Promoting Effect of the Core-Shell Structure of MnO₂@TiO₂ Nanorods on SO₂ Resistance in Hg⁰ Removal Process

Abstract

Sorbent of αMnO₂ nanorods coating TiO₂ shell (denoted as αMnO₂-NR@TiO₂) was prepared to investigate the elemental mercury (Hg⁰) removal performance in the presence of SO₂. Due the core-shell structure, αMnO₂-NR@TiO₂ has a better SO₂ resistance when compared to αMnO₂ nanorods (denoted as αMnO₂-NR). Kinetic studies have shown that both the sorption rates of αMnO₂-NR and αMnO₂-NR@TiO₂, which can be described by pseudo second-order models and SO₂ treatment, did not change the kinetic models for both the two catalysts. In contrast, X-ray photoelectron spectroscopy (XPS) results showed that, after reaction in the presence of SO₂, S concentration on αMnO₂-NR@TiO₂ surface is lower than on αMnO₂-NR surface, which demonstrated that TiO₂ shell could effectively inhibit the SO₂ diffusion onto MnO₂ surface. Thermogravimetry-differential thermosgravimetry (TG-DTG) results further pointed that SO₂ mainly react with TiO₂ forming Ti(SO₄)O in αMnO₂-NR@TiO₂, which will protect Mn from being deactivated by SO₂. These results were the reason for the better SO₂ resistance of αMnO₂-NR@TiO₂.

1. Introduction

The emission of mercury from coal-fired power plants has drawn wide public concern in modern society. Mercury emissions are a long-term threat to human health and the environment because of extreme toxicity, persistence, and bioaccumulation. Therefore, controlling mercury emitted from coal-fired power plants has practical significance. Mercury in coal combustion flue gas is mainly present in three forms: Elemental mercury (Hg⁰), oxidized mercury (Hg²⁺), and particulate-bound mercury (Hg^p). Particulate-bound mercury (Hg^p) can be removed by electrostatic precipitators (ESP) and fabric filters (FF), while oxidized mercury (Hg²⁺) can be captured by wet flue gas desulfurization system (WFGD). However, existing air pollution control devices can hardly remove Hg⁰ due to its high volatility and low solubility.

Hg⁰ capture with specific adsorbents is a usual way to control Hg⁰ emissions from coal-fired power plants [1]. Activated cabon (AC) has been widely used for the adsorption of Hg⁰ in coal-fired flue gas [2,3]. However, a huge amount of AC needs to be injected into flue gas because of its low Hg⁰ capture capacity, which leads to a high operating cost of this technology. Sulfur or halogen modification can enhance adsorption ability of AC [4,5]. However, the injected AC is usually captured together with fly ash by particulate control device, and the Hg⁰ adsorbed on AC will influence the fly ash utilization [6]. Therefore, alternative economic sorbents with high Hg⁰ removal efficiency are necessary.

Oxides, such as CuO_x [7,8], FeO_x [9,10], CeO_x [11,12] and MnO_x [13,14,15], with high redox properties, exhibit great potential for Hg⁰ adsorption. Among these oxides, MnO_x is a commonly available and inexpensive material has received extensive attention due to the redox couples of Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ [16]. Electronic shift between the different valence states of Mn is active and leads to a high redox capacity. Stefano Cimino et al. [14] investigated the Hg⁰ removal performance of Mn/TiO₂ and found that Hg⁰ capture efficiency was about 57% at 70 °C. After modification by some other transition metal oxides, Mn-based materials, such as Mn-FeO_x [15], Mn-ZrO_x [17], Mn-CeO_x [18], and Mn-CuO_x [19] can remove Hg⁰ better. Furthermore, it has been reported that the shape and crystallographic phases of Mn based sorbents have serious effects on Hg⁰ removal performance. Xu et al. [20] synthesized three different crystallographic phases of MnO₂ and found that α-MnO₂ had the highest capacity due to its larger surface area and oxidizability. Chalkidis et al. [21] pointed out that MnO₂ nano-rods possessed good Hg⁰ removal capacity owing to the higher surface adsorbed oxygen species.

However, Mn-based sorbents usually have a poor SO₂ resistance as SO₂ can easily react with Mn, thereby forming MnSO₄ and leading to a largely suppressed Hg⁰ removal activity. Even a little amount of SO₂ will results in serious inhibited effects on Hg⁰ removal process. Our previous work has indicated that Ce-Zr modified Mn sorbent will be totally deactivated in 1h after the introduction of 50 ppm SO₂ due to SO₂ poisoning Mn forming MnSO₄ [22]. TiO₂ is a traditional way to enhance the SO₂ resistance of MnO_x [23] as TiO₂ can inhibit the deposition of sulfates on sorbents surface [24]. But the Hg⁰ removal activity of MnO_x/TiO₂ is unsatisfactory because the active component of Mn is still exposed in SO₂ atmosphere. Core-shell is a structure with active component core and supporting components shell. The shell can inhibit the interaction between SO₂ and sorbent surface and efficiently protect active component core [25]. Therefore, synthesizing a core-shell structure with MnO_x core and TiO₂ shell may obtain a better SO₂ resistance.

Inspired by this, αMnO₂ nanorods and αMnO₂ nanorods coating TiO₂ shell were synthesized in the present work to investigate the Hg⁰ removal efficiency in the presence of SO₂. Thermo-gravimetric (TG) and X-ray photoelectron spectroscopy (XPS) were performed to determine the role of SO₂ in the Hg⁰ oxidation and adsorption processes and a probable mechanism of SO₂ influence was deduced based on XPS and TG results. The kinetic model of the Hg⁰ adsorption process was examined as well.

2. Results and Discussion

2.1. Structure Characterization

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to investigate the morphologic and structural properties of αMnO₂-NR and αMnO₂-NR@TiO₂. Figure 1a,a’ show SEM and TEM images of αMnO₂-NR. It can be seen that αMnO₂-NR has a uniform nanorod structure with an average diameter of about 100 nm. As shown in Figure 1b, for αMnO₂-NR@TiO₂, the uniform nanorod structure is well-retained after being coated with TiO₂ and the packing state of this sample is similar to αMnO₂-NR. The surface of αMnO₂-NR@TiO₂ are rougher when compared to αMnO₂-NR, and the average diameter increases to 150 nm due to the TiO₂ coating. The average length of the αMnO₂-NR@TiO₂ is about 2–3 μm (shown in Figure 1c). As shown in Figure 1b’, an obvious dividing line can be detected between MnO₂ core and TiO₂ shell, and the shell with thickness of about 30 nm is well dispersed outside of the αMnO₂-NR.

N₂ sorption-desorption isotherms of the samples are shown in Figure 2. Both αMnO₂-NR and αMnO₂-NR@TiO₂ exhibit a type IV adsorption isotherm, according to the definition of IUPAC, which means that αMnO₂-NR and αMnO₂-NR@TiO₂ have a mesoporous structure. The surface areas, pore volumes, and average pore diameters of the sorbents are illustrated in

Table 1. Pore structure analysis of the sorbents.

Samples	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Average Pore Diameter (nm)
αMnO2-NR	29.103	0.192	5.428
αMnO2-NR@TiO2	32.985	0.207	4.186

. BET surface areas of the two sorbents are similar, suggesting that TiO₂ coating does not change the structure of αMnO₂-NR a lot. This result consists with SEM results.

X-ray diffractometer (XRD) patterns of the two catalysts are shown in Figure 3. All the peaks in XRD pattern of αMnO₂-NR and αMnO₂-NR@TiO₂ were indexed to cryptomelane type α-MnO₂ (JCPDS 44-0141, tetragonal, I4/m, a = b = 0.978 nm, c = 0.286 nm). The intensity of diffraction peaks for the two samples is almost the same. It means that TiO₂ shell does not influence the dispersion of αMnO₂-NR, which is great agreement with BET and SEM results.

2.2. Hg⁰ Adsorption

2.2.1. Hg⁰ Adsorption Performance

Breakthrough experiments were performed to investigate the Hg⁰ adsorption performance of the two sorbents. A blank test was also performed and the results is shown in Figure S1. It can be seen that the outlet Hg⁰ concentration is stable when no sorbent was loaded in the fixed-bed reactor. As shown in Figure 4, the Hg⁰ removal efficiency of αMnO₂-NR is about 92% at the beginning of the test and it decreases to 41% after 130 min reaction. When it comes to αMnO₂-NR@TiO₂, the Hg⁰ removal efficiency at the beginning of the test is about 81% which is lower than that of αMnO₂-NR. But it is about 43% at the end of the test suggesting a more stable removal activity. These results indicate that TiO₂ shell does not inhibit the Hg⁰ diffusion from gas phase to the surface of αMnO₂-NR.

Figure 5 shows the effects of SO₂ on Hg⁰ adsorption performance. For αMnO₂-NR, Hg⁰ removal efficiency sharply declines from 55% to 14% during the 35 min reaction, when SO₂ is injected into flue gas. However, for αMnO₂-NR@TiO₂, the downward trend of Hg⁰ removal efficiency is much slower and decreases from 76% to 43% in a 30 min test, and still has a Hg⁰ removal efficiency of 25% after 80 min. These results confirm that TiO₂ shell can inhibit the direct interaction between SO₂ and MnO₂ surface, which will efficiently protect MnO₂ core from SO₂ poisoning.

αMnO₂-NR@TiO₂ was used to investigate reusability for Hg⁰ removal. The results are shown in Figure 6. After 10 h Hg⁰ adsorption test, αMnO₂-NR@TiO₂ reaches a Hg⁰ adsorption equilibrium. And then, the sorbent was heated at 450 °C for 2 h to release the HgO on sorbent surface. It can be found that, after heated treatment, the Hg⁰ adsorption efficiency and capacity of αMnO₂-NR@TiO₂ recovers to its original level. After two recycling, it still shows a good Hg⁰ adsorption efficiency. Furthermore, SEM results of the fresh and used αMnO₂-NR@TiO₂ (shown in Figure S2) show that recycle have no effect on the microstructure. These results suggest an outstanding reusability of αMnO₂-NR@TiO₂. The Hg⁰ adsorption capacity of αMnO₂-NR@TiO₂ is 0.11 mg/g, it is good enough compared to other sorbents (shown in Table S1). The surface areas of the sorbents in the present work are relatively low thereby lowering the available surface active sites. αMnO₂-NR@TiO₂ with higher surface area will be studied in our following works, and may give a better Hg⁰ adsorption capacity.

Hg⁰ adsorption test of αMnO₂-NR@TiO₂ at different Hg⁰ concentration was also investigated and the results are shown in Figure S3. With a doubled Hg⁰ concentration, the breakthrough curve gets steep suggesting that αMnO₂-NR@TiO₂ will easily reach Hg⁰ adsorption equilibrium at a higher Hg⁰ concentration.

2.2.2. Structure-Activity Relationship

Fourier Transform Infrared Spectrometer (FTIR) was used to confirm the kind of surface active site for Hg⁰ adsorption. As can be seen in Figure 7, the peaks at 429, 503, and 700 cm⁻¹ correspond to Mn-O vibration [26], which becomes much weaker after reaction. It suggests that Mn-O group participates in Hg⁰ adsorption process. According to previous work, the surface active oxygen species in Mn-O group should be the active sites for Hg⁰ adsorption.

2.3. Models of Adsorption Kinetics

In order to better illustrate the Hg⁰ adsorption mechanisms of αMnO₂-NR and αMnO₂-NR@TiO₂, two popular models of pseudo-first order and pseudo-second order kinetic models, which have been widely used to investigate the adsorption process [27], were employed to fit the above experimental data. These two kinetic equations are displayed as follows [28]:

$$\lg (q_e-q_t)=\lg q_e-\frac{k_1}{2.303}t —\mathrm{pseudo}-\mathrm{first} \mathrm{order}$$

(1)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e{}^2}+\frac{1}{q_e} —\mathrm{pseudo}-\sec \mathrm{ond} \mathrm{order} \mathrm{kinetic}$$

(2)

where q_e and q_t are the adsorption capacity of Hg⁰ on the sorbents at equilibrium, and at reaction time t (min), respectively. The parameters k₁ (min⁻¹) and k₂ (g/(μg·min)) are the rate constants of the pseudo-first order, and pseudo-second order models, respectively.

The fitting results are shown in Figure 8, and the obtained values of correlation coefficient (R²) are summarized in

Table 2. Kinetic parameters (R²) of pseudo-first order and pseudo-second order models.

Kinetic Models	αMnO2-NR without SO2	αMnO2-NR@TiO2 without SO2	αMnO2-NR with SO2	αMnO2-NR@TiO2 with SO2
Pseudo-first (R2)	0.944	0.938	0.954	0.941
Pseudo-second (R2)	0.991	0.995	0.997	0.992

. The values of R² of the pseudo-second order model for αMnO₂-NR and αMnO₂-NR@TiO₂ are 0.991, and 0.995, respectively, which are higher than those of pseudo-first order kinetic model (0.944 and 0.938 for αMnO₂-NR and αMnO₂-NR@TiO₂). It indicates that the pseudo-second order model can better fit the experimental data and Hg⁰ removal process are dominantly controlled by chemisorption. After SO₂ introduction, the values of R² of the pseudo-second order model for αMnO₂-NR and αMnO₂-NR@TiO₂ are 0.997 and 0.992, which are still much higher than those of the pseudo-first order model. These results show that Hg⁰ adsorption process in the presence of SO₂ atmosphere are also dominantly controlled by chemisorption.

2.4. The Mechanism of SO₂ Effects on Hg⁰ Adsorption

XPS analysis was employed to explore the relative proportion of elements on the sample surface. The XPS spectra of Mn 2p, O 1s and S 2p for the fresh and used samples are shown in Figure 9. The surface atomic concentrations and surface atomic ratios are summarized in

Table 3. The surface atomic concentrations and the relative concentration ratios of samples based on XPS.

Samples	S	Mn4+/Mn	Oβ/O
αMnO2-NR (fresh)	3.17	37.8	26.0
αMnO2-NR@TiO2 (fresh)	2.27	33.4	24.7
αMnO2-NR (used)	4.97	34.0	22.8
αMnO2-NR@TiO2 (used)	2.66	33.0	20.0

.

Figure 9a shows the XPS spectra of Mn 2p. A doublet due to spin orbital coupling can be detected which corresponds to Mn 2p_3/2 (around 641.24 eV) and Mn 2p_1/2 (around 652.82 eV). Due to the high intensity of Mn 2p_3/2, it was fitted to give detail information of valence state of Mn and it can be separated into three peaks at 640.2–641.2 eV, 641.2–642.1 eV, and 642.2–643.4 eV corresponding to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively [29,30]. As shown in Table 3, the ratio of Mn⁴⁺/Mn is about 37.8% for the fresh αMnO₂-NR and it decreases to 33.4% after the SO₂ resistance test. Compared to αMnO₂-NR, Mn⁴⁺ content is almost constant for αMnO₂-NR@TiO₂ before, and after, SO₂ resistance test. These results indicate that, for αMnO₂-NR, Mn⁴⁺ is easily reduced to Mn²⁺ during SO₂ resistance process via the reaction between SO₂ and MnO₂ [31]. For αMnO₂-NR@TiO₂, the interaction between SO₂ and MnO₂ is inhibited by the TiO₂ shell structure, which can efficiently protect active component Mn⁴⁺ in the core.

Figure 9b shows O 1s XPS spectra. For the fresh catalysts, O 1s bands can be split into two peaks, corresponding to lattice oxygen (peak at 529.5 eV, denoted as O_α) and chemisorbed oxygen (peak at 530.8 eV, denoted as O_β), respectively [32]. Whereas, a new peak appears around 532.3 eV after SO₂ treatment, which corresponds to SO₄²⁻ (denoted as O_γ) [33]. The intensity of the peak around 532.3 eV for αMnO₂-NR@TiO₂ is weaker than that for αMnO₂-NR suggesting a lower amount of SO₄²⁻ on the used αMnO₂-NR@TiO₂ surface. Furthermore, the peaks of O_α and O_β in αMnO₂-NR have an obvious slight shift to higher binding energy after SO₂ treatment. It might be due to the formation of sulfate salts during the sulfating process [34].

To determine the above deduction, S 2p bands was further investigated and the results are shown in Figure 9c. For the fresh αMnO₂-NR and αMnO₂-NR@TiO₂, two peaks around 162.2 eV and 163.2 eV attributed to S²⁻ and S₂²⁻ can be detected [35,36], which may come from MnSO₄ (the precursor of MnO₂). But for the used αMnO₂-NR and αMnO₂-NR@TiO₂, two new peaks at about 168.8 eV and 170.0 eV are observed, which may be assigned to SO₄²⁻, and HSO₄⁻, respectively [37,38]. The peak intensity of the used αMnO₂-NR is much higher than that of αMnO₂-NR@TiO₂. As shown in Table 3, for αMnO₂-NR, the surface atomic concentrations of S increases from 3.17% to 4.97% after SO₂ teatment while it increases from 2.27% to 2.66% for αMnO₂-NR@TiO₂. These results confirm that TiO₂ shell can inhibit the S accumulation on catalyst surface.

To obtain more information about the SO₂ poisoning mechanism, Thermo-gravimetric-differential thermos-gravimetry (TG-DTG) was performed to investigate the weight loss of αMnO₂-NR and αMnO₂-NR@TiO₂ after SO₂ treatment, and the results are presented in Figure 10. It can be seen that the used αMnO₂-NR has an obvious weight loss step in the temperature range of 680−780 °C with a weight loss of about 2.4%, which can be attributed to manganese sulfate decomposition [39,40,41]. There is no weight loss step between 680−780 °C with respect to αMnO₂-NR@TiO₂, but there is a new weak step around 780–850 °C can be detected, and it may be due to the decomposition of Ti(SO₄)O [42]. This result demonstrates that SO₂ tends to react with titanium oxides instead of manganese oxides over αMnO₂-NR@TiO₂. Based on these results, TiO₂ shell can lead to the preferential adsorption of SO₂ on Ti surrounding forming Ti(SO₄)O to protect Mn active component from being deactivated.

3. Materials and Methods

3.1. Catalysts Preparation

The αMnO₂ nanorods were synthesized through a hydrothermal method [43]. KMnO₄ (2.5 g, AR) and MnSO₄·H₂O (1.05 g, AR) were dissolved in 80 mL distilled water. The mixed solution was transferred into a Teflon-line stainless steel autoclave, sealed, and kept in an oven at 160 °C for 12 h. After cooling to room temperature, the precipitates were filtered off, washed several times using deionized water and dried at 110 °C overnight. Finally, the product was calcined at 400 °C in a muffle furnace for 4 h and the obtained sample is denoted as αMnO₂-NR.

MnO₂@TiO₂ core-shell nanorods were synthesized through a versatile kinetics-controlled coating method [44]. αMnO₂-NR (0.075 g) and aqueous ammonia (0.28 mL, 28 wt.%) were dispersed in 100 mL absolute ethanol under ultrasound for 30 min. Afterwards, titanium tetrabutoxide (TBOT) (0.75 mL) was added drop-wise into the mixture and then kept at 45 °C for 24 h. The mixed solution was filtered, washed and dried at 60 °C for 12 h. Finally, the solid was calcined under flow air at 500 °C for 2 h to obtain the sample (denoted as αMnO₂-NR@TiO₂).

ALL reagents are from Aladdin company, Shanghai, China.

3.2. Hg⁰ Adsorption Experiments

The Hg⁰ removal test has been described in detail in our previous work [45]. The experimental reactor contains a gas distribution system, a Hg⁰ vapor generating device, a fixed-bed quartz reactor (ID = 8 mm), an online mercury analyzer and a tail gas treating unit. The mercury permeation tube was placed in a U-shape glass tube, which was immersed in a water bath at a constant-temperature (38 °C) to ensure a constant Hg⁰ permeation rate. The total gas flow was 600 mL/min, and the sorbent volume was generally 0.2 mL, resulting in a GHSV of 1.8 × 10⁵ h⁻¹. The concentrations of Hg⁰ and SO₂ were monitored by a VM-3000 online mercury analyzer (Mercury Instruments, München, German), and flue gas analyzer (KM950, Kane International Ltd., London, United Kingdom), respectively.

During each test, the Hg⁰ gas first bypassed the fixed-bed reactor, and then introduced into the reactor for 2 h to obtain a stable Hg⁰ concentration. Hg⁰ breakthrough ratio was quantified by the following formula,

$$Breakthrough ratio(\%)=\frac{C}{C_0^{}}\times 100\%$$

(3)

where C and C₀ represent the inlet and outlet Hg⁰ concentrations (μg/Nm³) in the fixed-bed reactor.

3.3. Characterization

The morphology and microstructure of the samples were observed using SEM (Nova NanoSEM 450, FEI) and TEM (Tecnai G2 F30 S-Twin, FEI). The surface areas and pore parameters of the samples were determined by Nitrogen adsorption/desorption method at liquid nitrogen temperature at −196 °C on an automated gas sorption analyzer (Autosorb-iQ-C, Quantachrome Instruments, Boynton Beach, FL, USA). The pore size and pore volume were derived from the desorption branches using the Barrette-Joynere-Halenda (BJH) model. The crystal structures of the samples were characterized by an XRD (XRD-7000S, SHIMADZU Corporation, Kyoto, Japan) operating at 40 kV and 100 mA using a Cu Kα radiation. The scanning range (2θ) was from 10° to 90° with a scan speed of 5°/min. The element (Mn, O, and Hg) valence state was analyzed by XPS (ESCALAB250 Thermo Fisher Scientific, Wilmington, DE, USA) with a monochromatic Al Kα source. The C 1s binding energy value of 284.8 eV was used to calibrate the observed spectra. TG was performed on TGA/DSC1 analyser (METTLER TOLEDO, Schwerzenbach, Switzerland), under a nitrogen flow of 20 mL/min, using a heating rate of 10 °C/min from room temperature to 900 °C (NETZSCH Corporation, Selb, Germany). DTG analysis was obtained based on residual weight of the sample with respect to time. FTIR spectra were obtained on a Nicolet Magana-IR 750 spectrometer to measure the surface groups of the samples (Thermo Nicolet Corporation, Madison, WI, USA).

4. Conclusions

αMnO₂-NR@TiO₂ was prepared by versatile kinetics-controlled coating method to compare with αMnO₂-NR in the Hg⁰ removal process. SEM, BET, and XRD results showed that TiO₂ shell did not change the structure of αMnO₂-NR. Therefore, the two sorbents had similar Hg⁰ removal performance in N₂ atmosphere. When SO₂ was introduced, αMnO₂-NR@TiO₂ had a much better performance than αMnO₂-NR. XPS and TG-DTG results showed that αMnO₂-NR@TiO₂ had lower surface S concentration after treatment of SO₂, and no manganese sulfate could be detected in αMnO₂-NR@TiO₂. It suggests that the TiO₂ shell can effectively protect MnO₂ from being deactivated by SO₂. Adsorption kinetic results showed that Hg⁰ adsorption process over both the two sorbents obeys pseudo-second order model with, or without, SO₂.