Tungsten-oxo-species deposited on alumina .1. Investigation of the nature of the tungstates deposited on the interface of the gamma-alumina/electrolyte solutions at various pH's

The mechanism of the deposition of the W-(vi) species from aqueous solutions on the gamma-alumina surface is refined in this work by investigating critical mechanistic points. A methodology recently developed to investigate the deposition of the Co-(II), Ni-(II), Cr-(VI), and Mo-(VI) on gamma-alumina has been applied. This methodology is based on the ''2pK/one site'' and ''triple-layer'' models and involves the writing down of various deposition equilibria, the derivation of the corresponding equations, the calculation of the amount of the deposited W-(vi) (through the calculated concentrations of the W-(vi) species formed on gamma-alumina) at various values of the impregnating parameters, the calculation of the variation, with pH, of the zeta-potential and of the difference in the H+ ions consumption by the support surface in the presence and absence of the W-(vi) species in the impregnating solution. The comparison of the calculated values of the aforementioned parameters with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of W-(vi) species on gamma-alumina. It was found that although eight different W-(vi) species are present in the impregnating solution under the conditions of deposition, the mechanism of deposition was proved to be quite simple: AlOH2+ + WO42- <-> AlOH2+...WO42- AlOH2+ + HW6O20(OH)(2)(5-) <-> AlOH2+...HW6O20(OH)(2)(5+) AlOH + WO42- <-> Al-O-(WO3)(-) + OH- 2AlOH + WO42- <-> Al-O-(WO2)-O-Al + 2OH(-). According to this mechanism only two W-(vi) species contribute to the whole deposition. These species move from the bulk solution to the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support particles and the impregnating solution and then are adsorbed on sites created in the IHP by protonated surface hydroxyl groups of the support. Moreover, the monomeric WO42- species, being in the IHP, may react with a single or a pair of adjacent neutral surface hydroxyl groups of the support resulting from the formation of a charged or uncharged W-(vi) species, respectively. Lateral intractions are exerted between W-(vi) species formed through water molecules also located in the IHP. The study of the variation of the saturation surface concentration of the species illustrated in the rhs of the above equilibria showed that in the pH range 10-6 the deposition occurs practically via reaction (equilibria [c] and [d]). The concentration of Al-O-(WO2)-O-Al and Al-O-(WO3-) species is maximized at pH's 7 and 6, respectively. In the pH range 6-5 both surface reaction and adsorption (equilibria [c] and [a]) contribute to the whole deposition process. The concentration of the AlOH2+...WO42- is maximized at pH 5. Finally in the pH range 5-3.5 the deposition takes place exclusively by adsorption (equilibria [a] and [b]). The concentration of the W-(vi) species illustrated in the rhs of the second equilibrium is maximized at pH 3.5. The selective deposition of the WO42- species with respect to the various polymeric species present in the impregnating suspension was attributed to the relatively low negative charge of these species and the negative potential developed at the IHP. (C) 1996 Academic Press Inc.