Photochemistry of 1,1,1-Trifluoroacetone on Rutile TiO2(110)
The ultraviolet (UV) photon-induced photodecomposition of 1,1,1-trifluoroacetone (TFA) adsorbed on the rutile TiO2(110) surface has been investigated with photon stimulated desorption (PSD), temperature programmed desorption (TPD) and density functional theory (DFT). TFA adsorbed molecularly on the reduced surface (8% oxygen vacancies) in states desorbing below 300 K with trace thermal decomposition observed in TPD. Adsorption of TFA on a preoxidized TiO2(110) surface (accomplished by pre-exposure with 20 L O2) led to formation of a new TFA desorption state at 350 K, assigned to decomposition of a TFA-diolate species ((CF3)(CH3)COO). No TFA photochemistry was detected on the reduced surface. UV irradiation of TFA on the oxidized surface depleted TFA in the 350 K state, with TFA molecules in other TPD states unaffected. PSD measurements reveal that both carbonyl substituents (CH3 and CF3), as well as CO, were liberated during UV exposure at 95 K. Post-irradiation TPD showed evidence for both acetate (evolving as ketene at 650 K) and trifluoroacetate (evolving as CO2 at 600 K) as surface-bound photodecomposition products. The CO PSD product was not due to adsorbed CO, to mass spectrometer cracking of a CO-containing PSD product, or from background effects, but originated from complete fragmentation of an unidentified adsorbed TFA species. Thermodynamic analysis using DFT indicated that the photodecomposition of the TFA-diolate was likely not driven by thermodynamics alone as both pathways (CH3+trifluoroacetate and CF3+acetate) were detected when thermodynamics shows a clear preference for only one (CF3+acetate). These observations are in contrast to the photochemical behavior of acetone, butanone and acetaldehyde on TiO2(110), where only one of the two carbonyl substituent groups was observed, with a stoichiometric amount of carboxylate containing the other substituent left on the surface. We conclude that fluorination significantly alters the electronic structure of adsorbed carbonyls on TiO2(110) in such a way as to promote multiple channels of photofragmentation. Factors that dictate the partitioning between the three TFA channels are not related to photon energy (above that of the TiO2 band gap), but likely to the electronic structure of the charge transfer excited state.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1000624
- Report Number(s):
- PNNL-SA-69471; ISSN 1932--7455; 35219; 35193; KC0302010; TRN: US1100154
- Journal Information:
- Journal of Physical Chemistry C, 114(40):16900-16908, Vol. 114, Issue 40; ISSN 1932--7447
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ACETALDEHYDE
ACETATES
ACETONE
ADSORPTION
CARBONYLS
DESORPTION
ELECTRONIC STRUCTURE
EXCITED STATES
FLUORINATION
FRAGMENTATION
FUNCTIONALS
IRRADIATION
KETENES
MASS SPECTROMETERS
PHOTOCHEMISTRY
PHOTONS
PYROLYSIS
RUTILE
THERMODYNAMICS
photocatalysis
titanium dioxide
surface science
Environmental Molecular Sciences Laboratory