Environmental electron microscopy (ETEM) for catalysts with a closed E-cell with carbon windows

Intrinsic catalytic structure of gold nanoparticles supported on TiO2

Despite the fragility of TiO(2) under electron irradiation, the intrinsic structure of Au/TiO(2) catalysts can be observed by environmental transmission electron microscopy. Under reaction conditions (CO/air 100 Pa), the major {111} and {100} facets of the gold nanoparticles are exposed and the particles display a polygonal interface with the TiO(2) support bounded by sharp edges parallel to the 〈110〉 directions.

Image resolution and sensitivity in an environmental transmission electron microscope

An environmental transmission electron microscope provides unique means for the atomic-scale exploration of nanomaterials during the exposure to a reactive gas environment. Here we examine conditions to obtain such in situ observations in the high-resolution transmission electron microscopy (HRTEM) mode with an image resolution of 0.10nm. This HRTEM image resolution threshold is mapped out under different gas conditions, including gas types and pressures, and under different electron optical settings, including electron beam energies, doses and dose-rates. The 0.10nm resolution is retainable for H(2) at 1-10mbar. Even for N(2), the 0.10nm resolution threshold is reached up to at least 10mbar. The optimal imaging conditions are determined by the electron beam energy and the dose-rate as well as an image signal-to-noise (S/N) ratio that is consistent with Rose's criterion of S/N≥5. A discussion on the electron-gas interactions responsible for gas-induced resolution deterioration is given based on interplay with complementary electron diffraction (ED), scanning transmission electron microscopy (STEM) as well as electron energy loss spectroscopy (EELS) data.

Self-aligned wet-cell for hydrated microbiology observation in TEM

This paper describes a Self-Aligned Wet (SAW) cell suitable for direct-cell or bacteria incubation and observation in a wet environment inside a transmission electron microscope. This SAW cell is fabricated by a bulk-micromachining process and composed of two structurally complementary counterparts (an out-frame and an in-frame), where each contain a silicon nitride film based observation window. The in- and out-frames can be self-aligned via a mechanism of surface tension from a bio-sample droplet without the aid of positioning stages. The liquid chamber is enclosed between two silicon nitride membranes that are thin enough to allow high energy electrons to penetrate while also sustaining the pressure difference between the TEM vacuum and the vapor pressure within the liquid chamber. A large field of view (150 μm × 150 μm) in a SAW cell is favored and formed from a larger sized observation window in the out-frame, which is fabricated using a unique circular membrane formation process. In this paper, we introduce a novel design to circumvent the challenges of charging/heating problems in silicon nitride that arise from interactions with an electron beam. This paper also demonstrates TEM observations of D. Radiodurans growth in a liquid environment within a thicker chamber (20 μm) within a SAW cell.

Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

Improvement of windowed type environmental-cell transmission electron microscope for in situ observation of gas-solid interactions

We have developed an improved, windowed type environmental-cell (E-cell) transmission electron microscope (TEM) for in situ observation of gas-solid interactions, such as catalytic reactions at atmospheric pressure. Our E-cell TEM includes a compact E-cell specimen holder with mechanical stability, resulting in smoother introduction of the desired gases compared with previous E-cell TEMs. In addition, the gas control unit was simplified by omitting the pressure control function of the TEM pre-evacuation chamber. This simplification was due to the successful development of remarkably tough thin carbon films as the window material. These films, with a thickness of <10 nm, were found to withstand pressure differences >2 atm. Appropriate arrangement of the specimen position inside the E-cell provided quantitatively analyzable TEM images, with no disturbances caused by the windowed films. As an application, we used this E-cell TEM to observe the dynamic shape change in a catalytic gold nanoparticle supported on TiO(2) during the oxidation of CO gas.

Development of a multifunctional TEM specimen holder equipped with a piezodriving probe and a laser irradiation port

The double-probe piezodriving specimen holder that was recently developed by some of the present authors is modified to introduce a laser irradiation port in one of its two arms. As a result, the new specimen holder consists of a piezodriving probe and a laser irradiation port, both of which can be three-dimensionally controlled by using piezoelectric elements and micrometers. While the piezodriving probe interacts with the specimen set in the holder in several ways, the laser beam causes photo-induced phenomena to occur. By performing electron holography using the new specimen holder, we demonstrate that it is possible to evaluate the change in the electric field resulting from the discharging effect of laser irradiation on organic photoconductors.

Dynamic adsorbate/reaction induced structural change of supported metal nanoparticles: heterogeneous catalysis and beyond

Our ever advancing abilities to examine nanosize metals and/or oxides with atomic and/or high temporal resolution have recently started to reveal much that is new about the behaviour of such systems. In many cases the notion of passive entities, upon which catalytic events occur, has been overturned, and with it many ideas that for a long time were axiomatic to the understanding of their behaviour. In its place a world of structuro-reactive dynamism is starting to appear wherein the atomic scale structure and reactivity are intimately tied to the nature of the environment being experienced. The aim of this tutorial review is to introduce the reader to these phenomena, to discuss how we might observe and categorise differing types of dynamic change, and to give some specific examples of where and how this fundamental structural dynamism can be tangibly linked to the reactive behaviour of heterogeneous catalysts.

Adsorption and reaction of methanol on supported palladium catalysts: microscopic-level studies from ultrahigh vacuum to ambient pressure conditions

We investigated the decomposition and (partial) oxidation of methanol on Pd based catalysts in an integrated attempt, simultaneously bridging both the pressure and the materials gap. Combined studies were performed on well-defined Pd model catalysts based on ordered Al(2)O(3) and Fe(3)O(4) thin films, on well-defined particles supported on powders and on Pd single crystals. The interaction of Pd nanoparticles and Pd(111) with CH(3)OH and CH(3)OH/O(2) mixtures was examined from ultrahigh vacuum conditions up to ambient pressures, utilizing a broad range of surface specific vibrational spectroscopies which included IRAS, TR-IRAS, PM-IRAS, SFG, and DRIFTS. Detailed kinetic studies in the low pressure region were performed by molecular beam methods, providing comprehensive insights into the microkinetics of the reaction system. The underlying microscopic processes were studied theoretically on the basis of specially designed 3-D nanocluster models containing approximately 10(2) metal atoms. The efficiency of this novel modelling approach was demonstrated by rationalizing and complementing pertinent experimental results. In order to connect these results to the behavior under ambient conditions, kinetic and spectroscopic investigations were performed in reaction cells and lab reactors. Specifically, we focused on (1) particle size and structure dependent effects in methanol oxidation and decomposition, (2) support effects and their relation to activity and selectivity, (3) the influence of poisons such as carbon, and (4) the role of oxide and surface oxide formation on Pd nanoparticles.