Hg⁰ Removal by a Palygorskite and Fly Ash Supported MnO₂-CeO₂ Catalyst at Low Temperature

Abstract

MnO₂-CeO₂/PG-FA catalysts were prepared by supporting MnO₂-CeO₂ to PG-FA and used to remove Hg⁰ in simulated flue gas. The results show that MnO₂-CeO₂/PG-FA catalyst had excellent and stable Hg⁰ removal activity, which was mainly due to the combination effect of the catalytic oxidation activity by MnO₂-CeO₂ and the adsorption ability by PG-FA. Mn8-Ce0.5/PG-FA (with 8.0% MnO₂ and 0.5% CeO₂ loading) catalyst showed the highest Hg⁰ removal efficiency at 140 °C and Hg⁰ removal efficiency could be maintained above 95% with the space velocity of 6000 h⁻¹ and Hg⁰ concentration of 160 μg/m³. O₂ promoted Hg⁰ removal by MnO₂-CeO₂/PG-FA catalyst, while SO₂ and H₂O had inhibitory effects. In the presence of O₂, the inhibitory effect of SO₂ and H₂O can be obviously weakened. MnO₂-CeO₂/PG-FA catalysts were characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption experiments (TPD). The results of SEM and XRD showed that the active components MnO₂-CeO₂ dispersed well on the surface of PG-FA support. The results of XPS and TPD show that the Hg⁰ removal process over MnO₂-CeO₂/PG-FA catalyst included adsorption and oxidation, HgO and HgSO₄ were generated and adsorbed on the catalyst. MnO₂-CeO₂/PG-FA catalyst also showed excellent regeneration performance after Hg⁰ removal.

1. Introduction

Mercury emission from coal combustion has become an important source of environmental mercury pollution and caused a worldwide concern due to its toxicity, bioaccumulation and persistence in the food chain and the ecological environment [1,2]. Governments around the world have successively promulgated laws and regulations to limit mercury emissions from coal-fired power plants. Currently, more than 140 countries and regions have signed The Minamata Convention on Mercury treaty to limit mercury emission and use [3].

Generally, there are three main forms of Hg in coal-fired flue gas: gaseous Hg⁰ (Hg⁰_g), gaseous Hg²⁺ (Hg²⁺_g) and Hg_p adsorbed on fly ash. Among them, Hg²⁺_g is soluble in water and can be easily removed by wet dust removal device or wet flue gas desulfurization device. Hg_p can be removed by dust removal device along with fly ash. Hg⁰, however, is difficult to remove by existing pollutant control devices due to its high volatility and insolubility in water [4]. Therefore, Hg⁰ is the main mercury form released to the atmosphere and becomes the focus and difficulty of mercury pollution control in coal-fired flue gas.

Activated carbon injection (ACI) technology has been widely studied and applied to remove mercury in coal-fired flue gas [5,6,7]. However, the application of ACI technology in coal-fired power plants showed that several questions must be solved. The disadvantages of ACI are the short contact time between activated carbon and flue gas, poor capacity and incapability to regenerate activated carbon, leading to high cost and its limited application [8]. Obviously, developing economical and effective Hg removal technology is required.

It was reported that metal oxides had high catalytic oxidation activity for Hg⁰, including V₂O₅, MnO₂, CeO₂, CuO, Mo₃O₄, etc., [9,10,11,12,13,14,15,16,17,18]. Among them, MnO₂ showed high activity for Hg⁰ oxidation at low temperature. CeO₂, as a variable valence oxide (Ce⁴⁺/Ce³⁺), had excellent oxygen storage and release functions. It also showed good oxidation activity and a certain anti-SO₂ poisoning ability. Meanwhile, palygorskite, as a natural porous chain-layered water-containing magnesium-rich aluminum silicate clay mineral, has a certain adsorption ability with thermal stability and can be used as good catalyst supports [19,20]. In this paper, a palygorskite (PG) and fly ash (FA) supported MnO₂ and CeO₂ catalyst (MnO₂-CeO₂/PG-FA) was prepared and used to remove Hg⁰ in simulated flue gas. The effect of MnO₂-CeO₂ loading, reaction temperature, flue gas components on Hg⁰ removal and the speciation of Hg adsorbed over MnO₂-CeO₂/PG-FA were studied, as well as the regeneration of MnO₂-CeO₂/PG-FA after Hg⁰ removal.

2. Results and Discussion

2.1. Characterization of the MnO₂-CeO₂/PG-FA Catalyst

Figure 1 shows the XRD results of PG-FA, Mn8/PG-FA, Ce0.5/PG-FA and Mn8Ce0.5/PG-FA catalysts. It can be seen that there were obvious peaks for Al₆Si₂O₁₃ (JPCDS 15-0776), (Mg, Al)₅(Si, Al)₈O₂₀(OH)₂•8H₂O (JPCDS 21-0958) and SiO₂ (JPCDS 46-1045) in PG-FA support. The XRD pattern of Ce0.5/PG-FA catalyst was completely consistent with the PG-FA support and no CeO₂ peak has been observed, which was mainly due to the low loading amount of CeO₂. Compared to the PG-FA support, the Mn8/PG-FA catalyst and Mn8Ce0.5/PG-FA catalyst showed MnO₂ peaks (JPCDS 24-0735), indicating that MnO₂ was successfully supported onto PG-FA support.

Figure 2 shows the SEM images of Mn8-Ce0.5/PG-FA catalyst. It can be seen that Mn8-Ce0.5/PG-FA catalyst had an obvious porous structure, which was facilitated for the diffusion of flue gas in catalyst particles. Besides, PG-FA support had a specific surface area of 100.99 m²/g, and it was beneficial to the loading and dispersion of MnO₂ and CeO₂ components, as well as the enhancement of adsorption capacity of catalysts.

2.2. Effect of MnO₂ and CeO₂ Loading on Hg⁰ Removal

Figure 3 shows Hg⁰ removal by MnO₂-CeO₂/PG-FA catalysts with different MnO₂ and CeO₂ loading at 140 °C in simulated flue gas (5% O₂, 5% H₂O, 0.15% SO₂, balance N₂, C_Hg⁰ = 160 μg/m³) for 5 h. It can be seen that PG-FA had low Hg⁰ removal capability and Hg⁰ removal efficiency was only about 50%. Supporting single 8% MnO₂ and 0.5% CeO₂ could obviously improve Hg⁰ removal capability, and Hg⁰ removal efficiency reached 90% and 83% for Mn8/PG-FA and Ce0.5/PG-FA, respectively. Supporting MnO₂-CeO₂ onto PG-FA further improved Hg⁰ removal capability, and Mn8-Ce0.5/PG-FA had the highest Hg⁰ removal capability and Hg⁰ removal efficiency was above 98%. Compared to single MnO₂ and CeO₂ catalyst, MnO₂-CeO₂ bimetallic catalyst had a higher Hg⁰ removal capability, which was mainly due to the co-effects of oxidation activity of MnO₂ and CeO₂, as well as the anti-SO₂ ability of CeO₂. Although the specific surface area of the Mn8/PG-FA and Mn8-Ce0.5/PG-FA catalyst decreased slightly after MnO₂ and CeO₂ loading, Hg⁰ removal capability still improved. The high Hg⁰ removal capability of MnO₂-CeO₂/PG-FA was mainly due to the oxidation activity of MnO₂ and CeO₂ for Hg⁰, as well as the adsorption ability of PG-FA. This was similar to Hg⁰ capture by V₂O₅/AC catalyst and MnO_x/PG catalyst in our previous researches [20,21].

2.3. Effect of Temperature on Hg⁰ Removal by MnO₂-CeO₂/PG-FA

Figure 4 shows Hg⁰ removal by PG-FA and Mn8-Ce0.5/PG-FA at different temperatures. It can be seen that Hg⁰ removal capability of PG-FA decreased as the temperature increased, which was mainly due to the decrease of adsorption effect by PG-FA for Hg⁰ removal. Mn8-Ce0.5/PG-FA showed high Hg⁰ removal capability in the temperature range of 120 to 200 °C, and Hg⁰ removal efficiencies were still above 85% after 280 min. Hg⁰ removal efficiency increased firstly and then decreased as the temperature increased from 120 to 200 °C, and the highest was at 140 °C. Since Hg⁰ removal process included oxidation and adsorption, the trend of Hg⁰ removal efficiency indicated that the effect extent of oxidation and adsorption on Hg⁰ removal over Mn8-Ce0.5/PG-FA was different at different temperatures [21]. The co-effect was the highest at 140 °C leading to the highest Hg⁰ removal efficiency.

2.4. Effect of Flue Gas Components on Hg⁰ Removal by MnO₂-CeO₂/PG-FA

Figure 5 shows the effects of flue gas components on Hg⁰ removal over Mn8-Ce0.5/PG-FA. It can be seen that O₂ had a promotion effect while SO₂ and H₂O showed an inhibition effect, which may be due to the competitive adsorption between Hg⁰ and SO₂ or H₂O. However, in the presence of O₂, Hg⁰ removal efficiency increased slightly, indicating that O₂ offset the inhibition effect of SO₂ and H₂O to a certain extent. Since Hg⁰ oxidation was mainly due to the lattice oxygen of metal oxides, the role of O₂ was to resume the oxidation activity of MnO₂-CeO₂ by replenishing O to the used MnO₂-CeO₂ sites after Hg⁰ oxidation [20,21,22]. Furthermore, in the presence of O₂, SO₂ and H₂O, Hg⁰ was oxidized to form HgSO₄ as shown in Figure 6.

2.5. Speciation of Hg Adsorbed over Mn8-Ce0.5/PG-FA

Temperature programmed desorption (TPD) experiments were conducted to identify the speciation of Hg adsorbed over Mn8-Ce0.5/PG-FA. Mn8-Ce0.5/PG-FA was firstly used to remove Hg⁰ in N₂ + O₂ + SO₂ + H₂O at 140 °C for 5 h and then was used for TPD experiments. Figure 6 shows the Hg release behaviors of the fresh and used Mn8-Ce0.5/PG-FA catalysts upon heating to 700 °C in Ar. It can be seen that the fresh Mn8-Ce0.5/PG-FA sample showed no Hg release during the TPD process. As for the used Mn8-Ce0.5/PG-FA sample, however, it showed three obvious Hg release peaks at about 150 °C, 480 °C and 540 °C, which could be attributed to Hg⁰, HgO and HgSO₄, respectively [23,24,25,26]. This indicated that the speciation of Hg adsorbed over Mn8-Ce0.5/PG-FA was mainly Hg²⁺ compounds, confirming the oxidation of Hg⁰ to Hg²⁺ by MnO₂ and CeO₂, as well as the reaction of Hg⁰ with O₂ and SO₂ to form HgSO₄.

XPS analyses were also conducted to identify the speciation of Hg adsorbed over Mn8-Ce0.5/PG-FA. Figure 7 shows the results of Mn 2p, Ce 3d and Hg 4f for fresh and used MnO₂-CeO₂/PG-FA catalyst. Compared to the fresh MnO₂-CeO₂/PG-FA sample, the amount of Mn⁴⁺ (643.4 eV for Mn 2p_2/3 and 655.2 eV for Mn 2p_1/3) and Ce⁴⁺ (u, u², u³, v, v², v³) decreased while Mn³⁺ (642.3 eV for Mn 2p_2/3 and 654.1 eV for Mn 2p_1/3) and Ce³⁺ (u¹, v¹) increased for the used MnO₂-CeO₂/PG-FA sample. Besides, it can be seen in Figure 7E that there was a peak at around 99.8 eV for the fresh MnO₂-CeO₂/PG-FA catalyst, which could be attributed to the SiO₂ of PG support [20,27]. As for the used MnO₂-CeO₂/PG-FA catalyst after Hg⁰ removal, a new peak at around 101.1 eV appeared (Figure 7F), which could be ascribe to Hg²⁺ of HgO and HgSO₄ [18,22]. These results confirm the reaction of Hg⁰ with MnO₂ and CeO₂, i.e., Hg⁰ was oxidized to Hg²⁺ while O of MnO₂-CeO₂ was consumed and Mn⁴⁺, Ce⁴⁺ were reduced to Mn³⁺, Ce³⁺. Meanwhile, adsorbed O₂ could replenish oxygen for Mn³⁺, Ce³⁺ to form MnO₂ and CeO₂ to resume their oxidation activity [21,22].

2.6. Hg⁰ Removal Process over MnO₂-CeO₂/PG-FA Catalyst

Based on the above experiments and characterization results, Hg⁰ removal over MnO₂-CeO₂/PG-FA catalyst might include adsorption and oxidation. Hg⁰ removal process over MnO₂-CeO₂/PG-FA can be summarized as follows and Figure 8:

• Hg⁰ (g) → Hg⁰ (ad)
• O₂ (g) → 2[O] (ad)
• 2CeO₂ → Ce₂O₃ + [O]
• 2MnO₂ → Mn₂O₃ + [O]
• Hg⁰ (ad) + [O] → HgO (ad)
• SO₂ (g) + [O] → SO₃ (ad)
• Hg⁰ (ad) + SO₃ (ad) + [O] → HgSO₄ (ad)
• SO₃ (ad) + HgO (ad) → HgSO₄ (ad)
• SO₃ (ad) + H₂O (ad) → H₂SO₄ (ad)
• H₂SO₄ (ad) + HgO (ad) → HgSO₄ (ad) + H₂O (ad)
• Ce₂O₃ + [O] → 2CeO₂
• Mn₂O₃ + [O] → 2MnO₂

Figure 8. Reactions of Hg⁰ over MnO₂-CeO₂/PG-FA.

Figure 8. Reactions of Hg⁰ over MnO₂-CeO₂/PG-FA.

2.7. Regeneration of MnO₂-CeO₂/PG-FA Catalyst after Hg⁰ Removal

The above results show that MnO₂-CeO₂/PG-FA catalyst had an excellent Hg⁰ removal capability at low temperature. To investigate the reusability of MnO₂-CeO₂/PG-FA catalyst, the Mn8-Ce0.5/PG-FA after Hg⁰ removal for 60 h with Hg⁰ removal efficiency of 76% was regenerated and reused for Hg⁰ removal again. The results of Mn8-Ce0.5/PG-FA, Mn8-Ce0.5/PG-FA-Re-400 °C and Mn8-Ce0.5/PG-FA-Re-500 °C for Hg⁰ removal are shown in Figure 9. It can be seen that the regenerated MnO₂-CeO₂/PG-FA catalyst still had a high and stable capability for Hg⁰ removal. Hg⁰ removal efficiency of Mn8-Ce0.5/PG-FA-Re-400 °C and Mn8-Ce0.5/PG-FA-Re-500 °C were still above 90% and 85% after 280 min, respectively. As the regeneration temperature increased from 400 to 500 °C, Hg⁰ removal efficiency decreased slightly. This may be due to the change of structure and chemical properties of MnO₂-CeO₂/PG-FA during regeneration, causing pores blockage and partial active sites loss on the surface of MnO₂-CeO₂/PG-FA catalyst at higher regeneration temperature. The MnO₂-CeO₂/PG-FA catalyst had excellent Hg⁰ removal activity and regeneration performance.

3. Materials and Methods

3.1. Catalysts Preparation

The PG and fly ash were mixed at a ratio (3:7), and then mixed with distilled water in a certain proportion (1 g:1.5 mL), dried at 110 °C for 6 h, then calcined at 300 °C for 6 h in air.

Table 1. The main chemical compositions of PG and FA.

Sample	Chemical Compositions
	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	K2O	MnO
PG	69.45	11.84	5.14	0.21	12.17	0.44	0.51	0.05
FA	48.29	21.64	3.05	20.33	1.74	0.98	1.66	0.04

shows the compositions of PG and FA. The obtained PG-FA samples were crushed and screened into 40–60 mesh. MnO₂-CeO₂/PG-FA catalysts were prepared by pore volume impregnation of PG-FA with an Mn(NO₃)₂ and Ce(NO₃)₃ aqueous solution (1 g:1.2 mL). According to the loading amount of MnO₂-CeO₂ in the prepared MnO₂-CeO₂/PG-FA catalyst, the FA-PG support was impregnated in an equal volume with 0.1650 g/mL of Mn(NO₃)₂ and 0.0095 g/mL Ce(NO₃)₃ solution. After impregnating, it was stewed at room temperature for 8 h, dried at 50 °C for 5 h and at 110 °C for 5 h in air. Then it was calcined at 300 °C for 3 h in air. Several MnO₂-CeO₂/PG-FA catalysts with different MnO₂-CeO₂ loading (wt.%) were prepared and named according to the weight percentage of MnO₂-CeO₂ in MnO₂-CeO₂/PG-FA. For example, Mn8-Ce0.5/PG-FA referred to MnO₂-CeO₂/PG-FA catalyst containing 8.0% MnO₂ and 0.5% CeO₂.

Table 2. Characterization of textural properties of the samples.

Sample	BET Area (m2/g)	Micropore Volume (cm3/g)	Average Diameter (nm)
PG-FA	100.99	0.35	13.86
Mn8/PG-FA	91.22	0.31	13.59
Mn8-Ce0.5/PG-FA	87.63	0.28	11.06

shows the properties of the PG-FA support, Mn8/PG-FA and Mn8-Ce0.5/PG-FA catalyst.

3.2. Hg⁰ Removal by MnO₂-CeO₂/PG-FA Catalyst

Hg⁰ removal experiments by MnO₂-CeO₂/PG-FA catalyst were carried out in a fixed-bed quartz reactor as shown in Figure 10. 0.5 g MnO₂-CeO₂/PG-FA catalyst was used for each experiment at 120–200 °C in simulated flue gas containing 5% O₂, 5% H₂O, 0.15% SO₂, 160 μg/m³ Hg⁰ and balance N₂. The total flow rate was 100 mL/min, corresponding to a space velocity of about 6000 h⁻¹. Hg⁰ was generated by an Hg⁰ permeation tube (VICI Metronics) immerged in a water bath. The whole gas tube for Hg⁰ delivery was kept at 120 °C to avoid the adsorption of Hg⁰. The Hg⁰ concentration in the inlet and outlet gas of the reactor was continuously measured by a RA-915M mercury analyzer (Lumex Instruments, Russia). Hg⁰ removal efficiency was defined as follows:

$${\mathrm{E}}_{\mathrm{Hg}}\left(\%\right)=\frac{{\mathrm{C}}_0-{\mathrm{C}}_1}{{\mathrm{C}}_0}\times 100\%$$

where C₀ and C₁ represent the Hg⁰ concentration at the inlet and outlet gas of the reactor, respectively.

Figure 10. Schematic diagram of Hg⁰ removal by MnO₂-CeO₂/PG-FA.

Figure 10. Schematic diagram of Hg⁰ removal by MnO₂-CeO₂/PG-FA.

3.3. Catalyst Regeneration

The used MnO₂-CeO₂/PG-FA catalyst after Hg⁰ removal was thermally regenerated in the fixed bed reactor in Figure 10. The sample was firstly heated to the regeneration temperature (300–500 °C) and maintain for 2 h in N₂, and then was heated at 350 °C for 2 h in an air atmosphere. After the regenerated MnO₂-CeO₂/PG-FA catalyst was naturally cooled to room temperature, simulated flue gas was switched and the regenerated MnO₂-CeO₂/PG-FA was used to remove Hg⁰ again.

3.4. Characterization

The N₂ adsorption-desorption tests were carried out by an Autosorb-iQ analyzer (Quantachrome, Boynton Beach, FL, USA). The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, the pore structure parameters were analyzed by the Barrett-Joyner-Halenda (BJH) method.

The morphologies of the MnO₂-CeO₂/PG-FA catalysts were performed on a scanning electron microscope (SEM) (JSM-7001F, JEOL, Akishima City, Japan).

The crystal phase structure of the MnO₂-CeO₂/PG-FA catalysts were characterized by XRD diffractometer (D8-ADVANCE-A25, Bruker, Karlsruhe, Germany), using Cu Kα rays and 10–80° scanning range. XRD diffraction data adopt step scan method (0.02°).

The X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi spectrometer (Thermo Fisher, Waltham, MA, USA) using an Al Kα X-ray source at room temperature. All binding energies (BE) were adjusted with the C 1s binding energy value of 284.6 eV.

Temperature-programmed desorption (TPD) experiments were conducted in a quartz tube reactor using 0.1 g sample. The sample was firstly used to remove Hg⁰ at 140 °C for 280 min, then cooled to the room temperature and swept with N₂ for 20 min, finally heated from room temperature to 700 °C with a heating rate of 10 °C/min. The outlet gas from the reactor was introduced into a KBH₄ solution to reduce the possibly existing Hg²⁺ to Hg⁰. The Hg⁰ concentration in the effluent gas after the KBH₄ solution was continuously measured by an on-line mercury analyzer (RA-915M, Lumex, St. Petersburg, Russia).

4. Conclusions

MnO₂-CeO₂/PG-FA catalyst had excellent Hg⁰ removal activity and stability, which was mainly due to the combination effect of the catalytic oxidation activity by MnO₂-CeO₂ and adsorption ability by PG-FA. Hg⁰ was oxidized to form HgO and HgSO₄, and then adsorbed on the MnO₂-CeO₂/PG-FA catalyst. The porous structure of the support PG-FA was conducive to the dispersion of the active component MnO₂-CeO₂ and the adsorption and oxidation of Hg⁰. Mn8-Ce0.5/PG-FA catalyst showed the highest Hg⁰ removal efficiency at 140 °C. O₂ exhibited a promoting effect on Hg⁰ removal, while SO₂ and H₂O showed an obvious inhibitory effect. Hg⁰ removal over MnO₂-CeO₂/PG-FA catalyst included adsorption, oxidation and reaction. The used Mn8-Ce0.5/PG-FA catalyst after Hg⁰ removal can be regenerated and its capability for Hg⁰ removal can be effectively recovered, and the Mn8-Ce0.5/PG-FA-Re-400 °C catalyst showed excellent Hg⁰ removal activity and regeneration performance.