Journal article

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 atm., temp.-programmed redn. in hydrogen (TPR), and soly. in dild. HNO3. Three types of species were obsd. with the max. peak temps. as follows: isolated monomeric species (?770-780 K), polymeric species (810 K), and bulk amorphous V2O5 (852 K). During the redn., the V:O bond of the monomeric species with tetracoordinated vanadium disappears as shown by diffuse reflectance IR Fourier transform spectroscopy. A formation of new hydroxyl groups with a basic character was obsd. The monomeric species was found to be chem. stable with respect to dild. HNO3. Bulk amorphous V2O5 and polymeric vanadia were sol. 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. Redn. by hydrogen of monolayer vanadia in a 0.2-ML V/TiO2 catalyst was studied by temp.-programmed redn. (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 detd. for the catalyst redn. (98+-5 kJ/mol). (c) 2002 Academic Press. [on SciFinder (R)]

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