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 Mn(1.7)Ga(1.3)O(4) 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 Mn(1.7)Ga(1.3)O(4+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 Mn(3+) 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 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 Mn(1.7)Ga(1.3)O(4+δ) was studied in detail by TPR, in situ XRD and XPS. It was found that the hydrogen reduction of Mn(1.7)Ga(1.3)O(4+δ) proceeds in two steps. In the first step, excess oxygen is removed, Mn(1.7)Ga(1.3)O(4+δ) → Mn(1.7)Ga(1.3)O(4). In the second step, Mn(3+) cations are reduced to Mn(2+) in the spinel structure with a release of manganese oxide as a single crystal phase, Mn(1.7)Ga(1.3)O(4) → Mn(2)Ga(1)O(4) + MnO.