilize nanoclusters of different metals and even metal salts. The approach builds on a Pickering stabilization effect inside a microemulsion system. We prove that the emulsion interface plays a critical role in the formation of nanoclusters, which are encapsulated in situ into a silica matrix. The resulting nanocapsule is characterized by a central cavity and a porous shell composed of a matrix of both silica and nanoclusters. This structure endows the nanoclusters simultaneously with high thermal stability, good biocompatibility, and excellent photostability, making them well suited for fundamental studies and practical applications ranging from materials chemistry, catalysis, and optics to bioimaging.

CuZrO3: If it exists it should be a sandwich.

CuZrO3 has been hypothesized to be a catalytic material with potential applications for CO2 reduction. Unfortunately, this material has received limited attention in the literature, and to the best of our knowledge the exact crystal structure is still unknown. To address this challenge, we utilize several different structural prediction techniques in concert, including the Universal Structure Predictor: Evolutionary Xtallography (USPEX), the Materials Project Structure Predictor, and the Open Quantum Materials Database (OQMD). Leveraging these structural prediction techniques in conjunction with Density-Functional Theory (DFT) calculations, we determine a possible structure for CuZrO3, which resembles a "sandwich" morphology. Our calculations reveal that this new structure is significantly lower in energy than a previously hypothesized perovskite structure, albeit it still has a thermodynamic preference to decompose into CuO and ZrO2. In addition, we experimentally tried to synthesize CuZrO3 based on literature reports and compared computational to experimental X-ray Diffraction (XRD) patterns confirming that the final product is a mixture of CuO and ZrO2. Finally, we conducted a computational surface energetics and CO2 adsorption study on our discovered sandwich morphology, demonstrating that CO2 can adsorb and activate on the material. However, these CO2 adsorption results deviate from previously reported results further confirming that the CuZrO3 is a metastable form and may not be experimentally accessible as a well-mixed oxide, since phase segregation to CuO and ZrO2 is preferred. Taken together, our combined computational and experimental study provides evidence that the synthesis of CuZrO3 is extremely difficult and if this oxide exists, it should have a sandwich-like morphology.

Optimization of High-Entropy Alloy Catalyst for Ammonia Decomposition and Ammonia Synthesis.

The successful synthesis of high-entropy alloy (HEA) nanoparticles, a long-sought goal in materials science, opens a new frontier in materials science with applications across catalysis, structural alloys, and energetic materials. Recently, a Co25Mo45Fe10Ni10Cu10 HEA made of earth-abundant elements was shown to have a high catalytic activity for ammonia decomposition, which rivals that of state-of-the-art, but prohibitively expensive, ruthenium catalysts. Using a computational approach based on first-principles calculations in conjunction with data analytics and machine learning, we build a model to rapidly compute the adsorption energy of H, N, and NHx (x = 1, 2, 3) species on CoMoFeNiCu alloy surfaces with varied alloy compositions and atomic arrangements. We show that the 25/45 Co/Mo ratio identified experimentally as the most active composition for ammonia decomposition increases the likelihood that the surface adsorbs nitrogen equivalently to that of ruthenium while at the same time interacting moderately strongly with intermediates. Our study underscores the importance of computational modeling and machine learning to identify and optimize HEA alloys across their near-infinite materials design space.

In situ environmental TEM observation of two-stage shrinking of Cu2O islands on Cu(100) during methanol reduction.

The structural dynamics of Cu catalyst regeneration from Cu2O under methanol is poorly understood. In situ Environmental TEM on Cu(100)-supported Cu2O islands reveals a transition from anisotropic to isotropic shrinking during reduction. Two-stage reduction is statistically supported and explained by preferential methanol reactivity on Cu2O nano-islands with DFT simulations.

H2/CO2 separations in multicomponent metal-adeninate MOFs with multiple chemically distinct pore environments.

Metal-organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn-Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H2/CO2 separation ability of bMOF-200. We examine the H2/CO2 separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H2/CO2 separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H2 and CO2 and a significantly improved H2/CO2 separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H2/CO2 separations.

Designing Open Metal Sites in Metal-Organic Frameworks for Paraffin/Olefin Separations.

Incorporating open metal sites (OMS) into metal-organic frameworks allows design of well-defined binding sites for selective molecular adsorption, which has a profound impact on catalysis and separations. We demonstrate that Cu(I) sites incorporated into MFU-4l preferentially adsorb olefins over paraffins. Density functional theory (DFT) calculations show that the OMS are independent, with no dependence of binding energy on olefin loading up to one olefin per Cu(I). Experimentally, increasing Cu(I) loading increased olefin uptake without affecting the binding energy, as predicted by DFT and confirmed by temperature-programmed desorption. The potential of this material for olefin/paraffin separation under ambient conditions was investigated by gas adsorption and column breakthrough experiments for an equimolar ratio of olefin/paraffin. High-grade propylene and ethylene (>99.999%) can be generated using temperature-concentration swing recycling from a Cu(I)-MFU-4l packed column with no measurable paraffin breakthrough.

Design of Copper-Based Bimetallic Nanoparticles for Carbon Dioxide Adsorption and Activation.

Cu-based nanoparticles (NPs) are promising candidates for the catalytic hydrogenation of CO2 to useful chemicals because of their low cost. However, CO2 adsorption and activation on Cu is not feasible. In this work we demonstrate a computational framework that identifies Cu-based bimetallic NPs able to adsorb and activate CO2 based on DFT calculations. We screen a series of heteroatoms on Cu-based NPs based on their preference to occupy a surface site on the NP and to adsorb and activate CO2 . We revealed two descriptors for CO2 adsorption on the bimetallic NPs, the heteroatom (i) local d-band center and (ii) electropositivity, which both drive an effective charge transfer from the NP to CO2 . We identified the CuZr bimetallic NP as a candidate nanostructure for CO2 adsorption and showed that although the Zr sites can be oxidized because of their high oxophilicity, they are still able to adsorb and activate CO2 strongly. Importantly, our computational results are verified by targeted synthesis, characterization, and CO2 adsorption experiments that demonstrate that i) Zr segregates on the surface of Cu, ii) Zr is oxidized to form a bimetallic mixed CuZr oxide catalyst, which iii) can strongly adsorb CO2 , whereas Cu NPs cannot. Overall our work highlights the importance of the generation of binding sites on a NP surface based on (catalyst) stability and electronic structure properties, which can lead to the design of more effective CO2 reduction catalysts.

The Developmental Toxicity of Complex Silica-Embedded Nickel Nanoparticles Is Determined by Their Physicochemical Properties.

Complex engineered nanomaterials (CENs) are a rapidly developing class of structurally and compositionally complex materials that are expected to dominate the next generation of functional nanomaterials. The development of methods enabling rapid assessment of the toxicity risk associated with this type of nanomaterial is therefore critically important. We evaluated the toxicity of three differently structured nickel-silica nanomaterials as prototypical CENs: simple, surface-deposited Ni-SiO2 and hollow and non-hollow core-shell Ni@SiO2 materials (i.e., ~1-2 nm Ni nanoparticles embedded into porous silica shells with and without a central cavity, respectively). Zebrafish embryos were exposed to these CENs, and morphological (survival and malformations) and physiological (larval motility) endpoints were coupled with thorough characterization of physiochemical characteristics (including agglomeration, settling and nickel ion dissolution) to determine how toxicity differed between these CENs and equivalent quantities of Ni2+ salt (based on total Ni). Exposure to Ni2+ ions strongly compromised zebrafish larva viability, and surviving larvae showed severe malformations. In contrast, exposure to the equivalent amount of Ni CEN did not result in these abnormalities. Interestingly, exposure to Ni-SiO2 and hollow Ni@SiO2 provoked abnormalities of zebrafish larval motor function, indicating developmental toxicity, while non-hollow Ni@SiO2 showed no toxicity. Correlating these observations with physicochemical characterization of the CENs suggests that the toxicity of the Ni-SiO2 and hollow Ni@SiO2 material may result partly from an increased effective exposure at the bottom of the well due to rapid settling. Overall, our data suggest that embedding nickel NPs in a porous silica matrix may be a straightforward way to mitigate their toxicity without compromising their functional properties. At the same time, our results also indicate that it is critical to consider modification of the effective exposure when comparing different nanomaterial configurations, because effective exposure might influence NP toxicity more than specific "nano-chemistry" effects.

Controlled Embedding of Metal Oxide Nanoparticles in ZSM-5 Zeolites through Preencapsulation and Timed Release.

We report a straightforward and transferrable synthesis strategy to encapsulate metal oxide nanoparticles (NPs) in mesoporous ZSM-5 via the encapsulation of NPs into silica followed by conversion of the NP@silica precursor to NP@ZSM-5. The systematic bottom-up approach allows for straightforward, precise control of both the metal weight loading and size of the embedded NP and yields uniform NP@ZSM-5 microspheres composed of stacked ZSM-5 nanorods with substantial mesoporosity. Key to the synthesis is the timed release of the embedded NPs during dissolution of the silica matrix in the hydrothermal conversion step, which finely balances the rate of NP release with the rate of SiO2 dissolution and the subsequent nucleation of aluminosilicate. The synthesis approach is demonstrated for Zn, Fe, and Ni oxide encapsulation in ZSM-5 but can be expected to be broadly transferrable for the encapsulation of metal and metal oxide nanoparticles into other zeolite structures.

Accurate amorphous silica surface models from first-principles thermodynamics of surface dehydroxylation.

Accurate atomically detailed models of amorphous materials have been elusive to-date due to limitations in both experimental data and computational methods. We present an approach for constructing atomistic models of amorphous silica surfaces encountered in many industrial applications (such as catalytic support materials). We have used a combination of classical molecular modeling and density functional theory calculations to develop models having predictive capabilities. Our approach provides accurate surface models for a range of temperatures as measured by the thermodynamics of surface dehydroxylation. We find that a surprisingly small model of an amorphous silica surface can accurately represent the physics and chemistry of real surfaces as demonstrated by direct experimental validation using macroscopic measurements of the silanol number and type as a function of temperature. Beyond accurately predicting the experimentally observed trends in silanol numbers and types, the model also allows new insights into the dehydroxylation of amorphous silica surfaces. Our formalism is transferrable and provides an approach to generating accurate models of other amorphous materials.

Stabilizing metal nanoparticles for heterogeneous catalysis.

Metal nanoparticles hold great promise for heterogeneous catalysis due to their high dispersion, large concentration of highly undercoordinated surface sites, and the presence of quantum confinement effects, which can drastically alter their reactivity. However, the poor thermal stability of nano-sized particles limits their use to low temperature conditions and constitutes one of the key hurdles towards industrial application. The present perspective paper briefly reviews the mechanisms underlying nanoparticle sintering, and then gives an overview of emerging approaches towards stabilizing metal nanoparticles for heterogeneous catalysis. We conclude by highlighting the current needs for further developments in the field.

Exceptional high-temperature stability through distillation-like self-stabilization in bimetallic nanoparticles.

Metal nanoparticles with precisely controlled size and composition are highly attractive for heterogeneous catalysis. However, their poor thermal stability remains a major hurdle on the way towards application at realistic technical conditions. Recent progress in this area has focused on nanostructured oxides to stabilize embedded metal nanoparticles. Here, we report an alternative approach that relies on synthesizing bimetallic nanoparticles with precise compositional control to obtain improved high-temperature stability. We find that PtRh nanoparticles with sufficiently high Rh content survive extended calcination at temperatures up to approximately 850 degrees C without significant sintering. For lower Rh content, sacrificial self-stabilization of individual nanoparticles through a distillation-like process is observed: the low-melting-point metal (Pt) bleeds out and the increasing concentration of the high-melting-point metal (Rh) leads to re-stabilization of the remaining nanoparticle. This principle of thermal self-stabilization should be broadly applicable to the development of multi-metallic nanomaterials for a broad range of high-temperature applications.