Structural and reactivity properties of Nb–MCM-41: Comparison with that of highly dispersed Nb₂O₅/SiO₂ catalysts

The molecular structures of Nb cations in mesoporous material Nb-MCM-41 and the supported Nb₂O₅/SiO₂ catalyst under hydrated and dehydrated conditions were investigated by in-situ Raman and UV-vis-NIR diffuse reflectance spectroscopies. Methanol oxidation was employed as a chemical probe reaction to examine reactivity/selectivity properties of these catalytic materials. The structural characterization techniques demonstrate that similar surface Nb oxide species are present in both Nb-MCM-41 and Nb₂O₅/SiO₂ catalysts, which are sensitive to the environmental conditions (ambient or dehydrated). However, the characterization results also suggest that some Nb atoms in Nb-MCM-41 may be incorporated within the siliceous framework. The spectroscopic results revealed that under dehydrated conditions, the Nb cations in Nb-MCM-41 and 1% Nb₂O₅/SiO₂ are predominantly composed of isolated NbO₄ units, while the Nb cations in the supported Nb₂O₅/SiO₂ catalysts possess polymerized NbO_x species and/or bulk Nb₂O₅ with increasing Nb loading. The catalytic results indicate that the dispersed Nb cations in both types of catalysts are active redox sites for methanol oxidation and exhibit similar reactivity and selectivity properties due to the presence of Nb-O-Si bonds in their structures.