Synthesis of W-modified CeO₂/ZrO₂ catalysts for selective catalytic reduction of NO with NH₃

Abstract

In this paper, a series of tungsten–zirconium mixed binary oxides (denoted as W_mZrO_x) were synthesized via co-precipitation as supports to prepare Ce_0.4/W_mZrO_x catalysts through an impregnation method. The promoting effect of W doping in ZrO₂ on selective catalytic reduction (SCR) performance of Ce_0.4/ZrO₂ catalysts was investigated. The results demonstrated that addition of W in ZrO₂ could remarkably enhance the catalytic performance of Ce_0.4/ZrO₂ catalysts in a broad temperature range. Especially when the W/Zr molar ratio was 0.1, the Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window of 226–446 °C (NO_x conversion rate > 80%) and its N₂ selectivity was almost 100% in the temperature of 150–450 °C. Moreover, the Ce_0.4/W_0.1ZrO_x catalyst also exhibited good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to a 100 ppm SO₂ atmosphere for 18 h. Various characterization results manifested that a proper amount of W doping in ZrO₂ was not only beneficial to enlarge the specific surface area of the catalyst, but also inhibited the growth of fluorite structure CeO₂, which were in favor of CeO₂ dispersion on the support. The presence of W was conducive to the growth of a stable tetragonal phase crystal of ZrO₂ support, and a part of W and Zr combined to form W–Zr–O_x solid super acid. Both of them resulted in abundant Lewis acid sites and Brønsted acid sites, enhancing the total surface acidity, thus significantly improving NH₃ species adsorption on the surface of the Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, the promoting effect of adding W on SCR performance was also related to the improved redox capability, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and abundant surface chemisorbed oxygen species. The in situ DRIFTS results indicated that nitrate species adsorbed on the surface of the Ce_0.4/W_0.1ZrO_x catalyst could react with NH₃ due to the activation of W. Therefore, the reaction pathway over the Ce_0.4/W_0.1ZrO_x catalyst followed both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms at 250 °C.

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1. Introduction

Selective catalytic reduction of nitrogen oxides with NH₃ (NH₃-SCR) has been widely employed for NO_x abatement applications in stationary and mobile sources.^1,2 During the past decades, V₂O₅–WO₃ (or MoO₃)/TiO₂ had been considered the most pervasive and efficient SCR catalysts.³ However, these catalysts still suffer from some inevitable shortcomings in practical application, such as a narrow operation temperature window (300–400 °C), toxicity of vanadium pentoxide and low N₂ selectivity at high temperatures.⁴ Given these disadvantages, great efforts have been made to develop environmentally friendly catalysts with a wide temperature window and high N₂ selectivity.

In recent years, some non-toxic SCR catalysts such as MnO_x, Fe₂O₃, CuO and CeO₂, have been extensively investigated in order to substitute vanadium-based catalysts.^5–8 Among them, cerium-based NH₃-SCR catalysts have attracted a lot of researchers' interest due to their high oxygen storage/release capacity and remarkable redox properties, which are significant to the oxidation of NO_x and the acceleration of NH₃-SCR reactions.⁹ However, pure CeO₂ catalysts exhibit poor thermal stability and are easy to sinter at high temperature. In addition, the high active surface oxygen of a pure CeO₂ catalyst results in NH₃ oxidation on the catalyst surface, especially at high temperature, leading to a decrease in SCR activity.^8,10 It is generally believed that acid sites are beneficial to suppress NH₃ oxidation and promoting NH₃ adsorption on the catalyst surface. Therefore, it should be feasible to enhance acid sites to improve NO_x conversion and N₂ selectivity of a CeO₂ catalyst.^11,12

Zirconia (ZrO₂) is an acid-based amphoteric oxide with excellent redox capability and high refractory property. Previous studies reported that the addition of ZrO₂ to CeO₂ led to an improvement on thermal stability and oxygen storage capacity.¹³ ZrO₂-supported CeO₂ catalysts exhibited good oxygen storage capacity and highly refractory property at the same time. It can utilize the large surface area of ZrO₂ to promote the dispersion of CeO₂ on catalyst surface. Previous studies showed that CeO₂/ZrO₂ catalysts possessed excellent NH₃-SCR activity at medium temperature.^14,15 Nonetheless, the low-temperature activity and SO₂ tolerance of CeO₂/ZrO₂ catalyst are still not very satisfactory, which hinders their industrial application.

As an important additive in traditional V-based catalysts, WO₃ has been recognized as an excellent “chemical” and “structural” promoter to improve SCR performance obviously.¹⁶ Previous studies have shown that the addition of WO₃ could enhance the adsorption and activation of NH₃ by increasing the surface acidity of the catalysts, which was beneficial to the improvement of NH₃-SCR activity.^17,18 Recently, Fang et al. prepared WO₃/Ce_0.65Zr_0.35O₂ catalyst by co-precipitation and impregnation method, it could obtain an excellent NH₃-SCR performance at 250–450 °C.¹⁹ Väliheikki et al. have proven that the WO₃/Ce_0.85Zr_0.15O₂ catalyst exhibited high SO₂ and H₂O resistance in the temperature range of 300–500 °C.²⁰ In these studies, WO₃ was usually used as a surface modifier to modify the catalyst surface. However, there are few reports about the incorporation of W into ZrO₂ to form binary metal oxide support for NH₃-SCR. Chen et al. reported that, the addition of W in ZrO₂ could enhance the total acidity and redox properties by forming W–Zr–O_x, which would greatly promote the SCR performance.^21,22 The authors considered that W–Zr–O_x solid super acid could be used as SCR support with a high surface area, which might enhance the catalytic activity of Ce/Zr catalysts greatly.

In this work, we focused on the effects of W doping in ZrO₂ on SCR performance of Ce/ZrO₂ catalyst. A series of Ce/WZrO_x catalysts were prepared by successive co-precipitation and impregnation methods. Catalytic performance tests showed that Ce/WZrO_x catalysts exhibited a much higher NO_x removal efficiency than that of Ce/ZrO₂ catalyst. Further, the effects of W doping in ZrO₂ were investigated in detail by using N₂ physisorption, XRD, Raman, SEM, TEM, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. Finally, the possible reaction mechanisms were also discussed to gain insights into the effect of WZrO_x solid super acid support on SCR reaction pathways.

2. Experimental

2.1 Catalyst preparation

A series of tungsten–zirconium oxides with various molar ratios of W/Zr were prepared by using the co-precipitation method. The typical synthesis process was as follows: a proper amount of Zr(NO₃)₄·5H₂O and (NH₄)₁₀H₂(W₂O₇)₆·xH₂O were dissolved in deionized water. Then the mixed solution was heated to 40 °C and held for 2 h under continuous magnetic stirring. Next, ammonia solution (25 wt%) was added dropwise to the above solution with vigorous stirring to adjust the solution pH to 10. The obtained precipitate was naturally cooled down to room temperature for 5 h and then filtered, and washed with deionized water until pH changed little. Afterwards, the precipitate was washed with anhydrous ethanol, and dried at 80 °C overnight. The collected solid was calcined at 550 °C in air for 3 h, and finally grounded into a fine powder. The prepared tungsten–zirconium mixed oxides were denoted as W_mZrO_x, where m represented the molar ratio of W/Zr (m = 0.025, 0.05, 0.1, 0.2). Pristine ZrO₂ was also prepared for reference by using the precipitation method.

Both Ce_0.4/ZrO₂ and Ce_0.4/W_mZrO_x catalysts were prepared by the impregnation method, where 0.4 represented the molar ratio of Ce/Zr. Firstly, a certain amount of Ce(NO₃)₃·6H₂O was dissolved in deionized water. Then a desired amount of ZrO₂ or W_mZrO_x powder was impregnated in the solution with strong stirring for 0.5 h. Next, the mixture continued to be stirred sufficiently at 80 °C in a water bath to evaporate the solvent. The solid was dried at 100 °C for 12 h, and calcined at 500 °C for 3 h in air. Finally, all catalysts were crushed and sieved to 40–60 mesh for testing.

2.2 Catalyst activity test

The SCR activity tests of these prepared catalysts were carried out in a fixed-bed quartz reactor (I.D. 6 mm) at atmospheric pressure with a catalyst dosage of 0.5 mL (40–60 mesh). SCR activity measurements were operated in a temperature range of 150–450 °C. The simulated gas consisted of 500 ppm NO, 500 ppm NH₃, 5 vol% O₂, 100 ppm SO₂ (when used) and N₂ as balance gas with a total flow rate of 500 mL min⁻¹. The corresponding gas hourly space velocity (GHSV) was 60 000 h⁻¹. The outlet concentrations of NO, NO₂, NH₃ and N₂O were monitored by an FTIR spectrometer (Antaris IGS, ThermoFisher Scientific) equipped with a heated low-volume multiple-path gas cell (2 m) and an MCT detector cooled by liquid nitrogen. Here NO_x referred to the sum of NO and NO₂. NO_x conversion efficiency and N₂ selectivity were calculated as follows:

2.3 Catalyst characterization

The textural properties of the prepared samples were measured using N₂ physisorption (ASAP 2020 PLUS, Micromeritics). The powder X-ray diffraction (XRD) patterns were performed on a diffractometer (TTRAX III, Rigaku, Japan) with a Cu-Kα radiation source (λ = 0.15406 nm) under 40 kV and 30 Ma. The Raman spectra of samples were carried out at a Raman Spectrometer (RM2000, Renishaw), using an Ar ion laser (532 nm) as the excitation source. The morphology of the samples was characterized by scanning electron microscopic (SEM, Tescan Mira4). The transmission electron microscopic (TEM) images were performed on FEI Talos F200X and the chemical analysis was obtained by energy dispersive X-ray spectrometer (EDS, Oxford Ultim Max65). X-ray photoelectron spectroscopy (XPS) measurement was obtained on a surface analysis photoelectron spectrometer (ESCALAB 250Xi, ThermoFisher Scientific) using Al Kα as a radiation source. Temperature programmed reduction with H₂ (H₂-TPR) experiments were operated on a chemisorption analyzer (Autochem II 2920, Micromeritics). Temperature programmed desorption of NH₃ (NH₃-TPD) experiments were operated on a chemisorption analyzer (Autochem II 2920, Micromeritics). In situ DRIFTS measurements were carried out by an FTIR spectrometer (Nicolet iS50, ThermoFisher Scientific) equipped with an MCT/A detector. The spectral resolution was 4 cm⁻¹ with co-addiction 64 scans.

3. Results and discussion

3.1 SCR performance

The catalytic performance of the prepared catalysts for NH₃-SCR of NO_x in the temperature range of 150–450 °C was tested, and the results were displayed in Fig. 1. It could be seen from Fig. 1(a) that W-doped ZrO₂ supports imposed significant impacts on SCR catalytic activities of Ce_0.4/ZrO₂ catalysts. Without W doping, Ce_0.4/ZrO₂ catalyst showed rather poor SCR activity in the whole temperature region with the maximum NO_x conversion of only about 56% at 370 °C, which was in accordance with our previous study.¹⁵ In contrast, Ce_0.4/W_mZrO_x catalysts exhibited much better catalytic activity in the test temperature range. With the increase of W/Zr molar ratio from 0.025 to 0.1, the promotional effect of W on SCR activity was observed over W-containing catalysts with dramatically increasing NO_x conversion and broadened operation temperature windows. However, further increasing W/Zr molar ratio to 0.2, SCR performance of Ce_0.4/W_0.2ZrO_x catalysts deteriorated obviously in the whole operating temperature, and NO_x conversion efficiency was only 41% at 226 °C. After all, Ce_0.4/W_0.1ZrO_x catalyst possessed the largest active temperature window (NO_x conversion rate > 80%) of 226–446 °C under GHSV of 60 000 h⁻¹. Fig. 1(b) showed the N₂ selectivity of Ce_0.4/ZrO₂ and Ce_0.4/W_mZrO_x catalysts. It could be seen that N₂ selectivity over Ce_0.4/ZrO₂ catalyst began to decline slowly when the reaction temperature was above 375 °C, and reduced to 95% at 450 °C. In contrast, all Ce_0.4/W_mZrO_x catalysts exhibited superior N₂ selectivity. It was close to 100% in the whole temperature range. The above results demonstrated that the doping of W in ZrO₂ supports could remarkably improve NH₃-SCR performance of Ce/Zr catalysts. Since the comprehensive performance of Ce_0.4/W_0.1ZrO_x catalyst was obviously better than other catalysts, comparative investigations between Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were conducted to elucidate the effect of W–Zr binary metal oxide support on NH₃-SCR performance.

Fig. 1 SCR performance test results of prepared catalysts: (a) NO_x conversion and (b) N₂ selectivity of Ce_0.4/ZrO₂ and Ce_0.4/W_mZrO_x catalysts. (c) SO₂ resistance test over Ce_0.4/W_0.1ZrO_x catalyst at 300 °C. (Reaction conditions: 0.5 mL catalyst, [NO] = [NH₃] = 500 ppm, [O₂] = 5 vol%, [SO₂] = 100 ppm (when used), balance with N₂, total flow rate = 500 mL min⁻¹ and GHSV = 60 000 h⁻¹).

It is well known that the common catalysts (vanadium-based) would gradually sinter and the catalytic performance decreased seriously after the SCR reactions. Therefore, the thermostability of catalyst was an important factor that must be considered in the practical application. To investigate the thermostability, Ce_0.4/W_0.1ZrO_x catalyst was cycled two times SCR reactions (as shown in Fig. S1†). It could be seen that there was no significant difference in the catalytic performance between two cycles. In addition, XRD, H₂-TPR and NH₃-TPD techniques were used for the fresh Ce_0.4/W_0.1ZrO_x and the used Ce_0.4/W_0.1ZrO_x (2^nd cycle) catalysts to investigate the effect of SCR reaction process on the structure, redox and surface acidity over Ce_0.4/W_0.1ZrO_x catalyst (as shown in Fig. S2–S4†). These results demonstrated that the Ce_0.4/W_0.1ZrO_x catalysts structure, redox and surface acidity were not significantly different before and after the SCR reactions. In other words, Ce_0.4/W_0.1ZrO_x catalyst exhibited excellent thermostability and its catalytic performance remained high even after treatment at high temperature.

Considered that the flue gas usually contained a certain concentration of SO₂ in practical cases, which would impose a significant impact on the deactivation of NH₃-SCR catalysts. Hence, the effect of SO₂ on NO_x conversion over Ce_0.4/W_0.1ZrO_x catalyst as a function of time was carried out at 300 °C, and the result was shown in Fig. 1(c). As 100 ppm SO₂ was introduced in the feeding gas, NO_x conversion efficiency of Ce_0.4/W_0.1ZrO_x catalyst began to decrease slowly, and reduced to 94% within the first 1 h. After stopping SO₂ injection, NO_x conversion kept still stable at ∼94%. The result indicated that Ce_0.4/W_0.1ZrO_x catalyst had an excellent tolerance to SO₂ at 300 °C, and the slight deactivation due to SO₂ poisoning was not irreversible.

3.2 Structural and textural characteristics

3.2.1 BET. N₂ adsorption–desorption isotherms of the prepared samples were collected to understand the textural properties of Ce_0.4/ZrO₂ and Ce_0.4/W_mZrO_x catalysts. It could be seen from Fig. 2 that all samples exhibited type-IV isotherms according to IUPAS, suggesting the presence of mesoporous materials.²³ The BET surface area, pore size and pore volume of Ce_0.4/ZrO₂ and Ce_0.4/W_mZrO_x catalysts were presented in Table 1. The specific surface area of Ce_0.4/ZrO₂ catalyst was 46.1 m² g⁻¹. With W/Zr molar ratio increasing from 0.025 to 0.1, the specific surface area over Ce_0.4/W_mZrO_x catalysts increased from 42.9 to 57.9 m² g⁻¹. It implied that a proper amount of W doping had an improving effect on specific surface area of Ce_0.4/ZrO₂ catalyst. Nevertheless, when further increasing W/Zr molar ratio from 0.1 to 0.2, the specific surface area of Ce_0.4/W_mZrO_x catalyst decreased sharply from 57.9 to 29.4 m² g⁻¹. It may be due to the excessive W causing the aggregation of active species (CeO₂) on the surface of Ce_0.4/W_0.2ZrO_x catalyst. Generally, the increase of the specific surface area could provide more reaction sites, thus improving SCR catalytic activity.^24–26 Although BET surface area of Ce_0.4/W_0.025ZrO_x (42.9 m² g⁻¹) and Ce_0.4/W_0.2ZrO_x (29.4 m² g⁻¹) catalysts was lower than that of Ce_0.4/ZrO₂ (46.1 m² g⁻¹) catalyst, they exhibited much better higher NH₃-SCR activity, indicating that the BET surface area might not play a key role in SCR reactions.

Fig. 2 N₂ adsorption–desorption isotherms of prepared catalysts.

Table 1 BET surface area and pore structure results of prepared catalysts

Catalysts	SBET (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)
Ce0.4/ZrO2	46.1	11.2	0.14
Ce0.4/W0.025ZrOx	42.9	9.1	0.11
Ce0.4/W0.05ZrOx	51.7	8.1	0.10
Ce0.4/W0.1ZrOx	57.9	6.5	0.09
Ce0.4/W0.2ZrOx	29.4	9.7	0.07

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3.2.2 XRD and Raman results. The XRD patterns of ZrO₂, W_0.1ZrO_x, Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x samples were illustrated in Fig. 3. It could be seen from Fig. 3(a) that pristine ZrO₂ exhibited characteristic peaks of monoclinic and tetragonal phases (PDF-ICDD 50-1089), respectively.^14,15 After the introduction of W, no characteristic diffraction peaks of monoclinic phase ZrO₂ could be found in the curve of W_0.1ZrO_x sample. It suggested that the addition of W in ZrO₂ support was conducive to the formation of a stable tetragonal phase crystal and inhibited the formation of the monoclinic phase. Previous studies proved that the tetragonal phase surface exhibited stronger acidity than that of the monoclinic phase.²⁷ It was beneficial to promote the adsorption of NH₃ species on catalyst surface, thus enhancing SCR reactions. However, no obvious WO₃ phase was detected in XRD pattern over W_0.1ZrO_x sample, suggesting that W was uniformly dispersed on the surface of support or entered into the ZrO₂ lattice. In order to further confirm the above results, W_0.1ZrO_x sample was tested by TEM and EDS. As showed in Fig. S5,† the TEM pattern of W_0.1ZrO_x sample only observed the lattice of ZrO₂ and did not detect the lattice belonging to WO₃. EDS scan mapping results showed that the W species were well dispersed on the support, as presented in Fig. S6.† Besides, compared to ZrO₂, the peak at ∼50.2° corresponded to tetragonal phase ZrO₂ in W_0.1ZrO_x sample shifted to a higher degree (as in Fig. 3(b)), indicating that the cell parameter of ZrO₂ had changed, which was listed in Table 2. It is noted that the radius of W⁶⁺ (0.65 Å) was smaller than that of Zr⁴⁺ (0.79 Å). W⁶⁺ was easier to enter into the ZrO₂ lattice, leading to the cell volume (V_cell = abc) over ZrO₂ sample from 67.01 ų decreased to 66.79 ų. This result showed that W had entered the lattice of ZrO₂ to form W–Zr-O_x solid super acid.^21,28 Therefore, the introduction of W species might exist in two forms: amorphous tungsten oxide and W–Zr-O_x solid super acid.

Fig. 3 (a) XRD patterns and (b) their enlarged views of ZrO₂, W_0.1ZrO_x, Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts.

Table 2 Cell parameter, cell volume and crystallite size of prepared samples

Samples	2θ/°	Cell parameter/Å		Cell volume/Å^3	Crystallite size/Å
		a = b	c
ZrO2	50.24	3.600	5.168	67.01	79
W0.1ZrOx	50.43	3.601	5.150	66.79	34
Ce0.4/ZrO2	50.11	3.599	5.157	66.84	85
Ce0.4/W0.1ZrOx	50.34	3.602	5.146	66.79	45

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After impregnation of CeO₂, the crystal structures of ZrO₂ and W_0.1ZrO_x in Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were the same as to their single supports, as shown in Fig. 3(a). Some diffraction peaks located at 28.6, 33.1, 47.4 and 56.4° could be identified over Ce_0.4/ZrO₂ catalyst, which was attributed to (111), (200), (220) and (311) planes of fluorite structure CeO₂ (PDF-ICDD 34-0394). As to Ce_0.4/W_0.1ZrO_x catalyst, the characteristic peaks corresponding to the crystalline phases of CeO₂ could also be detected, but the peak intensities became much weaker compared to those of Ce_0.4/ZrO₂ catalyst. This phenomenon indicated that the existence of W species could suppress the formation of fluorite structure CeO₂, leading to a decrease in the crystallite size. It was beneficial to obtain a highly-dispersed state of ceria oxide active species over W_0.1ZrO_x support, thus enhancing NH₃-SCR activity. Besides, the difference in BET surface area between Ce_0.4/ZrO₂ and W-containing samples could be interpreted by the crystal phase. From the XRD result, it could be seen that the introduction of W could inhibit the ZrO₂ phase transformation from a tetragonal phase to a denser monoclinic phase.^29,30 Moreover, an appropriate amount of W led to high dispersion of active species on catalyst surface. It was conducive to the increase of surface area for W-containing samples.

Raman characterization results were presented in Fig. 4. For pristine ZrO₂ sample, the band at 98, 187, 333, 380, 473, 560 and 613 cm⁻¹ were assigned to the Raman-active modes for monoclinic phase of ZrO₂, and other bands at 143, 315 and 641 cm⁻¹ were assigned to the tetragonal ZrO₂.¹³ As to W_0.1ZrO_x sample, the peaks at 147, 284, 315, 457 and 645 cm⁻¹ were typically characteristic peaks of tetragonal ZrO₂, and no Raman bands corresponding to the characteristic peaks of monoclinic phase were detected.¹⁵ Note that, two Raman bands corresponded to tetragonal phase over W_0.1ZrO_x sample had been shifted to 147 and 645 cm⁻¹ respectively, which might be attributed to a strong interaction between W and Zr in the form of W–Zr-O_x solid super acid. This result was well in accordance with the above XRD results.

Fig. 4 Raman results of ZrO₂ and W_0.1ZrO_x samples.

3.2.3 SEM. In order to determine their morphologies, ZrO₂, W_0.1ZrO_x, Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x samples were characterized by SEM (Scanning electron micrographs). The resultant micrographs were demonstrated in Fig. 5. As showed in Fig. 5(a) and (b), there was little difference in the morphology of ZrO₂ and W_0.1ZrO_x sample. After impregnation of CeO₂, the morphology changed on Ce_0.4/ZrO₂ catalyst seem to be less obvious, whereas there was almost no agglomeration on the surface of Ce_0.4/W_0.1ZrO_x catalyst. It was evident from Fig. 5(c) and (d) that the crystallite size on the surface of Ce_0.4/W_0.1ZrO_x catalyst was smaller than that of Ce_0.4/ZrO₂ catalyst. This was in line with the XRD results (Fig. 3), suggesting that existence of W species could significantly inhibit the crystallite size of oxide active species, thus improving the highly-dispersed state of ceria oxide species over W_0.1ZrO_x support.

Fig. 5 SEM images of ZrO₂ (a), W_0.1ZrO_x (b), Ce_0.4/ZrO₂ (c) and Ce_0.4/W_0.1ZrO_x (d).

3.2.4 XPS. The XPS spectra of Ce 3d, O 1s, Zr 3d and W 4f over Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were shown in Fig. 6, these absorbed peaks were calibrated against the C 1s peak standardized at 284.8 eV.³¹

Fig. 6 XPS spectra of (a) Ce 3d, (b) O 1s, (c) Zr 3d and (d) W 4f of prepared catalysts.

As shown in Fig. 6(a), the XPS spectra of Ce were fitted into 8 sub-peaks, in which two sub-bands marked in red represent 3d¹⁰4f¹ state of Ce³⁺, and the other ones marked in blue correspond to 3d¹⁰4f⁰ state of Ce⁴⁺.^32,33 The Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratios were calculated as the integral areas of the corresponding curves, and the results were listed in Table 3. Compared to Ce_0.4/ZrO₂ catalyst, the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) at the surface of Ce_0.4/W_0.1ZrO_x catalyst increased from 12.6% to 16.5%. The increase of Ce³⁺ content might be due to the interaction between cerium and the neighboring W atoms.¹⁶ Since the existence of Ce³⁺ species could induce a charge imbalance and unsaturated chemical bonds on the catalyst surface, it was conducive to improve redox properties and surface active oxygen content.^32,33 As there were abundant Ce³⁺ species at the surface of Ce_0.4/W_0.1ZrO_x catalyst, it was reasonable to obtain an enhancement effect on NO oxidation into NO₂, thus facilitating the fast SCR reaction in denitrification process.

Table 3 XPS data of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts

Samples	Surface atom concentration (%)				The relative molar ratio (%)
	Ce	Zr	O	W	Ce^3+/(Ce^3+ + Ce^4+)	Oα/(Oα + Oβ)
Ce0.4/ZrO2	17.9	6.8	75.3	—	12.6	41.7
Ce0.4/W0.1ZrOx	21.6	1.6	75.9	0.9	16.5	51.6

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The O 1s XPS information of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts was presented in Fig. 6(b). Two kinds of surface oxygen species were identified by performing a peak-fitting deconvolution. The peaks at a lower binding energy of 529.0–531.0 eV were assigned to surface lattice oxygen (donated as O_β), and the peaks at a higher binding energy of 531.0–533.0 eV were attributed to the surface chemisorbed oxygen (donated as O_α).⁷ Previous studies pointed out that, surface chemisorbed oxygen (O_α) was highly active in NO oxidation and NH₃ activation process due to its higher mobility than lattice oxygen (O_β).³⁴ The O_α/(O_α + O_β) ratios of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were calculated by the area integral of O_α and O_β curves. As shown in Table 3, the O_α/(O_α + O_β) ratio in Ce_0.4/ZrO₂ catalyst (12.6%) was much lower than that of Ce_0.4/W_0.1ZrO_x catalyst (16.5%). It was possible that the addition of W species resulted in the formation of low-valence state metal cations, thus producing a great deal of oxygen vacancies, charge unbalance and unsaturated chemical bonds on the surface of Ce_0.4/W_0.1ZrO_x catalyst.^17,18 This was also in favor of boosting NO oxidation to NO₂, promoting SCR reactions proceeding through a ‘fast SCR’ route.

Fig. 6(c) presented the Zr 3d XPS spectra of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts. There were two peaks at binding energy of 184.0–185.0 eV (Zr 3d_3/2) and 181.5–182.5 eV (Zr 3d_5/2), which corresponded to Zr⁴⁺ species.³⁵ Apparently, the peak intensities of Zr⁴⁺ for Ce_0.4/ZrO₂ catalyst were much higher than those for Ce_0.4/W_0.1ZrO_x catalyst. Moreover, the peaks of Zr⁴⁺ for Ce_0.4/ZrO₂ catalyst had been shifted to higher binding energy values. It was possibly due to the introduction of W, which resulted in W–Zr–O_x solid super acid at catalyst surface, arising a change in the electron density and lattice spacing of ZrO₂. The results were in accordance with the XRD and Raman results.

Fig. 6(d) presented the XPS spectra of W 4f over Ce_0.4/W_0.1ZrO_x catalyst. Spectrum deconvoluted into two doublets showed two chemical states of W on the surface of Ce_0.4/W_0.1ZrO_x catalyst. The spectral peaks at 35.7 and 37.8 eV corresponded to W 4f_7/2 and W 4f_5/2 respectively, which were attributed to W⁶⁺ state. Doublet with relatively low intensity (peaks at 34.1 and 36.9 eV) corresponded to W⁵⁺ state.³⁶

3.3 Redox properties

H₂-TPR experiments were performed to evaluate the redox properties of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts, and the results were shown in Fig. 7 and Table 4. Two distinctive peaks at 503 and 802 °C could be observed in H₂-TPR profiles of Ce_0.4/ZrO₂ catalyst, corresponding to the reduction of surface Ce⁴⁺ species to Ce³⁺ and bulk Ce⁴⁺ to Ce³⁺.^37,38 For Ce_0.4/W_0.1ZrO_x catalyst, there were three broad reduction peaks around 410, 520 and 792 °C, in which the first peak was assigned to the reduction of the surface Ce⁴⁺ species to Ce³⁺, the second one assigned to the reduction of W⁶⁺ to W⁵⁺, and the third peak assigned to the reduction of bulk Ce⁴⁺ to Ce³⁺.^17,18 Compared with Ce_0.4/ZrO₂ catalyst, the peak corresponding to the reduction of surface Ce⁴⁺ to Ce³⁺ over Ce_0.4/W_0.1ZrO_x catalyst had been shifted to a lower temperature (410 °C). It indicated that the surface Ce⁴⁺ species became more reducible after doping W species. Previous study reported that when host oxide (such as CeO₂) was reducible, the dopant might donate extra electrons to the host cations.³⁹ In view of this, it was very possible that W as dopant would donate electrons to adjacent Ce⁴⁺ species, resulting in a strong interaction between W and Ce, thus improving the redox properties of Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, H₂ consumption amount over Ce_0.4/W_0.1ZrO_x catalyst (2.34 mmol g⁻¹) was much higher than that of Ce_0.4/ZrO₂ catalyst (1.48 mmol g⁻¹). In other words, addition of W species in Ce_0.4/ZrO₂ catalyst support could greatly enhance the redox properties, which was an important factor for promoting SCR catalytic activity of Ce_0.4/W_0.1ZrO_x catalyst at low-temperature.

Fig. 7 H₂-TPR profiles of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts in the range of 100–900 °C.

Table 4 H₂-TPR results of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts

Catalysts	Reduction peak temperature (°C)			H2 consumption (mmol g^−1)
	Peak 1	Peak 2	Peak 3
Ce0.4/ZrO2	503	—	802	1.48
Ce0.4/W0.1ZrOx	410	520	792	2.34

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3.4 Surface acidity

Surface acidity of NH₃-SCR catalysts was one more critical factor in denitrification reaction. NH₃-TPD experiment was performed to probe the number of acid sites in Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts, and the results were presented in Fig. 8. The quantitative analysis results of total surface acidities were listed in Table 5. It could be seen from Fig. 8 that NH₃-TPD profiles of both Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts exhibited three desorption peaks, which were labeled as α, β and γ, respectively. The peak α was attributed to weak acid sites, the peak β was assigned to medium acid sites, and the peak γ was ascribed to strong acid sites.^21,40 As shown in Fig. 8, there were only slight differences in the peak positions of weak and medium acid sites between these two catalysts. However, the peak position of strong acid sites over Ce_0.4/W_0.1ZrO_x catalyst shifted to a much lower temperature compared to Ce_0.4/ZrO₂ catalyst. It could be ascribed to the formation of W–Zr–O_x solid super acid at the surface of Ce_0.4/W_0.1ZrO_x catalyst.²¹ As shown in Table 5, the total acid amounts at the surface of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were 1.42 and 1.94 mmol g⁻¹, respectively. The results demonstrated that, Ce_0.4/W_0.1ZrO_x catalyst possessed a superior total acidity over Ce_0.4/ZrO₂ catalyst. The doping W in ZrO₂ support effectively improved the surface acidity of Ce_0.4/ZrO₂ catalyst, which was beneficial to adsorb more NH₃ species, thus enhancing SCR performance.

Fig. 8 NH₃-TPD curves of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts in the range of 50–700 °C.

Table 5 NH₃-TPD results of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts

Catalysts	Peak center temperature (°C)			Total acidity (mmol g^−1)
	Peak α	Peak β	Peak γ
Ce0.4/ZrO2	122	258	468	1.42
Ce0.4/W0.1ZrOx	125	254	393	1.94

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3.5 In situ DRIFTS

3.5.1 NH₃ adsorption. NH₃-TPD experiment could determine the total amount of acid sites, but it failed to distinguish the acid sites (Brønsted acid sites and Lewis acid sites) and the adsorbed NH₃ species on catalyst surface. Here steady-state in situ DRIFTS experiments of NH₃ adsorption were carried out to ascertain the nature of acid sites and acquire more information about the surface acidity.

Fig. 9 showed the in situ DRIFTS spectra of NH₃ adsorption over Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts at different temperatures (100–350 °C). For Ce_0.4/ZrO₂ catalyst, after NH₃ adsorption, several bands were detected in the range of 1000–1800 cm⁻¹. The bands peaked at 1542 cm⁻¹ and 1152 cm⁻¹ were assigned to asymmetric and symmetric N–H bending vibrations of N–H bonds in coordinated NH₃ linked to Lewis acid sites.^41,42 The band peaked at 1358 cm⁻¹ could be ascribed to the amide (–NH₂) species.⁴³ Obviously, there was no obvious band corresponding to Brønsted acid sites in the in situ DRIFS spectra of NH₃ adsorption over Ce_0.4/ZrO₂ catalyst.

Fig. 9 In situ DRIFTS spectra of (a) Ce_0.4/ZrO₂ and (b) Ce_0.4/W_0.1ZrO_x catalysts at different temperatures. Condition: [NH₃] = 500 ppm and N₂ as balance gas.

For Ce_0.4/W_0.1ZrO_x catalyst, the in situ DRIFTS spectra of NH₃ adsorption over Ce_0.4/W_0.1ZrO_x catalyst were quite different from those for Ce_0.4/ZrO₂ catalyst. The NH₃ species adsorbed on Ce_0.4/W_0.1ZrO_x catalyst surface (1152 and 1542 cm⁻¹) were attributed to coordinated NH₃ on Lewis acid sites. But several new bands could also be detected: the bands peaked at 1585 cm⁻¹ and 1190, 1232 cm⁻¹ were assigned to asymmetric and symmetric bending vibrations of N–H bonds in coordinated NH₃ linked to Lewis acid sites, and the band peaked at 1431 cm⁻¹ was attribute to NH₄⁺ species on Brønsted acid sites.^24,42,44 Compared to Ce_0.4/ZrO₂ catalyst, much more NH₃ could be adsorbed on the surface of Ce_0.4/W_0.1ZrO_x catalyst, which was in accordance with NH₃-TPD results. This result suggested that the introduction of W species tremendously increased the amount of both Brønsted acid sites and Lewis acid sites on catalyst surface, thus significantly improving the adsorption of NH₃ species, which played a key role in NH₃-SCR process.⁴⁵

3.5.2 NO + O₂ co-adsorption. The in situ DRIFTS experiments of NO + O₂ co-adsorption over Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts were also performed to probe NO_x species adsorbed on catalyst surface at different temperatures. As shown in Fig. 10(a), for Ce_0.4/ZrO₂ catalyst, the intensity of band at 1190 cm⁻¹ decreased quickly with temperature increasing from 100 to 150 °C. Meanwhile, some new bands peaked at 1244, 1278, 1354 1533 and 1562 cm⁻¹ appeared obviously. The bands peaked at 1354 and 1383 cm⁻¹ could be assigned to M–NO₂ nitro compounds. The bands peaked at 1244, 1533 and 1562 cm⁻¹ could be ascribed to bidentate nitrates.^46,47 The bands peaked at 1190, 1278 and 1606 cm⁻¹ could be attributed to nitrosyl anion species, monodentate nitrate and gaseous NO₂ molecules, respectively.^33,40,42 The results showed that, the higher the reaction temperature was, the more nitrates could be detected on the surface of Ce_0.4/ZrO₂ catalyst.

Fig. 10 In situ DRIFTS spectra of (a) Ce_0.4/ZrO₂ and (b) Ce_0.4/W_0.1ZrO_x catalysts at different temperatures. Condition: [NO] = 500 ppm, [O₂] = 5 vol% and N₂ as balance gas.

As shown in Fig. 10(b), for Ce_0.4/W_0.1ZrO_x catalyst, several bands, bidentate nitrates (1244, 1533 and 1562 cm⁻¹), monodentate nitrate (1278 cm⁻¹) and bridged nitrate (1219 cm⁻¹), could also be detected after NO + O₂ adsorption, which could be assigned to adsorbed NO_x species.^48,49 The bands peaked at 1354, 1383 cm⁻¹ and 1606 cm⁻¹ were attributed to M-NO₂ nitro compounds and gaseous NO₂ molecules. Compared to in situ DRIFTS spectra of NO + O₂ co-adsorption over Ce_0.4/ZrO₂ catalysts, it was worth noting that the band intensity of adsorbed NO_x species on the surface of Ce_0.4/W_0.1ZrO_x catalyst was significantly weaker. Moreover, with the increase of reaction temperature, the band intensities of adsorbed NO_x species on the surface of Ce_0.4/W_0.1ZrO_x catalyst became weaker gradually. The above results indicated that the introduction of W species not only resulted in more Brønsted acid sites and Lewis acid sites formed on the surface of Ce_0.4/W_0.1ZrO_x catalyst, but also reduced the thermal stability of the inactive nitrate species, leaving more active sites available for the adsorption of NH₃ species. It was conducive to improving SCR performance.

3.5.3 Reactions of pre-adsorbed NH₃ species with NO + O₂. In situ DRIFTS experiments were carried out to investigate the reactions between pre-adsorbed NH₃ species and NO + O₂ over Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts at 250 °C. It could be seen from Fig. 11(a) that, after Ce_0.4/ZrO₂ catalyst being pre-adsorbed with NH₃ and then purged with N₂, there were two weak bands peaked at 1152 cm⁻¹ and 1542 cm⁻¹ corresponding to coordinated NH₃ linked to Lewis acid sites.^41,42 After switching to NO + O₂, the coordinated NH₃ species (1152 cm⁻¹ and 1542 cm⁻¹) were consumed within 3 and 10 min, respectively, indicating that both coordinated NH₃ species adsorbed on the surface of Ce_0.4/ZrO₂ catalyst could participate in SCR reactions.⁵⁰ After reacting for 5 min, the bands corresponding to NO₂ molecules (1606 cm⁻¹), bidentate nitrates (1533 and 1562 cm⁻¹), monodentate nitrate (1278 cm⁻¹), bidentate nitrates (1244 cm⁻¹) began to appear.^40,46,47 These nitrate species formed and accumulated on the surface of Ce_0.4/ZrO₂ catalyst due to the formation of inactive nitrate species. It resulted in less active sites for NH₃ adsorption, which was unfavorable for SCR reactions.⁵¹

Fig. 11 In situ DRIFTS of reactions between pre-adsorbed NH₃ species and NO + O₂ over (a) Ce_0.4/ZrO₂ and (b) Ce_0.4/W_0.1ZrO_x catalysts at 250 °C.

As shown in Fig. 11(b), after saturated adsorption of NH₃ for 30 min, several bands appeared in the spectra over Ce_0.4/W_0.1ZrO_x catalyst. The bands peaked at 1152, 1232, 1542 and 1585 cm⁻¹ on Lewis acid sites were attributed to NH₃ species, while the band peaked at 1431 cm⁻¹ on Brønsted acid sites was assigned to NH₄⁺.⁴⁴ After introduction of NO + O₂, all bands belonging to NH₃ species on Lewis acid sites and Brønsted acid sites decreased obviously in intensity. It could be seen that these NH₃ species had been completely substituted by nitrate species after 10 min. This result indicated that both coordinated NH₃ and NH₄⁺ species on Ce_0.4/W_0.1ZrO_x catalyst surface could act as reducing agents to reduce NO_x. Furthermore, the coordinated NH₃ species over Ce_0.4/W_0.1ZrO_x catalyst played a dominant role in SCR reactions, and the NH₄⁺ species was also involved in SCR reactions. As the doping of W to Ce_0.4/ZrO₂ catalyst resulted in more coordinated NH₃ and ionic NH₄⁺, both of them led to the improvement of NH₃-SCR performance.^21,24

3.5.4 Reactions of pre-adsorbed NO + O₂ with NH₃. A series of in situ DRIFTS experiments were performed to investigate the reactions between pre-adsorbed NO + O₂ species and NH₃ on the surface of Ce_0.4/ZrO₂ and Ce_0.4/W_0.1ZrO_x catalysts at 250 °C, and the results were displayed in Fig. 12.

Fig. 12 In situ DRIFTS of reactions between pre-adsorbed NO + O₂ species and NH₃ over (a) Ce_0.4/ZrO₂ and (b) Ce_0.4/W_0.1ZrO_x catalysts at 250 °C.

As shown in Fig. 12(a), after saturated pre-adsorption of NO + O₂ on the surface of Ce_0.4/ZrO₂ catalyst, several bands, monodentate nitrate (1278 cm⁻¹), bidentate nitrates (1244, 1533 and 1562 cm⁻¹), M–NO₂ nitro compounds (1383 cm⁻¹) and gaseous NO₂ molecules (1606 cm⁻¹), could be detected. After switching to NH₃, the bands corresponding to NO_x species hardly changed in 30 min, this result showed that the pre-adsorbed NO_x species on the surface of Ce_0.4/ZrO₂ catalyst hardly reacted with NH₃. In the meanwhile, with the injection of NH₃, no characteristic bands of NH₃ species appeared, which might be due to the occupation of active sites by stable nitrate/nitrite species. Thus, the poor NH₃-SCR activity of Ce_0.4/ZrO₂ catalyst might be related to the suppressive effect of nitrate/nitrite species on active sites.

Fig. 12(b) showed that, after pre-adsorbed with NO + O₂ on Ce_0.4/W_0.1ZrO_x catalyst surface, bridged nitrate (1219 cm⁻¹), monodentate nitrate (1278 cm⁻¹) and bidentate nitrates (1533 and 1562 cm⁻¹) were formed. Meanwhile, the bands corresponding to M–NO₂ (1383 cm⁻¹) and gaseous NO₂ molecules (1606 cm⁻¹) also appeared. After switching to NH₃, the intensities of the bands (1278, 1383, 1533 and 1562 cm⁻¹) assigned to adsorbed NO_x species decreased gradually, while other bands peaked at 1606 and 1219 cm⁻¹ disappeared in 1 and 3 min, respectively. From then on, Ce_0.4/W_0.1ZrO_x catalyst surface was mainly covered by adsorbed NH₃ species, which were in the form of coordinated NH₃ (1239 and 1665 cm⁻¹) bonded to Lewis acid sites and NH₄⁺ species (1431 cm⁻¹) bonded to Brønsted acid sites. This result showed that the adsorption of nitrate species on Ce_0.4/W_0.1ZrO_x catalyst surface could react with NH₃. Though the addition of W might inhibit the adsorption of nitrate species on Ce_0.4/W_0.1ZrO_x catalyst surface (see Fig. 10), the reactions between adsorbed nitrate species and NH₃ could still play an important role in NH₃-SCR of NO_x.

3.5.5 Discussion on reaction mechanism. The in situ DRIFTS results showed that NH₃ species pre-adsorbed on the surface of Ce_0.4/ZrO₂ catalyst could react with NO and had been completely consumed within a short time. On the other hand, NO_x species pre-adsorbed on the surface of Ce_0.4/ZrO₂ catalyst could not react with NH₃. Therefore, SCR reactions over Ce_0.4/ZrO₂ catalyst occurred only via Eley–Rideal (E–R) mechanism at 250 °C.

As to Ce_0.4/W_0.1ZrO_x catalyst, both Eley-Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms had been followed during NH₃-SCR reactions at 250 °C. Moreover, E–R rather than L–H mechanism was the dominant reaction pathway. The coordinated NH₃ species were considered the most important intermediates in E–R mechanism. Abundant Lewis acid sites had been formed on the surface of Ce_0.4/W_0.1ZrO_x catalyst due to the introduction of W, which promoted the generation of coordinated NH₃ species. Different from Ce_0.4/ZrO₂ catalyst, ionic NH₄⁺ species on Brønsted acid sites had been formed on the surface of Ce_0.4/W_0.1ZrO_x catalyst, which could further react with NO_x, thus providing a supplementary pathway for N₂ formation. As to L–H mechanism, the addition of W species favored the activation of adsorbed NO_x species, especially bridged nitrates and adsorbed NO₂, promoting the reactions between adsorbed NO_x species and NH₃.

4. Conclusion

In this work, W_mZrO_x-supported Ce-based catalysts have been prepared, and the effects of W doping in ZrO₂ on NH₃-SCR performance over Ce_0.4/W_mZrO_x catalysts have been investigated systematically. It was found that various W/Zr molar ratios imposed a distinctive impact on the SCR activity of the prepared Ce_0.4/W_mZrO_x catalysts. Compared to Ce_0.4/ZrO₂ catalyst, the addition of W in ZrO₂ promoted the catalytic performance in a broad temperature range. Especially, Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window (NO_x conversion rate > 80%) of 226–446 °C and nearly 100% N₂ selectivity. It was attributed to the enhanced redox property, W doping would lead to an increase in Ce³⁺ and O_α contents on the surface of Ce_0.4/W_0.1ZrO_x catalyst. Besides, Ce_0.4/W_0.1ZrO_x catalyst also had good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to 100 ppm SO₂ atmosphere for 18 h. The results showed that introduction of W in ZrO₂ resulted in a larger specific surface area, and formed more Brønsted acid sites and Lewis acid sites at the surface of Ce_0.4/W_0.1ZrO_x catalyst, which enhanced the total surface acidity. Moreover, the thermal stability of inactive nitrate species had also been reduced significantly, leaving more active sites available for the adsorption of NH₃ species. It was conducive to improving SCR performance. The in situ DRIFTS results indicated that coordinated NH₃ and ionic NH₄⁺ species were active intermediates, and bridging nitrates, monodentate nitrates and bidentate nitrates were involved in SCR reactions over Ce_0.4/W_0.1ZrO_x catalyst at 250 °C. Therefore, SCR reactions occurred over Ce_0.4/W_0.1ZrO_x catalyst might follow both Eley–Rideal (E–R) mechanism and Langmuir–Hinshelwood (L–H) mechanism.

Author contributions

Chenglong Li: conceptualization, investigation, writing–original draft, review and editing; Zhitao Han: conceptualization, validation, supervision, project administration, funding acquisition, writing, review and editing; Yuqing Hu: formal analysis, investigation, data curation; Tingjun Liu: formal analysis, investigation; Xinxiang Pan: project administration and funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (51779024, 51979045 and 52271356), Natural Science Foundation of Liaoning Province of China (2020MS130), Fundamental Research Funds for the Central Universities (3132019330), and Guangdong Province Natural Resources Project (2022-32).

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Footnote

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra04862k

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