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Keywords:

  • cerium;
  • nanostructures;
  • Raman spectroscopy;
  • surface chemistry;
  • vanadates

Abstract

Vanadia (VOx) supported on ceria (CeO2) nanocrystals with defined surface planes, which includes rods, cubes and octahedra, was synthesized and used to explore the effect of support surface structure on the speciation of surface vanadia. The vanadia structures on these ceria “nanoshapes” were identified by in situ visible and UV Raman spectroscopy as a function of loading and calcination temperature, and they include monomeric, dimeric, trimeric, polymeric vanadia, and eventually crystalline V2O5 and CeVO4 as vanadia loading increases. As expected, the faceted ceria nanocrystals provide a rather homogeneous platform for anchoring the vanadia. At low vanadia surface density, only monomeric vanadia exists on the ceria nanoshapes, in contrast to vanadia supported on polycrystalline CeO2 in which multiple vanadia species coexist. Formation of CeVO4 from the reaction between surface vanadia and ceria upon high temperature calcination was compared for the three ceria nanoshapes with similar surface vanadia density (≈1/4 monolayer). It was found that both the surface structure and the amount of defect sites on the ceria nanoshapes play major roles in the production of CeVO4. The easier formation of CeVO4 on ceria rods, compared with cubes or octahedra, is attributed to the rods’ lowest surface oxygen vacancy formation energy and largest amount of defect sites.