Characterization and SCR Performance of NanoStructured Iron-Manganese Oxides : Effect of Annealing Temperature

Nano-structured iron-manganese oxides composite was prepared by annealing of natural Mn-rich limonite at different temperatures (500, 600, 700, 800°C). Their SCR performances of NO removal by NH3 were evaluated, and SEM, TEM, XRD, XRF, BET, XPS, Raman, NH3-TPD were utilized to analyze the catalytic mechanism. The results indicated that after annealing at 500°C, the as-prepared iron-manganese oxides exhibited the best SCR performance in NO conversion and N2 selectivity (over 80%) at the temperature window of 150–300°C. An increase in annealing temperature from 500 to 800°C significantly increased the particle size resulting in the decrease of surface areas, active sites, and then SCR performances. The experimental results suggested the natural Mn-rich limonite was an excellent precursor for preparing iron-manganese oxides and possessed excellent SCR performance of NO removal by NH3. This study will provide a novel way for the preparation of SCR catalyst and exploit a new application field for natural limonite.

Besides the high NO conversion and N 2 selectivity, iron oxides and manganese oxides are significantly favorable catalysts among the vanadium-free catalysts due to low cost and high low-temperature activity, respectively.In the previous reports, iron oxide catalysts mostly contained α-Fe 2 O 3 (Liu et al., 2017), γ-Fe 2 O 3 (Yang et al., 2013;Huang et al., 2014) and rod-shaped Fe 2 O 3 (Mou et al., 2012).The results indicated that iron oxides catalyst exhibited an excellent SCR performance at a temperature range of 200-400°C.Mou et al. (2012) reported that rod-shaped α-Fe 2 O 3 kept a NO conversion of over 80% at the window ranged from 200 to 400°C, and confirmed γ-Fe 2 O 3 had better SCR activity than α-Fe 2 O 3 .Afterwards, Liu et al. (2013a) also verified both α-Fe 2 O 3 and γ-Fe 2 O 3 exhibited remarkably different NO conversion.The latter was better due to the presence of higher oxygen defect and the formation of few stable nitrates in the reaction process.Compared with iron oxides, manganese oxides are generally used as active composition supported on a carrier.The spent carrier materials included TiO 2 (Fang et al., 2014;Putluru et al., 2015), spinel (Yang et al., 2011;Yang et al., 2016), zeolite (Kim et al., 2012;Huang et al., 2016), and so forth.These results indicated that manganese oxides showed a remarkable activity for SCR at the temperature range between 100 to 300°C.Putluru et al. (2015) demonstrated that 25 wt% Mn0.75Fe0.25Ti-DPcatalyst prepared by deposition precipitation exhibited superior low-temperature SCR, and it was probably due to the presence of amorphous phases of manganese oxides and iron oxides with the properties of high surface area, high total acidity, acid strength and so on.
On the other hand, according to the previous report (Liu et al., 2013d), natural limonite widely spread on the earth and the crystal iron is usually substituted by aluminum, manganese, zinc, etc.It is normal for the coexistence of iron and manganese in limonite, and the amount of manganese depends on the genesis of limonite.In addition, natural limonite is a the eco-friendly, low-cost mineral material.According to the decomposition characteristic of limonite as the previously reported (Liu et al., 2013c), it was speculated that iron and manganese oxides with nano-porous structure and high surface area could be obtained by the thermal decomposition of limonite.Meanwhile, the iron and manganese oxides are believed to be of great potential in SCR of NO by NH 3 .Therefore, in this present study, natural Mn-rich limonite was used to prepare iron and manganese oxides for SCR catalysts.The effect of preparation temperature on the structure and SCR activity of the catalysts was investigated using the laboratory-scale catalytic system and characterization of XRD, N 2 adsorptiondesorption, XPS, SEM, TEM, Raman and NH 3 -TPD.The objectives are to prepare a eco-friendly, low-cost and higheffective SCR catalyst by the thermal decomposition of natural limonite, and to build the relationship between preparation temperature, pore texture and SCR activity of iron and manganese oxides catalysts.

Catalyst Preparation
The iron and manganese oxides (IMO) catalysts were prepared by annealing of limonite in air for 1 h at different temperatures (500, 600, 700, 800°C).Thermal treatment was carried out in a tube furnace with a programmable temperature controller.After annealing, catalysts were cooled down to room temperature and then sieved to 30-60 mesh (0.25-0.55 mm) for further characterization and activity evaluation.To simplify the sample labeling, LT (T = 500, 600, 700, 800) used to denote the catalyst was prepared from the annealing of limonite at T °C.

Catalytic Activity Testing
The testing of SCR performance of the as-prepared catalysts was carried out in a fixed-bed quartz tube reactor with an inner size of 6 mm and 0.25 g of catalyst was used each time.The simulated flue gases were composed of 1000 ppm of NH 3 , 1000 ppm of NO, 3% of O 2 , and balanced with Ar, and a gas flow rate of 300 mL min -1 was controlled by mass flow controllers (Sevenstar D08, Beijing).The catalytic activity testing was conducted in a temperature range of 100-450°C with a gas hourly space velocity (GHSV) of 72,000 h -1 .The temperature was regulated through a programmable temperature controller.The concentrations of NO, NO 2 , NH 3 , and N 2 O were monitored by a Fournier transform IR (FT-IR) spectrometer.The schematic diagram of the experimental set-up is displayed in Fig. 1.
As the SCR reaction reached a steady state, the gases concentration was recorded and then NO conversion and N 2 selectivity were calculated in terms of the following equations:

Catalyst Characterization
Crystal phases of samples were determined using an X-ray diffractometer (XRD, Dandong DS-2700) with 2θ ranged between 15° and 70° and a step size of 4° min -1 operated at 50 KV and 100 mA using Cu Kα radiation.
The specific surface area, pore volume and pore size distribution were measured based on N 2 adsorption-desorption at liquid nitrogen temperature with a Quantachrome (NOVA3000e) analyzer.The samples were degassed at 110°C for 24 h before analysis.
A scanning electron microscope (SEM, SU8020) and a transmission electron microscope (HRTEM, JEM-2100) were used to characterize the morphology of the samples.
An X-ray photoelectron spectroscopy (ESCALAB250Xi) was used to determine the Fe2p and Mn2p binding energies with Al Kα radiation.The binding energies were referenced to the C 1 s line at 284.7 eV.
Temperature-programmed desorption of NH 3 (NH 3 -TPD) experiments were used to characterize the surface acid of the prepared catalysts.NH 3 -TPD was carried out in a temperature of 50-500°C in a micro-reactor.The temperature was measured using a K-type thermocouple.Molecules from the outlet of the micro-reactor were monitored using a quadrupole mass spectrometer (Hiden QIC-20).Before desorption, the sample was saturated by NH 3 judging by 15 NH 3 signal at the temperature of 50°C.Afterwards, desorption was performed with a rate of 10 °C min -1 from 50 to 500°C.
Raman spectra of samples were carried out at room temperature using a confocal Raman spectrometer (LabRAM HR Evolution).Raman signal was excited using the 532 nm wavelength of an argon ion laser source.Typical acquisition time was 10 s and the wavenumber region was 100-1000 cm -1 with a spectral resolution of 1 cm -1 .

SCR Performance
The SCR performances of nano-sized iron-manganese oxides catalysts as a function of temperature from 100 to 450°C are shown in Fig. 2. The reaction temperature considerably influenced the NO conversion.In particular, the NO conversion increased firstly and then decreased with an increase of reaction temperature.The increasing reaction temperature favored the SCR reaction up to 300°C consequently resulting in the increase of NO conversion.The decrease of NO conversion is attributed to the oxidation of NH 3 at the temperature over 300°C, which can be deduced from the increase of NH 3 conversion as seen in Fig. 2. On the other hand, L500 exhibited a high NO conversion over 80% at the temperature window of 150-300°C, while L600 only had a NO conversion over 80% at 250°C.Meanwhile, the L700 and L800 presented a relatively low NO conversion (lower than 60%).Meanwhile, the N 2 selectivity of these samples as a function of temperature is also displayed in Fig. 3.It is not hard to find an increase of reaction temperature gradually decreased the N 2 selectivity, and an increase of annealing temperature slightly increased the N 2 selectivity.Even so, the N 2 selectivity was still more than 80% when the reaction temperature was not more than  300°C.These results indicate that the as-prepared catalyst displays a good performance and N 2 selectivity on SCR of NO by NH 3 and the annealing temperature also significantly affects the performance.The good performance is ascribed to the existence of active component of iron-manganese oxides.The performance of iron-manganese oxides had been documented in previous reports (Kim et al., 2012;Yang et al., 2013;Huang et al., 2014;Huang et al., 2016).Generally, iron oxides possessed a good performance at the temperature of 300-400°C, while manganese oxides exhibited a good performance at the temperature of 150-250°C.Moreover, iron oxides catalyst usually had a better N 2 selectivity than manganese oxides.In this study, the excellent reactivity of high NO conversion and N 2 selectivity in the medium-and-low temperature should ascribe to the presence of iron-manganese oxides.The Fe 2 O 3 derived from the decomposition of α-FeOOH is provided with large surface area (Liu et al., 2013b), namely active sites.The existence of manganese oxide also provided active component, which favored the oxidation of NO to NO 2 .The existence of NO 2 is beneficial for the fast SCR reaction (Eq.( 4)) which is an important process in low-temperature SCR.
The fast SCR reaction rate is much quicker than that of standard SCR reaction (Eq.( 5)) (Yang et al., 2011;Colombo et al., 2012;Yang et al., 2013;Mihai et al., 2014).It indicates that the SCR reactivity of the as-prepared iron-manganese oxides is considerably dependent on the surface area and pore structure.
On the other hand, as well known, Eqs. ( 6) and ( 7) are the common side reactions during the SCR.The increasing reaction temperature promoted the reaction of Eqs. ( 6) and (7).Combined with the results of NO and NH 3 conversion, it can be concluded that NH 3 was oxidized to NO and N 2 O during the reaction temperature over 300°C.In addition, as shown in Fig. 2, the relatively low annealing temperature favored the NO conversion.Therefore, the N 2 selectivity decreases with the increase of reaction temperature and annealing temperature.

XRD and XRF
To make the active component clear, XRD was utilized to characterize the phase composition of the catalyst.The XRD patterns of catalysts are shown in Fig. 4. A weak reflection at 2θ = 36.7° is found and identified as goethite.The weak reflection is intimately related with the crystallinity of goethite.These reflections at 2θ = 20.9°,26.7°, 50.2° and 2θ = 24.2°,33.2°, 35.7° can be observed and identified as quartz (SiO 2 ) and hematite (α-Fe 2 O 3 ), respectively.The intensity of hematite reflections increases with increasing anneal temperature, which contributes to the growth of hematite crystal.In detail, the particle sizes of L500, L600, L700 and L800 for (104) plane are 19.3, 23.7, 27.4, and 33.6 nm, which were obtained by the software of MDI Jade.Obviously, the increasing annealing temperature increases the size of α-Fe 2 O 3 , which reduces the active sites and results in the decrease of SCR performance.Besides, the results of XRF shows the limonite is composed of Fe 2 O 3 (67.18wt%), MnO (19.68 wt%) and SiO 2 (6.68 wt%).The iron oxide (Fe 2 O 3 ) is identified by XRD pattern, however the manganese oxide is not found.One possible reason is the high dispersion of manganese oxides with very small size.Anyway, the existence of α-Fe 2 O 3 and manganese oxides is proved by the XRD and XRF results, respectively.Meanwhile, the annealing temperature significantly influences the crystal size and consequently affects the SCR performance.

SEM and TEM of Raw Material
The SEM, TEM images and corresponding EDS of natural limonite are displayed in Fig. 5.Many lumps composed of a large number of short rod-like presenting a size of nanoscale are observed in the SEM image.Likewise, the rodlike substances with a size of about 10-30 nm are existent in TEM image.Besides, some acicular substances and aggregations with a low contrast are also found.Combining with EDS results and aforementioned XRD results, it can be concluded that the rod-like substance should be goethite, and manganese oxides accrete with goethite.The aggregation with a low contrast should be some clays which can be considered as a carrier to disperse iron and manganese oxides.
As discussed above, the natural limonite is composed of goethite, quartz, manganese oxides, and some clays.The thermal transformation of goethite generates hematite which played an important role in SCR of NO by NH 3 .The presence of manganese oxides considerably increased the low-temperature SCR activity of the samples, due to enhanced oxidation of NO to NO 2 .

Specific Surface Area and Pore Structure
To characterize the variation of pore structure in more detail, the surface area, pore volume, average pore size as Li et al., Aerosol and Air Quality Research, 17: 2328-2337, 20172332   oxides embodied the microporous and mesoporous properties deduced from results of N 2 adsorption-desorption curves, especially for L500 catalyst.The textural properties are also presented by the data of specific surface area, pore volume, and average pore size.As presented in Table 1, the surface area experiences an obvious decrease from 83.27 to 31.68 m 2 g -1 as the annealing temperature increases from 500 to 800°C, and meanwhile the average pore size increases from 10.82 to 36.6 nm.The pore size distribution of samples is presented in Fig. 7.As the annealing temperature is 500°C, the pore size distribution is mainly from 2.8 to 10 nm, consistent with the average pore size of 10.82 nm obtained from the N 2 -adsorption-desorption.As well known, catalysts with a large surface area are propitious to a catalytic reaction due to the more number of active sites.Thereby, excellent NO conversion was observed over L500, L600 catalyst under the experimental conditions.Moreover, the increasing order of SCR activity of the catalyst is as follows: L500 > L600 > L700 > L800.Therefore, it can be concluded that the variation of surface area is at least partly responsible for the fluctuation of SCR performance of the as-prepared catalysts.

XPS
The surface elements of catalysts were characterized using XPS, and the results are shown in Fig. 8. From the curves of Fe 2p, the binding energies of Fe 2p (711 and 724.5 eV) correspond well to Fe 3+ (Mihai et al., 2014).Similarly, the binding energies of Mn 2p (653.7 and 642.5 eV) correspond well to Mn 4+ (Zhang et al., 2015).Combined with the result of XRD and XRF, the Fe element exists as a state of α-Fe 2 O 3 and the Mn is mainly in the form of MnO 2 .
That is why the Mn-rich limonite after annealing exhibits a high SCR performance and N 2 selectivity at the mediumand-low temperature.

NH 3 -TPD
The NH 3 -TPD profiles of the four samples are shown in Fig. 9. Three NH 3 desorption peaks are clearly observed from 50 to 500°C for all samples.As reported previously (Liu et al., 2011c), the desorption of physisorbed NH 3 and Lewis acid sites adsorbed with NH 3 were found at 54°C and 193°C, respectively, while the desorption of NH 3 bound to strong Brönsted acid sites at a temperature over 350°C was also confirmed (Ayari et al., 2013;Mejri et al., 2016).Consequently, the large broad bands from 120 to 150°C are ascribed to the desorption of physisorbed NH 3 .The desorption peaks at about 240°C and 350°C are attributed to Lewis acid sites and Brönsted acid sites, respectively.In particular, it is speculated that NH 4 + and coordinated NH 3 formed over the prepared catalysts, especially for L500 and L600 catalysts, because NH 4 + bound to strong Brönsted acid sites and coordinated NH 3 bound to Lewis acid sites, respectively.In view of the quality analysis, in the whole temperature range L500 shows the largest NH 3 adsorption amount.The NH 3 adsorption amounts of L500, L600, L700, L800 are determined as 83700, 74600, 29900, 7600 µmol g -1 , respectively.Namely, the decrease of adsorption amount is in the following sequence: L500 > L600 > L700 > L800 which is in line with the NO conversion and surface area data of the four samples.Therefore, it is suggested that low annealing temperature inhibits the growth of active component, increases the surface area and enhances the SCR performance of NO by NH 3 .

Raman
Raman spectroscopy was used to further analysis the crystalline phase and valences of Mn in this study.The Raman spectra of samples are shown in Fig. 10.Previous studies (Bersani et al., 2000;Zoppi et al., 2010) showed that the bands appearing at 220, 290, 403, 500, 609, 650 cm -1 were commonly assigned to hematite.However, the band at 650 cm -1 is usually a low-intensity signal and the literature (León et al., 2004) reported that it was not the spectrum of a perfect hematite due to the lack of longrange order.On the other hand, earlier work (Buciuman et al., 1999;Hong et al., 2011) confirmed that the Raman spectrum of the MnO 2 appear in the range of 580-650 cm -1 .Thereby, the band at 650 cm -1 is ascribed to MnO 2 .This result is well in agreement with the previous XPS results.
The XPS and Raman results proved the existence of MnO 2 in the annealed products, which determined the excellent SCR performance of iron and manganese oxides.

CONCLUSION
Iron and manganese oxides with a nano size were obtained by annealing of natural limonite, a kind of naturally occurred and abundant reserved mineral material.The annealing temperature significantly influenced the surface physicochemical properties and resulted in the fluctuation of SCR performance.The particle size increases from 19.3 to 33.6 nm, and meanwhile the surface area decreased from 83.27 to 31.68 nm as the temperature increased from 500 to 800°C.Besides, the NH 3 adsorption amount also experienced an evident decrease from 83700 to 7600 µmol g -1 .The coexistence of iron and manganese oxides, larger surface area and strong surface activity enhanced the SCR reactivity of NO by NH 3 over L500 catalyst.In this present study, the as-prepared iron and manganese oxides exhibited an excellent performance in NO conversion and N 2 selectivity, and are proved to be an effective material as SCR catalyst.The results will provide a novel preparation method for the eco-friendly, low-cost and high-effective SCR catalyst and explore a new approach for the potential application of natural limonite.In addition, based on the experimental results, the investigation on resistance to SO 2 poisoning, lifetime, and synergetic efficiency between α-Fe 2 O 3 and MnO 2 will be carried out in future work.

Fig. 1 .
Fig. 1.Schematic diagram of SCR of NO by NH 3 over catalysts.

Table 1 .
Surface area, pore volume, and average pore size of different iron and manganese oxides.