The mechanism of retention of vanadium oxo-species at the “titanium oxide / aqueous solution" interface

Abstract

The mechanism of retention of vanadium oxo-species at the “titanium oxide/aqueous solution”
 interface was investigated over a wide pH range (4-9) and V(V) solution concentration (10-5-2×10-2 M)
 by combining equilibrium deposition experiments, potentiometric titrations, microelectrophoresis and
 “proton–ion” titration curves. It was inferred that the adsorbed V(V) oxo-species are retained inside
 the compact layer of the interface through hydrogen/coordinative bonds forming very probably innersphere
 complexes with the titania surface groups.

Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach

“…The structure of the V 4 O 12 4− tetramer ion is characterized by the di-oxo O=V=O termination configuration and tetrahedral coordination for each V atom, which is similar to the respective structural characteristics of the metavanadate chain, (VO 3 ) n n− . The observed 946 cm −1 band position agrees with the 936 cm −1 observed for molten NaVO 3 [64] and is assigned to the symmetric stretching, ν s , of the di-oxo V(=O) 2 unit. The antisymmetric counterpart mode, ν as , is obscured by the low-frequency wing of the symmetric mode, thereby resulting in an asymmetric shape for the 946 cm −1 band, which is much clearer at pH = 8 (Figure 9c), where the additional display of the Raman spectrum obtained with the VH configuration allows the exclusive presence of the ν as mode at ≈915 cm −1 .…”

“…The "proton-ion" curves are linear plots of the "amount of the H + ions adsorbed versus the amount of the V deposited" at constant pH and under conditions favoring complete deposition of the V(V) oxo-species at the titania surface. The last can be achieved by working at too low V concentrations and a very high solid/solution ratio [41,49,64]. A successful fitting of these curves is only possible if the right deposition mechanism is adopted in the simulation [49][50][51][52][53].…”

“…Proton-ion titrations and adsorption experiments data used for the quantitative modeling were drawn from ref. [64], where full experimental details about these experiments can be found.…”

“…Table A1 contains all the species for the H K V X O Y Z− /NaNO 3 /TiO 2 system, namely the surface (hydr)oxo groups of titania, the ion pairs of these groups with the ions of the indifferent electrolyte (Na + , NO 3 − ), and the deposited vanadium species. The formation of each surface species is defined in terms of components (columns), being components in solution, surface components (surface groups), and electrostatic components (exp(−FΨ i /RT), with i = 0, 1, and 2, standing for the corresponding planes of the interface [64]). The concentration [S] (mol L −1 ) of a surface species can be calculated by reading the table horizontally and using Equation (1).…”

“…Hence, in the present work, first, the speciation of the oxo-V(V) species in the precursor impregnation solutions is determined in conditions relevant to the follow up impregnation step by means of a model taking into account pertinent experimental information, and the precursor solution speciation is verified by Raman spectroscopy. Secondly (and more importantly), the type of interfacial deposition of oxo-V(V) species at the interface between TiO 2 (P25) and the precursor electrolyte solution is investigated and a prediction of the interfacial speciation of deposited oxo-V(V) species is reached based on experimental results previously reported [64]. Then, in situ Raman spectroscopy is used for verifying the interfacial speciation of the as-prepared by the EDF method wet H K V X O Y Z− /TiO 2 pastes and for monitoring the temperature-dependent evolution of the V(V) species at temperatures up to 430 • C.…”