Operando-SPM observation of the polyethylene formation
First direct in-situ experimental evidence of the ethylene polymerization pathway at the molecular level was observed by Guo et al., Science 375, 1188–1191 (2022). This was achieved by making use of the extreme mechanical stability of the SPM Aarhus 150 NAP, its fast data acquisition and the perfect tunability of the environment inside of its operando reactor cell.
Polyethylene is a common polymer material. Just look around and you will see containers, bottles, bags, toys, etc manufactured from this material everywhere. It consists of ethylene molecules, in which the monomers are linked together in the presence of a solid catalysts enabling a complex polymerization reaction. To understand these reaction pathways, research has been focusing on the determination of the formation rate and the composition of the resulting polymer. Spectroscopy has already widely been used to understand the catalysts. However extracting information about the processes on the individual, molecular connection sites, known as active centers, remained a challenge. Guo et al. have now demonstrated operando scanning tunneling microscopy (STM) to be perfectly suited by use of a model catalyst to visualize the processes on the active centers microscopically. To the paper on science.
Operando-STM and XPS experiments were performed using a multimethod SPECS system, specially designed for near ambient pressure (NAP) applications. The Iron carbide surface was characterized by performing angle resolved XPS experiments to check the chemical state and to estimate the thickness of the grown iron carbide overlayer. The sample has been kept at room temperature and exposed to ethylene gas at different pressures while continuously recording STM data and observing in-situ formation and growth of chains. STM measurements revealed that polymerization proceeds on a specific triangular site at the boundary between two carbide domains.