Surface and catalytic properties of NiO and Fe2O3 solids

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 NiO and Fe2O3 were determined from N-2 adsorption isotherms conducted at -196 degrees C for the different adsorbents preheated in air at temperatures in the range 300-800 degrees C. The catalytic activities exhibited in CO oxidation by O-2 on the various solids were investigated at temperatures varying between 150 degrees C and 400 degrees C. The effect of heating the NiO and Fe2O3 solids in CO and O-2 atmospheres at 175-275 degrees C on their catalytic activities was also studied. The results showed that increasing the calcination temperature in the range 300-800 degrees C resulted in a progressive decrease in the S-BET value of NiO and Fe2O3. The computed values of the apparent activation energy for the sintering of the oxides were 71 and 92 kJ/mol, respectively. The sintering of NiO and Fe2O3 took place mainly via a particle adhesion mechanism. The catalytic activity of NiO decreased progressively on increasing its calcination temperature from 300 degrees C to 800 degrees C, due to a decrease in its S-BET value and the progressive removal of excess O-2 which was present as non-stoichiometric NiO. This treatment also decreased the catalytic activity of Fe2O3. The decrease was, however, more pronounced when the temperature increased from 300 degrees C to 400 degrees C which was a result of the crystallization of the ferric oxide into the alpha-Fe2O3 phase. An increase in the calcination temperature for both oxides from 300 degrees C to 800 degrees C did not modify the mechanism of oxidation of CO by O-2 over the various solids but rather changed the concentration of catalytically active sites. Heating NiO and Fe2O3 in CO and O-2 atmospheres at 175-275 degrees C modified their catalytic activities, with Fe2O3 being influenced to a greater extent than NiO.