On-surface dehydrogenative lateral homo-coupling and aromatization of n-octane on Pt(111)

Technical Summary: On-surface dehydrogenative lateral homo-coupling and aromatization of n-octane on Pt(111)

Problem and Motivation
Aliphatic hydrocarbons, specifically normal alkanes ($n$-C$_n$H$_{2n+2}$), represent a naturally abundant carbon source. However, their intrinsic lack of reactivity due to saturated C(sp$^3$)−H bonds hinders their direct use as precursors for complex organic compounds without prior functionalization. While on-surface chemistry offers bottom-up strategies for synthesizing organic nanostructures, the controlled conversion of linear aliphatic precursors into cyclic and aromatic species remains challenging. Previous work has demonstrated linear C−C coupling on (110) surfaces but has struggled to achieve two-dimensional growth required for cyclic species due to surface ridges. Furthermore, the high temperatures needed to activate these reactions often lead to uncontrolled side reactions, such as reactant fragmentation. A fundamental understanding of the competing pathways—specifically intramolecular rearrangement (catalytic reforming) versus intermolecular coupling—is necessary to characterize the on-surface aromatization process.

Methodology
The study employs linear $n$-octane ($n$-C$_8$H$_{18}$) molecules adsorbed on a catalytic Pt(111) surface as a model system. Platinum was selected for its high selectivity toward dehydrogenation over C−C bond cleavage. The experimental approach combines:

• Ultra-High Vacuum (UHV) Deposition and Annealing: $n$-octane is deposited via physical evaporation. Samples are annealed at various temperatures (ranging from room temperature up to 1000 K) to induce thermal reactions.
• Scanning Tunneling Microscopy (STM): High-resolution and submolecular-resolution STM is used to image reactants, intermediates, and products at 77 K and room temperature. STM imaging simulations are performed to correlate experimental features with atomic structures.
• Ab Initio Calculations: Density Functional Theory (DFT) is utilized to calculate reaction intermediates, transition states (TS), and enthalpy barriers. These calculations model the reaction pathways for both intramolecular cyclization and intermolecular homo-coupling.
• Spectroscopy: Synchrotron-radiation X-ray photoelectron spectroscopy (XPS) is referenced to confirm the formation of unsaturated intermediates (n-octenyl groups) at lower temperatures.

Key Results
The investigation identifies two distinct reaction pathways occurring at temperatures above 600 K:

1. Intramolecular Cyclization and Aromatization:

   • Individual $n$-octane molecules undergo a multi-step dehydrogenation process. The reaction initiates with C−H bond cleavage at a methyl end, followed by backbone dehydrogenation to form unsaturated $n$-octenyl groups.
   • To cyclize, the linear adsorbate must bend to adopt a compact configuration. The rate-limiting step is the formation of an intramolecular C−C bond between the terminal carbon and the third carbon in the chain, associated with an enthalpy barrier of +2.8 eV.
   • This process yields a phenyl moiety, which subsequently undergoes further dehydrogenation to form ethylbenzene. Finally, the ethyl group is cleaved (via H intercalation and C$_2$H$_4$ release) to produce a planar benzene ring adsorbed on the Pt(111) hollow site.
   • The overall process is exothermic (-1.4 eV), but the high activation barrier results in a slow reaction rate, explaining the scarcity of unimolecular cyclization products compared to intermolecular aggregates in STM images.

2. Intermolecular Lateral Homo-coupling:

   • At temperatures above 650 K, thermal diffusion promotes the aggregation of adsorbates. When two suitably oriented $n$-octyl or $n$-octenyl radicals encounter each other, they undergo a lateral homo-coupling reaction.
   • The reaction initiates with the formation of a C−C bond between the activated methyl ends of two adjacent chains. This is followed by a sequential "zipper-like" propagation of C−C bond formation along the carbon backbones.
   • DFT simulations reveal that this pathway involves sequential dehydrogenation and the formation of two closed carbon rings, followed by a kinetically limiting step (barrier ≈2.8 eV) for the closure of the third fused ring.
   • The final product of this coupling is an anthracene-family molecule (specifically anthracene after the cleavage of a C$_2$H$_4$ fragment).
   • STM images confirm the formation of molecular dimers with elongated, lobed structures. Tip-induced manipulation demonstrates strong covalent interactions between the coupled chains, as the dimers can be displaced without dissociation.

Significance and Claims
The paper claims to provide molecular-level insight into the heterogeneous catalytic processes governing the generation of aromatic products and stable on-surface polycyclic species from simple alkanes. By combining STM with DFT, the authors elucidate the specific atomistic mechanisms for both the intramolecular cyclization of $n$-octane into benzene and the intermolecular homo-coupling into anthracene derivatives on Pt(111).

The study highlights that while intramolecular aromatization is thermodynamically favorable, it is kinetically hindered by high energy barriers, making intermolecular coupling a competitive and dominant pathway at elevated temperatures. The findings demonstrate that the Pt(111) surface not only catalyzes dehydrogenation but also facilitates the complex rearrangement required to transform linear aliphatic chains into polycyclic aromatic hydrocarbons (PAHs) and extended nanographenes. This work establishes a foundational understanding of the reaction networks involved in converting abundant aliphatic resources into functional aromatic nanostructures.