Platinum nanoparticle shape effects on benzene hydrogenation selectivity

Unprecedented selectivity behavior in the direct dehydrogenation of n-butane to n-butenes with similar active Pt nanoparticle size: unveiling structural and electronic characteristics of supported monometallic catalysts

In this work, supported Pt monometallic catalysts were prepared using oxide and carbon supports by conventional impregnation methods. Similar Pt metallic nanoparticle sizes (mean sizes about 1.8-2 nm) have been obtained using different Pt precursor loadings (0.3 to 5 wt%). For comparison, catalysts with larger nanoparticle sizes were prepared using the liquid phase reduction method. Characterization results indicate different electronic and structural characteristics for the Pt nanoparticles, comparing nanoparticles with similar and different sizes, implying that both the Pt loading and the preparation method affect the formation of different metallic phases. We used the direct dehydrogenation of n-butane to n-butenes reaction as a test reaction to study the catalytic behavior of the Pt nanoparticles obtained at different Pt atomic concentrations. Surprisingly, Pt catalysts with the lowest metallic loading show the highest selectivities to olefins. Besides, Pt catalysts supported on carbon materials showed higher selectivity to butenes than those supported on oxide materials, this was attributed to a higher electron density in the Pt active sites. Likewise, at low Pt loadings, the CNP-supported Pt nanoparticles could be confined at the defect in the nanotube structure as crystalline agglomerates of atoms with few layers or monolayers with very few surface adatom or stepped adatom nanostructures or simply as a group of atoms, thus creating active Pt sites that favor the dehydrogenation reaction over secondary reactions.

Ligands of Nanoparticles and Their Influence on the Morphologies of Nanoparticle-Based Films

Nanoparticle-based thin films are increasingly being used in various applications. One of the key factors that determines the properties and performances of these films is the type of ligands attached to the nanoparticle surfaces. While long-chain surfactants, such as oleic acid, are commonly employed to stabilize nanoparticles and ensure high monodispersity, these ligands often hinder charge transport due to their insulating nature. Although thermal annealing can remove the long-chain ligands, the removal process often introduces defects such as cracks and voids. In contrast, the use of short-chain organic or inorganic ligands can minimize interparticle distance, improving film conductivity, though challenges such as incomplete ligand exchange and residual barriers remain. Polymeric ligands, especially block copolymers, can also be employed to create films with tailored porosity. This review discusses the effects of various ligand types on the morphology and performance of nanoparticle-based films, highlighting the trade-offs between conductivity, structural integrity, and functionality.

An Outlook on Platinum-Based Active Ingredients for Dermatologic and Skincare Applications

Platinum-based materials exhibit a broad spectrum of biological activities, including antioxidant, anti-inflammatory, antimicrobial, and pro-collagen synthesis properties, making them particularly useful for various biomedical applications. This review summarizes the biological effects and therapeutic potential of platinum-based active ingredients in dermatological and skincare applications. We discuss their synthesis methods and their antioxidant, anti-inflammatory, antimicrobial, and collagen synthesis properties, which play essential roles in treating skin conditions including psoriasis and acne, as well as enhancing skin aesthetics in anti-aging products. Safety and sustainability concerns, including the need for green synthesis and comprehensive toxicological assessments to ensure safe topical applications, are also discussed. By providing an up-to-date overview of current research, we aim to highlight both the potential and the current challenges of platinum-based active ingredients in advancing dermatology and skincare solutions.

Advances and Challenges in Development of Transition Metal Catalysts for Heterogeneous Hydrogenation of Organic Compounds

Actual problems of development of catalysts for hydrogenation of heterocyclic compounds by hydrogen are summarized and discussed. The scope of review covers composites of nanoparticles of platinum group metals and 3d metals for heterogeneous catalytic processes. Such problems include increase of catalyst activity, which is important for reduction of precious metals content; development of new catalytic systems which do not contain metals of platinum group or contain cheaper analogues of Pd; control of factors which make influence on the selectivity of the catalysts; achievement of high reproducibility of the catalyst's performance and quality control of the catalysts. Own results of the authors are also summarized and described. The catalysts were prepared by decomposition of Pd0 and Ni0 complexes, pyrolysis of Ni2+ and Co2+ complexes deposited on aerosil and reduction of Ni2+ in pores of porous support in situ. The developed catalysts were used for hydrogenation of multigram batches of heterocyclic compounds.

Development of Electrochemical Cells and Their Application for Spatially Resolved Analysis Using a Multitechnique Approach: From Conventional Experiments to X-Ray Nanoprobe Beamlines

Real (electro)catalysts are often heterogeneous, and their activity and selectivity depend on the properties of specific active sites. Therefore, unveiling the so-called structure-activity relationship is essential for a rational search for better materials and, consequently, for the development of the field of (electro-)catalysis. Thus, spatially resolved techniques are powerful tools as they allow us to characterize and/or measure the activity and selectivity of different regions of heterogeneous catalysts. To take full advantage of that, we have developed spectroelectrochemical cells to perform spatially resolved analysis using X-ray nanoprobe synchrotron beamlines and conventional pieces of equipment. Here, we describe the techniques available at the Carnaúba beamline at the Sirius-LNLS storage ring, and then we show how our cells enable obtaining X-ray (XRF, XRD, XAS, etc.) and vibrational spectroscopy (FTIR and Raman) contrast images. Through some proof-of-concept experiments, we demonstrate how using a multi-technique approach could render a complete and detailed analysis of an (electro)catalyst overall performance.

Tuning the chemical composition of binary alloy nanoparticles to prevent their dissolution

The dissolution of nanoparticles under corrosive environments represents one of the main issues in electrochemical processes. Here, a model for alloying and protecting nanoparticles from corrosion with an anti-corrosive element (e.g. Au) is proposed based on the hypothesis that under-coordinated atoms are the first atoms to dissolve. The model considers the dissolution of atoms with coordination number ≤6 on A-B nanoparticles with different sizes, shapes, chemical compositions, and exposed crystallographic orientations. The results revealed that the nanoparticle's size and chemical composition play a key role in the dissolution, suggesting that a certain composition of an element with corrosive resistance could be used to protect nanoparticles. DFT simulations were performed to support our model on the dissolution of four types of atoms commonly found on the surface of Au0.20Pd0.80 binary alloys - terrace, edge, kink, and ad atoms. The simulations suggest that the less coordinated ad and kink Pd atoms on Au0.20Pd0.80 alloys are dissolved in a potential window between 0.26-0.56 V, while the rest of the Pd and Au atoms are protected. Furthermore, to show that a corrosion-resistant element can indeed protect nanoparticles, we experimentally investigated the electrochemical dissolution of immobilized Pd, Au0.20Pd0.80, and Au0.40Pd0.60 nanoparticles in a harsh environment. In line with the dissolution model, the experimental results show that an Au molar fraction of the nanoparticle of 0.20, i.e., Au0.20Pd0.80 binary alloy, is a good compromise between maximizing the active surface area (Pd atoms) and corrosion protection by the inactive Au.

Recent advances in nanoflowers: compositional and structural diversification for potential applications

In recent years, nanoscience and nanotechnology have emerged as promising fields in materials science. Spectroscopic techniques like scanning tunneling microscopy and atomic force microscopy have revolutionized the characterization, manipulation, and size control of nanomaterials, enabling the creation of diverse materials such as fullerenes, graphene, nanotubes, nanofibers, nanorods, nanowires, nanoparticles, nanocones, and nanosheets. Among these nanomaterials, there has been considerable interest in flower-shaped hierarchical 3D nanostructures, known as nanoflowers. These structures offer advantages like a higher surface-to-volume ratio compared to spherical nanoparticles, cost-effectiveness, and environmentally friendly preparation methods. Researchers have explored various applications of 3D nanostructures with unique morphologies derived from different nanoflowers. The nanoflowers are classified as organic, inorganic and hybrid, and the hybrids are a combination thereof, and most research studies of the nanoflowers have been focused on biomedical applications. Intriguingly, among them, inorganic nanoflowers have been studied extensively in various areas, such as electro, photo, and chemical catalysis, sensors, supercapacitors, and batteries, owing to their high catalytic efficiency and optical characteristics, which arise from their composition, crystal structure, and local surface plasmon resonance (LSPR). Despite the significant interest in inorganic nanoflowers, comprehensive reviews on this topic have been scarce until now. This is the first review focusing on inorganic nanoflowers for applications in electro, photo, and chemical catalysts, sensors, supercapacitors, and batteries. Since the early 2000s, more than 350 papers have been published on this topic with many ongoing research projects. This review categorizes the reported inorganic nanoflowers into four groups based on their composition and structure: metal, metal oxide, alloy, and other nanoflowers, including silica, metal-metal oxide, core-shell, doped, coated, nitride, sulfide, phosphide, selenide, and telluride nanoflowers. The review thoroughly discusses the preparation methods, conditions for morphology and size control, mechanisms, characteristics, and potential applications of these nanoflowers, aiming to facilitate future research and promote highly effective and synergistic applications in various fields.

Computational approach for structure generation of anisotropic particles (CASGAP) with targeted distributions of particle design and orientational order

The macroscopic properties of materials are governed by their microscopic structure which depends on the materials' composition (i.e., building blocks) and processing conditions. In many classes of synthetic, bioinspired, or natural soft and/or nanomaterials, one can find structural anisotropy in the microscopic structure due to anisotropic building blocks and/or anisotropic domains formed through the processing conditions. Experimental characterization and complementary physics-based or data-driven modeling of materials' structural anisotropy are critical for understanding structure-property relationships and enabling targeted design of materials with desired macroscopic properties. In this pursuit, to interpret experimentally obtained characterization results (e.g., scattering profiles) of soft materials with structural anisotropy using data-driven computational approaches, there is a need for creating real space three-dimensional structures of the designer soft materials with realistic physical features (e.g., dispersity in building block sizes) and anisotropy (i.e., aspect ratios of the building blocks, their orientational and positional order). These real space structures can then be used to compute and complement experimentally obtained characterization results or be used as initial configurations for physics-based simulations/calculations that can then provide training data for machine learning models. To address this need, we present a new computational approach called CASGAP - Computational Approach for Structure Generation of Anisotropic Particles - for generating any desired three dimensional real-space structure of anisotropic building blocks (modeled as particles) adhering to target distributions of particle shape, size, and positional and orientational order.

Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal

This study demonstrates the intercalation of single-atom Ni (NiSA ) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2 /Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH species on the ORR activity. Notably, the NiO2 /Ni1 /Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second-step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross-level demonstrations, a structure-Eads -ΔE correspondence for the proposed NiO2 /Nin /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping-depth effect exploration of the NiSA in the NiO2 /Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA /PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.

Structural Size Effect in Capped Metallic Nanoparticles

The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle. First, our electron diffraction measurements reveals that NiPt nanoparticles around 4 nm in diameter covered by a mixture of oleylamine and oleic acid (50:50) display a lattice parameter expansion around 2% when compared to the same particles without surfactant. Using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) techniques, we show that this expansion can not be explained by crystal defects, twinning, oxidation, or atoms insertion. Then, using covered NPs in the 4-22 nm size range, we show that the lattice parameter evolves linearly with the inverse of the NP size, as it is expected when a surface stress is present. Finally, the study is extended to pure nickel and pure platinum NPs, with different sizes, coated by different surfactants (oleylamine, trioctylphosphine, polyvinylpyrrolidone). The surfactants induce lattice parameter variations, whose magnitude could be related to the charge transfer between the surfactant and the particle surface.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Inserting Single-Atom Zn by Tannic Acid Confinement To Regulate the Selectivity of Pd Nanocatalysts for Hydrogenation Reactions

Precisely controlling the selectivity of nanocatalysts has always been a hot topic in heterogeneous catalysis but remains difficult owing to their complex and inhomogeneous catalytic sites. Herein, an effective strategy to regulate the chemoselectivity of Pd nanocatalysts for selective hydrogenation reactions by inserting single-atom Zn into Pd nanoparticles is reported. Taking advantage of the tannic acid coating-confinement strategy, small-sized Pd nanoparticles with inserted single-atom Zn are obtained on the O-doped carbon-coated alumina. Compared with the pure Pd nanocatalyst, the Pd nanocatalyst with single-atom Zn insertion exhibits prominent selectivity for the hydrogenation of p-iodonitrobenzene to afford the hydrodeiodination product instead of nitro hydrogenation ones. Further computational studies reveal that the single-atom Zn on Pd nanoparticles strengthens the adsorption of the nitro group to avoid its reduction and increases the d-band center of Pd atoms to facilitate the reduction of the iodo group, which leads to enhanced selectivity. This work provides new guidelines to tune the selectivity of nanocatalysts with guest single-atom sites.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Single Metal Atoms Embedded in the Surface of Pt Nanocatalysts: The Effect of Temperature and Hydrogen Pressure

Embedding energetically stable single metal atoms in the surface of Pt nanocatalysts exposed to varied temperature (T) and hydrogen pressure (P) could open up new possibilities in selective and dynamical engineering of alloyed Pt catalysts, particularly interesting for hydrogenation reactions. In this work, an environmental segregation energy model is developed to predict the stability and the surface composition evolution of 24 Metal M-promoted Pt surfaces (with M: Cu, Ag, Au, Ni, Pd, Co, Rh and Ir) under varied T and P. Counterintuitive to expectations, the results show that the more reactive alloy component (i.e., the one forming the strongest chemical bond with the hydrogen) is not the one that segregates to the surface. Moreover, using DFT-based Multi-Scaled Reconstruction (MSR) method and by extrapolation of M-promoted Pt nanoparticles (NPs), the shape dynamics of M-Pt are investigated under the same ranges of T and P. The results show that under low hydrogen pressure and high temperature ranges, Ag and Au—single atoms (and Cu to a less extent) are energetically stable on the surface of truncated octahedral and/or cuboctahedral shaped NPs. This indicated that coinage single-atoms might be used to tune the catalytic properties of Pt surface under hydrogen media. In contrast, bulk stability within wide range of temperature and pressure is predicted for all other M-single atoms, which might act as bulk promoters. This work provides insightful guides and understandings of M-promoted Pt NPs by predicting both the evolution of the shape and the surface compositions under reaction gas condition.

Continuous and Scalable Production of Platinum Nanocubes with Uniform and Controllable Sizes in Air-Free Droplet Reactors

Platinum (Pt) nanocrystals hold the key to a variety of catalytic applications, and those with a cubic shape are attractive as a reference catalyst due to their well-defined {100} facets on the surface. Here we demonstrate the use of droplet reactors as a viable platform for the continuous and scalable production of Pt nanocubes with uniform and controllable sizes. The synthesis was found to be sensitive to the O₂ from air because of the oxidative etching associated with the O₂/Br^- pair. As such, either silicone oil or an inert gas had to be employed as the carrier phase to keep the droplets isolated from air. By controlling the amounts of the precursor and halide ions, the edge length of the Pt nanocubes could be tuned from 5-7 nm. In the setting of a millifluidic device, the droplet reactors could be used to achieve a production rate as high as 31.8 mg min^-1, about 10-100 times greater than what has been reported in the literature. We also evaluated the electrocatalytic properties of the as-obtained Pt nanocubes toward the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). For the Pt nanocubes of 6 nm in edge length, they showed a specific activity of 0.27 mA cm^-2 toward ORR at 0.9 V and a specific activity of 0.96 mA cm^-2 toward MOR at the anodic potential.

Colloidal synthesis of nanoparticles: from bimetallic to high entropy alloys

At the nanoscale, the synthesis of a random alloy (i.e. without phase segregation, whatever the composition) by chemical synthesis remains a difficult task, even for simple binary type systems. In this context, a unique approach based on the colloidal route is proposed enabling the synthesis of face-centred cubic and monodisperse bimetallic, trimetallic, tetrametallic and pentametallic nanoparticles with diameters around 5 nm as solid solutions. The Fe-Co-Ni-Pt-Ru alloy (and its subsets) is considered a challenging task as each element has fairly different physico-chemical properties. Particles are prepared by temperature-assisted co-reduction of metal acetylacetonate precursors in the presence of surfactants. It is highlighted how the correlation between precursors' degradation temperatures and reduction potential values of the metal cations is the driving force to achieve a homogeneous distribution of all elements within the nanoparticles.

Electronic Metal-Support Interactions Between CuxO and ZnO for CuxO/ZnO Catalysts With Enhanced CO Oxidation Activity

Metal-support interaction has been one of the main topics of research on supported catalysts all the time. However, many other factors including the particle size, shape and chemical composition can have significant influences on the catalytic performance when considering the role of metal-support interaction. Herein, we have designed a series of CuxO/ZnO catalysts as examples to quantitatively investigate how the metal-support interaction influences the catalytic performance. The electronic metal-support interactions between CuxO and ZnO were regulated successfully without altering the structure of CuxO/ZnO catalyst. Due to the lower work function of ZnO, electrons would transfer from ZnO to CuO, which is favorable for the formation of higher active Cu species. Combined experimental and theoretical calculations revealed that electron-rich interface result from interaction was favorable for the adsorption of oxygen and CO oxidation reaction. Such strategy represents a new direction to boost the catalytic activity of supported catalysts in various applications.

Toward continuous production of high-quality nanomaterials using microfluidics: nanoengineering the shape, structure and chemical composition

Over the last decade, a multitude of synthesis strategies has been reported for the production of high-quality nanoparticles. Wet-chemical methods are generally the most efficient synthesis procedures since high control of crystallinity and physicochemical properties can be achieved. However, a number of challenges remain from inadequate reaction control during the nanocrystallization process; specifically variability, selectivity, scalability and safety. These shortcomings complicate the synthesis, make it difficult to obtain a uniform product with desired properties, and present serious limitations for scaling the production of colloidal nanocrystals from academic studies to industrial applications. Continuous flow reactors based on microfluidic principles offer potential solutions and advantages. The reproducibility of reaction conditions in microfluidics and therefore product quality have proved to exceed those obtained by batch processing. Considering that in nanoparticles' production not only is it crucial to control the particle size distribution, but also the shape and chemical composition, this review presents an overview of the current state-of-the-art in synthesis of anisotropic and faceted nanostructures by using microfluidics techniques. The review surveys the available tools that enable shape and chemical control, including secondary growth methods, active segmented flow, and photoinduced shape conversion. In addition, emphasis is placed on the available approaches developed to tune the structure and chemical composition of nanomaterials in order to produce complex heterostructures in a continuous and reproducible fashion.

Heterogeneous Trimetallic Nanoparticles as Catalysts

The development and application of trimetallic nanoparticles continues to accelerate rapidly as a result of advances in materials design, synthetic control, and reaction characterization. Following the technological successes of multicomponent materials in automotive exhausts and photovoltaics, synergistic effects are now accessible through the careful preparation of multielement particles, presenting exciting opportunities in the field of catalysis. In this review, we explore the methods currently used in the design, synthesis, analysis, and application of trimetallic nanoparticles across both the experimental and computational realms and provide a critical perspective on the emergent field of trimetallic nanocatalysts. Trimetallic nanoparticles are typically supported on high-surface-area metal oxides for catalytic applications, synthesized via preparative conditions that are comparable to those applied for mono- and bimetallic nanoparticles. However, controlled elemental segregation and subsequent characterization remain challenging because of the heterogeneous nature of the systems. The multielement composition exhibits beneficial synergy for important oxidation, dehydrogenation, and hydrogenation reactions; in some cases, this is realized through higher selectivity, while activity improvements are also observed. However, challenges related to identifying and harnessing influential characteristics for maximum productivity remain. Computation provides support for the experimental endeavors, for example in electrocatalysis, and a clear need is identified for the marriage of simulation, with respect to both combinatorial element screening and optimal reaction design, to experiment in order to maximize productivity from this nascent field. Clear challenges remain with respect to identifying, making, and applying trimetallic catalysts efficiently, but the foundations are now visible, and the outlook is strong for this exciting chemical field.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Reversible dehydrogenation and rehydrogenation of cyclohexane and methylcyclohexane by single-site platinum catalyst

Developing highly efficient and reversible hydrogenation-dehydrogenation catalysts shows great promise for hydrogen storage technologies with highly desirable economic and ecological benefits. Herein, we show that reaction sites consisting of single Pt atoms and neighboring oxygen vacancies (VO) can be prepared on CeO2 (Pt1/CeO2) with unique catalytic properties for the reversible dehydrogenation and rehydrogenation of large molecules such as cyclohexane and methylcyclohexane. Specifically, we find that the dehydrogenation rate of cyclohexane and methylcyclohexane on such sites can reach values above 32,000 molH2 molPt-1 h-1, which is 309 times higher than that of conventional supported Pt nanoparticles. Combining of DRIFTS, AP-XPS, EXAFS, and DFT calculations, we show that the Pt1/CeO2 catalyst exhibits a super-synergistic effect between the catalytic Pt atom and its support, involving redox coupling between Pt and Ce ions, enabling adsorption, activation and reaction of large molecules with sufficient versatility to drive abstraction/addition of hydrogen without requiring multiple reaction sites.

Evidencing the formation of Pt nano-islands on Cr2O3/Ag(111)

The present work reports on a comprehensive surface atomic structure investigation on the Pt/Cr2O3/Ag(111) model catalyst. Molecular beam epitaxy (MBE) was applied to achieve the Pt/Cr2O3 model system and in...

Correlating Surface Structures and Electrochemical Activity Using Shape-Controlled Single-Pt Nanoparticles

We report a method for synthesizing and studying shape-controlled, single Pt nanoparticles (NPs) supported on carbon nanoelectrodes. The key advance is that the synthetic method makes it possible to produce single, electrochemically active NPs with a vast range of crystal structures and sizes. Equally important, the NPs can be fully characterized, and, therefore, the electrochemical properties of the NPs can be directly correlated to the size and structure of a single shape. This makes it possible to directly correlate experimental results to first-principles theory. Because just one well-characterized NP is analyzed at a time, the difficulty of applying a theoretical analysis to an ensemble of NPs having different sizes and structures is avoided. In this article, we report on two specific Pt NP shapes having sizes on the order of 200 nm: concave hexoctahedral (HOH) and concave trapezohedral (TPH). The former has {15 6 1} facets and the latter {10 1 1} facets. The electrochemical properties of these single NPs for the formic acid oxidation (FAO) reaction are compared to those of a single, spherical polycrystalline Pt NP of the same size. Finally, density functional theory, performed prior to the electrochemical studies, were used to interpret the experimental results of the FAO experiments.

Tri-atomic Pt clusters induce effective pathways in a Cocore-Pdshell nanocatalyst surface for a high-performance oxygen reduction reaction

The crux of the hot topic concerning the widespread replacement of fuel cells (FCs) with traditional petrochemical energy is to balance improving the oxygen reduction reaction (ORR) and reducing the cost. The present study employs density functional theory (DFT) to investigate the effect of Pt ensemble size regulation from a single atom to full coverage on the physio-chemical properties, oxygen adsorption energies and overall ORR efficiency of bimetallic nanocatalysts (NCs) with a Co_core-Pd_shell structure. Our results reveal that the electronegativity difference and lattice strain between neighboring heteroatoms are enhanced to trigger a synergetic effect in local domains, with the Pt cluster size reduced from nanometers to subnanometers. They induce a directed and tunable charge relocation mechanism from deep Co to topmost Pt to optimize the adsorption energies of O₂/O* and achieve excellent ORR kinetics performance with minimum Pt usage but maximum Pt atom utilization (i.e., Pt₁ to Pt₃) compared with benchmark Pt(111). Such a dependency between the cluster size and corresponding ORR performance for the established Co@Pd-Pt_n system can be applied to accurately guide the experimental synthesis of ordered heterogeneous catalysts (e.g., other core@shell-clusters structures) toward low Pt, high efficiency and green economy.

Mesocrystals from Platinum Nanocubes

Platinum nanoparticles are widely known for their numerous electrochemical and catalytic applications. Enhanced or novel properties that may arise when ordering such particles in a highly defined manner, however, are still subject to ongoing research, as superstructure formation on the mesoscale is still a major challenge to be overcome. In this work, we therefore established a reproducible method to fabricate micrometer-sized superstructures from platinum nanocubes. Through small-angle X-ray scattering and electron diffraction methods we demonstrate that the obtained superstructures have a high degree of ordering up to the atomic scale and, therefore, fulfill all criteria of a mesocrystal. By changing the solvent and stabilizer in which the platinum nanocubes were dispersed, we were able to control the resulting crystal habit of the mesocrystals. Aside from mesocrystal fabrication, this method can be further utilized to purify nanoparticle dispersions by recrystallization with respect to narrowing down the particle size distribution and removing contaminations.

Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br^- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H₂ selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.

Synergetic effect dependence on activated oxygen in the interface of NiOx-modified Pt nanoparticles for the CO oxidation from first-principles

CO oxidation on NiO_x-modified Pt nanoparticles (NPs) was investigated by first-principles calculations and microkinetic methods. The binding energies of O₂ and CO on NiO_x/Pt suggest that CO adsorption is dominant and the CO oxidation mainly follows the Mars-van Krevelen (M-vK) mechanism. It was found that the interfacial O of NiO_x/Pt played a key role in the combination of adsorbed CO to O, as well as the O₂ dissociation. With a lower O vacancy formation energy, NiO_x/Pt_edge shows about four orders higher reaction rates than NiO_x/Pt₍₁₀₀₎. Microkinetic analysis suggests that the rate-determining step also depends on the active O at the interface. The calculations highlight the synergetic effect difference of NiO_x selectively deposited on the different sites of Pt NPs on the CO oxidation from the atomic reaction mechanism, and throws light on the high activity of CO oxidation on partially covered NiO_x/Pt_edge nanoparticles.

Facet Engineering of Pd Nanocrystals for Enhancing Photocatalytic Hydrogenation: Modulation of the Schottky Barrier Height and Enrichment of Surface Reactants

Metal cocatalyst loading is one of the most widely explored strategies in promoting photocatalytic solar energy conversion. Engineering surface-active facets of metal cocatalyst and exploring how they modulate the reactivity is crucial for the further development of advanced photocatalysts. In this work, through controlled hybridization of two-dimensional (2D) TiO₂ nanosheets with well-designed Pd nanocube (Pd NC) with exposed {100} facet and Pd nano-octahedron (NO) with exposed {111} facet, we unravel the distinct crystal facet effect of Pd cocatalyst in promoting the selective hydrogenation of nitroarenes to amines of TiO₂ photocatalyst. The activity tests show that the Pd NO with {111} facet is a more efficient cocatalyst than the Pd NC with exposed {100} facet. The prepared TiO₂-Pd NO composite displays a 900% enhancement of photocatalytic hydrogenation rate in comparison with bare TiO₂, while the TiO₂-Pd NC sample only shows a 200% photoactivity enhancement. Microscopic mechanism study discloses that the distinctive photoactivity improvement of Pd NO is ascribed to the concurrent modulation of the Schottky barrier height and enrichment of surface reactants: (i) the Pd NO with a lower Fermi level could result in steeper band bending of TiO₂ (i.e., higher Schottky barrier) than the Pd NC, which is more efficient in boosting interfacial separation and inhibiting the recombination of photoexcited charge pairs; and (ii) the {111} facet of Pd has higher nitroarenes adsorption ability and especially stronger hydrogen enrichment capability, thus accelerating the surface hydrogenation process and contributing to a higher reaction rate. This work emphasizes the rational facet control of cocatalysts for enhancing the photocatalytic hydrogenation performance.

Geometric and electronic properties of AulPtm (l + m ≤ 10) clusters: a first-principles study

The structural evolutions and electronic properties of AulPtm (l + m ≤ 10) clusters are investigated by using the first-principles methods. We use the inverse design of materials using the multi-objective differential evolution (IM2ODE) package to globally search the equilibrium structures and investigate the evolving trend from a two-dimensional structure to a three-dimensional structure on horizontal extension and vertical extension for AulPtm (l + m ≤ 10) clusters. The three-dimensional stable geometry of Au8Pt and Au8Pt2 is discovered for the first time in our work. We also notice that the equilibrium structures of AulPtm (l + m = 10 and l ≤ 8) tend to form a tetrahedral geometry and can be obtained by replacing the Au atom in the most stable structure of Aul+1Ptm-1 with the Pt atom, where Pt atoms assemble together and occupy the center of clusters and Au atoms prefer to lie on the vertex or edge position. The average binding energy (Eb) is mostly decided by Pt-Pt bond numbers, namely the numbers of Pt atoms, followed by Au-Pt bond numbers. The second-order energy difference (Δ2Ev and Δ2Eh) and the nearest-neighbor energy difference (Δ4Enn) show that Au6Pt, Au4Pt2, Au3Pt3, Au2Pt4 and AuPt7 clusters exhibit high relative physical stability, so we suggest that these clusters could be defined as the magic number clusters for AulPtm (l + m ≤ 10) clusters. The HOMO-LUMO energy gap (Eg), adiabatic ionization potential (AIP) and the adiabatic electron affinity (AEA) are also investigated to elaborate the relative electronic stability of all the clusters.

Using Reduction Kinetics to Control and Predict the Outcome of a Colloidal Synthesis of Noble-Metal Nanocrystals

Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the reduction kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief Viewpoint on the recent progress in leveraging reduction kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial reduction rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the reduction pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the preparation of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on reduction kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.

Correlating 3D Surface Atomic Structure and Catalytic Activities of Pt Nanocrystals

Active sites and catalytic activity of heterogeneous catalysts is determined by their surface atomic structures. However, probing the surface structure at an atomic resolution is difficult, especially for solution ensembles of catalytic nanocrystals, which consist of heterogeneous particles with irregular shapes and surfaces. Here, we constructed 3D maps of the coordination number (CN) and generalized CN (CN_) for individual surface atoms of sub-3 nm Pt nanocrystals. Our results reveal that the synthesized Pt nanocrystals are enclosed by islands of atoms with nonuniform shapes that lead to complex surface structures, including a high ratio of low-coordination surface atoms, reduced domain size of low-index facets, and various types of exposed high-index facets. 3D maps of CN_ are directly correlated to catalytic activities assigned to individual surface atoms with distinct local coordination structures, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reactions and CO electro-oxidation.

Solvent manipulation of the pre-reduction metal-ligand complex and particle-ligand binding for controlled synthesis of Pd nanoparticles

Understanding how to control the nucleation and growth rates is crucial for designing nanoparticles with specific sizes and shapes. In this study, we show that the nucleation and growth rates are correlated with the thermodynamics of metal-ligand/solvent binding for the pre-reduction complex and the surface of the nanoparticle, respectively. To obtain these correlations, we measured the nucleation and growth rates by in situ small angle X-ray scattering during the synthesis of colloidal Pd nanoparticles in the presence of trioctylphosphine in solvents of varying coordinating ability. The results show that the nucleation rate decreased, while the growth rate increased in the following order, toluene, piperidine, 3,4-lutidine and pyridine, leading to a large increase in the final nanoparticle size (from 1.4 nm in toluene to 5.0 nm in pyridine). Using density functional theory (DFT), complemented by 31P nuclear magnetic resonance and X-ray absorption spectroscopy, we calculated the reduction Gibbs free energies of the solvent-dependent dominant pre-reduction complex and the solvent-nanoparticle binding energy. The results indicate that lower nucleation rates originate from solvent coordination which stabilizes the pre-reduction complex and increases its reduction free energy. At the same time, DFT calculations suggest that the solvent coordination affects the effective capping of the surface where stronger binding solvents slow the nanoparticle growth by lowering the number of active sites (not already bound by trioctylphosphine). The findings represent a promising advancement towards understanding the microscopic connection between the metal-ligand thermodynamic interactions and the kinetics of nucleation and growth to control the size of colloidal metal nanoparticles.

Defect-assisted electronic metal-support interactions: tuning the interplay between Ru nanoparticles and CuO supports for pH-neutral oxygen evolution

Electronic metal-support interactions (EMSIs) comprise an area of intense study, the manipulation of which is of paramount importance in the improvement of heterogeneous metal nanoparticle (NP) supported catalysts. EMSI is the transfer of charge from the support to NP, enabling more effective adsorption and interaction of reactants during catalysis. Ru NPs on CuO supports show different levels of EMSI (via charge transfer) depending on their crystal structure, with multiple twinned NPs showing greater potential for EMSI. We use magnetron-assisted gas phase aggregation for the synthesis of batches of Ru NPs with different populations of single crystal and multiple twinned nanoparticles, which were deposited on CuO nanowires (NWs). The surface charging of the Ru-CuO catalysts was investigated by Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS). By doubling the population of multiple twinned NPs, the surface potential of the Ru-CuO catalysts increases roughly 4 times, coinciding with a similar increase in the amount of Ru⁴⁺. Therefore, tuning the amount of EMSI in a catalyst is possible through changing the population of multiple twinned Ru NPs in the catalyst. Increasing the amount of multiple twin NPs resulted in improved activity in the oxygen evolution reaction (a roughly 2.5 times decrease in the overpotentials when the population of multiple twinned NPs is increased) and better catalyst stability. This improvement is attributed to the fact that the multiple twin NPs maintained a metallic character under oxidation conditions (unlike single crystal NPs) due to the EMSI between the NP and support.

Cu oxide deposited on shape-controlled ceria nanocrystals for CO oxidation: influence of interface-driven oxidation states on catalytic activity

The polar CeO2 (100) surface facets contribute considerably to the formation of surface hydroxyl groups, which are necessary for selective, stable Cu1+ state loading and enhancement of CO oxidation activity.