Article pubs.acs.org/EF
Highly Efficient Nitric Oxide Absorption by Environmentally Friendly Deep Eutectic Solvents Based on 1,3-Dimethylthiourea Yongli Sun,†,‡ Guangsen Wei,† Xiaowei Tantai,*,† Zhaohe Huang,† Huawei Yang,† and Luhong Zhang† †
School of Chemical Engineering and Technology and ‡Tianjin Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: In this work, a series of 1,3-dimethylthiourea (1,3-DMTU)-derived environmentally friendly deep eutectic solvents (DESs) were reported and used for nitric oxide (NO) capture. The DESs were synthesized by mixing 1,3-DMTU with different hydrogen bond donors at different molar ratios, and their NO absorption performances were investigated. All of the DESs possessed excellent NO absorption performance. Notably, the absorption capacity of 1,3-DMTU/P4444Cl (3:1, molar ratio) DES is 4.25 mol/mol. 1,3-DMTU had a paramount effect on the performance of DESs, and the NO absorption capacity of these DESs increased with the increase of the molar ratio of 1,3-DMTU/P4444Cl in DESs. Moreover, the 1,3-DMTU-derived DESs showed good reusability; they could be recovered and reused 5 times. Through a combination of experimental results, quantum chemical calculation, proton nuclear magnetic resonance, and Fourier transform infrared spectroscopy investigation, the absorption mechanism was proposed and the easy deprotonation of 1,3-DMTU in DES was demonstrated to promote the bonding of NO.
1. INTRODUCTION Nitric oxide (NO), one of the main harmful substances in air pollution, can directly cause serious atmospheric pollution, such as acid rain, photochemical smog, and the formation of particulate matter with an aerodynamic diameter of up to 2.5 μm (PM2.5), which is a huge threat to human health and environmental balance. During the coal combustion process under high temperatures, NO accounts for 90% in the emission of nitrogen oxides.1 With the acceleration of the industrialization process, it is extremely significant and urgently necessary to reduce the emission of NO. Until now, much efforts have been focused on the development of effective NO removal technology, such as selective catalytic reduction (SCR),2 selective non-catalytic reduction (SNCR),3 liquid-phase complexation absorption (FeIIEDTA),4 photocatalytic decomposition,5 etc. SCR and SNCR are prevalent methods in the industry process, and therein, the denitration efficiency of SCR can reach up to 90%.6 Nevertheless, the drawbacks of them are obvious, such as high catalyst and operational cost, high working temperature, catalyst poisoning by sulfur compounds, and corrosion by NH3.7 Therefore, it is urgent for us to find a facile and less energy consumption method to eliminate the risk of NO. Over the past 2 decades, ionic liquids (ILs) have drawn a lot of attention owing to their unique characteristics, such as high thermal and chemical stability, extremely low volatility, nonflammability, and negligible vapor pressure.8,9 Furthermore, ILs can also be designed by changing the structures of their cations and anions for different applications.10 Until now, the application of ILs has extended to various fields, including catalytic reaction,11 extraction and separation,12 synthetic reaction,13 and gas absorption.14,15 In 2016, Wang et al.16 reported NO chemisorption by an azole-based ILs through multiple-site absorption, which was the first literature work that © XXXX American Chemical Society
reported the absorption of NO using ILs. Furthermore, the IL had a good absorption capacity for NO. Nevertheless, the high price and complicated preparation process still pose a major drawback in industrial applications of ILs. More recently, a new type of IL derivatives, named deep eutectic solvents (DESs), has aroused intense interests and been studied extensively.17 DESs, alternatives to ILs, have an ionic character but consist of a mixture of organic compounds, exhibiting an observably lower melting point than their individual components. Beyond that, DESs with high purity can be easily prepared at a considerably lower cost than ILs just by mixing hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA). Up to now, a lot of substances have been explored as HBD and HBA. For instance, various polyhydric alcohols, amides, organic acids, and sugars are chosen as HBDs,18 and quaternary ammonium salts19 and quaternary phosphonium salts20 are commonly used as HBAs. In 2011, Duan and his co-workers synthesized caprolactam tetrabutylammonium halide DESs to absorb NO. The DESs were used caprolactam as HBD and quaternary ammonium salts [tetrabutylammonium fluoride (N4444F), tetrabutylammonium chloride (N 4444 Cl), and tetrabutylammonium bromide (N4444Br)] as HBAs at different molar ratios. According to the results, CPL/N4444F DES (2:1, molar ratio) possessed the best NO absorption capacity, with just 0.16 mol/mol at 101.3 kPa and 338 K.21 In 2016, our team synthesized a series of DESs based on 1,3-dimethylurea (1,3-DMU) and tetrabutylphosphonium bromide (P4444Br), and their capacity for NO capture was investigated. The NO absorption capacity of DES (3:1, molar ratio) could reach a level at 1.173 mol/mol.22 Received: July 22, 2017 Revised: September 22, 2017
A
DOI: 10.1021/acs.energyfuels.7b02142 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels However, the absorption capacity failed to reach a satisfactory level. The exchange of oxygen by sulfur in carbonyl compounds obtained thiocarbonyl compounds, which may show notable differences in their physicochemical properties because of the greater bulk and polarizability of the sulfur atom and its decreased electronegativity.23 Kenneth et al.24 stated more charge transfer from nitrogen to sulfur in thioamide than from nitrogen to oxygen in amides, which could cause nitrogen electron deficiency. In 1,3-dimethylthiourea (1,3-DMTU), the charge transfer from nitrogen to sulfur causes the electron cloud density of the N atom to decline, and the poor electronegativity of the N atom can be easily deprotonated. On the basis of our previous work, 1,3-DMTU was very favorable for the NO absorption. Herein, in this work, a series of 1,3DMTU-based DESs were investigated and used for NO capture. First, the effect of DESs with various hydrogen bond receptors and molar content of 1,3-DMTU on NO absorption performance were studied. Subsequently, the effect of different temperatures on absorption capacity and repeatability were also discussed. Finally, Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR) spectra, theoretical calculation, and experimental results were used to clarify the mechanism of the NO absorption process.
Figure 1. Experimental diagram of NO absorption: (1) NO gas cylinder, (2) N2 gas cylinder, (3) digital thermostat water bath, (4) residual gas absorption bottle (NO is absorbed by H2O2 solution), (5) glass gas sampling tube, (6) gas rotor flow meters, and (7) pressure relief valve. absorption process can be finished until the weight remains constant, in which the NO absorption reached an equilibrium status. The saturated absorption of DES was regenerated in the vacuum drying oven at 80 °C and 10.1 kPa vacuum, and desorption of NO was determined by an electronic balance with an accuracy of ±0.0001 g. The weight change of desorption of NO was small after 1 h, and the regenerated DES was used to absorb NO again. This whole operating process was carried out 5 times to test the reusability of DES and recovery of NO gas. 2.5. Viscosity Determination. Viscosity of DESs was measured using the viscometer of Brookfield DV-II+ Pro (Brookfield Engineering Laboratories, Inc., Middleborough, MA, U.S.A.) at 30 °C. The reproducibility of the measurements was better than ±2.5%, and it was estimated that the data were accurate to ±5%. 2.6. Decomposition Temperature Determination. To obtain thermal stability of DES, the decomposition temperature was tested using thermogravimetric analysis (TGA, NETZSCH). 2.7. FTIR and 1H NMR Spectra Investigations. The virgin DES and NO-absorbed DES were measured by FTIR and 1H NMR to confirm the structure of DESs and investigate the NO absorption mechanism. The FTIR spectra of DESs were measured by a Nicolet 380 spectrophotometer (Thermo Electron Co.), using a typical thinfilm method in the wavenumber range from 400 to 4000 cm−1 under ambient conditions. A 500 MHz Bruker Avance III (Germany) was used to obtain the 1H NMR spectra of DESs.
2. EXPERIMENTAL SECTION 2.1. Materials. 1,3-DMTU, tetrabutylphosphonium chloride (P4444Cl), tetrabutylphosphonium bromide (P4444Br), N4444Cl, and N4444Br were purchased from J&K Chemical, Ltd., China. All reagents were obtained in the highest purity grade possible (≥99%) and directly used as received without further purification. NO gas (≥99.9%) and N2 (≥99.99%) gas were supplied by Tianjin Shengtang Specialty Gases Co., Ltd. 2.2. Preparation of DESs. 1,3-DMTU was chosen as HBD, and P4444Cl, P4444Br, N4444Cl, and N4444Br were chosen as HBAs. All DESs were synthesized following a similar route by mixing a certain molar ratio (mostly 1:1, 2:1, and 3:1) of HBD and HBA under stirring and heating at 60 °C for 1 h. The DESs were 1,3-DMTU/P4444Cl (1:1, molar ratio), 1,3-DMTU/P4444Cl (2:1, molar ratio), 1,3-DMTU/ P4444Cl (3:1, molar ratio), 1,3-DMTU/P4444Br (1:1, molar ratio), 1,3DMTU/N4444Cl (1:1, molar ratio), and 1,3-DMTU/N4444Br (1:1, molar ratio). All of DESs were used to study the absorption of NO. All synthesized DESs were dried under vacuum at 60 °C for more than 24 h to reduce the possible content of water. The water content was less than 0.2 wt %, which was determined by a Karl Fischer moisture titrator (AKF-2010, Hogon Co., Ltd.). 2.3. Experimental Apparatus. The NO absorption apparatus was mainly made up of the following two gas cylinders (N2 and NO), an electric-heated thermostatic water bath, a gas absorption tube (inner diameter of 10 mm and length of 100 mm), two gas flowmeters, and a residual gas absorption bottle. The NO absorption in different DESs by this experimental apparatus was measured from 30 to 70 °C at 101.3 kPa. The diagram of the experimental apparatus is depicted in Figure 1. 2.4. Absorption and Desorption of NO. In a typical absorption process, a kind of DES (about 1 g) as an absorbent was weighed in the gas absorption tube by an electronic balance and then the absorption experimental apparatus was loaded. After checking the air tightness, N2 at a flow ratio of 60 mL/min was used to drive away the air in this system for 30 min. After N2 was cut off by a valve, NO at atmospheric pressure was bubbled through into DES in the gas absorption tube at a rate of about 20 mL/min. The gas absorption tube was partly immersed in the electric-heated thermostatic water bath, so that the DES can keep a stationary temperature. The weight change of the gas absorption sample tube was determined at regular intervals (per 30 min) by an electronic balance with an accuracy of ±0.0001 g, which can be used to calculate the molar ratio of absorbed NO by DES. The
3. RESULTS AND DISCUSSION 3.1. Viscosities of DESs. The viscosities of prepared DESs were measured at 30 °C before NO bubbling. As shown in Table 1, there were significant differences among them. All of Table 1. Viscosity (cP) of Prepared DESs before NO Absorption DES
molar ratio
viscosity (cP)
1,3-DMTU/P4444Br 1,3-DMTU/N4444Cl 1,3-DMTU/N4444Br 1,3-DMTU/P4444Cl 1,3-DMTU/P4444Cl 1,3-DMTU/P4444Cl
1:1 1:1 1:1 1:1 2:1 3:1
245 512 1123 105 173 286
the DESs containing quaternary phosphonium salts exhibited a noticeably lower viscosity than that of DESs consisting of quaternary ammonium salts at the same molar ratio. Furthermore, among them, the DESs containing chlorine salts as HBA were also less viscous compared to bromine salts. In particular, 1,3-DMTU/P4444Cl (1:1) had the lowest viscosity of 105 cP. As for the different molar ratios of 1,3-DMTU/ B
DOI: 10.1021/acs.energyfuels.7b02142 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels P4444Cl DES, the viscosity became higher with the molar content of 1,3-DMTU increasing. 3.2. NO Absorption Using DES with Different HBAs. To investigate the effect of different HBAs on the absorption performance, the time-dependent absorption curves of NO at 303.15 K and 101.3 kPa were presented, as shown in Figure 2.
Figure 3. NO absorption by different molar ratios of 1,3-DMTU/ P4444Cl DESs at 30 °C.
absorption capacity of DES (3:1, molar ratio) reached up to 4.25 mol/mol and the absorption process lasted 8 h (Figure 3). 3.4. Effect of the Temperature on NO Absorption of DESs. To study the effect of the temperature on NO absorption of DESs, the 1,3-DMTU/P4444Cl DESs (1:1, 2:1, and 3:1, molar ratio) were tested from 30 to 70 °C at 101.3 kPa and all of the absorption processes were completed within 8 h. With the temperature increasing (Figure 4), the amount of NO
Figure 2. NO absorption by DESs with different HBAs at 303.15 K.
It was clear that the DESs with different HBAs possessed distinguishing NO absorption capacity. Interestingly, the DESs containing chlorine salts as HBAs performed much better absorption capacity than those containing bromine salts. For example, the capacity of 1,3-DMTU/P4444Cl (1:1) is 2.13 mol/ mol, while the absorption capacity of 1,3-DMTU/P4444Br (1:1) was only 1.13 mol/mol. Similar differences could also be observed in the case of 1,3-DMTU/N4444Cl (1:1) and 1,3DMTU/N4444Br (1:1). The absorption rate of DESs containing quaternary phosphonium salts was much faster than that of DESs containing quaternary ammonium salts. 1,3-DMTU/P4444Cl (1:1) DES reached absorption equilibrium within 6 h, which was 2 h faster than 1,3-DMTU/N4444Cl (1:1) DES. The result revealed that the viscosity increase would slow the absorption kinetics, and a longer time was needed to reach equilibrium. Consequently, low viscosity was favorable to accelerate the absorption rate. It is worth nothing that 1,3-DMTU/P4444Cl (1:1) DES had the smallest viscosity, the best absorption performance, and a higher absorption rate, which provided evidence that P4444Cl plays a vital role in the activation of 1,3DMTU toward NO absorption. 3.3. NO Absorption by Different Molar Ratios of 1,3DMTU/P4444Cl DESs. To further study the influence of composition of DESs upon the absorption performance, different molar ratios (from 1:1 to 3:1) of 1,3-DMTU/ P4444Cl DESs were synthesized and investigated for NO absorption. The dependence of the amounts of NO absorbed by different molar ratios of 1,3-DMTU/P4444Cl DESs at 30 °C and 101.3 kPa were shown in Figure 3. As seen, all of the absorption could be completed within 8 h at the condition of continuous bubbling NO. The saturated NO absorption capacity increased when the molar ratio of 1,3-DMTU/ P4444Cl increased in the DESs. For example, the molar ratio of absorbed NO/DES (1:1, molar ratio) was 2.10 mol/mol at 30 °C and 101.3 kPa, while under the same conditions, the
Figure 4. Effect of the temperature on NO absorption by different molar ratios of DESs.
in the DESs decreased continuously. For instance, the solubility of NO in 1,3-DMTU/P4444Cl DES (3:1, molar ratio) was 4.25 mol/mol at 30 °C, while it decreased to 1.78 mol/mol at 70 °C. Such a result means that a low temperature is favorable for achieving a high absorption amount of NO, while a high temperature is helpful for stripping of absorbed NO and regeneration of the absorbents. TGA was further used to investigate the thermostability of DESs. From the thermogravimetric curve in Figure 5, the results showed that the thermal decomposition temperature of 1,3-DMTU was 167 °C and that of 1,3-DMTU/P4444Cl (3:1, molar ratio) DES was 202 °C; the latter had good thermal stability. Besides, the DES shows a two-step weight loss. The first stage of the descent curve reaches at 315 °C with 51% C
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charge distribution of 1,3-DMU and 1,3-DMTU at the B3LYP/ 6-311G++ (d,p) level with the Gaussion 09 software program. The comparison based on the Mulliken population analysis of atomic charge is shown in Figure 7. It was clear that the N
Figure 7. Mulliken population atomic charges of (a) 1,3-DMU and (b) 1,3-DMTU. Figure 5. TGA of 1,3-DMTU and 1,3-DMTU/P4444Cl DES (3:1).
atom charge of the N−H bond was quite different between 1,3DMU and 1,3-DMTU. In 1,3-DMTU, the Mulliken population atomic charge of the N atom is −0.078 compared to −0.270 in 1,3-DMU. Moreover, it should be noted that charge transfer from nitrogen to sulfur in 1,3-DMTU was more than that from nitrogen to oxygen in 1,3-DMU; therefore, the electron delocalization degree of the N atom was much stronger. As a result, the electron cloud density of the N atom in 1,3-DMTU decreased, and thus, the N−H bond became weaker. This phenomenon was further confirmed by FTIR analysis of 1,3DMTU/P4444Cl (1:1). As shown in Figure 8, in comparison to
weight loss, which is equal to the 1,3-DMTU mass percent in the DES, while the second step is attributed to the thermal decomposition of 1,3-DMTU/P4444Cl. 3.5. Reusability of DES. The reusability of DES is an important criterion to evaluate the potential practical application. Thus, the reversibility of 1,3-DMTU/P4444Cl (3:1) DES was tested at 80 °C and 10.13 kPa, as shown in Figure 6, which was much lower than the decomposition
Figure 6. NO absorption by 1,3-DMTU/P4444Cl (3:1) DES for 5 cycles. NO absorption was carried out at 30 °C for 8 h in the first time and 5 h in others, and the desorption was performed at 80 °C and 10.13 kPa for 1 h.
Figure 8. FTIR spectra of the DES before and after absorption of NO.
our previous work, only one peak appeared at 3300 cm−1, which attributed to the N−H stretching of 1,3-DMTU.22 Interestingly, two peaks were found at 3251 and 3074 cm−1, which could be attributed to the N−H stretching of 1,3DMTU, while the peak at 3074 cm−1 indicated that the N−H bond of 1,3-DMTU could be partially deprotonated. To gain insight into the reaction mechanism between NO and DES, FTIR and 1H NMR spectra were recorded and the absorption process was further estimated by DFT calculation at the B3LYP/6-311G++ (d,p) level with the Gaussion 09 software program. As shown in the FTIR spectrum of 1,3DMTU/P4444Cl DES before absorption (Figure 8), the N−H stretching vibration (3251 and 3074 cm−1), C−H in −CH3
temperature of DES. The result demonstrated that NO absorption by 1,3-DMTU/P4444Cl (3:1) DES was reversible and the NO absorption capacity could be maintained more than 5 times, where the desorption residue was about 1.3 mol/ mol NO. Although saturated DES was not completely desorbed, it still performed a satisfactory desorption and repeatability. 3.6. Mechanism of NO Absorption. On the basis of the experimental results and our previous work, it was clear that the NO absorption capacity of 1,3-DMTU was much higher than that of 1,3-DMU. To investigate the performance difference between them, DFT calculation was used to compare the D
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Figure 9. Prediction of the vibration model after NO absorbed by DES.
(2958 cm−1), C−H in −CH2 (2931 and 2873 cm−1), N−H inplane bending vibration (1554 cm−1), C−N (1188 cm−1), and CS (1040 cm−1) could be observed. After the absorption of NO, significant changes could be detected. The N−H peak at 3074 cm−1 disappeared, which indicated that the N−H bond was dissociated. Furthermore, a series of new peaks appeared at 1674, 1369, 1292, and 999 cm−1 in the FTIR spectrum. Through comparison to DFT calculations (Figure 9), the new peaks at 1369 and 1292 cm−1 were mainly caused by ν(NO) in −N−NO and ν(N−O) in −N−N−O, respectively, while the other new peak at 999 cm−1 was attributed to ν(N−N) in −N−NONO. As for the new peak at 1674 cm−1, it should be ν(N−O) of the NO molecule, which was due to the physical dissolution in the DES.16 Besides, according to the 1H NMR spectra of DES before and after absorption of NO in Figure 10, there were two
obvious new peaks that appeared at δ = 3.46 and 3.29 ppm, which could be assigned to the methyl groups of the reaction product, namely, CH3−N and CH3−NH. Their distinct peak positions could be explained by the unsymmetrical structure of the reaction product. After absorption, one of CH3−NH was replaced by CH3−N−(NO−)−NO. The introduction of electron-withdrawing groups in 1,3-DMTU could not only improve the shielding effect but also cause the asymmetry on the electronic delocalization, resulting in the shifting of resonances of the two methyl groups, from 2.90 to 3.29 and 3.46 ppm, respectively. Therefore, in the absorption process, NO could be easily absorbed by 1,3-DMTU-based DESs as a result of the easy deprotonation of 1,3-DMTU in the DES system. The reaction of NO with amines to produce zwitterions had been reported for years.25,26 Early confusion over the structure of diazeniumdiolated diamines was resolved years later, when the zwitterionic nature of the polyamine/NO adducts was demonstrated and clearly shown that NO was added sequentially to polyamine.27,28 With the aid of quantum chemical calculations, FTIR, and 1H NMR analysis, a possible NO absorption mechanism can be presented in Figure 11. NO could be absorbed by one of CH3−NH in the 1,3-DMTU/ P4444Cl DES to form the structure of −N−(N−O−)−NO. 3.7. Comparison to Other DESs and Some ILs. Through the investigation of the present DESs as NO absorbents, we compared the absorption capacity of NO by the present DESs to that of other DESs as well as ILs reported in the literature, with the results listed in Table 2 on the basis of the molar ratio. It could be found that the present 1,3-DMTU/P4444Cl (3:1) DES demonstrates similar absorption capacity for NO as [P66614][Tetz]-functionalized ILs and is obviously superior to the other DESs. Moreover, the functionalized ILs suffer from multi-step synthesis and high cost. Thus, the present DESs are regarded as more promising NO absorbents as a result of their high absorption capacity, ease of synthesis, and reasonable cost.
Figure 10. 1H NMR spectra (500 MHz) of the DES before and after absorption of NO in CDCl3.
Figure 11. Possible NO absorption mechanism by 1,3-DMTU/P4444Cl DES. E
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Energy & Fuels Table 2. Comparison of NO Absorption Capacity of Present DESs to Those of Other DESs and ILs absorbent (molar ratio)
temperature (°C)
pressure (atm)
NO absorption (mol/mol)
reference
1,3-DMTU/P4444Cl (3:1) 1,3-DMTU/P4444Cl (2:1) 1,3-DMTU/P4444Cl (1:1) 1,3-DMTU/P4444Br (1:1) 1,3-DMTU/N4444Cl (1:1) 1,3-DMTU/N4444Br (1:1) 1,3-DMU/P4444Br (1:1) 1,3-DMU/P4444Br (2:1) 1,3-DMU/P4444Br (3:1) [P66614][Tetz] CPL/TBAF (2:1) CPL/N4444Cl(2:1) CPL/N4444Br (2:1)
30 30 30 30 30 30 30 30 40 30 65 70 70
1 1 1 1 1 1 1 1 1 1 1 1 1
4.25 3.18 2.13 1.13 2.05 1.00 0.66 0.92 1.17 4.52 0.16 0.13 0.09
this work this work this work this work this work this work our previous work22 our previous work22 our previous work22 Wang et al.16 Duan et al.21 Duan et al.21 Duan et al.21
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4. CONCLUSION In this contribution, 1,3-DMTU-derived DESs with low viscosity and high NO absorption capacity have been studied. The hydrogen bond receptor played an important role on the physicochemical property as well as absorption efficiency. Moreover, the content of 1,3-DMTU in DESs determined NO absorption capacity. Among those DESs, 1,3-DMTU/P4444Cl (3:1, molar ratio) had the highest NO absorption capacity, up to 4.25 mol/mol. The absorption process was an exothermic reaction, and absorption capacity decreased with the temperature increasing. In comparison to our previous work, the DESs based on 1,3-DMTU showed better absorption performance than 1,3-DMU-based DESs. Through a combination of quantum chemical calculations, FTIR, and 1H NMR investigation, we had a better understanding of the absorption mechanism of the studied DES in this work and confirmed that the easy deprotonation of 1,3-DMTU in the DES system promoted the absorption. In addition, the 1,3-DMTU/P4444Cl DES can be reused more than 5 times with little weight loss and maintains an almost unchanged absorption capacity. In summary, this work proposes novel DESs, which have the potential application for NO removal, and provides the strategy for enhancing the NO capture performance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaowei Tantai: 0000-0003-2925-0444 Huawei Yang: 0000-0002-3510-9407 Luhong Zhang: 0000-0002-7073-4793 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors are grateful for the financial support from the National Key R&D Program of China (2016YFC0400406).
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DOI: 10.1021/acs.energyfuels.7b02142 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.7b02142 Energy Fuels XXXX, XXX, XXX−XXX
