Lithium as a Modifier for Morphology and Defect Structure of Porous Magnesium Oxide Materials Prepared by Gel Combustion Synthesis



Defect rich MgO nanocrystals arranged in a hierarchic three-dimensional pore network were synthesized by using gel combustion synthesis (GCS). By adding Li to the combustion precursor, Li-induced changes in the morphology and defect structure of MgO could be studied systematically. At low Li loadings (up to 1 wt %), the three-dimensional pore network was resistant to temperatures up to 800 °C, even though the primary MgO nanoparticles had changed their morphology from on average 8 nm size {100} terminated nanocubes to up to 250 nm large complex polyhedral, exposing more and more {111} facets. At higher Li loadings, the primary MgO particles grow even further, to up to 500 nm, causing the three-dimensional pore network to collapse. After describing the GCS method, detailed structural characterizations of all of the materials synthesized were conducted by means of XRD, BET and pore size analysis, and electron microscopy. IR and thermogravimetric mass spectroscopy (TG-MS) in combination with XRD were used to investigate the formation and decomposition of carbonate species during synthesis and calcination. Diffuse reflectance UV/Vis (DR-UV/Vis) spectroscopy was used to characterize surface defects, such as low coordinated O2− ions at edges, corners, and kinks of the MgO surface. Bulk defects were studied by using electron paramagnetic resonance (EPR) spectroscopy. Morphology and defect concentration of the Li/MgO materials were found to be strongly dependent on the fuel-to-oxidizer ratio used in the combustion synthesis, the Li concentration, and the calcination atmosphere.