Intrinsic catalytic structure of gold nanoparticles supported on TiO2

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.

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.

Enhanced catalytic activity for CO oxidation by the metal-oxide perimeter of TiO2/nanostructured Au inverse catalysts

We report the effect of metal-oxide interfaces on CO oxidation catalytic activity with inverse TiO₂-nanostructured Au catalysts. The inverse nanocatalysts were prepared by depositing TiO₂via the liquid-phase immersion method on electrochemically synthesized Au nanostructure supports. The catalytic performance for CO oxidation was investigated using various amounts of Ti (i.e. 0.1-1.0 wt%) on two different morphologies of Au nanostructures (i.e. nanoporous and nanorod). In comparing the different Au morphologies, we found an overall higher TOF and lower activation energy for the TiO₂/nanoporous Au than those for the TiO₂/nanorod Au. In addition, the CO oxidation activity increased as the Ti content increased up to 0.5 wt% probably due to active TiO₂-Au interface sites enhancing CO oxidation via the supply of adsorption sites or charge transfer from TiO₂ to Au. However, a higher titania content (i.e. 1.0 wt% TiO₂) resulted in decreased activity caused by high surface coverage of TiO₂ decreasing the number of TiO₂-Au interface sites. These results implied that the perimeter area of the metal-oxide interface played a significant role in determining the catalytic performance for CO oxidation.

Unexpected refacetting of palladium nanoparticles under atmospheric N2 conditions

In situ TEM observations and DFT calculations reveal that the “inert” gas N₂ has the ability to modify the structure of metal nanoparticles.

Comparison studies of interfacial energetic and electronic properties of bimetallic AuCu/TiO2 hetero-structures from DFT calculations

The modification in which an Au co-catalyst is replaced with a bimetallic AuCu co-catalyst to build a TiO₂-based composite photocatalyst not only enhances the interaction of the metal layer with the TiO₂ substrate, but also promotes electron transfer and charge separation across the interface.

Classical strong metal-support interactions between gold nanoparticles and titanium dioxide

Supported metal catalysts play a central role in the modern chemical industry but often exhibit poor on-stream stability. The strong metal-support interaction (SMSI) offers a route to control the structural properties of supported metals and, hence, their reactivity and stability. Conventional wisdom holds that supported Au cannot manifest a classical SMSI, which is characterized by reversible metal encapsulation by the support upon high-temperature redox treatments. We demonstrate a classical SMSI for Au/TiO₂, evidenced by suppression of CO adsorption, electron transfer from TiO₂ to Au nanoparticles, and gold encapsulation by a TiO _x overlayer following high-temperature reduction (reversed by subsequent oxidation), akin to that observed for titania-supported platinum group metals. In the SMSI state, Au/TiO₂ exhibits markedly improved stability toward CO oxidation. The SMSI extends to Au supported over other reducible oxides (Fe₃O₄ and CeO₂) and other group IB metals (Cu and Ag) over titania. This discovery highlights the general nature of the classical SMSI and unlocks the development of thermochemically stable IB metal catalysts.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.

Redox processes in metal oxide surfaces can exhibit structure sensitivities which are difficult to uncover. Here, the authors use atomic-resolution imaging to demonstrate facet dependent alterations in the surfaces of supported vanadium oxide upon reduction and oxidation.

Adsorption of AuxCuy (x + y = 1, 2, 3) nanoclusters on the anatase TiO2(101) surface and their catalytic activity: a density functional theory study

The adsorption of Au_xCu_y (x + y = 1, 2, 3) nanoclusters on the anatase TiO₂(101) surface were studied in detail using density functional theory. The introduction of Cu atoms in Au clusters not only significantly reduces the cost, but also changes the catalytic process of Au/TiO₂, which may be helpful to some catalytic reactions.

Assessing and ameliorating the influence of the electron beam on carbon nanotube oxidation in environmental transmission electron microscopy

In this work, we examine how the imaging electron beam can induce damage in carbon nanotubes (CNTs) at varying oxygen gas pressures and electron dose rates using environmental transmission electron microscopy (ETEM). Our studies show that there is a threshold cumulative electron dose which brings about damage in CNTs in oxygen - through removal of their graphitic walls - which is dependent on O₂ pressure, with a 4-5 fold decrease in total electron dose per decade increase at a lower pressure range (10^-6 to 10^-5mbar) and approximately 1.3 -fold decrease per decade increase at a higher pressure range (10^-3 to 10⁰mbar). However, at a given pressure, damage in CNTs was found to occur even at the lowest dose rate utilized, suggesting the absence of a lower limit for the latter parameter. This study provides guidelines on the cumulative dose required to damage nanotubes in the 10^-7mbar to 10⁰mbar pressure regimes, and discusses the role of electron dose rate and total electron dose on beam-induced CNT degradation experiments.

In Situ Environmental TEM in Imaging Gas and Liquid Phase Chemical Reactions for Materials Research

Gas and liquid phase chemical reactions cover a broad range of research areas in materials science and engineering, including the synthesis of nanomaterials and application of nanomaterials, for example, in the areas of sensing, energy storage and conversion, catalysis, and bio-related applications. Environmental transmission electron microscopy (ETEM) provides a unique opportunity for monitoring gas and liquid phase reactions because it enables the observation of those reactions at the ultra-high spatial resolution, which is not achievable through other techniques. Here, the fundamental science and technology developments of gas and liquid phase TEM that facilitate the mechanistic study of the gas and liquid phase chemical reactions are discussed. Combined with other characterization tools integrated in TEM, unprecedented material behaviors and reaction mechanisms are observed through the use of the in situ gas and liquid phase TEM. These observations and also the recent applications in this emerging area are described. The current challenges in the imaging process are also discussed, including the imaging speed, imaging resolution, and data management.

On the role of the gas environment, electron-dose-rate, and sample on the image resolution in transmission electron microscopy

The introduction of gaseous atmospheres in transmission electron microscopy offers the possibility of studying materials in situ under chemically relevant environments. The presence of a gas environment can degrade the resolution. Surprisingly, this phenomenon has been shown to depend on the electron-dose-rate. In this article, we demonstrate that both the total and areal electron-dose-rates work as descriptors for the dose-rate-dependent resolution and are related through the illumination area. Furthermore, the resolution degradation was observed to occur gradually over time after initializing the illumination of the sample and gas by the electron beam. The resolution was also observed to be sensitive to the electrical conductivity of the sample. These observations can be explained by a charge buildup over the electron-illuminated sample area, caused by the beam–gas–sample interaction, and by a subsequent sample motion induced by electrical capacitance in the sample.

Bi-phasic titanium dioxide nanoparticles doped with nitrogen and neodymium for enhanced photocatalysis

Bi-phasic or multi-phasic composite nanoparticles for use in photocatalysis have been produced by a new synthetic approach. Sol-gel methods are used to deposit multiple layers of active material onto soluble substrates. In this work, a layer of rutile (TiO2) was deposited onto sodium chloride pellets followed by an annealing step and a layer of anatase. After dissolving the substrate, bi-phasic nanoparticles containing half anatase and half rutile TiO2; with "Janus-like" characteristics are obtained. Nitrogen and neodymium doping of the materials were observed to enhance the photocatalytic properties both under UV and white light irradiation. The unique advantage of this synthetic method is the ability to systematically dope separate sides of the nanoparticles. Nitrogen doping was found to be most effective on the anatase side of the nanoparticle while neodymium was found to be most effective on the rutile side. Rhodamine B dye was effectively photodegraded by co-doped particles under white light.

Dynamic In-Situ Experimentation on Nanomaterials at the Atomic Scale

With the development of in situ techniques inside transmission electron microscopes (TEMs), external fields and probes can be applied to the specimen. This development transforms the TEM specimen chamber into a nanolab, in which reactions, structures, and properties can be activated or altered at the nanoscale, and all processes can be simultaneously recorded in real time with atomic resolution. Consequently, the capabilities of TEM are extended beyond static structural characterization to the dynamic observation of the changes in specimen structures or properties in response to environmental stimuli. This extension introduces new possibilities for understanding the relationships between structures, unique properties, and functions of nanomaterials at the atomic scale. Based on the idea of setting up a nanolab inside a TEM, tactics for design of in situ experiments inside the machine, as well as corresponding examples in nanomaterial research, including in situ growth, nanofabrication with atomic precision, in situ property characterization, and nanodevice construction are presented.

Crystal-plane-dependent metal–support interaction in Au/TiO2

The metal–support interactions between Au and different TiO₂ crystal planes are investigated to identify their crystal-plane-dependent properties.

Superior catalytic performance of Ce1−xBixO2−δsolid solution and Au/Ce1−xBixO2−δfor 5-hydroxymethylfurfural conversion in alkaline aqueous solution

Ce_1−xBi_xO_2−δand Au/Ce_1−xBi_xO_2−δexhibited superior catalytic performance towards 5-hydroxymethylfurfural conversion.

In situ observation of facet-dependent oxidation of graphene on platinum in an environmental TEM

The facet-dependent oxidation reactionvia in situETEM.

Differently Shaped Au Nanoparticles: A Case Study on the Enhancement of the Photocatalytic Activity of Commercial TiO₂

In the present work, the influence of a gold nanoparticle's shape was investigated on the commercially available Evonik Aeroxide P25. By the variation of specific synthesis parameters, three differently shaped Au nanoparticles were synthetized and deposited on the surface of the chosen commercial titania. The nanoparticles and their composites' morphological and structural details were evaluated, applying different techniques such as Diffuse Reflectance Spectroscopy (DRS), X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). The influence of the Au nanoparticles' shape was discussed by evaluating their photocatalytic efficiency on phenol and oxalic acid degradation and by investigating the H₂ production efficacy of the selected composites. Major differences in their photocatalytic performance depending on the shape of the deposited noble metal were evidenced.

Oxidation and reduction processes of platinum nanoparticles observed at the atomic scale by environmental transmission electron microscopy

Oxidation and reduction of the surfaces of Pt nanoparticles were in situ examined in reactive gases (O2, CO and H2O vapor) by aberration-corrected environmental transmission electron microscopy. Atomic layers of Pt oxides were gradually formed on the surface of Pt nanoparticles at room temperature in O2. The surface Pt oxides were reduced to Pt promptly in both vacuum and gas including CO. We showed that H2O vapor suppressed the surface oxidation. The processes found in this study were induced by gases that were most likely activated by electron irradiation. The observation results provide atomistic insight into the oxidation and reduction process of the surface of Pt nanoparticles that is exposed to activated gases.

Atomic observation of catalysis-induced nanopore coarsening of nanoporous gold

Dealloyed nanoporous metals have attracted much attention because of their excellent catalytic activities toward various chemical reactions. Nevertheless, coarsening mechanisms in these catalysts have not been experimentally studied. Here, we report in situ atomic-scale observations of the structural evolution of nanoporous gold during catalytic CO oxidation. The catalysis-induced nanopore coarsening is associated with the rapid diffusion of gold atoms at chemically active surface steps and the surface segregation of residual Ag atoms, both of which are stimulated by the chemical reaction. Our observations provide the first direct evidence that planar defects hinder nanopore coarsening, suggesting a new strategy for developing structurally stable and highly active heterogeneous catalysts.

Advances in the environmental transmission electron microscope (ETEM) for nanoscale in situ studies of gas–solid interactions

This review highlights how ETEM technology advances have enabled new essential (structural) information that improve our understanding of nanomaterials' structure–property–function relationships.

Stepwise displacement of catalytically active gold nanoparticles on cerium oxide

Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO2) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO2 and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid.

Responsive hybrid nanosheets of hyperbranched poly(ether amine) as a 2D-platform for metal nanoparticles

Hybrid nanosheets of hyperbranched poly(ether amine) are demonstrated as a novel 2D-platform for metal nanoparticles, which makes metal nanoparticles transfer reversibly between oil and water phases.