Reduction of mixed Mn-Zr oxides: in situ XPS and XRD studies

A series of mixed Mn-Zr oxides with different molar ratios Mn/Zr (0.1-9) have been prepared by coprecipitation of manganese and zirconium nitrates and characterized by X-ray diffraction (XRD) and BET methods. It has been found that at concentrations of Mn below 30 at%, the samples are single-phase solid solutions (Mn x Zr 1− x O 2− δ ) based on a ZrO 2 structure. X-ray photoelectron spectroscopy (XPS) measurements showed that manganese in these solutions exists mainly in the Mn 4+ state on the surface. An increase in Mn content mostly leads to an increase in the number of Mn cations in the structure of solid solutions; however, a part of the manganese cations form Mn 2 O 3 and Mn 3 O 4 in the crystalline and amorphous states. The reduction of these oxides with hydrogen was studied by a temperature-programmed reduction technique, in situ XRD, and near ambient pressure XPS in the temperature range from 100 to 650 °C. It was shown that the reduction of the solid solutions Mn x Zr 1− x O 2− δ proceeds via two stages. During the first stage, at temperatures between 100 and 500 °C, the Mn cations incorporated into the solid solutions Mn x Zr 1− x O 2− δ undergo partial reduction. During the second stage, at temperatures between 500 and 700 °C, Mn cations segregate on the surface of the solid solution. In the samples with more than 30 at% Mn, the reduction of manganese oxides was observed: Mn 2 O 3 → Mn 3 O 4 → MnO. The reduction of the solid solutions Mn x Zr 1− x O 2− δ proceeds via two stages. During the first stage, the Mn cations incorporated into the solid solutions Mn x Zr 1− x O 2− δ undergo partial reduction. At the second stage, Mn cations segregate on the surface.