Study on NH 3 -SCR of Cerium-based Substances in Rare Earth Concentrates from Bayan Obo

In this paper, CePO 4 and CeCO 3 F were prepared by hydrothermal synthesis based on the ratio of bastnaesite to monazite in the process mineralogy of Baiyun Ebo rare earth concentrate. A comparison of the two treatments, ball milling and ball milling sulphation, revealed that the denitri ﬁ cation ef ﬁ ciency of the catalysts treated with ball milling sulphation increased by 20 percentage points, compared to those treated without sulphation, with denitri ﬁ cation ef ﬁ ciencies of up to 80%. The surface properties, redox properties and catalytic mechanism of the samples before and after the sulphation treatment were analyzed, by using XRD, NH 3 -TPD, H 2 -TPR, XPS and in situ IR characterization. The results showed that the CeF 3 diffraction peaks in the XRD patterns disappeared in the sulphated samples, NH 3 -TPD showed that the adsorption capacity of NH 3 on the surface of the samples was enhanced, and the introduction of sulphuric acid provided a large number of acidic sites on the catalyst surface, among which the Lewis acidic sites might be more favorable for the promotion of SCR reaction. The acidi ﬁ cation of sulphuric acid greatly increases the redox capacity of the catalyst, and the interconversion between Ce n+ was enhanced. XPS showed a signi ﬁ cant increase in the amount of adsorbed oxygen on the surface of the sample. The presence of -NO 2 , an important intermediate in the L-H mechanism, was also detected by IR analysis. reactant species during the L-H mechanism reaction were monodentate nitrate, bridged nitrate and NH 4+ species produced by NH 3 adsorption on the Brønsted acidic site of the catalyst surface.


INTRODUCTION
At present, coal-fi red thermal power generation is still the main mode of power generation in China, and fossil fuels such as coal release large amounts of NOx during the combustion process, and the environmental hazards caused by NOx have become the focus of longterm national governance 1 .The mainstream industrial fl ue gas denitrifi cation technology is NH 3 -SCR 2 , a process whereby NH 3 reacts selectively with NOx on the catalyst surface to form N 2 and H 2 O without consuming excess O 2 .Conventional denitrifi cation catalysts V 2 O 5 -WO 3 -MoO 3 /TiO 2 were toxic and suffer from a narrow activity temperature window(320-400 o C), and a complex preparation process 3 .It was therefore urgent to develop new NH 3 -SCR catalysts that were non-toxic, have a wide temperature window, and were simple to prepare.
The Cerium and other rare earth elements are the most widely studied catalysts for NH3-SCR denitrifi cation.The introduction of cerium can not only improve the catalytic performance of the catalyst 4, 5 , but also the potassium resistance, sulfur resistance, water-resistance of Catalystshas a signifi cant effect 6- 10 .It was well known, the Bayan Obo Mine was a super large rare earth deposit, and cerium was the main rare earth element.Cerium has a unique 4f electron structure which has good catalytic properties in catalytic reactions.Cerium has two oxidation valence states in its oxide CeO 2 , namely Ce 4+ and Ce 3+ , which can be converted to each other through the redox process, giving the oxide the ability to store and release oxygen, which was widely used in catalysis, and the redox activity of cerium-based catalysts can promote the rapid activation of NH 3 to reduce NOx and complete the redox cycle, which plays a crucial role in the NH 3- -SCR reaction process.In contrast, the number of acidic sites on the surface of rare earth concentrates was low, and need to additional modifi cation and treatment were required to enhance the adsorption capacity of NH 3 to improve the denitrifi cation effi ciency and N 2 selectivity of the NH 3 -SCR reaction.Therefore, acid modifi cation of rare earth concentrates was required.It was found that the sulphation of cerium-based catalysts resulted in improved catalytic performance, and enhanced resistance to alkali metal poisoning 11, 12, 13 .Currently, many scholars have used sulphuric acid solution to modify the catalysts, which greatly promoted the denitrifi cation performance of the catalysts and has good application prospects.Chen Wangsheng 14 et al. obtained sulphuric acid-modifi ed sinterite catalyst (SSOC) by using sulphuric acid solution to modify the sinterite catalyst (SOC).The results showed that the sintered ore was impregnated in a 5 mol/L sulphuric acid solution for 30 min, dried and calcined at 400 o C for 3 h, the SSOC catalysts were prepared to achieve 92% NOx conversion at 300 o C. Zhang Qiuling 15 et al.Acidifi cation of CeO 2 to improve its denitrifi cation performance, the results showed that, the SO 4 2-/CeO 2 catalyst with 2.5 wt.% SO 4 2-content exhibited more than 90% of NO conversion at 229-485 o C.
Therefore, in this study, based on previous research by scholars, sulphuric acid acidifi cation was used to treat rare earth concentrates.The denitrifi cation performance of the rare earth concentrate catalysts increased after sulphation, probably because sulphuric acid facilitates the removal of some impurity components from the samples and can further enhance the catalytic performance of the catalysts.However, due to the complex mineral cascade relationship of its rare earth concentrates, the existing characterization tools were unable to explore its catalytic mechanism.To investigate the specifi c mechanism of were sulphated by taking 5 g of the sample and adding 10mL of xmol/L sulphuric acid solution (x = 0, 0.4, 0.5, 0.6, 0.7, 1, 3), placed in a magnetic stirrer and stirred for 1h, dried and roasted.
The calculation formula of NOx conversion rate is as follows: Where η is the conversion rate of NOx under this working condition, (NOx)in is the inlet concentration of NO under this working condition, in ppm, (NOx)out is the outlet concentration of NO and NO 2 under this working condition, in ppm.Among them, NOx contains NO and NO 2 .

Characterization of the catalyst
It is heated by a riser furnace with a rated temperature of 1600 o C produced by Nanjing Boyuntong Instrument Technology Co., Ltd., and the GASMET-DX4000 Fourier Infrared Spectroscopy (FTIR) fl ue gas analyzer produced in Finland is used in conjunction with a data acquisition system for online measurement the smoke composition.The temperature-programmed desorption test of the catalyst was carried out with the PCA-1200 temperature-programmed chemical adsorption instrument produced by Beijing Biood Electronics Co., Ltd.Use Chinese PERSEE XD-3 X-ray diffraction (XRD) to analyze crystal form and mineral phase.X-ray photoelectron spectroscopy (XPS) using EDX1800E ray fluorescence spectrometer (Tianrui Instrument Co., Ltd.).VERTEX70 infrared spectrometer (including MCT detector and high-temperature micro-infrared accessories) from German Bruker company for in-situ infrared adsorption experiment.

Catalytic performance and surface properties
Figure 1 showed the activity test graph of the synthetic rare earth concentrate after ball milling.The results showed that the denitrifi cation capacity of the ball-milled samples remained stable in the temperature range of 250-400 o C. The best denitrifi cation capacity was found for the normal ball mill treated samples, with the de-action of the mineral catalyst NH 3 -SCR denitrifi cation, the role of the main mineral phases of the concentrate in the catalytic reaction needs to be investigated on a case-by-case basis, which in turn provides theoretical guidance for the high-value utilization of rare earth concentrates.
The two main mineral phases of Baiyun Ebo rare earth concentrate, bastnaesite and monazite, contain more than 95% of Ce.This paper was based on the two main minerals of Baiyun Ebo rare earth concentrate, bastnaesite and monazite.The CeCO 3 F and CePO 4 were synthesized, and sulphated according to the ratio of Ce in bastnaesite and monazite.By comparing the denitrifi cation performance and mechanism analysis before and after sulphation, the effect of sulphation on their catalytic performance was investigated to provide new ideas for the research of catalysts and to provide a reference value for the modifi cation of concentrate catalysts and mechanism research.

The basis of sample selection and its preparation method
The samples prepared in this paper were based on a cerium fl uorocarbon rare earth concentrate (hereinafter referred to as "concentrate") from Baiyun Ebo (see Table 1).From the composition of the main rare earth-like minerals in the concentrate, it appears that the ratio of bastnaesite to monazite was 13:7.Based on this, the synthesized CeCO 3 F and CePO 4 were mixed in proportion to each other and the mixing was done, by grinding, ordinary ball milling and high energy ball milling.The process parameters of the ball mill were 400 r/min, 10 min forward and reverse rotation interval and 1 h ball milling time.
Using the wet impregnation method, a certain amount of cerium nitrate hexahydrate, sodium fl uoride, sodium bicarbonate in aqueous solution, stirring impregnation 24 h, extraction, 80 o C drying oven, that is, CeCO 3 F sample.Dissolve a certain amount of cerium nitrate hexahydrate in distilled water, add phosphoric acid aqueous solution drop by drop, put in a hydrothermal kettle at 120 o C constant temperature stirring 1 h, drying, 500 o C roasting 2 h, that is, CePO 4 sample.The CeCO 3 F sample and CePO 4 sample were pretreated by ball milling and roasted to obtain sample 1, sample 2 and sample 3 (see Table 2 for specifi c treatments).The pretreated samples which indicated that a wider temperature window and that the higher catalytic activity was concentrated in the high-temperature range, which may be due to the introduction of sulphuric acid providing more Lewis acidic sites on the catalyst surface, resulting in an increase in activity in the high-temperature range.
To investigate the changes in the crystalline phase of the surface material before and after the sulphuric acid treatment, XRD analysis of the catalyst was carried out and the results are shown in Figure 3.As can be seen from the graphs, the mineral composition and crystalline shape of the samples produced signifi cant changes before and after the sulphate acidifi cation treatment.The 2Theta angle of some of the diffraction peaks was shifted, which indicated that the sulphuric acid acidifi cation may have caused lattice distortion in the synthetic rare earth concentrates, which required further calculation of the cell constants to prove.The minerals before sample acidifi cation are dominated by CeO 2 , CePO 4 and CeF 3 .It is known that CeCO 3 F was highly susceptible to decomposition to cerium oxides by high-temperature roasting.High-temperature decomposition was a key step in the use of rare earth concentrates for NH3-SCR denitrifi cation, as the excellent oxygen storage and release of cerium oxides can improve the redox capacity during the catalytic reaction.The diffraction peak of CeF 3 was detected in XRD, which indicated that some of the cerium elements were pulled by fl uorine elements to form CeF 3 during the decomposition of CeCO 3 F.This suggested that CeF 3 could be a hindrance to the NH 3 -SCR process, due to the inhibitory effect of fl uorine ions.After impregnation with 0.6 mol/L sulphuric acid, a variety of cerium sulphur oxide substances were generated.This substance was the product of the reaction of CeCO 3 F or an intermediate product of CeCO 3 F with sulphuric acid.Furthermore, the disappearance of the diffraction peak of CeF 3 was found, which indicated that sulphuric acid acidifi cation can effectively inhibit the process of cerium traction by fl uorine ions, and thus facilitate the reaction.The diffraction peaks of the CeO 2 species weakened in the presence of acid, which indicated that the homogeneous dispersion of the cerium oxide in the bulk phase, and facilitated the SCR reaction.
From the cell parameters in Table 3, it can be seen that the crystal plane spacing of the catalyst increased nitrifi cation rate remaining above 60%.The activity of the catalyst was also increased after the high-energy ball milling process, but the effect was less than that of normal ball milling.This may be because the pore channels inside the catalyst collapse at too high an energy level, causing the pores to become blocked and preventing the gas from adsorbing onto its surface, thus causing a reduction in activity.Based on the future practical use of the samples, the samples selected for this paper were pre-treated with a normal ball mill.
Figure 2a showed a comparison of the denitrifi cation capacity of samples that had undergone normal ball milling after acidifi cation with 0 mol/L, 1 mol/L and 3 mol/L sulphuric acids.The trend between 0mol/L and 1mol/L in the graph reveals that the denitrifi cation performance of both was similar, but there was an increase in the denitrifi cation effi ciency of the sample after 1 mol/L acidifi cation.From the results shown in Figure 2a, it can be analyzed that the optimum acid quantity for sulphuric acid acidifi cation should be in the range of 0-1 mol/L.Therefore, the treated sample with sulphuric acid concentration near 0.5 mol/L was selected for denitrifi cation performance analysis.As shown in Figure 2b, the best results were found for the sulphuric acid-treated samples with a 0.6 mol/L sulphuric acid concentration.The best denitrifi cation effi ciency was 80.13% at 0.6 mol/L sulphuric acid concentration, an increase of 20 percentage points compared to the non-acidifi ed sample.It was found that the activity of the 0.6 mol/L sulphuric acid-treated catalysts was less variable in the reaction temperature range of 300-400 o C, after the sulphation treatment.This indicated that the introduction of sulphuric acid causes lattice expansion, but the grain size decreases compared to pre-acidifi cation, which suggested that sulphuric acid inhibits grain growth, promotes activation of oxygen species and accelerates the SCR reaction.The increase in denitrifi cation efficiency after acidifi cation also provides evidence for the inhibitory effect of elemental fl uorine.

Absorption and desorption performance and chemical state
To investigate the number of acid centers as well as the acidic strength of the catalyst before and after sulphation, the catalyst was subjected to an experimental study of NH 3 -TPD.The adsorption and desorption properties of the samples were key to characterizing the catalytic performance of the samples, and it was evident from Figure 4 that both the pre-and post-acidifi cation samples produced a broad desorption peak around 200 o C.This desorption peak was attributed to the adsorption of NH 3 on the surface of the samples by physically adsorbed NH 3 , and NH 3 on the weak Brønsted acidic sites.From the peak intensity here, The acidifi ed sample was signifi cantly higher than the pre-acidifi ed sample, which suggested that the acidifi ed sample has a higher capacity to adsorb NH 3 on its surface.The possible reason for this was that the CeF 3 produced by the roasting of CeCO 3 F, occupies part of the cerium element, resulting in a reduction of the cerium sites and hence a lack of NH 3 adsorption capacity of the sample.After sulphuric acid acidifi cation, the catalyst showed two distinct desorption peaks around 300 o C and 570 o C. The peak around 300 o C can be attributed to the combination of NH 3 adsorbed on the catalyst surface with a moderately strong acidic site.
The peak around 570 o C was attributed to the binding of NH 3 to the Lewis acidic site on the catalyst surface.
It can be seen that both Brønsted and Lewis acidic sites on the catalyst surface were elevated after sulphuric acidifi cation, with the Brønsted acidic site being particularly elevated.Previous studies have shown that NH 4 + species produced by adsorption on the Brønsted acidic site were less thermally stable than NH 3 species bonded to the Lewis acidic site.Based on the activity test results, the excellent NOx conversions of the catalysts were all above 300 o C. Therefore it is likely that the catalytic activity was promoted by the emerging Lewis acidic sites on the catalyst surface, and the active component providing the Lewis acidic sites was likely to be the emerging metal sulphide oxide species.
Stronger redox properties can increase the oxidation rate and reaction activity of NO to NO 2 conversion.To investigate the redox ability of the catalyst before and after acidifi cation with sulphuric acid, the catalyst was characterized by H 2 -TPR.The results were shown in Figure 5, and the redox ability of the acidifi ed samples was signifi cantly increased.The reduction peaks of the samples before acidifi cation appeared at 650 o C and 810 o C. The reduction peaks at 650 o C belonged to the reduction of the bulk phase CeO 2 on the catalyst surface, and the reduction peaks at 810 o C belonged to the reduction of the bulk phase Ce 4+ to Ce 3+ on the catalyst surface.It can be found that the catalyst before acidifi cation has a poor redox ability, this was because the main component of monazite, CePO4, was very stable, and cerium cannot be easily oxidized or reduced, neither through the Ce 4+ /Ce 3+ transformation to achieve the redox cycle.However, it still has some catalytic activity, which was attributed to the fact that Ce can act as a defect center and produce some adsorption of NO and NO 2 , which generate active nitrate intermediates on the catalyst surface to participate in the reaction 16 .Not only that, CeFCO 3 also undergoes roasting to produce Ce 7 O 12 species, which promotes redox.After acidifi cation with sulphuric acid, the area of the reduction peak increases signifi cantly, which was probably due to the reduction peak of the metal sulphate.Metal sulphates are very thermally stable and may only decompose above 600 o C. The resulting metal sulphides and metal sulphate species may have some infl uence on the interconversion between Ce n+ in the original catalyst.The increase in peak area can also indicate that the introduction of sulphuric acid can promote the conversion between metal ions, making the formation of stronger electronic interactions between the Ce-O-Ce species and the sulphate group, The redox capacity of the catalyst was increased, and facilitating the catalytic reaction.Furthermore, in addition to promoting the valence conversion between other metal ions and enhancing the redox capacity, sulphate also has a certain redox capacity of its own, which jointly participates in the oxidation cycle reaction in the NH 3 -SCR reaction.
To better analyze the XPS profi le of Ce3d of the samples, we fi tted the samples to split peaks.The fi t of the sample before acidifi cation can be divided into seven peaks (as in Fig. 6a), where u1v1 was the XPS peak of Ce 3+ , and the rest were the XPS peaks of Ce 4+ 17, 18 .In Figure 6(a) it can be found that the diffraction peak intensity of Ce 4+ was signifi cantly higher than that of Ce 3+ , which indicates that Ce 4+ was the major oxidation state of cerium in the sample, while Ce 3+ in it mainly  promote the adsorption of NH 3 and NO species on the catalyst surface, which would facilitate the conversion of NO to NO 2 .After acidifi cation, the binding energy of Ce 3d was shifted to a higher level, which indicated a strong interaction between Ce and sulphate ions, which may promote the production of new species, as well as the valence conversion between ions, which was consistent with the results of the H 2 -TPR analysis.The introduction of sulphuric acid further promotes the SCR reaction process.
The spectra of the O1s fi tted to the split peaks of the samples before and after acidifi cation were shown in Figure 7.The samples before and after acidifi cation both fi tted two distinct peaks at binding energies of around 531 eV and 528 eV, attributed to adsorbed oxygen (Oα) and lattice oxygen (Oβ) on the catalyst surface respectively 19 .It can be found that the ratio of adsorbed oxygen to lattice oxygen on the surface of the catalyst before sulphation was not very different, and the content of adsorbed oxygen on the surface of the sample increases signifi cantly after the acidifi cation treatment.It was well known that the formation of adsorbed oxygen on the catalyst surface was due to the presence of oxygen vacancies, and since the activation process of both NH 3 and NO adsorption was transferred to the adsorbed oxygen via oxygen vacancies, this suggested that the introduction of sulphuric acid can increase the number of oxygen vacancies on the catalyst surface thereby promoting the increase of adsorbed oxygen, acted in the valence equilibrium, oxygen vacancy and unsaturated bond generation.The sample fi t after acidifi cation can be divided into 8 peaks (Fig. 6b), of which v1u1 was the XPS peak for Ce 3+ , and the rest were the Ce 4+ XPS peaks.It was evident that after acidifi cation of the catalysts with sulphuric acid, the peak of Ce 3+ was enhanced, which leads to an increase in chemisorbed oxygen or weakly bound oxygen species on the surface of the samples, and the increase in oxygen vacancies would and facilitating the conversion of NO to NO 2 .and the binding energy of O 1s was shifted towards high, which may be due to the interaction of oxygen in the sulphate with intermediate products of CeCO 3 F decomposition, which was consistent with the XRD results, producing species such as Ce 2 O 2 S 2 as sulphur oxides.

Catalytic mechanism analysis
In this experiment, the adsorption and activation of NH 3 on the catalyst surface were investigated by in situ infrared spectroscopy at different temperatures, and the presence of NH 3 species on the catalyst surface at different temperatures was studied.The results were shown in Figure 8a.In the range of 50 o C to 150 o C, infrared absorption peaks appear at 1622 cm -1 , 1482 cm -1 and 1420 cm -1 on the catalyst surface, where all three peaks were symmetrically deformed vibrational diffraction peaks of NH 4 + at the Brønsted acidic site 20 , and the infrared absorption peak at 1190 cm -1 was a diffraction peak of NH 3 absorption vibration at the Lewis acidic site.As the temperature increases, the diffraction peak of NH 4 + on the Brønsted acidic site disappears, while the diffraction peak of NH 3 on the L acidic site does not change signifi cantly, which indicated that when the temperature was below 150 o C, the surface of the catalyst sample was dominated by adsorption on the Brønsted acidic site with a small amount of adsorption on the Lewis acidic site.The symmetrically deformed vibrational diffraction peaks of NH 4 + on the Brønsted acidic sites almost disappear when the temperature was in the range of 200 o C to 400 o C. The infrared absorption peaks attributed to the NH 4 + species on the Brønsted acidic site at 1420 cm -1 were also present on the catalyst surface to a lesser extent, and participate in the reaction at different temperatures.In contrast, the infrared absorption peak at 1190 cm -1 for the NH 3 absorption vibration on the Lewis acidic site was relatively stable, with the strongest peaks for the adsorbed species at 350 o C, which indicated that the Lewis acidic site was predominant in the medium to high-temperature section.The above results indicated that both Brønsted and Lewis acids were involved in the reaction from 50 o C to 400 o C, with Brønsted acid adsorption dominating in the low-temperature range and Lewis acid adsorption in the medium to the high-temperature range.
The adsorption and activation processes of NO+O 2 species on the catalyst surface under different temperature conditions were subsequently investigated by in situ infrared spectroscopy.The form and variation of the presence of NOx species on the catalyst surface under different temperature conditions were discussed.From Figure 8b, it can be seen that in the range of 50-150 o C, an absorption peak appears on the catalyst surface at 1622 cm -1 , where the peak was attributed to the NO 2 species 21 , which can be found to be more strongly adsorbed in the low-temperature section.The absorption peak appearing at 1545 cm -1 was attributed to the bridged nitrate species, and the absorption peak appearing at 1261 cm -1 was attributed to the monodentate nitrate species.The NO adsorption products on the catalyst surface in the low-temperature section are dominated by NO 2 species and monodentate nitrate species.As the temperature increases, the NO 2 species and monodentate nitrate species decompose rapidly at 200 o C, which indicated that both were more sensitive to temperature and prone to pyrolysis reactions.When the temperature range was 200-400 o C, the infrared absorption peak at 1400 cm -1 belonged to the bidentate nitrate species.It can be found that as the temperature increases, the overall NO adsorption species was weaker in the high-temperature range, and only the bidentate nitrate species exists in a relatively stable state, which indicated that it occupies the main NO adsorption site.This also means that the adsorption products of NO in the low-temperature range of the SCR reaction were mainly NO 2 species and monodentate nitrate species, and in the medium to high-temperature range mainly dentate nitrate species were the main active species.Figure 9a showed the infrared profi le of the sample over time when the sample was pre-sorbed with NH 3 and then passaged with NO+O 2 .From the fi gure, it was found that when NH 3 was passed in for one hour, infrared absorption peaks appeared on the catalyst surface at 1480 cm -1 , 1416 cm -1 and 1185 cm -1 , where the peaks at 1480 cm -1 and 1416 cm -1 were diffraction peaks of NH 4 + species on the Brønsted acidic site, and the peak at 1185 cm -1 belonged to the diffraction peak of NH 3 on the Lewis acidic site 22 .When the NH 3 adsorbed species were stably present on the catalyst surface, NO+O 2 gas was passed in.From Figure 9a, it can be found that the diffraction peaks of NH 4 + at 1480 cm -1 and 1416 cm -1 belonged to the Brønsted acidic sites disappeared, after 2 min of NO+O 2 introduction.And the diffraction peaks of NH 3 at 1185 cm -1 belonged to the Lewis acidic sites weakened, which indicated that the NH 3 adsorbed species on the catalyst surface were mainly the NH 4 + species adsorbed on the Brønsted acidic sites.With increasing NO+O 2 infl ux time, all the NH 3 adsorbed species on the catalyst surface disappeared and stable nitrate species resulting from NO adsorption appeared.Two diffraction peaks appeared at 1347 cm -1 and 1278 cm -1 on the catalyst surface, both of which were free nitrate species and monodentate nitrate species respectively 23 , with peak intensities increasing with time.The depletion of ammonia species indicated their involvement in the catalytic reaction process and that the catalytic process was dominated by Brønsted acidic site adsorption.
Figure 9b showed the in situ IR spectra of the reaction between NH 3 and pre-adsorbed NO+O 2 species on the catalyst surface, at optimum denitrifi cation temperature conditions.When NO+O 2 was introduced into the reaction cell system for 60 min, nitrate species from NO adsorption were observed on the catalyst surface at 1519 cm -1 , 1420 cm -1 , 1126 cm -1 and 1322 cm -1 , and all species were stable on the catalyst surface, with the absorption peak at 1519 cm -1 being the -NO 2 species, which was an important intermediate product of the L-H mechanism.In addition, the absorption peaks at 1420 cm -1 and 1126 cm -1 belonged to the bridged nitrate species and 1322 cm -1 to the bidentate nitrate species.The diffraction peak of the bridged nitrate either disappeared or weakened after 2 min of NH 3 introduction, and at 5 min of NH 3 introduction, an absorption peak appeared on the catalyst surface at 1322 cm -1 attributed to the NH 4 + species at the Brønsted acidic site and disappeared at 20 min.At 2 min, a diffraction peak belonging to the Lewis acidic site was produced at 1207 cm -1 , and diminished at 10 min.It indicated that the catalytic process was most active with monodentate nitrate and bridged nitrate, which could react with ammonia species.It indicated that the reaction process on the catalyst surface followed the L-H mechanism and that the reactant species were monodentate nitrate, bridged nitrate and NH 4 + species produced by NH 3 adsorption on the Brønsted a cidic sites on the catalyst surface.
The proposed catalytic reaction mechanism is shown in Figure 10:  The sample prepared in this paper was consistent with the L-H mechanism in the catalytic mechanism, and the intermediate product-NO 2 absorption peak can be used as an important basis.As the better catalytic activity temperature interval was concentrated in the mid to high-temperature section.Combined with the IR analysis results, it was concluded that the catalytic activity in the low-temperature section of this sample was dominated by the Brønsted acidic sites, and the activity in the medium to high temperature was dominated by the Lewis acid.Another interesting fi nding was that, after the roasting of CePO 4 mixed with CeCO 3 F, the F element forms an inclusions layer on the catalyst surface, which hinders the promotion of Ce element in the catalytic process, The sulphation treatment can dissolve some of the F elements and expose more effective elements such as Ce.This fi nding provides an important basis for the catalytic process of Baiyun Ebo rare earth concentrates.

Figure 1 .
Figure 1.Denitration performance of samples with different pretreatment methods

Figure 3 .Figure 2 .
Figure 3. XRD comparison of samples before and after acidifi cation with sulfuric acid

Figure 4 .
Figure 4. NH 3 -TPD of samples before and after acidifi cation with sulfuric acid

Figure 8 .
Figure 8. In-situ infrared spectra of NH 3 and NO+O 2 adsorbed species on the catalyst surface under different temperature conditions

Figure 9 .
Figure 9. In-situ infrared spectra of the reaction of NH 3 and NO+O 2 pre-adsorbed species on the catalyst surface under the optimal reaction temperature

Figure 10 .
Figure 10.Schematic diagram of the catalytic process

Table 1 .
Main rare earth minerals and content in Bayan Obo fl uorocarbon-cerium rare earth concentrate

Table 2 .
Sample label names of different pretreatment methods

Table 3 .
Change of intercrystalline area and grain size before and after acidifi cation