The Catalytic Performance of CO Oxidation over MnOx-ZrO2 Catalysts: The Role of Synthetic Routes

Abstract

MnOx-ZrO2 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 °С; its increased activity is due to the formation of MnO2 oxide nanoparticles containing Mn4+ and low-temperature reducibility. An increase in the synthesis temperature leads to the formation of less active Mn2O3, 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 MnyZr1-yO2-x 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 Mn3O4, and a drop in catalytic activity.

Review on Medical Applications of Manganese Oxide (Mn2+, Mn3+, and Mn4+) Magnetic Nanoparticles

“…Wang et al [73] reported the preparation of different crystallographic phases γ-MnO 2 , Mn 2 O 3 , and Mn 3 O 4 at different calcination temperatures. Some of the manganese-based nanocomposites prepared using the coprecipitation method are MnO x -CeO 2 [74], MnO x -ZrO 2 [75], and Fe-Mn [76]. Thus, the size and morphology of nanoparticles prepared using coprecipitation method rely on the different reaction parameters like concentration of the precursor, ratio, and proportion of the solvent, temperature, stirring duration, calcination temperature, and duration.…”

Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2−δ Catalyst of CO Oxidation Reaction

“…The surface concentration of Mn atoms exceeded the stoichiometric value by 8 and 20% for the fresh and treated catalyst, respectively. For Mn x Zr 1−x O 2−δ oxide the surface was typically enriched with manganese cations, probably due to the unusual coordination of the Mn cation in the fluorite structure [37,63]. Along with the surface segregation of Mn cations, the average oxidation state of the Mn cations slightly increased from 2.77 to 2.84 after the pretreatment, due to the decrease in Mn 2+ and the increase in Mn 4+ cation fractions.…”

“…To eliminate the interfering influence of the presence of different phases, a single-phase mixed oxide containing a smaller amount of manganese Mn 0.2 Zr 0.8 O 2−δ was utilized to determine the correlations between activity and the nature of the active phases. The Mn 0.2 Zr 0.8 O 2−δ solid solution was treated in air, resulting in the partial decomposition of the initial oxide and exsolution of Mn ions from the lattice with the formation of highly dispersed MnO x on the surface of the oxide support [37]. Such an approach to catalyst preparation based on the decomposition of a solid solution has been developed recently [38].…”

“…Usually, the driving force for the exsolution process is a change in oxidation state and, correspondingly, the ionic radius of guest elements. In the case of Mn x Zr 1−x O 2−δ , the driving force of catalyst decomposition during calcination in air is the change in the oxidation state of the Mn cation in the volume of mixed oxide [37]. Pretreatment in a reductive atmosphere accelerates the reduction of the Mn cation to Mn 2+ and the migration of Mn cations to the surface of the mixed oxide [36].…”

Unraveling the Structural and Compositional Peculiarities in CTAB-Templated CeO2-ZrO2-MnOx Catalysts for Soot and CO Oxidation

“…A promising alternative comprises the catalysts based on oxides of transition metals (pristine and mixed oxides [8,9], spinels [10][11][12]). These materials combine a number of advantages, including availability, thermal stability, improved service period, resistance to catalytic poisons, etc.…”