J. Phys. Chem. C 2009, 113, 2063–2070
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Surface Chemistry of Organic Pollutants: Styrene, Ozone, and Water on TiO2(110)† R. G. Quiller,‡ L. Benz,§ J. Haubrich,§ M. E. Colling,§ and C. M. Friend*,‡,§ School of Engineering and Applied Sciences, HarVard UniVersity, 29 Oxford Street, Cambridge, Massachusetts 02138, and Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138 ReceiVed: August 2, 2008; ReVised Manuscript ReceiVed: October 27, 2008
The modification in reactivity of styrene by ozone and water was investigated on TiO2(110) as a model for heterogeneous reactions of hazardous pollutants using temperature programmed desorption. Styrene desorbs from TiO2(110) at 180, 270, and 340 K, in peaks tentatively assigned here to multilayer sublimation and desorption from Ti4+ in the first layer and defects (including step edges), respectively. Water and styrene compete for sites when these species are coadsorbed, resulting in a shift of the 270 K styrene desorption peak to 200 K. Styrene, however, is still able to bind to defects. Ozone preadsorption suppresses styrene desorption associated with defects but results in a new styrene desorption feature at 535 K. Ozone also promotes styrene oxidation to formaldehyde (325 K), benzaldehyde (380 K), and styrene epoxide (545 K). Two pathways for the reaction of ozone with styrene were identified. First, ozonolysis of the terminal CdC bond of styrene occurs, yielding benzaldehyde and formaldehyde, analogous to the gas-phase reaction. Second, a surfacemediated reaction converts styrene to styrene epoxide. The saturation behavior and thermal stability of the corresponding oxidizing species suggests it is possibly the result of surface-mediated decomposition of ozone into adsorbed O adatoms. Our results suggest that the heterogeneous thermal chemistry of styrene, and possibly nonpolar organic molecules in general, is dependent on moisture and ozone levels in the air, and that volatile organic compound (VOC) concentrations can be significantly affected by anthropogenic interfaces such as titania-containing paints and coatings. Introduction Metal oxides provide natural interfaces for heterogeneous reactions relevant to atmospheric chemistry and indoor air quality, originating in the environment from both natural and anthropogenic sources.1 These oxide surfaces can substantially change reaction rates important to both the production and remediation of atmospheric pollutants. TiO2(110) is a popular model for metal oxide reactions because its preparation is relatively straightforward and it is well-characterized.2 Furthermore, its applications are widespread and include gas sensing, coatings for degradation of pollutants on glass and in air filters, heterogeneous catalysis, photocatalysis, and corrosion-resistant coatings.2 Titania is particularly relevant in the study of the interaction of ambient gases with engineered surfaces because it is a commonly used optical coating and added as a whitener in commercial paints.3 Notably, the photocatalytic degradation of organic molecules on these surfaces has drawn considerable attention.2 Styrene is classified as a volatile organic compound (VOC)sa class of hazardous pollutants recognized formally by the Environmental Protection Agency (EPA).4 Styrene is a precursor to polystyrene and is a common indoor pollutant, emitted from computer printers,5,6 building materials,7 and tobacco smoke.8 Low-level exposures to such VOCs due to office equipment is known to cause mucous membrane irritation, headaches, and dry eyes, nose, and throat.9 Indoor styrene levels have been † Part of the special section “Physical Chemistry of Environmental Interfaces”. * Corresponding author. Tel.: +1-617-495-4052. Fax: +1-617-496-8410. E-mail:
[email protected]. ‡ School of Engineering and Applied Sciences. § Department of Chemistry and Chemical Biology.
measured to be ∼0.1 ppb (equivalent to a partial pressure of 7.6 × 10-8 torr);6 thus, the pressure range accessible in model ultrahigh vacuum studies lends insight into the chemistry of these pollutants. Such organic compounds are common in mixed aerosol particles where organic species form a coating over an inorganic core.10 The interaction of atmospheric pollutants with solid particles that may be present in aerosols is important to investigate because adsorption of organic species may alter the optical properties, wetting behavior, and heterogeneous chemistry of the aerosol particles.10 Water and ozone are additional species that are crucial in understanding indoor and environmental chemistry. Under ambient conditions, many oxide surfaces are hydroxylated and may also be covered with adsorbed water depending on the relative humidity and temperature.11 Moisture in the air makes it critical to understand the role of water in heterogeneous reactions. Interesting reactivity has been observed for fully oxidized polycrystalline TiO2 films in the presence of water: oxygen originating from adsorbed water is involved in the partial oxidation of styrene.12 Hydroxyl groups are proposed to be the active sites for the partial oxidation of styrene to styrene epoxide, which then undergoes a 1,2-hydride shift to isomerize to acetophenone, which desorbs above 450 K.12 On reduced films, Sykes et al. propose TiH is active for styrene hydrogenation to ethylbenzene.12,13 The selectivity strongly depends on the oxidation state of the surface with acetophenone and ethylbenzene being formed with 100% selectivity and conversions of 55% and 85% for the fully oxidized and reduced titania surfaces, respectively.12,13 Ozone is another air pollutant that is also emitted indoors from computer printers5 and air purifiers.14 Ozone levels vary significantly, but can be substantial. For example, in Southern
10.1021/jp806900j CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
2064 J. Phys. Chem. C, Vol. 113, No. 6, 2009 California there are reported indoor levels in homes of 13 ppb (9.9 × 10-6 torr) and outdoor levels of 37 ppb (2.8 × 10-5 torr).15 Indoor levels of up to 106 ppb (8 × 10-5 torr) were measured in Montreal16 and outdoor average levels of ozone were estimated to be >160 ppb (1.2 × 10-4 torr) in Mexico City.17 Even higher levels of ozone, ∼250 ppb (1.9 × 10-4 torr), have been reported in well-ventilated offices with air purifiers.14 Thus, heterogeneous reactions of ozone with VOCs are important to investigate, especially in areas where the partial pressure of ozone is relatively high. Styrene, in particular, forms secondary organic aerosols (SOA) via reaction with ozone and over 70% of urban pollution in some areas can be made of SOAs.18 SOAs are known to have negative implications for human health and climate change.18 Thus, it is important to also consider how ozone affects styrene chemistry on metal oxide surfaces. An understanding of the heterogeneous reactions of ozone, water, and styrene is important since ozone is specifically generated indoors as a means of oxidizing indoor pollutants that can be found in, e.g., tobacco smoke.19 Shaughnessy et al. found that high levels of ozone were needed to affect pollutant concentrations and that a decrease in alkenes including styrene were matched by increases in aldehydes. Oxidizing certain unsaturated hydrocarbons resulted in even more reactive or irritating products than their precursors.19 Their results motivate the need to understand alkene oxidation with ozone on titania in order to better understand the effect of heterogeneous chemistry on product formation. In response to this question, we have chosen to study the coadsorption of styrene, ozone, and water on TiO2(110) using temperature programmed desorption (TPD). In this work we describe the interplay between styrene, ozone, and water on rutile TiO2(110). The adsorption of styrene on the TiO2 is weakened by coadsorbed water, leading to a lower adsorption rate that may affect atmospheric concentrations of styrene. The adsorption of styrene also depends on the presence of defects on the surface. On the other hand, ozone oxidizes styrene to form benzaldehyde and styrene epoxide. Our studies show that the heterogeneous thermal chemistry of VOCs, such as styrene, depends strongly on the levels of water and ozone present as well as the morphology of the oxide surfaces available. Experimental Methods All experiments were performed in an ultrahigh vacuum chamber with a base pressure of 2 × 10-10 torr. The chamber is equipped with a quadrupole mass spectrometer (Pfeiffer Prisma QMS 200), an Auger electron spectrometer with cylindrical mirror analyzer (Perkin-Elmer model 15-155), and low-energy electron diffraction (LEED) optics (Physical Electronics model 15-180). The TiO2(110) crystal (Atomergic Chemetals Corp., 10 mm × 10 mm and 1.5 mm thick) was mounted to a tantalum plate using small molybdenum clips. It was conductively cooled by a liquid nitrogen reservoir and heated radiatively during desorption experiments using a tungsten filament for temperatures below 650 K. For temperatures in excess of 650 K, which were required during sample preparation, electron bombardment heating was employed by biasing the sample to +300 V and supplying a current to the tungsten filament. The temperature was monitored using a K-type chromel/alumel thermocouple junction that was fixed in a slot in the side of the crystal with a ceramic adhesive (Ceramabond 503, Aremco Inc.) known to have a similar coefficient of thermal expansion and thermal conductivity to TiO2.
Quiller et al. The sample was bulk reduced by heating at 850 K until it had a bluish tint. The surface was prepared by cycles of Ar+ sputtering (1 kV, 20 min, ∼2 µA sputter current) and annealing at 900 K. This was repeated until LEED showed a sharp (1 × 1) pattern and no impurities were detected using Auger electron spectroscopy (AES). Preparing TiO2(110) by vacuum annealing is known to produce a slightly defective surface with up to ∼7% bridge-bonded oxygen vacancies.2,20 Water dissociates exclusively at these Ti3+ sites, to form two bridge-bonded hydroxyls.21,22 Thus, the integrated amount of water formed from OH disproportionation at 460 K relative to the amount of molecular water desorbed from the monolayer at 270 K is a means of quantifying the defect density on the surface.23,24 These areas were approximated by subtracting a baseline associated with background water signal from the water desorption spectra and then integrating each peak over a local interval. Using this method, we determined that the Ti3+ defects comprise 5-7% of the surface after annealing in vacuum at 850 K for 5 min. Distilled H2O and styrene (Alfa Aesar, 99.5%) were purified using freeze-pump-thaw cycles, and the purity was confirmed using mass spectroscopy by condensing multilayers on the crystal and subsequently subliming the multilayer while monitoring species evolving from the surface. A directed doser was used for both water and styrene, so that the rise in chamber pressure during directed water and styrene dosing was 5 × 10-11 torr and 1 × 10-10 torr, respectively. The enhancement in the flux is estimated to be ∼145 for water and ∼130 for styrene. Water was exposed to the surface at a temperature of 110-120 K. We define saturation exposure of water and styrene as the minimum exposure required to observe a minor amount of multilayer sublimation while all other peaks are maximum in intensity. Ozone was prepared using an ozone generator and condensed in a silica trap held at 195 K following a method established elsewhere.25 Purity was monitored with the mass spectrometer while leaking the gas into the chamber. A major ion detected during ozone exposure is O2+ (32 amu) since ozone is known to fragment in the mass spectrometer ionizer and decompose on chamber walls. Several impurities were present in the ozone stream: N2, water, and formaldehyde. The ratio of each of these impurities to the O2+ signal was 13, 0.03, and 0.01, respectively. The amount of formaldehyde adsorbed on the surface during ozone exposures was less than 10% of the levels produced due to reactions as described below. None of these contaminants account for the observed effects of ozone. Ozone was exposed to the TiO2(110) crystal at 120 K using a direct doser to minimize decomposition prior to hitting the surface. The background pressure rise was 1 × 10-8 torr during O3 dosing. We define a 1 L ozone exposure as the exposure required to saturate a Au(111) crystal with oxygen,25 which we used as a calibration in separate experiments in the same chamber.26 Temperature programmed desorption (TPD) data using a linear heating rate of 2.4 K/s between 120 and 600 K were acquired with a computer-interfaced Pfeiffer Prisma QMS 200 mass spectrometer. The crystal was biased at -100 V throughout the experiments and during data collection to prevent any electron-induced reactions from the mass spectrometer filament. Relative product quantification for a substance, J, was estimated using the corrected total yields of each product ion, P, according to the following relation:
Surface Chemistry of Organic Pollutants
YJ ∝
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AP IJCPGPTP
where AP is the integrated area of the fragment ion peak, IJ is the ionization efficiency, GPis the multiplier gain, TP is the transmission coefficient, and CP is the fragment ion yield for the m/z that was scanned. As a reasonable approximation to the type of quadrupole filter we use, the transmission coefficients were taken from the UTI model 100c Operating Manual. Multiplier gain and ionization efficiency are given by the following equations:27
GP )
(
IJ ) 0.6
( 28m )
1⁄2
#electrons + 0.4 14
)
where m is the mass of the molecule or fragment that was scanned. The fragment ion yield is given by the ratio of the intensity of the ion that was scanned to the total intensity of the parent ion plus all other fragments, and was found using the NIST database. These calculations were used as a rough guide in quantifying the relative amounts of products formed in our studies. Results and Discussion Three styrene peaks centered at approximately 180, 270, and 340 K are observed during temperature programmed desorption following exposure of multilayers of styrene to vacuum-annealed TiO2(110) at 120 K (Figure 1). The peak at 180 K is attributed to sublimation of styrene multilayers since it increases linearly up to 20 times the dose required to saturate the other two peaks. The temperature for styrene sublimation from TiO2 reported here is within the range of values previously reported for Ag(100) (188 K)28 and Ag(111) (167 K).29 The desorption of all styrene below ∼400 K is in agreement with molecular styrene desorption from fully oxidized polycrystalline TiO2 films.13 No other products were observed based on a comprehensive scan for all m/z e 200 during temperature programmed measurements; only styrene (parent ion, m/z ) 104) and its fragments were detected. The highest temperature peak at 340 K is observed at the lowest styrene exposures and saturates prior to detection of styrene desorption at lower temperature. The 340 K peak is attributed to adsorption at minority defect sites since it saturates at low styrene exposures and the fact that the maximum corrected intensity of this peak corresponds to ∼50% of the number of Ti3+ vacancies, as determined by comparison to the amount of OH disproportionation associated with water exposure, which was used as a gauge of bridge-bonded oxygen vacancies (see experimental section). The lower temperature styrene peak is centered at 290 K for lower coverages (
