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Title: Electron-Scavenging Chemistry of Benzoquinone on TiO2(110)

Abstract

The chemistry of benzoquinone (BQ) on TiO2(110) was examined using temperature programmed desorption (TPD), electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). BQ adsorbs mostly molecularly on the clean surface, although EELS demonstrates that electrons from surface Ti3+ sites at oxygen vacancy sites (VO) are readily oxidized by the high electron scavenging ability of the molecule. In contrast, when the surface is covered with water, subsequently adsorbed BQ molecules that scavenge surface electrons also abstract H from surface OHbr groups to form hydroquinone (HQ), which desorbs at ~450 K. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1371974
Report Number(s):
PNNL-SA-116760
Journal ID: ISSN 1022-5528; 48287; KC0302010
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Topics in Catalysis; Journal Volume: 60; Journal Issue: 6-7
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Environmental Molecular Sciences Laboratory

Citation Formats

Henderson, Michael A., and Shen, Mingmin. Electron-Scavenging Chemistry of Benzoquinone on TiO2(110). United States: N. p., 2017. Web. doi:10.1007/s11244-016-0707-7.
Henderson, Michael A., & Shen, Mingmin. Electron-Scavenging Chemistry of Benzoquinone on TiO2(110). United States. doi:10.1007/s11244-016-0707-7.
Henderson, Michael A., and Shen, Mingmin. Mon . "Electron-Scavenging Chemistry of Benzoquinone on TiO2(110)". United States. doi:10.1007/s11244-016-0707-7.
@article{osti_1371974,
title = {Electron-Scavenging Chemistry of Benzoquinone on TiO2(110)},
author = {Henderson, Michael A. and Shen, Mingmin},
abstractNote = {The chemistry of benzoquinone (BQ) on TiO2(110) was examined using temperature programmed desorption (TPD), electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). BQ adsorbs mostly molecularly on the clean surface, although EELS demonstrates that electrons from surface Ti3+ sites at oxygen vacancy sites (VO) are readily oxidized by the high electron scavenging ability of the molecule. In contrast, when the surface is covered with water, subsequently adsorbed BQ molecules that scavenge surface electrons also abstract H from surface OHbr groups to form hydroquinone (HQ), which desorbs at ~450 K. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.},
doi = {10.1007/s11244-016-0707-7},
journal = {Topics in Catalysis},
number = 6-7,
volume = 60,
place = {United States},
year = {Mon Apr 03 00:00:00 EDT 2017},
month = {Mon Apr 03 00:00:00 EDT 2017}
}
  • 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
  • Low-temperature scanning tunneling microscopy (STM) has been used to study the adsorption of CO{sub 2} on rutile TiO{sub 2}(110) from 80 to 180 K. For low CO{sub 2} doses, two molecular adsorption sites with different binding energies are identified, which are effectively isolated from one another by an apparent activation barrier to their interconversion. We identify the less tightly bound adsorption site as CO{sub 2} adsorbed atop 5-fold coordinated titanium surface atoms (Ti{sub 5f}), without binding preferentially near oxygen vacancies. CO{sub 2} desorption from Ti{sub 5f} occurs at 140 K. The more strongly bound site involves molecular CO{sub 2} bindingmore » at bridging oxygen vacancies (V{sub O,br}). We observe two distinct configurations of V{sub O,br} bound CO{sub 2} molecules. Despite its being bound to the vacancy, CO{sub 2} does not dissociate thermally but remains intact up to the desorption temperature of {approx}175 K. At an elevated tunneling bias, the STM tip can selectively dissociate these CO{sub 2} molecules and thus trigger the healing of individual V{sub O,br}. At higher coverage, CO{sub 2} adsorption occurs predominantly at the more abundant Ti{sub 5f} sites, with the distribution of CO{sub 2} molecules being determined by interactions both along the [001] and [110] directions.« less
  • Electron-stimulated desorption (ESD) of H2, O2 and H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as function of film thickness and isotopic composition. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ~ 20 monolayer (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40 - 50 ML. Experiments using isotopically layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at ormore » near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on the TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semi-quantitatively reproduces the main features of the non-thermal reactions.« less
  • No abstract prepared.