Graphite-Mediated Oxidation of Coronene Adsorbates: A UHV Study

Abstract

Thin Coronene films on HOPG were oxidized by exposing them to atomic oxygen under ultra-high vacuum conditions. The products were probed by mass-resolved thermal desorption spectroscopy and X-ray photoelectron spectroscopy. The species subliming from thick oxidized films comprise predominantly epoxides C24H12O n (n ≤ 7), and the oxidation of monolayers yields different sublimable products, notably lactones and dilactonesthus providing clear evidence for a surface-mediated reaction mechanism. After on-top oxidation, roughly 7% of the Coronene monolayer can be sublimed as oxidized derivatives (limited by competing surface reactions, which lead to nondesorbable products). The desorption yield can be raised to 10% by depositing the Coronene monolayer onto preoxidized HOPG. On the basis of our observations and literature results concerning HOPG oxidation, we argue that lactones and dilactones can be generated by the reaction of Coronene adsorbates with epoxy and ether species formed on the graphite support. Our results obtained for the O/Coronene/HOPG model system may be relevant for the mechanism of carbon-based oxidation catalysis in general.

Comparative Study of Direct and Graphite-Mediated Oxidation of Large PAHs

“…For HBC monolayers, all oxides desorb in the same temperature range as unreacted HBCslightly below 750 K. Since the HOPG surface appears not to be significantly involved in the oxidation of HBC monolayers, it is not surprising that several epoxide products show desorption yields above 10% (see Figure 7B), which is significantly higher than that found for coronene (sub)monolayers. 17 In preoxidation experiments (Figure 7C), on the other hand, HBC behaves mostly like coronene. The dominant oxide is the lactone b, and we observed traces of quinones/dilactones (c + d).…”

“…We have recently probed the oxidation of thin coronene (C 24 H 12 ) films using mass-resolved temperature-programmed desorption (TPD) to characterize the volatile oxidic products generated under these conditions. 16,17 Interestingly, we observed significant amounts of intactly desorbing coronene oxides.…”

“…51 All thin-film samples were prepared by mass-selective ion beam soft-landing with the same ultra high vacuum (UHV) setup used for the coronene studies. 16,17 Briefly, the desired PAH was sublimed from a Knudsen effusion cell and subsequently ionized by electron impact (E el ≈ 70 eV). The resulting mixed molecular beam was split into anions, neutral molecules, and cations using an electrostatic deflector.…”

“…Detailed examples of this procedure can be found in our previous work on coronene. 16,17 In the latter study, 16 O/ 18 O labeling allowed us to identify epoxides, ethers, lactones, and quinones as the major reaction products of coronene with atomic oxygen (for a depiction of the corresponding molecular structures, see Figure 5). Up to seven oxygen atoms were observed to be sequentially addedwith addition sometimes also associated with loss of CO and H 2 .…”

“…Interestingly, (sub)monolayers (MLs) of coronene, also treated with O atoms in the same way, reacted to fundamentally different products than the thick films. 17 Based on oxygen isotope labeling experiments, we were able to assign these products as lactones (predominantly 2H-benzo [6,7]perylo [1,12-bca]pyran-2-one) and quinones (most likely 1,2-coronendione). 3.…”