Developing a Raman Spectrokinetic Approach To Gain Insights into the Structure–Reactivity Relationship of Supported Metal Oxide Catalysts

The elucidation of structure–reactivity relationships in supported metal oxide catalysts has proven to be a challenge that remains unresolved for various catalytic systems, especially for supported ternary metal oxide catalysts. The catalytic performance of these ternary multicomponent systems is controlled not only by their coverage (two- or three-dimensional MO x structures) but also by the ratio of the molar loading between the different MO x species. In this contribution, we show that by combining operando Raman spectroscopy and transient reaction kinetics, the redox reaction rates of a catalyst can be measured in real time directly from the Raman spectra. More importantly, such rates can be correlated to a specific catalytic site (i.e., that originating from the Raman signal) as opposed to an average overall reaction rate that can typically be obtained by mass spectrometry (MS) or gas chromatography (GC). This approach, defined as Raman spectrokinetic, was demonstrated by monitoring the Raman signal assigned to the vanadyl (VO) vibration (ca. ∼1032 cm–1) in a series of ternary V/Nb/SiO2 catalysts to ascertain the effect of niobia over the redox properties of vanadium oxide. The Raman spectrokinetic data show that the presence of niobia at the lowest loadings accelerates the formation of the VO group during oxidation. A temperature-programmed reduction (H2-TPR) study corroborates the aforementioned Raman spectrokinetic results. In addition, a similar redox trend, albeit varying in magnitude, is observed from the transient kinetic data obtained by mass spectrometry.