The Role of Nitrate on the Sol-Gel Spread Self-Combustion Process and Its E ﬀ ect on the NH 3 -SCR Activity of Magnetic Iron-Based Catalyst

: Sol-gel spread self-combustion is the burning of the complexing agent in dried gel and the oxidant. Meanwhile, high temperature takes place during the combustion process, which is harmful to the pore structure of the catalyst. The nitrate from metal nitrate precursors as an oxidant could participate in the spread of the self-combustion process. Therefore, the inﬂuence of nitrate from metal nitrate on the spread self-combustion of an iron–cerium–tungsten citric acid gel and its catalytic performance of NO x reduction were investigated by removing nitrate via the dissolution of washing co-precipitation with citric acid and re-introducing nitric acid into the former solution. It was found that the removal of nitrate contributes to enhancing the NH 3 –SCR activity of the magnetic mixed oxide catalyst. The NO x reduction e ﬃ ciency was close to 100% for Fe 85 Ce 10 W 5 –CP–CA at 250 ◦ C while the highest was only 80% for the others. The results of thermal analysis demonstrate that the spread self-combustion process of citric acid dried gel is enhanced by re-introducing nitric acid into the citric acid dissolved solution when compared with the removal of nitrate. In addition, the removal of nitrate helps in the formation of γ -Fe 2 O 3 crystallite in the catalyst, reﬁning the particle size of the catalyst and increasing its pore volume. The removal of nitrate also contributes to the formation of Lewis acid sites and Brønsted acid sites on the surface of the catalyst compared with the re-introduction of nitric acid. The in situ di ﬀ use reﬂectance infrared Fourier transform spectroscopy (DRIFTS) demonstrates that both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms exist over Fe 85 Ce 10 W 5 –CP–CA at 250 ◦ C with E–R as its main mechanism.


Introduction
Nitrogen oxide (NO x ) emitted from coal-fired power plants and automobile engines has a strong negative influence on the environment and human health [1][2][3][4][5][6]. Selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) is well known as the best available control technology (BACT) to reduce nitrogen oxides due to its high efficiency [7,8]. Meanwhile, there exists some drawbacks such as high cost, high-temperature conversion of SO 2 to SO 3 , the toxicity, and volatility of vanadium species for the commercial vanadium-based catalyst [9][10][11][12][13][14][15][16]. Due to the non-toxicity, low cost, environment, and the outstanding redox ability between Fe III and Fe II , a series of iron-based mixed oxide catalysts prepared through co-precipitation, sol-gel, and impregnation methods have been developed by many researchers [17][18][19][20]. However, the sol-gel spread self-combustion method, also called low-temperature combustion synthesis (LCS), which takes advantage of organics (citric acid, glucose, urea, and so on) as reactants and nitrates from metal nitrates as oxidants, has been widely used to obtain nano-particles, ultrafine powders, and metal oxide catalysts [21][22][23][24][25][26]. In our precious studies, a novel Fe-Ce-W mixed oxide catalyst synthesized via the citric acid sol-gel spread self-combustion method exhibited high catalytic activity of NO x reduction with high-dispersive γ-Fe 2 O 3 crystallite formed in it [27]. However, a large amount of heat is released during the spread self-combustion of the citric acid dried gel, and brings about a high temperature that acts on the obtained composite oxide catalyst, which might destroy the physical structure of catalyst, thereby influencing its catalytic performance of NO x reduction even though the duration of high temperature is short. Previous research has demonstrated that the amount of complexing agent as the fuel participated in the spread self-combustion of the dried gel, and affected the rapid oxidation of Fe II to Fe III [28]. Meanwhile, the oxygen from the surrounding air as an oxidant showed an enhancement effect on the spread self-combustion process of dried gel, which affected the structure properties of the obtained powder [29]. The nitrate from metal nitrate precursors as another oxidant could also participate in the spread self-combustion of dried gel. Therefore, it is necessary to investigate the influence of nitrate on the spread self-combustion of dried gel, especially its effect on the NH 3 -SCR activity over the above magnetic iron-cerium-tungsten mixed oxide catalyst.
Herein, in the present work, two kinds of magnetic iron-cerium-tungsten mixed oxide catalysts were synthesized via the spread self-combustion of citric acid gel by removing nitrate through the dissolution of washing co-precipitation with citric acid, and re-introducing nitric acid into the former citric acid dissolved solution, respectively. Thermo-gravimetric analysis (TG-DTG-DSC) was used to study the influence of nitrate on the combustion of citric acid gel. In addition, x-ray diffraction (XRD), N 2 -adsorption-desorption, x-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H 2 -TPR), temperature-programmed desorption (NH 3 -TPD), and scanning electron microscope (SEM) were eventually used to characterize the physical structural and chemical properties of the catalyst.

NH 3 -SCR Activity
As can be observed from Figure 1, magnetic Fe 85 Ce 10 W 5 -CP-CA synthesized through the spread self-combustion of the citric acid dried gel without the nitrate from the metal nitrate precursors exhibited excellent catalytic performance of NO x reduction at 150~400 • C, and more than 90% of NO x reduction was achieved at 225~400 • C and over, under a gaseous hourly space velocity (GHSV) of 60,000/h. Meanwhile, the re-introduction of nitric acid into the citric acid dissolved solution decreased the NH 3 -SCR activity of Fe 85 Ce 10 W 5 -CP-CA, and the enhancement of nitrate ions from 0.5 to 2.0 further decreased its catalytic performance. This demonstrates that the nitrate brought from the precursors of metal nitrate shows an inhibition on the NH 3 -SCR activity of the magnetic Fe 85 Ce 10 W 5 catalyst prepared through the spread self-combustion of citric acid gel. As shown in Table 1, the NO x conversion over per gram of Fe 85 Ce 10 W 5 -CP-CA at low temperature (125~200 • C) in one hour was still higher than those of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ), although the bulk density of Fe 85 Ce 10 W 5 -CP-CA was 0.7124 g/mL, which was higher than that of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ). Apparently, the re-introduction of nitric acid could participate in the spread self-combustion of citric acid dried gel, thereby decreasing the bulk density of Fe 85 Ce 10 W 5 -CP-CA. Therefore, the nitrate brought from the metal nitrate precursors shows an important role in the spread self-combustion of citric acid gel, thus affecting the physical structure and the redox properties of magnetic iron-cerium-tungsten mixed oxide catalyst. Herein, TG-DTG-DSC was used to investigate the combustion of the critic acid dried gels of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ).

Thermo-Gravimetric Analysis (TG-DTG-DSC)
Thermal analysis was carried out to investigate the relationship between the weight loss of the catalyst and temperature. The thermo-gravimetric (TG), differential thermo-gravimetric (DTG), and differential scanning calorimetry (DSC) traces of the Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0) citric acid gels were tested and the results are shown in Figure 2. The gel of Fe85Ce10W5-CP-CA(NA1.0) illustrates about 20% weight loss before approximately 140 °C, and a larger exothermic peak can be clearly observed from its DSC, which is mainly attributed to the decomposition of nitrate. However, the gel of Fe85Ce10W5-CP-CA shows two steps weight loss compared to the three steps weight loss of Fe85Ce10W5-CP-CA(NA1.0) gel. During the ignition, the gel of Fe85Ce10W5-CP-CA(NA1.0) presents a sharply spread self-combustion at about 140 °C with a large quantity of reddish brown gas released due to the decomposition of nitrate. Meanwhile, there exists an exothermic peak at 110~160 °C for Fe85Ce10W5-CP-CA(NA1.0) gel, or not an endothermic peak. This indicates that there exists a certain burning of citric acid at 110~160 °C, and this also enhances the main burning temperature of dried gel compared with that of Fe85Ce10W5-CP-CA with a major mass loss about 37% at 160~220 °C. Therefore, the presence of NO3 -helps with the burning or/and decomposing of the citric acid dried gel [30]. Finally, the exothermic peak at 240~350 °C with a smaller weight loss could be considered as the gradual decomposition of citrates (ferric citrate, etc.) for these two dried gels.

Thermo-Gravimetric Analysis (TG-DTG-DSC)
Thermal analysis was carried out to investigate the relationship between the weight loss of the catalyst and temperature. The thermo-gravimetric (TG), differential thermo-gravimetric (DTG), and differential scanning calorimetry (DSC) traces of the Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) citric acid gels were tested and the results are shown in Figure 2. The gel of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) illustrates about 20% weight loss before approximately 140 • C, and a larger exothermic peak can be clearly observed from its DSC, which is mainly attributed to the decomposition of nitrate. However, the gel of Fe 85 Ce 10 W 5 -CP-CA shows two steps weight loss compared to the three steps weight loss of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) gel. During the ignition, the gel of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) presents a sharply spread self-combustion at about 140 • C with a large quantity of reddish brown gas released due to the decomposition of nitrate. Meanwhile, there exists an exothermic peak at 110~160 • C for Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) gel, or not an endothermic peak. This indicates that there exists a certain burning of citric acid at 110~160 • C, and this also enhances the main burning temperature of dried gel compared with that of Fe 85 Ce 10 W 5 -CP-CA with a major mass loss about 37% at 160~220 • C. Therefore, the presence of NO 3 − helps with the burning or/and decomposing of the citric acid dried gel [30]. Finally, the exothermic peak at 240~350 • C with a smaller weight loss could be considered as the gradual decomposition of citrates (ferric citrate, etc.) for these two dried gels. Meanwhile, the decomposition temperature of the gradual citrates for Fe 85 Ce 10 W 5 -CP-CA is lower than those of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ). Thus, it can be concluded that the existence of nitrate could enhance the spread self-combustion of citric acid gel, which affects the physical structure and redox properties of magnetic iron-cerium-tungsten mixed oxide catalyst, thereby influencing its low-temperature NH 3 -SCR activity.
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 14 Meanwhile, the decomposition temperature of the gradual citrates for Fe85Ce10W5-CP-CA is lower than those of Fe85Ce10W5-CP-CA(NA1.0). Thus, it can be concluded that the existence of nitrate could enhance the spread self-combustion of citric acid gel, which affects the physical structure and redox properties of magnetic iron-cerium-tungsten mixed oxide catalyst, thereby influencing its lowtemperature NH3-SCR activity.

X-Ray Diffraction
The X-ray diffraction patterns of Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0) were measured and the results are shown in Figure 3. It can be noted that there exists some obvious sharp diffraction peaks at 2θ = 30.2°, 35.6°, 43.3°, 53.7°, 57.4°, 62.7° in the samples, which are attributed to γ-Fe2O3 crystallite (#25-1402), and the diffraction peak at 2θ = 28.9° could be assigned to CeO2 crystallite (#43-1002), according to the Joint Committee on Powder Diffraction Standards (JCPDS). This indicates that γ-Fe2O3 and CeO2 are the main crystallites of magnetic Fe-Ce-W mixed oxide catalysts [19,31]. Meanwhile, the intensity of diffraction peaks attributed to the γ-Fe2O3 crystallite in Fe85Ce10W5-CP-CA is stronger than that of Fe85Ce10W5-CP-CA(NA1.0). Therefore, the removal of nitrate promotes the formation of γ-Fe2O3 crystallite, which is usually thought to be an important active specie for NH3-SCR reaction [27]. The re-introduction of nitrate also weakens the crystallization of CeO2 in Fe85Ce10W5-CP-CA. Meanwhile, the average γ-Fe2O3 crystallite sizes of Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0) as calculated according to the Scherrer equation

X-Ray Diffraction
The X-ray diffraction patterns of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) were measured and the results are shown in Figure 3. It can be noted that there exists some obvious sharp diffraction peaks at 2θ = 30.2 • , 35.6 • , 43.3 • , 53.7 • , 57.4 • , 62.7 • in the samples, which are attributed to γ-Fe 2 O 3 crystallite (#25-1402), and the diffraction peak at 2θ = 28.9 • could be assigned to CeO 2 crystallite (#43-1002), according to the Joint Committee on Powder Diffraction Standards (JCPDS). This indicates that γ-Fe 2 O 3 and CeO 2 are the main crystallites of magnetic Fe-Ce-W mixed oxide catalysts [19,31]. Meanwhile, the intensity of diffraction peaks attributed to the γ-Fe 2 O 3 crystallite in Fe 85 Ce 10 W 5 -CP-CA is stronger than that of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ). Therefore, the removal of nitrate promotes the formation of γ-Fe 2 O 3 crystallite, which is usually thought to be an important active specie for NH 3 -SCR reaction [27]. The re-introduction of nitrate also weakens the crystallization of CeO 2 in Fe 85 Ce 10 W 5 -CP-CA. Meanwhile, the average γ-Fe 2 O 3 crystallite sizes of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) as calculated according to the Scherrer equation were 13.0 and 14.4 nm, respectively. Therefore, the re-introduction of nitrate causes a large amount of combustion heat to be released and results in a high temperature, thereby enlarging the particle size of Fe 85 Ce 10 W 5 -CP-CA. Then, the removal of nitrate helps in the formation of dispersive γ-Fe 2 O 3 crystallites during the spread self-combustion of citric acid gel and restrains the inter-particle agglomeration and growth of the magnetic iron-cerium-tungsten mixed oxide catalyst.
Catalysts 2020, 10, x FOR PEER REVIEW 5 of 14 were 13.0 and 14.4 nm, respectively. Therefore, the re-introduction of nitrate causes a large amount of combustion heat to be released and results in a high temperature, thereby enlarging the particle size of Fe85Ce10W5-CP-CA. Then, the removal of nitrate helps in the formation of dispersive γ-Fe2O3 crystallites during the spread self-combustion of citric acid gel and restrains the inter-particle agglomeration and growth of the magnetic iron-cerium-tungsten mixed oxide catalyst.

Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is a useful technique to study the surface morphology, shape, and macroscopic particle size of the catalyst. Typical SEM pictures of Fe85Ce10W5-CP-CA, Fe85Ce10W5-CP-CA(NA1.0), and their precursors before being ignited are shown in Figure 4. As shown in Figures 4A,B, the precursor of Fe85Ce10W5-CP-CA(NA1.0) shows stronger agglomeration than that of Fe85Ce10W5-CP-CA, and the combustion of dried gel at the presence of nitrate contributes to the agglomeration of the particles, and results in poor pore connectivity of Fe85Ce10W5-CP-CA(NA1.0), thus decreasing its pore volume. The particles in Fe85Ce10W5-CP-CA have an outstanding distribution, contributing to the smaller pores and larger pore volume, which is beneficial to mass transfer and diffusion. Meanwhile, the diffusion of reactant gases and product gases among the pores of catalysts is important for NH3-SCR reaction [31][32][33]. In addition, the removal of nitrate also decreases the particles' average diameter of Fe85Ce10W5-CP-CA as calculated from the SEM in Figures  4C,D, which is in accordance with the particle sizes calculated according to the Scherrer equation.

Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is a useful technique to study the surface morphology, shape, and macroscopic particle size of the catalyst. Typical SEM pictures of Fe 85 Ce 10 W 5 -CP-CA, Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ), and their precursors before being ignited are shown in Figure 4. As shown in Figure 4A,B, the precursor of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) shows stronger agglomeration than that of Fe 85 Ce 10 W 5 -CP-CA, and the combustion of dried gel at the presence of nitrate contributes to the agglomeration of the particles, and results in poor pore connectivity of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ), thus decreasing its pore volume. The particles in Fe 85 Ce 10 W 5 -CP-CA have an outstanding distribution, contributing to the smaller pores and larger pore volume, which is beneficial to mass transfer and diffusion. Meanwhile, the diffusion of reactant gases and product gases among the pores of catalysts is important for NH 3 -SCR reaction [31][32][33]. In addition, the removal of nitrate also decreases the particles' average diameter of Fe 85 Ce 10 W 5 -CP-CA as calculated from the SEM in Figure 4C,D, which is in accordance with the particle sizes calculated according to the Scherrer equation.  The porosity and the pore size distribution of the as-prepared two catalysts were determined using N2 adsorption-desorption. Figure 5 displays the N2 adsorption-desorption isotherms, the pore size distributions of Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0), and their NOx conversions over per surface area in one hour (mg/(m 2. h)). As can be observed, the isotherm of Fe85Ce10W5-CP-CA(NA1.0) can be recognized as a type IV N2 adsorption/desorption isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification, and it presents mainly meso-pores (2-50 nm), however, the hysteresis loops of Fe85Ce10W5-CP-CA(NA1.0) and Fe85Ce10W5-CP-CA are the H2 and H1 type [32,34], respectively. This demonstrates that the removal of nitrate promotes the formation of meso-pores in magnetic, and the Fe85Ce10W5-CP-CA catalyst shows uniform and regular meso-pores, which was confirmed by the results of the pore diameter distribution in Figure 5B [35]. Interesting, the Brunauer-Emmett and Teller (BET) surface area of Fe85Ce10W5-CP-CA was 90.85 m 2 /g, a little smaller than that of Fe85Ce10W5-CP-CA(NA1.0) (93.13 m 2 /g), as shown in Table 2. Usually, a large BET surface area is beneficial to enhance the catalytic ability of the catalyst. Thus, the NOx conversions at low-temperature over per surface area of catalysts in one hour were calculated, and it was found that Fe85Ce10W5-CP-CA showed a higher NOx conversion over per surface area in one hour than Fe85Ce10W5-CP-CA(NA1.0) at 150~200 °C.

N 2 Adsorption-Desorption
The porosity and the pore size distribution of the as-prepared two catalysts were determined using N 2 adsorption-desorption. Figure 5 displays the N 2 adsorption-desorption isotherms, the pore size distributions of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ), and their NO x conversions over per surface area in one hour (mg/(m 2 ·h)). As can be observed, the isotherm of Fe 85 Ce 10 W 5 -CP-CA(NA1.0) can be recognized as a type IV N2 adsorption/desorption isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification, and it presents mainly meso-pores (2-50 nm), however, the hysteresis loops of Fe85Ce10W5-CP-CA(NA1.0) and Fe85Ce10W5-CP-CA are the H2 and H1 type [32,34], respectively. This demonstrates that the removal of nitrate promotes the formation of meso-pores in magnetic, and the Fe85Ce10W5-CP-CA catalyst shows uniform and regular meso-pores, which was confirmed by the results of the pore diameter distribution in Figure 5B [35]. Interesting, the Brunauer-Emmett and Teller (BET) surface area of Fe 85 Ce 10 W 5 -CP-CA was 90.85 m 2 /g, a little smaller than that of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) (93.13 m 2 /g), as shown in Table 2. Usually, a large BET surface area is beneficial to enhance the catalytic ability of the catalyst. Thus, the NO x conversions at low-temperature over per surface area of catalysts in one hour were calculated, and it was found that Fe 85 Ce 10 W 5 -CP-CA showed a higher NO x conversion over per surface area in one hour than Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) at 150~200 • C.

X-ray photoelectron spectroscopy (XPS) and H2-temperature program reduction (H2-TPR).
To investigate the influence of nitrate on the elements' concentrations and chemical states on the surface of the catalyst, the XPS spectra of Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0) were carried out. As can be noted from Table 3, the re-introduction of nitrate decreased the concentrations of cerium and tungsten on the surface of Fe85Ce10W5-CP-CA, and increased its surface concentration of iron. Higher surface Ce concentration contributes to the excellent reduction ability, which is widely known to be conducive for NH3-SCR reaction [36]. To further study the influence of nitrate on the redox properties of the magnetic Fe-Ce-W mixed oxides catalyst, the H2-TPR curves of Fe85Ce10W5-CP-CA and Fe85Ce10W5-CP-CA(NA1.0) were obtained. The results in Figure 6 show that there exists one peak at the temperature range of 300~500 °C for these two samples, attributed to

X-ray Photoelectron Spectroscopy (XPS) and H 2 -Temperature Program Reduction (H 2 -TPR)
To investigate the influence of nitrate on the elements' concentrations and chemical states on the surface of the catalyst, the XPS spectra of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) were carried out. As can be noted from Table 3, the re-introduction of nitrate decreased the concentrations of cerium and tungsten on the surface of Fe 85 Ce 10 W 5 -CP-CA, and increased its surface concentration of iron. Higher surface Ce concentration contributes to the excellent reduction ability, which is widely known to be conducive for NH 3 -SCR reaction [36]. To further study the influence of nitrate on the redox properties of the magnetic Fe-Ce-W mixed oxides catalyst, the H 2 -TPR curves of Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) were obtained. The results in Figure 6 show that there exists one peak at the temperature range of 300~500 • C for these two samples, attributed to the reduction from Fe 2 O 3 to Fe 3 O 4 at the range of 300~400 • C and the further reduction of Fe 3 O 4 to FeO at about 500 • C [37][38][39]. Interesting, the re-introduction of nitrate enhanced the low-temperature reducibility of Fe 85 Ce 10 W 5 -CP-CA by increasing its surface concentration of iron species, which is confirmed by the results of XPS in Figure 5. Meanwhile, the re-introduction of nitrate decreased the NH 3 -SCR activity of the magnetic Fe 85 Ce 10 W 5 -CP-CA catalyst. the reduction from Fe2O3 to Fe3O4 at the range of 300~400 °C and the further reduction of Fe3O4 to FeO at about 500 °C [37][38][39]. Interesting, the re-introduction of nitrate enhanced the low-temperature reducibility of Fe85Ce10W5-CP-CA by increasing its surface concentration of iron species, which is confirmed by the results of XPS in Figure 5. Meanwhile, the re-introduction of nitrate decreased the NH3-SCR activity of the magnetic Fe85Ce10W5-CP-CA catalyst.  The peak position of NH3-TPD refers to the adsorption strength, and the peaks at 100~200, 200~350 and >350 °C are attributed to the weakly acidic site, the medium-strong acid site, and the strong acid site [40], respectively. From the results in Figure 7, it can be seen that there exists weak acid sites, medium acid site, and strong acid sites on the surface of the magnetic Fe-Ce-W mixed oxide catalysts. The NH3 species adsorbed on the weak acid sites and the medium acid sites were mainly assigned to the coordinated NH3 bound to Lewis acid sites and the partial ionic NH4 + bound to Brønsted acid sites [41][42][43]. The re-introduction of nitrate decreased the adsorption of NH3 on the magnetic Fe85Ce10W5-CP-CA catalyst, particularly decreasing the intensity of its weak and mediumstrong acidic sites. Combined with the results of XRD and SEM, we speculate that the re-introduction

NH 3 -Temperature Programmed Desorption (NH 3 -TPD)
The peak position of NH 3 -TPD refers to the adsorption strength, and the peaks at 100~200, 200~350 and >350 • C are attributed to the weakly acidic site, the medium-strong acid site, and the strong acid site [40], respectively. From the results in Figure 7, it can be seen that there exists weak acid sites, medium acid site, and strong acid sites on the surface of the magnetic Fe-Ce-W mixed oxide catalysts. The NH 3 species adsorbed on the weak acid sites and the medium acid sites were mainly assigned to the coordinated NH 3 bound to Lewis acid sites and the partial ionic NH 4 + bound to Brønsted acid sites [41][42][43]. The re-introduction of nitrate decreased the adsorption of NH 3 on the magnetic Fe 85 Ce 10 W 5 -CP-CA catalyst, particularly decreasing the intensity of its weak and medium-strong acidic sites. Combined with the results of XRD and SEM, we speculate that the re-introduction of nitrate results in a severe agglomeration of amorphous iron and tungsten species of the magnetic Fe 85 Ce 10 W 5 -CP-CA catalyst, and its acid center could be covered or decompose in this process, thereby decreasing the intensity of its weak and medium-strong acidic sites. Therefore, the re-introduction of nitrate modifies the surface structure of the magnetic Fe 85 Ce 10 W 5 -CP-CA catalyst, and shows better Catalysts 2020, 10, 314 9 of 14 adsorption of the NH 3 reactants than Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) at 125~250 • C, which is considered to be a key factor in improving the low-temperature NH 3 -SCR activity of the catalyst.
Fe85Ce10W5-CP-CA catalyst, and its acid center could be covered or decompose in this process, thereby decreasing the intensity of its weak and medium-strong acidic sites. Therefore, the reintroduction of nitrate modifies the surface structure of the magnetic Fe85Ce10W5-CP-CA catalyst, and shows better adsorption of the NH3 reactants than Fe85Ce10W5-CP-CA(NA1.0) at 125~250 °C, which is considered to be a key factor in improving the low-temperature NH3-SCR activity of the catalyst.

Catalytic Mechanism
To identify the presence of the adsorbed NH3 species in the SCR process on the surface of Fe85Ce10W5-CP-CA, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of three transient experiments were recorded under a steady-state condition, and the results are presented in Figure 8. As illustrated in Figure 8A, after NH3 adsorption and N2 purge at 250 °C, Fe85Ce10W5-CP-CA showed several bands located at 1188, 1406, 1587, 3256, and 3351 cm −1 . The bands located at 1188 and 1587 cm −1 can be attributed to the coordinated NH3 on the Lewis acid sites, and the band located at 1406 cm −1 can be ascribed to the ionic NH4 + bound to the Brønsted acid site, while the bands of 3256 and 3351 cm −1 corresponded to the N-H stretching modes of the coordinated NH3 connected to the Lewis acid sites [27,[44][45][46][47][48]. After the introduction of NO + O2 for 10 s, the adsorption peaks of the coordinated NH3 on the Lewis acid sites (located at 1188 and 1587 cm −1 ) and the N-H stretching modes of coordinated NH3 connected to the Lewis acid sites (located at 3256 and 3351 cm −1 ) disappeared, and the intensity of ionic NH4 + bound to the Brønsted acid site at 1406 cm −1 also became weakened. At the same time, the bidentate nitrates (1002 and 1547 cm −1 ), M-NO2 formed by the reaction between M-OH and NOx (1350 and 3639 cm −1 ) and the bridging nitrate (1618 cm −1 ) species appeared [44][45][46][47]. As shown in Figure 8B, after nitrogen oxide species adsorption and N2 purge at 250 °C, Fe85Ce10W5-CP-CA showed several bands ascribed to M-NO2 formed by the reaction between M-OH and NOx (1353 cm −1 ), bidentate nitrates (1560 cm −1 ), and bridging nitrate (1621 cm −1 ), respectively [48][49][50]. After the introduction of NH3 for 10 s, the intensity of adsorption peaks of M-NO2 (1353 cm −1 ) and bridging nitrate (1621 cm −1 ) became weakened, and the band at 1560 cm −1 Figure 7. NH 3 -temperature programmed desorption (NH 3 -TPD) profiles of the catalysts.

Catalytic Mechanism
To identify the presence of the adsorbed NH 3 species in the SCR process on the surface of Fe 85 Ce 10 W 5 -CP-CA, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of three transient experiments were recorded under a steady-state condition, and the results are presented in Figure 8. As illustrated in Figure 8A, after NH 3 adsorption and N 2 purge at 250 • C, Fe 85 Ce 10 W 5 -CP-CA showed several bands located at 1188, 1406, 1587, 3256, and 3351 cm −1 . The bands located at 1188 and 1587 cm −1 can be attributed to the coordinated NH 3 on the Lewis acid sites, and the band located at 1406 cm −1 can be ascribed to the ionic NH 4 + bound to the Brønsted acid site, while the bands of 3256 and 3351 cm −1 corresponded to the N-H stretching modes of the coordinated NH 3 connected to the Lewis acid sites [27,[44][45][46][47][48]. After the introduction of NO + O 2 for 10 s, the adsorption peaks of the coordinated NH 3 on the Lewis acid sites (located at 1188 and 1587 cm −1 ) and the N-H stretching modes of coordinated NH 3 connected to the Lewis acid sites (located at 3256 and 3351 cm −1 ) disappeared, and the intensity of ionic NH 4 + bound to the Brønsted acid site at 1406 cm −1 also became weakened. At the same time, the bidentate nitrates (1002 and 1547 cm −1 ), M-NO 2 formed by the reaction between M-OH and NO x (1350 and 3639 cm −1 ) and the bridging nitrate (1618 cm −1 ) species appeared [44][45][46][47]. As shown in Figure 8B, after nitrogen oxide species adsorption and N 2 purge at 250 • C, Fe 85 Ce 10 W 5 -CP-CA showed several bands ascribed to M-NO 2 formed by the reaction between M-OH and NO x (1353 cm −1 ), bidentate nitrates (1560 cm −1 ), and bridging nitrate (1621 cm −1 ), respectively [48][49][50]. After the introduction of NH 3 for 10 s, the intensity of adsorption peaks of M-NO 2 (1353 cm −1 ) and bridging nitrate (1621 cm −1 ) became weakened, and the band at 1560 cm −1 ascribed to the bidentate nitrates disappeared. The IR bands assigned to the coordinated NH 3 on the Lewis acid sites (1189 and 1587 cm −1 ), the ionic NH 4 + bound to the Brønsted acid site (1439 cm −1 ), and the N-H stretching modes of the coordinated NH 3 connected to the Lewis acid sites (3255 and 3364 cm −1 ) appeared [27,[47][48][49][50]. Figure 8C shows the experimental results of the introduction of NH 3 + NO+O 2 gases over Fe 85 Ce 10 W 5 -CP-CA at 250 • C. It can clearly be seen that the IR bands ascribed to the coordinated NH 3, the N-H stretching modes of the coordinated NH 3 and the ionic NH 4 + appeared, and their intensity gradually became stronger with the increase in the NH 3 + NO + O 2 introduction. However, the adsorption peaks at about 1350 and 1562 cm −1 ascribed to M-NO 2 and bidentate nitrate also appeared and then quickly vanished when NH 3 + NO + O 2 gases were introduced into the reaction tank. Therefore, it can be concluded that the reaction between the adsorbed NH 3 species with gaseous NO + O 2 or the adsorbed NO x species might occur over Fe 85 Ce 10 W 5 -CP-CA at 250 • C, obeyed both E-R and L-H mechanisms, and E-R was its main mechanism.
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 14 ascribed to the bidentate nitrates disappeared. The IR bands assigned to the coordinated NH3 on the Lewis acid sites (1189 and 1587 cm −1 ), the ionic NH4 + bound to the Brønsted acid site (1439 cm −1 ), and the N-H stretching modes of the coordinated NH3 connected to the Lewis acid sites (3255 and 3364 cm −1 ) appeared [27,[47][48][49][50]. Figure 8C shows the experimental results of the introduction of NH3 + NO+O2 gases over Fe85Ce10W5-CP-CA at 250 °C. It can clearly be seen that the IR bands ascribed to the coordinated NH3, the N-H stretching modes of the coordinated NH3 and the ionic NH4 + appeared, and their intensity gradually became stronger with the increase in the NH3 + NO + O2 introduction. However, the adsorption peaks at about 1350 and 1562 cm −1 ascribed to M-NO2 and bidentate nitrate also appeared and then quickly vanished when NH3 + NO + O2 gases were introduced into the reaction tank. Therefore, it can be concluded that the reaction between the adsorbed NH3 species with gaseous NO + O2 or the adsorbed NOx species might occur over Fe85Ce10W5-CP-CA at 250 °C, obeyed both E-R and L-H mechanisms, and E-R was its main mechanism.   O were dissolved in the de-ionized water by keeping their molar ratio at 85:10:5. After magnetic stirring evenly, this solution was titrated into ammonia water of 2 mol/L until the pH value of 9-10. The obtained precipitation was fully washed by the de-ionized water to reduce the nitrate ion in it, and then a certain amount of citric acid was added into the washed precipitation with the molar ratio of citric acid/(Fe + Ce + W) as 1.0. After being stirred for 3 h under water bath at 50 • C, the citric acid dissolved solution were treated by microwave irradiation for 10 min with 36.4% power (microwave irradiation 8 s, 14 s suspended for a cycle with full power) using a household microwave oven (EG8MEA6-NR, 2.45 GHz, 800 W) to ignite, and this catalyst is denoted as Fe 85 Ce 10 W 5 -CP-CA. The preparation of the catalyst by re-introducing nitric acid into the citric acid dissolved solution is similar to the preparation of Fe 85 Ce 10 W 5 -CP-CA, which can be expected by adding a certain amount of nitric acid into the citric acid dissolved co-precipitation precursor before the water bath, which can be denoted as Fe 85 Ce 10 W 5 -CP-CA (NA x , x = 0.5, 1.0, 2.0), where x is the molar ratio of nitric acid/NO 3 − in both Fe(NO 3 ) 3 and Ce(NO 3 ) 3 .

Catalytic Measurement and Characterization
The catalytic measurements of NO x abatement with NH 3 were carried out in a one-dimensional transversely fixed quartz reactor. The simulated gas consisted of 1000 ppm NH 3 , 1000 ppm NO, 3 vol.% O 2 , and the balanced N 2 with the total flow of 2000 mL/min. The samples used in each experiment were 2 mL with a gas hourly space velocity (GHSV) of 60,000 h −1 . The concentrations of NO x and O 2 were monitored via a flue gas analyzer (Model 60i, Thermo Fisher Scientific Co. Ltd., Waltham, MA, USA). NO x conversion is defined by the following equation: The thermal decomposition properties of the precursor before being ignited were determined with a thermal gravimetric analyzer (Netzsch, Selb, Germany, STA449 F3) under an air atmosphere. The surface morphology of the catalyst was measured on a SEM (Japan, Shimadzu). In addition, the physicochemical properties of the samples were also characterized via XRD, N 2 adsorption-desorption, XPS, H 2 -TPR, and NH 3 -TPD as the same as our previous research [27]. The average crystallite sizes of γ-Fe 2 O 3 in Fe 85 Ce 10 W 5 -CP-CA and Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ) were calculated according to the Scherrer equation: where k is the shape factor (k = 0.9); λ is the wavelength of radiations; and β is the FWHM (full width at half maxima). Finally, the NH 3 -SCR mechanism over Fe 85 Ce 10 W 5 -CP-CA at 250 • C was acquired by in situ diffuse reflection infrared Fourier transform spectroscopy (in situ DRIFTS).

Conclusions
The nitrate brought from metal nitrates could participate in the spread self-combustion of citric acid dried gel. The removal of nitrate contributes to optimize the structure properties and acid sites of the magnetic Fe-Ce-W mixed oxide catalyst. The removal of nitrate helps in the formation of dispersive gamma-Fe 2 O 3 in Fe 85 Ce 10 W 5 -CP-CA, accelerating the crystallite rate and refining the particle size, and it shows a wider pore size distribution than Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ). At the same time, the removal of nitrate also enhances the concentrations of cerium/tungsten on the surface of the magnetic catalyst and its low-temperature adsorption of NH 3 . Therefore, the re-introduction of nitrate decreases the low-temperature NH 3 -SCR activity of Fe 85 Ce 10 W 5 -CP-CA, and both E-R and L-H mechanisms exist over Fe 85 Ce 10 W 5 -CP-CA at 250 • C.