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.
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S.A.B acknowledges a postdoctoral fellowship from the Alexander von Humboldt foundation. S.A.B and A.S acknowledge funding by the Max Planck Society and A.S. by the BMBF EPR-on-a-Chip network project (03SF0565). The authors thank M. Teucher (MPI CEC) for providing the data in Fig. 9.
The authors declare no competing interests.
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- Active species
A key catalyst state in the catalytic cycle that enables catalytic activity, typically existing at or prior to the rate-determining step.
Molecules with one or more unpaired electrons.
- Ligand field
The electronic field of a ligand that affects the valance orbital degeneracy of the metal centre within crystal/ligand field theory.
- Relaxation times
The times characterizing the return of an excited spin system to the ground state, which has a direct consequence on the spectral shape. Short relaxation times broaden the lines, in some cases beyond detection, and vice versa. The longest relaxation times are typically found at cryogenic temperatures.
- Freeze quenching
Rapid cooling of the sample during a reaction, such as by immersing a sample tube in a cryogenic liquid or spraying an aerosol of reaction solution onto a cryogenic surface.
- Spin traps
Organic molecules that react with radicals to form more stable radicals.
- Dielectric losses
Losses of microwave energy stored in the resonator due to electric dipole excitations resulting in heating of the system.
- Analyte lifetime
The lifetime of the species to be analysed.
- Rate-determining step
The slowest step of a catalytic cycle consisting of several reaction steps.
- Catalytic turnover
A single pass of a catalytic cycle after which the catalyst returns to its initial state, from which it can enter into the next cycle.
An evacuated multi-walled quartz glass vessel to reduce heat transfer due to thermal conduction. Metal layers on the glass to supress radiative losses cannot be used inside microwave cavities.
- Coherent spectral assignment
The coherent assignment of spectra from the same sample obtained from different spectroscopic experiments when the different data sets can be explained by the same chemical reaction.
An electrochemical technique on which a potential difference is applied and the current flow is measured as a function of time.
- Zeeman interaction
The interaction of an unpaired electron with the external magnetic field.
Magnetic resonance splitting patterns in which a signal is split into two lines of the same intensity.
In the context of a spin trap, radical adducts are the molecules resulting from reaction of a radical with a spin trap.
- Zero-field splitting (ZFS) constants
In spin systems with more than one unpaired electron, magnetic interaction between the latter splits the otherwise degenerated spin states already in the absence of an external magnetic field. D and E are a measure for this splitting and depend on the anisotropy of the paramagnetic centre, being zero for isotropic centres.
- Free electron g value
The g value of a free electron in a vacuum, ge = 2.0023.
- Helmholtz layer
(Also known as the electrical double layer). A build up by the ions of a solution being bound to a charged surface.
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Bonke, S.A., Risse, T., Schnegg, A. et al. In situ electron paramagnetic resonance spectroscopy for catalysis. Nat Rev Methods Primers 1, 33 (2021). https://doi.org/10.1038/s43586-021-00031-4