- Email: [email protected]
Investigation on removal of NO and Hg0 with different Cu species in Cu-SAPO-34 zeolites
Catalysis Communications 119 (2019) 91–95
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Investigation on removal of NO and Hg0 with different Cu species in CuSAPO-34 zeolites ⁎
Yu Wanga, Wenzhe Sia,b, , Yue Penga, Junhua Lia, a b
T
⁎
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Elemental mercury oxidation NH3-SCR Cu-SAPO-34 zeolites
A series of Cu-SAPO-34 zeolites with different Cu contents were prepared by one-pot synthesis method. The NH3-SCR performance and the oxidation of elemental mercury (Hg0) efficiency over the Cu-SAPO-34 samples were tested respectively. With the increase of Cu loadings, the amount of Cu2+ and CuO were both increased, wherein the former promoted SCR reaction and the latter accelerated the Hg0 oxidation. What's more, the ratio of Cu2+/CuO was also increased. The NO conversion rate and Hg0 removal efficiency could be over 80% on the high Cu catalyst at 150–300 °C.
1. Introduction With the development of industry, mercury (Hg) has become a highly toxic heavy metal that causes serious human health problems [1–3]. Hg emission from coal burning is the most significant one among the main anthropogenic sources [4,5]. There are three main existence forms of Hg in coal-fired flue gas, element Hg (Hg0), oxidized Hg (Hg2+), and particle-bound Hg (Hgp) [6]. As we know, Hg2+ and Hgp could be relatively easily captured by conventional pollution control facilities [7,8]. However, it is difficult to remove Hg0 because of its low solubility in water and weak interaction on adsorbent particles [9,10]. Therefore, it is of great importance to find out a solution to remove Hg0 efficiently. Currently, adsorption and oxidation are two main technologies for the eliminatory of Hg0 [11–15]. However, adsorption technology is limited for wide applications owing to several disadvantages, such as higher cost, poor capacity, narrow operation temperature range, and slow regeneration and adsorption rates [16,17]. Conversely, Hg2+ (oxidized by Hg0) is facile to be removed through currently available pollution control devices, which makes oxidation method more feasible to some practical applications. In addition, some commercial catalysts used in selective catalytic reduction (SCR) of NOx with NH3 have been proven to exhibit some catalytic activities on the oxidation of Hg0 [18]. This provides us a thought on exploring a suitable catalyst which not only shows higher NOx removal efficiency, but also can promote the Hg0 removal. The operation temperature range for most SCR catalysts is
300–500 °C, during which the decomposition of Hg2+ (especially HgCl2) is easy to take place. However, the oxidation of Hg0 usually takes place at low temperature [19]. Hence it is necessary to develop attractive catalysts with high efficiencies both on DeNOx and Hg0 removal in low temperature range. Cu-SAPO-34 zeolite is one of the Cu/Chabazite catalysts which expresses superior activity on NH3-SCR reaction in a wide operation temperature range [20,21]. There exist several types of copper species in Cu-SAPO-34, such as isolated Cu2+ and Cu+ ions and CuO. For SCR reactions, the active sites are likely the isolated Cu ions inside the pores [22], while CuO is helpful to the oxidation of Hg0 to Hg2+ [23]. If the contents of Cu ions and CuO in Cu-SAPO-34 could be regulated in a proper ratio, it might be a promising catalyst for the removal of NOx and Hg0 with high performance. Copper species are traditionally introduced in small pore zeolites by ion-exchange method [24–26]. However the loading of Cu is quite limited because of the small pore openings and the restriction of ionexchanged capacity [27]. Recently, Corma et al. prepared Cu-SAPO-34 by “one-pot” synthesis method, in which the copper-amine complexes were introduced into the catalyst as templet directly [28,29]. Cu-SAPO34 prepared by this method contains much higher Cu content, and it is favorable to adjust the ratio of Cu cations and CuO. Herein, it is reported that the Cu-SAPO-34 catalysts prepared by one-pot synthesis method was used to remove NOx and Hg0 respectively. The NO conversion rate and Hg removal efficiency are both above 80% in 150–300 °C by adjusting the content of Cu loadings. Our
⁎ Corresponding authors at: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China. E-mail addresses: [email protected] (W. Si), [email protected] (J. Li).
https://doi.org/10.1016/j.catcom.2018.10.019 Received 5 August 2018; Received in revised form 3 October 2018; Accepted 19 October 2018 Available online 30 October 2018 1566-7367/ © 2018 Elsevier B.V. All rights reserved.
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
electron paramagnetic resonance spectrometer. Hydrogen temperature-programmed reduction (H2-TPR) experiments were performed on a chemisorption analyzer. Temperatureprogrammed desorption (TPD) of Hg0 studies were carried out in a fixed-bed quartz reactor equipped with an on-line Hg analyzer. More details on characterization could be seen in the Supporting Information.
Table 1 Starting compositions in the gels, Cu/Al ratio, Cu content and BET specific surface area of the samples. sample
Al/P/Si/Cu-TEPA/ DEA/H2O
Cu/Al molar ratio ICP
Cu1 Cu2 Cu3 Cu4 a b c
1/0.8/0.3/0.05/0.95/ 40 1/0.8/0.3/0.1/0.9/40 1/0.8/0.3/0.2/0.8/40 1/0.8/0.3/0.3/0.7/40
a
XPS
b
Cu wt (%) ICP
a
SBET (m2/g)
0.057
0.033
2.504
578
0.112 0.17 0.26
0.054 0.072 0.089
5.027 7.411 10.44
506 471 422
c
3. Results and discussion 3.1. Structure and BET area
Obtained from ICP results. Calculated from XPS results. Calculated by BET method.
Fig. S1 shows the XRD results of the samples in the 2θ range of 5–40°. The XRD patterns of the four catalysts with different Cu loadings could be corresponded well with the featured peaks (9.5°, 15.96°, 17.92°, 20.05°, 25.82°, 30.92° and 31.2°) of typical CHA structure [30]. The high intensity of the XRD peaks could suggest a good crystallinity. For Cu1 and Cu2 samples, no featured peaks of CuO (35.6° and 38.8°) could be observed, which meant that the Cu species were highly dispersed and the dominant existence form of Cu species were Cu cations. However, for Cu3 and Cu4 samples, the featured peaks of CuO appeared, demonstrating that CuO particles larger than 3 nm were aggregated on the external surface of Cu3 and Cu4 samples with the increase of Cu loading. The BET surface areas of the four samples are listed in Table 1. The surface areas of these samples are in the range of 422–578 m2/g, lower than the usual value of H-SAPO-34 (600 m2/g). In addition, with the increase of Cu loading, the BET areas of these samples decreased gradually, and Cu4 sample possessed the smallest BET area among the four samples. This indicates that the loading of Cu could decline the BET areas of the samples. The CuO particle existed on the surface of the sample might block the CHA cage, resulting in the decrease of the BET area.
research may suggest a new route for combined removal of Hg0 and NOx from flue gas in the future. 2. Experimental 2.1. Synthesis Copper-amine complex (Cu2+ with tetraethylenepentamine, CuTEPA) and diethylamine (DEA) were used as the organic structure directing agents (OSDAs). All chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. The chemical compositions of the reaction gels are given in Table 1. The loading of Cu could be adjusted by the different ratio of Cu-TEPA and DEA. In a typical synthesis procedure, phosphoric acid was firstly dissolved in deionized water. Then alumina and silica sources were added into the solution and stirred for 2 h. At last, CuSO4·5H2O and TEPA were introduced to the above gel under continuous stirring for 12 h. The resulting mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for 2 days. After filtered and washed with deionized water, the product was dried at 80 °C overnight. Then the powders were calcined at 550 °C for 4 h in air to remove the remained organic species. The samples were denoted as Cu1-Cu4 in the sequence of the increase of Cu contents, respectively.
3.2. Cu species XPS and ICP measurements were conducted to elucidate the effect of the Cu loading on the distribution of different Cu species, and the estimated Cu/Al atomic ratios on the surface and in the bulk are presented in Table 1. For all these samples, the surface Cu/Al ratios (0.033–0.089) are lower than those in the bulk (0.057–0.26), which implies that the Cu species existed in the samples are mainly in the form of Cu cations inside the Cu-SAPO-34 pores. Fig. 1 illustrates the Cu 2p XPS spectra of the samples, and the binding energy of the Cu 2p transition peak could provide information about the state of Cu species on the surface. The asymmetrical Cu 2p3/2 transition peak can be
2.2. Catalytic activity evaluation The catalytic activity for the SCR reaction was measured in a fixed bed quartz reactor (i.d. 6 mm) with 0.1 g catalyst. The total gas flow was 200 ml/min, containing 500 ppm of NO, 500 ppm of NH3, 5% of O2, and N2 (balance gas). The activity test was carried out from 150 to 550 °C. The concentrations of NH3 and NOx were monitored by a MKS MultiGas 2030 HS FTIR spectrometer. The Hg0 removal efficiency of the catalyst was also measured in a fixed-bed quartz reactor with 0.2 g catalyst. The concentration of Hg0 was collected by an RA-915 M cold vapor atomic absorption spectroscopy mercury analyzer (Lumex Co., Ltd., Russia). The vapor-phase Hg0 was generated by passing N2 flow through a permeation tube. The total gas flow was 200 ml/min, containing 250 μg/m3 of Hg0, 5% of O2, 5 ppm of HCl (when needed) and N2 (balance gas). Further details are described in the Supporting Information. 2.3. Characterization The crystal structure of the three samples was characterized by powder X-ray diffraction (XRD). The element contents were analyzed by inductively coupled plasma (ICP)-Optical Emission spectrometer. Surface areas of the samples were obtained by N2 adsorption–desorption at −196 °C. The surface atom composition and oxidation state of elements on the surface of the samples were determined by X-ray photoelectron spectroscopy (XPS). The EPR data were acquired on an
Fig. 1. Cu 2P XPS spectra of the samples. 92
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
Fig. 3. SCR activity of the Cu-SAPO-34 and V1W5Ti samples in the range of 150–550 °C. Reaction conditions: The total gas flow was 200 ml/min, containing 500 ppm of NO, 500 ppm of NH3, 5% of O2, and N2 (balance gas). The catalyst was 0.1 g.
Fig. 2. H2-TPR profiles of the samples.
deconvoluted. The peaks located at 932.9 eV and 935.6 eV are attributed to CuO and Cu2+, respectively. With the increase of Cu loading, the amount of CuO and Cu2+ are both increased. And the molar ratio of CuO/Cu2+ are also calculated. This ratio decrease from Cu1 to Cu4, which suggests that the raising rate of Cu2+ content was much faster than that of CuO. H2-TPR experiments were carried out to further investigate the change of Cu species over the samples with different Cu loading, and Fig. 2 shows the TPR profiles. According to the literature, the H2 consumption peaks in the temperature range of 175–400 °C could be generally divided into two parts: the peaks located at low temperature assigned to reduction of isolated Cu2+ to Cu+, and the peaks at high temperature assigned to reduction of CuO species to Cu0 [29,31]. In addition, the H2 consumption signal appeared at much higher temperature is attributed to the reduction of Cu+ to Cu0 [32]. Thus, there should be three types of Cu species in Cu-SAPO-34 samples, including external surface CuO, isolated Cu2+ and Cu+ ions inside the samples. However, as shown in Fig. 2, the reduction peak of Cu+ is hardly observed in these samples, which might be owing to the high stability or the little amount of the Cu+ species after the one-pot process. With the increase of Cu loading, the reduction peaks gradually shift towards lower temperature, indicating an increment on the redox property from Cu1 to Cu4. The amount of Cu2+ and CuO became larger as the increase of Cu loading, in agreement with the XPS results. EPR is an effective approach to identify the hyperfine structure of isolated Cu2+ ions [33]. As the EPR spectra shown in Fig. S2, all the samples exhibited the characteristic signal of isolated Cu2+ at g// =2.391 (four hyperfine splitting peaks originated from the interaction of the Cu nuclear and unpaired electron). This illustrates that the isolated Cu2+ species existed in these four Cu-SAPO-34 catalysts are in the same environment, which are octahedrally coordinated to three framework oxygen atoms and three water molecules in Cu-SAPO-34. Furthermore, the intensity of the EPR signal enhanced in the sequence of Cu1 < Cu2 < Cu3 < Cu4, which also indicates an increment on the contents of isolated Cu2+ ions. This result is in good accordance with the H2-TPR and XPS results.
increase of Cu loading. Thus the sample with higher Cu loading possesses better catalytic activity in low temperature range. When it comes to high temperature, unselective oxidation of NH3 will take place caused by the CuO on the surface, which is enhanced with the increase of the Cu loading. So the SCR activities of the catalysts gradually turn worse with the increase of Cu loading. However, the increase trend of CuO becomes slow, compare to Cu2+, which is already confirmed by TPR and XPS results. So the decrease trend of the SCR performance at high temperature becomes slow. Fig. S3 exhibits excellent N2 selectivity of all the four samples, which implies that few by-products (NO2 and N2O) were generated during the SCR process. 3.4. Hg0 adsorption properties Hg0 adsorption property of the catalyst is a key factor for the oxidation of Hg0. In order to investigate this property, Hg0-TPD experiments were carried out. Fig. 4 shows that there exist three desorption peaks 120 °C, 280 °C and 500 °C. The peak at about 120 °C is attributed to the decomposition of Hg2O and weak physical adsorption of Hg0, and the peak at about 280 °C comes from the chemical adsorption of Hg0, which is similar to the results in the literature [19]. In addition, it is reported that the decomposition of HgO usually appears above 400 °C [34]. As shown in Fig. 4, the TPD profiles illustrates that the decomposition peak of HgO for Cu-SAPO-34 samples appeared at approximately 500 °C. During the whole TPD experiment, no additional oxygen was introduced to the atmosphere. Thus the oxygen in HgO might come
3.3. SCR performance As shown in Fig. 3, the NO conversion of the samples was tested, and a commercial 1%V2O5–5%WO3-TiO2(VIW5Ti)was also tested as contrast. It can be found that all the four Cu-SAPO-34 samples exhibit excellent catalytic activities in the temperature range of 200–350 °C, obviously better than VIW5Ti. As the TPR results shown, the redox property of the catalysts at low temperature is improved with the
Fig. 4. Hg0-TPD profiles of the samples. 93
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
temperature is higher than 250 °C. This phenomenon may be caused by two factors. On one hand, the adsorption of Hg0 on the catalyst surface could influence the oxidation reaction. When the temperature increases from 100 to 200 °C, the reaction could enhance between the adsorbed Hg0 and the Oads on the catalyst surface. However, the Hg0 adsorption ability of the catalysts becomes lower when the temperature increase to higher than 250 °C, leading to a decrease on the oxidation efficiency. On the other hand, HgO could decompose into Hg and O2 at high temperature, also doing harm to the catalytic performance. [19] As HCl is commonly present in coal-derived flue gas composition, the effect of gaseous HCl in the reaction atmosphere was also investigated over Cu4 sample. When 5 ppm of HCl was added, the activity of the catalyst was greatly improved (Fig. 5b). Within 200–350 °C, the Hg0 oxidation efficiency of Cu4 sample was even over 95%. In the HCllacking condition, the oxidation products are Hg2O (< 150 °C) and HgO (> 300 °C). When HCl is introduced in the reaction mixture, Hg2O or HgO is easier to react with HCl to form HgCl or HgCl2, which then could promote the oxidation reaction. 4. Conclusions A series of Cu-SAPO-34 zeolites with different Cu contents were prepared by one-pot synthesis method using copper-amine complex as templet directly. Then the NH3-SCR performance and the oxidation of Hg0 efficiency over the thus obtained Cu-SAPO-34 samples were tested respectively. It is commonly recognized that isolated Cu2+, Cu+ ions and CuO are co-existed in Cu-SAPO-34. For SCR reactions, the active sites are likely the isolated Cu ions inside the pores, whereas CuO is the active component for the oxidation of Hg0. With the increase of Cu loadings, Cu2+ and CuO were both increased, and the ratio of Cu2+/ CuO was also increased. By adjusting the amount of Cu loadings, the catalysts exhibit excellent SCR activities in the temperature range of 200–350 °C. When they were used in Hg0 oxidation, the catalytic efficiency could reach above 80% over the sample with high Cu content at 150–300 °C. This work may provide new ideas for combined removal of Hg0 and NOx from coal-derived flue gas streams in the future.
Fig. 5. (a) Hg0 oxidation performance of the Cu-SAPO-34 and V1W5Ti samples and (b) the effect of HCl on the Hg0 oxidation performance over Cu4 sample. Reaction conditions: The total gas flow was 200 ml/min, containing 250 μg/m3 Hg0, 5% O2, 5 ppm HCl (when needed) and N2 (balance gas). The catalyst was 0.2 g.
Acknowledgements This work was financially supported by the National Natural Science Fund of China (Grant Nos. 21325731 and 51478241) and the National High-Tech Research and Development (863) Program of China (Grant No. 2013AA065304).
from the Olatt and residual Oads of the samples. With the increase of Cu loading, more HgO was decomposed to Hg and O2, indicating that the oxidability for the catalysts increased from Cu1 to Cu4. The strongest peak for Cu4 catalyst suggests that it is reasonably expected to possess strong potential for the adsorption and oxidation of Hg0.
Appendix A. Supplementary data
3.5. Hg0 oxidation performance
Characterization details, catalytic activity measurement method, and additional results are shown. Supplementary data to this article can be found online at doi: https://doi.org/10.1016/j.catcom.2018.10.019.
Hg0 oxidation performances of the four Cu-SAPO-34 catalysts were investigated in the temperature range of 100–400 °C (Fig. 5a). VIW5Ti catalyst was also tested as contrast. It is found that except for Cu1 sample, the Hg0 oxidation performance of other Cu-SAPO-34 catalysts are much better than that of V1W5Ti. With the increase of Cu loading, the catalytic activity of Hg0 oxidation is gradually improved. However, the increase trend of catalytic activity becomes slow. The Hg0 removal efficiencies for Cu3 and Cu4 were almost the same, especially at low temperature. For Hg0 oxidation, the main active component of CuSAPO-34 catalyst is CuO. As the TPR and XPS results indicate, compared to Cu2+, the increase trend of CuO becomes slow with the increase of Cu loading. The amount of CuO on the surface of the high Cu loading sample reach a threshold value. No more CuO could be generated on the catalyst by increasing the amount of Cu loading. So the catalytic performances on Hg0 removal for Cu3 and Cu4 samples are nearly the same. In addition, from the Hg0 oxidation conversion profiles it can be observed that the catalytic activity of the samples increases from 100 to 200 °C and then decreases dramatically when the
References [1] T.M. Bisson, L.C.W. MacLean, Y. Hu, Z. Xu, Characterization of mercury binding onto a novel brominated biomass ash sorbent by X-ray absorption spectroscopy, Environ. Sci. Technol. 46 (2012) 12186–12193. [2] Y. Liu, T.M. Bisson, H. Yang, Z. Xu, Recent developments in novel sorbents for flue gas clean up, Fuel Process. Technol. 91 (2010) 1175–1197. [3] J. Yang, Y. Zhao, X. Guo, H. Li, J. Zhang, C. Zheng, Removal of elemental mercury from flue gas by recyclable CuCl2 modified magnetospheres from fly ash. Part 4. Performance of sorbent injection in an entrained flow reactor system, Fuel 220 (2018) 403–411. [4] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A. Benson, Status review of mercury control options for coal-fired power plants, Fuel Process. Technol. 82 (2003) 89–165. [5] Z. Yang, H. Li, X. Liu, P. Li, J. Yang, P.-H. Lee, K. Shih, Promotional effect of CuO loading on the catalytic activity and SO2 resistance of MnOx/TiO2 catalyst for simultaneous NO reduction and Hg0 oxidation, Fuel 227 (2018) 79–88. [6] G. Skodras, I. Diamantopoulou, G. Pantoleontos, G.P. Sakellaropoulos, Kinetic studies of elemental mercury adsorption in activated carbon fixed bed reactor, J. Hazard. Mater. 158 (2008) 1–13.
94
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
N2O decomposition: a comparison with CuZSM-5, J. Catal. 217 (2003) 100–106. [22] L. Wang, W. Li, G. Qi, D. Weng, Location and nature of Cu species in Cu/SAPO-34 for selective catalytic reduction of NO with NH3, J. Catal. 289 (2012) 21–29. [23] X. Fan, C. Li, G. Zeng, X. Zhang, S. Tao, P. Lu, S. Li, Y. Zhao, The effects of Cu/ HZSM-5 on combined removal of Hg0 and NO from flue gas, Fuel Process. Technol. 104 (2012) 325–331. [24] M. Moliner, C. Martínez, A. Corma, Synthesis strategies for preparing useful small pore zeolites and zeotypes for gas separations and catalysis, Chem. Mater. 26 (2014) 246–258. [25] M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Cu-SSZ-39, an active and hydrothermally stable catalyst for the selective catalytic reduction of NOx, Chem. Commun. 48 (2012) 8264–8266. [26] J. Hun Kwak, H. Zhu, J.H. Lee, C.H.F. Peden, J. Szanyi, Two different cationic positions in Cu-SSZ-13? Chem. Commun. 48 (2012) 4758–4760. [27] L. Ren, L. Zhu, C. Yang, Y. Chen, Q. Sun, H. Zhang, C. Li, F. Nawaz, X. Meng, F.S. Xiao, Designed copper-amine complex as an efficient template for one-pot synthesis of Cu-SSZ-13 zeolite with excellent activity for selective catalytic reduction of NOx by NH3, Chem. Commun. 47 (2011) 9789–9791. [28] R. Martínez-Franco, M. Moliner, C. Franch, A. Kustov, A. Corma, Rational direct synthesis methodology of very active and hydrothermally stable Cu-SAPO-34 molecular sieves for the SCR of NOx, Appl. Catal., B 127 (2012) 273–280. [29] R. Martínez-Franco, M. Moliner, P. Concepcion, J.R. Thogersen, A. Corma, Synthesis, characterization and reactivity of high hydrothermally stable Cu-SAPO34 materials prepared by “one-pot” processes, J. Catal. 314 (2014) 73–82. [30] M.M.J. Treacy, J.B. Higgins, CHA - Chabazite, K-exchanged, in: M.M.J. Treacy, J.B. Higgins (Eds.), Collection of Simulated XRD Powder Patterns for Zeolites, 5th ed., Elsevier Science B.V, Amsterdam, 2007, pp. 112–113. [31] L. Xie, F. Liu, X. Shi, F.-S. Xiao, H. He, Effects of post-treatment method and Na cocation on the hydrothermal stability of Cu–SSZ-13 catalyst for the selective catalytic reduction of NOx with NH3, Appl. Catal., B 179 (2015) 206–212. [32] Z. Chen, C. Fan, L. Pang, S. Ming, P. Liu, D. Zhu, J. Wang, X. Cai, H. Chen, Y. Lai, T. Li, Direct synthesis of submicron Cu-SAPO-34 as highly efficient and robust catalyst for selective catalytic reduction of NO by NH3, Appl. Surf. Sci. 448 (2018) 671–680. [33] T. Yu, T. Hao, D. Fan, J. Wang, M. Shen, W. Li, Recent NH3-SCR mechanism research over Cu/SAPO-34 catalyst, J. Phys. Chem. C 118 (2014) 6565–6575. [34] X.S. Wu, H.M. Shao, X.X. Yao, S.S. Jiang, D.W. Wang, Z.H. Wu, Y.M. Cai, L.J. Shen, Z. Wu, Synthesis of the superconducting thin film of HgBa2Ca2Cu3O8+δ, Appl. Phys. Lett. 68 (1996) 1723–1725.
[7] J.C.S. Chang, S.B. Ghorishi, Simulation and evaluation of elemental mercury concentration increase in flue gas across a wet scrubber, Environ. Sci. Technol. 37 (2003) 5763–5766. [8] H. Yang, Z. Xu, M. Fan, A.E. Bland, R.R. Judkins, Adsorbents for capturing mercury in coal-fired boiler flue gas, J. Hazard. Mater. 146 (2007) 1–11. [9] G. Luo, H. Yao, M. Xu, X. Cui, W. Chen, R. Gupta, Z. Xu, Carbon nanotube-silver composite for mercury capture and analysis, Energy Fuel 24 (2010) 419–426. [10] W. Chen, Y. Pei, W. Huang, Z. Qu, X. Hu, N. Yan, Novel effective catalyst for elemental mercury removal from coal-fired flue gas and the mechanism investigation, Environ. Sci. Technol. 50 (2016) 2564–2572. [11] Y. Liu, D.J.A. Kelly, H. Yang, C.C.H. Lin, S.M. Kuznicki, Z. Xu, Novel regenerable sorbent for mercury capture from flue gases of coal-fired power plant, Environ. Sci. Technol. 42 (2008) 6205–6210. [12] A.P. Jones, J.W. Hoffmann, D.N. Smith, T.J. Feeley, J.T. Murphy, DOE/NETL's phase II mercury control technology field testing program: preliminary economic analysis of activated carbon injection, Environ. Sci. Technol. 41 (2007) 1365–1371. [13] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609. [14] A. Suarez Negreira, J. Wilcox, DFT Study of Hg oxidation across vanadia-titania SCR catalyst under flue gas conditions, J. Phys. Chem. C 117 (2013) 1761–1772. [15] A. Suarez Negreira, J. Wilcox, Role of WO3 in the Hg oxidation across the V2O5–WO3–TiO2 SCR catalyst: A DFT study, J. Phys. Chem. C 117 (2013) 24397–24406. [16] J.-E. Jung, D. Geatches, K. Lee, S. Aboud, G.E. Brown, J. Wilcox, First-principles investigation of mercury adsorption on the α-Fe2O3(11̅02) surface, J. Phys. Chem. C 119 (2015) 26512–26518. [17] J.-E. Jung, S. Liguori, A.D. Jew, G.E. Brown, J. Wilcox, Theoretical and experimental investigations of mercury adsorption on hematite surfaces, J. Air Waste Manage. Assoc. 68 (2018) 39–53. [18] Z. Mei, Z. Shen, Z. Mei, Y. Zhang, F. Xiang, J. Chen, W. Wang, The effect of Ndoping and halide-doping on the activity of CuCoO4 for the oxidation of elemental mercury, Appl. Catal. B 78 (2008) 112–119. [19] Y. Shao, J. Li, H. Chang, Y. Peng, Y. Deng, The outstanding performance of LDHderived mixed oxide Mn/CoAlOx for Hg0 oxidation, Catal. Sci. Technol. 5 (2015) 3536–3544. [20] T. Ishihara, M. Kagawa, F. Hadama, Y. Takita, Copper ion-exchanged SAPO-34 as a thermostable catalyst for selective reduction of NO with C3H6, J. Catal. 169 (1997) 93–102. [21] B.I. Palella, M. Cadoni, A. Frache, H.O. Pastore, R. Pirone, G. Russo, S. Coluccia, L. Marchese, On the hydrothermal stability of CuAPSO-34 microporous catalysts for
95
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Investigation on removal of NO and Hg0 with different Cu species in CuSAPO-34 zeolites ⁎
Yu Wanga, Wenzhe Sia,b, , Yue Penga, Junhua Lia, a b
T
⁎
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Elemental mercury oxidation NH3-SCR Cu-SAPO-34 zeolites
A series of Cu-SAPO-34 zeolites with different Cu contents were prepared by one-pot synthesis method. The NH3-SCR performance and the oxidation of elemental mercury (Hg0) efficiency over the Cu-SAPO-34 samples were tested respectively. With the increase of Cu loadings, the amount of Cu2+ and CuO were both increased, wherein the former promoted SCR reaction and the latter accelerated the Hg0 oxidation. What's more, the ratio of Cu2+/CuO was also increased. The NO conversion rate and Hg0 removal efficiency could be over 80% on the high Cu catalyst at 150–300 °C.
1. Introduction With the development of industry, mercury (Hg) has become a highly toxic heavy metal that causes serious human health problems [1–3]. Hg emission from coal burning is the most significant one among the main anthropogenic sources [4,5]. There are three main existence forms of Hg in coal-fired flue gas, element Hg (Hg0), oxidized Hg (Hg2+), and particle-bound Hg (Hgp) [6]. As we know, Hg2+ and Hgp could be relatively easily captured by conventional pollution control facilities [7,8]. However, it is difficult to remove Hg0 because of its low solubility in water and weak interaction on adsorbent particles [9,10]. Therefore, it is of great importance to find out a solution to remove Hg0 efficiently. Currently, adsorption and oxidation are two main technologies for the eliminatory of Hg0 [11–15]. However, adsorption technology is limited for wide applications owing to several disadvantages, such as higher cost, poor capacity, narrow operation temperature range, and slow regeneration and adsorption rates [16,17]. Conversely, Hg2+ (oxidized by Hg0) is facile to be removed through currently available pollution control devices, which makes oxidation method more feasible to some practical applications. In addition, some commercial catalysts used in selective catalytic reduction (SCR) of NOx with NH3 have been proven to exhibit some catalytic activities on the oxidation of Hg0 [18]. This provides us a thought on exploring a suitable catalyst which not only shows higher NOx removal efficiency, but also can promote the Hg0 removal. The operation temperature range for most SCR catalysts is
300–500 °C, during which the decomposition of Hg2+ (especially HgCl2) is easy to take place. However, the oxidation of Hg0 usually takes place at low temperature [19]. Hence it is necessary to develop attractive catalysts with high efficiencies both on DeNOx and Hg0 removal in low temperature range. Cu-SAPO-34 zeolite is one of the Cu/Chabazite catalysts which expresses superior activity on NH3-SCR reaction in a wide operation temperature range [20,21]. There exist several types of copper species in Cu-SAPO-34, such as isolated Cu2+ and Cu+ ions and CuO. For SCR reactions, the active sites are likely the isolated Cu ions inside the pores [22], while CuO is helpful to the oxidation of Hg0 to Hg2+ [23]. If the contents of Cu ions and CuO in Cu-SAPO-34 could be regulated in a proper ratio, it might be a promising catalyst for the removal of NOx and Hg0 with high performance. Copper species are traditionally introduced in small pore zeolites by ion-exchange method [24–26]. However the loading of Cu is quite limited because of the small pore openings and the restriction of ionexchanged capacity [27]. Recently, Corma et al. prepared Cu-SAPO-34 by “one-pot” synthesis method, in which the copper-amine complexes were introduced into the catalyst as templet directly [28,29]. Cu-SAPO34 prepared by this method contains much higher Cu content, and it is favorable to adjust the ratio of Cu cations and CuO. Herein, it is reported that the Cu-SAPO-34 catalysts prepared by one-pot synthesis method was used to remove NOx and Hg0 respectively. The NO conversion rate and Hg removal efficiency are both above 80% in 150–300 °C by adjusting the content of Cu loadings. Our
⁎ Corresponding authors at: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China. E-mail addresses: [email protected] (W. Si), [email protected] (J. Li).
https://doi.org/10.1016/j.catcom.2018.10.019 Received 5 August 2018; Received in revised form 3 October 2018; Accepted 19 October 2018 Available online 30 October 2018 1566-7367/ © 2018 Elsevier B.V. All rights reserved.
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
electron paramagnetic resonance spectrometer. Hydrogen temperature-programmed reduction (H2-TPR) experiments were performed on a chemisorption analyzer. Temperatureprogrammed desorption (TPD) of Hg0 studies were carried out in a fixed-bed quartz reactor equipped with an on-line Hg analyzer. More details on characterization could be seen in the Supporting Information.
Table 1 Starting compositions in the gels, Cu/Al ratio, Cu content and BET specific surface area of the samples. sample
Al/P/Si/Cu-TEPA/ DEA/H2O
Cu/Al molar ratio ICP
Cu1 Cu2 Cu3 Cu4 a b c
1/0.8/0.3/0.05/0.95/ 40 1/0.8/0.3/0.1/0.9/40 1/0.8/0.3/0.2/0.8/40 1/0.8/0.3/0.3/0.7/40
a
XPS
b
Cu wt (%) ICP
a
SBET (m2/g)
0.057
0.033
2.504
578
0.112 0.17 0.26
0.054 0.072 0.089
5.027 7.411 10.44
506 471 422
c
3. Results and discussion 3.1. Structure and BET area
Obtained from ICP results. Calculated from XPS results. Calculated by BET method.
Fig. S1 shows the XRD results of the samples in the 2θ range of 5–40°. The XRD patterns of the four catalysts with different Cu loadings could be corresponded well with the featured peaks (9.5°, 15.96°, 17.92°, 20.05°, 25.82°, 30.92° and 31.2°) of typical CHA structure [30]. The high intensity of the XRD peaks could suggest a good crystallinity. For Cu1 and Cu2 samples, no featured peaks of CuO (35.6° and 38.8°) could be observed, which meant that the Cu species were highly dispersed and the dominant existence form of Cu species were Cu cations. However, for Cu3 and Cu4 samples, the featured peaks of CuO appeared, demonstrating that CuO particles larger than 3 nm were aggregated on the external surface of Cu3 and Cu4 samples with the increase of Cu loading. The BET surface areas of the four samples are listed in Table 1. The surface areas of these samples are in the range of 422–578 m2/g, lower than the usual value of H-SAPO-34 (600 m2/g). In addition, with the increase of Cu loading, the BET areas of these samples decreased gradually, and Cu4 sample possessed the smallest BET area among the four samples. This indicates that the loading of Cu could decline the BET areas of the samples. The CuO particle existed on the surface of the sample might block the CHA cage, resulting in the decrease of the BET area.
research may suggest a new route for combined removal of Hg0 and NOx from flue gas in the future. 2. Experimental 2.1. Synthesis Copper-amine complex (Cu2+ with tetraethylenepentamine, CuTEPA) and diethylamine (DEA) were used as the organic structure directing agents (OSDAs). All chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. The chemical compositions of the reaction gels are given in Table 1. The loading of Cu could be adjusted by the different ratio of Cu-TEPA and DEA. In a typical synthesis procedure, phosphoric acid was firstly dissolved in deionized water. Then alumina and silica sources were added into the solution and stirred for 2 h. At last, CuSO4·5H2O and TEPA were introduced to the above gel under continuous stirring for 12 h. The resulting mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for 2 days. After filtered and washed with deionized water, the product was dried at 80 °C overnight. Then the powders were calcined at 550 °C for 4 h in air to remove the remained organic species. The samples were denoted as Cu1-Cu4 in the sequence of the increase of Cu contents, respectively.
3.2. Cu species XPS and ICP measurements were conducted to elucidate the effect of the Cu loading on the distribution of different Cu species, and the estimated Cu/Al atomic ratios on the surface and in the bulk are presented in Table 1. For all these samples, the surface Cu/Al ratios (0.033–0.089) are lower than those in the bulk (0.057–0.26), which implies that the Cu species existed in the samples are mainly in the form of Cu cations inside the Cu-SAPO-34 pores. Fig. 1 illustrates the Cu 2p XPS spectra of the samples, and the binding energy of the Cu 2p transition peak could provide information about the state of Cu species on the surface. The asymmetrical Cu 2p3/2 transition peak can be
2.2. Catalytic activity evaluation The catalytic activity for the SCR reaction was measured in a fixed bed quartz reactor (i.d. 6 mm) with 0.1 g catalyst. The total gas flow was 200 ml/min, containing 500 ppm of NO, 500 ppm of NH3, 5% of O2, and N2 (balance gas). The activity test was carried out from 150 to 550 °C. The concentrations of NH3 and NOx were monitored by a MKS MultiGas 2030 HS FTIR spectrometer. The Hg0 removal efficiency of the catalyst was also measured in a fixed-bed quartz reactor with 0.2 g catalyst. The concentration of Hg0 was collected by an RA-915 M cold vapor atomic absorption spectroscopy mercury analyzer (Lumex Co., Ltd., Russia). The vapor-phase Hg0 was generated by passing N2 flow through a permeation tube. The total gas flow was 200 ml/min, containing 250 μg/m3 of Hg0, 5% of O2, 5 ppm of HCl (when needed) and N2 (balance gas). Further details are described in the Supporting Information. 2.3. Characterization The crystal structure of the three samples was characterized by powder X-ray diffraction (XRD). The element contents were analyzed by inductively coupled plasma (ICP)-Optical Emission spectrometer. Surface areas of the samples were obtained by N2 adsorption–desorption at −196 °C. The surface atom composition and oxidation state of elements on the surface of the samples were determined by X-ray photoelectron spectroscopy (XPS). The EPR data were acquired on an
Fig. 1. Cu 2P XPS spectra of the samples. 92
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
Fig. 3. SCR activity of the Cu-SAPO-34 and V1W5Ti samples in the range of 150–550 °C. Reaction conditions: The total gas flow was 200 ml/min, containing 500 ppm of NO, 500 ppm of NH3, 5% of O2, and N2 (balance gas). The catalyst was 0.1 g.
Fig. 2. H2-TPR profiles of the samples.
deconvoluted. The peaks located at 932.9 eV and 935.6 eV are attributed to CuO and Cu2+, respectively. With the increase of Cu loading, the amount of CuO and Cu2+ are both increased. And the molar ratio of CuO/Cu2+ are also calculated. This ratio decrease from Cu1 to Cu4, which suggests that the raising rate of Cu2+ content was much faster than that of CuO. H2-TPR experiments were carried out to further investigate the change of Cu species over the samples with different Cu loading, and Fig. 2 shows the TPR profiles. According to the literature, the H2 consumption peaks in the temperature range of 175–400 °C could be generally divided into two parts: the peaks located at low temperature assigned to reduction of isolated Cu2+ to Cu+, and the peaks at high temperature assigned to reduction of CuO species to Cu0 [29,31]. In addition, the H2 consumption signal appeared at much higher temperature is attributed to the reduction of Cu+ to Cu0 [32]. Thus, there should be three types of Cu species in Cu-SAPO-34 samples, including external surface CuO, isolated Cu2+ and Cu+ ions inside the samples. However, as shown in Fig. 2, the reduction peak of Cu+ is hardly observed in these samples, which might be owing to the high stability or the little amount of the Cu+ species after the one-pot process. With the increase of Cu loading, the reduction peaks gradually shift towards lower temperature, indicating an increment on the redox property from Cu1 to Cu4. The amount of Cu2+ and CuO became larger as the increase of Cu loading, in agreement with the XPS results. EPR is an effective approach to identify the hyperfine structure of isolated Cu2+ ions [33]. As the EPR spectra shown in Fig. S2, all the samples exhibited the characteristic signal of isolated Cu2+ at g// =2.391 (four hyperfine splitting peaks originated from the interaction of the Cu nuclear and unpaired electron). This illustrates that the isolated Cu2+ species existed in these four Cu-SAPO-34 catalysts are in the same environment, which are octahedrally coordinated to three framework oxygen atoms and three water molecules in Cu-SAPO-34. Furthermore, the intensity of the EPR signal enhanced in the sequence of Cu1 < Cu2 < Cu3 < Cu4, which also indicates an increment on the contents of isolated Cu2+ ions. This result is in good accordance with the H2-TPR and XPS results.
increase of Cu loading. Thus the sample with higher Cu loading possesses better catalytic activity in low temperature range. When it comes to high temperature, unselective oxidation of NH3 will take place caused by the CuO on the surface, which is enhanced with the increase of the Cu loading. So the SCR activities of the catalysts gradually turn worse with the increase of Cu loading. However, the increase trend of CuO becomes slow, compare to Cu2+, which is already confirmed by TPR and XPS results. So the decrease trend of the SCR performance at high temperature becomes slow. Fig. S3 exhibits excellent N2 selectivity of all the four samples, which implies that few by-products (NO2 and N2O) were generated during the SCR process. 3.4. Hg0 adsorption properties Hg0 adsorption property of the catalyst is a key factor for the oxidation of Hg0. In order to investigate this property, Hg0-TPD experiments were carried out. Fig. 4 shows that there exist three desorption peaks 120 °C, 280 °C and 500 °C. The peak at about 120 °C is attributed to the decomposition of Hg2O and weak physical adsorption of Hg0, and the peak at about 280 °C comes from the chemical adsorption of Hg0, which is similar to the results in the literature [19]. In addition, it is reported that the decomposition of HgO usually appears above 400 °C [34]. As shown in Fig. 4, the TPD profiles illustrates that the decomposition peak of HgO for Cu-SAPO-34 samples appeared at approximately 500 °C. During the whole TPD experiment, no additional oxygen was introduced to the atmosphere. Thus the oxygen in HgO might come
3.3. SCR performance As shown in Fig. 3, the NO conversion of the samples was tested, and a commercial 1%V2O5–5%WO3-TiO2(VIW5Ti)was also tested as contrast. It can be found that all the four Cu-SAPO-34 samples exhibit excellent catalytic activities in the temperature range of 200–350 °C, obviously better than VIW5Ti. As the TPR results shown, the redox property of the catalysts at low temperature is improved with the
Fig. 4. Hg0-TPD profiles of the samples. 93
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
temperature is higher than 250 °C. This phenomenon may be caused by two factors. On one hand, the adsorption of Hg0 on the catalyst surface could influence the oxidation reaction. When the temperature increases from 100 to 200 °C, the reaction could enhance between the adsorbed Hg0 and the Oads on the catalyst surface. However, the Hg0 adsorption ability of the catalysts becomes lower when the temperature increase to higher than 250 °C, leading to a decrease on the oxidation efficiency. On the other hand, HgO could decompose into Hg and O2 at high temperature, also doing harm to the catalytic performance. [19] As HCl is commonly present in coal-derived flue gas composition, the effect of gaseous HCl in the reaction atmosphere was also investigated over Cu4 sample. When 5 ppm of HCl was added, the activity of the catalyst was greatly improved (Fig. 5b). Within 200–350 °C, the Hg0 oxidation efficiency of Cu4 sample was even over 95%. In the HCllacking condition, the oxidation products are Hg2O (< 150 °C) and HgO (> 300 °C). When HCl is introduced in the reaction mixture, Hg2O or HgO is easier to react with HCl to form HgCl or HgCl2, which then could promote the oxidation reaction. 4. Conclusions A series of Cu-SAPO-34 zeolites with different Cu contents were prepared by one-pot synthesis method using copper-amine complex as templet directly. Then the NH3-SCR performance and the oxidation of Hg0 efficiency over the thus obtained Cu-SAPO-34 samples were tested respectively. It is commonly recognized that isolated Cu2+, Cu+ ions and CuO are co-existed in Cu-SAPO-34. For SCR reactions, the active sites are likely the isolated Cu ions inside the pores, whereas CuO is the active component for the oxidation of Hg0. With the increase of Cu loadings, Cu2+ and CuO were both increased, and the ratio of Cu2+/ CuO was also increased. By adjusting the amount of Cu loadings, the catalysts exhibit excellent SCR activities in the temperature range of 200–350 °C. When they were used in Hg0 oxidation, the catalytic efficiency could reach above 80% over the sample with high Cu content at 150–300 °C. This work may provide new ideas for combined removal of Hg0 and NOx from coal-derived flue gas streams in the future.
Fig. 5. (a) Hg0 oxidation performance of the Cu-SAPO-34 and V1W5Ti samples and (b) the effect of HCl on the Hg0 oxidation performance over Cu4 sample. Reaction conditions: The total gas flow was 200 ml/min, containing 250 μg/m3 Hg0, 5% O2, 5 ppm HCl (when needed) and N2 (balance gas). The catalyst was 0.2 g.
Acknowledgements This work was financially supported by the National Natural Science Fund of China (Grant Nos. 21325731 and 51478241) and the National High-Tech Research and Development (863) Program of China (Grant No. 2013AA065304).
from the Olatt and residual Oads of the samples. With the increase of Cu loading, more HgO was decomposed to Hg and O2, indicating that the oxidability for the catalysts increased from Cu1 to Cu4. The strongest peak for Cu4 catalyst suggests that it is reasonably expected to possess strong potential for the adsorption and oxidation of Hg0.
Appendix A. Supplementary data
3.5. Hg0 oxidation performance
Characterization details, catalytic activity measurement method, and additional results are shown. Supplementary data to this article can be found online at doi: https://doi.org/10.1016/j.catcom.2018.10.019.
Hg0 oxidation performances of the four Cu-SAPO-34 catalysts were investigated in the temperature range of 100–400 °C (Fig. 5a). VIW5Ti catalyst was also tested as contrast. It is found that except for Cu1 sample, the Hg0 oxidation performance of other Cu-SAPO-34 catalysts are much better than that of V1W5Ti. With the increase of Cu loading, the catalytic activity of Hg0 oxidation is gradually improved. However, the increase trend of catalytic activity becomes slow. The Hg0 removal efficiencies for Cu3 and Cu4 were almost the same, especially at low temperature. For Hg0 oxidation, the main active component of CuSAPO-34 catalyst is CuO. As the TPR and XPS results indicate, compared to Cu2+, the increase trend of CuO becomes slow with the increase of Cu loading. The amount of CuO on the surface of the high Cu loading sample reach a threshold value. No more CuO could be generated on the catalyst by increasing the amount of Cu loading. So the catalytic performances on Hg0 removal for Cu3 and Cu4 samples are nearly the same. In addition, from the Hg0 oxidation conversion profiles it can be observed that the catalytic activity of the samples increases from 100 to 200 °C and then decreases dramatically when the
References [1] T.M. Bisson, L.C.W. MacLean, Y. Hu, Z. Xu, Characterization of mercury binding onto a novel brominated biomass ash sorbent by X-ray absorption spectroscopy, Environ. Sci. Technol. 46 (2012) 12186–12193. [2] Y. Liu, T.M. Bisson, H. Yang, Z. Xu, Recent developments in novel sorbents for flue gas clean up, Fuel Process. Technol. 91 (2010) 1175–1197. [3] J. Yang, Y. Zhao, X. Guo, H. Li, J. Zhang, C. Zheng, Removal of elemental mercury from flue gas by recyclable CuCl2 modified magnetospheres from fly ash. Part 4. Performance of sorbent injection in an entrained flow reactor system, Fuel 220 (2018) 403–411. [4] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A. Benson, Status review of mercury control options for coal-fired power plants, Fuel Process. Technol. 82 (2003) 89–165. [5] Z. Yang, H. Li, X. Liu, P. Li, J. Yang, P.-H. Lee, K. Shih, Promotional effect of CuO loading on the catalytic activity and SO2 resistance of MnOx/TiO2 catalyst for simultaneous NO reduction and Hg0 oxidation, Fuel 227 (2018) 79–88. [6] G. Skodras, I. Diamantopoulou, G. Pantoleontos, G.P. Sakellaropoulos, Kinetic studies of elemental mercury adsorption in activated carbon fixed bed reactor, J. Hazard. Mater. 158 (2008) 1–13.
94
Catalysis Communications 119 (2019) 91–95
Y. Wang et al.
N2O decomposition: a comparison with CuZSM-5, J. Catal. 217 (2003) 100–106. [22] L. Wang, W. Li, G. Qi, D. Weng, Location and nature of Cu species in Cu/SAPO-34 for selective catalytic reduction of NO with NH3, J. Catal. 289 (2012) 21–29. [23] X. Fan, C. Li, G. Zeng, X. Zhang, S. Tao, P. Lu, S. Li, Y. Zhao, The effects of Cu/ HZSM-5 on combined removal of Hg0 and NO from flue gas, Fuel Process. Technol. 104 (2012) 325–331. [24] M. Moliner, C. Martínez, A. Corma, Synthesis strategies for preparing useful small pore zeolites and zeotypes for gas separations and catalysis, Chem. Mater. 26 (2014) 246–258. [25] M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Cu-SSZ-39, an active and hydrothermally stable catalyst for the selective catalytic reduction of NOx, Chem. Commun. 48 (2012) 8264–8266. [26] J. Hun Kwak, H. Zhu, J.H. Lee, C.H.F. Peden, J. Szanyi, Two different cationic positions in Cu-SSZ-13? Chem. Commun. 48 (2012) 4758–4760. [27] L. Ren, L. Zhu, C. Yang, Y. Chen, Q. Sun, H. Zhang, C. Li, F. Nawaz, X. Meng, F.S. Xiao, Designed copper-amine complex as an efficient template for one-pot synthesis of Cu-SSZ-13 zeolite with excellent activity for selective catalytic reduction of NOx by NH3, Chem. Commun. 47 (2011) 9789–9791. [28] R. Martínez-Franco, M. Moliner, C. Franch, A. Kustov, A. Corma, Rational direct synthesis methodology of very active and hydrothermally stable Cu-SAPO-34 molecular sieves for the SCR of NOx, Appl. Catal., B 127 (2012) 273–280. [29] R. Martínez-Franco, M. Moliner, P. Concepcion, J.R. Thogersen, A. Corma, Synthesis, characterization and reactivity of high hydrothermally stable Cu-SAPO34 materials prepared by “one-pot” processes, J. Catal. 314 (2014) 73–82. [30] M.M.J. Treacy, J.B. Higgins, CHA - Chabazite, K-exchanged, in: M.M.J. Treacy, J.B. Higgins (Eds.), Collection of Simulated XRD Powder Patterns for Zeolites, 5th ed., Elsevier Science B.V, Amsterdam, 2007, pp. 112–113. [31] L. Xie, F. Liu, X. Shi, F.-S. Xiao, H. He, Effects of post-treatment method and Na cocation on the hydrothermal stability of Cu–SSZ-13 catalyst for the selective catalytic reduction of NOx with NH3, Appl. Catal., B 179 (2015) 206–212. [32] Z. Chen, C. Fan, L. Pang, S. Ming, P. Liu, D. Zhu, J. Wang, X. Cai, H. Chen, Y. Lai, T. Li, Direct synthesis of submicron Cu-SAPO-34 as highly efficient and robust catalyst for selective catalytic reduction of NO by NH3, Appl. Surf. Sci. 448 (2018) 671–680. [33] T. Yu, T. Hao, D. Fan, J. Wang, M. Shen, W. Li, Recent NH3-SCR mechanism research over Cu/SAPO-34 catalyst, J. Phys. Chem. C 118 (2014) 6565–6575. [34] X.S. Wu, H.M. Shao, X.X. Yao, S.S. Jiang, D.W. Wang, Z.H. Wu, Y.M. Cai, L.J. Shen, Z. Wu, Synthesis of the superconducting thin film of HgBa2Ca2Cu3O8+δ, Appl. Phys. Lett. 68 (1996) 1723–1725.
[7] J.C.S. Chang, S.B. Ghorishi, Simulation and evaluation of elemental mercury concentration increase in flue gas across a wet scrubber, Environ. Sci. Technol. 37 (2003) 5763–5766. [8] H. Yang, Z. Xu, M. Fan, A.E. Bland, R.R. Judkins, Adsorbents for capturing mercury in coal-fired boiler flue gas, J. Hazard. Mater. 146 (2007) 1–11. [9] G. Luo, H. Yao, M. Xu, X. Cui, W. Chen, R. Gupta, Z. Xu, Carbon nanotube-silver composite for mercury capture and analysis, Energy Fuel 24 (2010) 419–426. [10] W. Chen, Y. Pei, W. Huang, Z. Qu, X. Hu, N. Yan, Novel effective catalyst for elemental mercury removal from coal-fired flue gas and the mechanism investigation, Environ. Sci. Technol. 50 (2016) 2564–2572. [11] Y. Liu, D.J.A. Kelly, H. Yang, C.C.H. Lin, S.M. Kuznicki, Z. Xu, Novel regenerable sorbent for mercury capture from flue gases of coal-fired power plant, Environ. Sci. Technol. 42 (2008) 6205–6210. [12] A.P. Jones, J.W. Hoffmann, D.N. Smith, T.J. Feeley, J.T. Murphy, DOE/NETL's phase II mercury control technology field testing program: preliminary economic analysis of activated carbon injection, Environ. Sci. Technol. 41 (2007) 1365–1371. [13] A.A. Presto, E.J. Granite, Survey of catalysts for oxidation of mercury in flue gas, Environ. Sci. Technol. 40 (2006) 5601–5609. [14] A. Suarez Negreira, J. Wilcox, DFT Study of Hg oxidation across vanadia-titania SCR catalyst under flue gas conditions, J. Phys. Chem. C 117 (2013) 1761–1772. [15] A. Suarez Negreira, J. Wilcox, Role of WO3 in the Hg oxidation across the V2O5–WO3–TiO2 SCR catalyst: A DFT study, J. Phys. Chem. C 117 (2013) 24397–24406. [16] J.-E. Jung, D. Geatches, K. Lee, S. Aboud, G.E. Brown, J. Wilcox, First-principles investigation of mercury adsorption on the α-Fe2O3(11̅02) surface, J. Phys. Chem. C 119 (2015) 26512–26518. [17] J.-E. Jung, S. Liguori, A.D. Jew, G.E. Brown, J. Wilcox, Theoretical and experimental investigations of mercury adsorption on hematite surfaces, J. Air Waste Manage. Assoc. 68 (2018) 39–53. [18] Z. Mei, Z. Shen, Z. Mei, Y. Zhang, F. Xiang, J. Chen, W. Wang, The effect of Ndoping and halide-doping on the activity of CuCoO4 for the oxidation of elemental mercury, Appl. Catal. B 78 (2008) 112–119. [19] Y. Shao, J. Li, H. Chang, Y. Peng, Y. Deng, The outstanding performance of LDHderived mixed oxide Mn/CoAlOx for Hg0 oxidation, Catal. Sci. Technol. 5 (2015) 3536–3544. [20] T. Ishihara, M. Kagawa, F. Hadama, Y. Takita, Copper ion-exchanged SAPO-34 as a thermostable catalyst for selective reduction of NO with C3H6, J. Catal. 169 (1997) 93–102. [21] B.I. Palella, M. Cadoni, A. Frache, H.O. Pastore, R. Pirone, G. Russo, S. Coluccia, L. Marchese, On the hydrothermal stability of CuAPSO-34 microporous catalysts for
95