CuAl2O4 formation and its effect on α-Al2O3 phase evolution on calcination of metal ion doped boehmite xerogels

The formation characteristics of CuAl2O4 from sol–gel boehmite doped with Cu2+ion and also the effect of dopants like Cu2+, Ti4+ and Ce4+ on calcination and concentration dependent structural evolution of α-Al2O3 from xerogels derived from boehmite sol are studied. The structural evolutions of various phases are studied by PXRD, FTIR, UV–vis (diffused reflectance) and EPR spectral techniques. Even though the formation temperature of α-Al2O3 from undoped boehmite is around 1200 °C, we observed significantly early and quantitative conversion of boehmite into α-Al2O3 along with the formation of CuAl2O4 spinel at 1000 °C in 4 and 8 mol.% Cu2+ ion doped samples owing to the nucleation effect and growth process initiated by the in situ generated spinel phase. However, no indication of such quantitative conversion in the case of 2% doped sample is seen presumably because of the retarding effect of transition aluminas on spinel formation. Temperature dependent phase evolution of CuAl2O4 spinel could be monitored by diffuse reflectance, EPR and PXRD, which show some interesting features associated with the change from CuO6 to CuO4 polyhedra. Dopants like Ti4+ and Ce4+ do not show any nucleation and facilitation effects in the formation of α-Al2O3 from boehmite xerogels.