: Tailored hetero-aggregates from flame-spray pyrolysis for heterogeneously catalysed transient reactions

Project Leader :

Prof. Dr.-Ing. habil. Lutz Mädler
University of Bremen

Prof. Dr.-Ing. Robert Güttel
University of Ulm

Hetero-contacts, that are the interaction loci between two different solid materials, play a key role in heterogeneous catalysis, where they are typically designated as metal-support interactions (MSI). In many applications in that field, metal nanoparticles are immobilized on support particles, in order to control their size and spatial distribution as well as to enhance their stability. Support materials are often not inert and new interface phenomena arise, which may improve the catalytic performance drastically. For instance, active sites at the interfacial perimeter may form, which facilitate the activation of reactants. Furthermore, spill-over effects of activated or intermediate species between support and active material are known to occur. Hence, controlling and understanding the hetero-contacts in solid catalysts opens the opportunity to tailor and enhance the catalytic performance significantly.

The synthesis of highly defined model catalysts required for the isolation of such specific material functions with traditional wet-chemical methods is still challenging. Consequently, the project uses multi-flame-spray-pyrolysis as single step gas phase synthesis route, where primary particle formation is followed by controlled mixing and hetero-aggregation, allowing precise tuning of hetero-contact properties. Specifically, the number and chemical nature of the hetero-contacts as well as the primary particle size and the chemical nature of hetero-contacts is controlled. In order to distinguish spillover at the hetero-contact from mere adsorption/desorption effects, the triple-flame spray pyrolysis (TFSP) is established as a novel technique to generate ABC model systems, where the number and size of hetero-contacts between the components A, B and C can be tuned individually.

Consequently, the project aims to deduce the correlation between the properties of the hetero-aggregates and the catalytic performance under transient conditions. For that purpose, well defined hetero-aggregates with tailored properties are synthesized and characterized extensively through BET, XRD, TPX and TEM. The mixing quality with respect to cluster sizes and hetero-contact numbers is achieved analysing HAADF-STEM images using convolutional neural networks in an established collaboration in the SPP2289. Subsequently, the impact of the tailored properties on catalytic activity, sorption capacity and deactivation behaviour under dynamic conditions for appropriate chemical test reactions using the periodic transient kinetics (PTK) method is investigated. Catalytic test reactions emphasize hydrogenation of CO₂ into CH₄ and CO, since it involves various gaseous species (e.g., CO₂, CO, H₂, CH₄ and H₂O) exhibiting characteristic individual transient responses. This way, the individual responses of the reaction components are correlated with the hetero-contact properties and the synthesis parameters.