Efficient Photocatalytic Removal of NO in Indoor Air ... - ACS Publications

Apr 30, 2009 - Center for Environmental Technology and Management,. The Hong Kong Polytechnic University, Hong Kong,. People's Republic of China; and ...
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Environ. Sci. Technol. 2009, 43, 4143–4150

Efficient Photocatalytic Removal of NO in Indoor Air with Hierarchical Bismuth Oxybromide Nanoplate Microspheres under Visible Light Z H I H U I A I , †,†,‡ W I N G K E I H O , † S H U N C H E N G L E E , * ,†,† A N D L I Z H I Z H A N G * ,‡ Department of Civil and Structural Engineering, Research Center for Environmental Technology and Management, The Hong Kong Polytechnic University, Hong Kong, People’s Republic of China; and Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China

Received February 11, 2009. Revised manuscript received April 15, 2009. Accepted April 16, 2009.

In this study, hierarchical bismuth oxybromide (BiOBr) nanoplate microspheres were used to remove NO in indoor air under visible light irradiation. The BiOBr microspheres were synthesized with a nonaqueous sol-gel method by using bismuth nitrate and cetyltrimethyl ammonium bromide as the precursors. On degradation of NO under visible light irradiation (λ > 420 nm) at 400 part-per-billion level, which is typical concentration for indoor air quality, these nonaqueous sol-gel synthesized hierarchical BiOBr microspheres exhibited superior photocatalytic activity to the chemical precipitation synthesized counterpart BiOBr bulk powder and Degussa TiO2 P25 as well as C doped TiO2. The excellent catalytic activity and the long-term activity of nonaqueous sol-gel synthesized BiOBr microspheres were attributed to their special hierarchical structure, which was favorable for the diffusion of intermediates and final products of NO oxidation. Ion chromatograph results confirmed that nitric acid was produced on the surface of BiOBr microspheres during the photooxidation of NO in gas phase. This work suggests that the nonaqueous sol-gel synthesized BiOBr nanoplate microspheres are promising photocatalytic materials for indoor air purification.

Introduction Recently, more and more attention has been paid to indoor air quality (IAQ) with increasing awareness of the public environment and health, especially in urban cities (1, 2). A number of techniques are well-established for the purification of polluted air. Traditional methods such as physical adsorption, biofiltration, and thermal catalysis are not economically feasible at low pollutant concentrations of partper-billion (ppb) levels, which are typical concentration for indoor air quality (3). As an ambient temperature catalytic process, photocatalysis has gained considerable attention in view of solar energy conversion and environmental cleaning, * Address correspondence to either author. Phone/fax: +852-2766 6011 (S.L. and L.Z.). E-mail: [email protected] (S.L.); zhanglz@ mail.ccnu.edu.cn (L.Z.). † The Hong Kong Polytechnic University. ‡ Central China Normal University. 10.1021/es9004366 CCC: $40.75

Published on Web 04/30/2009

 2009 American Chemical Society

as well as purification of indoor air pollutants even at low concentrations (4-6). For instance, TiO2 immobilized on different substrates, such as activate carbon and glass fibers, can photocatalytically degrade indoor air pollutants at parts per billion levels in a flow system under UV light irradiation (3, 7-9). Although TiO2 has proven to be the most excellent photocatalyst for the oxidative decomposition of many compounds, its relatively wide band gap limits its further application in the visible light region (750 nm > λ > 400 nm), which accounts for 43% of the incoming solar energy (5, 6). Thus, it is appealing to develop visible light sensitive photocatalysts active enough for practical application (10, 11). Nowadays, two strategies have been employed in the design of visible-light-driven photocatalysts. One involves the chemical modifications on a UV-active photocatalyst (for example, TiO2), including dye sensitization or doping of foreign elements or coupling with a narrow band gap semiconductor, etc (12-19). The other is to develop stable single-phase photocatalysts active under visible light. For example, ZnWO4 (20), BaBiO3 (21), Bi5FeTi3O5 (22), NiGa2O4 (23), Sr2Sb2O7 (24), LiBi4M3O14 (M ) Nb, Ta) (25), HNb3O8 (26), BiFeO3 (27), CdIn2S4 (28), and InNbO4 (29)have been reported to show activity for the photodegradation of organic pollutants or/and water splitting under visible light irradiation. Bismuth oxybromide (BiOBr), an important V-VI-VII ternary compound, has attracted increasing interest recently because of its unique and excellent electrical, magnetic, optical, and luminescent properties, in addition to be a new visible light responding photocatalyst (30-34). Usually, nanostructured materials exhibit characteristic optical and physical properties that are substantially different from those of the corresponding bulk materials, and their properties depend on the size, morphology, and dimensionality (4). As expected, various approaches, including solid-state or melting reaction, hydrothermal method, solution phase synthesis, and microemulsion polymerization, have been employed for the fabrication of BiOBr nanostructures with various morphologies (31-34). These approaches led to a characteristic crystalline structure, which thus showed significant differences in their performance. For example, our group recently developed a general one-pot nonaqueous sol-gel method to prepare BiOX (X ) Cl, Br, I) nanoplate micropheres with bismuth nitrate and the corresponding potassium halide as the precursors (31). Li’s group synthesized ternary bismuth oxyhalide crystalline nanobelts and nanotubes by using convenient hydrothermal methods (35). Deng and his coworkers reported the preparation of two-dimensional (2D) single-crystalline bismuth oxyhalides nanoplates, nanosheets, and microsheets via a wet chemistry approach (36). Despite these advances, the diversity of desired geometry for BiOBr nanocrystals with high photocatalytic activity still needs to be greatly expanded to meet the ever-in-creasing demand. In this work, we synthesize hierarchical BiOBr nanoplate microspheres with an ethylene glycol based nonaqueous sol-gel method by using CTAB as the Br precursor instead of KBr and investigate the potential of the as-obtained BiOBr nanoplate microspheres to remove NO pollution at typical concentration level in indoor air under visible light irradiation. It is interesting to find that as-prepared BiOBr microspheres show very excellent photocatalytic NO removal ability under visible light irradiation. More importantly, the BiOBr microspheres are stable and can keep long-term activity after multiple photocatalytic removal runs. VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Experimental Section

Sample Preparation. Bi(NO3)3 · 5H2O was obtained from National Medicines Corporation Ltd., China, cetyltrimethyl ammonium bromide (CTAB), and ethylene glycol (EG) were purchased from Sigma-Aldrich Hong Kong, China. All the chemicals were of analytical grade and used as received without further purification. Nanostructured hierarchical BiOBr microspheres were synthesized by a nonaqueous sol-gel method. In a typical synthesis, 0.1 mmol of Bi(NO3)3 · 5H2O was added into an EG solution containing stoichiometric amounts of CTAB. The mixture was stirred for 0.5 h at room temperature, and then poured into a 20 mL Teflonlined stainless autoclave until 80% of the autoclave volume was filled. The autoclave was heated at 180 °C for 12 h under autogenously pressure, and then air cooled to room temperature. The resulting precipitates were collected and washed with ethanol and deionized water thoroughly and dried at 50 °C in air. For the purpose of comparison, BiOBr bulk powders were also prepared by a chemical precipitation method (Supporting Information (SI) Figure S1). Characterization. X-ray diffraction (XRD) patterns were obtained on a Philips Xpert System X-ray diffractometer with Cu Ka radiation (λ ) 1.54178 Å). Scanning electron microscopy images were performed on a JEOL 6490 scanning electron microscope. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were performed on a JEOL JEM-2010 electron microscope operating at 200 kV. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon-copper grids. Furthermore, the obtained powders deposited on a copper grid were observed by a high-resolution transmission electron microscope. UV-vis diffuse reflectance spectra (DRS) were recorded at room temperature with the Cary 300 UV-visible spectrophotometer equipped with an integrated sphere. X-ray photoemission spectroscopy (XPS) was recorded on a PHI 5600 multitechnique system with a monochromatic Al KR source (Physical Electronics) operated at 150 W (15 kV, 10 mA). The nitrogen adsorption and desorption isotherms at 77 K were measured using a Micrometritics ASAP2010 system after samples were vacuum-dried at 473 K overnight. Nitrate and nitrite ions were performed by using an ion chromatograph (IC, Dionex LC20) equipped with an AS14 column; the mobile phase of a mixture of 1.8 mM Na2CO3 and 1.7 mM NaHCO3 was used at a flow rate of 1.20 mL/min; the volume of samples was 20 µL. Photocatalytic Experiments. The photocatalytic experiments for the removal of NO in air were performed at ambient temperature in a continuous flow reactor. The volume of the rectangular reactor which was made of stainless steel and covered with Saint-Glass was 4.5 L (10 × 30 × 15 cm (H × L × W)). One sample dish containing the 0.2 g catalyst powders were placed in the middle of the reactor. A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar-light source. The lamp was vertically placed outside the reactor above the sample dish. Four mini-fans were fixed around the lamp to avoid the temperature rise of the flow system. The integrated UV intensity in the range 310-400 nm was 720 ( 10 µW/cm2. The catalyst samples were prepared by coating an aqueous suspension of our sample onto a dish with a diameter of 5.0 cm. The dishes containing the photocatalyst were pretreated at 60 °C until complete removal of water in the suspension and then cooled to room temperature. NO gas was selected as the target pollutant for the photocatalytic degradation at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with traceable National Institute of Stands and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air 4144

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stream supplied by a zero air generator (Thermo Environmental Inc. model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 4 L min-1 by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases, and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The removal rate (%) of NO was the ratio of the concentration of NO in the feeding stream and the concentration of NOx in the outlet stream. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of photocatalyst. During photocatalytic reaction, the intermediates and final products (nitrate and nitrite ions) remaining on the catalyst powders were extracted by immersing the samples into deionized water (10 mL), and were measured with an IC.

Results and Discussion Characterization of BiOBr Catalysts. The power X-ray diffraction (XRD) pattern provides crystallite size and phase information of the nonaqueous sol-gel synthesized sample (Figure 1). The diffraction peaks could be indexed to a tetragonal phase of BiOBr with lattice constants of a ) 3.915, c ) 8.076 (JCPDS card no. 73-2061). The observed broadening of the diffraction peaks is ascribed to the nanocrystalline nature of the samples. No other diffraction peaks are found, indicating that pure tetragonal phase BiOBr could be synthesized with the nonaquous sol-gel method by using bismuth nitrate and cetyltrimethyl ammonium bromide as the bismuth and bromide sources, respectively. The surface element composition of the nonaqueous sol-gel synthesized BiOBr sample was studied by X-ray photoelectron spectroscopy (XPS) (Figure 2). The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s line to 284.5 eV. The survey XPS spectrum (Figure 2a) reveals that the nonaqueous sol-gel synthesized sample is composed of elements of Bi, O, Br, and C. The carbon peak could be attributed to the adventitious carbon on the surface of the sample. Two strong peaks at 158.65 and 163.85 eV in the high-resolution spectra are assigned to Bi 4f7/2 and Bi 4f5/2, respectively, which is characteristic of Bi3+ in BiOBr (Figure 2b) (37). The Br 3d peaks are associated with binding energy of 68.20 eV (Figure 2c). Meanwhile, the high-resolution of O 1s XPS spectra are also recorded (Figure 2d), which can be fitted by two peaks at binding energies of 530.0 and 531.5 eV, respectively. The dominant peak at 530.0 eV is characteristic of oxygen in BiOBr, and the other peak at around 531.4 eV suggests the presence of other components such as OH, H2O, and carbonate species adsorbed on the surface. Both XRD and XPS results confirm that the nonaqueous sol-gel synthesized sample is pure BiOBr. Figure 3 shows scanning electron micrographs (SEM) of the nonaqueous sol-gel synthesized BiOBr sample. The SEM image at low magnification shows that sample consists of polydispersive microspheres with diameters ranging from 2 to 5 µm (Figure 3a). The surfaces of the microspheres are rough as they are composed of numerous radically grown nanoplates. The nanoplates interweave together to form a flower-like hierarchical spheres (Figure 3b). Furthermore, energy dispersive X-ray (EDX) spectroscopy analysis reveals that carbon, bismuth, bromine, and oxygen elements coexist in the nonaqueous sol-gel synthesized BiOBr microspheres (Figure 3c), where carbon comes from the surface adventitious carbon from atmosphere and the conductive adhesive

FIGURE 1. XRD patterns of the as-prepared samples.

FIGURE 2. XPS spectra of the as-prepared samples, (a) survey of the sample, (b) Bi 4f, (c) Br 3d, and (d) O 1s. tape for SEM measurement. The average atomic ratio of O/Br/ Bi is almost 1.15:1:1.04 (inset of Figure 3c), indicating the product is oxygen-rich. The excess oxygen might arise from the surface OH, adsorbed H2O, and carbonate species on the surface. The microstructure and morphology of the nonaqueous sol-gel synthesized BiOBr products were further investigated by transmission electron microscopy (TEM). The representative bright-field TEM image of as-prepared samples

at low magnification confirms that the samples are of microspheres structures (SI Figure S2). The TEM images at high magnification further confirm that the microspheres are made up of many nanoplates, and these nanoplates are highly organized to form radial arrays from the center to the surface of these micropheres, suggesting that the individual nanoplates were formed simultaneously and then connected together as they continued to grow. The corresponding VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. SEM images at low magnification (a) and high magnification (b), and EDX pattern (c) of the as-obtained BiOBr samples.

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FIGURE 4. Plots of the decrease in NO concentration vs irradiation time in the presence of different photocatalysts. (a) UV-visible light irradiation, (b) Visible light irradiation, and dependence of ln(C/C0) on irradiation time under UV-visible light (c) and visible light irradiation (d), respectively. selected area electron diffraction (SAED) on different spheres or different positions of a given single microsphere are essentially identical and reveal that the BiOBr microspheres are polycrystalline. In general, the photocatalytic activity of the catalyst is closely related to its band structure. The UV-vis diffuse reflectance spectrum (DRS) of the nonaqueous sol-gel synthesized BiOBr microspheres displays a broad absorbance in the visible region (SI Figure S3). This permits the resulting BiOBr microspheres to respond to a wide range of solar spectrum. The bands with steep edges are attributed to band gap transitions. The band gap of BiOBr could be estimated from the tangent line in the plot of the square root of Kubelka-Munk fuctions against photon energy (38), as seen in the inset in SI Figure S3. The band gap is thus estimated to be 2.5 eV, close to the value reported in the literatures (31, 34). BiOBr microspheres with such an energy band gap are very attractive for utilizing more visible light for photocatalysis than anatase TiO2 with a wide band gap of about 3.2 eV. Photocatalytic Degradation of NO. The ability of the nonaqueous sol-gel synthesized BiOBr microspheres to remove NO in indoor air was investigated in order to check their potential for indoor air purification. Figure 4 shows the variation of NO concentration (C/C0) with irradiation time under different experimental conditions. As a comparison, direct photolysis of NO, and photocatalytic removal of NO with a chemical precipitation synthesized BiOBr were also performed under identical conditions. The preparation and characterization results of chemical precipitation synthesized BiOBr were provided in the Supporting Information. It was found that NO could not be photolyzed under both UV-vis

and visible light irradiation (Figure 4a and b). Under UV-visible light, the removal of NO reached 45% in 10 minutes in the presence of the nonaqueous sol-gel synthesized BiOBr microspheres, whereas the degradation of NO on the chemical precipitation synthesized BiOBr bulk powders was merely 13%, revealing the superior photocatalytic activity of the nonaqueous sol-gel synthesized BiOBr microspheres under UV-visible light (Figure 4a). The NO removal efficiency of the nonaqueous sol-gel synthesized BiOBr microspheres was also much higher than the famous photocatlayst Degussa TiO2 P25, which could only remove 8% NO in 10 minutes under UV-visible light irradiation (3). More interestingly, the nonaqueous sol-gel synthesized BiOBr microspheres also show high photocatalytic activity even under visible light (λ > 420 nm), as shown in Figure 4b. The removal rates were about 30 and 8% for the nonaqueous sol-gel synthesized BiOBr microspheres and the chemical precipitation synthesized BiOBr bulk powders, respectively. Moreover, the visible light photocatalytic activity of the nonaqueous sol-gel synthesized BiOBr microspheres was even significantly higher than that of C-doped TiO2 (25% of NO removal in 10 minutes) (3). For a clearly quantitative comparison, we used the Langmuir-Hinshelwood model (L-H) to describe the rates of photocatalytic destruction of NO (39). Here, the initial photocatalytic degradation of NO was recognized to follow mass-transfer-controlled first-order kinetics approximately as a result of low concentration target pollutants, as evidenced by the linear plot of ln(C/C0) versus photocatalytic reaction time t. The initial rate constant of the NO degradation over the nonaqueous sol-gel synthesized BiOBr microspheres under UV-visible light irradiation is estimated to be 0.0841 min-1, faster than that over the VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (a) The stability of BiOBr microspheres in multiple runs of degradation of NO; (b) The Plots of the decrease in NO concentration vs prolonged irradiation time. chemical precipitation synthesized BiOBr bulk powders (0.0167 min-1) (Figure 4c). Similarly, the photocatalytic removal of NO over the nonaqueous sol-gel synthesized BiOBr microspheres under visible light irradiation (0.0614 min-1) is significantly faster than 0.0094 min-1 over the chemical precipitation synthesized BiOBr bulk powders (Figure 4d). The higher photocatalytic activity of BiOBr microspheres than the precipitation synthesized counterpart bulk powders could mainly be attributed to the hierarchical structure with a high surface area. It is reported that the hierarchical structure was favorable for the diffusion of intermediates and could enhance the photocatalytic activity (40), and the BET surface area of the nonaqueous sol-gel synthesized BiOBr microspheres (21.1 m2/g) is much higher than that of the precipitation synthesized bulk powder (1.5 m2/g) (SI Figure S4). The better photocatalytic performance of BiOBr than that of TiO2 should be attributed to its efficient absorption of visible light because of the suitable band gap. The stability of a photocatalyst is important for its practical application. It was reported that the N-doped TiO2 and sulfide photocatalysts sometimes suffer from instability under repeated use. Comparing with aqueous-phase photocatalytic 4148

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reaction, an important disadvantage existing in gas-phase photocatalytic reactions is that the intermediates generated by photocatalysis would accumulate on the surface of the photocatalyst to deactivate the photocatalyst during the photocatalytic process in the gas phase, whereas water is able to remove reaction intermediates from the photocatalyst surface in aqueous-phase photocatalytic reaction system, which can alleviate the deactivation of photocatalyst (41). XRD analysis of the used BiOBr microspheres sample showed that the crystal structure of the used photocatalyst was not changed after the photocatalytic reaction with NO (SI Figure S5), suggesting its phase stability. To further test the stability of the nonaqueous sol-gel synthesized BiOBr microspheres on photocatalytic NO removal, we carried out the multiple runs of photocatlaytic experiment with the used BiOBr microspheres (Figure 5a). It was interesting to find the BiOBr microspheres catalyst only exhibited slight deactivation after six cycles of repeated experiments. In addition, a prolonged photodegradation experiment confirmed that BiOBr microspheres were not significantly deactivated during the photocatalytic oxidation of NO of concentration at the indoor

air level (Figure 5b), suggesting that the BiOBr microspheres are promising for indoor air purification under visible light irradiation. The photocatalytic oxidation of gaseous NO has been proposed to involve reactions displayed in eqs 1-4, in which nitrogen monoxide reacted with reactive radicals to produce HNO2 and HNO3 (42, 43). Therefore, the measurement of the NO3- and NO2- species production can be used to determine the major process for the photocatalytic oxidation of NOx molecules with the nonaqueous sol-gel synthesized BiOBr microspheres. The amount of NO3- and NO2- collected with deionized water from the used BiOBr microshperes were monitored by IC, where each plot was acquired by repeating separated experiments in which the illumination stopped at a given time (SI Figure S6). It was found the amount of NO3progressively grew near linearly with reaction time, after approximately 22 h of UV-visible light degradation, the amount of NO3- collected from the used BiOBr microspheres reached 21.17 µmol, revealing the continuous production of HNO3 during the photocatalytic process (SI Figure S6a). However, the amount of NO2- increases at the initial stage, and then keeps almost constant at 0.08 µmol regardless of the prolonged illumination (SI Figure S6b). The amount of produced NO2- is much lower than that of produced NO3-, indicating that the oxidation of NO to NO3- is the major process for the oxidation of gaseous NO over BiOBr microspheres. At the initial photocatalytic oxidation, the amount of NO3- was very low; the deactivation of photocatalyst by NO3- could not play an important role. As the degradation proceeded, more NO3- would be generated, which worsen photocatalytic performance of the BiOBr microspheres in the late stage of photocatalytic removal of NO (Figure 5). It was reported that the photocatalytic activity of the TiO2 thin film decreased with the accumulation of HNO3 on the surface (43). Similarly, these final product (HNO2 and HNO3) absorbed on the surface of the photocatalyst are the reason for the slight decrease of photocatalytic activity of the nonaqueous sol-gel synthesized BiOBr microspheres. This kind of slight deactivation could be easily regenerated by washing the photocatalyst with water according to our experiments. NO + 2·OH f NO2 + H2O

(1)

NO2 + ·OH f NO3 + H

(2)

NO + NO2 + H2O f 2HNO2

(3)

NOx + ·O2 f NO3

(4)

Considering the nitrogen mass balance between the amount of produced NO3- and NO2- ([NOx-]rc) and the amount of NOx (NO2 and NO) removed from air ([NOx]rm, µmol), [NOx-]rc and [NOx]rm at time t (min) can be calculated. It was found that the amount of removed NOx- determined from the IC analysis was somewhat lower than that calculated (SI Table S1). The reason could be attributed to the release of HNO2 or HNO3 into humidified air (RH 75%), and the generation of new species undetectable by NOx chemiluminescent analyzer or IC.

Acknowledgments This work was supported by the Research Grants Council of Hong Kong (Poly U 5204/07E) and the Hong Kong Polytechnic University (GYX0L, GYF08 and GYX75). This work was also partially supported by National Science Foundation of China (Grants 20673041 and 20777026), National Basic Research Program of China (973 Program) (Grant 2007CB613301), and Program for New Century Excellent Talents in University (Grant NCET-07-0352).

Supporting Information Available Preparation of BiOBr bulk materials; TEM images, UV-vis diffuse reflectance absorption spectra, and N2 adsorptiondesorption isotherms of BiOBr microspheres and BiOBr bulk powders; XRD patterns of the BiOBr microspheres after the reaction with NO; The amount of NO3-1 and NO2- recovered by extraction with water from the BiOBr microspheres and detected by IC vs reation time, respectively; the calculated and measured [NOx-]rc values. This material is available free of charge via the Internet at http://pubs.acs.org.

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