The impact of Fe/Mn loads on the surface property of rare earth mineral

After using the mixed acid system to modify the surface of rare earth minerals, two transition metals (Fe and Mn) are loaded on the surface of the rare earth minerals by hydrothermal method to improve the NH3-SCR (Selective catalytic reduction) activity. In this paper, a variety of characterization means like SEM, XRD, BET, XPS, H2-TPR (Temperature-programmed reduction), NH3-TPD (Temperature-programmed desorption) and NO-TPD are used to study and analyze the surface properties of rare earth minerals before and after loading transition metals. Its catalytic performance was further measured in a simulated flue gas installation. The results show that the active elements on the surface of rare earth minerals after loading transition metals are mainly oxides. The specific surface area of rare earth minerals is effectively improved, the crystallinity of the active material is reduced, and the element distribution is more uniform. In addition, a composite structure of Fe-Ce and Mn-Ce is formed on the surface of rare earth minerals, and the oxygen vacancies and adsorption sites on the surface of the minerals are significantly improved. The NOX conversion rate for Fe-loaded mineral catalysts reached 89.1% at 300 ℃. The NOX conversion rate for Mn-loaded mineral catalysts reached 92.3% at 250 ℃. Generally speaking, the NH3-SCR activity of rare earth mineral can be improved by loading transition metals (Fe and Mn).


Introduction
At present, NH 3 -SCR catalysts based on rare earth metal oxides have become a research hotspot.The catalyst prepared by the transition metal oxide doped with rare earth metal oxide has a good improvement of its catalytic efficiency and tolerance.Currently, it is widely known that catalysts with Mn-Ce, Fe-Ce and Mn-Fe-Ce composite metal oxides as major active components can exhibit excellent catalytic performance at low temperature [1][2][3].The reason is that elements such as Fe and Mn on the surface promote the refinement of Ce crystals and form a solid solution structure with Ce.Ce 2 O 3 and CeO 2 are usually found to co-exist in Ce oxides.They may differ in crystallographic form, among which +3 is hexagonal while +4 is face-oriented cubic [4,5].Oxygen vacancies are formed during the transition metal valence state conversion process, where Ce plays a role of oxygen storage, promoting the reaction of adsorbed NH 3 and NO on the surface to form -NH 2 and -NO 3 , and finally react to form N 2 and H 2 O.
Taking natural minerals as raw materials and select proper elements for loading has become a hotspot in the development of green and environmentally-friendly materials.The researches were initially launched about fluorocarbon calcium perovskite [6], Qingyang manganese ore [7] and natural zeolite [8].After processing rare earth perovskite into nanoparticles, as the specific surface area increases, the surface structure and properties also change.Due to size effect, the bonding state and electronic state on the surface of nanoparticles are different from those in the interior, together incomplete atom coordination on the surface, increase its active positions, thus changing its catalytic performance.Most manganese exists as manganese oxide in roasted manganese ore, whose crystal form and catalytic effect have been significantly improved after being loaded with Ti/Ce/Fe.The catalytic efficiency of natural zeolite is enhanced by loading elements like Ce, Fe and Mn on its surface, which is mainly attributable to its large surface area.
The rare earth mineral in Baiyun Obo Mining District is rich in metal elements like La, Ce, Pr, Nd (La 2 O 3 15%, Ce 2 O 3 71%, Nd 2 O 3 6%, Pr 2 O 3 5%) and a small amount of transition metal elements.It also has a lot of active substances required for Ce-based SCR catalysts.The direct preparation of NH 3 -SCR catalyst from Bayan Obo rare earth minerals avoids the cumbersome process of mineral purification and re-preparation compared with pure material synthesis catalysts, saving a lot of money and manpower.Natural rare earth minerals use their own natural framework and the synergy between elements to exert high-strength tolerance and stability in the catalytic process.However, the elements in natural minerals have various forms and complex accompanying structures, which do not have the structural properties of catalyst carriers.Structurally, natural rare earth mineral contains SiO 2, CaF and CaO, which can be dissolved through chemical treatment so as to shape a complex pore structure.In terms of components, natural rare earth mineral contains a variety of rare earth elements and transition metals while being characterized by complex embedded and enveloped structure, for which there is synergy between metal elements.However, its catalytic activity is restricted because some elements are not sufficiently rich.In this project, we take advantage of the rare earth mineral itself (with Ce oxides) and treat the surface of the natural rare earth mineral to increase its specific surface area and provide space for the loading of transition metals.Artificially forming a metal solid solution on the surface of rare earth mineral.Finally, we can prepare a green and efficient rare earth mineral catalyst.

Sample pretreatment
A certain amount of rare earth mineral was separately packed and reserved with a particle size of 400 mesh after crushing, grinding, sifting and drying.The processing of rare earth mineral mainly includes: raw ore (S1), mixed acid modified raw ore (S2), Fe-loaded rare earth mineral (S3), and Mn-loaded rare earth mineral (S4) are mixed in a high temperature and high pressure heating axe (130 ℃, 2 h).The specific ingredients are shown in table 1:

Test method of catalyst activity
Test instrument mainly consisted of quartz tube, vertical tube furnace, sampler, Fourier infrared spectrum flue gas analyzer and computer data acquisition system.The vertical tube furnace with a rated temperature of 1,600 ℃ manufactured by Nanjing Boyuntong Instrument Technology Co. Ltd and MoSi 2 with an inner diameter of 20 mm and a length of 1.2 m (model No. 1800) were used for heating.The Fourier transform infrared spectroscopy (FTIR) flue gas analyzer originated in Finland (model No. GASMET-DX4000) and data acquisition system were used together for online measurement of flue gas composition.1g samples were weighted for each test, and a certain amount of quartz cotton was weighed and placed in the heating section of quartz tube, which acted as a reaction bed used to support catalyst.
Before experiment, vertical tube furnace was heated from room temperature to experimental temperature at a heating rate of 10 ℃ min −1 .The reaction gas was fed in for 30 min.When experimental temperature and gas concentrations were in a stable state, test samples were quickly poured into quartz tube to heat constant temperature zone, and used to calculate the denitrification efficiency of the catalyst based on the following formula:

Analysis on catalytic performance
In the process of investigating the physical and chemical properties of rare earth minerals, it is found that when the particle size of rare earth minerals becomes smaller, its specific surface area becomes larger.In order to explore the influence of the specific surface area changes of rare earth minerals with different particle sizes on the catalytic activity, the NO X conversion rate was tested, as shown in figure 1(a) (constant temperature: 450 ℃).
The activity of rare earth minerals of different particle sizes did not show NO X conversion before 10 min, because it takes a certain time for rare earth minerals to enter the test environment for activation.After 20 min, the NO X conversion rates of rare earth minerals with different particle sizes tended to be stable, indicating that natural minerals have certain tolerance.Rare earth minerals with a particle size larger than 400 mesh have the strongest catalytic conversion ability, indicating that the particle size of rare earth minerals is one of the main factors in NO X conversion ability.It also proves that the increase in specific surface area is beneficial to improve the NO X conversion capacity of rare earth minerals.
The NO X conversion rates of different samples at different temperatures are shown in figure 1(b).S1 is rare earth mineral(raw ore); S2 is acid-treated rare earth mineral; S3 is acid-treated rare earth mineral loaded with Fe; S4 is acid-treated rare earth mineral loaded with Mn.The NO X conversion rates of S1 and S2 both reached the maximum at 450 ℃.The NO X conversion rate of S2 was slightly higher than S1, but the catalytic reaction temperature was both high.It shows that the mixed acid modification of rare earth minerals increases the specific surface area, and exposes the active components on the surface of the rare earth minerals, which promotes the occurrence of catalytic reactions.The NO X conversion rate of S3 at 300 ℃ reached 89.1%, and the NO X conversion rate of S4 at 250 ℃ reached 92.3%.It can be seen from the figure that the peak temperature of the NO X conversion rate of S3 and S4 has decreased.The content and types of the surface elements of the Fe/Mn loaded rare earth minerals have increased, and the active sites on the surface of the rare earth minerals have also increased, which is one of the main factors to improve the NO X conversion rate.

Analysis on microcosmic appearance and structure
SEM was applied to investigate the morphological changes of samples during treatment, and the results were shown as figure 2. It can be seen from the figure 2 that the particle sizes for raw ores were generally around 20 μm with flat and polygonal surfaces.However, cracks appeared on the surface of mineral after it was soaked in acid, indicating that acid leaching caused some particles to be dissociated and broken, which further resulted in larger surface area, thus providing a certain room for loading and increasing the access possibility among Ce, Fe and Mn in the rare earth mineral.After acid treatment, Fe-loaded rare earth mineral formed a wrapped and embedded structure, spherical and netted accumulation particles appeared on the originally coarse surface.So it can be concluded that rare earth mineral and Fe oxide were artfully combined during loading, and Fe oxides were successfully loaded on the surface of rare earth mineral.The Mn oxide loaded on the surface of the acidtreated rare earth mineral forms a villiform overlay structure, which was produced by reaction between Mn oxides and the substances on the surface of rare earth mineral.The special Fe-Ce and Mn-Ce structures between metal elements were formed while the specific surface area of rare earth mineral was increased by these two loading methods.The BET test results of the rare earth minerals with different treatments are shown in table 2. The average pore size of S1 is relatively large but the specific surface area is relatively small.Combined with the scanning electron microscope, it can be seen that the surface of the raw ore is smooth and flat, and the pore structure is not obvious, so the specific surface area of the raw ore is small.The mixed acid treatment erodes the surface of rare earth minerals and forms porous surfaces.The structure increases the specific surface area of S2 and increases the average pore diameter, which not only provides more attachment points for the loaded transition metal elements, but also increases the acidic sites on the surface of rare earth minerals.Compared with S1 and S2, the surface area of S3 and S4 is significantly increased.The loading of Fe and Mn leads to changes in the crystal forms of S3 and S4.A small pore structure is formed on the surface during high-temperature calcination, which increases the specific surface area and decreases the average pore diameter.

Analysis of different element content in different samples
The Ce, Fe, and Mn in different treated rare earth minerals were tested by ICP-OES, as shown in figure 3. It can be seen from the changes of elements in S1 and S2 that the content of Fe and Ce increased significantly after acid treatment, which indicates that acid leaching can remove impurities on the surface of rare earth minerals, thereby increasing the relative content of Fe and Ce.The content of Fe in S3 increased significantly because the sample was loaded with Fe and the content of Ce decreased slightly.The content of Mn in S4 is significantly increased, and the loading form of Mn is the same as Fe.For S3 and S4, the introduction of Fe and Mn decreases  the relative content of Ce.Because Fe and Mn are evenly embedded on the surface of rare earth minerals, part of Ce is wrapped and not completely exposed, which leads to a decrease in the relative content of Ce. .The peak baseline of S4 is greatly improved, but no diffraction peak of MnO 2 is found, indicating that MnO 2 is evenly dispersed on the surface of rare earth minerals.The dispersion of Ce 7 O 12 on the surface of rare earth minerals has also been improved [11].The combined form of Mn and rare earth minerals is different from the combined form of Fe and rare earth minerals.It does not form a package but is evenly embedded on the surface of rare earth minerals.Both the loading of Fe and Mn changed the  state of the surface elements of rare earth minerals.Fe, Mn and Ce 7 O 12 on the surface of rare earth minerals form a complex structure of wrapping and embedding.The formation of these structures promotes the improvement of the catalytic performance of rare earth minerals.

Adsorption characteristics of NH 3 on the surface of mineral catalysts
In order to further explore the influencing factors of NH 3 -SCR reaction, NH 3 -TPD tests were performed on four groups of samples as shown in figure 5. Since the catalytic activity test temperature (<500 ℃) in this experiment is relatively low, the high temperature characteristic peaks (>500 ℃) are not studied here.According to the desorption peak area, it can be seen that the adsorption amount of NH 3 is S1<S2<S3<S4.The oxides of Fe and Mn are highly dispersed on the surface of the rare earth minerals and form composite oxides, indicating that the rare earth minerals provide part of the active components while taking on the carrier.It can be seen from the figure that all samples have two characteristic peaks in this temperature range, mainly concentrated around 150 ℃ and 400 ℃.The first desorption peak is attributable to the NH 3 adsorbed at the weakly acidic sites on the surface of rare earth minerals, which is easy to desorb at low temperatures.The second desorption peak is related to the -NH 2 species, which is mainly formed by NH 3 adsorption on the strong acidic sites [12].Compared with S1 and S2, the desorption peaks of S3 and S4 in the low temperature section are significantly increased, indicating that Fe and Mn loaded on the surface of rare earth minerals can increase weak acid sites and promote the adsorption of NH 3 by weak acid sites [13,14].

Characteristics of NO adsorption on the surface of mineral catalysts
In order to explore the adsorption performance of different samples for NO, this paper conducted a NO-TPD test, and the results are shown in the figure 6.According to the desorption peak area, it can be seen that the adsorption amount of NO is S1<S2<S3<S4.The raw ore(S1) appears a desorption peak in near 550 ℃.
The NO adsorption capacity of rare earth minerals after mixed acid treatment is slightly increased.The main reason is that the acid treatment exposes the active materials on the surface of the rare earth minerals, which provides more alkaline sites for the reaction.At the same time, the specific surface area is increased, which promotes adsorption and activation of NO.The NO adsorption capacity of earth mineral being loaded with Fe(S3) and Mn (S4) was significantly improved [15].Composite metal oxides of Fe-Ce and Mn-Ce are formed in S3 and S4.NO enters the composite structure of Fe-Ce and Mn-Ce, and NO is adsorbed after forming a broken bond with e-.When e-in the crystal form is completely exhausted, the NO adsorption capacity will disappear.The addition of Fe and Mn provides more alkaline sites for the reaction, and the peak at 550 ℃ is attributed to the decomposition of nitrates formed by the oxidation of NO.It can be concluded that the increase of specific surface area [16] and the effect of multi-metal oxide can promote the adsorption and activation of NO [17].[19], and at the same time Ce 4+ →Ce 3+ [20].Compared with S2, the peak shapes of S3 and S4 are significantly enhanced and move to the low temperature area.It can be concluded that the element valence conversion and electron transfer of Fe-Ce and Mn-Ce will form oxygen vacancies on the surface of rare earth minerals, which promote the mutual conversion between surface adsorbed oxygen and lattice oxygen, thereby improving the rare earth minerals redox performance.

Analysis of element valence
In order to explore the changes of element valence states in different samples, this paper carried out XPS testing on the three elements Ce, Fe, and Mn in the samples, the results are shown in figure 8. Ce in S1 mainly exists in the form of CeCO 3 F, so Ce 3+ accounts for a larger proportion.The Ce in S2 and S3 mainly exists in the form of Ce 7 O 12 , so Ce 4+ accounts for a larger proportion.Ce plays the role of oxygen storage here.Under hypoxic conditions, it will release lattice oxygen and become surface adsorbed oxygen.The Ce 4+ in the mineral is partially converted into Ce 3+ and oxygen vacancies are formed.Under oxygen-rich conditions, the adsorbed oxygen on the surface will be converted into lattice oxygen for storage, and Ce 3+ will be converted into Ce 4+ , thereby improving the redox capacity of rare earth minerals [21].The Fe in S1 mainly exists in the form of Fe 2 O 3 , so Fe 3+ accounts for a relatively large proportion.Fe 2 O 3 exists in rare earth minerals and forms a continuous structure with CeCO 3 F. S3 is loaded with Fe 2 O 3 , but there are both Fe 3+ and Fe 2+ in S3 [22], mainly due to the composite structure formed between Fe-Ce.The Ce in S3 is converted from Ce 3+ to Ce 4+ , which promotes the conversion of Fe 3+ to Fe 2+ .This follows the conservation of charge, thus completing the valence conversion and electron transfer.The Mn content in rare earth minerals is small, so XPS analysis is only performed for Mn in S4.It can be observed that Mn 2+ , Mn 3+ and Mn 4+ exist simultaneously in S4, among which Mn 3+ takes up a larger proportion.S4 is loaded with MnO 2 , Ce in S4 is transformed from Ce 3+ to Ce 4+ .According to the conservation of charge, Mn is transformed from Mn 4+ to Mn 3+ and Mn 2+ [23].The conversion of valence states of Fe, Mn, and Ce will cause the surface electron imbalance of rare earth minerals, forming oxygen vacancies and unsaturated chemical bonds, which promote the mutual conversion of adsorbed oxygen on the mineral surface and lattice oxygen.The combined action of Fe-Ce and Mn-Ce improves the oxidation-reduction ability of rare earth minerals, which is beneficial to the improvement of catalytic performance.

Conclusions
In this paper, rare earth minerals are used to prepare NH 3 -SCR catalyst active powders with high efficiency and simple processes.The main reason for the increase in catalytic activity of rare earth minerals treated with mixed acid is that acid treatment increases the specific surface area of rare earth minerals and exposes active materials on the surface.The loading of Fe and Mn metal oxides in rare earth minerals significantly increases the specific surface area of rare earth minerals.At the same time, Fe and Mn modify the pores of rare earth minerals, increasing the amount of micropores on the surface of rare earth minerals, and further improve the pore structure, thereby providing a place for catalytic reactions.The loading of Fe and Mn on the surface of rare earth minerals greatly improve the adsorption capacity of NH 3 and NO, form a state of co-existence of multiple elements and valence.The synergy of Mn-Ce and Fe-Ce produces more oxygen vacancies and adsorption sites, thereby improving catalytic activity.The NO X conversion rate of Fe-loaded rare earth minerals reached 89.1% at 300 ℃, and the Mn-loaded rare earth minerals reached 92.3% at 250 ℃.Overall, the loading of Fe/Mn is beneficial to increase the NH 3 -SCR activity of rare earth minerals.

Figure 1 .
Figure 1.Analysis on NO X conversion rate of rare earth mineral catalyst.(a) Catalytic efficiency of rare earth minerals with different particle sizes; (b).Catalytic efficiency of different treatments of rare earth minerals).

Figure 2 .
Figure 2. Analysis of SEM on the surface of rare earth mineral.

3. 4 .
Crystal phase change of mineral catalysts treated in different ways The XRD results of different treatments of rare earth minerals are shown in figure 4. The composition of S1 is complex, mainly including CeCO 3 F, CePO 4 , CaF 2 , and Ca 5 (PO 4 )F.Among them, CeCO 3 F and CaF 2 have high crystallinity and occupy the main components in minerals.In order to better expose the active substances on the surface of rare earth minerals, S2 was prepared by mixed acid treatment.It can be seen in the figure that a new substance Ce 7 O 12 (CeO 2 /Ce 2 O 3 ) [9] appeared in S2, and the peak shape of CeCO 3 F was significantly weakened, indicating that part of the CeCO 3 F in the mineral was decomposed into Ce 7 O 12 during the roasting process.Because CePO 4 is relatively stable, it does not participate in the decomposition reaction.The weak diffraction peak of Fe 2 O 3 in S3 indicates that Fe 2 O 3 is evenly dispersed on the surface of rare earth minerals during the calcination process.At the same time, the diffraction peaks of Ce 7 O 12 are partially covered, indicating that Ce 7 O 12 and Fe 2 O 3 form a package structure [10]

Figure 3 .
Figure 3. Relative changes of elements in differently treated mineral catalysts.

Figure 4 .
Figure 4. Crystal phase change of mineral catalysts treated in different ways.

Figure 5 .
Figure 5. Characteristics of NH 3 adsorption on the surface of differently treated mineral catalysts.

3. 7 .
Oxidation-reduction characteristics of mineral catalyst surfaces with different treatments In order to explore the redox performance of different samples, this paper conducted a H 2 -TPR test, and the results are shown in the figure 7. The area of the H 2 reduction peak is S1<S2<S3<S4.The reduction peak of raw ore(S1) at 400 ℃−600 ℃ corresponds to the decomposition of CeCO 3 F, namely Ce 3+ →Ce 4+ .The reduction peak at 600 ℃−800 ℃ corresponds to Ce 4+ →Ce 3+ .The reduction peak of the sample treated with mixed acid (S2) at 500 ℃−700 ℃ corresponds to Ce 4+ →Ce 3+ , which is mainly due to the existence of Ce in the S2 is in the form of Ce 7 O 12 .Compared with S1, the reduction peak obviously moves to the low temperature area.The reduction peak of S3 at 400 ℃−500 ℃ corresponds to the conversion of Fe 2 O 3 to Fe 3 O 4. The reduction peak appearing at 500 ℃−700 ℃ corresponds to Fe 3 O 4 →FeO→Fe [18], and at the same time Ce 4+ →Ce 3+ .This reduction peak is a composite peak of Fe-Ce valence state conversion, indicating that a composite oxide is formed between Fe-Ce.The reduction peak of S4 at 400 ℃ corresponds to the conversion of MnO 2 to Mn 3 O 4 .The reduction peak appearing at 500 ℃−700 ℃ corresponds to Mn 3 O 4 →MnO→Mn

Figure 6 .
Figure 6.Analysis of NO-TPD on the surface of rare earth mineral.

Figure 7 .
Figure 7. Oxidation-reduction characteristics of mineral catalyst surfaces with different treatments.

Figure 8 .
Figure 8. Analysis of XPS on the surface of rare earth mineral.

Table 1 .
Preparation and test schemes for rare earth mineral samples.

Table 2 .
Specific surface area of differently treated mineral catalysts.