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Evaluation of Molecular Fragmentation in Polycyclic Aromatic Hydrocarbons by Time-of-Flight Secondary Ion Mass Spectrometry
RISE Research Institutes of Sweden, Materials and Production, Product Realisation Methodology.ORCID iD: 0000-0002-2696-7215
ExxonMobil Research and Engineering Company, USA.
Instituto Mexicano del Petroleo, Mexico.
Schlumberger-Doll Research, USA.
2023 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 37, p. 7071-Article in journal (Refereed) Published
Abstract [en]

In time-of-flight secondary ion mass spectrometry (TOF-SIMS), ionized molecules and molecular fragments (secondary ions) are generated in collisions of high-energy ions (primary ions) with a solid sample surface. Mass spectra of the emitted secondary ions are typically used to identify molecular species and to determine their spatial distribution on the sample surface. Here, we extend this application in a TOF-SIMS study of a series of polycyclic aromatic hydrocarbons (PAHs) where we focus on the fragmentation of these molecules, with the purpose of better understanding the fragmentation patterns of heavy aromatic molecules in petroleum. For all PAHs, the collision process generated (i) a series of smaller cation fragments and (ii) cations close in size to the original PAH (molecular cations). Stark differences are measured for various PAHs regarding the abundance of smaller fragments versus molecular cations. Observation of hydrogen-deficient (H-deficient) cation fragments indicates the formation of polyynes and allenes. For PAHs producing higher fractions of small cation fragments, these ions are surprisingly hydrogen rich (H-rich). The H/C ratio of fragments does not scale with the fraction of Clar sextet carbon, nor with energies of low-lying electronic transitions. Free radical cation fragments tend to be suppressed. For sufficiently large fragments, aromatic cations appear to be formed and include some free radical aromatics. There is ample production of molecular ions with loss of a single carbon atom or a methine group, which corresponds to the reduction of a 6-membered aromatic ring to a 5-membered ring. There is some enhancement of free radical molecular cations due to the corresponding formation of neutral polyynes. Fragment anions are also produced with a strong preference for very H-deficient carbon clusters, in some cases being the same as carbon cluster anions observed in space. Comparisons of PAH TOF-SIMS spectra with those of asphaltenes are discussed in detail.

Place, publisher, year, edition, pages
American Chemical Society , 2023. Vol. 37, p. 7071-
Keywords [en]
Aromatization, Free radicals, Hydrogen, Ion sources, Molecules, Organic polymers, Positive ions, Secondary emission, Secondary ion mass spectrometry, High-energy ions, Ionized molecules, Molecular cations, Molecular fragmentation, Molecular fragments, Polyynes, Primary ions, Sample surface, Secondary ions, Time of flight secondary ion mass spectrometry, Polycyclic aromatic hydrocarbons
National Category
Analytical Chemistry
Identifiers
URN: urn:nbn:se:ri:diva-64952DOI: 10.1021/acs.energyfuels.3c00098Scopus ID: 2-s2.0-85159623217OAI: oai:DiVA.org:ri-64952DiVA, id: diva2:1764932
Note

 Vetenskapsrådet, VR, 2019-03731; Funding text 1: Financial support for this work was provided by the Swedish Research Council, grant no 2019-03731 (PS).

Available from: 2023-06-09 Created: 2023-06-09 Last updated: 2023-06-09Bibliographically approved

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Sjövall, Peter

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