Preparation of Cu-BTC Supported Fly Ash-Based Catalyst and Research on Low-Temperature SCR Denitration Performance

The novel Cu-BTC/FA materials were synthesized with Cu-BTC as the active component and solid y ash as the catalyst carrier. then developed as the NO removing catalysts for low temperature selective catalytic reduction (SCR) with NH 3 . The physic-chemical properties of catalyst samples were characterized by multiple techniques, such as SEM, XRD, FTIR, XPS, TPR and Raman. The results show that: (1) The active component Cu-BTC loading ratio is 9%, the denitration performance is excellent and has strong stability. Under this condition, the catalyst showed the highest NH 3 -SCR activity, giving 94% NO conversion. (2) The catalysts show the good cyclic and stability in SCR denitration under 200 ℃ . (3) It still shows high NO removal rate and good stability under the condition of SO 2 , and the denitration rate is as high as 78.36%. (4) There is a certain degree of resource integration for carrier y ash, which not only treats air pollutants to a certain extent, but also achieves the purpose of solid waste recycling. y ash as an active component to prepare a Cu-BTC/FA catalyst. Use a variety of characterization methods to analyze, determine the morphology, structure and properties of the catalyst, and analyze its impact on the denitration performance. The results show: The optimal Cu-BTC/FA was prepared and it was proved to have excellent catalytic activity in the temperature range of 100-300 ℃ . When the active component Cu-BTC loading ratio is 9%, the denitration effect is as high as 94%. The denitration performance is excellent and has strong stability. The Cu-BTC/FA catalyst also shows a high NO removal rate and good stability under the condition of SO 2 . The catalyst has good stability and cyclability. After 6 tests, the activity of the catalyst can still be maintained about 72%. Also, stability test of up to 6 h is carried out, and its denitration eciency is maintained above 75%. According to XPS and H 2 -TPR test results, the main metal active species of the catalyst is Cu 2+ /Cu + . When the temperature continues to rise, the main active


Introduction
The waste gas pollution of coal chemical enterprises has the characteristics of a wide range of emissions, a large amount of emissions, and the waste gas often contains toxic and harmful substances, which form a certain obstacle to the overall development of the social economy [1][2][3]. At the same time, it has a great negative impact on the living environment of mankind. The crude coal gas produced by the coking furnace passes through the condenser and the raw gas produced enters the gas boiler [4]. The main pollutants produced are sulfur dioxide (SO 2 ), nitrogen oxides (NO x ) [5]. The discharge of these pollutants into the atmosphere has caused serious environmental problems. Therefore, the treatment of NOx is imminent [6][7].
At present, the most widely used NOx removal technology is selective catalytic reduction method in which the reducing agent selectively reacts with the NOx in the ue gas in the presence of a catalyst, and nally reduces the NOx to harmless N 2 and H 2 O [8 -9]. The reaction temperature of this method is low, generally not higher than 400°C [10]. It is currently recognized at home and abroad with a wide range of applications and better denitration performance [11].
As a key factor for SCR denitration, catalyst is also an important reason for the success or failure of the entire catalytic reaction. In commercial operation, a catalyst with good performance not only has strong Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js stability, high NO conversion rate and wide temperature window, but also has good sulfur and water resistance [12][13]. According to the different active components of the catalyst, the catalyst is divided into noble metals catalysts, molecular sieve catalysts, metal oxide catalysts. Noble metals have good stability and good low-temperature catalytic activity, and high denitration e ciency [14].
At present, the more mature catalysts for industrial applications are mainly the mixture of vanadiumtitanium base and molybdenum trioxide or tungsten trioxide [15]. This catalyst is mainly active in the temperature range of 380-450℃, and its pores will be formed by side reactions when the temperature is lower than 420℃ [16][17][18]. The blockage of ammonium sulfate causes a decrease in catalytic activity, and it is easy to leak vanadium-containing compounds into the atmosphere at high temperatures, which reduces the denitri cation performance and also causes secondary pollution to the environment [19][20].
Therefore, it is imperative to develop a catalyst with good low temperature effect and high NO removal rate.
Metal-organic framework (MOFs) materials are a kind of open crystal framework materials with adjustable pore size and permanent porous structure that are assembled by metal-containing units and organic ligands under certain conditions [21][22]. Because of its ultra-high speci c surface area, strong adsorption capacity, multifunctional metal center and modi able pores, it is widely used in many elds [23][24]. Among them, in the catalytic process, on the one hand, the high speci c surface of the MOFs material is conducive to the adsorption and enrichment of the substrate molecules around the active center. On the other hand, the structural properties of inorganic-organic hybrid MOFs provide the possibility to form one or more catalytic centers in a single pore. Therefore, MOFs are a new type of solid catalytic material with broad application prospects [25].
There are certain researches on the application of metal-organic framework to SCR denitration. At present, the prepared MOFs are compressed for denitration and have a good treatment effect, but the preparation time is too long, the preparation amount is small, and the cost is too high. Therefore, the bottleneck encountered at present is how to industrialize it.
As a kind of solid waste with relatively large output, y ash needs to be resolved in a reasonable manner. Fly ash is a special powder mixed with regular particles and porous particles, inorganic and organic substances, hollow particles and solid particles [26]. It is a char-like particle with a complex shape and a porous surface formed due to the volatilization and chemical reaction of the carbon particles during the combustion process. The speci c surface area is about 0.8-2.4 m 2 g −1 . Fly ash is mainly composed of SiO 2 and a small amount of CaO, Al 2 O 3 , Fe 2 O 3 , MgO, and has stable physical and chemical properties and a more complex composition structure [27]. The application of y ash to the preparation of catalytic materials is an important way to realize its high added value utilization. The change of its composition and structure has a great impact on the performance of the catalyst. Different active components and structures correspond to different reaction types and have a very wide range of adaptability [28][29]. Due to the relatively stable physical and chemical properties of y ash, more and more researchers use it as a catalyst or catalyst carrier for research. Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js In this paper, a hydrothermal method is used to prepare Cu-BTC materials, with solid waste y ash as the carrier. Cu-BTC is used as the active component and loaded on y ash to prepare Cu-BTC/FA catalyst for low-temperature selective denitration. It not only improves the utilization rate of y ash, but also reduces pollution. The purpose of improving the environment and secondary use. And most importantly, it provides a feasible basis for the industrial application of y ash ue gas denitration loaded with Cu-BTC, which has strong practical signi cance.

Preparation of Cu-BTC/FA catalyst
Mix y ash and Ca-based bentonite in a 4:1 ratio, add 20 wt% deionized water to knead into a dough, put it into the catalyst molding extrusion device, extrude it, and place it in a 105°C electric blast drying oven for drying 30 min to obtain a columnar y ash carrier. Weigh 0.5 g y ash carrier, load a certain amount of Cu-BTC, and dry it in a vacuum drying oven for 10min to obtain Cu-BTC/FA catalyst. The preparation of Cu-BTC/FA catalyst are shown in Fig. 1.

Catalyst evaluation device
The activity evaluation of the catalyst was carried out in a vertical denitration experimental device.
Simulated gas was introduced to observe the changes of NO data. Fig. 2 is the catalyst evaluation device and experimental ow chart.
The calculation method of the denitration rate is:

Experimental characterization
XRD: The sample was tested by XD-3 X-ray diffraction analyzer to obtain an XRD pattern.
XPS: The X-ray photoelectron spectrometer model used in this paper was: ESCALAB250 (manufactured by Thermo Fisher Scienti c, USA). The instrument had a sensitivity of 180 KCPS, an energy resolution of 0.45eV (Ag), and an image resolution of 3µm. The results of the analysis were all corrected with C1s.  It can be seen that the catalyst prepared by Cu-BTC supported y ash has a higher denitration effect. The denitration rate of the traditional y ash blank catalyst is about 20%. Cu-BTC/FA has the best denitration performance, the denitration rate gradually stabilizes, up to 94%. The best loading ratio of the catalytic activity of Cu-BTC/FA catalysts is:9%. The reason is that the load is too small, the active sites are not evenly loaded, so the denitration effect is not optimal, but when the load is too large, the active sites occupy more positions, and the load will appear piled up, which will affect the denitration effect. Therefore, the optimal loading ratio for preparing Cu-BTC/FA is 9%. When 600 ppm SO 2 is introduced, the activity of the catalyst is affected, and the removal rate of NO is reduced. After stopping SO 2 , the removal rate is rebounded. The results show that the Cu-BTC/FA catalyst exhibits a higher NO removal rate and good stability under the condition of SO 2 . that the catalyst still maintains a denitration effect of up to 70%. At the same time, a stability test of up to 6 h is carried out, and its denitration e ciency is maintained above 75%. Therefore, it shows that the catalyst has good stability and cyclability. The results show that SO 2 reacts preferentially with the active components of the catalyst, occupying active sites and introducing new impurities, which is consistent with the decrease in the denitration effect of the Cu-BTC/FA catalyst caused by the introduction of SO 2 . the peaks at 1644 cm −1 , 1447 cm −1 , and 1371 cm −1 disappeared. The reason is that the three peaks are mainly the active component Cu -BTC peak, and in the catalytic reaction, the characteristic peak of the active component reacts with NH 3 or NO, causing the peak to weaken or even disappear. When SO 2 is introduced, compared with the infrared spectrum of Cu-BTC/FA after denitration, there are almost no new peaks, and the peak shapes are basically similar, which further proves that the sulfur resistance is good [31]. This can be well combined with XRD to explain its denitration mechanism. Figure 6 is the electron micrograph of the Cu-BTC/FA catalyst before and after denitration and after sulfur resistance. Figure 6(a) is the Cu-BTC/FA (fresh) catalyst, 6(b) is the Cu-BTC/FA (inactived) catalyst, and 6(c) is the Cu-BTC/FA (desulfurization) catalyst. It can be seen that the Cu-BTC/FA (inactived) catalyst has reduced the gaps between particles, agglomeration has occurred on the surface of the catalyst, and the distribution of surface components is not uniform even before the reaction. The reason is that the ammonium salt produced during the SCR reaction covers the surface of the catalyst, reducing the gap between the particles and the pore structure of the catalyst. After a certain amount of SO 2 is introduced into the system, the active components in the catalyst will react with the element S to form sulfur salts, which makes the reduction of the pore structure of the catalyst surface more obvious. Therefore, there are still small particles on the catalyst surface as fresh catalysts. The reduction of the voids between the particles comes from the aggregation of sul de salts and sulfate salts.

XPS
In order to further reveal the element composition and valence state of the Cu-BTC/FA catalyst surface, the Cu-BTC/FA catalyst with better denitration and anti-sulfur effects was characterized by XPS. As shown in Fig. 7, the XPS full spectrum con rmed that the catalyst surface contains Cu, O and C elements. The characteristic peak position of C 1s is 284.6 eV as the measurement standard for XPS characterization.
It can be seen from the Cu2p XPS spectrum that the Cu2p binding energy of the Cu-BTC/FA catalyst is composed of Cu2P 3/2 , Cu2P 1/2 , and the characteristic peaks of 895 eV and 979 eV are attributed to Cu2P 3/2 , Cu2P 1/2 . The sample has an obvious satellite peak at the binding energy of 938.8-944.2 eV, which is a characteristic peak of Cu 2 + d 8 orbital electron arrangement, indicating that the surface of the sample contains Cu 2+ species. It is reported in the literature that in the SCR denitration reaction, NH 3 and NO molecules are easily adsorbed on the Cu 2+ and Cu + on the surface of the catalyst to react [32][33]. At the same time, Cu 2+ and Cu + are also considered to be the main metal active sites in the SCR reaction. Fitting the Cu2P 3/2 and Cu2P 1/2 peaks respectively, it can be found that the sample surface contains Cu species, that is, Cu 2+ and Cu + coexist, and the relative content of the two-valence species is not much different. After the reaction, the positions of the main peaks all shifted to high binding energy, and the shift was small after anti-sulfur denitration [34]. It is speculated that SO 2 binds to the surface-active sites of the metal organic framework, resulting in a small shift of the main peak.
Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js Fitting the O1s spectra before and after the reaction showed that there were two peaks at 532 eV and 530.8 eV. The peak at 532 eV mainly exists in the form of O 2− or O − , corresponding to the surface adsorbed oxygen species O ß , the peak at 530.8 eV mainly exists in the form of O 2− , corresponding to the lattice oxygen species O α . According to literature reports, adsorbed oxygen is the most active reactive species in catalytic reactions and can signi cantly promote the oxidation-reduction reaction on the catalyst surface in SCR reactions. Based on the area integration of these two peaks, it can be calculated that the relative concentration of O ß /(O α +O ß ) in the Cu-BTC/FA material is 29.9%. Because the surface adsorbed oxygen O ß has better uidity, and can be used as an electron acceptor to react with the reaction gas NO. Therefore, Cu-BTC/FA with a higher O ß /(O α +O ß ) ratio is presumed to have higher SCR denitration performance [35]. This explains the excellent denitration performance of the catalyst in the above activity test. reduction peaks before and after the reaction. Combined with the Cu-BTC thermogravigram, it can be seen that the catalyst sample has only one obvious reduction peak in the low temperature range of 0-350°C, and the peak that appears is caused by the reduction of Cu 2 + to Cu + . When the temperature is higher than 400°C, the Cu-BTC structure will collapse [36]. The reduction peak of the catalyst after the reaction disappears at 0-350°C. The reason is that the characteristic peak of Cu-BTC participates in the catalytic reaction during the denitration process, resulting in the disappearance of the characteristic peak. The three obvious reduction peaks appearing at temperatures above 500°C are mainly caused by the reduction of other elements (Si, Al, etc.) in the catalyst carrier y ash. trimellitic acid is completely deprotonated. The characteristic peaks at 1097cm −1 and 1001cm −1 can be attributed to the vibrational stretching peak of the C=C bond on the benzene ring, and the characteristic peak near 825cm −1 is the bending vibration peak of the C-H bond on the benzene ring. The above results show that the Cu-BTC material prepared by hydrothermal synthesis method is successfully synthesized and the structure is complete.

NO reaction process
Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js Based on the XPS and H 2 -TPR characterization results, it is speculated that the NH 3 -SCR reaction process with Cu-BTC/FA as the catalyst is as follows: The reaction diagram is shown in Fig. 9.
In the low temperature stage, that is, the reaction (1) and (2)

Conclusion
The solid waste y ash was used as a carrier, and Cu-BTC was loaded on the surface of columnar y ash as an active component to prepare a Cu-BTC/FA catalyst. Use a variety of characterization methods to analyze, determine the morphology, structure and properties of the catalyst, and analyze its impact on the denitration performance. The results show: The optimal Cu-BTC/FA was prepared and it was proved to have excellent catalytic activity in the temperature range of 100-300 ℃. When the active component Cu-BTC loading ratio is 9%, the denitration effect is as high as 94%. The denitration performance is excellent and has strong stability. The Cu-BTC/FA catalyst also shows a high NO removal rate and good stability under the condition of SO 2 . The catalyst has good stability and cyclability. After 6 tests, the activity of the catalyst can still be maintained about 72%. Also, stability test of up to 6 h is carried out, and its denitration e ciency is maintained above 75%. According to XPS and H 2 -TPR test results, the main metal active species of the catalyst is Cu 2+ /Cu + . When the temperature continues to rise, the main active

Con icts of interest
There are no con icts to declare.

Figure 1
The preparation of Cu-BTC/FA catalyst Figure 2 Catalyst evaluation device   Possible reaction mechanisms for NO reduction by NH3