Unprecedented view of a single catalyst nanoparticle at work
X-rays reveal compositional changes on active surface under reaction conditions

Date:
October 1, 2021
Source:
Deutsches Elektronen-Synchrotron DESY
Summary:
A research team has been using high-intensity X-rays to observe a
single catalyst nanoparticle at work. The experiment has revealed
for the first time how the chemical composition of the surface
of an individual nanoparticle changes under reaction conditions,
making it more active.

This study marks an important step towards a better understanding
of real, industrial catalytic materials.



FULL STORY ==========================================================================
A DESY-led research team has been using high-intensity X-rays to observe a single catalyst nanoparticle at work. The experiment has revealed for the
first time how the chemical composition of the surface of an individual nanoparticle changes under reaction conditions, making it more active. The
team led by DESY's Andreas Stierle is presenting its findings in the
journal Science Advances. This study marks an important step towards a
better understanding of real, industrial catalytic materials.


========================================================================== Catalysts are materials that promote chemical reactions without being
consumed themselves. Today, catalysts are used in numerous industrial processes, from fertiliser production to manufacturing plastics. Because
of this, catalysts are of huge economic importance. A very well-known
example is the catalytic converter installed in the exhaust systems
of cars. These contain precious metals such as platinum, rhodium and
palladium, which allow highly toxic carbon monoxide (CO) to be converted
into carbon dioxide (CO2) and reduce the amount of harmful nitrogen oxides (NOx).

"In spite of their widespread use and great importance, we are still
ignorant of many important details of just how the various catalysts
work," explains Stierle, head of the DESY NanoLab. "That's why we have
long wanted to study real catalysts while in operation." This is not
easy, because in order to make the active surface as large as possible, catalysts are typically used in the form of tiny nanoparticles, and the
changes that affect their activity occur on their surface.

Surface strain relates to chemical composition In the framework of the
EU project Nanoscience Foundries and Fine Analysis (NFFA), the team
from DESY NanoLab has developed a technique for labelling individual nanoparticles and thereby identifying them in a sample. "For the study,
we grew nanoparticles of a platinum-rhodium alloy on a substrate in the
lab and labelled one specific particle," says co-author Thomas Keller
from DESY NanoLab and in charge of the project at DESY. "The diameter
of the labelled particle is around 100 nanometres, and it is similar
to the particles used in a car's catalytic converter." A nanometre is
a millionth of a millimetre.

Using X-rays from the European Synchrotron Radiation Facility ESRF in
Grenoble, France, the team was not only able to create a detailed image
of the nanoparticle; it also measured the mechanical strain within its
surface. "The surface strain is related to the surface composition, in particular the ratio of platinum to rhodium atoms," explains co-author
Philipp Plessow from the Karlsruhe Institute of Technology (KIT), whose
group computed strain as a function of surface composition. By comparing
the observed and computed facet- dependent strain, conclusions can be
drawn concerning the chemical composition at the particle surface. The different surfaces of a nanoparticle are called facets, just like the
facets of a cut gemstone.

When the nanoparticle is grown, its surface consists mainly of platinum
atoms, as this configuration is energetically favoured. However, the
scientists studied the shape of the particle and its surface strain
under different conditions, including the operating conditions of an
automotive catalytic converter. To do this, they heated the particle
to around 430 degrees Celsius and allowed carbon monoxide and oxygen
molecules to pass over it. "Under these reaction conditions, the rhodium
inside the particle becomes mobile and migrates to the surface because
it interacts more strongly with oxygen than the platinum," explains
Plessow. This is also predicted by theory.

"As a result, the surface strain and the shape of the particle change,"
reports co-author Ivan Vartaniants, from DESY, whose team converted
the X-ray diffraction data into three-dimensional spatial images. "A facet-dependent rhodium enrichment takes place, whereby additional
corners and edges are formed." The chemical composition of the surface,
and the shape and size of the particles have a significant effect
on their function and efficiency. However, scientists are only just
beginning to understand exactly how these are connected and how to
control the structure and composition of the nanoparticles. The X-rays
allow researchers to detect changes of as little as 0.1 in a thousand
in the strain, which in this experiment corresponds to a precision of
about 0.0003 nanometres (0.3 picometres).

Crucial step towards analysing industrial catalyst maerials "We can now,
for the first time, observe the details of the structural changes in such catalyst nanoparticles while in operation," says Stierle, Lead Scientist
at DESY and professor for nanoscience at the University of Hamburg.

"This is a major step forward and is helping us to understand an entire
class of reactions that make use of alloy nanoparticles." Scientists
at KIT and DESY now want to explore this systematically at the new Collaborative Research Centre 1441, funded by the German Research
Foundation (DFG) and entitled "Tracking the Active Sites in Heterogeneous Catalysis for Emission Control (TrackAct)." "Our investigation is an
important step towards analysing industrial catalytic materials," Stierle points out. Until now, scientists have had to grow model systems in the laboratory in order to conduct such investigations. "In this study,
we have gone to the limit of what can be done. With DESY's planned
X-ray microscope PETRA IV, we will be able to look at ten times smaller individual particles in real catalysts, and under reaction conditions." ========================================================================== Story Source: Materials provided by
Deutsches_Elektronen-Synchrotron_DESY. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Young Yong Kim, Thomas F. Keller, Tiago J. Goncalves, Manuel Abuin,
Henning Runge, Luca Gelisio, Jerome Carnis, Vedran Vonk, Philipp N.

Plessow, Ivan A. Vartaniants, Andreas Stierle. Single alloy
nanoparticle x-ray imaging during a catalytic reaction. Science
Advances, 2021; 7 (40) DOI: 10.1126/sciadv.abh0757 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211001152720.htm

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