Effect of Mg and Al on manganese oxides as catalysts for VOC oxidation

Abstract

Manganese mixed oxides with the compositions Mn-Mg-Al-O and Mn-O were synthesized by the self-combustion method. The effects of Mg and Al incorporation on the structural, textural, redox and catalytic properties of the manganese oxides were evaluated. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption, temperature-programed desorption of oxygen (O2-TPD), temperature-programmed reduction (H2-TPR), oxygen storage capacity (OSC), 18O/16O oxygen isotope exchange, and their catalytic performance was evaluated in the oxidation of toluene, ethyl acetate and 2-propanol. The analyses showed that the use of a glycine/nitrate ratio equal to 0.56 in the self-combustion favors the formation of the Mn3O4 phase and the incorporation of Mg and Al into the structure stabilizes this phase after a thermal process. In addition, the surface area increases, the particle size decreases, and the presence of electrophilic oxygens is favored, which increases the adsorption sites, exposes the active centers and facilitates the occurrence of redox cycles, leading to a better catalytic performance in the total conversion of the oxygenated compounds on the Mn-Mg-Al-O mixed oxide. In the oxide that only contains manganese, the presence of nucleophilic oxygens with great mobility was observed, leading to its good activity in the oxidation of the VOCs.The catalytic tests showed that a high oxygen exchange capacity leads to lower temperatures of conversion of toluene, which suggests that, for the oxidation of aromatic compounds, the lattice oxygen plays a fundamental role in the catalytic activity. In contrast, in the oxidation of oxygenated compounds that require lower temperatures of conversion, the existence of surface oxygens such as O2− and O− favors the catalytic process and therefore ensures superior activity in the transformation of ethyl acetate and 2-propanol on the Mn-Mg-Al-O mixed oxide.