Cr Doping MnO - ACS Publications - American Chemical Society

Nov 19, 2018 - Cr doping can enlarge the BET surface area of MnOx when Cr was doped ...... controversy of dental amalgam use and mercury waste disposa...
0 downloads 0 Views 9MB Size
Download PDF
Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Cr Doping MnOx Adsorbent Significantly Improving Hg0 Removal and SO2 Resistance from Coal-Fired Flue Gas and the Mechanism Investigation Dan Zhang,†,‡ Li’an Hou,†,§ Guanyi Chen,*,† Anchao Zhang,*,∥ Fahui Wang,∥ Ruirui Wang,∥ and Chengwei Li∥ †

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China School of Civil Engineering, Henan Polytechnic University, Jiaozuo 454003, China § Rocket Force University of Engineering, Xi’an 710025, China ∥ School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454003, China

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/16/18. For personal use only.

‡

ABSTRACT: A series of binary Mn-based materials (Fe-MnOx, Cu-MnOx, CrMnOx, and Mo-MnOx) were prepared by the coprecipitation method, and the mechanism for elemental mercury (Hg0) removal of Cr-MnOx in the absence and presence of SO2 was investigated. The physicochemical structure properties of the fresh as well as spent Cr-MnOx samples were well characterized by the methods of N2 physisorption, XRD, SEM-EDS, FTIR, H2-TPR, XPS, and TPD. The experiment results revealed that the loading of Cr into/onto MnOx yielded above 90% of Hg0 removal efficiency in the presence of SO2 at 120 °C. However, the appearance of SO2 would lead to the formation of chromium sulfate/sulfite and manganese sulfate/sulfite although much superior SO2 resistance was obtained. According to the N2 physisorption and XRD characterization results, Cr doping can enlarge the BET surface area of MnOx when Cr was doped into/ onto MnO2, and the MnCrO4 mixed oxide material was produced. Moreover, the H2-TPR result exhibited that Cr loading into/onto MnOx enhanced the redox property of Cr-MnOx mixed oxides and many more easily reduced species appeared in the mixture. The larger surface area and an abundance of reactive species could benefit from excellent performances of Hg0 removal and superior SO2 resistance although the DFT calculation showed almost the same adsorption energies of Hg0 on MnO2 (110) and Cr-MnO2 (110) surfaces. The DFT calculation exhibited that the adsorption energy of SO2 on the surface of Cr-MnO2 (110) was much lower than that on MnO2 (110), implying that Cr doping into MnO2 would decrease the adsorption behavior of SO2, thereby improving its SO2 resistance performance. fired power plants, but high cost and low utilization rate limit its application in power plants.15−17 Metal oxides are supposed to be new alternative materials due to their higher mercury removal efficiency and lower application cost.18−21 In a variety of metal oxide catalysts or adsorbents for Hg0 removal, Mn-based catalysts, such as MnOx/TiO2,22 MnCeO2−ZrO2,23 Co−Mn−Ce/TiO2,24 MnOx-CeO2/γ-Al2O3,25 Ce-MnOx/Ti-PILCs,26 Fe−Sn−MnOx,27 Mn−Ce/TiO2,28 CoxMn5/TiCey,9 and Ru−Mn−Ti,29 have received extensive attention due to their superior performances. It was reported that the activity of catalyst/adsorbent for Hg0 removal depended on the presence of strong oxidizing gases such as HCl or Cl2 gas in the coal-fired flue gas.23,26 While in fact the concentration of HCl or Cl2 in real flue gas is rather low,30 especially for some Chinese coals. Unfortunately, the SO2 tolerance performance of Mn-based materials or pure

1. INTRODUCTION Anthropogenic mercury emission from coal-fired flue gas into the atmosphere has attracted wide public attention because of their toxicity, persistence, and bioaccumulation of methyl mercury that transformed from emitted mercury.1 In August, 2017, the “Minamata Convention on Mercury” was in force. This is a new global convention in the field of environment and health in the past decade that prompted the government to take concrete measures to control anthropogenic mercury pollution.2,3 The main species presented in flue gas normally included elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp).4 In comparison to Hg2+ and HgP, Hg0 is much difficult to be removed due to its poor solubility and high volatility. The discharge of Hg0 into the atmosphere is harmful to human health.5,6 Therefore, it is particularly urgent to develop the control technology of mercury emission from coal-fired boilers. In recent years, various technologies for Hg0 removal have been developed, such as metal oxides catalytic oxidation7−10 and adsorbents injection.11−14 Activated carbon injection is recognized as a useful method for Hg0 emission in some coal− © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 3, 2018 November 19, 2018 November 19, 2018 November 19, 2018 DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research manganese oxides (MnOx) is frequently unsatisfactory.27 According to the low chlorine content and the high sulfur content of Chinese coal,31 it is necessary to develop superior Mn-based catalysts/adsorbents with higher sulfur resistance. Thus, in this work, four binary Mn-based materials (FeMnOx, Cu-MnOx, Cr-MnOx, and Mo-MnOx) were prepared by the coprecipitation method. The effect of SO2 on Hg0 removal using the binary Mn-based adsorbents under different flue gas atmospheres was elaborately studied. To investigate the promotional mechanism of complex metal oxide on the performances of Hg0 removal and SO2 resistance, the physicochemical structure properties of the fresh and spent Cr-MnOx samples were well characterized by N2 physisorption, X-ray diffraction (XRD), scanning electron micrograph and energy dispersive X-ray spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), hydrogen gas temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) analysis. Moreover, a Density Functional Theory (DFT) calculation was also implemented to further explore the possible mechanism of Cr doping MnOx adsorbents for enhanced Hg0 removal and SO2 resistance.

carried out for 10 h in the absence and presence of 500 ppm of SO2 (using 0.3 g of fresh adsorbent), where the used or spent adsorbents were denoted as CrMn-H and CrMn-HS (with a 1:1 Cr/Mn molar ratio), respectively. Here, the letter “H” stands for the spent sample that was reacted under baseline (BL) condition, and the letters “HS” represent the spent sample that was reacted under BL and SO2 environment. 2.3. Adsorbent Characterization. The BJH pore size distribution, BET surface area, and pore volume of the adsorbents were measured using a Quantachrome Autosorb IQ porosity and surface area analyzer. X-ray diffraction (XRD) patterns were implemented on a Bruker D8 Advance instrument. SEM-EDS measurement was carried out using a Quanta 250 instrument to observe the changes of the compositions of samples. FTIR spectra were measured using a Magna-IR 750 instrument. H2-TPR curves were obtained using an Auto Chem II 2920 adsorption apparatus. XPS experiments were conducted to probe the change of elements of the samples on a Thermo ESCALAB 250XI spectrometer. SO2-TPD and Hg0-TPD experiments were similarly implemented using 0.10 g of the spent adsorbent on a temperatureprogrammed oven with its outlet connected to a flue gas analyzer or a VM-3000 mercury analyzer, respectively. A detailed description about SO2-TPD experiment can be seen in our previous study.29 2.4. Method of DFT Calculation. In order to further explore the adsorption mechanism of Hg0 and SO2 species on MnO2 and Cr-MnO2 (CrMn) surfaces, Density Functional Theory (DFT) calculations were carried out. The spin restricted DFT calculation was carried out using the CASTEP program.33 The DFT plane-wave (PW) pseudopotential method was used for geometry optimization. The generalized gradient optimization (GGA) + Perdew Burke Ernzerhof (PBE) function was employed to approximate the exchangecorrelation energy. Ultrasoft pseudopotential was carried out to depict the electronic interaction. A cutoff energy of 380 eV and a Monkhorst−Pack of 2 × 2 × 2 were used. A vacuum gap of 10 Å was intended to avoid layer-to-layer interference. The convergence energy of 5.0 × 10−6 eV/atom, maximum force of 0.01 eV/Å, and maximum displacement tolerance of 0.002 Å were employed for the calculations.33,34 The adsorption energy (Eads) for Hg0 or SO2 was calculated as follows

2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. The adsorbents were prepared by a facile coprecipitation method. In a typical preparation route, Fe(NO3)3·9H2O (0.05 mol) and Mn(NO3)2 (0.15 mol) with a molar ratio of 1:3 were dissolved in 100 mL of distilled water to obtain a mixed solution. Some amounts of potassium hydroxide (KOH) solution (2.0 mol/L) were then added dropwise to the mixture until the pH value equaled around 10. Afterward, the mixture was continuously stirred in a water bath (50 °C) for 2 h and then was rinsed repeatedly with distilled water until the pH value finally approached 7. Finally, the filtered precipitate was dried at 120 °C for 12 h and calcined at 500 °C for 5 h in air conditioning. The obtained powder was pressed, ground, sieved to 40−60 mesh, and designated to Fe-MnOx (marked as FeMn). In the same manner, Cu-MnOx (CuMn), Cr-MnOx (CrMn), and Mo-MnOx (MoMn) adsorbents were synthesized by replacing Fe(NO3)3·9H2O with Cu(NO3)2·3H2O, Cr(NO3)3·9H2O, and (NH4)6Mo7O24·4H2O, respectively. Samples with different Cr/ Mn molar ratios (1:3, 2:3, 1:1, 3:2, and 3:1) were also prepared to understand the effect of compositions on Hg0 removal and SO2 resistance. For comparison, pure MnOx and CrOx materials were also synthesized using a similar route. 2.2. Adsorbent Performance Test. The experiments for Hg0 removal were implemented in a fixed bed quartz tube reactor under atmospheric pressure. Detailed descriptions about experimental devices and procedures could be found in our previous literature.32 In the present work, the total flow through the reactor, inlet Hg0 concentration, and used adsorbent mass were 1.5 L/min, 55 μg/m3, and 0.3 g, respectively. The Hg0 removal efficiency (η) was calculated according to the following equation η (%) =

Hg0in

ΔHg 0 Hg 0 in

=

Hg0out

Hg 0 in − Hg 0 out Hg 0 in

Eads = E(substrate−adsorbate) − Esubstrate − Eadsorbate

(2)

where E(substrate−adsorbate), Esubstrate, and Eadsorbate stand for the total energies of the substrate plus adsorbate system, substrate, and isolated adsorbate at their equilibrium geometries, respectively. A negative Eads value corresponds to a stable adsorbate/substrate system. The smaller the value, the greater the adsorption energy is. The change of atomic Mulliken charge before and after adsorption was also studied. Note that a positive value for the net charge suggests a loss of electrons. The MnO2 (110) and CrMn (110) surfaces are the most thermodynamically stable structure and catalytically reactive surfaces,34 thus geometry optimization and energy calculations were based on the MnO2 (110) and CrMn (110) surfaces in the present work. For the MnO2 (110) model, there are 5-fold coordinated Mn (Mn5) and 6-fold coordinated Mn (Mn6), which are alternately formed rows. Three kinds of coordinative oxygen atoms are included:35 the bridging oxygen atom Obr coordinated with two Mn6 atoms, the Os atom coordinated with two Mn5 and one Mn6 atoms, and the Oh atom in the middle of the layer (Figure 1a). The surface is employed by

× 100% (1) 0

where and stand for the Hg concentrations of the inlet and the outlet from the reactor (μg/m3), respectively. To further explore changes of the chemical structure of the samples before and after Hg0 removal, the experiments were B

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Models of MnO2 (110) or CrMn (110) surface with top view (a) and side view (b). The red and purple spheres represent the O atom and Mn atom (similarly hereinafter), respectively.

eight-layer slabs of atoms and the bottom layers are fixed in their bulk positions, while the top layers of atoms are allowed to relax (Figure 1b).34 For the CrMn (110) simulation model, the manganese atom Mn5 or Mn6 on the surface of MnO2 (110) in the model is replaced by a Cr atom, respectively. The Cr doped atoms after optimization have different degrees of shift, and the positional stability of Cr atoms instead of Mn5 is better, and the substitution is available in the literature.36,37

SO3 to form HgSO4, thereby slightly enhancing Hg0 removal activity.38 While with the SO2 continuous injection, reactive oxygen and MnOx itself would react with SO2 to generate stable and inert manganese sulfate, resulting in its inferior activity. A similar result also appeared for FeMn samples. However, it was interesting that the Hg0 removal efficiency of CuMn significantly decreased from 90% to about 8% and then gradually increased to 45%. The Hg0 removal efficiency of the CrMn sample maintained above 90% in the absence and presence of SO2. Zhang et al.39 studied the activity of Hg0 removal using the Ce0.1-Zr-MnO2 adsorbent under the condition of 5% O2 + 50 ppm of SO2 and showed that Hg0 removal efficiency reached about 70%. Moreover, with the addition of the SO2 gas, Hg0 removal efficiency of CrMn decreased to a lower degree than that over the Mn-based catalyst in Wang et al.25 studies, meaning that Cr loading enhanced sulfur dioxide tolerance of the adsorbents. By comparison, CrMn adsorbent exhibited much excellent Hg0 removal performance. Thus, CrMn material was chosen for the later study. Moreover, the experiments for detecting the distribution of oxidation efficiency and adsorption efficiency were performed under BL conditions based on the Ontario Hydro Method (OHM).40,41 The Hg0 removal reaction was carried out at 120 °C for 60 min. From Figure 3, we can find that MnOx, CrOx,

3. RESULTS AND DISCUSSION 3.1. Hg0 Removal Performance. 3.1.1. Hg0 Removal Efficiencies of Different Samples. Figure 2 exhibits the Hg0

Figure 2. Hg0 removal efficiency of different Mn-based adsorbents.

removal efficiency over different binary Mn-based adsorbents at 120 °C under BL and BL+SO2 conditions. For clearly understanding the effect of SO2 on Hg0 removal, the tests were first carried out for 1 h with SO2 absence, then 500 ppm of SO2 was injected for another 1 h, and then the SO2 stream was cut off for the last 1 h. We can obviously observe that the Hg0 removal efficiency of MoMn adsorbent maintained at about 20% in the researched 3 h both in the absence and presence of SO2, indicating that the Hg0 removal activity of MoMn adsorbent was not affected by the addition of SO2, but its Hg0 removal performance was poor. The Hg0 removal efficiencies of pure MnOx, CrMn, CuMn, and FeMn are all above 80% under BL environment. When SO2 was fed into the reaction stream, the Hg0 removal efficiency of MnOx slightly increased from 83% to 87% and then gradually decreased to about 70%. The slightly improved Hg0 removal efficiency in the initial minutes after SO2 addition can be due to the reaction of SO2 and the reactive oxygen to produce SO3 and then HgO with

Figure 3. Distribution of oxidation and adsorption efficiencies over the samples.

and CrMn samples can remove about 78%, 60%, and 96% of Hg0 through adsorption, and only about 2−4% of Hg0 escaped to the environment, demonstrating that a large amount of trapped mercury was stored on the surface.40−42 3.1.2. Effect of Cr Content on Hg0 Removal Activity. It was reported that the chemical components of a sample frequently affected not only the crystallinity and morphology but also its C

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research redistribution and aggregation,43 thereby further influencing its oxidation performance. Thus, it is essential to study the effect of Cr loading on the Hg0 removal and SO2 resistance for mixed metal oxides. Figure 4 shows the Hg0 removal efficiency of

could be easily poisoned by SO2 to generate sulfate; therefore, a negative influence on Hg0 removal appeared because of the higher thermostability of sulfate species.29 With the increase of reaction time and the concentration of SO2, the activity for Hg0 removal gradually decreased due to Mn-based materials being vulnerable to SO 2 . 45 In particular, when SO 2 concentration increased to 1500 ppm, Hg0 removal performance started to decrease significantly from 120 to 180 min, which can be due to a large amount of SO2 presence that exacerbated the poisoning of CrMn adsorbent. Yet even so, CrMn adsorbents also showed much superior Hg0 removal performance when SO2 concentration was lower than 500 ppm. 3.1.4. Effect of Reaction Temperature on Hg0 Removal. Figure 6 shows the Hg0 removal efficiency of CrMn adsorbents

Figure 4. Effect of Cr content on Hg0 removal efficiency.

various adsorbents with different Cr/Mn molar ratios at 120 °C. In the first 1 h without SO2 presence, the Hg0 removal efficiency of CrMn adsorbent increased first and then decreased with the increase of Cr loading. In particular, the Hg0 removal efficiency decreased significantly to about 70% when the Cr/Mn molar ratio was equal to 3:2 and 3:1. CrMn (1:3), CrMn (2:3), and CrMn (1:1) adsorbents exhibited slightly enhanced performances when 500 ppm of SO2 was injected in the following 2 h, while CrMn (3:2) and CrMn (3:1) adsorbents presented a significant enhancement on Hg0 removal. The observation clearly showed that SO2 played a somehow promotional role in Hg0 removal. By comparison, it can be concluded that CrMn (1:1) adsorbent exhibited the highest activity and that nearly 96% of Hg0 was removed in the absence and presence of SO2. 3.1.3. Effect of SO2 Concentration on Hg0 Removal. Figure 5 presents the effect of SO2 concentration on Hg0 removal efficiency over CrMn adsorbents with the Cr/Mn molar ratio of 1:1 at 120 °C. It was observed that the addition of SO2 resulted in a slight increase of Hg0 removal at lower SO2 concentrations (under 1000 ppm) probably due to the appearance of HgSO4 as discussed above. According to the literature,1,22,44 the reactive sites of the Mn-based adsorbent

Figure 6. Effect of reaction temperature on Hg0 removal over CrMn.

with the Cr/Mn molar ratio of 1:1 under different reaction temperatures. It was clear that the Hg0 removal efficiencies of the CrMn samples were all higher than 95% at 50−150 °C. However, when the reaction temperature was raised to 200 °C, the removal efficiency of Hg0 was greatly lowered.19 The results showed that a higher reaction temperature was not conducive to a higher Hg0 removal activity, probably due to the rapid reaction of CrMn with SO2 at a higher reaction temperature. 3.2. Characterization of Adsorbents. 3.2.1. N2 Physisorption, XRD, and SEM-EDS Analyses. The N2 adsorption− desorption isotherm analysis was implemented to investigate the pore structure parameters of pure MnOx, pure CrOx, and CrMn adsorbents with the Cr/Mn molar ratio of 1:1 (similarly hereinafter) as well as the used CrMn-H and CrMn-HS materials. As shown in Table 1, the BET surface area and total pore volume of as-prepared CrMn were 44.63 m2/g and 0.287 cm3/g, which were almost 2.1 times and 1.7 times higher than Table 1. Textural Properties of the Samples

Figure 5. Effect of SO2 concentration on Hg0 removal over CrMn. D

sample

BET surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

MnOx CrOx CrMn CrMn-H CrMn-HS

21.48 35.19 44.63 43.08 37.38

0.164 0.170 0.287 0.225 0.198

132.0 131.4 146.5 149.2 135.1

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 7. BET adsorption−desorption analysis of the fresh and spent adsorbents (The insert gives the changes of BET surface area.).

sponded to the characteristics of MnO2 (JCPDS 24-0735, 2θ = 25.4°, 34.1°, 36.2°, 42.2°, 51.3°, 54.1°, 64.8°, 67.0°, and 73.5°).46 The diffraction peaks of CrOx were mainly located at 2θ = 24.5°, 33.7°, 37.9°, 44.5°, 48.1°, 54.9°, and 65.3°,47 and the peaks were well indexed to the Cr2O3 phase (JCPDS 821484) with a six party structure. It was interesting to notice that the main characteristic peaks of MnCrO448,49 were nearly the same as pure MnOx except for a small decrease in intensity and even the molar ratio of Cr to Mn was as high as 50% in this sample, suggesting the mixture of MnCrO4 material was formed due to an intimate combination between the two materials in the preparation process. Compared with the fresh CrMn adsorbent, the XRD diffraction peaks of used CrMn-H exhibited almost no change, revealing that the amount of oxidized mercury species was probably too low to be detected by XRD or the crystal phase of CrMn sample did not change during the Hg0 removal process. However, as for the spent CrMn-HS, the characteristic peaks shifted toward a lower diffraction angle in comparison to the fresh CrMn sample. Furthermore, a slight peak located at 2θ = 38.2° (JCPDS 82-14842) corresponding to chromium sulfate appeared.47,50,51 The observation obviously validated the reaction of CrMn material with SO2. To investigate the changes of element composition after Hg0 removal, the SEM-EDS experiment was carried out. As shown in Figure 9, CrMn and CrMn-H mainly consisted of Mn, Cr, and O. As expected, the S element appeared in the CrMn-HS adsorbent due to the presence of SO2 in the reaction stream. The carbon (C) element presented in the three samples should be ascribed to adventitious contaminants from the environment.52 However, the Hg element was not detected probably because of its lower contents in CrMn-H and CrMn-HS adsorbents. In addition, the amounts of Cr and Mn in CrMn-H were lower than those of CrMn, and the molar ratios of Cr and Mn decreased by 0.43% and 0.33%, respectively, indicating the reaction between MnOx and/or CrOx and Hg0. By contrast, CrMn-HS also exhibited a lower content of Cr and Mn, and the molar ratio of Cr and Mn greatly decreased by 0.58% and 1.06%, respectively, implying an intense chemical reaction between metal oxides and SO2. Moreover, the prominent decrease of the Mn molar ratio perhaps suggested that the Mn atom was easily involved in the sulfurization process, producing more manganese sulfate.38 Simultaneously, from the SEM images, we can find that in comparison to CrMn with

that of MnOx, respectively. The large specific surface area and porous volume of CrMn could be helpful to adsorb Hg0 and also provide a greater number of reactive sites for the adsorption process. As for used CrMn-H or CrMn-HS, their surface area and pore volume were all slightly reduced, implying the reactions of CrMn with Hg0 or Hg0 and SO2. Figure 7 exhibits the N2 adsorption−desorption isotherm and the pore size distribution curve of the samples. As shown in Figure 7a, the five samples all showed the type IV with a H1 hysteresis loop, indicating the appearance of mesoporous structures. The pore-size distributions shown in Figure 7b revealed a bimodal distribution with two wide peaks centered at 2.2 and 40 nm for MnOx. Similarly, a bimodal distribution also appeared for CrOx; however, the peak intensity of CrOx located at 2.2 nm was much higher than that located at 35 nm, suggesting a different pore structure between MnOx and CrOx. Therefore, CrMn adsorbent showed a bimodal distribution with a small peak centered at 2.2 nm and an intense peak located at 21 nm (Figure 7b). Moreover, the pore size distribution of CrMn-H and CrMn-HS slightly changed because of the reaction of CrMn with Hg0 in the absence and presence of SO2. Figure 8 shows the XRD pattern of three selective samples as well as the spent CrMn materials. The results indicated that the MnOx exhibited nine diffraction peaks, which corre-

Figure 8. XRD patterns of the MnOx, CrOx, CrMn, CrMn-H, and CrMn-HS samples. E

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 9. SEM-EDS of the samples (a) CrMn, (b) CrMn-H, and (c) CrMn-HS.

mixed oxide. Compared with CrMn, CrMn-H displayed almost no change in absorption bands. However, a new absorption band located at 1118 cm −1 clearly appeared, which corresponded to the ν (S = O) vibration of the surface substance55 or molecular chemisorption of SO2, confirming the existence of surface sulfates. 3.2.3. H2-TPR Analysis. Figure 11 shows the H2-TPR profiles of the samples. To evidently observe the changes of

a porous and smooth structure, CrMn-H and CrMn-HS showed a somewhat dense surface, which is in accordance with N2 physisorption results. 3.2.2. FTIR Analysis. FTIR analysis was conducted to further investigate the changes of chemical bonds of the fresh and spent samples (Figure 10). The bands at 3363 and 1635 cm−1

Figure 10. FTIR spectra of the adsorbents.

were assigned to the ν (O−H) stretching vibration peaks of the adsorbed water and hydroxyl groups, respectively.18 The characteristic absorption bands of MnOx located at 953 and 574 cm−1 corresponded to the Mn−O bonds in the structure,53 and the absorption bands of CrOx at 600 cm−1 corresponded to the Cr−O bonds.54 It was clear that the absorption bands spanned 400−1000 cm−1 spectra of CrMn showed a significant change compared with that of CrOx and MnOx, which was probably due to the formation of the CrMn

Figure 11. H2-TPR patterns of the samples.

reduction temperature and reduction peaks in intensity, H2TPR profiles of MnOx, CrMn, and CrMn-H were amplified in the right part of Figure 11. It was apparent that a broad peak spanned at 330−470 °C appeared for the pure MnOx, which corresponded to the reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+.56 CrOx showed two intense reduction peaks at about F

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 12. XPS patterns of the samples.

392 and 491 °C, corresponding to the reduction of dispersed Cr5+ to Cr4+ and Cr4+ to Cr3+ species, respectively.57 While the reduction peaks of CrMn within 300−560 °C were much wider than that of MnOx and CrOx, the reduction area of CrMn was significantly larger than that of MnOx. The observation evidently implied that the addition of Cr into/ onto MnOx changed their redox properties by the interaction between MnOx and CrOx species. The H2-TPR profile of spent CrMn-H exhibited almost no change except for a slight shift to a higher reduction temperature compared with that of CrMn, which might be due to the reaction of MnOx/CrOx species with Hg0 that leads to a reduction of oxidation activity.29,38 The CrMn-HS sample exhibited a sharp H2 reduction peak located at 548 °C and a weak shoulder peak at 600 °C. Obviously, the reduction peak

of CrMn-HS shifted to a higher temperature due to the presence of sulfate, indicating that the reduction ability of the spent adsorbent weakens greatly at low temperature, which also confirmed the previous XRD and SEM-EDS results. The reduction processes could correspond to the reducing behaviors of Cr2(SO4)338,58 and MnSO459 as follows: Cr2(SO4 )3 + 15H 2 → 2Cr + 3H 2S + 12H 2O

(3)

2MnSO4 + 5H 2 → MnS + MnO + 5H 2O + SO2

(4)

3.2.4. XPS Analysis. To further reveal the surface chemical compositions and valence state changes of the elements in the fresh samples and spent samples, XPS analysis was implemented. The XPS spectra of the samples are presented in Figure 12, and the surface atomic ratios of the main G

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 2. Surface Atomic Ratio of the Samples Based on the XPS Result surface atomic ratio (%) sample

Mn4+/Mn

Mn3+/Mn

Mn2+/Mn

MnOx CrOx CrMn CrMn-H CrMn-HS

37.80

35.19

27.01

41.56 43.12 41.25

39.73 37.18 37.55

Cr5+/Cr 26.95 24.13 23.06 22.31

18.71 19.70 21.02

Cr4+/Cr 48.72 46.50 44.94 44.93

Cr3+/Cr

Oβ/O

Oα/O

24.33 29.37 32.00 32.76

21.50 14.51 24.40 26.95 49.08

78.50 85.49 73.60 73.05 50.92

Figure 13. Profiles of Hg0-TPD (a) and SO2-TPD (b) of spent CrMn samples.

of Mn4+/Mn and Cr5+/Cr decreased and the surface atomic concentrations of Mn2+/Mn and Cr3+/Cr increased after Hg0 removal in the absence and presence of SO2, suggesting that higher valence states of Mn and Cr were reduced due to the oxidation of Hg0. Simultaneously, the Oβ/O ratios of CrMn-H and CrMn-HS samples increased from 24.40% to 26.95 and 49.08% because of the reaction of metal oxides with Hg0 or/ and SO2 to generate mercuric oxides, manganese sulfate, and chromium sulfate that accumulated on the adsorbents. As expected, Hg and S elements were detected by XPS. As shown in Figure 12d, the XPS spectrum of Hg was located at about 100 and 104 eV, which were the characters of Hg 4f7/2 and Hg 4f5/2 from mercuric oxide or mercuric sulfate, respectively.32 By comparison, the Hg 4f5/2 peaks of CrMnHS shifted to higher BE than that of CrMn-H, which implied that the presence of SO2 in the flue gas could lead to the production of some amount of mercuric sulfates besides mercuric oxide.39,64 Figure 12e shows the XPS spectrum of S 2p, and it was clear that the S 2p spectrum consisted of two types of sulfur. The lower BE at 168.7 eV was assigned to the SO32− species, and the higher BE at 169.8 eV corresponded to the SO42− species.51,65 Thus, the manganese sulfate/sulfite and chromium sulfate/sulfite (or even mercuric sulfate) were probably present on the surface of CrMn-HS. 3.2.5. Hg0-TPD and SO2-TPD Analyses. To further identify the mercury species and sulfur species deposited on the adsorbents surface after Hg0 removal in the presence of SO2, the Hg0-TPD and SO2-TPD experiments of three spent samples obtained at different reaction temperatures were performed (Figure 13). As shown in Figure 13a, Hg0 concentration that desorbed from CrMn-HS-50 greatly increased at about 260 °C, reached a peak at 320 °C, and then gradually decreased with the temperature increasing. The Hg0 should be derived from desorption of physically adsorbed species of Hg0 and the weakly bond adsorptive Hg0 species. In contrast to CrMn-HS-50, the Hg0 desorption behaviors of CrMn-HS-120 and CrMn-HS-200 exhibited higher desorption

elements based on the XPS results were listed in Table 2. In Figure 12a, the XPS spectra of Mn 2p could be separated into three peaks referred to as Mn4+ at 642.2−643.4 eV, Mn3+ at 641.2−642.1 eV, and Mn2+ at 640.2−641.2 eV,50,60 respectively. In comparison to MnOx, the peaks of Mn 2p1/2 and Mn 2p3/2 of CrMn moved to higher binding energy (BE), implying that the crystal structure of the CrMn changed after Cr doping, which was consistent with the results of the previous FTIR and H2-TPR analyses. As shown in Table 2, the Mn4+/Mn atom concentration on the CrMn surface increased from 37.80% to 41.56%, which indicated that Cr doping can lead to an enrichment of manganese atoms on the CrMn surface, thereby enhancing the oxidation of the CrMn sample. Figure 12b gives the XPS spectra of Cr. According to the literature,61 the spectra of Cr 2p1/2 and Cr 2p3/2 could be divided into three peaks, respectively. The higher peaks at 577.6−578.9 eV and 585.3−585.9 eV were attributed to Cr5+, the peaks at 576.5−577.2 eV and 586.5−586.9 eV corresponded to Cr3+, and the lower peaks at 575.3−576.0 eV and 587.9−588.5 eV were ascribed to Cr2+.47,62 The main peaks Cr 2p1/2 and Cr 2p3/2 in CrMn also shifted to higher BE in comparison to CrOx, which was similar to that observed in MnOx and CrMn samples, implying the formation of the Mn−Cr mixed oxide. Figure 12c gives the O 1s peaks of the samples. The separating peak located at lower BE referred to the lattice oxygen (Oα) such as O22− or O−,47 while the peak at higher BE corresponded to the chemisorbed oxygen (Oβ), such as O2−.63 It was obvious that the main peak of O 1s in CrMn also varied to higher BE due to the mixture of CrOx and MnOx. Moreover, as shown in Table 2, the Oβ/O ratio significantly increased from 14.51% of CrOx to 24.40% of CrMn. The enhanced content of Oβ could improve the performance of Hg0 removal due to its flexible mobility. As for the spent CrMn-H and CrMn-HS samples, the intensities of Mn 2p and Cr 2p were decreased slightly due to the reaction of MnOx or/and CrOx and Hg0 or/and SO2. From Table 2, we can observe that the surface atomic concentrations H

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 14. Adsorption structures of Hg0 on the surface of the p(2×2) MnO2 (110) surface: (a) Obr-top site of Hg adsorption; (b) Obr-bridge site of Hg adsorption; (c) Obr/s-hollow site of Hg adsorption; (d) Obr-top site of SO2 adsorption; (e) Obr-bridge site of SO2 adsorption; (f) Obr/s-hollow site of SO2 adsorption. The pink and yellow colors represent Hg atom and S atom (similarly hereinafter), respectively.

Table 3. Adsorption Energy, Bond Length, and Mulliken Charge of Hg0 and SO2 Adsorbed on the Surface of p(2×2) MnO2 (110) MnO2 (110) (2×2) surface cell (Hg adsorption)

MnO2 (110) (2×2) surface cell (SO2 adsorption)

Ads-site

Eads (kJ/mol)

RHg−O (Å)

QHg (e)

Ads-site

Eads (kJ/mol)

RS−O (Å)

QS (e)

a (top) b (bridge) c (hollow)

−19.26 −23.36 −23.05

2.417 2.542/2.543 2.387/3.151/3.153

0.27 0.33 0.27

d (top) e (bridge) f (hollow)

−47.88 −193.29 −78.43

1.624 1.600/1.601 1.694/3.425/3.322

1.91 2.32 1.66

temperatures, and both of them started to release Hg0 at 250 °C and finished the desorption behavior at 550 °C. According to the previous studies,29,38 the Hg0 in these two samples can be due to decomposition of chemically adsorbed Hg species, Hg2O and HgO. It was clear that the contents of Hg0 desorbed from CrMn-HS-120 and CrMn-HS-200 came from higher desorption temperature (above 370 °C), while most of Hg0 desorbed from CrMn-HS-50 was from lower desorption temperature (below 370 °C), reflecting that the majority of the mercury species onto CrMn-HS-120 and CrMn-HS-200 existed as Hg2O and HgO and chemisorption was dominant when the reaction temperature was higher than 120 °C. Figure 13b shows SO2-TPD curves of the three spent samples. The three samples all showed one big SO2 desorption peak that spanned from 580 to 900 °C in the whole heating process. It was evident that these SO2 originated from the decomposition of MnSO4 or/and CrSO4 that deposited on the CrMn surface, which further verified the results of XPS and H2-TPR. The result also indicated that SO2 will react with MnOx and CrOx to produce stable sulfates even at a lower reaction temperature of 50 °C.

3.3. DFT Study. To further understand the positive influence of Cr loading on the performances of Hg0 removal and SO2 resistance and competitive adsorption relationship between Hg0 and SO2 on the CrMn adsorbent, two typical models of MnO2 (110) and Cr-MnO2 (110) (CrMn (110) for short) were established and calculated using the DFT method. A single adsorbed atom was initially placed on the surface of p(2×2)MnO2 (110) and p(2×2)CrMn (110). Parameters of adsorption energy, bond lengths, Mulliken charge, and partial density of states (PDOS) were analyzed. The optimized structures of Hg0 and SO2 on top, bridge, and hollow sites of the MnO2 (110) surface are presented in Figure 14a-f, respectively. The adsorption energy, bond length, and Mulliken charge of Hg0 and SO2 on the three active sites of the MnO2 (110) surface are shown in Table 3. The adsorption energies (Eads) for Hg0 are in the sequence of Obrbridge > Obr/s-hollow > Obr-top with Eads within 19.26−23.36 kJ/mol. The Obr-bridge site is the most stable adsorption site, in which the Hg−O bond length is 2.542 Å and the amount of charge transfer is 0.33 e, indicating that Hg0 is chemisorbed on the reactive oxygen sites of the surface. These calculation results are nearly consistent with the research results of Wang I

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 15. Adsorption structures of Hg0 on the p(2×2) CrMn (110) surface: (a) Obr-bridge site of Hg adsorption and (b) Obr-bridge site of SO2 adsorption. The blue color stands for the Cr atom.

Table 4. Adsorption Energies, Bond Lengths, and Mulliken Charge for Hg0 and SO2 Adsorption on the p(2×2) CrMn (110) Surface CrMn (110) (2×2) surface cell (Hg adsorption)

CrMn (110) (2×2) surface cell (SO2 adsorption)

Ads-site

Eads (kJ/mol)

RHg−O (Å)

QHg (e)

Ads-site

Eads (kJ/mol)

RS−O (Å)

QS (e)

bridge

−21.86

2.536/2.630

0.31

bridge

−95.85

1.600/1.601

1.88

et al.35 and Zhang et al.,66 where Zhang et al. reported that the length of the Hg−O bond was 2.692 Å and the charge transfer was 0.274 e. However, due to the difference of the basis set employed, the maximum adsorption energy for Hg0 is 78.32 kJ/mol, which is higher than that of the present study. As shown in Table 3, the charges of adsorbed mercury atoms are in the range of 0.27−0.33 e, which means that electrons are transferred from Hg0 to lattice oxygen. In addition, many electrons are moved from the Hg atom to the Obr-bridge compared with the Obr-top and Obr/s-hollow, implying that Hg0 is readily adsorbed on the Obr-bridge sites on the surface of MnO2 (110), which is in agreement with the above adsorption energy results. From Table 3, it shows that the adsorption energies for SO2 are in the sequence of Obr-bridge > Obr/s-hollow > Obr-top, and the adsorption energies of SO2 on the MnO2 (110) surface are in the range of 47.88−193.9 kJ/mol, which means SO2 is chemisorbed in three sites. The charges of adsorbed S atoms in three stable configurations are within 1.66−2.31 e, and the electrons move from the S atom to the Obr-bridge instead of the Obr-top and Obr/s-hollow. It is observed that the adsorption of SO2 is realized mainly through the S atom and the surface O atom, and the S atom is more readily adsorbed on Obr-bridge sites of the MnO2 (110) surface. It can be concluded the most stable binding sites for Hg0 and SO2 are both Obr-bridge sites. The adsorption energy for SO2 is much larger than that of Hg0 on the surface of MnO2 (110). Therefore, SO2 may have intense competitive adsorption with Hg0 on MnO2, which would have a negative influence on the removal of Hg0 on MnO2 material. Because the Obr-bridge of MnO2 (110) was the most reactive site for Hg0 and SO2 adsorption, the discussion below mainly focused on the adsorption of Hg0 and SO2 on the Obrbridge site of the CrMn (110) surface. The optimized structures of Hg0 and SO2 on bridge sites of the CrMn (110) surface are presented in Figure 15a and b, respectively.

The adsorption energies, bond lengths, and Mulliken charge for Hg0 and SO2 on CrMn (110) are listed in Table 4. By comparison, it is found that the adsorption energy of Hg0 on CrMn (110) is nearly the same as that of Hg on the MnO2 (110) surface. However, the adsorption energy of SO2 on the surface of CrMn (110) is much lower than that of SO2 on MnO2 (110), indicating that the presence of Cr into/onto the MnO 2 (110) surface would significantly decrease the adsorption behavior of SO2. The PDOS of the most concerned atoms is also studied to understand the possible adsorption mechanism of Hg0 and SO2 onto MnO2 (110) and CrMn (110) surfaces. Figure 16a shows the PDOS of Hg and O atoms before and after adsorption. In the process of Hg0 adsorption, the p-orbital of the O atom would be stanchly hybridized with the d- and s-orbitals of the Hg atom at around −5.7, −4.8, and −2.9 eV, respectively. In Figure 16b for SO2 adsorption at the Obr-bridge site, the sorbital of the O atom and the s- and p-orbitals of the S atom are hybridized at about −18.6, −9.4, −7.6, and −6.0 eV, respectively. This would result in a strong competitive adsorption of Hg0 and S atoms on the surface of MnO2.67 When the Mn5 atom on the MnO2(110) surface is replaced by a Cr atom, the calculation result shows that the d-orbital of the Hg atom is strongly resonant with the p-orbital of the O atom at about −4.8, −2.5, and −1.2 eV (Figure 16c). In addition, the s-orbital of the S atom is sturdily resonant with the s-orbital of O at approximately −22.6 and −9.2 eV (Figure 16d). From the calculation above, we can find that although the doping of Cr would not significantly enhance the adsorption energy of the Hg0 atom on the CrMn (110) surface, it can weaken the binding of the SO2 molecule with the lattice oxygen around the doping atoms. 3.4. Mechanism of Cr Loading on Hg0 Removal and SO2 Resistance. In combining the experimental and characterization results and DFT calculations, the possible positive influences of Cr loading on enhanced Hg0 removal and J

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 16. PDOS for the surface system before and after (a) Hg0 adsorption at Obr-bridge sites on the p(2×2) MnO2 (110) surface; (b) SO2 adsorption at Obr-bridge sites on the p(2×2) MnO2(110) surface; (c) Hg0 adsorption at Obr-bridge sites on the p(2×2) CrMn (110) surface; and (d) SO2 adsorption at Obr-bridge sites on the p(2×2) CrMn (110) surface.

superior SO2 tolerance are illustrated in Figure 17. According to the N2 physisorption and XRD results (the middle section of Figure 17), Cr doping can enlarge the BET surface area of MnOx due to the formation of mixed MnCrO4 material. H2TPR results exhibited that the addition of Cr into/onto MnOx would enhance the redox performance of CrMn mixed oxide and many more easily reduced species appeared in the mixture. Thus, the larger surface area and sufficient reactive species would be beneficial for higher Hg0 removal activity although

the DFT calculation shows almost the same adsorption energies of Hg0 on MnO2(110) and CrMn (110) surfaces. According to the literature,68 the Hg0 removal over CrMn mixed oxide corresponded to the Mars−Maessen mechanism (the left part of Figure 17). First, gaseous Hg0 is physically adsorbed onto the reactive oxygen species (O*) (It is known from the DFT analysis that the main position is Obr-bridge.) near the metal Mn or Cr through collision. It is confirmed by the orbital hybridization that the adsorption of Hg(ads)* is K

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 17. Possible mechanisms for enhanced Hg0 removal and poisoning by SO2.

sulfates even at a lower reaction temperature of 50 °C. The large specific surface area, abundant porous volume, and sufficient reactive species of CrMn could be helpful to Hg0 removal. The DFT calculations indicated that the most stable adsorption sites for Hg0 and SO2 were both Obr-bridge sites of MnO2 (110). The adsorption energy of SO2 on the MnO2 (110) surface was much higher than that of Hg0 on the MnO2 (110) surface. Thus SO2 may have an intense competitive adsorption with Hg0 on MnO2, thereby resulting in lower Hg0 removal activity and SO2 poisoning. The adsorption energy of SO2 on the CrMn (110) surface was much lower than that on MnO2 (110), and the charge transfer of the S atom to the Obrbridge around the Mn atom was much more, suggesting that manganese sulfate would be easily generated in comparison with chromium sulfate and higher SO2 resistance could be obtained.

chemisorption, which is consistent with previous analysis results of Hg0-TPD and SO2-TPD. Then, the O* receives electrons from Hg(ads)* and further gives the electrons to Mn or Cr atoms adjacent to O*, and the consumed O* can be supplemented by O2. The high valence of Mn or/and Cr in the crystal structure can obtain more electrons from the active oxygen sites, which is advantageous for the removal of Hg0.69 When SO2 is injected into the flue gas (the right section of Figure 17), SO2 is preferentially adsorbed on the adjacent O* of Mn or Cr to form SO32− or SO42−39 and further to produce manganese sulfate/sulfite or chromium sulfate/sulfite, which have been confirmed by FTIR, H2-TPR, and XPS analyses. From the DFT calculation results, the adsorption energy of SO2 on the MnO2 (110) surface is significantly higher than that of SO2 on the CrMn (110) surface, and the charge transfer from the S atom to the Obr-bridge around the Mn atom in the MnO2 (110) structure is much higher (2.32 e) than that (1.88 e) of the CrMn (110) structure. Therefore, much more manganese sulfate could be deposited on the surface of the MnO2 adsorbent than that on the CrMn surface, which is chemically stable and difficult to decompose.6,70 Therefore, the loading of Cr into/onto MnOx could improve the performance of Hg0 removal and SO2 tolerance.

■

AUTHOR INFORMATION

Corresponding Authors

*Phone/Fax: 86-22-87402075. E-mail: [email protected] (G.C.). *Phone/Fax: 86-391-3987511. E-mail: [email protected] (A.Z.). ORCID

4. CONCLUSIONS Compared with CuMn, FeMn, MoMn, and pure MnO2, the resulting CrMn material exhibited the highest Hg0 removal activity and SO2 tolerance, and the optimal atomic ratio of Cr to Mn was 1:1. Cr loading into/onto MnOx material not only can improve the performance of Hg0 removal but also can significantly enhance its SO2 resistance. The physisorption was the main reason for Hg0 removal at 50 °C, while chemisorption was dominant when the reaction temperature was higher than 120 °C. SO2 will react with MnOx and CrOx to produce stable

Anchao Zhang: 0000-0002-0704-6736 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51676064, 51306046, 51676138), the Young Core Instructor Project in the Higher Education Institutions of Henan Province (2016GGJS-038), and the L

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

(19) Zhao, Y.; Ma, X.; Xu, P.; Wang, H. Elemental mercury removal from flue gas by CoFe2O4 catalyzed peroxymonosulfate. J. Hazard. Mater. 2018, 341, 228−237. (20) Zhao, L.; Li, C.; Du, X.; Zeng, G.; Gao, L.; Zhai, Y.; Wang, T.; Zhang, J. Effect of Co addition on the performance and structure of V/ZrCe catalyst for simultaneous removal of NO and Hg0 in simulated flue gas. Appl. Surf. Sci. 2018, 437, 390−399. (21) Zhang, X.; Shen, B.; Shen, F.; Zhang, X.; Si, M.; Yuan, P. The behavior of the manganese-cerium loaded metal-organic framework in elemental mercury and NO removal from flue gas. Chem. Eng. J. 2017, 326, 551−560. (22) Cheng, J.; Wang, X.; Li, Y.; Ning, P. Adsorption and catalytic oxidation of mercury over MnOx/TiO2 under low-temperature conditions. Ind. Eng. Chem. Res. 2017, 56 (49), 14419−14429. (23) Zhou, Z.; Liu, X.; Liao, Z.; Shao, H.; Lv, C.; Hu, Y.; Xu, M. Manganese doped CeO2-ZrO2 catalyst for elemental mercury oxidation at low temperature. Fuel Process. Technol. 2016, 152, 285−293. (24) Shen, B.; Zhu, S.; Zhang, X.; Chi, G.; Patel, D.; Si, M.; Wu, C. Simultaneous removal of NO and Hg0 using Fe and Co co-doped MnCe/TiO2 catalysts. Fuel 2018, 224, 241−249. (25) Wang, P.; Su, S.; Xiang, J.; You, H.; Cao, F.; Sun, L.; Hu, S.; Zhang, Y. Catalytic oxidation of Hg0 by MnOx-CeO2/gamma-Al2O3 catalyst at low temperatures. Chemosphere 2014, 101, 49−54. (26) He, C.; Shen, B.; Chen, J.; Cai, J. Adsorption and oxidation of elemental mercury over Ce-MnOx/Ti-PILCs. Environ. Sci. Technol. 2014, 48 (14), 7891−7898. (27) Xu, H.; Xie, J.; Ma, Y.; Qu, Z.; Zhao, S.; Chen, W.; Huang, W.; Yan, N. The cooperation of FeSn in a MnOx complex sorbent used for capturing elemental mercury. Fuel 2015, 140, 803−809. (28) Ma, Y.; Mu, B.; Yuan, D.; Zhang, H.; Xu, H. Design of MnO2/ CeO2-MnO2 hierarchical binary oxides for elemental mercury removal from coal-fired flue gas. J. Hazard. Mater. 2017, 333, 186−193. (29) Zhang, A.; Zhang, Z.; Lu, H.; Liu, Z.; Xiang, J.; Zhou, C.; Xing, W.; Sun, L. Effect of promotion with Ru addition on the activity and SO2 resistance of MnOx-TiO2 adsorbent for Hg0 removal. Ind. Eng. Chem. Res. 2015, 54 (11), 2930−2939. (30) Li, H.; Wang, Y.; Wang, S.; Wang, X.; Hu, J. Removal of elemental mercury in flue gas at lower temperatures over Mn-Ce based materials prepared by co-precipitation. Fuel 2017, 208, 576− 586. (31) Hu, Y.; Cheng, H. Control of mercury emissions from stationary coal combustion sources in China: Current status and recommendations. Environ. Pollut. 2016, 218, 1209−1221. (32) Zhang, A.; Zheng, W.; Song, J.; Hu, S.; Liu, Z.; Xiang, J. Cobalt manganese oxides modified titania catalysts for oxidation of elemental mercury at low flue gas temperature. Chem. Eng. J. 2014, 236, 29−38. (33) Mei, H. Y.; Pang, Y.; Liu, D. Y.; Cheng, N.; Zheng, S.; Song, Q.; Wang, M. Electronic and mechanic properties of trigonal boron nitride by first-principles calculations. Phys. E 2018, 101, 16−21. (34) Zhang, B.; Liu, J.; Zhang, J.; Zheng, C.; Chang, M. Mercury oxidation mechanism on Pd(100) surface from first-principles calculations. Chem. Eng. J. 2014, 237, 344−351. (35) Wang, Z.; Liu, J.; Yang, Y.; Liu, F.; Ding, J. Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst. Proc. Combust. Inst. 2018, No. 2018, 1−9. (36) Chen, H.; Li, X.; Wan, R.; Kao-Walter, S.; Lei, Y. A DFT study of the electronic structures and optical properties of (Cr, C) co-doped rutile TiO2. Chem. Phys. 2018, 501, 60−67. (37) Thakur, J.; Saini, H. S.; Singh, M.; Reshak, A. H.; Kashyap, M. K. Quest for magnetism in graphene via Cr- and Mo-doping: A DFT approach. Phys. E 2016, 78, 35−40. (38) Chen, G.; Zhang, D.; Zhang, A.; Zhang, Z.; Liu, Z.; Hou, L. a. CrOx-MnOx-TiO2 adsorbent with high resistance to SO2 poisoning for Hg0 removal at low temperature. J. Ind. Eng. Chem. 2017, 55, 119− 127. (39) Zhang, X.; Li, Z.; Wang, J.; Tan, B.; Cui, Y.; He, G. Reaction mechanism for the influence of SO2 on Hg0 adsorption and oxidation with Ce0.1-Zr-MnO2. Fuel 2017, 203, 308−315.

Outstanding Youth Science Foundation of Henan Polytechnic University (J2016-1) is gratefully acknowledged.

■

REFERENCES

(1) Li, H.; Wu, C.-Y.; Li, Y.; Li, L.; Zhao, Y.; Zhang, J. Impact of SO2 on elemental mercury oxidation over CeO2-TiO2 catalyst. Chem. Eng. J. 2013, 219, 319−326. (2) Hilson, G.; Zolnikov, T. R.; Ortiz, D. R.; Kumah, C. Formalizing artisanal gold mining under the Minamata convention: Previewing the challenge in Sub-Saharan Africa. Environ. Sci. Policy 2018, 85, 123− 131. (3) Mackey, T. K.; Contreras, J. T.; Liang, B. A. The Minamata Convention on Mercury: Attempting to address the global controversy of dental amalgam use and mercury waste disposal. Sci. Total Environ. 2014, 472, 125−129. (4) Xu, W.; Wang, H.; Zhou, X.; Zhu, T. CuO/TiO2 catalysts for gas-phase Hg0 catalytic oxidation. Chem. Eng. J. 2014, 243, 380−385. (5) Svoboda, K.; Hartman, M.; Syc, M.; Pohorely, M.; Kamenikova, P.; Jeremias, M.; Durda, T. Possibilities of mercury removal in the dry flue gas cleaning lines of solid waste incineration units. J. Environ. Manage. 2016, 166, 499−511. (6) Liu, Y.; Zhou, J.; Zhang, Y.; Pan, J.; Wang, Q.; Zhang, J. Removal of Hg0 and simultaneous removal of Hg0/SO2/NO in flue gas using two Fenton-like reagents in a spray reactor. Fuel 2015, 145, 180−188. (7) Li, J.; Chen, J.; Yu, Y.; He, C. Fe-Mn-Ce/ceramic powder composite catalyst for highly volatile elemental mercury removal in simulated coal-fired flue gas. J. Ind. Eng. Chem. 2015, 25, 352−358. (8) Zhou, Z.; Liu, X.; Hu, Y.; Liao, Z.; Cheng, S.; Xu, M. An efficient sorbent based on CuCl2 loaded CeO2-ZrO2 for elemental mercury removal from chlorine-free flue gas. Fuel 2018, 216, 356−363. (9) Li, H.; Wang, S.; Wang, X.; Tang, N.; Pan, S.; Hu, J. Catalytic oxidation of Hg0 in flue gas over Ce modified TiO2 supported Co-Mn catalysts: Characterization, the effect of gas composition and cobenefit of NO conversion. Fuel 2017, 202, 470−482. (10) Huang, W.; Qu, Z.; Chen, W.; Xu, H.; Yan, N. An enhancement method for the elemental mercury removal from coal-fired flue gas based on novel discharge activation reactor. Fuel 2016, 171, 59−64. (11) Thompson, J. S.; Pavlish, J. H.; Hamre, L. L.; Jensen, M. D.; Smith, D.; Podwin, S.; Brickett, L. A. Sorbent injection into a slipstream baghouse for mercury control: Project summary. Fuel Process. Technol. 2009, 90 (11), 1392−1399. (12) Zhao, B.; Zhang, Z.; Jin, J.; Pan, W. P. Simulation of mercury capture by sorbent injection using a simplified model. J. Hazard. Mater. 2009, 170 (2−3), 1179−1185. (13) Dombrowski, K.; Richardson, C.; Padilla, J.; Fisher, K.; Campbell, T.; Chang, R.; Eckberg, C.; Hudspeth, J.; O’Palko, A.; Pletcher, S. Evaluation of low ash impact sorbent injection technologies for mercury control at a Texas lignite/PRB fired power plant. Fuel Process. Technol. 2009, 90 (11), 1406−1411. (14) Yang, J.; Zhao, Y.; Guo, X.; Li, H.; Zhang, J.; Zheng, C. 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 2018, 220, 403− 411. (15) Wu, J.; Zhao, Z.; Huang, T.; Sheng, P.; Zhang, J.; Tian, H.; Zhao, X.; Zhao, L.; He, P.; Ren, J.; Gao, K. Removal of elemental mercury by Ce-Mn co-modified activated carbon catalyst. Catal. Commun. 2017, 93, 62−66. (16) Xu, W.; Hussain, A.; Liu, Y. A review on modification methods of adsorbents for elemental mercury from flue gas. Chem. Eng. J. 2018, 346, 692−711. (17) Zhang, S.; Zhao, Y.; Wang, Z.; Zhang, J.; Wang, L.; Zheng, C. Integrated removal of NO and mercury from coal combustion flue gas using manganese oxides supported on TiO2. J. Environ. Sci. 2017, 53, 141−150. (18) Wang, T.; Li, C.; Zhao, L.; Zhang, J.; Li, S.; Zeng, G. The catalytic performance and characterization of ZrO2 support modification on CuO-CeO2/TiO2 catalyst for the simultaneous removal of Hg0 and NO. Appl. Surf. Sci. 2017, 400, 227−237. M

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research (40) Jampaiah, D.; Ippolito, S. J.; Sabri, Y. M.; Reddy, B. M.; Bhargava, S. K. Highly efficient nanosized Mn and Fe codoped ceriabased solid solutions for elemental mercury removal at low flue gas temperatures. Catal. Sci. Technol. 2015, 5 (5), 2913−2924. (41) Jampaiah, D.; Ippolito, S. J.; Sabri, Y. M.; Tardio, J.; Selvakannan, P. R.; Nafady, A.; Reddy, B. M.; Bhargava, S. K. Ceria−zirconia modified MnOx catalysts for gaseous elemental mercury oxidation and adsorption. Catal. Sci. Technol. 2016, 6 (6), 1792−1803. (42) Qiao, S.; Li, J.; Qu, Z.; Liu, P.; Yan, N.; Jia, J.; Chen, J. Adsorption and Catalytic Oxidation of Gaseous Elemental Mercury in Flue Gas over MnOx/Alumina. Ind. Eng. Chem. Res. 2009, 48 (7), 3317−3322. (43) Zhao, B.; Yi, H.; Tang, X.; Li, Q.; Liu, D.; Gao, F. Copper modified activated coke for mercury removal from coal-fired flue gas. Chem. Eng. J. 2016, 286, 585−593. (44) Zhao, Y.; Han, Y.; Guo, T.; Ma, T. Simultaneous removal of SO2, NO and Hg0 from flue gas by ferrate (VI) solution. Energy 2014, 67, 652−658. (45) Zhang, A.; Zhang, Z.; Shi, J.; Chen, G.; Zhou, C.; Sun, L. Effect of preparation methods on the performance of MnO x -TiO 2 adsorbents for Hg0 removal and SO2 resistance. J. Fuel Chem. Technol. 2015, 43 (10), 1258−1266. (46) Li, J.; Chen, J.; Ke, R.; Luo, C.; Hao, J. Effects of precursors on the surface Mn species and the activities for NO reduction over MnOx/TiO2 catalysts. Catal. Commun. 2007, 8 (12), 1896−1900. (47) Chen, Z.; Yang, Q.; Li, H.; Li, X.; Wang, L.; Chi Tsang, S. CrMnOx mixed-oxide catalysts for selective catalytic reduction of NOx with NH3 at low temperature. J. Catal. 2010, 276 (1), 56−65. (48) Qiu, M.; Zhan, S.; Zhu, D.; Yu, H.; Shi, Q. NH3-SCR performance improvement of mesoporous Sn modified Cr-MnOx catalysts at low temperatures. Catal. Today 2015, 258, 103−111. (49) Wang, G.; Zhang, J.; Liu, L.; Zhou, J. Z.; Liu, Q.; Qian, G.; Xu, Z. P.; Richards, R. M. Novel multi-metal containing MnCr catalyst made from manganese slag and chromium wastewater for effective selective catalytic reduction of nitric oxide at low temperature. J. Cleaner Prod. 2018, 183, 917−924. (50) Chen, Z.; Li, X.; Gao, X.; Jiang, Y.; LÜ , Y.; Wang, F.; Wang, L. Selective catalytic reduction of NOx with NH3 on a Cr-Mn mixed oxide at low temperature. Chin. J. Catal. 2009, 30 (1), 4−6. (51) Yan, D.-j.; Yu, Y.; Huang, X.-m.; Liu, S.-j.; Liu, Y.-h. Poisoning effect of SO2 on Mn-Ce/TiO2 catalysts for NO reduction by NH3 at low temperature. J. Fuel Chem. Technol. 2016, 44 (2), 232−238. (52) Sun, H.; Zhao, S.; Ma, Y.; Wu, J.; Liang, P.; Yang, D.; Zhang, H. Effective and regenerable Ag/4A zeolite nanocomposite for Hg0 removal from natural gas. J. Alloys Compd. 2018, 762, 520−527. (53) Tian, Y.; Li, D.; Liu, J.; Wang, H.; Zhang, J.; Zheng, Y.; Liu, T.; Hou, S. Facile synthesis of Mn3O4 nanoplates-anchored graphene microspheres and their applications for supercapacitors. Electrochim. Acta 2017, 257, 155−164. (54) Ren, Z.; Xu, X.; Wang, X.; Gao, B.; Yue, Q.; Song, W.; Zhang, L.; Wang, H. FTIR, Raman, and XPS analysis during phosphate, nitrate and Cr(VI) removal by amine cross-linking biosorbent. J. Colloid Interface Sci. 2016, 468, 313−323. (55) Zhang, S.; Zhao, Y.; Yang, J.; Zhang, Y.; Sun, P.; Yu, X.; Zhang, J.; Zheng, C. Simultaneous NO and mercury removal over MnOx/ TiO2 catalyst in different atmospheres. Fuel Process. Technol. 2017, 166, 282−290. (56) Sun, P.; Guo, R.; Liu, S.; Wang, S.; Pan, W.; Li, M. The enhanced performance of MnOx catalyst for NH3-SCR reaction by the modification with Eu. Appl. Catal., A 2017, 531, 129−138. (57) Fridman, V. Z.; Xing, R.; Severance, M. Investigating the CrOx/ Al2O3 dehydrogenation catalyst model: I. identification and stability evaluation of the Cr species on the fresh and equilibrated catalysts. Appl. Catal., A 2016, 523, 39−53. (58) Ma, Y.; Cheng, R.; Li, J.; Zhong, L.; Liu, Z.; He, X.; Liu, B. Effect of Mo-modification over Phillips CrOx/SiO2 catalyst for ethylene polymerization. J. Organomet. Chem. 2015, 791, 311−321.

(59) Zhuang, K.; Zhang, Y.; Huang, T.; Lu, B.; Shen, K. Sulfurpoisoning and thermal reduction regeneration of holmium-modified Fe-Mn/TiO2 catalyst for low-temperature SCR. J. Fuel Chem. Technol. 2017, 45 (11), 1356−1364. (60) Xu, H.; Ma, Y.; Huang, W.; Mei, J.; Zhao, S.; Qu, Z.; Yan, N. Stabilization of mercury over Mn-based oxides: Speciation and reactivity by temperature programmed desorption analysis. J. Hazard. Mater. 2017, 321, 745−752. (61) Chen, J.; Zhang, X.; Arandiyan, H.; Peng, Y.; Chang, H.; Li, J. Low temperature complete combustion of methane over cobalt chromium oxides catalysts. Catal. Today 2013, 201, 12−18. (62) Wang, A.; Guo, Y.; Gao, F.; Peden, C. H. F. Ambienttemperature NO oxidation over amorphous CrOx-ZrO2 mixed oxide catalysts: Significant promoting effect of ZrO2. Appl. Catal., B 2017, 202, 706−714. (63) Wu, Z.; Jin, R.; Liu, Y.; Wang, H. Ceria modified MnOx/TiO2 as a superior catalyst for NO reduction with NH3 at low-temperature. Catal. Commun. 2008, 9 (13), 2217−2220. (64) Gao, F.; Tang, X.; Yi, H.; Li, J.; Zhao, S.; Wang, J.; Chu, C.; Li, C. Promotional mechanisms of activity and SO2 tolerance of Co- or Ni-doped MnOx-CeO2 catalysts for SCR of NOx with NH3 at low temperature. Chem. Eng. J. 2017, 317, 20−31. (65) Ma, Z.; Yang, H.; Liu, F.; Zhang, X. Interaction between SO2 and Fe-Cu-Ox/CNTs-TiO2 catalyst and its influence on NO reduction with NH3. Appl. Catal., A 2013, 467, 450−455. (66) Zhang, B.; Liu, J.; Zheng, C.; Chang, M. Theoretical study of mercury species adsorption mechanism on MnO2(110) surface. Chem. Eng. J. 2014, 256, 93−100. (67) Yang, Y.; Liu, J.; Zhang, B.; Liu, F. Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2O4. J. Hazard. Mater. 2017, 321, 154−161. (68) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82 (2−3), 89−165. (69) Xie, J.; Qu, Z.; Yan, N.; Yang, S.; Chen, W.; Hu, L.; Huang, W.; Liu, P. Novel regenerable sorbent based on Zr-Mn binary metal oxides for flue gas mercury retention and recovery. J. Hazard. Mater. 2013, 261, 206−213. (70) Zhou, C.; Sun, L.; Zhang, A.; Ma, C.; Wang, B.; Yu, J.; Su, S.; Hu, S.; Xiang, J. Elemental mercury (Hg0) removal from containing SO2/NO flue gas by magnetically separable Fe2.45Ti0.55O4/H2O2 advanced oxidation processes. Chem. Eng. J. 2015, 273, 381−389.

N

DOI: 10.1021/acs.iecr.8b04857 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX