Simultaneous NO reduction and Hg0 oxidation over La0.8Ce0

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Kinetics, Catalysis, and Reaction Engineering

Simultaneous NO reduction and Hg0 oxidation over La0.8Ce0.2MnO3 perovskite catalysts at low temperature Jianping Yang, Mingguang Zhang, Hailong Li, Wenqi Qu, Yongchun Zhao, and Junying Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01431 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Industrial & Engineering Chemistry Research

Simultaneous NO reduction and Hg0 oxidation over La0.8Ce0.2MnO3 perovskite catalysts at low temperature Jianping Yang 1, Mingguang Zhang 1, Hailong Li 1, *, Wenqi Qu 1, Yongchun Zhao 2, Junying Zhang 2 1

School of Energy Science and Engineering, Central South University, Changsha 410083, China

2

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

University of Science and Technology, Wuhan 430074, China ※

Corresponding

author,

Email:

[email protected],

86-731-88879863

1 ACS Paragon Plus Environment

TEL:

86-18670016725,

FAX:

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2) perovskite catalysts were employed for simultaneous NO and Hg0 removal. The perovskite structure is beneficial for low temperature catalysis. The substitution of A-site cations with cerium (Ce) cations significantly improved the catalytic activity of perovskite catalyst. 90% NO conversion and 98% Hg0 oxidation was attained using La0.8Ce0.2MnO3 catalyst at 200°C. Hg0 oxidation posed negligible effect on NO reduction. Compared to the N2 plus 4% O2 atmosphere, Hg0 oxidation was significantly facilitated by selective catalytic reduction (SCR) atmosphere. The enhancement in Hg0 oxidation was probably attributed to NO2 originated from NO. Furthermore, a possible reaction mechanism was proposed, in which surface oxygen, Mn4+, and Ce4+ contributed to NO and Hg0 removal. Such knowledge provides useful information for the development of effective and economical NO and Hg0 removal technology for coal-fired power plants. Keywords. Perovskite; Mercury; Nitric oxide; Flue gas


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1. Introduction Coal combustion emits large amounts of nitrogen oxides (NOx) and mercury (Hg). NOx, presents mainly as NO, can cause acid rain, ozone depletion, photochemical smog 1. Hg is of extreme toxicity, persistence, and bio-accumulation, which causes serious environmental and health hazards

2, 3

.

Selective catalytic reduction (SCR) with ammonia (NH3) has been commercialized for controlling NOx emission in power plants. A variety of Hg removal technologies including adsorption catalytic oxidation

15-22

4-14

and

have been developed. Among these technologies, activated carbon injection

(ACI) was the most commercially available technology. However, the current control devices for NOx and Hg0 were separated, leading to a high operating cost. Thus, it was attractive to simultaneously remove multiple pollutants using the air pollutant control devices (APCDs). Elemental mercury (Hg0) is the primary Hg form in flue gas, because the high-volatility and water-insolubility makes it difficult to be removed using the APCDs. Accrodingly, a promising strategy is to maximize the amount of water soluble oxidized mercury (Hg2+) in flue gas, which can greatly enhance the co-beneficial removal capacity of APCDs. Recent studies indicate that the SCR catalysts exhibited co-beneficial Hg0 oxidation capacity 23-27. Generally, the Hg0 oxidation ability of commercial vanadia based catalysts was relatively low and significantly depended on the HCl content in flue gas 28. Additionally, although the commercial SCR catalysts presented high NO reduction activity at high temperature (300-430 °C), there were still several drawbacks related to these catalysts. For instance, the high operating temperature window required the SCR catalysts to be located upstream of the air preheater, where high concentration fly ash wore and blocked the SCR catalysts. To avoid the deactivation of SCR catalysts by fly ash, it was essential to develop SCR catalysts with the optimal operating temperature at 100-200 °C. Many 3 ACS Paragon Plus Environment


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metal oxides were used for NO reduciton by NH3 at low temperature 29-35. Among these, manganese (Mn) oxides presented optimal NO reduction activity 36. However, the optimal operating temperature for Mn based catalysts was still as high as 250 °C

34, 37

, limiting its application at the tail-end of

electrostatic precipitator (ESP). Several studies demonstrates that the perovskite type Mn based oxides presented excellent NO reduction performance at low temperature structure was beneficial for NO reduction

38-42

. Compared to other Mn based oxides, the perovskite

41, 42

. Additionally, the perovskite oxides also presented

excellent Hg0 oxidation performance even in the absence of HCl

17, 18, 43, 44

. However, the best

temperature windows of NO reduction (200-250 °C) and Hg0 oxidation (100-150 °C) over perovskite oxides were unmatched. Thus, it was essential to modify the perovskite oxides for integrated removal of NO and Hg0 into one SCR reactor. The general chemical formula of perovskite is ABO3. In general, the A-site is rare-earth or alkaline-earth metal ions and the B-site is occupied by transition metal ions 18. Substitutiing A- and/or B-site cations by other cations could generate oxygen vacancy 45

, which led to abundant surface oxygen and hence significantly enhanced catalytic activity

38-42

.

Accordingly, the enhanced catalytic activity especially at low temperatures warrants the integrated removal of NO and Hg0 into one device. However, whether A- and B-site cations substitutions can make the optimal operating temperature of NO and Hg0 removal into a same window is unknown. Furthermore, understanding the interactions between NO reduction and Hg0 oxidation is also urgently needed to facilitate simulaneous removal of NO and Hg0. In this work, La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2) catalysts were prepared and employed for simulaneous NO and Hg0 removal. The effects of A- and B-site cations substitutions in perovskite oxide on NO and Hg0 removal were investigated. The 4 ACS Paragon Plus Environment

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interactions between NO reduction and Hg0 oxidation were studied. Moreover, the involved mechanism for NO and Hg0 removal was also explored. 2. Experimental 2.1 Catalyst preparation La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2) catalysts were synthesized by a sol-gel method

18

Ca(NO3)2·4H2O,

Ce(NO3)4·6H2O,

Sr(NO3)2·4H2O,

. Given amounts of La(NO3)3·6H2O, Mn(NO3)2·6H2O, Cu(NO3)2·3H2O,

Co(NO3)2·6H2O,

and

Fe(NO3)3·9H2O were firstly dissolved in deionized water. Citric acid monohydrate aqueous solution was then added into the above aqueous solution. The dosage of citric acid monohydrate was same as the metal ions mole amount. The solution was kept stirring at 80 °C for 2 h to obtain clear viscous liquid (sol). The sol was placed at an oven (100 °C) for 10 h to obtain dry gel. After that, the dry gel was ground and calcined at 400 °C for 2 h and at 750 °C for another 4 h. The solid sample was finally ground and sized to 100-120 meshes. 2.2 Characterization The crystalline phase of catalyst was analyzed using X-ray diffraction (XRD, X’Pert PRO diffractometer), which performed by a Cu-Kα radiation source (λ=0.15406 nm) at 40 kV and 40 mA. The diffraction patterns were collected from 10° to 90° (2θ) and the step size was 0.02 °/s. The surface property of catalyst was studied by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi). The binding energy was calibrated by the C 1s binding energy value (284.8 eV). 2.3 Catalytic activity tests The catalytic performances of catalysts were tested by a bench-scale experimental system (Figure 1), which contained a fixed-bed reactor, a flue gas feeding system, a Hg0 permeation system, a flue 5 ACS Paragon Plus Environment


gas analyzer, and a Hg analyzer. The flue gas consisted of N2, 0%/4% O2, 300/500/1000 ppm NO, 300/500/1000 NH3, 20/50/100 µg·m−3 Hg0, which total flow rate was 0.6/1.2 L· min-1. Different gases were feed by cylinders. the flow rate of which were controlled using mass flowmeter. A Hg0 permeation tube (VICI Metronics) kept at 40 °C constantly provided Hg0 vapor, which was brought in the experimental system by pure N2. In each test, 50/100 mg of catalyst was loaded at the reactor (φ=10/15 mm) and the temperature was controlled by a tubular furnace. The NO and Hg0 concentration was monitored by a flue gas analyzer (OPTIMA7, YORK Instrument) and an online Hg analyzer (RA-915+, Ohio-Lumex), respectively. The experimental conditions are shown in Table 1. In Set I experiments, the NO and Hg0 removal performances of LaMnO3 were studied. The performances of LaOx and MnOx were also studied as reference. In Set II, the effects of A- and B-site cations substitutions for La1-xAxMn1-yByO3 on the removal of NO and Hg0 were studied. In Set III, the effects of temperature and gaseous hourly space velocity (GHSV) on NO and Hg0 removal over La0.8Ce0.2MnO3. Set IV experiments were performed to study the simultaneous NO and Hg0 removal performance of La0.8Ce0.2MnO3 at 200 °C under the GHSV of 30000 h-1. In Set V, the influence of Hg0 oxidation process on NO reduction was studied. The instantaneous influence of Hg0 vapor on NO reduction was firstly studied by adding different content of Hg0 into flue gas. The accumulated impact of deposited Hg species on catalyst surface originated from Hg0 oxidation on NO reduction was also evaluated by pretreated catalysts under different atmospheres. In Set VI, the effect of NO reduction on Hg0 oxidation was studied. Firstly, the influence of typical SCR reactants, i.e. NO and NH3, on Hg0 oxidation were studied with an intention to study the instantaneous impact of NO reduction on Hg0 oxidation. Then the Hg0 oxidation performance of pretreated catalysts by NO and/or NH3 were studied so as to investigate the 6 ACS Paragon Plus Environment

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accumulated impact of deposited SCR product and/or intermediates on Hg0 oxidation. Additionally, a desorption test was conducted to interpret the inhibitory role of NH3 in Hg0 oxidation. The fresh catalyst was firstly pretreated in the atmosphere containing 50 µg/m3 Hg0 at 30 °C for 2 h to obtain a Hg-loaded catalyst. Then, a N2 stream containing NH3 was passed through the Hg-loaded catalyst. To deeply explore the reaction mechanism, the surface chemical composition of fresh and spent catalyst after NO and Hg0 removal was compared on the basis of XPS tests. The catalyst was firstly pretreated by passing N2, 4% O2, 500 ppm NO, 500 ppm NH3 and 50 µg/m3 Hg0 at 200 °C for 10 h, and subsequently analyzed by XPS. Meanwhile, the Hg species on the catalyst was also confirmed by a temperature programmed desorption (TPD) experiment. The catalyst pretreatment method for TPD experiment was same as that for the XPS test. The TPD experiment was conducted from room temperature to 700 °C, with a heating rate of 10 °C/min. The desorbed mercury was carried into the mercury analyzer by N2 (250 mL/min). Prior to the tests, the inlet concentration of NO ( NOin ) and Hg0 ( Hg in0 ) was firstly guaranteed with less than 5% fluctuation for more than 30 min. After that, the flue gas was through the catalyst. The outlet concentration of NO ( NO out ) and Hg0 ( Hg 0out ) was measured when the concentration reached stabilization for 30 min,. When the N2 stream containing 50 µg·m−3 Hg0 passed through the catalysts, the catalysts were saturated by Hg0 in 5 min. Thus, the loss of Hg0 should be attributed to the Hg0 oxidation over catalysts. Nevertheless, the catalysts were firstly pretreated by Hg0 under N2 atmosphere at 30 °C until achieved saturation to avoid the possible interference of Hg0 adsorption before each experiment. The NO reduction (ENO) and Hg0 oxidation efficiency (EHg) of catalyst was calculated by the following equations:



ENO =

NOin − NO out × 100% NOin

(1)

EHg =

Hg in0 − Hg 0out ×100% Hg 0in

(2)

3. Results and discussion 3.1 Characterization of catalysts Figure 2 shows the XRD patterns of La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2). The characteristic diffraction peaks of LaMnO3 catalyst were assigned to the perovskite phase of LaMnO3 with orthorhombic structure (JCPDS PDF No.75-0440). The additional segregated phases like LaOx and MnOx or starting materials were not detected. By introducing Ca, Sr, and Ce into A-site and Cu, Co, and Fe into B-site, only single LaMnO3 perovskite phase with orthorhombic structure was observed in the synthesized catalysts. This suggests that these cations were completely incorporated into the LaMnO3 perovskite structure. Similar ionic radio of Sr2+ (0.118 nm), Ca2+ (0.100 nm), Ce4+ (0.101 nm) and La3+ (0.103 nm) allowed these ions incorporating into perovskite A-site, while similar ionic radio of Cu2+ (0.062 nm), Co3+ (0.054nm), Fe3+ (0.055 nm) and Mn4+ (0.053 nm) caused the substitution of these ions into perovskite B-site 40. Figure 3 shows the Mn 2p XPS spectra of La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2). Two peaks assigning to Mn 2p3/2 and Mn 2p1/2 were observed on the LaMnO3 at 635-660 eV. The Mn 2p3/2 broad included two peaks at 642.0 and 644.4 eV, which were assigned to Mn3+ and Mn4+, respectively. Apparently, the Mn species also consisted of Mn3+ and Mn4+ after substituting of La by Ca, Sr, Ce, Co, Cu and Fe in LaMnO3 perovskite catalyst. To quantitatively analyze the effect of ions substitution on the valence change of Mn species, the concentrations of Mn3+ and Mn4+ were calculated by deconvolution of their characteristic peaks. 8 ACS Paragon Plus Environment

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Comparing with the pristine LaMnO3, the Mn4+/Mn3+ ratio for La0.8Ce0.2MnO3 and La0.8Sr0.2MnO3 increased significantly, while the amount of Mn4+ did not change obviously after doping Ca, Cu, Co and Fe (Table 2). Since high-valence Mn species was beneficial for Hg0 oxidation 32, substituting La in LaMnO3 perovskite by Ce and Sr might improve its catalytic activity. Figure 4 shows the O 1s XPS spectra of La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2). The oxygen species in all samples consisted of lattice oxygen (Oα) and chemisorbed oxygen (Oβ). The quantitative calculation of oxygen concentration indicates that the Oβ content in A- and B-site substituted perovskite was much higher than the pristine LaMnO3 (Table 2). Generally, the Oβ content was related to the amount of oxygen vacancy 46. This suggests that the ions substitution of La and Mn in LaMnO3 created oxygen vacancies, thereby causing a increase in the Oβ content

47

. The surface oxygen, especially Oβ, is beneficial for low temperature catalysis

48

.

Therefore, substituting ions of La and Mn in LaMnO3 might facilitate the NO reduction and Hg0 oxidation.

3.2 NO and Hg0 removal performance of LaMnO3 Figure 5 (a) shows that about 90% of EHg was attained over LaMnO3 catalyst at 150 °C under N2 plus 4% O2 atmosphere. However, the EHg greatly decreased once the temperature deviated from 150 °C. As a reference, the Hg0 oxidation performances over La2O3 and MnOx were also studied. As shown in Figure 5(a), LaOx presented almost no Hg0 oxidation activity at the entire reaction temperature range. The highest EHg for MnOx was about 35%. These results fully demonstrate that the combination of La2O3 and MnOx to form perovskite structure resulted in synergy for Hg0 oxidation. Moreover, it could be deduced that Mnn+ is more favorable for Hg0 oxidation than Lan+ in the LaMnO3 perovskite catalyst. 9 ACS Paragon Plus Environment


Figure 5(b) shows the NO reduction performances of LaMnO3, LaOx, and MnOx under SCR atmosphere. As shown, the LaMnO3 catalyst presented superior NO reduction activity compared to LaOx and MnOx. About 80% of ENO was attained at 200 °C under a GHSV as high as 30000 h-1, which was about 10 times of that in a real power plant SCR reactors 47. Compared to the commercial SCR catalysts 25-27, 49, 50, the NO reduction performance of perovskite catalyst was superior regardless of operation temperature and catalyst dosage. The low optimal operation temperature allowed the perovskite catalyst to be located at the tail-end of ESP, which could avoid the deactivation of high-concentration fly ash. Meanwhile, the perovskite catalyst could achieve almost the same performance with a much less dosage. Thus, the perovskite catalysts presented potential to be used as a low temperature SCR catalyst for integrated removal of NO and Hg0.

3.3 Effect of A and B sites substitution on NO and Hg0 removal Substitution of La with Ca led to an obvious decrease of ENO and EHg (Figure 6(a)). Both ENO and EHg for La0.8Ca0.2MnO3 was about 50%, which was less than that for the LaMnO3. Although the total oxygen and Oβ content increased after doping Ca in LaMnO3 (Table 2), the catalytic activity was significantly weakened. This suggests that, besides the surface oxygen, Mn species in B-site of perovskite also acted as an active sites for NO and Hg0 removal. Since doping Ca caused the decrease of Mn content on the catalyst surface, the catalytic activity was weakened. Substituting La with Sr and Ce resulted in an increase of ENO and EHg. Particularly, the EHg and ENO for La0.8Ce0.2MnO3 were 93.8% and 85.5%, respectively. Ce substitution greatly enhanced the catalytic performance of the La0.8Ce0.2MnO3 catalyst. The XPS analysis (Table 2) showed that the partial substitution of La cations with Ce cations significantly increased the amount of chemisorbed oxygen (Oβ). This was due to the shift between Ce3+/Ce4+ and Mn3+/Mn4+ in catalysts, which was favorable 10 ACS Paragon Plus Environment

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to generate abundant oxygen vacancies and active oxygen. Thus, the high-valence Ce4+ and Mn4+ together with bulk active oxygen both contributed to improving the NO and Hg0 removal activity. It is well known that the cations in B-site play a dominant role in catalytic process 40. However, the substitution of B-site cations in the LaMnO3 with Cu, Co and Fe cations did not improve but even weakened the NO and Hg0 removal activity (Figure 6(b)). As shown in Table 2, after doping Cu, Co, and Fe in LaMnO3 catalyst, the Oβ content increased about 20%-25%. This might because substituting Mn cations in perovskite B-site generated oxygen vacancies, hence leading to abundant surface oxygen. However, the Mn content decreased 3.6%-5.1% because Mn cations was partially substituted by other cations. As stated above, the abundant active oxygen and high-valence Mn was beneficial for catalytic activity. However, the NO reduction and Hg0 oxidation efficiency decreased obviously after doping Cu, Co and Fe in LaMnO3. Thus, the promotional effect of increasing Oβ on catalytic activity did not offset the weakening caused by the decreasing Mn, even though the increasing amount (20%-25%) of Oβ was about five times than the decreasing amount (3.6%-5.1%) of Mn species. This could further demonstrate that the surface oxygen and Mn species in perovskite B-site were both responsible for NO and Hg0 removal, and Mn should be more efficient in catalysis compared to surface oxygen. Moreover, the catalytic activity of Mn was much superior than that of Cu, Co, and Fe incorporated in the perovskite B-site.

3.4 NO reduction and Hg0 oxidation performance of La0.8Ce0.2MnO3 Figure 7(a) shows that the Hg0 oxidation performance of La0.8Ce0.2MnO3 was excellent in a wide temperature window. EHg about 95% was attained at 100-250 °C under N2 plus 4% O2 atmosphere with a GHSV of 30000 h-1, and then sharply decreased to approximately 50% at 300 °C. The Hg0 oxidation over the La0.8Ce0.2MnO3 should follow the Mars-Maessen (M-M) mechanism. The gaseous 11 ACS Paragon Plus Environment


Hg0 was firstly bound over the catalyst and subsequently oxidized to mercuric oxide (HgO) by the surface oxygen. Surface oxygen was consumed to generate oxygen vacancies, and gaseous O2 could recover these vacancies. As a result, the gaseous O2 maintained a high Hg0 removal activity. However, high temperature weakened the adsorption of Hg0, limiting the subsequent Hg0 oxidation through the M-M mechanism. Moreover, as the raise of temperature, the HgO adsorbed on the catalyst surface might be desorbed, hence causing the decrease of EHg. ENO significantly increased as the raise of temperature from 100 to 250 °C, then slightly decreased at 300 °C. About 85% of ENO was attained over the La0.8Ce0.2MnO3 in 200-250 °C with a GHSV of 30000 h-1 under SCR atmosphere. With the decrease of GHSV to 20000 h-1 and further to 10000 h-1, ENO of 90% and EHg of 98% were achieved, as shown in Figure 7(b).

3.5 Simultaneous removal of NO and Hg0 To investigate the simultaneous NO and Hg0 removal ability of La0.8Ce0.2MnO3, a relative long time test was carried out under SCR plus 50 µg/m3 Hg0 atmosphere. Figure 8 shows that the simultaneous NO and Hg0 removal efficiency slightly decreased in the overall reaction process. After 14 h test, above 80% EHg and ENO were still obtained. Moreover, since the GHSV was much higher than that in a real SCR unit, higher NO and Hg0 removal efficiency could be attained in practical application.

3.6 Interaction between NO reduction and Hg0 oxidation 3.6.1 Impact of Hg0 oxidation process on NO reduction As shown in Figure 9 (a), similar ENO was attained regardless the addition of 0 or 20 or 200 µg/m3 Hg0 into the SCR atmosphere, implying that the instantaneous effect of Hg0 on NO reduction was unobvious. This was due to the Hg0 concentration (ppb level) in flue gas was thousands of times 12 ACS Paragon Plus Environment

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lower than the NO concentration (ppm level). To explore the effect of Hg0 oxidation product accumulated on catalyst surface during prolong use on NO reduction, the La0.8Ce0.2MnO3 was firstly pretreated in different atmospheres and then used for NO reduction. Figure 9(b) shows that the ENO for catalysts pretreated in SCR plus Hg0 and SCR atmosphere were similar to each other. This implies that little Hg0 oxidation product accumulated on the catalyst in SCR atmosphere or the effect of Hg species deposited on the catalyst is insignificant for NO reduction. The decrease of ENO for catalysts after pretreated in SCR plus Hg0 and SCR atmospheres was probably due to the deactivation by SCR reaction itself rather than Hg0 oxidation product. However, as presented in Figure 9(b), a pretreatment of the La0.8Ce0.2MnO3 catalyst by 4% O2 plus Hg0 for 10 h significantly limited its NO reduction activity. ENO further decreased as the increase of pretreatment time. Figure 9(c) shows that a large number of mercury deposited on the catalyst after pretreated in the 4% O2 plus Hg0 atmosphere. The accumulated amount of Hg increased as the increase of pretreatment time. However, negligible Hg accumulated on the catalyst after pretreated in a SCR plus Hg0 atmosphere. These results indicate that the accumulation of Hg species on catalyst probably inhibited NO reduction. SCR atmosphere alleviated the Hg deposition on catalyst. Therefore, the Hg0 oxidation process had negligible impact on NO reduction during a simultaneous NO and Hg0 removal process.

3.6.2 Impact of NO reduction process on Hg0 oxidation Figure 10(a) shows that NO significantly enhanced Hg0 oxidation. When adding 50 ppm NO into N2 atmosphere, EHg increased from 20.4% to 66.1%. This is because the gaseous NO might be adsorbed on the catalyst and subsequently oxidized by surface oxygen to some active nitrite species like NO2, which were active for Hg0 oxidation to form HgO, Hg2(NO3)2, and Hg(NO3)2 51, 52. This is in agreement with Figure 10(b) that the NO pretreated catalyst exhibited superior Hg0 oxidation 13 ACS Paragon Plus Environment


performance compared to the fresh catalyst. Nevertheless, when increasing the NO concentration to 500 ppm, the Hg0 oxidation activity was not further improved. This might because the amount of surface oxygen for accelerating the formation of active nitrite species is limited compared to NO in the gas flow. The presence of gaseous O2 could replenish the consumed surface oxygen, hence facilitating the Hg0 oxidation by NO. Adding 4% O2 to 300 ppm NO caused more Hg0 oxidation compared to that without O2. This is also in line with Figure 10(b) that the NO plus O2 pretreated catalyst was better than the NO pretreated catalyst and fresh catalyst in Hg0 oxidation. As stated above, NO was converted into active nitrite species over catalyst and hence promoted Hg0 oxidation. Meanwhile, gaseous O2 significantly affected the formation of active nitrite species from NO. Figure 10(b) shows that NH3 played an inhibitive role in Hg0 oxidation whether O2 was presence or not. Even containing 4% O2 in the steam, the EHg decreased to about 64.1% with the addition of 300 ppm NH3. When the NH3 concentration increased from 300 ppm to 500 and 1000 ppm, a more obvious decrease in EHg was observed. The inhibitive effects of NH3 were due to the following aspects: (1) the surface oxygen consumption by NH3; (2) the competition for active sites between NH3 and Hg0

34, 35, 53

. Figure 10(a) shows that O2 could somewhat alleviate the inhibition of NH3 on

Hg0 oxidation, suggesting that NH3 consumed surface active oxygen. To confirm the second hypothesis, a Hg0 desorption experiment by NH3 was performed over Hg-loaded catalyst. Figure 10(c) show that large number of Hg0 was desorbed when a stream containing 300 ppm NH3 was passed through the catalyst. This suggests that NH3 competed active adsorption sites with Hg0; meanwhile, compared to Hg0, NH3 exhibited higher binding affinity on active sites. In summary, the weakening of Hg0 oxidation by NH3 was resulted from the surface oxygen consumption and competition for active sites with Hg0. 14 ACS Paragon Plus Environment

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When NO, NH3 and O2 co-existed, Hg0 oxidation efficiency of 90% was obtained. This suggests that the promotional effect of NO could offset the inhibition of NH3. Figure 10(b) that the NO+NH3+O2 pretreated catalyst present better Hg0 oxidation performance compared to fresh catalyst. This suggests that active species were generated from the reactant of NO reduction process, which could promote the Hg0 oxidation. In this way, the La0.8Ce0.2MnO3 might be promising for integrated removal of NO and Hg0 at low temperature.

3.7 The involved reaction mechanism In order to explore the NO and Hg0 removal mechanism, the catalyst surface chemistry was investigated by XPS. Figure 3(d) and Figure 11(b) shows the Mn 2p XPS spectra of fresh and spent La0.8Ce0.2MnO3 catalysts, respectively. As shown, the amount of Mn4+ decreased while the amount of Mn3+ increased after NO and Hg0 removal. This indicates that part of Mn4+ was reduced to Mn3+. Figure 11(a) and Figure 11(c) shows the Ce 3d spectra of fresh and spent La0.8Ce0.2MnO3, respectively. Ce on the La0.8Ce0.2MnO3 consisted of Ce3+ (u1 and v1) and Ce4+ (u, u2, u3, v, v2 and v3), while Ce4+ was the dominant cerium species. The presence of Ce3+ could generate vacancies and unsaturated chemical bonds 32, hence forming abundant surface chemisorbed oxygen. After NO and Hg0 removal, the Ce4+/Ce ratio decreased from 83.5% to 73.1%. This suggests the occurrence of redox reaction between Ce4+ and Ce3+. The O 1s spectra of fresh and spent La0.8Ce0.2MnO3 catalysts are shown in Figure 4(d) and Figure 11(d). The change of the Oβ/O ratio (from 50.5 to 45.8) suggests that Oβ participated in the NO and Hg0 removal reactions. Figure 11(e) shows the N 1s XPS spectra of spent La0.8Ce0.2MnO3, and NH4+ (400.11 eV) and NO2 (406.51 eV) were detected. NO2 could enhance the oxidation of Hg0. The reduction of NO could be also promoted, since NO2 exhibited a faster reaction rate with NH3 compared to NO 36. 15 ACS Paragon Plus Environment


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The Hg 4f XPS spectra of the spent La0.8Ce0.2MnO3 catalysts are shown in Figure 11(f). Because the peaks corresponding to La and Hg species are difficult to be identified, a Hg-TPD experiment for spent catalyst was further performed. Figure 12 shows that large amount of Hg desorbed at 210-500 °C. The peak at 480 °C was corresponded to HgO

54

. Because the Mn 2p, Ce 4d and O 1s

state changed obviously, the peaks at 235 and 305 °C might be attributed to the recombination of Hg oxides like MnHgO3 and CeHgO3. On the basis of the above analysis, possible reaction mechanisms for NO and Hg0 removal were proposed. The NO reduction proceeded as following: Firstly, gas-phase NH3 and NO was adsorbed on the catalyst surface and converted into NH4+ and NO2, respectively. Then, NH4+ reacted with NO2 and NO to generate NO2[NH4+]2, which further decomposed to N2 and H2O

35

. The Hg0 oxidation

was as following: Firstly, Hg0 was bound with the Mn4+ and Ce4+ cations to form adsorbed mercury (Hg(ad)). Subsequently, the Hg(ad) was oxidized by Mn4+ and Ce4+ cations

55

. During this process,

Mn4+ and Ce4+ cations were reduced to Mn3+ and Ce3+ cations. Meanwhile, Figure 10(a) shows that O2 significantly enhanced the Hg0 oxidation. Thus, it could demonstrate that Oβ participated in Hg0 oxidation, and the consumed Oβ was replenished by O2. The above analysis indicates the Hg0 oxidation product was HgO and some recombination compounds of Hg oxide. Additionally, Hg0(ad) could be oxidized by NO2 forming Hg(NO3)2. The Hg0 oxidation process can be interprated as follows: Hg0(g) + Mn4+ → Mn4+-Hg(ad)

(3)

Hg0(g) + Ce4+ → Ce4+-Hg(ad)

(4)

Mn4+-Hg(ad) → Mn3+Hg(ad)+

(5)

Ce4+-Hg(ad) → Ce3+Hg(ad)+

(6) 16

ACS Paragon Plus Environment

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O2(g) → 2Oβ

(7)

Hg(ad)+Oβ → HgO(ad)

(8)

Mn3+Hg(ad) + + 3Oβ → Mn4+Hg2+O3(ad) (MnHgO3)

(9)

Ce3+Hg(ad) + + 3Oβ → Ce4+Hg2+O3(ad) (CeHgO3)

(10)

Hg(ad) + 2NO2 + O2 → Hg(NO3)2

(11)

Hg(ad) + NO2 → HgO + NO

(12)

4. Conclusions The simultaneous NO and Hg0 removal performance of La1-xAxMn1-yByO3 (A = Ca, Sr, and Ce, B = Cu, Co, and Fe, x=0/0.2, y=0/0.2) catalysts were investigated systematically. Among of these perovskite catalysts, La0.8Ce0.2MnO3 exhibited the best NO and Hg0 removal performance. The NO and Hg0 removal efficiency was 90% and 98% at 200 °C, respectively. Negligible amount of mercury was deposited on the catalyst surface during integrated NO and Hg0 removal. Accordingly, the inhibition of Hg0 oxidation on NO reduction was not obvious. NH3 inhibited the Hg0 oxidation, while NO could partially offset the inhibition of NH3 under SCR atmosphere, since NO2 derived from NO oxidation was efficient for Hg0 oxidation. Surface oxygen, Mn4+, and Ce4+ were all responsible for the high NO and Hg0 removal activity. Such knowledge indicates that the La0.8Ce0.2MnO3 might be promising low-temperature SCR catalyst for simultaneous NO and Hg0 removal.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51476189, 51776227) and Natural Science Foundation of Hunan Province, China (2018JJ1039, 2018JJ3675).

References 17 ACS Paragon Plus Environment


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Table 1 Experimental conditions

Experiment

Sample

Set I

Set III

MnOx, LaOx LaMnO3 LaMnO3, La0.8Ca0.2MnO3 La0.8Sr0.2MnO3, La0.8Ce0.2MnO3 LaMn0.8Cu0.2O3, LaMn0.8Co0.2O3 LaMn0.8Fe0.2O3 La0.8Ce0.2MnO3

Set IV

La0.8Ce0.2MnO3

Set V Set VI

La0.8Ce0.2MnO3 La0.8Ce0.2MnO3

Set VII

La0.8Ce0.2MnO3

Set VIII

La0.8Ce0.2MnO3

Set II

Flue gas components

Temperature (°C)

GHSV (h-1)

N2+4% O2+50 µg/m3 Hg0 N2+4%O2+500ppm NO+500 ppm NH3 N2+4% O2+50 µg/m3 Hg0 N2+4% O2+500ppm NO+500 ppm NH3

100, 150, 200, 250, 300 200

30000

N2+4% O2+50 µg/m3 Hg0 N2+4% O2+500ppm NO+500ppm NH3 N2+4% O2+50 µg/m3 Hg0 N2+4% O2+500ppm NO+500ppm NH3

100, 150, 200, 250, 300 200

30000

N2+4% O2+500ppm NO+500ppm NH3+ 50 µg/m3 Hg0 N2 + 4% O2 + 500 ppm NO + 500 ppm NH3+ 0/20/200 µg/m3 Hg0 Pretreated with SCR+0/50 µg/m3 Hg0 for 10h, and then SCR Pretreated with N2+4% O2 +50 µg/m3 Hg0 for 10h, and then SCR N2 + 50 µg/m3 Hg0; N2 + 4% O2 + 50 µg/m3 Hg0 N2 + 4% O2 + 50/300/500 ppm NO + 50 µg/m3 Hg0 N2 + 300 ppm NH3 + 50 µg/m3 Hg0 N2 + 4% O2 + 300/500/1000 ppm NH3 + 50 µg/m3 Hg0 N2 + 4% O2 + 500 ppm NO + 500 ppm NH3 + 50 µg/m3 Hg0 Pretreated with N2 + 0/4% O2+ 500 ppm NO + 0/500 ppm NH3 for 2 h, and then N2

SCR atmosphere: N2 + 4%O2 + 500 ppm NO + 500 ppm NH3 25


30000

200 200

10000 20000 30000 30000 60000

200

60000

200

60000

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 41

Table 2 Molar concentration of surface atoms on the catalysts (detected by XPS) Sample

Molar concentration of surface atoms (%) La

LaMnO3

19.6

La0.8Ca0.2MnO3

17.1

La0.8Ce0.2MnO3

15.0

La0.8Sr0.2MnO3

14.6

LaMn0.8Co0.2O3

14.0

LaMn0.8Cu0.2O3

17.4

LaMn0.8Fe0.2O3

16.8

Spent La0.8Ce0.2MnO3

16.8

Ca

Ce

Sr

Co

Cu

Fe

7.1 4.1 3.6 9.0 6.3 5.0 4.9

Mn (%)

O (%)

Mn

O

Mn3+/Mn

Mn4+/Mn

Oα/O

Oβ/O

20.3

60.1

84.4

15.6

79.2

20.8

15.3

60.5

83.1

16.9

53.3

46.7

17.8

63.1

79.9

20.1

49.5

50.5

19.0

62.8

74.0

26.0

46.5

53.5

16.4

60.6

84.1

15.9

58.3

41.7

15.1

61.2

83.5

16.5

53.6

46.4

16.7

61.5

82.9

17.1

58.1

41.9

19.1

59.2

82.8

17.2

54.2

45.8

26


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List of figures: Figure 1 Schematic diagram of the experimental apparatus Figure

2

XRD

patterns

of

LaMnO3,

La0.8Ca0.2MnO3,

La0.8Sr0.2MnO3,

La0.8Ce0.2MnO3,

LaMn0.8Cu0.2O3, LaMn0.8Co0.2O3, and LaMn0.8Fe0.2O3 (△ represents LaMnO3)

Figure 3 XPS spectra of fresh samples over the spectral regions of Mn 2p (a) LaMnO3, (b) La0.8Ca0.2MnO3, (c) La0.8Sr0.2MnO3, (d) La0.8Ce0.2MnO3, (e) LaMn0.8Cu0.2O3, (f) LaMn0.8Co0.2O3, (g) LaMn0.8Fe0.2O3

Figure 4 XPS spectra of fresh samples over the spectral regions of O 1s (a) LaMnO3, (b) La0.8Ca0.2MnO3, (c) La0.8Sr0.2MnO3, (d) La0.8Ce0.2MnO3, (e) LaMn0.8Cu0.2O3, (f) LaMn0.8Co0.2O3, (g) LaMn0.8Fe0.2O3

Figure 5 (a) NO reduction and (b) Hg0 oxidation performance of different catalysts (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2. GHSV=30000 h-1)

Figure 6 NO reduction and Hg0 oxidation performance of different catalysts (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2. Reaction temperature: 200 °C, GHSV=30000 h-1)

Figure 7 NO reduction and Hg0 oxidation performance of La0.8Ce0.2MnO3 at (a) different temperature with GHSV of 30000 h-1 and (b) different GHSV at 200 °C (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2)

Figure 8 Simultaneous efficiencies of NO reduction and Hg0 oxidation over La0.8Ce0.2MnO3 in SCR + Hg0 atmosphere (Reaction condition: 50 µg/m3 Hg0, NO=NH3=500 ppm, 4% O2, reaction temperature: 200 °C, GHSV = 30000 h-1)

Figure 9 (a) Effect of Hg0 on NO conversion in SCR + Hg0 atmosphere, (b) NO reduction on 27 ACS Paragon Plus Environment


pretreated catalyst, (c) Hg content in pretreated catalyst (Reaction condition (a) and (b): 50 µg/m3 Hg0, NH3 = NO = 500 ppm, 4% O2, reaction temperature: 200 °C, GHSV = 60000 h-1 )

Figure 10 (a) Effect of NO and/or NH3 on Hg0 oxidation, (b) Hg0 removal efficiency of pretreated catalysts at N2 atmosphere, (c) desorption of Hg0 from catalyst by NH3 (Reaction temperature: 200 °C, GHSV=60000 h-1)

Figure 11 (a) Ce 3d XPS spectra of fresh La0.8Ce0.2MnO3 and (b) Mn 2p, (c) Ce 3d, (d) O 1s, (e) N 1s, (f) Hg 4f XPS spectra of spent La0.8Ce0.2MnO3

Figure 12 Temperature programmed desorption (TPD) spectra of mercury species


Page 28 of 41

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Figure 1 Schematic diagram of the experimental apparatus



Figure

2

XRD

patterns

of

LaMnO3,

La0.8Ca0.2MnO3,

La0.8Sr0.2MnO3,

LaMn0.8Cu0.2O3, LaMn0.8Co0.2O3, and LaMn0.8Fe0.2O3 (△ represents LaMnO3)


Page 30 of 41

La0.8Ce0.2MnO3,

Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 XPS spectra of fresh samples over the spectral regions of Mn 2p (a) LaMnO3, (b) La0.8Ca0.2MnO3, (c) La0.8Sr0.2MnO3, (d) La0.8Ce0.2MnO3, (e) LaMn0.8Cu0.2O3, (f) LaMn0.8Co0.2O3, (g) LaMn0.8Fe0.2O3

(a)

(b)

(c)

(d)

(e)

(f)

(g) 31 ACS Paragon Plus Environment


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Figure 4 XPS spectra of fresh samples over the spectral regions of O 1s (a) LaMnO3, (b) La0.8Ca0.2MnO3, (c) La0.8Sr0.2MnO3, (d) La0.8Ce0.2MnO3, (e) LaMn0.8Cu0.2O3, (f) LaMn0.8Co0.2O3, (g) LaMn0.8Fe0.2O3

(a)

(b)

(c)

(d)

(e)

(f)

(g)


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Figure 5 (a) NO reduction and (b) Hg0 oxidation performance of different catalysts (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2. GHSV=30000 h-1)



Figure 6 NO reduction and Hg0 oxidation performance of different catalysts (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2. Reaction temperature: 200 °C, GHSV=30000 h-1)

(a)

(b)


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Figure 7 NO reduction and Hg0 oxidation performance of La0.8Ce0.2MnO3 at (a) different temperature with GHSV of 30000 h-1 and (b) different GHSV at 200 °C (NO reduction: NO=NH3=500 ppm, 4% O2; Hg0 oxidation: 4% O2)

(a)

(b)



Figure 8 Simultaneous efficiencies of NO reduction and Hg0 oxidation over La0.8Ce0.2MnO3 in SCR + Hg0 atmosphere (Reaction condition: 50 µg/m3 Hg0, NO=NH3=500 ppm, 4% O2, reaction temperature: 200 °C, GHSV = 30000 h-1)


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Figure 9 (a) Effect of Hg0 on NO conversion in SCR + Hg0 atmosphere, (b) NO reduction on pretreated catalyst, (c) Hg content in pretreated catalyst (Reaction condition (a) and (b): 50 µg/m3 Hg0, NH3 = NO = 500 ppm, 4% O2, reaction temperature: 200 °C, GHSV = 60000 h-1 )

(a)

(b)

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Figure 10 (a) Effect of NO and/or NH3 on Hg0 oxidation, (b) Hg0 removal efficiency of pretreated catalysts at N2 atmosphere, (c) desorption of Hg0 from catalyst by NH3 (Reaction temperature: 200 °C, GHSV=60000 h-1)

(a)

(b)

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Figure 11 (a) Ce 3d XPS spectra of fresh La0.8Ce0.2MnO3 and (b) Mn 2p, (c) Ce 3d, (d) O 1s, (e) N 1s, (f) Hg 4f XPS spectra of spent La0.8Ce0.2MnO3

(a)

(b)

(c)

(d)

(e)

(f)



Figure 12 Temperature programmed desorption (TPD) spectra of mercury species


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Abstract Graphic


Simultaneous NO reduction and Hg0 oxidation over La0.8Ce0

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