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.
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.
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
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.
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.
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.
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.
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.
In addition, in collaboration with K. Zbigniew from the Polish
Academy of Science we aim to correlate visual shape changes
of particles with the contour length distribution (CLD) obtained
by integral in situ XRD measurements.
TEM grid micro reactors for ambient and high
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 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.
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.