Structure of Vanadium Oxide Supported on Ceria by Multiwavelength Raman Spectroscopy

Abstract

The structure of vanadium oxide species supported on ceria (VO_<i>x</i>/CeO₂) 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_<i>x</i> species was revealed on the polycrystalline ceria support. VO_<i>x</i> species can coexist on ceria surface in the structure of monomer, dimer, trimer, polymer, crystalline V₂O₅, and CeVO₄ as a function of VO_<i>x</i> 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_<i>x</i> 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 (H₂¹⁸O) and V¹⁶O_<i>x</i> species, leading to very facile isotopic oxygen exchange between the two even at room temperature. During H₂ reduction, both the VO_<i>x</i> species and the ceria support can be reduced with ceria surface being more reducible. The reducibility of various dispersed VO_<i>x</i> species scales with their polymerization degree, that is, polymer > trimer > dimer > monomer. The reoxidation of reduced VO_<i>x</i> 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_<i>x</i> species on ceria under hydrated, dehydrated, reduced, and regenerated conditions provides a basis for understanding the vanadia-ceria catalysis.