Effect of Ce on the acidity of LaMnO3 / Fe2O3 catalyst in NH3-SCR at low temperature

doi:10.21203/rs.3.rs-3806870/v1

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Effect of Ce on the acidity of LaMnO3 / Fe2O3 catalyst in NH3-SCR at low temperature

https://doi.org/10.21203/rs.3.rs-3806870/v1

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A series of La_1 − xCe_xMnO₃/Fe₂O₃ catalysts were prepared by citric acid method, and their catalytic denitration efficiency and water and sulfur resistance were tested., XRD, BET, H₂-TPR, NH₃-TPD, XRF, TG and In situ DRIFTS are used to characterize the crystal structure redox properties of the catalysts. The results show that Fe₂O₃ catalyst loading LaMnO₃ can significantly enhance the catalytic denitrification ability of the catalyst at low and medium temperatures. When the doping amount of LaMnO₃ is 40%, the catalyst effect is the best, and the maximum efficiency is 94% at 240℃. The optimal temperature activity window is expanded by 30.8% compared with Fe₂O₃ catalyst at 240℃ to 330℃. The flue gas temperature corresponding to the optimal catalytic denitration efficiency is reduced by 30℃. Ce doping can improve the distribution of active substances on the catalyst surface and further improve the catalytic denitration efficiency at low temperature. La_0.6Ce_0.4MnO₃/Fe₂O₃ has the highest catalytic denitrification efficiency, reaching the highest efficiency of 96% at 240℃, which is about 4% higher than that of undoped catalyst. In addition, Ce doping significantly weakened the formation of sulfate on the catalyst surface, and La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst showed excellent sulfur resistance and water resistance.

NOx removal

NH3-SCR

Fe2O3

LaMnO3

Ce doping

Nitrogen oxides (NO_x) produced by stationary and flowing sources are one of the main gaseous pollutants in the atmospheric environment, and the main components of nitrogen oxides polluting the atmosphere are NO₂ and NO.¹ NO_x emission sources in the atmosphere can include natural sources (lightning, volcanoes, natural biomass combustion, soil or marine microbial activities, etc.) and man-made sources (fossil fuel combustion, biomass fuel combustion, etc.).^2,3 According to Miiller et al.⁴, anthropogenic sources account for three quarters of total global NO_x emissions, and this proportion is increasing year by year. With NO_x emission exceeding the standards, the environment is constantly destroyed, and people's health is also seriously harmed. In addition, NO_x are prone to a series of chemical reactions with volatile organic compounds (VOCs), resulting in a series of major environmental problems such as haze, acid rain and photochemical haze.^5,6

At present, the NO_x produced by fixed source combustion is mainly controlled through the following three approaches: pre-combustion pretreatment, low nitrogen treatment during combustion, and post-combustion flue gas de-NO_x treatment.⁷ NH₃ selective catalytic reduction technology (NH₃-SCR), one of the flue gas denitrification technologies, is considered to be the most effective technology for NO_x emission control. ^8–10 The basic principle of SCR is that within a suitable temperature range, some reducing gases (NH₃) selectively react with NO_x to generate non-toxic N₂ and H₂O through the catalysis of catalysts.¹¹ Catalyst plays an important role in the NH₃-SCR reaction process and is the core of the reaction process.¹² At present, V₂O₅-WO₃ (MoO₃)/TiO₂ catalysts are relatively mature in commercial use, and the activity temperature window of such catalysts is 300–400℃. The catalyst has certain satisfactory advantages, such as high denitrification efficiency and stability, strong SO₂ resistance and high NO_x conversion rate.^13–14 However, commercial V₂O₅‐WO₃/TiO₂ catalysts have some drawbacks, including low temperature catalytic activity, relatively narrow active temperature window (300–400℃), biological toxicity of V₂O₅, secondary contamination and high cost of catalyst processing. These limit the further use of V₂O₅‐WO₃/TiO₂ in NH₃‐SCR.^15–17 Therefore, it is imperative to develop vanadium-free and environmentally friendly catalysts for low temperature NH₃‐SCR reactions.

Metal oxide catalyst has attracted much attention from scholars. It has been found that Fe₂O₃-based catalyst has sufficient redox performance between Fe³⁺ and Fe²⁺, and has high denitrification activity and nitrogen selectivity at medium and high temperature.^18–21 Japan steel industry company ²² found that using Fe₂O₃ without carrier as catalyst for NH₃-SCR denitrification, the denitrification efficiency reached 90% in the temperature window between 400℃ and 450℃. Yao et al.^23,24 found that the denitrification efficiency of γ-Fe₂O₃ in fluidized bed was better at 200–290℃, and the maximum efficiency reached 90% at 250℃. Compared with vanadium and titanium catalyst, it has the advantages of wide sources, low price, no pollution to the environment, high temperature resistance, waste catalyst easy to dispose, etc., and has been widely used in NH₃-SCR reaction. γ-Fe₂O₃ will be converted to α-Fe₂O₃ at 320℃,^25–26 the catalytic activity of γ-Fe₂O₃ is higher than α-Fe₂O₃ at 150–300℃. Liu et al. ²⁷ compared the catalytic activity of α-Fe₂O₃ and γ-Fe₂O₃ phases in SCR, and concluded that the catalytic activity of the latter was higher than that of the former in the temperature range of 150℃ to 300℃, but the formed nitrate easily blocked the active site and reduced the catalyst activity. Yang et al.^28,29 found through experiments that the denitrification efficiency of γ-Fe₂O₃ is higher than that of α-Fe₂O₃, and pointed out that γ-Fe₂O₃ will transition to α-Fe₂O₃ when the temperature is higher than 300℃, resulting in a decline in efficiency. Fe₂O₃ has good catalytic denitrification performance, but the activity window of pure Fe₂O₃ is narrow, and the catalytic denitrification effect is poor at low temperature. Therefore, increasing the thermal stability of γ-Fe₂O₃ and enhancing its activity at low temperature is beneficial to broaden the activity window of catalyst. At present, scholars focus on the research of shifting the temperature window of iron oxide catalysts towards the low temperature segment, and many methods have been proposed to improve the low temperature activity and anti-SO₂ performance of the catalysts.

Perovskite compounds are used as catalysts in NH₃-SCR reaction because of their good catalytic oxidation activity and stable structure. Zhang et al.³⁰ studied the reaction mechanism of lanthanum manganese oxides and found that in SCR, the chemisorption of ammonia by catalyst is a rate-determined step, and this mechanism is essentially the Langmuir-Hinshelwood mechanism, that is, the reaction between ammonium ion and nitrite at low temperature. Gao et al.³¹ prepared perovskite-type catalyst by doping Sr and Ce on the basis of LaMnO₃ catalyst for purifying diesel vehicle exhaust NO_x. The study found that the catalyst had a higher conversion rate of NO_x, and can maintain certain catalytic activity in sulfur environment. Wang³² prepared a series of catalysts by loading LaMnO₃ on hematite with citric acid method, observed the surface microscopic appearance and tested the denitrification performance of the catalysts. It was found that Ce could enhance the adsorption capacity of NH₃, O₂ and NO as reactants, and improve the redox capacity. Iron oxide has the characteristics of nanoporous structure, good catalytic reduction performance and large specific surface area, but its catalytic denitrification performance at low temperature remains to be developed.

Considering that LaMnO₃ has a perovskite structure and has good catalytic activity at low temperature, a series of LaMnO₃/ Fe₂O₃ catalysts were prepared in this paper using Fe₂O₃ as the carrier and LaMnO₃ as the active substance. In addition, it has been found that partial substitution of La in perovskite LaMnO₃ with heterovalent atoms can generate cation vacancy or lattice oxygen vacancy in perovskite structure, thus improving the activity of catalyst.^33–36 CeO₂ has good oxygen storage capacity and redox performance. Using Ce to transform NH₃-SCR into catalyst has been the object of interest to many researchers. On the one hand, the addition of Ce can improve the oxygen concentration on the catalyst surface and the thermal stability of the carrier; on the other hand, the Ce doped catalyst can effectively inhibit the active catalyst components of sulfation.³⁷ Sun³⁸ replaced the LaMnO₃ catalyst with Ce and Fe respectively, and found that the addition of Ce ion improved the activity of lattice oxygen in the catalyst, thus promoting the oxidation activity of NO in the catalyst. In addition, in the presence of SO₂, the addition of Fe ion could delay the sulfur poisoning of Co ion. Qi et al.³⁹ prepared MnO_x-CeO₂ catalyst by co-precipitation method, and found that Ce doping and modification of manganese composite oxide can enhance the catalytic activity of the catalyst at low temperature. Wei et al.⁴⁰ prepared the Ce doped modified Mn-based catalyst by co-precipitation method, and found that the NO_x removal rate of the catalyst reached more than 90% in the temperature range of 180 ~ 300℃. In addition, DFT was used to simulate the Mn-Ce/TiO₂ catalyst at the atomic structure level, and it was found that SO₂ was preferentially adsorbed on the Ce atom as sulfate. The adsorption with other active sites is reduced. France et al.⁴¹ found that FeMnO_x catalyst doped with Ce showed superior resistance to water and sulfur, and showed better recovery characteristics after stopping pollutants. In previous DFT studies, it was found that Ce can form a stable composite oxide catalyst on the surface of γ-Fe₂O₃, improve the redox capacity of the surface of Fe₂O₃, and an appropriate amount of Ce can enhance its adsorption of NO and NH₃.⁴² In this paper, A catalyst La_1 − xCe_xMnO₃/Fe₂O₃ was constructed by doping or replacing La ion in LaMnO₃ composite oxide with Ce element with relatively active chemical properties. The catalyst was studied by XRF, XRD, BET, H₂-TPR, NH₃-TPR TG characterization methods.

2.1 Experimental Equipment

This article conducted de-NO_x activity tests on the catalyst, and the entire experiment was conducted on a fixed bed reactor.

Figure 1 shows the evaluation system of SCR catalytic denitration. The system consists of gas distribution system, reaction system and flue gas analysis system. In the distribution system, pressure valves and flowmeters are installed on each cylinder. O₂ and NO are first mixed and transported to the reaction system through the mixer, and NH₃ flows into the SCR reaction chamber as an independent gas path. The reaction system is mainly composed of a tube furnace and quartz tube. The gas will complete the catalytic denitrification process in the catalyst layer. The gas after catalytic denitration reaction is sampled and analyzed by flue gas analyzer.

2.2 Catalyst Preparation

LaMnO₃/Fe₂O₃ catalyst was prepared by citric acid method. A variety of chemical reagents are used to make catalysts: γFe₂O₃, La(NO₃)₃, Mn(NO₃)₆ (50wt,%), C₆H₈O₇·nH₂O, Ce(NO₃)₃·6H₂O. Experimental equipment: precision electronic balance, drying oven, Muffle furnace, constant temperature magnet heating agitator, beaker, glass rod and other chemical glass instruments.

The calculated La(NO₃)₃, Mn(NO₃)₆ (50wt, %) and C₆H₈O₇·nH₂O were dissolved in deionized water and stirred for 0.5 h, quantitative Fe₂O₃ particles are then put into the solution. The mixture is then heated to 80℃ and stirred until it becomes a gel. The resulting gel was then dried in a 105°C oven for 24 h. Finally, the samples were roasted at 400°C for 3 h.

2.3 Experimental Condition

The following are the standard operating conditions for simulated smoke: φ (O₂) = 3%, φ (NO) = 0.05%, φ (SO₂) = 0.02%, n (NH₃) : n (NO) = 1:1, balance gas: N₂, GHSV: 8500h^− 1.

Nine test temperature points were selected, which were 90℃, 120℃, 150℃, 180℃, 210℃, 240℃, 270℃, 300℃ and 330℃.

Stay at each measuring point for 10 min and record data after the test value is stable.

The NO_x conversion was calculated by the following equations:

$${\text{N}\text{O}}_{\text{x}} \text{c}\text{o}\text{n}\text{v}\text{e}\text{r}\text{s}\text{i}\text{o}\text{n}= \frac{{\left[{\text{N}\text{O}}_{\text{x}}\right]}_{\text{i}\text{n}}-{\left[{\text{N}\text{O}}_{\text{x}}\right]}_{\text{o}\text{u}\text{t}}}{{\left[{\text{N}\text{O}}_{\text{x}}\right]}_{\text{i}\text{n}}} \times 100\text{%}$$

where [NO_x]_in, [NO_x]_out denote the steady state concentration of NO at the inlet and outlet, respectively.

3.1 Effect of LaMnO₃ Loading on Catalytic de-NO_x Activity

The amount of active sites on the surface of the catalyst will be affected by the content of the active substance, and then affect the catalytic performance of the catalyst. The NO_x conversion efficiency of seven catalysts is shown in Fig. 2. The catalysts were impregnated with citric acid complex and calcined at 400℃ with a particle size of 35–65 mesh. The figure shows the de-NO_x efficiency of pure Fe₂O₃ and LaMnO₃, as well as the loading rates of 10%, 20%, 30%, 40%, and 50%, respectively. It can be seen from the figure that the denitration efficiency of the six catalysts first increases with the increase of flue gas temperature, and then decreases with the increase of flue gas temperature. γ-Fe₂O₃ catalyst has a certain catalytic denitration efficiency, but the highest denitration efficiency only reached about 80%, and the denitration efficiency is not high in the low temperature region (90℃-270℃). The denitration efficiency curves of Fe₂O₃ catalysts loaded with different amounts of LaMnO₃ are similar to those of unloaded catalysts, showing a trend of first increasing and then decreasing. At low temperature (< 240℃), the efficiency of the five supported catalysts is higher than that of the single γ-Fe₂O₃ catalyst without supported active substance. When the flue gas temperature is 210℃, the denitration efficiency of the five supported catalysts has reached about 80%. The denitration efficiency of 20% catalyst can reach 95% at 270℃. Under 240℃, the performance of 40%LaMnO₃/Fe₂O₃ is the best, and the highest efficiency can reach 93%. The optimum temperature activity window of γ-Fe₂O₃ catalyst is 240℃-330℃, and of 40%LaMnO₃/Fe₂O₃ is 200℃-330℃. It can be seen that the optimal temperature activity window of the catalyst is widened after loading, and the maximum catalytic denitration efficiency is also improved to a certain extent.

3.2 Effect of Ce Doping on Catalytic de-NO_x Activity of LaMnO₃/Fe₂O₃

LaMnO₃ belongs to perovskite and has ABO₃ type structure, in which La is the A-site ion and plays the role of supporting molecular structure, and Mn corresponds to Brønsted acid site (BAS) ion and plays the main active role. Studies have shown that the selection of highly active rare-earth elements for doping or substituting A-site ions can cause lattice defects of LaMnO₃ or abnormal valence state of B-site ions, play the catalytic denitrification role of rare-earth elements, and thus improve the catalytic activity of LaMnO₃. The catalyst was doped with Ce, Cr, Mg and Cu in this study.

Figure 3 shows four kinds of catalysts prepared with Ce, Cr and other elements doped. It can be seen that the change trend of denitration efficiency of catalysts is similar, which increases first and then decreases. Among them, the efficiency of the two groups doped with Mg and Cu is poor in the test temperature range, while the efficiency of the two groups doped with Cr and Ce is improved to different degrees. At 120–180℃, the denitration efficiency of 40%La_0.9Cr_0.1MnO₃/Fe₂O₃ is higher than that of 40%La_0.9Ce_0.1MnO₃/Fe₂O₃. However, the efficiency of 40%La_0.9Cr_0.1MnO₃/Fe₂O₃ is 3%-5% lower than that of the undoped group at 210℃. 40%La_0.9Ce_0.1MnO₃/Fe₂O₃ has an efficiency of 65% at 90℃, which is 6% higher than that of the undoped group. At 180℃, the efficiency is 78%, which is about 5% higher than that of the undoped group and above 180 ℃, the efficiency is higher than 40%La_0.9Cr_0.1MnO₃/Fe₂O₃. As a whole, the low temperature catalytic effect of 40%La_0.9Ce_0.1MnO₃/Fe₂O₃ is better, and the highest efficiency is 92% at 270℃.

In order to investigate the effect of different Ce doping levels, this study tested the efficiency of catalysts with Ce doping levels of 0.1,0.2,0.3,0.4, as shown in Fig. 4. It can be seen that the activity of catalysts doped with different Ce chemical amounts increased in different degrees (< 270℃), and the curves all showed a trend of first increasing and then decreasing with the increase of temperature. When the temperature is lower than 270℃, the effect of the four catalysts is improved greatly, but when the temperature is higher than 270℃, the catalyst has no obvious change, which indicates that Ce element has good catalytic desorption performance at low temperature. Among them, the denitration efficiency of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ can reach 80% at 150℃, and the maximum efficiency of 96% at 240℃, it is about 4% higher than undoped catalyst; the optimum temperature activity window is 150–300℃, and that of undoped catalyst is 210–300℃. The temperature range of the optimum activity range is widened to low temperature by 60℃. This indicates that Ce can not only enhance the catalytic activity of catalysts at low temperature, but also broaden the temperature window of catalysts.

3.3 SO₂ Resistance of the Catalysts

The actual flue gas composition is relatively complex and has SO₂, which has a great influence on the catalyst and is easy to form large particles (NH₄)₂SO₄ and NH₄HSO₄ on its surface, resulting in catalyst poisoning and deactivation.⁴³ In the experiment, SO₂ and H₂O were added under the original working condition to test the sulfur resistance characteristics, as shown in Fig. 5a. The simulated flue gas temperature is 240℃. When the conversion rate of NOx is stable, 200ppm SO₂ and 5%H₂O are injected. It can be observed that Fe₂O₃ itself has poor sulfur resistance. The NO_x conversion of Fe₂O₃ decreases significantly (to 30%) when 200ppm SO₂ and 5%H₂O is added into the feed gas, and the NO_x conversion is less than 50% after SO₂ and H₂O are cut off. When SO₂ and H₂O were added (75min), the NO_x conversion of 40%LaMnO₃/Fe₂O₃ catalyst decreased from 96–58% (130min) within 100min, and could be maintained within 450 min. Compared with pure Fe₂O₃, 40%LaMnO₃/Fe₂O₃ catalyst has better sulfur and water resistance in pollutant flue gas environment. After cutting off SO₂ and H₂O, it can recover to about 70%. The results indicated that pollutants caused irreversible inactivation to a certain extent, and the possible reason was that ammonium sulfate or metal sulfate was formed on the catalyst surface, which made the catalyst surface permeability worse.

The NO_x conversion of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst under a single pollutant is shown in Fig. 5b. On the one hand, the conversion of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst decreased continuously within 150min after the introduction of SO₂, remained stable after reaching 60%. On the other hand, the conversion decreased to 73% within 75min after the introduction of H₂O. It is not difficult to find that 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst has a slow deactivation rate in the environment of single SO₂, while the deactivation degree is low in the environment of single H₂O, and the recovery property is strong.

The catalytic NO_x conversion of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst is not significantly different from that of 40%LaMnO₃/Fe₂O₃ catalyst, but the advantages of the catalyst can be reflected in the sulfur and water resistance. When pollutants (SO₂ and H₂O) are added, the NO_x conversion between 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst and 40%LaMnO₃/Fe₂O₃ catalyst is about 96%. After the introduction of pollutants, the conversion of the two catalysts slowly decreases. Among them, 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst decreased to stable within 100min, and 40%LaMnO₃/Fe₂O₃ catalyst also decreased to stable after about 200min. It can be seen that 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst decreased slowly, showing strong resistance to SO₂ and H₂O. After pollutants are cut off, 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst can recover to 70%, which is about 5% higher than 40%LaMnO₃/Fe₂O₃ catalyst, indicating that Ce doping delays the catalyst poisoning to a certain extent, and improves the irreversible inactivation caused. The possible reason is that the addition of Ce alleviates the competitive adsorption of SO₂ and H₂O at the active site on the catalyst surface, and sulfate substances produced at low temperature can play part of the role of active sites.⁴⁴ At present, only partial explanation can be made. In order to explain the mechanism of sulfur resistance and water resistance, further research work is being done.

4.1 Structural Characterization of x%LaMnO₃/Fe₂O₃

In order to study the physical state and morphology of the catalyst, XRD analysis was conducted, and the results were shown in Fig. 6a. 30%LaMnO₃/Fe₂O₃ shows strong diffraction peaks at 2θ = 30°, 35°, 43°, 53°, 57°, 64° and 75°. 40%LaMnO₃/Fe₂O₃ shows strong diffraction peaks at 2θ = 30°, 32°, 35°, 43°, 45°, 53°, 57°, 64°, 68° and 75°. By comparing the diffraction curve with the standard PDF, it is found that after doping a certain amount of LaMnO₃, the diffraction peaks of LaMnO₃ and Fe₂O₃ appear at the same time. Compared with 30%LaMnO₃/Fe₂O₃, the diffraction peak of Fe₂O₃ at 2θ = 30° and 64° becomes weaker and narrower, while the diffraction peak of LaMnO₃ appears at 2θ = 32° and 45°, indicating that LaMnO₃ in crystal form is formed on the catalyst surface. 30%LaMnO₃/Fe₂O₃ only has the diffraction peak of Fe₂O₃ but no other peaks, indicating that the active substance is distributed evenly on the catalyst surface and may exist in an amorphous form, which has a certain promotion effect on the denitrification activity of low-temperature SCR. Compared with 40%LaMnO₃/Fe₂O₃, the LaMnO₃ diffraction peak is stronger and the peak width is narrower when 2θ = 32°, 45° and 68°, which indicates that the distribution uniformity of the active substance on the catalyst surface becomes worse, this may be the reason for the poor catalytic denitration efficiency.

The XRD patterns of 40%LaMnO₃/Fe₂O₃ catalysts with different Ce doping amounts are shown in Fig. 6b. It can be seen that 40%La_0.9Ce_0.1MnO₃/Fe₂O₃ presents strong diffraction peaks at 2θ = 30°, 32°, 35°, 43°, 45°, 53°, 57°, 64° and 75°. Compared with 40%LaMnO₃/Fe₂O₃, the intensity of partial diffraction peaks gradually becomes weaker and the peak width becomes wider. The results showed that LaMnO₃ was evenly distributed in the catalyst with Ce element.

In samples 40%La_0.8Ce_0.2MnO₃/Fe₂O₃ and 40%La_0.7Ce_0.3MnO₃/Fe₂O₃, diffraction peaks of composite oxides are almost not detected, indicating that the lattice of LaMnO₃ composite oxides is damaged after the addition of Ce element. In addition, the LaMnO₃ diffraction peak of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ curve disappears completely, which indicates that with the increase of Ce doping amount, the active component gradually changes from crystal form to amorphous form, which is conducive to the highly uniform dispersion of active substance on the carrier. It is also explained that Ce doping can improve the catalytic activity of catalysts.

The specific surface area and pore structure of the catalyst were analyzed as shown in Table 1. With the increase of LaMnO₃ loading, the specific surface area increased from 33.601m²/g to 54.720m²/g, pore volume increased from 0.110m³/g to 0.145m³/g, and pore size increased from 100.692 nm to 158.273nm. The increase of specific surface area is conducive to the increase of contact area between catalyst and reaction gas, which is conducive to the progress of SCR reaction. In addition, with the increase of pore volume, the internal diffusion resistance of the reaction gas in the catalytic reaction process is relatively reduced, which is conducive to the adsorption of the reaction gas on the catalyst surface, thus improving the SCR reaction rate.

The specific surface area and pore structure of the catalyst were analyzed as shown in Table 2. With the increase of Ce doping amount, the specific surface area of the catalyst increased from 48.452m²/g to 53.510m²/g, pore volume increased from 0.114m³/g to 0.144m³/g, and pore size increased from 136.201nm to 136.580nm. The increase of specific surface area, pore volume and pore diameter is conducive to the adsorption and diffusion of gas within the catalyst, which also explains the doping of Ce element is conducive to the improvement of denitrification efficiency of the catalyst to a certain extent.

Table 1

BET data of x%LaMnO₃/Fe₂O₃
x%	Specific surface area	Pore volume	Hole diameter
0%	33.601	0.110	100.692
10%	39.157	0.120	106.459
20%	41.146	0.138	103.475
30%	43.482	0.140	129.025
40%	48.452	0.144	136.201
50%	54.720	0.145	138.273

Table 2

BET data of 40%La_1 − xCe_xMnO₃/Fe₂O₃
40%La_1 − xCe_xMnO₃/Fe₂O₃	Specific surface area	Pore volume	Hole diameter
0	48.452	0.114	136.201
0.1	48.406	0.114	136.347
0.2	49.551	0.117	137.509
0.3	51.151	0.132	137.330
0.4	53.510	0.144	136.580

Figure 7a shows the surface morphology of Fe₂O₃ magnified 10K times. It can be seen that there are more spherical particles on the surface of Fe₂O₃, some particles are agglomerated, and there are fewer voids. Figure 7b shows the 40%LaMnO₃/Fe₂O₃ catalyst. It can be seen that after the catalyst is loaded with LaMnO₃, the voidage increases, the surface is relatively loose, and the particle uniformity improves, but the particle agglomeration phenomenon still exists locally. Figure 7c shows 40%La_0.6Ce_0.4MnO₃/Fe₂O₃. Compared with Fig. 7b, the surface diameter is significantly larger, and the catalyst surface is loose and porous. The increase of specific surface area and the uniform distribution of particles are conducive to the contact, adsorption and activation of the catalyst surface with the reaction gas, which is favorable for the catalytic reaction process of NH₃-SCR. Although 40%La_1 − xCe_xMnO₃/Fe₂O₃ has a high efficiency in catalytic denitrification experiments, it can be found that the local surface particle agglomeration and voidage are less, and the active substances cannot be uniformly loaded on the catalyst surface, which also indicates that the impregnation method for catalyst needs to be improved.

The citric acid impregnation method has the advantages of high utilization rate of active components, low cost and simple production method, but the active components are easy to migrate. Therefore, it is necessary to investigate the load of active substances. As can be seen from the Fig. 8, when the load is 10%, 20% and 30%, the actual load is 9.408%, 19.370% and 28.608%. When the load was 40%, the actual load was 37.029%, while when the load was 50%, the actual load was only 42.821%. The results indicated that citric acid impregnation method was not suitable for the preparation of catalyst with large loading capacity. The oxides of other trace elements, such as ZrO₂, SO₃ and P₂O₅, were also detected in the prepared catalyst, and the sum of oxide content was less than 3%.

4.2 Catalytic Performance of 40%La_1-xCe_xMnO₃/Fe₂O₃ When Reducing NO

The H₂-TPR distribution of 40%La_1 − xCe_xMnO₃/Fe₂O₃ is shown in Fig. 9. Three reduction peaks appear in all three catalysts, which are related to the continuous reduction of Fe₂O₃→Fe₃O₄→FeO→Fe.^44–46 The reduction behavior of Fe₂O₃ changed obviously after loading active substance. The reduction peak area increased at 402℃, and a new reduction peak appeared at 370℃. This is because the addition of Ce provides a new active site on the catalyst surface. According to the integration, as the doping amount of Ce increases from 0.3 to 0.4, the area of the first reduction peak and the second reduction peak of the catalyst gradually increases and moves towards low temperature (the first reduction peak :402℃→398℃→391℃, the second reduction peak:540℃→538℃→512℃). In addition, 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ reduction peak corresponds to the lowest desorption temperature. This indicates that 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ has the strongest reducing ability at low temperature. The above results show that LaMnO₃ can improve the catalytic denitrification capacity of the catalyst, and the addition of Ce can improve the catalytic capacity of catalyst at low temperature. This further explains why the catalytic effect of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ is higher than other catalysts.

The NH₃-TPD of La_1 − xCe_xMnO₃/Fe₂O₃ is shown in Fig. 10. As we all know, the thermal stability of NH₃ adsorbed at Brønsted acid site is lower than that of NH₃ coordinated with LAS. Thus, desorption peaks below 300 ℃ are attributed to the desorption of NH⁴⁺ from the Brønsted acid site(BAS), while peaks above 300 ℃ are attributed to the desorption of NH₃ coordinated with Lewis acid(LAS). It can be found from the spectrum that LaMnO₃/Fe₂O₃ has three desorption peaks at 248℃, 415℃ and 440℃. La_0.8Ce_0.3MnO₃/Fe₂O₃ and La_0.7Ce_0.4MnO₃/Fe₂O₃ have two desorption peaks, and the acid distribution is typical bimodal ammonia desorption curve. The acid distribution is typical bimodal ammonia desorption curve. The desorption peak of LaMnO₃/Fe₂O₃ at 248℃ belongs to the weak acid position, while the desorption peak at 415℃ and 440℃ belongs to the strong acid position. When the Ce doping amount was 0.3, two strong acid sites became one strong acid site (445℃). When the doping amount continued to increase (x = 0.4), the desorption peak attributed to the strong acid site shifted significantly (450℃), and the peak area of both weak acid site and strong acid site increased. This indicates that Ce doping can change the intensity and quantity of Brønsted acid sites and Lewis acid sites of catalysts. The La_0.6Ce_0.4MnO₃/Fe₂O₃ Lewis acid site is stronger, which also explains why the catalytic denitration efficiency of La_0.6Ce_0.4MnO₃/Fe₂O₃ is the highest among the three catalysts.

4.3 Chemisorption Properties of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃

In situ DRIFT spectra of NH₃ adsorption and activation on the catalyst surface changing with time are shown in Fig. 11a. At 50℃, the adsorption peaks of NH₃ on the catalyst surface mainly appeared at 3684, 1760, 1540, 1419, 1355 and 1250cm^− 1. 45min after NH₃ was introduced, the vibration peaks at 1540 cm^− 1 and 1250 cm^− 1 were weakened. The vibration peaks at 1540 cm^− 1 and 1250 cm^− 1 are attributed to the antisymmetric variable Angle vibrations (δas(NH₃)) and symmetric variable Angle vibrations (δs(NH₃)) of the coordination NH₃ adsorbed at the Lewis acid site (LAS). ²⁶ According to the vibration peak area of the spectrum, the surface of catalyst is mainly LAS. The peak at 3684 cm^− 1 was attributed to the hydroxyl group produced at the Brønsted acid site (BAS) on the catalyst surface. Peaks at 1760 cm^− 1 and 1463 cm^− 1 were attributed to NH⁴⁺ species bound to BAS. ^47–49 Peaks at 1540, 1362 and 1250 cm^− 1 were attributed to NH₃ species adsorbed to LAS. According to the integration, the peak area at 1760 cm^− 1 and 1463 cm^− 1 gradually increased after NH₃ began to flow into the area, while the vibration peaks showed a weakening trend after N₂ purge. Among them, the vibration peak at 1760 cm^− 1 showed a larger weakening trend, while the peak at 1463 cm^− 1 showed little weakening trend. This indicates that the BAS at 1463 cm^− 1 is stronger than that at 1760 cm^− 1. The adsorption peak at 1362 cm^− 1 is attributed to NH₂, the active intermediate product produced by adsorption and dehydrogenation of NH₃. ^50–52 The adsorption peak at 3684 cm^− 1 is due to the formation of NH⁴⁺ by the consumption of surface acidic hydroxyl by NH₃, and the resulting adsorption peak at 1463 cm^− 1 belongs to the binding of NH⁴⁺ at the Brønsted acid site. Infrared spectra of quasi-steady-state adsorption of NH₃ on the catalyst surface at 50℃-400℃ is presented in Fig. 11b. It can be seen that as the temperature goes from 50℃ to 400℃, the infrared adsorption peak at 1362 cm^− 1 which is attributed to the LAS starts at 150℃, and the peak area is basically unchanged, while the adsorption peak at 3684, 1760, 1540 and 1463 cm^− 1 decreases at different rates. This indicates that the adsorption of acid sites at 1362 cm^− 1 is relatively stable, while the strength of other sites is not strong. The coverage of NH₃ molecules at acid sites on the surface deteriorates with the increase of temperature.

It has been reported that the adsorption capacity of NO and O₂ together on Fe₂O₃ at low temperature is stronger than that of NO alone.²⁶ In situ DRIFTS spectra of transient adsorption of NO and O₂ on the catalyst surface at 50℃ is presented in Fig. 12a. At 50℃, the main infrared adsorption peaks on the catalyst surface are 1772, 1548, 1424 and 1350 cm^− 1. It can be seen that when NO and O₂ were first introduced into the catalyst surface, obvious infrared adsorption peaks appeared at 1424 cm^− 1 and 1350 cm^− 1, indicating that the acid site was adsorbed at a faster rate. The adsorption capacity of NO and O₂ at 1548, 1424 and 1350 cm^− 1 increased with the increase of catalyst adsorption time. According to reports, the band 1622 − 1548 cm^− 1 is mainly produced by NO and lattice oxygen in the catalyst, which belongs to the typical N-O antisymmetric stretching vibration region, belonging to nitrate or adsorption state NO₂.^53–57 1548 cm^− 1 was attributed to the v_as(NH₃) vibration mode of the v₃ cleavage of the undented nitrate, and the infrared peaks at 1424 and 1350 cm^− 1 were attributed to the undented nitrate and nitro compounds.^58,59 It can be seen that the main adsorption products of NO and O₂ on the surface are single-tooth nitrate and nitro compounds. A small infrared peak appeared at 1770 cm^− 1 about 30min after gas injection, which was attributed to Fe²⁺-N = O species. After N₂ purge, strong adsorption peaks were still found at 1424 and 1350 cm^− 1, which may be because when N₂ purge the catalyst surface, NO was oxidized by Fe³⁺ to generate NO₂, which returned to the catalyst surface to generate nitro, resulting in enhanced peak at 1424 cm^− 1. In situ DRIFTS spectra of quasi-steady-state adsorption at programmed temperature on the catalyst surface is presented in Fig. 12b. The main products of catalyst surface adsorption were nitrate (1424 cm^− 1) and nitro compound (1350 cm^− 1). It can be seen that the characteristic peaks of nitrate (1424 cm^− 1) and nitro compounds (1350 cm^− 1) showed an increasing trend with the increase of temperature, indicating that more nitrate substances were generated after further oxidation of nitrite and nitro. The characteristic peak area at 1548 cm^− 1 decreases significantly at 100℃, indicating that the weakly adsorbed single-tooth nitrate and adsorbed NO₂ produce desorption on the catalyst surface. This may be due to the involvement of NO₂ in the rapid SCR reaction, resulting in a significant reduction. There is literature indicating that when NO₂ is present, nitric acid will be generated to further generate NH₄NO₃. Under the action of surface acidic sites, NH₄NO₃ will be reduced by NO to NH₄NO₂, which is extremely unstable and can be decomposed into N₂ and H₂O at 100 ℃, which is a rapid SCR reaction⁶⁰. Therefore, for this catalyst, strong oxidizability can lead to the production of NO₂ and promote the SCR reaction process.

The reaction of NH₃ with NO and O₂ at 240℃ is shown in Fig. 13. The main infrared adsorption peaks on the catalyst surface at 270℃ are 3265, 1781, 1540, 1412 and 1355 cm^− 1. It can be seen that, with the influx of NO and O₂, ammonia species are consumed. Meanwhile, the peak value at 1540 cm^− 1 increases at 40min of reaction, while the peak value at 1412 cm^− 1 decreases. The peak at 1540 cm^− 1 was monodontic nitrate, which belonged to nitrate species. The peak at 1412 cm^− 1 was attributed to NH⁴⁺. The peak at 3265 cm^− 1 was attributed to NH₃ adsorbed at the L acid site. Although there was the presence of monondonate nitrate in the spectrum, no characteristic peak of bridging nitrate was detected, indicating that the pre-adsorbed NH₃ was not completely consumed and NH₃ still participated in the reaction on the catalyst surface. It was found from the spectrum that the thermal stability of the Lewis acid site was still stronger than that of the Brønsted acid site under the two different pre-adsorbed SCR reactions, so the enhancement of the Brønsted acid site played a key role in improving the catalytic activity at low temperature.⁶¹

4.4 SO₂-tolerance Mechanism of 40%La_0.7Ce_0.4MnO₃/Fe₂O₃

In order to understand the formation of ammonium sulfate salts on the catalyst surface in SO₂ and H₂O environment, TG characterization was performed on fresh and 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalysts after sulfur toxicity resistance at 240℃, and the results were shown in the Fig. 14. HAADF image and EDS mappings of used Fe₂O₃ and used 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ are as show in Fig. 15. It can be seen that compared with the fresh catalyst, the mass of the poisoned catalyst gradually loses during the heating process, indicating the presence of sulfate on the surface of the catalyst. Previous studies have found that there are two ways for SO₂ to cause catalyst poisoning. On the one hand, NH₄HSO₄ generated by SO₂ reacts with NH₃ in the environment of H₂O is attached to the catalyst surface, resulting in the active site being covered; on the other hand, SO₂ can directly generate relatively stable metal sulfate with metal oxides.^61,62

Weightlessness of catalyst can be divided into three steps. It is attributed to the loss of H₂O on the catalyst surface at 50–200℃.⁶³ In the second step, 200–500℃ is attributed to the decomposition of (NH₄)₂SO₄ and NH₄HSO₄.⁶⁴ In the third step, 500–800℃ is attributed to the decomposition of Fe₂ (SO₄)₃. It can be seen that the mass loss of used catalysts is significantly increased compared with that of fresh catalysts. In the second stage, the weight loss of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst before and after the experiment is ~ 0.62%, while the weight loss of Fe₂O₃ catalyst before and after the experiment is ~ 0.68%, indicating that compared with pure Fe₂O₃, the surface of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst has less sulfate. At 500 ~ 800℃, the weight loss difference of Fe₂O₃ catalyst before and after use decreases gradually, indicating that Fe₂ (SO₄)₃ decomposition occurs at this temperature. The weight loss degree of used 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst is lower than that of used Fe₂O₃ catalyst, indicating that there is more Fe₂ (SO₄)₃ on the surface of supported catalyst. On the other hand, in the third stage, the weight loss rate of 40%LaMnO₃/ Fe₂O₃ catalyst before and after the experiment is lower than 40%La_0.6Ce_0.4MnO₃/ Fe₂O₃ catalyst, indicating that the doping of Ce element improves the formation of Fe₂ (SO₄)₃, this is consistent with simulation results from previous studies.⁶⁵

The catalyst was prepared by citric acid method, when 40%LaMnO₃ was loaded onto Fe₂O₃ catalyst, the catalytic denitration effect was the best, and the highest efficiency reached 94% at 240℃. In the range of 240 ℃ to 330℃, the optimum temperature activity window is increased by 30.8% compared with Fe₂O₃ catalyst. LaMnO₃ can enhance the catalytic effect of Fe₂O₃ catalyst at low temperature. The flue gas temperature corresponding to the optimal catalytic denitration efficiency is reduced by 30℃. LaMnO₃ can be dispersed on the surface of Fe₂O₃. Compared with Fe₂O₃, the specific surface area, pore volume and pore diameter of 40%LaMnO₃/Fe₂O₃ catalyst show a larger trend.

The Ce doped 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst improved the agglomeration and reduced the pore space on the catalyst surface. At low temperature, the adsorption peaks of NH₃ on the catalyst surface mainly appear at 3684, 1760, 1540, 1419, 1355 and 1250cm^− 1. The surface of the catalyst is mainly composed of Lewis acid site and a small amount of Brønsted acid site, among which the acid site at 1362cm^− 1 is relatively stable. The main adsorption products of NO and O₂ on catalyst surface are mono-dentate nitrate and nitro compound. As the temperature increases, nitrite and nitro group undergo further oxidation to produce more nitrate substances. In the case of two different pre-adsorbed SCR reactions, the thermal stability of Lewis acid site is still stronger than that of Brønsted acid site.

The enhancement of Brønsted acid site plays a key role in improving the catalytic activity at low temperature. The doping of Ce alleviated the competitive adsorption of SO₂ and H₂O at the active site on the catalyst surface, delayed the catalyst poisoning to a certain extent, and improved the irreversible inactivation caused by SO₂ and H₂O. 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst can recover to about 70% after 100min after stopping the pollutant inlet. It was found in TG results that the coating of less sulfate substances may be one of the reasons why 40%La_0.6Ce_0.4MnO₃/Fe₂O₃ catalyst has stronger sulfur and water resistance.

Acknowledgement

This work was supported by Natural Science Foundation of Shandong Province, China, (ZR2021QE295) and Natural Science Foundation of Shandong Province, China, (ZR2023ME2116).

Authors

Wencong Hao - College of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266000, China

Yongchao Xu - College of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266000, China

Zongliang Zuo - Key Laboratory of Waste Heat Utilization and Energy Saving of Shandong Province, College of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266000, China

Haohan Sha - Key Laboratory of Waste Heat Utilization and Energy Saving of Shandong Province, College of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266000, China

Notes The authors declare no competing financial interest.

Declaration statements

I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Author contributions:Dongdong Ren provided the idea, The formation or evolution of overall research objectives and objectives.Wencong Hao carried out analysis of research data, synthesizing statistical, mathematical, computational, or other forms of application of techniques.Dongdong Ren is a financial supporter of the publication's project.Wencong Hao conducted research and investigation processes, especially conducting experiments, or collecting data/evidence.Ren dongdong contributed methodology and modeling.Siyi Luo, Zongliang Zuo provided research materials, materials, computational resources or other analytical tools.Ren dongdong supervised and led the planning and execution of research activities, including external guidance to the core team.Wencong Hao created the presentation part of the work, especially the visualization/data presentation.Wencong Hao created and publishes works, especially the first draft.Dongdong Ren contributed comments or revisions before publication.

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Effect of Ce on the acidity of LaMnO3 / Fe2O3 catalyst in NH3-SCR at low temperature

Status:

Version 1

Abstract

Figures

1. INTRODUCTION

2. EXPERIMENTAL

2.1 Experimental Equipment

2.2 Catalyst Preparation

2.3 Experimental Condition

3. SECTION

3.1 Effect of LaMnO₃ Loading on Catalytic de-NO_x Activity

3.2 Effect of Ce Doping on Catalytic de-NO_x Activity of LaMnO₃/Fe₂O₃

3.3 SO₂ Resistance of the Catalysts

4. RESULTS AND DISCUSSION

4.1 Structural Characterization of x%LaMnO₃/Fe₂O₃

4.2 Catalytic Performance of 40%La_1-xCe_xMnO₃/Fe₂O₃ When Reducing NO

4.3 Chemisorption Properties of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃

4.4 SO₂-tolerance Mechanism of 40%La_0.7Ce_0.4MnO₃/Fe₂O₃

5. CONCLUSIONS

Declarations

References

Additional Declarations

Status:

Version 1

Effect of Ce on the acidity of LaMnO3 / Fe2O3 catalyst in NH3-SCR at low temperature

Status:

Version 1

Abstract

Figures

1. INTRODUCTION

2. EXPERIMENTAL

2.1 Experimental Equipment

2.2 Catalyst Preparation

2.3 Experimental Condition

3. SECTION

3.1 Effect of LaMnO3 Loading on Catalytic de-NOx Activity

3.2 Effect of Ce Doping on Catalytic de-NOx Activity of LaMnO3/Fe2O3

3.3 SO2 Resistance of the Catalysts

4. RESULTS AND DISCUSSION

4.1 Structural Characterization of x%LaMnO3/Fe2O3

4.2 Catalytic Performance of 40%La1-xCexMnO3/Fe2O3 When Reducing NO

4.3 Chemisorption Properties of 40%La0.6Ce0.4MnO3/Fe2O3

4.4 SO2-tolerance Mechanism of 40%La0.7Ce0.4MnO3/Fe2O3

5. CONCLUSIONS

Declarations

References

Additional Declarations

Status:

Version 1

3.1 Effect of LaMnO₃ Loading on Catalytic de-NO_x Activity

3.2 Effect of Ce Doping on Catalytic de-NO_x Activity of LaMnO₃/Fe₂O₃

3.3 SO₂ Resistance of the Catalysts

4.1 Structural Characterization of x%LaMnO₃/Fe₂O₃

4.2 Catalytic Performance of 40%La_1-xCe_xMnO₃/Fe₂O₃ When Reducing NO

4.3 Chemisorption Properties of 40%La_0.6Ce_0.4MnO₃/Fe₂O₃

4.4 SO₂-tolerance Mechanism of 40%La_0.7Ce_0.4MnO₃/Fe₂O₃