• Unprecedented view of a single catalyst

    From ScienceDaily@1:317/3 to All on Fri Oct 1 21:30:46 2021
    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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