C Kiely

Electron microscopy: New views of catalysts

Developments in electron microscopy are generating more realistic views of catalysts, allowing optimization of their structure to improve their performance.

Adoption of near-coincident-site lattice orientations by contacting monolayer rafts of metallic nanoparticles with different superlattice periodicities

A hexagonal raft of monodisperse alkane-thiol-stabilized Au nanoparticles has been self-assembled from solution on to an amorphous C substrate and then subsequently a second layer of monodisperse but differently sized gold nanoparticles deposited on top of the first. Detailed analysis of electron micrographs obtained from various regions of this bilayer revealed the presence of several distinct epitaxial interface structures. A simple near-coincident-site lattice model is used to rationalize the existence of the observed characteristic nanoparticle interface structures.

Advantages of Cs-correctors for Spectrometry in STEM

Recent theoretical calculations and practical experiments have proven that high-angle annular dark-field (HA-ADF) imaging is significantly improved due to the incident probe refined by a spherical aberration corrector (Cs-corrector) in scanning transmission electron microscopes (STEMs) [1]. The Oak Ridge group has achieved the sub-Å image resolution using a 300 keV Cs-corrected STEM [2]. For microanalysis via electron energy-loss spectrometry (EELS) and/or X-ray energy dispersive spectrometry (XEDS), the major benefit due to the Cs corrector is also improvement of spatial resolution in analysis. For the EELS analysis, it is possible to achieve atomic-level spatial resolution even in conventional STEM instruments and further improvements have been demonstrated by the Cs-corrected STEM [3]. Figure 1 shows a HA-ADF STEM image of Si<110> (a) and an EELS spectrum around Si K edge (at 1839 eV) (b) obtained with the beam current of 50 pA in a 200 keV JEM-2200FS STEM/TEM at Lehigh University, which is equipped with a CEOS STEM Cs-corrector. It is possible to measure higher core-loss spectrum within a reasonable acquisition time (10 s in this case) while maintaining the atomic resolution (1.36 Å).

Measuring Population Distributions and Catalytic Hierarchy of the Active Species in Gold on Metal Oxide Catalysts for Low Temperature CO Oxidation

Supported gold on metal oxide (MOx) catalysts have been investigated extensively by many groups over recent years for their activity in low temperature CO oxidation. Materials prepared by co-precipitation, deposition precipitation and sol-immobilization methods have all been studied and found to display varying levels of activity for this reaction. Furthermore, such materials when examined using high resolution TEM and aberration corrected scanning transmission electron microscopy show considerable variations in their nanostructures [1]. So far, Au nano-particles [2,3], monolayer & bilayer sub-nm Au clusters [4,5], extended Au rafts [6] and even highly dispersed Au atoms [7] have been identified in such catalysts and implicated as potential active species in the low temperature CO oxidation reaction (see Figure 1). The relative populations of the various supported Au species present in such Au/MOx catalysts depends critically on the preparation method and precise synthesis conditions used. Naturally this structural complexity and catalytic variability has led to considerable debate over the nature of the active species/sites in such Au/MOx catalysts.

Structural characterization of Niobium Phosphate Catalysts used for the Oxidative Dehydrogenation of Ethane to Ethylene

Vanadium phosphorus oxides (V-P-O) represent the most well studied heterogeneous complex oxide catalyst system. Thousands of articles have been published regarding their preparation routes, the nature of the active vanadium phosphate phase and their reaction mechanisms. A related group of compounds, niobium phosphates (Nb-P-O), are now also beginning to receive some attention from academic researchers.

Grain Boundary Complexions in TiO 2 Bicrystals Doped with CuO

The kinetic properties of grain boundaries play a crucial role in determining the processing, properties and performance of crystalline materials. A new concept of grain boundary complexions that relates the kinetic properties and structures of grain boundaries has recently been proposed [1, 2]. With increasing temperature or doping activity, a grain boundary (GB) can undergo coupled structural and chemical thermodynamically stable (or metastable) interfacial phases are term , 2].

Evaluation and Structural Characterization of DuPont V-P-O/SiO2 Catalysts

For several decades vanadyl pyrophosphate, (VO)2P2O7, has been used commercially as a catalyst for the partial oxidation of n-butane into maleic anhydride (MA) [1]. However due to the moderate specificity to MA production, many research groups still continue to investigate the V-P-O catalyst system with the aim of further improving its overall performance. Now a specialized form of V-P-O catalyst has been developed by DuPont for use in a fluidized bed reactor, which consists of a V-P-O core encased by a mechanically protective porous silica shell. Selected research centers worldwide have received samples of this material for analysis, aiming at combining the results generated, in an effort to further understand the functionality of the V-P-O catalyst.

Microstructural Investigations of High Productivity Au-Pd Catalysts for the Synthesis of Hydrogen Peroxide via Direct Combination of H2 and O2

There is considerable interest in using hydrogen peroxide (H2O2) as an oxygen source in the fine chemicals industry due to the environmentally benign by-product of such reactions: water. However, current production methods of H2O2 render it more costly than other common oxygen donors. The direct combination of H2 and O2 to form H2O2 is a highly desirable but difficult reaction to bring about because the reaction conditions that are required to create H2O2 also drive the decomposition of H2O2 to water. Recent work on H2O2 production has focused on Pd based catalysts operating under highly dangerous explosive conditions (elevated temperature and pressure) for hydrogen [1]. In an effort to efficiently produce H2O2 by direct combination of H2 and O2 in a safer manner, nanocrystals of Au, Pd, and Au-Pd alloy were impregnated via an incipient wetness method onto αFe2O3 particles. The catalysts were subsequently calcined in air at 400 C and then reduced in H2 at 500 C. When catalytically tested under non-explosive conditions (low pressure, low temperature), it was found that significant H2O2 production was achieved when a 1:1 ratio of Au to Pd was used.

What are the Limitations in the Characterization of Self-Assembled Metamaterials using Advanced Microscopy Techniques?

C.J. Kiely,* M. Watanabe,* A. Burrows,* P. Clasen,* M.P. Harmer,* B.Rodriguez-Gonzalez,** L. Liz-Marzan,** I. Hussain,*** J. Fink*** and M. Brust *** * Center for Advanced Materials and Nanotechnology, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA. ** Departmento de Quimica Fisica, Universidad de Vigo, 36200, Vigo, Spain. *** Center for Nanoscale Science, University of Liverpool, Liverpool, Merseyside, L69 7ZD, UK. The controlled manipulation of materials on the nanometer scale is an important task for the production and precision positioning of ever more compact electronic, optical and magnetic components. It is now possible to create thin films, nanowires and 3-D supercrystals by exploiting the order inducing chemical interactions that are inherent to a particular system. For instance, ordered structures comprised of monosized ligand stabilized nanoparticles (of most materials) can be ‘self-assembled’ from a solution of their components simply by evaporating a drop of such solution onto a suitable substrate. The current state-of-the-art is the production of