Chang Wan Han

Highly Stable Bimetallic AuIr/TiO2 Catalyst: Physical Origins of the Intrinsic High Stability against Sintering

It has been a long-lived research topic in the field of heterogeneous catalysts to find a way of stabilizing supported gold catalyst against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO2 synthesized by sequential deposition-precipitation. To reveal the physical origin of the high stability of AuIr/TiO2, we used aberration-corrected scanning transmission electron microscopy (STEM), STEM-tomography, and density functional theory (DFT) calculations. Three-dimensional structures of AuIr/TiO2 obtained by STEM-tomography indicate that AuIr nanoparticles on TiO2 have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO2 showed that the presence of Ir near the TiO2 surface increases the adhesion energy of the bimetallic slabs to the TiO2 and the attractive interactions between Ir and TiO2 lead to higher stability of AuIr nanoparticles as compared to Au nanoparticles.

Migration of Single Iridium Atoms and Tri-iridium Clusters on MgO Surfaces: Aberration-Corrected STEM Imaging and Ab Initio Calculations.

To address the challenge of fast, direct atomic-scale visualization of the migration of atoms and clusters on surfaces, we used aberration-corrected scanning transmission electron microscopy (STEM) with high scan speeds (as little as ∼0.1 s per frame) to visualize the migration of (1) a heavy atom (Ir) on the surface of a support consisting of light atoms, MgO(100), and (2) an Ir3 cluster on MgO(110). Sequential Z-contrast images elucidate the surface transport mechanisms. Density functional theory (DFT) calculations provided estimates of the migration energy barriers and binding energies of the iridium species to the surfaces. The results show how the combination of fast-scan STEM and DFT calculations allow visualization and fundamental understanding of surface migration phenomena pertaining to supported catalysts and other materials.

Participation of interfacial hydroxyl groups in the water-gas shift reaction over Au/MgO catalysts

Au/MgO and Au/Mg(OH)2 catalysts were prepared and used as model systems to study the participation of the Au–support interface in the water-gas shift reaction (WGS). Au/MgO and Au/Mg(OH)2 showed similar WGS kinetics, consistent with a similar WGS reaction mechanism. However, Au/MgO had a lower apparent reaction order with respect to H2O and was identified as having a higher specific WGS rate compared with Au/Mg(OH)2 at the same average Au particle size. The focus of the work is on Au/MgO, where we observed a correlation between the hydroxyl group coverage and WGS rate. The measured kinetic isotope effect, DFT calculations, and operando FTIR for that catalyst are all consistent with surface carboxyl formation as the rate-determining step. Comparisons of hydroxyl group coverage with and without Au suggest that the formation of OH groups is strongly influenced by the presence of Au and likely to be highest at the Au–MgO interface, as supported by theoretical calculations. Temperature programmed reaction shows that Au is necessary to catalyze reaction of the surface OH groups with CO. This work confirms the importance of the metal support interface in WGS catalysis and suggests that the unique chemistry at the interface offers both an explanation of catalyst behaviour and a new opportunity to design materials with improved function for additional catalytic applications.

Implementation of Sparse Image Acquisition in a Conventional Scanning Transmission Electron Microscope

STEM is one of the foremost methods for nanoscale characterization of materials, but high current densities can cause damage in electron-beam sensitive materials. Here, we report our recent approach to implement a sparse acquisition in STEM mode executed by a random sparse-scan and a signal processing algorithm called model-based iterative reconstruction (MBIR) [1]. In this method, a small portion (such as 5%) of randomly chosen unit sampling areas (i.e. electron probe positions), which corresponds to pixels of a STEM image of the specimen are scanned with an electron probe to obtain a sparse image. Sparse images are then reconstructed using the MBIR inpainting algorithm to produce an image of the specimen at the original resolution that is consistent with an image obtained using conventional scanning methods. Experimental results for down to 5% sampling show consistency with the full STEM image acquired by the conventional scanning method.

Dynamic Investigation of Metal-support Interactions in Heterodimer Nanoparticles by in situ Transmission Electron Microscopy

Catalysts are critical components of many current technologies and essential components of sustainable energy systems ranging from fuel cells and batteries to turning biomass into useful chemicals or fuels by enabling reactions to be guided quickly and efficiently along desirable pathways rather than those that are inefficient or lead to unwanted byproducts. Fundamental investigations of catalyst structures and the mechanisms of catalytic reactions requires characterization imaging of surfaces at the atomic scale and probing the structures and energetics of the reacting molecules as they function under reaction condition on varying time and length scales. High-angle annular dark field (HAADF) STEM is an indispensable technique for analyzing heterogeneous catalysts, in particular those comprising high atomic number (Z) metallic nanoparticles (NPs) dispersed on low-Z supports. Readily interpretable atomic-scale Z-contrast imaging with high spatial resolution spectroscopy, including X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), allows researchers to investigate structural information such as dimensions, morphologies, and size distribution of catalytic NPs as well as their material chemistry (e.g., chemical composition, bonding of catalytic particles with supports at interface, etc.).

Dynamic Aberration-corrected STEM of Bimetallic Nanocatalysts during Surface Diffusion

Fundamental investigations of catalyst structures and the mechanisms of catalytic reactions requires characterization imaging of surfaces at the atomic scale and probing the structures and energetics of the reacting molecules as they function under reaction condition on varying time and length scales. Highangle annular dark field (HAADF) STEM is an indispensable technique for analyzing heterogeneous catalysts, in particular those comprising high atomic number (Z) metallic nanoparticles (NPs) dispersed on low-Z supports [1]. Readily interpretable atomic-scale Z-contrast imaging with high spatial resolution spectroscopy, including X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), allows researchers to investigate structural information such as dimensions, morphologies, and size distribution of catalytic NPs as well as their material chemistry (e.g., chemical composition, bonding of catalytic particles with supports at interface, etc.).

In situ Environmental TEM and DFT Studies on the Highly Stable AuIr Bimetallic Catalyst

Since the report by Haruta et al. in 1987 of high catalytic activity of supported nanocrystalline Au catalysts in CO oxidation at low temperature [1], Au catalysts have attracted significant amount of interest from researchers. Despite the remarkable activity of Au catalysts in various oxidation processes, a wide range of uses of Au catalysts in industry is limited due to the lack of stability against sintering [2,3]. In this regard, stabilization of supported Au nanoparticles (NPs) is of utmost importance in the field of Au catalysis. With the purpose of increasing the stability of Au catalysts, various methods for increasing the stability have been developed [4,5]. One of the successful methods for increasing the stability is through adding a second metal into Au NPs [6–9]. Specifically, R. Zanella et al. reported that AuIr bimetallic NPs on TiO2 support show enhanced stability and activity compared to Au on TiO2 [7]. Considering several reports of the improved catalytic activity as well as on the improved stability of supported Au-based bimetallic catalysts, it can be expected that Au catalysts for industrial applications will be comprise bimetallic NPs rather than pure Au. Therefore, the mechanism behind the increased stability of Au-based bimetallic catalysts needs to be clearly understood. Despite the importance of Aubased bimetallic catalysts, however, the in situ investigations of their dynamic behaviors during the “real” catalytic process have been limited.

A Correlative Study of HRTEM, HAADF-STEM, and STEM-EELS Spectrum Imaging for Biphasic Electrochemically Active TiO 2

Nanocrystalline TiO2 polymorphs have been widely studied as candidate materials for the energy harvesting and storage applications, including photovoltaic devices and rechargeable batteries due to its excellent performances as well as low cost [ 1–3] . Interestingly, it has been reported that a mixture of the polymorphs shows improved photocatalytic activity compared to that of a single TiO2 polymorph [ 4,5] . D. C. Hurum et al. found the improved activity of the biphasic TiO2, Degussa P25 a mixture of anatase and rutile, is due to the synergetic charge transfer effect between them [6]. Recently, we also found that the synergetic effect in a biphasic TiO2 anode significantly improves the capacity and the rate performance of lithium ion batteries. Considering the fact that the biphasic form of TiO2 outperforms single polymorphic phases in the energy-related applications, various forms of biphasic materials would be further developed. Since the polymorphs in the biphasic materials have identical composition with each other, distribution of the constituent polymorph in a biphasic material is not readily obtainable at high spatial resolution. Therefore, most of reports just provide relative amount of constituent phases determined by powder X-ray diffraction and/or Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) images with low magnification in which the spatial distribution of the polymorphs are not clearly shown [5,7,8]. In this respect, a systematic approach to characterize the biphasic materials is essential.

Secondary signal imaging (SSI) electron tomography (SSI-ET): A new three-dimensional metrology for mesoscale specimens in transmission electron microscope

We have demonstrated a new electron tomography technique utilizing the secondary signals (secondary electrons and backscattered electrons) for ultra thick (a few μm) specimens. The Monte Carlo electron scattering simulations reveal that the amount of backscattered electrons generated by 200 and 300keV incident electrons is a monotonic function of the sample thickness and this causes the thickness contrast satisfying the projection requirement for the tomographic reconstruction. Additional contribution of the secondary electrons emitted from the edges of the specimens enhances the visibility of the surface features. The acquired SSI tilt series of the specimen having mesoscopic dimensions are successfully reconstructed verifying that this new technique, so called the secondary signal imaging electron tomography (SSI-ET), can directly be utilized for 3D structural analysis of mesoscale structures.

In Situ HAADF-STEM Imaging and Tomography of AuIr Bimetallic Catalysts

High-angle annular dark field (HAADF) STEM is an indispensable technique for analyzing heterogeneous catalysts, in particular those comprising high atomic number (Z) metallic nanoparticles (NPs) dispersed on low-Z supports. Readily interpretable atomic-scale Z-contrast imaging with high spatial resolution spectroscopy, including X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), allows researchers to investigate structural information such as dimensions, morphologies, and size distribution of catalytic NPs as well as their material chemistry (e.g., chemical composition, bonding of catalytic particles with supports at interface, etc.).