Catalytic Properties of Pd Modified Cu / SAPO-34 for NO x Removal from Diesel Engine

The Cu/SAPO-34 catalysts with different Cu contents were prepared by in situ hydrothermal synthesis. The selected Cu/SAPO34 was modified by impregnating 1 wt% Pd(NO 3 ) 3 . The morphology and structure of the samples were characterized via XRD and SEM techniques. The effects of Cu contents and the Pd modification on the de-NO x activity of the samples were investigated through the selective catalytic reduction by C 3 H 6 and NH 3 . The Cu contents do not change the skeleton structure of the SAPO-34 crystalline and the Cu/SAPO-34 catalysts with Cu/Si ratios of 0.05, 0.1, and 0.2 have better de-NO x activity than other catalysts.The addition of Pd can improve the de-NO x activity of the Cu/SAPO-34 catalysts. The maximum of NO conversion of samples with Pd could reach 90%. Besides, the effect of aging treatment for Cu/SAPO-34 catalysts with and without Pd on the de-NO x activity was also investigated. The results indicated that the Cu/SAPO-34 catalysts modified by Pd have better antiaging performance than raw samples.


Introduction
NO x can react with hydrocarbons (HC) and produce photochemical smog, which is increasingly highlighted and to be concerned about [1][2][3].Selective catalytic reduction (SCR) is an effective method for controlling the emission of NO x , which has already been used in industry [4][5][6].The development of high efficiency catalyst, as the key technology of SCR, has received more and more concerns [7].Depending on high thermal and hydrothermal stability [8][9][10][11], metal-SAPO-34 has a broad application prospect.The proper pore structure makes it exhibit excellent properties in the region of diesel engine exhaust purification [12].As is known to all, the dispersion of the active sites is closely related to the metal loading for the supported catalysts.When the loading amount of metal is much too high, it could aggregate easily during the calcination process.Therefore, there should be a suitable amount of active components, neither too much nor too little, loading on the support.Meunier et al. [13] found that a maximum NO x conversion could be available on Ag/Al 2 O 3 when the Ag loading was 1%; however, the conversion decreased obviously when Ag loading was 2%.Horiuchi et al. [14] reported that the highest activity could be achieved over Co/Al 2 O 3 when Co loading was 0.5%.These results indicate that the active metal content of the catalysts is very important for the catalytic activity.
As a kind of low-cost active constituent for the SCR of NO, Cu has drawn the attentions of many researchers [15,16], but diesel exhaust is often at high temperatures (>650 ∘ C) and the antiaging capacity is required [17,18].Morever, as one of the noble metals, Pd is a promising candidate for HC-SCR in excess oxygen [19][20][21].In this paper, a series of Cu/SAPO-34 catalysts were prepared by hydrothermal synthesis.The effect of Cu content and Pd modifying on the de-NO x activity were investigated.The morphology and structure of the samples were characterized using XRD and SEM.In addition, the influence of the aging treatment of the prepared catalysts was studied.

Preparation of Catalysts.
The Cu/SAPO-34 catalysts with different Cu contents were prepared by insitu hydrothermal synthesis, and the details were as follows: CuO, H 3 PO 4 , Al(OH) 3 , and silica gel were used as the sources of Cu, P, Al, and Si, respectively.Morpholine (C 4 H 9 NO) was selected as the template.The crystallization gel was prepared according to the mole ratio of  CuO : 0.2 SiO 2 : 0.92 Al(OH) 3 : 0.9 H 3 PO 4 : 1.25 C 4 H 9 NO : 50 H 2 O : 0.01 HF ( = 0, 0.005, 0.01, 0.02, 0.04, and 0.08).Firstly, H 3 PO 4 was added to deionized water, followed by CuO at 80 ∘ C while stirring until dissolved completely.And then, Al(OH) 3 , silica gel, morpholine, and HF were added.The initial gel was loaded into the stainless reaction kettles equipped with a polytetrafluorethylene liner of 200 mL and crystallized for 72 h at 190 ∘ C. The product was washed and then calcined.Finally, the Cu/SAPO-34 catalysts were obtained.And the preparation method had been described in our previous paper [22].
The Pd modified Cu/SAPO-34 catalysts were prepared with pore volume impregnation.The details were as follows: the Cu/SAPO-34 catalysts were impregnated by Pd(NO 3 ) 2 solution(1 mol/L) overnight to give a 0.5% Pd loading, and then dried at 120 ∘ C and calcined at 600 ∘ C for 3 h.

Characterization.
The XRD patterns of powder samples were obtained on a Japanese Rigaku D/MAX2500 diffractometer at 45 kV and 100 mA with CuK radiation ( = 0.154 nm).The scanning rate and range were 8 ∘ /min and 5 ∘ -85 ∘ , respectively.
The SEM images were obtained by a Japanese Jeol Jsm-6700 F at 10 kV.The samples were covered with a thin gold layer before scanning.
The Cu content was measured using an atomic absorption spectroscopy (AAS; Varian AA240FS, USA) with a 324.7 nm Cu testing wavelength, operating at 3.0 mA lamp current and a fuel gas of C 2 H 2 (1700 mL/min).

Activity and Antiaging Capacity
Test.The activities of the Cu/SAPO-34 catalysts for the selective catalytic reduction (SCR) of NO x by C 3 H 6 or NH 3 at atmospheric pressure were determined using a fixed-bed flow microreactor.The devices were composed of a gas-way equipped with a flow controller, fixed-bed quartz reactor (Φ = 8.5 mm), a temperature controller, and a detection system.A total flow rate of 100 mL/min was used for the catalytic activity runs.The feed gas consisted of 0.6% NO, 0.6% C 3 H 6 or 0.6% NH 3 , and 5% O 2 with He as the balance gas.The catalyst powder (40-60 mesh) was placed into the center of the quartz reactor.
Firstly, He gas flow of 100 mL/min was used to sweep off for 2 h in order to eliminate the N 2 residue in the reactor and then switched to the simulated exhaust.The reactor was heated up to 50 ∘ C, holden for 0.5 h, and then risen to 600 ∘ C at a rate of 7 ∘ C/min.NO, NO 2 , O 2 , and C 3 H 6 in the gases before and after the catalytic reaction were analyzed simultaneously online by gas chromatograph (GC-9890A, Shanghai Linghua Instrument Company Limited) equipped with column Porapak Q for separating N 2 O and CO 2 and column molecular sieve 5A for separating N 2 , O 2 and CO.
The aging treatment is also carried out using the abovementioned fixed-bed flow microreactor.Catalyst samples were exposed to a stream of gases containing 0.03% SO 2 and 10% vapor balanced with Ar with the total rate of 400 mL/min at the temperature of 720 ∘ C for 10 h.Then, the same way as described above was used to test the aged catalyst activity.
NO conversion was calculated using the following equation: where  is the conversion of NO and  1 and  0 are the concentrations of NO x (NO, NO 2 , and N 2 O) before and after the reaction, respectively.1 ((a): C 3 H 6 -SCR and (b): NH 3 -SCR).When the reductant was C 3 H 6 , the NO conversions of all catalysts were below 70% in the temperature range of 100-600 ∘ C; Particulary, that of the catalyst without Cu was below 20%.It can be also seen that the NO conversions of the catalysts with Cu/Si atom ratios 0.05, 0.1 of and 0.2 were higher than these with Cu/Si atom ratios of 0.025 and 0.4.The NO conversions of all Cu/SAPO-34 catalysts increased as the temperature increases from 100 ∘ C to 600 ∘ C.Moreover, the NO conversions of all Cu/SAPO-34 catalysts increased sharply at low temperature, while it increased slowly at high temperature.

Results and Discussion
When the reductant was NH 3 , the NO conversions of all catalysts were much higher than using C 3 H 6 as the reductant in the temperature range of 100-600 ∘ C. Particulary, at 600 ∘ C, the NO conversions of the Cu/SAPO-34 catalysts can reach to about 96% using NH 3 as the reductant, while it only reached 65% using C 3 H 6 as the reductant.In addition, like C 3 H 6 , the NO conversions of the catalysts with Cu/Si atom ratios from 0.05 to 0.2 were higher than these with Cu/Si atom ratios of 0.025 and 0.4.
On the basis of the above results, it can be concluded that as the Cu/Si atom ratios of the crystallization gel were in the range of 0.05-0.2, the de-NO x activity of the Cu/SAPO-34 catalysts was better than others.
The XRD patterns of the Cu/SAPO-34 catalysts with different Cu contents are shown in Figure 2. It can be obviously found that all samples have SAPO-34 characteristic peaks around 2 = 9.45 ∘ ∼9.65 ∘ , 16.0 ∘ ∼16.2 ∘ , 17.85 ∘ ∼18.15 ∘ , 20.55 ∘ ∼ 20.9 ∘ , 24.95 ∘ ∼25.4 ∘ , and 30.5 ∘ ∼30.7 ∘ , indicating that SAPO-34 was successfully formed using in-situ hydrothermal synthesis.As the Cu content increases, the peaks position of all the samples do not change.It indicates that the amount of  Cu content has no effect on the CHA structure of SAPO-34.Besides, it should be noted that the peaks became higher and sharper as the SAPO-34 was modified by Cu because the introduction of Cu into the framework could reduce the crystal defects and increase the crystallinity [23][24][25][26].It is also found that no peaks related to CuO and Cu 2 O species appeared in the XRD spectrum of catalyst, indicating that the Cu loadings were low or the Cu species were not big enough to be detected [27,28].
The SEM images of the Cu/SAPO-34 catalysts with different Cu contents are displayed in Figure 3.The morphology features of the Cu/SAPO-34 catalysts with different Cu contents were compared with those without Cu.It can be found that all the samples were cubic crystals with similar averaged sizes (0.6-2 m), which suggested that the addition of Cu did not change the CHA structure and crystal morphology of SAPO-34.It is consistent with the XRD results.When the Cu/Si atom ratios were 0 and 0.025, there was no random-shaped material around the cubic crystals.Some random-shaped materials appeared as the Cu content in Cu/SAPO-34 catalysts increased.When the Cu/Si atom ratio was 0.2, the surface of the crystals was nearly covered with the random-shaped material.When the Cu/Si atom ratio was 0.4, big cubic SAPO-34 crystals were formed with the random-shaped material.It should be noted that the relative crystallinity of all the samples was above 92% (Table 1).It means that the random-shaped material was not copper oxide and they could be SAPO-34 crystals with different crystal morphologies.Thus, it can be deduced that the high Cu content could change the morphology of SAPO-34 crystal.
For clarifying the effect of different Cu contents in Cu/SAPO-34 explicitly, the physical properties of samples are summarized in Table 1.It is notable that the Cu/Si atom ratios of Cu/SAPO-34 catalysts were quite different from these of precursor solution, but the trend was inconsistent.It indicated that only a part of Cu could enter into the pore and the skeleton of SAPO-34.In addition, it can be seen from Table 1 that the range of grain sizes of the Cu/SAPO-34 catalysts became smaller as the Cu/Si ratio increases from 0 to 0.2.The smaller grain sizes may be beneficial to the catalytic activity.The sample  (Cu/Si = 0.2) has the highest crystallinity and the smallest grain sizes among the six samples, which is consistent with its high de-NO x activity.

Activity Test of Pd Modified
Cu/SAPO-34.For further improving the de-NO x activity of the Cu/SAPO-34 catalysts, the noble metal Pd was introduced into the Cu/SAPO-34 catalysts with Cu/Si = 0.05 and 0.2 by impregnated using Pd(NO 3 ) 2 as precursor.The Pd loading on catalyst was 0.5% (denoted as PdCu/SAPO-34). Figure 4 illustrates the NO conversions of the PdCu/SAPO-34 catalysts.The experimental GHSV is 40,000 h −1 , which is close to the real diesel engine exhaust.When C 3 H 6 was used as reducing agent and the temperature was below 300 ∘ C, the NO conversions of the PdCu/SAPO-34 catalysts were similar to the samples without Pd.However, when the temperature rises from 300 ∘ C to 600 ∘ C, the advantage of Pd became significant.The NO conversions of PdCu/SAPO-34 catalysts were much higher than these of Cu/SAPO-34 catalysts and it can reach to around 70%.It is also found that the de-NO x activities of the PdCu/SAPO-34 catalysts with Cu/Si = 0.2 were superior to these of Cu/Si = 0.05, especially, at the higher temperature.
When NH 3 was used as reducing agent, the NO conversions of the PdCu/SAPO-34 catalysts were similar to those of the Cu/SAPO-34 catalysts, especially, at the low temperature.The de-NO x activities of the PdCu/SAPO-34 catalysts with Cu/Si = 0.2 were higher than these with Cu/Si = 0.05, and the maximum of the NO conversion can reach to 90%.That is because the reaction between NH 3 and NO with low activation energy is quite easy [29].According to the previous reports [30], the ammonium ion would react with the NO-Pd 2+ species to give N 2 .Similarly, the activity of the Pd modified Cu/SAPO-34 samples was superior to those without Pd, especially, at the high temperature.decreased sharply after aging for C 3 H 6 -SCR, especially, when the temperature is in the range of 200-600 ∘ C. For NH 3 -SCR, the NO conversions of the two kinds of Cu/SAPO-34 catalysts decreased slightly at low temperature, but it decreased significantly as the temperature was above 200 ∘ C, particularly, for the sample with Cu/Si = 0.05.

Antiaging Performance of Catalysts
Figure 5 illustrates the XRD patterns of the Cu/SAPO-34 catalysts before and after aging.The intensities of the characteristic peaks weakened after aging compared with the fresh samples, which indicated that the aging process had a negative effect on the crystallinity.It may be the reason for the decrease of the de-NO x activity.
In order to investigate the effect of aging on the morphology and structure of the Cu/SAPO-34 catalysts, the samples before and after aging were characterized by SEM and the results are shown in Figure 6.Comparing with the fresh samples, more random-shaped materials were found around the crystal particles; the smooth and regular crystal grains were partly destroyed, which leads to the decrease of the NO conversion of the Cu/SAPO-34 catalysts.

PdCu/SAPO-34
Catalysts.The activity test of the PdCu/SAPO-34 catalysts before and after aging in C 3 H 6 and NH 3 is also shown in Figures 4(b) and 4(d).The NO aged samples had more random-shaped materials which were the fragment of SAPO-34 crystals.It suggested that more SAPO-34 crystals had been broken after aging treatment, which resulted in the decrease of the NO conversion of the PdCu/SAPO-34 catalysts.
It can be concluded that the de-NO x activity of the Cu/SAPO-34 and PdCu/SAPO-34 catalysts had a decrease due to the rupture of SAPO-34 crystals resulted by the aging treatment.But the antiaging performance of PdCu/SAPO-34 catalysts was better than Cu/SAPO-34 catalysts.

Conclusions
The Cu/SAPO-34 catalysts were successfully prepared by insitu hydrothermal synthesis.The results of XRD and SEM analysis indicated that the addition of Cu does not change the CHA structure of SAPO-34 crystal and the prepared Cu/SAPO-34 catalysts were cubic crystals with similar averaged sizes (0.6-2 m).But the high Cu content could change the morphology of the SAPO-34 crystals.The Cu/SAPO-34 catalysts with Cu/Si = 0.05, 0.1, and 0.2 had better de-NO x activities than other Cu/SAPO-34 and SAPO-34 catalysts.
The antiaging performance of the Cu/SAPO-34 and PdCu/SAPO-34 catalysts was studied and the experiment results suggested that the de-NO x activity of the Cu/SAPO-34 and PdCu/SAPO-34 catalysts had a decrease due to the rupture of SAPO-34 crystals resulted by the aging treatment.However, the addition of Pd could improve the de-NO x activity and antiaging performance of the catalysts.

4 Figure 2 :
Figure 2: XRD patterns of SAPO-34 catalysts with different Cu contents in the crystallization gel.

Table 1 :
Physical properties of the Cu/SAPO-34 catalysts with different Cu contents.