CHEMISTRY OF PHOSPHOMOLYBDATE ADSORPTION ON ALUMINA SURFACES .1. THE MOLYBDATE ALUMINA SYSTEM

The adsorption of ammonium heptamolybdate (AHM) on γ-Al2O3 and α-AlOOH has been studied by using FTIR and Raman spectroscopies, TPR, and EXAFS. It is shown that molybdate adsorption on alumina cannot be explained in terms of electrostatic binding to protonated surface hydroxyls but can be rationalized by assuming (i) a reaction between the molybdate and basic surface OH groups, leading to the decomposition of the adsorbing molecule/ion, and (ii) physisorption on coordinatively unsaturated (cus) Al3+ sites. In the AHM/α-AlOOH case, surface precipitation, presumably of (NH4)2Mo3O10, takes place at surprisingly low initial AHM concentrations. The presence of (divalent) cations can influence the shape of the AHM/γ-Al2O3 isotherm but does not affect the ultimate loadings that can be achieved through processes (i) and (ii). © 1990 American Chemical Society.