Unsupported nanostructured Mn oxides obtained by Solution Combustion Synthesis: Textural and surface properties, and catalytic performance in NOx SCR at low temperature

• Nanostructured MnOx catalysts for low temperature NOx SCR were obtained by SCS.
• Different calcination temperatures and fuel/oxidizer ratios were investigated.
• The developed MnOx materials gave good performance as unsupported catalysts.
• The activity was mostly affected by surface Mn oxidation and labile oxygen species.

Unsupported nanostructured MnOx catalysts obtained by SCS were tested in NOx SCR in the low temperature range. The (satisfactory) catalytic behaviour of the SCS samples has been analysed and explained by considering some physico-chemical properties of the synthesized catalysts, as studied by X-Ray powder Diffraction; N2 isotherms at −196 °C; Field Emission Scanning Electron Microscopy; X-ray photoelectron Spectroscopy, Temperature Programmed Reduction and NH3 Temperature Programmed Desorption. Three commercial (microstructured) MnOx powders were used as reference of pure phases. It has been shown that the catalytic behaviour of the studied catalysts mostly depends on the average oxidation state of surface Mn species, rather than on the specific surface area of the catalysts. The most active and selective catalyst results to be a sample obtained at 400 °C in “fuel-rich” conditions, which was characterized by the highest Mn4+/Mn3+ ratio and, accordingly, exhibited the highest content of surface oxygen (Oα) species.

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