Rationally tailored redox ability of Sn/γ-Al₂O₃ with Ag for enhancing the selective catalytic reduction of NO_x with propene

Abstract

The development of excellent selective catalytic reduction (SCR) catalysts with hydrocarbons for lean-burn diesel engines is of great significance, and a range of novel catalysts loaded with Sn and Ag were studied in this work. It was found that the synergistic effects of Sn and Ag enabled the 1Sn5Ag/γ-Al₂O₃ (1 wt% Sn and 5wt% Ag) to exhibit superior C₃H₆-SCR performance. The de-NO_x efficiency was maintained above 80% between 336 and 448 °C. The characterization results showed that the presence of AgCl crystallites in the 1Sn5Ag/γ-Al₂O₃ catalyst helped its redox ability maintain an appropriate level, which suppressed the over-oxidation of C₃H₆. Besides, the number of surface adsorbed oxygen (O_α) and hydroxyl groups (O_γ) were enriched, and their reactivity was greatly enhanced due to the coexistence of Ag and Sn. The ratio of Ag⁰/Ag⁺ was increased to 3.68 due to the electron transfer effects, much higher than that of Ag/γ-Al₂O₃ (2.15). Lewis acid sites dominated the C₃H₆-SCR reaction over the 1Sn5Ag/γ-Al₂O₃ catalyst. The synergistic effects of Sn and Ag facilitated the formation of intermediates such as acetates, enolic species, and nitrates, and inhibited the deep oxidation of C₃H₆ into CO₂, and the C₃H₆-SCR mechanism was carefully proposed.

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1. Introduction

Nitrogen oxides (NO_x) are one of the main pollutants that cause atmospheric pollution such as acid rain, photochemical smog, and global warming, and cause serious harm to the ecological environment and human health.^1–3 Automotive exhaust is an important source of NO_x emissions, of which diesel vehicle exhaust contributes around 90%. The large amount of NO_x emissions has been one of the main reasons for the ozone pollution in many urban areas in China in recent years.⁴ At present, the selective catalytic reduction technology (SCR) using NH₃ as a reducing agent has been commercially applied for diesel exhaust gas denitrification under lean-burn conditions.⁵ However, NH₃-SCR technology requires addition of urea to generate ammonia, which increases operating costs, and NH₃ slip usually causes secondary pollution.

As an alternative, the technology of removing NO_x by hydrocarbons (HCs) has attracted extensive attention, because no extra reducing agent needs to be supplemented, and the HC pollutants could be removed simultaneously. The saturated hydrocarbons would preferentially react with oxygen and showed poor activity, by contrast, the unsaturated hydrocarbons, such as propene could preferentially react with NO_x under certain conditions.^6–8 However, C₃H₆-SCR technology generally has problems such as poor low-temperature activity and narrow temperature window, and the core of this problem lies in the design of catalysts. Optimizing the acidity and redox properties of the catalyst through bimetallic active components has been proved to be an effective method to improve the low-temperature de-NO_x activity. More et al.⁹ reported that Au adding onto Ag/Al₂O₃ effectively lowered the temperature window of C₃H₆-SCR, and the de-NO_x efficiency reached nearly 100% at 350 °C. However, the high cost of the noble metals as active components restrains its application. Besides, it was also found that 7% Mg-doped Ag/Al₂O₃ could effectively improve the C₃H₆-SCR efficiency below 400 °C,¹⁰ which suggests that the co-loading of noble metal and base metal is a feasible approach for C₃H₆-SCR activity promotion. The rational design of superior bi-metal supported catalysts for C₃H₆-SCR and the structure–performance relationships and reaction mechanism of the catalyst are worthy of being further explored.

Due to the abundant reserves and low price, transitional metal-based catalysts such as Cu, Sn, Fe, Co, and Mn have been widely studied for C₃H₆-SCR.^11–19 In addition, due to the strong oxidizing ability of precious metals and excellent NO_x removal performance, Ag, Pt, Pd and other precious metals are also widely used in various industrial flue gases purification.^20,21 Among them, SnO₂ has been found to be rich in surface oxygen vacancies and acidic sites and showed great potential. Kung et al.²² and Liu et al.²³ studied the C₃H₆-SCR performance of SnO₂/Al₂O₃ catalysts and found that the Sn loading has a significant effect on the de-NO_x efficiency, and it reached a maximum value of 83% at 450 °C. Lai et al.²⁴ prepared a single-layer supported SnO₂/beta catalyst and the de-NO_x efficiency reached 85% at 500 °C. It was found that the number of beneficial surface oxygen vacancies increased due to the electron transfer and interfacial interaction between Sn⁴⁺ and H-beta support. Besides, Zhang et al.¹¹ studied the C₃H₆-SCR activity of SnO₂/ZSM-5 catalyst, which reached the highest de-NO_x efficiency of ca. 80% at around 450 °C. The inferior catalytic activity in the low-temperature range is mainly due to its weak oxidation ability for C₃H₆, leading to its limited practical application. As one of the relatively cheap noble metals, supported Ag-based catalysts have also been extensively studied for C₃H₆-SCR. Chaieb et al.²⁵ and Wang et al.²⁶ studied the C₃H₆-SCR performance of Ag/Al₂O₃, and found that the de-NO_x efficiency reached about 70% at 450 °C. He et al.^27,28 found that 4 wt% Ag loading amounts improved the de-NO_x efficiency, reaching about 90% at 450 °C. However, unlike Sn-based catalysts, the main factor that restrains the SCR activity of Ag-based catalysts in the low-temperature range results from its strong oxidizing ability, which facilitates C₃H₆ to deeply oxidized into CO₂ instead of reactive organic intermediates to react with NO_x. Therefore, a combination of Sn and Ag is possibly conducive to a broadened temperature window. In addition, Wang et al.²⁹ reported that 2% AgCl/Al₂O₃ catalyst achieved almost 100% de-NO_x efficiency between 300 and 500 °C with the reducing gases of C₃H₆ and H₂. They found that AgCl nanoparticles could promote the production of formats and enolic species. Due to the fact that the oxidation ability of AgCl nanoparticles was much weaker than that of Ag₂O, the presence of small amounts of AgCl particles might be conducive to suppressing the deep oxidation process of C₃H₆. Inspired by this, in addition to the preparation of Ag and Sn bimetallic supported catalysts, the present study will also introduce AgCl nanoparticles through using tin chloride salts to optimize the acidity and redox properties of the catalysts, thus further widening the low-temperature window of C₃H₆-SCR.

Herein, a series of γ-Al₂O₃ supported catalysts with different Sn/Ag ratios were prepared and their activity was tested in this work. Furthermore, the physicochemical properties of the catalysts were characterized by N₂ adsorption–desorption, scanning electron microscope (SEM), high resolution-transmission electron microscope (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), pyridine infrared (Py-IR), H₂-temperature programmed reduction (H₂-TPR), and O₂-temperature programmed desorption (O₂-TPD). Besides, in situ diffuse reflectance Fourier transform infrared spectroscopy (in situ DRIFTS) was carried out to explore the catalytic reaction mechanism. The structure–activity relationships of the 1Sn5Ag/γ-Al₂O₃ catalyst and its C₃H₆-SCR mechanism were fully explored.

2. Experimental

2.1 Catalyst preparation

The catalysts tested in this work were prepared by wet impregnation method. As an example, 1Sn5Ag/γ-Al₂O₃ catalyst was prepared as follows: an appropriate amount of fresh γ-Al₂O₃ was dried in an oven at 120 °C for 6 h and then calcined in a muffle furnace at 500 °C for 2 h for subsequent use. 0.0234 g SnCl₄·5H₂O and 888 μL standard Ag(NO)₃ solution (1.022 mol L⁻¹) were dissolved in 20 mL deionized water to prepare the impregnation solution, and then 1 g pre-treated γ-Al₂O₃ powder was add, and stir vigorously in a water bath at 80 °C for about 2 h until it is perfectly dry. The catalyst was then transferred into the oven and dried at 80 °C for 6 h. Finally, the catalyst was placed in a tube furnace and calcined at a temperature of 600 °C under air atmosphere for 6 h to obtain 1Sn/γ-Al₂O₃. After that, a certain amount of AgNO₃ was dissolved in deionized water, then 1Sn/γ-Al₂O₃ was dosed, and the above procedures were repeated to obtain the 1Sn5Ag/γ-Al₂O₃ catalyst.

2.2 Catalyst characterization

N₂ adsorption–desorption was carried out with an automatic specific surface and porosity analysers (Micromeritics TriStar II 3020) at −196 °C. SEM was performed with a Zeiss SUPRA-55 field emission scanning electron microscope. HR-TEM was tested with the American FEI Talos F200S transmission electron microscope. XRD was measured using an X-ray diffractometer (Rigaku Ultima IV). The incident ray light source is a copper target, the test angle is 5–120°, and the step size is 0.02°. XPS was carried out with an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha) equipment with monochromatic Al–K X-ray radiation at 250 W. H₂-TPR was carried out on the gas chromatography equipped with a thermal conductivity detector (TCD). 200 mg of the catalysts were placed in a U-type reaction tube, and the temperature was from room temperature to 300 °C at a ramp of 10 °C min⁻¹ under O₂ flow for 30 min to remove surface adsorbed species. Then 30 mL per min N₂ was purged for 30 min and cooled to 50 °C. The temperature was increased to 750 °C at a heating rate of 5 °C min⁻¹ in 5% H₂/N₂, and the signals were continuously recorded. O₂-TPD was also performed similarly. 200 mg of the catalysts were placed in a U-type reaction tube, and the temperature was from room temperature to 300 °C at a ramp of 10 °C min⁻¹ under O₂ flow for 30 min to remove surface adsorbed species. Subsequently, 30 mL per min N₂ was purged for 30 min to remove unstable surface adsorption of O₂ and cooled to 50 °C. Finally, the sample was heated from 30 °C to 600 °C at a rate of 5 °C min⁻¹ under an N₂ atmosphere. Py-IR and in situ DRIFTS were obtained on a Nicolet iS50 spectrometer equipped with in situ reaction cell (Harrick HVC-DRP-5), and the scan resolution was 4 cm⁻¹. After pre-treatment in the Ar atmosphere at 400 °C for 1 h, the catalyst was reacted with pyridine vapor at 300 °C for 30 min, and the corresponding IR spectrum was recorded after stabilizing for 10 min. Prior to the in situ DRIFTS tests, the catalyst was pre-treated in the in situ cell at 300 °C for 30 min under air, and then cooled to ambient temperature. Subsequently, the temperature was increased to 300 °C at 10 °C min⁻¹, and the corresponding components of NO/N₂, C₃H₆/N₂, and O₂ were purged as needed, and the IR spectrum was recorded continuously.

2.3 Catalytic performance evaluation

The catalyst was sieved to 40–60 mesh and packed in quartz tubes (4 mm i.d.). The model flus gas was composed of 0.05 vol% NO, 0.3 vol% C₃H₆, 5 vol% O₂, and balanced with N₂. The NO_x and C₃H₆ conversion were calculated in terms of the eqn (1) and (2), respectively, and the CO_x selectivity was calculated in terms of the eqn (3):where the subscript ‘in’ and ‘out’ represents volume concentration at the steady state. The total flow rate of 240 mL min⁻¹, and the corresponding weight hourly space velocity (WHSV) was 36 000 mL g⁻¹ h⁻¹. NO_x concentrations were measured with an online NO_x analyzer (Thermo Fisher, 42i-LH), and C₃H₆, CO and CO₂ concentrations were measured by gas chromatography (Fuli, GC 9790II).

3. Results and discussion

3.1 Texture properties

XRD patterns for 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, 1Sn5Ag/γ-Al₂O₃ catalysts are presented in Fig. 1, where diffraction peaks observed for all the catalysts at 2θ = 19.6°, 32.2°, 37.5°, 39.6°, 46.0°, 60.1°, and 66.7° were ascribed to γ-Al₂O₃ (JCPDS #02-1420), indexed to (111), (220), (311), (222), (400), (511), and (440) crystal planes, respectively. This also suggests that the loading of Ag and Sn active components did not disrupt the crystalline structure of the support. In addition, no diffraction peaks belonging to the corresponding metals or oxides of Ag and Sn were detected on any of the catalysts, indicating that Ag and Sn species remained in the amorphous state on the γ-Al₂O₃ support. It was noted that the 1Sn5Ag/γ-Al₂O₃ catalyst showed significantly different diffraction peaks at 2θ = 27.6°, 32.1°, 46.2°, 54.8°, 57.5°, and 76.8° from the other two catalysts, which were all ascribed to the diffraction peaks of AgCl crystalline. This should be related to the fact that the precursor of tin used for catalyst preparation was SnCl₄ as mentioned in Section 2.1. The specific surface area loss for 1Sn5Ag/γ-Al₂O₃ catalyst might be related to the formation of AgCl crystallites as well.

Fig. 1 XRD patterns of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts.

N₂ adsorption–desorption was carried out to obtain the specific surface area and pore structure of the catalysts, the results of which are shown in Table 1 and Fig. 2. It can be seen that the loading of Sn or Ag had little effect on the specific surface area, and the values of 1Sn/γ-Al₂O₃ and 5Ag/γ-Al₂O₃ were very close to the γ-Al₂O₃ support (180.10 m² g⁻¹). However, the catalysts loaded with both Sn and Ag (1Sn5Ag/γ-Al₂O₃) were found to show an obvious surface area loss, and it exhibited the lowest specific surface area (172.2 m² g⁻¹). This is possibly due to the precipitation of Ag and Sn species into the channel of γ-Al₂O₃, resulting in a reduction in the specific surface area of the catalysts.³⁰ The change in pore volume and pore size of the catalysts showed a slight decrease in pore volume and pore size after loading. As shown in Fig. 2a, the adsorption–desorption isotherms of the 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts were all of type IV, and the hysteresis loops were of type H2b. This suggests the mesoporous structure of the catalysts, and the loading of Sn and Ag had little effect on the pore structure. The pore size distributions of the three catalysts were also similar, with the pore sizes mainly in the mesoporous range of 2–30 nm (Fig. 2b).

Table 1 The specific surface area, pore volume, and pore size of γ-Al₂O₃, 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts

Catalysts	SBET (m^2 g^−1)	Pore volume^a (cm^3 g^−1)	Pore size^b (nm)
a BJH desorption cumulative volume of pores.b BJH desorption average pore diameter.
γ-Al2O3	180.1	0.68	10.2
1Sn/γ-Al2O3	186.2	0.58	8.3
5Ag/γ-Al2O3	177.8	0.55	9.9
1Sn5Ag/γ-Al2O3	172.2	0.54	9.5

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Fig. 2 N₂ adsorption–desorption isotherms (a) and pore diameter distribution (b) of γ-Al₂O₃, 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts.

Fig. 3 demonstrates the micrographs of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃, and energy dispersive spectrometer (EDS) mapping of 1Sn5Ag/γ-Al₂O₃. Small particles belonging to Sn and Ag species were observed, and the particle sizes mainly fell into the range of 20–50 nm (Fig. 3a–f). EDS mapping of 1Sn5Ag/γ-Al₂O₃ detected a uniform distribution of Ag, Sn, and Cl elements on the catalyst surface (Fig. 3g–i), indicating a high dispersion of Ag and Sn species on the surface of 1Sn5Ag/γ-Al₂O₃ catalyst. Besides, it was noted that the 1Sn5Ag/γ-Al₂O₃ had rod-like components with larger size that were distinct from 1Sn/γ-Al₂O₃ and 5Ag/γ-Al₂O₃. In combination with the XRD results, it was speculated that these rod-like components were probably AgCl species. Furthermore, the states of Ag and Sn species of the 1Sn5Ag/γ-Al₂O₃ catalyst were determined by HR-TEM, and the representative micrographs are shown in Fig. 3j–l. Both γ-Al₂O₃ support exposing the (111) crystal plane and AgCl crystallites exposing the (220) crystal plane could be clearly observed, which corresponded well to the XRD patterns presented in Fig. 1. Besides, many areas with instinct lattice fringe were also found. This implies that a mass of amorphous phases also existed in the catalyst. Combined with the catalyst preparation and XRD results, it was deduced that the amorphous phases were highly dispersed oxides of Sn and Ag. To sum up, the pore structure and morphological characteristics of the catalysts were little affected by Sn and Ag loading, indicating that the increased de-NO_x activity of 1Sn5Ag/γ-Al₂O₃ might be mainly due to changes in its chemical properties.

Fig. 3 SEM images of 1Sn/γ-Al₂O₃ (a and b), 1Sn5Ag/γ-Al₂O₃ (c and d), and 5Ag/γ-Al₂O₃ (e and f) catalysts, EDS mapping of 1Sn5Ag/γ-Al₂O₃ catalyst (g–i), and TEM graphs of 1Sn5Ag/γ-Al₂O₃ catalyst (j–l).

3.2 Chemical states of surface elements

High-resolution XPS was performed to determine the chemical states of the elements on the surface of catalysts. The spectra of O 1s, Sn 3d, and Ag 3d of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts, and their deconvolution results are presented in Fig. 4. The Sn 3d spectrum on 1Sn/γ-Al₂O₃ catalyst (Fig. 4a), where the binding energies at 495.35 and 495.35 eV are 3d_5/2 and 3d_3/2 resulting from Sn 3d spin–orbit coupling, respectively. This indicates that the Sn species mainly exists in the form of Sn⁴⁺,²⁴ which were active sites in C₃H₆-SCR reaction.³¹ However, when Ag and Sn were co-loaded, the binding energies of Sn 3d_3/2 and Sn 3d_5/2 of 1Sn5Ag/γ-Al₂O₃ shifted from 495.35 eV to 495.5 eV and 486.65 eV to 486.8 eV, respectively. This indicated that the Sn 3d electron cloud density of 1Sn5Ag/γ-Al₂O₃ catalyst decreased, which was possibly due to the electron transfer from Sn to Ag species, and the strong electronegativity of Cl⁻ might contribute as well.

Fig. 4 XPS spectra of Sn 3d (a), Ag 3d (b), and O 1s (c) of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts.

Two XPS characteristic peaks of Ag 3d, 3d_5/2 and 3d_3/2, were observed, and they could be deconvoluted into Ag⁺ and Ag⁰ (ref. 29) (Fig. 4b). The ratio of Ag⁰/Ag⁺ was calculated to be 2.15 for 5Ag/γ-Al₂O₃ and 3.68 for 1Sn5Ag/γ-Al₂O₃, respectively. The coexistence of Ag and Sn species on the surface of the 1Sn5Ag/γ-Al₂O₃ resulted in a significant increase in the relative Ag⁰ content, which was deduced to be the electron cloud transfer from Sn to Ag species. The existence of Ag⁰ on the surface of 5Ag/γ-Al₂O₃ catalyst was resulted from the strong metal-support interaction (SMSI) effects. In combination with the XRD results, it was concluded that the presence of Ag on the surface of the 5Ag/Al₂O₃ was in the amorphous states of metallic Ag⁰ and Ag₂O. The presence of Ag on the surface of the 1Sn5Ag/γ-Al₂O₃ was in the amorphous states of metallic Ag⁰, Ag₂O and AgCl crystalline. The highly dispersed state of Ag⁰ species had a positive effect on the SCR activity. Xu et al. found that metallic silver species were conducive to the partial oxidation of hydrocarbons and form active enolic species, thus the low-temperature catalytic efficiency was improved.³² More et al.⁹ also reported similar positive effects of Ag⁰ species for NO_x removal.

O 1s was deconvoluted into three peaks at 530.9, 533.1, and 532.2 eV, corresponding to surface lattice oxygen (denoted as O_β), surface adsorbed oxygen (denoted as O_α), and surface hydroxyl (denoted as O_γ), respectively.³³ It is found that the relative ratios of O_α (0.33) and O_γ (0.15) for 1Sn5Ag/γ-Al₂O₃ were greatly increased compared to that of 1Sn/γ-Al₂O₃ and 5Ag/γ-Al₂O₃. It is well known that O_α species with high mobility are more favorable for the SCR process.³⁴ Besides, the O_γ species were also important active sites for the adsorption and activation of C₃H₆ and NO, especially in the low temperature range.¹³ The increased relative ratios of O_α and O_γ species on the surface of 1Sn5Ag/γ-Al₂O₃ made great contributions to its de-NO_x activity improvement.

3.3 The mobility of surface oxygen species

The mobility of surface oxygen species on the surface of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃ and 1Sn5Ag/γ-Al₂O₃ catalysts were further studied by H₂-TPR and O₂-TPD, and the results are shown in Fig. 5. Due to the high temperature pretreatment of the Al₂O₃ support and its strong Al–O bond, no significant reduction peaks should be observed in the temperature range tested. As presented in Fig. 5a, 1Sn/γ-Al₂O₃ showed only one H₂ reduction peak at 200 °C, which could be ascribed to Sn⁴⁺ to Sn²⁺ reduction peak. Two reduction peaks were observed at 91 °C and 123 °C for 5Ag/γ-Al₂O₃, probably corresponding to the Ag⁺ reduction of AgCl and Ag⁺ reduction of Ag₂O, respectively. In contrast, a broad reduction peak at 187 °C was observed for the 1Sn5Ag/γ-Al₂O₃, as well as a reduction peak at 533 °C. The peak at 187 °C should be an overlap of the Sn⁴⁺ reduction peak and the Ag⁺ reduction peak, while the peak at 533 °C was ascribed to the Sn²⁺ to Sn⁰ reduction peak.²³ Compared with 5Aag/γ-Al₂O₃, there was a significant increase in the number of reducible species for 1Sn5Ag/γ-Al₂O₃, but the reducibility was weakened. This indicates that a higher number of surface oxygen species on 1Sn5Ag/γ-Al₂O₃ could participate in the SCR reaction, but with a weaker mobility.

Fig. 5 H₂-TPR (a) and O₂-TPD (b) profiles of 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts.

As shown in Fig. 5b, several O₂ desorption bands were clearly observed for the catalysts tested. It is generally considered that the desorption bands between 50 and 300 °C corresponded to surface adsorption oxygen species, and the bands between 300 and 600 °C corresponded to surface lattice oxygen species.³⁵ There was a broad band centered at 396 °C of 1Sn/γ-Al₂O₃, with the highest desorption temperature among the three catalysts, and this suggests that the surface oxygen species mobility of 1Sn/γ-Al₂O₃ was inferior to that of 5Ag/γ-Al₂O₃ and 1Sn5Ag/γ-Al₂O₃. By contrast, there was a broad desorption band centered at 209 °C of 5Ag/γ-Al₂O₃, indicating the high mobility of its surface adsorbed oxygen species. This also explains why it has the strongest ability to deeply oxidize C₃H₆ into CO₂. Besides, no desorption peaks attributed to lattice oxygen were detected for 5Ag/γ-Al₂O₃, indicating the inferior mobility of surface lattice oxygen species and they were difficult to participate in the SCR reaction. In comparison to the 5Ag/γ-Al₂O₃, the amount of adsorbed oxygen species on the surface of 1Sn5Ag/γ-Al₂O₃ increased, and the amount of total oxygen species desorbed was the largest. Besides, it is noted that the surface adsorbed oxygen desorption temperature of 1Sn5Ag/γ-Al₂O₃ shifted to 190 °C and became weaker, but a new surface lattice oxygen desorption band (296 °C) appeared. The decrease in the number of adsorbed oxygen species on the surface of 1Sn5Ag/γ-Al₂O₃ inhibited the over-oxidation of C₃H₆, while the high mobility of surface lattice oxygen was conducive to partial oxidation of C₃H₆.³⁶ Therefore, 1Sn5Ag/γ-Al₂O₃ catalyst showed better de-NO_x efficiency. The combined results of O₂-TPD and H₂-TPR showed that the redox ability of the catalyst was effectively regulated by co-loading Sn and Ag. The coexistence of Sn and Ag promoted the oxidation of C₃H₆ and NO to acetates, nitrates and other important intermediates on 1Sn5Ag/γ-Al₂O₃, but inhibited the over-oxidation of C₃H₆ to CO₂, thus the de-NO_x efficiency was improved.

3.4 Surface acidity properties

Py-IR was carried out to identify the nature of surface acid sites, which is another determining factor for C₃H₆-SCR performance, and the results of 1Sn5Ag/γ-Al₂O₃ catalyst are shown in Fig. 6. The bands at 1450 and 1575 cm⁻¹ were assigned to the pyridine molecule (Py-L)³⁷ adsorbed on the Lewis acid sites, and the band at 1612 cm⁻¹ was assigned to Lewis–Brønsted acid complex,^18,37 the band at 1612 cm⁻¹ was assigned to the adsorbed pyridine ion (Py-H⁺).³⁸ It is worth noting that compared with the number of Lewis acid sites, Brønsted acid sites were extremely lacking. There were only a few Brønsted acid sites on the surface of the 5Ag/γ-Al₂O₃ catalyst, and none for 1Sn5Ag/γ-Al₂O₃. Some researchers reported that the Brønsted acid sites were indispensable for C₃H₆-SCR reaction, because they played a key role in C₃H₆ activation and forming related intermediates.^36,39 However, Lewis acids were found to be essential for NO adsorption, which was a key step in C₃H₆-SCR,⁴⁰ and the adsorption strength was stronger than on Brønsted acid sites.⁴¹ Hence, it was deduced that for the 1Sn5Ag/γ-Al₂O₃ catalyst, the adsorption and activation of C₃H₆ and NO were mainly achieved at the Lewis acid sites, i.e., the Lewis acids dominated C₃H₆-SCR reaction. Besides, Lin et al.⁴² reported that Lewis acid sites were responsible for deep oxidation of C₃H₆. The 5Ag/γ-Al₂O₃ catalyst had the largest number of Lewis acid sites, which explains why it exhibited the highest C₃H₆ conversion. In comparison, total acid content and acid strength of 1Sn5Ag/γ-Al₂O₃ greatly decreased, thus inhibited C₃H₆ over-oxidation into CO₂, which was beneficial to the improvement of the de-NO_x efficiency. A similar phenomenon was also observed for C₃H₆-SCR over Mg–Ag/Al₂O₃ catalyst.¹⁰ The above studies revealed that the improved de-NO_x efficiency for 1Sn5Ag/γ-Al₂O₃ catalyst was largely attributed to the synergistic effect between Sn and Ag species, which positively changed their chemical states, the mobility of surface oxygen species and surface acidity for C₃H₆-SCR.

Fig. 6 DRIFTS of pyridine adsorption on 5Ag/γ-Al₂O₃, 1Sn/γ-Al₂O₃, and 1Sn5Ag/γ-Al₂O₃ catalysts at 300 °C.

3.5 Catalytic activity

Ag species with various amounts were loaded onto 1Sn/γ-Al₂O₃, and the results are shown in Fig. 7a. It is seen that the de-NO_x efficiency was somewhat suppressed after the introduction of 1 wt% Ag component onto 1Sn/γ-Al₂O₃. As the Ag loading was increased 5 wt%, the overall de-NO_x efficiency reached the highest level. The 1Sn5Ag/γ-Al₂O₃ catalyst showed the widest temperature window of T₈₀ (the temperature at which 80% de-NO_x efficiency was achieved), which was 336–448 °C, and it reached a maximum de-NO_x efficiency of 88% at 400 °C. At 350 °C, the 1Sn5Ag/γ-Al₂O₃ catalyst achieved ca. 50% higher de-NO_x activity than 1Sn/γ-Al₂O₃, and ca. 15% higher than 5Ag/γ-Al₂O₃. Several representative metal oxides supported catalysts were compiled in Table 2 and the de-NO_x efficiency at 350 °C was compared with 1Sn5Ag/γ-Al₂O₃ catalyst. However, a further increase of Ag loading to 10 wt% resulted in a significant decrease in de-NO_x efficiency. This might be attributed to the high Ag loading resulting in a large amount of C₃H₆ being over-oxidized into CO₂ and thus less involved in NO_x reduction. Based on the above results, there were obvious synergistic effects between the Sn and Ag components supported on γ-Al₂O₃, and the best de-NO_x efficiency was achieved when the loading amount of Sn and Ag was 1 wt% and 5 wt%, respectively.

Fig. 7 The de-NO_x efficiency (a), C₃H₆ conversion (b), and CO_x selectivity (c) over 1Sn/γ-Al₂O₃, 5Ag/γ-Al₂O₃, 1Sn1Ag/γ-Al₂O₃, 1Sn5Ag/γ-Al₂O₃, and 1Sn10Ag/γ-Al₂O₃ catalysts. Reaction conditions: 0.05% NO, 0.3% C₃H₆, 5% O₂, and N₂ balance.

Table 2 De-NO_x efficiency of representative metal oxides supported catalysts

Catalysts	Reaction conditions/vol%	WHSV/mL g^−1 h^−1	T80/°C	De-NOx efficiency at 350 °C/%	Ref.
SnO2/Beta	0.05% NO, 0.05% C3H6, 5% O2	60 000	470	13	24
SnO2/ZSM-5	0.05% NO, 0.05% C3H6, 5% O2	60 000	446	35	11
SnO2/Al2O3	0.05% NO, 0.05% C3H6, 5% O2	60 000	448	32	23
5%Ag/Al2O3	0.08% NO, 0.1714% C3H6, 10% O2, 10% H2O	20,0000	425	23	26
1.6%Au/Al2O3	0.21% H2, 0.0385% NO, 0.04% C3H6, 8% O2	40 000	—	40	44
2%Ag–CeZr	0.2% NO, 0.2% C3H6, 10% O2, 10% H2	45 000	300	83	45
Cu0.71Fe0.29-600c	0.05% NO, 0.1% C3H6, 2% O2	30 000	—	60	18
Cu/SO4^2−/Al–Ce-PILC	0.22% NO, 0.12% C3H6, 2% O2	30 000	—	47	42
This work	0.05% NO, 0.3% C3H6, 5% O2	36 000	336	85	—
		60 000	—	77

---

Fig. 7b and c demonstrate the C₃H₆ conversion and CO_x selectivity. It is seen that the 5Ag/γ-Al₂O₃ catalyst achieved the highest conversion for the catalytic oxidation of C₃H₆, and the CO₂ selectivity was close to 100% at 300 °C. However, the deep oxidation of C₃H₆ was unfavorable for its participation in the SCR process. In the C₃H₆-SCR process, the reaction of C₃H₆ with NO_x and the direct oxidation of C₃H₆ are in competition, and the NO_x reduction mainly depends on the intermediate species such as acetates generated by the partial oxidation of C₃H₆.⁴³ Higher ratio of C₃H₆ being deeply oxidized to CO₂ resulted in less C₃H₆ for NO_x reduction and thus deteriorated the de-NO_x efficiency. It is seen that the C₃H₆ conversion of 1Sn5Ag/γ-Al₂O₃ catalyst at 300 °C was reduced by ca. 70% compared to that of 5Ag/γ-Al₂O₃ (Fig. 7b). The total CO_x selectivity was ca. 75%, indicating that a large number of organic intermediates might have been generated on the surface of 1Sn5Ag/γ-Al₂O₃ catalyst. For the 1Sn1Ag/γ-Al₂O₃ catalyst, the CO_x selectivity was the lowest overall among all samples tested (Fig. 7c), although its C₃H₆ conversion was slightly higher than that of 1Sn/γ-Al₂O₃. Moreover, the de-NO_x efficiency was also lower than that of 1Sn/γ-Al₂O₃. This suggests that the oxidation of C₃H₆ possibly generated a certain number of intermediates over 1Sn1Ag/γ-Al₂O₃, and they exhibited inferior reactivity with NO_x. As for 1Sn10Ag/γ-Al₂O₃ catalyst, the decrease in de-NO_x efficiency might be due to the higher amount of C₃H₆ over-oxidation after the Ag loading increases. The above results suggest that the synergistic effects between a certain proportion of Ag and Sn components maintained the oxidation of C₃H₆ at a suitable level, so that more reactive organic intermediates would be generated to participate in the SCR reaction, thus effectively improving the de-NO_x efficiency.

3.6 Reaction mechanism

Firstly, the adsorption characteristics of C₃H₆ and NO molecules on the 1Sn5Ag/γ-Al₂O₃ catalyst were studied, and the corresponding FTIR bands at 30 °C are presented in Fig. 8. When C₃H₆ and O₂ were introduced, five bands emerged between 1200 and 1800 cm⁻¹. The band observed at 1383 cm⁻¹ was assigned to formate species.¹⁵ The band at 1406 cm⁻¹ was assigned to enolic species.²⁹ The bands at 1456 and 1593 cm⁻¹ were assigned to acetate species.⁴⁶ The band at 1654 cm⁻¹ was assigned to the δ(OH) hydroxyl group of surface adsorbed water.⁴⁷ Besides, the broad band between 3000 and 3200 cm⁻¹ was ascribed to ν(C–H) of C₃H₆, and 3200–3500 cm⁻¹ was ascribed to hydrogen-bonded OH groups.³⁶ The above results indicate that C₃H₆ could be easily oxidized over 1Sn5Ag/γ-Al₂O₃ catalyst, and acetates were the main intermediates.

Fig. 8 C₃H₆ + O₂ adsorption at 30 °C and NO + O₂ adsorption at 30 °C over 1Sn5Ag/γ-Al₂O₃ catalyst. Reaction conditions: 0.05% NO, 0.3% C₃H₆, 5% O₂, and N₂ balance.

When NO and O₂ were introduced, four bands were observed between 1200 and 1800 cm⁻¹. The band at 1244 cm⁻¹ was assigned to bridged nitrates. The bands at 1298 and 1554 cm⁻¹ were assigned to bidentate nitrates.¹⁶ The bands at 1654 cm⁻¹ and 3200–3500 cm⁻¹ were assigned to surface hydroxyl groups. This indicates that NO could be easily oxidized over 1Sn5Ag/γ-Al₂O₃ catalyst to generate bridged nitrate and bidentate nitrate. In addition, it was noted that the surface hydroxyl groups (originated from the synergistic effects between Sn and Ag based on the XPS results) were largely consumed after the introduction of NO. Therefore, the presence of surface hydroxyl groups helped to promote the formation of key intermediate nitrates, and then effectively improve the low-temperature de-NO_x activity.

The transient reaction experiments were carried out to further explore the C₃H₆-SCR mechanism of 1Sn5Ag/γ-Al₂O₃ catalyst. At first, C₃H₆ and O₂ were pre-adsorbed on the surface of 1Sn5Ag/γ-Al₂O₃ catalyst at 300 °C, and then NO was introduced. The results of the bands changes at different time durations are shown in Fig. 9a. The bands at 1296, 1456, 1545 cm⁻¹ were assigned to ν(C–C), ν_s(COO), and ν_as(COO) of acetates.^26,27,48 Comparing with the FTIR spectra at 30 °C, it was found that the thermal stability of formates was quite high (1383 cm⁻¹), but the acetates would partially disappear (1456 and 1593 cm⁻¹) as the temperature was increased to 300 °C. Besides, it was noted that the bands at 1296 and 1545 cm⁻¹ were continuously consumed after NO was introduced, following the E-R mechanism. In contrast, formates were almost inert and did not react with NO. Fig. 9b show normalized peak height changes of the band at 1296 and 1545 cm⁻¹ with the time of NO feeding. It is seen that the acetates were quickly consumed within 10 min, indicating that acetates were key active intermediates involved in the C₃H₆-SCR reaction over 1Sn5Ag/γ-Al₂O₃ catalyst. Acetates with high reactivity were also detected over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, and being consumed at a similar rate to NO_x.^49–51 The band at 1456 cm⁻¹ was possibly ascribed to the overlapping of acetates and monodentate nitrates, because monodentate nitrates would accumulate with the continuous flow of NO while acetates should be consumed.

Fig. 9 In situ DRIFTS of pre-adsorbed C₃H₆ and O₂ at 300 °C and followed by a flow of NO over 1Sn5Ag/γ-Al₂O₃ catalyst at different time durations (a), and normalized peak height changes of the band at 1545 and 1296 cm⁻¹ (b); In situ DRIFTS of pre-adsorbed NO and O₂ at 300 °C and followed by a flow of C₃H₆ at different time durations (c), and normalized peak height changes of the band at 1298 and 1554 cm⁻¹ (d). Reaction conditions: 0.05% NO, 0.3% C₃H₆, 5% O₂, and N₂ balance.

Subsequently, NO and O₂ were pre-adsorbed on the surface of 1Sn5Ag/γ-Al₂O₃ catalyst at 300 °C, and then C₃H₆ was introduced. The results of the bands changes at different time durations are shown in Fig. 9c. The bands at 1298 and 1456 cm⁻¹ were assigned to monodentate nitrates, and the band at 1554 cm⁻¹ was assigned to bidentate nitrates. Comparing with the FTIR spectra at 30 °C, it was found that the bridge nitrates was not thermally stable, and it could be converted into monodentate and bidentate nitrates at elevated temperature. It is also noted in the FTIR bands belonging to monodentate nitrate (1456 cm⁻¹) and bidentate nitric acid (1554 cm⁻¹) were gradually consumed as C₃H₆ was introduced, following the E-R mechanism. The negative band assigned to adsorbed water at 1654 cm⁻¹ was gradually weakened with the introduction of C₃H₆, which was related to the formation of the R–NO₂ intermediates (1651 cm⁻¹).¹⁴ Fig. 9d demonstrate the consumption rates of monodentate nitrates and bidentate nitrates as C₃H₆ was introduced, and both were consumed ca. 80% within 30 min. Besides, no accumulation of acetates was observed. Combined with the results in Fig. 9a, it suggests that the reactivity between gaseous C₃H₆ and adsorbed nitrates was higher than that of gaseous NO and adsorbed acetates. This is also consistent with the Py-IR results that the Lewis acid sites dominate nitrate species formation and activation. Besides, it is speculated that the nitrate species adsorbed on the hydroxyl group on the surface of the catalyst also participates in the SCR reaction, because with the introduction of C₃H₆, the band at 3258 cm⁻¹ assigned to the surface hydroxyl group weakened gradually, and water (3720 cm⁻¹) was formed at the same time.

Besides, NO + C₃H₆ + O₂ was introduced simultaneously between 200 and 500 °C, and the results are shown in Fig. 10. A new band at 2325 cm⁻¹ emerged, and it was assigned to NCO species.⁵² NCO was also a key active intermediate for Ag/Al₂O₃ catalysts,⁵² which was formed through the reaction between acetates and nitrates, and was easily further oxidized into CO₂ and N₂. Xu et al.⁴⁶ and Guo et al.¹² also reported that NCO species played an important role in improving C₃H₆-SCR process. The band assigned to NCO did not appear under the pre-adsorption reaction conditions, indicating that the SCR reaction of the 1Sn5Ag/γ-Al₂O₃ catalyst not only followed the E-R mechanism, but also followed the L-H mechanism. Besides, the band assigned to NCO at 300 °C and below was very weak. With the increase of the reaction temperature, the amount of NCO generated increases, indicating that the formation of NCO was quite limited by the reaction kinetics at low temperature. The low amount NCO produced was responsible for the inferior de-NO_x efficiency. With the further increase of the reaction temperature, the monodentate and bidentate nitrates accumulated and could not be further converted, thus deteriorated the de-NO_x efficiency. The band assigned to water at 3545 and 3720 cm⁻¹ became stronger with the increase of temperature, which might be related to the SCR reaction and the direct oxidation of C₃H₆.

Fig. 10 In situ DRIFTS spectra of 1Sn5Ag/γ-Al₂O₃ catalyst in a flow of NO + C₃H₆ + O₂ from 200 to 500 °C. Reaction conditions: 0.05% NO, 0.3% C₃H₆, 5% O₂, and N₂ balance.

Based on the results of catalyst characterizations and in situ DRIFTS, the C₃H₆-SCR reaction mechanism over 1Sn5Ag/γ-Al₂O₃ catalyst was proposed and depicted in Scheme 1. The Lewis acid sites were mainly responsible for the C₃H₆-SCR reaction. A majority of C₃H₆ would therefore be partially oxidized into adsorbed formates, acetates and enolic species, and NO would be oxidized into monodentate and bidentate nitrates, on 1Sn5Ag/γ-Al₂O₃ catalyst. The presence of AgCl particles and hydroxyl species helped to promote the reactions. The reaction mainly occurred between adsorbed acetates and nitrates to form NCO species, and gaseous C₃H₆ with adsorbed monodentate and bidentate to form R–NO₂ species, and CO₂ and N₂ would be finally generated.

Scheme 1 Proposed reaction mechanism of C₃H₆-SCR over 1Sn5Ag/γ-Al₂O₃ catalyst.

4. Conclusion

In this work, a series of Sn/γ-Al₂O₃ and SnAg/γ-Al₂O₃ catalysts with different loadings were prepared and tested, and it was found that the 1Sn5Ag/γ-Al₂O₃ catalyst exhibited the best C₃H₆-SCR activity, which maintained above 80% de-NO_x efficiency between 336 and 448 °C. The synergistic effects of Sn and Ag improved the C₃H₆-SCR activity effectively. The characterizations found that the improvement of the de-NO_x activity of 1Sn5Ag/γ-Al₂O₃ was closely related to the formation of AgCl nanoparticles, the change of Ag valence state, and the improvement of the mobility of surface oxygen species. The surface oxygen species contributed to the adsorption and activation of gaseous C₃H₆ and NO, and the highly dispersed metallic Ag species could promote the partial oxidation of C₃H₆ to enolic species with high reactivity. The presence of AgCl nanoparticles adjusted the redox ability of 1Sn5Ag/γ-Al₂O₃ to a moderate level and inhibited the deep oxidation of C₃H₆, which was essential for C₃H₆-SCR. The Lewis acid sites were mainly responsible for C₃H₆-SCR over 1Sn5Ag/γ-Al₂O₃ catalyst, and the reaction followed both E-R and L-H mechanism. The main reactions occurred between adsorbed monodentate/bidentate nitrates and gaseous C₃H₆, as well as acetates and nitrates, then key intermediates such as NCO and R–NO₂ would be generated. Finally, they would convert into harmless N₂ and CO₂. This work reveals the synergistic effects of noble and base metals (Ag and Sn) for promoting C₃H₆-SCR performance, which provided new theoretical ideas and guidance for developing new HC-SCR catalysts.

Author contributions

Ning Li: investigation, writing – original draft, data curation; Tiantian Zhang: formal analysis; Zuliang Wu, Jing Li, Wei Wang, and Jiali Zhu: investigation; Shuiliang Yao: supervision; Erhao Gao: conceptualization, writing – review & editing, 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 funded by Natural Science Foundation of Jiangsu Province, China (Grant No. BK20210857), Scientific Research Foundation of Jiangsu Provincial Education Department, China (Grant No. 21KJB610006), and Leading Innovative Talent Introduction and Cultivation Project of Changzhou City, China (Grant No. CQ20210083). The authors also would like to thank shiyanjia lab for the support of XPS test.

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