Nonstoichiometric oxygen in Mn-Ga-O spinels: reduction features of the oxides and their catalytic activity

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 degrees C. The study revealed the presence of excess oxygen in low-temperature oxides synthesized at 600-800 degrees C. The occurrence of superstoichiometric oxygen in the structure of Mn1.7Ga1.3O4+ 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 degrees 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 degrees C. For oxides prepared at 600 and 800 degrees C, catalytic activity was 0.0278 and 0.0048 cm(3) (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+ was studied in detail by TPR, in situ XRD and XPS. It was found that the hydrogen reduction of Mn1.7Ga1.3O4+ proceeds in two steps. In the first step, excess oxygen is removed, Mn1.7Ga1.3O4+ 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.