Fritz-Haber-Institut der Max-Planck-Gesellschaft  

Inorganic Chemistry – Electron Microscopy Group
  

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Electron Microscopy Group:
Marc Willinger
 
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Ernst Ruska
 
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Analysis of the structure – property relationship


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.

Particle-Support Interactions

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 particle-support interaction (PSI) 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.

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 study charging and discharging in.lithium ion battery materials and to obtain localized chemical information in nanostructured catalyst materials. Furthermore, we investigate effects of functionalization on the electronic structure for different carbon supports.











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



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.

Dynamic processes of heterogeneous catalysts



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 for example in the interaction of silver with oxygen at temperatures relevant for the ethylene epoxidation or methanol oxidation. It is also used to study the metal catalyzed chemical vapor deposition of thin carbon layers and graphene using copper, nickel and platinum catalysts.

In-situ TEM

For the study of dynamic structural changes of working catalyst at the nanometre scale, we use a commercial in situ TEM holder equipped with an environmental cell (a nanoreactor). The holder is combined with a home built gas feeding and a mass spectrometer for gas analysis. The environmental TEM cell allows monitoring gas-solid interactions under relevant catalytic conditions.
The nanoreactor itself is a microelectromechanical system (MEMS) that comprises two chemically inert E-chips and electron transparent windows. The gases are confined within a 5 micron space around the sample in an EDS compatible geometry.

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.


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.



(to see the video, click on the image)

Sequence of images recorded during
the heating of Ni particles in an atmosphere
of C2H2:H2 (1:1) at 200 mbar show
the process of metal dusting.



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