The effect of the terminal groups on the nature of the films formed by the thermal decomposition of carboxylic acids on copper is studied in ultrahigh vacuum using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). The influence of the presence of vinyl or alkynyl terminal groups and chain length is studied using heptanoic, octanoic, 6-heptenoic, 7-octenoic, 6-heptynoic and 7-octynoic acids. The carboxylic acids form strongly bound carboxylates following adsorption on copper at room temperature, and thermally decompose between ∼500 and 650 K. Previous work has shown that this occurs by the carboxylate plane tilting towards the surface to eliminate carbon dioxide and deposit a hydrocarbon fragment. The fragment can react to evolve hydrogen or form oligomeric species on the surface, where the amount of carbon increases for carboxylic acids that contain terminal functional groups that can anchor to the surface. These results will be used to compare with the carbonaceous films formed by the mechanochemical decomposition of carboxylic acids on copper, which occurs at room temperature. This is expected to lead to less carbon being deposited on the surface than during thermal decomposition.
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Adsorption and reaction pathways of 7-octenoic acid on copper
The surface structure and reaction pathways of 7-octenoic acid are studied on a clean copper substrate in ultrahigh vacuum using a combination of reflection–absorption infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning-tunneling microscopy, supplemented by first-principles density functional theory calculations. 7-Octenoic acid adsorbs molecularly on copper below ∼260 K in a flat-lying configuration at low coverages, becoming more upright as the coverage increases. It deprotonates following adsorption at ∼300 K to form an η 2 -7-octenoate species. This also lies flat at low coverages, but forms a more vertical self-assembled monolayer as the coverage increases. Heating causes the 7-octenoate species to start to tilt, which produces a small amount of carbon dioxide at ∼550 K and some hydrogen in a peak at ∼615 K ascribed to the reaction of these tilted species. The majority of the decarbonylation occurs at ∼650 K when CO 2 and hydrogen evolve simultaneously. Approximately half of the carbon is deposited on the surface as oligomeric species that undergo further dehydrogenation to evolve more hydrogen at ∼740 K. This leaves a carbonaceous layer on the surface, which contains hexagonal motifs connoting the onset of graphitization of the surface.
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- Award ID(s):
- 2020525
- NSF-PAR ID:
- 10234885
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 23
- Issue:
- 10
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 5834 to 5844
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d1cp00167a
