Elemental Mercury Capture from Syngas by Novel High-Temperature

Mar 24, 2015 - Haitao Zhao , Gang Yang , Xiang Gao , Chengheng Pang , Sam Kingman , Edward Lester , Tao Wu. Fuel 2016 181, 1089-1094 ...
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Elemental Mercury Capture from Syngas by Novel High-Temperature Sorbent Based on Pd−Ce Binary Metal Oxides Wenhui Hou, Jinsong Zhou,* Shulin You, Xiang Gao, and Zhongyang Luo State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, P. R. China ABSTRACT: CeO2 was doped into Pd/Al2O3 using an impregnation method to suppress the interference of H2S and to improve the ability to capture elemental mercury (Hg0) from syngas at high temperatures. As a result, the Hg0 capture performance was generally enhanced by the incorporation of Ce as a result of the consumption of H2S. The individual presence of H2 or CO resulted in the formation of Pd metal responsible for Hg0 capture and active ceria in reduced form for H2S removal, thus promoting the Hg0 removal. Meanwhile, H2S could react with CO to form COS, which competed with Hg0 for active sites, hence inhibiting Hg0 capture. The effect of HCl on Hg0 capture over PCA sorbents was found to be slightly promotional, because a small amount of Hg0 was oxidized to HgCl2. More importantly, the spent PCA sorbents could be easily reactivated, indicating that this material is a prospective candidate for Hg0 control from syngas. stability.14−16 In our previous study, we found that Pd/Al2O3 sorbents with a Pd loading value of 8% exhibited a high removal activity at 270 °C and investigated the effects of syngas conditions on Hg0 capture over Pd/Al2O3 sorbents. We also found that the presence of H2S could cause severe interference with Hg0 adsorption as a result of the consumption of Pd.17 As is well-known, cerium oxide has been shown to be an attractive option for removing H2S in highly reducing syngas and is considered to be a second-generation sorbent for hightemperature desulfurization.18,19 Furthermore, in strongly reducing gas at high temperature, the reduced form of ceria, CeOn (n < 2), is superior to CeO2 in H2S removal.20 CeO2 has also been proposed as a promising sorbent for mercury capture from syngas at 150 °C.21 Thus, we anticipate that the inhibitory effect of H2S on Hg0 capture could be mitigated by the incorporation of Ce into Pd/Al2O3, forming Pd−CeO2/Al2O3 mixed oxides. The combination of CeO2 and Pd might be very effective for mercury capture even in extremely adverse environments in the presence of H2S. However, no research on high-temperature Hg0 capture from syngas over Pd−Ce/Al mixed oxides has been reported to date. In this work, Ce was incorporated into Pd/Al2O3 to suppress the interference of H2S on Hg0 capture at high temperatures. In addition, the possible mechanisms involved in Hg0 capture are discussed. The optimum doping content of Ce in the sorbents was explored, and the effects of syngas components on the performance of the sorbent were also investigated. Finally, the regenerability of the sorbent was evaluated over several regeneration cycles. The aim of this work was to develop a competitive and effective sorbent that is favorable for Hg0 capture from syngas at high temperatures.

1. INTRODUCTION Mercury is a global pollutant that attracts considerable attention because of its long-range transport across intercontinental distances, its ability to bioaccumulate in ecosystems and the food chain, its extreme persistence in the atmosphere once anthropogenically introduced, and its significant negative impacts on human and environmental health.1,2 In January 2013, more than 140 countries adopted the world’s first legally binding treaty, the Minamata Convention on Mercury, to protect public health and the environment by reducing worldwide mercury emissions.3 Over the past decade, coal gasification has gained significant interest as a source of electric power generation because of its proven mitigating effects on the environment relative to those of other combustion technologies. However, Hg is normally released in a gaseous form at high temperatures during the gasification process. Hg0 is regarded as the dominant mercury species in the syngas generated from gasification, accounting for about 50−97% of the total, because Hg0 is difficult to oxidize in reducing environments by gas-phase reactions.4 In addition, the capture of Hg0 is relatively difficult as compared to that of oxidized (Hg2+) or particulate-bound (Hgp) mercury because of its low solubility in water.5,6 Recently, the capture of Hg0 from coal combustion flue gas has been widely investigated. Unfortunately, there is a lack of reports on the removal of Hg0 from syngas. Compared with that in combustion flue gas, Hg0 capture in syngas is desired at high temperature (>200 °C) to provide improvements in the overall thermal energy efficiency for power generation gasification systems. A number of sorbents, particularly activated carbons (ACs), have exhibited high mercury removal efficiencies and can be used to capture Hg0. However, these sorbents cannot remain active at elevated temperatures in coal gasification systems.7−9 Fortunately, Pd has been identified as the most promising candidate for Hg0 capture because of its high amalgamation enthalpy of all metals and regeneration potential.10−13 For industrial applications, Pd is usually supported on Al2O3, which can provide high mechanical strength and high thermal © 2015 American Chemical Society

Received: Revised: Accepted: Published: 3678

November 10, 2014 January 31, 2015 March 24, 2015 March 24, 2015 DOI: 10.1021/ie504447j Ind. Eng. Chem. Res. 2015, 54, 3678−3684

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2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. Pd−Ce/Al bimetallic sorbents with different CeO 2 loadings were synthesized by a coimpregnation method using palladium nitrate, cerium nitrate aqueous solution, and γ-Al2O3 particles. After being stirred for 0.5 h, the mixture was dried at 100 °C for 12 h and calcined in air at 500 °C for 2 h. Finally, all samples were pressed and then sieved at 80 mesh. The loading (wt %) of Pd in all of the samples was 8%, which was reported in our previous research to be the optimum value for Hg0 capture.17 The sorbents are denoted as PCxA, in which P represents Pd; Ce represents CeO2; and x, ranging from 0.2 to 1.5, represents the CeO2/ Al2O3 mass ratio. To evaluate the Hg0 capture performance of the modified sorbents, Pd (8%)/Al2O3 (PA) and Ce/Al2O3 (CA) with the optimum CeO2/Al2O3 mass ratio of 1.0 were also prepared by the same method for comparison. 2.2. Sorbent Characterization. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractionmeter (X’Pert PRO system) with Cu Kα radiation (λ = 1.541 Å). The Brunauer−Emmett−Teller (BET) surface area was determined on a gas sorption analyzer (Micromeritics 2020 instrument). All samples were degassed at 200 °C before measurements under a vacuum. Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker Vector 22 FTIR spectrometer over the range of 400−4000 cm−1 with a resolution of 4 cm−1. Temperature-programmed reduction in H2 (H2 TPR) was carried out using a chemisorption instrument (Micromeritics, Autochem 2920). The TPR profile was obtained by passing a 10% H2/He flow (30 mL/min) through the samples (0.1 g) from room temperature to 900 °C at a rate of 10 °C/min. The sample was preheated in an Ar flow at 200 °C for 2 h before measurements. 2.3. Mercury Removal Measurement. The laboratoryscale experimental system (Figure 1) used for Hg0 capture was

by a temperature-controlled tubular furnace. The adsorption temperature was 270 °C, which was reported in our previous research to be the optimum temperature for Hg0 removal.17 The sorbent layer was supported by quartz wool, which was demonstrated to have no ability for mercury conversion and capture.22 The total gas flow rate was kept at 1.2 L·min−1, and a gas hourly space velocity (GHSV) of approximately 120000 h−1 (except as stated otherwise) was obtained. The temperatures of all lines through which the gas flow containing gas-phase Hg0 passed were controlled at 120 °C to prevent any possible condensation of mercury. A bypass line was used to ensure the stability of the inlet Hg concentration at the beginning of the tests. When the fluctuation in Hg0 concentration was maintained within ±5% for more than 30 min, the syngas flow was switched to the reactor line. The experimental conditions for three sets of experiments are summarized in Table 1. Set I experiments were performed to Table 1. Experimental Conditions set I

sample(s)

II

PA, PC0.2A, PC0.5A, PC1.0A, PC1.5A PC1.0A

III

PC1.0A

gas components (1.2 L·min−1)

temperature (°C)

N2 + 0/100 ppm of H2S

270

N2 + individual syngas components (H2/CO/H2S/HCl), SSG (dry), SSG (8% H2O) 30% H2, 20% CO, 100 ppm of H2S, 1 ppm of HCl

270 270

evaluate the sulfur-tolerance performance of the sorbents with the addition of CeO2 and to determine the optimal CeO2/ Al2O3 mass ratio. Therefore, only 100 ppm of H2S was introduced into the gas flow at 270 °C. The experiments in set II were conducted to investigate the effects of gas conditions on Hg0 capture over the optimal PC1.0A sorbents by adding individual syngas components to the N2 flow. Temperatureprogrammed desorption (TPD) experiments at temperatures ranging from 50 to 650 °C were performed in the same quartz system on some selected samples after the Hg0 adsorption experiments with or without the addition of 1 ppm of HCl. In set III, the regeneration performance of the PC1.0A sorbent was studied at high GHSVs of 120000 and 180000 h−1 under the simulated syngas. It should be noted that little Hg2+ was observed when the gas stream was flowed through Pd−Ce/Al bimetallic sorbents under the present conditions. Therefore, the reduced amount of Hg0 was considered to be captured by the sorbent. The Hg0 removal efficiency (η) over Pd−Ce/Al bimetallic sorbents was calculated according to the equation ⎛ [Hg T]out ⎞ ⎟ × 100% Ecap (%) = ⎜1 − [Hg T]in ⎠ ⎝

Figure 1. Fixed-bed reaction system. 21

T

similar to that described in our previous research. It consisted of a simulated syngas feed system, a Hg0 vapor-generating device, a vertical down-flow quartz reactor (10-mm i.d.), and a mercury continuous emissions monitor (MS-1A/DM-6B, Nippon Instrument Corporation, Osaka, Japan). A small flow of N2 (200 mL) was flowed through the permeation tube in the Hg0 vapor-generating device to generate a feed of Hg0 with a constant concentration (∼70 μg/m3). In each test, 0.15 g of the sample was positioned inside the quartz tube, which was heated

(1)

T

where [Hg ]in and [Hg ]out represent the total amounts of input and output Hg0, respectively, which were calculated by integrating the real-time Hg0 concentration at the inlet and outlet, respectively, of the reactor over 2 h. 2.4. Sorbent Regeneration. After the mercury capture measurements under simulated syngas, the sorbents were heated to 400 °C and flushed with pure N2 carrier gas for 1 h in the same experimental device for regeneration. The Hg0 captured on the surface of the sorbent was considered to be 3679

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inactive at this high temperature. This result is in accordance with our earlier research, in which only negligible Hg0 was captured on the Ce-based sorbent when the reaction temperature exceeded 200 °C.21 No obvious fluctuation in Ecap was observed for PCxA sorbents with CeO2/Al2O3 mass ratios ranging from 0.2 to 1.0 under pure N2 condition, and a slight decrease of Ecap was obtained as the CeO2/Al2O3 mass ratio was further increased to 1.5. As displayed in Table 2, high

released completely at this temperature through the decomposition of the Pd−Hg amalgam. It should be noted that regeneration at higher temperatures can accelerate the release of captured Hg0; however, it can also negatively affect the morphology of the Pd supported on Al2O3.

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. XRD patterns of the synthesized samples are presented in Figure 2. For PA, the

Table 2. Physical Properties of Synthesized Samples sample x x x x x

= = = = =

0 0.2 0.5 1.0 1.5

surface area (m2/g)

pore volume (m3/g)

average pore size (nm)

199.36 116.95 113.02 119.20 86.02

0.391 0.294 0.187 0.211 0.188

7.836 7.078 6.607 7.093 8.726

loadings of CeO2 (x > 1.0) resulted in an obvious reduction in the surface area of the PCxA sorbent, hence causing a slight decrease of Ecap. It was also concluded that the surface area of the sorbents was at least partly responsible for the Hg0 removal performance. It can been seen that addition of CeO2 enhanced the removal of Hg0; for example, an approximately 10.1% increase in Hg0 removal efficiency over PC0.5A was obtained as compared to that over PA under H2S plus N2 atmosphere. PCxA sorbents with higher CeO2 loadings showed stronger sulfur-tolerance performance until the CeO2/Al2O3 mass ratio reached 1.0, with Ecap being greater than 30% at 270 °C. PC1.0A showed the highest Hg0 removal efficiency, which was likely due to an excellent sulfur removal performance resulting from a relatively higher surface area.26 Further increasing the CeO2/Al2O3 mass ratio to 1.5 also resulted in a slight decrease of Ecap. It is worth mentioning that this test merely aimed to investigate the sulfurtolerance performance of the PCxA sorbents; therefore, only H2S was introduced into the gas stream. The prohibitive effect of H2S could be alleviated by adding H2 to the gas flow, as described in section 3.4. 3.3. Mechanism for Enhanced Hg0 Capture Activity. In previous research, H2S was found to have a significantly prohibitive effect on Hg0 capture as a result of the consumption of palladium species, and the pathway was proposed to be17

Figure 2. XRD profiles of (a) PA, (b) PC0.2A, (c) PC0.5A, (d) PC1.0A, (e) PC1.5A, (f) CA.

diffraction line of PdO was detected, implying that PdO crystallites were formed. In all PCxA or CA samples, crystalphase CeO2 peaks were also observed, and the CeO2 peaks became more intense and sharper as the CeO2/Al2O3 mass ratio was increased from 0.2 to 1.5. No diffraction peaks assigned to the PdO or Pd crystal phase were observed, suggesting that Pd species (PdO or Pd) existed in amorphous or highly dispersed form. These results are in good accordance with those reported in the literature.23 Usually, palladium on the surface of Al2O3 exists in at least two different states, and Pd metal is considered to be responsible for Hg0 capture, especially at high temperatures.24,25 3.2. Activity of PCxA Sorbents. A comparison of the Hg0 removal efficiencies over various PCxA sorbents at 270 °C with and without H2S injection is presented in Figure 3. For CA sorbent, less than 10% Ecap at 270 °C under each set of conditions was observed; thus, this sorbet was considered to be

PdS + H 2S → PdS + H 2

(2)

PdO + H 2S → PdS + H 2O

(3)

0

Therefore, the Hg capture activity could be significantly improved if H2S were removed by the Ce species. In this work, the H2S removal activity of PCxA sorbents was also tested to demonstrate the better tolerance of PC1.0A to H2S. Figure 4 shows the outlet H2S concentrations over various PCxA sorbents during the activity tests. When the gas flow was switched to the reactor line at approximately 20 min, H2S was captured by the sorbent for different levels. Increasing the CeO2/Al2O3 mass ratio from 0.2 to 1.0 was found to be favorable for H2S removal. The lowest H2S concentration dropped from 52 ppm for PC0.2A to 1 ppm for PC1.0A. With a further increase of the CeO2/Al2O3 mass ratio, Ecap exhibited a minor decrease. As expected, PC1.0A provided the best H2S removal performance, which could also be ascribed to the relatively higher surface area. This result indicates that the Hg0

Figure 3. Mercury removal efficiencies over PCxA sorbents under N2 and N2 + H2S atmospheres. 3680

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Figure 4. H2S removal curves of PCxA sorbents at 270 °C under N2 atmosphere.

Figure 6. Effect of gas conditions on Hg0 capture over PC1.0A at 270 °C.

capture activity was at least partly impacted by the H2S removal performance of the sorbent. In this work, FTIR experiments were performed to gain insights into the nature of the functional groups formed on the sorbent surface. Figure 5 shows the FTIR spectra measured on

2CeO2 + H 2S + H 2 = Ce2O2 S + H 2O

(4)

2CeO2 + H 2 = Ce2O3 + H 2O

(5)

Ce2O3 + H 2S = Ce2O2 S + H 2O

(6)

In addition, the PdO species were reduced to elemental Pd by H2, replenishing the active Pd available to capture Hg0 under reducing conditions. Therefore, more Hg0 can be captured by forming a Pd−Hg amalgam with the addition of H2. In this study, the reduction of PdO species with H2 was demonstrated by H2 TPR experiments. As shown in Figure 7,

Figure 5. FTIR spectra of (a) fresh PA, (b) fresh PCA, (c) H2S-treated PA, and (d) H2S-treated PCA.

the PCA sorbent before and after pretreatment by 0.1% H2S balanced in N2 at 270 °C for 2 h. The peak at 1385 cm−1 was observed for the PCA sorbent pretreated with H2S, and it was assigned to lattice stretches of Ce 2 O 2 S.27 The result demonstrates that a fraction of the H2S was consumed by the CeO2 doped in the sorbents. Therefore, the addition of CeO2 enhanced the removal of Hg0. 3.4. Effect of H2. As is well-known, H2 is one of most important species in syngas; hence, evaluating its effect on Hg0 removal performance of PCxA sorbents is very necessary. In this study, H2 was found to enhance Hg0 capture. As shown in Figure 6, when 10% H2 balanced in N2 was passed through the PC1.0A sorbent, Ecap was found to be 90.5%, which was higher than the 76.0% Hg0 removal under a pure N2 atmosphere. Further increasing the H2 concentration to 30% resulted in an even higher Ecap value of 94.3%. This excellent performance of Hg0 removal was attributed to Hg amalgamation on the surface of the PC1.0A sorbent.28 It should be noted that Ecap was also observed to be 88.7% even when 100 ppm of H2S was added to syngas containing 30% H2. It is very likely that the reduced form of ceria, CeOn (n < 2), was more active than CeO2 in H2S removal, improving the ability of palladium to resist H2S poisoning.20 The possible reactions occurring in the presence of H2 can be summarized as follows:

Figure 7. H2 TPR profiles of PC1.0A.

the peaks in the temperature range from 300 to 450 °C were considered to represent the reduction of surface oxygen on the surface of PC1.0A, whereas the peaks above 700 °C can be attributed to the reduction of bulk Ce. The peak was found to shift to a lower temperature with the addition of Ce, which could be due to the interaction between CeO2 and PdO species.29 A negative peak at about 85 °C was ascribed to the decomposition of PdHx and the desorption of weakly bonded hydrogen.30 A peak located at about 66 °C for the PC1.0A sorbent was obtained owing to the shift of PdO to Pd. These results demonstrated that PdO species could be easily reduced to metallic Pd under the experimental conditions. This is in line with previous research that the reduction of PdO can take place in a hydrogen atmosphere, and it is believed to proceed through the reactions31 2PdO + H 2 → Pd 2O + H 2O 3681

(7) DOI: 10.1021/ie504447j Ind. Eng. Chem. Res. 2015, 54, 3678−3684

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Industrial & Engineering Chemistry Research Pd 2O + H 2 → 2Pd + H 2O

HCl can promote Hg0 oxidation over metal oxides, particularly CeO2, as a result of surface oxygen, even without the aid of gasphase O2.1,35−37 Therefore, HCl enhanced the removal of Hg0 over the PCA sorbents in the absence of O2. The results also imply that it is difficult for PCA sorbent to capture Hg0 at temperatures high than 280 °C. Nevertheless, it is important to note that formation of Pd−Hg amalgam was still the primary mechanism responsible for Hg0 capture because HgCl2 was considered completely desorbed at 300 °C and the mercury released at higher temperatures came from the decomposition of Pd−Hg.38 As summarized below, several reactions are likely to be responsible for the promotional effect of HCl39,40

(8)

3.5. Effect of CO. A promotional effect of CO on Hg0 capture was also found over PCA sorbent. The presence of 5% CO caused an increase in Ecap from 76.0% under a pure N2 atmosphere to 83.5%, whereas 20% CO further improved Ecap to 86.5%. Similarly, the promotional effect due to the reduction of PdO species can also be demonstrated by the CO TPR experiment, and the results, which were in agreement with the previous findings, are not discussed in this study.32 It was worth mentioning that the combination of 20% CO and 100 ppm of H2S resulted in an Ecap value of only 80.5%, which can be ascribed to two causes: (1) H2S consumed the active component Pd for Hg0 capture, thus reducing the Ecap value over PCA sorbent, and (2) CO reacted with H2S to form COS, which competed with Hg0 for active sites and was adsorbed strongly on the surface of palladium, hence inhibiting Hg0 capture. The reactions involved in Hg0 capture can be summarized as follows:33 PdO + CO → Pd + CO2

(9)

H 2S + CO → H 2 + COS

(10)

Hg(g) → Hg(ad)

(11)

2HCl + O* → 2Cl* + H 2O

(12)

Cl* + Hg(ad) → HgCl

(13)

HgCl + Cl* → HgCl2

(14)

where O* and Cl* represents the surface oxygen and active chlorine species, respectively, on the surface of PCA sorbents. 3.7. Effect of H2O. H2O is one of the main components in syngas and has been found to reduce the activities of various metal oxide sorbents and/or catalysts attributed to competitive adsorption.41 The results in Figure 4 show that H2O inhibits Hg0 capture over PCA sorbents. It can be seen that Ecap decreased by 9.1% when the dry flow was switched to 8% H2O in simulated syngas. H2O has been reported to prohibit the adsorption of effective flue gas species such as NOx and HCl, which are not present or crucial for Hg0 capture under gasification conditions. Therefore, the effect of H2O on Ecap over PCA sorbents was slight, indicating that the PCA sorbent is a good candidate for Hg0 capture from syngas. 3.8. Regeneration of Spent PCA. In our previous study, Hg0 could not be captured at 400 °C, which was appropriate for sorbent regeneration. In this work, several cycles of capture− regeneration at high GHSVs of 120000 and 180000 h−1 were carried out to evaluate the regenerability of PCA sorbents. As shown in Figure 9, the Ecap value of the sorbent decreased slightly after the first regeneration, which was attributed to incomplete Hg release and degradation of palladium at 400 °C.42 However, the Ecapvalue remained constant thereafter over several regeneration cycles. It is worth mentioning that this decrease of Ecap over PCA sorbent was minor compared with that in previous investigations on gold-loaded AC sorbents. In

3.6. Effect of HCl. In this study, the effect of HCl on Hg0 capture over PCA sorbents was found to be slightly promotional. Ecap was observed to increase from 76.0% in pure N2 to 81.5% in the presence of 1 ppm of HCl. However, no obvious increase in Ecap was obtained by further increasing the HCl concentration to 10 ppm, indicating that no more HCl was needed for Hg0 removal and that the promotional effect of HCl was slight and limited under the experimental conditions. To identify the mercury species captured on the surface of the PCA sorbents and evaluate their desorption characteristics, TPD experiments were carried out under pure N2 conditions at a flow rate of 1 L/min. It can be seen from Figure 8 that PCA

Figure 8. Hg concentration profiles during a TPD experiment.

sorbent after adsorption under a HCl atmosphere started to significantly release Hg0 at approximately 200 °C and peaked at 300 °C, whereas the line corresponding to the sorbent used under the N2 conditions started at 280 °C and was centered at approximately 390 °C, indicating that the mercury species retained in the two cases were different. The mercury desorption peak at high temperature was attributed to the decomposition of the Pd−Hg amalgam, and another at low temperature appeared to be in accordance with that of HgCl2,34 indicating that some HgCl2 was formed and that the formation of Pd−Hg amalgam was not the unique mechanism involved in Hg0 capture in the presence of HCl. It has been reported that

Figure 9. Mercury capture efficiency over PCA sorbent during five cycles of capture−regeneration. 3682

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(8) Wu, S.; Azhar Uddin, M.; Sasaoka, E. Characteristics of the removal of mercury vapor in coal derived fuel gas over iron oxide sorbents. Fuel 2006, 85, 213. (9) He, P.; Wu, J.; Jiang, X. M.; Pan, W. G.; Ren, J. X. Effect of SO3 on elemental mercury adsorption on a carbonaceous surface. Appl. Surf. Sci. 2012, 258, 8853. (10) Granite, E. J.; Pennline, H. W. Method for high temperature mercury capture from gas streams. U.S. Patent 7,033,419 B1, 2006. (11) Pennline, H. W.; Granite, E. J. Sorbents for gasification processes. In Mercury Removal from Coal-Derived Gases; Wiley-VCH: Weinheim, Germany, 2014; pp 357−374. (12) Uffalussy, K. J.; Miller, J. B.; Howard, B. H.; Stanko, D. C.; Yin, C. R.; Granite, E. J. Arsenic Adsorption on Copper−Palladium Alloy Films. Ind. Eng. Chem. Res. 2014, 53, 7821. (13) Munson, C.; Indrakanti, P.; Ramezan, M.; Granite, E.; Tennant, J. Evaluation of palladium-based sorbents for trace mercury removal in electricity generation. Int. J. Clean Coal Energy 2014, 3, 65. (14) Rupp, E. C.; Granite, E. J.; Stanko, D. C. Method for Detection of Trace Metal and Metalloid Contaminants in Coal-Generated Fuel Gas Using Gas Chromatography/Ion Trap Mass Spectrometry. Anal. Chem. 2010, 82, 6315. (15) Poulston, S.; Granite, E. J.; Pennline, H. W.; Hamilton, H.; Smith, A. Palladium based sorbents for high temperature arsine removal from fuel gas. Fuel 2011, 90, 3118. (16) Rupp, E. C.; Granite, E. J.; Stanko, D. C. Laboratory scale studies of Pd/γ-Al2O3 sorbents for the removal of trace contaminants from coal-derived fuel gas at elevated temperatures. Fuel 2013, 108, 131. (17) Hou, W. H.; Zhou, J. S.; Yu, C. J.; You, S. L.; Gao, X.; Luo, Z. Y. Pd/Al2O3 Sorbents for Elemental Mercury Capture at High Temperatures in Syngas. Ind. Eng. Chem. Res. 2014, 53, 9909. (18) Li, R.; Krcha, M. D.; Janik, M. J.; Roy, A. D.; Dooley, K. M. Ce− Mn Oxides for High-Temperature Gasifier Effluent Desulfurization. Energy Fuels 2012, 26, 6765. (19) Guo, B.; Chang, L. P.; Xie, K. C. Desulfurization Behavior of Cerium−Iron Mixed Metal Oxide Sorbent in Hot Coal Gas. Ind. Eng. Chem. Res. 2014, 53, 8874. (20) Cheah, S.; Carpenter, D. L.; Magrini-Bair, K. A. Review of Midto High-Temperature Sulfur Sorbents for Desulfurization of Biomassand Coal-Derived Syngas. Energy Fuels 2009, 23, 5291. (21) Zhou, J. S.; Hou, W. H.; Qi, P.; Gao, X.; Luo, Z. Y.; Cen, K. F. CeO2−TiO2 Sorbents for the Removal of Elemental Mercury from Syngas. Environ. Sci. Technol. 2013, 47, 10056. (22) Li, J. F.; Yan, N. Q.; Qu, Z.; Qiao, S. H.; Yang, S. J.; Guo, Y. F.; Liu, P.; Jia, J. P. Catalytic Oxidation of Elemental Mercury over the Modified Catalyst Mn/α-Al2O3 at Lower Temperatures. Environ. Sci. Technol. 2010, 44, 426. (23) Feio, L.; Hori, C. E.; Damyanova, S.; Noronha, F. B.; Cassinelli, W. H.; Marques, C.; Bueno, J. The effect of ceria content on the properties of Pd/CeO2/Al2O3 catalysts for steam reforming of methane. Appl. Catal. A: Gen. 2007, 316, 107. (24) Ivanova, A. S.; Slavinskaya, E. M.; Gulyaev, R. V.; Zaikovskii, V. I.; Stonkus, O. A.; Danilova, I. G.; Plyasova, L. M.; Polukhina, I. A.; Boronin, A. I. Metal−support interactions in Pt/Al2O3 and Pd/Al2O3 catalysts for CO oxidation. Appl. Catal. B 2010, 97, 57. (25) Baltrus, J. P.; Granite, E. J.; Pennline, H. W.; Stanko, D.; Hamilton, H.; Rowsell, L.; Poulston, S.; Smith, A.; Chu, W. Surface characterization of palladium−alumina sorbents for high-temperature capture of mercury and arsenic from fuel gas. Fuel 2010, 89, 1323. (26) Baltrus, J. P.; Granite, E. J.; Stanko, D. C.; Pennline, H. W. Surface characterization of Pd/Al2O3 sorbents for mercury capture from fuel gas. Main Group Chem. 2008, 7, 217. (27) Luo, T.; Vohs, J. M.; Gorte, R. J. An examination of sulfur poisoning on Pd/ceria catalysts. J. Catal. 2002, 210, 397. (28) Poulston, S.; Hyde, T. I.; Hamilton, H.; Mathon, O.; Prestipino, C.; Sankar, G.; Smith, A. EXAFS and XRD characterization of palladium sorbents for high temperature mercury capture from fuel gas. Phys. Chem. Chem. Phys. 2010, 12, 484.

summary, the mercury capture capacity can be maintained at a relatively high level over several cycles of capture−regeneration, which makes the PCA sorbent economically viable for Hg0 capture at elevated temperatures, even though it is more expensive than AC.

4. CONCLUSIONS CeO2 was doped into Pd/Al2O3 using an impregnation method to suppress the interference of H2S and to improve the ability to capture elemental mercury (Hg0) from syngas at high temperatures. As a result, the Hg0 capture performance was generally enhanced by the incorporation of Ce as a result of the consumption of H2S. Pd−Ce/Al sorbents (PCA) with a CeO2/ Al2O3 mass ratio of 1.0 showed strong resistance to H2S, which was ascribed to the excellent sulfur removal performance. The combination of CeO2 and Pd resulted in a significant synergy for Hg0 capture from syngas. The addition of 30% H2 or 20% CO to the N2 flow resulted in a significant promotional effect on Hg0 capture as a result of the formation of Pd metal species and reduced ceria. The catalytic oxidation of Hg0 in the presence of HCl enhanced the Hg0 capture. More importantly, the mercury capture capacity could be maintained at a relatively high level over several cycles of capture−regeneration, which makes the PCA sorbent economically viable for Hg0 capture at elevated temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87952041. Fax: +86-571-87951616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Fund of China (No. 51176171) and the National Science Foundation for Distinguished Young Scholars of China (No. 51125025).

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REFERENCES

(1) Li, H. L.; Wu, C. Y.; Li, Y.; Zhang, J. Y. CeO2−TiO2 Catalysts for Catalytic Oxidation of Elemental Mercury in Low-Rank Coal Combustion Flue Gas. Environ. Sci. Technol. 2011, 45, 7394. (2) Wang, S.; Zhang, L.; Zhao, B.; Meng, Y.; Hao, J. Mitigation Potential of Mercury Emissions from Coal-Fired Power Plants in China. Energy Fuels 2012, 26, 4635. (3) Rodriguez-Perez, J.; Lopez-Anton, M. A.; Diaz-Somoano, M.; Garcia, R.; Martinez-Tarazona, M. R. Regenerable sorbents for mercury capture in simulated coal combustion flue gas. J. Hazard. Mater. 2013, 260, 869. (4) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89. (5) Lim, D. H.; Wilcox, J. Heterogeneous Mercury Oxidation on Au(111) from First Principles. Environ. Sci. Technol. 2013, 47, 8515. (6) Liu, M. M.; Hou, L. A.; Xi, B. D.; Zhao, Y.; Xia, X. F. Synthesis, characterization, and mercury adsorption properties of hybrid mesoporous aluminosilicate sieve prepared with fly ash. Appl. Surf. Sci. 2013, 273, 706. (7) Baltrus, J. P.; Granite, E. J.; Rupp, E. C.; Stanko, D. C.; Howard, B.; Pennline, H. W. Effect of palladium dispersion on the capture of toxic components from fuel gas by palladium-alumina sorbents. Fuel 2011, 90, 1992. 3683

DOI: 10.1021/ie504447j Ind. Eng. Chem. Res. 2015, 54, 3678−3684

Article

Industrial & Engineering Chemistry Research (29) Yue, L.; He, C.; Zhang, X. Y.; Li, P.; Wang, Z.; Wang, H. L.; Hao, Z. P. Catalytic behavior and reaction routes of MEK oxidation over Pd/ZSM-5 and Pd-Ce/ZSM-5 catalysts. J. Hazard. Mater. 2013, 244, 613. (30) Sangeetha, P.; Shanthi, K.; Rao, K.; Viswanathan, B.; Selvam, P. Hydrogenation of nitrobenzene over palladium-supported catalysts Effect of support. Appl. Catal. A: Gen. 2009, 353, 160. (31) Padmasri, A. H.; Venugopal, A.; Krishnamurthy, J.; Rao, K.; Rao, P. K. Novel calcined Mg−Cr hydrotalcite supported Pd catalysts for the hydrogenolysis of CCl2F2. J. Mol. Catal. A: Chem. 2002, 181, 73. (32) Satsuma, A.; Osaki, K.; Yanagihara, M.; Ohyama, J.; Shimizu, K. Activity controlling factors for low-temperature oxidation of CO over supported Pd catalysts. Appl. Catal. B 2013, 132, 511. (33) Rupp, E. C.; Granite, E. J.; Stanko, D. C. Catalytic formation of carbonyl sulfide during warm gas clean-up of simulated coal-derived fuel gas with Pd/γ-Al2O3 sorbents. Fuel 2012, 92, 211. (34) Uddin, M. A.; Ozaki, M.; Sasaoka, E.; Wu, S. TemperatureProgrammed Decomposition Desorption of Mercury Species over Activated Carbon Sorbents for Mercury Removal from Coal-Derived Fuel Gas. Energy Fuels 2009, 23, 4710. (35) Poulston, S.; Granite, E. J.; Pennline, H. W.; Myers, C. R.; Stanko, D. P.; Hamilton, H.; Rowsell, L.; Smith, A.; Ilkenhans, T.; Chu, W. Metal sorbents for high temperature mercury capture from fuel gas. Fuel 2007, 86, 2201. (36) A, A.; Presto; Granite, E. J. Noble metal catalysts for mercury oxidation in utility flue gas. Platinum Met. Rev. 2008, 52, 144. (37) Granite, E. J.; Myers, C. R.; King, W. P.; Stanko, D. C.; Pennline, H. W. Sorbents for Mercury Capture from Fuel Gas with Application to Gasification Systems. Ind. Eng. Chem. Res. 2006, 45, 4844. (38) Ozaki, M.; Uddin, M. A.; Sasaoka, E.; Wu, S. Temperature programmed decomposition desorption of the mercury species over spent iron-based sorbents for mercury removal from coal derived fuel gas. Fuel 2008, 87, 3610. (39) Presto, A. A.; Granite, E. J.; Karash, A.; Hargis, R. A.; O’Dowd, W. J.; Pennline, H. W. A Kinetic Approach to the Catalytic Oxidation of Mercury in Flue Gas. Energy Fuels 2006, 20, 1941. (40) Presto, A. A.; Granite, E. J. Survey of Catalysts for Oxidation of Mercury in Flue Gas. Environ. Sci. Technol. 2006, 40, 5601. (41) Li, H. L.; Wu, C. Y.; Li, Y.; Zhang, J. Y. Superior activity of MnOx−CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures. Appl. Catal. B 2012, 111, 381. (42) Ballestero, D.; Gomez-Gimenez, C.; Garcia-Diez, E.; Juan, R.; Rubio, B.; Izquierdo, M. T. Influence of temperature and regeneration cycles on Hg capture and efficiency by structured Au/C regenerable sorbents. J. Hazard. Mater. 2013, 260, 247.

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