Chemical States and Redox Properties of Mn/CeO₂−TiO₂ Nanocomposites Prepared by Solution Combustion Route

Abstract

The catalytically important Mn/CeO2−TiO2 (MCT) solid solutions were synthesized by solution combustion technique using glycine, urea, or PEG as fuel. These samples along with those prepared by coprecipitation and wet impregnation methods were characterized for its structural, textural and redox properties using various physical and spectroscopic techniques, viz., powder X-ray diffraction, high-resolution transmission electron microscopy, temperature-programmed reduction, diffuse reflectance UV−visible, electron paramagnetic resonance, and X-ray photoelectron spectroscopy. The samples prepared by the solution combustion method showed high thermal stability, even at temperatures of 1323 K, whereas the samples synthesized by conventional routes lost its structural integrity at higher temperatures due to fast sintering. The combustion and coprecipitation methods stabilize Mn species both in +2 and +3 oxidation states, while in the MCT sample prepared by wet impregnation, Mn ions are present in +3 state exclusively. The latter possesses highly reversible redox properties, whereas the samples prepared by combustion and coprecipitation techniques show quasireversible redox behavior. On comparing the physicochemical properties of MCT solid solutions with that of Mn/CeO2, we elucidate that Ti incorporation into the ceria matrix alters the redox properties of both cerium and manganese ions. Though the nature of the fuel did not have significant effect on the oxidation states of Mn species stabilized in the solid solutions, the relative concentration of Mn2+ and Mn3+ ions and their existence in the bulk or on the surface of the material did vary with the nature of the fuel used in the combustion process. These variations could be attributed to the intrinsic properties of combustion process, viz., the adiabatic/real flame temperature and mode of combustion and its duration. Employing PEG as fuel in the combustion process, one can obtain thermally stable nanocrystalline MCT samples with high phase purity.

Activity of MnOx–CeO2 catalysts in combustion of low concentrated methane

“…It partially overlaps with the CT bands described above. The symmetry disorder around Mn 3+ ions in previously described oxide clusters leads to the presence of additional absorption bands centered about 370 and 785 nm [33].…”

“…Location of maximum absorption varied in the range of 310-350 nm and was slightly higher than that observed in the case of pure CeO 2 [30][31][32], confirming interaction between CeO 2 and manganese ions within the MnO x -CeO 2 system leading to the formation of Ce 1-x Mn x O 2 solid solutions postulated earlier. Another broad band at around 455-489 nm can be attributed to d-d transitions, 5 T 2 ← 5 E 2 of Mn 3+ ions (3d 4 configuration), occurring in small MnO x clusters with octahedral coordination of Mn centers [32][33][34][35]. It partially overlaps with the CT bands described above.…”

Morphology control and photocatalytic characterization of yttrium-doped hedgehog-like CeO2

“…When fabricated samples are irradiated under UV light, electrons are excited from the O 2p valence band to the conduction band (Ce 4f ) resulting in the formation of hole and electron pairs [32], and the holes easily get trapped on the oxygen ions [33,34]. When the number of oxygen vacancies is higher, the oxygen ion mobility becomes higher [35]. Transportation mobility of lattice oxygen ions is beneficial for the separation of photogenerated electrons and holes.…”

Photocatalytic oxidative desulfurization of dibenzothiophene under simulated sunlight irradiation with mixed-phase Fe2O3 prepared by solution combustion

“…Solution combustion synthesis is a fast and easy approach that allows the formation of homogeneous solid solutions in preparing nanomaterials. 33 Using this approach to synthesize iron oxides, ferric nitrate was used as the mother salt, while urea, glycine, ammonium nitrate, or hydrazine were used as fuels. [34][35][36][37][38] Due to the nitrate and fuel having an exothermic combustion reaction, the heat can cause the formation of iron oxide nanopowders.…”