Preparation of Cu-Al/SiO2 Porous Material and Its Effect on NO Decomposition in a Cement Kiln

Nitrogen oxide (NOx) emissions have attracted much attention for increasing concern on the quality of the atmospheric environment. In view of NO decomposition in the cement production process, the preparation of Cu-Al/SiO2 porous material and its effect on NO decomposition were studied, and the denitrification mechanism was proposed in this paper. The NO decomposition performance of the Cu-Al/SiO2 porous material was tested via the experimental setup and infrared spectrometer and micro gas chromatography (GC). The result shows that the Cu-Al/SiO2 porous material with the template of cetyltrimethylammonium bromide (CTAB) had a better NO decomposition rate than materials with other templates when the temperature was above 500 °C, and NO decomposition rate could approach 100% at high temperatures above 750 °C. Structure analysis indicates that the prepared Cu-Al/SiO2 material structure was a mesoporous structure. The X-Ray Diffraction (XRD) and Ultraviolet–visible spectrophotometry (UV–Vis) results of the denitrification product show that the Cu-Al/SiO2 material mainly decomposed to Cu2O and Si2O, and the CuO decomposed to Cu2O and O2 at high temperature. The Cu(I)O was considered as the active phase. The redox process between Cu(II)O and Cu(I)O was thought to be the denitrification mechanism of the Cu-Al/SiO2 porous material.


Preparation of Porous Materials
The Cu-Al/SiO 2 porous materials were synthesized by hydrothermal method. In addition, the Cu-Al/SiO 2 porous materials were prepared with different templates such as cetyltrimethylammonium bromide (CTAB), tetrapropylammonium hydroxide (TPAOH), ammonium hydroxide (NH 3 ·H 2 O), and zeolite seed crystal in this paper.
The sodium hydroxide solution was dropped into silica sol under vigorous stirring with a magnetizer (solution A). In addition, the template was mixed with the aluminum sulfate solution (solution B). After stirring for half an hour, solution B was slowly dropwise added into the solution A; finally, the copper sulfate solution was added. The obtained molar composition of gel mixture was SiO 2 : 0.02Al 2 O 3 : 0.05CuO: 0.1CTAB: 0.3NaOH: 45H 2 O. During the process, sulfuric acid solution was used to ensure that the pH of the solution was about 10~12. The solution was stirred at room temperature for 3 h, and poured into a reaction kettle, then the reaction kettle was put in an oven at 160 • C for 24 h. After that, the reaction kettle was cooled in the air, the solid product was separated by centrifugation, and washed with deionized water and ethanol 3 times. The final product was dried at 100 • C for 24 h and then calcined at 550 • C for 5 h.

Characterization
The NO decomposition reaction was carried out in a laboratory-scale quartz fixed bed reactor heated by an electric control heated oven, as shown in Figure 1. NO decomposition reaction was performed on a fixed-bed quartz tube reactor containing 2 mL porous material. The thermocouple was inserted into the center of the reactor to measure the test temperature. The total flow rate was 300 mL/min. The reactant gas condition was 1000 ppm NO and He balance gas. The materials and reactant gas were heated at a heating rate of 15 • C/min from ambient temperature to 1000 • C. The concentrations of NO, NO 2 , and N 2 O were continually monitored by a TENSOR 27 FT-IR spectrometer (Bruker, Karlsruhe, Germany). In addition, the NO decomposition rate was calculated by Equation (1): NO decomposition rate = NO in -NO out -NO 2 out -N 2 O out [NO] in ⁄ ×100% (1) The specific surface area, N2 adsorption desorption curves, and pore size distribution were determined by BET using a TriStarII 3020 gas sorption analyzer (Mike, Georgia, USA). All the samples were fully dried at 90 °C for 1 h and then degassed at 350 °C for 3 h in vacuum prior to the BET surface measurements.
The crystal structures of the prepared porous materials were determined by X-ray diffraction (XRD). XRD patterns were recorded on a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, German) equipped with the Cu-Kα radiation source (λ = 0.15405 nm), operating at 40 kV and 40 mA. In addition, the scanning angle (2θ) was taken over a range of 10°-60°. The diffraction patterns were used for phase identification of crystalline materials or crystalline grains in powder. Crystalline phases were identified by comparison with the reference data cited from the International Center for Diffraction Data (ICDD) files. However, it should be reminded that this technique can only evaluate the crystalline phases and any amorphous phases could not be detected. The TEM was obtained with JEOL-2010 transmission electron microscope (JEOL, Tokyo, Japan). The porous material powders were dispersed in alcohol and kept in an ultrasonic bath for 10 min, and then the well-prepared suspension was deposited on a carbon-covered Cu grid to be tested. UV-Visible diffuse reflectance spectra of the samples were recorded using a JASCO V-650 spectrophotometer (JASCO, Tokyo, Japan).  Figure 2 displays the relationship between NO decomposition rate of the porous materials and the temperature ranging from 300 to 1000 °C, with the denitrification data collected in an interval of 50 °C. The porous materials were prepared with different templates (TPAOH, CTAB, NH3·H2O, zeolite seed crystal and no template added) by hydrothermal method. It can be seen that the NO decomposition rate of Cu-Al/SiO2 porous material increased with the increasing of the denitrification The specific surface area, N 2 adsorption desorption curves, and pore size distribution were determined by BET using a TriStarII 3020 gas sorption analyzer (Mike, Georgia, USA). All the samples were fully dried at 90 • C for 1 h and then degassed at 350 • C for 3 h in vacuum prior to the BET surface measurements.

Denitrification Performance and Structure
The crystal structures of the prepared porous materials were determined by X-ray diffraction (XRD). XRD patterns were recorded on a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, German) equipped with the Cu-Kα radiation source (λ = 0.15405 nm), operating at 40 kV and 40 mA. In addition, the scanning angle (2θ) was taken over a range of 10 • -60 • . The diffraction patterns were used for phase identification of crystalline materials or crystalline grains in powder. Crystalline phases were identified by comparison with the reference data cited from the International Center for Diffraction Data (ICDD) files. However, it should be reminded that this technique can only evaluate the crystalline phases and any amorphous phases could not be detected. The TEM was obtained with JEOL-2010 transmission electron microscope (JEOL, Tokyo, Japan). The porous material powders were dispersed in alcohol and kept in an ultrasonic bath for 10 min, and then the well-prepared suspension was deposited on a carbon-covered Cu grid to be tested. UV-Visible diffuse reflectance spectra of the samples were recorded using a JASCO V-650 spectrophotometer (JASCO, Tokyo, Japan). Figure 2 displays the relationship between NO decomposition rate of the porous materials and the temperature ranging from 300 to 1000 • C, with the denitrification data collected in an interval of 50 • C. The porous materials were prepared with different templates (TPAOH, CTAB, NH 3 ·H 2 O, zeolite seed crystal and no template added) by hydrothermal method. It can be seen that the NO decomposition Materials 2020, 13, 145 4 of 11 rate of Cu-Al/SiO 2 porous material increased with the increasing of the denitrification reaction temperature. Cu-Al/SiO 2 porous material with CTAB and TPAOH have better denitrification efficiency than Cu-Al/SiO 2 porous material with NH 3 ·H 2 O and zeolite seed crystal during the temperature range 500-1000 • C. The porous material with CTAB had more than 60% NO decomposition rate at 600-1000 • C, while the NO decomposition rate at 300−500 • C was lower than 20%, and the NO decomposition rate could approach 99% at high temperatures above 750 • C. However, the porous material with TPAOH could reach 60% NO decomposition rate at 750-1000 • C. Compared with the Cu-Al/SiO 2 porous material with TPAOH, NO decomposition rate of the Cu-Al/SiO 2 porous material with CTAB was increased by 63.1% and 10.7% at 650 and 750 • C, respectively, indicating that the denitrification efficiency was enhanced with a broader operation temperature window due to the template of CTAB. On the whole, the NO decomposition rate of the Cu-Al/SiO 2 porous material with the templates of CTAB had the best denitrification efficiency, compared to that of porous materials with other templates.  Figure 2. The effect of porous materials with different templates on the NO decomposition rate (Blank, tetrapropylammonium hydroxide (TPAOH), cetyltrimethylammonium bromide (CTAB), ammonium hydroxide (NH3·H2O), and Zeolite were used, respectively, instead of the Cu-Al/SiO2 porous materials with no template added, TPAOH, CTAB, NH3·H2O, and zeolite seed crystal).

Denitrification Performance and Structure
The Cu-Al/SiO2 materials have different porous properties with different templates. Thus, the pore properties of the Cu-Al/SiO2 porous materials with varying templates were obtained from N2 adsorption-desorption isotherms at −196 °C. The results of BET surface area, pore volume, and pore diameter estimated by the Barrett-Joyner-Halenda (BJH) model are presented for Cu-Al/SiO2 porous materials with different templates in Table 1. The results show that the porous materials with organic template reagent had higher specific surface area than those with inorganic template agent. The Cu-Al/SiO2 porous material with CTAB reached the largest specific surface area, the biggest cumulative pore volume, and the smallest average pore diameter. The specific surface area, pore volume, and the pore diameter were about 392.27 m 2 /g, 0.4876 cm 3 /g, and 4.3119 nm, respectively. Compared with the Cu-Al/SiO2 porous material with CTAB, the specific surface area of Cu-Al/SiO2 porous material with TPAOH was decreased to 135.27 m 2 /g, and the pore diameter was increased to 7.55 nm. Figure  3 shows the N2 adsorption-desorption isotherms of Cu-Al/SiO2 porous materials with no template added, TPAOH, CTAB, NH3.H2O, and zeolite seed crystal, separately. The template provides key functions in the process of synthesizing the pore structure material, namely in the roles of structure orientation, space filling, and balancing skeleton charge, and so on. From Figure 3, we can see that the N2 amount absorption of the Cu-Al/SiO2 porous material with CTAB was much larger than those of materials with other templates. The pore structure of Cu-Al/SiO2 porous materials was confirmed by N2 adsorption-desorption isotherms. The N2 adsorption-desorption isotherm of porous material with CTAB shows that the adsorption-desorption hysteresis loop appeared in the middle segment. The Cu-Al/SiO2 porous material with CTAB had the typical IV type N2-adsorption isotherm curves, Figure 2. The effect of porous materials with different templates on the NO decomposition rate (Blank, tetrapropylammonium hydroxide (TPAOH), cetyltrimethylammonium bromide (CTAB), ammonium hydroxide (NH 3 ·H 2 O), and Zeolite were used, respectively, instead of the Cu-Al/SiO 2 porous materials with no template added, TPAOH, CTAB, NH 3 ·H 2 O, and zeolite seed crystal).
The Cu-Al/SiO 2 materials have different porous properties with different templates. Thus, the pore properties of the Cu-Al/SiO 2 porous materials with varying templates were obtained from N 2 adsorption-desorption isotherms at −196 • C. The results of BET surface area, pore volume, and pore diameter estimated by the Barrett-Joyner-Halenda (BJH) model are presented for Cu-Al/SiO 2 porous materials with different templates in Table 1. The results show that the porous materials with organic template reagent had higher specific surface area than those with inorganic template agent. The Cu-Al/SiO 2 porous material with CTAB reached the largest specific surface area, the biggest cumulative pore volume, and the smallest average pore diameter. The specific surface area, pore volume, and the pore diameter were about 392.27 m 2 /g, 0.4876 cm 3 /g, and 4.3119 nm, respectively. Compared with the Cu-Al/SiO 2 porous material with CTAB, the specific surface area of Cu-Al/SiO 2 porous material with TPAOH was decreased to 135.27 m 2 /g, and the pore diameter was increased to 7.55 nm. Figure 3 shows the N 2 adsorption-desorption isotherms of Cu-Al/SiO 2 porous materials with no template added, TPAOH, CTAB, NH 3 .H 2 O, and zeolite seed crystal, separately. The template provides key functions in the process of synthesizing the pore structure material, namely in the roles of structure orientation, space filling, and balancing skeleton charge, and so on. From Figure 3, we can see that the N 2 amount absorption of the Cu-Al/SiO 2 porous material with CTAB was much larger than those of materials with other templates. The pore structure of Cu-Al/SiO 2 porous materials was confirmed by N 2 adsorption-desorption isotherms. The N 2 adsorption-desorption isotherm of porous material with CTAB shows that the adsorption-desorption hysteresis loop appeared in the middle segment. The Cu-Al/SiO 2 porous material with CTAB had the typical IV type N 2 -adsorption isotherm curves, and the material had the inflection point and hysteresis loop, implying the material's pores were well distributed and narrow (also shown in the pore size distribution curve, the distribution of pore diameter was mainly in the range 2~5 nm), which indicate that the material structure was a mesoporous structure. Thus, the pore structure of the Cu-Al/SiO 2 porous material with CTAB enlarged the contact area with NO gas, which contributed to the denitrification performance. and the material had the inflection point and hysteresis loop, implying the material's pores were well distributed and narrow (also shown in the pore size distribution curve, the distribution of pore diameter was mainly in the range 2~5 nm), which indicate that the material structure was a mesoporous structure. Thus, the pore structure of the Cu-Al/SiO2 porous material with CTAB enlarged the contact area with NO gas, which contributed to the denitrification performance.  A transmission electron microscope (TEM) was used to study the microstructure of Cu-Al/SiO2 porous material with the template of CTAB (as shown in Figure 4). The TEM micrograph in Figure 4 demonstrated the uniform one-dimensional mesoporous channel structure of Cu-Al/SiO2 porous material. It was notable that the uniform porosity illustrated by TEM analysis was in line with the quite narrow pore size distribution determined by N2 adsorption (see Figure 3). Representative TEM images of the Cu-Al/SiO2 porous material with a highly ordered hexagonal structure could be clearly observed in the image taken with the electron beam parallel to the pore direction ( Figure 4a). Additionally, the micrograph that was taken with the electron beam perpendicular to the pores showed both the channels and the framework (Figure 4b). The well-ordered mesoporous structure was observed clearly and it could hardly find nanoparticles or clusters of metal (Cu) oxides outside the mesoporous channels, indicating the copper was incorporated into the Si-O and Al-O framework in Cu-Al/SiO2 material. A transmission electron microscope (TEM) was used to study the microstructure of Cu-Al/SiO 2 porous material with the template of CTAB (as shown in Figure 4). The TEM micrograph in Figure 4 demonstrated the uniform one-dimensional mesoporous channel structure of Cu-Al/SiO 2 porous material. It was notable that the uniform porosity illustrated by TEM analysis was in line with the quite narrow pore size distribution determined by N 2 adsorption (see Figure 3). Representative TEM images of the Cu-Al/SiO 2 porous material with a highly ordered hexagonal structure could be clearly observed in the image taken with the electron beam parallel to the pore direction (Figure 4a). Additionally, the micrograph that was taken with the electron beam perpendicular to the pores showed both the channels and the framework (Figure 4b). The well-ordered mesoporous structure was observed clearly and it could hardly find nanoparticles or clusters of metal (Cu) oxides outside the mesoporous channels, indicating the copper was incorporated into the Si-O and Al-O framework in Cu-Al/SiO 2 material.  As the component of the Cu-Al/SiO2 porous material was similar to the cement composition, it did not negatively influence the performance of cement. In combination with the cement production process, it is suitable to add the Cu-Al/SiO2 porous material into the preheater with the cement raw materials. NO would be removed in the preheater and precalciner. The products of the denitrification reactions goes into the rotary kiln with the cement raw material, and, finally, contributes to the cement clinker in the rotary kiln. During the whole process, the Cu-Al/SiO2 porous material is not only a denitrification material but also can be a component of the cement clinker.
In order to ensure the denitrification performance of the Cu-Al/SiO2 porous material in cement kiln, cement raw materials (CRM) mixed with low content (3%~5%) of Cu-Al/SiO2 porous materials with the template of CTAB were tested. It can be seen, from Figure 5, that the NO decomposition rate increased with the increase of the proportion of the Cu-Al/SiO2 porous material in the mixture material. In addition, the NO decomposition rate increased with the increase of the temperature from 500 to 1000 °C. When a small amount of Cu-Al/SiO2 porous material (5%) was added into cement raw materials, the NO decomposition rate of the mixture material was more than 80% at 800-1000 °C, while the NO decomposition rate was lower than 50% at 500−700 °C. NO decomposition rate increased quickly when the temperature was above 750 °C, and the decomposition rate of all the materials could reach as high as 80% above 900 °C. When a small amount of Cu-Al/SiO2 porous material (5%) was added into cement raw materials, the NO decomposition rate could reach over 80% above 800 °C, and the NO decomposition rate could approach 90% at the temperature of 900 °C.  As the component of the Cu-Al/SiO 2 porous material was similar to the cement composition, it did not negatively influence the performance of cement. In combination with the cement production process, it is suitable to add the Cu-Al/SiO 2 porous material into the preheater with the cement raw materials. NO would be removed in the preheater and precalciner. The products of the denitrification reactions goes into the rotary kiln with the cement raw material, and, finally, contributes to the cement clinker in the rotary kiln. During the whole process, the Cu-Al/SiO 2 porous material is not only a denitrification material but also can be a component of the cement clinker.
In order to ensure the denitrification performance of the Cu-Al/SiO 2 porous material in cement kiln, cement raw materials (CRM) mixed with low content (3%~5%) of Cu-Al/SiO 2 porous materials with the template of CTAB were tested. It can be seen, from Figure 5, that the NO decomposition rate increased with the increase of the proportion of the Cu-Al/SiO 2 porous material in the mixture material. In addition, the NO decomposition rate increased with the increase of the temperature from 500 to 1000 • C. When a small amount of Cu-Al/SiO 2 porous material (5%) was added into cement raw materials, the NO decomposition rate of the mixture material was more than 80% at 800-1000 • C, while the NO decomposition rate was lower than 50% at 500−700 • C. NO decomposition rate increased quickly when the temperature was above 750 • C, and the decomposition rate of all the materials could reach as high as 80% above 900 • C. When a small amount of Cu-Al/SiO 2 porous material (5%) was added into cement raw materials, the NO decomposition rate could reach over 80% above 800 • C, and the NO decomposition rate could approach 90% at the temperature of 900 • C.  As the component of the Cu-Al/SiO2 porous material was similar to the cement composition, it did not negatively influence the performance of cement. In combination with the cement production process, it is suitable to add the Cu-Al/SiO2 porous material into the preheater with the cement raw materials. NO would be removed in the preheater and precalciner. The products of the denitrification reactions goes into the rotary kiln with the cement raw material, and, finally, contributes to the cement clinker in the rotary kiln. During the whole process, the Cu-Al/SiO2 porous material is not only a denitrification material but also can be a component of the cement clinker.
In order to ensure the denitrification performance of the Cu-Al/SiO2 porous material in cement kiln, cement raw materials (CRM) mixed with low content (3%~5%) of Cu-Al/SiO2 porous materials with the template of CTAB were tested. It can be seen, from Figure 5, that the NO decomposition rate increased with the increase of the proportion of the Cu-Al/SiO2 porous material in the mixture material. In addition, the NO decomposition rate increased with the increase of the temperature from 500 to 1000 °C. When a small amount of Cu-Al/SiO2 porous material (5%) was added into cement raw materials, the NO decomposition rate of the mixture material was more than 80% at 800-1000 °C, while the NO decomposition rate was lower than 50% at 500−700 °C. NO decomposition rate increased quickly when the temperature was above 750 °C, and the decomposition rate of all the materials could reach as high as 80% above 900 °C. When a small amount of Cu-Al/SiO2 porous material (5%) was added into cement raw materials, the NO decomposition rate could reach over 80% above 800 °C, and the NO decomposition rate could approach 90% at the temperature of 900 °C.

Denitrification Mechanism
According to the results of the denitrification performance of Cu-Al/SiO 2 porous materials with different templates (TPAOH, CTAB, NH 3 ·H 2 O and ZSM seed crystal), the Cu-Al/SiO 2 porous material with CTAB obtained the best NO decomposition rate above 500 • C. Therefore, in order to further study the denitrification mechanism of the Cu-Al/SiO 2 porous material, under the gas condition of 1000 ppm NO and balance gas He, experiments were carried out with fixed bed reactor system, and the reaction gas after denitrification was analyzed with GC (Agilent 490 Micro GC, California, USA). The NO decomposition rate of the Cu-Al/SiO 2 porous material with the template of CTAB was calculated by Equation (1). In addition, the gas flow rate was 100 mL/min, and the reaction temperature was increased from 100 to 1000 • C at 15 • C/min. The denitrification result of the Cu-Al/SiO 2 porous material with CTAB is shown in Figure 6.

Denitrification Mechanism
According to the results of the denitrification performance of Cu-Al/SiO2 porous materials with different templates (TPAOH, CTAB, NH3·H2O and ZSM seed crystal), the Cu-Al/SiO2 porous material with CTAB obtained the best NO decomposition rate above 500 °C. Therefore, in order to further study the denitrification mechanism of the Cu-Al/SiO2 porous material, under the gas condition of 1000 ppm NO and balance gas He, experiments were carried out with fixed bed reactor system, and the reaction gas after denitrification was analyzed with GC (Agilent 490 Micro GC, California, USA). The NO decomposition rate of the Cu-Al/SiO2 porous material with the template of CTAB was calculated by Equation (1). In addition, the gas flow rate was 100 mL/min, and the reaction temperature was increased from 100 to 1000 °C at 15 °C/min. The denitrification result of the Cu-Al/SiO2 porous material with CTAB is shown in Figure 6. The amount of different gas is proportional to the peak area, thus Figure 6a illustrates that peak area of different gases changed along with the temperature. As can be seen in Figure 6a, the amount of NO decreased sharply and, correspondingly, the amount of N2 and O2 increased at high temperature (above 500 °C). In addition, there was basically no NO2 and N2O produced in the whole process. The result tested by micro GC is coincident with that of detected by FTIR (as shown in Figure  6c). There was NO absorption peak (1803~1962 cm −1 ) in reaction gas after denitrification at 100 °C. In addition, the NO absorption peak disappeared and basically no NO2 absorption peak (1550~1650 cm −1 ) and N2O absorption peak (2160~2262 cm −1 ) appeared in reaction gas after denitrification at 1000 °C. The amount of different gas is proportional to the peak area, thus Figure 6a illustrates that peak area of different gases changed along with the temperature. As can be seen in Figure 6a, the amount of NO decreased sharply and, correspondingly, the amount of N 2 and O 2 increased at high temperature (above 500 • C). In addition, there was basically no NO 2 and N 2 O produced in the whole process. The result tested by micro GC is coincident with that of detected by FTIR (as shown in Figure 6c). There was NO absorption peak (1803~1962 cm −1 ) in reaction gas after denitrification at 100 • C. In addition, the NO absorption peak disappeared and basically no NO 2 absorption peak (1550~1650 cm −1 ) and N 2 O Materials 2020, 13, 145 8 of 11 absorption peak (2160~2262 cm −1 ) appeared in reaction gas after denitrification at 1000 • C. According to the result of Figure 6a, NO decomposition rate could be calculated by Equation (1). Therefore, NO decomposition rate of Cu-Al/SiO 2 porous material with the template of CTAB was shown in Figure 6b. NO decomposition rate reached to nearly 40% at low temperature of 350 • C, and increased quickly above 500 • C. In addition, the NO decomposition rate of Cu-Al/SiO 2 porous material with CTAB could approach 100% at high temperatures above 750 • C. After the denitrification test from 100 to 1000 • C, NO decomposition rate of Cu-Al/SiO 2 porous material was tested when the temperature was kept at 1000 • C for 60 min, as shown in Figure 6d. The gaseous products of the reaction were analyzed by micro GC. The GC result shows that there was no nitrogen oxides left in the gaseous products, thus NO decomposition rate was still nearly 100% in the 60 min.
The phase analysis of denitrification product was analyzed by X-ray diffractometer and Ultraviolet-visible spectroscopy (UV-Vis) (shown in Figure 7). It can be seen from Figure 7a, XRD phase analysis indicates that the denitrification products mainly were cristobalite and cuprite. According to the result of Figure 6a, NO decomposition rate could be calculated by Equation (1). Therefore, NO decomposition rate of Cu-Al/SiO2 porous material with the template of CTAB was shown in Figure 6b. NO decomposition rate reached to nearly 40% at low temperature of 350 °C, and increased quickly above 500 °C. In addition, the NO decomposition rate of Cu-Al/SiO2 porous material with CTAB could approach 100% at high temperatures above 750 °C. After the denitrification test from 100 to 1000 °C, NO decomposition rate of Cu-Al/SiO2 porous material was tested when the temperature was kept at 1000 °C for 60 min, as shown in Figure 6d. The gaseous products of the reaction were analyzed by micro GC. The GC result shows that there was no nitrogen oxides left in the gaseous products, thus NO decomposition rate was still nearly 100% in the 60 min. The phase analysis of denitrification product was analyzed by X-ray diffractometer and Ultraviolet-visible spectroscopy (UV-Vis) (shown in Figure 7). It can be seen from Figure 7a, XRD phase analysis indicates that the denitrification products mainly were cristobalite and cuprite. The The result of the UV-Vis absorbance analysis (Figure 7b) reveals that the maximum absorbance of Cu2O in the visible light range (400-800 nm) occurred at around 470 nm, which is consistent with nanometer Cu2O [23][24][25]. The results of XRD and UV-Vis indicate that during the increasing of the temperature, the porous structure of the Cu-Al/SiO2 porous material collapsed, and Cu-Al/SiO2 porous material decomposed to Cu2O, Si2O, and very little aluminum oxide. CuO in the porous material decomposed to Cu2O and O2 at high temperature. The results of XRD and UV-Vis analysis show that the CuO in Cu-Al/SiO2 porous material decomposed to Cu2O and O2, and then the material reached a high NO decomposition rate at high temperature. Therefore, the Cu(I)O was considered as the active phase, and the denitrification mechanism of the Cu-Al/SiO2 porous material was proposed, being that the NO decomposition is a redox process between Cu(II)O and Cu(I)O [26]. Several excellent reviews [27][28][29] have been published for establishing the reaction mechanism of NO decomposition over Cu-zeolites. The denitrification mechanism of the Cu-Al/SiO2 porous material is thought to be similar to that of the Cu-zeolites in this paper. As for the reaction mechanism of the NO decomposition, various experimental and theoretical models have been suggested, and considerable evidence has been provided to indicate that Cu(I) species participate in the NO decomposition reaction [30][31][32][33][34]. There are some discrepancies among the concepts of the form of the intermediate active sites. However, in our opinion, there is no doubt that the NO decomposition is a redox process. The results of XRD and UV-Vis analysis show that the CuO in Cu-Al/SiO 2 porous material decomposed to Cu 2 O and O 2 , and then the material reached a high NO decomposition rate at high temperature. Therefore, the Cu(I)O was considered as the active phase, and the denitrification mechanism of the Cu-Al/SiO 2 porous material was proposed, being that the NO decomposition is a redox process between Cu(II)O and Cu(I)O [26]. Several excellent reviews [27][28][29] have been published for establishing the reaction mechanism of NO decomposition over Cu-zeolites. The denitrification mechanism of the Cu-Al/SiO 2 porous material is thought to be similar to that of the Cu-zeolites in this paper. As for the reaction mechanism of the NO decomposition, various experimental and theoretical models have been suggested, and considerable evidence has been provided to indicate that Cu(I) species participate in the NO decomposition reaction [30][31][32][33][34]. There are some discrepancies among the concepts of the form of the intermediate active sites. However, in our opinion, there is no doubt that the NO decomposition is a redox process. Thus, Figure 8 described the proposed denitrification mechanism of the Cu-Al/SiO 2 porous material at high temperature [31]. As we can see in Figure 8, during the process of heating, a pair of Cu(II) ions would lose an oxygen ion above 500 • C, a pair of Cu(I) ions were generated, and a Cu(I) active center was formed. Then, two NO molecules were adsorbed on the pair of Cu(I) ions, and an electron of Cu(I) ion was easily lost and went into the NO antibonding orbital, which formed an intermediate of adsorbed anion pairs (Cu 2+ -NO − ). NO molecules took one electron from the Cu(I) ions, then bond length of N−O became longer, and the bond of N-O became easier to break, and finally the anion decomposed to N 2 and O 2 molecules. Cu(II) ion was formed due to losing an electron of the Cu(I) ion. Then Cu(I) ions active site was re-generated from the Cu(II) ion easily at temperatures above 500 • C. This denitrification mechanism is consistent with experimental results, thus the denitrification mechanism of Cu-Al/SiO 2 was thought to be that the NO decomposition is a redox process between Cu(II)O and Cu(I)O.
Materials 2020, 13, x FOR PEER REVIEW 9 of 11 Thus, Figure 8 described the proposed denitrification mechanism of the Cu-Al/SiO2 porous material at high temperature [31]. As we can see in Figure 8, during the process of heating, a pair of Cu(II) ions would lose an oxygen ion above 500 °C, a pair of Cu(I) ions were generated, and a Cu(I) active center was formed. Then, two NO molecules were adsorbed on the pair of Cu(I) ions, and an electron of Cu(I) ion was easily lost and went into the NO antibonding orbital, which formed an intermediate of adsorbed anion pairs (Cu 2+ -NO − ). NO molecules took one electron from the Cu(I) ions, then bond length of N−O became longer, and the bond of N-O became easier to break, and finally the anion decomposed to N2 and O2 molecules. Cu(II) ion was formed due to losing an electron of the Cu(I) ion. Then Cu(I) ions active site was re-generated from the Cu(II) ion easily at temperatures above 500 °C. This denitrification mechanism is consistent with experimental results, thus the denitrification mechanism of Cu-Al/SiO2 was thought to be that the NO decomposition is a redox process between Cu(II)O and Cu(I)O.

Conclusions
The Cu-Al/SiO2 porous material with the template of cetyltrimethylammonium bromide (CTAB) had a better NO decomposition rate than materials with other templates when the temperature was above 500 °C, and the NO decomposition rate could approach 100% at high temperatures above 750 °C. The structure analysis indicates that the prepared Cu-Al/SiO2 material structure was a mesoporous structure, and average pore diameter was 4.3119 nm. When adding a small amount of Cu-Al/SiO2 porous material (5%) into cement raw materials, the NO decomposition rate still could reach over 80% above 800 °C. In addition, the NO decomposition rate increased with the incremental addition of proportions of Cu-Al/SiO2 porous material. The XRD and UV-Vis results of the denitrification product show that the Cu-Al/SiO2 porous material mainly decomposed to Cu2O and Si2O. The CuO in the Cu-Al/SiO2 porous material decomposed to Cu2O and O2 at temperatures above 500 °C. The Cu(I)O was considered as the active phase. The redox process between Cu(II)O and Cu(I)O was thought to be the denitrification mechanism of the Cu-Al/SiO2 porous material.

Conclusions
The Cu-Al/SiO 2 porous material with the template of cetyltrimethylammonium bromide (CTAB) had a better NO decomposition rate than materials with other templates when the temperature was above 500 • C, and the NO decomposition rate could approach 100% at high temperatures above 750 • C. The structure analysis indicates that the prepared Cu-Al/SiO 2 material structure was a mesoporous structure, and average pore diameter was 4.3119 nm. When adding a small amount of Cu-Al/SiO 2 porous material (5%) into cement raw materials, the NO decomposition rate still could reach over 80% above 800 • C. In addition, the NO decomposition rate increased with the incremental addition of proportions of Cu-Al/SiO 2 porous material. The XRD and UV-Vis results of the denitrification product show that the Cu-Al/SiO 2 porous material mainly decomposed to Cu 2 O and Si 2 O. The CuO in the Cu-Al/SiO 2 porous material decomposed to Cu 2 O and O 2 at temperatures above 500 • C. The Cu(I)O was considered as the active phase. The redox process between Cu(II)O and Cu(I)O was thought to be the denitrification mechanism of the Cu-Al/SiO 2 porous material.