WEB Direct insight into the reactivity of Pt nanoparticles in CO Oxidation by operando TEM and the impact of electron dose rate on their coarsening
In general, heterogeneous catalysts are metastable materials that adapt to the chemical potential of the environment. The in situ formed phases fluctuate locally and are often only stable under working conditions. Recent developments in operando transmission electron microscopy (TEM) techniques allow to study catalysts in their working environment and to visualize on the local scale these in situ generated structures. However, the applied electron dose rates needed to pinpoint these changes in different gas environments, in particular for high resolution imaging, could have a significant impact on the final observations. In this study, we will show the influence of different morphologies and sizes of Pt nanoparticles (NPs) on their performances in the oxidation of CO and unravel the response of Pt NPs in different gaseous environments, such as CO, O2 and gas mixtures of (CO:O2:He = 1:5:19) to different electron beam dose rates. The investigated Pt NPs were prepared either in situ by the thermal decomposition of tetraamineplatinum(II) nitrate at 400°C (method 1) or ex situ by sputtering the Pt NPs directly on the microelectromechanical systems (MEMS) chip (method 2). Using operando TEM approach we were able to compare conversion rates in CO oxidation reaction of two differently prepared Pt catalysts. Pt NPs prepared by method 1 were found to have a very broad size distribution and irregular shapes, while the ones prepared by method 2 exhibited more spherical shapes with rather uniform and narrow size distribution over the entire area. Pt NPs prepared by method 2 were found to be more active and reach full conversion at 400°C compared to Pt NPs prepared by method 1 where a maximum conversion of 63% was achieved. The ignition point seems to be almost independent of the method of preparation and was found at temperatures of 382°C and 392°C respectively. Subsequent HAADF STEM imaging was conducted to study influence of the electron beam dose rates on Pt NPs prepared by method 2 at 400°C in the pressure range between 700 and 1000 mbar. The results show that Pt NPs prepared by method 2 are most stable in a pure oxygen environment. This is evidenced by the much higher dose rate that causes changes in the NPs perimeter and density, compared to environments consisting of pure CO or gas mixture of CO, O2 and He. In summary, the size and shape of Pt NPs were found to be essential for the conversion in MEMS cells, but leave the ignition point almost unaffected.