Chapter 1. In situ electron paramagnetic resonance (EPR) – a unique tool for analysing structure and reaction behaviour of paramagnetic sites in model and real catalysts

Abstract

The majority of catalytic reactions involves reduction and/or reoxidation steps in which electrons are transferred between catalysts and substrates. EPR spectroscopy can sensitively probe the local environment of paramagnetic catalytic sites as well as their behavior in catalytic redox processes since it can be applied under a wide range of conditions. After a short summary of the most important application examples of in situ EPR in redox catalysis, the main features of EPR spectra such g and A matrices and approaches of their evaluation are presented using model systems that contain Au atoms deposited on MgO single crystal surfaces. To illustrate the versatility of in situ EPR for deriving structurereactivity relationships in catalysis, two application examples are presented in more detail:(1) Analysis of heterogeneous supported VO x /TiO 2 catalysts during oxidative dehydrogenation of propane in the gas phase and (2) Study of photocatalytic water splitting over a homogeneous catalytic system comprising an iridium photosensitizer complex and an iron carbonyl catalyst.

Operandomonitoring of mechanisms and deactivation of molecular catalysts

“…The relaxation properties of transition metals are highly sensitive to temperature, which further influences the signal intensity of paramagnetic species. 43 Hence operando EPR is preferably carried out under harsh conditions, for instance the above cases were measured below 200 K. However, Brückner et al were able to perform operando EPR measurements of Cr in ethylene tetramerization at 40 °C and up to 14 bar ethylene pressure. 44 These results gave hints to a deactivation pathway forming Cr (I) species.…”

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

“…Moreover, pulsed rf schemes have been used to coherently drive EPR excitations in single atoms (12). These developments open the way for further applications of EPR-STM, including the storage and retrieval of quantum information from surface spins (13), measurements of the relaxation time of single-molecule magnets (14,15), and the characterization of active sites and intermediate reaction species in catalysis (16,17).…”

Quantitative Cu Counting Methodologies for Cu/SSZ-13 Selective Catalytic Reduction Catalysts by Electron Paramagnetic Resonance Spectroscopy

“…Important to catalysis, this technique is quantitative: double integration of the EPR signal allows quantification of the EPR-active Cu species. Furthermore, the correlations between g-values and the hyperfine coupling constants provide useful information about the coordination environments around copper ions. − In the past few years, researchers have utilized in situ EPR to study Cu-gas interactions relevant to SCR and have made important discoveries . Particularly, Mossin and collaborators utilized EPR to quantify various Cu-containing species in Cu/CHA and to observe differences in reactivity between the two Z 2 Cu species shown in Scheme …”