Platinum nanoparticles are widely used as catalysts in many important fields such as the chemical industry, fuel cell technology and automotive exhaust gas purification. At the nanoscale, crystal defects have a strong influence on the properties and functionality of materials. Therefore, for the design of catalysts, it is essential to predict the structural evolution of nanocrystals during their catalytic reaction. However, until now, imaging and characterizing the structure of defects inside and in three dimensions (3D) of a nano-catalyst has remained a challenge.
The catalytic converter in automobiles is designed to trap polluting CO particles emitted by the combustion engine and then release them as CO
2, a less toxic gas. On the occasion of a collaboration
[1], researchers at IRIG have used an innovative technique to observe at the nanoscale the defects that form in the platinum catalysts used in catalytic converters.
Indeed, thanks to
coherent X-ray diffraction imaging performed at ESRF in Grenoble, it was possible to characterize the deformations of platinum nanoparticles in a laboratory reactor that reproduces the operation of a catalytic converter. This X-ray beam is intense and focused enough to measure a single Pt nanoparticle. Thus, the 3D images allow to reveal defects in the form of a
twin migration, and to measure precisely their displacements during the gas reaction, with a high resolution (a few pm). The images also show a change in morphology (faceting) of the Pt nanoparticle during the reaction (
Figure).
Figure. The 3D images show a change in morphology (facet) of the platinum nanoparticle during the reaction. Credit CEA
These results have been complemented by simulation calculations (
density functional theory) which show that the twin migration is correlated with the change in the energies of the surfaces exposed to CO. The facet formation and the twin migration evidence the high mobility and diffusivity of Pt atoms during the reaction. Moreover, X-ray imaging has the advantage to be an in situ and non-invasive characterization technique. This type of imaging is therefore well adapted to the dynamic study of the structure of nanometric materials (deformation, morphology, facets, defects) under the effect of external parameters (pressure, temperature, gas) and in reactive environments. However, for each material studied, it could be tedious to analyze the hundreds of 3D images generated by this technique. This is why the researchers found it clever to combine image processing with a neural network type algorithm
[2]. Thanks to this approach, it was possible to identify linear defects (dislocations) in nanoparticles in an automated way. This is the first time that this original method has been used for the benefit of large-scale materials science (big data) in the recognition of defects from three-dimensional structural data.
Thus, 3D X-ray imaging is proving to be ideally suited to study changes in the nanostructure of nano-catalysts during physicochemical reactions. It thus offers new perspectives to observe the behavior of defects in confined crystals, which opens the way to defect-engineering in nanocatalysts.
Coherent X-ray diffraction imaging: is a "lensless" technique for 2D or 3D reconstruction of the image of nanoscale structures such as nanotubes, nanocrystals, porous nanocrystalline layers, defects, potentially proteins, and more.
Cristal twining: twinned structures may form during crystal nucleation, growth, phase transformations, recrystallization.
Density Functional Theory or DFT: a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body systems, in particular atoms, molecules, and the condensed phases.