Operando
Raman spectroscopy was performed to understand the mechanism
of a microwave (MW)-enhanced fixed-bed catalytic reaction. An MW operando
Raman spectroscopy system was constructed by integrating a Raman spectroscopy
probe (785 nm) with a single-mode (TM110) MW cavity resonator
equipped with a semiconductor MW generator. MW heating enhanced the
dehydration of 2-propanol to diisopropyl ether and propylene by +54
to +60% under the air flow condition. In the argon flow condition,
the conversion of 2-propanol was further enhanced by +71 to +83% with
>99.9% selectivity toward propylene when MW heating was used. Operando
Raman spectroscopy revealed that MW irradiation enhances the reduction
of the WO3 surface and exhibits intense light emission,
especially under the argon flow condition. The enhanced generation
of WO3–x
species on the surface
of the WO3 catalyst should be, therefore, a key factor
in the enhancement of the dehydration of 2-propanol under MW irradiation.
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 WO 3 → WO 3−x . That is, the redox state of the WO 3 catalyst could be detected using the resonance frequency. The oxidation state of the WO 3 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 WO 3 catalyst. Moreover, the temporal changes in the oxidation state of the WO 3 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.
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