Coupling Reactions of Formaldehyde on Rutile TiO$_{\mathrm{2}}$(110)

We have studied reactions of formaldehyde on reduced rutile TiO$_{\mathrm{2}}$(110) surfaces using scanning tunneling microscope (STM). Images from the same area at viable temperatures (75 $-$ 170 K) show two coupling reactions channels. Formaldehyde adsorbed on bridging-bonded oxygen vacancy (V$_{\mathrm{O}})$ couples with Ti-bound CH$_{\mathrm{2}}$O form a diolate species, which stays stable at room temperature. Consequently, exposure of formaldehyde at room temperature leads to diolate as the majority species on the surface and no V$_{\mathrm{O}}$-bound formaldehyde is observed. STM images directly visualize a second coupling reaction channel, a low-temperature channel. Two V$_{\mathrm{O}}$-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above 215 K. This coupling reaction heals both the V$_{\mathrm{O}}$ sites indicating formation and desorption of ethylene. 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 V$_{\mathrm{O}}$ coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposures between 215 and 300 K.