High Resolution ESR Spectroscopy for Catalysis Research

Lead Research Organisation: Cardiff University
Department Name: Chemistry

Abstract

Electron Spin Resonance (ESR) spectroscopy is a magnetic resonance technique used in the analysis of any system containing unpaired electrons. It is therefore an extremely powerful, advanced and versatile tool for the study of paramagnetic compounds and free radicals. However, one drawback of ESR that is often encountered at the common microwave frequency (9.5 GHz) adopted by most commercial instruments, is the overlapping signals observed for radicals with low g-anisotropy, particularly in disordered systems, or the complex spectra arising from paramagnetic systems containing more than one unpaired electron. As a result, a significant amount of information on the identity of the radical can be lost and the spectra themselves can be very difficult to interpret. This information can be more readily extracted, and the ease of spectral analysis considerably simplified, by performing the ESR measurements at higher frequencies (94 GHz, referred to as W-band ESR). W-band ESR is regarded as a powerful complimentary spectroscopic technique with unique capabilities for providing information on the structure and dynamics of paramagnetic systems, by offering higher resolution and electronic insights into systems bearing unpaired electrons. High resolution ESR is thus an essential compliment to the traditional ESR tool-box in providing a more complete description of the spin Hamiltonian and the detailed characterization of reactive free radicals in the liquid phase and paramagnetic complexes in the solid state and frozen solution.

In this project, we will install a continuous wave (CW) W-band ESR spectrometer in Cardiff University, which will be specifically set-up for projects in catalysis research. The CW instrument will enable us to perform more advanced ESR measurements of paramagnetic metal centres, surface and bulk defects, localised electrons, dopants, spin labels/probes and free radicals (of relevance to homogeneous or heterogeneous catalytic systems), primarily in solids & liquids, at the gas-solid and liquid-solid interface, at high and low temperatures and under photolysis conditions. Catalysis represents the ultimate challenge for any technique, with reactions occurring at specific atomic sites, at fast times scales and often in complex media. This requires the utilization of advanced spectroscopic methodologies that can probe not only the changes in electronic structure, symmetry, spin states and coordination numbers, but also that can reveal insights into the dynamics and nature of the reactive intermediates involved in the catalytic cycle. The W-band ESR instrument can access all of this key information for catalytic systems bearing unpaired electrons, and thus will provide an important additional capability for catalysis research in the UK.

The instrument will provide the catalysis community access to an ESR instrument dedicated for research projects in homogeneous and heterogeneous catalysis, namely in the detection of reactive oxygen species in heterogeneous catalysis, understanding the involvement of surface bound radicals and defects in catalysis and photocatalysis, investigating the role of redox active centres in catalysis & the characterization of homogeneous organometallic catalysis, and characterising the transition metal ions doped into microporous materials and confined environments. These project work-packages will be undertaken through collaborations with key stakeholders and project partners in the Cardiff Catalysis Institute (CCI), the UK Catalysis Hub, the GW4 Alliance of Universities and other UK catalysis groups and industrial partners. The instrument will be managed by the PI and CoI, is supported by Cardiff University and Bruker UK Limited, and will add an essential additional capacity to the UK scientific equipment infrastructure. The insights gained in this project will ultimately be used to develop the next generation of improved, cheaper and more efficient catalysts.

Planned Impact

Who will benefit from this research?
Free radicals and redox reactions are common in nature. Many radicals are present in the air around us, in the foods we consume, and in the many products we use on a daily basis. Most foods contain chemical additives ('anti-oxidants') whose role is to minimize the adverse effects of these free radicals. There is even evidence to suggest that several degenerative diseases may involve deleterious free radical processes. However, it is wrong to think that all free radicals or paramagnetic species produce adverse effects. Indeed many essential redox reactions occurring in the body actually rely on the participation of free radicals, while the hydroxyl radical acts as the primary 'cleaning agent' in the atmosphere. In chemistry and biology, numerous chemical transformations are dependent on the participation of paramagnetic species and radicals, particularly in catalysis. Despite their importance, the role of these radicals is exceedingly complex and still not fully understood. The importance of catalysis itself cannot be underestimated as a subject and for its contributions to the UK economy, since it will play an essential role for solutions to major problems in society (energy, environment, quality of life). To develop better catalysts and to understand their mode of action, one needs better tools to characterize the intrinsic mechanisms at faster time scales, the intermediates at higher resolution or the catalyst structure under relevant conditions. All these insights can be achieved using advanced ESR techniques. In this Strategic Equipment bid, the high resolution ESR instrument will provide an important enabling tool for revealing a molecular basis for understanding catalytic reactions. This instrument and the associated expertise at Cardiff will provide an important tool for the wider UK catalysis community, particularly the numerous catalysis research groups at the local, regional and national level. Furthermore, owing to the importance of free radicals and paramagnetic species in many systems, the wider academic and industrial community conducting research in free radicals, may also benefit from the new spectrometer.

How will they benefit?
The primary mode by which the key users of this Strategic Equipment bid will benefit, is through the creation of academic (and industrial) networks around the thematic area of paramagnetism in catalysis, whereby users can access state-of-the-art ESR facilities and expertise covering all aspects of radicals, defects or paramagnetic centres involved in catalytic reactions. These networks will be developed with key stakeholders through engagement with the CCI, UK Catalysis Hub and GW4 Alliance of Universities. The instrument will be installed in a dedicated refurbished laboratory to facilitate ease of user access and operation (i.e. sample preparation and treatment on site) and secondment of PhD/PDRA personnel will be encouraged. In this way, we will provide our key beneficiaries not only with important spectroscopic data on their samples (collaboration & coproduction), which will contribute to improvements in catalysts design and function that may be subsequently exploited for economic gain, but also provide them with direct 'hands-on' experience and training on the instrument, thereby improving their skills (capacity & involvement). Understanding the nature and behaviour of a modern catalyst (operating at extreme conditions, in complex phases and with mixed components) is exceedingly challenging, and requires access not only to advanced instrumentation but also to specialist practitioners capable of operating the spectrometers and analysing the result data, in order to understand the catalytic mechanism and thus make better catalysts. This instrument will serve the UK catalysis community in this endeavour and thus help to deliver improved catalysts (exploitation & application).

Publications

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Description One of the key highlights to date, has been the detection of subtle twists in the environment surrounding copper centres in a series of complexes. The subtle distortions to the structure could only be resolved at these higher resolving frequencies. A publication is currently in preparation to describe the finding.
Exploitation Route As the work is ongoing, it is too early to state this. It should also be noted that the delivery of the spectrometer was considerably delayed, arriving in May 2018 (despite the grant funding starting in May 2017). These are very advanced instruments, and inevitably delays occur in the manufacture and delivery of the instruments.
Sectors Chemicals