In situ electron paramagnetic resonance spectroscopy for catalysis

In situ catalysis studies seek insight into species present under reaction conditions to elucidate reaction mechanisms and understand the atomistic details of the active catalyst, both of which are key to optimizing catalyst reactivity and processes. Many reactions follow radical mechanisms, and many catalysts adopt paramagnetic states within their catalytic cycles where the systems exhibit species with unpaired electrons, which provide a sensitive handle to probe their geometric and electronic structure. Electron paramagnetic resonance (EPR) spectroscopy directly probes these unpaired electrons to characterize molecular radicals as well as determine transition metal ion oxidation states and coordination geometries. Here, we introduce the concept of EPR followed by the methodology for in situ EPR studies and discuss high-temperature gas–solid reactions, molecular catalysis, photocatalysis and electrocatalysis. The broad applicability of the approaches is demonstrated through case studies in each area, with a focus on unravelling catalytic mechanisms. We also discuss data sharing and reproducibility issues as well as limitations to the technique. Finally, we identify directions for development to guide interested researchers towards evolving areas including miniaturization and high-frequency analysis. This Primer on in situ electron paramagnetic resonance spectroscopy describes various experimental set-ups to acquire spectral information on the paramagnetic state of chemical species with unpaired electrons present during catalytic reactions, with the goal of unravelling catalytic mechanisms and optimizing catalyst activity.