Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

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Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Structural and electrochemical properties of the proton / γ-MnO2 system

http://ematweb.cmi.ua.ac.be/emat/pdf/1786.pdf

Oxidation state and chemical shift investigation in transition metal oxides by EELS

Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Mn2O3–Fe2O3 powders have been prepared by a coprecipitation method. The pure compounds have been characterized as constituted of α-Fe2O3 (haematite) and α-Mn2O3 (bixbyite) while the mixed oxides are constituted of a mixture of haematite- and bixbyite-structure solid solutions. The observed reciprocal solubilities calculated from the XRD patterns approach the thermodynamic ones. α-Mn2O3 is more active than α-Fe2O3 as catalyst for both propane and propene oxidation. However, α-Mn2O3 is less active than Mn3O4 powder. Propene oxidation is in all cases very selective to CO2 while propane oxidation gives rise to significant amounts of propene on α-Fe2O3. Mn2O3–Fe2O3 powders are slightly more active than α-Mn2O3 as combustion catalysts. The selectivities to propene upon propane oxidation decrease with increasing Mn content.

Characterization of manganese and iron oxides as combustion catalysts for propane and propene

Thermal decomposition of managanese oxyhydroxide

Water desorption from a γ-phase manganese dioxide has been studied by gravimetric temperature programmed desorption (TPD) in static water vapor environments. Desorption peaks due to the presence of physisorbed and “structured” hydrogen-bonded water (peak I), chemisorbed molecular water (IIa) and hydroxyl groups (IIb) have been identified. Additional hydroxyl groups were introduced into the interior of the solid by reaction with hydrazine at room temperature. Their effect on the TPD spectra was studied.

F.L. Tye

Papers

Thermal decomposition of managanese oxyhydroxide

Temperature programmed desorption studies on γ-phase manganese dioxide in static water vapor environments