Comprehensive Evaluation of Mercury ... - ACS Publications

Article pubs.acs.org/IECR

Comprehensive Evaluation of Mercury Photocatalytic Oxidation by Cerium-Based TiO2 Nanofibers Lulu Wang, Yongchun Zhao,* and Junying Zhang* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: Efficient and economical technologies are essential to the control of mercury, the emission of which imposes serious health concerns and environmental risks. Photocatalysis is an attractive method for reducing mercury emissions. Considering that titania is widely applied in the photodegradation of toxic contaminants, this study investigated the removal of mercury over ceriumbased titania nanofibers (CBTs) at low temperature. According to the results, in the atmosphere containing SO2, both catalyst and UV proposed adverse effect on Hg0 oxidation. The competition between SO2 and Hg0 for active sites and the formation of cerium sulfate are responsible for the deactivation of Hg0 removal capacity. More interestingly, without O2, NO and HCl still exerted a superior promoting effect on mercury removal. Entirely different from the properties under SO2, UV and catalyst both facilitated Hg0 oxidation with the existence of HCl. Meanwhile, effects of the copresence of NO and SO2 on Hg0 removal were further investigated. NO was the most dominant gas component enhancing the removal capacity. Considerable high removal efficiency (>80%) was observed in the presence of 300 ppm of NO and 400 ppm of SO2. These indicate that combining photocatalysis technology with CBTs is a promising strategy to oxidize mercury under low-rank coal combustion flue gas in which the concentration of HCl is relatively low. In addition, Lee and co-workers10−13 deduced that during the irradiation by UV light, active sites were generated on a titania particle surface to adsorb Hg and thus form a complex (TiO2·HgO(complex)) on TiO2. Consequently, the Hg-containing TiO2 nanoparticles were readily removed when the flue gases passed through APCDs. However, the large bandgap energy of 3.2 eV limits the application of TiO2, which could only be excited by UV light (λ < 365 nm) irradiation. Subsequently, many modification techniques have been investigated to enlarge the application of TiO2, such as noble metal loading,14 transition metal ion doping,15 and metal and nonmetal codoping.16 Compared with noble metal catalysts, the lowercost transition metal catalysts also exhibit good performance. Kaluza9,17 found that elemental mercury can be photo-oxidized by titania, zinc oxide, tin oxide, and cerium oxide, and the maximum excitation wavelengths have been tabulated. CeO2, which is abundant, nontoxic, and inexpensive, emerges as an attractive candidate. However, pure CeO2 is thermally unstable, and fortunately, this shortcoming could be overcome through the introduction of Ti4+, which would help enhance the stability by forming cerium-based titania (CBT).18 Traditionally, coal-fired flue gas contains some constituents such as O2, SO2, NOx, HCl, and water vapor. These

1. INTRODUCTION Due to the extreme toxicity, persistence, and bioaccumulation, emission of mercury has become a major environmental issue that attracts global concerns. Coal combustion has been targeted as one of the most significant anthropogenic sources of mercury emissions to the atmosphere.1,2 Together, three countries, China, India, and the United States, are responsible for about 65.39% of the global emission of mercury from coal burning.3−5 Established on March 19, 2013, Minamata Convention on Mercury is a significant boost to prevent releases of mercury with global and legally binding agreements between countries.6 Among the different forms of mercury present in the coal-fired atmosphere, gaseous elemental mercury (Hg0) is more difficult to remove using currently available air pollution control devices (APCDs) due to its high volatility, inertness, and insolubility in water.7 Unfortunately, Hg0 is the dominant mercury species emitted to the atmosphere; i.e., Hg0 contributed 66−94% of total mercury released from coal-fired power plants in China.8 This arouses urgent needs for efficient and economical technologies to reduce Hg0 emission. To meet these needs, titanium dioxide (TiO2) has been used to remove Hg0 effectively in the presence of UV.9 Hg0 was adsorbed and oxidized on the activated TiO2 surface by hydroxyl (OH) radicals generated under UV irradiation: Ti−OH• + Hg → HgOH + OH• → HgO + H 2O © 2017 American Chemical Society

Received: Revised: Accepted: Published:

(1) 3804

December March 22, March 22, March 22,

26, 2016 2017 2017 2017 DOI: 10.1021/acs.iecr.6b04995 Ind. Eng. Chem. Res. 2017, 56, 3804−3812

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Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of mercury oxidation fixed bed system.

components are believed to play crucial roles on Hg0 removal due to the complex reactions involved. However, their effects are still enigmatic. By calculations, Xie et al.19 demonstrated that an addition of 300 ppm of NO only exhibited a very slight promotional effect on mercury removal, while further increase in the concentration caused no significant variation. In contrast, Chen20 deduced that NO drastically reduced Ecap of mercury under various light irradiations. A pilot-scale photocatalytic Hg0 emission control system was installed at a real combustion facility, and results showed that lower ηHg was due to the various acid gases present.21 As a consequence, a systematic study of the effects of those components, including NO, on Hg0 photocatalytic removal is essential. It is worthy of study due to the emissions of these compounds, especially SO2, NOx, and mercury, that pose significant challenges to the environment and public health. Meanwhile, the interaction mechanisms among UV, catalyst, and flue gas are complicated, which need to be further identified. Considering the problems mentioned above, the present study aims to (i) elucidate the photocatalytic properties of Hg0 removal over CBTs, (ii) clarify the mechanisms involved, and (iii) explain the synergistic effects of acidic gases on Hg0 photocatalytic oxidation. Also, the study proposes possible strategies for the simultaneous removal of SO2, NO, and mercury by photocatalysis technology.

atmosphere. The obtained solution was then loaded into a syringe and ready for electrospinning. The detailed operating procedures have been described in our earlier studies.22 Additionally, the resulting nanofibers were calcined at 400 °C in static air. The crystalline structures of samples were identified through X-ray powder diffractometer (XRD, PANalytical B.V.X’Pert PRO). The BET surface areas of samples were obtained via N2 isothermal adsorption (Micromeritics ASAP 2000). Further analysis regarding functional groups was carried out with X-ray photoelectron spectroscopy (XPS, AXISULTRA DLD-600W) as well as Fourier transform infrared spectrometry (FTIR, VERTEX 70). 2.2. Experimental Apparatus and Procedure. As shown in Figure 1, the test unit consists of a gas feeding system, a fixed bed reactor, and a set of analytical instruments including flue gas and Hg0 analyzers. Flow rates of all the individual flue gas components were accurately controlled by mass flow controllers with a total flow rate of 1 L min−1. Hg0 vapor was introduced into the gas mixing chamber using high-purity N2 (99.999%) from a Hg0 permeation tube immersed in a water bath at fixed temperature, and the initial concentration was 50 μg·m−3. Experiments were performed in a fixed-bed reactor consisting of an inner quartz tube and an outer quartz tube. Irradiations were performed with a low-pressure UV lamp (Philips, TUV PL-S 9W, Japan), which was placed in the inner tube. The primary wavelength of light emitted by the lamp was 253.7 nm. The light intensity was 3 mW/cm2, while the UV energy density (Ed) was fixed at 540 J/L. Cold air was generated by an air pump efficiently to cool the lamp and help maintain a constant reaction temperature at 120 °C. Effluent gas from the photochemical reactor flowed through an impinger containing 10% NaOH(aq). Furthermore, a moisture trap was installed downstream of the impinger to remove water vapor from the gas stream and prevent interference in mercury detection. The Hg0 concentration was continuously monitored using a VM-3000 mercury vapor monitor (Mercury Instrument, Germany). Changes in the concentrations of NO, NO2, and SO2 were analyzed by a flue gas analyzer (OPTIMA7, YORK INSTRUMENTS LTD). Finally, the exhaust was passed through a carbon trap before released into a fume hood. All

2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Analysis. In this work, the cerium-based TiO2 nanofibers (shorted by CBTs) were obtained by an electrospinning method. The design of CBTs exhibits the distinct advantages of large surface area, high crystallinity, and excellent geometrical flexibility, which favors photocatalytic activity. The precursor solution was a mixture of tetrabutyl titanate (TBOT) with ethanol and acetic acid. The volume ratio of them was 1:5:2. Subsequently, cerium nitrate and an appropriate amount of polyvinyl pyrrolidone (Mw = 1 300 000) were slowly dissolved in the above solution and stirred for several hours to obtain a homogeneous mixture. The loading value of Ce on the CBTs was defaulted as 0.3%, which presented optimal Hg0 removal activity in air combustion 3805

DOI: 10.1021/acs.iecr.6b04995 Ind. Eng. Chem. Res. 2017, 56, 3804−3812

Article

Industrial & Engineering Chemistry Research Table 1. Experimental Conditions for Activity Measurement of the Catalystsa experiment set set set set set set set set

reaction conditions

I II III IV V VI VII VIII

CBTs, UV, N2 + 0, 4%, 8% O2. (i) without catalyst, UV; (ii) without UV, CBTs; (iii) UV, CBTs, N2/O2/SO2. (i) CBTs, UV, N2 + 400, 800, 1200 ppm of SO2; (ii) CBTs with Hg0 preloaded, pure N2. (i) N2 + 50, 100, 200, 300 ppm of NO; (ii) BFG + 300 ppm of NO, CBTs, UV. (i) without catalyst, UV; (ii) without UV, CBTs; (iii) UV, CBTs, N2/O2/HCl. (i) N2 + 30, 50 ppm of HCl; (ii) BFG + 50 ppm of HCl, CBTs, UV. CBTs, UV, N2 + 2%, 4% H2O. (i) 1200 ppm of SO2 + 50−300 ppm of NO; (ii) 300 ppm of NO + 400−1200 ppm of SO2, CBTs, UV.

Temperature was 120 °C. BFG (baseline flue gas): 4% O2/N2 mixture, inlet Hg0 concentration was about 50 μg·m−3, and the dosage of catalyst in each experiment was 0.3 g. a

3. RESULTS AND DISCUSSION 3.1. Interference of Flue Gas Constituents on Mercury Analyzer. To investigate the influence of flue gas constituents on Hg0 measurement under UV light irradiation, various kinds of gases were introduced to the reaction system successively. As shown in Figure 2, in the entire experimental duration, the

connecting tubes were made of Teflon to avoid corrosion and minimize mercury species adsorption.23 Furthermore, it should not be ignored that duplicated experiments were conducted to guarantee the reproducibility and validity of the results. Eight sets of experiments were carried out, as summarized in Table 1. Individual gas or combinations of the gases were added to the baseline blend. Set I was designed to figure out the effect of O2 on Hg0 removal. In set II and set III, Hg0 oxidation properties in the presence of SO2 were investigated. Sets IV− VII were conducted to identify the effect of NO, HCl and H2O on Hg0 oxidation, respectively. The aim of set VIII was to confirm the coexistence of NO and SO2 on Hg0 oxidation, which would help elucidate the interactions between SO2, NO, and Hg0. 2.3. Data Processing. Before the testing of Hg0 removal over the nanofibers, blank experiments were conducted to eliminate any possible interference from the system. The results showed interference in Hg0 detection by the empty reactor was verified to be negligible. Furthermore, during each test, the gas stream first bypassed the fixed-bed reactor until a stable inlet concentration of Hg0 was obtained. The Hg0 removal efficiency (η) was quantified by the following formula: η (%) =

C Hg 0

inlet

− C Hg 0

C Hg 0

outlet

× 100 Figure 2. Interference of flue gas constituents on mercury measurement.

(2)

inlet

where CHg0inlet and CHg0outlet represent the concentrations of Hg0 measured at the inlet and outlet of the reactor, respectively. Photon utilization efficiency is the primary factor that determines the effectiveness of photolytic and photocatalytic oxidation processes. Therefore, an appropriate analysis of the photocatalytic oxidation of Hg0 should be carried out in terms of quantum yield: Φ=

no. molecules decomposed A = no. photons absorbed B

instantaneous value of Hg0 fluctuated at 0 ± 0.4 μg·m−3, which is well smaller than the detection limit of the automatic mercury analyzer. This indicates that the interference of flue gas on Hg0 analyzer under UV light was negligible. We could further infer that the variations of Hg0 observed in this study were due to the interaction between catalyst, Hg0, and flue gas atmosphere. It is noted that a catalyst and a sorbent are closely related species and that a catalyst typically first adsorbs mercury before oxidizing it, resulting in initial transient removals of mercury observed by the analyzer.24−26 The BET surface area of CBTs was 258.67 m2/g in the present study. 3.2. Effect of Individual Flue Gas Constituents on Hg0 Photocatalytic Oxidation. The various flue gas components individually flowed into the reactor. Adding the acidic/oxidizing gas constituent one at a time into the carrier gas caused variation in Hg0 oxidation capacity. Effects of flue gas atmosphere on Hg0 oxidation and the reaction pathways over CBTs are summarized in Figure 3, in which BFG was defined as the O2/N2 mixture. The quantum yield (QY, the number of photons utilized for a desired chemical reaction divided by the number of photons absorbed by the catalyst) for TiO2 was

(3)

In the present study, A=

B=

ηC in,Hg 0Qt M

NA

(4)

StI hυ

(5) 0

0

3

Where Cin,Hg is the inlet concentration of Hg (μg/m ), Q the flue gas flow (L/min), t the running time per each experiment (in minutes), M the atomic mass of Hg (g/mol), and NA the Avogadro constant. For B, S is the irradiated area (cm2), h the Planck constant, υ the frequency of light irradiation, and I the light intensity. I could be measured by the light radiometer. 3806


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Figure 3. Effect of individual flue gas constituents on Hg 0 photocatalytic oxidation at 120 °C. Figure 4. Photocatalytic properties in the presence of SO2. Inset is the S 2p spectra of spent catalyst.

16.26%. In particular, the addition of 0.3% Ce into TiO2 achieved a higher quantum yield of Hg0 reduction (44.69%). 3.2.1. Effect of O2 on Hg0 Oxidation. About 38.66% Hg0 removal efficiency was achieved under pure N2 gas flow. Surface oxygen played a significant role in this condition. As ceria-based oxides acquire their oxygen storage capability via a Ce4+ ⇔ Ce3+ process,27 the loss of Hg0 under this condition was due to the reaction between Hg0 and stored oxygen, which was a combination of lattice oxygen and chemisorbed oxygen. Overall, the UV radiation and the abundant surface oxygen on the catalyst were responsible for the performance under N2 atmosphere. When 4% O2 was introduced to the N2 gas stream, η increased to around 89.95%. Further increase in the concentration leads to higher η. It is not difficult to find that O2 played a significant role in Hg0 oxidation over CBTs. Gasphase O2 regenerated the lattice oxygen and replenished the chemisorbed oxygen,28 which had been consumed in the photochemical reaction, and thus enhanced Hg0 removal. 3.2.2. Effect of SO2 on Hg0 Oxidation. Apart from SO2, UV and catalyst could also affect Hg0 oxidation. Experiments were designed to reveal the possible interaction between them, as shown in Figure 4. In the atmosphere containing SO2, CBTs had adverse effects on Hg0 oxidation. It should be noted that UV also decreased η, which is in contrast to the widely accepted view that UV irradiation could activate sample and facilitate Hg0 removal. To explore the involved mechanism, semiquantitative XPS analysis of the spent catalyst was conducted (inset of Figure 4). With the irradiation of UV light, the peak found at 168.7 eV was stronger than that without UV, which was assigned to sulfate (SO42−). In other words, more SO2 reacted with catalyst in the presence of UV, resulting in the deactivation of Hg0 oxdiation performance. It can be seen from Figure 3 that SO2 suppressed Hg0 oxidation. When 800 ppm of SO2 was added to N2, η decreased from 30.34% to 25.55% and further decreased to 24.15% with SO2 content increasing up to 1200 ppm. It is reasonable to conclude that Hg0 oxidation over CBTs coincides with the Langmuir−Hinshelwood mechanism, where reactive sites on catalyst surface react with adjacently adsorbed Hg0 to form Hg2+. And this is in satisfactory agreement with the findings in literature.29 First, a desorption experiment was designed to illustrate the negative impact of SO2, the results of which are

shown in Figure 5. CBTs were saturated by Hg0 at 120 °C with N2 as the carrier gas for several hours. During this process, the

Figure 5. Desorption of Hg0 from catalyst by SO2.

Hg0 concentration displaying on the mercury analyzer decreased at first and then rose up slowly, which was a result of adsorption. Therefore, when mercury concentration was the same as the original value, it indicated that the catalyst had reached its largest adsorption capacity. When this steady state lasted for some time, Hg0 was cut off and adding SO2 at the same time. A spike was observed, as depicted in Figure 5. Without the feeding of SO2, Hg0 concentration just decreased directly. It indicates that SO2 inhibited Hg0 adsorption. Therefore, it is not difficult to find that the competition adsorption between Hg0 and SO2 was responsible for the deactivation of CBTs. More encouragingly, after adding SO2, the Hg0 concentration increased gently, just from 50 μg·m−3 to 60.7 μg·m−3, which is quite smaller than other literature30 and makes our CBTs so unique. It can be deduced that our sample could reach its adsorption saturation quickly. This performance is well in line with some studies.31 Second, SO2 might also react with the CBTs to form thermally stable cerium sulfate, which is responsible for the 3807


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Industrial & Engineering Chemistry Research deactivation of Hg0 removal capacity. XPS spectra of S 2p were then performed to provide more specific information, and results are described in Figure 6. Compared with the fresh

Figure 7. FTIR spectra of the spent catalyst.

bond NO2 and NO3 species, which would facilitate Hg0 oxidation on the surface of the catalyst.35 Subsequently, the band at 1654 cm−1 might be assigned to nitrate species.36 It can be clearly seen that the detection of NO2-containing species, generated from the reaction between NO and active surface oxygen, may be responsible for the Hg0 oxidation over CBTs. This is consistent with the experimental results. Meanwhile, the formation of nitrate happened in the reaction of these species. This might be connected with a series of reactions, such as those listed below:

Figure 6. XPS spectra of S 2p for fresh and spent catalyst.

catalyst, the S peaks for spent samples mainly centered at 168.7 eV, which can be assigned to sulfate (SO4 2−). As a consequence, SO42− species have been considered as strongbinding species that will inhibit Hg0 oxidation. 3.2.3. Effect of NO on Hg0 Oxidation. To probe into the influence of NO on mercury removal, Figure 3 also compares Hg0 removal efficiencies in pure N2 and NO/N2 mixture. First, this test indicates that η was 53.10% for the gas flow containing 50 ppm of NO. Further increase in the concentration to 300 ppm resulted in an even higher η of 78.10%. The presence of Ce4+ on CBTs was reported to accelerate the oxidation of NO to NO2.32 Second, in the presence of O2, a significantly higher η was obtained. With the aid of O2, more adsorbed NO can be oxidized on the CBTs to form abundant active species like NO+ and NO2, which are active for Hg0 oxidation.33 Moreover, the consumed surface oxygen could be replenished by the gasphase O2. Interestingly, η increased up to almost 100% (99.27% actually) when just 4% O2 was introduced to gas flow. Furthermore, this number is also higher than that in the 4%O2/ N2 mixture, indicating again that NO possessed a promoting effect on Hg0 oxidation. To identify the possible mechanisms, the species of flue gas components at the outlet of the reactor were detected by an online gas analyzer. About 39 ppm of NO2 was detected. Moreover, additional experiments were conducted. During the additional tests, NO2 concentrations at the outlet of the reactor were determined by gas analyzer with Hg0 feeding flowed over catalysts and free of Hg0, respectively. Results showed that the concentration of NO2 free of Hg0 was higher than that with Hg0 feeding. It is reasonable to conclude that NO2 could cause Hg0 oxidation. This is entirely consistent with Huang’s point of view, which evidenced that low concentration of NO2 could heterogeneously react with Hg0.34 Besides the above tests, wide survey FTIR spectra were adopted to identify the functional groups, as summarized in Figure 7. When oxygen was introduced, the peak at 1384 cm−1 sharply increased, which could be assigned to nitrite or nitrate species. The broad peak in the range of 1458−1577 cm−1 could be regarded as a series of successive peaks, and they could be assigned to NO2-containing species, like chemisorbed NO2, O-

NO(g) + O2 → NO2 (g) + O

(6)

Hg 0(g) + O → HgO

(7)

Hg 0(g) + 2O + 2NO2 → Hg(NO3)2

(8)

Analysis of XPS spectra was conducted to provide more specific information, and the results were depicted in Figure 8. The O 1s peak of fresh catalyst is resolved into three component peaks, which achieved an acceptable fit in the XPS spectra. The primary peak at ∼529.85 eV, ∼532.02 eV, and ∼533.44 eV can be assigned to lattice oxygen in TiO2, surface hydroxyl groups, and adsorbed oxygen, respectively. Compared with fresh catalyst, peak positions of the spent catalyst under NO are basically the same. After addition of O2 to the atmosphere, the areas of the primary peaks changed apparently, indicating the change of relative concentration. As depicted in Figure 8 c, the concentrations of surface hydroxyl groups and adsorbed oxygen increased, which may originate from the replenishment of gas-phase O2. The XPS spectra of Ce 3d can be divided into eight peaks; the bands labeled u1 and v1 represent the 3d104f1 initial electronic state corresponding to Ce3+, while the peaks labeled u, u2, u3, v, v2, and v3 represent the 3d104f0 state of Ce4+ ions.37 There is no denying that the coexistence of Ce3+/Ce4+ can provide advantageous charge imbalance and give rise to oxygen vacancies.38 On the other hand, ceria-based oxides acquire their oxygen storage capacity via this redox reaction;27 thus the lattice oxygen defects over CBTs surface can also be improved. 3.2.4. Effect of HCl on Hg0 Oxidation. HCl is also found crucial to Hg0 removal.39,40 Figure 9 depicts the interaction between UV, HCl, and catalyst on Hg0 oxidation. Quite different from the properties under SO2, UV and CBTs both 3808


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Figure 8. XPS of O 1s and Ce 3d for (a) fresh catalyst, (b) spent catalyst under NO, (c) spent catalyst under NO/O2.

denotes an active chlorine species for oxidizing Hg0. Under this condition, the possible reaction between CBTs and HCl possibly occurs through31 2CeO2 + 2HCl ↔ Ce2O3 + 2[Cl] + H 2O

(9)

0

With the aid of O2, the Hg removal efficiency reached 95.88%, indicating that CBTs are extremely efficient for Hg0 heterogeneous oxidation by 50 ppm of HCl. By inference, the Deacon process may well explain Hg0 oxidation in the presence of HCl, which was also enlighted by Du et al.45 Hg0 reacted with active chlorine species to form Hg2+, which could be adsorbed onto catalyst surface46,47(e.g., eqs 10−12). 4HCl + O2 → 4[Cl] + 2H 2O

(10)

Hg 0 + [Cl] → HgCl

(11)

HgCl + [Cl] → HgCl2 Figure 9. Photocatalytic properties in the presence of HCl.

(12) 0

3.2.5. Effect of H2O on Hg Oxidation. Water vapor is regarded as an inevitable component of coal combustion flue gas. It has been found to hinder Hg removal,48,49 and the same inhibitive effect was observed over CBTs in this work. The introduction of 2% H2O into pure N2 resulted in a negligible reduction of removal efficiency. An addition of 4% H2O led to about 3% declination of η. The negative impact was to some extent due to the competition between water vapor and Hg0 for active sites, which restrained the Hg0 adsorption. This competitive adsorption for active sites may also prohibit the adsorption of reactive species such as NO and [Cl] in the actual flue gas system, which are more important components affecting Hg0 oxidation. Additionally, the adsorbed water vapor may react with SO3 and CeO2 to form sulfate, which could cover the surface of the catalysts and hinder the Hg0 removal by deactivating the catalysts to a certain extent.50 Nevertheless, it should be noted that the reduction of η in the presence of H2O over CBTs was minor, indicating that our product has excellent resistance to H2O. Furthermore, these tests were to investigate the effects of individual flue gas components on Hg0 oxidation. Thus, only 0.30 g of CBTs was used in each test. Accordingly, this disadvantage could be offset by using more catalysts. Fortunately, the electrospinning method we adopted to prepare samples is a highly versatile method that allows the fabrication of continuous fibers with

enhance Hg0 oxidation with the existence of HCl. HCl was reported to sharply promote Hg0 oxidation, even when the catalyst had been saturated with adsorbed mercury.40,41 Xie et al.19 and Wang et al.42 provided a fundamental perspective into HCl and believed that Hg0 is not directly oxidized by HCl since it is already in a reduced state. Under pure N2 atmosphere, the Hg0 removal efficiency over the catalyst was even lower in the presence of HCl. Niksa et al.43 further demonstrated that Hg0 oxidation by HCl requires the existence of oxygen. But in the present study, the effect of HCl on Hg0 oxidation was investigated in the presence of O2 alone, HCl alone, and both HCl and O2, as summarized in Figure 3. In the presence of HCl alone, 30 ppm of HCl balanced in N2 resulted in η of 75.42%, which is obviously higher than that under pure N2. Lattice oxygen and/or chemisorbed oxygen was responsible for the transformation of HCl to Cl2 or other active chlorine species, which promoted Hg0 oxidation.19,44 More interestingly, Ce-based oxides acquire their oxygen storage capability via the Ce4+ ⇔ Ce3+ process,27 which is significant for Hg0 removal by HCl in the absence of O2. η increased rapidly when HCl concentration further increased to 50 ppm, which is almost the same as the result in the Hg/O2/N2 atmosphere. Increasing HCl content resulted in additional generations of [Cl], which 3809


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Industrial & Engineering Chemistry Research diameters ranging from micrometer to nanometer.51 In addition, the promotional effects of O2, NO, and HCl in a simulated flue gas atmosphere, as previously demonstrated, may outweigh the negative impact of H2O. This is very favorable for application in the actual flue gas system, where water vapor is unavoidable. 3.3. Effects of the Coexistence of SO2 and NO on Hg0 Photocatalytic Oxidation. SO2 and NO are simultaneously presented in the actual flue gas. However, their concentrations will change with coal species and combustion conditions of the boiler.52 Importantly, they have significant influences on the Hg0 removal.43 Furthermore, understanding the interactions between them is an essential part of the simultaneous removal of SO2, NO, and mercury through an integrated process. So a series of experiments with various concentrations of NO and SO2 were carried out. As depicted in Figure 10, when 1200 ppm of SO2 and 50 ppm of NO were introduced to the reaction system, η

Figure 11. Hg0 oxidation with 300 ppm of NO and various SO2 concentrations.

by the variation of SO2 concentration when NO coexisted with SO2, suggesting that the competition reaction between SO2/ NO and Hg0 for the oxidants was not serious. NO was the most dominant gas component enhancing the Hg0 oxidation capacity for CBTs in the tested flue gases. Results again showed that the presence of NO and SO2 improved the Hg0 oxidation. Furthermore, the enhancements caused by the coexistence of these two acidic gases were more significant than those by individual component. On the basis of the analysis above, influences of the coexistence of NO and SO2 on Hg0 oxidation were revealed. 300 ppm of NO and 400 ppm of SO2 were sufficient for the removal of Hg0, so the optimal parameters were also established. It should be emphasized that the tests were conducted without HCl. In other words, Hg0 can be efficiently oxidized over CBTs in the absence of HCl. This distinguishing feature is of tremendous value for Hg0 oxidation under lowrank coal combustion flue gas, which usually comes with low concentration of HCl. Knowledge learned from this part would help elucidate the synergistic effects of acidic gases on Hg0 photocatalytic oxidation. Such studies are of fundamental importance in developing effective pollution control technologies and devices in which the simultaneous removal of SO2, NO, and Hg0 through an integrated process is possible.

Figure 10. Hg0 oxidation with 1200 ppm of SO2 and various NO concentrations.

significantly increased to 41.76%. Further increase in NO concentration to 300 ppm resulted in an even higher efficiency of 81.44%. NO again was observed to significantly enhance the Hg0 oxidation capacity of CBTs, which is well in line with the trend in the presence of NO alone (Figure 3). Moreover, the results also show that the average removal efficiency of Hg0 in the coexistence of SO2 and NO is better than that in the presence of SO2 alone. A similar phenomenon was also found in previous studies,53 and they indicated that Hg0 secondary volatile was effectively prevented, since NO was eventually oxidized to NO3−, which would react with Hg0 to form more stable Hg(NO3)2 or Hg(NO3)2·H2O compared with Hg2+. A further experiment was also conducted with specific NO content and various SO2 concentrations (Figure 11). Baselines 1 and 2 represented pure N2 atmosphere and 300 ppm of NO/ N2 mixture, respectively. According to the results, the coexistence of NO and 400 ppm of SO2 led to a better performance (80.83%) than that from baseline 1 (24.54%) and comparable to that from baseline 2 (78.10%). This means that about 400 ppm of SO2 in flue gas is for the benefit of removing Hg0. Although SO2 alone showed a deteriorating effect on Hg0 oxidation, the removal efficiency of Hg0 was almost unaffected

4. CONCLUSIONS This work has elucidated the influences of flue gas components on Hg0 removal by photocatalysis technology and clarified the mechanisms involved. Results reported herein demonstrate that in the atmosphere containing SO2, both CBTs and UV had negative effect on Hg0 oxidation. The competition between SO2 and Hg0 for active sites and the formation of cerium sulfate are both responsible for the deactivation of Hg0 removal in the presence of SO2. In contrast, the detection of NO2-containing species generated from the reaction promoted Hg0 conversion. Quite different from the properties under SO2, UV and CBTs enhanced Hg0 oxidation with the existence of HCl. Moreover, effects of coexistence of NO and SO2 on Hg0 removal were further investigated. The results indicate that NO significantly enhanced the Hg0 oxidation capacity of CBTs. In the absence of HCl, the competition reaction between SO2/NO and Hg0 for the oxidants was not serious. This distinguishing feature is of tremendous value for Hg0 removal under low-rank coal 3810


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Industrial & Engineering Chemistry Research combustion flue gas. Consequently, it is a promising method to enhance Hg0 conversion over CBTs under UV irradiation. Coal-derived flue gas is a complex mixture containing fly ash particles, oxygen, moisture, carbon monoxide, and many acid gases. A typical untreated flue gas derived from the combustion of a U.S. low sulfur eastern bituminous coal can contain 5−7% H2O, 3−4% O2, 15−16% CO2, 1 ppb total of Hg, 20 ppm of CO, 10 ppm of hydrocarbons, 100 ppm of HCl, 800 ppm of SO2, 10 ppm of SO3, 500 ppm of NOx, and balance N2.5,54−56 There can be potential impacts of other acid gases such as SO3, as well as carbon monoxide and particulates, upon Hg oxidation, and this could be a fruitful area for future study and research. Also, the potential deactivation of the catalyst over time by flue gas species such as SO2, SO3, and Hg also merits further examination.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04995. Details on XRD and TEM analysis of samples. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.Z.: e-mail, [email protected]. *J.Z.: e-mail, [email protected]. ORCID

Lulu Wang: 0000-0003-4646-9974 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Key Basic Research and Development Program (Grant 2014CB238904), the National Key Technologies R&D Program (2016YFB0600604), the National Natural Science Foundation of China (NSFC) (Nos. U1610110, 41672148), and the Foreign Science and Technology Cooperation of Hubei Province (2016AHB021). The authors express their thanks to the China Scholarship Council (CSC) and the Department of Analytical and Testing Center at Huazhong University of Science and Technology for providing financial support and the sample analysis, respectively.

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REFERENCES

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