Surface properties of Ni-Al mixed oxide catalysts

A series of Ni-Al mixed oxides having different compositions was prepared by coprecipitation and by impregnation. The solids obtained were heated in air at 450–1000 °C. The surface properties, namely, S_BET, S_t, V_p and $\text{r}$, of each adsorbent were determined from nitrogen adsorption isotherms. The change in S_BET as a function of calcination temperature permitted the calculation of the apparent energy of activation of sintering.

The results obtained revealed that the catalysts prepared by coprecipitation had higher surface areas than those obtained by impregnation. The mixed oxide solids seemed to be composed of wide pores. However, the pores of the adsorbents prepared by impregnation were narrower than those measured for the adsorbents prepared by coprecipitation. Also, the specific surface area depended on the NiO/Al₂O₃ ratio. The presence of Al₂O₃ increased the degree of dispersion of NiO grains leading to a significant increase in its surface area, with effective retardation of sintering of the NiO phase. The apparent energy E_s of activation of sintering indicated that the increase in alumina content was followed by a corresponding increase in the magnitude of E_s.