THE ADSORPTION OF METAL OXYANIONS ON ALUMINA

The adsorption of perrhenate, permanganate, molybdate, tungstate, and chromate on alumina was studied at pH 4.5. The effect of nitrate in the impregnation solution on the adsorption of these ions was examined as well as the effect of washing the freshly prepared catalysts. The uptake of perrhenate and permanganate decreased with increasing nitrate in the impregnation solution; the uptake of molybdate, tungstate, and chromate was unaffected. Washing the catalysts almost completely removed perrhenate, permanganate, and chromate, but allowed a significant amount of molybdate and tungstate to remain on the alumina. It is concluded that tungstate and molybdate adsorb on two types of surface sites, producing loosely and tightly bound surface species. From the amounts of molybdate and tungstate adsorbed, and from EXAFS results from washed and unwashed Mo/Al2O3 samples, both loosely and tightly bound molybdate and tungstate are polymeric in nature. The results are discussed in terms of electrostatic and surface reaction models. © 1990.