S. Moldovan, S. Carenco, D. Porthault, C. Sanchez, J.-J. Gallet, F. Bournel, G. Olivieri, O. Ersen
Institut de Physique et Chimie des Matériaux de Strasbourg,
Keywords: TEM, nanocatalysts
Summary:Core-shell structures constituted by individual cobalt borides (CoB) nanoparticles embedded in boride shells are potential candidates for methantion reactions. However, only little is known on this system characteristic, particularly its behavior under reaction conditions. As compared to cobalt nanoparticles which exhibit rather constant catalytic activity, in the case of CoB nanostructures, one assists at a constant evolution of the activity, which cannot be entirely explained by the presence of Co atoms and/or metallic clusters on the surface of the boride shell. The environmental transmission electron microscopy under its different modes (TEM, STEM-HAADF, etc) offers nowadays elegant solutions for assessing the materials behavior under reactions conditions, with particular applications in catalysis. When combined with other « operando » methodologies such as XPS or XAFS, it pictures a complete image of systems behavior from a morphological, structural and chemical point of views. As applied to CoB, the study of system changes under different reduction and/or reactional conditions allows particularly exploring the mechanisms of catalyst activation and comportment during reaction. Under Ar flow at 150°C, the system shows the presence of Co atoms and atomic clusters within the boride shell. The hydrogen flow treatment at 400°C leads to the expected Co reduction, accompanied by Co fragmentation. The reduced Co nanoparticles migrate from the agglomerates centers to the outer boride shells. Under CO2 flow at 400°C the microstructure remains stable; the initial microstructure is not recovered. These observations explain the increasing catalytic activity of the CoB-B core shell systems and backup the complementary operando spectroscopy analyses carried out under the Near-Ambient Pressure XPS de la ligne TEMPO B au synchrotron SOLEIL which anticipated the CoB decomposition during reaction. These findings show the high potential of the operando methodologies for probing materials behavior in the course of reactions. Among the state of the art techniques, the Environmental TEM offers the huge advantage of being able to provide a not only complete image of the systems microstructural evolution under reactions, but a complete chemical analysis of the systems involved, via the spectroscopy analyses or the set-up of residual gas analyzers for the reaction products.