Design Strategies for CeO2–MoO3 Catalysts for DeNOx and Hg0

Sep 30, 2015 - Promotional Effects of Ti on a CeO2–MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3. Yang Geng , Xiaoling Chen , ...
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Design Strategies for CeO2−MoO3 Catalysts for DeNOx and Hg0 Oxidation in the Presence of HCl: The Significance of the Surface Acid−Base Properties Huazhen Chang,*,†,‡ Qingru Wu,‡ Tao Zhang,‡ Mingguan Li,‡ Xiaoxu Sun,‡ Junhua Li,*,‡,§ Lei Duan,‡,§ and Jiming Hao‡,§ †

School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing 100084, China § State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: A series of CeMoOx catalysts with different surface Ce/Mo ratios was synthesized by a coprecipitation method via changing precipitation pH value. The surface basicity on selective catalytic reduction (SCR) catalysts (CeMoOx and VMo/Ti) was characterized and correlated to the durability and activity of catalyst for simultaneous elimination of NOx and Hg0. The pH value in the preparation process affected the surface concentrations of Ce and Mo, the Brunauer-Emmett-Teller (BET) specific surface area, and the acid−base properties over the CeMoOx catalysts. The O 1s Xray photoelectron spectroscopy (XPS) spectra and CO2temperature programmed desorption (TPD) suggested that the surface basicity increased as the pH value increased. The existence of strong basic sites contributed to the deactivation effect of HCl over the VMo/Ti and CeMoOx catalysts prepared at pH = 12. For the CeMoOx catalysts prepared at pH = 9 and 6, the appearance of surface molybdena species replaced the surface −OH, and the existence of appropriate medium-strength basic sites contributed to their resistance to HCl poisoning in the SCR reaction. Moreover, these sites facilitated the adsorption and activation of HCl and enhanced Hg0 oxidation. On the other hand, the inhibitory effect of NH3 on Hg0 oxidation was correlated with the competitive adsorption of NH3 and Hg0 on acidic surface sites. Therefore, acidic surface sites may play an important role in Hg0 adsorption. The characterization and balance of basicity and acidity of an SCR catalyst is believed to be helpful in preventing deactivation by acid gas in the SCR reaction and simultaneous Hg0 oxidation. catalyst to improve its catalytic performance for NOx and Hg0 removal.13,14 Although the emergence of quantitative acidic sites (which are very important in the NH3−SCR reaction) upon HCl addition were observed, the deactivation effect of HCl on SCR catalysts is irreversible. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of metal chlorides over the SCR catalyst.14,15 Nam and coworkers16−18 found that the CuHM catalyst could be poisoned rapidly when HCl existed, especially with H2O at the same time. The deactivation mechanism was developed on the basis of the identification of deactivation precursors by means of surface characterization.

1. INTRODUCTION To date, considerable efforts have been focused on the development of a novel efficient catalyst over a wide temperature range (150−450 °C) for DeNOx and elemental mercury (Hg0) oxidation from the flue gas of many industrial boilers.1−5 Ce-based oxides, which possess excellent oxygen storage capacities and superior redox properties, are interesting as potential selective catalytic reduction (SCR) catalysts.6−9 It was found that the doping of Mo and W into CeO2 could lead to an increase in the acidity and surface area, which apparently further enhanced the SCR activity at moderate temperatures.7,10 However, compared with the state-of-the-art SCR formula of V2O5−WO3(MoO3)/TiO2, problems still exist for the application of a wide temperature catalyst, i.e., the inferior N2 selectivity at high temperatures and the deactivation effects of H2O, HCl, and SO2 in the flue gases.11,12 An important issue in NH3−SCR is the investigation of the poisoning effect of acidic gases (such as SO2 and HCl) on SCR © XXXX American Chemical Society

Received: May 22, 2015 Revised: September 1, 2015 Accepted: September 30, 2015

A

DOI: 10.1021/acs.est.5b02520 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Hg0-TPD was carried out in a fixed-bed quartz reactor with typically 0.05 g of catalyst. The sample was first pretreated in N2 at 400 °C for 1 h. Then, it was kept at 100 °C for the adsorption of Hg0 for 15 h and then purged in N2 for 2 h. Finally, the catalyst was heated to 600 °C at a fixed rate of 10 °C per min in a flow of 200 mL/min N2. More details on characterization could be seen in the Supporting Information.

On the other hand, it is commonly recognized that the existence of HCl will effectively promote Hg0 oxidation.19 The removal of Hg0 will be strongly facilitated if Hg0 is oxidized to the water-soluble Hg2+ species by an upstream SCR catalyst.2 Kamata and co-workers20 believed that HCl adsorbed on active surface sites and reacted with Hg0 to produce oxidized mercury species. They also found that a supported MoO3 catalyst exhibited a much higher Hg0 oxidation activity compared to other metal oxides.21 In the presence of HCl, the high oxygen storage capacity of CeO2 was considered to be the main contributor to the excellent Hg0 oxidation activity over Cebased catalysts, such as Ce/TiO2.3 Due to its promotion of Hg0 oxidation and deactivation of the SCR reaction, the interactions between HCl and SCR catalysts need to be given more attention for the simultaneous removal of NOx and Hg0. Although the favorable SCR performance of the CeMoOx catalyst has been reported, a systematic investigation on the synthesis of these types of catalysts and their acid−base properties relating to SCR and the mercury removal efficiency in the presence of HCl have not been previously attempted. In the present work, an extensive investigation has been carried out, with the intention of correlating the acid−base properties and redox capability with the NH3−SCR and Hg0 oxidation performances over a CeMoOx catalyst.

3. RESULTS 3.1. Effect of HCl and H2O on SCR Catalytic Performance. The effects of HCl and H2O on the SCR catalytic performance (at 200 °C) of the CeMoOx and VMo/Ti catalysts are shown in Figure 1. The NOx conversion was detected after

Figure 1. Effect of 200 ppm of HCl and 5% H2O on SCR activity over CeMoOx and VMo/Ti catalysts at 200 °C, GHSV = 70 000 h−1.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. The CeMoOx mixed oxides catalysts (the molar ratio of Ce/Mo = 1:0.5 in the preparation procedure) were synthesized from cerous nitrate and ammonium molybdate(VI) by a coprecipitation method.9 For the purpose of controlling the pH, H2C2O4, (NH4)2CO3, and ammonia−water solutions were applied as precipitators. The catalysts are described as CeMopHx, where x means the final pH value of the solution in the preparation procedure, for example, CeMopH9. The VMo/Ti catalyst was synthesized by the impregnation method. The V2O5 loading was 1 wt %, and MoO3 is 5 wt %.9 2.2. Catalytic Performance. The NH3−SCR and Hg0 removal activity, as well as the NH3 and NO oxidation performance, was tested in a fixed-bed reactor with typically 0.15 g of catalyst. The following conditions were used in the SCR reaction: 500 ppm of NH3, 500 ppm of NO, 5% O2, and the balance of N2. To simulate the poisoning effect of a low concentration of HCl over a long time, a relatively high concentration of HCl (200 ppm) was applied to accelerate the deactivation. The Hg0 concentrations were measured with an online Lumex RA 915 M Hg analyzer. The components in the inlet and outlet gas were estimated by an FT-IR gas analyzer. The NO x conversion was estimated on the basis of NO x concentrations in the inlet and outlet gas. 2.3. Catalyst Characterization. The CO2-temperature programmed desorption (TPD) was carried out using a Micromeritics AutoChem II 2920 chemisorption analyzer. First, approximately 0.06 g of catalyst was pretreated at 500 °C in a flow of He for 1 h. The chemisorption of CO2 was performed at 70 °C with 30% CO2/He (25 cm3 min−1) for 1 h, and weakly adsorbed CO2 was purged at the same temperature in He for 50 min. Finally, the catalyst was heated to 900 °C at a fixed rate of 10 °C per min in He, with a regular flow rate of 25 cm3 min−1. The quantity of desorbed CO2 was estimated by a TCD detector.

exposure to HCl and H2O at a steady state for at least 10 h. It could be seen that the NOx conversions were affected slightly by H2O. The NOx conversion of the VMo/Ti catalyst decreased from 80% to 37% with the addition of 200 ppm of HCl. It indicated that the activity of the VMo/Ti catalyst for NOx removal was significantly suppressed when HCl and H2O were simultaneously present in the flue gas. The addition of HCl also showed suppression effects on the SCR performance of the CeMopH12 catalyst, that NOx conversion decreased to 65% after the addition of HCl. However, the performances of the CeMopH9 and CeMopH6 were very different. The NOx conversion showed nearly no change after the introduction of HCl, revealing that these two catalysts had good resistance to HCl in the presence of H2O. The NOx conversion was 70% over the CeMopH9 catalyst, which was the highest among the four catalysts at 200 °C. All of the CeMoOx catalysts prepared at different pH values showed better SCR performance than the VMo/Ti catalyst in the presence of HCl and H2O. 3.2. Hg0 Oxidation Performance. To make good use of the cobenefit of existing SCR catalysts of Hg0 removal with a low level of HCl in flue gas, the Hg0 oxidation behaviors were investigated over CeMoOx and VMo/Ti samples, and the results of Hg0 conversions vs temperature are shown in Figure 2. The CeMoOx catalysts prepared at different pH values exhibited excellent Hg0 oxidation performance with a space velocity of 2.0 × 105 h−1 (see Figure 2a). The results indicated that the sequence of catalytic activity for Hg0 oxidation is CeMopH9 > CeMopH6 > CeMopH12 throughout the tested temperature range. The CeMopH9 catalyst even showed better Hg0 oxidation activity than the VMo/Ti catalyst at high temperatures (see Figure S1). The superior Hg0 oxidation activity over the CeMopH9 catalyst demonstrated that the preparation conditions affect the surface properties, which further influence the Hg0 oxidation efficiency. B

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demonstrated that the crystal grain growth was affected by the pH value (see Table 1). The smallest crystal size was observed on the CeMopH9 catalyst (7.08 nm). Compared to the CeMopH6 sample, the decrease in crystal size could be regarded as an important reason for the increasing surface area of the CeMopH9 catalyst. For all of the three catalysts, the catalytic activity increased as the crystal size decreased. The surface and bulk atomic concentrations of Ce and Mo in these catalysts were characterized and calculated from the X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) analyses, respectively. As exhibited in Table 1, the surface atomic concentration of Ce increased but the concentration of Mo decreased as the pH value increased from 6 to 12. Meanwhile, the surface atomic ratio of Ce/Mo increased gradually from 2.93 to 5.95 for the same pH increase. These results are in agreement with the literature, which indicates that the content of MoO3 in mixed oxides system gradually decreases with an increase in the pH of coprecipitation.22 Cerium ions may undergo complexation and hydrolysis depending on the pH of the solution and the concentration of the ion.23 It was reported that the Ce(OH)4 compound was likely to precipitate predominately at pH > 4.24 The states and structures of the surfaces of the CeMoOx samples strongly depend on the preparation conditions.25 3.4. Surface Basicity. 3.4.1. XPS. The O 1s XPS spectra of different catalysts are displayed in Figure 3a. The sub-band at 533.0−533.6 eV could be assigned to adsorbed H2O on the surface of the catalyst {He, 2014 #2749}. The sub-band at ca. 530.4−530.6 eV could be attributed to the surface of chemisorbed oxygen (Oα), for example, hydroxyl (OH−) oxygen.26 The sub-band at 529.5−529.6 eV could be ascribed to the lattice oxygen O2− (Oβ). From the relative peak area of Oα, it can be concluded that ca. 33.0%, 36.5%, and 37.4% of the total amount of surface oxygen could be attributed to Oα on CeMopH6, CeMopH9, and CeMopH12, respectively.26 As the surface of chemisorbed oxygen is active in oxidation reactions, the higher Oα ratio in CeMopH9 and CeMopH12 would facilitate the oxidation of NO and Hg0. The superior redox property of CeMopH9 and CeMopH12 could also be proved by H2-TPR (see Figure S4). The improved NO oxidation activity by the superior redox property of CeMopH9 can be evidenced by Figure S5. Meanwhile, the binding energy of the Oβ band changes monotonously with the preparation pH value. The increase of pH improves the nucleophilicity of surface oxygen. It is reported that electron pair donation becomes stronger as the O 1s BE decreases; thus, the basic strength of the oxygen increases.27 3.4.2. CO2-TPD. To elucidate the influence of the pH value during preparation on the surface basicity, the CeMoOx and VMo/Ti samples were characterized by CO2-TPD (see Figure 3b). In CO2-TPD, the peak temperatures and areas provide information for the intensity and quantity of basic sites.27 The

Figure 2. Hg0 conversion as a function of temperature over CeMoOx catalysts with different preparation pH values (a); the effect of 200 ppm of NH3 on Hg0 oxidation over CeMopH9 and VMo/Ti catalysts at different temperatures (b). Reaction conditions: 110 μg m−3 Hg0, 20 ppm of HCl, 5% O2, and N2 balance.

In the flue gas of an SCR plant, NH3 is unavoidable for Hg0 oxidation by the SCR catalysts. The effects of NH3 on Hg0 oxidation over CeMopH9 and VMo/Ti catalysts are shown in Figure 2b. At 150 °C, it could be seen that the Hg0 oxidation activity decreased dramatically after addition of 200 ppm of NH3 over CeMopH9. At 250 °C, the Hg0 oxidation was also affected over the same catalyst, as only 78% of Hg0 was oxidized. Meanwhile, the addition of NH3 also showed an obvious inhibition effect on Hg0 oxidation over VMo/Ti catalyst, and much lower Hg0 oxidation rates were obtained compared with the CeMopH9 catalyst. The effect of NH3 on Hg0 oxidation was in accordance with previous studies.2,19 The experiment on simultaneous removal of NOx and Hg0 was performed (see Figure S2). In order to discuss the correlation between the catalytic activity and physical and chemical performance of these catalysts, these catalysts were further described by means of characterization. 3.3. Physical Properties and Atomic Concentration of Ce and Mo. The results of BET specific surface areas are displayed in Table 1. The data show that the BET specific surface areas of the CeMopH6 (53.0 m2 g−1) and CeMopH9 (54.9 m2 g−1) samples are almost the same. The CeMopH12 sample showed the highest surface area (92.1 m2 g−1) among three catalysts. This revealed that pH value in the preparation process was an important parameter that affected the physical properties. An increasing pH value enlarged the surface areas of the CeMoOx catalysts. In our previous study, MoO3 showed a much smaller specific surface area, and the surface area obviously increased with the addition of Ce.9 The XRD profiles did not show any intense or sharp peaks for molybdenum oxides. All of the peaks in the XRD profiles of the CeMoOx catalysts could be ascribed to CeO2 with cubic fluorite structure (see Figure S3), indicating that the crystal structures of the CeMoOx catalysts were not influenced by the pH values. From the crystal size calculated from XRD data, it

Table 1. BET Specific Surface Area and Atomic Concentrations of CeMoOx Mixed Oxide Catalysts bulk atomic concentrationb (wt %)

a

surface atomic ratioc (mol %)

samples

BET surface area (m2 g−1)

crystal sizea (nm)

Ce

Mo

Ce

Mo

Ce/Mo

CeMopH6 CeMopH9 CeMopH12

53.0 54.9 92.1

8.09 7.08 7.32

62.67 69.09 69.17

9.20 6.34 5.01

19.79 22.11 22.71

6.75 4.44 3.82

2.93 4.98 5.95

Calculated from XRD results. bFrom ICP. cCalculated from XPS results. C

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Figure 4. NH3-TPD curves of CeMoOx catalysts prepared at different pH values.

literature.29 At higher pH values, chemically bonded −OH was likely to be introduced onto the CeMoOx catalyst, and additional Brønsted acid sites could be produced.30,31 As a result, the amount of NH3 adsorption increased with improved surface acidity, which favors the SCR reaction accordingly. 3.6. Effect of NH3 on Hg0 Adsorption Capacity. The Hg0 adsorption capacities of the CeMoOx samples were characterized by Hg0-TPD (see Figure 5). For all three of the catalysts, there was one strong desorption peak at approximately 300−400 °C. This peak might be ascribed to the strongly bound Hg species. The peak shifted to higher temperature on the CeMopH9 catalyst, indicating that the interaction between Hg0 and this sample was stronger than the interactions with the other two catalysts. Meanwhile, the amount of desorbed Hg0 was much larger on this catalyst. The results showed that a good Hg0 adsorption capability was obtained over the CeMopH9 catalyst, which might be due to its specific surface properties caused by the Ce/Mo distribution. It is believed that the species of Hg adsorbed on the surface of the catalyst play an important role in Hg0 oxidation.32 The Hg0 adsorption capability of CeMopH9 might contribute to its excellent performance in Hg0 removal. For the purpose of investigating the effect of NH3 on Hg0 adsorption, Hg0-TPD was also performed under similar conditions to the first set of experiments, except that the Hg0 adsorption was carried out in the presence of NH3. In the presence of NH3, the amount of adsorbed Hg0 significantly decreased over all three of the catalysts. Meanwhile, the Hg0 desorption peak shifted to a slightly lower temperature over all of the catalysts, indicating that the strength between the adsorbed Hg and the catalysts was also affected by NH3. The suppression effect of NH3 on Hg0 adsorption was consistent with the effect of NH3 on Hg0 oxidation activity, further demonstrating that the adsorption of Hg0 was an important step in Hg0 oxidation. The effect of NH3 on Hg0 adsorption will be discussed later.

Figure 3. O1s XPS spectra of CeMoOx catalysts (a); CO2-TPD curves of CeMoOx and VMoTi catalysts (b).

CO2-TPD spectra of the CeMoOx catalysts consist of two sets of peaks at 130−160 and 500−710 °C, indicating that two types of basic sites exist on these catalysts. As the pH increased from 6 to 12, the peaks shifted to higher temperatures, which revealed that the basicities of the catalysts were enhanced. The peaks for the CeMopH12 catalyst are much stronger than for the other two samples, which might be ascribed to the presence of strong basic sites (i.e., basic O2− 28) on this sample. The VMo/Ti catalyst showed two peaks, one strong peak at 645 °C and one weak peak at 421 °C. The existence of basic O2− was also possible on the VMo/Ti catalyst. The results indicated that the basicity of the VMo/Ti catalyst is stronger than those of the CeMopH6 and CeMopH9 catalysts. The presence of basic sites with the appropriate strength would favor the adsorption of HCl during Hg0 oxidation and correspondingly promote Hg0 removal.2 However, stronger basic sites are more likely to react with HCl and forms metal chlorides, which leads to the deactivation of CeMopH12 and VMo/Ti catalysts, affecting their performance in SCR reaction and Hg0 oxidation.17 3.5. Surface Acidity. In order to elucidate the influence of the pH value on the surface acidity, NH3-TPD experiments were carried out on CeMoOx catalysts. As exhibited in Figure 4, for the CeMopH6 catalyst, the desorption of NH3 occurs with a broad shape throughout a wide temperature range (70−350 °C) with a maximum near 150 °C. This suggests the presence of more than one type of adsorbed NH3 species, with each type having a different thermal stability. NH3 desorption on the other two samples occurs within nearly the same temperature range, but the NH3 adsorption amount increased with increasing pH values. The amount of NH3 adsorption over the CeMopH9 sample was 2.4 times of that adsorbed over the CeMopH6 sample. The fact that molybdenyls acts as an adsorption site for ammonia has been verified in the

4. DISCUSSION 4.1. Significance of the Surface Basicity for the NOx Removal and Hg0 Oxidation. In previous studies, it has been proved that HCl could obviously enhance Hg0 oxidation activity, but a SCR catalyst is easily deactivated by HCl.10,14,29,33 The poisoning effect of HCl on SCR catalysts has been investigated by researchers. It is believed that the evaporation of Cu2+ from the CuHM sample is the main reason for its deactivation by HCl.18 The modification of the Si/Al proportion and the introduction of additives to the catalyst D

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Figure 5. Hg0-TPD curves (in the absence/presence of NH3) of CeMoOx catalysts prepared at different pH values. (a) CeMopH6; (b) CeMopH9; (c) CeMopH12.

catalysts (see Table 1). Terminal MoO and Mo−O−X (X = Mo or Ce) of the surface molybdena species could be identified by Raman spectra (see Figure S6). The expense of the surface OH by replacement of the surface molybdena species has been evidenced before.36 Thus, the strengths of basic sites were much lower over the CeMopH9 and CeMopH6 catalysts (peaks at 500−550 °C in CO2-TPD). The decrease of the surface ratio of Ce/Mo and weakened basicity might contribute to the HCl resistance over these two samples in the SCR reaction. In addition, the strength and distribution of the basic sites of the catalyst are very important for the NOx removal and Hg0 oxidation. It is believed that adsorbed NO2 could react with coordinated NH3 through the SCR reaction below 300 °C (Langmuir−Hinshelwood mechanism).12,37 The adsorption of NO and formation of adsorbed NO2 on the CeO2−MoO3/ TiO2 catalyst has been evidenced by Liu et al.10 The basic sites should be significant active centers for the adsorption of NO. Meanwhile, the adsorption of HCl is a very important step in Hg0 oxidation, and the activity is influenced by the presence of active chlorinated species.20 Evidence of HCl adsorption onto the surface of the catalyst was obtained by employing surface analysis methods.38 It is suggested that HCl is first adsorbed on active centers over catalysts, and reactive chlorine species are generated during the following step:

were effective ways to improve the resistance to HCl of the sample. Particularly, the addition of Ce onto the catalyst significantly enhanced the stableness of the sample by the formation of Ce−Cl compounds, which diminish the electronegativity of the sample.17 In our present work, the enhancement of HCl resistance could also be found on the CeMoOx catalysts under the NH3− SCR reaction conditions. In the presence of HCl and H2O, the performances of the CeMoOx samples were much better than that of the VMo/Ti, whereas the presence of HCl showed negative effects on the NOx conversion over CeMopH12, on which the surface atomic ratio of Ce is the highest among the catalyst series. It seems the surface atomic ratio of Ce is not the determining factor for HCl tolerance over a catalyst. Recently, we proposed that basic sites exist on SCR catalysts, and the strength and distribution of these basic sites would affect their resistance to acidic gases.28 Modification of the basic sites on surface of the catalyst can be an effective way for improving HCl tolerance. It was proven that the addition of Na, which might provide strongly basic sites, would result in suppressed deactivation from HCl poisoning for Rh active sites over the Rh/Al2O3 catalyst.34 The strengths and distribution of basic sites over CeMoOx and VMo/Ti catalysts were characterized by O 1s XPS spectra and CO2-TPD (see Figure 3). The binding energy of the Oβ band decreased monotonously (from 529.7 to 529.4) as the preparation pH value increased from 6 to 12. According to literature,27 the basic strength of the oxygen increases as the O 1s BE decreases due to the stronger electron pair donation. In the literature, four types of basic sites (very strong, strong, medium, and weak) over the catalysts surface could be distinguished by the temperature ranges from the peaks of CO2-TPD.35 In this work, the CO2-TPD results showed that strong basic sites exist over VMo/Ti (peak at 645 °C); meanwhile, much stronger basic sites were characterized on the CeMopH12 catalyst (peak at 702 °C). Du et al.36 suggested that basic hydroxyl groups associated with Ce existed on the CeO2 surface (Ce4+−OH), and the basic hydroxyl group decreased upon the dispersion of MoO3 onto the CeO2 surface. The high surface Ce/Mo ratio facilitates formation of Ce4+− OH over CeMopH12. These sites facilitated interactions between HCl and the catalysts, and the SCR activity was obviously inhibited by HCl over CeMopH12 catalysts (see Figure 1). Decreasing the precipitation pH value resulted in a lower surface ratio of Ce/Mo over CeMopH9 and CeMopH6

2HCl (g) + O* → 2Cl* + H 2O

where O* represents chemisorbed or surface lattice O on the sample.3 If very strong basic centers exist, the catalyst is more likely to be deactivated by HCl following this reaction: HCl + M−O* → M−Cl + OH* (medium basic site)

The formation of OH* has been reported in our previous work.28 The intermediates formed after exposure to HCl were determined by the strength of the basic surface sites. Although it was difficult to quantitatively determine the intensities of basic sites when the catalysts would be deactivated by HCl, it was clear that the existence of relatively weak basic sites would favor resistance to HCl under SCR reaction conditions and Hg0 removal over SCR catalysts (see Figures 1 and 2). In the present work, it was proposed that at least the following three types of basic sites could be distinguished for the interaction between HCl and catalyst: strong basic sites that lead to deactivation of the SCR catalyst by HCl and medium along with weak basic sites that both facilitate HCl adsorption E

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Environmental Science & Technology for Hg0 oxidation and NO adsorption for the NH3−SCR reaction (especially at low temperatures39). The elimination of Hg0 by HCl-pretreated CeMopH9 catalyst has been verified by experiment (see Figure S7). Combined with the results in Hg0TPD (see Figure 5), it could be concluded that HCl adsorbed on the catalyst and generated reactive chlorine species during the Hg0 oxidation process. The Hg0 could also adsorb on the catalyst, and the adsorbed species reacted with reactive chlorine species. 4.2. Effect of NH3 on Hg0 Adsorption. The effect of NH3 on Hg0 oxidation has been investigated by many researchers. In the literature,2,20 it was believed that the Hg0 oxidation performance was suppressed in the presence of NH3. Both laboratory- and full-scale studies noted that increasing the NH3 concentration caused the desorption of Hg0 from the surface of the catalyst.19 However, the mechanism behind this inhibitory effect of NH3 on Hg0 oxidation remains controversial. Kamata et al.20 suggested that NH3 competes for the active catalyst sites with HCl, and NH3 adsorption may predominate over the adsorption of HCl when both are present. Yan and other researchers2 observed that NH3 and Hg0 strongly compete for sites on the surface without HCl. The inhibition effect of NH3 on Hg0 adsorption was also confirmed by our experiments (see Figures 5 and S8). In the present work, the Hg0 oxidation of all of the catalysts was also considerably inhibited by the addition of NH3. The trend of the activity in the presence of NH3 was different from that observed in its absence. In the presence of NH3, the inhibition of Hg0 adsorption implied the existence of competitive adsorption between NH3 and Hg0 over all of the samples (see Figure 5). Over an SCR catalyst, the most significant type of active center for NH3 adsorption is an acidic site (Brønsted or Lewis acidic site).39 It could be concluded that acidic sites were also important centers for Hg0 adsorption. The Hg0 could be considered to be a “basic” probe in this step. It adsorbed onto acidic sites and reacted with adsorbed HCl species or gaseous HCl in the absence of NH3. Whereas, when both Hg0 and NH3 exist in flue gas, the acidic sites tend to be occupied by NH3. Additionally, it was believed that the effect of NH3 on HCl adsorption remains a problem because they could easily react in the gas phase. The consumption of HCl by reacting with NH3 also contributed to the inhibitory effect of NH3 on Hg0 oxidation in the presence of HCl. Therefore, from the point of the simultaneous removal of NO and Hg0, it is better that the SCR catalyst is divided into the following two distinct “zones”:19 a NOx elimination zone (denoted as the “SCR zone”) with abundant NH3 and the Hg0 oxidation zone (denoted as the “Hg0oxi zone”) afterward, where there are basic and acidic sites available for HCl and Hg0 adsorption. The role of basic and acidic sites in the SCR and Hg0oxi zones for the simultaneous removal of NOx and Hg0 can be illustrated in Scheme 1. Simultaneous removal of NOx and Hg0 was performed in the present study (see Figure S2), and the results showed that the CeMopH9 catalyst was a superior candidate for simultaneous NOx and Hg0 elimination. In summary, the pH of the preparation procedure defines the atomic ratios of Ce and Mo and consequently the acid−base properties and reducibility behavior of the CeMoOx catalyst. The poisoning effect of HCl was correlated with the surface basicity of different catalysts. The strength of the basic sites also affected the adsorption of HCl for Hg0 oxidation. NH3 and Hg0 strongly compete for sites on the surface of the catalyst. Acidic sites were believed to be important centers for Hg0 adsorption.

Scheme 1. Role of Basic and Acidic Sites in the SCR Zone and the Hg0oxi Zone for Simultaneous Removal of NOx and Hg0 a

a

“S” stands for strong basic site.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02520. Characterization method and additional results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-62512572. E-mail: [email protected] (H. Chang). *Tel.: +86-10-62771093. E-mail: [email protected] (J. Li). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program, Grant No. 2013CB430005) and the National Natural Science Foundation of China (Grant Nos. Nos. 21307071, 21325731, 21577173).

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REFERENCES

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DOI: 10.1021/acs.est.5b02520 Environ. Sci. Technol. XXXX, XXX, XXX−XXX