Surface and catalytic properties of Mn2O3/MgO system doped with ZnO

ABSTRACT

The effects of calcination temperature and doping of Mn2O3/MgO system with ZnO on its surface and catalytic properties were investigated. The techniques employed were nitrogen adsorption at -196°C. XRD and H2O2 decomposition at 30-50°C. Pure and variously doped solids were prepared by wet impregnation method using manganese nitrate, magnesium basic carbonate and zinc nitrate. The prepared solids were calcined at 400, 600, 700, 900 and 1000°C. The amount of Mn2O3 was fixed at 20 mol% for all solids. The dopant concentration was changed between 0.75 and 3 mol% ZnO. The results revealed that manganese oxides interacted with magnesium oxide to yield crystallized magnesium manganates at temperature starting from 400°C. Pure and doped solids precalcined at 400°C consisted of MgO and MgMnO3 phases. The degree of crystallinity of the detected phases increased with increasing the calcination temperature to 600°C with detection of poorly crystalline Mn2O3. Furthermore, ZnO-doping of the system investigated followed by calcination at 400°C and 600°C resulted in decreasing the intensity of the diffraction lines with subsequent decrease the detected phases, in their crystallite size and increases their surface areas [SBET] up to a certain extent of dopant added. ZnO doping hinders the formation of MgMnO3 phase at 400 and 600°C. At 700°C, Mn2O3, Mg6MnO8 and Mg2MnO4 phases were detected. At 900 and 1000°C, only, well crystalline Mg6MnO8 and Mg2MnO4 phases were detected for all the solids. The doping process carried out at 400 and 600°C increased effectively the catalytic activity of the system under investigation reaching a maximum limit at 1.5 mol% ZnO. The increase in dopant concentration above this limit decreased the catalytic activity which remained greater than those measured for the pure solids calcined at the same temperatures. The presence of 1.5 mol ZnO brought about an increase of 191% and 144% of the catalytic activity of the solids calcined at 400 and 600°C, respectively. The doping process did not affect the activation energy values of the catalyzed reaction but rather increased the concentration of active sites involved in the catalyzed reaction without changing their energetic nature