Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu-Exchanged Zeolites

In the past, Cu-oxo or -hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site's stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu-exchanged zeolite SSZ-13. Phase diagrams developed from first-principles suggest that Cu-hydroxy or Cu-oxo dimers are stabilized when O2 or N2O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu-oxo dimers can convert twice as much methane to methanol compared to Cu-hydroxyl dimers. Our theoretical models rationalize how Cu-di-oxo dimers can convert up to two methane molecules to methanol, while Cu-di-hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure-activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol.

Actuating Liquid Crystals Rapidly and Reversibly by Using Chemical Catalysis

Microtubules and catalytic motor proteins underlie the microscale actuation of living materials, and they have been used in reconstituted systems to harness chemical energy to drive new states of organization of soft matter (e.g., liquid crystals (LCs)). Such materials, however, are fragile and challenging to translate to technological contexts. Rapid (sub-second) and reversible changes in the orientations of LCs at room temperature using reactions between gaseous hydrogen and oxygen that are catalyzed by Pd/Au surfaces are reported. Surface chemical analysis and computational chemistry studies confirm that dissociative adsorption of H2 on the Pd/Au films reduces preadsorbed O and generates 1 ML of adsorbed H, driving nitrile-containing LCs from a perpendicular to a planar orientation. Subsequent exposure to O2 leads to oxidation of the adsorbed H, reformation of adsorbed O on the Pd/Au surface, and a return of the LC to its initial orientation. The roles of surface composition and reaction kinetics in determining the LC dynamics are described along with a proof-of-concept demonstration of microactuation of beads. These results provide fresh ideas for utilizing chemical energy and catalysis to reversibly actuate functional LCs on the microscale.

Atomically Dispersed Zn/Co-N-C as ORR Electrocatalysts for Alkaline Fuel Cells

Hydrogen fuel cells have drawn increasing attention as one of the most promising next-generation power sources for future automotive transportation. Developing efficient, durable, and low-cost electrocatalysts, to accelerate the sluggish oxygen reduction reaction (ORR) kinetics, is urgently needed to advance fuel cell technologies. Herein, we report on metal-organic frameworks-derived nonprecious dual metal single-atom catalysts (SACs) (Zn/Co-N-C), consisting of Co-N4 and Zn-N4 local structures. These catalysts exhibited superior ORR activity with a half-wave potential (E1/2) of 0.938 V versus RHE (reversible hydrogen electrode) and robust stability (ΔE1/2 = -8.5 mV) after 50k electrochemical cycles. Moreover, this remarkable performance was validated under realistic fuel cell working conditions, achieving a record-high peak power density of ∼1 W cm-2 among the reported SACs for alkaline fuel cells. Operando X-ray absorption spectroscopy was conducted to identify the active sites and reveal catalytic mechanistic insights. The results indicated that the Co atom in the Co-N4 structure was the main catalytically active center, where one axial oxygenated species binds to form an Oads-Co-N4 moiety during the ORR. In addition, theoretical studies, based on a potential-dependent microkinetic model and core-level shift calculations, showed good agreement with the experimental results and provided insights into the bonding of oxygen species on Co-N4 centers during the ORR. This work provides a comprehensive mechanistic understanding of the active sites in the Zn/Co-N-C catalysts and will pave the way for the future design and advancement of high-performance single-site electrocatalysts for fuel cells and other energy applications.

Molecular-scale Insights into Cooperativity Switching of xTAB Adsorption on Gold Nanoparticles

Quantifying adsorption behaviors is crucial for various applications such as catalysis, separation, and sensing, yet it is generally challenging to access in solution. Here, we report a combined experimental and computational study of the adsorption behaviors of alkyl-trimethylammonium bromides (xTAB), a class of ligands important for colloidal nanoparticle stabilization and shape control, with various alkyl chain lengths x on Au nanoparticles. We use density functional theory (DFT) to calculate xTAB binding energies on Au{111} and Au{110} surfaces with standing-up and lying-down configurations, which provides insights into the adsorption affinity and cooperativity differences of xTAB on these two facets. We demonstrate the key role of van der Waals interactions in determining the xTAB adsorption behavior. These computational results predict and explain the experimental discovery of xTAB's adsorption behavior switch from stronger affinity, negative cooperativity to weaker affinity, positive cooperativity when the concentration of xTAB increases in solution. We also show that in the standing-up configuration, bilayer adsorption may occur on both facets, which can lead to different differential binding energies and consequently adsorption crossover between the two facets when the ligand concentration increases. Our combined experimental and computational approaches demonstrate a paradigm for gaining molecular-scale insights into adsorbate-surface interactions.

Thermal Stability of Au Rhombic Dodecahedral Nanocrystals Can Be Greatly Enhanced by Coating Their Surface with an Ultrathin Shell of Pt

Rhombic dodecahedral nanocrystals have been considered particularly difficult to synthesize because they are enclosed by {110}, a low-index facet with the greatest surface energy. Recently, we demonstrated the use of seed-mediated growth for the facile and robust synthesis of Au rhombic dodecahedral nanocrystals (AuRD). While the unique shape and surface structure of AuRD are desirable for potential applications in plasmonics and catalysis, respectively, their high surface energy makes them highly susceptible to thermal degradation. Here we demonstrate that it is feasible to greatly improve the thermal stability with some sacrifice to the plasmonic properties of the original AuRD by coating their surface with an ultrathin shell made of Pt. Our in situ electron microscopy analysis indicates that the ultrathin Pt coating can increase the thermal stability from 60 up to 450 °C, a trend that is also supported by the results from a computational study.

Elucidating Molecular-Scale Principles Governing the Anchoring of Liquid Crystal Mixtures on Solid Surfaces

Computational chemistry calculations are broadly useful for guiding the atom-scale design of hard-soft material interfaces including how molecular interactions of single-component liquid crystals (LCs) at inorganic surfaces lead to preferred orientations of the LC far from the surface. The majority of LCs, however, are not single-component phases but comprise of mixtures, such as a mixture of mesogens, added to provide additional functions such as responsiveness to the presence of targeted organic compounds (for chemical sensing). In such LC mixtures, little is understood about the near-surface composition and organization of molecules and how that organization propagates into the far-field LC orientation. Here, we address this broad question by using a multiscale computational approach that combines density functional theory (DFT)-based calculations and classical molecular dynamics (MD) simulations to predict the interfacial composition and organization of a binary LC mixture of 4'-cyano-4-biphenylcarbolxylic acid (CBCA) and 4'-n-pentyl-4-biphenylcarbonitrile (5CB) supported on anatase (101) titania surfaces. DFT calculations determine the surface composition and atomic-scale organization of CBCA and 5CB at the titania surface, and classical MD simulations build upon the DFT description to describe the evolution of the near-surface order into the bulk LC. A surprising finding is that the 5CB and CBCA molecules adopt orthogonal orientations at the anatase surface and that, above a threshold concentration of CBCA, this mixture of orientations evolves away from the surface to define a uniform far-field homeotropic orientation. These results demonstrate that molecular-level knowledge achieved through a combination of computational techniques permits the design and understanding of functional LC mixtures at interfaces.

Fast and Non-equilibrium Uptake of Hydrogen by Pd Icosahedral Nanocrystals

We report for the first time that Pd nanocrystals can absorb H via a "single-phase pathway" when particles with a proper combination of shape and size are used. Specifically, when Pd icosahedral nanocrystals of 7- and 12-nm in size are exposed to H atoms, the H-saturated twin boundaries can divide each particle into 20 smaller single-crystal units in which the formation of phase boundaries is no longer favored. As such, absorption of H atoms is dominated by the single-phase pathway and one can readily obtain PdHx with anyx in the range of 0-0.7. When switched to Pd octahedral nanocrystals, the single-phase pathway is only observed for particles of 7 nm in size. We also establish that the H-absorption kinetics will be accelerated if there is a tensile strain in the nanocrystals due to the increase in lattice spacing. Besides the unique H-absorption behaviors, the PdHx (x=0-0.7) icosahedral nanocrystals show remarkable thermal and catalytic stability toward the formic acid oxidation due tothe decrease in chemical potential for H atoms in a Pd lattice under tensile strain.

Adsorbate-Induced Adatom Formation on Lithium, Iron, Cobalt, Ruthenium, and Rhenium Surfaces

Recent experimental and theoretical studies have demonstrated the reaction-driven metal-metal bond breaking in metal catalytic surfaces even under relatively mild conditions. Here, we construct a density functional theory (DFT) database for the adsorbate-induced adatom formation energy on the close-packed facets of three hexagonal close-packed metals (Co, Ru, and Re) and two body-centered cubic metals (Li and Fe), where the source of the ejected metal atom is either a step edge or a close-packed surface. For Co and Ru, we also considered their metastable face-centered cubic structures. We studied 18 different adsorbates relevant to catalytic processes and predicted noticeably easier adatom formation on Li and Fe compared to the other three metals. The NH3- and CO-induced adatom formation on Fe(110) is possible at room temperature, a result relevant to NH3 synthesis and Fischer-Tropsch synthesis, respectively. There also exist other systems with favorable adsorbate effects for adatom formation relevant to catalytic processes at elevated temperatures (500-700 K). Our results offer insight into the reaction-driven formation of metal clusters, which could play the role of active sites in reactions catalyzed by Li, Fe, Co, Ru, and Re catalysts.

Hydroformylation of pyrolysis oils to aldehydes and alcohols from polyolefin waste

Waste plastics are an abundant feedstock for the production of renewable chemicals. Pyrolysis of waste plastics produces pyrolysis oils with high concentrations of olefins (>50 weight %). The traditional petrochemical industry uses several energy-intensive steps to produce olefins from fossil feedstocks such as naphtha, natural gas, and crude oil. In this work, we demonstrate that pyrolysis oil can be used to produce aldehydes through hydroformylation, taking advantage of the olefin functionality. These aldehydes can then be reduced to mono- and dialcohols, oxidized to mono- and dicarboxylic acids, or aminated to mono- and diamines by using homogeneous and heterogeneous catalysis. This route produces high-value oxygenated chemicals from low-value postconsumer recycled polyethylene. We project that the chemicals produced by this route could lower greenhouse gas emissions ~60% compared with their production through petroleum feedstocks.

Gas-phase microactuation using kinetically controlled surface states of ultrathin catalytic sheets

Biological systems convert chemical energy into mechanical work by using protein catalysts that assume kinetically controlled conformational states. Synthetic chemomechanical systems using chemical catalysis have been reported, but they are slow, require high temperatures to operate, or indirectly perform work by harnessing reaction products in liquids (e.g., heat or protons). Here, we introduce a bioinspired chemical strategy for gas-phase chemomechanical transduction that sequences the elementary steps of catalytic reactions on ultrathin (<10 nm) platinum sheets to generate surface stresses that directly drive microactuation (bending radii of 700 nm) at ambient conditions (T = 20 °C; Ptotal = 1 atm). When fueled by hydrogen gas and either oxygen or ozone gas, we show how kinetically controlled surface states of the catalyst can be exploited to achieve fast actuation (600 ms/cycle) at 20 °C. We also show that the approach can integrate photochemically controlled reactions and can be used to drive the reconfiguration of microhinges and complex origami- and kirigami-based microstructures.

Formation of active sites on transition metals through reaction-driven migration of surface atoms

Adopting low-index single-crystal surfaces as models for metal nanoparticle catalysts has been questioned by the experimental findings of adsorbate-induced formation of subnanometer clusters on several single-crystal surfaces. We used density functional theory calculations to elucidate the conditions that lead to cluster formation and show how adatom formation energies enable efficient screening of the conditions required for adsorbate-induced cluster formation. We studied a combination of eight face-centered cubic transition metals and 18 common surface intermediates and identified systems relevant to catalytic reactions, such as carbon monoxide (CO) oxidation and ammonia (NH₃) oxidation. We used kinetic Monte Carlo simulations to elucidate the CO-induced cluster formation process on a copper surface. Scanning tunneling microscopy of CO on a nickel (111) surface that contains steps and dislocations points to the structure sensitivity of this phenomenon. Metal-metal bond breaking that leads to the evolution of catalyst structures under realistic reaction conditions occurs much more broadly than previously thought.

Optical method for quantifying the potential of zero charge at the platinum-water electrochemical interface

When an electrode contacts an electrolyte, an interfacial electric field forms. This interfacial field can polarize the electrode's surface and nearby molecules, but its effect can be countered by an applied potential. Quantifying the value of this countering potential ('potential of zero charge' (pzc)) is, however, not straightforward. Here we present an optical method for determining the pzc at an electrochemical interface. Our approach uses phase-sensitive second-harmonic generation to determine the electrochemical potential where the interfacial electric field vanishes at an electrode-electrolyte interface with Pt-water as a model experiment. Our method reveals that the pzc of the Pt-water interface is 0.23 ± 0.08 V versus standard hydrogen electrode (SHE) and is pH independent from pH 1 to pH 13. First-principles calculations with a hybrid explicit-implicit solvent model predict the pzc of the Pt(111)-water interface to be 0.23 V versus SHE and reveal how the interfacial water structure rearranges as the electrode potential is moved above and below the pzc. We further show that pzc is sensitive to surface modification; deposition of Ni on Pt shifts the interfacial pzc in the cathodic direction by ~360 mV. Our work demonstrates a materials-agnostic approach for quantifying the interfacial electrical field and water orientation at an electrochemical interface without requiring probe molecules and confirms the long-held view that the interfacial electric field is more intense during hydrogen electrocatalysis in alkaline than in acid.

Sensing Gas Mixtures by Analyzing the Spatiotemporal Optical Responses of Liquid Crystals Using 3D Convolutional Neural Networks

We report how analysis of the spatial and temporal optical responses of liquid crystal (LC) films to targeted gases, when performed using a machine learning methodology, can advance the sensing of gas mixtures and provide important insights into the physical processes that underlie the sensor response. We develop the methodology using O₃ and Cl₂ mixtures (representative of an important class of analytes) and LCs supported on metal perchlorate-decorated surfaces as a model system. Although O₃ and Cl₂ both diffuse through LC films and undergo redox reactions with the supporting metal perchlorate surfaces to generate similar initial and final optical states of the LCs, we show that a three-dimensional convolutional neural network can extract feature information that is encoded in the spatiotemporal color patterns of the LCs to detect the presence of both O₃ and Cl₂ species in mixtures and to quantify their concentrations. Our analysis reveals that O₃ detection is driven by the transition time over which the brightness of the LC changes, while Cl₂ detection is driven by color fluctuations that develop late in the optical response of the LC. We also show that we can detect the presence of Cl₂ even when the concentration of O₃ is orders of magnitude greater than the Cl₂ concentration. The proposed methodology is generalizable to a wide range of analytes, reactive surfaces, and LCs and has the potential to advance the design of portable LC monitoring devices (e.g., wearable devices) for analyzing gas mixtures using spatiotemporal color fluctuations.

Ordering Transitions of Liquid Crystals Triggered by Metal Oxide-catalyzed Reactions of Sulfur Oxide Species

Liquid crystals (LCs), when supported on reactive surfaces, undergo changes in ordering that can propagate over distances of micrometers, thus providing a general and facile mechanism to amplify atomic-scale transformations on surfaces into the optical scale. While reactions on organic and metal substrates have been coupled to LC-ordering transitions, metal oxide substrates, which offer unique catalytic activities for reactions involving atmospherically important chemical species such as oxidized sulfur species, have not been explored. Here, we investigate this opportunity by designing LCs that contain 4'-cyanobiphenyl-4-carboxylic acid (CBCA) and respond to surface reactions triggered by parts-per-billion concentrations of SO₂ gas on anatase (101) substrates. We used electronic structure calculations to predict that the carboxylic acid group of CBCA binds strongly to anatase (101) in a perpendicular orientation, a prediction that we validated in experiments in which CBCA (0.005 mol %) was doped into an LC (4'-n-pentyl-4-biphenylcarbonitrile). Both experiment and computational modeling further demonstrated that SO₃-like species, produced by a surface-catalyzed reaction of SO₂ with H₂O on anatase (101), displace CBCA from the anatase surface, resulting in an orientational transition of the LC. Experiments also reveal the LC response to be highly selective to SO₂ over other atmospheric chemical species (including H₂O, NH₃, H₂S, and NO₂), in agreement with our computational predictions for anatase (101) surfaces. Overall, we establish that the catalytic activities of metal oxide surfaces offer the basis of a new class of substrates that trigger LCs to undergo ordering transitions in response to chemical species of relevance to atmospheric chemistry.

Phase-Controlled Synthesis of Ru Nanocrystals via Template-Directed Growth: Surface Energy versus Bulk Energy

Despite the successful control of crystal phase using template-directed growth, much remains unknown about the underlying mechanisms. Here, we demonstrate that the crystal phase taken by the deposited metal depends on the lateral size of face-centered cubic (fcc)-Pd nanoplate templates with 12 nm plates giving fcc-Ru while 18-26 nm plates result in hexagonal closed-packed (hcp)-Ru. Although Ru overlayers with a metastable fcc- (high in bulk energy) or stable hcp-phase (low in bulk energy) can be epitaxially deposited on the basal planes, the lattice mismatch will lead to jagged hcp- (high in surface energy) and smooth fcc-facets (low in surface energy), respectively, on the side faces. As the proportion of basal and side faces on the nanoplates varies with lateral size, the crystal phase will change depending on the relative contributions from the surface and bulk energies. The Pd@fcc-Ru outperforms the Pd@hcp-Ru nanoplates toward ethylene glycol and glycerol oxidation reactions.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Solution-Phase Synthesis of PdH0.706 Nanocubes with Enhanced Stability and Activity toward Formic Acid Oxidation

Palladium is one of the few metals capable of forming hydrides, with the catalytic properties being dependent on the elemental composition and spatial distribution of H atoms in the lattice. Herein, we report a facile method for the complete transformation of Pd nanocubes into a stable phase made of PdH_0.706 by treating them with aqueous hydrazine at a concentration as low as 9.2 mM. Using formic acid oxidation (FAO) as a model reaction, we systematically investigated the structure-catalytic property relationship of the resultant nanocubes with different degrees of hydride formation. The current density at 0.4 V was enhanced by four times when the nanocubes were completely converted from Pd to PdH_0.706. On the basis of a set of slab models with PdH(100) overlayers on Pd(100), we conducted density functional theory calculations to demonstrate that the degree of hybrid formation could influence both the activity and selectivity toward FAO by modulating the relative stability of formate (HCOO) and carboxyl (COOH) intermediates. This work provides a viable strategy for augmenting the performance of Pd-based catalysts toward various reactions without altering the loading of this scarce metal.

Identifying hydroxylated copper dimers in SSZ-13 via UV-vis-NIR spectroscopy

Potential active sites for the conversion of methane to methanol in Cu-exchanged SSZ-13 are identified using a combination of experimentally measured UV-vis-NIR spectroscopy and theoretical modeling.

Facile Synthesis of Palladium-Based Nanocrystals with Different Crystal Phases and a Comparison of Their Catalytic Properties

A relatively unexplored aspect of noble-metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core-shell nanocrystals featuring polymorphism, with the core made of hexagonally close-packed (hcp)-Ru while the Pd shell takes either an hcp or face-centered cubic (fcc) phase. The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking the hcp phase whereas the thicker ones revert to fcc. The injection rate provides an experimental knob for controlling the phase, with one-shot and drop-wise injection of the Pd precursor corresponding to fcc-Pd and hcp-Pd shells, respectively. When these nanocrystals are tested as catalysts toward formic acid oxidation, the Ru@Pd_hcp nanocrystals outperform Ru@Pd_fcc in terms of both specific activity and peak potential. Density functional theory calculations are also performed to elucidate the origin of this performance enhancement.

Steam-created grain boundaries for methane C-H activation in palladium catalysts

[Figure: see text].

Coupling the chemical reactivity of bimetallic surfaces to the orientations of liquid crystals

The development of responsive soft materials with tailored functional properties based on the chemical reactivity of atomically precise inorganic interfaces has not been widely explored. In this communication, guided by first-principles calculations, we design bimetallic surfaces comprised of atomically thin Pd layers deposited onto Au that anchor nematic liquid crystalline phases of 4'-n-pentyl-4-biphenylcarbonitrile (5CB) and demonstrate that the chemical reactivity of these bimetallic surfaces towards Cl₂ gas can be tuned by specification of the composition of the surface alloy. Specifically, we use underpotential deposition to prepare submonolayer to multilayers of Pd on Au and employ X-ray photoelectron and infrared spectroscopy to validate computational predictions that binding of 5CB depends strongly on the Pd coverage, with ∼0.1 monolayer (ML) of Pd sufficient to cause the liquid crystal (LC) to adopt a perpendicular binding mode. Computed heats of dissociative adsorption of Cl₂ on PdAu alloy surfaces predict displacement of 5CB from these surfaces, a result that is also confirmed by experiments revealing that 1 ppm Cl₂ triggers orientational transitions of 5CB. By decreasing the coverage of Pd on Au from 1.8 ± 0.2 ML to 0.09 ± 0.02 ML, the dynamic response of 5CB to 1 ppm Cl₂ is accelerated 3X. Overall, these results demonstrate the promise of hybrid designs of responsive materials based on atomically precise interfaces formed between hard bimetallic surfaces and soft matter.

Atomistic insights into the nucleation and growth of platinum on palladium nanocrystals

Despite the large number of reports on colloidal nanocrystals, very little is known about the mechanistic details in terms of nucleation and growth at the atomistic level. Taking bimetallic core-shell nanocrystals as an example, here we integrate in situ liquid-cell transmission electron microscopy with first-principles calculations to shed light on the atomistic details involved in the nucleation and growth of Pt on Pd cubic seeds. We elucidate the roles played by key synthesis parameters, including capping agent and precursor concentration, in controlling the nucleation site, diffusion path, and growth pattern of the Pt atoms. When the faces of a cubic seed are capped by Br-, Pt atoms preferentially nucleate from corners and then diffuse to edges and faces for the creation of a uniform shell. The diffusion does not occur until the Pt deposited at the corner has reached a threshold thickness. At a high concentration of the precursor, self-nucleation takes place and the Pt clusters then randomly attach to the surface of a seed for the formation of a non-uniform shell. These atomistic insights offer a general guideline for the rational synthesis of nanocrystals with diverse compositions, structures, shapes, and related properties.

Janus Nanocages of Platinum-Group Metals and Their Use as Effective Dual-Electrocatalysts

Janus nanocages with distinctive platinum-group metals on the outer and inner surfaces can naturally catalyze at least two different reactions. Here we report a general method based on successive deposition and then selective etching for the facile synthesis of such nanocages. We have fabricated 11 different types of Janus nanocages characterized by a uniform size and well-defined {100} facets, together with porous, ultrathin, asymmetric walls up to 1.6 nm thick. When tested as dual-electrocatalysts toward oxygen reduction and evolution reactions, the Janus nanocages based on Pt and Ir exhibited superior activities depending on the thickness and relative position of the metal layer. Density functional theory studies suggest that the alloy composition and surface structure of the nanocages both play important roles in enhancing the electrocatalytic activities by modulating the stability of key reaction intermediates.

Physical Transformations of Noble-Metal Nanocrystals upon Thermal Activation

ConspectusThe last two decades have witnessed the successful development of noble-metal nanocrystals with well-controlled properties for a variety of applications in catalysis, plasmonics, electronics, and biomedicine. Most of these nanocrystals are kinetically controlled products greatly deviated from the equilibrium state defined by thermodynamics. When subjected to elevated temperatures, their arrangements of atoms are expected to undergo various physical transformations, inducing changes to the shape, morphology (hollow vs solid), spatial distribution of elements (segregated vs alloyed/intermetallic), internal structure (twinned vs single-crystal), and crystal phase. In order to optimize the performance of these nanocrystals in various applications, there is a pressing need to understand and improve their thermal stability.By integrating in situ heating with transmission electron microscopy or X-ray diffraction, we have investigated the physical transformations of various types of noble-metal nanocrystals in real time. We have also explored the atomistic detail responsible for a physical transformation using first-principles calculations, providing insightful guidance for the development of noble-metal nanocrystals with augmented thermal stability. Specifically, solid nanocrystals were observed to transform into pseudospherical particles favored by thermodynamics by reducing the surface area while eliminating the facets high in surface energy. For nanocrystals of relatively large in size, a single-crystal lattice was more favorable than a twinned structure. When switching to core-shell nanocrystals, the elevation in temperature caused changes to the elemental distribution in addition to shape transformation. The compositional stability of a core-shell nanocrystal was found to be strongly dependent on the shape and thus the type of facet expressed on the surface. For hollow nanocrystals such as nanocages and nanoframes, their thermal stabilities were typically inferior to the solid counterparts, albeit their unique structure and large specific surface area are highly desired in applications such as catalysis. When a metastable crystal structure was involved, phase transition was also observed at a temperature close to that responsible for shape or compositional change. We hope the principles, methodologies, and mechanistic insights presented in this Account will help the readers achieve a good understanding of the physical transformations that are expected to take place in noble-metal nanocrystals when they are subjected to thermal activation. Such an understanding may eventually lead to the development of effective methods for retarding or even preventing some of the transformations.

Mechanism of methanol synthesis on Ni(110)

Planewave density functional theory (DFT-PW91) calculations are employed to study the methanol synthesis through CO₂ and CO hydrogenation, as well as the two side reactions: the water gas shift (WGS) reaction and the formic acid formation, on Ni(110).

Computational Methods in Heterogeneous Catalysis

The unprecedented ability of computations to probe atomic-level details of catalytic systems holds immense promise for the fundamentals-based bottom-up design of novel heterogeneous catalysts, which are at the heart of the chemical and energy sectors of industry. Here, we critically analyze recent advances in computational heterogeneous catalysis. First, we will survey the progress in electronic structure methods and atomistic catalyst models employed, which have enabled the catalysis community to build increasingly intricate, realistic, and accurate models of the active sites of supported transition-metal catalysts. We then review developments in microkinetic modeling, specifically mean-field microkinetic models and kinetic Monte Carlo simulations, which bridge the gap between nanoscale computational insights and macroscale experimental kinetics data with increasing fidelity. We finally review the advancements in theoretical methods for accelerating catalyst design and discovery. Throughout the review, we provide ample examples of applications, discuss remaining challenges, and provide our outlook for the near future.

Eliminating dissolution of platinum-based electrocatalysts at the atomic scale

A remaining challenge for the deployment of proton-exchange membrane fuel cells is the limited durability of platinum (Pt) nanoscale materials that operate at high voltages during the cathodic oxygen reduction reaction. In this work, atomic-scale insight into well-defined single-crystalline, thin-film and nanoscale surfaces exposed Pt dissolution trends that governed the design and synthesis of durable materials. A newly defined metric, intrinsic dissolution, is essential to understanding the correlation between the measured Pt loss, surface structure, size and ratio of Pt nanoparticles in a carbon (C) support. It was found that the utilization of a gold (Au) underlayer promotes ordering of Pt surface atoms towards a (111) structure, whereas Au on the surface selectively protects low-coordinated Pt sites. This mitigation strategy was applied towards 3 nm Pt₃Au/C nanoparticles and resulted in the elimination of Pt dissolution in the liquid electrolyte, which included a 30-fold durability improvement versus 3 nm Pt/C over an extended potential range up to 1.2 V.

Author Correction: Eliminating dissolution of platinum-based electrocatalysts at the atomic scale

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Combining Computational Modeling with Reaction Kinetics Experiments for Elucidating the In Situ Nature of the Active Site in Catalysis

Microkinetic modeling based on density functional theory (DFT) derived energetics is important for addressing fundamental questions in catalysis. The quantitative fidelity of microkinetic models (MKMs), however, is often insufficient to conclusively infer the mechanistic details of a specific catalytic system. This can be attributed to a number of factors such as an incorrect model of the active site for which DFT calculations are performed, deficiencies in the hypothesized reaction mechanism, inadequate consideration of the surface environment under reaction conditions, and intrinsic errors in the DFT exchange-correlation functional. Despite these limitations, we aim at developing a rigorous understanding of the reaction mechanism and of the nature of the active site for heterogeneous catalytic chemistries under reaction conditions. By achieving parity between experimental and modeling outcomes through robust parameter estimation and by ensuring coverage-consistency between DFT calculations and MKM predictions, it is possible to systematically refine the mechanistic model and, thereby, our understanding of the catalytic active site in situ.Our general approach consists of developing ab initio informed MKM for a given active site and then re-estimating the energies of the transition and intermediate states so that the model predictions match quantities measured in reaction kinetics experiments. If (i) model-experiment parity is high, (ii) the adjustments to the DFT-derived energetics for a given model of the active site are rationalized within the errors of standard DFT exchange-correlation functionals, and (iii) the resultant MKM predicts surface coverages that are consistent with those assumed in the DFT calculations used to initialize the MKM, we conclude that we have correctly identified the active site and the reaction mechanism. If one or more of these requirements are not met, we iteratively refine our model by updating our hypothesis for the structure of the active site and/or by incorporating coverage effects, until we obtain a high-fidelity coverage-self-consistent MKM whose final kinetic and thermodynamic parameters are within error of the values derived from DFT.Using the catalytic reaction of formic acid (FA, HCOOH) decomposition over transition-metal catalysts as an example, here we provide an account of how we applied this algorithm to study this chemistry on powder Au/SiC and Pt/C catalysts. For the case of Au catalysts, on which the FA decomposition occurred exclusively through the dehydrogenation reaction (HCOOH → CO₂+H₂), our approach was used to iteratively refine the model starting from the (111) facet until we found that specific ensembles of Au atoms present in sub-nanometer clusters can describe the active site for this catalysis. For the case of Pt catalysts, wherein both dehydrogenation (HCOOH → CO₂ + H₂) and dehydration (HCOOH → CO + H₂O) reactions were active, our approach identified that a partially CO*-covered (111) surface serves as the active site and that CO*-assisted steps contributed substantially to the overall FA decomposition activity. Finally, we suggest that once the active site and the mechanism are conclusively identified, the model can subsequently serve as a high-quality basis for designing specific goal-oriented experiments and improved catalysts.

Site-dependent reactivity of MoS2 nanoparticles in hydrodesulfurization of thiophene

The catalytically active site for the removal of S from organosulfur compounds in catalytic hydrodesulfurization has been attributed to a generic site at an S-vacancy on the edge of MoS2 particles. However, steric constraints in adsorption and variations in S-coordination means that not all S-vacancy sites should be considered equally active. Here, we use a combination of atom-resolved scanning probe microscopy and density functional theory to reveal how the generation of S-vacancies within MoS2 nanoparticles and the subsequent adsorption of thiophene (C4H4S) depends strongly on the location on the edge of MoS2. Thiophene adsorbs directly at open corner vacancy sites, however, we find that its adsorption at S-vacancy sites away from the MoS2 particle corners leads to an activated and concerted displacement of neighboring edge S. This mechanism allows the reactant to self-generate a double CUS site that reduces steric effects in more constrained sites along the edge.

Binding of Organophosphorus Nerve Agents and Their Simulants to Metal Salts

Nerve agents (NAs) pose a great threat to society because they are easy to produce and are deadly in nature, which makes developing methods to detect, adsorb, and destroy them crucial. To enable the development of these methods, we report the use of first principles electronic structure calculations to understand the binding properties of NAs and NA simulants on metal salt surfaces. We report calculated Gibbs free binding energies (G_BE) for four NAs (tabun (GA), sarin (GB), soman (GD), and venomous X (VX)) and five NA simulants (dimethyl methylphosphonate (DMMP), dimethyl chlorophosphate (DMCP), trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and di-isopropyl methylphosphonate (DIMP)) on metal perchlorate and metal nitrate salts using density functional theory. Our results indicate a general trend in the binding strength of NAs and NA simulants to metal salt surfaces: MDCP < DMCP < GA < GD ≈ GB < TMP < VX ≈ DMMP < DIMP. Based on their binding properties on salt surfaces, we identify the most effective simulant for each of the studied NAs as follows: DMCP for GA, TMP for GB and GD, and DMMP for VX. To illustrate the utility of the binding energies calculated in our study, we address the design of NA sensors based on the competitive binding of NAs and liquid crystalline compounds on metal salts. We compare our results with previous experimental findings and provide a list of promising combinations of liquid crystal and metal salt systems to selectively and sensitively detect NAs. Our study highlights the great value of computational chemistry for designing selective and sensitive NA sensors while minimizing the number of very dangerous experiments involving NAs.