Abstract
A series of MnxOy - Al2O3 materials have been prepared either by spray-drying (SD) of Al/Mn precursor solutions or by multiple wet-impregnations of two different Mn precursors on various alumina supports. The two Mn precursors used in the preparation were manganese nitrate (Mn(NO3)2.4H2O) and manganese acetate (Mn(CH3COO)2.4H2O). To study properties relevant for the sorption performance, the calcined materials were characterized using several methods. Textural properties (N2 sorption) of the Mn loaded samples show significant differences when compared to the parent supports. For the samples prepared by multiple wet-impregnations this is due to the relatively high Mn loading (up to 30 wt. %) and the additional heat treatment. For the sample prepared by SD it is the presence of Mn species that modifies the textural properties and affects the way the final material is formed. X-ray diffraction measurements show that the ratio of Mn2O3/Mn3O4 depends on the Mn precursor used when the samples are prepared by the multiple wet-impregnation method. For the samples prepared by the SD method the use of a specific Mn precursor has no influence on the ratio of Mn2O3/Mn3O4 in the final material. UV-Raman spectra strongly indicate the presence of MnO2 on the surface of all MnxOy - Al2O3 sorbents. MnO2 was not detected by XRD. The presence of new MnxOy species was also detected by Raman spectroscopy and their existence is ascribed to a specific interaction between MnxOy and Al2O3. TPR data show there are three main reduction steps most likely due to the reduction of MnO2 to MnO through intermediate Mn2O3 and Mn3O4. The data, however, indicates that a new species is formed at high temperature (above approx. 550 °C). It is proposed that the additional features observed in the TPR profiles might be due to the formation of the spinel structure MnAl2O4.
