Performance of Mn-Ce co-doped siderite catalysts in the selective catalytic reduction of NOx by NH3

WEI Yu-liang, GUI Ke-ting, LIU Xiang-xiang, LIANG Hui, GU Shao-chen, REN Dong-dong

(Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China)

Abstract: Siderite, rich in the transition elements, is an idea material to prepare the catalysts for the selective catalytic reduction (SCR) of NOx by NH3. In this work, siderite was doped with Mn and Ce and the performance of Mn-Ce co-doped siderite catalysts in the removal of NOx (de-NOx) by SCR with NH3 was then investigated. The results illustrate that FeCO3 as the main component of siderite can be converted into Fe2O3 by calcination at 450 ℃. The doping of siderite with Mn and Ce can enhance the surface area and acidity of siderite and reduce the thermal stability of ammonium sulfate formed on the catalyst surface. As a result, the Mn-Ce co-doped siderite catalysts exhibit high efficiency in the de-NOx by SCR and high resistance against sulfur. Over the 3%Mn1%Ce-siderite catalyst, high NOx conversion (>90%) is achieved in the temperature window of 180-300 ℃; moreover, the NOx conversion remains above 75% even after introducing SO2 for 7.5 h.

Key words: Mn-Ce co-doping; siderite; selective catalytic reduction; de-NOx; sulfur resistance

At present, the development of catalysts for the removal of NOx(de-NOx) from the flue gas by the selective catalytic reduction (SCR) of NOxwith NH3at low temperature has become a hot research spot in the field of air pollution control[1-3]. Since the electrons in the d orbital of the cations of the transition metal (like Fe and Mn) oxides are unsaturated, when used as the catalyst in the de-NOxby SCR, the transition metal oxides can easily extract electrons from NH3and form NH2radicals and H+ions, promoting the SCR reaction. Therefore, the catalysts with Fe and Mn as the active components show high activity in SCR at low temperature[4-7]. For instance, Ellmers et al[8]found that Fe-ZSM-5 was highly active in de-NOxby SCR at low temperature. Yao et al[9]reported that the iron-based catalyst displayed high de-NOxactivity at 200-290 ℃. Wu et al[10,11]observed that the de-NOxefficiency over MnOx/TiO2catalyst exceeded 90% at 150 ℃ and reached 95% at 200 ℃. Besides, it was also found that doping the catalyst with Ce could not only improve the de-NOxefficiency, but also inhibit the sulfation of the active component[12-17]. As Fe and Mn oxides together are excellent medium for the low temperature de-NOxby SCR, it was then expected that the doping of Mn-Fe catalyst with Ce can further promote its resistance against SO2.

On the other hand, siderite is a widely distributed mineral, in which FeCO3appears as the major component and Mn element is present in a small amount. Taking the advantages of both Fe and Mn oxides and possible synergy between two elements, siderite could be an idea material to prepare the catalysts for SCR with high de-NOxefficiency at low temperature. To further improve the low temperature catalytic activity and sulfur resistance, the calcined siderite was doped with Mn and Ce. The performance of Mn-Ce co-doped siderite catalysts in the de-NOxby SCR was then investigated in a fixed bed reactor. Furthermore, the pore structure, crystal phase and surface acidity were characterized by means of XRF, N2sorption, XRD and TGA, to build a relation between the catalyst structure and performance.

1 Experimental

1.1 Catalyst preparation

Siderite was doped with the Mn and Ce elements through mixing and stirring, as illustrated in Figure 1. Briefly, siderite was first crushed and ground by a ball mill and then sieved to 120-180 mesh powder. A certain amount of siderite powder was then calcined and converted from FeCO3to Fe2O3. After that, the calcined siderite powder was put into a mixed solution of Mn(NO3)2(50%) and Ce(NO3)2; the mixture was then stirred for 2.0-3.0 h and dried at 100 ℃ for 3 h, to obtain the Mn and Ce modified siderite powder. Finally, the Mn and Ce doped siderite powder was calcined again at 450 ℃ for 5 h in a muffle furnace; the resultant catalysts are labeled asx%Mny%Ce-siderite, on the basis of the mass fraction of each active elements. For comparison, single Mn doped siderite catalyst was also prepared following the same procedures without adding Ce(NO3)2in the impregnation solution.

Figure 1 Flow chart for the preparation of Mn and Ce co-doped siderite catalyst

1.2 Catalytic activity measurements

The activity of siderite catalysts in the de-NOxby SCR was tested in a fixed bed reactor, as shown in Figure 2. NO, NH3, O2, N2, and so on (≥ 98%) purchased from Nanjing Special Gas Co. were used to simulate the flue gas; N2was used as the balance gas, NO and O2as the reaction gases, and NH3as the reducing agent. N2, NO and O2were premixed in a mixing chamber and then preheated to certain temperature; it was then mixed with NH3before feeding into the reactor.

Figure 2 Schematic diagram of the device for the catalytic tests in the SCR of NOx by NH3

The feed gas had a flow rate of 1.5 L/min, with a gaseous hourly space velocity (GHSV) of 10000 h-1; the concentrations of NO and NH3in the feed were both 5.0×10-4by molar fraction and the content of O2was 3%. To test the sulfur resistance, a certain fraction of SO2(99.99%) was introduced into the reactor for 7.5 h. The flue gas and effluent was measured by a flue gas analyzer (RBR, Germany, Model ecom-J2KN, ±5 (0-200) or ±5% (other range)). The de-NOxefficiency, viz., the conversion of NOx(xNOx), was calculated by formula(1)

xNOx= (cin-cout)/cin× 100%

(1)

wherecinandcoutdenote the concentrations of NOxat the inlet and outlet of the reactor, respectively.

1.3 Catalytic characterization

The siderite catalysts used in this work were characterized by XRF, nitrogen physisorption, XRD, NH3-TPD and TGA, to determine the elemental composition, surface area, pore structure, crystal composition and morphology of the catalysts as well as the adsorption capacity of NH3and thermal stability of sulfate on the catalyst surface.

2 Results and discussion

2.1 Effects of calcination temperature on the catalytic performance of siderite

The calcination temperature (400, 450 and 500 ℃ for 5 h) has a great influence on the nature of the active components, the textural properties, and the catalytic performance of siderite in the de-NOxreactions by SCR, as shown in Figure 3. Obviously, the siderite catalyst calcined at 450 ℃ exhibits the highest activity in SCR; the conversion of NOxis higher than 90% in the temperature range of 240-300 ℃. In contrast, the maximum NOxconversion over the siderite catalyst calcined at 400 ℃ does not exceed 80%, whilst the maximum NOxconversion over the siderite catalyst calcined at 500 ℃ is only 87%.

Figure 3 Conversion of NOx for the de-NOx reaction by SCR with NH3 over the siderite catalysts calcined at different temperatures

Table 1 gives the textural properties of various siderite catalysts calcined at different temperatures. FeCO3, as the main component of siderite, may experience the reaction of 3FeCO3=Fe3O4+2CO2+CO during the calcination; the releasing of gases produced by calcination can create and enlarge the passages in the catalyst. As a result, the calcination treatment can greatly improve the porous structure and enhance the surface area. Fe3O4formed by calcination at low temperature (400 ℃) is catalytically less active in the de-NOxreaction by SCR. At a higher calcination temperature, Fe3O4is oxidized to Fe2O3with higher de-NOxactivity; therefore, the NOxconversion over the siderite catalyst calcined at 450 ℃ is greatly improved. However, a further increase in the calcination temperature (500 ℃) may lead to the agglomeration of crystals, collapse of pores and decrease in the surface area, which are detrimental to the catalytic performance of siderite in the de-NOxreaction by SCR. The siderite catalyst obtained by calcination at 450 ℃ displays a large surface area, a large pore volume, and a high activity in the de-NOxreaction by SCR.

Table 1 Textural properties of the siderite catalysts calcined at different temperatures

2.2 Characteristics of the Mn doped siderite catalyst

To investigate the effect of Mn doping on the de-NOxefficiency of siderite catalyst, a series of Mn-doped siderite catalysts with different Mn mass fractions (0, 1%, 2% and 3%) were prepared; the NOxconversions over various Mn-modified siderite catalysts are shown in Figure 4.

Obviously, four catalysts are rather different in the initial de-NOxactivity at 90 ℃; the NOxconversion over the 3%Mn-siderite catalyst is 30.5%, nearly twice of that over the original unmodified siderite catalyst. At 90-210 ℃, the NOxconversion over the Mn-modified siderite catalyst is significantly improved with an increase in the Mn doping amount. In particular, the NOxconversions over the 3%Mn-siderite and 2%Mn-siderite catalysts exceed 90% at 180 ℃, whereas the NOxconversions over 1%Mn-siderite and unmodified siderite catalysts are less than 90%. Among four catalysts, the 3%Mn-siderite catalyst exhibits the best de-NOxefficiency at low temperature and has a wide active temperature window. Furthermore, the NOxconversion over 3%Mn-Siderite exceeds 90% at 180-300 ℃ and reaches 98% at 240 ℃. In contrast, the unmodified siderite catalyst displays the lowest de-NOxefficiency among four catalysts; the NOxconversion reaches 90% only at 240 ℃. Current results indicate that the doping with Mn is beneficial to improving the low-temperature SCR de-NOxactivity of the siderite catalyst; in particular, with a mass fraction of below 3% for Mn doping, the low-temperature catalytic de-NOxefficiency of the siderite catalyst increases considerably with an increase in the Mn content.

Figure 4 Conversion of NOx for the de-NOx reaction by SCR with NH3 over the Mn-modified siderite catalysts with different Mn doping amounts■: siderite; ●: 1%Mn-siderite; ▲: 2%Mn-siderite; ▼: 3%Mn-siderite

Table 2 gives the elemental composition of various Mn-modified siderite catalysts determined by XRF. The actual contents of Mn in the siderite, 1%Mn-siderite, 2%Mn-siderite and 3%Mn-siderite catalysts are approximately 3%, 4%, 5% and 6%, respectively, because the siderite catalysts contain certain content of Mn elements. The mass percentage of Mn increases with the increase of the doped Mn content; in contrast, the mass percentage of other elements in the Mn-modified siderite catalyst is reduced after doping with Mn. Although the doping amount of Mn is small, it may have a great influence on the catalytic performance of siderite, as Mn is a kind of metal element with high low-temperature SCR de-NOxactivity and the doped Mn species is mainly present on the catalyst surface.

The textural properties of various Mn-modified siderite catalysts are shown in Table 3. With an increase in the Mn doping content, the surface area of Mn-modified siderite catalyst increases and the pore size decreases. It suggests that the doping of siderite with a proper amount of Mn can enhance the surface area and improve distribution of active components, which is beneficial to the adsorption of the reaction gas on the catalyst surface and promote the de-NOxreaction by SCR.

Table 2 XRF analysis results of various Mn-modified siderite catalysts

Table 3 Textural properties of various Mn-modified siderite catalysts

Figure 5 shows the XRD patterns of the unmodified siderite and 3%Mn-siderite catalysts. Two catalysts are substantially identical in the crystal form; both show strong diffraction peaks at 23°, 33°, 36°, 44°, 50°, 57° and 63°. By comparing with standard PDF, they are attributed toα-Fe2O3and MnO2. Meanwhile, Figure 5 also illustrates that the intensity of the characteristic peak forα-Fe2O3in the 3%Mn-siderite catalyst is obviously weaker than that in the unmodified siderite catalyst; however, the diffraction angles are not shifted. It means that the crystal lattice ofα-Fe2O3is not damaged after doping with Mn, but the degree of crystallization is reduced. Besides, the increase in the proportion of amorphousα-Fe2O3may makeα-Fe2O3more dispersed on the catalyst surface, which is beneficial to the SCR de-NOxreaction.

Figure 5 XRD patterns of the unmodified siderite (a) and 3%Mn-siderite (b) catalysts

As the unmodified siderite catalyst also contains a small amount of Mn, the diffraction peaks for the manganese oxides are observed in the XRD patterns of both unmodified siderite and 3%Mn-siderite catalysts; moreover, the diffraction peaks of 3%Mn-siderite catalyst is even weaker, suggesting that the Mn species introduced in the 3%Mn-siderite catalyst is mainly dispersed on the catalyst surface and in an amorphous form. Due to the stronger interaction between Mn and Fe in the 3%Mn-siderite catalyst, the agglomeration of MnO2is inhibited, leading to a higher dispersion of the active MnO2species on the surface of 3%Mn-siderite catalyst and subsequently, a higher SCR de-NOxactivity for the Mn-modified siderite catalyst.

2.3 Characteristics of the Mn-Ce co-doped siderite catalyst

To explore the effect of Mn-Ce co-doping on the performance of the modified siderite catalyst, it is necessary to know the separated influence of Mn and Ce doping amounts on the catalytic performance. Above results indicate that the optimum doping amount of Mn is 3%. Hence, the siderite catalysts were first modified with a cerium mass fraction of 0.5%, 1% and 1.5%, as shown in Figure 6(a). Obviously, with the increase of the Ce doping amount, the low-temperature de-NOxefficiency increases, whereas the high-temperature efficiency decreases. With a Ce doping amount of 1%, the temperature window for the de-NOxby SCR with a NOxconversion of higher than 90% is widest, from 180 to 270 ℃, indicating that the optimum doping amount of Ce is 1%.

The semi-orthogonal method was used to investigate the effect of Mn-Ce co-doping on the activity of the modified siderite catalyst. Above results suggest that the optimum doping amounts of Mn and Ce are 3% and 1%, respectively; therefore, the experimental group setting was then arranged as listed in Table 4. All the catalysts were calcined at 450 ℃ and the activity test results are shown in Figure 6(b). Obviously, the NOxconversion in the whole temperature range of 90-330 ℃ increases first and then decreases with an increase in the reaction temperature; the maximum NOxconversion exceeds 90% for all six catalysts.

Figure 6 Conversion of NOx for the de-NOx reaction by SCR with NH3 over the Mn and Ce modified siderite catalysts(a): siderite doped with Ce; (b): siderite co-doped with Mn and Ce

Table 4 Experimental group setting for the investigation of the effect of Mn and Ce co-doping on the performance of modified siderite catalyst

Figure 6(b) illustrates that six catalysts are rather different in their de-NOxefficiency at 90 ℃. The NOxconversion over the 3%Mn1.5%Ce-siderite catalyst reaches 66.1%, whereas that over the unmodified siderite catalyst is only 15.8%. With the increase of Mn and Ce doping amount, the low-temperature SCR de-NOxefficiencies of the Mn-Ce co-doped siderite catalysts increase greatly in comparison with that of the unmodified siderite catalyst, whereas the high-temperature SCR de-NOxefficiencies reduce slightly, except the 3%Mn1%Ce-siderite catalyst.

When the doping amount of Ce is 1%, the SCR de-NOxefficiency of the Ce and Mn co-modified catalysts remain at the same level with the increase of the Mn doping amount from 1% to 2%; however, an increase of the Mn doping amount from 2% to 3% lead to a considerable increase of the NOxconversion at a temperature below 270 ℃; the temperature window with a NOxconversion of higher than 90% over the 3%Mn1%Ce-siderite catalyst is extended to 180-300 ℃ and the highest NOxconversion reaches 99% at 240 ℃.

In contrast, when the doping amount of Mn is 3%, the de-NOxefficiency increases greatly both in the low temperature range and in the medium temperature range with an increase in the Ce doping amount from 0.5% to 1%. However, a further increase of Ce doping amount from 1% to 1.5% may lead a decrease in the de-NOxefficiency at a temperature above 150 ℃; the temperature window of 3%Mn1.5%Ce-siderite is then reduced to 180-240 ℃ and the maximum NOxconversion drops to 95% at 240 ℃.

It can be then concluded that the 3%Mn1%Ce-siderite catalyst doped with 3% Mn and 1% Ce has the widest temperature window and the highest NOxconversion. In addition, compared with the Mn-Ce catalysts supported on activated carbon[18,19], the 3%Mn1%Ce-siderite catalyst achieves a better de-NOxefficiency at a lower Mn and Ce doping amount. Moreover, the 3%Mn1%Ce-siderite catalyst can get the desired catalytic effect at a lower oxygen content in the feed, in comparison with the TiO2supported Mn-Ce catalyst[20]. This may be ascribed to the fact that siderite itself contains the active Fe and Mn substances; after modification with Mn and Ce, the quantity of active sites on the catalyst increases considerably.

Figure 7 shows the XRD patterns of various siderite catalysts co-doped with Mn and Ce. Five Mn-Ce co-doped siderite catalysts exhibit approximately the same crystal phase, with strong diffraction peaks at 23°, 26°, 33°, 36°, 42°, 50° and 63°. In addition, the 1%Mn1%Ce-siderite and 3%Mn1.5%Ce-siderite catalysts show a diffraction peak at 32°. Compared with the standard PDF card, the diffraction peaks should be attributed toα-Fe2O3and MnO2.

Figure 7 XRD patterns of various siderite catalysts co-doped with Mn and Cea: 1%Mn1%Ce-siderite; b: 2%Mn1%Ce-siderite; c: 3%Mn1%Ce-siderite; d: 3%Mn1.5%Ce-siderite; e: 3%Mn0.5%Ce-siderite

Meanwhile, it seems that the characteristic peaks ofα-Fe2O3for 3%Mn1%Ce-siderite are less intense, indicating the high dispersion ofα-Fe2O3. In contrast, theα-Fe2O3diffraction peaks for the 1%Mn1%Ce-siderite catalyst is most intense, suggesting the highest crystallinity and low dispersion ofα-Fe2O3on the surface of 1%Mn1%Ce-siderite. Besides, the MnO2diffraction peak intensities of all five catalysts are rather weak, indicating the high dispersion of MnO2on the catalyst surface. Meanwhile, no diffraction peaks for CeOxare observed, illustrating that the doped cerium oxide exists mainly in an amorphous form and is uniformly dispersed on the catalyst surface. All these are beneficial to the SCR de-NOxreaction.

Figure 8 shows the NH3-TPD profiles of various siderite catalysts co-doped with Mn and Ce. All five Mn and Ce co-modified siderite catalysts display a NH3desorption peak at 65-70 ℃ corresponding to the weak acid sites. Besides, the 1%Mn1%Ce-siderite catalyst shows a NH3desorption peak at 445 ℃ which is attributed to strong acid sites, whereas the NH3desorption peaks of other four catalysts for the strong acid sites appear at 470-480 ℃. This indicates that the acid strength of 1%Mn1%Ce-siderite catalyst is relatively weaker than those of other four catalysts. However, five catalysts are rather different in the area of NH3desorption peaks; in particular, the 3%Mn1%Ce-siderite catalyst displays a notably larger NH3desorption peak than that other four catalysts. That is, the NH3adsorption capacity of five Mn-Ce co-doped siderite catalysts follows the order of 3%Mn1%Ce-siderite > 3%Mn1.5%Ce-siderite > 2%Mn1%Ce-siderite > 3%Mn0.5%Ce-siderite > 1%Mn1%Ce-siderite. The surface acidity of the catalysts with the same Ce doping amount increases with the increase of Mn doping amount, which may be ascribed to the synergy between Mn and Ce, consistent with their de-NOxactivity by SCR.

Figure 8 NH3-TPD profiles of various siderite catalysts co-doped with Mn and Cea: 3%Mn1%Ce-siderite; b: 3%Mn1.5%Ce-siderite; c: 2%Mn1%Ce-siderite; d: 3%Mn0.5%Ce-siderite; e: 1%Mn1%Ce-siderite

2.4 Sulfur resistance of the Mn-Ce co-doped siderite catalyst

The resistance of Mn-Ce co-doped siderite catalysts against sulfur in the SCR de-NOxreaction was considered at 210 ℃ by introducing 0.01% SO2in the feed, as shown in Figure 9. Obviously, when 0.01% SO2is introduced, the NOxconversion over the unmodified siderite catalyst decreases considerably with the reaction time and stabilizes gradually at about 40% after introducing SO2for 7 h. In contrast, the NOxconversion over the 3%Mn1%Ce-siderite catalyst decreases much more gently after introducing SO2and levels off at 76.3% after reaction in the presence of SO2for 6 h. It indicates that the Mn-Ce co-coping is beneficial to improving the sulfur resistance of the siderite catalyst in SCR. After the addition of Ce, SO2preferentially forms sulfate on CeO2, which can weaken the poisoning effect on the active sites of metal oxides such as Fe and Mn on the siderite catalyst surface. Moreover, Ce can also reduce the thermal stability of the ammonium sulfate on the catalyst surface and promote the decomposition of sulfate[21,22]. Unfortunately, it seems that the sulfur poisoning of the siderite catalyst is irreversible; after removing SO2from the feed, the NOxconversions over two catalysts can not come back to the original values before introducing SO2.

The thermal stability of sulfate salt deposited on the spent catalysts after the 7.5 h sulfur resistance test were considered by thermogravimetric analysis (TGA), as shown in Figure 10.

Figure 9 Sulfur resistance of the unmodified siderite (a) and 3%Mn1%Ce-siderite (b) catalysts in the SCR de-NOx reactions

Figure 10 TGA profiles of the spent siderite (a) and 3%Mn1%Ce-siderite (b) catalysts after the sulfur resistance tests for the SCR de-NOx reaction

During the heating process, both the unmodified siderite and 3%Mn1%Ce-siderite catalysts show significant weight losses at 50-180 ℃ and above 650 ℃; the former is attributed to dehydration and the latter is the decomposition of metal sulfate. Furthermore, at 180-600 ℃, two catalysts also show a slight weight loss, especially at 200-450 ℃, due to the decomposition of ammonium sulfate. During the SCR de-NOxreaction, SO2reacts with NH3, forming ammonium hydrogen sulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4)[23,24]; the decomposition temperatures of (NH4)2SO4and NH4HSO4are in general above 240 and 290 ℃, respectively[25,26]. Figure 10 illustrates that the weight loss of the spent 3%Mn1%Ce-siderite catalyst in this temperature range is much less than that of the unmodified siderite catalyst. It indicates that the Mn-Ce co-doping can reduce thermal stability of NH4HSO4and (NH4)2SO4formed on the catalyst surface and promote the decomposition of the sulfate; as a result, the Mn-Ce co-modified siderite catalyst exhibits better resistance against sulfur in the SCR de-NOxreaction.

3 Conclusions

Owing to the presence of transition elements, siderite calcined at 450 ℃ shows a good activity in the SCR de-NOxreaction; the NOxconversion is higher than 90% at 240-300 ℃. At the optimum calcination temperature of 450 ℃, FeCO3as the main component in siderite can be oxidized to Fe2O3with high SCR de-NOxactivity and meanwhile the agglomeration of active components is unobvious.

The performance of siderite catalyst in SCR is notably improved by increasing the Mn doping amount, which can enhance the catalyst surface area and the dispersion of active components. Over the 3%Mn-siderite catalyst, the NOxconversion is above 90% at 180-300 ℃ and reaches 98% at 240 ℃.

The performance of siderite catalyst in SCR can be greatly enhanced by co-doping with Mn and Ce. The 3%Mn1%Ce-siderite catalyst exhibits the best low-temperature activity in the SCR de-NOxreaction, owing to the large surface area, high active substance dispersion and strong surface acidity. The temperature window for the 3%Mn1%Ce-siderite catalyst with a NOxconversion of higher than 90% is extended to 180-300 ℃ and the highest NOxconversion reaches 99% at 240 ℃.

Doping with Ce can improve the sulfur resistance of the siderite catalyst. The doping of siderite with Ce can weaken the poisoning effect of SO2on the active sites of Fe and Mn oxides, reduce the thermal stability of ammonium sulfate on the catalyst surface and promote the decomposition of sulfate. For the SCR de-NOxreaction at 210 ℃ over the 3%Mn1%Ce-siderite catalyst, the NOxconversion remains above 75% after introducing 0.01% SO2in the feed for 7.5 h, much higher than the NOxconversion (40%) over the unmodified siderite catalyst under the same conditions.