Surface and Catalytic Properties of NiO and Fe 2 O 3 Solids

Abstract

The surface characteristics, viz., the specific surface area S_BET, the total pore volume V_p and the mean pore radius r̄, of NiO and Fe₂O₃ were determined from N₂ adsorption isotherms conducted at −196°C for the different adsorbents preheated in air at temperatures in the range 300–800°C. The catalytic activities exhibited in CO oxidation by O₂ on the various solids were investigated at temperatures varying between 150°C and 400°C. The effect of heating the NiO and Fe₂O₃ solids in CO and O₂ atmospheres at 175–275°C on their catalytic activities was also studied.

The results showed that increasing the calcination temperature in the range 300–800°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°C to 800°C, due to a decrease in its S_BET value and the progressive removal of excess O₂ 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°C to 400°C which was a result of the crystallization of the ferric oxide into the α-Fe₂O₃ phase. An increase in the calcination temperature for both oxides from 300°C to 800°C did not modify the mechanism of oxidation of CO by O₂ over the various solids but rather changed the concentration of catalytically active sites. Heating NiO and Fe₂O₃ in CO and O₂ atmospheres at 175–275°C modified their catalytic activities, with Fe₂O₃ being influenced to a greater extent than NiO.