C05: Doped Real and Model Transition Metal Oxide Catalysts from Pulsed Laser Post-Processing

Prof. Dr.-Ing. Stephan Barcikowski

In the first funding period, the pulsed laser post-processing (PLPP) method was advanced via a novel flat-jet experimental design to gradually increase the defect density in colloidal transition metal oxide nanoparticles and investigate how the defect density affects the respective catalytic activity while the phase purity and BET surface area were maintained. By this single-pulse-per-particle method, the improved alkaline ORR activity of a laser-post processed Co₃O₄ spinel was correlated to an increased occupancy of Co²⁺ on tetrahedral defect sites. In the second funding period, the established method and knowledge base for precisely controlling the defect density in colloidal oxide nanoparticles will be expanded by laser-based doping of heteroatoms (Fe, Mn, V) that is required to understand how doped heteroatoms affect the formation or annihilation of active sites.

Due to the complexity of real-structure catalysts, careful control experiments and internal standards are mandatory to identify cross-correlations from different surface terminations. Consequently, the laser-based doping of colloidal real-structure will be complemented by a 2D-material processing scheme, where arrays of laser-induced lateral surface structures are inscribed into thin-film model catalysts that exhibit predetermined crystal facets and are immersed in the aqueous dopant metal cation salt solution during laser processing. This allows differential catalytic studies at laterally doped structure arrays, written with predefined laser intensity and number of laser pulses per spot. This approach automatically delivers internal control at non-irradiated areas, relevant for differential catalytical tests. In combination with advanced surface characterization methods (Area B), specially resolved catalytic testing methods (e.g., SECCM, Area A), and theoretical models this differential model catalysis approach using dopant arrays will allow the identification of active sites and the respective correlation with reaction mechanisms. By also doping colloidal real-structure oxide catalysts in the flat jet setup and the respective investigation of the electrocatalytic (A02) and liquid phase oxidation (A01) catalysis with ensembles as well as single nanoparticles (e.g., in A02) the developed reaction mechanisms will be generalized to bridge the gap between model and real-structure catalysis. Consequently, the project will pave the way for 1) identifying active sites from structure-sensitive spectroscopic methods in Area B as well as theory projects on model catalysts in Area A that are required to 2) understand reaction mechanisms with real-structure catalysts in the respective catalytic systems of Area A.

(Figure: Overview of the main contributions of C05 throughout the second funding period of the CRC. (A) provision of doped real-structure catalyst nanoparticles (Co₃O₄) with gradual surface densities and distributions of single or mixed cation dopants (Ni²⁺, Fe³⁺, Mn⁴⁺, V⁵⁺). (B) Provision of doped Co₃O₄ thin-film model catalysts with different surface terminations and inscribed arrays of respective dopants, densities, and distributions).