Tracking Site-Specific C–C Coupling of Formaldehyde Molecules on Rutile TiO₂(110)

Direct imaging of site-specific reactions of individual molecules as a function of temperature has been a long-sought goal in molecular science. Here, we report the direct visualization of molecular coupling of formaldehyde on reduced rutile TiO₂(110) surfaces as we track the same set of molecules using scanning tunneling microscopy (STM) when the temperature is increased from 75 to 170 K. Our previous study showed that formaldehyde preferably adsorbs at bridging-bonded oxygen (O_b) vacancy (V_O) defect site. Herein, images from the same area as the temperature is increased show that V_O-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate intermediate. Consequently, exposure of formaldehyde at room temperature leads to diolate as the majority species on the surface, and no V_O-bound formaldehyde is observed. The diolate species are the key reaction intermediates in the formation of ethylene reported in previous ensemble-averaged studies