Self-activated surface dynamics in gold catalysts under reaction environments

Crystal plane orientation-dependent surface atom diffusion in sub-10-nm Au nanocrystals

Surface atom diffusion is a ubiquitous phenomenon in nanostructured metals with ultrahigh surface-to-volume ratios. However, the fundamental atomic mechanism of surface atom diffusion remains elusive. Here, we report in situ atomic-scale observations of surface pressure-driven atom diffusion in gold nanocrystals at room temperature using high-resolution transmission electron microscopy with a high-speed detection camera. The topmost layer of atoms on (001) plane initially diffuse in a column-by-column manner. As diffusion proceeds, the remaining atomic columns collectively inject into adjacent underlayer, accompanied by nucleation of a surface dislocation. In comparison, atoms on (111) plane directly diffuse to the base without collective injection. Quantitative calculations indicate that these crystal plane orientation-dependent atom diffusion behaviors contribute to the larger diffusion coefficient of (111) plane compared to (001) plane in addition to the effect of diffusion activation energy. Our findings provide valuable insights into atomic mechanisms of diffusion-dominant morphology evolution of nanostructured metals and guide the design of nanostructured materials with enhanced structural stability.

Temperature-driven evolution of ceria-zirconia-supported AuPd and AuRu bimetallic catalysts under different atmospheres: insights from IL-STEM studies

The evolution of the structure and composition of the system of particles in two Ce0.62Zr0.38O2-supported bimetallic catalysts based on Au and a 4d metal (Ru or Pd) under high temperature conditions and different reducing and oxidizing environments has been followed by means of Identical Location Scanning Transmission Electron Microscopy (IL-STEM). As an alternative to in situ microscopy, this technique offers valuable insights into the structural modifications occurring in chemical environments with the characteristics of a macro-scale reactor. By tracking exactly the same areas on a large number of metallic entities, it has been possible to reveal the influence of particle size and the nature of the redox environment on the temperature-driven mobilization of the different metals involved. Thus, oxidizing environments evidenced a much higher capacity to mobilize the three metals, preferentially Au. Moreover, the typical storage conditions (under air) of catalysts during the prolonged exposure time has been proved to induce significant modifications in these bimetallic systems, even at room temperature. Regardless of the type of redox environment, bimetallic systems showed better thermal resistance, which demonstrates a beneficial effect of the second metal. In summary, IL-STEM is an invaluable and complementary methodology for characterizing heterogeneous catalysts under realistic reaction conditions and is within the reach of most laboratories.

Particle Size-Controlled Oxygen Reduction and Evolution Reaction Nanocatalysts Regulate Ru(bpy)32+'s Dual-potential Electrochemiluminescence for Sandwich Immunoassay

Multiple signal strategies remarkably improve the accuracy and efficiency of electrochemiluminescence (ECL) immunoassays, but the lack of potential-resolved luminophore pairs and chemical cross talk hinders their development. In this study, we synthesized a series of gold nanoparticles (AuNPs)/reduced graphene oxide (Au/rGO) composites as adjustable oxygen reduction reaction and oxygen evolution reaction catalysts to promote and modulate tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)₃²⁺)'s multisignal luminescence. With the increase in the diameter of AuNPs (3 to 30 nm), their ability to promote Ru(bpy)₃²⁺'s anodic ECL was first impaired and then strengthened, and cathodic ECL was first enhanced and then weakened. Au/rGOs with medium-small and medium-large AuNP diameters remarkably increased Ru(bpy)₃²⁺'s cathodic and anodic luminescence, respectively. Notably, the stimulation effects of Au/rGOs were superior to those of most existing Ru(bpy)₃²⁺ co-reactants. Moreover, we proposed a novel ratiometric immunosensor construction strategy using Ru(bpy)₃²⁺'s luminescence promoter rather than luminophores as tags of antibodies to achieve signal resolution. This method avoids signal cross talk between luminophores and their respective co-reactants, which achieved a good linear range of 10^-7 to 10^-1 ng/ml and a limit of detection of 0.33 fg/ml for detecting carcinoembryonic antigen. This study addresses the previous scarcity of the macromolecular co-reactants of Ru(bpy)₃²⁺, broadening its application in biomaterial detection. Furthermore, the systematic clarification of the detailed mechanisms for converting the potential-resolved luminescence of Ru(bpy)₃²⁺ could facilitate an in-depth understanding of the ECL process and should inspire new designs of Ru(bpy)₃²⁺ luminescence enhancers or applications of Au/rGOs to other luminophores. This work removes some impediments to the development of multisignal ECL biodetection systems and provides vitality into their widespread applications.

Frustrations of supported catalytic clusters under operando conditions predicted by a simple lattice model

The energy landscape with a number of close minima separated by low barriers is a well-known issue in computational heterogeneous catalysis. In the framework of the emerging out-of-equilibrium material science, the navigation through such involved landscapes is associated with the functionality of materials. Current advancements in the cluster catalysis has brought and continues to bring essential nuances to the topic. One of them is the possibility of frustration of the catalytic centre under operando conditions. However, this conjecture is difficult to check either experimentally or theoretically. As a step in this direction, as-simple-as-possible lattice model is used to estimate how the supposed frustrations may couple with the elementary reaction and manifest themselves at the macroscopic scale.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

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.

Preparation and application of porous materials from coal gasification slag for wastewater treatment: A review

In recent years, coal gasification has been gradually promoted as clean technology, and coal gasification slag (CGS) emissions have increased accordingly. CGS, including coarse slag and fine slag, is rich in SiO₂ and Al₂O₃ and has pozzolanic activity, and thus CGS can be regarded as a cheap source of aluminosilicate. Also, CGS, especially the fine slag, usually contains higher contents of residual carbon which has a large specific surface area and low volatility and hence can be considered as a favorable precursor of activated carbon. Benefiting from these characteristics, CGS can be used to prepare high value-added porous materials, such as zeolite, mesoporous silica, carbon-silicon composite, and porous ceramics, and the obtained structures accommodate both sufficient adsorption capacity and low cost. Here, we review the research advances in characteristics of CGS and preparation methods of CGS-based porous materials, as well as their adsorption performance of heavy metal ions, organic dyes, ammonia nitrogen, and other water pollutants. The current studies indicate that CGS-derived adsorbents are effective and economical alternatives for removing aqueous pollutants. In addition, further research prospects on CGS-based porous materials are proposed.

Structure reconstruction of metal/alloy in reaction conditions: a volcano curve?

Recent in situ works have shown extensive evidence of the dramatic and reversible structure reconstructions of metal and alloy materials in reaction conditions. The reconstructions are of primary interest because they could lead to alternative catalytic mechanisms during real reactions. However, how the catalyst structure evolves under the pressures relevant to industrial applications (>1 atm) is so far unexplored. In our recent works, we have developed multiscale theoretical models to give reliable and precise predictions of the equilibrium shapes of metal nanoparticles and of the segregation properties of alloy surfaces at a given temperature and gas pressure. The theoretical predictions have been successfully used in interoperations of various in situ experimental observations. In this work, we applied these methods to study the detailed structural information of metal NPs and of bimetallic alloys at the temperature from 300 to 1000 K and the gas pressure from 10 to 10⁷ Pa. The results show, in some cases, both the gas-induced shape change and the gas-induced segregation change are maximized when the gas adsorption is 'just right'. The fraction of the low-coordinated sites of the metal NP shows a volcano-like curve with pressure at a constant temperature. A similar volcano shape could also be found in the plot of the environmental segregation energy as functions of temperature and pressure. The similar gas effects at low pressure and at high pressure indicate the structural information obtained in laboratory environments (<1 atm) could be of use to understanding the catalysts structure reconstruction in industrial conditions (>1 atm).

Developments and advances in in situ transmission electron microscopy for catalysis research

Recent developments and advances in in situ TEM have raised the possibility to study every step during the catalysts' lifecycle. This review discusses the current state, opportunities and challenges of in situ TEM in the realm of catalysis.

From Chip Size to Wafer-Scale Nanoporous Gold Reliable Fabrication Using Low Currents Electrochemical Etching

We report a simple, scalable route to wafer-size processing for fabrication of tunable nanoporous gold (NPG) by the anodization process at low constant current in a solution of hydrofluoric acid and dimethylformamide. Microstructural, optical, and electrochemical investigations were employed for a systematic analysis of the sample porosity evolution while increasing the anodization duration, namely the small angle X-ray scattering (SAXS) technique and electrochemical impedance spectroscopy (EIS). Whereas the SAXS analysis practically completes the scanning electronic microscopy (SEM) investigations and provides data about the impact of the etching time on the nanoporous gold layers in terms of fractal dimension and average pore surface area, the EIS analysis was used to estimate the electroactive area, the associated roughness factor, as well as the heterogeneous electron transfer rate constant. The bridge between the analyses is made by the scanning electrochemical microscopy (SECM) survey, which practically correlates the surface morphology with the electrochemical activity. The results were correlated to endorse the control over the gold film nanostructuration process deposited directly on the substrate that can be further subjected to different technological processes, retaining its properties. The results show that the anodization duration influences the surface area, which subsequently modifies the properties of NPG, thus enabling tuning the samples for specific applications, either optical or chemical.

Dynamic co-catalysis of Au single atoms and nanoporous Au for methane pyrolysis

Nanocatalysts and single-atom catalysts are both vital for heterogeneous catalysis. They are recognized as two different categories of catalysts. Nevertheless, recent theoretical works have indicated that Au nanoparticles/clusters release Au single atoms in CO oxidation, and they co-catalyze the oxidation. However, to date, neither experimental evidence for the co-catalysis nor direct observations on any heterogeneous catalysis process of single-atom catalysts are reported. Here, the dynamic process of nanoporous Au to catalyze methane pyrolysis is monitored by in situ transmission electron microscopy with high spatial–temporal resolutions. It demonstrates that nanoporous Au surfaces partially disintegrate, releasing Au single atoms. As demonstrated by DFT calculation, the single atoms could co-catalyze the reaction with nanoporous Au. Moreover, the single atoms dynamically aggregate into nanoparticles, which re-disintegrate back to single atoms. This work manifests that under certain conditions, the heterogeneous catalysis processes of nanocatalysts and single-atom catalysts are not independent, where their dynamic co-catalysis exists.

Nanocatalysts and single‐atom catalysts are generally considered as two categories with distinct performances. Here, in situ TEM study of catalytic methane pyrolysis over nanoporous Au reveals a highly dynamic process where co‐catalysis exists among various catalyst forms.

Solid-to-liquid phase transitions of sub-nanometer clusters enhance chemical transformation

Understanding the nature of active sites is crucial in heterogeneous catalysis, and dynamic changes of catalyst structures during reaction turnover have brought into focus the dynamic nature of active sites. However, much less is known on how the structural dynamics couples with elementary reactions. Here we report an anomalous decrease in reaction free energies and barriers on dynamical sub-nanometer Au clusters. We calculate temperature dependence of free energies using ab initio molecular dynamics, and find significant entropic effects due to solid-to-liquid phase transitions of the Au clusters induced by adsorption of different states along the reaction coordinate. This finding demonstrates that catalyst dynamics can play an important role in catalyst activity.

Visualizing Progressive Atomic Change in the Metal Surface Structure Made by Ultrafast Electronic Interactions in an Ambient Environment

Understanding the atomic and molecular phenomena occurring in working catalysts and nanodevices requires the elucidation of atomic migration originating from electronic excitations. The progressive atomic dynamics on metal surface under controlled electronic stimulus in real time, space, and gas environments are visualized for the first time. By in situ environmental transmission electron microscopy, the gas molecules introduced into the biased metal nanogap could be activated by electron tunneling and caused the unpredicted atomic dynamics. The typically inactive gold was oxidized locally on the positive tip and field-evaporated to the negative tip, resulting in the atomic reconstruction on the negative tip surface. This finding of a tunneling-electron-attached-gas process will bring new insights into the design of nanostructures such as nanoparticle catalysts and quantum nanodots and will stimulate syntheses of novel nanomaterials not seen in the ambient environment.

Reversible gas-solid reaction in an electronically-stimulated palladium nanogap

We investigated a nanogap between a pair of palladium electrode tips with gas (nitrogen, hydrogen, and oxygen) and a biasing voltage using in situ atomic resolution environmental transmission electron microscopy (ETEM). We found an unexpected gas-solid (nitrogen-palladium) reaction that occurs on the surface of the positive electrode tip. A palladium nitride compound was synthesized with gaseous nitrogen at low pressure at room temperature. The nitridation of palladium was previously reported and predicted to occur only under high pressure and at high temperature. The reaction in ETEM apparatus was reversible with the change in the magnitude of an electric field in the nanogap. Additionally, the asymmetrical surface dynamics on the pair of electrode tips in gas (nitrogen, hydrogen, and oxygen) were revealed by ETEM observation. It is likely that the electrons in the gap induce the reversible reaction. This study has opened a new route toward creating nanoscale materials because the creation, stabilization, and annihilation of the material in a nanogap can be controlled electrically and electronically on demand for various applications.

Differences in the Catalytic Behavior of Au-Metalized TiO2 Systems During Phenol Photo-Degradation and CO Oxidation

For this present work, a series of Au-metallized TiO2 catalysts were synthesized and characterized in order to compare their performance in two different catalytic environments: the phenol degradation that occurs during the liquid phase and in the CO oxidation phase, which proceeds the gas phase. The obtained materials were analyzed by different techniques such as XRF, SBET, XRD, TEM, XPS, and UV-Vis DRS. Although the metallization was not totally efficient in all cases, the amount of noble metal loaded depended strongly on the deposition time. Furthermore, the differences in the amount of loaded gold were important factors influencing the physicochemical properties of the catalysts, and consequently, their performances in the studied reactors. The addition of gold represented a considerable increase in the phenol conversion when compared with that of the TiO2, despite the small amount of noble metal loaded. However, this was not the case in the CO oxidation reaction. Beyond the differences in the phase where the reaction occurred, the loss of catalytic activity during the CO oxidation reaction was directly related to the sintering of the gold nanoparticles.

Isomerizations of a Pt4 cluster revealed by spatiotemporal microscopic analysis

We now report the first direct observation of the fluxional nature in which the four-atomic platinum cluster (Pt4) randomly walks through several isomers. Time-lapse analysis by a Cs-corrected transmission electron microscope allowed us to acquire the atomic coordinates at a sub-angstrom space resolution and 0.2 s time resolution for each cluster isomer. The analysis revealed that the isomerization follows a simple first-order kinetic model.