Photochemistry and Gas-Phase FTIR Spectroscopy of Formic Acid Interaction with Anatase Ti18O2 Nanoparticles

The photoinduced isotope exchange between O-18 atoms in the lattice of vacuum-calcinated solid (TiO2)-O-18 and O-16 atoms of formic acid and its photoproducts was studied with gas-phase high-resolution Fourier transform infrared absorption spectroscopy. The rotation-vibration absorption spectra of all products from the photochemical reactions of formic acid were measured over a broad infrared spectral range and used to quantify the time-dependent isotope exchange between the oxygen atoms on solid (TiO2)-O-18 and the oxygen atoms in gaseous (HCOOH)-O-16-O-16 and the isotopologues of CO2, CO, and H2O. It was found that formic acid did not exchange oxygen with titania during adsorption and decomposition processes; strongly bonded formate species blocked active sites and thereby inhibited the exchange between CO2 and (TiO2)-O-18. Similar blocking was observed by adsorbed water. The isotopologues (COO)-O-16-O-18 and (CO2)-O-18 are the products of the spontaneous exchange of oxygen atoms in (CO2)-O-16 and the active sites on (TiO2)-O-18 that are unblocked during the irradiation of the surface by UV photons. The (CO)-O-18 molecules are a product of the UV decomposition of (COO)-O-16-O-18 or (CO2)-O-18 that is formed during the spontaneous exchange process.