Novel Method for Preparing Controllable Nanoporous α-Fe2O3 and its Reactivity to SCR De-NOx

Controllable nanoporous hematite (CNH, α-Fe2O3) was prepared by thermal treatment of α-FeOOH at different temperatures, and the SCR activity of the prepared α-Fe2O3 was evaluated. XRD (X-ray diffraction), HRTEM (highresolution transmission electron microscopy), N2 adsorption-desorption, XPS (X-ray photoelectron spectroscopy), and NH3-TPD (NH3-temperature programmed desorption) were utilized to characterize the catalysts. The results indicated that after thermal treatment at less than 600°C, the α-Fe2O3 catalysts exhibited excellent NO conversion that was higher than 80% in a temperature range from 300 to 400°C. CNH400 with a relatively large surface area of 47.24 m g and many surface hydroxyl groups did not exhibit a substantially improved SCR activity even though it exhibited the best SCR activity. Therefore, the L-H mechanism was not the main reaction route for SCR of NO by α-Fe2O3. The increasing crystallization of α-Fe2O3 decreased the SCR activity, indicating that a decrease in surface oxygen defects was important for the SCR of NO and fitting the E-R mechanism. The NH3-TPD and XPS (O1s) results confirmed this hypothesis. This study provides an approach for the design and preparation of a controllable nanoporous α-Fe2O3 catalyst for SCR of NO by NH3.

The SCR activities of different Fe-based oxide catalysts (i.e., iron oxide and Fe-based catalysts) have been reported, and different reaction mechanisms have been discussed. The reported iron oxide catalysts primarily contained α-Fe 2 O 3 (Liu et al., 2013a), γ-Fe 2 O 3 (Huang et al., 2013;Yang et al., 2013), rod-shaped Fe 2 O 3 (Mou et al., 2012) and Fe-based catalyst including Fe-ZSM-5 (Brandenberger et al., 2010), Fe/HBEA , Fe-Ti-V spinel , Fe-Ti spinel (Yang et al., 2016), and Fe-Mn oxides . The results indicated that the Fe-based oxide catalyst exhibit excellent SCR performance in a temperature range of 200-400°C. Mou et al. (2012) reported that rod-shaped α-Fe 2 O 3 could maintain a NO conversion of more than 80% in a temperature range from 200°C to 400°C, and γ-Fe 2 O 3 exhibited a better SCR activity than α-Fe 2 O 3 . In addition, Yang et al. (2013) confirmed that both α-Fe 2 O 3 and γ-Fe 2 O 3 exhibit remarkably different NO conversions. The latter was better due to additional oxygen defects and the formation of few stable nitrates. To improve the stability, operation temperature window and resistance to poisoning, different additives and supports have been employed. The addition of WO 3 can improve the thermal stability and catalytic activity of Fe 2 O 3 /ZrO 2 (Chen et al., 1995;Apostolescu et al., 2006).
The Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms have been extensively cited to discuss the reaction route of the SCR of NO. For Fe-based catalysts, the main reaction mechanism is influenced by the type of catalyst and reaction temperature with a focus on the importance of Brönsted and Lewis acid sites. The standard SCR process and rough mechanism are as follows: where the formation of adsorbed NO 2 is the rate-controlling step with respect to the L-H mechanism. * 1, 2, 3 denoted different adsorption sites. For pure iron oxides, the E-R mechanism, which involves the conversion of NO by reacting amide intermediates and gaseous NO, has been proposed. As previously mentioned, the iron oxide and Fe-based catalyst exhibit good SCR activity, and the reaction mechanism has been discussed in detail. However, nanoporous α-Fe 2 O 3 and the effect of the pore texture on the NO conversion has not been previously reported. α-FeOOH is a type of hydroxyl-rich iron oxyhydroxyl (Liu et al., 2014), which is different from Fe-based catalysts. In addition to these bulky groups, α-FeOOH still has a complicated surface structure, especially the distribution of adsorbed water/hydroxyl groups, which are illustrated in a previous report (Haibo et al., 2013). Therefore, α-FeOOH was chosen as a precursor to prepare nanoporous α-Fe 2 O 3 due to its thermal decomposition characteristics at different temperatures. The objective of this study was to prepare controllable nanoporous hematite (CNH, α-Fe 2 O 3 ) by thermal treatment of α-FeOOH and evaluate the SCR activity of different CNH catalysts. The effect of the preparation temperature on the porous texture of the newly formed α-Fe 2 O 3 was investigated, and the relationship between the pore texture and the SCR activity is discussed relative to the NO conversion as well as the TEM, N 2 adsorptiondesorption, XPS and NH 3 -TPD characterization.

Catalyst Preparation
The nanoporous α-Fe 2 O 3 (CNH) catalysts were prepared by thermal treatment of α-FeOOH in air for 1 h at various temperatures (400, 500, 600, 700, 800°C). Thermal treatment was carried out in a tube furnace with a temperatureprogrammed controller using 10 °C min -1 as the heating rate. The particle size was controlled between 0.25 mm (60 mesh) and 0.38 mm (40 mesh) by sieving. To simplify the sample labels, an abbreviation was used. CNH400 refers to the α-Fe 2 O 3 catalyst obtained by thermal treatment of α-FeOOH at 400°C.

Catalyst Activity Testing
The SCR of NO by NH 3 was carried out in a fixed-bed quartz tube reactor with an inner size of 8 mm, and 1 g of catalyst was used each time. The typical reactant gas contained 500 ppm of NO, 500 ppm of NH 3 , 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 carried out in a temperature range of 150-400°C with a gas hourly space velocity (GHSV) of 72,000 h -1 . The temperature was regulated via a temperature-programmed controller. The concentration of NO, NO 2 , NH 3 , and N 2 O were monitored using a Fournier transform IR (FTIR) spectrometer. The schematic diagram of the experimental setup is shown in Fig. 1.
As the SCR reaction reached a steady state, the ratio of NO conversion and N 2 selectivity were calculated using the following equations: Catalyst Characterization X-ray diffraction (XRD) measurements were carried out on a Rigaku D/max diffractometer using Cu Kα radiation (50 kV and 100 mA). The diffraction patterns were recorded between 15° and 70° with a step size of 4 ° min -1 .
A high-resolution transmission electron microscope (HRTEM, JEM-2100) was employed to characterize the morphology and pore structure of the catalysts.
The specific surface area, pore volume and pore size distribution were measured by N 2 adsorption-desorption at liquid nitrogen temperature with a Quantachrome NOVA 3000e analyzer. The samples were first degassed at 110°C for 24 h.
Temperature-programmed desorption of NH 3 (NH 3 -TPD) was employed to characterize the surface acidity of the asformed α-Fe 2 O 3 . NH 3 -TPD was carried out at 50-600°C in a microreactor. The temperature was measured using a Ktype thermocouple. The molecules from the outlet of the microreactor were monitored using a quadrupole mass spectrometer (Hiden QIC-20, m/z = 15). Prior to desorption, the sample was saturated with NH 3 based on the m/z = 15 signal at a temperature of 50°C. Then, desorption was performed at a rate of 10 °C min -1 from 50 to 600°C.

SCR Activity of the CNH Catalyst
The NO conversions over CNH catalysts with different reaction temperatures of 150, 200, 250, 300, 350, and 400°C are shown in Fig. 2(a). Similar to other iron oxides catalyst, the reaction temperature had a considerable effect on the NO conversion over the CNH catalysts. The NO conversion increased dramatically as the reaction temperature increased for all the selected catalysts. In addition, the NO conversion was significantly dependent on the preparation temperature of CNH. In particular, the increase in the reaction temperature favored the conversion of NO. Moreover, under the same reaction temperature, as the thermal treatment temperature decreased, the NO conversion increase, as shown in Fig. 2. The NO conversion reached 81.1-95.3% over CNH400 and CNH500, respectively, in contrast to 34-38.8% over CNH800 when the reaction temperature was 350-400°C. As the reaction temperature decreased to 300°C, the NO conversion over CNH400 and CNH500 remained at 80% but decreased to 52.98% and 21.98% over CNH700 and CNH800, respectively. These results indicated that the increase in the thermal treatment temperature went against the NO conversion. Therefore, the thermal treatment temperature is responsible for the remarkable difference in the SCR activity of the different CNH catalysts. The N 2 selectivity of all the catalysts gradually decreased above 300°C, and the N 2 selectivity of CNH400, CNH500, and CNH600 was higher than 80%, as shown in Fig. 1(b).

XRD
The XRD patterns of α-FeOOH and the thermally treated α-FeOOH are shown in Fig. 3. The reflections at 2θ = 21.3°, 33.3°, and 36.7° as well as 2θ = 24.2, 35.7°, and 39.3° were identified as goethite (α-FeOOH) and hematite (α-Fe 2 O 3 ), respectively. The reflections of goethite disappeared and were replaced by those of hematite, indicating that the goethite was transformed into hematite when the thermal treatment temperature was more than 400°C. The decomposition of goethite, which occurs at more than 225°C, has been was previously reported (Rendon et al., 1983;Serna and Iglesias, 1986;Liu et al., 2013b). However, the intensity of the hematite reflections increased as the thermal treatment temperature increased, and well-defined hematite was obtained after thermal treatment at 800°C. The increasing temperatures contributed to increased crystallization of hematite. The existence of slightly remnant hydroxyl (-OH) hampered the crystallization of hematite until a high temperature was reached (i.e., 800°C in this study) (Schulze, 1984;Schulze and Schwertmann, 1987). In addition, the crystallization degree of hematite was inferred based on the increasing particle size of the (104) plane from 29 to 66 nm and the (110) plane from 36 to 56 nm as the temperature increased from 400°C to 800°C. The average particle size was calculated using the Scherrer equation.
Based on the XRD patterns and SCR reactivity, α-Fe 2 O 3 was confirmed to be the active component for the SCR of NO, and the SCR reactivity was significantly affected by the pretreatment temperature. The SCR activity of α-Fe 2 O 3 has been documented during the past three decades, and the corresponding mechanism has been extensively studied (Brandenberger et al., 2010;Li et al., 2011;Mou et al., 2012b;Liu et al., 2013a;Boroń et al., 2015). In addition, both the L-H and E-R mechanisms have been extensively discussed for the SCR of NO reaction over different Febased oxide (iron oxides and Fe-based) catalysts Liu et al., 2013a;Mejri et al., 2016), where the E-R mechanism has been proposed as the main reaction route (Apostolescu et al., 2006;Liu et al., 2013a). Briefly, NH 3 is adsorbed on the surface of the iron oxides to form NH 4 + and coordinated NH 3 followed by the reaction with gaseous NO to afford N 2 and H 2 O. The amount of NH 4 + and coordinated NH 3 were positively correlated to the amount of Bronsted and Lewis sites, respectively. In this study, α-Fe 2 O 3 , which was obtained by thermal treatment of α-FeOOH due to dihydroxylation, inevitably caused variation in the amount of surface hydroxyl groups as well as the pore structure at different temperatures. In detail, the peak area corresponding to the hydroxyl group (3000-3800 cm -1 ) decreased from 2.90 to 0.45 as the temperature increased from 400°C to 800°C (results not shown here). The peak area was calculated by integration of the FT-IES results. The variation in both the surface hydroxyl and pore structure affected the surface acid-base properties. The increasing thermal treatment temperature favored the transformation of α-FeOOH to α-Fe 2 O 3 , resulting in a decrease in surface hydroxyl groups (i.e., surface acidic properties) and affects the SCR activity. Therefore, the decrease in surface hydroxyl groups is a possible reason for the decrease in the NO conversion.

TEM
To obtain representative images of nanoporous α-Fe 2 O 3 as well as the effects of the temperature on the pore structure, HRTEM was employed to characterize CNH400, CNH500, CNH600, CNH700 and CNH800. The TEM images of α-FeOOH and thermally treated α-FeOOH (i.e., α-Fe 2 O 3 ) at different temperatures are shown in Figs. 4(1) and 4(2). The spacing of the (104), (110) and (012) crystal planes was calculated and identified as the crystal plane of α-Fe 2 O 3 using the XRD results. Therefore, α-FeOOH was transformed into α-Fe 2 O 3 , and the newly formed α-Fe 2 O 3 retained the original shape of α-FeOOH, as shown in Fig. 4(1a). However, many pores can be observed in the images of CNH400, CNH500 and CNH600 compared to the extremely few pores observed for CNH700 and CNH800. The pore was labeled with a red dashed line, as shown in Fig. 4(1d), and based on the TEM images, the pore distribution of CNH400, CNH500 and CNH600 is provided in Fig. 4(1b). A single crystal was selected to number and measure each sample. The pore size distributions of CNH400, CNH500 and CNH600 were centered at 0-3, 12-15 and 21-24 nm, respectively, indicating a shift to larger pore sizes as the temperature increased from 400°C to 600°C. In combination with the NO conversion shown in Fig. 2, the decrease in the pore as well as the increase in the pore size may decrease the surface area, resulting in a decrease in reactive sites as well as a decrease in the NO conversion.
size can be controlled by adjusting the temperature. The increasing temperature decreased the number of pores and increased the pore size, which most likely decreased the NO conversion due to a decrease in the number of reactive sites.

Specific Surface Area and Pore Structure
To gain additional insight into the variation in the pore structure, the surface area, pore volume, average pore size and pore distribution were characterized using a pore size and surface area analyzer. The N 2 adsorption-desorption curves of five samples are shown in Fig. 5. All the isotherms were similar and close to type IV (i.e., porous property). However, at a high relative pressure, the hysteresis loop and amount of absorbed N 2 decreased as the thermal treatment temperature increased, indicating a decrease in the surface area. This result was in good agreement with the TEM images. In particular, the newly formed α-Fe 2 O 3 embodied the microporous and mesoporous properties deduced from the results of the TEM images, N 2 adsorption-desorption curves and the pore distribution (Fig. 6), especially for the CNH400 catalyst. The textural properties were also represented by the specific surface area, pore volume, and average pore size data. As shown in Table 1, the surface area and pore volume of this series of catalysts decreased with increasing temperature, and the average pore size increased to 13.1 nm and then slightly decreased. The decrease in the former was due to a decrease in the number of pores and the increase in the pore size. In addition, the pore size and surface area of Fe 2 O 3 prepared by thermal treatment at different temperatures were reproducible.
Catalysts with a large surface area are favorable to a catalytic reaction due to additional active sites. Therefore, excellent NO conversion was observed over the CNH400, CNH500, and CNH600 catalysts at 300°C. Moreover, at a reaction temperature of 250°C, the SCR activity of the catalysts increased in the following order: CNH800 < CNH700 < CNH600 < CNH500 < CNH400. However, the decrease in the surface area was not proportionate to the decrease in the NO conversion at a reaction temperature of 250°C. Therefore, the surface area had a considerable effect on the NO conversion, especially at low temperature, but was not the only factor.

XPS
To determine the state of oxygen on the surface of the catalysts, five types of catalysts were characterized using XPS, and the results are shown in Fig. 7. The O 1s band of the five samples exhibit one high peak with a large shoulder peak, and the intensity decreased with increasing thermal treatment temperature. To elaborate the O 1s band, Fig. 7 was deconvoluted under the same conditions. After deconvolution, the main peak at a lower binding energy (530-530.4 eV) and the shoulder peak at a lower binding energy (531.3-531.7 eV) were more obvious. To the best of our knowledge, the former peak was due to the lattice oxygen O 2-(denoted as O β ), and the latter peak corresponded to surface adsorbed oxygen (denoted as O α ) including defect oxide or surface hydroxyl groups (Liu et al., 2011a, c;Liu et al., 2013a). As shown in Fig. 7, a decrease in both types of peak areas as the thermal treatment temperature increased was observed, especially at 800°C. O α are considered to be the active sites of the SCR of NO by providing acid sites to adsorb NH 3 molecules to form NH 4 + and coordinated NH 3 . In this study, thermal treatment led to dehydroxylation and restructuring of the iron oxide, resulting in the formation of defect oxygen. The increasing temperature also caused deep dehydroxylation and crystallization of α-Fe 2 O 3 , decreasing the surface area and the number of surface hydroxyl groups, increasing the crystallization degree and decreasing the O α . Therefore, the decrease in O α most likely leads to the decrease in the NO conversion.
In particular, the reaction between NH 4 + due to the adsorption of NH 3 molecules on Brönsted acid sites and monodentate nitrate due to the oxidation of NO in the presence of O 2 has been proposed as the main reaction process at temperatures lower than 200°C (i.e., L-H mechanism) (Liu et al., 2011c). However, the E-R mechanism replaces the L-H mechanism at temperatures higher than 200°C (Apostolescu et al., 2006;Boroń et al., 2015).
In this study, α-Fe 2 O 3 exhibited excellent SCR activity at reaction temperatures higher than 200°C. Therefore, the E-R mechanism is regarded as the main reaction route for these catalysts. Therefore, the Brönsted acid sites do not play an important role in the catalytic SCR of NO by NH 3 . In fact, CNH400 did not exhibit excellent SCR activity compared to the other catalysts even though CNH400 had the largest number of surface hydroxyl groups. Therefore, the existence of hydroxyl groups does not substantially improve the SCR activity. In addition, it is important to note that O α substantially decreased as the thermal treatment temperature reached 800°C, and the NO conversion over CNH800 was remarkably low compared to that of other catalysts. The higher temperature favored an improved degree of crystallization of α-Fe 2 O 3 , decreasing the oxygen defect. Therefore, the decrease in oxygen defects directly decreased the SCR activity of α-Fe 2 O 3 . In addition, these results confirmed that the main route for the SCR of NO over iron oxide catalysts is consistent with the E-R mechanism rather than the L-H mechanism.

NH 3 -TPD
The reaction mechanism of the SCR of NO could follow either the L-H mechanism or the E-R mechanism. Here, the activation of NH 3 is the key factor in the reaction process. To characterize the adsorption property of NH 3 on the surface of the newly formed α-Fe 2 O 3 catalysts, NH 3 -TPD was performed, and the curves are shown in Fig. 8. Moreover, the amount of adsorbed NH 3 was determined based on the NH 3 -TPD curves. Three NH 3 desorption peak was clearly observed from 50°C to 500°C. As previously reported (Liu et al., 2011c), the desorption of physisorbed NH 3 and Lewis acid sites with adsorbed NH 3 were observed at 54°C and 193°C, respectively. In addition, the desorption of NH 3 bound to strong Brönsted acid sites at a temperature of more than 350°C was confirmed (Ayari et al., 2013;Mejri et al., 2016). Therefore, the relatively higher desorption temperature of NH 3 in this study was due to the NH 3 bound to strong Brönsted acid sites derived from Fe-OH. The hydroxyl group resulted from the incomplete decomposition of α-FeOOH. The lower temperature contributed to physisorbed NH 3 and Lewis acid sites with adsorbed NH 3 . The desorption temperature was higher than that previously mentioned (i.e., 54 and 193°C). The porous structure of α-Fe 2 O 3 was also taken into consideration. The porous structure shown in the TEM images inhibited the desorption of physisorbed NH 3 , which resulted in a high desorption temperature. The porous property also contributed to a large adsorption amount, and the amount decreased as the number of pores decrease. In particular, CNH400 exhibited the maximum NH 3 adsorption and desorption amounts. The calculated amount of adsorbed NH 3 increased in the following order: CNH800 (56.8 µmol g -1 ) < CNH700 (71.2 µmol g -1 ) < CNH600 (177.4 µmol g -1 ) < CNH500 (296.9 µmol g -1 ) < CNH400 (362.7 µmol g -1 ).

CONCLUSIONS
Nanoporous hematite (α-Fe 2 O 3 ) with different specific surface areas were prepared by thermal treatment of α-FeOOH. As the temperature increased from 400°C to 800°C, the surface area, average pore size and amount of adsorbed NH 3 varied from 47.24 m 2 g -1 , 5.89 nm and 56.8 µmol g -1 , respectively, to 3.27 m 2 g -1 , 11.76 nm and 362.7 µmol g -1 , respectively. The increasing thermal treatment temperature increased the average pore size and decreased the amount of adsorbed NH 3 . In addition, the XPS results indicated that the increased temperature improved the crystallization of α-Fe 2 O 3 and decreased the surface oxygen defects. With respect to the surface activity, the newly formed α-Fe 2 O 3 exhibited excellent SCR activity, especially at reaction temperatures of more than 300°C. In combination with previous results, the E-R mechanism was proposed as the main route for the SCR of NO over α-Fe 2 O 3 in this study. Goethite (α-FeOOH) naturally occurs on the surface of the Earth and can be easily obtained. This use of this type of natural mineral as the precursor for α-Fe 2 O 3 will significantly decrease the catalyst cost and expand the application field of goethite. This study confirmed the SCR activity of the newly formed α-Fe 2 O 3 that was derived from the decomposition of α-FeOOH and provided a theoretical principle for the application of natural goethite in the SCR of NO by NH 3 .