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. 2022 Oct 12;7(42):37694–37704. doi: 10.1021/acsomega.2c04603

Low-Dose Element-Doped CeCrM/TiO2 (M = La, Cu, Fe, LaCu, LaFe) Catalyst for Low-Temperature NH3-SCR Process: Synergistic Effect of LaCu/LaFe

Guilin Mu 1, Shuai Liu 1, Qingjie Liu 1, Shanshan Liu 1,*, Xintang Zhang 1,*
PMCID: PMC9608399  PMID: 36312429

Abstract

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A series of CeCrM/TiO2 (M = La, Cu, Fe, LaCu, LaFe) catalysts were prepared via impregnation method and are employed as low-temperature NH3-SCR catalysts. The present study investigates the low-dose element doping on the TiO2-supported catalyst to improve the NH3-SCR performance. And CeCrLaCu/TiO2 exhibited the best catalytic performance (NO conversion approaching 100% at 260–420 °C). The characterization results show that the synergistic effect of LaCu and LaFe on the catalytic performance was more obvious than that of Cu, Fe, and La alone. The doping of LaCu/LaFe decreased the specific surface area of the catalyst but increased the dispersion, surface acidity, and reducibility of the catalyst. Moreover, LaCu/LaFe promoted the formation of valence state distribution and oxygen vacancy content on the surface of the catalyst. There were more Ce3+, Cr6+, Cu+, and oxygen adsorbed on the surface of the CeCrLaCu/TiO2 catalyst. H2-TPR analysis showed that the synergistic effect of LaCu was more likely to promote the reduction of Cr and Cu and increase the reduction degree of metal oxides. However, Fe is easier to coordinate with La, thus improving the redox performance of the catalyst. Compared with CeCrLaFe/TiO2, the ammonia adsorption capacity of CeCrLaCu/TiO2 is better. Therefore, the synergistic effect of LaCu can promote the reaction performance of NH3-SCR better.

1. Introduction

Nitrogen oxides (NOx) derived from the burning of fossil fuels such as coal, oil, etc., are harmful to human beings and the environment. As one of the main polluting gases, NOx not only causes acid rain and photochemical pollution but is also a major component of PM 2.5.1 Thus, it is imperative to control the emission of harmful NOx. In recent years, selective catalytic reduction of ammonia (NH3-SCR) has been widely used in NOx removal. In this process, NH3 acts as a reducing agent to reduce NOx to harmless N2 and H2O with the action of the catalyst, which is the core of NH3-SCR technology. At present, the commercial catalyst for NH3-SCR is mainly a V2O5-based catalyst, which shows excellent catalytic performance in the NH3-SCR reaction in the temperature range of 300–400 °C. However, the catalyst has disadvantages, such as narrow operating temperature window, toxicity of VOx species, inactivation of alkali metals, and decrease of N2 selectivity at high temperatures.2 Therefore, non-V2O5-based catalysts have been extensively studied.

Currently, anatase TiO2, Al2O3, molecular sieves, and carbon-based materials are usually used as denitrification catalyst supports. The supports are important for the SCR reaction. A good support can disperse the active components well. Moreover, the interaction of supports and active components also has an important effect on the catalytic performance.3 Therefore, anatase TiO2 has been widely studied due to its low cost, good sulfur resistance, nontoxicity,4 and rich Lewis acid sites.5

In addition, catalysts with polymetallic oxides as active components can improve the NH3-SCR performance due to the synergistic effect between components, which has also been widely studied.6−8 Ce-based catalysts are expected to be potential substitutes for V2O5-based catalysts. CeO2 is widely used as NH3-SCR catalysts for excellent redox ability of Ce4+/Ce3+ reversible redox pair, abundant surface oxygen vacancy, and high oxygen storage capacity.9−13 The oxidation state of chromium oxide (CrOx) is variable, especially for exposed chromium ions, and the most common valence states are Cr3+ and Cr5+.14 The presence of Cr makes the surface acidity and reducibility of the CeZr catalyst stronger, and more ad-NOx and ad-NH3 on the catalyst surface are conducive to the NH3-SCR reaction.15−17 Furthermore, Ce can promote the conversion between Cr3+ and Cr6+ and increase the proportion of lattice oxygen, which improves the activity of the catalyst.18 Thus, the interaction between Ce and Cr can improve the redox property and accelerate the redox circle, which are beneficial to the improvement of NH3-SCR activity.

However, the mass percentages of Cr and Ce in most of the catalysts prepared by researchers are greater than 5%,19 and some even exceed 30%.20 The low-dose CeCr/Ti catalyst has limited redox performance and acidity, which is insufficient to show excellent NH3-SCR activity. To optimize the catalytic performance, the supported catalyst can be doped with some transition metals to improve the catalytic activity through the synergistic effect of multiple metals.21 Due to the synergistic effect between metals, the bimetallic electron transport cycle serves as a bridge to accelerate the electron transfer from NO molecules to O2 molecules, promoting the generation of NO2 and accelerating the fast-SCR.22 Lanthanide is a rare earth element abundant in China. La-doped catalysts have been widely studied in SCR reactions due to their excellent oxygen storage capacity and unique redox properties.23,24 Studies have shown that La3+ could be partly integrated into ceria fluorite lattice to induce an expansion of the crystal cell dimensions, which concomitantly induces more lattice defects and improves the lattice oxygen mobility.25 Yi et al.26 studied the effects of rare earth element (La, Ce) modification on oxidation performance, and the results showed that La, Ce modification could promote the oxidation performance of adsorbents and reduce the pore diameter of the adsorbents. Therefore, the interaction between Ce and La is also conducive to the improvement of NH3-SCR activity.27

The state of Cu species has a great impact on the SCR activity of the CeO2–CuO catalyst,28 and Cu2+ can improve the low-temperature activity, while CuO would decrease the high-temperature activity. The interaction between Cu and Ce can reduce the crystallinity of CeO2, enhance the oxygen storage capacity and acidity of the catalyst, and increase the types of Ce3+ species.29 Thus, the interaction between Cu and Ce is helpful to improve the surface acidity and redox performance of the catalyst. In addition to copper, the presence of Fe can increase the surface area, enhance the redox properties, increase the number of surface acid sites, and increase the surface adsorbed oxygen.30 Zhang et al.31 reported that Fe3+ replaces Ce4+ to form Ce–Fe solid solution. This change of crystal phase structure facilitated the homogeneous dispersion of the active component on the catalyst surface, increased acid sites and specific surface area, and exposed more oxygen vacancies. Ge et al.32 found that the addition of Cr greatly increased the specific surface area, the number of weak and medium-strong acid sites, and the ratio of Fe3+/Fe2+ on the surface of the Fe/AC catalyst, thus promoting the low-temperature SCR activity of the catalyst. Therefore, the interaction between Fe and Ce or Cr helps to improve the redox capacity and the number of acid sites on the catalyst.

The composition of coal-fired flue gas in China is complex, often containing components such as SO2 and H2O, which will seriously damage the activity and stability of the NH3-SCR catalyst. Furthermore, the deactivation of the low-temperature DeNOx catalyst was more severe when SO2 and H2O were present at the same time, suggesting that SO2 and H2O appeared to exhibit synergistic toxicity in catalyst poisoning.33 For example, the metal element Cu easily reacts with sulfuric acid generated by H2O and SO2, resulting in the lack of active components and additives, and the sulfate species generated on the surface of the catalyst consumes the active Cu2+ species and occupies the main active sites, thereby reducing nitric acid. The types of salt species and intermediates lead to bad NH3-SCR activity.34 In this study, it can be known from XPS that after adding La (CeCrLaCu/TiO2), Cu2+ decreases and Cu+ increases, so it can reduce the formation of sulfate with sulfuric acid, thereby inhibiting poisoning. Hao et al. concluded with the XPS results that part of Fe2+ was oxidized to Fe3+ during the reaction, and the Fe2+ species had strong redox ability, which could explain the reason for the decrease in activity after the reaction. In addition, SO2 and H2O induce Ce transformation from Ce4+ to Ce3+ on the catalyst surface, which increases the amount of chemisorbed oxygen. Due to these factors, the addition of Ce and Fe species contributes to the excellent tolerance of the catalyst to SO2 and H2O.35 According to the change of Ce ion content in XPS in this paper, the Ce3+ content in CeCrLaCu/TiO2 is the highest (58%), which increases the content of chemisorbed oxygen as described in the literature, thereby improving the water resistance and sulfur resistance of the CeCrLaCu/TiO2 catalyst. Denitration catalyst poisoning is one of the key problems leading to substandard emission in flue gas during the denitration process. It is of great significance to improve the catalytic performance of SCR denitration catalysts, reduce the use cost of denitration catalysts, and avoid its poisoning.

Hydrothermal stability refers to the property of catalytic cracking catalysts in industrial application. At high temperatures and in the presence of water vapor, the structure is not damaged and has the ability to maintain sufficient equilibrium activity. However, hydrothermal aging usually occurs on molecular sieve catalysts. Under high-temperature hydrothermal aging conditions of 650 °C and above, the structure of molecular sieves, the properties of active species, and other physical and chemical properties will still undergo significant changes, which may cause irreversible loss of catalysts.36 There are also few studies on the hydrothermal stability of composite metal oxides. In this paper, the hydrothermal stability of the catalyst was also investigated by the NO conversion rate after hydrothermal aging.

In this paper, the low-dosed CeCrMx/TiO2 (M = La, Cu, Fe, LaCu, LaFe) catalyst was prepared by an impregnation method, and its low-temperature NH3-SCR performance was investigated. The promotion of metal doping on NH3-SCR over the Ce0.004Cr0.003Mx/TiO2 catalyst was studied by N2 physical adsorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), and temperature-programmed desorption by NH3 (NH3-TPD). The effects of different metal doping on the redox performance and NH3 adsorption performance of the catalyst were discussed, and the optimum ratio was selected. The synergistic effect of bimetallic elements of the LaCu/Fe metal-doped catalyst on catalytic activity was also elucidated.

2. Results and Discussion

2.1. Performances of NH3-SCR

NH3-SCR performance of CeCr/TiO2, CeCrCu/TiO2, CeCrFe/TiO2, CeCrLa/TiO2, CeCrLaCu/TiO2, and CeCrLaFe/TiO2 catalysts was evaluated, and the results are illustrated in Figure 1. With the increase of temperature, the NO conversion of the catalysts increases, and CeCrM/TiO2 (M = La, Cu, Fe) reaches the maximum NO conversion (98.9%) at 340 °C. Then, the NO conversion of the CeCrM/TiO2 (M = Cu, Fe) catalyst decreases rapidly, while the NO conversion of the CeCrLa/TiO2 catalyst remains stable in the temperature range of 340–420 °C. Compared with CeCrFe/TiO2 catalysts, CeCrLa/TiO2 catalysts have better high-temperature activity and a wider operating temperature window, which indicates that the addition of La is more favorable for the high-temperature activity (as shown in Figure 1a). And the addition of Cu, compared with Fe and La, produced a limited improvement in the low-temperature activity of the catalyst in the temperature range of 140–260 °C. However, the modification effects of La, Fe, and Cu are limited.

Figure 1.

Figure 1

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NO conversion of (a) all catalysts, (b) CeCr/TiO2 and CeCrLa/TiO2 catalysts, (c) CeCrCu/TiO2 and CeCrLaCu/TiO2 catalysts, and (d) CeCrFe/TiO2 and CeCrLaFe/TiO2 catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 5%, N2 balance, GHSV = 20,000 h–1.

To further improve the low-temperature activity of the catalyst, considering the excellent redox performance of Cu and Fe, the CeCr/Ti catalyst was doped with LaCu and LaFe, respectively. As shown in Figure 1c, the NO conversion curve of CeCrLaCu/TiO2 is shifted toward lower temperature by 80 °C compared to CeCrLa/TiO2 (Figure 1b). In addition, 100% NO conversion is maintained until 420 °C, the temperature at which the conversion starts to decrease. The CeCrLaFe/TiO2 catalyst (Figure 1d) reaches the maximum NO conversion (99%) at 300 °C. When the temperature exceeds 380 °C, the NO conversion drops rapidly to reach 70% at 460 °C. Consequently, the CeCrLaCu/TiO2 and CeCrLaFe/TiO2 catalysts effectively broadened the NO conversion at low temperatures. This indicates that the introduction of La can greatly improve the low-temperature catalytic activity of catalysts (Figure 1b–d), especially in the catalysts containing Cu and Fe. In addition, it was found that the interaction between La and Cu was more conducive to NH3-SCR performance at low temperatures.

2.2. Physicochemical Properties

2.2.1. XRD Analysis

The XRD patterns of the catalysts are shown in Figure 2. All samples display diffraction peaks at 25.3, 37.8, 48.1, 53.9, 55.1, 62.8, 68.7, 70.4, and 75.1°, which correspond to anatase TiO2 phase peaks [PDF-JCPDS 89-4921]. No independent diffraction peaks related to CeOx, CrOx, LaOx, CuOx, and FeOx were observed in samples. This indicates that the active components are in a highly dispersed state or the amount of active component is below the detection threshold. The good dispersion facilitated good contact of the active components with each other and led to strong interactions between Ti and other metal oxides.37 Compared with the diffraction peaks of CeCrFe/TiO2, CeCrCu/TiO2, and CeCrLa/TiO2 catalysts, the weakened diffraction peak intensities of CeCrLaCu/TiO2 and CeCrLaFe/TiO2 catalysts indicate that the interaction of La and Cu/Fe reduces the crystallinity of the catalysts, which is in agreement with the previous report.38 According to the Debye–Scherrer’s formula, the crystalline size is calculated and listed in Table 1. The crystal size increased in the following sequence: CeCrLa/TiO2 <CeCrLaFe/TiO2 <CeCrLaCu/TiO2 <CeCrFe/TiO2 <CeCrCu/TiO2. The particle size of the catalyst doped with La is smaller than that without La, indicating that La doping inhibited the crystalline growth of anatase TiO2.9 The CeCrCu/TiO2 and CeCrFe/TiO2 catalysts with lower NO conversion had larger crystallite size, while the CeCrLaCu/TiO2 catalyst with the highest NO conversion had smaller crystallite size. The results show that the small crystallite size can promote the NO conversion of the catalyst, but it does not always increase with decreasing crystallite size.

Figure 2.

Figure 2

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XRD patterns of all samples.

Table 1. XRD and BET Results of All of the Samples.
samples crystal sizea (nm) BET surface area (m2/g) pore volume (cm3/g) average pore diameter (nm)
CeCrCu/TiO2 9.1 84.2 0.24 11.1
CeCrFe/TiO2 9.0 117.8 0.28 10.1
CeCrLa/TiO2 8.2 81.8 0.20 9.1
CeCrLaCu/TiO2 8.6 77.1 0.19 9.7
CeCrLaFe/TiO2 8.4 85.0 0.20 9.1
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a

Calculated by Debye–Scherrer.

2.2.2. N2 Adsorption–Desorption Behavior

The N2 adsorption–desorption isotherms of the catalysts are presented in Figure 3. As shown in Figure 3a, the samples showed isotherms of type III, which exhibited hysteresis loops mostly of type H3, indicating that these samples contained mesopores (2–50 nm). The specific surface area, pore volume, and pore size of all samples are summarized in Table 1. A larger surface area and smaller average pore size contribute to the higher physisorption ability of the catalyst.39 Compared with CeCrCu/TiO2 and CeCrFe/TiO2 catalysts, the CeCrLa/TiO2 catalyst has a smaller specific surface area and average pore size, but the CeCrLa/TiO2 catalyst has better NO conversion. The addition of composite metal oxides has little effect on the pore volume but has a greater effect on the specific surface area and average pore size. Compared with CeCrLa/TiO2, the addition of the LaCu composite metal oxide leads to a decrease in the specific surface area of the catalyst, while the LaFe composite metal oxide increases the specific surface area of the catalyst. Furthermore, the CeCrLaCu/TiO2 catalyst with the best catalytic activity has the smallest specific surface area and pore volume, indicating that the specific surface area is not the key factor limiting the catalytic activity.

Figure 3.

Figure 3

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(a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions of the samples.

2.2.3. SEM Analysis

SEM test was carried out to observe the morphology of all samples. It can be observed in Figure 4 that there is no significant difference in the morphology of the five catalysts. The catalysts are in the form of particles and they are aggregated. However, the surface of the CuCeCrLa/TiO2 catalyst is relatively smooth compared to other catalysts. The results show that the addition of an appropriate amount of La and Cu improved the dispersity of the active components and reduced the agglomeration.

Figure 4.

Figure 4

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SEM images obtained for (a) CeCrCu/TiO2, (b) CeCrFe/TiO2, (c) CeCrLa/TiO2, (d) CeCrLaCu/TiO2, and (e) CeCrLaFe/TiO2.

2.2.4. XPS Analysis

The O 1s, Ti 2p, Ce 3d, Cr 2p, Cu 2p, and Fe 2p photoelectrom profiles of all samples are shown in Figure 5, and their relative atomic concentrations are listed in Table 2.

Figure 5.

Figure 5

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(A) O 1s, (B) Ti 2p, (C) Ce 3d, (D) Cr 2p, (E) Cu 2p, (F) Fe 2p, and (G) La 3d XPS spectra of the samples (a) CeCrCu/TiO2, (b) CeCrFe/TiO2, (c) CeCrLa/TiO2, (d) CeCrLaCu/TiO2, and (e) CeCrLaFe/TiO2.

Table 2. Surface Composition Contents and Redox Properties for the Different Catalysts.
  surface atomic contenta (%)
   percent of valence stateb (%)
 total H2 consumptionc(10–5 mmol/g) H2 consumptiond (10–5 mmol/g)
samples Ce Cr La Cu Fe O Ti Ce3+/Ce Cr6+/Cr Cu+/Cu Fe2+/Fe Oα/O   I II III IV V
CeCrCu/TiO2 0.2 1.5   0.3   59.6 38.4 35 54 55   25 33.4 1.8 3.6 10.2 9.3 8.5
CeCrFe/TiO2 0.2 1.6     0.3 59.9 38.1 43 63   55 27 33.3 2.5 9.3 10.0 11.5  
CeCrLa/TiO2 0.2 1.6 0.1     60.5 37.8 44 61     27 23.2 2.5 3.3 7.6 9.8  
CeCrLaCu/TiO2 0.2 1.5 0.1 0.3   59.6 38.4 58 68 61   28 46.9 3.9 9.8 11.4 9.7 12.1
CeCrLaFe/TiO2 0.2 1.6 0.1   0.3 60.1 37.8 46 63   23 26 34.3 2.6 2.5 6.2 10.7 12.3
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a

The H2 consumption over catalysts.

b

The total H2 consumption over catalysts.

c

The content of various elements determined by XPS.

d

The percent of valence state on the surface of catalysts.

The O 1s XPS spectra of all catalysts are shown in Figure 5A. The spectra of O 1s could be spilt into two peaks: lattice oxygen (BE = 530.0–530.4 eV, denoted as Oβ) and chemisorbed oxygen (BE = 531.6–532.2 eV, denoted as Oα).40,41 The content of surface adsorbed oxygen over these catalysts is calculated as Oα/(Oα+Oβ) and listed in Table 2. The mobility of the surface chemisorbed oxygen is stronger than the lattice oxygen; thus, it is easier to participate in the NH3-SCR reaction.42−44 In general, the high Oα/Oβ relative concentration ratio on the catalyst surface could be related to excellent SCR activity.45 However, it is obvious from Table 2 that there is a slight difference in Oα over catalysts. The chemical adsorption oxygen species on the CeCrLaCu/TiO2 surface were the most, indicating that the doping of LaCu was more conducive to the generation of chemical adsorption oxygen sites on the catalyst surface. As shown in Figure 5A, according to (c, d, e), the peaks of Oα and Oβ over CeCrLaCu/TiO2 and CeCrLaFe/TiO2 catalysts shifted to lower binding energy compared to the CeCrLa/TiO2 catalyst, suggesting the strong interaction among La and Cu/Fe. According to (b, c, e), the peaks of Oα and Oβ over CeCrLaFe/TiO2 catalysts shifted to lower binding energy compared to CeCrLa/TiO2 and CeCrFe/TiO2 catalysts, indicating that there is some kind of synergy between La and Fe. According to (a, c, d), the peaks of Oα and Oβ over CeCrLaCu/TiO2 catalysts shifted to lower binding energy compared to CeCrLa/TiO2 and CeCrCu/TiO2 catalysts, indicating that there is a certain kind of synergy between La and Fe. This is consistent with previous studies.46

XPS of Ti 2p can be fitted into two peaks (Figure 5B). The peaks located at around 458.9 and 464.7 eV could be the result of the orbitals of Ti 2p3/2 and Ti 2p1/2, respectively. This indicated that Ti existed in the Ti4+ oxidation state.47,48 Compared to other catalysts, the Ti 2p peak position of the CeCrLaFe/TiO2 catalyst was shifted to the right by approximately 0.2 eV, indicating that some of the Ti4+ ions were reduced to Ti3+ ions. The redox cycle of Ti4+/Ti3+ is hypothesized to be another reason for the high SCR performance.49

The Ce 3d XPS spectra of the catalysts are presented in Figure 5C. The Ce 3d bands are deconvoluted by the curve-fitting procedure,9,48,50 and the Ce 3d XPS spectra could be separated into seven peaks and “u” and “v” were attributed to Ce 3d3/2 and Ce 3d5/2, respectively. The two peaks labeled u′ and v′ could be assigned to Ce3+, with the remaining peaks of u, u″, v, v″, and v‴ belonging to Ce4+.51 It is generally believed that Ce3+ is related to the appearance of oxygen vacancies. One electron lost by Ce4+ combines with O2 to form O2– (eq 1), which oxidizes NO to NO2 (eq 2). Therefore, the more Ce3+, the more NO2 is produced, providing the possibility of a low-temperature L–H mechanism.

2.2.4. 1
2.2.4. 2

where “□” represents an empty position originating from the removal of O2– from the lattice.52 The ratio of Ce3+/Ce for the samples could be calculated from the results of XPS analysis, and the results are listed in Table 2. Obviously, the Ce3+/Ce ratio of the CeCrLa/TiO2 catalyst is higher than that of the CeCrM/TiO2 (M = Cu, Fe) catalyst, which indicates that the introduction of La promotes the conversion of Ce4+ to Ce3+. However, the doping of bimetallic oxides led to the increase of Ce3+/Ce ratio in CeCrLaCu/TiO2 and CeCrLaFe/TiO2 catalysts, indicating that the synergistic effect of LaCu or LaFe played an important role in the formation of Ce3+.

The XPS spectra for Cr 2p of all samples are given in Figure 5D and show that two distinct peaks correspond to Cr3+ (577.6–578.8 eV) and Cr6+ (579.8–581.0 eV).14,15 It has been reported that the addition of Cr could facilitate the SCR performance by the valence change between Cr6+ and lower oxidized states (Cr5+, Cr3+, and Cr2+).16,53 It is obvious that most Cr species in catalysts exist in the form of Cr6+. It is noteworthy that the Cr6+/Cr ratio (Table 2) of CuCrLaCu/TiO2 (68%) is higher than those of CeCrCu/TiO2 (54%) and CeCrLa/TiO2 (61%). The Cr6+/Cr ratio of CeCrLaFe/TiO2 (63%) was similar to those of CeCrLa/TiO2 (61%) and CeCrFe/TiO2 (63%), indicating that the doping of LaCu was conducive to the formation of Cr6+ and the synergistic effect of LaCu was stronger than that of LaFe.

The XPS spectra of Cu 2p of the catalysts are shown in Figure 5E. Both peaks of 932.2 eV and 952.1 eV ought to be attributed to Cu+ species, and the peaks at 953.1 eV and 933.1 eV should be ascribed to Cu2+.54 As shown in Table 2, the ratio of Cu+/Cu on the surface of the CeCrLaCu/TiO2 catalyst is slightly higher than that of the CeCrCu/TiO2 catalyst. The doping of La leads to the increase of the Cu+/Cu ratio on the surface, indicating that the synergistic effect of LaCu contributes to the formation of surface Cu+.

The Fe element was present in the catalyst mainly in two valence states, Fe2+ and Fe3+, and was attributed to BE = 702.9 eV, 716.1 eV and BE = 711.6 eV, 733.8 eV, respectively, in Figure 5F. The content of Fe2+ and Fe3+ in the catalysts was closely related to the formation of oxygen vacancies in the catalyst. From reactions (3, 4), it is known that Fe2+ and Fe3+ produce O2– during the mutual conversion process. The effect is the same as that of Ce3+. Therefore, the greater the Fe2+ content, the more the oxygen adsorption site, and the better the denitration activity of the catalyst.

2.2.4. 3
2.2.4. 4

The relative contents of Fe2+/Fe were calculated and are listed in Table 2. Compared with the CeCrFe/TiO2 catalyst, the CeCrLaFe/TiO2 catalyst has lower Fe2+, but its catalytic performance is better than the CeCrFe/TiO2 catalyst. Due to the lower ionization energy of La2+→La3+, electrons are easy to be lost and may be more easily transferred from La to the active component.55 The incorporation of La facilitates the transformation from low-valence species (Fe2+) to the stable high-valence species (Fe3+).56

Figure 5G shows the La 3d spectra. Both La 3d3/2 and La 3d5/2 have double peaks, which meant that companion peaks appear. Due to the ionization of electrons in the inner shell layers of La 3d3/2 and La 3d5/2, the transfer of 2p valence electrons from the La-coordinated oxygen to the 4f vacant orbital of La results in the splitting of the La 3d characteristic peak and the resultant vibrational companion peaks of La 3d3/2 and La 3d5/2. So, the change in the intensity of the companion peak can reflect the ability of the 2p electrons of oxygen to give La.57 Compared with CeCrLa/TiO2 catalysts, CeCrLaFe/TiO2 and CeCrLaCu/TiO2 catalysts have stronger companion peak intensities, indicating that Fe/Cu promotes the electron supply of O 2p. The electron transfer on the catalyst surface is of great significance for NH3-SCR. From the XPS analysis in this paper, it is known that the incorporation of La promotes the transfer of electrons between the active components. Doping of LaFe/LaCu is by surface charge transfer doping.58 And the LaCu/LaFe dopant is in surface contact with the TiO2 carrier, and the former is used to transfer charges to the carrier based on the difference in the ability of the former to gain and lose electrons.

2.2.5. H2-TPR Analysis

The redox property of the samples was determined by H2-TPR analysis, and the results are shown in Figure 6. The H2 consumption values of all samples are shown in Table 2. Five peaks appear from 250 to 750 °C in the H2-TPR profile of CeCrCu/TiO2. It has been reported that CuOx species have a great degree of reduction capacity at lower temperatures.59 Thus, two reduction peaks of CuOx could be seen at 339 and 422 °C, respectively, which are due to two reduction steps, that of CuO into Cu2O and Cu2O into Cu;60,61 the reduction peak appearing at 483 °C belongs to the reduction of Cr6+ → Cr3+;62 the peak at 550 °C is attributed to Ce4+→Ce3+;31 and the peak at 631 °C is attributed to crystal lattice CeO2/Ce2O3 and Cr3+ → Cr0.62,6362,63 For the CeCrFe/TiO2 catalyst, the reduction peaks appearing at 570 °C belong to the reduction of Cr6+ → Cr3+ and Fe2O3 → Fe3O4.64 The reduction peak at 630 °C belongs to the reduction of FeO→Fe0 and the transformations of surface CeO2/Ce2O3.65 The H2 reduction peaks at 695 °C may be due to overlapping of transformations of surface CeO2/Ce2O3 and Cr3+→Cr0.66 The reduction peaks at 763 °C were attributed to lattice oxygen contained in bulk CeO2.62 As for CeCrLa/TiO2, the reduction peak can be divided into four peaks. The reduction peaks appearing at 565 °C belong to the reduction of Cr6+ → Cr3+;67 the peaks at 617 and 707 °C are respectively attributed to transformations of surface CeO2/Ce2O3 and crystal lattice CeO2/Ce2O3;68 and the peak at 664 °C might be assigned to Cr3+ → Cr0.69 After the introduction of La, the peak positions move to a higher temperature and the peak area gradually decreases, indicating that the reducibility of the CeCrLa/TiO2 catalyst decreases.

Figure 6.

Figure 6

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H2-TPR profiles of the catalysts.

The reduction profiles of the CeCrLaCu/TiO2 catalyst are composed of five main hydrogen consumption steps. Compared with the CeCrCu/TiO2 catalyst, the reduction peak position of the CeCrLaCu/TiO2 catalyst is basically unchanged. And compared with the CeCrLa/TiO2 catalyst, the reduction peak position of the CeCrLaCu/TiO2 catalyst shifted to a lower temperature, such as the reduction temperature of Ce4+ → Ce3+ and Cr6+ → Cr3+ decreases. This indicates that Ce and Cr become more reducible, which can be attributed to the synergistic effect of LaCu. At the same time, the introduction of La increases the hydrogen consumption of Cu and Cr, indicating that the synergistic effect of LaCu is more conducive to improving the redox performance of the catalyst. Similarly, compared with CeCrLa/TiO2 and CeCrFe/TiO2 catalysts, the reduction peak position of CeCrLaFe/TiO2 catalysts shifted to lower temperatures, and the introduction of La increases the reduction peak area of Fe (321 and 424 °C) and Cr (523 and 672 °C), indicating that the synergistic effect between LaFe is beneficial to improve redox performance of catalysts, which is consistent with the result of the improved low-temperature activity of CeCrLaFe/TiO2.

2.2.6. NH3-TPD Analysis

NH3-TPD tests were performed, and the results are shown in Figure 7. The total NH3 adsorption capacity is summarized in Table 3. From Figure 7, three peaks appear from 100 to 500 °C in the NH3-TPD profile of each catalyst. The peaks at the range of 150–250, 250–400, and 400–550 °C refer to weak acid, medium-strength acid, and high-strength acid, respectively.70,71 According to the literature,72 the medium-strength and high-strength acid sites are effective for NO reduction at moderate to high temperature. The NH3 uptake of these catalysts in descending order is CeCrLaCu/TiO2 > CeCrLaFe/TiO2 > CeCrLa/TiO2 > CeCrFe/TiO2 > CeCrCu/TiO2. The NH3 adsorption capacity of the catalyst is proportional to the catalytic performance, indicating that the NH3 adsorption capacity plays an important role in the catalytic system.73,74 Compared with CeCrLa/TiO2 and CeCrCu/TiO2 catalysts, the NH3 adsorption capacity of CeCrLaCu/TiO2 and CeCrLaFe/TiO2 catalysts was significantly improved, indicating that the synergistic effect of LaCu/LaFe promoted the surface acidity of the catalysts, which could provide more ad-NH3 species for the NH3-SCR reaction. Compared with the CeCrLaFe/TiO2 catalyst, the CeCrLaCu/TiO2 catalyst has stronger NH3 adsorption capacity, so it is more conducive to NH3-SCR performance.

Figure 7.

Figure 7

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NH3-TPD profiles of all samples.

Table 3. NH3 Adsorption Capacity of the Catalysts.
    acidity (mmol/g)
samples total NH3 uptake (mmol/g) weak moderate strong
CeCrCu/TiO2 0.130 0.038 0.032 0.060
CeCrFe/TiO2 0.132 0.035 0.041 0.056
CeCrLa/TiO2 0.133 0.051 0.020 0.062
CeCrLaCu/TiO2 0.173 0.045 0.050 0.078
CeCrLaFe/TiO2 0.164 0.041 0.060 0.063
Open in a new tab

3. Catalytic Hydrothermal Aging

As shown in Figure 8, when the temperature is less than 300 °C, the activities of all catalysts have a sharp decrease; this may be due to the reduction of catalyst active sites due to hydrothermal aging.75 It can be concluded that hydrothermal aging has a great influence on low temperature catalysis. CeCrLaCu/TiO2-H and CeCrLaFe/TiO2-H still maintain good activity while the temperature is greater than 300 °C, and the NO conversion drops from 99.5 to 95%, which shows that the synergistic effect of LaCu and LaFe is better for hydrothermal stability. In general, hydrothermal aging has an acceptable effect on catalysts in this article (Figure 8).

Figure 8.

Figure 8

Open in a new tab

NO conversion of catalysts after hydrothermal aging. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 5%, N2 balance, GHSV = 20,000 h–1.

4. Conclusions

In this paper, a series of CeCrM/TiO2 (M = Cu, Fe, La, LaCu, LaFe) catalysts were prepared. By mixing low-dose components, a good low-temperature denitrification catalyst CeCrLaCu/TiO2 was selected. The catalyst has excellent NH3-SCR performance in a wide temperature window. XRD analysis showed that the synergistic effect of LaCu and LaFe could effectively reduce the aggregation of active species and promote the high dispersion of active components on the surface of TiO2. The addition of LaCu and LaFe changed the redox state and surface species concentration of the catalysts. The synergistic effect of LaCu/LaFe promoted the formation of Ce3+ and Cr6+, and the synergistic effect of LaCu was stronger. The CeCrLaCu/TiO2 catalyst has the highest relative atomic ratio (Ce3+/Cen+, Cr6+/Cr, Cu+/Cu, and Oα/Oβ). It is beneficial for redox of reactive gas on the catalyst surface. The results of H2-TPR and NH3-TPD showed that the synergistic effect of LaCu improved the redox capacity and surface acidity of the catalyst, which was an important reason for the excellent performance of the NH3-SCR catalyst.

5. Experimental Section

5.1. Catalyst Preparation

5.1.1. Preparation of Supports

A certain amount of metatitanic acid (TiO(OH)2, Xiya Reagent, China, 98%) was calcined at 500 °C for 3 h in air.

5.1.2. Preparation of Catalysts

The catalyst was prepared by the impregnation method. TiO2 powder and specific amounts of Ce(NO3)3·6H2O, Cr(NO3)3·9H2O, La(NO3)3·6H2O, Cu(NO3)2·6H2O, and Fe(NO3)3·9H2O were mixed together in absolute ethanol, and the mixture was stirred in a magnetic stirrer at room temperature for 6 h. The mixture was aged at room temperature for 24 h and then dried at 80 °C for 12 h, followed by calcination at 450 °C for 4 h in a muffle furnace. CeOx, CrOx, LaOx, and CuOx (FeOx) mixed metal oxides are obtained after calcination. The solid sample was crushed and sieved to 40–60 mesh and was recorded as CeCrMx/TiO2 (M = La, Cu, Fe, LaCu, LaFe). The active ingredient ratio of catalysts Ce/Cr/La/Fe/Cu was 4:3:2:8:8.

5.1.3. Preparation of Hydrothermal Aging Catalysts

The hydrothermal aging conditions are as follows: 5% CO2, 5% H2O, 5% O2, and N2 as equilibrium gases, 700 °C for 12 h.

5.2. Catalyst Characterization

5.2.1. X-ray Diffraction (XRD)

An X-ray diffractometer (Rigaku Ultima IV) was used in determining the catalyst crystal structures using these parameters: scanning at 2θ range from 10 to 80° and radiation at 40 mA and 40 kV with Cu Kα (λ = 0.15406 nm).

5.2.2. Brunauer–Emmett–Teller (BET) Surface Area Analysis

A Micromeritics ASAP 2460 was used to perform the test of N2 adsorption–desorption for the measurement of the textural characteristics of the catalysts at a liquid nitrogen temperature of −196 °C. To remove physically adsorbed species, the catalysts were pre-degassed for 10 h at 200 °C before each analysis. The pore size distribution and specific surface area of the catalysts were calculated using the Barrett–Joyner–Halenda (BJH) method and the Brunauer–Emmet–Teller (BET) equation.

5.2.3. Scanning Electron Microscopy (SEM)

An FEI Apreo was used to examine the morphology of the catalysts. The sample was dispersed on the conductive adhesive and then sprayed gold to observe.

5.2.4. X-Ray Photoelectron Spectroscopy (XPS)

An Escalab 250Xi with a monochromatic Al-Kα target (hυ = 1486.6 eV) was used to obtain X-ray photoelectron spectroscopy (XPS) of the samples. Before the test, the samples were outgassed in a UHV chamber (<10–7 Pa). The calibration of binding energies of O 1s, Ti 2p, Ce 3d, Cr 2p, Cu 2p, and Fe 2p was obtained using the C 1s peak (284.8 eV). Peak fitting is done using XPSPEAK 4.1 with a Shirley background.

5.2.5. Temperature-Programmed Reduction by Hydrogen (H2-TPR)

A chemisorption apparatus (Micromeritics ChemiSorb 2720) with 0.2 g of catalyst was used to collect H2-TPR profiles. Before that, the samples were pretreated in a pure Ar atmosphere for 1 h at 300 °C and then cooled to 50 °C for half an hour to mix the gas 10% H2/Ar. The TPR profiles of the catalysts were collected when the baseline remained unchanged by heating the samples from 50 to 800 °C at a rate of 10 °C/min, with an aggregation rate of 25 mL/min with 10% H2/Ar. Thermal conductivity detectors were taken in continuously monitoring H2 consumption.

5.2.6. Temperature-Programmed Desorption by Ammonia (NH3-TPD)

A chemisorption apparatus (Micromeritics Auto Chem II 2920) with 0.2 g of catalyst was used to collect NH3-TPD profiles. Before that, the catalysts were preheated for 1 h at 300 °C in a helium flow and then cooled to 100 °C for 5% NH3/He 1 h. The physically adsorbed NH3 was removed by purging He for another 1 h. When the samples were heated at a rate of 10 °C/min from 100 to 500 °C, with an aggregation rate of 50 mL/min with He, the profiles of the catalysts were recorded.

5.3. Catalytic Performance Measurement

In this work, a fixed-bed flow reactor containing 3 mL of catalyst of 40–60 mesh was taken for testing SCR activities. The reaction condition was controlled as follows: 1000 ppm NO, 1000 ppm NH3, 5% O2, and the balance gas is N2. The gas hourly space velocity (GHSV) was approximately 20,000 h–1 with a sum rate of 1000 mL/min of the feed gas. The NO concentration was measured with a signal NOx analyzer (KM945, KANE, U.K.). The following equation was used to calculate NO conversion

5.3. 5

Acknowledgments

The authors thank Prof. Zhang and Dr. Liu for their help in designing the experiment and writing the paper and also thank S.L. and Q.L. for their help in the process of the experiment. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

The authors declare no competing financial interest.

Notes

No conflict of interest exits in the submission, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously in whole or in part.

References

  1. Ren S.; Guo F. Q.; Yang J.; Yao L.; Zhao Q.; Kong M. Selection of carbon materials and modification methods in low-temperature sintering flue gas denitrification. Chem. Eng. Res. Des. 2017, 126, 278–285. 10.1016/j.cherd.2017.08.022. [DOI] [Google Scholar]
  2. Chu S.; Zhou W.; Zhang C.; Zheng Y.; Liu Y.; Liu Y. Relationship between the structure and catalytic performance of MoS2 with different surfactant-assisted syntheses in the hydrodesulfurization reaction of 4,6-DMDBT. RSC Adv. 2020, 10, 7600–7608. 10.1039/C9RA08407J. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Tong J. B.; Lin Y.; Liu S. L.; Wen J. T.; Liu Y. Y. Recent progress in the support of hydrodesulfurization catalysts. Chem. Ind. Eng. Prog. 2014, 33, 1170–1179. [Google Scholar]
  4. Liu X. Y.; Zhang P.; Jia Y. Y.; Tang Z. H.; Liu G. L.; Wu S. F. Research progresses of TiO2 supported manganese-ceriumcatalyst for low-temperature SCR denitration. Environ. Prot. Chem. Ind. 2020, 40, 26–31. [Google Scholar]
  5. Ganjkhanlou Y.; Janssens T. V. W.; Vennestrøm P. N. R.; Mino L.; Paganini M. C.; Signorile M.; Bordiga S.; Berlier G. Location and activity of VOx species on TiO2 particles for NH3-SCR catalysis. Appl. Catal., B 2020, 278, 119337 10.1016/j.apcatb.2020.119337. [DOI] [Google Scholar]
  6. Thirupathi B.; Smirniotis P. G. Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures. Appl. Catal., B 2011, 110, 195–206. 10.1016/j.apcatb.2011.09.001. [DOI] [Google Scholar]
  7. Saeidi M.; Hamidzadeh M. Co-doping a metal (Cr, Mn, Fe, Co, Ni, Cu, and Zn) on Mn/ZSM-5 catalyst and its effect on the catalytic reduction of nitrogen oxides with ammonia. Res. Chem. Intermed. 2017, 43, 2143–2157. 10.1007/s11164-016-2751-0. [DOI] [Google Scholar]
  8. Guo L. L.; Liu L.; Zhu X. L.; Zhang Q.; Li C. Y. Effect of Mg/Al molar ratios on NO reduction activity of CO using Ce-La/MgAl2O4-x catalysts. J. Fuel Chem. Technol. 2017, 45, 723–730. 10.1016/S1872-5813(17)30036-1. [DOI] [Google Scholar]
  9. Hou X. X.; Chen H. P.; Liang Y. H.; Wei Y. L.; Li Z. Q. La Modified Fe-Mn/TiO2 Catalysts to Improve SO2 Resistance for NH3-SCR at Low-Temperature. Catal. Surv. Asia 2020, 24, 291–299. 10.1007/s10563-020-09309-1. [DOI] [Google Scholar]
  10. Liu C.; Chen L.; Li J.; Ma L.; Arandiyan H.; Du Y.; Xu J.; Hao J. Enhancement of activity and sulfur resistance of CeO2 supported on TiO2-SiO2 for the selective catalytic reduction of NO by NH3. Environ. Sci. Technol. 2012, 46, 6182–6189. 10.1021/es3001773. [DOI] [PubMed] [Google Scholar]
  11. Gao R. H.; Zhang D. S.; Maitarad P.; Shi L. Y.; Rungrotmongkol T.; Li H. R.; Zhang J. P.; Cao W. G. Morphology-Dependent Properties of MnOx/ZrO2–CeO2 Nanostructures for the Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. C 2013, 117, 10502–10511. 10.1021/jp400984z. [DOI] [Google Scholar]
  12. Sun P.; Guo R. T.; Liu S. M.; Wang S. X.; Pan W. G.; Li M. Y.; Liu S. W.; Liu J.; Sun X. Enhancement of the low-temperature activity of Ce/TiO2 catalyst by Sm modification for selective catalytic reduction of NOx with NH3. Mol. Catal. 2017, 433, 224–234. 10.1016/j.mcat.2016.12.025. [DOI] [Google Scholar]
  13. Chen L.; Li J. H.; Ge M. F.; Ma L.; Chang H. Z. Mechanism of Selective Catalytic Reduction of NOx with NH3 over CeO2-WO3 Catalysts. Chin. J. Catal. 2011, 32, 836–841. 10.1016/S1872-2067(10)60195-7. [DOI] [Google Scholar]
  14. Su J.; Yao W. Y.; Liu Y.; Wu Z. B. The impact of CrOx loading on reaction behaviors of dichloromethane (DCM) catalytic combustion over Cr-O/HZSM-5 catalysts. Appl. Surf. Sci. 2017, 396, 1026–1033. 10.1016/j.apsusc.2016.11.083. [DOI] [Google Scholar]
  15. Wang Z. Y.; Guo R. T.; Guan Z. Z.; Shi X.; Pan W. G.; Fu Z. G.; Qin H.; Liu X. Y. The promotion effect of Cr additive on CeZr2Ox catalyst for the low-temperature selective catalytic reduction of NOx with NH3. Appl. Surf. Sci. 2019, 485, 133–140. 10.1016/j.apsusc.2019.04.199. [DOI] [Google Scholar]
  16. Cai W.; Zhong Q.; Zhang S. L.; Zhang J. X. Effects of Cr on the NO oxidation over the ceria–zirconia solid solution. RSC Adv. 2013, 3, 7009–7015. 10.1039/c3ra40226f. [DOI] [Google Scholar]
  17. Cai W.; Zhong Q.; Yu Y.; Dai S. Correlation of morphology with catalytic performance of CrO/Ce0.2Zr0.8O2 catalysts for NO oxidation via in-situ STEM. Chem. Eng. J. 2016, 288, 238–245. 10.1016/j.cej.2015.12.009. [DOI] [Google Scholar]
  18. Lu M.; Hou H. L.; Wei C. Y.; Guan X. H.; Wei W.; Wang G. S. Preparation of Quasi-MIL-101(Cr) Loaded Ceria Catalysts for the Selective Catalytic Reduction of NOx at Low Temperature. Catalysts 2020, 10, 140. 10.3390/catal10010140. [DOI] [Google Scholar]
  19. Chen D.; Cen C.; Feng J.; Yao C.; Xiong Y.; et al. Co-catalytic effect of Al-Cr pillared montmorillonite as a new SCR catalytic support. J. Chem. Technol. Biotechnol. 2016, 91, 2842–2851. 10.1002/jctb.4899. [DOI] [Google Scholar]
  20. Gao E.; Huang B.; Zhao Z.; Pan H.; Zhang W.; Li Y.; Bernards M. T.; He Y.; Shi Y. Understanding the co-effects of manganese and cobalt on the enhanced SCR performance for MnxCo1-xCr2O4 spinel-type catalysts. Catal. Sci. Technol. 2020, 10, 4752–4765. 10.1039/D0CY00872A. [DOI] [Google Scholar]
  21. Li F.; Xie J.; Qi K.; Gong P.; He F. Evaluating the Intermetallic Interaction of Fe or Cu Doped Mn/TiO2 Catalysts: SCR Activity and Sulfur Tolerance. Catal. Lett. 2019, 149, 788–797. 10.1007/s10562-018-2613-z. [DOI] [Google Scholar]
  22. Jiang L. J.; Liu Q. C.; Zhao Q.; Ren S.; Kong M.; Yao L.; Meng F. Promotional effect of Ce on the SCR of NO with NH3 at low temperature over MnOx supported by nitric acid-modified activated carbon. Res. Chem. Intermed. 2018, 44, 1729–1744. 10.1007/s11164-017-3194-y. [DOI] [Google Scholar]
  23. Bin F.; Wei X. L.; Li B.; Hui K. S. Self-sustained combustion of carbon monoxide promoted by the Cu-Ce/ZSM-5 catalyst in CO/O2/N2 atmosphere. Appl. Catal., B 2015, 162, 282–288. 10.1016/j.apcatb.2014.07.007. [DOI] [Google Scholar]
  24. Gao Q.; Han S.; Ye Q.; Cheng S. Y.; Kang T. F.; Dai H. X. Effects of Lanthanide Doping on the Catalytic Activity and Hydrothermal Stability of Cu-SAPO-18 for the Catalytic Removal of NOx (NH3-SCR) from Diesel Engines. Catalysts 2020, 10, 336. 10.3390/catal10030336. [DOI] [Google Scholar]
  25. Lu P.; Li C. T.; Zeng G. M.; He L. J.; Peng D. L.; Cui H. F.; Li S. H.; Zhai Y. B. Low temperature selective catalytic reduction of NO by activated carbon fiber loading lanthanum oxide and ceria. Appl. Catal., B 2010, 96, 157–161. 10.1016/j.apcatb.2010.02.014. [DOI] [Google Scholar]
  26. Yi H. H.; Yang K.; Tang X. L.; Zhao S. Z.; Gao F. Y.; Huang Y. H.; Shi Y. R.; Xie X. Z.; Zhang R. C. Promoting Simultaneous Desulfurization and Denitrification Performance of Al2O3@TiO2 Core–Shell Structure Adsorbents by Enhancing Oxidation Performance: Modification by Rare Earth Elements (La, Ce, and Y), Reaction Temperature, and Oxygen Concentration. Ind. Eng. Chem. Res. 2019, 58, 5423–5431. 10.1021/acs.iecr.8b06051. [DOI] [Google Scholar]
  27. Zhang D. J.; Ma Z. R.; Wang B. D.; Sun Q.; Xu W. Q.; Zhu T. Effects of MO (M=Mn, Cu, Sb, La) on V–Mo–Ce/Ti selective catalytic reduction catalysts. J Rare Earth 2020, 38, 157–166. 10.1016/j.jre.2019.02.016. [DOI] [Google Scholar]
  28. Guo R.-t.; Zhen W.; Pan W.; Zhou Y.; Hong J.; et al. Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method. J. Ind. Eng. Chem. 2014, 20, 1577–1580. 10.1016/j.jiec.2013.07.051. [DOI] [Google Scholar]
  29. Guan B.; Jiang H.; Peng X.; Wei Y.; Liu Z.; Chen T.; Lin H.; Huang Z. Promotional effect and mechanism of the modification of Ce on the enhanced NH3-SCR efficiency and the low temperature hydrothermal stability over Cu/SAPO-34 catalysts. Appl. Catal., A 2021, 617, 118110 10.1016/j.apcata.2021.118110. [DOI] [Google Scholar]
  30. Schill L.; Putluru S. S. R.; Schill L.; Jensen A. D. Effect of Fe doping on low temperature deNOx activity of high-performance vanadia anatase nanoparticles. Catal. Commun. 2014, 56, 110–114. 10.1016/j.catcom.2014.07.015. [DOI] [Google Scholar]
  31. Zhang K.; Wang J.; Guan P.; Li N.; Gong Z.; Zhao R.; Luo H.; Wu W. Low-temperature NH3-SCR catalytic characteristic of Ce-Fe solid solutions based on rare earth concentrate. Mater. Res. Bull. 2020, 128, 110871 10.1016/j.materresbull.2020.110871. [DOI] [Google Scholar]
  32. Ge T. T.; Zhu B. Z.; Sun Y. L.; Song W. Y.; Fang Q. L.; Zhong Y. X. Investigation of low-temperature selective catalytic reduction of NOx with ammonia over Cr-promoted Fe/AC catalysts. Environ. Sci. Pollut. Res. Int. 2019, 26, 33067–33075. 10.1007/s11356-019-06301-9. [DOI] [PubMed] [Google Scholar]
  33. Pan S. W.; Luo H. C.; Li L.; Wei Z. L.; Huang B. C. H2O and SO2 deactivation mechanism of MnOx/MWCNTs for low-temperature SCR of NOx with NH3. J. Mol. Catal. A 2013, 377, 154–161. 10.1016/j.molcata.2013.05.009. [DOI] [Google Scholar]
  34. Chen L.; Ren S.; Xing X. D.; Yang J.; Yang J.; Wang M. M.; Chen Z. C.; Liu Q. C. Low-Cost CuX Catalyst from Blast Furnace Slag Waste for Low-Temperature NH3-SCR: Nature of Cu Active Sites and Influence of SO2/H2O. ACS Sustainable Chem. Eng. 2022, 10, 7739–7751. 10.1021/acssuschemeng.2c02098. [DOI] [Google Scholar]
  35. Hao S. J.; Cai Y. D.; Sun C. Z.; Sun J. F.; Tang C. J.; Dong L. High Resistance of SO2 and H2O over Monolithic Mn-Fe-Ce-Al-O Catalyst for Low Temperature NH3-SCR. Catalysts 2020, 10, 1329. 10.3390/catal10111329. [DOI] [Google Scholar]
  36. Zhang Y. N.; Zhu H. C.; Zhang T.; Li J.; Chen J. J.; Peng Y.; Li J. H. Revealing the Synergistic Deactivation Mechanism of Hydrothermal Aging and SO2 Poisoning on Cu/SSZ-13 under SCR Condition. Environ. Sci. Technol. 2022, 56, 1917–1926. 10.1021/acs.est.1c06068. [DOI] [PubMed] [Google Scholar]
  37. Xie S. Z.; Li L. L.; Jin L. J.; Wu Y. H.; Liu H.; Qin Q. J.; Wei X. L.; Liu J. X.; Dong L. H.; Li B. Low temperature high activity of M (M = Ce, Fe, Co, Ni) doped M-Mn/TiO2 catalysts for NH3-SCR and in situ DRIFTS for investigating the reaction mechanism. Appl. Surf. Sci. 2020, 515, 146014 10.1016/j.apsusc.2020.146014. [DOI] [Google Scholar]
  38. Ren J.; Ren M. J.; Wang D. L.; Lin J. Y.; Li Z. Mechanism of microwave-induced carbothermic reduction and catalytic performance of Cu/activated carbon catalysts in the oxidative carbonylation of methanol. J. Therm. Anal. Calorim. 2015, 120, 1929–1939. 10.1007/s10973-015-4519-y. [DOI] [Google Scholar]
  39. Han F.; Xu C.; Sun W. Z.; Yu S. T.; Xian M. Effective removal of salicylic and gallic acids from single component and impurity-containing systems using an isatin-modified adsorption resin (vol 7, pg 23164, 2017). RSC Adv. 2017, 7, 26370. 10.1039/C7RA90064C. [DOI] [Google Scholar]
  40. Wang S. H.; Fan C.; Zhao Z. Q.; Liu Q.; Xu G.; Wu M. H.; Chen J. J.; Li J. H. A facile and controllable in situ sulfation strategy for CuCeZr catalyst for NH3-SCR. Appl. Catal., A 2020, 597, 117554 10.1016/j.apcata.2020.117554. [DOI] [Google Scholar]
  41. Gao E. H.; Su G. J.; Zhang W.; Bernards M. T.; He Y.; Pan H.; Shi Y. Surface lattice oxygen activation via Zr4+ cations substituting on A(2+) sites of MnCr2O4 forming ZrxMn1-xCr2O4 catalysts for enhanced NH3-SCR performance. Chem. Eng. J. 2020, 380, 122397 10.1016/j.cej.2019.122397. [DOI] [Google Scholar]
  42. Li S. H.; Huang B. C.; Yu C. L. A CeO2-MnOx core-shell catalyst for low-temperature NH3-SCR of NO. Catal. Commun. 2017, 98, 47–51. 10.1016/j.catcom.2017.04.046. [DOI] [Google Scholar]
  43. Yao X. J.; Ma K. L.; Zou W. X.; He S. G.; An J. B.; Yang F. M.; Dong L. Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature. Chin. J. Catal. 2017, 38, 146–159. 10.1016/S1872-2067(16)62572-X. [DOI] [Google Scholar]
  44. Yao X. J.; Kong T. T.; Yu S. H.; Li L. L.; Yang F. M.; Dong L. Influence of different supports on the physicochemical properties and denitration performance of the supported Mn-based catalysts for NH3-SCR at low temperature. Appl. Surf. Sci. 2017, 402, 208–217. 10.1016/j.apsusc.2017.01.081. [DOI] [Google Scholar]
  45. Peng Y.; Li K. Z.; Li J. H. Identification of the active sites on CeO2–WO3 catalysts for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study. Appl. Catal., B 2013, 140-141, 483–492. 10.1016/j.apcatb.2013.04.043. [DOI] [Google Scholar]
  46. Yang J.; Li Z. F.; Yang C. L.; Ma Y. Y.; Li Y. Y.; Zhang Q.; Song K.; Cui J. X. Significant promoting effect of La doping on the wide temperature NH3-SCR performance of Ce and Cu modified ZSM-5 catalysts. J. Solid State Chem. 2022, 305, 122700 10.1016/j.jssc.2021.122700. [DOI] [Google Scholar]
  47. Lee K. J.; Kumar P. A.; Maqbool M. S.; Rao K. N.; Song K. H.; Ha H. P. Ceria added Sb-V2O5/TiO2 catalysts for low temperature NH3 SCR: Physico-chemical properties and catalytic activity. Appl. Catal. B. 2013, 142–143, 705–717. 10.1016/j.apcatb.2013.05.071. [DOI] [Google Scholar]
  48. Liu S. S.; Wang H.; Wei Y.; Zhang R. D. Core-shell structure effect on CeO2 and TiO2 supported WO3 for the NH3-SCR process. Mol. Catal. 2020, 485, 110822 10.1016/j.mcat.2020.110822. [DOI] [Google Scholar]
  49. Xu Q.; Su R. G.; Cao L.; Li Y. Q.; Yang C. Y.; Luo Y.; Street J.; Jiao P. C.; Cai L. L. Facile preparation of high-performance Fe-doped Ce-Mn/TiO2 catalysts for the low-temperature selective catalytic reduction of NOx with NH3. RSC Adv. 2017, 7, 48785–48792. 10.1039/C7RA07854D. [DOI] [Google Scholar]
  50. Cheng J.; Song L. Y.; Wu R.; Li S. N.; Sun Y. M.; Zhu H. T.; Qiu W. G.; He H. Promoting effect of microwave irradiation on CeO2-TiO2 catalyst for selective catalytic reduction of NO by NH3. J. Rare Earth 2020, 38, 59–69. 10.1016/j.jre.2019.04.014. [DOI] [Google Scholar]
  51. Wang Q. L.; Zhou J. J.; Zhang J. C.; Zhu H.; Feng Y. H.; Jin J. Effect of Ceria Doping on the Catalytic Activity and SO2 Resistance of MnOx/TiO2 Catalysts for the Selective Catalytic Reduction of NO with NH3 at Low Temperatures. Aerosol Air Qual. Res. 2020, 20, 477–488. 10.4209/aaqr.2019.10.0546. [DOI] [Google Scholar]
  52. Wang R. H.; Hao Z. F.; Li Y.; Liu G. Q.; Zhang H.; Wang H. T.; Xia Y. G.; Zhan S. H. Relationship between structure and performance of a novel highly dispersed MnOx on Co-Al layered double oxide for low temperature NH3-SCR. Appl. Catal., B 2019, 258, 117983 10.1016/j.apcatb.2019.117983. [DOI] [Google Scholar]
  53. Yu Q.; Manfred R.; Li L. D.; Kong F. X.; Wu G. J.; Guan N. J. The promotional effect of Cr on catalytic activity of Pt/ZSM-35 for H2-SCR in excess oxygen. Catal. Commun. 2010, 11, 955–959. 10.1016/j.catcom.2010.03.021. [DOI] [Google Scholar]
  54. Wang X.; Su J.; et al. Promotional synergistic effect of Cu and Nb doping on a novel Cu/Ti-Nb ternary oxide catalyst for the selective catalytic reduction of NOx with NH3. Appl. Catal., B 2018, 220, 234–250. 10.1016/j.apcatb.2017.08.021. [DOI] [Google Scholar]
  55. Hou X.; Chen H.; Liang Y.; Wei Y.; Li Z. La Modified Fe-Mn/TiO2 Catalysts to Improve SO2 Resistance for NH3-SCR at Low-Temperature. Catal. Surv. Asia 2020, 24, 291–299. 10.1007/s10563-020-09309-1. [DOI] [Google Scholar]
  56. Liu T. K.; Wei L. Q.; Yao Y. Y.; Dong L. H.; Li B. La promoted CuO-MnOx catalysts for optimizing SCR performance of NO with CO. Appl. Surf. Sci. 2021, 546, 148971 10.1016/j.apsusc.2021.148971. [DOI] [Google Scholar]
  57. Li N.; Chen Z. D.; Bai X. R.; Hou L. M.; Jiao K. L.; Wu W. F. Study on the mechanism of synthetic (Ce,La)CO3F sulfuric acid acidification and NH3-SCR loaded with Mn and Fe. RSC Adv. 2021, 11, 19943–19955. 10.1039/D1RA02788C. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Ma L. P.; Ren W. C.; Cheng H. M. Progress in Surface Charge Transfer Doping of Graphene. Acta Phys.-Chim. Sin. 2022, 38, 2012080. [Google Scholar]
  59. Ali S.; Chen L. Q.; Li Z. B.; Zhang T. R.; Li R.; Bakhtiar S. H.; Leng X. S.; Yuan F. L.; Niu X. Y.; Zhu Y. J. Cux-Nb1.1-x (x = 0.45, 0.35, 0.25, 0.15) bimetal oxides catalysts for the low temperature selective catalytic reduction of NO with NH3. Appl. Catal., B 2018, 236, 25–35. 10.1016/j.apcatb.2018.05.014. [DOI] [Google Scholar]
  60. Putluru S. S. R.; Mossin S.; Riisager A.; F Ehrmann R. Heteropoly acid promoted Cu and Fe catalysts for the selective catalytic reduction of NO with ammonia. Catal. Today 2011, 176, 292–297. 10.1016/j.cattod.2010.11.087. [DOI] [Google Scholar]
  61. Fan J.; Ning P.; Wang Y. C.; Song Z. X.; Liu X.; Wang H. M.; Wang J.; Wang L. Y.; Zhang Q. L. Significant promoting effect of Ce or La on the hydrothermal stability of Cu-SAPO-34 catalyst for NH3-SCR reaction. Chem. Eng. J. 2019, 369, 908–919. 10.1016/j.cej.2019.03.049. [DOI] [Google Scholar]
  62. Wang Z. Y.; Guo R. T.; Guan Z. Z.; Shi X.; Pan W. G.; Fu Z. G.; Qin H.; Liu X. Y. The promotion effect of Cr additive on CeZr2Ox catalyst for the low-temperature selective catalytic reduction of NOx with NH3. Appl. Surf. Sci. 2019, 485, 133–140. 10.1016/j.apsusc.2019.04.199. [DOI] [Google Scholar]
  63. Liu H.; Wei L.; Yue R.; Chen Y. CrOx-CeO2 binary oxide as a superior catalyst for NO reduction with NH3 at low temperature in presence of CO. Catal. Commun. 2010, 11, 829–833. 10.1016/j.catcom.2010.03.002. [DOI] [Google Scholar]
  64. Yuan E. H.; Li M.; Yang M. H.; Huang X. S.; Zhang K.; Han W. L.; Tang Z. C.; Liu Z. W. Encapsulation of ultra-small Cu-Fe into ZSM-5 zeolites for NH3-SCR with broad reaction-temperature ranges. Microporous Mesoporous Mater. 2022, 331, 111675 10.1016/j.micromeso.2021.111675. [DOI] [Google Scholar]
  65. Jiang L. J.; Liang Y.; Liu W. Z.; Wu H. L.; Aldahri T.; Carrero D. S.; Liu Q. C. Synergistic effect and mechanism of FeOx and CeOx co-doping on the superior catalytic performance and SO2 tolerance of Mn-Fe-Ce/ACN catalyst in low-temperature NH3-SCR of NOx. J. Environ. Chem. Eng. 2021, 9, 106360 10.1016/j.jece.2021.106360. [DOI] [Google Scholar]
  66. Cai W.; Zhao Y. X.; Chen M. D.; Jiang X. M.; Wang H.; Ou M.; Wan S. P.; Zhong Q. The formation of 3D spherical Cr-Ce mixed oxides with roughness surface and their enhanced low-temperature NO oxidation. Chem. Eng. J. 2018, 333, 414–422. 10.1016/j.cej.2017.10.002. [DOI] [Google Scholar]
  67. Gao E. H.; Huang B.; Zhao Z. L.; Pan H.; Zhang W.; Li Y. N.; Bernards M. T.; He Y.; Shi Y. Understanding the co-effects of manganese and cobalt on the enhanced SCR performance for Mn(x)Co(1-x)Cr(2)O(4) spinel-type catalysts. Catal. Sci. Technol. 2020, 10, 4752–4765. 10.1039/D0CY00872A. [DOI] [Google Scholar]
  68. Yang C. X.; Zhang K. X.; Zhang Y. K.; Peng G. J.; Yang M.; Wen J. J.; Xie Y.; Xia F. T.; Jia L. J.; Zhang Q. L. An environmental and highly active Ce/Fe-Zr-SO42– catalyst for selective catalytic reduction of NO with NH3: The improving effects of CeO2 and SO4. J. Environ. Chem. Eng. 2021, 9, 106799 10.1016/j.jece.2021.106799. [DOI] [Google Scholar]
  69. Jia Y.; Yang J. H.; Jiang J.; Huang M. Y.; Guo L. N.; Zhang C. W.; Yuan J.; Gu M. Y. Low temperature SCR de-NOx performance and mechanism of heteropolyacid VPO-Cr-PEG/TiO2 catalyst. Catal. Commun. 2021, 153, 106299 10.1016/j.catcom.2021.106299. [DOI] [Google Scholar]
  70. Li S. C.; Huang W. J.; Xu H. M.; Chen T. J.; Ke Y.; Qu Z.; Yan N. Q. Alkali-induced deactivation mechanism of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO by NH3 in aluminum hydrate calcining flue gas. Appl. Catal., B 2020, 270, 118872 10.1016/j.apcatb.2020.118872. [DOI] [Google Scholar]
  71. Xu H. D.; Wang Y.; Cao Y.; Fang Z. T.; Lin T.; Gong M. C.; Chen Y. Q. Catalytic performance of acidic zirconium-based composite oxides monolithic catalyst on selective catalytic reduction of NOx with NH3. Chem. Eng. J. 2014, 240, 62–73. 10.1016/j.cej.2013.11.053. [DOI] [Google Scholar]
  72. Chitsazi H.; Wu R.; Zhang N. Q.; He J. D.; Zhang G. Z.; He H. Enhancement of Low-Temperature NH3-SCR Catalytic Activity and H2O Resistance Ability Over MnOx/TiO2 Catalyst by Expanded Graphite. Catal. Lett. 2020, 150, 2688–2694. 10.1007/s10562-020-03170-7. [DOI] [Google Scholar]
  73. Zhou X. M.; Huang X. Y.; Xie A. J.; Luo S. P.; Yao C.; Li X. Z.; Zuo S. X. V2O5-decorated Mn-Fe/attapulgite catalyst with high SO2 tolerance for SCR of NOx with NH3 at low temperature. Chem. Eng. J. 2017, 326, 1074–1085. 10.1016/j.cej.2017.06.015. [DOI] [Google Scholar]
  74. Wu X.; Wang R. N.; Du Y. L.; Li X. J.; Meng H.; Xie X. M. NOx removal by selective catalytic reduction with ammonia over hydrotalcite-derived NiTi mixed oxide. New J. Chem. 2019, 43, 2640–2648. 10.1039/C8NJ05280H. [DOI] [Google Scholar]
  75. Chen L.; Wang D.; Wang J. D.; Weng D.; Cao L. Hydrothermal and sulfur aging of CeTi/CeWTi catalysts for selective catalytic reduction of NOx with NH3. J Rare Earth 2019, 37, 829–836. 10.1016/j.jre.2018.10.021. [DOI] [Google Scholar]

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