Low-Temperature Reductive Coupling of Formaldehyde on Rutile TiO₂(110)

Abstract

The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO₂(110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (V_O) defect site. V_O-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two V_O-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above ∼215 K. This coupling reaction heals both V_O sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty V_O sites upon dissociation of the C–O bond in V_O-bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction products on the surface can only account for a half of the consumption of the initial V_O coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposure between 215 and 300 K