Desulfurization-Related Surface Chemistry on Two-Dimensional Silica Films: Adsorption of Thiophene and Short-Chain Alkanes on Silicatene

The kinetics of thiophene and alkane adsorption on silicatene (i.e., on two-dimensional atomically thin and crystalline silica films) was studied by employing (multimass) thermal desorption spectroscopy (TDS) at ultrahigh vacuum. The monoatomic SiO₂ film is grown on a Mo(112) support. Thiophene is a standard probe molecule for hydrodesulfurization (HDS); alkanes typically form as reaction products; Mo is a catalyst for HDS. The samples were characterized by Auger electron spectroscopy and low-energy electron diffraction as well as the silica film additionally by water TDS. The silicatene film is hydrophobic. All probe molecules adsorbed nondissociatively on silicatene. Two TDS peaks in the monolayer range are evident as well as a condensation structure. Thiophene binds with 42.6 ± 0.5 kJ/mol in the limit of zero coverage. Using a Redhead analysis and assumed pre-exponential of 1 × 10¹³/s, the (low coverage) binding energy of the alkanes (isobutane, n-pentane, and n-heptane) increases from 34.1 to 46.8 (±0.5) kJ/mol with the chain length. A more sophisticated data analysis (direct inversion of the Polanyi-Wigner equation) reveals a chain-length-dependent pre-exponential and binding energy. In addition, the adsorption dynamics of butane was characterized by molecular beam scattering. The adsorption probabilities and binding energies are systematically smaller for silicatene than for polycrystalline bulk silica wafer.