De-sintering action of phosphorus in alumina–aluminium phosphates (AAP) containing transition-metal ions (M = Cr, Mn, Fe, Co, Ni, Cu, Zn)

Abstract

The de-sintering action of phosphorus added in different amounts to alumina containing 20% of a second cation M (M = Cr, Mn, Fe, Co, Ni, Cu, Zn) has been quantified from measurements of the surface areas of the solids. The solids were prepared by co-precipitation with NH₃ from solutions containing calculated amounts of metal nitrates and phosphoric acid and by heating at 600 °C. The atomic ratio of the elements in the obtained solids was Al : P : M = 100 : x : 20 where x= 0, 4.5, 9, 18, 36, 72 and 144. The surface areas of the solids were measured by the B.E.T. method. The results showed that the maximum de-sintering action of phosphorus, as estimated from the change of the surface area of the solids, appeared at x= 9. Thereafter the surface area decreased, at a different pace for each cation, and at x= 36 it returned to the original value at x= 0. Further addition of phosphorus decreased the surface area even further and at x= 144 it fell almost to zero, depending on the second cation. The strongest sintering ability was exhibited by Cu and Mn and the smallest by Cr, Fe and Ni, with the rest occupying intermediate positions. An explanation of the results is attempted by taking into account the enthalpies of formation of the corresponding metal phosphates and aluminates formed in each case.