Structure of vanadium oxide supported on ceria by multiwavelength Raman spectroscopy

The structure of vanadium oxide species supported on ceria (VO _x/CeO ₂) was investigated under various conditions by in situ multiwavelength Raman spectroscopy, IR spectroscopy, isotopic labeling, and temperature-programmed reduction (TPR). For the first time, the detailed structure of dehydrated VO _x species was revealed on the polycrystalline ceria support. VO _x species can coexist on ceria surface in the structure of monomer, dimer, trimer, polymer, crystalline V ₂O ₅, and CeVO ₄ as a function of VO _x loading. These species interact strongly with both the defect sites and labile surface oxygen of ceria, passivating the redox property of ceria. Under ambient condition, the dispersed VO _x species are hydrated into polyvanadate species that can be reversibly dehydrated back to the original structure forms. The ceria support with defect sites facilitates the interaction between water (H ₂ ¹⁸O) and V ¹⁶O _x species, leading to very facile isotopic oxygen exchange between the two even at room temperature. During H ₂ reduction, both the VO _x species and the ceria support can be reduced with ceria surface being more reducible. The reducibility of various dispersed VO _x species scales with their polymerization degree, that is, polymer > trimer > dimer > monomer. The reoxidation of reduced VO _x species is found to proceed via ceria lattice oxygen instead of the gas phase oxygen where ceria acts as an oxygen buffer. The revealed structure evolution of surface VO _x species on ceria under hydrated, dehydrated, reduced, and regenerated conditions provides a basis for understanding the vanadia-ceria catalysis.