Imaging Surface Reactions of Formaldehyde on TiO$_{2}$

Formaldehyde is involved in many surface catalytic and photo-catalytic reactions on metal oxides. We studied surface reactions of formaldehyde on reduced TiO$_{2}$(110) surfaces using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen vacancy (V$_{\mathrm{O}})$ defect sites. Bias-dependent STM images suggest the bonding configurations of the Ti-bound CH$_{2}$O and the V$_{\mathrm{O}}$-bound CH$_{2}$O. The isothermal time dependent images show the rotation of V$_{\mathrm{O}}$-bound CH$_{2}$O and the two diffusion channels of formaldehyde at different temperatures. We also directly observed the formation of formaldehyde dimmer.