Electron Microscope group

Analysis of the structure – property relationship

A: HAADF STEM images of Pt on a modified carbon B: FeO encapsulated Pt particle on a Fe3O4 support.

We use state-of-the art analytical electron microscopes to „see“ the atomic arrangement, to „identify“ the atomic species and to „collect“ information about the resulting electronic structure.

Ultimate spatial resolution combined with the simultaneous acquisition of spectroscopic data are used to guide the synthesis of new catalysts as well as to monitor structural developments induced under catalytic conditions ex-situ down to the atomic level.

The aim is to get an insight in the relation between structure and activity, to understand catalyst-support interactions and to identify key properties that are required for the formation of specific active surface species under reaction conditions.

The information obtained at the local atomic scale is complemented by integral techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM). A pre-requisiste for harvesting all this local information is an excellent sample preparation. Amongst others, we have implemented routines, which make use of a focused ion beam (FIB), ion milling and/or ultramicrotomy.

Metal-Support Interaction (MSI)

In heterogeneous catalysis, the active component of a catalyst is often dispersed on a high surface area support. The interaction between dispersed particles and support has a strong influence on the properties of a catalyst and is therefore part of our current research.
The metal-support interaction (MSI) influences the stability of the supported particles against sintering and effects the electronic structure of the supported particles.
For metal nanoparticles on oxide supports, strong metal-support interaction (SMSI) can lead to decoration or encapsulation of metal particles by the oxide. The encapsulation usually suppresses catalytic activity, but, in certain cases, it results in an enhanced reactivity and unparalleled selectivity.
We investigate metals on oxide supports and the interactions between different components in intergrowth structures as well as (noble) metal particles on different types of carbon supports.

Fe L3,2 edge position for LiFePO4 (red) and FePO4 (blue); removal of Li from the host structure leads to a change of the Fe oxidation state from 2+ to 3+ observed by a shift of the L3 edge position from ~709 eV to 711 eV.
Electronic Structure

A transmisson electron microscope provides ideal conditions for precise and well defined scattering experiments. The energy loss due to inelastic scattering events between the electron beam and the atoms of the sample can be detected by a spectrometer. Electron energy loss spectrometry (EELS) provides localized information about the electronic structure. We use EELS to obtain localized chemical information in nanostructured catalyst materials. Furthermore, we investigate local compositional differences of bulk, surface, and defects.

Our MaxNet Energy project represents an example of how the interaction of different TEM techniques has led to a knowledge gain on selected materials relevant for energy conversion.
In collaboration with ThermoFischer Scientific, we are currently optimizing and standartizing TEM research in order to fit to the needs of chemists.

Dynamic processes of heterogeneous catalysts

Snapshots recorded during in-situ low pressure metal catalyzed CVD growth of graphene on copper in the ESEM at 1000°C and 2·10-2 Pa.
Surface Reactions by Environmental SEM

For in-situ studies of dynamic processes at the micrometer scale, we use a modified environmental scanning electron microscope (ESEM). The instrument is equipped with a heating stage, a gas feeding system with mass flow controllers and a mass spectrometer. The set-up allows direct observation of reaction induced morphological changes

TEM grid micro reactors for ambient and high pressure reactions

For the study of reaction induced changes of catalysts at the atomic scale we have developed TEM grid micro-reactors. They were designed to allow a close coupling of analytical transmission electron microscopy with catalytic reactions. Microscopic amounts of catalyst on an inert TEM grid can be exposed to relevant catalytic conditions and subsequently transferred via glove box and vacuum transfer holder from the reactor into the TEM without contact to ambient air. A highly sensitive proton transfer-reaction mass spectrometer is used to monitor catalytic activity. Using this set-up we are able to monitor structural and compositional modifications of catalyst particles that are induced under well-defined and catalytically relevant conditions

The TEM grid micro-reactor (top) and a HAADF STEM image of the M1 phase that is studied using this set-up.

The knowledge retrieved by ex situ, in situ and quasi in situ TEM will enlighten our synthetic project, in which aim to decipher and tailor important surface states..

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