Kinetic isotope effects and product analysis of tertiary alcohols as mechanistic probes to decipher reaction paths of heterogeneous photocatalysis in different environments

• Applicants: Professor Dr. Martin Elsner; Professor Dr. Ulrich Heiz
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Analytical Chemistry ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 575759682

Photocatalysis is receiving enormous interest in fundamental and applied research with promises for renewable fuels, water and air purification, and novel synthetic routes for (fine) chemicals. Contrasting this vibrant research activity, underlying chemical transformation mechanisms remain surprisingly unconstrained. For example, even a canonical consensus mechanism in the photocatalysis of titania surfaces (the “electrochemical-type” mechanism: a set of coupled by independent redox half reactions) has recently been disproven by detailed mechanistic studies of model systems in a vacuum environment. Inspired by this breakthrough, we aim to advance the mechanistic understanding of photocatalytic processes (i) by bridging the gap between model and applied systems by moving to ambient condition and liquid phase; (ii) to this end, by introducing a dedicated set of experimental reactors; (iii) by establishing a dedicated set of model reactants to probe into putative photocatalytic synthesis routes; (iv) by introducing the analysis of kinetic isotope effects as powerful yet underexplored observable to unravel transition states and intermediates as branching points in heterogeneous photocatalysis. This new research avenue is only accessible by combining the expertise of different scientific fields, in particular, the analytical approach of compound-specific isotope analysis by gas chromatography isotope ratio mass spectrometry (Elsner group) and the study and interpretation of heterogeneous photocatalytic transformations under defined conditions in different environments (Heiz group). While our approach holds prospects for multiple photocatalytic materials and transformations, this proposal (a) concentrates on the specific and selective disproportionation of tertiary alcohols as a thermally inaccessible reaction of potential synthetic interest; (b) builds on recent discoveries in the Heiz group using titania-based photocatalysts to bring forward testable hypotheses of reaction pathways and selectivities. By combining dedicated product studies and kinetic isotope measurements, we aim to probe for selective C-C bond cleavages as well as ring-opening reactions, dimerizations of alkyl moieties, and changes in the rate-determining reaction steps under different experimental conditions. This strategy will enable us to bridge the gap and extend our mechanistic understanding across three different chemical environments, namely vacuum, ambient gas, and liquid phase. By applying our general approach to this reaction system of specific interest, we aim to lay the foundation for future investigations: to explore new synthetic routes using heterogeneous photocatalysis and to elucidate general reaction mechanisms on promising photocatalytic materials.

• DFG Programme: Research Grants