Investigation of Various Pd Species in Pd/BEA for Cold Start Application

Abstract

A series of Pd/BEA catalysts with various Pd loadings were synthesized. Two active Pd²⁺ species, Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺, on exchanged sites of zeolites, were identified by in situ FTIR using CO and NH₃ respectively. Higher NO_x storage capacity of Z⁻-Pd²⁺-Z⁻ was demonstrated compared with that of Z⁻-Pd(OH)⁺, which was caused by the different resistance to H₂O. Besides, lower Pd loading led to a sharp decline of Z⁻-Pd(OH)⁺, which was attributed to the ‘exchange preference’ for Z⁻-Pd²⁺-Z⁻ in BEA. Based on this research, the atom utilization of Pd can be improved by decreasing Pd loading.

1. Introduction

The exhaust regulation on NO_x emissions is getting more stringent [1]. Presently, NH₃ selective catalytic reduction (SCR) [2] and NO_x storage reduction (NSR) [3] are widely used for NO_x removal. However, standard NO_x aftertreatment technologies fail to function efficiently at low temperatures, which results in a large proportion of the tailpipe NO_x emission [4]. Meanwhile, high efficiency internal combustion engines require new and/or improved technologies which specifically address their low exhaust temperatures. In response to difficulties of low temperature emissions control, numerous efforts are underway to develop catalysts that light-off at temperatures below 150 °C. Passive NO_x adsorbers (PNAs) could play a critical role in enabling high efficiency advanced combustion systems.

Recently, Pd/zeolite serving as passive NO_x adsorbers (PNAs) was first proposed by Chen et al. [4]. This catalyst is emerging as effective passive NO_x adsorbent technology because of its NO_x storage/release capabilities, resistance to sulfur poisoning and hydrothermal deactivation [4,5,6,7,8]. Due to these excellent characteristics, Pd/zeolite has attracted great interest recently and has been further optimized [6,7,8,9,10,11,12,13,14,15,16,17,18].

Several recent studies show that isolated Pd ions in Pd/zeolite are the main active sites for NO_x trapping [6,7,11,13,16]. It is reported that there are nine skeletal T sites with various chemical environments in BEA [19]. So, isolated Pd ions located in various positions of zeolite framework are likely to be formed. As reported by Gao et al. and Giordanino et al. [20,21,22], two kinds of isolated Cu species (Cu²⁺ and [Cu(OH)]⁺) are identified on various framework positions of Cu/SSZ-13, which indicates that species of isolated cations may be influenced by their locations in zeolites. They also mentioned that Cu species were significantly affected by Cu loading. So, various isolated Pd species may co-exist in Pd/BEA, and Pd loading may be capable of influencing the content of them. Actually, two kinds of active isolated Pd ions (bare Pd²⁺ and Pd(OH)⁺) have been observed by Zheng et al. [10] and Khivantsev et al. [12]. However, they did not point out the difference in adsorption between these two species.

Therefore, these two kinds of isolated Pd ions were further studied in this research. A series of in situ FTIR experiments in CO and NH₃ were carried out, and a semiquantitative method was adopted to distinguish these two species. Based on this method, the NO_x storage capacity of each isolated Pd species was compared. Further, by increasing the proportion of isolated Pd species with higher NO_x storage capacities, the atom utilization of Pd can be improved.

2. Results

2.1. Ex-Situ FTIR

Ex-situ FTIR spectra of each sample are exhibited in Figure 1. Compared with 0-Pd (Figure 1a), an extra vibration at 926 cm⁻¹ is observed on the FTIR spectrum of 1-Pd. This peak is attributed to the vibration distortion of the skeletal T–O–T bond due to the strong interaction of Pd ions [4]. This is an obvious piece of evidence for the existence of Pd ions on the exchange sites of zeolites. Peaks at 926 cm⁻¹ appear on spectra of 0.2-Pd, 0.5-Pd and 2-Pd, too (Figure 1b). So, the existence of Pd ions on exchange sites in these samples is confirmed.

Compared with 1-Pd (Figure 1a), the peak at 926 cm⁻¹ disappears in the spectrum of Na-1-Pd, which indicates the elimination of isolated Pd ions on exchange sites. This is firm evidence of which isolated Pd ions in Na-1-Pd have been completely exchanged by Na⁺ ions through the titration process. So, Pd loaded on Na-1-Pd mainly exists in form of Pd oxidations.

2.2. Na⁺ Titration

Na⁺ titration was adopted to measure the content of isolated Pd ions in each sample [23], and the proportion of isolated Pd ions in total Pd loading (marked as Isolated Pd/Pd loading in

Table 1. The content of Pd ions on exchange sites of each sample.

Catalyst	0-Pd	0.2-Pd	0.5-Pd	1-Pd	2-Pd
Isolated Pd (wt %)	0.00	0.22	0.53	0.87	0.91
Isolated Pd/Pd loading (%)	0.00	95.7	96.4	77.7	42.7

) was further calculated. Considering that Pd is sensitive to Cl [24,25], the titration process was carried out in a NaNO₃ solution. Besides, this measurement is believed to have no negative effect on zeolite structures since the titration process was carried out in mild conditions and no calcination was done. As shown in Table 1, lower Pd loading leads to less isolated Pd²⁺ content, whereas the proportion of isolated Pd ions in total Pd loading becomes larger. This phenomenon indicates that the increase of Pd loading leads to the formation of more Pd oxidations (PdOx), which is consistent with Jaeha Lee et al.’s study [11].

2.3. CO In Situ FTIR

Pd species were further probed by CO using in situ FTIR, and the result is displayed in Figure 2. Peaks below 2100 cm⁻¹ are attributed to CO signals on metallic Pd (Pd⁰) formed via CO reduction [10,26], among which Pd⁰−CO (atop) are found at 2098 cm⁻¹, 2076 cm⁻¹ and Pd₂⁰−CO (bridging) are found at 1951 cm⁻¹. Besides, the peak at 2117 cm⁻¹ is attributed to the C-O vibration on Pd⁺ [10,27]. As reported by Vu et al. [9], Pd⁺ is formed by the CO reduction of ion-exchanged Pd species [9]. CO signals on isolated Pd²⁺ species are observed above 2100 cm⁻¹. The peak at 2152 cm⁻¹ with a shoulder peak at 2137 cm⁻¹ is attributed to the C-O vibration on isolated Pd²⁺ bonded with the hydroxy of zeolites (marked as Z⁻-Pd²⁺-Z⁻) [10]. The peak at 2179 cm⁻¹ is attributed to the vibration of C-O adsorbed by another kind of isolated Pd²⁺ [10], which was first determined as Z⁻-Pd(OH)⁺ by Okumura et al. [28].

In short, two kinds of isolated Pd²⁺ (Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺) were identified in 0.2-Pd, 0.5-Pd, 1-Pd and 2-Pd. Note that all spectra in Figure 2 were obtained when the steady state had been achieved, and corresponding peaks of Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺ were still observed. So, these two isolated Pd²⁺ species cannot be completely reduced by CO at this temperature, which indicated that the reduction reaction is a reversible one. As shown in Figure S1, the existence of Z⁻-Pd²⁺-Z⁻ in 1-Pd-80 is also demonstrated by CO whereas no Z⁻-Pd(OH)⁺ is observed. Since the complete reduction of Z⁻-Pd(OH)⁺ cannot be achieved by CO, there is only one isolated Pd²⁺ species, Z⁻-Pd²⁺-Z⁻, in 1-Pd-80 (see Figure S1).

2.4. Catalyst Evaluation

Profiles of the NO adsorption stage are shown in Figure 3. NO_x storage capacities are calculated by the integration of negative peaks on these profiles, and the dead volume has been subtracted. The result is displayed in

Table 2. NO_x storage capacities in NO_x, CO, H₂O, O₂ and CO₂.

Catalyst	0-Pd	0.2-Pd	0.5-Pd	1-Pd	2-Pd
NOx storage capacity (μmol/gcat)	3.7	21.6	47.5	53.3	50.1

. Note that samples with the same Pd loading as much as 1 wt % (see Figure S2) exhibit entirely different NO_x storage capacities (53.3 μmol/gcat and 9.9 μmol/gcat respectively), which indicates that only part of Pd loaded on samples is efficient. As shown in Figure 3, NO_x storage capacities of 0-Pd and Na-1-Pd are very low. So, Brønsted hydroxyl group and PdOx are inefficient active centers for NO_x storage, whereas they do trap NO_x in this condition as reported [10,27]. Since the NO_x storage capacity of 1-Pd is much higher, isolated Pd²⁺ ions are likely to be the main active sites for NO trapping.

2.5. NH₃ In Situ FTIR

Acidity over samples is probed by NH₃ with in situ FTIR, and the result is exhibited in Figure 4. The peak at 1463cm⁻¹ observed in both spectra of 1-Pd and 0-Pd (Figure 4a) is attributed to the vibration of NH⁴⁺ in Brønsted hydroxyl groups (NH⁴⁺-B) [29]. In the spectrum of 1-Pd, two additional peaks at 1625 cm⁻¹ and 1313cm⁻¹ corresponding to the vibration of NH₃ on Lewis acid (NH₃-L) [30,31] is observed. Nevertheless, these two peaks do not exist in Na-1-Pd in which isolated Pd²⁺ ions (Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺) are replaced by Na⁺ completely. So, it is reasonable to believe that Lewis acid is generated from Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺ species.

As shown in Figure S3, there is only one peak at 1625cm⁻¹ corresponding to the vibration of NH₃-L observed in the spectrum of 1-Pd-80. Since there is only one kind of isolated Pd²⁺, Z⁻-Pd²⁺-Z⁻, in this sample as discussed above, the peak at 1625cm⁻¹ should be assigned to the vibration of NH₃-L originated from Z⁻-Pd²⁺-Z⁻. In this case, the peak at 1313cm⁻¹ should be attributed to the vibration of NH₃-L originating from Z⁻-Pd(OH)⁺.

Since Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺ are capable of being probed by NH₃, the ratio of the height of the peaks at 1625 cm⁻¹ (P_Z⁻-_Pd²⁺-_Z⁻) and 1313 cm⁻¹ (P_Z⁻-_Pd(OH)⁺) in Figure 4b can be defined as the relative content between these two isolated Pd²⁺ species. Considering that extinction coefficients for NH₃ molecules adsorbed on Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺ are both constants, they have no effect on the tendency of peak height ratios. So, the extinction coefficient is not considered in this part [32,33,34]. The result is shown in Figure 5a. It is worth nothing that P_Z⁻-_Pd(OH)⁺/P_Z⁻-_Pd²⁺-_Z⁻ rises in parallel with the increase of isolated Pd²⁺, which means that the higher content of isolated Pd²⁺ leads to a much more obvious increase of Z⁻-Pd(OH)⁺ than that of Z⁻-Pd²⁺-Z⁻. So, isolated Pd²⁺ in 0.2-Pd mainly exists in the form of Z⁻-Pd²⁺-Z⁻, whereas a large amount of Z⁻-Pd(OH)⁺ ions are formed in 2-Pd.

Mole ratios of NO_x adsorbed to isolated Pd²⁺ for each sample (marked as NO/Pd²⁺) are calculated via data in Table 1 and Table 2. Figure 5b is plotted by P_Z⁻-_Pd(OH)⁺/P_Z⁻-_Pd²⁺-_Z⁻ on the horizontal axis and NO_x/Pd²⁺ on the vertical. As reported, NO_x is believed to be trapped in the form of Pd^ƍ+-NO [12,15], so the maximum NO_x/Pd²⁺ ratio is 1 in theory. However, the downward trend between P_Z⁻-_Pd(OH)⁺/P_Z⁻-_Pd²⁺-_Z⁻ and NO_x/Pd²⁺ means that higher P_Z⁻-_Pd(OH)⁺/P_Z⁻-_Pd²⁺-_Z⁻ leads to lower NO_x storage capacity of isolated Pd²⁺, which indicates that the NO_x storage capacity of Z⁻-Pd(OH)⁺ is obviously lower than that of Z⁻-Pd²⁺-Z⁻ in this condition.

A further discussion is given below so as to give a reasonable explanation from the perspective of the structure-function relationship.

In BEA zeolite, nine skeletal T sites with various chemical environments are determined [19]. Among these T sites, T5 and T6 are demonstrated to have the lowest Al substitution energy [35]. Thus, Al substitution on these two sites (marked as Al_T5 and Al_T6) is preferential, and the amount of Al_T5 and Al_T6 is larger than that of Al atoms on the other T sites in BEA. Besides, exchange sites formed on these two Al atoms are more active due to the lowest deprotonation energy [36] of them. Meanwhile, Al_T5 and Al_T6 are located in the meta-position of the same five-membered ring [37], and the co-ion-exchange of protons on these two exchange sites formed on Al_T5 and Al_T6 is feasible. These characteristics of BEA can explain the ‘exchange preference’ for Z⁻-Pd²⁺-Z⁻ formed in Pd/BEA with lower content of isolated Pd ions to some extent. Besides, with the increase of Pd loading, more exchange sites are taken up. Limited by the amount of protons which are capable of being exchanged, Z⁻-Pd(OH)⁺ ions, which take up less exchange sites than Z⁻-Pd²⁺-Z⁻, are preferred to be formed. So, higher content of isolated Pd²⁺ leads to much more obvious increase of Z⁻-Pd(OH)⁺ than that of Z⁻-Pd²⁺-Z⁻.

As discussed above, the maximum NO_x/Pd²⁺ ratio is 1 in theory. As reported by Khivantsev et al. [12], H₂O molecules can occupy NO_x storage sites due to strong hydration of isolated Pd²⁺ species. Since isolated Pd²⁺ is the main active site for NO_x storage, the NO_x/Pd²⁺ ratio of Pd/zeolite-based PNA materials will significantly smaller than 1 in the presence of H₂O. However, as shown in Figure 5b, NO_x/Pd²⁺ of 0.2-Pd is as much as 1, which indicates that isolated Pd²⁺ in this sample is not occupied by H₂O. As discussed above, isolated Pd²⁺ in 0.2-Pd mainly exists in the form of Z⁻-Pd²⁺-Z⁻. Thus Z⁻-Pd²⁺-Z⁻ may be insensitive to H₂O in this condition. Furthermore, Z⁻-Pd(OH)⁺ probably tends to hydrate due to the hydrogen bond interaction between H₂O and hydroxy. Accordingly, the difference in NO_x storage capacities of Z⁻-Pd(OH)⁺ and Z⁻-Pd²⁺-Z⁻ is probably caused by the different resistance to H₂O in this condition.

The atom utilization of Pd can be represented by mole ratios of NO_x adsorbed to total Pd loading. According to Figure 6, the Pd utilization of 0.2-Pd is as much as 100% whereas that of 2-Pd is only 25%. So, it is obvious that lower Pd loading benefits to the increase of Pd utilization. Besides, Khivantsev et al. [13] have reported that the Pd utilization of the 1 wt% Pd/SSZ-13 (Si/Al ratio = 6) sample is 100%, which is much higher than that of 1-Pd (Si/Al ratio = 16) using BEA as a support. Accordingly, zeolite structure and Si/Al ratio can also influence the Pd utilization. In this part, only the influence of Pd loading will be discussed.

As discussed above, lower Pd loading leads to lower content of isolated Pd²⁺, whereas the proportion of isolated Pd ions in total Pd loading becomes larger. On the on hand, a larger proportion of isolated Pd ions in total Pd loading is beneficial for the improvement of Pd utilization, since isolated Pd ions are identified as the main active center for NO_x storage. On the other hand, less isolated Pd²⁺ results in a preference for the formation of Z⁻-Pd²⁺-Z⁻ which exhibits higher NO_x storage capacity. Thus, the atom utilization of Pd can be improved by decreasing Pd loading.

Nevertheless, lower Pd loading also leads to less NO_x storage capacities of unit mass of Pd/BEA. Note that the coating amount of catalysts is limited in order to obtain acceptable back pressure. Accordingly, to determine optimum Pd loading for low temperature NO_x adsorption, the NO_x storage capacity and the atom utilization of Pd should be both considered. As shown in Figure 6, 0.2-Pd and 0.5-Pd exhibits much higher Pd atom utilization than that of the other two samples. Meanwhile, Pd atom utilization of 0.5-Pd is 92%, which is only slightly lower than that of 0.2-Pd. However, the NO_x storage capacity of 0.5-Pd is larger than twice of that of 0.2-Pd. So, 0.5wt% should be determined as the optimum Pd loading for Pd/BEA (Si/Al ratio = 16) served as PNA material.

3. Materials and Methods

3.1. Catalyst Preparation

H-BEA zeolite (Si/Al = 16.2) was supplied by Novel Chemistry. Pd/BEA samples with various Pd loading were prepared by incipient wetness impregnation, and Pd(NO₃)₂ solution (15.47wt% Pd, Heraeus Materials Technology Shanghai Ltd., Shanghai, China) was used. Then, the powders obtained were dried under ambient temperature followed by a 4-h calcination at 550 °C in air. Finally, samples were stabilized at 750 °C for 12 h in air with 10% H₂O. Pd loading of each sample was detected by inductively coupled plasma (ICP) analysis (USA Agilent 5100 ICP-OES, Santa Clara, CA, USA). Si/Al ratios were measured by X-ray fluorescence (XRF, ThermoFisher PERFORM’X, Waltham, MA, USA) analysis. Detailed information is exhibited in

Table 3. Pd loading of samples.

Catalyst	0-Pd	0.2-Pd	0.5-Pd	1-Pd	2-Pd	Na-1-Pd
ICP Pd (wt %)	0	0.23	0.55	1.12	2.13	0.25

. In the following text, samples are abbreviated as x-Pd, where “x” represents the Pd loading of corresponding samples. Besides, the sample treated by Na⁺ titration was named as Na-1-Pd.

3.2. Catalyst Characterization

Na⁺ titration was used to quantify isolated Pd ions as reported by Ogura et al. [23]. Each sample was mixed with NaNO₃ (Purity above 99.0%, Tianjin Jiangtian Chemical Technology Co., Ltd., Tianjin, China) solution (0.1M) at 80 °C. The solution was stirred for 4 h followed by suction filtration. At last, the powders obtained were washed by deionized water and dried at 100 °C for 6 h. The whole process above was repeated three times. By analyzing the change of Pd loading in each sample after the titration process, the content of isolated Pd can be measured.

NO_x storage capacities were measured by standard catalyst evaluation tests carried out in a plug flow reactor system. 0.25 g sample (60–80 mesh) mixed with 0.75 g quartz (60–80 mesh) was loaded in a quartz reactor with a thermocouple. The sample was oxidized in the flow of 10% O₂/N₂ for 30 min at 500 °C and was cooled to 80 °C in N₂. Then the flow (200 ppm NO_x, 200 ppm CO, 5% CO₂, 5%H₂O, 10% O₂, balanced with N₂) mixed in the bypass in advance was introduced into the reactor. The NO_x adsorption stage continued for 3 min. After that, the sample was heated to 500 °C with a ramping rate of 10 °C/min in the flow of N₂. Gas concentrations were measured by an online MKS MultiGas 2030 FTIR gas analyzer in the whole process. Besides, a space velocity of 28,800 h⁻¹ was adopted.

Ex-situ FTIR was carried out on a Nicolet iS10 FTIR spectrometer equipped with a liquid N₂ cooled mercury cadmium telluride (MCT) detector to identify the existence of Pd ions. The sample (20 mg) was pressed into a self-supporting wafer with a diameter of 13 mm and was inserted into a cell sealed with ZnSe windows connected with a gas manifold. KBr was used to obtain background spectra. All samples, as well as KBr, were oxidized in the flow of 10% O₂/N₂ for 30 min at 500 °C in advance. Spectra were obtained at 200 °C in N₂.

In situ FTIR was carried out on a Nicolet iS10 FTIR spectrometer, too. Test temperatures and feed compositions varied according to the needs of different experiments, and detailed information will be reported below.

4. Conclusions

In this work, two isolated Pd²⁺ ions, Z⁻-Pd²⁺-Z⁻ and Z⁻-Pd(OH)⁺, on exchange sites of zeolites are confirmed as the main active sites for NO trapping in cold-start applications. Lower Pd loading leads to a lower content of isolated Pd²⁺ whereas the proportion of isolated Pd ions in total Pd loading becomes larger. Lower content of isolated Pd²⁺ further leads to a sharp decline of Z⁻-Pd(OH)⁺, which is attributed to the ‘exchange preference’ for Z⁻-Pd²⁺-Z⁻ in BEA. Besides, the higher NO_x storage capacity of Z⁻-Pd²⁺-Z⁻ is demonstrated compared with that of Z⁻-Pd(OH)⁺, which is caused by the different resistance to H₂O. In conclusion, the atom utilization of Pd can be improved by using lower Pd loading. 0.5wt% should be determined as the optimum Pd loading for Pd/BEA (Si/Al ratio = 16) serving as PNA material among all samples.