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Title: Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage

Abstract

The selectivity of trimethyl acetate (TMA) photodecomposition on TiO2(110) as a function of O2 pressure and TMA coverage was probed at room temperature (RT) using isothermal mass spectrometry (ISOMS) and scanning tunneling microscopy (STM). The selectivity of TMA photodecomposition on TiO2(110) is sensitive to the initial TMA coverage and the O2 pressure. TMA bridge bonds to the surface via the carboxylate end of the molecule in a manner consistent with the binding of other carboxylate species (e.g., formate and acetate) on TiO2 surfaces. Under all conditions, photodecomposition of TMA was initiated via hole reaction with the electron in carboxylate's ? system resulting in opening of the O-C-O bond angle, and formation of CO2 and a t-butyl radical by cleavage of the C-C bond between these groups. The CO2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O2 partial pressure is <1x10-10 torr, the TMA photodecomposition results in a near 1:1 yield of isobutene (i-C4H8) and isobutane (i-C4H10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO2(110) surface. In the presence of O2, the photodecomposition selectivitymore » switches from initially i-C4H8 to a mixture of i-C4H8 and i-C4H10 and then back to predominately i-C4H8. The latter selectivity change occurs at the point at which void regions form and grow in the TMA overlayer. At this point, the photodecomposition rate accelerates and the reaction occurs preferentially at the interface between the TMA-rich and TMA-void regions on the surface. These results illustrate both the changing dynamics of a typical photooxidation reaction on TiO2, and how factors such as O2 pressure and TMA coverage, impact the photooxidation reaction selectivity. We also present results that suggest the rate of photodecomposition of monodentate carboxylates is greater than that of bidentate (bridging) carboxylates. This implies that the structural arrangement of Ti cation sites on the surface is an important issue that influences photocatalytic rates on TiO2.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
876845
Report Number(s):
PNNL-SA-47680
Journal ID: ISSN 0021-9517; JCTLA5; 11105; KC0302010; TRN: US200608%%78
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Catalysis; Journal Volume: 238; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACETATES; PHOTOLYSIS; OXIDATION; TITANIUM OXIDES; CATALYTIC EFFECTS; OXYGEN; PRESSURE DEPENDENCE; Photocatalysis; TiO2(110); trimethyl acetic acid; surface science; Environmental Molecular Sciences Laboratory

Citation Formats

Henderson, Michael A., White, J M., Uetsuka, H, and Onishi, Hiroshi. Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage. United States: N. p., 2006. Web. doi:10.1016/j.jcat.2005.12.004.
Henderson, Michael A., White, J M., Uetsuka, H, & Onishi, Hiroshi. Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage. United States. doi:10.1016/j.jcat.2005.12.004.
Henderson, Michael A., White, J M., Uetsuka, H, and Onishi, Hiroshi. Wed . "Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage". United States. doi:10.1016/j.jcat.2005.12.004.
@article{osti_876845,
title = {Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage},
author = {Henderson, Michael A. and White, J M. and Uetsuka, H and Onishi, Hiroshi},
abstractNote = {The selectivity of trimethyl acetate (TMA) photodecomposition on TiO2(110) as a function of O2 pressure and TMA coverage was probed at room temperature (RT) using isothermal mass spectrometry (ISOMS) and scanning tunneling microscopy (STM). The selectivity of TMA photodecomposition on TiO2(110) is sensitive to the initial TMA coverage and the O2 pressure. TMA bridge bonds to the surface via the carboxylate end of the molecule in a manner consistent with the binding of other carboxylate species (e.g., formate and acetate) on TiO2 surfaces. Under all conditions, photodecomposition of TMA was initiated via hole reaction with the electron in carboxylate's ? system resulting in opening of the O-C-O bond angle, and formation of CO2 and a t-butyl radical by cleavage of the C-C bond between these groups. The CO2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O2 partial pressure is <1x10-10 torr, the TMA photodecomposition results in a near 1:1 yield of isobutene (i-C4H8) and isobutane (i-C4H10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO2(110) surface. In the presence of O2, the photodecomposition selectivity switches from initially i-C4H8 to a mixture of i-C4H8 and i-C4H10 and then back to predominately i-C4H8. The latter selectivity change occurs at the point at which void regions form and grow in the TMA overlayer. At this point, the photodecomposition rate accelerates and the reaction occurs preferentially at the interface between the TMA-rich and TMA-void regions on the surface. These results illustrate both the changing dynamics of a typical photooxidation reaction on TiO2, and how factors such as O2 pressure and TMA coverage, impact the photooxidation reaction selectivity. We also present results that suggest the rate of photodecomposition of monodentate carboxylates is greater than that of bidentate (bridging) carboxylates. This implies that the structural arrangement of Ti cation sites on the surface is an important issue that influences photocatalytic rates on TiO2.},
doi = {10.1016/j.jcat.2005.12.004},
journal = {Journal of Catalysis},
number = 1,
volume = 238,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}
  • Organic photooxidation on TiO2 invariably involves the coexistence of organic species with oxygen on the surface at the same time. In the case of acetone and oxygen, both species exhibit their own interesting photochemistry on TiO2, but interdependences between the two are not understood. In this study, a rutile TiO2(110) surface possessing 7% surface oxygen vacancy sites is used as a model surface to probe the relationship between O2 photodesorption and acetone photodecomposition. Temperature programmed desorption (TPD) and photon stimulated desorption (PSD) measurements indicate that coadsorbed oxygen is essential to acetone photodecomposition on this surface, however the form of oxygenmore » (molecular and dissociative) is not known. The first steps in acetone photodecomposition on TiO2(110) involve thermal activation with oxygen to form an acetone diolate ((CH3)2COO) species followed by photochemical decomposition to adsorbed acetate (CH3COO) and an ejected CH3 radical that is detected in PSD. Depending on the surface conditions, O2 PSD is also observed during the latter process. However, the time scales for the two PSD events (CH3 and O2) are quite different, withthe former occurring at ~10 times faster than the latter. By varying the preheating conditions or performing pre-irradiation on an O2 exposed surface, it becomes clear that the two PSD events are uncorrelated. That is, the O2 species responsible for O2 PSD is not a significant participant in the photochemistry of acetone on TiO2(110) and likely originates from a minority form of O2 on the surface. The CH3 and O2 PSD events do not appear to be in competition with each other suggesting either that ample charge carriers exist under the experimental conditions employed or that different charge carriers or excitation mechanisms are involved.« less
  • Integral, isothermal heats of adsorption were measured for O{sub 2} on UHP Pt powder and SiO{sub 2{minus}} supported Pt, with the latter providing an average crystallite size range of 1.3-22.3nm. The energy changes that occur during the titration of this chemisorbed oxygen by dihydrogen were also measured. The average Q{sub ad} value of 67.9{plus minus}8.9 kcal/mol for well-dispersed Pt on Davison 57 silica compared to that of 52.6{plus minus}6.1 kcal/mol for the Pt powder indicates that a small increase may occur on very small Pt crystallites. The enthalpy of titration was relatively constant, with a value of{Delta}H{sub titr} = {minus}26.7{plusmore » minus}4.2 kcal/mol H{sub 2} obtained for all the Pt/SiO{sub 2} samples compared to {minus}24.5{plus minus}3.5 kcal/mol H{sub 2} for the Pt powder. The use of a thermodynamic cycle not only gave an approximate value of 11 kcal/mol for the heat of adsorption of water on SiO{sub 2}, but also indicated that all the water desorbed from the Pt surface at 300 K thus allowing a complete monolayer of hydrogen to form.« less
  • In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups that are formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2 (110). The electronic structure of an oxygen vacancy defect on TiO2 (110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO2 photocatalysts, being Ti3? in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure ofmore » these sites. Temperature programmed desorption (TPD) measurements show that O2 adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO2 (110) to form an unidentified intermediate that decomposes to generate terminal OH groups at non-defect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti3?) is oxidized. Both vibrational and electronic EELS results indicate that the reaction between O2and OHbr occurs at about 230 K. Detailed TPD experiments in which the precoverage of water was varied indicate that O2 need not chemisorb to cation sites on the TiO2 (110) surface in order for the reaction between O2 and OHbr to occur, which implies a direct interaction between weakly bound O2 and the OHbr groups. In agreement with this conclusion, we find that second layer water, which selectively hydrogen-bonds to bridging O and OH groups, blocks the reaction of O2with OHbr groups and prevents oxidation of the vacancy-related Ti3? electronic state.« less