Enabled by DENSsolutions Climate system in correlation with other TEM characterization techniques
Noble metal nanoparticles stabilized on oxide supports are an important class of catalysts that are used in many applications such as fuel cells, exhaust gas treatment and energy conversion. It is well known that an interaction occurs between the nanoparticles and their oxide support which affects the catalytic activity called ‘strong metal-support interaction’ (SMSI). SMSI is a surface phenomena in which the migration of partially reduced oxide species, from the oxide support, covers the nanoparticle and thereby alters the chemisorption and catalytic properties. It can give rise to desired synergistic effects and increased selectivity. Now, using in situ TEM combined with other analytical techniques and theoretical modelling, researchers at ETH Zurich have been able to create a real time view of the SMSI phenomena.
Controlling the sample environment
Reductive pre-treatment of catalysts by heating, resulting in SMSI, has been known to alter the selectivity of oxide supported nanoparticles since the late 1970’s. However, the exact influence of the different parameters like temperature and gas concentration were still unknown. But now, thanks to the DENSsolutions Climate G+ system, researchers are able to determine the immediate effect of these parameters in increasing detail. The Climate G+ system provides a nano-reactor, containing the catalyst sample, that can be placed in any TEM* and gives the researcher unprecedented control over the sample environment in terms of temperature and gas parameters.
The in situ TEM experiments performed for this research required multiple switching between hydrogen and oxygen environments at 600 °C. This made the Climate G+ system, that is used on the JEM-ARM 300F at ETH Zurich, ideal for this research.
Evolution and dynamic structural changes of the overlayer in SMSI. A platinum particle on a titania support in the first exposure to H2 at 600 °C (a,b) and the subsequent atmosphere change to O2 at 600 °C (c), a switch to H2 (d) and then a switch to O2 again (e), and interpretation of the phenomena based on the combined results of in situ transmission electron microscopy, in situ X-ray photoemission spectroscopy, and in situ powder X-ray diffraction (f–j). Insets for c–e show a magnified image of the overlayer structure observed. Scale bar is 5 nm.
In situ TEM, using gas and heating, is a powerful characterization technique to obtain atomistic, real time, information about the SMSI phenomena. To derive a holistic view of SMSI and the role of hydrogen and oxygen within this process. The in situ TEM results have been combined with ambient pressure XPS and in situ powder XRD experimental results. Finally, theoretical density functional theory (DFT) modelling was used to support the conclusions about how SMSI actually works.