CeO2–TiO2 Sorbents for the Removal of Elemental Mercury from

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CeO2−TiO2 Sorbents for the Removal of Elemental Mercury from Syngas Jinsong Zhou,* Wenhui Hou, Pan Qi, Xiang Gao, Zhongyang Luo, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: A series of CeO2−TiO2 (CeTi) sorbents with different CeO2/TiO2 mass ratios were prepared by an impregnation method and employed to remove elemental mercury (Hg0) in simulated syngas. The CeTi sorbents with a CeO2/TiO2 mass ratio of 0.2 exhibited superior Hg0 removal efficiency from 80 to 150 °C, which could be ascribed to the greater amount of surface chemisorbed oxygen resulted from Ce3+ on the sample surface. H2S was the most effective syngas component responsible for Hg0 removal. The use of 400 ppm H2S resulted in 98% Hg0 removal efficiency under the experimental conditions. H2 and CO had a negligible effect on the efficiency of Hg removal. In the presence of H2S, a prohibitive effect of HCl and NH3 on Hg0 removal was observed because of the consumption of the surface oxygen. Water vapor also inhibited Hg0 removal due to competitive adsorption with H2S. Hg0 removal over CeTi sorbents was proposed to follow the Eley−Rideal mechanism, in which active surface sulfur reacts with gas-phase Hg0. This large oxygen storage capacity of CeTi sorbents is quite favorable to H2S catalytic oxidation and Hg0 emission control in an extremely reducing environment, such as when there is a deficiency of O2.

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INTRODUCTION Mercury is a pollutant of concern because of its toxicity, volatility, persistence, and bioaccumulation in the environment.1 Mercury circulates in the atmosphere for up to a year, and hence can be widely dispersed and transported thousands of kilometers.2 Coal-fired power plants are the major anthropogenic mercury emission sources in China and the U.S. because of the huge coal consumption for power generation.3,4 The Ministry of Environmental Protection of the People’s Republic of China promulgated new emission standard of air pollutants for power plants, which specified a threshold of 0.03 mg/m−3 for mercury and its compounds after January 1, 2015.5 Due to its toxicity, the U.S. Congress included mercury and its compounds in the 1990 Clean Air Act Amendments as a hazardous air pollutant.6 In December 2011, final standards were issued for limiting mercury, acid gases, and other toxic species from coal-fired power plants under the U.S. Environmental Protection Agency’s (EPA’s) Mercury and Air Toxics Standards (MATS) ruling.7 Recently, coal gasification has been gaining popularity as a source of power generation due to the ready global availability of coal and its mitigated environmental effects relative to other combustion technologies.8 Fuel gas generated from coal gasification also contains Hg0. It has been reported that a reducing environment is not favorable for Hg oxidation via gasphase reactions alone and that the ratio of the Hg0 in the mercury compounds evolved from coal by gasification is larger than that in the combustion of the coal.1,9,10 To date, most of © 2013 American Chemical Society

the research activities have focused on the removal and conversion of Hg0 from coal combustion flue gas. In contrast, there is a lack of information available on the capture of Hg0 under gasification conditions.11 It has been reported that activated carbon, particularly activated carbon impregnated with chlorine, sulfur, and iodine, showed a high mercury removal efficiency, but it is expensive for industrial applications.12,13 Along with mercury, another byproduct produced during gasification is H2S, which has received more attention. Iron-, titanium-, chromium-, and vanadium-based catalysts were reported to have high potentials in selective oxidation of H2S to elemental sulfur at relatively low temperatures, which is believed to proceed through the following overall equation:14,15 H 2S +

1 1 O2 → Sn + H 2O 2 n

(1)

It should be noted that Hg0 can be oxidized by sulfur to form HgS. These observations suggest that Hg0 in coal-derived fuel gas may be removed efficiently based on the possible interactions among H2S, mercury, and metal oxides.16 Furthermore, the used sorbent waste needs no further treatment because HgS is a stable form of mercury.17,18 Received: Revised: Accepted: Published: 10056

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Corporation, Japan) was employed to measure the Hg0 and Hg2+ concentrations. Three sets of experiments were conducted, and the experimental conditions and compositions of the simulated syngas are summarized in Table S1 (Supporting Information). Set I experiments aimed at finding the optimal CeO2/TiO2 ratio and operating temperature. Because hot-gas cleanup continues to present many challenges such as operation and stability issues, researchers have more recently focused efforts on warm-gas cleanup, with temperatures ranging from 150 to 370 °C.27 In this work, the mercury removal activities over CeTi sorbents with different CeO2/TiO2 ratios were evaluated under simulated syngas (30% H2, 20% CO, 400 ppm H2S, 10 ppm HCl, 100 ppm NH3, and approximately 50 μg/m−3 Hg0) at reaction temperatures from 30 to 250 °C. In Set II, the effects of individual gas components on Hg0 removal and the reaction pathways were studied. The experiments were conducted on Ce0.2Ti in the presence of individual flue gases (balanced in N2 or H2S plus N2) at 150 °C. Set III experiments were designed to identify the possible mechanism involved in Hg0 removal using H2S pretreated Ce0.2Ti (400 ppm H2S balanced in N2 passed through Ce0.2Ti at 150 °C for 10 min; then, the sample was flushed by a pure N2 gas flow at the same temperature for 30 min). At the beginning of the tests for mercury removal activity, the gas stream bypassed the reactor and the inlet gas was sampled to ensure a stable Hg0 feed concentration (Hg0in). After the system had established stable and consistent mercury feeding, the Hg0-containing syngas was added to the reactor, and the mercury monitor measured the mercury concentration in the outlet flue gas. It should be noted that no obvious Hg2+ was observed in the gas flow downstream of the CeTi sorbent. Therefore, for our reporting of the results, the loss of Hg0 was considered to occur through adsorption, and the Hg0 removal efficiency (η) over CeTi sorbents can be defined using eq 3.

However, the selective oxidation of hydrogen sulfide using oxygen from air is necessary, which may seriously limit the catalytic activity in a reducing environment.14,19 Thus, developing an efficient and low-cost method for mercury removal from syngas is an urgent problem. As is well-known, cerium, a rare earth element of the lanthanide series, has received considerable attention due to its prominent ability to store/release oxygen as an oxygen reservoir via the redox shift between Ce4+ and Ce3+ under oxidizing and reducing conditions, respectively.20,21 Matsumoto22 reported that the following reversible reaction describes storing and releasing oxygen from CeO2. CeO2 ↔ CeO2(1 − x) + xO2

where

0 ≤ x ≤ 0.25

(2)

However, pure CeO2 is thermally unstable. CeO2 loading on TiO2 was developed to overcome this problem by forming a CeO2−TiO2 (CeTi) mixed oxide, which has been extensively studied for emission control.23,24 In addition, there is a potential for these sorbents to be regenerated via a route that directly produces elemental sulfur, which reduces the complexity of the regeneration step.25 On the basis of the above analysis, CeTi sorbents may be candidates for the capture of Hg0 even in extremely adverse conditions such as a deficiency of O2. To date, no research on Hg0 adsorption over CeTi sorbents from syngas has been reported. In this work, CeTi sorbents prepared by an impregnation method were employed to remove Hg0 in simulated syngas. The effects of the sorbent composition and individual syngas components on Hg0 adsorption were investigated. The purposes of this study are (1) to develop a sorbent for the removal of Hg0 from syngas and (2) to reveal the removal mechanisms of Hg0 using CeTi sorbents.

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EXPERIMENTAL SECTION Sorbent Preparation.26 The CeTi sorbents were prepared by an impregnation method using TiO2 nanoparticles and a cerium nitrate aqueous solution. The samples are denoted as CexTi, where Ce represents CeO2, Ti represents TiO2, and x, in the range of 0.01 to 0.5, represents the mass ratio of CeO2/ TiO2. Please refer to the sorbent preparation in the Supporting Information for further details. Characterization of Sorbents. Please refer to the material characterization methods in the Supporting Information. Mercury Removal Measurement. All of the adsorption experiments were performed in a fixed-bed quartz reactor (i.d. 9 mm), which was located inside a temperature controlled tubular furnace, as shown in Figure S1 (Supporting Information). An Hg0 permeation tube (VICI Metronic, Inc. U.S.A.) was used to generate a constant quantity of Hg0 vapor (∼50 μg/m−3), which was introduced to the inlet of the gas mixer. Other simulated syngas components including H2, CO, H2S, HCl, NH3, and the balance N2 were supplied by gas cylinders and were precisely controlled by mass flow controllers. Water vapor was generated using a heated water bubbler. A typical sample mass of 0.2 g and a total gas flow rate of 1.2 L·min−1 were used during the experiments, which corresponded to a gas hourly space velocity(GHSV) of approximately 120 000 h−1. All the gas components were mixed and preheated to the desired temperature and then passed through the quartz reactor. All Teflon lines that mercury passed through were heated up to 120 °C to avoid adsorption of mercury on the inner surface. A mercury continuous emission monitor (DM-6A/MS-1A, Nippon Instrument

η=

0 Hg 0in − Hg out

Hg 0in

× 100 (3)

Hg0in and Hg0out represent the total input and output of Hg0 that was calculated by integrating the real-time data of Hg0 over the two hours, respectively.

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RESULTS AND DISCUSSION Characterization of Sorbents. The BET surface areas of pure TiO2, pure CeO2, and the CeTi sorbents are listed in Table S2 (Supporting Information). The surface area of CeO2 was greater than that of TiO2. Furthermore, the surface area of the Ce−Ti mixed oxides was larger than that of pure TiO2 or CeO2, except for 0.5Ti, but no clear trend between surface area and CeO2/TiO2 ratio was observed. The XRD spectra of different samples are shown in Figure S2 (Supporting Information). Crystalline phases were identified by comparison with ICDD files (anatase TiO2, 21-1272; rutile TiO2, 21-1276; CeO2, 34-0394). For pure CeO2, only strong diffraction peaks indicative of cubic CeO2 (2θ = 28.57, 33.10, 47.53, 56.38) were observed in the XRD pattern. For pure TiO2, both anatase TiO2 (2θ = 25.28, 37.80, 48.05, 53.89, 55.10) and rutile TiO2 (2θ = 27.45, 41.23) were detected, and the anatase titania was the dominating phase. In all CeTi samples, anatase TiO2 was also found to be the predominant form, and the intensity of the peaks due to TiO2 decreased with the increase of Ce loadings, indicating that the crystallinity of 10057

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pure TiO2, no obvious Hg0 removal was observed, and the η in the entire temperature range was less than 10%. When CeO2 was doped into TiO2 sample, the Hg0 removal activities can be significantly enhanced, for example, approximately 45.6% Hg0 removal over Ce0.01Ti was obtained at 120 °C. Higher Ce loading showed a high Hg0 removal efficiency until the mass ratio of CeO2/TiO2 reached 0.2, with η higher than 80% from 80 to 150 °C. Further increasing the mass ratio of CeO2/TiO2 to 0.5 resulted in a slight decrease of the Hg0 removal efficiency because of the sintering effect.24 As for the pure CeO2, the maximum Hg0 removal efficiency was only approximately 63.4% at 120 °C. Similar results could be obtained from the work by Xu,35 in which CeTi catalysts had a higher catalytic activity than pure TiO2 and pure CeO2 for selective catalytic reduction of NOx by NH3. Ce0.2Ti sorbent exhibited the highest Hg0 removal activity, which could be ascribed to the greater amount of chemisorbed oxygen on the surface resulted from Ce3+ on the sorbent surface.29 For all CeTi sorbents, η increased with temperature from 30 to 120 °C and then decreased dramatically when the temperature further increased from 150 to 250 °C. Effect of Individual Syngas Components. The effects of individual syngas components on the removal of Hg0 were examined, and the results are summarized in Figure 2.

the anatase phase TiO2 deteriorates.28 No cubic CeO2 phase was observed in Ce0.01Ti and Ce0.1Ti, suggesting that Ce existed as highly dispersed or amorphous species, which are more active than the crystalline phase for the catalytic process.29 As the mass ratio of CeO2/TiO2 increased from 0.2 to 0.5, the diffraction line of CeO2 became apparent and grew sharper. This implies that sintering took place and CeO2 crystallites were formed. This is in accordance with previous reports showing that cubic CeO2 was observed when the mass ratio of CeO2/TiO2 exceeded 0.2 in CeTi catalysts prepared by an impregnation method.30 Moreover, the CeO2 peaks became much wider and weaker with the increase in the TiO2 content. This was most likely due to the incorporation of Ti atoms into the CeO2 lattice inhibiting the crystal growth of the cubic phase, because the radius of the Ti4+ ions (0.068 nm) is smaller than that of the Ce4+ ions (0.094 nm), which has been confirmed by refs 31 and 32. The XPS spectra of Ti 2p, Ce 3d, and O 1s for different samples are shown in Figure S3 (Supporting Information). Please refer to ref 24 for further details about Figure S3(1) and Figure S3(2). As shown inFigure S3(3), the O 1s peaks could be fitted by three peaks referenced to the lattice oxygen at approximately 529.3−530.0 eV (OA), chemisorbed oxygen and/or weakly bonded oxygen species at approximately 531.3−531.9 eV (OB), and surface oxygen in hydroxyl and/or surface adsorbed water at approximately 532.6 eV (OC).26,33 Satisfactory fitting results were obtained, and the surface atomic concentrations of the three oxygen species were calculated accordingly, as shown in Table S2 (Supporting Information). With the addition of Ce, (OB+OA)/OT increased. This increase was attributed to the presence of Ce3+, which could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the sample surface. Usually, the chemisorbed oxygen on the sample surface is the most active oxygen and plays an important role in the oxidation reaction.23,34 Ce0.2Ti, with the highest surface lattice oxygen and chemically adsorbed oxygen/weakly bound oxygen concentration showed the best Hg0 removal performance, as will be demonstrated in a later section. Performance of CeTi Sorbents. Hg0 removal efficiencies of CeTi sorbents with different CeO2/TiO2 ratios at temperatures ranging from 30 to 250 °C are shown in Figure 1. For

Figure 2. Effect of individual syngas components on Hg0 remvoal at 150 °C.

Effect of H2S. As shown in Figure 2, when a gas steam containing Hg0/N2 without any other components was passed through the fresh Ce0.2Ti sorbent, some mercury was removed. Similar phenomena were observed by Li et al.32 The minor loss of Hg0 on the Ce0.2Ti can be explained by the Mars−Maessen mechanism,36,37 in which adsorbed Hg0 reacts with surface oxygen (including lattice oxygen and chemisorbed oxygen) to form HgO directly. It is well-known that H2S is one of the most important pollutants that must be removed from syngas. In this study, H2S exhibited a promotional effect on Hg0 removal over CeTi sorbent. When 40 ppm H2S was introduced to the gas flow, η was observed to be 67.0%, which is much higher than that observed under a pure N2 atmosphere. An η of 98.1% was observed when the H2S concentration further increased to 400 ppm, indicating that H2S is extremely helpful for facilitating Hg0 removal by CeTi sorbents. Although the

Figure 1. Comparison of mercury removal efficiencies of the CeTi sorbents under simulated syngas as a function of temperature. 10058

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N2 or N2 + H2S gas flow, indicating that either CO or H2 has a negligible effect on Hg removal at 150 °C. Much of the research conducted to date uses ceria, which is presumed to remove H2S according to the following reactions:25,40

significant promotion of Hg0 removal using H 2S was demonstrated, H2S cannot oxidize Hg0 directly because it is already at a reduced state. The reducing environment is not favorable for Hg oxidation via gas-phase reactions alone.38 Hg0 removal by H2S requires the assistance of oxygen on the CeTi sorbent surface.11,12 Ce3+ species on the CeTi sorbent surface were at least partly responsible for the high Hg0 removal activity because the surface chemisorbed oxygen attributed to Ce3+ has been reported to be the most active oxygen.24 To study the H2S effect on mercury adsorption, Ce0.2Ti was chosen to continue the mercury adsorption test with and without H2S injection at 150 °C. The addition of 400 ppm H2S into the N2 caused a rapid decrease of Hg0 from 0.78 to approximately 0.02 for feed Hg0, as demonstrated in Figure 3.

Reduction of cerium oxide: 2CeO2 + H 2 = Ce2O3 + H 2O

Sulfidation of cerium oxide:

Furthermore, when the H2S feed was ceased, Hg removal still continued, and the Hg0 concentration increased back to the same level as that observed under the N2 condition 80 min later. The same phenomenon can be observed after adding and removing H2S again. From these results, it may be concluded that some surface species, for example, adsorbed elemental sulfur, which formed from H2S and accumulated over the sorbent, contributed to the Hg removal even after the H2S feed was stopped. Considering the low mercury removal efficiency at reactor temperatures above 200 °C, it is likely that the elemental sulfur, which has an approximate melting point of 116 °C,39 is volatile at high temperatures and thus part of it may be released from the reactor. It is very likely that lattice oxygen and/or chemisorbed oxygen supported the transformation of H2S to active surface sulfur, which is active at 150 °C and can react with Hg0 to form stable HgS.11,12 The possible heterogeneous reactions over CeTi sorbents are proposed to be as follows: (4)

S(ad) + Hg → HgS

(5)

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

(7)

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

(8)

As is well-known, the shift between CeO2 and Ce2O3 can generate additional chemisorbed oxygen or weakly adsorbed oxygen species, which are favorable for oxidation processes.20,21 This insensitivity indicates CeTi sorbent has large oxygen storage capacity, and only a fraction of the stored oxygen has been consumed in Hg0 oxidation during our experimental period though the shift between CeO2 and Ce2O3 generated additional chemisorbed oxygen. Therefore, only a negligible increase of Hg0 removal resulted with the addition of H2 and CO at 150 °C. Effect of HCl. It is well-known that HCl is present in syngas, but the effect of HCl on the Hg0 removal performance of CeTi sorbents is not yet well understood. As shown in Figure 2, HCl exhibited a prohibitive effect on Hg0 removal over Ce0.2Ti sorbent. The addition of 10 ppm HCl to a gas flow containing 400 ppm H2S balanced in N2 resulted in a significant decrease of η from 98.1% to 82.7%. A further increase in the HCl concentration to 25 ppm resulted in an even lower η of 58.6%. This implies that (a) HCl consumed the surface oxygen41 that is responsible for Hg0 removal in the presence of H2S or (b) HCl inhibited Hg0 adsorption on active sulfur.42 The possible heterogeneous reactions over CeTi catalysts are proposed to be as follows32,37,43

Figure 3. Hg0 removal efficiency with and without H2S injection.

H 2S(g) + O* → S(ad) + H 2O

(6)

2HCl + O* → 2Cl* + H 2O

(9)

Cl* + Hg 0 → HgCl

(10)

HgCl + Cl* → HgCl 2

(11)

where O* represents chemisorbed or lattice oxygen on the surface of CeTi catalysts, which can be consumed by HCl, and Cl* denotes an active chlorine species for oxidizing Hg0. The reaction proceeds through an intermediate product, HgCl. HgCl is subsequently oxidized by other active chlorine species, but the reaction rate is usually slow, especially in reducing and low-temperature environments.44 The gaseous HCl molecules are adsorbed on the CeO2 catalyst to form active chlorine species. In this process, HCl consumed the surface oxygen that is responsible for Hg0 removal in the presence of H2S. In this work, HCl-pretreated Ce0.2Ti sorbents were employed for Hg0 removal under an atmosphere of 400 ppm H2S plus N2 at 150 °C to demonstrate the consumption of surface oxygen by HCl. Before the experiment, Ce0.2Ti was first pretreated at 150 °C under a flow of 10 ppm HCl balanced in N2 for 30 min. As shown in Figure 4, Ce0.2Ti sorbents exhibited poor Hg0 removal performance after pretreatment by HCl, indicating that the surface oxygen was consumed during the pretreatment process. It is likely that these adsorbed species derived from HCl suppressed the production of the active surface sulfur S(ad) in this study. However, it is not possible to directly investigate the effect of HCl on the production of the

S(ad) is active surface sulfur, and O* is surface oxygen of the CeTi sorbent. Effect of H2 and CO. H2 and CO are the main components in syngas, so it is very important for industrial application to investigate their effect on the mercury removal activities of CeTi sorbents. 10 As shown in Figure 2, no obvious difference was detected when 30% H2 and/or 20% CO was introduced to 10059

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adsorption on CeO2 most probably takes place according to the following steps: NH3(g) → NH3(ad)

(12)

NH3(ad) + O* → NH 2(ad) + OH(ad)

(13)

Figure 5. Desorption of Hg0 by adding HCl at 150 °C.

Gaseous NH3 molecules are adsorbed on the CeO2 surface to form coordinated NH3 and NH2. In this process, NH3 consumed the surface oxygen that is responsible for Hg0 removal. It should be noted that this series of tests merely aimed to study the effect of NH3; thus, only 40 ppm H2S was added. The inhibitory effect of NH3 can be mitigated by using more H2S; that is, an η larger than 95.0% was obtained when 400 ppm H2S was added under the same gas conditions. Effect of H2O. H2O inevitably exists in syngas, and it has been reported to inhibit Hg0 oxidation and removal over metalor metal oxide- based catalysts due to competitive adsorption on active sites.32,46 However, the CeTi catalyst exhibited strong resistance to H2O poisoning.26,32 In this work, H2O was found to inhibit Hg removal over CeTi sorbents. As shown in Figure 2, under N2 condition using 400 ppm H2S, η decreased from 98.1 to 81.6% when the gas was switched from dry to humid (8% H2O). The Hg0 removal efficiency became lower when the H2O concentration further increased to 16%. It has been reported that a high concentration of H2O would cover the catalytic active center and diminish the adsorption of HCl.42 Therefore, it is very likely that water vapor inhibited the adsorption of H2S, which is a crucial gas component responsible for Hg0 removal in this work. However, compared with previous research on carbon-based materials and noncarbon sorbents,47,48 CeTi sorbent was relatively less affected by H2O, indicating that the CeTi catalyst has good resistance to H2O. Identification of Hg0 Removal Mechanism. To identify the reaction mechanism involved in Hg0 adsorption, Ce0.2Ti was first pretreated at 150 °C by H2S (Ce0.2Ti was exposed to 400 ppm H2S balanced with N2 for 10 min; then, the sorbent was flushed by a pure N2 gas flow at the same temperature for 10 min) and then used for Hg0 adsorption at different temperatures under a N2 atmosphere. As shown in Figure 6, Hg0 was captured by the H2S-pretreated sorbent when the gas steam started to pass through the sorbent at 20 min. Sorbent pretreated by H2S exhibited much better Hg0 adsorption performance than nonpretreated sorbent, indicating that it was

atmosphere, a significant increase in the Hg0 concentration was observed after turning off the Hg0 and adding 25 ppm HCl at the same time. For the other experiment, no obvious increase in the Hg0 concentration was observed after adding 25 ppm HCl and turning off the Hg0 and H2S at the same time. The result demonstrates that both Hg0 and HCl competed for the active sites, and HCl adsorbs more easily and more strongly to the active sites under a pure N2 atmosphere. However, HCl cannot inhibit Hg0 adsorption onto the active sites in the presence of H2S. This also implies that the mercury species on the CeTi sorbent were stable. Effect of NH3. It has been reported that NH3 inhibits Hg0 oxidation and removal over Ce-based catalysts.29 In this work, NH3 was also found to inhibit Hg removal over CeTi sorbents. As illustrated in Figure 2, the addition of 100 ppm NH3 to 40 ppm H2S balanced with N2 resulted in a significant decrease of η from 67.0% to 8.3%. Qi et al.45 proposed that NH3

Figure 6. Hg0 breakthrough curves over H2S pretreated Ce0.2Ti sorbent under pure N2 atmosphere.

Figure 4. Hg0 breakthrough curves over HCl pretreated Ce0.2Ti sorbent.

active surface sulfur because separation of the active surface sulfur and the common surface sulfur produced from the CeTi sorbent is difficult. In addition, the inhibition of Hg0 adsorption by HCl in the presence of H2S has been eliminated over the Ce0.2Ti sorbent. A 0.2 g sample of Ce0.2Ti was first pretreated at 150 °C under a flow of 50 μg/m−3 Hg0 and 50 μg/m−3 Hg0 plus 400 ppm H2S balanced in N2 for 3 h. As shown in Figure 5, in a pure N2

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the surface sulfur species that reacted with Hg0. Furthermore, the Hg0 removal efficiency increased as the temperature increased from 30 to 100 °C. Generally, higher temperatures accelerate chemical reactions. Therefore, it is very likely that Hg0 adsorption over Ce0.2Ti sorbent follows the Eley−Rideal mechanism, in which active surface sulfur generated from H2S reacts with gas-phase Hg0 to form HgS.11,12,49 However, when the temperature was higher than 200 °C, only negligible Hg0 could be adsorbed on the CeTi sorbent, as shown in Figure 1. It is very possible that elemental sulfur on the sorbent surface is volatilized at high temperature and that the active component for capturing Hg decreases. Accordingly, the Hg0 removal efficiency was low at high temperatures. This study revealed the possibility of Hg0 removal from syngas using CeTi sorbents. Such knowledge is of fundamental importance in developing effective pollution control technologies in which Hg0 removal at low temperatures is possible. Hence, further study to enhance the Hg0 removal efficiency of CeTi sorbents at elevated temperatures is warranted.

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(8) Park, C.; Kim, B. The optimization of low-rank coal grinding for transport coal gasification by robust design. Fuel 2012, 95, 282−286. (9) 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 (17−18), 3610−3615. (10) Yoshiie, R.; Nishimura, M.; Moritomi, H. Influence of ash composition on heavy metal emissions in ash melting process. Fuel 2002, 81 (10), 1335−1340. (11) Wu, S.; Ozaki, M.; Uddin, M. A.; Sasaoka, E. Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride. Fuel 2008, 87 (4−5), 467−474. (12) 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 (2), 213−218. (13) Aravind, P. V.; de Jong, W. Evaluation of high temperature gas cleaning options for biomass gasification product gas for solid oxide fuel cells. Prog. Energy Combust. 2012, 38 (6), 737−764. (14) León, M.; Jiménez-Jiménez, J.; Jiménez-López, A.; RodríguezCastellón, E.; Soriano, D.; López Nieto, J. M. Vanadium oxide-porous phosphate heterostructure catalysts for the selective oxidation of H2S to sulphur. Solid State Sci. 2010, 12 (6), 996−1001. (15) Koyuncu, D.; Yasyerli, S. Selectivity and stability enhancement of iron oxide catalyst by ceria incorporation for selective oxidation of H2S to sulfur. Ind. Eng. Chem. Res. 2009, 48 (11), 5223−5229. (16) Liu, W.; Vidic, R. D.; Brown, T. D. Impact of flue gas conditions on mercury uptake by sulfur-impregnated activated carbon. Environ. Sci. Technol. 2000, 34 (1), 154−159. (17) Cathum, S.; Velicogna, D.; Obenauf, A.; Dumouchel, A.; Punt, M.; Brown, C. E.; Ridal, J. Detoxification of mercury in the environment. Anal. Bioanal. Chem. 2005, 381 (8), 1491−1498. (18) Wei, L.; Liao, P.; Wu, H.; Li, X.; Pei, F.; Li, W.; Wu, Y. Toxicological effects of cinnabar in rats by NMR-based metabolic profiling of urine and serum. Toxicol. Appl. Pharmacol. 2008, 227 (3), 417−429. (19) Zhang, H.; Zhao, J. T.; Fang, Y. T.; Huang, J. J.; Wang, Y. Catalytic oxidation and stabilized adsorption of elemental mercury from coal-derived fuel gas. Energy Fuels 2012, 26 (3), 1629−1637. (20) Li, P.; Xin, Y.; Li, Q.; Wang, Z. P.; Zhang, Z. L.; Zheng, L. R. Ce−Ti amorphous oxides for selective catalytic reduction of NO with NH3: Confirmation of Ce−O−Ti active sites. Environ. Sci. Technol. 2012, 46 (17), 9600−9605. (21) Wang, L. X. J. Magnetic nanoscaled Fe3O4/CeO2 composite as an efficient fenton-like heterogeneous catalyst for degradation of 4chloropheno. Environ. Sci. Technol. 2012, 46 (18), 10145−10153. (22) Matsumoto, S. I. Recent advances in automobile exhaust catalysts. Catal. Today 2004, 90 (3−4), 183−190. (23) Liu, C. X.; Chen, L.; Li, J. H.; Ma, L.; Arandiyan, H.; Du, Y.; Xu, J. Y.; Hao, J. M. Enhancement of activity and sulfur resistance of CeO2 supported on TiO2−SiO2 for the Selective catalytic reduction of NO by NH3. Environ. Sci. Technol. 2012, 46 (11), 6182−6189. (24) Gao, X.; Jiang, Y.; Zhong, Y.; Luo, Z.; Cen, K. The activity and characterization of CeO2-TiO2 catalysts prepared by the sol−gel method for selective catalytic reduction of NO with NH3. J. Hazard. Mater. 2010, 174 (1−3), 734−739. (25) Cheah, S.; Carpenter, D. L.; Magrini-Bair, K. A. Review of midto high-temperature sulfur sorbents for desulfurization of biomass-and coal-derived syngas. Energy Fuels 2009, 23 (11), 5291−5307. (26) Chen, L.; Li, J.; Ge, M.; Zhu, R. Enhanced activity of tungsten modified CeO2/TiO2 for selective catalytic reduction of NOx with ammonia. Catal. Today 2010, 153 (3−4), 77−83. (27) Pavlish, J. H.; Hamre, L. L.; Zhuang, Y. Mercury control technologies for coal combustion and gasification systems. Fuel 2010, 89 (4), 838−847. (28) Fang, J.; Bi, X. Z.; Si, D. J.; Jiang, Z. Q.; Huang, W. X. Spectroscopic studies of interfacial structures of CeO2−TiO2 mixed oxides. Appl. Surf. Sci. 2007, 253 (22), 8952−8961. (29) Li, H. L.; Wu, C. Y.; Li, Y.; Zhang, J. Y. Superior activity of MnOx−CeO2/TiO2 catalyst for catalytic oxidation of elemental

ASSOCIATED CONTENT

S Supporting Information *

Information regarding sorbent preparation, characterization of sorbents, a summary of the experimental conditions (Table S1), and the BET surface area and surface atomic concentration of the CeTi sorbents (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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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 National Natural Science Fund of China (No. 51176171) and the nonprofit specific environmental research fund (No. 200909024).

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REFERENCES

(1) 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 (2−3), 89−165. (2) Zheng, Y.; Jensen, A. D.; Windelin, C.; Jensen, F. Review of technologies for mercury removal from flue gas from cement production processes. Prog. Energy Combust. 2012, 38 (5), 599−629. (3) Yan, N. Q.; Chen, W. M.; Chen, J.; Qu, Z.; Guo, Y. F.; Yang, S. J.; Jia, J. P. Significance of RuO2 modified SCR catalyst for elemental mercury oxidation in coal-fired flue gas. Environ. Sci. Technol. 2011, 45 (13), 5725−5730. (4) Fu, X.; Feng, X.; Sommar, J.; Wang, S. A review of studies on atmospheric mercury in China. Sci. Total Environ. 2012, 421−422, 73− 81. (5) Ministry of Environmental Protection (MEP) of China. Emission Standard of Air Pollutants for Thermal Power Plants, GB 13223-2011; MEP of China: Beijing, China, 2011. (6) U.S. Environmental Protection Agency. The Clean Air Act Amendments of 1990, Section 114; US EPA; Government Printing Office: Washington, DC, 1990. (7) U.S. Environmental Protection Agency. Mercury and Air Toxics Standards. http://www.epa.gov/airquality/powerplanttoxics/actions. html. (accessed June 17, 2013). 10061

dx.doi.org/10.1021/es401681y | Environ. Sci. Technol. 2013, 47, 10056−10062

Environmental Science & Technology

Article

mercury at low flue gas temperatures. Appl. Catal., B 2012, 111, 381− 388. (30) Xu, W.; Yu, Y.; Zhang, C.; He, H. Selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst. Catal. Commun. 2008, 9 (6), 1453−1457. (31) Luo, M. F.; Chen, J.; Chen, L. S.; Lu, J. Q.; Feng, Z.; Li, C. Structure and redox properties of CexTi1−xO2 solid solution. Chem. Mater. 2001, 13 (1), 197−202. (32) 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 (17), 7394−7400. (33) Zhang, Y.; Yang, M.; Dou, X. M.; He, H.; Wang, D. S. Arsenate adsorption on an Fe−Ce bimetal oxide adsorbent: Role of surface properties. Environ. Sci. Technol. 2005, 39 (18), 7246−7253. (34) Yang, S. X.; Zhu, W. P.; Jiang, Z. P.; Chen, Z. X.; Wang, J. B. The surface properties and the activities in catalytic wet air oxidation over CeO2−TiO2 catalysts. Appl. Surf. Sci. 2006, 252 (24), 8499− 8505. (35) Xu, W. Q.; He, H.; Yu, Y. B. Deactivation of a Ce/TiO2 Catalyst by SO2 in the selective catalytic reduction of NO by NH3. J. Phys. Chem. C 2009, 113 (11), 4426−4432. (36) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel sorbents for mercury removal from flue gas. Ind. Eng. Chem. Res. 2000, 39 (4), 1020−1029. (37) Presto, A. A.; Granite, E. J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol. 2006, 40 (18), 5601−5609. (38) Lu, D. Y.; Granatstein, D. L.; Rose, D. J. Study of mercury speciation from simulated coal gasification. Ind. Eng. Chem. Res. 2004, 43 (17), 5400−5404. (39) Lu, H.; Greenwood, P.; Chen, T. S.; Liu, J. Z.; Peng, P. A. The separate production of H2S from the thermal reaction of hydrocarbons with magnesium sulfate and sulfur: Implications for thermal sulfate reduction. Appl. Geochem. 2012, 27 (1), 96−105. (40) Kobayashi, M.; Flytzani-Stephanopoulos, M. Reduction and sulfidation kinetics of cerium oxide and Cu-modified cerium oxide. Ind. Eng. Chem. Res. 2002, 41 (13), 3115−3123. (41) Zhuang, Y.; Laumb, J.; Liggett, R.; Holmes, M.; Pavlish, J. Impacts of acid gases on mercury oxidation across SCR catalyst. Fuel Process. Technol. 2007, 88 (10), 929−934. (42) Li, H. L.; Wu, C. Y.; Li, Y.; Li, L. Q.; Zhao, Y. C.; Zhang, J. Y. Role of flue gas components in mercury oxidation over TiO2 supported MnOx−CeO2 mixed-oxide at low temperature. J. Hazard. Mater. 2012, 243, 117−123. (43) Guo, Y. F.; Yan, N. Q.; Yang, S. L.; Qu, Z.; Wu, Z. B.; Liu, Y.; Liu, P.; Ha, J. P. Conversion of elemental mercury with a novel membrane delivery catalytic oxidation system (MDCOs). Environ. Sci. Technol. 2011, 45 (2), 706−711. (44) Zhao, Y. X.; Mann, M. D.; Pavlish, J. H.; Mibeck, B.; Dunham, G. E.; Olson, E. S. Application of gold catalyst for mercury oxidation by chlorine. Environ. Sci. Technol. 2006, 40 (5), 1603−1608. (45) Qi, G. S.; Yang, R. T.; Chang, R. MnOx−CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal., B 2004, 51 (2), 93−106. (46) Li, Y.; Murphy, P. D.; Wu, C. Y.; Powers, K. W.; Bonzongo, J. Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas. Environ. Sci. Technol. 2008, 42 (14), 5304−5309. (47) Yan, R.; Ng, Y. L.; Liang, D. T.; Lim, C. S.; Tay, J. H. Benchscale experimental study on the effect of flue gas composition on mercury removal by activated carbon adsorption. Energy Fuels 2003, 17 (6), 1528−1535. (48) Zhang, L.; Zhuo, Y. Q.; Du, W.; Tao, Y.; Chen, C. H.; Xu, X. C. Hg removal characteristics of noncarbon sorbents in a fixed-bed reactor. Ind. Eng. Chem. Res. 2012, 51 (14), 5292−5298. (49) Wang, J. C.; Zhang, Y. P.; Han, L. N.; Chang, L. P.; Bao, W. R. Simultaneous removal of hydrogen sulfide and mercury from simulated syngas by iron-based sorbents. Fuel 2013, 103 (SI), 73−79.

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