Abstract
Mn-La catalysts were prepared by solid-state reaction (SSR) and co-precipitation methods (CPM). The effects of preparation methods on denitration and SO2/H2O resistance of the catalysts were investigated. The structure and physicochemical properties of the catalysts were characterized by XRD, BET, H2-TPR, NH3-TPD and XPS. The results show that La doping decreases the crystallinity of MnOx and increases the specific surface area and pore volume of MnOx. The bond cooperation of Mn-O-La promotes the dispersion of Mn on the catalyst surface, while the highly dispersed Mn are easier to be reduced. The reduction temperature of the catalyst moves to lower temperature, and the redox ability is improved. After La doping, the amount of Brønsted acid and total acid increase on the catalyst surface, and the concentration of M
Science Press
Nitrogen oxides are one of the main atmospheric pollutants, which not only cause environmental problems such as acid rain and photochemical smog, but also cause serious harm to human health. Selective catalytic reduction of NOx by NH3-selective catalytic reduction (NH3-SCR) is currently the most widely used technology for removing NOx from flue gas. The V2O5-WO3(MoO3)/TiO2 series catalyst with an active tempe-rature window between 300 and 400 °C is a commonly used commercial SCR catalyst. It is usually placed before the desulfurization and dust removal device. During the operation, V2O5-WO3(MoO3)/TiO2 catalyst is subjected chronically to the scouring of flue gas containing high concentration of dust and SO2, so it is easy to cause catalyst attrition and poisoning and shorten the service life of the catalyst. The low temperature SCR technology disposed after the desulfurization and dust removal device can effectively avoid the above problems. However, the flue gas temperature is greatly reduced (<200 °C) under this condition, and the denitration effect of the vanadium-based catalyst is not satisfactory. Therefore, the development of a new low-temperature and high-efficiency SCR catalyst has become the focus.
The transition metal element Mn shows excellent activity of denitration at low temperatures. This is mainly due to the special valence layer electron configuration of Mn element (3d54s2), which makes the valence state of Mn change widely. The valence states of manganese include +2, +3, +4, +5 and some non-integer valence. Different valence states of Mn can realize mutual conversion to produce redox, which can promote the reduction of NO, thus promoting SCR reactio
Single Mn based SCR catalyst is easy to be poisoned and inactivated in flue gas containing SO2. The addition of one or more rare earth elements can effectively improve the surface area, redox performance and surface acid strength of the catalyst, and then significantly improve the denitration activity and sulfur resistance of the catalyst. Rare earth metal lanthanum (La) has stable performance, which can promote the dispersion and reduction of active components of the catalyst and improve the migration ability of lattice oxygen, which is conducive to the catalytic reaction. H
Because of the advantages of high selectivity, high yield and simple process, solid state reaction has become one of the main methods to prepare new solid materials. However, the traditional solid-state reaction is usually carried out at high temperatures, which has the disadvantages of high energy consumption, serious equipment corrosion, large particle size and uneven distribution. Co-precipitation method can directly obtain homogeneous nano powder materials through various chemical reactions in solution, which can realize uniform doping at the molecular level. Compared with the solid-state reaction, its chemical reaction is easier to carry out, and it only requires a lower synthesis temperature, so it is easy to prepare nano powder materials with small particle size and uniform distribution. Therefore, Mn-La composite oxide catalysts were prepared by solid state reaction and co-precipitation in this study. The structure and physicochemical properties of the catalyst were characterized by XRD, BET, H2-TPR, NH3-TPD and XPS. The low-temperature denitration activity and SO2/H2O resistance of the catalyst were also investigated.
A certain amount of manganese acetate was ground in a dry mortar for 0.5 h. Then it was placed in an oven at 80 °C for 24 h. The obtained samples were put into a muffle furnace and calcined at 500 °C for 4 h. Finally, black Mn oxide powder was obtained, which was labeled as MnOx (

Fig.1 Synthesis procedure of MnOx
Manganese acetate of 24.51 g and lanthanum acetate of 5.27 g (Mn/La molar ratio is 6:1) were weighed and ground in a dry mortar for 0.5 h. Then it was placed in an oven at 80 °C for 24 h. The samples were put into a muffle furnace and calcined at 500 °C for 4 h. Finally, the black Mn-La powder was obtained, which was labeled as MnLa-SSR (

Fig.2 Synthesis procedure of MnLa-SSR
Manganese acetate of 24.51 g and lanthanum acetate of 5.27 g (Mn/La molar ratio is 6:1) were weighed, and the volume was adjusted to 150 mL with deionized water. The pH value of the mixed solution was adjusted to 11 with ammonia water, and stirred for 0.5 h The precipitate was washed with deionized water until pH was neutral, and then dried in an oven at 110 °C until the water evaporated completely to obtain solid powder. The obtained solid powder was put into a muffle furnace and calcined at 500 °C for 4 h. Finally, black Mn-La powder was obtained, which was labeled as MnLa-CPM (Fig.3).

A D8 advance X-ray diffractometer produced by German Bruker was used to characterize the catalyst. The test conditions were: current in the tube of 40 mA, voltage in the tube of 40 kV, Kα as the radiation source, Cu as the target, scanning range 2θ=10°~80°, step=0.02°.
The N2 physical adsorption test of the catalyst was carried out by the Autosorb-iQ physical adsorption instrument of Quantachrome Company of the United States to determine the specific surface area and pore structure characteristics of the catalyst. First, a certain mass of sample (0.25~0.3 g) was heated and evacuated at 240 °C for 2 h, and then tested under liquid nitrogen (-196 °C). The specific surface area was calculated using the multi-point BET equation, and the pore size and pore volume were measured using the BJH method.
The H2-TPR characterization of the catalyst was carried out by the Auto Chem II 2920 type chemical adsorption instrument of Mike Corporation. The sample (50 mg, particle size of 0.3~0.45 mm) was pretreated at 300 °C for 0.5 h in pure Ar atmosphere, and then cooled to 50 °C. Under the condition of 10vol% H2/Ar (20 mL/min), the temperature was programmed to 900 °C at a rate of 10 °C/min. The H2 consumption during this process was also detected by the TCD thermal conductivity detector.
The NH3-TPD characterization of the catalyst was carried out by the Auto Chem II 2920 type chemical adsorption instrument of Mike Corporation. The sample (100 mg, particle size of 0.3~0.45 mm) was pretreated at 400 °C for 0.5 h in a pure He (20 mL/min) atmosphere, and then cooled to 50 °C. Next, the sample was fed with 5vol% NH3/N2 (20 mL/min) for 0.5 h for NH3 adsorption, then purged with pure He atmosphere for 1 h, and finally programmed to 550 °C at a rate of 10 °C/min. The attached NH3 was detected by the TCD thermal conductivity detector.
The Thermo Scientific ESCALAB 250Xi X-ray photo-electron spectrometer of American Thermoelectric Group was used for XPS characterization of the catalyst. The vacuum degree of the analysis chamber was 8×l
The activity evaluation of the catalyst was carried out in a stainless steel SCR fixed reactor. The temperature was controlled by the external heating of the tube furnace. The inside of the tube furnace was a stainless steel tube where the catalyst was placed. The stainless steel mesh prevents the catalyst from leaking. The schematic diagram of the specific experimental device is shown in
(1) |

Fig.4 Schematic diagram of catalyst activity evaluation device

Fig.5 shows the XRD pattrerns of catalysts prepared by different methods. It can be seen that the intensity and width of diffraction peaks of samples prepared by different methods are quite different. The typical diffraction peaks of Mn2O3 (JCPDS No. 24-0508) appear at 18.8°, 23.1°, 28.4°, 32.9°, 35.6°, 38.2°, 45.1°, 49.3°, 55.1° and 65.7° in pure MnOx catalyst. The crystal growth of single MnOx is faster, and Mn2O3 has high crystallinity and large particle size. Therefore, the diffraction peaks show high intensity, the half peak width is smaller and the peak type is sharper. However, after adding La, the peak width of Mn2O3 increases, and the characteristic peak of MnLa-CPM catalyst prepared by co-precipitation method has the widest peak and the least intensity. The reason is that L


Fig.6 and Fig.7 show the N2 adsorption-desorption isotherm and pore size distribution of the catalyst, respectively. It can be seen from Fig.6 that MnOx sample displays a type I isotherm, and the sharp rise at relatively low pressure (P/P0<0.01) is relevant to the presence of micropore
The specific surface area and pore volume are important factors affecting the catalytic reaction. The increase of specific surface area is beneficial to the dispersion of active species and the adsorption of reactants, and it also increases the number of surface active sites to a certain extent. The increase of pore volume is beneficial to the adsorption and diffusion of reaction medium, which increases the contact opportunity between the catalyst and the reaction medium, thus increasing the catalytic reaction rate.

The redox ability of the catalyst is one of the important factors affecting the catalytic performance of the catalyst in the NH3-SCR reaction. It directly affects the oxidation of NO to NO2, and further promotes the reaction through “fast SCR
In NH3-SCR reaction, NH3 is an alkaline reaction gas, which is firstly adsorbed on the acid site of the catalyst for activation, and then participates in SCR reaction. Therefore, the type and strength of acid sites on the surface of the catalyst determine the denitration activity of the catalyst to a certain exten

The Mn 2p orbital electron spectra of the catalyst are shown in Fig.10a. It can be seen that there are two main peaks at about 642 and 653 eV for each catalyst, corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. From the peak fitting, it can be seen that the two main peaks are composed of M

Table 2 Atomic concentration on the catalyst surface (at%)
The La 3d orbital electron spectra of the catalyst are shown in Fig.10b. The binding energies of La 3d5/2 and La 3d3/2 are 834.6~834.8 and 851.2~851.4 eV, respectivel
The O1s orbital electron spectra of the catalyst are shown in Fig.10c. It can be seen that there are two characteristic peaks at 530 and 531.5 eV, which are attributed to lattice oxygen (

Fig.11 shows the denitration activity curves of catalysts prepared by different methods. It can be seen that with the increase of temperature, the NO conversion rate of each catalyst also increases, but when the temperature rises to a certain extent, the NO conversion rate gradually decreases. This is mainly due to the fact that NH3, as a reducing agent, is oxidized to NO at high temperatures. The amount of reducing agent decreases, the content of NO increases and the denitration efficiency decreases. Pure MnOx catalyst has the worst denitration activity and the highest denitration efficiency is about 70%. The denitration efficiency of the catalyst with La element is significantly improved, and the highest denitration efficiency is more than 90%. The order of denitration activity of catalysts prepared by different methods is as follows: MnLa-CPM>MnLa-SSR>MnOx. The main reason for the poor denitration performance of the single metal catalyst is that sintering occurs easily in the preparation process of the single metal catalyst, which leads to the decrease of the dispersion and specific surface area of the catalysts. On the one hand, element doping can inhibit the sintering of the catalyst in the preparation process, so that the active components of the catalyst can obtain a good dispersion and a large specific surface area; on the other hand, the mixture of metal oxides forms after doping metal elements, which has a new crystal structure and can improve the low-temperature denitration activity of catalyst

Although the low-temperature NH3-SCR device is arranged after desulfurization and dust removal, there are still a certain amount of SO2 and H2O in the flue gas. Therefore, the influence of SO2 and H2O on the performance of the catalyst is one of the important factors to evaluate the advantages and disadvantages of the catalyst. 10vol% H2O and 0.03vol% SO2 are introduced into the simulated flue gas at 180 °C to study the SO2/H2O poisoning resistance performance of the catalyst. Fig.12 shows that when 10vol% H2O and 0.03vol% SO2 are added, the denitration efficiency of pure MnOx catalyst decreases significantly, from 70% to 40%, with a decrease of 30%. However, the NO conversion rate of MnLa-SSR and MnLa-CPM catalysts is decreased by 20% (MnLa-SSR catalyst decreases from 90% to 70%, and MnLa-CPM catalyst decreases from 100% to 80%). The results show that H2O and SO2 have serious toxicity to the catalyst, and MnLa-CPM catalyst has better resistance to SO2/H2O poisoning in the presence of H2O and SO2. This is due to the fact that the active components are evenly dispersed in the preparation process of co-precipitation, forming low crystallinity or amorphous structure, and the interaction between Mn and La is strong, and then more active sites are forme
MnOx species are the main active components of the catalyst, and the performance of MnOx species is closely related to SCR activity. The characterization and analysis of the catalysts before and after lanthanum doping show that La is doped in the synthesis process of MnOx catalyst by co-precipitation method. LaOx enters the complex network structure to form La-O-Mn bond, which restricts the transportation of MnOx, and then controls the size of MnOx particles in MnOx/LaOx composite oxide. Because Mn atom is replaced by La atom, the lattice is deformed, the specific surface area of the catalyst is greatly increased, the pore size is refined, and the dispersion of Mn and La active components is improved. According to the acid characterization results, the addition of La increases the content of Brønsted acid on the surface of the catalyst. These acids are weaker than those on MnOx species, and the adsorbed NH3 is easier to desorb. Therefore, it is helpful to improve the activity of the catalyst for low temperature denitration. In addition, La entering the lattice of manganese oxide, causes the electron imbalance on the surface of the catalyst, and thus the number of oxygen vacancies and unsaturated chemical bonds is increased, which provides a new adsorption site for adsorbed oxygen, and increases the concentration of chemically adsorbed oxygen on the surface. Through the redox cycle on the surface of the catalyst, the adsorbed oxygen will be continuously converted into active oxygen, which can improve the catalytic oxidation performance of NO to NO2 at low temperatures, and then increase the concentration of NO2 on the catalyst surface, and finally enhance the “rapid SCR reaction” of the catalyst. At the same time, the addition of rare earth element La can also improve the low-temperature oxidation-reduction ability of the catalyst. In conclusion, the addition of rare earth element La can improve the low-temperature SCR denitration performance of manganese oxide catalyst.
Many scholars have done a lot of research on the denitration mechanism of single manganese oxide catalyst, and most of them think that the catalytic process of Mn is more in line with Eley-Rideal mechanism. They believe that gaseous NH3 is adsorbed on Lewis acid sites on the catalyst surface to form coordinated NH3, and the coordinated NH3 forms NH2 groups under the action of hole oxygen on the catalyst surface. These surface hole oxygen is formed by the valence state conversion and electron transfer between MnO2 and Mn2O3. After that, NH2 group will react with NO in gas phase to form the key intermediate product NH2NO, which will be decomposed into N2 and H2O. The detailed steps of these reactions are as follows:
(2) |
(3) |
(4) |
(5) |
At the same time, it is considered that the addition of La increases the content of Brønsted acid on the catalyst surface because of the formation of La-O-Mn bond and -OH group after doping La into MnOx. NH3 is adsorbed on the newly added Brønsted acid site, which is oxidized to form bidentate nitrate. The reaction of bidentate nitrate with coordinated NH3 forms a new acid site, which leads to the decrease of the concentration of bidentate coordination nitrate and the removal of NO by another way. The specific reaction steps are as follows:
(6) |
![]() | (7) |
(8) |
(9) |
1) Compared with pure MnOx, the addition of La significantly increases the denitration activity of the catalyst.
2) Among the Mn-La catalysts prepared by different methods, the Mn-La prepared by co-precipitation method shows the best denitration activity. The denitration efficiency is close to 100% at 80 °C and maintains above 90% in the temperature range of 80~340 °C. In the presence of H2O and SO2, MnLa-CPM catalyst also shows good resistance to SO2/H2O poisoning. This is mainly because the components in MnLa-CPM catalyst are uniformly dispersed and exist in the form of low crystallinity or amorphous structure, with a large specific surface area and pore volume, strong redox ability and high surface acid content.
3) The higher concentration of M
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