Atomic Resolution Scanning Transmission Electron Microscope Imaging of The In Situ Synthesis of Nanoparticle Catalysts

The shape and size of metallic nanoparticle catalysts have been shown to control the activity and selectivity for many chemical reactions. Most industrial catalysts are produced by wet impregnation and calcination, due to the scalable nature of this synthesis technique. However, a greater control of the composition, size and shape of the resulting nanoparticles is highly desirable. Understanding the interactions of metal ions with the surfaces in the wet impregnation solution, as well as better understanding of the drying behavior in hydrogen at elevated temperature would be highly desirable to support the production of the improved industrial catalysts needed to address the current energy emergency.<br/>Our group have been developing in situ 2D heterostructure cells for atomic resolution transmission electron microscopy (TEM) imaging and analysis, using a combination of graphene, hexagonal boron nitride and MoS2 layers to trap liquid and gas pockets and enable new functionality such as liquid-liquid mixing [1]. We have used these 2D heterostructure liquid cells to investigate the dynamic processes that occur at a solid-liquid interfaces as metal ions from solution interact with a solid support at atomic resolution.[2] For example to demonstrate the use of liquid phase imaging to probe the preferred resting sites for platinum atoms on molybdenum disulfide.[2]<br/>We have also used in situ gas cell TEM to investigate how the morphology is determined by the wet impregnation synthesis parameters, as well as how the starting structure and composition determines the evolution of industrial supported nanoparticle catalysts during activation heat treatment.[3]<br/>In situ TEM studies are often limited to 2D imaging, especially for highly active nanoparticle systems like PtNi, where the particle composition and morphology can be used tune the performance for the oxygen reduction reaction.[4] We have shown that the single particle reconstruction method, which is widely used in cryogenic TEM imaging of proteins, is a valuable means to probe the three dimensional structural evolution of inorganic nanoparticles.[5] This approach averages over particles present in the image with different orientation to build up a tomographic reconstruction at much lower radiation dose than is required for conventional tilt series tomography.[5] We have shown that this opens up the single particle reconstruction technique to allow 3D visualization at different time points during in a synthesis process or catalytic reaction.[6] This approach could be brought to the atomic scale through harnessing the improved imaging performance achievable with new in situ cell designs.<br/><br/>[1] Kelly et al, Advanced Materials (2021) 33, 2100668;<br/>[2] Clark et al. Nature (2022) 609, 942;<br/>[3] Prestat et al ChemPhysChem (2017), 18, 2151 and unpublished work<br/>[4] Leteba et al Nano Lett.(2021), 21, 9, 3989–3996<br/>[5] Wang et al Nano Lett. (2019), 19, 2, 732–738<br/>[6] Wang et al Small (2023) in press