Pratibha L. Gai

Environmental high resolution electron microscopy and applications to chemical science

Abstract An environmental cell high resolution electron microscope (EHREM) has been developed for in situ studies of dynamic chemical reactions on the atomic scale. It allows access to metastable intermediate phases of catalysts and to sequences of reversible microstructural and chemical development associated with the activation, deactivation and poisoning of a catalyst. Materials transported through air can be restored or recreated and samples damaged, e.g. by dehydration, by the usual vacuum environment in a conventional electron microscope can be preserved. A Philips CM30 HRTEM/STEM system has been extensively modified in our laboratory to add facilities for in situ gas-solid reaction studies in controlled atmospheres of gas or vapor at pressures of 0–50 mbar, instead of the regular TEM high vacuum environment. The integrated new environmental cell capability is combined with the original 0.23 nm TEM resolution, STEM imaging (bright field/annular dark field) and chemical and crystallographic microanalyses. Regular sample holders are used and include hot stages to > 1000°C. Examples of applications include direct studies of dynamic reactions with supported metal particle catalysts, the generation of defects and structural changes in practical complex oxide catalyst systems under operating conditions and carbon microstructures.

Solid-State Defect Mechanism in Vanadyl Pyrophosphate Catalysts: Implications for Selective Oxidation

High-resolution and in situ electron microscopy of vanadyl pyrophosphate catalysts reacted in alkane (n-butane) and other reducing environments have shown evidence for surface structure modifications accompanied by two sets of symmetry-related extended defects. Defect analysis reveals that the defects are formed by pure (glide) shear mechanism. The defect mechanism suggests the presence of basal (coplanar) anion vacancies, associated with Lewis acid centers, at oxygen sites linking corner-sharing phosphorus tetrahedra and vanadyl octahedra in the active plane. These in-plane defect sites may be key to the activation of the alkane, especially in the dehydrogenation.

Highly active and selective supported iron oxide nanoparticles in microwave-assisted N-alkylations of amines with alcohols

Highly active and stable supported iron oxide nanoparticles show excellent activities and switchable selectivities to target products in the microwave-assisted N-alkylation of amines with alcohols.

Advances in atomic resolution in situ environmental transmission electron microscopy and 1Å aberration corrected in situ electron microscopy

Advances in atomic resolution in situ environmental transmission electron microscopy for direct probing of gas‐solid reactions, including at very high temperatures (∼2000°C) are described. In addition, recent developments of dynamic real time in situ studies at the Angstrom level using a hot stage in an aberration corrected environment are presented. In situ data from Pt/Pd nanoparticles on carbon with the corresponding FFT/optical diffractogram illustrate an achieved resolution of 0.11 nm at 500°C and higher in a double aberration corrected TEM/STEM instrument employing a wider gap objective pole piece. The new results open up opportunities for dynamic studies of materials in an aberration corrected environment. Microsc. Res. Tech., 2009.

Structural systematics in boron-doped single wall carbon nanotubes

We report atomic level high resolution transmission electron microscopy (HRTEM), electron nanodiffraction and nano-electron energy loss spectroscopy (nano-EELS) of boron-doped carbon nanostructures obtained by laser ablation of Co/Ni/B-doped carbon targets. The observations provide direct evidence for structural systematics and atomic structural defects as a function of the B content in the target. Targets with low B concentration (below 3 at%), produced ropes of single wall carbon nanotubes (SWCNTs) with no detectable boron present in the SWCNT ropes. However, unintended N-doping of the curved honeycomb lattice was observed in the 2.5 at% B sample (which is attributed to the possible presence of small amounts of N in the targets or reaction environments), with striking consequences for doping of heteroatoms within the hexagonal lattice of the graphene layer. At higher B concentration (3.5 at% and higher), there are significant changes in the nanostructure, which exhibits defective graphite layers and a small number of double wall carbon nanotubes (DWCNTs). At the higher B concentration, boron-doping is evidenced in the form of very small amorphous B clusters trapped in graphite-like defective sites.

Development of Wet Environmental TEM (Wet-ETEM) for In Situ Studies of Liquid-Catalyst Reactions on the Nanoscale

We present the development of in situ wet environmental transmission electron microscopy (wet-ETEM) for direct probing of controlled liquid-catalyst reactions at operating temperatures on the nanoscale. The first nanoscale imaging and electron diffraction of dynamic liquid hydrogenation and polymerization reactions in the manufacture of polyamides reported here opens up new opportunities for high resolution studies of a wide range of solution-solid and solution-gas-solid reactions in the chemical and biological sciences.

On the Structural Origin of the Catalytic Properties of Inherently Strained Ultrasmall Decahedral Gold Nanoparticles

A new mechanism for reactivity of multiply twinned gold nanoparticles resulting from their inherently strained structure provides a further explanation of the surprising catalytic activity of small gold nanoparticles. Atomic defect structural studies of surface strains and quantitative analysis of atomic column displacements in the decahedral structure observed by aberration corrected transmission electron microscopy reveal an average expansion of surface nearest neighbor distances of 5.6%, with many strained by more than 10%. Density functional theory calculations of the resulting modified gold d-band states predict significantly enhanced activity for carbon monoxide oxidation. The new insights have important implications for the applications of nanoparticles in chemical process technology, including for heterogeneous catalysis.

Developments in in situ Environmental Cell High-Resolution Electron Microscopy and Applications to Catalysis

High-resolution in situ controlled-environment electron microscopy (environmental cell TEM (ETEM) or ECELL) instrumentation and techniques, and some of the key applications to dynamic reaction studies in catalysis, are reviewed. Developments over the past decade or so have led to the novel development of ETEM for in situ studies on the atomic scale of operating catalysts under controlled environments. The powerful ETEM technique enables direct access to the important, but often metastable with respect to temperature and gas atmosphere, intermediate phases in dynamic catalysis processes. Unique insights are provided into reaction mechanisms and the sequences of microstructural and nanochemical evolution of catalyst active site structures associated with selectivity and activity, and potential deactivation and poisoning. The examples demonstrate the pivotal role of ETEM in understanding, developing and controlling novel catalysts and processes. The latest developments include wet-ETEM for in situ dynamic studies of liquid--solid reactions in polymerization and molecular electronics applications.

Environmental high resolution electron microscopy of gas‐catalyst reactions

Environmental electron microscopy has become an important scientific method for fundamental studies of dynamic chemical reaction processes in heterogeneous catalysis and of catalytic growth of carbon nanotubes. Outstanding contributions are resulting from the ability to observe gas‐catalyst surface reactions in situ, on the atomic scale. A great deal of structural and chemical information including lattice modification of working catalysts is possible. This is key to understanding novel reaction processes, including release mechanism of structural oxygen in oxide catalysts in selective oxidation of hydrocarbons and to designing improved catalysts. This brief survey of the recent spectacular developments in environmental high resolution electron microscopy shows that new opportunities are being opened up in catalysis.

Silver carbonate nanoparticles stabilised over alumina nanoneedles exhibiting potent antibacterial properties

A simple method of preparing Ag2CO3 nanoparticles utilising high area gamma-alumina nanoneedles has been developed; these are promising antimicrobial agents against diverse bacterial strains.