Introduction

Nitrogen oxide (NOx) is one of the key factors causing air pollution, and its main components are nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O)1,2,3. The respiratory system and health of humans and animals can be seriously threaten by these harmful gases4. Meanwhile, it also causes great harm to the atmospheric environment, such as acid rain and photochemical smog and other severe natural phenomena that seriously damage the atmospheric environment5,6,7. The sources of nitrogen oxides are emissions from industrial production processes, human activities and the burning of fossil fuels, of which fuel combustion emissions account for more than 90% of the total emissions8,9,10.

Catalytic removal is considered to be one of the most promising methods for complete reduction of NOx11. Selective catalytic reduction (SCR) is the most mature, effective and widely used flue gas denitrification technology in the world12. The oxidizing NOx in the flue gas can react with the reducing gas to effectively remove NOx. Due to the NH3 is easy to obtain, good reducibility, low price, etc., so it is most commonly used in the reduction reaction gas of nitrogen oxides in industrial emissions13,14,15. Under the action of catalyst, NH3 can react with NOx in flue gas to form N2 and H2O16. Some catalysts can not only promote the reaction, but also increase the selectivity of the reaction to achieve the purpose of controlling the reaction product17.

At present, commercial SCR catalysts are still mainly V2O5 and MnOx based. However, the V2O5 is volatile at high temperature and has biological toxicity, and N2 has poor selectivity above 300 °C, which will produce toxic gases such as N2O, resulting in secondary pollution18,19,20. In recent years, some studies have shown that SCR catalysts based on Cu have satisfactory catalytic activity. Chen et al. prepared Cu/MCM-22 catalyst by impregnation method, and the results showed that the catalyst exhibit excellent hydrothermal resistance due to the doping of Cu element21. Liu et al. prepared Cu/TiO2 catalysts added with Eu by sol–gel method, and found that the growth of crystals can be inhibited by the interaction of Eu and Cu, thus, promoting the improvement of reduction ability22. Chen et al. synthesized Cu-Ce-La-SSZ-13 molecular sieve catalyst by one-step method, and the results showed that the catalyst had 97% denitrification activity at 200–340 °C23. Meanwhile, some relevant researchers also paid attention to SCR catalysts for cupric oxides. Dong et al. prepared Cu/TiO2 catalysts and found that Cu coordinated with four adjacent oxygen atoms on the tetrahedral TiO2 (101) crystal surface, resulting in high catalytic activity24. In addition, Yu et al. used CuSO4 as the precursor system to prepare Cu/TiO2 catalyst in order to improve catalytic activity, and the presence of S-OH on the surface was favorable for improving catalytic activity25. However, Cu-based catalysts have relatively ideal applications in molecular sieve catalysts, but the activity of Cu-oxide catalysts is not very ideal.

Cerium based catalysts have attracted much attention because of their economy, excellent oxygen storage capacity and excellent REDOX performance26. Actually, there are still some problems in the application of pure CeO2/TiO2 catalysts in SCR. Although CeO2 has a good ability to store and release oxygen due to the mutual transformation between Ce4+ and Ce3+ valence states, studies have shown that its total acid content on the surface is only 89 μmol/g, and the surface acidity is weak, which can not effectively adsorb NH327,28. Lu et al. preparation of SCR catalyst by loading MnCe composite oxide on HZSM-5 molecular sieve. The results showed that the catalyst had a wide active temperature window of 200–400 °C in NH3-SCR reaction, and the conversion rate of NO was > 80%. Meanwhile, more Lewis acid sites were the main reason for the high activity of the catalyst29. Ye et al. prepared a series of MnOx-CeO2 catalysts and found that toluene inhibited the Lewis acid site on the catalyst surface, resulting in a significant decline in catalyst activity30. Thus, how to increase the surface acidity of CeO2-based catalyst is the key to improve the activity of catalyst.

Therefore, the effect of Cu on the structure of CeO2/TiO2 catalyst is studied by X-ray diffraction (XRD), transmission electron microscope (TEM) and pyridine infrared radiation (py-IR). Meanwhile, analysis techniques such as temperature programmed desorption (TPD) and in situ diffuse reflection infrared spectroscopy (DRIFTs) are used to investigate the effect of Cu doping on NH3-SCR catalytic performance of CeO2/TiO2 catalyst was investigated.

Results and discussion

Structure characterization

The XRD patterns of the four catalyst samples are shown in Fig. 1. It can be found that the main phase is anatase TiO2, which is consistent with the research of Liu and Jiang31,32. It indicates that catalyst was supported by TiO2, and the addition of CeO2 has no significant effect on the structure of TiO2. The characteristic peak of CeO2 was not observed, indicating that Ce entered the crystal lattice of TiO2 or existed on the surface of TiO2 in a highly dispersed form33. Furthermore, compared with CeTi samples, the peak strength of anatase TiO2 in Cu-doped CeTiCu0.1, CeTiCu0.2 and CeTiCu0.3 samples was significantly reduced, indicating that Cu doping promoted the interaction between several components, thereby reducing the crystallinity of the catalyst. This is beneficial to increase the specific surface area of the catalyst34. Figure S1 shows the TEM morphology of the catalyst samples. It can be seen that the catalysts are all nanoparticles agglomerated together, the size of which is about 6–10 nm, indicating that Cu doping has no effect on the overall morphology of the catalyst samples.

Fig. 1
figure 1

The XRD patterns of the CeTi, CeTiCu0.1, CeTiCu0.2 and CeTiCu0.3 catalysts (a); Optimized structures of (b,c) TiO2 (001) and (d,e) Ce, Cu-TiO2 (001) surface.

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The (001) surface of anatase TiO2 has been proved to possess excellent reactivity35,36. So, a (3 × 3 × 1) slab model of TiO2 (001) is cleaved from anatase TiO2 unit cell for DFT calculations, as shown in Fig. 1b,c. CeO2 crystal is not detected by XRD results on the CeO2-TiO2 system in the experiment, which proves that the Ce elements entered the crystal lattice of TiO2. Thus, the CeO2/TiO2 (001) model is constructed by replacing a Ti atom in the TiO2 (001) slab center with a Ce atom. To investigate the effect of Cu doping on NH3 adsorption over CeO2/TiO2 (001) surface, we construct Cu-CeO2/TiO2 (001) surface by replacing a Ti atom in the CeO2/TiO2 (001) surface with a Cu atom. The corresponding optimized structure is shown in Fig. 1d,e.

Figure 2 shows the high-resolution TEM (HRTEM) image of a selected region of the sample. It can be seen that the D-spacing of (101) and (103) is determined to be 3.58 Å and 2.71 Å, respectively, which is larger than the D-spacing of (101) and (103) for the standard TiO2, the values of 3.46 Å and 2.39 Å. This indicates that part of Ce and Cu enter the TiO2 lattice.

Fig. 2
figure 2

The TEM image of CeTiCu0.3 (a), HRTEM of CeTiCu0.3 (b–d).

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N2 adsorption and desorption curve of catalyst was studied, its specific surface area was calculated by BET, and its average pore diameter was analyzed by BJH method, the results are shown in Fig. 3 and Table 1. The N2 absorption and desorption curves of the four samples all belong to typical type IV, and the hysteresis ring is H1, indicating that the synthesized catalyst is an ordered mesoporous material37,38. And doping of Cu on the CeTi catalyst has no obvious effect on the structure. The CeTi catalyst sample has the smallest N2 adsorption capacity, and its specific surface area is 86.96 m2/g, pore volume is 0.089 cm3/g, and its average pore size is 4.49 nm. In Cu-doped CeTi catalyst samples, the adsorption amount of N2 is significantly increased, and the same increasing of specific surface area. The specific surface area of CeTiCu0.1 and CeTiCu0.2 samples were 116.23 m2/g and 119.21 m2/g, and the pore volume was 0.118 cm3/g and 0.119 cm3/g, respectively, indicating no significant difference between the two samples. When the Cu doping amount is 0.3, the specific surface area of the sample is 130.80 m2/g. From the perspective of average pore diameter, with the increase of Cu doping amount, the average pore diameter of the three samples doped with Cu decreases gradually. The average pore diameter of the three samples doped with Cu is 4.2 nm, 3.75 nm and 3.66 nm, respectively. The increase of specific surface area of Cu doping catalyst is mainly related to the variation of crystallinity of the sample39.

Fig. 3
figure 3

The N2 adsorption and desorption curves of the catalyst. (a) CeTi, (b) CeTiCu0.1, (c) CeTiCu0.2, (d) CeTiCu0.3, the pore diameter of (a1) CeTi, (b1) CeTiCu0.1, (c1) CeTiCu0.2, (d1) CeTiCu0.3

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Table 1 The specific surface area, pore volume and average pore size of different samples.
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Catalytic activity

The catalytic activity of the four catalyst samples was tested, and the results were shown in Fig. 4. It can be seen that the NO conversion of CeTi sample was the lowest, while the that of CeTiCu0.1 and CeTiCu0.2 samples was increased, and there was no significant difference between them, and CeTiCu0.3 showed the best catalytic activity. SCR activity was assessed using T50 and T90 (temperatures at 50% or 90% NO conversion). The T50 of CeTi sample is 200 °C, while the T50 of CeTiCu0.3 is 150 °C, indicating that Cu doping significantly improves the catalytic activity at low temperature. Meanwhile, the T90 of CeTiCu0.3 is about 260 °C, while the maximum NO conversion of other samples is lower than 90%. The results show that Cu doping can significantly broaden the temperature window of SCR catalyst and enhance the catalytic activity of SCR.

Fig. 4
figure 4

NO conversion (a) and Arrhenius plots (b) and N2 selective of different catalyst samples in the SCR reaction.

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Figure 4b shows the Arrhenius diagram of the four catalysts in the NH3-SCR reaction, from which the apparent activation energy (Eapp) values of the corresponding catalysts are calculated40. The Eapp of CeTi catalyst is the largest, which is 23.14 kJ/mol. With the doping of Cu, the Eapp of the catalyst becomes smaller and smaller, from 22.54 kJ/mol of CeTiCu0.1 to 19.04 kJ/mol of CeTiCu0.3. These results indicate that Cu doping improves the kinetic conditions of the reaction. Meanwhile, the N2 selectivity of the different catalysts was shown in Fig. 4c, as can be seen that the N2 selectivity of CeTiCu0.3 was above 96% in the whole testing temperature range of 50–550 °C. This is consistent with the catalytic activity of the sample.

Catalytic mechanism

The acid content is the key to the performance of SCR catalyst, so the acid analysis of four samples was carried out by using pyridine-infrared spectroscopy. The sample pre-adsorbing pyridine was scanned by 1000–2000 cm−1, and the results were shown in Fig. 5. 1445 cm−1 and 1615 cm−1 belong to pyridine adsorbed at Lewis acid site of catalyst sample, while 1543 cm−1 and 1635 cm−1 correspond to pyridine adsorbed at Brønsted acid site41,42. The absorption peak near 1490 cm−1 is the pyridine adsorbed by Lewis and Brønsted acidic sites43. It can be found that the Lewis acid is the main acidic site of CeTi catalyst and Cu-doped CeTi catalyst. However, Cu doping significantly enhanced the acidic sites of catalyst samples, especially the Lewis acidic sites, especially in the temperature of 350 °C. The adsorption of pyridine by CeTiCu0.3 at two temperature points did not produce significant differences, indicating that the catalyst has a wider temperature window and is more suitable for medium–low temperature SCR reaction. In the NH3-SCR reaction, NH3 is first adsorbed at the Brønsted or Lewis acid site of the catalyst, and then reacts with gaseous NO or adsorbed NO44. More acid sites are favorable for NH3 adsorption and activation, which may lead to enhanced catalytic activity.

Fig. 5
figure 5

Py-IR spectra of CeTiCu catalysis after desorption pyridine at (a) 200 °C and (b) 350 °C, Optimized structure for the adsorsption of NH3 on (c) Lewis acid sites and (d) Brønsted acid sites over Ce, Cu-TiO2 (001) surface.

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Moreover, the adsorption statuses of NH3 on the surface of Cu-CeO2/TiO2 (001) are calculated by DFT calculations. Optimized structures for the adsorption of NH3 on the Lewis acid site and Brønsted acid site over Cu-CeO2/TiO2 (001) surface are shown in Fig. 5c,d. The adsorption energies of NH3 on the Lewis and Brønsted acid sites are − 4.21 eV and − 1.33 eV, respectively. These adsorption energies of NH3 on the two sites are negative, indicating that the adsorption process of NH3 on Cu-CeO2/TiO2 (001) surface is exothermic and can proceed spontaneously.

The amounts of Brønsted and Lewis acid sites are list in Table 2. It can be seen that the surface acid content of CeO2/TiO2 catalyst without Cu doping is the lowest. With the doping of Cu, the Brønsted and Lewis acid sites on the catalyst surface are significantly increased. The change trend of 200 °C and 350 °C is basically the same.

Table 2 The amounts of Brønsted and Lewis acid sites.
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The valence states of the elements on the catalyst surface were characterized by XPS. Figure 6a shows the XPS spectrum of survey, and Fig. 6b–e shows the spectrum of Ce 3d, O 1 s, Cu 2p and Cu LMM, respectively. The corresponding analysis results are shown in Table 3. Figure 6a shows that the sample consists of Ce, Ti, O, and Cu. Figure 6b shows the XPS spectrum of Ce 3d. It can be seen that the Ce 3d spectrum consists of 4 pairs of spin orbit bipeaks, including 8 peaks45. The peaks labeled v1 and u1 represent the initial electronic state of 3d104f1 belonging to Ce3+, and the other peaks represent the initial electronic state of 3d104f0 belonging to Ce4+46. Figure 6c shows the spectrum of O 1 s XPS. It can be seen that three peaks are unfolded at 529.7 eV, 531.9 eV and 533.4 eV, which belong to lattice oxygen (denoted Olat), surface chemisorbed oxygen (denoted Osur) and hydroxyl or adsorbed water (denoted Oads), respectively47,48. The content of surface chemisorbed oxygen Osur can be used to characterize the number of surface oxygen vacancies and defect sites49. The surface adsorbed oxygen has a high mobility, which makes NO have a high activity in the oxidation–reduction reaction during the SCR process50. Therefore, the oxygen vacancy derived from it is conducive to reducing the N–O bond energy, which plays a positive role in the SCR reaction. Figure 6d shows the distribution of Cu on the surface of the catalyst. It can be found in the figure that the peak at 934.5 eV and the oscillating satellite at 953.5 eV indicate the presence of Cu2+, while the peak at 932 eV belongs to Cu+51. Therefore, it is inferred that CuO and Cu2O are the main Cu forms. Meanwhile, the Fig. 6e shows the spectrum of Cu LMM. It can be seen that almost all Cu exists in the form of Cu2+ in CeTiCu0.1 sample, and the Cu+ content increases gradually with the increase of Cu doping amount. Some studies have shown that a higher Cu+/(Cu0 + Cu+ + Cu2+) ratio is conducive to improving the adsorption capacity of the catalyst to the reaction gas.

Fig. 6
figure 6

XPS spectra of (a) survey, (b) Ce 3d, (c) O 1 s, (d) Cu 2p and (e) Cu LMM.

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Table 3 Distribution information of chemical elements on the surface of catalyst samples.
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Table 3 shows the distribution information of various elements on the catalyst surface. With the doping of Cu, Ce3+/Ce in samples increased from 23.09% to 29.42%, and the proportion of Cu+ also gradually increased. There are two main reasons for this phenomenon: On the one hand, mainly due to the electron transfer between Ce2+ and Ce3+ caused by Cu doping (Cu2+/Cu+ and Ce4+/Ce3+ formed Cu2+, Ce3+), which reduces the concentration of Ce3+52. On the other hand, the introduction of Cu will lead to lattice distortion. In order to alleviate the lattice distortion effect, Ce4+ will be converted to Ce3+ by the following reaction: 4Ce4+ + O2− forms 2Ce4+ + 2Ce3+ + ○ + O2 (where ○ represents oxygen vacancy). Meanwhile, it can be seen that oxygen vacancies are also formed in this process.

In order to study the mechanism of the influence of Cu doping on the catalytic activity of samples, NH3-TPD was carried out on the surface acidity of four samples, the results are shown in Fig. 7. In general, the peak in the low temperature range corresponds to the desorption of NH3 at the Lewis acid site, and the other peak above the high temperature is assigned to the Brønsted acid site53. It can be seen that NH3 can be adsorbed on the Lewis and Brønsted acid sites of catalyst. The order of adsorption capacity of NH3 by catalyst is that of CeTiCu0.3 > CeTiCu0.2 > CeTiCu0.2 > CeTi, which is basically the same as the activity order of SCR. The desorption peak strength in the low temperature region of the four groups of catalysts is higher, indicating that the Lewis acid is the main acid sites of the sample. Meanwhile, Cu doping can indeed increase the Brønsted acid site of the sample, but the increase effect is far less obvious than the increase effect on Lewis acid site. This is mutually verified with the previous results of py-IR. The total acid content of catalyst obtained by NH3-TPD is shown in Table 4.

Fig. 7
figure 7

NH3-TPD of different samples.

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Table 4 Total acid content of catalyst obtained by NH3-TPD.
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In addition, a large adsorption capacity appears at 150–350 °C, and then there is no obvious NH3 desorption peak at high temperature. However, with the doping of Cu, it can be obviously seen that the adsorption capacity of NH3 increases significantly, especially in the range of 150–500 °C. Therefore, it is speculated that Cu doping is conducive to the adsorption of NH3 on the catalyst surface.

CeTiCu0.3 sample has a large NH3 adsorption capacity and the best catalytic performance. During the NH3-SCR reaction, the adsorbed NH3 can react with gaseous NOx in the Eley–Rideal (E-R) mechanism or adsorbed NOx in the Langmuir–Hinshelwood (L–H) mechanism54. In order to understand the control mechanism of the catalyst, the transient reaction of CeTi and CeTiCu0.3 samples was studied by in situ DRIFTs. When NO + O2 is transferred to the catalyst that pre-adsorbed NH3, as shown in Fig. 8a and c, it can be seen that the coordination NH3 near 1600 cm−1 caused by the Lewis acid site, gradually disappeared with the introduction of NO and O2. However, by doping Cu in the catalyst, the surface acid site is increased, which proved by the NH3-TPD. it can be seen that the coordination NH3 near 1600 cm−1, 1400 cm−1 and 1183 cm−1, which is attributed to the Lewis acid site55, is rapidly consumed at the beginning of the reaction. Due to the increase of Lewis acid sites on the catalyst surface, more NH3 was adsorbed than CeTi samples. It is also similar to the improvement of catalyst activity through the establishment of complex oxide systems. Ma et al. established a composite oxide system catalyst for CeFeCuTiO, the results show that the modification can adjust surface acid distribution, increase surface oxygen content and oxidation reduction cycle56. In the CeO2-TiO2 catalyst, almost the same reaction mechanism is displayed26. NH3 adsorbed on the surface of CeO2-TiO2 catalyst has strong activity and can react with gaseous NO quickly. However, the doping of Cu further increases the Lewis acidic site, which can be accelerated the reaction. However, the absorption peak of NH4+ at the Brønsted acid site is not significant. This is consistent with the previous results of py-IR results, the catalyst is dominated by Lewis acidic sites. This indicates that NH3 is mainly adsorbed with the catalyst surface in a coordination state and is active in SCR reaction through E-R mechanism, while NH4+ has no significant reactivity.

Fig. 8
figure 8

In situ DRIFTs spectra recorded at 250 °C of NO + O2 adsorption on pre-adsorbed NH3 of (a) CeTi and (c) CeTiCu samples, NH3 adsorption on pre-adsorbed NO + O2 of (b) CeTi and (d) CeTiCu samples.

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When NH3 is transferred to the catalyst that pre-adsorption NO + O2, as shown in Fig. 8b and d. The reaction process of CeTi sample was almost the same as that of CeTiCu 0.3 sample, and Cu doping had no obvious effect. It can be seen in the figure that the corresponding bands of bidentate nitrate adsorbed at 1576 cm−1 and linear nitrate adsorbed at 1431 cm−1 appear in the infrared spectrum57. After the introduction of NH3, the bidentate nitrate absorption band (1576 cm−1) rapidly disappeared, while the NO2 absorption band (1626 cm-1) appeared, mainly due to the reduction of the bidentate nitrate to NO258. It can be seen that in the infrared spectrum at 1 min, the absorption peak of NO2 is the strongest, and it is not completely consumed over time. This is mainly because NO2 needs to combine with NH4+ in the ionic state and complete the reduction59. The py-IR analysis shows that the low Brønsted acid content of the catalyst leads to the low NH4+ content adsorbed on the catalyst surface. The results suggested that not all adsorbed NOx was capable of reacting with adsorbed NH3. Wang et al.60 and Liu et al.26 also observed the similar phenomenon.

The above results show that NH3 is adsorbed on the surface of CeTiCu catalyst in coordination form and reacts with gaseous NOx during SCR reaction. NH3-TPD, of course, confirms this conclusion. The reactions are mainly E-R mechanism. First, due to the doping of Cu, a large number of acidic sites are generated, and NH3 is adsorbed at these acidic sites, which has high reactivity. Subsequently, NH3 adsorbed at the acid site is activated by ceric oxygen to form NH2 substance. Then NH2 reacts with NO in the gaseous state to form N2 and H2O. Finally, Ce3+ is oxidized to Ce4+ by O2 and electron transfer occurs. Thus completing the cycle. Therefore, we conclude that Cu doping mainly improves the adsorption performance of NH3. The doping of Cu makes more NH3 adsorbed on the catalyst surface with coordination bonds and react with gaseous NOx, the reaction mechanism of CeTiCu catalyst was shown in Fig. 9.

Fig. 9
figure 9

Schematic diagram of SCR reaction mechanism of CeTiCu catalyst.

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Conclusions

In this study, the structure and properties of Cu-doped CeO2/TiO2 were studied. Cu doping has no significant effect on the phase composition and microstructure of CeO2/TiO2 catalyst. However, the acid sites on the catalyst surface were significantly enhanced, especially the Lewis acid sites. CeTiCu0.3 showed excellent catalytic performance at 200–450 °C. This is mainly due to the increase in the number of Lewis acidic sites, which improves the adsorption performance of NH3 coordination state and is conducive to the SCR reaction following the E-R mechanism. In conclusion, doping appropriate Cu can significantly improve the denitrification performance of CeO2/TiO2 catalyst.

Materials and methods

Catalyst synthesis

The raw materials used in this study included tetrabutyl titanate, anhydrous ethanol, glacial acetic acid, nitric acid, deionized water, cerium nitrate hexahydrate and copper nitrate trihydrate form Macklin Shanghai, China. All raw materials were used as received.

6.8 ml tetrabutyl titanate was mixed with 17 ml anhydrous ethanol to obtain solution A. Solution B was obtained by stirring 17 ml anhydrous ethanol, a certain amount of deionized water, 3.4 ml glacial acetic acid, 2 ml of cerium nitrate solution with 3 mol/L and a certain amount of copper nitrate. Adding with nitric acid to adjust the pH value of solution B to 2.5. Then, the solution B was added to solution A by drops at a stirring speed of 250 ~ 500 r/min. The complete gel was prepared by 70 °C water bath. The solid gel was dried at 80 °C for 4 h and sintered at 500 °C for 5 h to obtain CuCeTi composite catalyst. The synthesis process of CeTiCu catalyst is shown in Fig. 10. The amount of raw materials used in the synthesis of the four samples is shown in Table S1.

Fig. 10
figure 10

Schematic diagram of the CeTiCu catalyst synthesis process.

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Catalyst characterization

Characterization of microstructure and phase composition

The phase composition was determined by the X-ray diffractometer (XRD, Rigaku, SmartLab) at a 2θ range of 10–50°and a speed of 4°/min, with a voltages and currents of 40 kV and 30 mA. Transmission electron microscope (TEM, JEM-2100 F, JEOL, Japan) was used to observe the microstructures of the catalysts.

Characterization of physical and chemical properties

N2 adsorption–desorption isotherm test (Micromeritics, ASAP2460) was performed on the catalysts at 77 K after vacuum degassing at 350 °C for 5 h. The specific surface areas (SBET) of the samples were calculated using the Brunauer–Emmett–Teller (BET) equation. The mesopore size distribution in the samples was calculated using the Barret-Joyner-Halenda (BJH) method. The surface composition and relative content of catalyst were analyzed by X-ray electron spectrometer (XPS, Thermo, ESCALAB 250). Al Kα was used as the radiation source and C1s peak (B.E. = 284.8 eV) was used as the calibration binding energy.

Evaluation of catalyst acid sites

Temperature programmed desorption (NH3-TPD) was used to analyze the acidity of the catalyst surface. The 100 mg catalyst was placed in the sample pool and pre-treated with high purity He at 500 °C for 30 min. NH3 was adsorbed at room temperature for 60 min, and then He was used to purge for 30 min to remove the physically adsorbed NH3. It is heated to 800 °C at a heating rate of 10 °C/min under He atmosphere. Pyridine adsorbed IR spectroscopy (py-IR) was carried out using an FT-IR spectrometer (Bruker, Tensor27). 50 mg sample was first pretreated at 350 °C in a vacuum for 2 h, followed by adsorption of pyridine vapor at room temperature for 30 min. The sample was then heated to 200 °C or 350 °C and maintained for 1 h to desorb pyridine. After that, the sample was cooled to room temperature, and the py-IR spectra were recorded at a resolution of 1 cm-1. The amounts of Brønsted and Lewis acid sites were calculated from Py-IR spectra by following equations.

$$C(pyridine on B sites)=1.88 IA(B)\frac{{R}^{2}}{W}$$
(1)
$$C(pyridine on L sites)=1.42 IA(L)\frac{{R}^{2}}{W}$$
(2)

where C = concentration (mmol/g catalyst), IA (B or L) = integrated absorbance of B or L band (cm−1), R = radius of catalyst disk (cm), W = weight of disk (mg).

In situ diffuse reflectance infrared Fourier transform spectroscopy

Fourier transform infrared spectroscopy equipped with MCT detector is used to detect in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). The catalyst samples were placed in the reaction tank and pre-treated at 350 °C for 1 h in N2 atmosphere. During the cooling process, the background spectrum is collected at the desired temperature. Subsequently, 1000 ppm NH3 + N2 was absorbed for 1 h, after saturation adsorption, N2 was pursed for 1 h, and then 1000 ppm NO + 5%O2 + N2 was injected, the spectrum was scanned every 1 min, and the reaction was carried out for 15 min. The two kinds of gases were exchanged and infrared spectra were collected again.

Evaluation of catalytic activity

The catalyst samples were placed in a quartz tube reactor at a test temperature of 50–550° C with an inlet speed of 200 mL/min, corresponding to a gas space velocity (GHSV) of 60,000 mL/(g∙h). The contents of gas were 500 ppm NH3, 500 ppm NO, 10 vol% O2, and N2 in balance. The gas components were continuously monitored using the FT-IR spectrometer (Bruker, Tensor27). NO conversion rate (%) and N2 selectivity (%) were calculated using Eqs. (3) and (4).

$$NO \;conversion \left(\%\right)=\frac{{C}_{NO}^{in}-{C}_{NO}^{out}}{{C}_{NO}^{in}}$$
(3)
$${N}_{2} \;selectivity \left(\%\right)=\left(1-\frac{2{C}_{{N}_{2}O}^{out}}{{C}_{NO}^{in}+{C}_{{NH}_{3}}^{in}-{C}_{NO}^{out}-{C}_{{NH}_{3}}^{out}}\right)$$
(4)

The kinetic analysis method is described in detail in the Text S-1 in Supplementary Information.

Density functional theory calculations

The calculation of the DFT is described in detail in the Text S-1 in Supplementary Information.