Supported transition metal nanoparticles – what makes a good catalyst?

Abstract

Catalysts are crucial for the production of fuels, chemicals and materials and for energy storage and conversion. They often consist of transition metal nanoparticles (<10 nm) as an active phase, supported on a porous material that provides stability to the system, but has 3D open porosity to allow diffusion of reactants and products. The effectiveness of a catalyst is a combination of three qualities: activity, selectivity and stability. All three are not only determined by the composition of the metal nanoparticle, but also greatly depend on factors such as the exact particle size (and particle size distribution), a redistribution of atoms within the nanoparticle under reaction conditions, the interaction between the metal nanoparticle and the support, and small amounts of additives (‘promoters’) which can modify for instance the electronic properties or provide specific sites.

We study the performance of supported metal nanoparticles using 3D model supports (ordered mesoporous oxides and structural carbons) as a key tool. They allow mimicking realistic catalyst systems and conditions (100-400 ^oC, 1-50 bar pressure, H₂, CO, CO₂, alkenes, H₂O, O₂  feeds), while their highly defined morphology and surface properties facilitate high precision in the variation of individual structural parameters, characterization and data interpretation.

In this presentation I will highlight a few recent examples of research based on relatively noble metal catalysts, Au, Ag and Cu, used for industrially relevant processes.^1-5 Relatively little is known about for instance the intrinsic effect of metal particle size, and the mechanisms of particle growth under real reaction conditions in 3D supported systems. We varied particle parameters such as size, size distribution, and average particle density, but also collective properties such as the nanospatial distribution of the metal nanoparticles over the support, and support pore size, window size, connectivity (for instance 1D or 3D porosity), and surface. Our aim is to unravel the interplay of different structural parameters, and obtain a more systematic understanding of what determines the activity, stability and selectivity of catalysts under realistic reaction conditions.

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