Low-Medium Temperature-Selective Catalytic Reduction of NO with NH3 over a Mn/Co-MOF-74 Catalyst.

To realize the selective catalytic reduction of NO at low-medium temperatures and avoid secondary pollution, a highly active catalyst Mn/Co-MOF-74 was synthesized. X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric analysis, Brunauer-Emmett-Teller method, and scanning electron microscopy were employed to analyze the physicochemical properties of catalysts with different Mn/Co molar ratios and conjecture about the difference in the catalytic activity. Meanwhile, the effects of the molar ratio of Mn/Co, catalyst dosage, catalyst synthesis conditions, GHSV, and temperature on the NO conversion efficiencies were investigated and found that an optimal NO conversion efficiency of 93.5% was obtained at 200-225 °C. In the end, the stability of Mn/Co-MOF-74 was investigated and found that the catalyst has better sulfur and water resistance, and the NO conversion mechanism was speculated on the basis of characterizations and literature data.

Introduction

As one of the main pollutants released by the burning of fossil fuels, nitrogen oxide (NO) is the main target of flue gas treatment in coal-fired power plants. If its emissions cannot be effectively controlled, it will exacerbate a series of air pollution problems such as acid rain and photochemical smog,[1−5] thereby threatening the ecosystem and human health. By now, NH3-selective catalytic reduction (NH3-SCR) is not only the most widely used NO control technology in coal-fired power plants but also in other industries such as glass, steel, cement, and so forth.[6−10] However, this technology has exposed many troubles in practical applications: the wear, blockage, poisoning, and deactivation of the catalyst occur frequently because the catalyst is usually placed in high sulfur and dusty areas to maintain the reaction temperature; meanwhile, when the power plant is at a low load, the lower flue gas temperature (200–250 °C) can hardly meet the temperature required by SCR, unless an external heat source is introduced.[11−13] Hence, it is of practical significance and commercial value to develop medium- and low-temperature SCR catalysts, which can not only arrange the SCR catalyst backward to reduce the hazards of dust and sulfur dioxide but also expand the reaction temperature and application range of NH3-SCR.

Liu et al.[14] synthesized mesoporous Ce0.8W0.2O by the mechanochemical self-assembly strategy, and a removal efficiency of 100% and a N2 selectivity of 97% in a wide temperature range of 225–400 °C were achieved, better than those of Ce0.8W0.2O prepared by the sol–gel method. Meanwhile, mesoporous Ce0.8W0.2O also has excellent sulfur and water resistances. Although the removal efficiency is reduced by 15% after the addition of SO2 + H2O, it can quickly return to 95% after stopping the flow. In recent years, metal–organic framework (MOF) materials have been widely used in the field of environmental protection due to their large specific surface area, high porosity, controllable pore size, and modifiability.[15−19] In terms of NH3-SCR over MOF materials, researchers had done a lot of work: Sun et al. prepared the Cu-BTC (BTC = benzene-1, 3, 5-tricarboxylate) catalyst and found that it had better selective adsorption of NO in multicomponent flue gas, which is essential for the occurrence of the SCR reaction;[20] Wang et al.[21] synthesized the MIL-100(Fe) (iron(III) 1,3,5-benzenetricarboxylate hydrate) catalyst by the hydrothermal method and utilized it to perform the SCR experiments at 200–300 °C, in which more than 80% removal efficiency was achieved, and the highest removal efficiency 99% was obtained at 275 °C. Further research adopted MIL-100(Fe) and CeO2 as the carrier and active ingredient to prepare 0.08IM-CeO2/MIL-100(Fe) catalysts, and the characterization results proved that the presence of CeO2 successfully improved the oxygen absorption capacity of the composite catalyst, which is why the effective range of the catalytic temperature can be expanded to 175–300 °C. Sun et al.[22] prepared a Ni-MOF catalyst that can maintain the effective crystal structure below 440 °C using the hydrothermal synthesis method, which successfully improved the service life of the MOF catalyst. In particular, more than 90% NO removal efficiency was achieved in a wide temperature range of 275–400 °C. Overall, although the MOF catalyst has many advantages, its sulfur resistance and water resistance need to be improved.

According to literature, manganese-based materials are the most widely used medium- and low-temperature SCR catalysts. Kapteijn et al.[23] used manganese oxides with different valence states to conduct a series of NO removal experiments, and the results indicated that the valence of Mn has a great influence on the catalytic activity. Meanwhile, researchers also found that the catalytic activity of Mn-based catalysts decreases in the order of Mn4+ > Mn3+ > Mn2+ at 177 °C. In addition, Tian et al.[24] used different methods to prepare MnO2 in the state of nanoparticles, nanorods, and nanotubes and studied the effects of different morphologies on the denitrification efficiency and found that the removal efficiency of all catalysts at GHSV of 36 000 h–1 increased with the rising of the reaction temperature, among which the denitrification efficiency of nanorod MnO was the highest, reaching more than 90% at 250–300 °C and close to 100% at 300 °C. It must be pointed out that the shortcomings of the MNO catalyst are also obvious; the sulfur resistance of the catalyst is not strong and irreversible, and the N2 selectivity decreases with the increase of the reaction temperature.[25] Fortunately, these defects can be overcome by doping with other elements to form binary or even ternary composite catalysts.

At present, several related studies have shown that doping Co can improve the physicochemical properties of Mn-based catalysts.[26] Boningari et al.[27] investigated the NO removal efficiency using the MnCo/TiO2 catalyst and found that the addition of Co indeed improved the NO removal efficiency. Shi et al.[28] explored the sulfur resistance of Mn–Co nanorods and found that the addition of Co significantly enhanced the sulfur resistance; when 100 ppm of SO2 was added into the reactor, the NO removal efficiency only dropped by 9%. The goal of this paper is to combine the advantages of Mn, Co, and MOF and propose a novel low–medium temperature NO removal method to avoid the above drawbacks. In detail, we will synthesize a novel Mn/Co-MOF-74 catalyst by the microwave method, and the formation and crystallization only take 4 h. Meanwhile, the influence of synthesis conditions and reaction factors on the NO removal efficiency will be investigated; the sulfur and water resistances of the catalyst and N2 selectivity would also be evaluated. Finally, the NO removal mechanism will be speculated based on the serial characterization of reaction products and catalysts.

Experimental Section

Chemicals

All the chemicals were of analytical grade and purchased from Kermel Reagent Manufactories (Tianjin, China). MnCl2·4H2O (99%), Co(NO3)2·6H2O (99%), and 2,5-dihydroxyterephthalic acid (DHTP, 99%) were used for preparing the catalyst. Deionized water, anhydrous ethanol, and methanol were used for dissolving and washing.

Synthesis of Catalysts

The MOF-74 catalyst loaded with manganese and cobalt oxides (named as Mn/Co-MOF-74) was prepared by the microwave hydrothermal method. In detail, MnCl2·4H2O and Co(NO3)2·6H2O were mixed with DHTP (0.1333 g), DMF (53 mL), CH3CH2OH (8.5 mL), and H2O (8.5 mL); then, the resulting solution was put into a Teflon-lined autoclave and heated at 130 °C in a microwave hydrothermal apparatus (XH-800S) for 4 h. After cooling to room temperature, the mother liquor was filtered out, and the products were washed and soaked in methanol for 1 h. Then, the methanol solvent was decanted and replaced every 1 h during the next 6 h, until the supernatant was transparent. The sediment was removed from the bottom and dried in an air-drying oven at 80 °C for12 h.

Characterization

Characterizations of the catalysts were performed by various methods: X-ray diffraction (XRD, Bruker D8 Advance) in the range of 10–90° with a scanning speed of 4/min was carried out to determine the composition of the catalyst. Thermogravimetric analysis (TGA, TA7000) was adopted to analyze the thermal stability of the catalyst. A scanning electron microscope equipped with a Hitachi S-4800 (Japan) system was used to observe the microstructure of the catalyst, and X-ray photoelectron spectroscopy (XPS, PHI X-tool) calibrated by the C 1s peak at 284.6 eV was conducted to investigate the surface chemical composition and elemental speciation. Besides, nitrogen adsorption and desorption isotherms obtained using the Brunauer–Emmett–Teller (BET) method (Quantachrome SI) were employed to analyze the surface area of the catalyst.

Experimental Procedures

As illustrated in Figure , the experimental system mainly consists of simulated flue gas generation, catalytic reduction, and tail gas detection system. The core part is the catalytic reduction device, in which the selective catalytic reduction experiments of NO with NH3 were conducted in a fixed-bed quartz reactor with approximately 0.2 g of catalyst. Before the start of the experiment, the catalysts were pretreated in nitrogen at 250 °C for 3 h, whose purpose was to remove the residual organic solvents on the surface of the catalyst. N2, NO, NH3, and SO2 cylinder gases (1–4, 99.99%, North Special Gas Co., Ltd., China) controlled by mass controllers (7) were employed to simulate the flue gas, and water vapor was generated by using N2 as the carrier gas to purge the liquid water heated by the electrical heater (6). The total flue gas flow rate of the fixed-bed quartz reactor with an inner diameter of 10 mm (10) was 300 mL/min, and the typical composition of the reactant gas was as follows: 500 ppm NO, 500 ppm NH3, 6% O2, 500 ppm SO2 (only when used), and 5% H2O (only when used). After the reaction was completed, the gas passed through the absorption bottle (11) and drying tower (12), and the concentrations of NO, SO2, and O2 were measured using a multifunctional flue gas analyzer (13) (detection limit of 1 ppm/m3, ECOM-J2KN, German RBR company). In this paper, the NO removal efficiency (η) and N2 selectivity were obtained from the following equation

Figure 1

Schematic diagram of the experimental apparatus. (1–5) N2, NO, SO2, O2, and NH3 gas cylinders; (6) electrical heater; (7) water; (8) mass flow meters; (9) mixer; (10) fixed-bed reactor; (11) absorption bottle; (12) drying tower; and (13) multifunctional flue gas analyzer.

Results and Discussion

Characterization of Catalysts

In order to analyze the crystalline phase composition of the synthesized catalyst, XRD was performed, and the results are shown in Figure . Four characteristic peaks at 2θ = 6.7, 11.7, 24.4, and 25.7° were all observed in the spectra of the catalysts with different Mn and Co ratios, consistent with the literature data,[29−31] indicating that the synthesis of Mn/Co-MOF-74 was successful. Besides, the strength of the characteristic peak appeared to show more obvious difference with the change of the ratio of Mn and Co, proving that Mn and Co elements had been loaded successfully. It is worth noting that when the Mn/Co ratios are 3:1 and 2:1, respectively, a group of strong peaks in the range of 2θ = 18∼21° are observed, and the corresponding substances are determined as MnO2 and Mn2O3 by referring to literature[32,33] because in the microwave hydrothermal synthesis process, when the Mn content is too high, the oxidation and crystallization are likely to occur on the surface of the catalyst, and the manganese component eventually exists in the stable state of MnO2 and Mn2O3. Besides, when the Mn/Co ratios are 3:1 and 2:1, respectively, the characteristic peaks of Co3O4 also appear at 27.5 and 32.9°. Overall, it can be concluded that the molar ratio of Mn/Co is a key synthesis factor, and oxides will be formed on the surface of the catalyst and affect the NO removal efficiency when the content of Mn or Co is too high.

Figure 2

XRD patterns of Mn/Co-MOF-74.

XPS was also used to explore the surface elemental composition and valence of the Mn/Co-MOF-74 catalyst. As depicted in Figure a, it can be found that the Co element presents four characteristic peaks within 770–815 eV, and the height of the characteristic peak changes with the content of Co in the catalyst. According to literature reports,[34,35] for Co 2p, the two main peaks at 781.7 and 797.6 eV are attributed to Co3+ and Co2+, respectively, and the peaks at about 803.2 and 786.5 eV are attributed to the satellite peaks. This shows that the Co element exists on the surface of the catalyst in two valences: Co2+ and Co3+. As shown in Table , the relative atomic percentage concentration of Co3+/Co is higher than that of Co2+/Co. Generally, Co3+ is much more reactive than Co2+ because of its stronger redox ability.

Figure 3

XPS spectra of Co (a), Mn (b), and O (c).

Table 1

Relative Atomic Percentages of Different Valence Elements

percent of valence state % (binding energy, eV)
catalysts	Mn4+	Mn3+	Co3+	Co2+	Mn4+/Mn3+	Co3+/Co2+
Mn/Co = 3:1	76.47	23.53	65.72	34.28	3.25	1.92
Mn/Co = 2:1	72.59	27.41	65.13	34.87	2.65	1.87
Mn/Co = 1:1	62.49	37.51	65.81	34.19	1.67	1.92
Mn/Co = 1:2	72.18	27.82	66.51	33.49	2.59	1.99
Mn/Co = 1:3	72.85	27.15	67.48	32.52	2.68	2.08

The Mn 2p spectra of five catalysts are presented in Figure b, which are mainly divided into two categories: the peak centers at 654.3 and 643.1 eV can be explained as Mn4+, and the peak at 641.7 eV is assigned to Mn3+.[36−41] As shown in Table , the percentage of Mn4+/Mn on Mn/Co-MOF-74 is higher than that of Mn3+/Mn. According to the reports, Mn4+ participates in the reversible oxidation–reduction reaction cycle, which can improve the ability of NO to oxidize to NO2, being beneficial for promoting the rapid SCR reaction.[42] It can be seen from Figure c that the types of O on the catalyst surface are mainly divided into two types: the peak centers at 532.3–532.6 eV are explained as O in C–OH, and another oxygen peak at 531.7–531.9 eV is assigned to C=O.[43,44] Overall, it can be concluded that the main surface active substances of Mn/Co-MOF-74 are Co3+ and Mn4+; abundant Mn4+ and Co3+ are presented on the surface of the Mn/Co-MOF-74 catalyst, which is considered to be one of the factors for increasing the catalytic activity.

To determine the structural change and thermal stability of the prepared Mn/Co-MOF-74 catalyst, TG experiments were performed, and the results are shown in Figure . The mass loss of Mn/Co-MOF-74 catalysts with different ratios is roughly similar. For the catalyst with Mn/Co = 1:1, three significant stages of mass loss were observed during the experiment:[45] in the first stage, at 30–150 °C, the catalyst mass loss was 16.5%, mainly due to H2O and methanol on the catalyst surface; in the 150–316 °C stage, the mass loss of the catalyst was about 13.1%, mainly due to the solvent DMF on the surface of the catalyst. The final stage is at 316–576 °C, in which the structure of the catalyst begins to collapse, and it is completely destroyed at 576 °C. The above results indicate that the activation of the catalyst at 200 °C before the experiment can effectively remove the residual solvent on the surface of the catalyst, exposing more active sites of the catalyst, which is beneficial to the SCR reaction.

Figure 4

TG curves of the Mn/Co-MOF-74 catalyst.

The textural properties of Mn/Co-MOF-74 catalysts were measured at −196 °C and summarized in Table It can be found that with the change of the molar ratio, the catalyst with Mn/Co = 1:1 has the largest BET surface area and pore volume and the smallest pore size, which are 212.84 m/g, 0.24 cm3, and 3.94 nm, respectively.

Table 2

Summary of the Textural Parameters of the Catalysts

catalysts	pore volume (cm3/g)	BET surface area (m2/g)	pore size (nm)
Mn/Co = 3:1	0.06	35.99	6.60
Mn/Co = 2:1	0.04	59.44	6.03
Mn/Co = 1:1	0.24	212.84	3.94
Mn/Co = 1:2	0.09	72.82	5.01
Mn/Co = 1:3	0.11	103.74	4.19

Pore size distribution (Figure ) has shown that the Mn/Co-MOF-74 catalyst is a mesoporous material, and the pore size is mainly distributed in the range 3–6 nm. The pore volume of Mn/Co = 1:1 is larger than the other ratio catalysts. Combined with the XRD results, there are some oxides like MnO2 and Mn2O3 or Co3O4 on the surface of Mn/Co-MOF-74 catalysts with other molar ratios; this may be the reason for the reduction of the specific surface area. When Mn/Co = 1:1, the two metal elements exist in a coordinated form in the catalyst; the mesoporous structure of the oxide covering the catalyst is not formed on the catalyst surface, so the specific surface area of Mn/Co = 1:1 is the largest. The unique mesoporous structure with a large BET surface area helps to improve the gas adsorption capacity of the catalyst and enhance the mass-transfer rate between the active sites and NH3 or NO, thereby improving the NO removal efficiency.

Figure 5

Pore size distribution of Mn/Co-MOF-74 catalysts.

The morphology of Mn/Co-MOF-74 catalysts with different molar ratios was observed by scanning electron microscopy (SEM), and the results with different magnifications are shown in Figure . The catalyst with a molar ratio of Mn/Co = 3:1 has a layered lumpy morphology, and the overall particle size of the catalyst is relatively large. Combined with the XRD results, it is speculated that the generated MnO2 and Mn2O3 cover the surface and make the particles bigger. When the Co content gradually increases to Mn/Co = 1:1, the particles of the catalyst change from large into small. In addition, it can be seen from Figure c that the surface of the catalyst has obvious edges, corners, and a regular shape, which can improve the catalytic activity. However, as the Co content further increased, the particle size of the catalyst became larger again, and some deposits appeared on the surface. XRD characterization results suggest that the deposit is Co3O4, which can cover the active sites on the catalyst surface, being unconducive to the catalytic reduction of NO.

Figure 6

SEM images of Mn/Co-MOF-74 catalysts: (a,b) Mn/Co = 3:1; (c,d) Mn/Co = 2:1; (e,f) Mn/Co = 1:1; (g,h) Mn/Co = 1:2; and (i,j) Mn/Co = 1:3.

Effect of Synthesis Conditions

In order to study the effect of catalyst synthesis conditions and experimental conditions on the NO removal efficiency and facilitate the subsequent industrial applications, a series of experiments were designed and carried out. It is generally believed that the synthesis conditions, especially the temperature and time, have greater impacts on the catalytic removal experiments. The effects of synthesis temperature and time on the NO removal efficiency of the Mn/Co-MOF-74 catalyst were first investigated.

It can be seen from Figure a that the Mn/Co-MOF-74 catalyst synthesized by the microwave method exhibits a NO removal efficiency of more than 80% at 175–250 °C. The catalyst with a synthesis temperature of 130 °C has the highest NO removal efficiency, 93.5%, at 200 °C. The effect of synthesis time on NO removal is illustrated in Figure b; it is not difficult to find that the catalyst synthesized in 4 h has the highest removal efficiency at each temperature, followed by 2 and 6 h. With the increasing synthesis time, the metal oxides gradually form and crystallize. Once the synthesis time is insufficient, the synthesis of oxides is incomplete; however, the oxides will form on the surface of the catalyst if the synthesis time is too long, which would reduce the catalytic activity of the catalyst. Therefore, in this paper, the optimal synthesis time is selected as 4 h.

Figure 7

Effect of synthesis conditions. (a) Synthesis temperature and (b) synthesis time (flue gas flow is 600 mL/min, NO=NH3 = 500 ppm, O2 = 6%, and catalyst dosage = 0.2 g).

Effect of Molar Ratio of Mn/Co

According to the results of XRD, SEM, and BET, the catalyst with a molar ratio of Mn/Co = 1:1 has an optimal catalyst structure. To verify this result, the effect of different Mn/Co molar ratios on the removal efficiency of NO was performed. It can be seen from Figure that when the temperature is in the range of 125–200 °C, for the catalysts with molar ratios Mn/Co = 3:1, Mn/Co = 2:1, and Mn/Co = 1:1, the NO removal efficiency increases as the temperature rises. When the temperature is between 200 and 225 °C, the NO removal efficiency reaches the highest level and then drops. In contrast, the NO removal efficiency using catalysts with molar ratios of Mn/Co = 1:2 and Mn/Co = 1:3 does not show obvious change. In particular, the highest efficiency occurred when the Mn/Co ratio was 1:1 and the reaction temperature was 200 °C, which is consistent with the analysis result of the above characterizations: the catalyst with a molar ratio of Mn/Co = 1:1 has the optimal structure and higher NO removal efficiency.

Figure 8

Effect of the molar ratio of Mn/Co (flue gas flow is 600 mL/min, NO=NH3 = 500 ppm, O2 = 6%, and catalyst dosage = 0.2 g).

Effect of Catalyst Dosage and GHSV

It is generally considered that the catalyst dosage has a greater effect on the removal efficiency; therefore, the effect of catalyst dosage on the NO removal efficiency is investigated in this work, and the results are shown in Figure . It can be found that as the dosage increases from 0.1 to 0.2 g, the NO removal efficiency increases significantly from 77.6 to 93.5%. When the catalyst dosage increases from 0.2 to 0.3 g, the NO removal efficiency does not change so much. This indicates that with the increase of the catalyst dosage, the NO removal efficiency is improved, although this improvement is limited. Considering the economics of the experiment, the optimal dosage of the catalyst is chosen as 0.2 g.

Figure 9

Effect of the catalyst dosage and GHSV (flue gas flow is 600 mL/min, NO=NH3 = 500 ppm, O2 = 6%, and temperature = 200 °C).

The effect of GHSV on removal efficiency is also shown in Figure . It can be found that the removal efficiency is the highest when GHSV is 45859.9 h–1. The increase of GHSV would reduce the contact time between NO/NH3 and Mn/Co-MOF-74. When GHSV was too fast, the flue gas could not fully contact the catalyst, and the catalytic reaction was not completed, resulting in the decrease of removal efficiency. Therefore, the optimal GHSV in the experiment is 45859.9 h–1.

N2 Selectivity of Mn/Co-MOF-74

N2O and NO2 are the byproducts of the SCR reaction. It is meaningful to test the amount of N2O and NO2 produced by the Mn/Co-MOF-74 catalyst at different temperatures and calculate the N2 selectivity. It can be seen from Figure that almost no NO2 was generated during the entire experiment, no N2O is generated at 125–225 °C, and a small amount of N2O is released at 250 °C; currently, the N2 selectivity is 94.7%. In general, the catalyst could maintain the high N2 selectivity and NO removal efficiency.

Figure 10

N2 selectivity and N2O generation amount (flue gas flow is 600 mL/min, NO=NH3 = 500 ppm, O2 = 6%, and catalyst dosage = 0.2 g).

Stability of Mn/Co-MOF-74

Stability and sulfur and water resistances are important indicators for the long-term use of low-temperature denitrification catalysts. As shown in Figure , the NO removal efficiency was stabilized at 90% in 16 h without SO2 and H2O, but when SO2 and H2O were added, the NO removal efficiency dropped rapidly to 66.7% and then stabilized, partly due to the competitive adsorption of SO2 and H2O with the reactants, which affected the active sites on the catalyst surface. Another reason was that SO2, H2O, and NH3 would generate sulfites on the surface of the catalyst, covering the catalyst active sites and decreasing the NO removal efficiency. Fortunately, after cutting off the supplements of SO2 and H2O, the NO removal efficiency gradually recovered to 74%, proving that the Mn/Co-MOF-74 catalyst has good sulfur and water resistances.

Figure 11

Stability test of Mn/Co-MOF-74 (flue gas flow is 600 mL/min, NO=NH3 = 500 ppm, O2 = 6%, SO2 = 500 ppm, H2O = 5%, catalyst dosage = 0.2 g, and temperature = 200 °C).

Conversion Mechanism

Figure compares the XRD and XPS characterizations of the fresh and spent (reacting at 200 °C for 10 h) catalysts. It can be found from Figure a that although some impurity peaks appeared on the catalyst after the reaction, the characteristic peaks of the catalyst at 2θ = 6.7, 11.7, 24.4, and 25.7° did not change significantly, indicating that the structure of the catalyst was not destroyed. Figure b–d indicates that the element types and valence states on the catalyst surface before and after the reaction did not change, but the results in Table showed that the ratio of Mn4+/Mn3+ and Co3+/Co2+ in the catalyst decreased after the reaction, indicating that Mn4+ and Co3+ in NH3-SCR play a leading role.

Figure 12

Fresh and spent catalysts: (a) XRD patterns and (b–d) Co XPS (b), Mn XPS (c), and O XPS (d) spectra.

Table 3

Relative Atomic Percentages of Different Valence Elements

percent of valence state % (binding energy, eV)
catalysts	Mn4+	Mn3+	Co3+	Co2+	Mn4+/Mn3+	Co3+/Co2+
fresh	60.38	39.62	66.99	33.01	1.52	2.03
spent	55.54	44.46	65.41	34.59	1.25	1.89

Figure shows the NH3-TPD curves of Mn/Co-MOF-74 (Mn/Co = 1:1) at 100–350 °C. There is a clear NH3 desorption peak on the surface of the Mn/Co-MOF-74 catalyst. The peaks can be divided into three peaks at 171.4 °C (831.1 μmol/g), 236.7 °C (381.0 μmol/g), and 296.9 °C (763.4 μmol/g) (Table ), indicating that there are three types of acidic centers in Mn/Co-MOF-74 and that Mn/Co-MOF-74 has a strong NH3 adsorption capacity.

Figure 13

NH3-TPD curves of Mn/Co-MOF-74 (Mn/Co = 1:1).

Table 4

Amount of Desorption of NH3 in TPD of Catalysts

peak number	temperature (°C)	quantity (μmol/g)
1	171.4	813.1
2	236.7	381.0
3	296.9	763.4

Based on the characterization results and related literature, the mechanism of selective catalytic reduction of NO at a low–medium temperature over the Mn/Co-MOF-74 catalyst is speculated as follows: (1) first, gaseous NH3(g) in the flue gas is adsorbed and converted to adsorbed NH3(a) by the active sites on the catalyst surface, as depicted in eq , which is the prerequisite and key to the catalytic reaction; (2) then, the adsorbed NH3(a) reacts with the active sites on the catalyst surface containing Co3+ and Mn4+ to form the intermediates NH2(a) and H radicals, by which the reduced Co2+ and Mn3+ ions are generated, as depicted in eqs and 3. It is worth noting that, according to the relevant reports on the Eley–Rideal (E–R) mechanism, NH2(a) is an important intermediate in the selective catalytic reduction of NO.

(3) Finally, as depicted in eqs and 7, the removal products N2 and H2O are produced via the reaction between the intermediate NH2(a) and gaseous NO; meanwhile, the reduced Co2+ and Mn3+ ions would also be oxidized to Co3+ and Mn4+ by oxygen in the flue gas (eqs and 6), thereby ensuring the continuation of the selective catalytic reduction of NO.

Conclusions

A series of bimetallic Mn/Co-MOF-74 catalysts were successfully synthesized via the microwave hydrothermal method, and the experimental results showed that Mn/Co-MOF-74 with a molar ratio of Mn/Co = 1:1 exhibited the optimal catalytic performance; a 93.5% NO removal efficiency was obtained at 200–225 °C, with a catalyst dosage of 0.2 g, a synthesis temperature of 130 °C, and a synthesis time of 4 h. The abovementioned excellent characteristics could be attributed to the smooth mesoporous structure and the atomic ratios of Mn4+/Mn3+ and Co3+/Co2+ on the catalyst surface, which played important roles in improving the NO removal efficiency at a low temperature. Furthermore, the Mn/Co-MOF-74 catalyst also exhibited excellent sulfur and water resistances, confirming the feasibility of using bimetal MOF as a new catalyst for the SCR reaction.