Physicochemical properties of Mn Incorporated Spinels
This thesis is aimed at tuning chemical and physical properties of spinels via a novel approach of manganese incorporation. Oxide spinels benefit from 1) hosting diverse cations at 4 and 6-coordinated sites; and 2) dual functionality of shared anionic oxygen in electron transport and surface reactivity. In other words, redox reactions over transition metal oxide spinels take advantage of charge transport in the bulk and mass exchange phenomena on the surface. Both rates of bulk charge transport and surface mass exchange depend on anion-cation interaction. Variability of manganese oxidation state and the impact of Jahn-Teller active Mn3+O6 are the most significant features affecting bulk properties as well as surface reactivity. This thesis establishes a meaningful correlation between activation energy of electron hopping and the rate of oxygen evolution reaction. Both features follow a volcano plot versus eg occupancy at octahedral site in the range of 0-2. Such a correlation is one of the novelties brought about by this thesis. The effect of calcination and post calcination on the structural and electrical properties of MgFe2O4 is investigated. An oxygen free atmosphere is introduced as the most suitable environment to collect densified crystallites of magnesium ferrite at 850 ˚C. A DC electrical conductivity instrument was developed to evaluate the mechanism of electron hopping under controlled atmospheres. The instrumental approach allows us to evaluate the electron exchange interaction managed by cation-anion interactions. In situ evaluation of redox kinetics of such ceramic material is another feature of the instrument, providing novel insights into additional chemical and electrical properties. Both single and multi-phase approaches of manganese incorporation in MgFe2O4 are considered here, along with applications pertaining to catalysis, electrocatalysis, energy storage, and sensors. In this thesis, an advanced composite of porous Mn3O4 in dense MgFe2O4 matrix is introduced and the effect of Mn diffusion into ferrite spinel is further investigated.