Abstract
The subject of this study was the content of oxygen in mixed oxides with the spinel structure Mn1.7Ga1.3O4 that were synthesized by coprecipitation and thermal treatment in argon at 600–1200 °C. The study revealed the presence of excess oxygen in “low-temperature” oxides synthesized at 600–800 °C. The occurrence of superstoichiometric oxygen in the structure of Mn1.7Ga1.3O4+δ oxide indicates the formation of cationic vacancies, which shows up as a decreased lattice parameter in comparison with “high-temperature” oxides synthesized at 1000–1200 °C; the additional negative charge is compensated by an increased content of Mn3+ cations according to XPS. The low-temperature oxides containing excess oxygen show a higher catalytic activity in CO oxidation as compared to the high-temperature oxides, the reaction temperature was 275 °C. For oxides prepared at 600 and 800 °C, catalytic activity was 0.0278 and 0.0048 cm3 (CO) per g per s, and further increase in synthesis temperature leads to a drop in activity to zero. The process of oxygen loss by Mn1.7Ga1.3O4+δ was studied in detail by TPR, in situ XRD and XPS. It was found that the hydrogen reduction of Mn1.7Ga1.3O4+δ proceeds in two steps. In the first step, excess oxygen is removed, Mn1.7Ga1.3O4+δ → Mn1.7Ga1.3O4. In the second step, Mn3+ cations are reduced to Mn2+ in the spinel structure with a release of manganese oxide as a single crystal phase, Mn1.7Ga1.3O4 → Mn2Ga1O4 + MnO.
Highlights
The ability of manganese cations to change their oxidation state makes Mn-containing oxides efficient catalysts for various processes: oxidation of hydrocarbons, CO, and VOCs1–6 and the selective reduction of NOx with NH3.7Of special interest are Mn-containing oxides with a spinel structure that possess a high thermal stability and are able to intercalate additional cations
The catalytic properties of Mncontaining oxides were examined in different processes: Mn– Co oxides are promising for the oxidation of CO, propene,[8] and benzyl alcohol,[9] as materials for fuel cells and oxygen reduction/ evolution electrocatalysts;[10,11] Ga–Co–Mn can be employed for visible-light-driven water oxidation;[12] Mn–Al spinels are precursors of the active component of catalysts for the oxidation of CO and hydrocarbons;[13] Zn–Mn–Al oxides are used for the reduction of nitrobenzene to nitroazobenzene;[14] and Mn–Fe as Fenton catalysts toward catalytic degradation of highly concentrated methylene blue.[15]
Nonstoichiometry of the metal/oxygen ratio is commonly implemented in the spinel structure via the cationic vacancies with preservation of the closest oxygen packing, A1Àx1B2Àx2[]yO4; in this case, the chemical formula can be written as AB2O4+d, it follows that the structure includes superstoichiometric oxygen, which under certain conditions can pass to the gas phase and become reactive
Summary
The ability of manganese cations to change their oxidation state makes Mn-containing oxides efficient catalysts for various processes: oxidation of hydrocarbons, CO, and VOCs1–6 and the selective reduction of NOx with NH3.7Of special interest are Mn-containing oxides with a spinel structure that possess a high thermal stability and are able to intercalate additional cations. Noteworthy is the fact that the lattice parameter of spinel at 450 C (8.492(2) A) exceeds the parameters of the high-temperature oxides under consideration, 8.481(1)–8.478(1) A (1000 and 1200 C), the indicated samples do not contain excess oxygen (according to the TG and TPR data). The reduction is accompanied by changes in the lattice parameter, which testify to structural rearrangements caused by the removal of excess oxygen and the release of manganese cations with the formation of MnO oxide.
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