Catalyst Nanoparticles

Heterogeneous catalysts commonly comprise nanometric metal particles anchored on inert carbon or metal oxide supports, e.g. alumina, and the outstanding catalytic properties of PGMs makes them the material of choice for many applications. For instance, 65% of the world production of Pt is used for catalysts distributed across a range of industries, including petroleum refineries, fine chemicals production, auto emission control catalysts, electrolysers and fuel cells.

The use of nanoparticles in these catalytic applications is essential, not only to maximize the surface/bulk ratio and therefore optimize performance, but also to minimize cost when using precious metals. However, it is well established that catalytic behavior is highly sensitive to particle size, morphology and surface structure, as well as particle-support interactions, which are important in the design of active catalysts. The effect of the size is primarily due to the increase of low coordinated atoms (e.g. corner sites) with decreasing particle size, which raises the adsorption energy of reactants and lowers the activation energy for the reaction.

Size-selected clusters offer an ultimate level of precision in the design of catalytic particles. They are aggregates with nuclearity controllable in the range from 2 to ~20,000 atoms (diameters of 0.2 – 20 nm) and prototypical “building blocks” in nanotechnology. Clusters have been shown to exhibit “magic numbers”, corresponding to closed shells of atoms or of electrons with particular stability. Clusters also exhibit physical and chemical properties which differ drastically from the corresponding individual atoms or bulk solids and depend acutely on cluster nuclearity – especially in the “non-scalable” regime (N < a few hundred). Examples are ionisation potential, polarisability, electron affinity, and, most significantly, chemical reactivity and selectivity. The discovery of the magic number phenomenon leads to the concept that size-selected clusters represent a “third dimension” to the periodic table – new materials can be synthesised from “artificial atoms” of selected sizes and therefore properties.

 Image Pt-clusters (002) Image Pd-Al3O2 (002)

Figure:
Electron microscopy images of nanoparticles produced in CritCat. (left) Size-selected Pt clusters produced by the cluster beam source and (b) Pd nanoparticles produced by flame pyrolysis on alumina support.