Characterization of surface vanadia forms on V/Ti-oxide catalyst via temperature-programmed reduction in hydrogen and spectroscopic methods

Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML) of VOx have been characterized by FT-Raman spectroscopy under controlled atmosphere, temperature-programmed reduction in hydrogen (TPR), and solubility in diluted HNO3. Three types of species were observed with the maximum peak temperatures as follows: isolated monomeric species (less than or equal to770-780 K), polymeric species (810 K), and bulk amorphous V2O5 (852 K). During the reduction, the V=O bond of the monomeric species with tetracoordinated vanadium disappears as shown by diffuse reflectance infrared Fourier transform spectroscopy. A formation of new hydroxyl groups with a basic character was observed. The monomeric species was found to be chemically stable with respect to diluted HNO3. Bulk amorphous V2O5 and polymeric vanadia were soluble in HNO3 and removed from the surface. The state of vanadium in the oxidized catalysts was mainly pentavalent, as shown by XPS, and did not change after acid treatment. Reduction by hydrogen of monolayer vanadia in a 0.2-ML V/TiO2 catalyst was studied by temperature-programmed reduction (TPR) at different heating rates. A one-site kinetic model is able to account for the TPR data, in spite of the presence of the monomeric and polymeric species. This indicates that these species could be considered equal with respect to the interaction with hydrogen. The activation energy was determined for the catalyst reduction (98 +/- 5 kJ/mol). (C) 2002 Elsevier Science.