Binding of Formic Acid on Anatase TiO2(101)

The adsorption of formic acid (FA) and the formation of formate species on metal oxide surfaces are of great interest in catalysis. Formic acid is used to probe the adsorption properties of surface sites, and formates are common intermediates in many catalytic reactions. Here we focus on the interaction of FA with a prototypical anatase TiO2(101) surface by using a combination of scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), electron-stimulated desorption (ESD), and density functional theory (DFT) to assess the coverage-dependent evolution of different FA-derived surface species at low temperatures (80-240 K). We find that isolated FA adsorbs at 80 K in both monodentate (MD) and bidentate (BD) configurations on top of undercoordinated Ti sites (Ti-5c). The MD form is likely deprotonated and readily converts upon annealing to deprotonated bridging BD on two neighboring Ti-5c sites. DFT calculations show that molecularly bound MD FA is metastable and readily converts to a deprotonated state in the proximity of subsurface oxygen vacancy. The stability of the MD species increases as the availability of the paired Ti-5c sites for BD formation diminishes at high coverages. Upon surface saturation, a mixed configuration of alternating MD and BD species with a coverage of 2/3 FA/Ti-5c represents the most favorable configuration. This is in contrast with the adsorption of FA on rutile TiO2(110) and many other oxides, where bidentate formate species dominate.