Monitoring transition metal ions (TMI) in oxide catalysts during (re)action: the power of operando EPR

Abstract

Tailoring the properties of transition metal oxide catalysts so as to provide optimum performance for the desired reaction is a major goal in catalysis research that can only be reached by deep understanding of structure–reactivity relationships of active sites. Operando EPR is a versatile tool for that purpose since it provides simultaneous information on valence state, coordination geometry and electronic interactions of paramagnetic TMI in relation to activity and selectivity of the catalysts. This is demonstrated by illustrative examples comprising the action of different TMI: (i) structure and function of V ions in vanadia-based catalysts used for the oxidative dehydrogenation of propane to propene (ODP) and the selective oxidation of toluene to benzaldehyde; (ii) activity and stability of Cr sites in supported chromium oxides during dehydrogenation of propane; (iii) structure and function of Mn ions in MnO_x storage catalysts for selective catalytic reduction of NO_x. Benefits arising from combined operando EPR/UV-vis/on-line GC studies are also shown. This combination provides an excellent opportunity to monitor the reaction-dependent interconversion of diamagnetic and paramagnetic TMI.