On the origin of the catalytic activity of gold nanoparticles for low-temperature CO oxidation

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Strategies to improve hydrogen activation on gold catalysts

Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.

Manganese-oxide-supported gold catalyst derived from metal-organic frameworks for trace PCl3 oxidation in an organic system

Polysilicon is widely used in the field of semiconductors and solar energy. Trichlorosilane feedstocks that are used to produce polysilicon in the mainstream production process contain PCl3 impurities that have adverse effects on the quality of the polysilicon. Traditional methods for dephosphorization cannot achieve the effect of complete removal, whereas oxidizing PCl3 to POCl3 in the presence of oxygen for removal via adsorption is a promising and appealing route for establishing a dephosphorization process; it has a high phosphorous removal rate due to the strong Lewis-base property of POCl3 in comparison with PCl3. In this work, we synthesized an active catalyst with an active interface between Au nanoparticles (NPs) and a manganese-oxide support (Mn3O4) by calcination of a corresponding composite, where Au NPs were embedded uniformly in a metal-organic framework (MOF). The catalyst shows a significantly active catalytic performance for trace PCl3 oxidation in an organic system that is an imitation of a trichlorosilane system, with a 99.13% yield of POCl3 in an 80 °C and 0.6 MPa reaction environment. The structure-performance-mechanism analysis shows that the possible reaction and catalytic mechanism is PCl3 oxidation by interface lattice oxygens, which bridge the Au NPs and the support, in a Mars van Krevelen (MvK) process; this process was promoted by the interaction between the Au NPs and Mn3O4 in terms of charge transfer and chemical potential changes. This work provides an effective way to dephosphorize trichlorosilane feedstocks in the polysilicon industry and gives guidance for constructing an efficient catalyst via the study of the structure and mechanism.

Electrochemical investigations and antimicrobial activity of Au nanoparticles photodeposited on titania nanoparticles

Titanium oxide nanopowder (TiO2 NPs) was synthesized via anodization in 0.7 M perchloric acid then annealed in nitrogen at 450 °C for 3 h to prepared the Titanium Oxide Nitrogen annealed nanoparticles (TiO2 NPs-N2) powder as catalytic support. Using a photodeposition process, gold was added with isopropanol as a sacrificial donor and H[AuCl4] acid, producing gold nanoparticles on nitrogen-annealed titanium oxide nanoparticles (Au-NPs on TiO2-NPs-N2). The mass loading of Au NPs was 2.86 × 10-4 (g/cm2). TEM images of Au NPs on TiO2-NPs-N2 suggest circular particles with a tendency to agglomerate. Cyclic voltammetry (CV) was used to investigate the electrocatalytic performance of the Au NPs/TiO2-NPs-N2 catalysts in ferrocyanide, KOH, and H2SO4, and the results were compared to those of a polycrystalline Au electrode that is readily accessible in the market. In KOH, H2SO4, and (2 M KOH + 0.1 M glycerol) solutions, the Au NPs/TiO2-NPs-N2 electrode displayed a startlingly high electrocatalytic performance. Using CV, the electrocatalytic oxygen reduction reaction (ORR) of Au NPs/TiO2-NPs-N2 and Au-NPs against glycerol oxidation in basic media was studied. The results indicated that Au NPs/TiO2-NPs-N2 is a promising support material for improving the electrocatalytic activity for acidic and basic oxidation. The electrode made of Au NPs/TiO2-NTs-N2 has steady electrocatalytic activity and may be reused repeatedly. TiO2 NPs and Au NPs/TiO2NPs-N2 showed satisfactory antibacterial activity against some human pathogenic bacteria using the disc diffusion method.

In Situ Incorporation of Atomically Precise Au Nanoclusters within Zeolites for Ambient Temperature CO Oxidation

Preserving ultrasmall sizes of metal particles is a key challenge in the study of heterogeneous metal-based catalysis. Confining the ultrasmall metal clusters in a well-defined crystalline porous zeolite has emerged as a promising approach to stabilize these metal species. Successful encapsulation can be achieved by the addition of ligated metal complexes to zeolite synthesis gel before hydrothermal synthesis. However, controlling the metal particle size during post-reduction treatment remains a major challenge in this approach. Herein, an in situ incorporation strategy of pre-made atomically precise gold clusters within Na-LTA zeolite was established for the first time. With the assistance of mercaptosilane ligands, the gold clusters were successfully incorporated within the Na-LTA without premature precipitation and metal aggregation during the synthesis. We have demonstrated that the confinement of gold clusters within the zeolite framework offers high stability against sintering, leading to superior CO oxidation catalytic performance (up to 12 h at 30 °C, with a space velocity of 3000 mL g-1 h-1).

Mechanism and dynamics of Baeyer-Villiger oxidation of furfural to maleic anhydride in presence of H2O2 and Au clusters

CONTEXT

The increasing demand for fuels and chemicals in the world has prompted the exploration of various forms of renewable energy resources. Using C5-based furfural as the platform to replace the fossil energy resources is greatly attractive because of its abundance and environmental friendliness. Here we study the activity, selectivity, and possible reaction pathways for the Baeyer-Villiger oxidation of furfural over small Au clusters using hydrogen peroxide as oxidant. Furfural reacts with hydrogen peroxide in the presence of the catalysts with 93% selectivity towards maleic anhydride. Natural population analysis, frontier molecular orbital analysis, and spectroscopic analysis are used to illustrate the interaction mechanism between C5H4O2, H2O2, and Au. Reaction pathways leading to the formation of maleic anhydride are also explored. The reaction of C5H4O2 with H2O2 in the absence of a catalyst bears a relatively high transition state energy barrier of 2.98 eV for the first step involving absorption of H atom of H2O2 on the -OH group of C5H4O2. This is in agreement with the blank experiment where there were rare oxidation products observed in the absence of the metal cluster catalysts. On the other hand, transition state energies in the presence of the Au metal clusters are lower and the most feasible pathway is where the substrate and H2O2 co-bind on the Au catalyst and H2O2 molecule transfers an oxygen to the substrate, leading to the cleavage of the O-O bond.

METHODS

DFT calculations were done with B3PW91 functional. 6-311G(df, p) basis set was used for C, O, and H and aug-cc-pVDZ-PP was used for gold atoms. Gaussian 09 software was used for the calculations. Multiwfn 3.7 dev was used for the quantum theory of atoms-in-molecules (QTAIM) investigations.

Field Effect Transistor with Nanoporous Gold Electrode

Nanoporous gold (NPG) has excellent catalytic activity and has been used in the recent literature on this issue as a sensor in various electrochemical and bioelectrochemical reactions. This paper reports on a new type of metal-oxide-semiconductor field-effect transistor (MOSFET) that utilizes NPG as a gate electrode. Both n-channel and p-channel MOSFETs with NPG gate electrodes have been fabricated. The MOSFETs can be used as sensors and the results of two experiments are reported: the detection of glucose and the detection of carbon monoxide. A detailed comparison of the performance of the new MOSFET to that of the older generation of MOSFETs fitted with zinc oxide gate electrodes is given.

Integrated in situ spectroscopic studies on syngas production from partial oxidation of methane catalyzed by atomically dispersed rhodium cations on ceria

Catalytic reforming of methane to produce syngas is an important strategy for producing value-added chemicals. The conventional reforming catalyst relies on supported nickel nanoparticles. In this work, we investigated singly dispersed Rh cations anchored on a CeO₂ catalyst (Rh₁/CeO₂) for high activity and selectivity towards the production of syngas via partial oxidation of methane (POM) in the temperature range of 600-700 °C. The yields of H₂ and CO at 700 °C are 83% and 91%, respectively. The anchored Rh₁ atoms on CeO₂ of Rh₁/CeO₂ are in the cationic state, and on an average each Rh₁ atom coordinates with 4-5 surface lattice oxygen atoms of CeO₂. Compared to inert CeO₂ for POM, via the incorporation of single-atom sites, Rh₁ modifies the electronic state of oxygen atoms proximal to the Rh₁ atoms and thus triggers the catalytic activity of CeO₂. The high activity of single-atom catalyst Rh₁/CeO₂ suggests that the incorporation of single atoms of transition metals to the surface of a reducible oxide can modulate the electronic state of proximal anions of the oxide support toward forming an electronic state favorable for the selective formation of ideal products.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO₂ catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO₂ support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO₂. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO₂, which can be crucial for supported metal catalysts in biomass conversion.

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.

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Facet Selectivity of Cetyltrimethyl Ammonium Bromide Surfactants on Gold Nanoparticles Studied Using Molecular Simulations

We have studied facet selectivity of cetyltrimethyl ammonium bromide (CTAB) surfactants of varying alkyl tail lengths (C₁₇TAB and C₁₀TAB) during their adsorption on a spherical gold metal nanoparticle (MNP) using umbrella sampling and well-tempered metadynamics techniques in molecular simulations. We show that the surfactants strongly adsorb with their alkyl tails wrapped around the MNP. The adsorption morphologies are dictated by the strong preference of the polar head group of the surfactants to adsorb on to the atoms that lie between the facets of the MNP, that is, in the vicinity of low-coordinated gold atoms. The alkyl tails do not display any strong facet preference. Owing to the longer alkyl tails, C₁₇TAB molecules pack together better than the C₁₀TAB molecules in the adsorbed state on the MNP. These findings suggest that the regions near the edges of the facets and low-coordinated atoms are expected to be preferentially covered with the adsorbed surfactants.

Metal-Support Interaction and Charge Distribution in Ceria-Supported Au Particles Exposed to CO

Understanding how reaction conditions affect metal-support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure-activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal-support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO₂ samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au⁺ atoms in contact with the ceria substrate and neutral Au⁰ atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Au^δ- atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy.

Single-Atom-Based Catalysts for Photocatalytic Water Splitting on TiO2 Nanostructures

H2 generation from photocatalytic water splitting is one of the most promising approaches to producing cost-effective and sustainable fuel. Nanostructured TiO2 is a highly stable and efficient semiconductor photocatalyst for this purpose. The main drawback of TiO2 as a photocatalyst is the sluggish charge transfer on the surface of TiO2 that can be tackled to a great extent by the use of platinum group materials (PGM) as co-catalysts. However, the scarcity and high cost of the PGMs is one of the issues that prevent the widespread use of TiO2/PGM systems for photocatalytic H2 generation. Single-atom catalysts which are currently the frontline in the catalysis field can be a favorable path to overcome the scarcity and further advance the use of noble metals. More importantly, single-atom (SA) catalysts simultaneously have the advantage of homogenous and heterogeneous catalysts. This mini-review specifically focuses on the single atom decoration of TiO2 nanostructures for photocatalytic water splitting. The latest progress in fabrication, characterization, and application of single-atoms in photocatalytic H2 generation on TiO2 is reviewed.

Adsorption and Activation of O2 on Small Gold Oxide Clusters: the Reactivity Dominated by Site-Specific Factors

We experimentally explored adsorption and activation of O₂ on small anionic clusters Au_xO_y^- containing one to five gold atoms and between one and three oxygen atoms using an instrument including a magnetron sputtering cluster source, a micro flow reactor running at low temperature, and a time-of-flight mass spectrometer. Some species, including AuO^-, one isomer of Au₂O₂^-, Au₃O^-, one isomer of Au₃O₃^-, and Au₅O₂^-, can adsorb an O₂ molecule. We theoretically explored the structures of these active species and the inert ones appearing in the experiment by combining a structure search strategy based on the genetic algorithm and the density functional theory (DFT) calculations. Impressively, all active species observed in the experiment have a -O-Au site, in which the gold atom is a dangling or a vertex atom. Each -O-Au site can strongly adsorb one O₂ with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O₂ is significantly activated by accepting one electron with one of its π_2p* orbitals. With no exception, all oxygen sites and the -O-Au-Au sites in Au_xO_y^- are inert. Analyses on the density of states (DOS) of representative species well interpret the physical origins of the activity of -O-Au and the inertness of -O-Au-Au. The observations that site-specific factors dominate the reactivity of gold oxide clusters with O₂ are in contrast to what happens in the reactions of Au_n^- with O₂, where clusters' reactivity is completely determined by their global spins and electron detachment energies. The new conclusions in this work offer a reference to understand the crucial O₂ activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.

Synergetic Catalysis of Magnetic Single-Atom Catalysts Confined in Graphitic-C3N4/CeO2(111) Heterojunction for CO Oxidization

Magnetic single-atom catalysts (MSAC), due to the intrinsic spin degree of freedom, are of particular importance relative to other conventional SAC for applications in various catalytic processes, especially in those cases that involve spin-triplet O₂. However, the bottleneck issue in this field is the clustering of the SAC during the processes. Here using first-principles calculations we predict that Mn atoms can be readily confined in the interface of the porous g-C₃N₄/CeO₂(111) heterostructure, forming high-performance MSAC for O₂ activation via a delicate synergetic mechanism of charge transfer, mainly provided by the p-block g-C₃N₄ overlayer mediated by the d-block Mn active site, and spin selection, preserved mainly through active participation of the f-block Ce atoms and/or g-C₃N₄, which effectively promotes the CO oxidization. Such a recipe is also demonstrated to be valid for V- and Nb-MSACs, which may shed new light on the design of highly efficient MSACs for various important chemical processes wherein spin-selection matters.

Single Ni active sites with a nitrogen and phosphorus dual coordination for an efficient CO2 reduction

Transition metal single-atom catalysts (SACs) have emerged as a research hotspot in CO₂RRs. However, tuning the electronic configuration of a metal single-atom by employing new heteroatoms still remains a challenge. Herein, a carbon matrix loaded with a N and P co-coordinated Ni single-atom (denoted as Ni-NPC) was prepared for an efficient CO₂RR. XANES and EXAFS were conducted to explore the coordination environment and charge distribution of the Ni-NPC catalyst. DFT calculations indicated that the Ni atom gained electrons from the P atom, and the Ni-NPC sample had a decreased energy barrier of +0.97 eV after doping with P atoms, which was favorable to overcome the limiting-step bottleneck for promoting CO₂RR. Due to the rich Ni atomic active sites and superior P-doping effect, Ni-NPC exhibited a maximum FE_CO of 92% with a high current density of 22.6 mA cm^-2 at -0.8V vs. RHE, which was far superior to those of NC, NPC and Ni-NC catalysts. Moreover, both the FE_CO and current density of the Ni-NPC catalyst remained stable for more than 16 h at -0.8 V vs. RHE, indicating a high stability for long-term CO₂RR experiments.

High-Performance Ligand-Protected Metal Nanocluster Catalysts for CO2 Conversion through the Exposure of Undercoordinated Sites

Previous experimental breakthroughs reveal the potential to create novel heterogeneous catalysts for the electroreduction of CO2 to a high-value product CO using ligand-protected Au-based nanoclusters. Since the chemical composition and geometric structures have been precisely defined, it is possible to adopt robust design guidelines for the development of practical catalysts and to fundamentally elucidate the underlying reaction mechanism. In this short review, the computational progress made to understand the experimentally observed reduction process on the following subset of materials—Au25(SR)18−, Au24Pd(SR)18, Au23(SR)16− and Au21Cd2(SR)16−—is described. A significant finding from our first-principles mechanistic studies is that CO2 conversion on the fully ligand protected nanoclusters is thermodynamically unfavorable due to the very weak binding of intermediates on the surface region. However, the reaction becomes feasible when either Au or S sites are exposed through the removal of a ligand. The results particularly point to the role of undercoordinated S sites in the creation of highly functional heterogeneous catalysts that are both active and selective for the CO2 conversion process. The incorporation of dopants could significantly influence the catalytic reactivity of the nanoclusters. As demonstrated in the case of the monopalladium substitution in Au25(SR)18−, the presence of the foreign atom leads to an enhancement of CO production selectivity due to the greater stabilization of the intermediates. With the Cd substitution doping of Au23(SR)16−, the improvement in performance is also attributed to the enhanced binding strength of the intermediates on the geometrically modified surface of the nanocluster.

Colloidal Polydopamine Beads: A Photothermally Active Support for Noble Metal Nanocatalysts

Polydopamine (PDA) is a unique bioinspired synthetic polymer that integrates broadband light absorption, efficient photothermal transduction, and versatile surface-adhesion functions in a single material entity. Here, we utilize colloidal PDA beads in the submicron particle size regime as an easily processable and photothermally active support for sub-10 nm Pd nanocatalysts to construct a multifunctional material system that allows us to kinetically boost thermal catalytic reactions through visible and near-infrared light illuminations. Choosing the Pd-catalyzed nitrophenol reduction by ammonium formate as a model transfer hydrogenation reaction exhibiting temperature-dependent reaction rates, we demonstrate that interfacial molecule-transforming processes on metal nanocatalyst surfaces can be kinetically modulated by harnessing the thermal energy produced through photothermal transduction in the PDA supports.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.

Laser-assisted synthesis of Au aerogel with high-index facets for ethanol oxidation

Gold (Au) can be used as an ideal metal electrocatalyst for ethanol and glucose oxidation reactions due to its high performance-to-cost ratio. In this paper, the Au aerogel with high-index facets was synthesized by using the laser ablation in liquid technology, which can improve the electrocatalytic activity of Au. The as-prepared Au aerogel showed excellent mass activity and specific activity toward ethanol oxidation reaction, which are 4.6 times and 2.1 times higher than Au/C, respectively. The 3D porous nature and rich defect of the Au aerogel provide more active sites. In addition, the high-index facets with under-coordinated atoms enhance the adsorption of ethanol and glucose molecules, thus improving the intrinsic catalytic activity of Au aerogel. The effect of high-index facets has also been investigated by density functional theory calculations. Furthermore, the Au aerogels also show good electrocatalytic activity and stability toward glucose oxidation reaction. These results are conducive to promote the practical application of Au in electrocatalysis.

Theoretical Study on Carbon Monoxide Adsorption on Unsupported and γ-Al2O3-Supported Silver Nanoparticles: Size, Shape, and Support Effects

Nanoparticles (NPs) supported on metal oxides exhibit high catalytic activities for various reactions. The shape and oxidation state of such NPs, which are related to the catalytic activity, are often determined by the support. Herein, we conducted a density functional theory study on isolated silver (Ag) NPs and two types of Ag-NPs supported on gamma-aluminum oxide (γ-Al₂O₃). First, carbon monoxide (CO) adsorption on the isolated Ag NPs was investigated for decahedra (D _5h ), icosahedra (I _h ), and cuboctahedra (O _h ) of various sizes. I _h and O _h NPs showed moderate size dependence, whereas D _5h NPs showed high size dependence when the height was below 1.4 nm. The enhancement of CO adsorption on D _5h NPs was attributed to the presence of superatomic states. Next, we performed geometrical optimization of Ag₅₄/γ-Al₂O₃(110) with a decahedral shape. Two types of structures were obtained: amorphous Ag₅₄(A) and locally fivefold symmetrical Ag₅₄(B) structures. Both NPs on γ-Al₂O₃(110) were found to be positively charged, but electron transfer to the support occurred only from the Ag atoms at the two bottom layers, and the upper part of NPs was relatively neutral. The enhancement of CO adsorption on Ag₅₄(B) disappeared due to loss of the high symmetry. In turn, the moderate size dependence of neutral isolated NPs can be applied.

Sputtering onto liquids: a critical review

Sputter deposition of atoms onto liquid substrates aims at producing colloidal dispersions of small monodisperse ultrapure nanoparticles (NPs). Since sputtering onto liquids combines the advantages of the physical vapor deposition technique and classical colloidal synthesis, the review contains chapters explaining the basics of (magnetron) sputter deposition and the formation of NPs in solution. This review article covers more than 132 papers published on this topic from 1996 to September 2021 and aims at providing a critical analysis of most of the reported data; we will address the influence of the sputtering parameters (sputter power, current, voltage, sputter time, working gas pressure, and the type of sputtering plasma) and host liquid properties (composition, temperature, viscosity, and surface tension) on the NP formation as well as a detailed overview of the properties and applications of the produced NPs.

Methanol carbonylation to acetaldehyde on Au particles supported by single-layer MoS2grown on silica

Homogenous single-layer MoS₂films coated with sub-single layer amounts of gold are found to isolate the reaction of methanol with carbon monoxide, the fundamental step toward higher alcohols, from an array of possible surface reactions. Active surfaces were prepared from homogenous single-layer MoS₂films coated with sub-single layer amounts of gold. These gold atoms formed clusters on the MoS₂surface. A gas mixture of carbon monoxide (CO) and methanol (CH₃OH) was partially converted to acetaldehyde (CH₃CHO) under mild process conditions (308 kPa and 393 K). This carbonylation of methanol to a C₂species is a critical step toward the formation of higher alcohols. Density functional theory modeling of critical steps of the catalytic process identify a viable reaction pathway. Imaging and spectroscopic methods revealed that the single layer of MoS₂facilitated formation of nanoscale gold islands, which appear to sinter through Ostwald ripening. The formation of acetaldehyde by the catalytic carbonylation of methanol over supported gold clusters is an important step toward realizing controlled production of useful molecules from low carbon-count precursors.

Recent advancements and future submissions of silica core-shell nanoparticles

The core-shell silica-based nanoparticles (CSNPs) possess outstanding properties for developing next-generation therapeutics. CSNPs provide greater surface area owing to their mesoporous structure, which offers a high opportunity for surface modification. This review highlights the potential of core-shell silica-based nanoparticle (CSNP) based injectable nanotherapeutics (INT); its role in drug delivery, biomedical imaging, light-triggered phototherapy, Plasmonic enhancers, gene delivery, magnetic hyperthermia, immunotherapy, and potential as next-generation theragnostic. Specifically, the conceptual crosstalk on modern synthetic strategies, biodistribution profiles with a mechanistic view on the therapeutics loading and release modeling are dealt in detail. The manuscript also converses the challenges associated with CSNPs, regulatory hurdles, and their current market position.

Coalescence and shape oscillation of Au nanoparticles in CO2 hydrogenation to methanol

Recently, there has been renewed interest in Au nanoparticle (Au NP) catalysts owing to their high selectivity for CO₂ hydrogenation to methanol. However, there is still limited knowledge on the main factors of the catalytic activity and product selectivity of Au NPs. To address this issue, we utilized in situ transmission electron microscopy to observe the evolution of Au NP catalysts during CO₂ hydrogenation to methanol at 260 °C under ambient pressure. During the reaction, Au NPs sized ≤5 nm coalesced rapidly, forming stable Au NPs sized 5-10 nm with oscillating shapes. The first-principles calculations demonstrated that the adsorption of the reactant gas CO₂ is the main factor in inducing the coalescence of Au NPs, and CO and/or H₂O adsorption generated by the reaction caused the oscillation of the Au NP shape. Furthermore, the adsorption of various gas molecules resulted in continuous changes in the structure of the catalyst active center. In this study, the in situ observation of the dynamic evolution of the Au NP morphology is important in understanding the structural transformation of Au NP catalysts at the nanometer scale and determining the active site motifs under the reaction conditions. Moreover, this would allow us to further understand the size effect and the dynamic evolution behavior of the active center of Au NP catalysts, thereby providing a new idea for the development and application of new catalysts and strong theoretical support for heterogeneous catalytic reaction mechanisms.

Gold nanoparticles with tailored size through ligand modification for catalytic applications

The active sites of catalysts can be tuned by using appropriate organic moieties. Here, we describe a facile approach to synthesise gold nanoparticles (AuNPs) using various Au(I) precursors. The core size of these AuNPs can be precisely tailored by varying the steric hindrance imposed by bound ligands. An interesting relationship is deduced that correlates the steric hindrance around the metal to the final size of the nanoparticles. The synthesised AuNPs are immobilised onto TS-1 zeolite (Au/TS-1) with minimal change in the final size of the AuNPs. The catalytic performance of Au/TS-1 catalyst is evaluated for the direct gas phase epoxidation of propylene with hydrogen and oxygen, an environmentally friendly route to produce propylene oxide. The results indicate that smaller AuNPs exhibit enhanced catalytic activity and selectivity. Furthermore, this synthetic approach is beneficial when tailored synthesis of gold nanoparticles of specific sizes is required.

Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

Nanomaterial-based artificial enzymes (or nanozymes) have attracted great attention in the past few years owing to their capability not only to mimic functionality but also to overcome the inherent drawbacks of the natural enzymes. Numerous advantages of nanozymes such as diverse enzyme-mimicking activities, low cost, high stability, robustness, unique surface chemistry, and ease of surface tunability and biocompatibility have allowed their integration in a wide range of biosensing applications. Several metal, metal oxide, metal-organic framework-based nanozymes have been exploited for the development of biosensing systems, which present the potential for point-of-care analysis. To highlight recent progress in the field, in this review, more than 260 research articles are discussed systematically with suitable recent examples, elucidating the role of nanozymes to reinforce, miniaturize, and improve the performance of point-of-care diagnostics addressing the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to the end user) criteria formulated by World Health Organization. The review reveals that many biosensing strategies such as electrochemical, colorimetric, fluorescent, and immunological sensors required to achieve the ASSURED standards can be implemented by using enzyme-mimicking activities of nanomaterials as signal producing components. However, basic system functionality is still lacking. Since the enzyme-mimicking properties of the nanomaterials are dictated by their size, shape, composition, surface charge, surface chemistry as well as external parameters such as pH or temperature, these factors play a crucial role in the design and function of nanozyme-based point-of-care diagnostics. Therefore, it requires a deliberate exertion to integrate various parameters for truly ASSURED solutions to be realized. This review also discusses possible limitations and research gaps to provide readers a brief scenario of the emerging role of nanozymes in state-of-the-art POC diagnosis system development for futuristic biosensing applications.

Rational Design of Single-Atom Site Electrocatalysts: From Theoretical Understandings to Practical Applications

Atomically dispersed metal-based electrocatalysts have attracted increasing attention due to their nearly 100% atomic utilization and excellent catalytic performance. However, current fundamental comprehension and summaries to reveal the underlying relationship between single-atom site electrocatalysts (SACs) and corresponding catalytic application are rarely reported. Herein, the fundamental understandings and intrinsic mechanisms underlying SACs and corresponding electrocatalytic applications are systemically summarized. Different preparation strategies are presented to reveal the synthetic strategies with engineering the well-defined SACs on the basis of theoretical principle (size effect, metal-support interactions, electronic structure effect, and coordination environment effect). Then, an overview of the electrocatalytic applications is presented, including oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, oxidation of small organic molecules, carbon dioxide reduction reaction, and nitrogen reduction reaction. The underlying structure-performance relationship between SACs and electrocatalytic reactions is also discussed in depth to expound the enhancement mechanisms. Finally, a summary is provided and a perspective supplied to demonstrate the current challenges and opportunities for rational designing, synthesizing, and modulating the advanced SACs toward electrocatalytic reactions.