Low-Temperature Reductive Coupling of Formaldehyde
on Rutile TiO<sub>2</sub>(110)
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Abstract
The
formation and coupling of methylene upon dissociation of formaldehyde
on reduced TiO<sub>2</sub>(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<sub>O</sub>) defect site. V<sub>O</sub>-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<sub>O</sub>-bound formaldehyde molecules can couple and form Ti-bound
species, which desorbs above ∼215 K. This coupling reaction
heals both V<sub>O</sub> sites indicating the formation and the desorption
of ethylene. We also directly observed the diffusion of methylene
groups to nearby empty V<sub>O</sub> sites upon dissociation of the
C–O bond in V<sub>O</sub>-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<sub>O</sub> coverage, which further supports the desorption
of the coupling reaction product, ethylene, after formaldehyde exposure
between 215 and 300 K