Article pubs.acs.org/IECR
Effect of Promotion with Ru Addition on the Activity and SO2 Resistance of MnOx−TiO2 Adsorbent for Hg0 Removal
Anchao Zhang,*,† Zhihui Zhang,† Hao Lu,† Zhichao Liu,† Jun Xiang,‡ Changsong Zhou,‡ Weibo Xing,† and Lushi Sun*,‡ †
School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454003, P. R. China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
‡
ABSTRACT: In order to investigate the promotional effects of ruthenium (Ru) addition on the structure, activity, and SO2 resistance of MnOx−TiO2 mixed-oxide adsorbent for Hg0 removal at lower temperature, the Ru modified MnOx−TiO2 (RMT) adsorbent, MnOx−TiO2 (MT), and RuO2−TiO2 (RT) were synthesized by the sol−gel method. The fresh and spent adsorbents were characterized by N2 physisorption, XRD, FTIR, H2-TPR, XPS, TG-DSC, and SO2-TPD. It was found that the addition of Ru oxide not only can increase the activity of Hg0 removal but also can significantly enhance their SO2 resistance performance. The amorphous structure and some synergistic effects among Ru, Mn and oxygen species may play key roles in promoting Hg0 removal and superior performance of SO2 tolerance. Although RMT exhibited an excellent SO2 resistance performance for Hg0 capture, some active sites would be destroyed with as high as 2500 ppm of SO2 introduced. The sulfate species deposited on the surface of poisoned adsorbent and the consumption of active MnOx and RuO2 by SO2 had a negative activity for Hg0 removal. The enhanced effect of Ru modified MnOx−TiO2 adsorbent on SO2 resistance could be derived from the selective formation of sulfate species on Ru. The possible mechanism of Ru addition on the SO2 resistance of MnOx−TiO2 mixed-oxide adsorbent has also been proposed.
1. INTRODUCTION Mercury has seriously harmful effects on the central nervous system and can cause many diseases, such as pulmonary and renal failure, respiratory damage, blindness, and chromosome damage.1 Coal-fired utility boilers are major anthropogenic sources of mercury emissions.2−4 In China, about 38% of mercury emission comes from coal combustion.5,6 To reduce the atmospheric pollutant emission, the newly revised Chinese Emission Standard of Air Pollutants for Thermal Power Plants (GB 13223-2011) ruled that, from January 1, 2015, the emission limits for mercury and its compounds would be compulsorily executed.7 Mercury from coal-fired flue gas is in three forms: elemental (Hg0), oxidized (Hg2+), and particlebound (Hgp).6 Among them, Hg0 is very difficult to control from flue gas because of its high volatility and low solubility in water.1,6 Therefore, controlling the release of Hg0 from coalfired power plants is challenging work for environmental protection. Many technologies have been studied to capture Hg0 from flue gas. Generally, they can be divided into two major kinds. One is the method of adsorption by using activated carbons (ACs) and modified adsorbents. Cl, Br, I, S, and metal oxides are generally employed as modifiers.1,3,8−11 The other is Hg0 catalytic oxidation method by using metal oxides12−15 or noble metal,16 strong oxidizing solution,17 and photocatalysis.18 Metal oxides have been considered one of the most promising alternatives3 because of its lower cost. Particularly, manganese oxide (MnOx) adsorbents/catalysts3,6,8−13,17,18 had attracted an astonishing amount of attention for Hg0 removal owing to their excellent activity, easy manufacturing, and lower cost compared with noble metal-based catalysts. However, Mn-based adsorbents were sensitive to SO2 poisoning because highly active © 2015 American Chemical Society
MnO2 preferred to produce an inactive manganese sulfate (MnSO4).6 Several methods have been investigated to enhance the sulfur tolerance of the metal oxide adsorbents/catalysts. The presence of a second element like Mo,14 Cu,19 Ce,20 Sn,21 and Ru14,15 would provide a first reasonable way to design the adsorbents/ catalysts with better sulfur tolerance than the monometal oxide materials, as the additive metal would change their properties.22 Guo et al.14 observed that the addition of transition metals of Mo and Ru on MnOx modified MDCOs obviously improved the conversion of Hg0 to Hg2+ and the inhibition of SO2 for Hg0 conversion was insignificant. Furthermore, as reported,15 the RuO2 modified SCR catalyst displayed a remarkable tolerance to SO2, and RuO2 not only showed rather high catalytic activity on Hg0 oxidation by itself but also appeared to be well cooperative with the commercial SCR catalyst for Hg0 conversion. Because of the excellent performance of the Deacon reaction over Ru-based catalyst, Yan et al.15 assumed that Hg0 can be adsorbed on virgin cus-Ru sites and then chlorinated by atomic chlorine on the near cus-Ru sites, and the gaseous Hg0 can even react with atomic chlorine directly. Therefore, it seemed that the presence of HCl is very important for good activity of Hg0 removal over Ru-based catalyst. In our previous study,23 it was demonstrated that the addition of Ru into MnOx−TiO2 material could significantly promote the Hg0 removal activity and SO2 tolerance with no HCl presence. Because of the fact of lower chlorine and higher Received: Revised: Accepted: Published: 2930
January 15, 2015 March 3, 2015 March 4, 2015 March 4, 2015 DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
Article
Industrial & Engineering Chemistry Research
Ltd. China). For quality assurance, the mercury analyzer was calibrated before each test. The Hg0 removal efficiency over an adsorbent was calculated as follows:
sulfur contents in Chinese coal, superior SO2 tolerance adsorbents with enhanced Hg0 removal performance without the assistance of HCl are still being explored. However, so far as is known, there were few reports on the SO2 poisoning of Mnbased adsorbent especially on the novel Ru modified MnOx− TiO2 material. Hence, further efforts are being made to clarify the mechanism of SO2 resistance in both the academic and the industrial fields. In the present work, the Ru modified MnOx−TiO2 (RMT) adsorbent, MnOx−TiO2 (MT), and RuO2−TiO2 (RT) were prepared by the sol−gel method and investigated for Hg0 removal with or without SO2 conditions. The fresh and spent adsorbents were well characterized by means of N 2 physisorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), hydrogen gas temperatureprogrammed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric and differential scanning calorimetry (TG-DSC), and SO2 temperature-programmed desorption (SO2-TPD). On the basis of the results, the physicochemical structures of fresh and spent adsorbents were concluded and the SO2 tolerance mechanism was proposed.
η = (1 − Cout /C in) × 100
(1)
where η is the Hg removal efficiency (%) and where Cin and Cout represent the Hg0 concentrations corresponding to the inlet and outlet (μg/m3), respectively. To understand the effect of SO2 on physical and chemical structures of the fresh adsorbents in depth, the poisoned experiments (about 1.0 g adsorbent) were also implemented for Hg0 removal in the environment of 2500 ppm of SO2 and BL condition for 600 min. The three spent samples were denoted as MTS, RMTS, and RTS. Here, the letter “S” represented the poisoned adsorbent. 2.3. Physicochemical Characterization. The specific surface area and pore parameter of the sample were determined by N2 physisorption method at liquid-nitrogen temperature (77 K) on a surface area and porosimeter (Tristar II 3020, Micromeritics Instrument Corporation, USA). The specific surface area of the sample was measured by using Brunauer− Emmett−Teller (BET) method, and the pore volume and pore size distribution were calculated by Barrett−Joiner−Halenda (BJH) method. XRD measurements were carried out on an X-ray diffractometer (Bruker D8 Advance, Bruker Corporation, Germany) using Cu Kα radiation as X-ray source. The scanning range (2θ) was from 10° to 80° with a step size of 0.02°. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The crystallite sizes of TiO2 for the samples were estimated by the Scherrer’s equation. FTIR spectra were recorded on a Magna-IR 750 apparatus (Nicolet Instrument Corporation, USA) to study the changes of structure and surface chemistry of a sample using the KBr pressed pellet technique. The transmittance spectrum was measured with the wavenumber range of 400−4000 cm−1. H2-TPR experiments were conducted on an AutoChem II 2920 adsorption instrument (Micromeritics Instrument Corporation, USA) equipped with a TCD detector. At the beginning of the H2-TPR measurement, the sample (∼100 mg) was first pretreated in Ar condition at 200 °C for 2 h and cooled to 50 °C. Then the treated sample was reduced from 50 to 800 °C at a linear heating rate of 10 °C/min in 15% H2/Ar with a flow rate of 30 mL/min. The surface compositions of the adsorbents before and after sulfation were investigated by XPS using a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scientific, USA) with an Al Kα X-ray source (hν = 1486.6 eV) and a pass energy of 30 eV. The C 1s line at 284.6 eV was used as a reference for the binding energy (BE) calibration. The spectra were fitted by the program XPS Peak 4.1. TG-DSC analyses of the fresh and spent adsorbents were also performed on a SDT Q600 thermogravimetric analyzer (TA Instrument Corporation, USA). Constant mass of the sample was used in order to avoid the effect of variation in sample weight on peak shape and temperature. The temperature was raised from room temperature to 900 °C with a linear heating rate of 10 °C/min. SO2-TPD experiments were implemented on a temperatureprogrammed oven by using 0.50 g spent adsorbent. In a typical measurement, after pretreatment in N2 condition at 150 °C for 30 min, the fresh sample was saturated with SO2 (800 ppm in BL) at a flow rate of 1 L/min for 2 h. Then desorption was 0
2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. The adsorbents were synthesized by the sol−gel method. All chemicals used are of analytical grade. First, anhydrous ethanol (0.2 mol), acetic acid (0.18 mol), and deionized water (0.55 mol) were uniformly mixed at room temperature. A certain amount of RuCl3·xH2O was added into the above solution under vigorous stirring (denoted as solution A). Second, aqueous solutions of manganese nitrate, anhydrous ethanol (0.5 mol), and tetrabutyl titanate (0.1 mol) were blended under rapid stir (denoted as solution B). Then solution A was slowly added into the solution B with vigorous stirring until the black sol was yielded. After aging for 4 days at room temperature, the sample was exposed in the bright sunlight for another 4−6 days to form a xerogel. Afterwards, the xerogel was crushed and dried at 105 °C for 12 h. Finally the material was ground into power and calcined at 500 °C for 5 h in air to obtain Ru modified MnOx− TiO 2 adsorbent, which was designated as RMT. For comparison, MnOx−TiO2 and RuO2−TiO2 samples were also made by using the same procedure and denoted as MT and RT, respectively. The molar ratios of Ru/Ti and Mn/Ti were selected as 2% and 10%. The corresponding Ru loadings in RMT and RT were about 2.2 and 2.4 wt %, respectively. Before Hg0 removal test, the above materials were pressed, crushed, and sieved to 60−80 mesh. 2.2. Activity Test. The Hg0 removal tests were carried out in a fixed-bed quartz tube reactor (i.d. 10 mm) under atmospheric pressure. A detailed description of the experimental setup was given elsewhere.24 To explore the mechanism of SO2 resistance of Ru modified MnOx−TiO2 adsorbent and eliminate the effect of multiple factors caused by other flue gas components (such as NO, HCl and H2O) on Hg0 removal, baseline (BL) flue gas, which only contained 6% O2, 12% CO2, and a balance of N2, and 800 ppm of SO2 (when used) were considered. The total flow rate of passing through the fixed-bed column was 1.0 L/min. The inlet Hg 0 concentration (Cin) was 55 ± 2 μg/m3, and about 0.50 g of sample was used if there was no special specification. The vapor-phase Hg0 concentrations of influent and effluent were determined continuously using a SG-921 cold vapor atomic absorption spectroscopy (CVAAS) mercury analyzer (Jiangfen 2931
DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
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decreased with further temperature rising above 150 °C, which can be due to the combination effect of adsorption inhibition of Hg0 and desorption of adsorbed HgO.24 The Hg0 removal over Mn-based oxide adsorbent can be interpreted by the Mars− Maessen mechanism,3 where Hg0 bonds with lattice oxygen and/or chemisorbed oxygen of the adsorbent to form weakly bonded Hg−O−Mn−Ox−1 species and then formed HgO. The consumed lattice oxygen and/or chemisorbed oxygen can be replenished by gaseous O2. Generally, different catalysts/ adsorbents would demonstrate different adsorption strength; thus, the oxidized mercury could be removed with various pyrolysis temperature. In the case of RT, Hg0 removal efficiency gradually declined with an increase of reaction temperature, and low temperature was beneficial for higher Hg0 removal efficiency. The above phenomena indicated that the reaction temperature played an important role in Hg0 removal, and the optimized reaction temperature was around 150 °C. Moreover, it was evident that the addition of Ru into MT adsorbent showed a remarkable promotion on Hg0 removal, indicating a synergistic effect between Mn and Ru. 3.2. Positive Effect of Ru Addition for SO2 Resistance. Fuel gas often inevitably contains a small amount of SO2. As is well-known, SO2 can react with manganese oxide and ruthenium oxide to produce their sulfate compounds.25,26 To intuitively investigate the effect of Ru addition on the activity and SO2 resistance of a MnOx−TiO2 material, the Hg0 removal experiments were first performed in the absence of SO2 for about 25 min and then 800 ppm of SO2 was injected. Figure 2 exhibits Hg0 removal efficiencies of the prepared adsorbents in the absence of SO2 and that in the presence of SO2 at 150 °C.
performed by heating the spent sample from 50 to 900 °C at an increasing rate of 15 °C/min in N2 environment with the same flow rate. The concentration of SO2 was monitored using a KM9106 flue gas analyzer (Kane International Limited, U.K.).
3. RESULTS AND DISCUSSION 3.1. Promotional Effect of Ru Addition on Hg0 Removal. Figure 1 shows the effect of reaction temperature
Figure 1. Effect of reaction temperature on Hg0 removal efficiency.
on Hg0 removal efficiencies over MT, RMT, and RT adsorbents. To obtain a relatively constant Hg0 removal efficiency, the reaction time employed was fixed 60 min. Obviously, Hg0 removal efficiencies over MT and RMT first increased with the reaction temperature increasing and then
Figure 2. Hg0 removal efficiencies by (a) MT, (b) RMT, and (c) RT. 2932
DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
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Industrial & Engineering Chemistry Research As observed, under BL condition, MT, RMT, and RT demonstrated much better Hg0 removing activities. When 800 ppm of SO2 was introduced into the BL stream, Hg0 removal efficiencies over MT and RT gradually decreased, while Hg0 removal efficiency of RMT was maintained above 90% for 200 min. Without O2, the addition of SO2 into flow gas somehow increased Hg0 removal efficiency of MT. However, the Hg0 removal efficiency over RT decreased dramatically to lower than 20% in 30 min, indicating that the presence of O2 can remarkably improve the SO2 resistance of RT. The results suggested that the addition of Ru into MnOx−TiO2 adsorbent not only can increase the activity of Hg0 removal but also can enhance its SO2 resistance performance. Considerating the higher activity of RMT adsorbent on Hg0 removal and excellent SO2 resistance performance, it is reasonable and essential to research the effects of other flue gas compositions (such as NO, HCl, and H2O) on Hg0 removal in further study. 3.3. Results of Characterization. 3.3.1. N2 Physisorption. The textural properties of the three adsorbents are shown in Table 1. It was clear that both the BET surface area and pore
Table 2. Crystallite Sizes of the Fresh and Spent Samples
BET surface area (m2·g−1)
pore volume (cm3·g−1)
pore size (nm)
MT RMT RT
81.47 62.82 67.44
0.220 0.193 0.164
10.75 12.26 9.64
TiO2 crystallite size by XRD (nm)
MT RMT RT MTS RMTS RTS
12.2 13.5 14.7 12.2 13.8 15.1
sample, evident diffraction peaks ascribable to RuO2 (JCPDS 40-1290) were observed. The appearance of RuO2 diffraction peaks indicated the formation of bulk RuO2 with high crystallinity.29,30 In the case of RMT, the peaks corresponding to RuO2 became weak or even disappeared, suggesting that the introduction of MnOx could lead to a good dispersion of Ru oxide. Compared with the fresh adsorbents, none of possible sulfate species was observed in MTS, RMTS, and RTS, which revealed that either the formed sulfate species were too low in quantity to be detected by XRD or might exist as surface sulfate or amorphous sulfate.31 Furthermore, the TiO2 crystallite sizes of the fresh samples, as calculated from the XRD data, were in the range of 12−15 nm (Table 2). After sulfation the TiO2 crystallite sizes were slightly increased because of the formation of sulfate species. A similar phenomenon was observed by Deshmukh et al.32 3.3.3. FTIR. The FTIR method was used to further verify the sulfur species on the surface of the spent samples. In Figure 4,
Table 1. Textural Properties of the Samples sample
sample
volume of MT decreased after the addition of ruthenium oxide, which can be attributed to agglomeration of the metal oxide onto TiO2 surface. Although the BET surface area of RMT was lower than those of MT and RT, it exhibited higher Hg0 removal efficiency, forecasting that the synergistic effect between Ru and Mn species appeared and the BET surface area and pore volume did not play a key role in Hg0 removal. 3.3.2. XRD. Figure 3 shows the XRD patterns of the fresh and used samples, and Table 2 lists the corresponding
Figure 4. FTIR spectra for the fresh and spent adsorbents.
the absorption bands at 3400 and 1635 cm−1 were attributed to O−H stretching vibration peak of the adsorbed water and the bending vibration band of the surface O−H groups, respectively.33 For MT sample, the absorption band at 457 cm−1 was due to the Mn−O asymmetric stretching.34 However, in the case of RMT, this band vanished, implying that an intimate reaction existed among Mn, Ru, and O as stated in XRD. After Hg0 removal tests with SO2 presence, significant changes happened in all spectra. Vast negative absorption bands at about 3400 and 1635 cm−1 were observed, suggestive of an enhancement of surface chemisorbed water after sulfation. New bands corresponding to SO42− appeared at about 1120 cm−1 on MTS, RMTS, and RTS, indicating the formation of sulfate.35,36 Additionally, two very weak bands at 1370 and 1040 cm−1 were discernible in RTS, which may be assigned to molecularly chemisorbed SO2.
Figure 3. XRD patterns of the fresh and spent samples.
crystallite sizes, which were calculated from the (111) plane of TiO2 using Scherrer’s equation. As described above, the used samples had been tested for Hg0 removal under atmosphere of BL + 2500 ppm of SO2 for 600 min. We can clearly observe that the fresh absorbents, as well as the spent materials, contained the main anatase phase TiO2 (JCPDS 21-1272) and a little amount of rutile phase (JCPDS 76-0318).27 No peaks corresponding to MnOx species appeared for MT and RMT, implying much higher dispersion of MnOx,28 while for the RT 2933
DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
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Industrial & Engineering Chemistry Research 3.3.4. H2-TPR. Figure 5 presents the H2-TPR profiles of MT, RMT, RT, and their spent samples. MT exhibited two weak
the last one might be assigned to RuS and RuS2 reducing according to the following reactions:26 (3)
Ru(SO4 )2 + 8H 2 → RuS2 + 8H 2O
(4)
RuS + H 2 → Ru + H 2S
(5)
RuS2 + 2H 2 → Ru + 2H 2S
(6)
For RMTS, the addition of Ru resulted in a rather big shift of the reduction peak of manganese sulfate to lower temperature region in comparison to that of MTS. The noticeable and broad shoulder peak centered at 352 °C referred to the reduction of Ru4+ in ruthenium sulfate species to metallic Ru0.39 The features indicated that sulfate species may be formed preferentially on Ru rather than Mn. Thus, the deactivation of the active site on RMT was impeded to a certain extent by Ru addition. 3.3.5. XPS. The XPS technique was performed to probe the transformation of the surface chemical composition in the absence and presence of SO2 under BL condition. Results of XPS analysis for all samples are reported in Figure 6. The resultant surface elemental concentration and surface atomic ratio are summarized in Table 3. Figure 6a gives the XPS spectra of Ti 2p. Two well-defined characteristic peaks assigned to Ti 2p1/2 at about 464.2 eV and Ti 2p3/2 at around 458.5 eV appeared for all samples, showing Ti mainly being in the Ti4+ oxidation state.11,17,40 The intensity of Ti 2p3/2 peak increased with the presence of Ru, which was possibly owing to an improvement in the electron affinity of Ti and Ru. The binding energies (BEs) of Ti 2p1/2 and Ti 2p3/2 for MTS, RMTS, and RTS shifted to higher BEs in comparison with their fresh adsorbents. The shift in BE should be owed to the formation of sulfated titanium,20,41 while a closer examination revealed that the changing magnitude of the main Ti 2p peaks in RMTS was the smallest among them, indicating that the addition of Ru into MnOx-TiO2 could inhibit TiO2 being sulfated. The XPS results for Mn 2p in Mn-containing adsorbents are shown in Figure 6b. The Mn 2p region consisted of a spin− orbit doublet with Mn 2p1/2 located at 653.4 eV and Mn 2p3/ 2 centered at 641.7 eV, revealing the characteristic of a mixedvalence of Mn3+ and Mn4+.17,42 The intensities of Mn 2p1/2 and Mn 2p3/2 of RMT greatly increased as a result of the introduction of Ru. Correspondingly, its surface Mn atomic concentration and surface Mn4+/Mn atomic ratio were also enhanced (Table 3), which would be beneficial for higher Hg0 capture. This suggested that the addition of Ru could lead to an enrichment of Mn on the RMT surface. After Hg0 reaction in the presence of SO2, the intensities of Mn 2p peaks in MTS and RMTS adsorbents significantly decreased because of the appearance of MnSO4. The surface Mn atomic concentration and surface Mn/Ti atomic ratio for MTS also markedly descreased, with surface atomic concentration of Mn varying from 3.61% to 1.74% and Mn/Ti varying from 15.9% to 8.0%. This was in line with the previous literature.20 It can be inferred that Mn was sulfated by SO2 to produce the manganese sulfate on the spent adsorbents. As a result, the sulfate species deposited on the surface of poisoned adsorbent and the consumption of active MnOx by SO2 had a negative effect on Hg0 removal. For O 1s (Figure 6c), two kinds of oxygen species were observed on the fresh and spent samples. The peaks at
Figure 5. H2-TPR profiles for the samples.
reducing peaks at 290 and 526 °C, which could be attributable to the two-step reduction of MnO2. The first step corresponded to MnO2 → Mn3O4, and the second step was assigned to Mn3O4 → MnO.37 According to the literature,30 the reduction peak of Ru modified adsorbent/catalyst at lower temperature was attributed to the reduction of well-dispersed or bulk crystallized RuO2 to Ru0. The higher temperature reduction peak was ascribed to the reduction of RuOx with Ru−support interaction. Thus, the low temperature reduction (below 200 °C) for RT could be assigned to the reduction of highly dispersed amorphous RuO2, and the high temperature reduction corresponded to the bulk amorphous RuO2 with Ru−Ti interaction.30 Since ruthenium oxide can be reduced at lower temperature than manganese oxide, metallic ruthenium could act as reduction nuclei via smooth spreading of hydrogen over the manganese species. Hence, compared with MT, the reducing peaks of RMT adsorbent significantly shifted to lower temperature. The result was consistent with the previous reports that indicated that the promotion of noble metals leads to a great decrease of reduction temperature.38 Furthermore, it was worthy to note that a sharp signal in RMT attributed to the reduction peak of RuO2 was observed at about 210 °C, implying an increase of well-dispersed amorphous RuO2. Thus, it can be concluded that the introduction of Ru into MT adsorbent can bear some new centers for oxygen storage and enhance the redox property at low temperature via the synergistic reaction among Ru, Mn, and oxygen. After sulfation in the process of Hg0 removal, the H2-TPR profiles of the spent adsorbents showed significant variation compared with that of the fresh samples. A broad and intense peak with a maximum at 495 °C appeared on MTS, which was ascribed to the reduction of manganese sulfate species as follows:25 2MnSO4 + 5H 2 → MnS + MnO + 5H 2O + SO2
RuSO4 + 4H 2 → RuS + 4H 2O
(2)
The H2-TPR profile of RT displayed four successive reducing peaks: the first three could correspond to the reduction of ruthenium−sulfate species, such as RuSO4 and Ru(SO4)2, and 2934
DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
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Figure 6. XPS spectra of (a) Ti 2p, (b) Mn 2p, (c) O 1s, (d) Ru 3d, (e) S 2p, and (f) Hg 4f for the samples.
Table 3. Surface Compositions of the Samples Based on XPS Results surface atomic concentration (%) sample
Ti
Mn
O
MT RMT RT MTS RMTS RTS
22.67 21.96 26.13 21.53 22.68 26.32
3.61 3.66
55.75 53.07 54.42 58.17 55.69 58.09
1.74 1.72
Ru 1.09 1.16 0.87 0.84
C
surface atomic ratio (%) S
17.97 20.22 18.29 15.33 16.79 12.93
3.19 2.21 1.79
Hg
0.04 0.04 0.03
Mn/Ti
Mn4+/Mn
Oβ/(Oα + Oβ)
15.9 16.7
51.3 52.8
8.0 7.6
60.0 58.0
35.9 30.5 20.9 52.1 44.1 29.6
BEs. This can be explained by the consumption of manganese or/and ruthenium oxygen by SO2.19 Interestingly, the contents of surface adsorbed oxygen Oβ for MTS, RMTS, and RTS clearly increased and their main peaks shifted to higher BE compared with that of the fresh materials, which was also observed by Du,19 Sheng,31 and Gao36 et al. This indicated that the sulfation not only could introduce an extra component of O in sulfate but also could enhance the content of surface chemisorbed water.36,41
approximately 529.5 eV were regarded as lattice oxygen (denoted as Oα) existed in metal oxides, and the one at around 531.3 eV corresponded to chemisorbed oxygen (denoted as Oβ), such as O22− or O− from oxide defects or hydroxyl-like groups.40,41 As listed in Table 3, compared with RT, MT and RMT contained a large amount of Oβ. As reported,40 the surface chemisorbed oxygen Oβ might be one of the active sites for Hg0 removal due to its flexible mobility. After sulfation, the intensities of Oα in the spent adsorbents weakened. Meanwhile, the main peaks of Oα shifted to higher 2935
DOI: 10.1021/acs.iecr.5b00211 Ind. Eng. Chem. Res. 2015, 54, 2930−2939
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Industrial & Engineering Chemistry Research Figure 6d shows the XPS results of Ru element. The Ru 3d5/ 2 spectra were positioned at about 281.0 eV, which was identified as the photoemission from RuO2, indicating the formation of the RuO2 phase. The overlapping of C 1s and Ru 3d3/2 peaks at about 285 eV made it complicated to ascertain the BE of Ru 3d3/2. The main peak at 284.6 eV could be ascribed to the spectra of contaminated C 1s,43 which were possibly picked up during exposure in air or from the drying process. The shoulders positioned at 286.6 eV were related to the spectra of Ru 3d3/2 belonging to Ru4+ species.44 As observed, the intensity of Ru 3d3/2 + C 1s substantially decreased after Hg0 removal test in the presence of SO2. Meanwhile, the relative content of Ru4+ species was enlarged, which might be due to the enrichment of Ru(SO4)2 on the surface of RMTS and RTS. The S 2p spectrum is shown in Figure 6e. The photoelectron line appeared at 168.6 eV for the spent adsorbents, which was attributed to sulfate species.45,46 The inhibitory effect over the adsorbents was caused by the SO2 adsorbing on the adsorbents surface and oxidizing to SO42−. Figure 6e and Table 3 obviously show that the sulfur content of MTS was much higher than that of RMTS, implying that the doped Ru could reduce the formation of sulfate species. The XPS spectra of Hg 4f for MTS, RMTS, and RTS are also given in Figure 6f. Two typical peaks were demonstrated at 100.9 and 102.3 eV for Hg 4f7/2, which could correspond to Hg2+ species from HgO and HgSO4, respectively.17 By comparing the intensities, one would find that the Hg 4f peak of RMTS was much broader and intensive than that of MTS and RTS, suggesting once again its excellent activity on Hg0 removal. Therefore, from the above, it can be concluded that although RMT exhibited an excellent SO2 tolerance for Hg0 capture, some active sites would be destroyed with as high as 2500 ppm of SO2 presence; thus, the deactivation of RMT poisoned by SO2 still occurred. 3.3.6. TG-DSC and SO2-TPD. To elucidate the deactivating agents formed on the spent surface, the TG-DSC analyses were performed for selective samples. As given in Figure 7a, the TG curves of MT and RMT presented three weight losses during the heating process. The first one appearing at about 100 °C was assigned to the desorption of the physically adsorbed water,20 corresponding to the first endothermic peak in the DSC curve (Figure 7b). The second loss from 250 to 350 °C could be related to the decomposition of residual solvent and/ or unbound stabilizing acid and phase transformation of MnOx from the sample. The small exothermal peak in the DSC curve appearing at about 300 °C was due to the decomposition of organic substances in the samples.47 The last one emerging at 720 °C was due to the phase transformation of anatase TiO2 and MnOx.48 The maximum mass losses of MT and RMT were approximately 4.0% and 3.5%, respectively. The difference in weight loss was accompanied by a broadly endothermic valley in the DSC curve centered at around 500 °C, which might be due to a succession of phase transformations of formed MnOx. In the cases of MTS and RMTS, in the entire heating process, the TG curves showed that the total weight losses were about 13.6% and 10%, respectively, significantly higher than their fresh samples. This also clearly demonstrated that the addition of Ru could greatly reduce the formation of sulfate substance on adsorbent surface. The mass losses of the spent adsorbents occurred at four stages. The first two stages were the same as the fresh samples. In the third stages centered at around 500 °C, the weight losses could be assigned to the decomposition/desorption of sulfates, such as TiO2−SO42−
Figure 7. TG (a) and DSC (b) curves for MT, MTS, RMT, and RMTS.
and/or Ru(SO4)2. The presence of a distinct region of sulfate loss in MTS was similar to the observation of Baraket et al.,49 who reported that the sulfate removal starts at 425 °C and ends at 550 °C in sulfated titania solid (TiS). From the result of SO2TPD (vide infra), it was reasonable to speculate that the decomposition of Ru(SO4)2 in RMTS appeared. In the fourth stage (680−800 °C), the weight loss was attributable to the decomposition of MnSO4.50 The decomposition process was correspondingly accompanied by absorption of heat as shown in the DSC curve (Figure 7b). The fact that no peaks were detected in the XRD patterns implied that MnSO4 may be in the form of surface sulfation or an amorphous sulfate.25 As reported in much of the literature,6,8,11−14 it is known that MnO2 itself is an active reaction site for effective Hg0 removal. However, once MnO2 is poisoned by SO2 to produce MnSO4, the proposed Mars−Maessen mechanism can hardly come true. To validate the above speculation and clarify the strength and property of sulfate species after sulfation, the SO2-TPD tests were also carried out. As shown in Figure 8, the production of SO2 from MTS started at about 600 °C and reached the maximum SO2 concentration at around 750 °C along with a shoulder at 665 °C. The shoulder can probably be assigned to the decomposition of sulfated titania (TiO2−SO42−), and the strong SO2 desorption peak corresponded to the decomposition of MnSO4.25 The only smaller and broader desorption peak in RTS was attributed to the decompositions of Ru(SO4)2 and sulfated titania. The SO2-TPD profile of RMTS presented a similar shape like MTS. Furthermore, comparison of the SO2TPD patterns of MTS and RMTS indicated that the amount of SO2 released and the temperature of reaching maximum peak decreased, suggesting that the addition of Ru would reduce the formation of manganese sulfate and an interaction happened among Mn, Ru, and oxygen. This observation was also in accordance with that of TG-DSC and XPS analyses. 2936
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poisoned adsorbent by using the method of calcination was difficult at lower temperature. Moreover, the damaged adsorbent would not be restored to initial even in the presence of O2. This had been demonstrated in our previous study.24 RT poisoned by SO2 proceeded with the same manner. However, the destroyed active site of RT could be partially restored by gaseous O2 as observed in Figure 2c. König et al.51 studied the behavior of methane synthesis and sulfur removal over a Ru catalyst and reported that regeneration of sulfur-poisoned Ru catalysts by O2 is feasible without the detrimental formation of sulfates. Therefore, the introduction of O2 might result in a transformation of RuSO4 to Ru(SO4)2 and produce RuO2 as Scheme 1(reaction III): Figure 8. SO2-TPD curves for MTS, RMTS, and RTS.
2RuSO4 + O2 → Ru(SO4 )2 + RuO2
3.4. The Mechanism Study. According to the Hg0 removal results and characterizations, the mechanism for Hg0 removal over the prepared samples and their deactivation by SO2 could be elucidated as Scheme 1. As reported3,4,6 and shown in Scheme 1(reaction I), gaseous Hg0 was first adsorbed on Mn sites to form adsorbed Hg0 (Hg0ads). Hg0ads was oxidized to HgO by active lattice oxygen or chemisorbed oxygen. Simultaneously, the oxidation of Hg0 would result in the reduction of Mn4+ to Mn3+. The consumed lattice oxygen or chemisorbed oxygen was supplemented by gaseous O2. RMT and RT materials also had the same mechanism for Hg0 removal as MT. When SO2 was introduced, it would be preferentially adsorbed on the Mn and Ti sites to form manganese sulfate25 and sulfated titania20,41 because of its higher affinity with metal and competitive adsorption ability, which would occupy and damage the active sites simultaneously. As revealed from Figures 7 and 8, the decomposition temperature of manganese sulfate was as high as 700 °C; thus, the regeneration of
(7)
The behavior of SO2 impacted on RMT surface can be described as in Scheme 1(reaction IV). As can be observed from the results of obtained by H2-TPR and SO2-TPD, the existing Ru in RMT would preferentially attract SO2 to produce Ru sulfate and protect the adjacent Mn4+. The formation of RuSO4 would attract the O connected with Mn and extend their bond length, resulting in the reduction of Mn4+ to Mn3+. Because of an excellent redox property of Mn, the exposed Mn3+ would be easily oxidized by gas-phase O2 and transform to Mn4+ species. Accordingly, in comparison with MTS, fewer sulfate species were generated and deposited on the surface of RMTS under the same condition. Moreover, the formed RuSO4 can also be activated by gaseous O2 to create RuO2 and Ru(SO4)2 as presented above. Thus, the addition of Ru can greatly improve the SO2 tolerance of Mn-based adsorbent. Being a novel adsorbent with higher Hg0 removal and SO2 tolerance, further insight into the behavior of the adsorptions of SO2 and O2 in real time will be examined by in situ DRIFTS technique.
Scheme 1. Possible Reaction Pathway for Hg0 Removal and Poisoned by SO2
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4. CONCLUSIONS In summary, Hg0 removal efficiencies over MT and RMT first increased with the reaction temperature increasing and then decreased with further temperature rising above 150 °C. Hg0 removal efficiency of RT gradually declined with an increase of reaction temperature, and low temperature was beneficial for its higher Hg0 removal efficiency. The presence of Ru into MT adsorbent exhibited a remarkable promotion on Hg0 removal and SO2 resistance due to some synergistic effects among Mn, Ru, and oxygen and an improvement of the redox property. The sulfate species deposited on the surface of poisoned adsorbent and the consumption of active MnOx and RuO2 by SO2 had a negative activity for Hg0 removal. The redox ability of MT significantly decreased after sulfation. The existing Ru in RMT will preferentially attract the SO2 to produce RuSO4 and prevent the reduction of the adjacent Mn4+ by SO2. The formed RuSO4 might be activated by gaseous O2 to create RuO2.
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AUTHOR INFORMATION
Corresponding Authors
*A.Z.: phone/fax, +86 391 3987511; e-mail, anchaozhang@ 126.com. *L.S.: phone/fax, +86 27 87545526; e-mail, sunlushi@mail. hust.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This study was financially supported by the National Natural Science Foundation of China (Grants 51306046, 51376073, and 21176098), the Research Project of Chinese Ministry of Education (Grant 113045A), and the Power Engineering and Engineering Thermophysics of Key Discipline of Henan in China (2012).
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