Effect of Copper Precursors on the Activity and Hydrothermal Stability of Cu^II−SSZ−13 NH₃−SCR Catalysts

Abstract

A series of Cu^II−SSZ−13 catalysts are prepared by in-situ hydrothermal method using different copper precursors (Cu^II(NO₃)₂, Cu^IISO₄, Cu^IICl₂) for selective catalytic reduction of NO by NH₃ in a simulated diesel vehicle exhaust. The catalysts were characterized by X−ray diffraction (XRD), scanning electron microscope (SEM), X−ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption, hydrogen-temperature-programmed reduction (H₂−TPR), ammonia temperature-programmed desorption (NH₃−TPD), and ²⁷Al and ²⁹Si solid state Nuclear Magnetic Resonance (NMR). The Cu^II−SSZ−13 catalyst prepared by Cu^II(NO₃)₂ shows excellent catalytic activity and hydrothermal stability. The NO conversion of Cu^II−SSZ−13 catalyst prepared by Cu^II(NO₃)₂ reaches 90% at 180 °C and can remain above 90% at a wide temperature range of 180–700 °C. After aging treatment at 800 °C for 20 h, the Cu^II−SSZ−13 catalyst prepared by Cu^II(NO₃)₂ still exhibits above 90% NO conversion under a temperature range of 240–600 °C. The distribution of Cu species and the Si/Al ratios in the framework of the synthesized Cu^II−SSZ−13 catalysts, which determine the catalytic activity and the hydrothermal stability of the catalysts, are dependent on the adsorption capacity of anions to the cation during the crystallization process due to the so called Hofmeister anion effects, the NO₃⁻ ion has the strongest adsorption capacity among the three kinds of anions (NO₃⁻, Cl⁻, and SO₄²⁻), followed by Cl^– and SO₄^2– ions. Therefore, the Cu^II−SSZ−13 catalyst prepared by Cu^II(NO₃)₂ possess the best catalytic ability and hydrothermal stability.

1. Introduction

The emission of nitrogen oxides is a major cause of unhealthy air quality and is strictly regulated in many places. To meet regulations, controlling the emission of nitrogen oxides from diesel exhaust is one important topic in catalysis [1,2]. The selective catalytic reduction of NO_x by ammonia (NH₃-SCR) is one of the most effective approaches to convert NO_x due to its high fuel economy, and high denitrification efficiency [3,4]. The core issue of this technology is the development of environmentally friendly SCR catalysts with high activity, wide operating temperature window, and excellent hydrothermal stability. Prior to the application of molecular sieve catalysts, NO_x abatement technology relied primarily on V₂O₅-WO₃/TiO₂ as a NO_x removal catalyst [5,6]. In recent years, the Cu^II−SSZ−13 molecular sieve used as NH₃-SCR catalysts has become a hot topic due to its wide temperature window, high N₂ selectivity, and excellent hydrothermal stability [7,8,9]. In addition, Cu^II−SSZ−13 shows its superiority in hydrothermal stability under moderate aging temperatures (e.g., 750–800 °C) compared to other well-known Cu molecular sieves (e.g., Cu-ZSM-5, Cu-beta, and Cu-Y) [10].

Recently, the Cu^II−SSZ−13 catalysts are usually synthesized by hydrothermal method and subsequent ion exchange process. Han et al. [11] prepared Cu^II−SSZ−13 catalyst using N, N, N-trimethyl-1-adamantane ammonium hydroxide (TMAda-OH) as template by microwave, dynamic and static hydrothermal methods, respectively. Shishkiny et al. [12] synthesized functionalized Cu^II−SSZ−13 catalyst by introducing Cu and Fe ions into the Na-SSZ-13 catalyst during the ion exchange process. The industrial application of SSZ-13 molecular sieve catalyst is limited due to the high costs of TMAda-OH template. Ren et al. [13] developed an in situ method to synthesize Cu^II−SSZ−13 using low-cost copper teraethylenepentamine (Cu-TEPA) as template, which reduced economic cost the Cu^II−SSZ−13 catalyst. Zhang et al. [14] compared Cu^II−SSZ−13 catalysts prepared by ion-exchange (Cu^II−SSZ−13−I) and in-situ synthesis methods (Cu^II−SSZ−13−O), and found that Cu^II−SSZ−13−O showed higher DeNO_x activity and stronger Lewis acid site strengths than Cu^II−SSZ−13−I. Cu^II−SSZ−13 was prepared by selecting a novel low-temperature solid-state ion-exchange (LT−SSIE) method using Cu^IIAc₂ and Cu^II(NO₃)₂ as Cu precursors, showing comparable SCR activity and hydrothermal stability to the traditional solution ion-exchanged Cu^II−SSZ−13 with a similar Cu loading [15]. Paolucci et al. [16] have worked out a scenario that is based on the formation of the Cu−ammonia complex Cu(NH₃)₂⁺ mechanism shows that the actual active sites for NO_x reduction are not static structures but dynamic and come and go as the reaction proceeds. Borfecchia et al. [17] provide a comprehensive overview on the structural complexity of Cu−CHA materials, it has been shown that the active site in the low temperature NH₃−SCR catalyst is a mobile Cu-molecular entity that ‘‘lives in symbiosis’’ with an inorganic solid framework. Only in the high temperature NH₃−SCR regime do the mobile Cu species lose their ligands and find docking sites at the internal walls of the molecular sieve framework. Marberger et al. [18] monitored the evolution of various Cu species in Cu^II−SSZ−13 and experimentally identified crucial characteristics of the SCR catalyst in the low-temperature regime: the rate-limiting re-oxidation of Cu^I(NH₃)₂ is strongly influenced by NH₃ inhibition; the active Cu^II(NH₃)₄ species are mainly formed below 250 °C; and the modification of the active sites with increasing temperatures is driven by the loss of NH₃ coordinating Cu species.

It should be noted that the synthesis of molecular sieves via hydrothermal method is a multi-variable process. The crystallization temperature, duration, dynamic/static, as well as the prescriptions of anions and cations have important influences on the physicochemical properties of the final molecular sieve products, which further determine the performances of the catalysts. In the process of in-situ synthesis of Cu^II−SSZ−13 catalyst, the distribution of metal cations on the molecular sieve framework determines its structure properties and catalytic performance. The metal cations can affect the aggregation state, condensation rate, and colloidal state of silicate materials in the formation in the reaction system [19]. The anions also have influences on the synthesis of silicon-alumina molecular sieves. It has been reported that the crystallization time of ZSM−5 and TS−1 can be regulated by the oxyacid anion [20]. In the early days, Hofmeister studied the ability of different salt solutions to denature proteins, and called the relative effectiveness of anions to produce different specificities on a wide range of phenomena the Hofmeister anion effect [21]. Alexander V [22] investigated a series of Hofmeister anions that may be arranged in the following sequence: SO₄²⁻ > Cl⁻ ~ NO₃⁻ with decreasing zeolite Beta formation potency from left to right. The Hofmeister anion effect can influence the surface activity of the mesoporous materials. The adsorption ability of anion to the cation: NO₃⁻ > Cl⁻ > SO₄²⁻ [23,24]. Anion species will change hydrolysis rate of silicate liquid precursor and the size of template agent micelle, which finally results in the differences of the surface properties and the morphology of molecular sieve catalyst after calcination. In short, the anion can change hydrolysis rate of precursor and the structure of colloid formed by the template agent, which affects the framework structure, crystal structure, morphology, and the catalytic properties of the final Cu^II−SSZ−13 molecular sieve.

In this work, Cu^II−SSZ−13 catalysts were prepared by in-situ hydrothermal synthesis method for NO selective catalytic reduction with NH₃ in simulated diesel vehicles exhaust. Three kinds of copper precursors (Cu^II(NO₃)₂, Cu^IISO₄, Cu^IICl₂) were used to investigate the effect of anions on the structure, catalytic activity, and hydrothermal stability of Cu^II−SSZ−13 catalysts. The NO conversion, N₂ selectivity, and the structure characterization of the fresh and aged Cu^II−SSZ−13 catalysts were investigated in detail.

2. Results and Discussion

2.1. Catalytic Activity

Figure 1 shows the conversion of NO, N₂ selectivity, and N₂O yield of fresh and aged Cu^II−SSZ−13 catalysts. As shown in Figure 1a, the Cu^II−SSZ−13 catalysts prepared by three kinds of copper precursors all exhibit excellent catalytic activity. Compared with the other samples, the NO conversion of the F−NO₃ sample reaches 90% at 180 °C and has a widest temperature window (180–700 °C). The Cu^II−SSZ−13 catalysts prepared by three kinds of copper precursors declines dramatically after aging at 800 °C for 20 h as shown in Figure 1b. However, the NO conversion of the A−NO₃ sample exceeds 90% ranging from 140 to 620 °C and just declines a little at the temperature range of 240–600 °C.

Figure 1c,d shows that the ammonia oxidation ability of the fresh and aged Cu^II−SSZ−13 catalysts is almost the same over below 300 °C. The stoichiometric relationship between ammonia and NO_x conversion is 1:1 indicating that standard SCR occurs at this temperature range. However, at the high temperature range, the ammonia conversion rate is significantly higher than the NO_x conversion rate, indicating that the ammonia oxidation reaction has occurred [25,26].

The three fresh catalysts exhibit 100% N₂ selectivity in the range of 100–200 °C and 440–700 °C as shown in Figure 1d. While, F−NO₃ sample has the lowest N₂O yield (<15 ppm) among the three samples. Moreover, the amounts of N₂O produced in all aged Cu^II−SSZ−13 catalysts are obviously decreased after 300 °C (Figure 1e). The A−NO₃ sample has lower N₂O production (<12 ppm), and shows 100% N₂ selectivity in the range of 100–180 °C and 340–700 °C. The high N₂ selectivity of Cu^II−SSZ−13 catalysts is affected by the type of molecular sieve structure and ammonia oxidation ability [10,27]. The formation of N₂O is mainly related to NO_x reacting with proton-adsorbed NH₃ in which NH₄NO₃ that forms on protonic sites could slowly decompose [28].

2.2. Structure and Morphology

2.2.1. X-ray Diffraction (XRD) Results

The X−ray diffraction (XRD) results of all Cu^II−SSZ−13 catalysts are shown in Figure S1. The fresh and aged samples exhibit characteristic diffraction peaks of SSZ-13 at 2θ = 9.5°, 14.0°, 16.1°, 17.8°, 20.7°, 25.0°, 30.7° [2,29,30]. The fresh samples show perfect crystal structure and high crystallinity, among which the peaks of the F−Cl sample is stronger than that of F−NO₃ and F−SO₄ samples (Figure S1a). It can be inferred that the copper precursors can affect the crystallinity and regularity of the fresh samples. No distinct diffraction peaks can be observed for the A−SO₄ sample (Figure S1b), which means that the crystal structure of the A−SO₄ sample is severely destroyed. The Cu^II−SSZ−13 prepared by the Cu^II(NO₃)₂ sample has the smallest change of characteristic peak after aging, which means the hydrothermal stability of these samples are the best. However, there are no diffraction peaks of CuO observed before and after hydrothermal aging. It means that the Cu content is low, or the Cu species are well-dispersed [4].

The grain size parameters with scanning angle near 20° for all Cu^II−SSZ−13 catalysts are shown in

Table 1. Grain size parameters of Cu^II−SSZ−13 catalysts before and after hydrothermal aging treatment.

Samples	Angle (°)	FWHM	Diameter (nm)	Area (m2)
F−Cl	20.93	0.26	30.51	3024
A−Cl	20.96	0.29	27.19	1925
F−NO3	20.69	0.29	27.93	2620
A−NO3	20.87	0.34	23.64	1761
F−SO4	20.69	0.26	31.21	2639
A−SO4	-	-	-	-

. The particle size of F−SO₄ and F−Cl samples are 31.21 and 30.51 nm, respectively, while that of F−NO₃ sample is 27.93 nm. It indicated that F−NO₃ sample has small grains, which may be due to the fact that adsorption capacity of NO₃⁻ anions on cationic is much larger than those of the SO₄²⁻ and Cl⁻ anions. NO₃⁻ ions easily lose water molecules in the process of crystallization or directly utilize water molecules to dissolve other substances on the interface. Its large molecular weight has little effect on surface active agent charge, leading to form small spherical micelles and finally generate small SSZ−13 grain size. The SO₄²⁻ ions frequently lead to a higher surface tension and a salting-out effect (aggregation of cations), which may influence the entering of copper ions to the pores of SSZ−13 and gathering on the support surface [23]. After aging, the diameter size of all the samples reduced. The grain parameters of the sample A−SO₄ was not calculated because there are no obvious diffraction peaks observed over the scanning angle from 5° to 50°. The particle size of the A−NO₃ and A−Cl samples are 23.64 and 27.19 nm, respectively. It shows that the grain size of the A−NO₃ sample is also the smallest. The grain size of the F−Cl sample decreases to a smaller extent after aging, which can be used to explain why the Cu^II(NO₃)₂ sample has strong anti-aging ability.

2.2.2. N₂ Adsorption Results

In order to investigate the effect of different copper precursors on the pore structure of Cu^II−SSZ−13, Cu^II(NO₃)₂, Cu^IISO₄ and Cu^IICl₂ samples were selected for structural analysis. The results are shown in

Table 2. Pore structure results of Cu^II−SSZ−13 catalysts before and after hydrothermal aging treatment.

Samples	SBET (m2/g)	V (cm3/g)	D (nm)
F−Cl	363.99	0.15	0.77
A−Cl	242.15	0.10	0.67
F−NO3	301.64	0.13	0.65
A−NO3	145.84	0.08	0.56
F−SO4	233.44	0.12	1.07
A−SO4	11.64	0.01	0.49

. The specific surface areas of F-NO₃ and F−Cl samples are more than 300 m²/g, while that of F−SO₄. sample is only 233.44 m²/g. The pore volume and pore size of the F−Nit. sample are 0.13 cm³/g and 0.65 nm, respectively. After aging, the specific surface area of all samples reduced to varying degrees. The specific surface area and pore volume of the A−SO₄ sample decrease significantly, which is consistent with SSZ−13 peaks disappearance in the XRD. The pore volume and pore size of the A−NO₃ and A−Cl samples are slightly reduced. It indicates that the Cu^II(NO₃)₂ and Cu^IICl₂ samples are beneficial to strengthen the stability of the molecular sieve catalyst framework structure, and improve the anti-aging properties of the catalysts. Moreover, the pore size of the Cu^II(NO₃)₂ sample before and after aging are the smallest, showing that it has an excellent shape-selective role during the process of gas reaction.

2.2.3. Scanning Electron Microscope (SEM) Results

Figure 2 shows scanning electron microscope (SEM) pictures of Cu^II−SSZ−13 catalysts before and after aging. The grain size of F−SO₄ and F−Cl samples are relatively large and gather together in clusters. However, the F−NO₃ sample presents a neat and angular cubic crystal grain size, large quantity, and good dispersion. Furthermore, the grain sizes of the entire sample are damaged after aging. The A−SO₄ and A−Cl samples have no obvious cubic grain size. The agglomeration phenomenon is very serious. Although the grain size of the Cu^II(NO₃)₂ sample is slightly damaged, it can still maintain complete cubic shape and relatively uniform distribution.

To sum up, during the process of crystal nucleus growth, the adsorption capacity of different anions on the template micelles is very different due to the effect of Hofmeister anion effects, which has a direct impact on the formation and growth of crystal grain [23]. It causes the diverse skeletal-structure properties of Cu^II−SSZ−13, thereby making the catalyst form different morphologies.

2.2.4. Hydrogen-Temperature-Programmed Reduction (H₂−TPR) Results

The hydrogen-temperature-programmed reduction (H₂−TPR) profiles of Cu^II−SSZ−13 catalysts are displayed in Figure 3. It can be seen that the copper precursors have a significant effect on the distribution of Cu species in Cu^II−SSZ−13. There are several types of cationic sites in CHA: in [31,32,33] the H₂ reduction peaks at 180–200 and 200–280 °C stand for the reduction of isolated Cu^II ions to Cu^I ions in the eight ring and CHA cages, respectively; the H₂ reduction peaks at 280–500 °C are assigned to the reduction of the stable Cu^II in double six-rings. The reduction peaks of 500–1000 °C are due to the reduction of Cu^I to Cu⁰. As shown in Figure 3, the H₂−TPR process of isolated Cu^II on Cu^II−SSZ−13 catalysts are divided into two steps: reduction of Cu^II→Cu^I at low temperature (<500 °C) and Cu^I→Cu⁰ at high temperature (>500 °C) [14]. The fresh samples are shown in Figure 3a, the peak at around 240 °C is assigned to the reduction of isolated Cu^II→Cu^I in the CHA cage. Different reduction peaks areas appear at 375 °C, indicating the presence of different isolated Cu^II ions in the stable six-membered ring. At 500–1000 °C, two different types reduction peaks of Cu^I into Cu⁰ appear in three samples, which can be unstable Cu^I at low temperature (550 °C) and stable Cu^I at high temperature (850 °C) [30,34].

After hydrothermal aging, the aged sample’s H₂ reduction peak decrease and move to the higher temperature, indicating the oxidation capacity of Cu species is weakened (Figure 3b). However, the A−NO₃ sample still has significant reduction peaks at 550 and 813 °C. The H₂ reduction peak of at 813 °C represents the reduction of extremely stable Cu^I ions. It is worth noting that the reduction temperature of this kind of stable Cu^I ion is when the structure of the molecular sieve begins to collapse [30,34]. This is consistent with the XRD results. The A−NO₃ sample has higher stable Cu^I content and therefore it can maintain a stable skeleton structure. The H₂ consumption of Cu species in the fresh and aged catalysts is shown in

Table 3. H₂ consumption of Cu^II−SSZ−13 catalysts before and after hydrothermal aging treatment.

Samples	H2 Consumption (μmol g−1)				Total H2 Consumption (μmol g−1)
	CuII→CuI (CHA Cages)	CuII→CuI (D6R)	CuII→CuI (Total)	CuI→Cu0
F−Cl	53.69	28.71	82.40	163.34	245.74
A−Cl	4.32	25.56	29.88	32.89	62.77
F−NO3	34.75	136.46	171.21	152.76	323.97
A−NO3	25.74	111.43	137.17	79.50	216.67
F−SO4	28.61	87.35	115.96	133.87	249.83
A−SO4	4.17	58.48	62.65	0	62.65

, the content of isolated Cu^II ions in the F−NO₃ and A−NO₃ samples are much higher than the Cu^IICl₂ and Cu^IISO₄ samples. Due to the destruction of the skeleton structure of the A−SO₄ sample, the amount of Cu^I is significantly reduced. During the NH₃−SCR reaction, the catalytic performance of the Cu^II−SSZ−13 catalysts is determined by the reducibility of active metal species in molecular sieves. Many researchers have shown that isolated Cu^II ion is the main active component [35,36]. Hence, the conversion of Cu^II−SSZ−13 catalysts will be decreased when the account of isolated Cu^II ion is lower. Due to the difference of copper precursors, anions may affect the distribution of copper species in the samples. The Pauling radium increased in the order of NO₃⁻ (1.79 Å) ≈Cl⁻ (1.80 Å) < SO₄²⁻ (2.30 Å) [23]. Larger anions from the copper precursors can inhibit the entering of copper ions into the SSZ−13 pore during the formation of the colloidal process. However, the copper ions from the precursors with small anions ions like NO₃⁻ are more accessible to enter the pore, which generates more isolated Cu^II, and exhibits excellent NH₃−SCR activity.

2.2.5. Ammonia Temperature-Programmed Desorption (NH₃−TPD) Results

The acid sites of the molecular sieve catalyst are beneficial to adsorption and activation of NH₃, which is one of the key steps for the reduction of NO by NH₃ molecules [2,37]. The ammonia temperature-programmed desorption (NH₃−TPD) spectra of the Cu^II−SSZ−13 catalysts are shown in Figure 4. The peak at 130 °C (Peak A) and 168 °C (Peak B) are attributed to the physical adsorption and weakly adsorbed NH₃ on the weak Lewis acid sites, respectively [38,39]. The peaks at 258 °C (Peak C) and above 420 °C (Peak D) belong to NH₃ adsorbed on strong Lewis acid sites originating from the isolated Cu^II ions and NH₃ adsorbed on Brønsted acid sites, respectively.

Table 4. The adsorbed NH₃ amount of Cu^II−SSZ−13 catalysts before and after hydrothermal aging treatment.

Samples	Adsorbed NH3 Amount (mmol g−1)				Total Amount (mmol g−1)
	Physical Adsorption	Weak Lewis Acid Sites	Strong Lewis Acid Sites	Brønsted Acid Sites
F−Cl	0.18	0.23	0.33	0.45	1.19
A−Cl	0.08	0.10	0.17	0.21	0.56
F−NO3	0.20	0.21	0.32	0.54	1.27
A−NO3	0.14	0.19	0.21	0.28	0.82
F−SO4	0.18	0.23	0.31	0.47	1.19
A−SO4	0.09	0.11	0.16	0.23	0.59

shows the adsorption amounts of NH₃ on the fresh and aged samples by deconvolution of the NH₃−TPD curves. The total acid amounts of all fresh samples are similar. Additionally, the adsorption amount of ammonia decreases and the ammonia oxidation reaction has occurred during temperature increase, causing the activity of the catalyst to decrease. After hydrothermal aging, the NH₃ adsorption peaks all decreased due to the destruction of the CHA structure and the loss of the isolated Cu^II ions, which is demonstrated by the results of H₂−TPR and XRD characterization. Many studies have shown that the chemisorption of NH₃ plays a significant role in the NH₃−SCR performance [2,30,40].

2.2.6. X-ray Photoelectron Spectroscopy (XPS) Results

The Cu^II−SSZ−13 catalysts are analyzed by Cu_2p X−ray photoelectron spectroscopy (XPS) in Figure 5. The fresh and aged samples have a main peak and a satellite peak of Cu 2p_3/2 (930–939 and 940–948 eV) and Cu 2p_1/2 (950–955 and 960.0–968 eV). The Cu 2p_3/2 peaks at 935.9 eV are attributed to the isolated Cu^II species. The characteristic peak at 933.2 eV is ascribed to the Cu^I species [41,42].

The Cu^II/Cu_sur ratios of all fresh sample are reduced after aging as shown in

Table 5. Cu 2p_3/2 XPS curve-fitting results of Cu^II−SSZ−13 catalysts before and after hydrothermal aging treatment.

Samples	Cusur (wt%)	CuII/Cusur	CuII/CuI	Sisur (wt%)	Alsur (wt%)	Si/Alsur
F−Cl	0.56	0.44	0.63	6.59	3.42	1.93
A−Cl	0.29	0.15	0.35	6.30	2.66	2.37
F−NO3	0.34	0.30	0.61	6.36	2.44	2.61
A−NO3	0.57	0.17	0.39	6.66	2.95	2.26
F−SO4	0.26	0.38	0.52	6.94	2.50	2.78
A−SO4	0.77	0.24	0.35	6.65	3.36	1.98

. The isolated Cu^II species facilitate the reduction of NO_x at low temperature, which trigger the decrease of catalytic activity of the aged samples [37]. The Cu and Al contents of the A−SO₄ sample increase significantly and can be due to the serious destruction of the framework. However, the Cu and Al contents of A−Cl sample decrease obviously. According to the results of the ²⁷Al Nuclear Magnetic Resonance (NMR) spectra, the signal invisible of octahedral aluminum ions in the fresh and aged catalysts prepared by Cu^IICl₂ is because that paramagnetic Cu ions may interact more strongly with the forming octahedral aluminum, which cause the decrease of surface Cu and Al species [10,43]. Furthermore, the change in XPS curve of Cu^II(NO₃)₂ sample is the least before and after aging. The Si/Al decreases slightly on the surface after aging, which may be due to aging inducing slight de-alumination. Therefore, a small portion of aluminum migrates to the surface, which maintains good anti-aging properties of the skeleton structure.

2.2.7. NMR Results

In order to further study the effects of different copper precursors on the chemical environment of Si and Al in Cu^II−SSZ−13 catalysts, and the effect of hydrothermal aging treatment, ²⁹Si NMR and ²⁷Al NMR were performed for the fresh and aged catalysts. It can be seen from the ²⁷Al NMR spectra of Figure 6a–c that fresh samples show significant Al³⁺ tetrahedral coordination framework features that appear at ∼58.7 ppm [10,11]. The F−SO₄ and F−NO₃ show one relatively small new resonance peak at ∼−1.3 ppm, which come down the extra-framework Al in octahedral [44]. It is worth noting that hydrothermal aging at 800 °C for 20 h can result in some tetrahedral aluminum transferred into penta-coordinated extra-framework aluminum structure. The resonance peak at −1.3 ppm almost disappears. Note also that the portion of Al detached from the molecular sieve framework does not appear at ∼−1.3 ppm. This portion of Al stays adjacent to paramagnetic Cu sites and thus invisible to NMR [43,45]. To better compare the changes of the Al coordination structure before and after the hydrothermal aging treatment, the tetrahedral Al signal area of the fresh sample is taken as a unity. The amount of framework Al in the aged sample is normalized to reveal the dealumination during the hydrothermal treatment. The proportion of tetrahedral Al in A−Cl sample is 0.74 indicating evident dealumination. In addition, the A−SO₄ sample has the strongest pentacoordinated extra-framework aluminum structure resonance peak. The proportion of tetrahedral Al is 0.85. However, the resonance peak intensity of Al³⁺ tetrahedral coordination structure of the A-Nit. sample at 58.7 ppm is sharper. The proportion of tetrahedral Al in A−NO₃ sample is 0.94, which means that the hydrothermal aging process causes little damage to the Cu^II(NO₃)₂ sample skeleton structure. It is consistent with the results of XRD. The Cu^II(NO₃)₂ sample exhibits slight skeleton dealumination and maintains stable SSZ−13 framework structure. It is further confirmed that the Cu^II(NO₃)₂ sample has better resistance to hydrothermal aging.

As seen in Figure 6d–f, wide ²⁹Si-NMR peaks with maxima at −99.8, −105.3, and −110.8 ppm are registered for Cu^II−SSZ−13 catalysts. These peaks are related to the following types: the structure of Si(2Al), the Si(1Al), and Si(0Al) [46,47]. The F−NO₃ sample contains more Si (2Al) and Si (2Al) and Si (1Al) structures. After aging, the peak at −105.3 ppm of three samples is weakened. The Si(1Al) structure of A−SO₄ is severely damaged, and the F−Cl sample contains more Si (0Al) structure. However, two peaks at 105.3 and 110.8 ppm become similar shoulder peaks for the A−Nit sample. Si(1Al) and Si (0Al) structures occupy a larger proportion. The catalytic properties of molecular sieves are mainly depending on the skeleton structure and surface acidity. The Si coordination structures have a significant effect on the acid intensity, which is enhanced in the order of Si(0Al) < Si(4Al) < Si(3Al) < Si(2Al) < Si(1Al) [4,27,48]. It is consistent with the results of NH₃−TPD.

3. Materials and Methods

3.1. Catalyst Preparation

The Cu^II−SSZ−13 catalysts were prepared by in-situ hydrothermal synthesis method using three kinds of copper precursors (Cu^II(NO₃)₂, Cu^IISO₄, Cu^IICl₂). The copper complex (Cu−TEPA) was used as the template. An appropriate amount of silica sol (30.4%), sodium metaaluminate (99%), sodium hydroxide (96%), tetraethylenepentamine (TEPA, 95%), and deionized water, were mixed at the molar ratios of 2.5Al₂O₃:2.5Na₂O:7.4SiO₂:147.7H₂O:1.47Cu−TEPA [49,50]. The formed gel was successively transferred into three 100 mL PTFE (polytetrafluoroethylene) autoclaves and crystallized at 140 °C for 72 h. The products were washed by deionized water, and dried at 100 °C for 12 h. Then, the samples was treated by ion exchange with a certain amount of NH₄NO₃ (1 mol/L) solution for 12 h at 80 °C. Finally, the samples were calcined at 550 °C for 8 h. The obtained Cu^II−SSZ−13 catalyst using Cu^IICl₂, Cu^II(NO₃)₂, and Cu^IISO₄ solution as precursor were labeled as F−Cl, F−NO₃, and F−SO₄, respectively.

3.2. Hydrothermal Aging Treatment of Catalysts

To investigate the hydrothermal stability of the prepared Cu^II−SSZ−13 catalysts, they were aged in a fixed bed reactor containing 10 vol% H₂O under Ar atmosphere at 800 °C for 20 h. The aged catalysts were labeled as A−Cl, A−NO₃, and A−SO₄, respectively.

3.3. Catalyst Activity Evaluation

The catalytic activity was evaluated using a temperature-programmed fixed bed reactor at a gaseous hourly space velocity (GHSV) of 40,000 h⁻¹. As shown in Figure S2, the gaseous hourly space velocity (GHSV) of 40,000 h⁻¹ is preferentially used as the optimal space velocity for the F−NO₃ sample. The reactor contained 0.05% NO, 0.05% NH₃, 5 vol% O₂, and 10 vol% H₂O. He was used as balance gas. The total flow rate 400 mL min⁻¹. The concentrations of NO, NH₃, N₂O, and NO₂ gases were measured in situ by a NICOLET IS10 FTIR spectrometer. The NO conversion and the N₂ selectivity were calculated based on the following equation:

$$X_{\mathrm{NO}}=\frac{C_{\mathrm{NO},\mathrm{in}} {-C}_{\mathrm{NO},\mathrm{out}}}{C_{\mathrm{NO},\mathrm{out}}}\times 100\%,$$

$$S_{{\mathrm{N}}_2}=\left(1-\frac{{2C}_{{\mathrm{N}}_2\mathrm{O},\mathrm{out}}{+C}_{{\mathrm{NO}}_2,\mathrm{out}}}{C_{\mathrm{NO},\mathrm{in}} {+C}_{{\mathrm{NH}}_3,\mathrm{in}}{-C}_{\mathrm{NO},\mathrm{out}} {-C}_{{\mathrm{NH}}_3,\mathrm{out}}}\right)\times 100\%.$$

3.4. Catalyst Characterization

The XRD analysis was carried out on a DX−2700 diffractometer (Dangdong, Dandong Aolong Ray Instrument Group Co., Ltd. China) with Cu Kα radiation (λ = 1.54184 nm) at 40 kV and 30 mA. The dates were collected with a scanning speed of 8°·min⁻¹ that ranged from 5° to 60°. The SEM images of samples were obtained via a Japanese Jeol JSM-7001F instrument (JEOL, Japan) at 10 kV. H₂−TPR and NH₃−TPD experiments were carried out on the Autochem II 2920 instrument (Micromeritics, Norcross, GA, USA). For H₂−TPR measurement, 100 mg of sample was set into a quartz reactor and pretreated at 300 °C for 0.5 h in Ar atmosphere. After cooling to room temperature (25 °C), the gas was switched to a 10 vol% H₂−Ar mixture gas with the temperature increased from 50 to 1000 °C at a rate of 10 °C min⁻¹. For NH₃−TPD measurement, the pretreatment was as same as the H₂−TPR process and then exposed to 10% NH₃/He at 50 °C for 30 min, followed by a temperature ramp to 600 °C at a rate of 10 °C min⁻¹. The pore structure analysis of the samples was acquired on a JW-BK122W N₂ adsorption-desorption instrument (Beijing, JWGB Sci. & Tech. Co., Ltd., China) by using liquid nitrogen at 77 K (−196 °C). XPS measurements were performed on an AXIS ULTRA DLD electron spectrometer (Kratos corporation, UK) with a monochromatic Al Kα (hv = 1486.6 eV) as excitation source. ²⁷Al and ²⁹Si solid state NMR spectra were collected by an Avance Ⅲ 600 MHz instrument (Bruker, UK). ²⁹Si MAS NMR spectra were recorded at a rate of 5 kHz and 7 mm probe. ²⁷Al MAS NMR spectra were recorded at a rate of 13 kHz using a 4 mm probe.

4. Conclusions

In summary, the Cu^II−SSZ−13 catalysts were prepared by in-situ hydrothermal synthesis method using various copper precursors (Cu^II(NO₃)₂, Cu^IISO₄, Cu^IICl₂) for NO selective catalytic reduction with NH₃ in simulated diesel vehicle exhausts. The Cu^II(NO₃)₂ sample not only exhibits outstanding catalytic activity and N₂ selectivity, but also has higher anti-aging properties. The F−NO₃ sample exceeded 90% conversion at a wide range of 180~700 °C. After aging for 20 h at 800 °C, the A−NO₃ sample still exhibits above 90% NO conversion in the temperature range of 240–600 °C. The difference of deNO_x activity and anti-aging performance can be tuned by copper precursor due to the anion’s effect. The Pauling radium of the anion affects the distribution of copper species in the Cu^II-SSZ-13 catalyst. The larger anions could impede the entry of copper ions into the pores and gathered on the out surface of the molecular sieve. In addition, the adsorption capacity of different anions to the cations will affect the nucleation, and the growth process of SSZ−13 can affect the crystallization process. More isolated Cu^II ions are adsorbed in the framework of the Cu^II−SSZ−13 catalyst and served as active sites for the NH₃−SCR reaction resulting in excellent catalytic activity. Furthermore, after hydrothermal aging treatment, the Cu^II(NO₃)₂ sample remains stable in the SSZ-13 framework resulting in maintaining its high catalytic activity. Our work can help to understand the effect of anions originating from copper precursors on the structure and catalytic property and rational design of a Cu^II−SSZ−13 catalyst for NO selective catalytic reduction with NH₃ in diesel vehicle exhausts.