Surface and catalytic properties Of MoO3/Al2O3 system doped with Co3O4

Thermal solid-solid interactions in cobalt treated MoO3/AI(2)O(3) system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH)(3), ammonium molybdate and cobalt nitrate solutions, drying at 100 degrees C then calcination at 300, 500, 750 and 1000 degrees C. The amount of MoO3, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co3O4. Surface and catalytic properties of various solid samples precalcined at 300 and 500 degrees C were studied using nitrogen adsorption at -196 degrees C, conversion of isopropanol at 200-500 degrees C and decomposition of H2O2 at 30-50 degrees C. The results obtained revealed that pure mixed solids precalcined at 300 T consisted of AlOOH and MoO3 phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO3 and CoMoO4 compounds. The rise in calcination temperature to 500 T resulted in complete conversion of AlOOH into very poorly crystalline gamma-Al2O3. The further increase in precalcination temperature to 750 degrees C led to the formation of Al-2(MoO4)(3), kappa-Al2O3 besides CoMoO4 and un-reacted portion Of Co3O4 in the samples rich in cobalt oxide. Pure M003/Al-103 preheated at 1000 degrees C composed of MoO3-alpha Al2O3 solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO4 and CoAl2O4 compounds. The increase in calcination temperature of pure and variously doped solids from 300 to 500 degrees C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 degrees C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 degrees C increased its catalytic activity in H2O2 decomposition. This increase, measured at 300 degrees C, attained 25.4- and 12.9-fold for the solids precalcined at 300 and 500 degrees C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the catalysts precalcined at 300 degrees C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol. The activation energies of H2O2 decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide. (c) 2005 Elsevier Ltd. All rights reserved.