Cu Based Dilute Alloys for Tuning the C2+ Selectivity of Electrochemical CO2 Reduction.

Electrochemical CO2 reduction is a promising technology for replacing fossil fuel feedstocks in the chemical industry but further improvements in catalyst selectivity need to be made. So far, only copper-based catalysts have shown efficient conversion of CO2 into the desired multi-carbon (C2+) products. This work explores Cu-based dilute alloys to systematically tune the energy landscape of CO2 electrolysis toward C2+ products. Selection of the dilute alloy components is guided by grand canonical density functional theory simulations using the calculated binding energies of the reaction intermediates CO*, CHO*, and OCCO* dimer as descriptors for the selectivity toward C2+ products. A physical vapor deposition catalyst testing platform is employed to isolate the effect of alloy composition on the C2+/C1 product branching ratio without interference from catalyst morphology or catalyst integration. Six dilute alloy catalysts are prepared and tested with respect to their C2+/C1 product ratio using different electrolyzer environments including selected tests in a 100-cm2 electrolyzer. Consistent with theory, CuAl, CuB, CuGa and especially CuSc show increased selectivity toward C2+ products by making CO dimerization energetically more favorable on the dominant Cu facets, demonstrating the power of using the dilute alloy approach to tune the selectivity of CO2 electrolysis.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites.

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts.

The dissociation of H2 is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H2-D2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H2-D2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H2 is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H2-D2 exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H2 pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H2-D2 exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Ultra-low-density digitally architected carbon with a strutted tube-in-tube structure.

Porous materials with engineered stretching-dominated lattice designs, which offer attractive mechanical properties with ultra-light weight and large surface area for wide-ranging applications, have recently achieved near-ideal linear scaling between stiffness and density. Here, rather than optimizing the microlattice topology, we explore a different approach to strengthen low-density structural materials by designing tube-in-tube beam structures. We develop a process to transform fully dense, three-dimensional printed polymeric beams into graphitic carbon hollow tube-in-tube sandwich morphologies, where, similar to grass stems, the inner and outer tubes are connected through a network of struts. Compression tests and computational modelling show that this change in beam morphology dramatically slows down the decrease in stiffness with decreasing density. In situ pillar compression experiments further demonstrate large deformation recovery after 30-50% compression and high specific damping merit index. Our strutted tube-in-tube design opens up the space and realizes highly desirable high modulus-low density and high modulus-high damping material structures.

Mechanistic and Electronic Insights into a Working NiAu Single-Atom Alloy Ethanol Dehydrogenation Catalyst.

Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multipronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective nonoxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge that increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogeneous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step toward developing structure-function relationships that inform the design of new catalysts.

Evaluating the stability and activity of dilute Cu-based alloys for electrochemical CO2 reduction.

Cu-based catalysts currently offer the most promising route to actively and selectively produce value-added chemicals via electrochemical reduction of CO2 (eCO2R); yet further improvements are required for their wide-scale deployment in carbon mitigation efforts. Here, we systematically investigate a family of dilute Cu-based alloys to explore their viability as active and selective catalysts for eCO2R through a combined theoretical-experimental approach. Using a quantum-classical modeling approach that accounts for dynamic solvation effects, we assess the stability and activity of model single-atom catalysts under eCO2R conditions. Our calculations identify that the presence of eCO2R intermediates, such as CO*, H*, and OH*, may dynamically influence the local catalyst surface composition. Additionally, we identify through binding energy descriptors of the CO*, CHO*, and OCCO* dimer intermediates that certain elements, such as group 13 elements (B, Al, and Ga), enhance the selectivity of C2+ species relative to pure Cu by facilitating CO dimerization. The theoretical work is corroborated by preliminary testing of eCO2R activity and selectivity of candidate dilute Cu-based alloy catalyst films prepared by electron beam evaporation in a zero-gap gas diffusion electrode-based reactor. Of all studied alloys, dilute CuAl was found to be the most active and selective toward C2+ products like ethylene, consistent with the theoretical predictions. We attribute the improved performance of dilute CuAl alloys to more favorable dimerization reaction energetics of bound CO species relative to that on pure Cu. In a broader context, the results presented here demonstrate the power of our simulation framework in terms of rational catalyst design.

Dynamic restructuring drives catalytic activity on nanoporous gold-silver alloy catalysts.

Bimetallic, nanostructured materials hold promise for improving catalyst activity and selectivity, yet little is known about the dynamic compositional and structural changes that these systems undergo during pretreatment that leads to efficient catalyst function. Here we use ozone-activated silver-gold alloys in the form of nanoporous gold as a case study to demonstrate the dynamic behaviour of bimetallic systems during activation to produce a functioning catalyst. We show that it is these dynamic changes that give rise to the observed catalytic activity. Advanced in situ electron microscopy and X-ray photoelectron spectroscopy are used to demonstrate that major restructuring and compositional changes occur along the path to catalytic function for selective alcohol oxidation. Transient kinetic measurements correlate the restructuring to three types of oxygen on the surface. The direct influence of changes in surface silver concentration and restructuring at the nanoscale on oxidation activity is demonstrated. Our results demonstrate that characterization of these dynamic changes is necessary to unlock the full potential of bimetallic catalytic materials.

Post-print UV curing method for improving the mechanical properties of prototypes derived from two-photon lithography.

Two photon polymerization (TPP) is a precise, reliable, and increasingly popular technique for rapid prototyping of micro-scale parts with sub-micron resolution. The materials of choice underlying this process are predominately acrylic resins cross-linked via free-radical polymerization. Due to the nature of the printing process, the derived parts are only partially cured and the corresponding mechanical properties, i.e. modulus and ultimate strength, are lower than if the material were cross-linked to the maximum extent. Herein, post-print curing via UV-driven radical generation, is demonstrated to increase the overall degree of cross-linking of low density, TPP-derived structures.

Catalyst design for enhanced sustainability through fundamental surface chemistry.

Decreasing energy consumption in the production of platform chemicals is necessary to improve the sustainability of the chemical industry, which is the largest consumer of delivered energy. The majority of industrial chemical transformations rely on catalysts, and therefore designing new materials that catalyse the production of important chemicals via more selective and energy-efficient processes is a promising pathway to reducing energy use by the chemical industry. Efficiently designing new catalysts benefits from an integrated approach involving fundamental experimental studies and theoretical modelling in addition to evaluation of materials under working catalytic conditions. In this review, we outline this approach in the context of a particular catalyst-nanoporous gold (npAu)-which is an unsupported, dilute AgAu alloy catalyst that is highly active for the selective oxidative transformation of alcohols. Fundamental surface science studies on Au single crystals and AgAu thin-film alloys in combination with theoretical modelling were used to identify the principles which define the reactivity of npAu and subsequently enabled prediction of new reactive pathways on this material. Specifically, weak van der Waals interactions are key to the selectivity of Au materials, including npAu. We also briefly describe other systems in which this integrated approach was applied.

Integrated Framework to Model Microstructure Evolution and Decipher the Microstructure-Property Relationship in Polymeric Porous Materials.

Unraveling the microstructure-property relationship is crucial for improving material performance and advancing the design of next-generation structural and functional materials. However, this is inherently challenging because it requires both the comprehensive quantification of microstructural features and the accurate assessment of corresponding properties. To meet these requirements, we developed an efficient and comprehensive integrated modeling framework, using polymeric porous materials as a representative model system. Our framework generates microstructures using a physics-based phase-field model, characterizes them using various average and localized microstructural features, and evaluates microstructure-aware properties, such as effective diffusivity, using an efficient Fourier-based perturbation numerical scheme. Additionally, the framework incorporates machine learning methods to decipher the intricate microstructure-property relationships. Our findings indicate that the connectivity of phase channels is the most critical microstructural descriptor for determining effective diffusivity, followed by the domain shape represented by curvature distribution, while the domain size has a minor impact. This comprehensive approach offers a novel framework for assessing microstructure-property relationships in polymer-based porous materials, paving the way for the development of advanced materials for diverse applications.

Planar defect nucleation and annihilation mechanisms in nanocontact plasticity of metal surfaces.

The incipient contact plasticity of metallic surfaces involves nucleation of crystalline defects. The present molecular dynamics simulations and nanoindentation experiments demonstrate that the current notion of nanocontact plasticity in fcc metals does not apply to high-strength bcc metals. We show that nanocontact plasticity in Ta-a model bcc metal-is triggered by thermal and loading-rate dependent (dynamic) nucleation of planar defects such as twins and unique {011} stacking fault bands. Nucleation of different planar defects depending on surface orientation leads to distinct signatures (pop ins) in the nanoindentation curves. Nanoscale plasticity is then ruled by an outstanding dynamical mechanism governing twin annihilation and subsequent emission of linear defects (full dislocations). While this investigation concerns Ta crystals, the present are landmark findings for other model bcc metals.

ALD functionalized nanoporous gold: thermal stability, mechanical properties, and catalytic activity.

Nanoporous metals have many technologically promising applications, but their tendency to coarsen limits their long-term stability and excludes high temperature applications. Here, we demonstrate that atomic layer deposition (ALD) can be used to stabilize and functionalize nanoporous metals. Specifically, we studied the effect of nanometer-thick alumina and titania ALD films on thermal stability, mechanical properties, and catalytic activity of nanoporous gold (np-Au). Our results demonstrate that even only 1 nm thick oxide films can stabilize the nanoscale morphology of np-Au up to 1,000°C, while simultaneously making the material stronger and stiffer. The catalytic activity of np-Au can be drastically increased by TiO(2) ALD coatings. Our results open the door to high-temperature sensor, actuator, and catalysis applications and functionalized electrodes for energy storage and harvesting applications.

Scalable Gas Diffusion Electrode Fabrication for Electrochemical CO2 Reduction Using Physical Vapor Deposition Methods.

Electrochemical CO2 reduction (ECR) promises the replacement of fossil fuels as the source of feedstock chemicals and seasonal storage of renewable energy. While much progress has been made in catalyst development and electrochemical reactor design, few studies have addressed the effect of catalyst integration on device performance. Using a microfluidic gas diffusion electrolyzer, we systematically studied the effect of thickness and the morphology of electron beam (EB) and magnetron-sputtered (MS) Cu catalyst coatings on ECR performance. We observed that EB-Cu outperforms MS-Cu in current density, selectivity, and energy efficiency, with 400 nm thick catalyst coatings performing the best. The superior performance of EB-Cu catalysts is assigned to their faceted surface morphology and sharper Cu/gas diffusion layer interface, which increases their hydrophobicity. Tests in a large-scale zero-gap electrolyzer yielded similar product selectivity distributions with an ethylene Faradaic efficiency of 39% at 200 mA/cm2, demonstrating the scalability for industrial ECR applications.

Refractive index matched polymeric and preceramic resins for height-scalable two-photon lithography.

Nanofabrication techniques that can generate large and complex 3D structures with nanoscale features are becoming increasingly important in the fields of biomedicine, micro-optics, and microfluidics. Direct laser writing via two-photon polymerization (DLW-TPP) is one such technique that relies on nonlinear absorption of light to form nanoscale 3D features. Although DLW-TPP provides the required nanoscale resolution, its built height is often limited to less than a millimetre. This height limitation is driven by the need to tightly focus the laser beam at arbitrary depths within the photopolymer. This requirement necessitates matching the photopolymer's refractive index to specific values but the required techniques have not been disseminated widely in the open scientific literature. To address this knowledge gap, we test two universal, different approaches to generate refractive index-matched polymeric and preceramic resins and demonstrate their performance by printing of fine submicron features in 3D structures as tall as 2.5 mm. Specifically, we achieve index-matching by mixing commercially-available resins or covalent modification of functional monomers. This work investigates the relationship of voxel shape to RI mismatch, and presents tuning of RI through mixing and covalent modification to a nonconventional material system of preceramic resin which has never been demonstrated before. We demonstrate the material flexibility by generating 3D silicon oxycarbide structures from preceramic resists while simultaneously eliminating the part-height limitation of conventional DLW-TPP.

Synthesis of graphene aerogel with high electrical conductivity.

We report the synthesis of ultra-low-density three-dimensional macroassemblies of graphene sheets that exhibit high electrical conductivities and large internal surface areas. These materials are prepared as monolithic solids from suspensions of single-layer graphene oxide in which organic sol-gel chemistry is used to cross-link the individual sheets. The resulting gels are supercritically dried and then thermally reduced to yield graphene aerogels with densities approaching 10 mg/cm(3). In contrast to methods that utilize physical cross-links between GO, this approach provides covalent carbon bonding between the graphene sheets. These graphene aerogels exhibit an improvement in bulk electrical conductivity of more than 2 orders of magnitude (∼1 × 10(2) S/m) compared to graphene assemblies with physical cross-links alone (∼5 × 10(-1) S/m). The graphene aerogels also possess large surface areas (584 m(2)/g) and pore volumes (2.96 cm(3)/g), making these materials viable candidates for use in energy storage, catalysis, and sensing applications.

Effect of surface chemistry on the stability of gold nanostructures.

Understanding the role of surface chemistry in the stability of nanostructured noble-metal materials is important for many technological applications but experimentally difficult to access and thus little understood. To develop a fundamental understanding of the effect of surface chemistry on both the formation and stabilization of self-organized gold nanostructures, we performed a series of controlled-environment annealing experiments on nanoporous gold (np-Au) and ion-bombarded Au(111) single-crystal surfaces. The annealing experiments on np-Au in ambient ozone were carried out to study the effect of adsorbed oxygen under dynamic conditions, whereas the ion-bombarded Au single-crystal surfaces were used as a model system to obtain atomic-scale information. Our results show that adsorbed oxygen stabilizes nanoscale gold structures at low temperatures whereas oxygen-induced mobilization of Au surface atoms seems to accelerate the coarsening under dynamic equilibrium conditions at higher temperatures.

On the Network Topology of Cross-Linked Acrylate Photopolymers: A Molecular Dynamics Case Study.

A reactive molecular dynamics approach is used to simulate cross-linking of acrylate polymer networks. By employing the same force field and reactive scheme and studying three representative multifunctional acrylate monomers, we isolate the importance of the nonreactive moieties within these model monomers. Analyses of reactive trajectories benchmark the estimated gel points, cyclomatic character, and spatially resolved cross-linking tendencies of the acrylates as a function of conversion. These insights into the similarities and differences of the polymerization and resulting networks suggest molecular mechanics as a useful tool in the rational design of photopolymerization resins.

Integration of Fullerenes as Electron Acceptors in 3D Graphene Networks: Enhanced Charge Transfer and Stability through Molecular Design.

Here, we report a concept that allows the integration of the characteristic properties of [60]fullerene in 3D graphene networks. In these systems, graphene provides high electrical conductivity and surface area while fullerenes add high electron affinity. We use molecular design to optimize the interaction between 3D graphene networks and fullerenes, specifically in the context of stability and charge transfer in an electrochemical environment. We demonstrated that the capacity of the 3D graphene network is significantly improved upon the addition of C60 and C60 monoadducts by providing additional acceptor states in the form of low-lying lowest unoccupied molecular orbitals of C60 and its derivative. Guided by experimental results and first-principles calculations, we synthesized and tested a C60 monoadduct with increased stability by strengthening the 3D graphene-C60 van-der-Waals interactions. The synthesis method and stabilization strategy presented here is expected to benefit the integration of graphene-C60 hybrid materials in solar cell and charge storage applications.

Origins and Implications of Interfacial Capacitance Enhancements in C60-Modified Graphene Supercapacitors.

Understanding and controlling the electrical response at a complex electrode-electrolyte interface is key to the development of next-generation supercapacitors and other electrochemical devices. In this work, we apply a theoretical framework based on the effective screening medium and reference interaction site model to explore the role of electrical double-layer (EDL) formation and its interplay with quantum capacitance in graphene-based supercapacitors. In addition to pristine graphene, we investigate a novel C60-modified graphene supercapacitor material, which promises higher charge-storage capacity. Beyond the expected enhancement in the quantum capacitance, we find that the introduction of C60 molecules significantly alters the EDL response. These changes in EDL are traced to the interplay between surface morphology and charge localization character and ultimately dominate the overall capacitive improvement in the hybrid system. Our study highlights a complex interplay among surface morphology, electronic structure, and interfacial capacitance, suggesting general improvement strategies for optimizing carbon-based supercapacitor materials.

Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing.

Monolithic nanoporous metals, derived from dealloying, have a unique bicontinuous solid/void structure that provides both large surface area and high electrical conductivity, making them ideal candidates for various energy applications. However, many of these applications would greatly benefit from the integration of an engineered hierarchical macroporous network structure that increases and directs mass transport. We report on 3D (three-dimensional)-printed hierarchical nanoporous gold (3DP-hnp-Au) with engineered nonrandom macroarchitectures by combining 3D printing and dealloying. The material exhibits three distinct structural length scales ranging from the digitally controlled macroporous network structure (10 to 1000 µm) to the nanoscale pore/ligament morphology (30 to 500 nm) controlled by dealloying. Supercapacitance, pressure drop, and catalysis measurements reveal that the 3D hierarchical nature of our printed nanoporous metals markedly improves mass transport and reaction rates for both liquids and gases. Our approach can be applied to a variety of alloy systems and has the potential to revolutionize the design of (electro-)chemical plants by changing the scaling relations between volume and catalyst surface area.