In-depth structural and analytical study of the washcoating layer of a Mn-Cu monolithic catalyst using STEM-FIB, EDX and EELS. Insights into stability under working conditions

Abstract

The texture, structure, composition, and electronic state of new and used monolithic Mn-Cu catalysts were studied. Characterisation was performed at different scales and spatial resolution by combining STEM-FIB and imaging techniques (STEM-HAADF) and spectroscopy (STEM-EDX and EELS of space resolution). A thickness of the alumina layer that varies between 1 and 2 µm was observed, while the thickness of the Mn-Cu catalyst layer varies linearly with the number of charges between 1 µm and 2 µm, for 1 and 2 charges, respectively. In both fresh and used catalysts, the migration of both phases was observed to a different extent, depending on the number of charges of the Mn-Cu phase. This was associated with the synthesis conditions or the catalytic behaviour in the combustion of the n -hexane. The STEM-EDX analyses showed a homogeneous distribution of Mn and Cu, with values like the theoretical one. At the nanoscale, the SR-EELS analyses detected the presence of nanometric patches with lower Cu content, chemically inhomogeneous with a higher Mn oxidation state than the rest of the material. The advanced characterisation techniques used in this work allowed the precise identification of part of the constitutive phases of the Mn-Cu system generated in-situ on the monolith. Among them, manganese oxides with different oxidation states (MnO, Mn3O4, Mn2O3), Mn3Cu3O8 and CuO were detected.