Surface characteristics of the pure and Li2O-doped MoO3/Al2O3 system

Two series of MoO3/Al2O3 solids, having the nominal compositions 0.2MoO(3):Al2O3 and 0.5MoO(3):Al2O3, were prepared by impregnating finely powdered Al(OH), samples with calculated amounts of ammonium molybdate solutions. The solids thus obtained were dried at 120 degrees C and then calcined in air at temperatures varying between 400 degrees C and 1000 degrees C. The doped samples were prepared by treating Al(OH)(3) with LiNO3 solutions prior to impregnation with ammonium molybdate, The dopant concentrations employed were 1.5 and 6.0 mol% Li2O, respectively. The surface characteristics, viz. the specific surface area (S-BET), the total pore volume (V-p) and the mean pore radius ((r) over bar) of the various pure and doped solids were measured from nitrogen adsorption isotherms conducted at -196 degrees C. The S-BET data measured for different adsorbents calcined at various temperatures enabled the apparent activation energy for sintering (Delta E-s) to be determined for all the adsorbents investigated. The results obtained reveal that the S-BET value of the pure and doped solids decreased on increasing the calcination temperature in the range 400-1000 degrees C, The decrease was, however, more pronounced when the calcination temperature increased from 500 degrees C to 700 degrees C due to the formation of Al-2(MoO4)(3). Lithium oxide doping decreased the S-BET value of the solid samples investigated and also decreased the activation energy for sintering to an extent proportional to the amount of dopant present. The sintering process for the pure and doped solids proceeds, mainly, via a particle adhesion mechanism.