Bin Zhang1,2, Dong Chen 1,2, Haibo Liu1,2, Xuehua Zou1,2, Tianhu Chen1,2

Key Laboratory of Nano-minerals and Pollution Control of Anhui Higher Education Institutes, Hefei University of Technology, Hefei 230009, China
Institute of Environmental Minerals and Materials, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China


Received: March 12, 2020
Revised: April 14, 2020
Accepted: April 14, 2020
Download Citation: ||https://doi.org/10.4209/aaqr.2020.03.0099 

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Cite this article:

Zhang, B., Chen, D., Liu, H., Zou, X. and Chen, T. (2020). Selective Catalytic Reduction of NO With NH3 over High Purity Palygorskite-supported MnO2 with Different Crystal Structures. Aerosol Air Qual. Res. 20: 1155-1165. https://doi.org/10.4209/aaqr.2020.03.0099


HIGHLIGHTS

  • Several MnO2 with different crystal structure have been supported on high-purity palygorskite
  • The catalytic activities were influenced by purified effect of palygorskite.
  • The catalytic activities of MnO2/PG with different crystal structures were quite different.
  • α-MnO2/PG catalyst has an optimum catalytical performance among MnO2/PG catalysts.
 

ABSTRACT


In order to find efficient catalysts for low-temperature selective catalytic reduction with ammonia (NH3-SCR), several MnO2 with different crystal structure have been supported on high-purity palygorskite (Pal) by a hydrothermal method. The effect of MnO2 crystal structures on the NH3-SCR performance of the catalysts was investigated. All catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, Raman spectrum, Thermo gravimetric analysis/Differential thermo gravimetric (TG/DTG), ammonia-temperature programmed desorption (NH3-TPD), hydrogen-temperature programmed reduction (H2-TPR), Transmission electron microscope (TEM) and X-ray energy dispersive spectroscopy (EDS) and X-ray photoelectron spectrometer (XPS). The presented results suggested that α-MnO2/Pal catalyst exhibit the highest catalytic activity among four type MnO2/Pal catalysts in the temperature range of 50–400°C, own promising stability during a 24 h continuous denitration experiment. Furthermore, the crystal structure affords α-MnO2/Pal with the highest specific surface area and more acidic sites, which is beneficial for the SCR reaction. It has improved the NH3 adsorption ability and catalytic activity of the catalyst.


Keywords: Palygorskite; MnO2; Selective catalytic reduction; NO; Crystal structure.


INTRODUCTION


Nitrogen oxides (NOx) can result in acid rain, photochemical smog and ozone depletion, it has become a global environmental concern question (Yung et al., 2007; Liu et al., 2015a; Yu et al., 2015). NH3-SCR is the most commonly used and effective denitration technology in modern thermal power plants, accounting for about 95% of the worldwide flue gas denitration devices, with NO conversion reaching over 90% (Pappas et al., 2016; Boningari et al., 2018; Wang et al., 2019a). Transition metal catalysts gradually show bright prospects because of their high resistant performance to poisoning and sintering (Du et al., 2019; He et al., 2019; Wang et al., 2020). In order to reduce costs, it is necessary to develop composite catalysts with higher catalytic activity at low temperatures.

The Mn based catalysts own substantial low-temperature activity, besides, the material sources are world wide access, so the catalysts have a good prospect of industrial application (Hua et al., 2020; Wu et al., 2020). In addition, as the valence state of Mn species in the catalyst increases, its SCR catalytic activity becomes better. An earlier report showed that MnO2 own the highest NO conversion among the MnOx catalyst include of MnO2, Mn5O8, Mn2O3 and Mn3O4 (Kapteijn et al., 1994). Moreover, the morphology and pore structure of MnO2 have significant influence on its catalytic activity, there are several kinds of crystal structures in MnO2, and the most common ones are α-, β-, γ- and δ-MnO2. Different crystal structures have different functions in adsorption ability, oxygen functional groups and redox activity (Cheng et al., 2006; Liang et al., 2008; Zhang et al., 2015). In the past few decades, different forms of manganese dioxide have received attention and research in catalytic conversion of CO, NO, formaldehyde and ethanol (Liang et al., 2008; Zhang et al., 2015; Chen et al., 2016; Liu et al., 2016; Yang et al., 2020). However, there is less research on the relationship between the crystal structure and catalytic activity of SCR catalysts. Importantly, the substantial porous structure, is a vital factor in SCR reaction (Wang et al., 2019b). Palygorskite is recognized as a typical low-cost clay mineral. Its well-developed pore structure and large specific surface area make it a potential catalyst support for many catalytic reactions and a good carrier for many catalysts (Lin et al., 2017b; Liu et al., 2018; Liu et al., 2019). It may be implied that manganese dioxides loaded Palygorskite catalyst may have a greatly improved removal efficiency for NOx. Furthermore, the main composition of raw Pal clay were Pal, quartz, dolomite and a few impurities (Qiu et al., 2015).

Therefore, there is great need to study: 1) the purification effect of raw Palygorskite; 2) the relationship between the SCR activity and crystal structures of MnO2 in MnO2/Pal catalysts to synthesize a novel MnOx catalyst for SCR. In this study, four crystal forms of manganese dioxide were loaded on highly purified Palygorskite, subsequently the denitration performance and N2 selectivity of Pal-supported catalyst in NH3-SCR were evaluated, and the characterizations were applied for the catalyst, the reaction mechanism was investigated. In order to further study with Pal as the carrier, is expected to achieve industrial application of forming low temperature SCR denitration catalyst preparation.


EXPERIMENTAL SECTION



Purified Palygorskite

200 g of raw Pal mine powder (guanshan Palygorskite clay mine, mingguang city, anhui province) was added into 1.5 L deionized water, ultrasonic dispersion 30 min. Then added 2 L deionized water, removed the magnetic material with the magnetic bar until there is no obvious trace on the magnetic bar. The following 2 g sodium hexametaphosphate and 1.5 L deionized water were added, stirring 30 min, then static settlement 12 h. The upper mixed liquid was placed in the 5 L measuring cylinder, then added deionized water to 5 L, and the substratum sedimentation was poured and settled for 12 h. The mixture was transferred from the measuring cylinder to the large beaker, 2 g CaCl2 was added and stirred until it is sticky, centrifugal, and took the sediment into the oven to dry. After that, 1% mass ratio of dilute hydrochloric acid was added for acidification, the mixed solution was stirred for 24 h at 60°C. Subsequently, the purified Palygorskite clay (In the following sections, the purified Palygorskite clay was labeled as Pal) was acquired after filtration, washing and drying at 100°C for 2 h.


Preparation of Catalysts

The synthetic method of α-MnO2/Pal, 5.09 g Pal was added into 100 ml deionized water, stirring 2 h ensure Pal spread evenly, then 0.53 g MnSO4∙H2O, 1.25 g KMnO4 were dissolved in deionized water and then 2 ml concentrated HNO3 added slowly, after that stirring overnight. The solution was transferred into a Teflon reactor (200 mL) then heated to 100°C and held isothermally for 24 h. After cooling down, the sediment was centrifuged, washed five times, then dried at 100°C overnight, and finally calcined in a muffle furnace at 400°C for 2 h. Ultimately, α-MnO2/Pal was achieved by grinding to 40 to 60 mesh sieving for the following experiment. The procedures for the preparation of other crystal structures catalysts were similar to that of α-MnO2/Pal:

For β-MnO2/Pal, 4.63 g Pal was added into 100 mL deionized water, stirring 2 h, then 1.69 g MnSO4∙H2O and 2.28 g (NH4)2S2O8 were added slowly, subsequently heated to 140°C and held isothermally at 140°C for 12 h in an oven.

For γ-MnO2/Pal, 9.26 g Pal was added into 100 mL deionized water, stirring 2 h, then 3.37 g MnSO4∙H2O and 4.57 g (NH4)2S2O8 were added slowly, subsequently heated to 90°C and held isothermally at 90°C for 24 h in an oven.

For δ-MnO2/Pal, 5.86 g Pal was added into 100 mL deionized water, stirring 2 h, then 0.80 g (NH4)2C2O4 and 2.00 g KMnO4 were added slowly, subsequently heated to 90°C and held isothermally at 90°C for 10 h in an oven.


Characterization

In order to test the purification effect of Palygorskite clay, a X-ray fluorescence (XRF, Shimadzu XRF-1800) catalysis was carried out for Palygorskite clay before and after purification. X-ray diffraction (XRD) was investigated by a DX-2700 diffractometer fitted with a nickel-filtered Cu Ka radiation source and operating at a voltage of 40 kV and 30 mA. The specific area (BET) analyses of the catalysts were determined by a Quantachrome NOVA 3000e apparatus. The pore size distribution and pore volume were calculated from the desorption branch of N2 adsorption isotherm using the BJH method. TG curves were performed on a TG/DTA7300 instrument (EXSTA-SII, Seiko Instruments Inc. Japan). H2-TPR analyses was carried out in a quartz tube reactor with a mass spectrograph for detection of H2, 0.1 g catalyst was heated from 50 to 800°C under 5% H2/Ar. NH3-TPD was also tested using the above equipment. Raman scattering spectra were recorded on HORIBA HR spectrometer with 532 nm line of argon ion laser as an excitation source. XPS data were acquired by an ESCALAB 250Xi electron spectrometer using Mg K radiation (1253.6 eV). All element binding energies were calibrated relative to the C 1s peak from carbon contamination at 284.6 eV. The physicochemical properties of the catalysts were determined by the micrographs obtained on a JEM-2100F transmission electron microscope (TEM). Elemental distribution mapping for the particles was obtained with the aid of TEM integrated with X-ray energy dispersive spectroscopy (EDS).


Activity Test

The SCR activity of different structural MnO2/Pal catalysts were evaluated in a fixed-bed quartz tubular reactor (i.d. 8 mm) and catalyst (100 mg, 40–60 mesh) was placed in the center of the reactor. The mixture gas consisted of 500 ppm NO, 500 ppm NH3, 3 vol% O2 and with Ar as balance gas. The total flow rate was 300 mL min–1 and keeping a gas hourly space velocity (GHSV) of 64000 h–1 in the whole time. In detail, the procedure is as follows: the system was firstly inlet the reaction gas for 20 min at room temperature. Then the samples were heated to 50°C and mantain 20 min. After that, the catalysts were heated at 10°C min–1 from 50°C up to 400°C. The concentrations of NOx in inlet and outlet gas were measured by a flue gas analyzer (KANE, KM9106), respectively. NO conversion is defined as:


NOconversion = (NOinlet – NOoutlet)/NOinlet × 100%                      (1)


(NO and NO2 were the only nitrogen-containing oxides detected).


RESULTS AND DISCUSSION



XRF

The purification result investigated by XRF was shown in Table 1. From the table, we found that after the acid leaching purification, the content of SiO2 increased while the content of MgO decreased; besides the CaO was completely removed and the burning loss decreased. After the hydrostatic sedimentation and acid leaching purification, dolomite and other impurities can be effectively removed and the sample of Pal with a higher purity can be obtained. It can be used as the catalyst carrier for the subsequent study of NH3 SCR denitration.


XRD

The XRD patterns of purified Pal and MnO2/Pal were shown in Fig. 1. After purification, we found that the diffraction peak ascribed to dolomite disappeared while the intensity of the quartz diffraction peak decreased significantly. In the meantime, the Pal exhibited more prominent diffraction peaks compared with the raw Palygorskite mineral, indicating that the purification had a good efficiency. The diffraction peaks in 8.4°, 13.7°, 16.3°, 19.8°, 20.9°, 27.6° and 34.4° were in good agreement with the characteristic peaks of Palygorskite (JSPDS 31-783). The XRD patterns of the four catalysts have no obvious corresponding crystal manganese dioxide peaks, and compared with purified Pal, the reflections strength of MnO2/Pal catalysts significantly decreased. It may be related to the hydrothermal synthesis method destroying the structure of Pal and highly distribution manganese dioxide with poor crystallinity on Pal. Combined with TEM and Raman spectroscopy analysis below, the main reason is that manganese dioxide has uniform load on Pal and poor crystallinity. 

Fig. 1. The XRD patterns of purified Pal and MnO2/Pal.Fig. 1. The XRD patterns of purified Pal and MnO2/Pal.


TG/DTG

Thermogravimetric analysis can be used to identify the phase of manganese dioxides loaded onto Pal. TG-DTG analyses were respectively performed on the Pal after purification and the four prepared catalysts, as shown in Fig. 2. below. The typical TG curves show four weight losses in 91°C, 212°C, 437°C and 668 C respectively. It indicates that there are four states of dehydration in Pal, namely, caused by loss of water adsorbed on outer surface, pore channel, water of coordination crystal and structure respectively (Frost and Ding, 2003). In general typical MnO2 within the scope of room temperature to 900°C will react through two-step pyrolysis, namely MnO2 is decomposed into Mn2O3 at first, and then decomposed to Mn3O4, corresponding to the decomposition temperature of about 483°C and 799°C (Terayama and Ikeda, 1983). The weight loss ratios of the two catalysts, α-MnO2/Pal and δ-MnO2/Pal, with potassium permanganate as the precursors, are significantly higher than the other two catalysts.This may be related to some K+ ions into the MnO2 pore channel, reducing the stability of its crystalline structure (Meher and Rao, 2012). The δ-MnO2, α-MnO2, γ-MnO2, and β-MnO2 show layered structure, 2 × 2 tunneled structure, 2 × 1 tunneled structure, and 1 × 1 tunneled structure, respectively. The main reason of δ-MnO2 and α-MnO2 less stable than β-MnO2 was due to the easy collapse of layer or large tunnel. 

Fig. 2. The TG/DTG patterns of purified Pal and MnO2/Pal, (a) α-MnO2/Pal, (b) β-MnO2/Pal, (c) γ-MnO2/Pal, (d) δ-MnO2/Pal, (e) Pal.Fig. 2. The TG/DTG patterns of purified Pal and MnO2/Pal, (a) α-MnO2/Pal, (b) β-MnO2/Pal, (c) γ-MnO2/Pal, (d) δ-MnO2/Pal, (e) Pal.


Raman

Raman spectroscopy was applied to further identify the crystal structure of MnO2 among MnO2/Pal catalysts. The Raman spectra of MnO2/Pal catalysts was shown in Fig. 3, several bands were recognized from α-MnO2/Pal at 149, 400, 510, 560 and 641 cm–1. The band at 560 cm–1 was attributed to the Mn-O lattice vibration in α-MnO2(Gao et al., 2009; Sinha et al., 2013). The weak band at 641 cm–1 belonged to Mn3O4, which was due to the local overheating of the samples during the spectrum acquisition (Chen et al., 2016). Through the special diffraction bands measured by Raman spectroscopy and the lattice spacing measured by TEM result in section 3.8 below, we confirmed the synthesis of α-MnO2. For β-MnO2/Pal, the band at 640 cm–1 was attributed to MnO6 octahedral stretching vibration (Liang et al., 2008). γ-MnO2 was considered to be formed by the random symbiosis of pyrolusite (β-MnO2) and ramsdellite (R-MnO2). From the Raman spectra of γ-MnO2/Pal, we could find the band at 637 cm–1 was similar to above β-MnO2. In addition, the bands at 382, 486 and 577 cm–1 were attributed to the R-MnO2 (Zhang et al., 2015). The weak band at 502 cm–1 and two strong bands at 639 and 561 cm–1 were belong to δ-MnO2 of δ-MnO2/Pal (Chen et al., 2016). 

Fig. 3. The Raman spectra of MnO2/Pal catalysts.Fig. 3. The Raman spectra of MnO2/Pal catalysts.


BET

A BET analysis was performed on different MnO2/Pal catalysts and Pal after purification. The analysis results were summarized in the following Table 2. From this table, we find α-MnO2/Pal own the maximum specific surface area, β-MnO2/Pal own the highest total pore volume and average pore size while the lowest specific surface area. Compared with the purified Pal, MnO2/Pal catalysts own higher total pore volume and average pore size. Except α-MnO2/Pal, the specific surface area of other MnO2/Pal catalysts were reduced. After purification, we found that Pal own large specific surface area and was suitable as a catalyst carrier.

The loading of different crystal structures of MnO2 onto Pal can explain the decrease of BET surface area. However, compared with the BET characterization results of pure MnO2 with different crystal structures, MnO2/Pal own larger specific surface area (Lin et al., 2017a), which may be related to the contribution of purified Pal (Zhao et al., 2016). 


Catalyst Activity Test and the Selective of N2

As shown in Fig. 4, after purification, it can be seen that Pal almost have no catalytic effect in low temperature, until at 250°C the catalytic effect appears, then starts to rise, up to 20% at 400°C. All the four catalysts loaded with manganese dioxide have good catalytic performances at medium temperature, and their catalytic activity reaches the highest at 250°C, which is consistent with previous researches (Chen et al., 2016; Jia et al., 2016; Yang et al., 2020). In the temperature ranging from 150–300°C, the α-MnO2/Pal shows promising catalytic activity (NO conversion rate was over 97%) towards NO reduction with O2. Interestingly, except δ-MnO2/Pal, the other MnO2/Pal catalysts during 250–300°C are all close to 100% NO conversion. In comparison to the above catalysts, δ-MnO2/Pal shows higher NO conversion before 250°C. Yet, the NO conversion of δ-MnO2/Pal decreases significantly at high temperature period. Compared with pure phase manganese dioxide, the activity intensity was close, and the order of activity intensity of different crystal forms of manganese dioxide was consistent (Luo et al., 2016). The catalytic activity was similar to pure manganese dioxide, which was achieved by using Pal supported manganese dioxide. According to the comprehensive characterization analysis, the catalytic activity of α-MnO2/Pal is higher than the other three catalysts. 

Fig. 4. Catalytic activities of purified Pal and MnO2/Pal.Fig. 4. Catalytic activities of purified Pal and MnO2/Pal.

Fig. 5 shows the results of N2 selectivity of purified Pal and MnO2/Pal catalysts. Purified Pal afforded highly impressive N2 selectivity, which were over 99% at different temperatures. Therefore, we can infer that Pal as the catalyst carrier had no effect on the N2 selectivity of catalyst. N2 selectivity of four Pal supported catalysts decreased before 350°C then increased with the rising of temperature, obviously α-MnO2/Pal exhibited the better N2 selectivity than others. Fig. 6 shows the stability experiment of α-MnO2/Pal catalyst at 250°C. After 24 h continuous denitration experiment, the denitration efficiency of the catalyst can still be maintained at about 99%, which shown that the MnO2 supported on high-purity Pal own satisfactory stability. 

Fig. 5. The N2 selectivity for purified Pal and MnO2/Pal.Fig. 5. The N2 selectivity for purified Pal and MnO2/Pal. 

Fig. 6. The stability experiment of α-MnO2/Pal catalyst at 250°C.
Fig. 6. The stability experiment of α-MnO2/Pal catalyst at 250°C.

Through the above experiments, it was found that the optimal catalyst was α-MnO2/Pal among the four catalysts. Therefore, it was tested for sulfur dioxide and water vapor tolerance. The results are shown in Fig. 7. The experimental conditions were used as follows: 250°C, 500 ppm NH3 and NO, 3% O2, Ar balance, and GHSV of 64000 h–1. After 200 ppm SO2 was introduced in, NO conversion dropped rapidly to below 40% after 20 minutes and then gradually decreased to about 3%. After switching SO2 off, NO removal rate slightly increased to about 5%. When 6% volume fraction of water vapor was introduced, the NO removal rate gradually decreased, and finally reached a steady state at about 70%. When cutting off water vapor, NO removal rate gradually restored to the initial value. It can be seen that the deactivation effect of SO2 on the catalyst is irreversible, while water vapor is reversible. Finally, we investigated the combined impacts of H2O and SO2 on SCR activity of α-MnO2/Pal. The NO conversion rate has a slight decrease at first and could maintain NO conversion above 90% after 1 hour. Nevertheless, the NO removal rate gradually decreased until it reached to 0%. When the H2O and SO2 inlet gas were turn off, the NO conversion rate gradually increases to about 10%. 

Fig. 7. Effect of H2O and SO2 on the selective catalytic reduction activity of α-MnO2/Pal catalysts.Fig. 7. Effect of H2O and SO2 on the selective catalytic reduction activity of α-MnO2/Pal catalysts.


NH3-TPD and H2-TPR

As we known that the surface acid sites of the catalysts make an important impact in the removal of NO by NH3-SCR at low temperature (Liu et al., 2015b). As shown in Fig. 8, the NH3-TPD curves of the α-MnO2/Pal only contained one single main peak in 150°C. Compared with other catalysts, the peak position shifted to right for an offset of 20°C and the intensity was far more than other kinds of catalysts. According to previous reports, 130–150°C of NH3 desorption peak attributed to weak acid Bronsted acid sites of adsorption, 480–665°C of NH3 desorption peak assigned to acid adsorption Lewis acid sites (Jin et al., 2010; Lee et al., 2012; Zhang et al., 2014). As a Pal supported catalyst, in 480 and 665°C, δ-MnO2/Pal had two obvious acid sites, compared with the front weak acid sites its strength lower. δ-MnO2/Pal showed a peak in 640°C belong to strong acid site, β- and γ-MnO2/Pal both own one peak in 560°C belong to strong acid site. The desorption peak position in the NH3-TPD curve reflected the strength of the acid site of the catalyst, while the peak area corresponds to the adsorption amount of ammonia (Bates et al., 2014; Cao et al., 2015). The peak area of α-MnO2/Pal was significantly greater than that of the other three catalysts, and the weak acidic site was mainly responsible for it, which was more favorable for NH3-SCR reaction. 

Fig. 8. NH3-TPD patterns of purified Pal and MnO2/Pal.Fig. 8. NH3-TPD patterns of purified Pal and MnO2/Pal.

The H2-TPR patterns of MnO2/Pal catalysts were showed in Fig. 9. We can find all catalysts have two obvious H2 reduction peaks, which can be explained by the gradual reduction of MnO2 to MnO (Tong and Huang, 2012). By contrast, the following ellipticity-temperature profiles are shown to evaluate the abundant Mn4+, Mn3+ and Oα species. β-MnO2/Pal and γ-MnO2/Pal catalysts own two reduction peaks appear in similar positions of 367°C and 376°C, meanwhile high-temperature reduction peaks were located at 521°C and 526°C, respectively. According to the characterization results of XRD, TG and Raman, the typical process of reduction was MnO2 to Mn3O4 and then to MnO (Alvarez-Galvan et al., 2003). The reduction peaks of δ-MnO2/Pal were observed at 414°C, 517°C, respectively, and the strength of the two peaks were lower than former catalysts. α-MnO2/Pal had a main reduction peak at 300°C while a weak peak at 462°C. It should be attributed to the reduction of MnO2 to Mn2O3 and Mn3O4, finally transformed to MnO. Due to the lowest reduction temperature, α-MnO2/Pal was regarded as the optimal catalyst. 

Fig. 9. H2-TPR patterns of MnO2/Pal catalysts.Fig. 9. H2-TPR patterns of MnO2/Pal catalysts.


TEM

Figs. 10(a)–10(d) showed the TEM images of the different catalysts. For these images, the one-dimensional rod-shaped substance was ascribed to Pal, at the same time the surface of Pal was coated by flocculent MnO2. We can find although MnO2 particles were in nano size, nonetheless, their dispersion in MnO2/Pal were different. It is clear that α-MnO2 (Fig. 10(a)) has good dispersibility on the surface of Pal. Compared with Figs. 10(b) and 10(d), there was a certain degree of agglomeration of MnO2 on surface of β- and γ-MnO2/Pal catalysts, with the particle size between 50 and 200 nm. Then, δ-MnO2 was also highly dispersed on the surface of Pal as shown in Fig. 10(c). In addition, it was found that the pores on the surface of catalysts could be seen more clearly after purification, which might be due to the fact that the impurities were removed. The dispersion of different crystal forms of manganese dioxide just confirms the direction of special surface area values with different loaded MnO2 as shown in Table 2. The high-resolution image and EDS spectrum of α-MnO2/Pal is shown in Figs. 10(e) and 10(f), where the crystal edge of α-MnO2 is found, and the lattice spacing is 2.1 Å. This is consistent with Raman spectrum characterization (Fig. 3). Through Raman spectroscopy and TEM analysis, we confirmed the successful synthesis of α-MnO2. The EDS result (Fig. 10(f)) confirmed that Si, Mg, Al, Fe, O and other elements were attributed to Pal, Mn and O elements attributed to MnO2, and Cu element to the TEM sample slide. We calculated loading content of Mn element reaches the theoretical value. 

Fig. 10. TEM images pattern of MnO2/Pal catalysts (a) α-MnO2/Pal, (b) β-MnO2/Pal, (c) γ-MnO2/Pal, (d) δ-MnO2/Pal, (e) the high-resolution image and (f) EDS spectrum of α-MnO2/Pal.Fig. 10. TEM images pattern of MnO2/Pal catalysts (a) α-MnO2/Pal, (b) β-MnO2/Pal, (c) γ-MnO2/Pal, (d) δ-MnO2/Pal, (e) the high-resolution image and (f) EDS spectrum of α-MnO2/Pal.


XPS

XPS spectroscopy analyze was used to identify the MnO2/Pal catalysts surface species of Mn and O. It is clear that the Mn 2p 3/2 peaks of β-MnO2/Pal right shift about 0.5 eV while other catalysts were located near 642.2 eV (Fig. 11(a)), and similar shift was occurred in the following O 1s peak (Fig. 11(c)). This is caused by the unique crystal structure of β-MnO2 (Zhang et al., 2015; Chen et al., 2016). The percents of surface Mn3+/Mn4+ molar ratios were listed in Table 3. α-MnO2/Pal own the highest Mn3+/Mn4+ molar ratio whereas the δ-MnO2/Pal showed the lowest, which is consistent with the H2-TPR result in section 3.7.

XPS spectra of O 1s was shown in Fig. 11(c), the profiles of MnO2/Pal catalysts were fitted to three peaks. Binding energies at 529.6 ± 0.1 eV was ascribed to surface adsorbed oxygen (denoted as Oα) and 531.0 ± 0.2 eV belong to lattice oxygen with manganese (denoted as Oβ). Peaks near 532.4 eV were assigned to surface hydroxyl species and chemisorbed water (denoted as OOH) (Li et al., 2016). Unlike the high content of lattice oxygen in pure phase MnO2 (Liang et al., 2008), the surfaces of the prepared different crystalline MnO2/Pal catalysts are covered by mostly hydroxyl oxygen, because the carrier Pal contains a large amount of hydroxyl oxygen (Chen et al., 2011; Xie et al., 2016; Yang et al., 2020). As we known that Oα performs high activities and makes an important impact in reduction reaction because of its higher mobility than Oβ (Zhao et al., 2018). Table 3 showed the percent of molar ratios Oα/Oβ. The value was decrease in the following sequence: α- > β- > γ- > δ-MnO2, which consistent with the catalytic activity results in section 3.6. This indicating that the catalytic activity was nearly correlated with the surface Oα species concentration. 

Fig. 11. XPS spectra of α-, β-, γ- and δ-MnO2/Pal catalysts: (a) Mn 2p and (b) Mn 2p 3/2 (c) O 1s.Fig. 11. XPS spectra of α-, β-, γ- and δ-MnO2/Pal catalysts: (a) Mn 2p and (b) Mn 2p 3/2 (c) O 1s. 





CONCLUSIONS


Considering the catalytic performance, selecting an optimum support for MnO2 catalysts plays a major role in NO reduction behaviour. In our work, after the hydrostatic sedimentation and acid leaching purification, impurities can be effectively removed and a higher-purity palygorskite can be obtained. After the purification process, several MnO2 with different crystal structure have been supported on high-purity palygorskite by hydrothermal method. All four catalysts show good NO removal efficiency at 150–300°C, the α-MnO2/Pal exhibits the best SCR catalytic performance (NO conversion rate was over 97%) and N2 selectivity. Besides, α-MnO2/Pal after the addition of SO2 and H2O in the feed gas could exert an adverse effect on the NOx conversion. TG analysis revealed that different hydroxyl structures of catalysts in SCR reaction. NH3-TPD and H2-TPR analysis indicated that α-MnO2/Pal catalyst own more weak acid sites and redox ability than other catalysts. The higher specific surface areas of catalyst, uniformly distributed MnO2 particles, and percentage of Mn3+/Mn4+ molar ratio, oxygen-containing groups and surface adsorbed oxygen, which results in the excellent catalytic performance of α-MnO2/Pal.


DECLARATION OF COMPETING INTEREST


The authors declare no competing interests.


ACKNOWLEDGEMENTS


This study was financially supported by the National Natural Science Foundation of China (No. 41672040). 


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