Real-Time Facile Detection of the WO₃ Catalyst Oxidation State under Microwaves Using a Resonance Frequency

Microwaves (MWs) are often used to enhance various heterogeneous catalytic reactions. Here, we demonstrate real-time monitoring of a catalyst's oxidation state in a microwave catalytic reaction using a resonance frequency. The changes in the catalyst's oxidation state during the reaction induced changes in the resonance frequency in the cavity resonator. The resonance frequency was not affected by 2-propanol adsorption, while the frequency decreased with the reduction of WO₃ → WO_3-x. That is, the redox state of the WO₃ catalyst could be detected using the resonance frequency. The oxidation state of the WO₃ catalyst was then directly observed by the resonance frequency during the dehydration reaction of 2-propanol by microwaves as a model reaction. Resonance frequency monitoring revealed that the enhanced dehydration of 2-propanol by microwaves was attributable to the reduction of the WO₃ catalyst. Moreover, the temporal changes in the oxidation state of the WO₃ catalyst detected by the resonance frequency coincided with that observed by operando Raman spectroscopy. Therefore, real-time resonance frequency monitoring allowed facile detection of the bulk catalyst oxidation state under microwaves without using any spectroscopic apparatus.