| Summary: | MnOx-ZrO<sub>2</sub> catalysts prepared by co-precipitation and vacuum impregnation were calcined at 400–800 °C and characterized by powder X-ray diffraction, textural studies, high-resolution transmission electron microscopy, temperature-programmed reduction, X-ray absorption near edge structure, and X-ray photoelectron spectroscopy. The catalytic activity was tested in the CO oxidation reaction. The activity of the co-precipitated samples exceeds that of the catalysts prepared by vacuum impregnation. The characterization studies showed that the nature of the active component for the catalysts obtained by co-precipitation differs from that of the catalysts obtained by impregnation. In the impregnation series, the most active catalyst was obtained at a temperature of 400 °C; its increased activity is due to the formation of MnO<sub>2</sub> oxide nanoparticles containing Mn<sup>4+</sup> and low-temperature reducibility. An increase in the synthesis temperature leads to the formation of less active Mn<sub>2</sub>O<sub>3</sub>, catalyst sintering, and, accordingly, deterioration of the catalytic properties. In the case of co-precipitation, the most active CO oxidation catalysts are formed by calcination at 650–700 °C in air. In this temperature interval, on the one hand, a Mn<sub>y</sub>Zr<sub>1−y</sub>O<sub>2−x</sub> solid solution is formed, and on the other hand, a partial separation of mixed oxide begins with the formation of highly dispersed and active MnOx. A further increase in temperature to 800 °C leads to complete decomposition of the solid solution, the release of manganese cations into Mn<sub>3</sub>O<sub>4</sub>, and a drop in catalytic activity.
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