Adsorption and Cyclotrimerization Kinetics of C₂H₂ at a Cu(110) Surface

Abstract

The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C 2 H 2 island formation and adsorbed C 4 H 4 , which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C 2 H 2 diffusion is determined to be 43 ± 1 kJ/mol. The activation barrier for formation of the C 4 H 4 intermediate is found to be 84 ± 6 kJ/mol and the barrier for benzene formation 5 ± 3 kJ/mol lower. Barriers to diffusion and formation of C 4 H 4 and C 6 H 6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C 2 H 2 islands is included.

Mechanisms of Graphene Growth on Metal Surfaces: Theoretical Perspectives

“…Such a possibility has been demonstrated in a surface assisted cyclo‐dehydrogenation experiment . Consistently, it was found that benzene can be formed by cyclotrimerization of C 2 H 2 on Cu(110) surface, which is an exothermic reaction with a moderate activation barrier of 0.79 eV . It is thus reasonable to assume that dehydrogenation is a reaction involved during most of or even the whole graphene growth process.…”

“…moderate activation barrier of 0.79 eV. [ 45 ] It is thus reasonable to assume that dehydrogenation is a reaction involved during most of or even the whole graphene growth process.…”

Review of CVD Synthesis of Graphene

“…Moreover, carbon dimers containing hydrogen are very unfavorable on a surface with low adsorption energies, even in defects, and desorb or immediately decompose even at very low temperatures, as demonstrated in temperature‐programmed desorption (TPD) and thermal desorption spectroscopy (TDS) experiments . Complementary studies of acetylene (CH≡CH) aromatization over transition metals also demonstrated that the benzene ring is not stable at the metallic surface . Therefore, in the case of Cu, Reaction D with z = 0 should be considered in carbon deposition, this moment being the most probable to complete the dehydrogenation and the formation of CC bonds with sp 2 ‐hybridization.…”

Adsorption of small hydrocarbons on rutile TiO2(110)

“…This linear increase in adsorption energy with the chain length has been interpreted in terms of bonding configurations in which the alkane chains lie parallel to the metal surface [2,[20][21][22][23]. For unsaturated hydrocarbons such as ethylene and acetylene, their adsorption on metal surfaces in general can be described within the Dewar-Chatt-Duncanson chemisorption model, in which the bonding is formed via a π-donation from the adsorbate into empty metal-states coupled with back-donation from the metal into the empty orbital of the hydrocarbon [24][25][26][27][28]. This bonding configuration entails a rehybridization of the carbon atoms, which results in a C-C bond elongation and an upward "bending" of the hydrogen atoms from the surface [24][25][26][27][28].…”