Real-time observation of hydrocarbon polymerization

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 CH_x monomers, the typical strong adsorption of small molecules at the step edges could trap the CH_x 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.

Fig.1a (top) shows the C 1s spectrum acquired after exposing the Ni(111) surface to 45 Langmuir (L) of ethylene at 343 K. A main peak at 283.3 eV is observed, together with two features at higher binding energies, shifted by ≈ 350 meV and ≈ 700 meV, respectively. The main peak corresponds to the acetylene adiabatic transition, while the other components are attributed to transitions to excited vibrational states. Low-temperature STM (LT-STM) images reveal the presence on the Ni(111) surface of adsorbed acetylene molecules (Fig.1b). These results indicate the surface-catalyzed partial dehydrogenation of ethylene into acetylene. Fig.1a (bottom) shows additional XPS components at 283.9 eV and 284.5 eV occurring at a much larger ethylene exposure (> 600 L). LT-STM images reveal the presence of adsorbed acetylene, co-adsorbed with the elongated chains (Fig.1c), revealing that the two newly arising XPS components could be ascribed to the formation of polyacetylene oligomers (C₂H₂)_n. The two C 1s components at 283.9 eV and 284.5 eV (Fig.1a, bottom) are attributed to carbon atoms within the formed chains and to the passivated end of the oligomer chains, respectively.

Figure 1: Growth of hydrocarbon chains upon ethylene partial dehydrogenation into acetylene (C₂H₂) and subsequent polymerization on Ni(111) at 343 K. (a) High-resolution XP spectra of the C 1s core level after different doses of ethylene. (b,c) LT-STM images acquired in the same conditions used for the XPS experiments in (a).

To elucidate the mechanism responsible for the polyacetylene chain growth, we performed in-situ scanning tunneling microscopy (STM) studies, showing that polymerization does not proceed by chain formation at the substrate step sites, but occurs almost exclusively on the flat terraces. At step sites, hydrocarbon chains form, but remain anchored and therefore do not detach. The observed inactivity of the step sites for the growth of the polyacetylene oligomers is attributed to a strong binding of the hydrocarbon fragments, causing self-poisoning.

Complementary near-ambient pressure XPS experiments show remarkably resembling products of the on-surface reaction, indicating that the self-poisoning of the step edges of the catalyst observed under UHV conditions may be valid up to the mbar pressure regime. As the Ni(111) and Co(0001) surfaces exhibit similar catalytic properties in promoting C-C coupling and hydrocarbon dehydrogenation, the reported acetylene growth mechanism could be relevant also for cobalt-based catalysts.

This research was conducted by the following research team:

Zhiyu Zou¹, Alessandro Sala^1,2, Mirco Panighel¹, Ezequiel Tosi³, Paolo Lacovig³, Silvano Lizzit³, Mattia Scardamaglia⁴, Esko Kokkonen⁴, Cinzia Cepek¹, Cristina Africh¹, Giovanni Comelli^1,2, Sebastian Günther⁵ and Laerte L. Patera^5,6

¹ CNR-IOM Materials Foundry Institute, Trieste, Italy
² Department of Physics, University of Trieste, Trieste, Italy
³ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁴ MAX IV Laboratory, Lund University, Lund, Sweden
⁵ Department of Chemistry, Technical University of Munich, Garching, Germany
⁶ Institute of Physical Chemistry, University of Innsbruck, Innsbruck, Austria

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