Combined (S)TEM-FIB Insight into the Influence of the Preparation Method on the Final Surface Structure of a Co3O4/La-Modified-CeO2 Washcoated Monolithic Catalyst

A detailed structural and chemical characterization study has been performed, both at the micrometer and nanometer scale levels, throughout the different steps of preparation of a catalytic device consisting in Co3O4/La-modified-CeO2/Al2O3 powders washcoated onto a honey-comb-type cordierite monolith. High-resolution transmission electron microscopy (HREM) images obtained from the initial catalyst powder allowed us identifying the formation of 50-100 nm crystallites of a Co3O4 phase in contact with bundles of much smaller CeO(2)nanocrystallites. For the catalytic device, backscattered electron images and X-ray energy dispersive spectroscopy (XEDS) elemental maps acquired in a scanning electron microscope (SEM) provided quantitative estimate of key parameters of the washcoated monolith as the thickness of the catalyst coating and the percentage of the monolith internal surface covered by the active catalytic components. Electron transparent cross-section lamellae of the monolith coating were also prepared by the focused ion beam (FIB) technique. Subnanometer resolution (scanning) transmission electron microscopy (STEM)-XEDS elemental maps acquired from the FIB lamellae revealed the occurrence of a final patchy-like surface structure, which could not be expected on the basis of the structural information obtained for the initial powder, made up by two types of chemically distinctive regions: one corresponding to a two-layer structure which exposed Co3O4 nanocrystallites at the top, in size as those in the powder, and large ceria aggregates underneath, and a second type of regions constituted only by aggregates of bare ceria nanocrystals. This component separation imposes a well-defined structure and composition to the surface of the monolith, which the most routinely used SEM characterization techniques failed to detect. Further comparison with STEM data coming from the initial catalyst powders and the calcined washcoating suspension suggests that this particular structure is linked to processes taking place both during the impregnation and drying of the washcoating suspension on the monolith walls. This study reveals the key role of the combined FIB-STEM approach to monitor the eventual influence of the preparation steps on the actual surface of catalytic monolithic devices-a question not properly considered up to now.