Molecular Coupling of Formaldehyde on Rutile TiO$_2$(110)

Molecular coupling of formaldehyde is imaged by variable temperature scanning tunneling microscope (VT-STM) on Rutile TiO$_{2}$(110) surfaces in our study. Two different molecular coupling reactions are identified via time-elapsed sequences of STM images. A formaldehyde bounded at a bridging oxygen vacancy site (V$_{\mathrm{O}}$-bound CH$_{2}$O) couples with Ti-bound CH$_{2}$O to form a static and stable diolate at room temperature and low temperatures. Also, two V$_{\mathrm{O}}$-bound CH$_{2}$O molecules couple with each other to form a Ti-bound species and desorb above 215 K. This coupling reaction heals both the V$_{\mathrm{O}}$ sites indicating formation and desorption of ethylene. In addition, we directly observe the diffusion of methylene groups to nearby empty V$_{\mathrm{O}}$ sites formed upon dissociation of the C-O bond in V$_{\mathrm{O}}$-bound formaldehyde, which suggests that the ethylene formation is via coupling of the methylene groups. These results will be discussed with previously reported chemistry of formaldehyde on TiO$_{2}$ surface.