Thermal chemistry and photochemistry of hexafluoroacetone on rutile TiO2(110)

Abstract

The ultraviolet (UV) photon-induced decomposition of hexafluoroacetone ((CF₃)₂CO; HFA) adsorbed on the rutile TiO₂(110) surface was investigated using photon stimulated desorption (PSD) and temperature programmed desorption (TPD). HFA adsorbs both molecularly and dissociatively on the reduced TiO₂(110) surface. The initial ∼0.2 ML (where 1 ML equates to the cation site density of the ideal surface) coverage of HFA thermally decomposes resulting in the formation of adsorbed trifluoroacetate groups, with further HFA exposure resulting in molecular adsorption. No evidence was found for HFA photochemistry on the reduced surface. HFA adsorbed and desorbed molecularly on a pre-oxidized TiO₂(110) surface with only a minor amount (∼1%) of thermal decomposition in TPD. A new adsorption state at 350 K was assigned to the reversible formation of a photoactive HFA-diolate species [(CF₃)₂COO]. UV irradiation depleted the 350 K state, resulting in evolution of CF₃, CO, and CO₂ in the gas phase and formation of surface bound trifluoroacetate groups. ¹⁸O isotope scrambling experiments showed that the ejected CO₂ was from photodecomposition of the HFA-diolate species while the CO photoproduct was not. These results are in contrast to the photochemical behavior of acetone, butanone and acetaldehyde on TiO₂(110), where UV irradiation resulted in the gas phase ejection of one of the carbonyl substituent groups as well as a stoichiometric amount of carboxylate left on the surface. We conclude that fluorination alters the electronic structure of adsorbed carbonyls on TiO₂(110) in such a way as to promote complete fragmentation of the adsorbed carbonyl complex to form gas phase CO₂ as well as to open up additional photodissociation pathways leading to CO production.