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The role of ESR spectroscopy in advancing catalytic science: some recent developments

Posted on 27. August, 2015.

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Paramagnetic species, i.e. those containing unpaired electrons, are involved as reactive intermediates in many of the mechanisms devised for catalytic processes. Since these tend to have an intrinsically fleeting existence, to detect and characterise them is difficult. They may take the form, for example of free radicals, paramagnetic redox sites on catalytic surfaces, or be a part of the essential functional centre of an enzyme. 

Paramagnetic molecules are sometimes artificially incorporated onto the surfaces of catalysts, or within nanopores, to probe molecular mobilities, or the local electrostatic nature of the catalytic environment. As this review aims to demonstrate, electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR) spectroscopy, is an extremely powerful method by which unpaired
electrons may be detected in a variety of media, since it affords considerable detection sensitivity and the ability to characterise the molecular species involved, from the determination of their associated g-factors and nuclear hyperfine couplings. 

The review begins with studies of enzymes and their role directly in biological systems, and then discusses investigations of various artificially created catalysts with potential human and environmental significance, including zeolites. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, the degradation of environmental pollutants, hydrocarbon conversions, fuel cells, ionic liquids and sensor devices employing graphene. Studies of muonium-labelled radicals in zeolites are also reviewed, as a means for determining the dynamics of transient radicals in these nanoporous materials.

Read the full review in Progress in Reaction Kinetics and Mechanism, Volume 40, Number 3, 2015, pp. 201-248.

Keywords: catalysis, unpaired electrons, radicals, matrix isolation, spintrapping, TiO2, water oxidation, water splitting, graphene, graphene oxide, hydrocarbon conversions, ionic liquids, fuel cells, muon, muonium, enzymes, ESR, EPR, dendrimer, dendron, zeolite, spin-relaxation.

Author: Christopher J. Rhodes

Image: Structure of tolb in complex with a peptide of the colicin e9 t-domain.