The porous material Fe2O3-Cr2O3 usually employed as catalyst was
prepared at several calcination temperature and for several calcination times.
The study has been carried out by means of multiple physical-chemical
analytical techniques for characterising the materials, but also relied on
monitoring and correlating the textural, diffusional, mechanical, crystallographic
and catalytic properties. The textural properties indicate intermediate calcination
times (1-4 h) and calcination temperatures (723-773 K) to be appropriate; the
best catalytic activity is obtained for the samples calcined at 773 K for 1 h, while
the crystallographic results limit to maximum 4 h of calcination. The mechanical
properties advocate for using a slightly lower calcination temperature than
773 K, while experimental results for BET surface area found the highest value
at 746 K (Fig.). Generally, the time of calcination does not affect greatly the
diffusional properties, but has significant influence on the textural ones;
however, the temperature of calcination has stronger influence upon the
diffusional properties. Based on the Knudsen number (Kn), diffusion is
controlled by Knudsen mechanism, which influences the catalytic activity that
increases with the diffusional velocity through a highly porous material
characterised by a large surface area and dominated by small pores with radii
dimensions below 200 Å.

Fig. The dependence on the temperature of calcination of the axial mechanical strength and of the distribution of the BET surface area, after 4 hours of calcination