Article pubs.acs.org/EF
Thermal Regeneration of Manganese Supported on Activated Carbons Treated by HNO3 for Desulfurization Yong-Jun Liu,†,‡ Yi-Fan Qu,† Jia-Xiu Guo,*,†,‡,§ Xue-Jiao Wang,† Ying-Hao Chu,†,‡,§ Hua-Qiang Yin,†,‡,§ and Jian-Jun Li†,‡,§ †
College of Architecture and Environment, Sichuan University, Chengdu 610065, Sichuan, China National Engineering Technology Research Center for Flue Gas Desulfurization, Chengdu 610065, Sichuan, China § Sichuan Provincial Environmental Protection Environmental Catalysis and Materials Engineering Technology Center, Chengdu 610065, Sichuan, China ‡
ABSTRACT: Manganese supported on activated carbons treated by HNO3 (Mn/NAC) was prepared using an excessive impregnation method and calcined at 650 °C, and the deactivation and recovery factors of Mn/NAC for desulfurization were investigated. The results showed that fresh catalyst calcined at 650 °C has breakthrough sulfur capacity of 141.8 mg/g and breakthrough time of 300 min and that the catalysts thermally regenerated at different temperatures under N2 atmosphere exhibit different removal capacity of SO2. After the catalysts undergo the first thermal regeneration at 650 °C, the catalysts have breakthrough sulfur capacity of 144.9 mg/g and breakthrough time of 299 min. These values are close to those of the fresh catalysts, suggesting that active sites can be recovered almost completely. In the following cycles, the SO2 removal capacity of the regenerated catalysts gradually decreases, indicating that active sites reduce gradually. The fresh catalyst has 710 m2/g specific surface area and 0.404 cm3/g total pore volume with 0.262 cm3/g micropore volume; after desulfurization, the specific surface area and micropore pore volume of the sample decrease to 612 m2/g and 0.220 cm3/g, respectively. The regenerated catalysts at different temperatures have different texture, but the first regenerated catalysts at 650 °C still has an 800 m2/g specific surface area and 0.448 cm3/g total pore volume with 0.291 cm3/g micropore volume. These values decrease with the increase of the number of regeneration cycles. Both sulfates and manganese oxides such as MnO and Mn3O4 are detected in the regenerated catalysts, and with the increase of the number of regeneration cycles, average crystalline size of MnO increase from 29.8 to 40.3 nm, indicating that sulfates are partially decomposed in N2 atmosphere and reduced by neighboring C atoms. After desulfurization, the relative content of CO and C−O decrease while that of OC−O is almost unchanged, indicating that CO and C−O play a role in the desulfurization reaction. Thermal regeneration can recover CO and change its relative content, while the unreduced sulfates increase with the increase of the number of regeneration cycles and accumulate in the catalysts, leading to a gradual decrease of SO2 removal capacity.
1. INTRODUCTION The burning of coal is a main source of atmospheric pollution in China, and sulfur dioxide (SO2) is one of the major pollutants. To resolve the problem of SO2 pollutants, many engineers and researchers are working to improve the current flue gas desulfurization technology and to develop new technology. Activated carbon (AC) is often used as a medium for desulfurization, and AC modified by metals or metal oxides can significantly improve SO2 removal capacity.1,2 Davini has found that different metal precursors show different catalytic activities.3 However, sulfur is deposited on the catalyst surface as the reaction progresses, resulting in a decline of SO2 removal ability. The recovery of activity must occur. Generally, the desulfurization activities of the used activated carbon-supported catalysts could be recovered using water and steam washing regeneration or thermal regeneration under a different atmosphere.4 Water soak cleaning may remove the sulfuric acid which covers the pores and active components to recover the activity, but the loss of active components and the deficiency of metal sulfate regeneration limit their applications. Studies have shown that the used catalysts can also be thermally regenerated in reductive gases such as hydrogen (H2),5,6 ammonia (NH3),7,8 and carbon monoxide (CO).5 This method © 2015 American Chemical Society
can effectively recover the catalyst activities, and active components have hardly been lost. However, these reductive gases are expensive, and some are flammable. It is reported that activated carbon-supported copper oxides can be regenerated efficiently in inert gas such as nitrogen (N2) in the range of temperatures from 260 to 480 °C, and the adjacent surface oxygen complexes on the carbon supports play the role of the reducing agents to reduce copper sulfate (CuSO4) to copper (Cu).9 Thermal regeneration at high temperature will require high energy consumption, but it can overcome the disadvantages of water regeneration and recover the desulfurization performance of activated carbon-supported metal oxide catalysts. It can use waste heat in flue gas to regenerate catalysts and reduce energy consumption, which is feasible in the process. Therefore, thermal regeneration in an inert gas such as N2 still has commercial feasibility and attracts a great deal of attention. Recently, our studies have found that the coexistence of MnO and Mn3O4 in manganese-based activated carbon Received: November 26, 2014 Revised: February 19, 2015 Published: February 19, 2015 1931
DOI: 10.1021/ef502655k Energy Fuels 2015, 29, 1931−1940
Article
Energy & Fuels
Figure 1. Experimental apparatus of SO2 removal: (1) blower, (2) N2/SO2, (3) N2, (4) flow meter, (5) blender, (6) buffer bottle, (7) saturated humidifier, (8) circulating water bath, (9) three-way valve, (10) reactor, (11) sampling bottle, and (12) absorption device. dried at 110 °C for 6 h, they were thermally treated at 280, 350, 400, 500, and 650 °C for 2 h in N2 (99.99%) atmosphere to recover their SO2 removal ability. The regenerated catalysts were designated as CTn, where, T is the regeneration temperature and n is the regeneration cycle. 2.2. Activity Evaluation. The SO2 removal capacity of catalysts was measured in a fixed-bed microreactor with 16 mm inner diameter under atmosphere pressure. The experimental apparatus is presented in Figure 1. The reactor contained 16 g of catalysts with a height of 200 mm. A gas mixture, simulated from flue gas, contained 0.23% SO2, 10.2% water vapor, 10% O2, and N2 as the balance. Gases were separately controlled using rotor flow meters before they entered the blender. The water vapor was introduced into the reactor by the gas mixtures passing a humidifier. The gas space velocity (SV) was 1657 h−1. The bed temperature was 80 °C. The gas mixtures before and after the reactor passed through a solution. This solution contained 3% H2O2. NaOH solution (0.01 mol/L) was used to titrate the formed H2SO4 in this solution. Bromcresol green-methyl red was used as an indicator to determine the ending point.11 When the outlet SO2 concentration reached 200 mg/m3, the catalyst bed could be considered as having reached the breakthrough point. At this moment, the cumulative working time is considered as the breakthrough time, and the corresponding cumulative amount of SO2 removal (milligrams of SO2 per gram catalyst) is determined as the breakthrough sulfur capacity. SO2 removal efficiency (%) is obtained by analyzing the SO2 inlet and outlet concentration.
catalysts exhibits the best SO2 removal ability, but manganese (Mn) oxides are gradually transformed into MnO2 during the reaction process.10 However, the information about the SO2 adsorption−regeneration cycles on carbon-supported manganese oxide catalysts is relatively scarce in the literature. To achieve higher regeneration efficiency, it is of great importance to clarify the reason for the loss of SO2 removal capacity. However, the changes of metal species and surface functional groups in deactivated metal-supported catalyst and regenerated catalysts as well as the reasons for the impaired activity of regenerated catalyst are rarely studied. The aim of this study is to explore the changes of manganese species and surface functional groups in used and regenerated activated carbonsupported manganese-based catalysts, which could provide a more reasonable theory to explain the catalytic activity and establish a feasible way to regenerate exhausted manganesebased activated carbon catalysts. In this paper, manganese supported on activated carbons treated by nitric acid (HNO3) (Mn/NAC) is prepared using an excessive impregnation method and calcined at 650 °C. SO2 removal capacity of fresh and thermally regenerated catalyst is evaluated in a fixed bed under the simulated flue gas. The deactivation and recovery factors of Mn/NAC are studied using N2 adsorption−desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and Fourier transform infrared spectroscopy (FTIR). The effects of the regenerative cycles on manganese species and functional groups are also investigated.
SO2 removal efficiency =
SO2(inlet) − SO2(outlet) SO2(inlet)
2.3. Catalyst Characterization. N2 adsorption−desorption isotherms were measured on an AUTOSORB-IQ (Quantachrome Instruments, United States) apparatus at −196 °C. Prior to measurement, the sample was degassed with heating to 200 °C under vacuum for 10 h. The surface area (SBET) of samples was calculated from the adsorption isotherm data using the Brunauer− Emmett−Teller (BET) equation at relative pressure (P/P0) between 0.05 and 0.35. The total pore volume (Vtotal) was calculated from N2 volume at P/P0 = 0.95, and the micropore volume was determined via the Dubinin−Radushkevich (D-R) equation. The mesopore volume was estimated from total pore volume subtracting the micropore volume. The pore size distribution was determined from N 2 adsorption isotherms at −196 °C with the nonlocal density functional theory (NDFT). X-ray powder diffraction (XRD) experiments were conducted on a DX-2007 diffractometer (Haoyuan Instruments Ltd., Dandong, Liaoning, China) using Cu Kα radiation (λ = 0.1542 nm) and operating at 40 kV and 30 mA to determine the crystal structure of the catalyst. Spectra were collected from 10° to 70°. The crystalline phases were identified by comparison with the reference data from the International Center for Diffraction Data (JCPDs). X-ray photoelectron spectroscopy data was obtained on a spectrometer (XSAM-800, KRATOS Co., UK) under ultrahigh vacuum (UHV). An Al Kα monochromatized radiation was employed as the X-ray source and operated at 12 kV and 15 mA. Energy
2. EXPERIMENTAL SECTION 2.1. Preparation and Regeneration of Catalysts. A commercial activated carbon (Xingtong Chemical Ltd., Henan, China) was ground and sieved into 10−20 mesh, which contained 5.18% ash. The activated carbon was treated using distilled water and dried at 110 °C for 12 h. After the carbon supports were completely immersed into 39 wt % HNO3 solution for 2 h at 60 °C, the samples were filtrated and washed using distilled water until the washing liquid became neutral. Finally, the carbon supports were dried at 110 °C for 12 h and designated as NAC, and the ash decreased to 3.62%. Catalysts were prepared by an excessive impregnation method. Mn(NO3)2 (AR grade; Kelong Chemical Reagent Factory, Chengdu, China) was used as a precursor. The carbon supports were immersed into Mn(NO3)2 precursor solution with an appropriate concentration to achieve 7 wt % Mn loading. First, the mixtures were stirred constantly for 80 min during impregnation and kept statically for 12 h. Second, the mixtures were heated at 60 °C with constant stirring until the liquid was totally eliminated. Finally, after the mixtures were dried at 110 °C for 12 h, they were calcined at 650 °C for 2 h in N2 (99.99%). The heating rate was 5 °C/min. The obtained catalyst contained 3.58% ash and was termed C-F. The catalyst after desulfurization was donated as C-D. After the used catalysts were 1932
DOI: 10.1021/ef502655k Energy Fuels 2015, 29, 1931−1940
Article
Energy & Fuels calibration was performed by recording the core level spectra of Au 4f7/2 (84.0 eV) and Ag 3d5/2 (368.3 eV). Peak areas, including satellites, were computed using a program which assumed Gaussian line shapes and flat background subtraction. The temperature-programmed reduction of H2 (H2-TPR) was performed in a quartz tubular microreactor to observe the reductive behavior on catalysts. A 70 mg sample was exposed to N2 with a 20 mL/min flow rate and heated from room temperature to 400 °C. After the sample was maintained at 400 °C for 60 min, it was cooled to room temperature. The sample was purged with a 5% H2−95% N2 mixture and heated from room temperature to 800 °C with a heating rate of 8 °C/min. The amount of H2 uptake during the reduction was detected by a thermal conductivity detector (TCD). Fourier transform infrared spectroscopy was performed on a Nicolet 6700 Fourier transform infrared spectrometer (Thermo Electron Co., United States) with KBr optics and a DTGS detector (Thermo Fisher Scientific Inc., United States). The catalyst samples were mixed with potassium bromide (KBr), ground, and pelletized. Samples in KBr contained 0.5% sample. The spectral range was 4000− 400 cm−1 with a resolution of 0.09 cm−1.
O2, followed by generating H2SO4 with H2O. C-F exhibits better SO2 removal capacity compared to NAC. Its breakthrough sulfur capacity is 141.8 mg/g, and its breakthrough time is 300 min, indicating that manganese component plays an important role in the SO2 removal process. This observation is consistent with the results of previous studies.10,12,13 In Figure 2, the used catalysts thermally regenerated at different temperatures under N2 atmosphere exhibit different removal capacity of SO2, which could be closely related with the recovered active phase of active components and surface functional groups. C280-1 has the worst SO2 removal capacity after the used catalysts are thermally treated at 280 °C in N2 atmosphere, indicating that active sites on the catalyst surface could not be recovered. With the increase of treatment temperatures, catalysts exhibit better desulfurization activities, suggesting that more active sites may be recovered. C650-1 exhibits the best SO2 removal capacity. This may indicate that the active phase of active components could be recovered when the catalysts are regenerated at 650 °C in N2 atmosphere. The conversions of manganese oxides are as follows: MnO2 ↔ Mn2O3 ↔ Mn3O4 ↔ MnO, showing that MnO2 is the most easily formed of the manganese oxides. Our previous study found that Mn2O3 is the main manganese oxide species after catalysts are calcined at 500 °C, and MnO and Mn3O4 coexist in the catalysts calcined at 650 °C.10 The coexistence of MnO and Mn3O4 shows the best SO2 removal capacity, whereas MnO2 is the worst.10 In Table 1, C650-1 has breakthrough sulfur capacity of 144.9 mg/g and breakthrough time of 299 min, values close to those of the fresh catalysts. This indicates that the SO2 removal capacity is almost completely recovered in the first regeneration cycle at 650 °C in N2 atmosphere. Multicycle regeneration and activity evaluation were conducted in order to study the SO2 removal capacity of catalysts regenerated at 650 °C. As shown in Table 1, after the second regeneration cycle, for C650-2, breakthrough sulfur capacity and breakthrough time of the catalysts decrease drastically to 101.7 mg/g and 223 min, respectively, and are only 70% of those of C-F, indicating that there are changes of surface chemical properties and Mn species. These two factors are of great importance to the SO2 removal capacity. In the following cycles, breakthrough sulfur capacity of C650-3 and C650-4 are 97.8 and 94.5 mg/g and less than that of C650-2 by 3.9 and 7.2 mg/g, respectively, suggesting that the surface chemical properties of the catalysts change further. 3.2. Study of Texture Properties. The pore size distribution of all samples are illustrated in Figure 3, and the texture parameters, including specific surface area (SBET), micropore volume, mesopore volume, and total pore volumes, are listed in Table 2. According to the IUPAC classification, a micropore is 50 nm. According to the literature,14 when pore size is in the range of 0.7 to 1 nm, it is classified as a supermicropore; when pore size is
