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Ceria-Modified Manganese Oxide/Titania Materials for Removal of Elemental and Oxidized Mercury from Flue Gas Juan He, Gunugunuri K. Reddy, Stephen W. Thiel,* Panagiotis G. Smirniotis,* and Neville G. Pinto† School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States ABSTRACT: High surface area ceria-titania materials were used as supports for manganese oxide for both warm-gas mercury capture and low temperature selective catalytic reduction. These materials exhibited excellent mercury capture capability at 175 °C. Increasing manganese loadings improves the mercury capacities. In the presence of SO2, only a small decrease in mercury capacity was observed for the CeO2TiO2 adsorbents. CeO2TiO2 adsorbents similarly showed excellent stability in the presence of CO and NO. It was also found that the CeO2TiO2 support can capture Hg0 and Hg2+ simultaneously from nitrogen at 175 °C; the total mercury capacities were high. BrunauerEmmenttTeller surface area measurements suggested that increasing manganese loading reduced the surface area due to pore blockage. X-ray diffraction measurements showed that MnOx is in an amorphous state on CeO2TiO2 materials. X-ray photoelectron spectroscopy (XPS) results indicate that the adsorbed mercury is present as both Hg0 and Hg2+ on these ceria-based materials. The XPS observations also suggest that the incorporation of titanium into the cubic lattice of ceria leads to the formation of more lattice oxygen atoms, leading to greater formation of Hg2+ on the CeO2TiO2 support.
1. INTRODUCTION Mercury is a highly toxic element that is released both naturally and as a result of human activity. The U.S. Environmental Protection Agency (EPA) has identified mercury as a Hazardous Air Pollutant.1 According to the EPA’s 2008 National Emissions Inventory, coal-fired electric power plants are the largest source of human caused mercury air emissions in the U.S. These power plants account for about 40% of total U.S. anthropogenic mercury emissions. EPA has proposed air toxics rules for coal-fired electric generating units, reducing atmospheric mercury emissions by 8 tons per year for existing units and 2.6 pounds per year for new units.2 Mercury vapor is emitted from power plants as a mixture of elemental mercury, oxidized mercury (typically as mercuric chloride when chlorine species are present), and mercury adsorbed to particulates.3 Each power plant has a different speciation profile, with the differences related primarily to the type of coal burned and air pollution control devices installed in the facility. Elemental mercury in flue gas is difficult to control. There are currently several technologies available to control mercury emissions, including sorbent injection, catalytic oxidation, photochemical oxidation, and air pollution control devices.47 Adsorption using activated carbon can remove 8098% of the gas-phase mercury, depending on temperature, contact time, flue gas composition, and the type and amount of activated carbon used.8,9 However, using activated carbon for elemental mercury capture is expensive.10 The combination of oxidation catalysts and flue gas desulfurization (FGD) is another effective mercury control strategy.11 Selective catalytic reduction (SCR) catalysts and metal oxides are mainly employed to remove NOx from flue gas but can, under certain conditions, promote the oxidation of Hg0 to Hg2+. r 2011 American Chemical Society
The highly water-soluble Hg2+ so formed can then be captured efficiently in wet FGD systems.12 Several studies have demonstrated the effectiveness of this process, with Hg2+ removal greater than 95% in the wet FGD operation.1315 Metal oxides can be used to catalyze the heterogeneous oxidation of Hg0.16 CuO, TiO2, and V2O5 in fly ash can also promote Hg0 conversion to HgCl2. Ghorishi et al.17 found that a fly ash containing CuO oxidized more than 90% of the Hg0 in simulated flue gas at 250 °C. The same group also found that V2O5 and TiO2 could oxidize 90% of the Hg0 at 350 °C when HCl is present.18 Recently, gold and palladium have been tested as potential mercury oxidation catalysts. Zhao et al.19 reported 4060% Hg0 oxidation using a gold catalyst in the presence of Cl2 at 200225 °C. Tests with iridium and iridium/HCl catalysts also showed 3040% Hg0 oxidation in a flue gas environment at 140 °C.20 Metal oxides have been considered as alternatives to traditional mercury sorbents. Granite et al.11 screened several metal oxides as adsorbents to remove elemental mercury. In the temperature range of 140280 °C, MnO2/Al2O3 showed an elemental mercury capacity of 2.4 mg g1, whereas Cr2O3/Al2O3 had a capacity of 1.23.1 mg g1; argon was the carrier gas in these studies. The Al2O3 support provided high surface area for collisions between gas-phase Hg0 and the metal oxides. Mei et al.21 developed a Co3O4 loaded activated carbon adsorbent that had a Hg0 capture efficiency of 60% at 75100 °C. A Mars-Maessen mechanism has been proposed to describe the chemisorption of mercury on metal oxide adsorbents.11 Received: September 11, 2011 Revised: October 28, 2011 Published: November 01, 2011 24300
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The Journal of Physical Chemistry C In a previous study, the investigators synthesized low-temperature SCR (LTSCR) catalysts consisting of manganese oxide supported on Hombikat TiO2; those catalysts showed 100% NO reduction using NH3 as reductant at a reaction temperature below 200 °C; these catalysts were also effective using carbon monoxide as a reductant.22,23 The same materials were also tested as adsorbents for mercury under warm-gas conditions, and it was found that MnOx/TiO2 low-temperature SCR (LTSCR) catalysts can also serve as elemental mercury adsorbents. Fixedbed adsorption tests using MnOx/TiO2 catalysts showed high capacities for elemental mercury (17.4 mg Hg0 g1 catalyst) at elevated temperatures of 175200 °C. The observed elemental mercury capacities were much higher than the capacities of commercially available activated carbons. Sulfur dioxide dramatically reduced both LTSCR activity and elemental mercury capture.24 To develop a sulfur dioxide tolerant catalyst, the same group synthesized a new LTSCR catalyst, manganese dioxide supported on ceria-titania. The use of the ceria-titania support was motivated by previous reports that CeO2TiO2 SCR catalysts showed some resistance to the effects of sulfur dioxide. In the presence of 200 ppm SO2, the CeO2TiO2 catalysts showed a slowly decreasing NO conversion at 350 °C but still had a high conversion (92.5%).25 This paper presents the results of a study of the use of ceriamodified manganese oxide/titania materials for warm-gas mercury removal. The support material, CeO2TiO2 was also evaluated for the simultaneous capture of elemental and oxidized mercury from gas phase; the effects of manganese loading and gas-phase sulfur dioxide, carbon monoxide, and nitrogen monoxide were determined. XPS results indicate that mercury adsorption occurred on the surface of these ceria-based materials. Both Hg2+ and Hg0 were present on the spent adsorbents; lattice oxygen atoms contributed to greater formation of Hg2+ on the CeO2TiO2 support.
2. MATERIALS AND METHODS 2.1. Preparation of Ceria-Titania Supported Manganese Oxide (MnOx/CeO2TiO2). The ceria and ceria-titania supports
were prepared from ammonium cerium(IV) nitrate (g98.5%, Fluka) and titanium(IV) chloride (99.0%, Alfa Aesar) precursors by hydrolysis with aqueous ammonia. To prepare the ceriatitania support, the ammonium cerium(IV) nitrate was dissolved in distilled water to the desired concentration. The titanium(IV) chloride solution was prepared by digesting TiCl4 in cold concentrated HCl and then diluting the concentrated solution with distilled water to the desired concentration. The precursor solutions were mixed. Dilute liquid ammonia was added dropwise with vigorous stirring until precipitation was complete. The solid product was filtered and washed several times with distilled water until no chlorides could be detected in the filtrate; this test was performed by adding Ag+ to the filtrate. The washed filter cake was dried at 120 °C for 12 h in an oven at atmospheric pressure and then calcined at 500 °C for 5 h in flowing air. The ceria support was prepared using a similar procedure. The ceria-titania supported manganese oxide material was prepared by wet impregnation. In a typical preparation, 50 mL of deionized water was mixed with 1 g of the support. The mixture was heated to 70 °C with continuous stirring. A measured quantity of the manganese nitrate precursor was then added to the solution, and the mixture was evaporated to dryness. The
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paste obtained was dried overnight at 110 °C and then calcined at 400 °C for 4 h in a continuous flow of air. 2.2. Materials Characterization. The materials synthesized using the procedures described above were characterized using X-ray diffraction (XRD), nitrogen adsorptiondesorption, temperature programmed reduction (TPR), ammonia temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). 2.2.1. X-ray Diffraction. X-ray powder diffraction patterns were recorded on a Siemens 500 diffractometer using a Cu Kα radiation source (wavelength 1.5406 Å). An aluminum holder was used to support the catalyst samples. The scanning range was 570° (2θ) with a step size of 0.05° and a step time of 1 s. The XRD phases present in the samples were identified with the help of JCPDS data files. 2.2.2. Nitrogen AdsorptionDesorption. The specific surface areas of the samples were determined by nitrogen physisorption using a Micromeritics 2360 instrument at liquid-nitrogen temperature (77 K), taking 0.162 nm2 as the molecular area of the nitrogen molecule. All samples were degassed at 200 °C under vacuum before analysis. Surface areas and pore volumes were determined from the adsorptiondesorption data using the BET method; pore diameters were determined using the BJH method. 2.2.3. X-ray Photoelectron Spectroscopy. The XPS measurements were performed on a Shimadzu ESCA 3400 spectrometer using Al Kα (1486.6 eV) radiation as the excitation source. Charging of the catalyst samples was corrected by setting the binding energy of the adventitious carbon (C 1s) at 284.6 eV. The XPS analysis was performed at ambient temperature and at pressures typically on the order of 17 37 8.5 3
100 ppm SO2
400 ppm NO, 400 ppm CO
100 ppm SO2, 400 ppm NO, 400 ppm CO
0
N/A
N/A
6.5 5.6
6.4 5.2
4.2 5.4
N/A
N/A
N/A
Inlet Hg0 concentration: 3050 ppbv; carrier gas N2; GHSV = 5000 h1. 24303
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Figure 5. Effect of SO2 on Hg0 capture efficiency using 20 wt % MnOx/ CeO2TiO2 at 175 °C. After the 1st breakthrough, SO2 was stopped. C = effluent mercury concentration; C0 = inlet mercury concentration; C0(Hg0) = 6080 ppbv; carrier gas N2; GHSV = 5000 h1, CSO2 = 100 ppm. Total mercury uptake is 6.3 mg g1.
Figure 6. Effluent concentration history for combined capture of Hg0 (red circle) and Hg2+ (9) using CeO2TiO2 support at 175 °C. C = effluent mercury concentration; C0 = inlet mercury concentration; C0(Hg0) = 3050 ppbv; C0(Hg2+) = 60 ppbv; carrier gas N2; GHSV = 5000 h1; total mercury uptake = 17 mg g1.
30 ppbv Hg0 and 60 ppbv Hg2+. The gas hourly space velocity (GHSV) was 5000 h1. Typical effluent concentration histories are shown in Figure 6. In the combined capture experiments, the elemental mercury capacity was 8.0 mg g1, while the oxidized mercury capacity was 8.39.1 mg g1. The elemental mercury capacity in the combined capture experiments (8.0 mg Hg0 g1) is similar to that observed for the capture of elemental mercury in the absence of oxidized mercury (8.59.1 mg Hg0 g1), suggesting that different adsorption sites may be involved in the capture of different mercury species. 3.4. Mechanism of Mercury Capture. 3.4.1. XPS Results for Fresh Materials. XPS measurements were performed on fresh and spent materials to determine the oxidation states of the elements in these materials and to understand nature of the interactions in the adsorbent system. The XPS spectra for O 1s, Ti 2p, Mn 2p, and Ce 3d for fresh CeO2, CeO2TiO2, 10 wt % MnOx/CeO2TiO2,
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Figure 7. O 1s XPS spectra of MnOx/CeO2TiO2 materials before mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/CeO2TiO2, (c) CeO2TiO2, and (d) CeO2.
Figure 8. Ti 2p XPS spectra of MnOx/CeO2TiO2 materials before mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/CeO2TiO2, and (c) CeO2TiO2.
and 20 wt % MnOx/CeO2TiO2 are presented in Figures 710. The corresponding binding energy values are presented in Table 4. As shown in Figure 7, the O 1s profile is generally broad and complex due to the nonequivalence of surface oxygen ions. The complex peak shape suggests that this peak is composed of multiple peaks arising from overlapping contributions of oxygen from ceria, titania, and MnOx in the samples. The O 1s binding energy values reported for CeO2, TiO2, and MnO2 are 530.1, 529.9, and 530 eV, respectively.26 The O 1s spectra obtained for fresh adsorbent samples are similar and signify the presence of several oxides. The 20 wt % Mn/CeO2TiO2 sample exhibited a broader O 1s spectrum than the other samples; this is likely due to the presence of crystalline MnO2 on the surface. XRD results confirm that 20 wt % MnOx/CeO2TiO2 contains crystalline MnO2. The Ti 2p spectra of fresh samples investigated in this study are presented in Figure 8. The Ti 2p photoemission spectrum of the CeO2TiO2 support exhibited typical XPS peaks in the range of 458.7459.1 eV for Ti 2p3/2; these agree well with the 24304
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The Journal of Physical Chemistry C values reported in the literature. For the MnOx/CeO2TiO2 samples, the binding energy of the Ti2p3/2 peak varied between 457.6 and 459.3 eV,27 within the range reported in the literature for stoichiometric TiO2 surfaces. Hence, it can be inferred from the XPS results that the titanium in the samples is primarily confined to its highest oxidation state (IV). The core level spectra of Mn 2p (Figure 9) exhibited two peaks in the range of 640658 eV. These peaks belong to the Mn 2p3/2
Figure 9. Mn 2p XPS spectra of MnOx/CeO2TiO2 materials before mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2 and (b) 10 wt % MnOx/CeO2TiO2.
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and Mn 2p1/2 electron spin states.24 These binding energy values suggest that Mn is in the +4 oxidation state. As expected the intensity of the peaks increased as the MnOx content increased from 10 wt % to 20 wt %. The CeO2 3d photoelectron peaks of fresh samples used in this study are shown in Figure 10. The assignment of CeO2 3d photoelectron peaks is difficult because of the complex nature of the spectra; this complexity occurs not only because of multiple oxidation states but also because of the mixing of Ce 4f levels and O 2p states during the primary photoemission process. This hybridization leads to multiplet splitting of the peaks into doublets, with each doublet showing further structure due to final state effects. On the basis of the work of Burroughs, Pfau and Schierbaum, and Creaser,2830 the Ce 3d spectrum can be assigned as follows. Two sets of spinorbital multiplets, corresponding to the 3d3/2 and 3d5/2 contributions are labeled as u and v, respectively. The peaks labeled v and v00 have been assigned to a mixing of the Ce 3d9 4f2 O 2p4 and Ce 3d9 4f1 O 2p5 Ce(IV) final states, and the peak denoted v000 corresponds to the Ce 3d9 4f0 O 2p6 Ce(IV) final state. On the other hand, the peaks v0 and v0 are assigned to the Ce 3d9 4f2 O 2p5 and Ce 3d9 4f1 O 2p6 states of Ce(III). The same assignment can be applied to the u structures, which correspond to the Ce 3d3/2 levels. The deconvoluted XPS pattern of CeO2 is presented in Figure 10a; the spectrum exhibits a total of six peaks, which are due to the presence of the Ce4+ oxidation state. No peaks corresponding to Ce3+ were observed for this sample. On the other hand, CeO2TiO2 exhibits contributions from both Ce4+ and Ce3+ oxidation states. After impregnation with 10 wt % MnOx the intensity of the v0 and u0 peaks increased. As the MnOx loading
Figure 10. Ce 3d XPS spectra of MnOx/CeO2TiO2 materials before mercury adsorption.(a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/ CeO2TiO2, (c) CeO2TiO2, and (d) CeO2. 24305
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Table 4. Summary of Binding Energy (eV) from XPS Analysis of Fresh and Spent Adsorbent Samples Ti 2p
CeO2
fresh
CeO2TiO2
O 1s
Ce 3d Ti 2p3/2 Ti 2p1/2
531.0
919.1
spent 531.3
917.1
fresh 530.9 spent 531.5
916.5 918.3
458.9 460.1
464.8 465.7
531.0
918.1
459.3
465.1
spent 531.1
918.1
459.6
465.4
530.8
918.8
458.8
464.4
spent 530.8
917.6
459.1
464.8
10 wt % MnOx/CeO2TiO2 fresh 20 wt % MnOx/CeO2TiO2 fresh
Mn 2p Mn 2p3/2 CeO2
fresh
CeO2TiO2
spent fresh
Hg 4f
Mn2p1/2
Hg 4f7/2
Hg 4f1/2
102.7
113.1
105.1
118.5
102.0
106.2
101.6
105.7
spent 10 wt % MnOx/
fresh
642.9
654.8
CeO2TiO2
spent
642.9
654.7
20 wt % MnOx/
fresh
642.7
653.8
CeO2TiO2
spent
642.4
653.9
Figure 11. O1s XPS spectra of MnOx/CeO2TiO2 materials after mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/CeO2TiO2, (c) CeO2TiO2, and (d) CeO2.
Table 5. Summary of UIII Percentages and Surface Atomic Ratios for Fresh and Spent Adsorbents % UIII CeO2 CeO2TiO2
Fresh
14.02
Spent
12.33
Fresh
13.9
Ti/Ce
Ce/Mn
Ti/Mn
0.154
Spent
7.97
1.896
10 wt % MnOx-CeO2TiO2
Fresh Spent
12.65 11.43
0.781 1.844
2.37 1.782
1.851 3.286
20 wt % MnOx-CeO2TiO2
Fresh
11.93
0.578
2.079
1.201
Spent
10.74
2.036
1.337
2.723
increased from 10 wt % to 20 wt %, the peak intensity further increased. Generally, the Ce3+ content is determined by calculating the area under the peaks u0 and v0 . However, the v0 line appears as a shoulder in the main v00 contribution, and so can be difficult to detect. The u0 line cannot be detected directly and is thus estimated from the intensity of v0 .31 Therefore, the ratio of the intensity of the u000 peak to the total area is used in the literature to determine the surface concentration of Ce3+ in the mixed oxide. According to literature reports, in pure ceria the area of the u000 peak is 14% of the total area.32 The percentage of the u000 peaks for the fresh samples before mercury adsorption are presented in Table 5. In pure ceria the value appeared to be 14.02%, in good agreement with the literature reports. In ceria-titania the value is 13.9%, which suggests that incorporation of Ti4+ into the cubic ceria lattice results in the formation of only small amounts of Ce3+. After impregnation with 10 and 20 wt % MnOx this value dropped to 12.65% and 11.93%, respectively, thus indicating that
Figure 12. Ti 2p XPS spectra of MnOx/CeO2TiO2 materials after mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/CeO2TiO2, and (c) CeO2TiO2.
MnOx impregnation enhances Ce3+ formation in the ceriatitania samples. 3.4.2. XPS Results: Mercury Adsorption. To investigate changes in the materials due to mercury adsorption, XPS measurements were performed on spent (mercury-loaded) samples. Figures 1115 show the O 1s, Ti 2p, Ce 3d, Mn 2p, and Hg 4f core level spectra; the corresponding binding energy values are presented in Table 4. As can be seen in Figure 11, mercury adsorption on these materials results in extensive broadening of the O1s peak. This broadening indicates that the oxygen is not easily accessible at the surface due to the adsorption of mercury on the surface. The Ti 2p spectra of spent samples investigated in this study are presented in Figure 12. The intensity of the Ti 2p peaks increased in spent materials. These results suggest surface enrichment of titanium as a result of mercury adsorption. Titania did not adsorb any mercury and remained exposed on the 24306
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The Journal of Physical Chemistry C surface, whereas both the ceria and the manganese oxide surfaces were covered by the mercury. The atomic ratios presented later in this section support this observation. The intensity of the Mn 2p peaks (Figure 13) decreased in spent materials compared to that of fresh materials due to adsorption of mercury on the surface. Interestingly, no shift in the peak position was observed for Mn 2p spectra after mercury adsorption. However, the broadness of the Mn 2p peaks indicates
Figure 13. Mn 2p XPS spectra of MnOx/CeO2TiO2 materials after mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2 and (b) 10 wt % MnOx/CeO2TiO2.
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the presence of multiple Mn species. This observation confirms that manganese may form compounds with the mercury but remains in the Mn4+ oxidation state. Ce 3d spectra for the spent adsorbent are shown in Figure 14. After mercury absorption, the Ce 3d spectra of pure ceria exhibit contributions from both the Ce3+ and the Ce4+ oxidation states. This change is attributed to the formation of HgO on the surface of the ceria; the Hg 4f spectra are discussed below. Since mercury was loaded onto the solid from the gas phase in the absence of gasphase oxygen and potential gas-phase oxygen donors, the investigators propose that mercury adsorbs on the ceria surface and reacts with lattice oxygen to form HgO and reducing some of the Ce4+ initially present to Ce3+. The Hg 4f spectra presented below are consistent with this hypothesis. For ceria-titania, the percentage of the u000 peak decreased from 13.93 to 7.97 (Table 5) after mercury adsorption, again suggesting that ceria-titania reacts with adsorbed mercury to form HgO and reduce a portion of the cerium. The data in Table 5 also indicate that more HgO was formed in ceria-titania than in pure ceria. This greater formation of HgO can be attributed to distortion of the O2‑ sub lattice due to the incorporation titanium in to the cubic ceria lattice in the ceriatitania mixed oxide. This distortion permits a higher mobility of the lattice oxygen, effectively making the lattice oxygen more available. This hypothesis is consistent with literature reports that the oxygen storage capacity of ceria-titania is much higher than that pure ceria.34 On the other hand, for the 10 and 20 wt % MnOx/CeO2TiO2 samples the percentage of u000 peak decreased from 12.65 to 11.43 and 11.93 to 10.74, respectively. These results suggest that in the MnOx impregnated samples most of the mercury adsorption takes place on the MnOx.
Figure 14. Ce 3d XPS spectra of MnOx/CeO2TiO2 materials after mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/ CeO2TiO2, (c) CeO2TiO2, and (d) CeO2. 24307
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Figure 15. Hg 4f XPS spectra of MnOx/CeO2TiO2 materials after mercury adsorption. (a) 20 wt % MnOx/CeO2TiO2, (b) 10 wt % MnOx/ CeO2TiO2, (c) CeO2TiO2, and (d) CeO2.
The Hg 4f spectra of all of the spent materials are presented in Figure 15. The Hg 4f spectra of all of the samples exhibit contributions from two components. The peak at lower binding energy is due to elemental mercury, and the peak at higher binding energy is due to HgO. The peak due to the HgO is larger for ceria-titania than for the other samples, due to the formation of more HgO in ceria-titania as explained above. The Hg 4f spectra results agree well with the Ce 3d spectra results. The atomic ratios of Ti/Ce, Ce/Mn, and Ti/Mn before and after mercury adsorption are presented in Table 5. Impregnation with MnOx decreases the atomic ratios of Ce/Ti and Ti/Mn due to the formation of MnO2 over the surface. The atomic ratios of Ti/Ce and Ti/Mn increased the after mercury adsorption; these results confirm the surface enrichment of titania during mercury adsorption. On the other hand, the atomic ratio of Ce/Mn decreased after mercury adsorption. 3.4.3. Mechanistic Implications. The mercury adsorption results presented above show that increasing MnOx content leads to an increase in the mercury adsorption capacity, but that the adsorption capacity is not proportional to the amount of MnOx. Elemental mercury adsorption and HgO compound formation were both observed for all the samples investigated in the present study. The cerium in ceria can easily occupy two oxidation states [CeO2 (Ce4+) T Ce2O3 (Ce3+)], thus allowing ceria to accommodate surface oxygen species (lattice oxygen). Consequently, ceria contains many lattice oxygen species on the surface. Mercury adsorbed on the ceria surface can react with the lattice oxygen to form HgO, thus reducing some of the CeO2 to Ce2O3. This reduction of cerium by mercury can be seen in the XPS results presented above. Thus, the following mechanism is
proposed for the chemisorption of elemental mercury on ceriabased materials: CeO2 T Ce2 O3 þ OðlatticeÞ Hg þ OðlatticeÞ f HgO The mercury adsorption results presented above also show that ceria-titania has a higher amount of HgO than the other samples. When Ti is incorporated into the cubic ceria lattice, it causes distortion of the O2 sub lattice. This distortion results in higher mobility of the lattice oxygen, making the lattice oxygen more available. In the MnOx-containing materials, the ceria is covered by MnOx so that some physical adsorption of elemental mercury is observed on the external surface of the adsorbent. When SO2 and mercury are simultaneously adsorbed on ceria and ceria-titania, mercury forms HgO with ceria, whereas SO2 only adsorbs physically. Hence, there is preferential adsorption of mercury on ceria relative to SO2, explaining the weak sensitivity of the mercury capacity of ceria and ceria-titania to the presence of SO2. Since mercury is adsorbed to the MnOx coating in the MnOx-containing samples, SO2 competes directly with mercury for the surface adsorption site, significantly reducing the mercury capacity of the MnOx-containing samples.
4. CONCLUSIONS New MnOx/CeO2TiO2 LTSCR catalysts are effective adsorbents for removing Hg0 from flue gas at 175200 °C. The manganese loading and overall gas composition have a significant influence on total mercury capacity. The MnOx/CeO2TiO2 24308
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The Journal of Physical Chemistry C adsorbents had large Hg0 capacities in an inert atmosphere, as high as 37 mg g1. Increasing manganese loading on the CeO2TiO2 support increased mercury capacities at the same conditions. However, changing the bed temperature from 175 to 200 °C reduced mercury adsorption. SO2 inhibits Hg0 adsorption on the MnOx, but CeO2TiO2 retains most of its Hg0 capacity in the presence of 100 ppm SO2. Simultaneous capture of Hg0 and Hg2+ at 175 °C was obtained using CeO2TiO2. The results of XRD and XPS analyses indicate that mercury adsorption occurs on the surface of catalysts, with adsorbed mercury present mainly as HgO. These results suggest that the sulfurtolerant MnOx/CeO2TiO2 materials might be technically feasible warm-gas mercury adsorbents.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.W.T.); Panagiotis.Smirniotis@ uc.edu (P.G.S.). Present Addresses †
Speed School of Engineering, University of Louisville, Louisville, Kentucky 40292, United States.
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the U.S. Department of Energy (Grant No. DE-FG26-06NT42712), and also thank Dr. James Boerio (University of Cincinnati) and Prof. M. Rodica and Prof. J. Jacek (University of Louisville) for assistance with the XPS measurements.
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