PNNL

Abstract

The formation and the coupling of methylene upon dissociation of formaldehyde on reduced TiO2(110) are studied using variable temperature scanning tunneling microscope (STM). In agreement with prior studies, formaldehyde preferably adsorbs to bridging-bonded oxygen vacancy (VO) defect site. VO-bound formaldehyde couples with Ti-bound CH2O forming a diolate species, which appears as the majority visible product on the surface at 300 K. Here, STM images directly visualize another low-temperature coupling reaction channel. Two VO-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above ~215 K. This coupling reaction heals both the VO sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty VO sites upon dissociation of the C-O bond in VO-bound formaldehyde, which suggests that the ethylene formation is via coupling of the methylene groups. Statistical analysis shows that the total visible reaction products observed on the surfaces can only account for a half of the consumption of the initial VO coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposures at 215 K and 300 K.