Surface Characteristics of the Pure and Li2O-doped MoO3/Al2O3 System

Abstract

Two series of MoO3/Al2O3 solids, having the nominal compositions 0.2MoO3: Al2O3 and 0.5MoO3:A12O3, were prepared by impregnating finely powdered Al(OH)3 samples with calculated amounts of ammonium molybdate solutions. The solids thus obtained were dried at 120°C and then calcined in air at temperatures varying between 400°C and 1000°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 (SBET), the total pore volume (VP) and the mean pore radius (r) of the various pure and doped solids were measured from nitrogen adsorption isotherms conducted at -196°C. The SBET data measured for different adsorbents calcined at various temperatures enabled the apparent activation energy for sintering (ΔE3) to be determined for all the adsorbents investigated. The results obtained reveal that the SBET value of the pure and doped solids decreased on increasing the calcination temperature in the range 400–1000°C. The decrease was, however, more pronounced when the calcination temperature increased from 500°C to 700°C due to the formation of Al2(MoO4)3. Lithium oxide doping decreased the SBET 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.