Simultaneous Removal of Elemental Mercury and NO from Flue Gas

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Simultaneous Removal of Elemental Mercury and NO from Flue Gas using CeO2 Modified MnOx/TiO2 Materials Juan He, Gunugunuri Krishna Reddy, Stephen W Thiel, Panagiotis G. Smirniotis, and Neville G. Pinto Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Jun 2013 Downloaded from http://pubs.acs.org on June 21, 2013

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Simultaneous Removal of Elemental Mercury and NO from Flue Gas using CeO2 Modified MnOx/TiO2 Materials

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, USA

Prepared for publication in Energy & Fuels

*

Corresponding Author: Stephen W. Thiel, School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012 USA. Email: [email protected] † Corresponding Author: Panagiotis G. Smirniotis, School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012 USA. Email: [email protected] ‡ Current Address: Neville G. Pinto, Speed School of Engineering, University of Louisville, Louisville, KY 40292 USA. 1


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Abstract The laboratory-scale simultaneous removal of elemental mercury (Hg0) and NO has been investigated using MnOx/TiO2, MnOx/CeO2-TiO2 and CeO2-TiO2 materials in the presence and absence of CO. Remarkably, these materials exhibit excellent NO removal performance and high Hg0 adsorption capacities both in single-component (NO or Hg0) tests and in combined NO and Hg0 removal experiments at 175°C. Interestingly, NO removal increased in the presence of CO due to the selective catalytic reduction (SCR) by CO over 20% MnOx/CeO2-TiO2; Hg0 adsorption did not inhibit SCR activity. In simultaneous removal tests, 20 wt% MnOx/CeO2-TiO2 effectively removed 9.4 mg Hg0 g-1 and 358 mg NO g-1 at 175°C. The surface areas of the TiO2 and CeO2-TiO2 materials decreased after impregnation with MnOx. CeO2-based materials have more lattice oxygen defects than TiO2, thus enhancing SCR in the presence of CO. Adsorbed Hg0 reacts with lattice oxygen to form HgO on the surface of the CeO2-based materials.

Keywords: Low temperature SCR; MnOx/CeO2-TiO2; Gas-phase Hg capture; NO removal; Adsorption

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1. Introduction Nitrogen oxides (NOx) and gas-phase mercury are major pollutants released from coalfired power plants. NOx emissions can contribute significantly to acid rain and urban photochemical smog. The US Environmental Protection Agency (US EPA) issued the Clean Air Interstate Rule (CAIR) in March 2005. This rule requires that NOx emissions in the Eastern United States be reduced by 60% from 2003 levels by 2015.1 Exposure to mercury, a neurotoxin, can harm the brain, heart, kidneys, lungs, and immune system of people of all ages.2 In November 2011, the US EPA issued a new Clean Air Mercury Rule that limits annual atmospheric mercury emissions from existing coal-fired electric generation units to 15 tons, beginning in 2018.3 Cost-effective control technologies for NOx and mercury are urgently needed. Current technology implements NOx and mercury control in separate units. Most widely used methods for NOx removal are liquid or solid adsorption4 and Selective Catalytic Reduction (SCR).5 There is also a direct NOx decomposition process that has not yet reached commercial application.6 Removal of elemental mercury (Hg0) is more difficult than removal of NOx. Sorbent injection using activated carbon or chemically-treated activated carbon is the most mature technology available to control Hg0 emissions from coal-fired power plants. 80-98% of the mercury can be captured by activated carbon, depending on temperature, contact time, flue gas composition, and the type and amount of activated carbon used.7, 8 However, using activated carbon for Hg0 capture is expensive.9 Some mercury emission controls can be achieved using existing air pollution control devices.10-12 Metal oxides and SCR catalysts, mainly employed to remove NOx from flue gas, can promote the oxidation of Hg0 to Hg2+. The highly water-soluble Hg2+ so formed can then be captured efficiently in wet-flue gas desulfurization (FGD) systems. 3


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Manganese dioxide has been reported to remove Hg0 from gas phase under a wide range of temperatures.13 Manganese-cerium mixed oxides (MnOx-CeO2) and MnOx-CeO2/TiO2 catalysts have shown increased low-temperature NO reduction with ammonia;14-16 the high redox activity on MnOx-CeO2/TiO2 contributed to the catalyst oxygen storage capacities and facilitated oxygen mobility on the catalyst surface.16 Wu et al.17, 18 reported that Ce modified MnOx/TiO2 catalyst was highly effective for low-temperature SCR of NO with NH3; the Ce doping greatly enhanced the SO2 resistance of the catalyst. The high cost of separate NOx and Hg control units is still a major concern for flue gas treatment for coal-fired power plants. For example, the cost to remove NO in flue gas with current SCR technologies is estimated to be $1,300 to $2,800 per ton of NO reduced;19 and the cost of mercury removal with activated carbon injection technology ranges from $33,000 to $55,000 per lb Hg captured.20 Integration of multiple treatment processes into a single unit has several advantages compared to treatment in separate process units. Since NO and CO are the typical components in the flue gas, and CO is one of the most common reductants for SCR, a mercury removal process based on SCR technology can provide great opportunities for simultaneous NO, CO, and Hg control. The authors previously reported the synthesis of catalysts consisting of manganese oxide materials for separate removal of NO and mercury.21, 22 Interestingly, MnOx/CeO2-TiO2 showed much higher mercury adsorption capacity (37 mg Hg0 g-1) than the MnOx/TiO2 (17 mg Hg0 g-1) and CeO2-TiO2 (8.5 mg Hg0 g-1) support. These observed mercury capacities are significantly greater than the capacities of commercially available activated carbon sorbents, 0.25-1.5 mg Hg0 g-1.23 These materials also exhibited excellent mercury capture in the presence of SO2. The study reported here investigated the use of these materials for simultaneous capture of Hg0 and NO. Experiments were completed for the removal of NO alone and simultaneous removal of NO and 4


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Hg removal both in the presence and in the absence of CO. Remarkably, the CeO2-TiO2 materials exhibit excellent single and simultaneous capture capacities. MnOx/CeO2-TiO2 materials show much higher NO removal in the presence of CO (334 mg g-1) than in the absence of CO (160 mg g-1) due to the SCR of NO; this behavior differs from that of MnOx/TiO2, which has higher NO capacity in the presence of CO (215 mg g-1), than in the absence of CO (148 mg g-1). MnOx/CeO2-TiO2 also exhibits greater Hg0 removal in the presence of CO (9.4 mg g-1) than in the absence of CO (5.1 mg g-1). X-ray Photoelectron Spectroscopy (XPS) results show that CeO2-based materials exhibit more lattice oxygen defects than do TiO2-based materials, which enhances SCR activity in the presence of CO.22 The results are consistent with previously reported mechanisms for NO and NO+CO reactions over CeO2 based materials.24

2. Experimental 2.1. Catalyst Materials and Characterization Techniques The synthesis and characterization of of the CeO2-TiO2 support and the MnOx/CeO2TiO2, and MnOx/TiO2 materials have been reported previously; 21, 22 the materials used in this study were from the same batches as the materials used in the previously reported work. The MnOx/CeO2-TiO2 and MnOx/TiO2 materials were prepared by wet impregnation. X-ray powder diffraction (XRD) and nitrogen adsorption/desorption measurement (BET) characterizations were completed for all the catalyst samples as reported previously.22 2.2 Fixed-Bed Removal of Elemental Mercury and Nitrogen Oxide The apparatus used for fixed-bed removal tests was described previously.21 Gas-phase Hg0 was generated using a Hg0 Dynacal® permeation tube (VICI Metronics Inc.). Nitrogen gas (prepurified level), nitric oxide (400 ppm NO balanced with N2) and carbon monoxide (1% CO 5


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balanced with N2) cylinders were used in the experiments. The tests were completed at 175°C and 200°C at a gas hourly space velocity (GHSV) of 5,000-10,200 h-1. The gas phase included one or more of the following components: 30-50 ppbv Hg0, 400 ppm NO, and 400 ppm CO; nitrogen was the balance gas. The inlet and outlet gas streams were analyzed for mercury concentration using a modified version of the Ontario Hydro method (ASTM D6784-02).25 The concentration of NO in gas streams was determined as described in Section 2.3. 2.3 Nitrogen Oxide Measurement by UV-Visible Spectroscopy A major challenge in performing the combined NO/Hg0 capture experiments was the determination of the NO concentration. Chemiluminescence is commonly used to quantify gasphase concentrations in SCR studies. However, it is desirable to avoid introducing mercury into the chemiluminscence detector. In addition, nitrogen, the principal component of flue gas, was used as the carrier gas for mercury. This nitrogen influences the accuracy for the NO analysis results. Consequently, a modified version of EPA Method 7C26 was used to determine gas-phase NO concentrations in this study. Briefly, NO was captured in liquid-filled impingers over a measured sampling period; the impinger liquid was then processed to convert all absorbed nitrogen oxides to nitrate. The processed solutions were then analyzed by UV-Vis spectroscopy to determine the amount of NO captured. Assay results for the inlet gas were consistent with the certified composition of the inlet gas. Typical calibration results can be found in the supplemental information. Absorbing (Impinger) solution. The absorbing solution was prepared by adding 2.8 mL concentrated H2SO4 (95.0-98.0%, Fisher Sci.) and 6 mL of 3% H2O2 solution (prepared from 30.9% H2O2, reagent A.C.S., Fisher Sci.) to a 1 L flask partially filled with DI H2O. The solution was then mixed well and diluted to a final volume of 1 L using DI H2O. 6


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Phenoldisulfonic acid solution. A solution of phenoldisulfonic acid, used to process the impinge solutions, was prepared by dissolving 25 g of pure white phenol crystal (reagent A.C.S., Fisher Scientific) in 150 mL concentrated sulfuric acid on a steam bath. After the solution cooled to room temperature, 75 mL of fuming sulfuric acid (reagent grade, 20% free SO2 basis, Sigma-Aldrich) were added. The solution was then heated to 100°C and held for 2 h. After preparation, the solution was stored in a dark stoppered bottle. Gas sampling. Gas sampling for NO quantitation was performed using the same impingers used for mercury quantification; the entire gas stream leaving the column flowed through an impinger containing 25 mL of the absorbing solution for 0.5 h. Processing of impinger solutions. After gas sampling was complete, the impinger was aged for at least two hours, after which it was agitated on a shaker table for two min. The contents of the impinger were then transferred to a 50 mL volumetric flask; the impinger was rinsed twice with 5 mL DI H2O and the rinse was also transferred to the volumetric flask. The pH was then adjusted to 9-12 by dropwise addition of 1N NaOH prepared from 98.4% NaOH solid (reagent A.C.S., Fisher Sci.), and the solution was diluted to the mark with water. A 25 mL aliquot of the diluted solution was evaporated to dryness. 2 mL of the phenoldisulfonic acid solution were added to the dried residue, along with 1 mL water and 4 drops of concentrated H2SO4 (96-98.0 w/w%, reagent A.C.S. Plus, Fisher Scientific). The solution was then heated for 3 min and then cooled to room temperature. After cooling, 20 mL of water were added and the pH was adjusted to 10 using concentrated ammonium hydroxide (29.31%, reagent A.C.S., Fisher Scientific.). The mixture was transferred into a 100 mL volumetric flask and diluted to volume. The concentration of the resulting solution was read using UV-Vis Spectrophotometer (Cary 50 Bio, Varian, Inc.), based on the absorbance at 415-420 nm range. 7


The measurement was

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calibrated using standard solutions prepared with KNO3 (99.6%, reagent A.C.S., Fisher Scientific).

2.4 In Situ FT-IR Characterization In situ FT-IR spectra were recorded using a Bio-Rad (FTS-40) Fourier transform instrument and a heatable IR cell connected to a conventional volumetric operator. The scans were collected at a scan speed of 5 kHz, resolution of 2.0, and an aperture opening of 2.0 cm-1. Sixteen scans were averaged for each normalized spectrum. Circular self-supporting thin wafers (8 mm diameter) consisting of 12 mg of material were used for the study. The wafers were placed in a high-temperature cell with CaF2 windows and purged in situ in the IR cell with prepurified grade helium (30 mL min-1, Wright Brothers) at 473 K for 2 h to remove any adsorbed impurities. Then, the samples were cooled to 323 K, and the selected gasses were introduced into the cell with a flow of 30 mL min-1 for 1 h at 323 K to ensure complete saturation of the sample. Physisorbed gases were removed by flushing the wafer with prepurified helium for 3 h at 373 K. Subsequently, the in situ FT-IR spectra were recorded at different temperatures.

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Results and Discussion

3.1 Materials Characterization The BET surface areas, pore diameters, and pore volumes of the supports and manganese-promoted adsorbents are presented in Table 1. The manganese-promoted samples exhibit lower surface area, lower pore volume and higher pore diameter than the supports. The loss of surface area is attributed to partial blockage of micro-pores of the support by the deposited manganese oxide. The X-ray diffraction patterns of the MnOx supported on TiO2 and 8


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CeO2-TiO2 catalysts exhibit peaks due to the MnOx, anatase TiO2 and CeO2-TiO2 solid solutions.22 These measurements indicate the presence of crystalline manganese oxide in the adsorbents. However, the low intensity of the lines due to manganese oxide indicates that the manganese oxide is highly dispersed over the supports. As reported previously, the XPS results show that CeO2-TiO2 and MnOx/CeO2-TiO2 contained both Ce3+ and Ce4+.22 After impregnation with MnOx, the Ce3+ content increased compared to that of the CeO2-TiO2 support.22 3.2 Single-Component Removal Prior to the fixed-bed removal tests, empty-column tests were performed as controls. No Hg0 or NO removal was observed in the control experiments; consequently any removal of Hg0 and/or NO reported in this study is attributed to the material being tested. Single-component tests were completed before the combined removal experiments. 3.2.1

Elemental Mercury Adsorption Removal of vapor-phase Hg0 in using supported MnOx materials has been discussed in

detail in previous studies.21, 22 Those experiments were conducted in an inert environment using nitrogen as a carrier gas at a gas hourly space velocity (GHSV) of 5000 h-1 and a bed temperature of 175ºC. The Hg0 capacities of the materials were calculated from the effluent concentration histories. Under the test conditions used, the Hg0 capacity of 20 wt% MnOx/CeO2-TiO2 was as high as 37 mg g-1; the CeO2-TiO2 support had an Hg0 capacity of 8.5 mg g-1; and 20 wt% MnOx/TiO2 had an Hg0 capacity of 17.4 mg g-1. These previously published capacities are used in the discussion below. 3.2.2

NO Removal Single-component tests were completed for fixed-bed NO removal using the 400 ppm

NO balanced with N2. The 20 wt% MnOx/CeO2-TiO2, CeO2-TiO2, and 20 wt% MnOx/TiO2 9


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materials were tested at 175°C; gas-phase NO concentrations were measured as described in Section 2.3 above. The NO removal capacities of CeO2 promoted MnOx/TiO2 materials calculated from the effluent concentration histories, shown in Figure 1, are included in Table 2 (a). For complete NO breakthrough, the 20 wt% MnOx/CeO2-TiO2 had a capacity of 160 mg NO g-1; the CeO2-TiO2 support had a capacity of 120 mg NO g-1; and the 20 wt% MnOx/TiO2 had a capacity of 148 mg NO g-1. These results show that the CeO2 promoted MnOx/TiO2 materials removed NO more effectively than the other materials tested; consequently, NO may inhibit mercury adsorption due to competition for surface binding sites. To investigate the influence of CO, a potential reductant in SCR, on NO removal capacity, experiments were performed over MnOx-based catalysts in the presence of 400 ppm CO and 400 ppm NO (balance N2). The experiments were performed at atmospheric pressure and 175ºC. Figure 2 shows the effluent concentration histories for NO capture in the presence of CO. The NO capacities obtained from the effluent concentration histories are summarized in Table 2 (a). Adding 400 ppm CO resulted in a significant increase in the NO removal capacity for 20 wt% MnOx/CeO2-TiO2, from 160 mg g-1 to 334 mg g-1. A similar increase in NO capacity was observed for CeO2-TiO2. This increase in NO removal suggests low-temperature SCR (LTSCR) activity. To test the hypothesis of SCR activity, another experiment was completed in which the 20 wt% MnOx/CeO2-TiO2 adsorbent was initially contacted with N2 carrying 400 ppm NO. After 85% breakthrough was observed, 400 ppm CO was added into inlet gas stream, providing a SCR environment for the adsorbent. The effluent concentration history for this experiment is shown in Figure 3. After 7×106 bed volumes, the total NO removal was 148 mg g-1, attributed to NO adsorption; after CO was introduced to the system, an additional 130 mg NO g-1 was removed. 10


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The total NO uptake (~280 mg g-1) in this experiment was higher than the NO capacity observed in the single-component experiment but lower than the NO removal observed in the when CO was present for the entire experiment. These data suggest that CO chemically interacted with NO on the surface of the materials, converting adsorbed NO to N2.27 Therefore, in the presence of CO, the NO capacity includes both adsorbed NO and NO that reacted with CO. These results show that CeO2-based mixed oxides are more effective for SCR of NO in the presence of CO than are the TiO2-supported catalysts. Literature reports suggest that the addition of CeO2 to the support or metal oxides efficiently promotes the CO + NO reaction.28, 29 It is well known that the higher oxygen storage capacity of CeO2-TiO2 compared to the TiO2 alone enhances the SCR activity.30-32 3.3 Simultaneous Removal of Hg0 and NO The main objective of this study is to achieve simultaneous capture of Hg0 and NO at elevated temperature using CeO2 promoted MnOx/TiO2 materials. In real applications, NO and CO are two typical components of flue gas, in addition to gas-phase mercury. Hence, the simultaneous removal of NO and Hg0 was tested both in the absence of CO and in the presence of 400 ppm CO. In the first group of experiments, 400 ppm NO and 30-50 ppbv Hg0 were both present in the inlet gas (balance N2); the GHSV was 10,200 h-1 and the temperature was 175ºC. The effluent concentration histories of NO and Hg0 are shown in Figure 4 ((a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2 and (c) CeO2-TiO2). The Hg0 and NO removals observed in these tests, summarized in Table 2 (b), suggest that simultaneous removal of Hg0 and NO at 175°C can be attributed to chemically chemisorption. Interestingly, the NO removal capacity of MnOx/CeO2-TiO2 decreased only slightly, from 160 to 152 mg g-1, in the presence of Hg0 while the Hg0 capacity decreased from 37 to 5.1 mg g-1 when NO was added. These results 11


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suggest that NO is adsorbed preferentially over Hg0 on the CeO2-TiO2 surface. Similar results were observed for the MnOx/TiO2 and CeO2-TiO2 materials. In another set of combined capture experiments at the same bed temperature, GHSV, and other experimental conditions, 400 ppm CO was introduced along with 400 ppm NO and 30-50 ppbv Hg0 (balance N2). The effluent concentration histories for NO and Hg0 are shown in Figure 5 ((a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2 and (c) CeO2-TiO2). The total Hg0 and NO uptakes are also included in Table 2 (b). In the absence of CO, there was a small reduction on the NO adsorption capacities of these materials when mercury was present. When CO was present, the NO removal capacity of MnOx/TiO2 increased from 148 to 225 mg g-1. This increase is attributed to the SCR reaction; the NO-CO reduction study has been published previously.33 There is not much change in the Hg0 capacity when CO is added to the gas phase. There is an even greater increase in the NO removal capacity of MnOx/CeO2-TiO2 after adding CO to the gas phase; the NO removal capacity increased from 152 to 358 mg g-1. These results suggest that the MnOx/CeO2-TiO2 has better SCR activity than MnOx/TiO2. This increase in NO removal is attributed to the presence of more lattice oxygen capacity, which enhances SCR activity. Moreover, the Hg0capacity increased from 5.1 to 9.4 mg g-1 in the presence of CO. Since NO is consumed by the SCR reaction over MnOx/CeO2-TiO2 there will be more sites available to absorb Hg0. The results obtained here show that the SCR activity of NO with CO is only slightly influenced by the presence of vapor-phase Hg0. Hence, the results of this study suggest the possibility of simultaneous removal of NO and Hg0 using CeO2 promoted MnOx/TiO2 materials at 175°C, with the presence of CO leading to increases NO removal capacity. Using MnOx/TiO2 as a representative adsorbent, in-situ FT-IR studies were completed to investigate the mechanism of the NO and NO+CO adsorption at 175 to 200 ºC. For NO 12


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adsorption on MnOx/TiO2 in the absence of CO, the in-situ FT-IR results showed that in the absence of O2, both mono-dentate and bi-dentate nitrates were present after the adsorption of NO.34 The peaks of the mono-dentate nitrate species were present at 1242, 1295, and 1498 cm-1, while the peaks at 1583 and 1610 cm-1 were due to bi-dentate nitrate species. This observation is consistent with literature reports.35 In the presence of O2, neither the FT-IR peak positions nor peak intensities changed significantly. The nitrate species observed in the FT-IR spectra suggest that lattice oxygen from the surface of the MnOx/TiO2 catalyst rather than gas phase O2 plays an important role in the formation of nitrates. CeO2 and TiO2 are primarily employed as oxygen storage components.36-38 Ramis et al. investigated the typical adsorption bands that were formed upon contact of NO with TiO2 at room temperature.35 The IR bands at 1622, 1545, 1322 and 1190 cm-1 are associated with nitrate ions or chelating nitrate species. The adsorption of NO also forms gaseous N2O in the regions 2300-2230 cm-1 and 1260-1220 cm-1, which are associated with the asymmetric and symmetric stretching frequencies, respectively, of N2O.39 This observation was confirmed by Hadjiivanov et al.40 Many studies have considered the influence of CeO2 on the adsorption process and identification of the adsorbed species. Philipp et al.41 investigated NO adsorption on CeO2 at different temperatures using Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy. At 200ºC, the FTIR spectra showed the formation of a strong band at 1162 cm-1, assigned to a chelating nitrite NO2-, and the formation of other nitrites, which were detected at 1411, 1097, 1021 and 974 cm-1. For binary oxides, Machida et al.42 used the in-situ FTIR diffuse reflectance spectrometry to demonstrate the adsorption and interactions of NO with MnOx-CeO2 binary oxides in the presence and absence of O2. Gaseous NO reacts with surface oxygen bound to Mn and Ce to form the oxidized adsorbates. The NO adsorbed by Mn could coordinate to a 13


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Ce4+-O2- pair in an adjacent site so that the cerium ions on the surface could form metal-NO complexes. Considering the surface oxygen vacancies, together with Ce3+ ions, more possible active sites for NO adsorption are expected. Based on this proposed mechanism, the observed NO adsorption on 20 wt% MnOx/CeO2TiO2, CeO2-TiO2, and 20 wt% MnOx/TiO2 materials in the absence of O2 is reasonable. The adsorbed NO species are stable, as no desorption is observed even while flushing with N2 gas. After adding CO to the system, these materials exhibited greater NO removal capacities in the simultaneous removal experiments, suggesting that SCR was occurring during the experiment. To investigate this possibility, in-situ FT-IR experiments were performed for the coadsorption of NO and CO on MnOx/TiO2 at room temperature and at 200ºC. The in-situ FT-IR spectra are shown in Figure 6. The strong peaks at 1632 and 1580 cm-1 are due to the formation of carbonates and adsorbed nitrates, confirming the individual adsorption of NO and CO; the peaks at 1289 and 2221 cm-1 are attributed to adsorbed N2O, indicating the formation of N2O during the experiment. It is proposed that N2O is formed by the reaction of adsorbed NO with adsorbed N atoms, and this adsorbed N2O can then react with adsorbed CO to produce N2 and CO2. Numerous studies of the adsorption mechanism of NO and CO onto ceria and metal doped ceria materials have been published.43, 44 Liu et al.45 performed an in-situ FT-IR study of NO+CO adsorption on to CeO2 catalysts and found that NO species adsorbed on ceria ranged from bridge nitrate (1613-1618 cm-1) to monodentate nitrate (1457-1475, 1230-1245, 1000-1020 cm-1) and bidentate nitrate (1540-1555, 1280-1281, 1010-1060 cm-1). Peaks corresponding to carbonates were found at 1075, 1351 and 1520 cm-1. Ilieva et al.46 synthesized gold-based catalysts on ceria and ceria-alumina materials that exhibited high activity with 100% selectivity 14


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to N2 by NO-CO reduction at 200ºC. Wang and Cheng et al. studied the activity of catalysts composed of NiO and CeO2 for the NO-CO reaction.24, 47 NiO/CeO2 showed the most active and complete conversion of NO and CO at 170-210 ºC in the absence of O2. DRIFTS spectra for the co-adsorption of NO and CO on NiO/CeO2 catalyst showed that the band for N2O appeared at 2237 cm-1, in agreement with the observation of N2O (2221 cm-1) on the MnOx/TiO2 material in this study. For MnOx/CeO2-TiO2, XPS analysis22 suggests that the active site for NO and Hg oxidation is related to the redox of Mn and the accessible lattice oxygen from MnOx and CeO2; the incorporation of Ti4+ ions into the cubic lattice of ceria leads to the formation of more lattice oxygen atoms, which also causes a significant increase of the SCR activity of metal ions dispersed on the CeO2-TiO2 support. From the in-situ FT-IR pattern obtained from NO+CO adsorption over MnOx/TiO2 and well reported NO+CO mechanism over CeO2 based materials,47 a similar reaction mechanism is proposed for NO+CO interaction in the absence of O2 over MnOx/CeO2-TiO2: when both NO and CO molecules are introduced into the MnOx/CeO2-TiO2, CO attacks the active oxygen, resulting in the formation of CO2 and surface oxygen vacancies; NO is adsorbed in combination with surface oxygen to form nitrate species. Potential pathways for production of N2 and CO2 on the catalysts surface have been widely reported.48-50 The adsorbed NO species then dissociate by the adjacent surface oxygen vacancies to form N* and O*; N* can react with either N* or NO to produce N2O or N2. The oxidation of CO is favored from the decomposition of N2O, resulting in the generation of N2 and CO2. In summary, in-situ FT-IR spectra for NO adsorption on MnOx/TiO2 catalysts exhibit similar peaks regardless of the presence of oxygen, suggesting that lattice oxygen in the sorbent is strongly involved in NO adsorption. The NO adsorption capacities of MnOx/TiO2 and 15


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MnOx/CeO2-TiO2 sorbents in the absence of CO are 142 and 160 mg g-1, respectively, suggesting that CeO2 has little or no influence on NOx adsorption in the absence of CO. Adding CO to the gas phase allows SCR reactions to proceed, resulting in an increase in the total NO removal; the total NO removal capacity increased more for MnOx/CeO2-TiO2 (from 160 to 334 mg g-1) than for MnOx/TiO2 (from 142 to 215 mg g-1). These results indicate that the presence of CeO2, increases SCR activity, due to the lattice oxygen present in CeO2. A mechanism for Hg adsorption onto Mn-based sorbents and Ce-based sorbents has been previously proposed based on the results of XPS studies.22 Cerium can occupy two oxidation states [CeO2 (Ce4+)↔Ce2O3 (Ce3+)], allowing ceria from the CeO2-TiO2 support to accommodate more surface lattice oxygen species. Consequently, Hg0 adsorbed on the ceria surface can react with the lattice oxygen to form HgO. The proposed mechanism is shown as follows (* denotes lattice oxygen): CeO2 ↔ Ce2O3 + O*

(1)

Hg + O* → HgO

(2)

Overall, the increased lattice oxygen resulting from CeO2 modification of MnOx/TiO2 improves both Hg0 and NOx removal capacity in both single-component and simultaneous removal tests.

4

Conclusions CeO2-modified MnOx/TiO2 materials have been shown to be effective for the

simultaneous capture of Hg0 and NO at the laboratory scale at 175ºC. In the presence of 400 ppm 16


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NO, both with and without 400 ppm CO, the MnOx/CeO2-TiO2, MnOx/TiO2, and CeO2-TiO2 adsorbents retained mercury capacities of 3.0-9.4 mg Hg0 g-1. For NO adsorption in the absence of CO and Hg0, the lattice oxygen from the surface plays an important role in the formation of nitrates. Adding CO to the system increased the NO removal capabilities; the observed behavior is consistent with the elementary reactions that might occur during SCR of NO using CO as a reductant over the surface of MnOx/CeO2-TiO2, MnOx/TiO2, and CeO2-TiO2 materials. Most importantly, in the presence of CO, some NO is consumed by SCR activity with CO over MnOx/CeO2-TiO2 and CeO2-TiO2 materials. The greater number of lattice oxygen sites available results in increased Hg0 capacities in the simultaneous removal process. These results suggest that the CeO2-modified MnOx/TiO2 might be technically feasible adsorbents for simultaneous NO and gas-phase mercury control.

Acknowledgements The authors gratefully acknowledge financial support from the US Department of Energy (Grant No. DE-FG26-06NT42712). The authors thank Dr. James Boerio (University of Cincinnati), Professor Jacek B. Jasinski and Rodica McCoy (University of Louisville) for their assistance with the XPS measurements.

REFERENCES

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(15) Chang, H.; Li, J.; Chen, X.; Ma, L.; Yang, S.; Schwank, J. W.; Hao, J. Effect of Sn on MnOx-CeO2 catalyst for SCR of NOx by Ammonia: Enhancement of Activity and Remarkable Resistance to SO2. Catalysis Communications 2012. (16) Sheng, Z.; Hu, Y.; Xue, J.; Wang, X.; Liao, W. A Novel Co-Precipitation Method for Preparation of Mn–Ce/TiO2 Composites for NOx Reduction with NH3 at Low Temperature. Environmental Technology 2012, 33, 2421-2428.

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(18) Wu, Z.; Jin, R.; Wang, H.; Liu, Y. Effect of Ceria Doping on SO2 Resistance of Mn/TiO2 for Selective Catalytic Reduction of NO with NH3 at Low Temperature. Catalysis Communications 2009, 10, 935-939.

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(20) Hoffmann, J.; Ratafia-Brown, J. Preliminary Cost Estimate of Activated Carbon Injection for Controlling Mercury Emissions from an Un-Scrubbed 500 MW Coal-Fired Power Plant, Final Report, prepared for US DOE/NETL, 2003.

(21) Ji, L.; Sreekanth, P. M.; Smirniotis, P. G.; Thiel, S. W.; Pinto, N. G. Manganese Oxide/Titania Materials for Removal of NOx and Elemental Mercury from Flue Gas. Energy & Fuels 2008, 22, 2299-2306.

(22) He, J.; Reddy, G. K.; Thiel, S. W.; Smirniotis, P. G.; Pinto, N. G. Ceria-Modified Manganese Oxide/Titania Materials for Removal of Elemental and Oxidized Mercury from Flue Gas. The Journal of Physical Chemistry C 2011, 49, 24300-24309.

(23) Lee, S.; Park, Y. Gas-Phase Mercury Removal by Carbon-Based Sorbents. Fuel Processing Technology 2003, 84, 197-206.

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(24) Cheng, X.; Zhu, A.; Zhang, Y.; Wang, Y.; Au, C. T.; Shi, C. A Combined DRIFTS and MS Study on Reaction Mechanism of NO Reduction by CO Over NiO/CeO2 Catalyst. Applied Catalysis B: Environmental 2009, 90, 395-404.

(25) ASTM D6784-02 Standard Test Method for Elemental, Oxidized, Particle-Bound and Total Mercury in Flue Gas Generated from Coal-Fired Stationary Sources (Ontario Hydro Method). 2008.

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(27) Sreekanth, P. M.; Smirniotis, P. G. Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts. Catalysis letters 2008, 122, 37-42.

(28) Holles, J. H.; Davis, R. J.; Murray, T. M.; Howe, J. M. Effects of Pd Particle Size and Ceria Loading on NO Reduction with CO. Journal of Catalysis 2000, 195, 193-206.

(29) Zhu, H.; Shen, M.; Kong, Y.; Hong, J.; Hu, Y.; Liu, T.; Dong, L.; Chen, Y.; Jian, C.; Liu, Z. Characterization of Copper Oxide Supported on Ceria-Modified Anatase. Journal of Molecular Catalysis A: Chemical 2004, 219, 155-164.

(30) Lee, S. M.; Park, K. H.; Hong, S. C. MnOx/CeO2-TiO2 mixed Oxide Catalysts for the Selective Catalytic Reduction of NO with NH3 at Low Temperature. Chemical Engineering Journal 2012,195-196, 323-331.

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(31) Jiang, X.; Lou, L.; Ding, G.; Chen, Y.; Zheng, X. The Active Species and Catalytic Properties of CuO/CeO2-TiO2 catalysts for NO + CO Reaction. Jounal of Material Science 2004, 39, 4663-4667.

(32) Reddy, B. M.; Khan, A.; Lakshmanan, P.; Aouine, M.; Loridant, S.; Volta, J. Structural Characterization of Nanosized CeO2-SiO2, CeO2-TiO2, and CeO2-ZrO2 Catalysts by XRD, Raman, and HREM Techniques. The Journal of Physical Chemistry B 2005, 109, 33553363.

(33) Sreekanth, P. M.; Smirniotis, P. G. Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts. Catalysis Letters 2008, 122, 37-42.

(34) Ettireddy, P. R.; Ettireddy, N.; Boningari, T.; Pardemann, R.; Smirniotis, P. G. Investigation of the Selective Catalytic Reduction of Nitric Oxide with Ammonia Over Mn/TiO2 Catalysts through Transient Isotopic Labeling and in Situ FT-IR Studies. Journal of Catalysis 2012, 292, 53-63.

(35) Ramis, G.; Lorenzelli, V.; Forzatti, P. Fourier Transform Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia on TiO2 Anatase. Applied catalysis 1990, 64, 243-257.

(36) Kim, S. S.; Lee, S. J.; Hong, S. C. Effect of CeO2 Addition to Rh/Al2O3 Catalyst on N2O Decomposition. Chemical Engineering Journal 2011, 169, 173-179.

(37) Li, W.; Yang, W.; Krist, K.; Regalbuto, J. R. Selective Adsorption of NO from Hot Combustion Gases by Ce-Doped CuO/TiO2. Energy Fuels 1997, 11, 428-432. 22


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(38) Yu, Q.; Wu, X.; Yao, X.; Liu, B.; Gao, F.; Wang, J.; Dong, L. Mesoporous Ceria–zirconia– alumina Nanocomposite-Supported Copper as a Superior Catalyst for Simultaneous Catalytic Elimination of NO–CO. Catalysis Communications 2011, 12, 1311-1317.

(39) Laane, J.; Ohlsen, J. R. Characterization of Nitrogen Oxides by Vibrational Spectroscopy. Progress in Inorganic Chemistry 1980, 27, 465.

(40) Hadjiivanov, K.; Knözinger, H. Species Formed After NO Adsorption and NO+O2 CoAdsorption on TiO2: An FTIR Spectroscopic Study. Physical Chemistry Chemical Physics 2000, 2, 2803-2806.

(41) Philipp, S.; Drochner, A.; Kunert, J.; Vogel, H.; Theis, J.; Lox, E. S. Investigation of NO Adsorption and NO/O2 Co-Adsorption on NOx-Storage-Components by DRIFTSpectroscopy. Topics in Catalysis 2004, 30/31, 235-238.

(42) Machida, M.; Uto, M.; Kurogi, D.; Kijima, T. MnOx-CeO2 Binary Oxides for Catalytic NOx Sorption at Low Temperatures. Sorptive Removal of NOx. Chemistry of Materials 2000, 12, 3158-3164.

(43) Chen, J.; Zhan, Y.; Zhu, J.; Chen, C.; Lin, X.; Zheng, Q. The Synergetic Mechanism between Copper Species and Ceria in NO Abatement Over Cu/CeO2 Catalysts. Applied Catalysis A: General 2010, 377, 121-127.

(44) Wilson, E. L.; Chen, Q.; Brown, W. A.; Thornton, G. CO Adsorption on the Model Catalyst Pd/CeO2-X(111)/Rh(111). Journal of Physical Chemistry.C 2007, 111, 14215-14222.

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(45) Liu, L.; Cao, Y.; Sun, W.; Yao, Z.; Liu, B.; Gao, F.; Dong, L. Morphology and Nanosize Effects of Ceria from Different Precursors on the Activity for NO Reduction. Catalysis Today 2011, 175, 48.

(46) Ilieva, L.; Pantaleo, G.; Ivanov, I.; Venezia, A. M.; Andreeva, D. Gold Catalysts Supported on CeO2 and CeO2–Al2O3 for NOx Reduction by CO. Applied Catalysis B: Environmental 2006, 65, 101-109.

(47) Wang, Y.; Zhu, A.; Zhang, Y.; Au, C. T.; Yang, X.; Shi, C. Catalytic Reduction of NO by CO Over NiO/CeO2 Catalyst in Stoichiometric NO/CO and NO/CO/O2 Reaction. Applied Catalysis B: Environmental 2008, 81, 141-149.

(48) Shan, J.; Zhu, Y.; Zhang, S.; Zhu, T.; Rouvimov, S.; Tao, F. Catalytic Performance and inSitu Surface Chemistry of Pure α-MnO2 Nanorods in Selective Reduction of NO and N2O with CO. The Journal of Physical Chemistry C 2013, 117, 8329.

(49) Stegenga, S.; van Soest, R.; Kapteijn, F.; Moulijn, J. A. Nitric Oxide Reduction and Carbon Monoxide Oxidation Over Carbon-Supported Copper-Chromium Catalysts. Applied Catalysis B: Environmental 1993, 2, 257-275.

(50) Pârvulescu, V. I.; Grange, P.; Delmon, B. Catalytic Removal of NO. Catalysis Today 1998, 46, 233-316.

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Table 1. Surface Area, Pore Volume and Pore Diameter of TiO2, MnOx/TiO2, CeO2-TiO2 and MnOx/CeO2-TiO2. Sample

Surface Area

Pore Volume

(m2 g-1)

(cm3 g-1)

TiO2a

309

0.36

44

20wt% MnOx/TiO2 a

204

0.23

64

CeO2-TiO2 b

270.6

0.41

58.4

20wt% MnOx/CeO2-TiO2 b

128.3

0.20

60.8

a

Reference (21)

b

Reference (22)

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Pore Diameter (Å)

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Table 2. Fixed-bed Adsorption Tests Using 20 wt% MnOx/TiO2, 20 wt% MnOx/CeO2-TiO2 and CeO2-TiO2 (a) Single-Component NO Removal; (b) Simultaneous Removal of Hg0 and NO . (a) Results of Single-Component NO Removal Tests Sample

400 ppm NO

400 ppm NO + 400 ppm CO

NO mg g-1

NO mg g-1

20 wt% MnOx/TiO2

148

215

20 wt% MnOx/CeO2-TiO2

160

334

CeO2-TiO2

120

195

(b) Results of Hg0 and NO Simultaneous Removal Tests Sample

30-50 ppbv Hg0

30-50 ppbv Hg0

400 ppm NO

400 ppm NO + 400 ppm CO

Hg0 mg g-1

NO mg g-1

Hg0 mg g-1

NO mg g-1

20 wt% MnOx/TiO2

3.9

140

3.4

205

20 wt% MnOx/CeO2-TiO2

5.1

152

9.4

358

CeO2-TiO2

3.0

85

3.8

134

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LIST OF FIGURES Figure 1. Effluent concentration history for NO adsorption in the absence of Hg0 using (■) 20 wt% MnOx/CeO2-TiO2, (●) CeO2-TiO2, and (▲)20 wt% MnOx/TiO2 materials at 175ºC. C = effluent NO concentration; C0 = inlet NO concentration; CNO= 400 ppm, balance N2; GHSV = 5000 h-1. Figure 2. Effluent concentration history for NO adsorption in the absence of Hg0 but in presence of CO using (■) 20 wt% MnOx/CeO2-TiO2, (●) CeO2-TiO2, and (▲)20 wt% MnOx/TiO2 materials at 175ºC. C = effluent NO concentration; C0 = inlet NO concentration, CNO= CCO = 400 ppm, balance N2; GHSV = 5000 h-1. Figure 3. Effect of CO on NO removal efficiency using 20 wt% MnOx/CeO2-TiO2 at 175ºC. After the 1st breakthrough, CO was added into gas flow. C = effluent NO concentration; C0 = inlet NO concentration; CNO= CCO = 400 ppm, balance N2; GHSV = 5000 h-1. Figure 4. Effluent concentration history for combined capture of NO (■) and Hg0 (●) at 175˚C; using (a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2, (c) CeO2/TiO2 . C = effluent concentration; C0 = inlet concentration; CHg0 = 30-50 ppbv, CNO= 400 ppm, balance N2; GHSV =10,200 h-1. Figure 5. Effluent concentration history for combined capture of NO (■) and Hg0 (●) in the presence of CO at 175˚C; using (a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2, (c) CeO2/TiO2. C = effluent concentration; C0 = inlet concentration; CHg0 = 30-50 ppbv, CNO = CCO = 400 ppm; GHSV =10,200 h-1.

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Figure 6. In-situ FT-IR spectra of adsorbed CO and NO over 20 wt% MnOx/TiO2 catalyst, at room temperature and 200ºC.

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Figure 1. Effluent concentration history for NO adsorption in the absence of Hg0 using (■) 20 wt% MnOx/CeO2-TiO2, (●) CeO2-TiO2, and (▲)20 wt% MnOx/TiO2 materials at 175ºC. C = effluent NO concentration; C0 = inlet NO concentration; CNO= 400 ppm, balance N2; GHSV = 5000 h-1.

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Figure 2. Effluent concentration history for NO adsorption in the absence of Hg0 but in presence of CO using (■) 20 wt% MnOx/CeO2-TiO2, (●) CeO2-TiO2, and (▲)20 wt% MnOx/TiO2 materials at 175ºC. C = effluent NO concentration; C0 = inlet NO concentration, CNO= CCO = 400 ppm, balance N2; GHSV = 5000 h-1.

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Figure 3. Effect of CO on NO removal efficiency using 20 wt% MnOx/CeO2-TiO2 at 175ºC. After the 1st breakthrough, CO was added into gas flow. C = effluent NO concentration; C0 = inlet NO concentration; CNO= CCO = 400 ppm, balance N2; GHSV = 5000 h-1.

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(a) 20 wt% MnOx/CeO2-TiO2

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(b) 20 wt% MnOx/TiO2

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(c) CeO2-TiO2 Figure 4. Effluent concentration history for combined capture of NO (■) and Hg0 (●) at 175˚C; using (a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2, (c) CeO2/TiO2 . C = effluent concentration; C0 = inlet concentration; CHg0 = 30-50 ppbv, CNO= 400 ppm, balance N2; GHSV =10,200 h-1.

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(a) 20 wt% MnOx/CeO2-TiO2

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(b) 20 wt% MnOx/TiO2

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(c) CeO2-TiO2 Figure 5. Effluent concentration history for combined capture of NO (■) and Hg0 (●) in the presence of CO at 175˚C; using (a) 20 wt% MnOx/CeO2-TiO2, (b) 20 wt% MnOx/TiO2, (c) CeO2/TiO2. C = effluent concentration; C0 = inlet concentration; CHg0 = 30-50 ppbv, CNO = CCO = 400 ppm; GHSV =10,200 h-1.

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Figure 6. In-situ FT-IR spectra of adsorbed CO and NO over 20 wt% MnOx/TiO2 catalyst, at room temperature and 200ºC.

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Supplemental information:

0.016 0.014 0.012 Abs at 415 nm

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0.010 0.008 0.006 0.004 0.002 0.0

0.2

0.4

0.6

0.8

NOx Conc. (mg)

Figure S.1. Typical UV/Vis spectrometer calibration graph using KNO3 working solution

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Simultaneous Removal of Elemental Mercury and NO from Flue Gas

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