C02: Flame Synthesis and -Modification of Perovskite Catalysts

Prof. Dr. Christof Schulz

In the first funding period, this project succeeded in synthesizing a wide variety of phase-pure perovskite and spinel materials in spray flames. These novel materials have contributed significantly to the scope of the CRC and widened the choice of materials with nominally identical composition but different synthesis “history”. In this previous work, however, limitations were found that prevented us from independently modifying composition and morphology. As both aspects have a high relevance for understanding heterogeneous oxidation catalysis in the scope of the CRC, it is important to overcome this limitation.

Based on the expertise gained so far, this project aims at decoupling the specific structural and (composition-related) electronic properties of nanoscale perovskites ABO₃ to provide “families” with well-defined composition, size, and morphology. To achieve this goal, we further advance to a staged two-step reaction process. Reaching a targeted cation composition is the goal of the first reaction stage, where substitutional doping of the A and B lattice sites is in the focus. Perovskite materials with large specific surface area will be synthesized via spray-flame synthesis. This method offers the advantage of synthesis under kinetic control due to extremely fast particle formation on the millisecond timescale with subsequent rapid cooling, thus enabling even the generation of metastable compounds. The goal is the direct synthesis of phase-pure materials in a single step through processing of solutions that contain the desired concentration ratios enabling the inclusion of a wide variety of cations. The process is highly flexible with respect to modifying the chemical composition of the products. A-site cations of interest are primarily lanthanum and strontium, while iron, cobalt, manganese, and vanadium will be investigated as B-site cations.

By adjusting the stoichiometry, bath-gas composition and the temperature distribution in the flame, both the oxygen content of the materials produced and their electronic conductivity are to be controlled. This can be done by simultaneous doping on both cation lattice sites and will be systematically investigated within the CRC. As these modifications can interfere with the first aim described above, the spray-flame process will be extended by a downstream “conditioning” through radial injection of further reactants in a way that ensures rapid mixing with the particle-laden flow. Both reducing and oxidizing conditions will be adjusted via the gas composition to control the oxygen partial pressure and the temperature in the particle growth zone independently. This will enable the modification of the degree of oxidation, the complete removal of carbonaceous contaminants, and particle size, crystallinity, and morphology through variations in the temperate–time history of the particles on the path through the reactor. These combined methods will provide highly-defined materials for investigation of the structure–composition–(catalytic) activity relationships together with virtually all experimental projects in the Areas A and B.

This flame-based materials modification will be further investigated based on model materials and compared to the complementary technique of laser-based post treatment. To this end, the second reaction stage described above will be decoupled from the first stage particle synthesis and continuous composition spreads from Project C04 will be exposed to flames of various stoichiometry, temperature, and elemental gas-phase composition and subsequently analyzed in terms of modifications in structure and catalytic activity (for a wide range of materials compositions on each single surface). Also, surfaces will be pre-treated with thin coatings containing foreign elements that will lead to doping as a result of rapid interaction with the flame. Information from these model systems will support the understanding of the inline two-stage particle synthesis and modification described above. Also, the interaction with flames (slow heating and cooling, supported through the presence of radicals) with laser-based processes (fast heating in inert environment; C05) will support the understanding of differences in post-treatment.

(Figure: Left: Primary spray-flame in the burner housing (center) used so far for materials synthesis. This flame will be combined with an additional inlet for secondary gases for reactive and thermal post-processing of the initially formed particles (demonstration arrangement: right. Plasma Process. Polym., 2020, 17, 1900245).