B03: Linking Charge Carrier Dynamics and Liquid Phase Oxidation Catalysis in Spinel and Perovskite Oxides

Prof. Dr. Gerd Bacher • Prof. Dr. R. Kramer Campen

A rational design of catalysts requires a detailed microscopic understanding of elementary kinetic processes occurring during catalytic reaction. Electrocatalysis of oxidation reactions demands the production of holes at the solid/liquid interface and a bulk phase that is sufficiently conductive to allow electrons to be extracted and employed in a reduction reaction at the electrochemical cell’s anode. Thermal oxidation catalysis needs the creation of nearby holes and electrons at the solid/liquid interface and low conductivity as the electron or hole should not diffuse apart. Viewed in this manner it is obvious that any mechanistic description of either type of catalysis requires the simultaneous description of charge carrier dynamics in the bulk catalyst and at the solid/liquid interface.

Our project goal is to identify the main kinetic bottlenecks of the activity in both, thermal and electrocatalysis, using in operando transient optical spectroscopy. During the first funding period the mutual relation between real structure and optical signatures has been elaborated. In this period, we will build on this foundation to characterize operando the dynamics of carriers in spinel and perovskite mixed oxides in bulk and at the solid/liquid interface and the chemical change they induce during thermal and electrocatalysis of water and 2-propanol oxidation. We will do so using transient absorption to characterize carrier dynamics in bulk, electronically resonant sum frequency generation and photoluminescence spectroscopy to characterize operando interfacial electronic structure and follow the resulting chemistry using vibrationally resonant sum frequency generation spectroscopy. In the electrocatalytic case we will additionally apply two photon photocurrent spectroscopies for extracting the optical response of particular sub-populations of holes. This suite of techniques will allow us to correlate optical observables of the entire journey from carrier creation to chemistry with steady-state reactivity induced by temperature or potential. Because charge generation – and thus chemistry – is stimulated optically, we will be able to follow changes in carrier population and chemical speciation at well-defined timescales ranging from seconds down to femtoseconds.

Both the OER and thermal 2-propanol oxidation require transferring electrons from bulk solution to charge separated states at the catalyst surface and either back to solution phase product species (thermal 2-propanol oxidation) or to the cathode (the OER). Project B03 considers these reactions on Co_xFe_3-xO₄ spinels and LaCo_xFe_1-xO₃ perovskites employing a suite of optical operando techniques: transient absorption, photoluminescence, Raman and several types of vibrationally and electronically resonant sum frequency generation spectroscopy. The combination of these methods enables the observation of every step in the electron's journey in real-time and thus provides a description of how elementary processes on timescales from femtoseconds to seconds influence catalyst activity.

(Figure: Link between structural and optical properties. (A) Raman spectra of Co_xFe_3-xO₄ thin films (left) and nanoparticles (right) with different Co/Fe ratio. (B) Normalized absorbance spectra of Co_xFe_3-xO₄ thin films (upper graph) and nanoparticles (lower graph) with different Co content x. Red lines show Fe-rich samples, green lines samples with approximately equal Co and Fe content and purple lines Co-rich samples. Absorption bands (ISCT, IVCT) are highlighted as shaded areas. (C) Transient absorption spectra of Co-rich Co_xFe_3-xO₄ nanoparticles in water. The lower graph shows absorbance spectra of Fe-rich (red) and Co-rich (blue) Co_xFe_3-xO₄ nanoparticles. The upper graph shows the corresponding transient absorption spectrum for Co-rich samples. Dotted lines indicate charge transfer processes between and within the different cation sites).