Unravelling Nanoscale Catalyst Dynamics Metallic Nanocatalysts: What Really Happens During Catalysis

A German research team has revealed how platinum–rhodium nanoparticles change during catalysis — showing that rhodium atoms migrate into the platinum core and that reaction rates depend on the particles’ facet orientation, paving the way for designing longer-lasting, more efficient nanocatalysts.

Nanoparticles measure less than one ten-thousandth of a millimetre in diameter and have enormous surface areas in relation to their mass. This makes them attractive as catalysts: metallic nanoparticles can facilitate chemical conversions, whether for environmental protection, industrial synthesis or the production of (sustainable) fuels from CO₂ and hydrogen.

Platinum (Pt) is one of the best-known metal catalysts and is used in heterogeneous gas phase catalysis for emission control, for example to convert toxic carbon monoxide in car exhaust gases from combustion engines into non-toxic CO₂. “Mixing platinum particles with the element rhodium (Rh) can further increase efficiency,” says Jagrati Dwivedi, first author of the publication. The location of the two elements plays an important role in this process. So-called core-shell nanoparticles with a platinum core and an extremely thin rhodium shell can help in the search for the optimal element distribution that can extend the lifetime of the nanoparticles.

Until now, however, little was known about how the chemical composition of a catalyst's surface changes during operation. A team led by Dr Thomas F. Keller, head of the microscopy group at Desy Nano Lab, has now investigated such crystalline Pt-Rh nanoparticles at Bessy II and gained new insights into the changes at the facets of the polyhedral nanoparticles.

The nanoparticles were first characterised and marked in their vicinity using scanning electron microscopy and atomic force microscopy at Desy Nano Lab. These markers were then used to analyse the same nanoparticles spectroscopically and image them microscopically simultaneously using X-ray light on a special instrument at Bessy II.

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The Smart instrument at the Fritz Haber Institute of the Max Planck Society enables X-ray photoemission electron microscopy (XPEEM) in a microscope mode. This makes it possible to distinguish individual elements with high spatial resolution, enabling the observation of chemical processes at near-surface atomic layers. ‘The instrument allows the chemical analysis of individual elements with a resolution of 5-10 nanometres, which is unique,’ says Thomas Keller. The investigation has shown that rhodium can partially diffuse into the platinum cores during catalysis: both elements are miscible at the typical operating temperatures of the catalyst. The mixing is enhanced in a reducing environment (H₂) and slowed down in an oxidising environment (O₂) without reversing the net flow of rhodium into platinum. ‘At higher temperatures, this process even increases significantly,’ explains Keller.

The reaction rates also depend on the orientation of the nanoparticles' facets. “They are particularly high on certain facets,” emphasises Jagrati Dwivedi: “Our facet-resolved study shows that rhodium oxidation is highest on facets with many atomic steps, where the atoms are most easily bound.” This detailed analysis of the oxidation behaviour will contribute to the further optimisation of such nanocatalysts, which can undergo irreversible changes during use.

Original Article: Spectro-Microscopy of Individual Pt–Rh Core–Shell Nanoparticles during Competing Oxidation and Alloying; ACS Nano; DOI:10.1021/acsnano.5c07668

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