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Sulfur Dioxide Adsorption on TiO2 Nanoparticles: Influence of Particle Size, Coadsorbates, Sample Pretreatment, and Light on Surface Speciation and Surface Coverage Jonas Baltrusaitis,† Pradeep M. Jayaweera,†,§ and Vicki H. Grassian*,†,‡ Department of Chemistry, and Department of Chemical and Biochemical Engineering, UniVersity of Iowa, Iowa City, Iowa 52242, United States, and Department of Chemistry, UniVersity of Sri Jayewardenepura, Nugegoda, Sri Lanka ReceiVed: September 14, 2010; ReVised Manuscript ReceiVed: NoVember 10, 2010
The adsorption of sulfur dioxide (SO2) on titanium dioxide (TiO2) nanoparticle surfaces at 296 K under a wide range of conditions has been investigated. X-ray photoelectron spectroscopy is used to investigate the surface speciation and surface coverage of sulfur-containing products on ca. 4 nm TiO2 anatase particles that remain on the surface following adsorption of SO2. The effects of various environmental conditions of relative humidity, molecular oxygen, and broadband UV/vis irradiation as well as sample pretreatment were found to impact the speciation of adsorbed SO2 as well as the saturation coverage. In particular, in the absence of light, the majority surface species upon SO2 adsorption is found to be adsorbed sulfite. Broadband UV/vis irradiation during sulfur dioxide adsorption leads to an increase (nearly 2-fold) in the amount of adsorbed sulfur species, as compared to experiments with no light, and results in the formation of adsorbed sulfate. The formation of sulfate was quantitative in the presence of molecular oxygen. New surface species including chemisorbed molecular SO2 were observed on samples that have been reduced in vacuum through argon ion sputtering. The total amount of adsorbed sulfur was impacted by surface hydroxyl group coverage and molecularly adsorbed water layer. Additionally, comparison of sulfur dioxide adsorption on 4 versus 32 nm sized anatase nanoparticles showed that surface saturation coverages of adsorbed sulfite on the 4 nm particles was almost twice that of 32 nm particles as measured by the S2p:Ti2p peak area ratios, thus showing an increase in the inherent adsorption capacity of the smaller particles. Proposed adsorption sites and mechanisms to account for the observed experimental data are discussed. Introduction SO2 is the principal sulfur-containing anthropogenic pollutant, which in the atmosphere can undergo various chemical transformations into sulfuric acid.1 Nearly one-half of the global SO2 emissions are converted to particulate sulfate by ozone and hydrogen peroxide.1 SO2 transformations can be homogeneous, gas phase or water droplet based, or heterogeneous, for example, occur on mineral aerosol surfaces. Previous studies have shown the importance of the chemical composition of the individual mineral aerosol components with respect to SO2 reactivity.2,3 It also has been proposed that ozone and molecular oxygen can oxidize adsorbed sulfite to adsorbed sulfate, especially on transition metals such as iron.1 Solar light could further contribute to the reactivity of SO2 in the atmosphere. Although there is little direct absorption of UV/vis light with gas-phase SO2 in the troposphere,4,5 when associated with mineral dust aerosol, especially semiconductor components such as TiO2, there can be new photochemical pathways for sulfur dioxide in the presence of solar light. Depending upon particle size and crystal structure, as well as the presence of certain surface crystallographic planes and dopants, the bandgap for TiO2 can range from 3.2 to 4.5 eV,6,7 within the solar spectrum. Thus, TiO2 in the atmosphere can be excited with light to produce an electron-hole pair that can * Corresponding author. E-mail:
[email protected]. † Department of Chemistry, University of Iowa. ‡ Department of Chemical and Biochemical Engineering, University of Iowa. § University of Sri Jayewardenepura.
facilitate the chemistry, especially redox, in the atmosphere in the presence of other gases such as water vapor and molecular oxygen.7 SO2 heterogeneous interactions with TiO2 surfaces have been studied previously under ultrahigh vacuum conditions on single crystal surfaces8-11 and nanostructures.12 Quantum chemical calculations have also been used to aid in this understanding.13,14 Reviews on the surface science of TiO2 and previous studies of SO2 surface chemistry on TiO2 in ultrahigh vacuum can be found elsewhere.7,15 While sulfite photochemistry on TiO2 in solution has been studied extensively primarily as a sacrificial reagent in water photodissociation,16-19 heterogeneous photochemistry of the SO2 adsorption products on TiO2 particles, important from an atmospheric chemistry perspective, has received less attention. Additionally, it should be noted that TiO2 nanomaterials are being used as outdoor and indoor coatings on glass windows and in solar panels. Recent studies have suggested that heterogeneous reactions on these nanomaterials may play a role in the chemistry of the atmosphere.20,21 Not only as coatings, but also because TiO2 nanomaterials are being used in a large number of applications (as gas sensors,22 photovoltaics,23 and catalysts for environmental remediation,24 often as a photocatalyst for the degradation of pollutants25-27) and make up a relatively large percentage of the commercially manufactured nanomaterials, it has also been suggested that these materials can make it into the atmospheric environment, thus becoming a part of the tropospheric aerosol.28 TiO2 with large surface area and pore volume has been shown to be a better adsorbent for SO2 relative to other metal oxides such as Al2O3, MgO, carbon,
10.1021/jp108759b 2011 American Chemical Society Published on Web 12/09/2010
Sulfur Dioxide Adsorption on TiO2 Nanoparticles and SiO2.27 Thus, for these reasons, chemical reactions of atmospheric gases with TiO2 nanoparticle surfaces are of great interest. The focus of the current study is on the surface chemistry of SO2 on TiO2 nanoparticles that are ca. 4 nm in diameter. Very small nanoparticles less than 10 nm in diameter are interesting from the perspective that as the surface to volume ratio decreases there are more reactive surface sites. For example, a ∼4 nm particle has ∼20% of its atoms at edge and corner sites. The edge and corner sites are highly coordinatively unsaturated sites. These coordinatively unsaturated sites have been shown to preferentially adsorb some molecules such as SO2.15 In this current study, X-ray photoelectron spectroscopy is used to investigate the surface speciation and surface coverage of sulfurcontaining products on 4 nm TiO2 anatase particles that remain on the surface following adsorption of SO2 under different environmental conditions of relative humidity, molecular oxygen, and broadband irradiation as well as sample pretreatment. Electron- and X-ray-based surface analytical techniques have been shown to be a powerful tool in the characterization of nanoparticle surfaces.29 The speciation of the adsorbed products is elucidated, and insights into SO2 photochemistry on TiO2 nanoparticle surfaces under different environmental conditions are provided. Comparisons to larger 32 nm TiO2 nanoparticles give further insight into the size dependent surface chemistry and the nature of adsorption sites on these nanoparticle surfaces. Experimental Methods XPS Analysis and Reaction Chamber. A custom-designed Kratos Axis Ultra X-ray photoelectron spectroscopy system was used to investigate the interaction of SO2 with nanoparticulate TiO2 surfaces under different environmental conditions. The experimental setup has been described in detail before.2,30 Briefly, the instrument used in this study has three chambers, (i) an ultra high vacuum (UHV) surface analysis chamber, (ii) a sample transfer antechamber, and (iii) a reaction chamber. The transfer antechamber is connected to both the analysis and the reaction chamber. The surface analysis chamber is equipped with monochromatic radiation at 1486.6 eV from an aluminum KR source using a 500 mm Rowland circle silicon single crystal monochromator. The X-ray gun was operated using a 15 mA emission current at an accelerating voltage of 15 kV. Low energy electrons were used for charge compensation to neutralize the sample. Survey scans were collected using the following instrument parameters: energy scan range of 1200 to -5 eV; pass energy of 160 eV; step size of 1 eV; dwell time of 200 ms; and an X-ray spot size of 700 × 300 µm. High-resolution spectra were acquired in the region of interest using the following experimental parameters: 20-40 eV energy window; pass energy of 20 eV; step size of 0.1 eV; and dwell time of 1000 ms. One sweep was used to acquire all the regions. The absolute energy scale was calibrated to the Cu 2p2/3 peak binding energy of 932.6 eV using an etched copper plate. From the surface analysis chamber, the sample was directly transferred to the auxiliary reaction chamber via the transfer antechamber by means of two sample transfer rods. The reaction chamber was fabricated from stainless steel and is approximately 3 L in volume and was retrofit with a leak valve, a pressure transducer (BOC Edwards WRG-S-NW35), a pumping system (Boc Edwards TIC), and two Pyrex glass windows. The pumping system consists of a rotary pump, a foreline trap (both BOC Edwards), and an EXT75DX turbomolecular pump with
J. Phys. Chem. C, Vol. 115, No. 2, 2011 493 60 L/s pumping capacity. The pumping system is separated from the reaction chamber by a hand valve (Granville-Philips Co.). For a typical XPS analysis, powdered samples were pressed into indium foil and mounted onto a copper stub. The copper stubs with the powdered samples were introduced into the transfer antechamber under ambient conditions and pumped to ∼5 × 10-7 Torr. Once this pressure was achieved, samples were introduced into the surface analysis chamber, which was maintained at a pressure in the 10-9 Torr range during analysis. For high temperature experiments, TiO2 sample was pressed into aluminum foil and heated in XPS analysis chamber to 773 K for 12 h. The temperature was controlled using a MICROMEGA temperature controller within (10 K and measured using a type K thermocouple. Before analysis, the sample was allowed to cool to room temperature. For ion beam sputtering experiments, TiO2 samples were bombarded with an argon ion beam (Minibeam I type ion gun). Typical parameters for the ion gun were 4.8 kV accelerating voltage, 15 mA filament current, with a background argon pressure of 1 × 10-7 Torr. To ensure that analysis of the sputtered region only was being subsequently probed with XPS, the ion beam was rastered over 2 mm × 2 mm area, which is significantly larger than the XPS scan area of 700 × 300 µm. TiO2 nanoparticle samples were typically sputtered for ∼5 min. In a typical experiment, TiO2 nanoparticle surfaces were exposed to gas-phase reactants in the reaction chamber in the following order: (1) SO2, (2) H2O vapor, and (3) O2. The time necessary for the introduction of each reactant was approximately 15 s. The resulting gas mixture was allowed to equilibrate with the sample for 30 min. The reacted sample was then transferred back to the analysis chamber for postreaction surface characterization. Additionally, the sample in the reaction chamber could be irradiated with a 500 W Hg lamp (Oriel Instruments housing model 66033) that is connected a power supply (Oriel Instruments model 68810). The lamp was also equipped with a water filter (Oriel Instruments) to minimize infrared radiation and heating of the sample. The light from the lamp was turned by a 90° turning mirror (Oriel Instruments, model 66215, full reflector, 200 nm to 30 µm primary range, 1.5 in. series) and entered the reaction chamber via a Pyrex window. The transmissivity of the Pyrex window was ∼80% at the wavelengths above ∼320 nm as discussed previously.30 A measured light intensity at the sample was 0.72 W/cm2. The measured temperature of the sample was ∼333 K (60 °C) in a vacuum and ∼308 K (35 °C) when 45% RH was present in the reaction chamber. Data Processing of Core Photoelectron Spectra. A Shirleytype background was subtracted from each spectrum to account for inelastically scattered electrons that contribute to the broad background. Transmission corrected relative sensitivity factor (RSF) values from the Kratos library were used for elemental quantification. CasaXPS software was used to process the XPS data.31 All spectra were calibrated using a well-defined Ti4+ binding energy of 459.3 eV.32 The reason for not using an adventitious carbon peak at 285.0 eV was because there was very little of it observed. When discernible, an adventitious C1s peak was ∼0.3 eV higher than the typically used value of 285.0 eV for calibration of the adventitious carbon signal when using the Ti4+ calibration instead.33 The S2p transition was fit to two peaks with a ratio of 2:1 for the 2p3/2 and 2p1/2 transitions, respectively. The S2p doublet was constrained to a separation energy of 1.2 eV with equivalent full-width at half-maximum (fwhm). In the Ti2p region, the
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Baltrusaitis et al. anatase (98%) but also contained a small amount of rutile (
