Solid-solid interactions in pure and Li2O-doped manganese and magnesium mixed oxides system

The solid - solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li2O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn2O3 and MgO were varied between 0.05: 1 to 0.5: 1. The mixed solids were calcined in air at 400 - 1000 degreesC. The techniques employed were DTA, XRD and H2O2 decomposition at 20 - 40 degreesC. The results obtained revealed that solid - solid interactions took place between the reacting solids at 600 - 1000 degreesC yielding magnesium manganates (Mg2MnO4, Mg6MnO8, MgMnO4 besides unreacted portions of MgO, Mn2O3 and Mn3O4). Li2O-doping (0.75 - 6 mol\%) of the investigated system followed by calcination at 600 and 800 degreesC decreased progressively the intensity of the diffraction lines of Mn2O3 (Bixbyite) with subsequent increase in the lattice parameter `a' of MgO to an extent proportional to the amount of Li2O added. This finding might suggest that the doping process enhanced the dissolution of Mn2O3 in MgO forming solid solution. This treatment led also to the formation of Li2MnO3. Furthermore, the doping with 3 and 6 mol\% Li2O conducted at 800 degreesC resulted in the conversion of Mn2O3 into Mn3O4, a process that took place at 1000 degreesC in absence of Li2O. The produced Li2MnO3 phase remained stable by heating at up to 1000 degreesC. Furthermore, Li2O doping of the investigated system at 400 - 1000 degreesC resulted in a progressive measurable increase in the particle size of MgO. The catalytic activity measurements showed that the increase in the molar ratio of Mn2O3 in the samples precalcined at 400 - 800 degreesC was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20 degreesC (k(20degreesC)) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800 degreesC, respectively. Li2O-doping of the samples having the formula 0.1 Mn2O3/MgO conducted at 400 - 600 degreesC brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k(20degreesC) due to Li2O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600 degreesC, respectively and opposite effect was found for the doped samples preheated at 800 degreesC.