High Resolution ESR Spectroscopy for Catalysis Research

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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.