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.

Graphical abstractNon-stoichiometric Mn-oxide (MnOx) with high specific surface area (525 m2 g−1) shows high catalytic activity in CO oxidation reaction.Figure optionsDownload full-size imageDownload 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.