The polymerization of hydrocarbons into linear chains lies at the heart of many industrially relevant chemical reactions. One prominent example is the alkene polymerization with the Ziegler Natta catalysts, which is responsible for 2/3 of the global production of polyolefins. Long-chain hydrocarbons can also be produced from syngas (a mixture of carbon monoxide and hydrogen) through the so-called Fischer-Tropsch synthesis (FTS), occurring typically on cobalt-based catalysts. This process is experiencing a renewed interest, especially in the context of modern power-to-gas and power-to-liquid plants.
From a microscopic point of view, the identification of the complex series of reaction steps involved in the polymerization of small molecules into long hydrocarbon chains is still under debate. Surface-science techniques have proved to be extremely powerful to explore the mechanisms of heterogeneous catalysis. However, due to the harsh reaction conditions of FTS, analysis using such techniques poses a real experimental challenge.
Though the step sites of the catalytic surface are also commonly assumed to promote the C-C coupling of CHx monomers, the typical strong adsorption of small molecules at the step edges could trap the CHx species, hindering the polymerization. This behavior can be understood in the framework of the Sabatier’s principle, stating that if the adsorption energy of the substrate is too low, then the catalytic activity is suppressed; if it is too large, then the product will not desorb and blocks the surface, leading to catalyst poisoning. Therefore, elucidating the actual role of the step sites is crucial for an in-depth atomistic understanding of the hydrocarbon chain growth process.
Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X-ray photoelectron spectroscopy (XPS) performed at the SuperESCA beamline of Elettra, the intermediate species and reaction products have been directly identified. This has been enabled by the high energy resolution (about 100 meV) of the instrument, allowing resolving vibrational fine structures.
Read more on the Elettra website