Formation, Migration, and Reactivity of Au–CO Complexes on Gold Surfaces

Optimizing low-temperature CO oxidation under realistic combustion conditions: The impact of CeO2 morphology on Au/CeO2 catalysts

The development of carbon monoxide oxidation catalysts for complex gas environments faces significant challenges in fire scenarios. Only a few representative gases are used as interfering components in simulated real smoke under laboratory conditions, which cannot accurately reflect the performance of catalysts in a real fire. Herein, Au/CeO2 catalysts with high activity were prepared by adjusting the morphology (rod, cube, polyhedron and irregular particles) and exposed crystal surface ratio of CeO2. Rod-like Au/CeO2 (Au/CeO2-NR) achieved 99 % CO conversion at 25 °C and demonstrated excellent water resistance. This excellent activity originates from the high oxygen vacancy concentration of the CeO2-NR and the interaction between Au species and the carrier. A testbed was established by connecting a steady-state tube furnace with a catalytic fixed-bed reactor to evaluate the CO elimination performance of the catalyst under realistic combustion conditions. Despite competitive adsorption of small molecules (H2O, acetone, etc.) on the active sites, Au/CeO2-NR eliminates carbon monoxide in real combustion atmospheres at only 60 °C. This study provides a method for evaluating the catalytic activity of CO in realistic environments, which is promising for practical use in application scenarios dealing with toxic fumes.

Dynamic phase transitions dictate the size effect and activity of supported gold catalysts

The landmark discovery of gold catalysts has aroused substantial interest in heterogeneous catalysis, yet the catalytic mechanism remains elusive. For carbon monoxide oxidation on gold nanoparticles (NPs) supported on ceria surfaces, it is widely believed that carbon monoxide adsorbs on the gold particles, while the reaction occurs at the gold/ceria interface. Here, we have investigated the dynamic changes of supported gold NPs with various sizes in a carbon monoxide oxidation atmosphere using deep potential molecular dynamics simulations. Our results reveal that the structure of tiny gold particles in carbon monoxide atmospheres becomes highly disordered and undergoes phase transition. Such a liquid-like structure provides massive reactive sites, enabling facile carbon monoxide oxidation on the solid-state gold NP rather than just at the gold/ceria interface. This result is further corroborated by catalytic experiments. This work sheds light on both the size effects and activity in noble metal catalysis and provides insights for the design of more effective nanocatalysts.

CO Oxidation Promoted by NO Adsorption on RhMn2O3- Cluster Anions

CO oxidation represents an important model reaction in the gas phase to provide a clear structure-reactivity relationship in related heterogeneous catalysis. Herein, in combination with mass spectrometry experiments and quantum-chemical calculations, we identified that the RhMn2O3- cluster cannot oxidize CO into gas-phase CO2 at room temperature, while the NO preadsorbed products RhMn2O3-[(NO)1,2] are highly reactive in CO oxidation. This discovery is helpful to get a fundamental understanding on the reaction behavior in real-world three-way catalytic conditions where different kinds of reactants coexist. Theoretical calculations were performed to rationalize the crucial roles of preadsorbed NO where the strongly attached NO on the Rh atom can greatly stabilize the products RhMn2O2-[(NO)1,2] during CO oxidation and at the same time works together with the Rh atom to store electrons that stay originally in the attached CO2- unit. The leading result is that the desorption of CO2, which is the rate-determining step of CO oxidation by RhMn2O3-, can be greatly facilitated on the reactions of RhMn2O3-[(NO)1,2] with CO.

Comprehensive Insights into the Family of Atomically Thin 2D-Materials for Diverse Photocatalytic Applications

2D materials with their fascinating physiochemical, structural, and electronic properties have attracted researchers and have been used for a variety of applications such as electrocatalysis, photocatalysis, energy storage, magnetoresistance, and sensing. In recent times, 2D materials have gained great momentum in the spectrum of photocatalytic applications such as pollutant degradation, water splitting, CO2 reduction, NH3 production, microbial disinfection, and heavy metal reduction, thanks to their superior properties including visible light responsive band gap, improved charge separation and electron mobility, suppressed charge recombination and high surface reactive sites, and thus enhance the photocatalytic properties rationally as compared to 3D and other low-dimensional materials. In this context, this review spot-lights the family of various 2D materials, their properties and their 2D structure-induced photocatalytic mechanisms while giving an overview on their synthesis methods along with a detailed discussion on their diverse photocatalytic applications. Furthermore, the challenges and the future opportunities are also presented related to the future developments and advancements of 2D materials for the large-scale real-time photocatalytic applications.

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Reduction-Controlled Atomic Migration for Single Atom Alloy Library

Picturing the atomic migration pathways of catalysts in a reactive atmosphere is of central significance for uncovering the underlying catalytic mechanisms and directing the design of high-performance catalysts. Here, we describe a reduction-controlled atomic migration pathway that converts nanoparticles to single atom alloys (SAAs), which has remained synthetically challenging in prior attempts due to the elusive mechanism. We achieved this by thermally treating the noble-metal nanoparticles M (M = Ru, Rh, Pd, Ag, Ir, Pt, and Au) on metal oxide (CuO) supports with H₂/Ar. Atomic-level characterization revealed such conversion as the synergistic consequence of noble metal-promoted H₂ dissociation and concomitant CuO reduction. The observed atomic migration pathway offers an understanding of the dynamic mechanisms study of nanomaterials formation and catalyst design.

Evolution of surface and sub-surface morphology and chemical state of exsolved Ni nanoparticles

Nanoparticle formation by dopant exsolution (migration) from bulk host lattices is a promising approach to generate highly stable nanoparticles with tunable size, shape, and distribution. We investigated Ni dopant migration from strontium titanate (STO) lattices, forming metallic Ni nanoparticles at STO surfaces. Ex situ scanning probe measurements confirmed the presence of nanoparticles at the H₂ treated surface. In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed reduction from Ni²⁺ to Ni⁰ as Ni dopants migrated to the surface during heating treatments in H₂. During Ni migration and reduction, the Sr and Ti chemical states were mostly unchanged, indicating the selective reduction of Ni during treatment. At the same time, we used in situ ambient pressure grazing incidence X-ray scattering (GIXS) to monitor the particle morphology. As Ni migrated to the surface, it nucleated and grew into compressed spheroidal nanoparticles partially embedded in the STO perovskite surface. These findings provide a detailed picture of the evolution of the nanoparticle surface and subsurface chemical state and morphology as the nanoparticles grow beyond the initial nucleation and growth stages.

Reaction-Intermediate-Induced Atomic Mobility in Heterogeneous Metal Catalysts for Electrochemical Reduction of CO2

Improving the activity and selectivity of heterogeneous metal electrocatalysts has been the primary focus of CO2 electroreduction studies, however, the stability of these materials crucial for practical application remains less...

High-Pressure Scanning Tunneling Microscopy

This is a Review of recent studies on surface structures of crystalline materials in the presence of gases in the mTorr to atmospheric pressure range, which brings surface science into a brand new direction. Surface structure is not only a property of the material but also depends on the environment surrounding it. This Review emphasizes that high/ambient pressure goes hand-in-hand with ambient temperature, because weakly interacting species can be densely covering surfaces at room temperature only when in equilibrium with a sufficiently high gas pressure. At the same time, ambient temperatures help overcome activation barriers that impede diffusion and reactions. Even species with weak binding energy can have residence lifetimes on the surface that allow them to trigger reconstructions of the atomic structure. The consequences of this are far from trivial because under ambient conditions the structure of the surface dynamically adapts to its environment and as a result completely new structures are often formed. This new era of surface science emerged and spread rapidly after the retooling of characterization techniques that happened in the last two decades. This Review is focused on the new surface structures enabled particularly by one of the new tools: high-pressure scanning tunneling microscopy. We will cover several important surfaces that have been intensely scrutinized, including transition metals, oxides, and alloys.

Ligand Stabilized Ni1 Catalyst for Efficient CO Oxidation

Supported single transition metal (TM₁ ) catalysts have attracted broad attention in academia recently. Still, their corresponding reactivity and stability under reaction conditions are critical but have not well explored at the fundamental level. Herein, we use density functional theory calculation and ab initio molecular dynamics simulation to investigate the role of reactants and ligands on the reactivity and stability of graphitic carbon nitride (g-C₃ N₄ ) supported Ni₁ for CO oxidation. We find out that supported bare Ni₁ atoms are only metastable on the surface and tend to diffuse into the interlayer of g-C₃ N₄ . Though Ni₁ is catalytically active at moderate temperatures, CO adsorption induced dimerization deactivates the catalyst. Hydroxyl groups not only are able to stabilize the supported Ni₁ atom, but also increase the reactivity by participating directly in the reaction. Our results provide valuable insights on improving the chemical stability of TM₁ by ligands without sacrificing the reactivity, which are helpful for the rational design of highly loaded atomically dispersed supported metal catalysts.

In situ formation of mononuclear complexes by reaction-induced atomic dispersion of supported noble metal nanoparticles

Supported noble metal nanoclusters and single-metal-site catalysts are inclined to aggregate into particles, driven by the high surface-to-volume ratio. Herein, we report a general method to atomically disperse noble metal nanoparticles. The activated carbon supported nanoparticles of Ru, Rh, Pd, Ag, Ir and Pt metals with loading up to 5 wt. % are completely dispersed by reacting with CH₃I and CO mixture. The dispersive process of the Rh nanoparticle is investigated in depth as an example. The in-situ detected I• radicals and CO molecules are identified to promote the breakage of Rh-Rh bonds and the formation of mononuclear complexes. The isolated Rh mononuclear complexes are immobilized by the oxygen-containing functional groups based on the effective atomic number rule. The method also provides a general strategy for the development of single-metal-site catalysts for other applications.

Supported noble metal nanoclusters and single-metal-sites catalysts are inclined to aggregate into particles. Here, the authors report a general method with CO and CH₃I to disperse the metal nanoparticles of Ru, Rh, Pd, Ag, Ir and Pt completely into single atoms with loading up to 5 wt%.

Catalytic CO Oxidation by Noble-Metal-Free Ni2VO4,5- Clusters: A CO Self-Promoted Mechanism

Catalytic CO oxidation is an important model reaction in gas-phase studies to provide a clear structure-reactivity understanding in related heterogeneous catalysis, whereas CO oxidation catalyzed by noble-metal (NM) free species has been scarcely reported, and the fundamental aspects are elusive. Herein a CO self-promoted mechanism of catalytic CO oxidation by O₂ mediated with the Ni₂VO_4,5^- clusters was experimentally identified and theoretically rationalized. The catalysis was characterized by mass spectrometry and quantum chemistry calculations. Ni₂VO₅^- can oxidize CO to generate an oxygen-deficient product Ni₂VO₄^-, which can only adsorb CO to give rise to Ni₂VO₄CO^-, and the oxidative reactivity of Ni₂VO₄^- can be boosted by the adsorbed  CO. This finding reinforces the significance that the attached CO can modify the electronic structure of the Ni₂ unit in Ni₂VO₄CO^- and make the Ni₂ unit behave like NM atoms to store the released electrons in an oxygen atom transfer process.

Neutral Au1-Doped Cluster Catalysts AuTi2O3-6 for CO Oxidation by O2

Oxide supported gold catalysts (e.g., Au/TiO₂) are of great significance in heterogeneous catalysis owing to their extraordinary catalytic activity. Study of heteronuclear metal oxide clusters (HMOCs, e.g., Au _xTi _yO _z ^q) is an important way to uncover the molecular-level mechanisms of gold catalysis in the related heterogeneous catalytic systems. However, the current studies of HMOCs are focused on charged clusters with little attention paid to neutral species. The reactivity study of neutral HMOCs is vital to have a comprehensive understanding of heterogeneous catalysis, but it is experimentally challenging because of the difficulty of cluster ionization and detection without fragmentation. Herein, benefiting from a homemade time-of-flight mass spectrometer coupled with a vacuum ultraviolet laser system, the reactivity of neutral Au₁-doped titanium oxide clusters AuTi₂O_3-6 in catalytic CO oxidation by O₂ has been successfully identified. The mechanistic details of the catalysis have been elucidated by quantum chemistry calculations. The crucial roles of the mobile AuCO species that can facilitate not only the process of CO oxidation but also the process of O₂ activation have been discovered in the cluster catalysis. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over Au/TiO₂, one type of the best studied gold catalysts.

Static Regulation and Dynamic Evolution of Single-Atom Catalysts in Thermal Catalytic Reactions

Single-atom catalysts provide an ideal platform to bridge the gap between homogenous and heterogeneous catalysts. Here, the recent progress in this field is reported from the perspectives of static regulation and dynamic evolution. The syntheses and characterizations of single-atom catalysts are briefly discussed as a prerequisite for catalytic investigation. From the perspective of static regulation, the metal-support interaction is illustrated in how the supports alter the electronic properties of single atoms and how the single atoms activate the inert atoms in supports. The synergy between single atoms is highlighted. Besides these static views, the surface reconstruction, such as displacement and aggregation of single atoms in catalytic conditions, is summarized. Finally, the current technical challenges and mechanistic debates in single-atom heterogeneous catalysts are discussed.

Size-dependent dynamic structures of supported gold nanoparticles in CO oxidation reaction condition

Gold (Au) catalysts exhibit a significant size effect, but its origin has been puzzling for a long time. It is generally believed that supported Au clusters are more or less rigid in working condition, which inevitably leads to the general speculation that the active sites are immobile. Here, by using atomic resolution in situ environmental transmission electron microscopy, we report size-dependent structure dynamics of single Au nanoparticles on ceria (CeO₂) in CO oxidation reaction condition at room temperature. While large Au nanoparticles remain rigid in the catalytic working condition, ultrasmall Au clusters lose their intrinsic structures and become disordered, featuring vigorous structural rearrangements and formation of dynamic low-coordinated atoms on surface. Ab initio molecular-dynamics simulations reveal that the interaction between ultrasmall Au cluster and CO molecules leads to the dynamic structural responses, demonstrating that the shape of the catalytic particle under the working condition may totally differ from the shape under the static condition. The present observation provides insight on the origin of superior catalytic properties of ultrasmall gold clusters.

Crystal-defect-induced facet-dependent electrocatalytic activity of 3D gold nanoflowers for the selective nanomolar detection of ascorbic acid

Understanding and exploring the decisive factors responsible for superlative catalytic efficiency is necessary to formulate active electrode materials for improved electrocatalysis and high-throughput sensing. This research demonstrates the ability of bud-shaped gold nanoflowers (AuNFs), intermediates in the bud-to-blossom gold nanoflower synthesis, to offer remarkable electrocatalytic efficiency in the oxidation of ascorbic acid (AA) at nanomolar concentrations. Multicomponent sensing in a single potential sweep is measured using differential pulse voltammetry while the kinetic parameters are estimated using electrochemical impedance spectroscopy. The outstanding catalytic activity of bud-structured AuNF [iAuNFp(Bud)/iGCp ≅ 100] compared with other bud-to-blossom intermediate nanostructures is explained by studying their structural transitions, charge distributions, crystalline patterns, and intrinsic irregularities/defects. Detailed microscopic analysis shows that density of crystal defects, such as edges, terraces, steps, ledges, kinks, and dislocation, plays a major role in producing the high catalytic efficiency. An associated ab initio simulation provides necessary support for the projected role of different crystal facets as selective catalytic sites. Density functional theory corroborates the appearance of inter- and intra-molecular hydrogen bonding within AA molecules to control the resultant fingerprint peak potentials at variable concentrations. Bud-structured AuNF facilitates AA detection at nanomolar levels in a multicomponent pathological sample.

Self-activated surface dynamics in gold catalysts under reaction environments

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Peculiar surface dynamics were found in the reaction environment for the oxidation of CO at room temperature, involving residual silver in the NPG leaves as well as gold and oxygen atoms, especially on {110} facets. The NPG is thus classified as a novel self-activating catalyst. The essential structure unit for catalytic activity was identified as Au–AgO surface clusters, implying that the NPG is regarded as a nano-structured silver oxide catalyst supported on the matrix of NPG, or an inverse catalyst of a supported gold nanoparticulate (AuNP) catalyst. Hence, the catalytically active structure in the gold catalysts (supported AuNP and NPG catalysts) can now be experimentally unified in low-temperature CO oxidation, a step forward towards elucidating the fascinating catalysis mechanism of gold.

Nanoporous gold (NPG) has gained significant attention, but its catalytically active structure has not yet been clarified. Here, the authors identify the catalytically active and dynamic structure in NPG by combining atomic-scale and microsecond-resolution environmental transmission electron microscopy with ab initio calculations.

CO oxidation on Ru-Pt bimetallic nanoclusters supported on TiO2(101): The effect of charge polarization

Pt-based catalyst is widely used in CO oxidation, while its catalytic activity is often undermined because of the CO poisoning effect. Here, using density functional theory, we propose the use of a Ru-Pt bimetallic cluster supported on TiO₂ for CO oxidation, to achieve both high activity and low CO poisoning effect. Excellent catalytic activity is obtained in a Ru₁Pt₇/TiO₂(101) system, which is ascribed to strong electric fields induced by charge polarization between one Ru atom and its neighboring Pt atoms. Because of its lower electronegativity, the Ru atom donates electrons to neighboring Pt. This induces strong electric fields around the top-layered Ru, substantially promoting the adsorption of O₂/CO + O₂ and eliminating the CO poisoning effect. In addition, the charge polarization also drives the d-band center of the Ru₁Pt₇ cluster to up-shift to the Fermi level. For surface O₂ activation/CO oxidation, the strong electric field and d-band center close to the Fermi level can promote the adsorption of O₂ and CO as well as reduce the reaction barrier of the rate-determining step. Meanwhile, since O₂ easily dissociates on Ru₁Pt₇/TiO₂(101) resulting in unwanted oxidation of Ru and Pt, a CO-rich condition is necessary to protect the catalyst at high temperature.

Dynamics of a plasmon-activated p-mercaptobenzoic acid layer deposited over Au nanoparticles using time-resolved SERS

Time-dependent SERS intensity recorded over a drop-coated coffee-ring pattern of p-MBA with gold colloids was investigated as a function of the specific laser power applied. Pure electromagnetic enhancement produced stochastic intensity variations of the whole SER spectra, which were mainly correlated with evolutions of the background intensity. Besides long-term, non-reversible spectral changes caused by plasmon-induced decarboxylation of p-MBA, transient original spectral profiles showing additional lines were also observed as the specific power reached 5.5 × 10(4) W cm(-2). An unprecedented qualitative and quantitative study of SERS intensity variations based on the complementary use of both extreme deviation and cross-correlation statistics is provided, which resulted in an improved understanding of SERS mechanisms. More precisely, cross-correlation analysis made it possible to follow the evolution of groups of modes assigned to one species or sharing the same symmetry while so-called individual events denote particular resonance structures, whose occurrence was tentatively related to a photo-thermally activated motion of the gold nanostructures.

Halogen-Adatom Mediated Phase Transition of Two-Dimensional Molecular Self-Assembly on a Metal Surface

Construction of tunable and robust two-dimensional (2D) molecular arrays with desirable lattices and functionalities over a macroscopic scale relies on spontaneous and reversible noncovalent interactions between suitable molecules as building blocks. Halogen bonding, with active tunability of direction, strength, and length, is ideal for tailoring supramolecular structures. Herein, by combining low-temperature scanning tunneling microscopy and systematic first-principles calculations, we demonstrate novel halogen bonding involving single halogen atoms and phase engineering in 2D molecular self-assembly. On the Au(111) surface, we observed catalyzed dehalogenation of hexabromobenzene (HBB) molecules, during which negatively charged bromine adatoms (Br^δ-) were generated and participated in assembly via unique C-Br^δ+···Br^δ- interaction, drastically different from HBB assembly on a chemically inert graphene substrate. We successfully mapped out different phases of the assembled superstructure, including densely packed hexagonal, tetragonal, dimer chain, and expanded hexagonal lattices at room temperature, 60 °C, 90 °C, and 110 °C, respectively, and the critical role of Br^δ- in regulating lattice characteristics was highlighted. Our results show promise for manipulating the interplay between noncovalent interactions and catalytic reactions for future development of molecular nanoelectronics and 2D crystal engineering.

Surface chemistry of group IB metals and related oxides

Catalytic surface chemistry of IB metals are reviewed with an attempt to bridge model catalysts and powder catalysts.

Morphology tuning of assembled Au–Cu nicotinate rings by ligand coordination and their use as efficient catalysts

In this work, cyclic annular Au–Cu nicotinates were synthesized through a facile one-pot method.

Disulfide-Bridged (Mo3S11) Cluster Polymer: Molecular Dynamics and Application as Electrode Material for a Rechargeable Magnesium Battery

Exploring novel electrode materials is critical for the development of a next-generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo3S11) clusters, has great potential as a rechargeable magnesium battery cathode. This material provided good reversible capacity, attributed to its unique structure with high flexibility and capability of deformation upon Mg insertion. Free-terminal disulfide moiety may act as the active site for reversible insertion and extraction of magnesium.

A DFT study of molecular adsorption on Au–Rh nanoalloys

Density functional theory calculations are performed to investigate both mixing and adsorption properties of 38-atom and 79-atom Au–Rh nanoalloys at the nanoscale.