Effects of thermal treatment and doping with cobalt and manganese oxides on surface and catalytic properties of ferric oxide

Abstract

The effects of doping and thermal treatment on surface and catalytic properties of pure ferric oxide solid were studied by means of thermal analyses (TG–DTG–DTA), X-ray powder diffractometry, BET analysis of nitrogen isotherms and hydrogen peroxide decomposition reaction at 20–50 °C. These techniques used for investigating the thermal behavior of pure and variously doped samples, crystalline bulk structure, specific surface areas and testing catalytic properties of pure and variously doped samples, respectively. A series of Co 3O 4–Fe 2O 3 and Mn 2O 3–Fe 2O 3 systems were prepared from cobalt nitrate, basic ferrous carbonate and manganese nitrate salts by impregnation method. Both of Co 3O 4 and Mn 2O 3 used as dopants (0.25–4.00 mol%) for pure Fe 2O 3. The pure and doped solids were conducted at 350, 550, 750 and 1000 °C. The results obtained revealed that the various solids heated at low temperature led to formation of free oxides. The doped solids calcined at high temperature resulted in formation of ferrite compounds (CoFe 2O 4 or MnFe 2O 4). The specific surface area of pure Fe 2O 3 progressively affected by amount of dopants and calcination temperature. It significantly increased with increasing amount of dopants for solids treated at 350 and 550 °C then it dramatically decreased with increasing the calcination temperature up to 750 °C. The increase was, however, more pronounced in case of Mn 2O 3-doping. The catalytic activity of pure Fe 2O 3 significantly increased with increasing the amount of cobalt or manganese oxides dopants. The observed increase in activity is attributed to increasing the concentration of catalytically active constituents and/or formation of new active sites. The measurement of activation energy of the catalytic reaction for pure and variously doped solids revealed that the doping process did not modify the energetic nature of the active sites involved in the catalyzed reaction. Furthermore, the catalytic activities of doped solids increased by increasing calcination temperature from 350 to 550 °C then it significantly decreased with increasing treatment temperature up to 750 °C. The progressive decrease in the activity is due to formation of inactive ferrite compounds and/or sintering processes.