Thermal dehydrogenation of n-alkane on Au(111) and Pt(111) surface

Thermal dehydrogenation of long n-alkane on Au(111) and Pt(111) surfaces is investigated by near the carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS) and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). On Au(111), dehydrogenation and formation of unsaturated bonds start at temperatures above 480 K, ending at 650 K. This is confirmed by the decrease in the 1 s → σ*CH/R resonance (σ*CH/R resonance) and the evolution of the π*CC resonance in the C K-NEXAFS spectra. The aliphatic or non-aromatic unsaturated bonds are generated at the early stage of the dehydrogenation on the Au(111), which is examined by the reaction with bromine. The dual peaks of the π*CC resonance at 284–286 eV suggest the formation of polycyclic aromatic hydrocarbons (PAHs). These PAHs exhibit a ribbon or rosary and planer shape by heating at 800 K, which is observed by the STM. In contrast, dehydrogenation begins at RT on Pt(111), and is almost completed at 480 K. π*CC resonance exhibits a main peak at 286 eV, which corresponds to a graphene skeleton, with a sub peak or a shoulder at 284.6 eV, which is attributed to a carbon atom at an edge or defect. The main peak intensity increases as the temperature is raised up to 820 K. STM images obtained after heating at 820 K show nanodots having a diameter of ca. 2–3 nm. The final products upon heating to 1000 K on Pt(111) are graphene nanoflakes with a size of tens of nanometers. The difference between the products is related to the difference between the reaction pathways on Au(111) and Pt(111) surfaces.