Characterization of Surface Vanadia Forms on V/Ti–Oxide Catalyst via Temperature-Programmed Reduction in Hydrogen and Spectroscopic Methods

Abstract

Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML) of VO x have been characterized by FT-Raman spectroscopy under controlled atmosphere, temperature-programmed reduction in hydrogen (TPR), and solubility in diluted HNO 3 . Three types of species were observed with the maximum peak temperatures as follows: isolated monomeric species (≤770-780 K), polymeric species (810 K), and bulk amorphous V 2 O 5 (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 HNO 3 . Bulk amorphous V 2 O 5 and polymeric vanadia were soluble in HNO 3 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/TiO 2 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).

DRIFTS and transient-response study of vanadia/titania catalysts during toluene partial oxidation

“…The peak is very similar to that obtained by reduction with hydrogen at 673 K. 9 The step of catalyst re-oxidation was performed by introduction of O 2 in the DRIFTS cell after purging with Ar. This operation led to rapid formation of new bands in the carbonyl region (Fig.…”

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5/TiO2 catalyst

“…It was found that vanadia on a titania surface forms different types of structures depending on the vanadia content and the preparation technique [27][28][29][30][31][32][33][34]. At vanadia content below 10 % of monolayer (ML), only a monomeric isolated structure exists, which has a tetrahedral coordination (VO 4 ) in dehydrated conditions [29,33].…”

Structure of Flame‐made Vanadia/Titania and Catalytic Behaviour in the Partial Oxidation of o‐Xylene

“…4 shows Raman spectra (solid line) of both dehydrated HE-FSP-made 10V40-16 and impregnated (10V65-9 imp, 10V40-16 imp) V 2 O 5 /TiO 2 . The flame-made catalyst exhibited a clear band at 1033 cm −1 that corresponds to monomeric VO x species [13]. as well as a band at 940 cm −1 confirming the presence of amorphous polymeric VO x species [32] stemming from the higher V coverage (16 V nm −2 ) of HE-FSP-made particles.…”

“…The band at around 800 cm −1 stems from the Si-O-Si vibration of the quartz window used in the in situ cell. At very low V coverage (1 V nm −2 , 1V93-1), a V=O stretching band at 1032 cm −1 attributed to monomeric VO x [13] was detected. With increasing V coverage up to 15% (w/w) V 2 O 5 , a band at around 920-940 cm −1 appeared that can be attributed to the V O-V bridges of the polymeric VO x species [6,17,[32][33][34][35].…”

“…At low loadings (up to 2-3 V nm −2 ), strongly bound monomeric VO x groups with three V-O bonds to the support and a terminal partly hydrated V=O bond are formed at ambient conditions [13]. At V coverage greater than about 3 V nm −2 , some V-O-support bridges rearrange themselves into V-O-V bridges [5,6,8] increasing the degree of polymerization.…”