Advances in the Mechanism of Deposition of MoO2-4 and Mo7O6-24 Species on the Surface of Titania Consisted of Anatase and Rutile
The mechanism of deposition of MoO2-4 and MoO7O6-24 from electrolytic solution on the surface of industrial titania consisting of anatase and rutile patches was further investigated following a recently developed methodology. The methodology is based on the "2pk/one site" and "triple-layer" models. It involves the writing down of various deposition equilibria, derivation of the corresponding equations, calculation of the amount of each of the deposited Mo(vi) species and the total amount of deposited Mo(vi) at various pH and concentrations, calculation of the variation with pH of the zeta potentials and of the difference in H+ ion consumption by the support surface in the presence and absence of MoO2-4 and Mo7O6-24 species in the impregnating solution. Comparison of the calculated parameters and variations mentioned above with the corresponding ones obtained by deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of the Mo(vi) species on the surface of titania. It was found that the deposition of MoO2-4 and Mo7O6-24 species on the surface of "anatase regions" of the industrial titania takes mainly place by the following equilibria in the pH range 9.0-4.6: $ \displaylines{ \hfill\hbox{Ti}^{\rm a}\hbox{OH} + \hbox{MoO}^{2-}_{4} \leftrightarrow \hbox{Ti}^{\rm a}\hbox{--O--}(\hbox{MoO}_3)^- + \hbox{OH}^-\hfill\cr \hfill\hbox{Ti}^{\rm a}\hbox{OH}^+_2 + \hbox{MoO}^{2-}_4 \leftrightarrow \hbox{Ti}^{\rm a}\hbox{OH}^+_2 \cdots \hbox{MoO}^{2-}_4\hfill\cr \hfill\hbox{Ti}^{\rm a}\hbox{OH}^+_2 + \hbox{Mo}_7\hbox{O}^{6-}_{24} \leftrightarrow \hbox{Ti}^{\rm a}\hbox{OH}^+_2 \cdots \hbox{Mo}_7\hbox{O}^{6-}_{24}.\hfill\cr } $ According to this mechanism, in the pH range 9-7 the deposition takes place exclusively through the first equilibrium, namely, by reaction of the MoO2-4 ions, located in the inner Helmholtz place (IHP) of the double layer developed between the support surface and the impregnating solution, and the neutral surface hydroxyls in the "anatase regions" of the support. The deposition through adsorption of the MoO2-4 and Mo7O6-24 species, on the protonated surface hydroxyls of the support, starts at pH 7 and 6.4, respectively. The same equilibria are mainly responsible for the deposition on the rutile patches of the support, but the extent of deposition in this case is too small. A preference for deposition of MoO2-4 with respect to Mo7O6-24 ions was observed in almost the whole pH range studied. This was attributed to the negative charge developed in the IHP, which inhibits the location in it of highly charged Mo7O6-24 species. Lateral, attractive, interactions are exerted between the deposited Mo(vi) species through water molecules being in the IHP. The intensity of these interactions is proportional to the charge of the deposited Mo(vi) species.
Bourikas
K
Department of Chemistry, University of Patras-Institute of Chemical Engineering and High Temperature Chemical Processes, ICE/HT-FORTH, Patras, GR-26 500, Greece
Spanos
N
Lycourghiotis
A