Correlation between Zeolitic SiO₂/Al₂O₃ Ratio and the Low-Temperature NOx Adsorption Capacity of Pd/SSZ-13 under Realistic Exhaust Conditions

Abstract

Passive NOx adsorber (PNA) is one of the important means to effectively reduce NOx emission control in the cold start of a diesel engine. A series of Pd/SSZ-13 catalysts with different SiO₂/Al₂O₃ ratios (11, 17, and 25) were prepared using the impregnation method. Furthermore, the effect of the SiO₂/Al₂O₃ ratio on the adsorption performance of Pd/SSZ-13 at low-temperature NOx under realistic exhaust conditions was studied. The results show that the Pd/SSZ-13 catalyst with a low SiO₂/Al₂O₃ ratio after loading Pd has a higher specific surface area and palladium ion content. There is a negative correlation between NOx adsorption performance and the SiO₂/Al₂O₃ ratio. After high-temperature heat treatment, the acid sites closely related to palladium species increase and palladium species will redisperse, producing more palladium ions. The palladium ions coexist in Pd/SSZ-13 in the form of Pd²⁺ and Pd⁺, among which Pd²⁺ is divided into two types: Z⁻Pd²⁺Z⁻ and Z⁻[Pd(II)OH]⁺. The NOx adsorption performance of the Pd/SSZ-13 catalyst was significantly improved, and the higher the SiO₂/Al₂O₃ ratio, the more obviously the advantage of NOx adsorption performance increased after heat treatment. The NOx adsorption kinetic model of the Pd/SSZ-13 catalyst under realistic exhaust conditions was most suitably described by the pseudo-first-order model.

1. Introduction

Nitrogen oxides (NOx) are generally considered the most important pollutants that threaten the natural environment and human health [1,2]. While the use of lean-burn engines could realize better fuel economy and lower the CO₂ emissions from vehicles, NOx reduction remains a severe issue due to the excessive supply of oxygen during the combustion process [3]. At present, ammonia-selective catalytic reduction (NH₃-SCR) technology is the most commercialized denitrification technology for removing NOx of lean-burn engine vehicle exhausts. However, NH₃-SCR technology can effectively remove NOx only at a high temperature of more than 200 °C. One of the main reasons is that ammonia is supplied through urea solution, and the amount of urea cannot be less than 200 °C, because precipitates such as biuret and cyanuric acid are formed at low temperatures [4,5,6]. With the environmental regulations becoming more stringent in recent years, the problem of cold start has attracted more and more attention in the field of catalysis. In order to solve the problem of NOx emission in the cold start-up phase, a passive NOx adsorber (PNA) is proposed [7,8]. The material can adsorb NOx (mainly in the form of NO) at lower temperatures (<200 °C) and desorb at higher temperatures (200~500 °C) [8]. The desorbed NOx is further removed by a downstream SCR catalyst, thus achieving the purpose of efficiently removing NOx at low temperatures [9,10].

In recent years, research into PNA materials has made considerable progress [1,11,12]. Among the reported PNA materials, Pd/SSZ-13 has excellent NO adsorption and desorption performance, so it has attracted widespread attention [12,13,14,15]. H-SSZ-13 has a CHA topological structure with a pore size of 0.38 × 0.38 nm. It is based on AlO₄ and SiO₄ tetrahedra, and is orderly arranged through the connection of oxygen atoms into a crystal structure with eight-membered ring channels and three-dimensional cross channels. CHA has a three-dimensional vertical channel structure that is straight and unhindered, which is favorable for Pd mobility [16]. Currently, many efforts have been made to investigate the NOx adsorption mechanism and key factors influencing the adsorption performance of Pd/SSZ-13 [17,18,19,20]. As reported, Pd precursors can be introduced into zeolites by ion exchange or impregnation to form different Pd species, such as PdO₂, PdO clusters, and Pd metal particles distributed on the surface of zeolites, and Pd ions at cation exchange sites in the framework of zeolites [17]. The cation exchange site in the zeolite framework is the main active site for NOx adsorption at low temperatures. High Al concentration is beneficial to the dispersion of palladium ions, which makes the catalyst have high NO adsorption capacity at low temperature [18,19]. The Si/Al ratio of zeolite can regulate the metal active center by affecting acidity, and also has a significant effect on the skeleton structure. Khivantsev et al. [19] found that the existence type of Pd species in Pd/SSZ-13 was closely related to the Si/Al ratio by HAADF-STEM. No particles were found on the surface of Pd/SSZ-13 when Si/Al = 6, but there were 1~2 nm PdO nanoparticles when Si/Al = 12 and agglomerated PdO particles when Si/Al = 30. Chen et al. [20] investigated the effect of crystal size on NOx adsorption and desorption over Pd/SSZ-13. The results showed that Pd/SSZ-13 with a smaller crystal size has more external surface acidic and higher Pdⁿ⁺ ion dispersion, showing excellent NOx adsorption performance. In addition, it was found that the NOx adsorption on Pd/SSZ-13 under ideal conditions conforms to the pseudo-second-order model. However, the Si/Al ratio effect on the NOx adsorption kinetics under realistic exhaust conditions over the Pd/SSZ-13 was rarely reported.

In this work, 0.8 wt% Pd/SSZ-13 zeolites with different SiO₂/Al₂O₃ ratios (11, 17, and 25) were prepared using the impregnation method. The NOx adsorption–desorption properties, the nature of Pd species, and acid sites were studied systematically. Finally, the NOx adsorption behavior was investigated through a kinetic study. It is expected that the effect of the SiO₂/Al₂O₃ ratio of Pd/SSZ-13 will extend to other zeolite-supported PNAs.

2. Results

2.1. Chemical Compositions and Structure Characteristics

The chemical compositions and structure properties of three Pd/SSZ-13 samples with different SiO₂/Al₂O₃ ratios were tested and are summarized in

Table 1. Textural properties and chemical composition of different catalysts.

Catalyst	Specific Surface Area (m2·g−1)	Total Pore Volume (cm3·g−1)	Average Pore Diameter (nm)	Pd Loadings (wt.%)	Content of Oxides in Catalysts (wt.%)
					Al2O3	SiO2
Pd/SSZ-13-11	737.527	0.3104	1.684	0.731	13.81	86.11
Pd/SSZ-13-17	699.371	0.3149	1.801	0.790	9.23	90.70
Pd/SSZ-13-25	676.834	0.3264	1.929	0.747	6.43	93.49
Pd/SSZ-13-11-A	701.719	0.3575	2.038	0.744	-	-
Pd/SSZ-13-17-A	672.604	0.3511	2.027	0.794	-	-
Pd/SSZ-13-25-A	653.965	0.3132	1.916	0.798	-	-

. The SiO₂/Al₂O₃ ratios of H-SSZ-13 materials were determined using XRF and the actual SiO₂/Al₂O₃ ratios were 10.6, 16.7, and 24.7, respectively. The actual palladium loading in different catalysts is in the range of 0.731~0.798%, which is slightly lower than the target loading. XRD results for all samples are shown in Figure S1. As shown in Figure S1, all three Pd/SSZ-13 samples display the well-crystallized CHA topology of the support zeolites [20]. Zeolite is a typical porous material with a regular skeleton structure, rich pores, and unique ion exchange centers, which makes it a good candidate to support and disperse exotic metal species. Figure S2 presents the adsorption–desorption isotherms of fresh and aged Pd/SSZ-13 with different SiO₂/Al₂O₃ ratios. The adsorption isotherms of Pd/SSZ-13 samples with different SiO₂/Al₂O₃ ratios are similar before and after aging, indicating that SiO₂/Al₂O₃ ratios have no significant effect on the pore types of Pd/SSZ-13. As shown in Figure S2, the isotherm rises sharply at very low P/P₀, which is closely related to the micropore filling effect. According to the classification of IUPAC, all Pd/SSZ-13 samples show type I isotherms, which are typical adsorption curves of microporous material. The adsorption–desorption isotherms of fresh Pd/SSZ-13 show an obvious hysteresis loop, especially when the P/P₀ is less than 0.3, which may be caused by capillary condensation and swelling. The phenomenon of hysteresis loop basically disappeared after aging. Table 1 shows the specific surface area and pore structure parameters of fresh and aged Pd/SSZ-13 with different SiO₂/Al₂O₃ ratios. It can be seen from this table that among the fresh samples, the Pd/SSZ-13 sample with a low SiO₂/Al₂O₃ ratio has the highest specific surface area of 737.527 m²/g. The specific surface area of Pd/SSZ-13 samples with ratios of 17 and 25 is relatively lower, at 699.371 m²/g and 676.834 m²/g, respectively. After aging, the specific surface area of the Pd/SSZ-13 samples decreased, while the total pore volume and average pore diameter increased. This is because the high temperature treatment leads to the disappearance of the catalyst micropore structure. However, there was almost no significant change in pore volume and average pore diameter of Pd/SSZ-13 samples (SiO₂/Al₂O₃ = 25) before and after aging. Compared with the Pd/SSZ-13 samples with a SiO₂/Al₂O₃ ratio of 11 and 17, the relatively higher SiO₂/Al₂O₃ ratio samples show the smallest specific surface loss, and the total pore volume and average pore diameter do not change obviously before and after aging, showing superior texture properties and thermal stability.

2.2. Zeolite Acidity Distribution

The distribution of zeolite acidity is a very important assistant for anchoring extraneous Pd species. In order to have a better understanding of the detailed acid site variation of different samples, peak-fitting deconvolutions were performed as shown in Figure 1. The peaks centered below 200 °C refer to the weakly hydrogen-bonded or physically adsorbed ammonium on zeolite [21,22,23]. The desorption peaks centered at approximately 200–350 °C and 350–700 °C can be assigned to the NH₃ adsorbed on zeolite hydroxyl groups and from the strong acid sites on zeolites, respectively [24]. H/SSZ-13 with a larger SiO₂/Al₂O₃ ratio possesses a higher amount of total acidity sites. After heat treatment, weak, medium, and strong acid sites are lost. Acidity loss is closely related to the high-temperature dehydrogenation of [AlO₄]⁻H⁺ sites to [AlO₄]⁰ sites through the dehydration and homolytic decomposition of hydroxyls [25,26].

Compared with fresh H/SSZ-13, the NH₃ desorption peaks for the fresh Pd/SSZ-13 shift to lower temperatures. Following the incorporation of Pd, an evidently extra signal was found in the original curve of Pd/SSZ-13 and this could be assigned to the NH₃ interacted with the Pd species dispersed in zeolite [21]. In order to better understand the detailed acid center changes in H-SSZ-13 of different SiO₂/Al₂O₃ ratios introduced by Pd, peak fitting deconvolution was carried out as shown in Figure 2, and the detailed peak integrals are compared in Table S1. Compared with H-SSZ-13 and Pd/SSZ-13, we found an interesting phenomenon that a new acid site appeared at 322 °C after the introduction of Pd, and the strong acid site corresponding to the desorption peak above 363 °C disappear. Similar results were found on Pd/SSZ-13-17 and Pd/SSZ-13-24 catalysts. This is because the stability of Pd species in molecular sieves will consume the strong acid sites, while the newly formed Pd species and agglomerated PdO species can also provide some new acid sites. After Pd was introduced into H-SSZ-13, the acidity was tuned to a large degree. The total amount of NH₃ desorption (acid sites) for Pd/SSZ-13-11, Pd/SSZ-13-17, and Pd/SSZ-13-25 were 651.3, 631.2, 478.3 μmol/g, respectively. Compared with Pd/SSZ-13, the NH₃ desorption peaks for the aged Pd/SSZ-13 shifted to lower temperatures. The total amounts of acid sites for them from a small to a large SiO₂/Al₂O₃ ratio were 272.5, 224.5, and 218.2 μmol/g. The corresponding acid loss rates were 58.2%, 64.4%, and 54.4%. Interestingly, we found that after aging, the area of NH₃ desorption peak which is closely related to palladium species significantly increased, which further indicated that heat treatment was beneficial to the formation of more Pdⁿ⁺ species.

2.3. Pd Distribution

The isolated Pd ions in the sample were determined by Na⁺ titration [25], and the number of PdO can be obtained by subtracting ionic Pdⁿ⁺ from ICP-OES-determined total Pd. As shown in Figure 3, the major Pd species were PdO, and the proportion of ionic Pdⁿ⁺ species in the Pd/SSZ-13 catalyst was about ~10%, which is significantly lower than that of Pd/ZSM-15-type PNA materials reported in the literature [25]. The possible explanation is that the small pore opening of SSZ-13 restricts the diffusion of cationic Pd to the zeolite channel, and the majority of Pd is deposited on the external surface of the zeolite. The higher SiO₂/Al₂O₃ ratio results in the proportion of isolated Pd ions in total Pd loading becoming lower. The Pdⁿ⁺/Pd ratio increased after heat treatment. The results showed that heat treatment promoted the formation of Pdⁿ⁺. It is worth noting that the higher the SiO₂/Al₂O₃ ratio of the Pd/SSZ-13 catalyst after heat treatment, the higher the Pdⁿ⁺ content.

CO-DRIFTS can be used to study the difference in Pd species in the catalysts. Figure 4 showed the dispersion and state of palladium in Pd/SSZ-13 samples with different SiO₂/Al₂O₃ ratios. The CO-DRIFTS profile of Pd/SSZ-13-11 was six peaks of CO adsorption. Four Pd²⁺−CO bands were found on the Pd/SSZ-13-11 sample, i.e., 2211, 2190, 2164, and 2145 cm⁻¹. Among them, the peaks at 2211/2190 cm⁻¹ are due to the adsorption of CO on Z⁻Pd²⁺Z⁻ sites; the peak at 2145 cm⁻¹ with a shoulder peak at 2164 cm⁻¹ is attributed to the Pd²⁺ (OH) (CO), which is formed by CO adsorption on Z⁻[Pd(II)OH]⁺ [17,27,28,29,30,31]. The shoulder peak at 2162 cm⁻¹ is formed because two palladium cations may reside in different locations on H-SSZ-13 [28]. The comparison of CO-DRIFTS spectra of catalysts with different SiO₂/Al₂O₃ ratios shows that the peak at 2162 cm⁻¹ has a high CO adsorbed signal intensity at high SiO₂/Al₂O₃ ratios. The peak at 2125 cm⁻¹ was related to CO adsorbed on “naked” Pd⁺ ions ligating with framework oxygens [17]. The weak vibration peak at 2110 cm⁻¹ belongs to Pd⁰-CO. Compared with the Pd/SSZ-13-11 samples, it was found that the peak intensity corresponding to Pd²⁺ in Pd/SSZ-13-17 is relatively weak, especially the corresponding Pd²⁺ (OH) (CO) peak at 2145 cm⁻¹. However, it is interesting that the intensity of shoulder peak at 2162 cm⁻¹ is enhanced. A similar phenomenon occurred in the Pd/SSZ-13-25 sample, and the peak intensity corresponding to CO on Z⁻Pd²⁺Z⁻ sites significantly decreased. This further shows that with the increase in SiO₂/Al₂O₃ ratio, the species content of Pdⁿ⁺ relatively decreases, which is basically consistent with the results of Na⁺ titration. Figures S3–S5 present CO-DRIFTS results with CO adsorption time collected on Pd/SSZ-13-11, Pd/SSZ-13-17, and Pd/SSZ-13-25, respectively. It can be seen that with the continuous introduction of CO, the CO adsorption peak area corresponding to 2211 and 2190 cm⁻¹ decreases obviously, while the peaks area at 2145 cm⁻¹ increases obviously and shifts to a high wavenumber. It is suggested that Pd³⁺ and Pd²⁺ linked to more active oxygen are more easily reduced by CO, resulting in more highly stable Pd²⁺ ions [17].

2.4. NOx Adsorption Desorption Capacity

Figure 5 shows that the profiles of NOx concentration changed with time in the continuous reaction gas feed at 120 °C. In the NOx adsorption curve, the negative peak below 200 ppm revealed the NO adsorption capacity. As can be seen from Figure 5a, when NO entered the reactor and began to be adsorbed, there was a significant difference in the NO adsorption saturation time of fresh Pd/SSZ-13 zeolites with different SiO₂/Al₂O₃ values, showing a trend of lower adsorption saturation time as the SiO₂/Al₂O₃ values increased. The Pd/SSZ-13-11 shows the largest NOx adsorption amount of 40.3 μmol·g⁻¹, which is higher than that of Pd/SSZ-13-17 (38.3 μmol·g⁻¹) and Pd/SSZ-13-25 (36.6 μmol·g⁻¹) materials, indicating that a smaller SiO₂/Al₂O₃ ratio is favorable for the low-temperature NO adsorption, which may be due to the rich Pdⁿ⁺ of the Pd/SSZ-13 samples with a low SiO₂/Al₂O₃ ratio. Huang et al. [25] also obtained a similar conclusion, namely, that compared to the Pd/Beta zeolites with an Si/Al value of 6, 25, and 260, the lower SiO₂/Al₂O₃ value sample showed the highest NOx adsorption capacity, which was attributed to the formation of PdO with increasing SiO₂/Al₂O₃ ratio. However, the NO saturated adsorption time on Pd/SSZ-13 materials increased after heat treatment, indicating that the heat treatment conditions can improve the NO adsorption capacity of Pd/SSZ-13. This may be closely related to the redispersion of palladium ions after TA treatment. The improvement in NOx adsorption capacity due to TA treatment is shown in Figure 6. A 20.0% (from 40.3 to 48.4 μmol·g⁻¹), 54.6% (from 38.3 to 59.2 μmol·g⁻¹), and 70.0% (from 36.6 to 62.1 μmol·g⁻¹) increase in NOx uptake is observed on Pd/SSZ-13 with an 11, 17, and 25 SiO₂/Al₂O₃ value. This phenomenon is basically consistent with the concentration of Pdⁿ⁺ obtained from Na⁺ titration results.

Previously, CO-FTIR has found that palladium ions in the catalyst mainly exist in the form of Pd²⁺ and Pd⁺, among which Pd²⁺ is divided into two types: Z⁻Pd²⁺Z⁻ and Z⁻[Pd(II)OH]⁺. Therefore, the NO storage paths of Pd/SSZ-13 under realistic exhaust conditions mainly include [17,27,28]:

Z⁻Pd²⁺Z⁻ + NO ↔ Z⁻Pd²⁺(NO)Z⁻

(1)

Z⁻[Pd(II)OH]⁺ + NO + CO ↔ [Pd(II)(NO)(CO)(OH)]⁺Z⁻

(2)

2[Pd(I)]⁺Z⁻ + 2NO ↔ 2[Pd(I)NO]⁺Z⁻

(3)

Figure 7 shows the temperature-programmed desorption curves of three samples after the saturation NOx adsorption at 120 °C. According to peak-fitting deconvolutions, four detailed peaks were derived from the original curve of the fresh sample. In the desorption temperature range, four peaks of the Pd/SSZ-13-11 sample at 221 °C, 332 °C, 390 °C, and 411 °C were observed, which were located in the ideal operation window of Cu-SCR catalysts [32,33]. In contrast to the minor NOx release below 300 °C, stored NOx mainly desorbed over 300~450 °C. The NOx desorption peak above 300 °C is mainly due to the decomposition of NO nitrosyl stored on the active palladium species [34]. According to the integral calculation of the NOx desorption peak (Table S2), the desorption peak areas of Pd/SSZ-13-11, Pd/SSZ-13-17, and Pd/SSZ-13-25 at a high temperature (>300 °C) are 7087.94, 6540.92, and 4789.57, respectively. The NOx adsorbed by the Z⁻Pd²⁺Z⁻ site has high stability, so it has a relatively high desorption temperature [31], which is basically consistent with the results of CO-DRIFTS.

After aging treatment, the NOx desorption amount increased clearly for Pd/SSZ-13. The total desorption peak area of Pd/SSZ-13-11, Pd/SSZ-13-17, and Pd/SSZ-13-25 increased by 32.78%, 59.50%, and 103.03%, respectively. The SiO₂/Al₂O₃ ratio has a significant impact on the thermal stability of Pd/SSZ-13 zeolites as PNA materials. For the aged samples, the Nox desorption temperature is intensely affected. As shown in Figure 7b, the desorption peaks shift to high temperatures. It is worth noting that the Nox adsorption over 180~200 °C can be caused by water release from the adsorption sites, as the adsorption competition of NO and H₂O is well known [17,35].

2.5. Kinetic Study

In order to better study the adsorption behavior of Nox on the Pd/SSZ-13 catalyst, the kinetics was discussed. At present, the low-temperature Nox adsorption kinetics of the Pd/SSZ-13 catalyst under realistic exhaust conditions are not known. Four chemical adsorption kinetic models, pseudo-first-order, pseudo-second-order, intra-particle diffusion, and Elovich, are widely used. The Nox breakthrough data of Pd/SSZ-13-11 at 120 °C was used to examine those models. The feed included 200 ppm NO, 230 ppm CO, 50 ppm C₃H₈, 10% O₂, 6% H₂O, 6% CO₂, and N₂ balance. The linearized form function for each model is described in Figure 8 and Table S3. The results show that the pseudo-first-order model is the most suitable to describe the Nox adsorption kinetic model of the catalyst in realistic exhaust conditions. The linearized equation of the pseudo-first-order model is expressed as:

$$\mathit{ln}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathit{ln}\left({\mathrm{q}}_{\mathrm{e}}\right)-{\mathrm{K}}_{\mathrm{I}}\mathrm{t}$$

where q_t and q_e are the amounts of adsorbed Nox at any time and equilibrium (μmol/g), respectively, and k_I is the rate constant of the pseudo-first-order adsorption. The integrated form becomes:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_{\mathrm{I}}\mathrm{t}}\right)$$

In chemical kinetics, the reaction rate constant k is the quantitative representation of chemical reaction rate. The kinetic parameters k_I are calculated as shown in the inserted table of Figure 8. The results show that the kinetic constant of fresh Pd/SSZ-13PNA is very close (1.295 × 10⁻²~1.686 × 10⁻² s⁻¹), indicating that the adsorption rate is independent of the SiO₂/Al₂O₃ ratio.

3. Experimental Section

3.1. Catalyst Preparation

H-SSZ-13 zeolite (SiO₂/Al₂O₃ = 11, 17 and 25) was obtained from CP Energy Material (Dalian) Co., Ltd., Dalian, China. The Pd(NO₃)₂ solution was obtained from Kunming Sino-Platinum Metals Catalysts Co., Ltd., Kunming, China. The Pd/SSZ-13 catalysts were prepared using wetness impregnation. The Pd(NO₃)₂ solution was slowly added to the H-SSZ-13 suspension, followed by drying at 120 °C overnight and calcinating at 550 °C in static air for 3 h. The final target Pd loading was 0.80 wt%. The obtained Pd-SSZ-13 was named Pd-SSZ-13-11, Pd-SSZ-13-17, and Pd-SSZ-13-25 according to the SiO₂/Al₂O₃ of support zeolite H-SSZ-13 with 10~11, 16~17, and 24~25, respectively. The sample was thermally aged in a muffle furnace at 800 °C for 10 h, respectively. The aged samples were labeled as Pd-SSZ-13-11-A, Pd-SSZ-13-17-A, and Pd-SSZ-13-25-A.

3.2. Characterization

The specific surface area and total pore volume of the catalyst samples were determined by means of nitrogen adsorption–desorption at liquid N₂ temperature (77 K) using a Autosorb-IQ2 apparatus (Quantachrome Instruments, Boynton Beach, FL, USA). The specific surface area was evaluated by the Brunauer–Emmett–Teller (BET) method. The total pore volume was calculated from the single nitrogen absorption amount at the P/P₀ of ~0.98. X-ray diffraction (XRD) patterns of samples were recorded on a Philips X’pert Pro diffractometer (Rigaku Corporation, Tokyo Metropolis, Japan) in reflection mode, with Cu Ka radiation in the 2θ range of 10–50°, with an increment step of 0.02°. The accelerating voltage was set at 36 kV, with 30 mA of working current.

The actual Pt loading was determined by inductively coupled plasma–optical emission spectroscopy (ICP-OES). NaCl titration was performed using the following procedure. The 100 mg catalyst was stirred in 0.1 M NaCl solution (100 mL) at 80 °C for 1 h. After separation by centrifuging, it was exchanged with fresh NaCl solution for secondary ion exchange, and then washed and dried. The amount of palladium retained in zeolites was quantified using ICP-OES, which enabled us to calculate the amount of exchangeable isolated palladium ions in these materials. The mass fraction of oxides such as Al₂O₃ and SiO₂ was determined using a Bruker S4 X-ray fluorescence spectrometer (Bruker Corporation, Billerica, MA, USA).

The ammonium temperature-programmed desorption (NH₃-TPD) experiment was carried out using CHEMBET3000 (Quantachrome Instruments, Boynton Beach, FL, USA) with a thermal conductivity detector. Before the experiments, the samples (100 mg) were pretreated at 500 °C for 1 h in a flow of 10% O₂/N₂, and then cooled to 50 °C. Next, NH₃ (10.0 vol%) flow was introduced into the system for 1 h, and the system was purged using a pure He flow for another hour to remove the physically adsorbed ammonia. The reactor temperature was ramped up to 800 °C at a ramp rate of 10 °C/min.

In situ DRIFTs experiments were carried out on a Thermo-Fisher Nicolet iS50 FTIR (Thermo Fisher Scientific, Waltham, MA, USA)with a mercury cadmium-telluride (MCT) detector. The spectrum collection range was 2000–2300 cm⁻¹, with a resolution of 4 cm⁻¹ and 32 scans. First, the catalyst was pretreated for 30 min in a 10% O₂/N₂ atmosphere at 500 °C, and then switched to N₂ to continue the treatment for 30 min. Adsorption took place in a 1% CO/N₂ atmosphere at 30 °C for 1 h and the spectrum was collected. During the test, the gas flow rate was 200 mL/min.

3.3. NOx Adsorption Desorption Test

The NOx adsorption–desorption test was performed using a continuous flow fixed-bed microreactor system. Before each experiment, the catalyst was pretreated in a 5% O₂/N₂ flow at 500 °C for 30 min. After being cooled to 120 °C, the gas flow was switched to reaction atmosphere (200 ppm NO, 230 ppm CO, 50 ppm C₃H₈, 10% O₂, 6% H₂O, 6% CO₂, and N₂ balance) for 300 s. Experiments were carried out at a gas hourly space velocity (GHSV) of 80,000 mL·h⁻¹·g⁻¹. The outlet gas concentrations were analyzed online using a 2030DBG2EZKS13T MultiGas FT-IR Analyzer purchased from MKS Instruments (Andover, MA, USA). For NO desorption TPD, the catalysts were pretreated at 500 °C in a 5% O₂/N₂ atmosphere for 30 min. Then, when the sample was cooled to 120 °C, the gas flow was switched to the reaction atmosphere (200 ppm NO, 230 ppm CO, 50 ppm C₃H₈, 10% O₂, 6% H₂O, 6% CO₂, and N₂ balance) for saturated adsorption. Finally, the reactor was heated up to 500 °C at a rate of 15 °C/min. The total gas flow rate and reaction atmosphere remained unchanged throughout the test. All tests were repeated three times to ensure the validity of the results.

4. Conclusions

A series of Pd/SSZ-13 catalysts with different SiO₂/Al₂O₃ ratios were prepared using the impregnation method. It was found that the catalysts with a low SiO₂/Al₂O₃ ratio had higher specific surface area and total acid content. After heat treatment, the acid sites closely related to palladium species increase, and palladium species will redisperse at high temperature, resulting in more palladium ions. The palladium ions coexist in Pd/SSZ-13 in the form of Pd²⁺ and Pd⁺, among which Pd²⁺ is divided into two types: Z⁻Pd²⁺Z⁻ and Z⁻[Pd(II)OH]⁺. The NOx adsorption performance of Pd/SSZ-13 catalyst was negatively correlated with the SiO₂/Al₂O₃ ratio, and the NOx adsorption performance of the catalyst was significantly improved after heat treatment. The higher the SiO₂/Al₂O₃ ratio, the more obvious the increase in NOx adsorption performance of the catalyst. The NOx adsorption kinetic model of the Pd/SSZ-13 catalyst under realistic exhaust conditions was most suitably described by the pseudo-first-order model, and the adsorption kinetic constant was in the range of 1.295 × 10⁻²~1.686 × 10⁻² s⁻¹. The properties of Pd/SSZ-13 materials with different SiO₂/Al₂O₃ ratios may be suitable for other analogous Pd/zeolite PNA materials. The dependence of Pdⁿ⁺ distribution on SiO₂/Al₂O₃ can guide the design of high-performance Pd/zeolite PNAs under realistic exhaust conditions.