Nanostructured MnOx as highly active catalyst for CO oxidation

Non-stoichiometric Mn-oxides (MnOx and MnOy) were prepared by temperature-programmed oxidation (TPO) of Mn-oxalates, MnC2O4·3H2O and MnC2O4·2H2O. Both oxides provide high specific surface areas (525 m2 g−1 and 385 m2 g−1, respectively) and identical CO oxidation reaction rates of 10−2 molecules nm−2 s−1 (0.017 μmolCO m−2 s−1) at 298 K. A “spinodal” transformation of oxalates into oxides was observed by transmission electron microscopy (TEM). The quantitative evaluation of TPO and temperature-programmed reduction with CO allowed x-values of 1.61, … , 1.67 to be determined for MnOx. The Mn oxidation state in MnOx was found to be 3.4 ± 0.1 by X-ray absorption near-edge structure analysis and X-ray photoelectron spectroscopy. In accordance with the high specific surface area and mixed-type I/IV adsorption isotherms of MnOx, high resolution TEM demonstrated the occurrence of nested micro-rod features along with nanocrystalline particles in the endings of the rods. After CO oxidation MnO and Mn3O4 phases were able to be identified in the regions between rods.

Non-stoichiometric Mn-oxide (MnOx) with high specific surface area (525 m2 g−1) shows high catalytic activity in CO oxidation reaction.Figure optionsDownload high-quality image (72 K)Download as PowerPoint slideHighlights
► Non-stoichiometric Mn-oxide, MnOx (x = 1.61–1.67) has been prepared via Mn-oxalate precipitation followed by temperature-programmed oxidation.
► MnOx shows very high specific surface area (525 m2 g−2) and is active in CO oxidation reaction below room temperature.
► Micro-XANES and XPS reveal the oxidation state of MnOx to be ∼3.4 ± 0.1.
► MnOx shows structural features similar to those of Mn5O8. In addition to micro-rods, typical for Mn5O8, HRTEM identifies nanocrystalline particles in the endings of the rods.
► MnOx has high potential for applications in oxidation catalysis.