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In situ electron paramagnetic resonance spectroscopy for catalysis

Abstract

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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Fig. 1: Crystal field splitting diagrams for CoII.
Fig. 2: Experimental set-ups for in situ EPR measurements during catalytic reactions.
Fig. 3: Origin of the shape of EPR spectra.
Fig. 4: EPR investigation of FeIII species in zeolites.
Fig. 5: Copper/TEMPO-catalysed selective oxidation of benzyl alcohol monitored by coupled in situ EPR/UV–Vis/ATR-IR/XAS.
Fig. 6: Photocatalytic water reduction in a homogeneous iron/iridium-containing system with triethylamine as a sacrificial agent.
Fig. 7: Heterogeneous photocatalytic formation of radicals from ozone for degradation of waste water pollutants.
Fig. 8: Reaction scheme for spin trapping radicals generated during SEC-EPR, and SEC-EPR of a cobalt oxide water oxidation catalyst correlating cobalt oxidation states with catalytic turnover.
Fig. 9: SEC-EPR of an immobilized alcohol oxidation catalyst.
Fig. 10: Thin film and liquid state EPRoC detection schemes.

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Acknowledgements

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.

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Correspondence to Angelika Brückner.

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Nature Reviews Methods Primers thanks G. Liu, M. Roessler, M. Self-Eddine, O. Schiemann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Glossary

Active species

A key catalyst state in the catalytic cycle that enables catalytic activity, typically existing at or prior to the rate-determining step.

Radicals

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.

Dewar

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.

Chronoamperometry

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.

Doublets

Magnetic resonance splitting patterns in which a signal is split into two lines of the same intensity.

Adducts

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

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